Revised 2023-06-09 at 14:25:43 UTC
Section: 31 [input.output] Status: Open Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2018-12-09
Priority: 3
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Discussion:
A third party test suite tries to exercise istream::ignore(N) with a negative value of N and expects that the implementation will treat N as if it were 0. Our implementation asserts that (N >= 0) holds and aborts the test.
I can't find anything in section 27 that prohibits such values but I don't see what the effects of such calls should be, either (this applies to a number of unformatted input functions as well as some member functions of the basic_streambuf template).
[ 2009-07 Frankfurt ]
This is related to LWG 255.
Move to NAD Future.
[LEWG Kona 2017]
Recommend Open: We agree that we should require N >= 0 for the selected functions
[2018-12-04 Reflector prioritization]
Set Priority to 3
Proposed resolution:
I propose that we add to each function in clause 27 that takes an argument, say N, of type streamsize a Requires clause saying that "N >= 0." The intent is to allow negative streamsize values in calls to precision() and width() but disallow it in calls to streambuf::sgetn(), istream::ignore(), or ostream::write().
[Kona: The LWG agreed that this is probably what we want. However, we need a review to find all places where functions in clause 27 take arguments of type streamsize that shouldn't be allowed to go negative. Martin will do that review.]
Section: 25.3.5.3 [input.iterators] Status: Open Submitter: Chris Jefferson Opened: 2004-09-16 Last modified: 2018-01-27
Priority: 2
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Discussion:
From comp.std.c++:
I note that given an input iterator a for type T, then *a only has to be "convertable to T", not actually of type T.
Firstly, I can't seem to find an exact definition of "convertable to T". While I assume it is the obvious definition (an implicit conversion), I can't find an exact definition. Is there one?
Slightly more worryingly, there doesn't seem to be any restriction on the this type, other than it is "convertable to T". Consider two input iterators a and b. I would personally assume that most people would expect *a==*b would perform T(*a)==T(*b), however it doesn't seem that the standard requires that, and that whatever type *a is (call it U) could have == defined on it with totally different symantics and still be a valid inputer iterator.
Is this a correct reading? When using input iterators should I write T(*a) all over the place to be sure that the object I'm using is the class I expect?
This is especially a nuisance for operations that are defined to be "convertible to bool". (This is probably allowed so that implementations could return say an int and avoid an unnessary conversion. However all implementations I have seen simply return a bool anyway. Typical implemtations of STL algorithms just write things like while(a!=b && *a!=0). But strictly speaking, there are lots of types that are convertible to T but that also overload the appropriate operators so this doesn't behave as expected.
If we want to make code like this legal (which most people seem to expect), then we'll need to tighten up what we mean by "convertible to T".
[Lillehammer: The first part is NAD, since "convertible" is well-defined in core. The second part is basically about pathological overloads. It's a minor problem but a real one. So leave open for now, hope we solve it as part of iterator redesign.]
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Mark as NAD Future. We agree there's an issue, but there is no proposed solution at this time and this will be solved by concepts in the future.
[2017-02 in Kona, LEWG recommends NAD]
Has been clarified by 14. By design. Ranges might make it go away. Current wording for input iterators is more constrained.
[2017-06-02 Issues Telecon]
Move to Open. This is very similar to 2962, possibly a duplicate.
Marshall to research
[2017-07 Toronto Thurs Issue Prioritization]
Priority 2; same as 2962.
Proposed resolution:
Rationale:
[ San Francisco: ]
Solved by N2758.
Section: 32 [re] Status: Open Submitter: Eric Niebler Opened: 2005-07-01 Last modified: 2020-07-17
Priority: 4
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Discussion:
A problem with TR1 regex is currently being discussed on the Boost developers list. It involves the handling of case-insensitive matching of character ranges such as [Z-a]. The proper behavior (according to the ECMAScript standard) is unimplementable given the current specification of the TR1 regex_traits<> class template. John Maddock, the author of the TR1 regex proposal, agrees there is a problem. The full discussion can be found at http://lists.boost.org/boost/2005/06/28850.php (first message copied below). We don't have any recommendations as yet.
-- Begin original message --
The situation of interest is described in the ECMAScript specification (ECMA-262), section 15.10.2.15:
"Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case ASCII letters as well as the symbols [, \, ], ^, _, and `."
A more interesting case is what should happen when doing a case-insensitive match on a range such as [Z-a]. It should match z, Z, a, A and the symbols [, \, ], ^, _, and `. This is not what happens with Boost.Regex (it throws an exception from the regex constructor).
The tough pill to swallow is that, given the specification in TR1, I don't think there is any effective way to handle this situation. According to the spec, case-insensitivity is handled with regex_traits<>::translate_nocase(CharT) — two characters are equivalent if they compare equal after both are sent through the translate_nocase function. But I don't see any way of using this translation function to make character ranges case-insensitive. Consider the difficulty of detecting whether "z" is in the range [Z-a]. Applying the transformation to "z" has no effect (it is essentially std::tolower). And we're not allowed to apply the transformation to the ends of the range, because as ECMA-262 says, "the case of the two ends of a range is significant."
So AFAICT, TR1 regex is just broken, as is Boost.Regex. One possible fix is to redefine translate_nocase to return a string_type containing all the characters that should compare equal to the specified character. But this function is hard to implement for Unicode, and it doesn't play nice with the existing ctype facet. What a mess!
-- End original message --
[ John Maddock adds: ]
One small correction, I have since found that ICU's regex package does implement this correctly, using a similar mechanism to the current TR1.Regex.
Given an expression [c1-c2] that is compiled as case insensitive it:
Enumerates every character in the range c1 to c2 and converts it to it's case folded equivalent. That case folded character is then used a key to a table of equivalence classes, and each member of the class is added to the list of possible matches supported by the character-class. This second step isn't possible with our current traits class design, but isn't necessary if the input text is also converted to a case-folded equivalent on the fly.
ICU applies similar brute force mechanisms to character classes such as [[:lower:]] and [[:word:]], however these are at least cached, so the impact is less noticeable in this case.
Quick and dirty performance comparisons show that expressions such as "[X-\\x{fff0}]+" are indeed very slow to compile with ICU (about 200 times slower than a "normal" expression). For an application that uses a lot of regexes this could have a noticeable performance impact. ICU also has an advantage in that it knows the range of valid characters codes: code points outside that range are assumed not to require enumeration, as they can not be part of any equivalence class. I presume that if we want the TR1.Regex to work with arbitrarily large character sets enumeration really does become impractical.
Finally note that Unicode has:
Three cases (upper, lower and title). One to many, and many to one case transformations. Character that have context sensitive case translations - for example an uppercase sigma has two different lowercase forms - the form chosen depends on context(is it end of a word or not), a caseless match for an upper case sigma should match either of the lower case forms, which is why case folding is often approximated by tolower(toupper(c)).
Probably we need some way to enumerate character equivalence classes, including digraphs (either as a result or an input), and some way to tell whether the next character pair is a valid digraph in the current locale.
Hoping this doesn't make this even more complex that it was already,
[ Portland: Alisdair: Detect as invalid, throw an exception. Pete: Possible general problem with case insensitive ranges. ]
[ 2009-07 Frankfurt ]
We agree that this is a problem, but we do not know the answer.
We are going to declare this NAD until existing practice leads us in some direction.
No objection to NAD Future.
Move to NAD Future.
[LEWG Kona 2017]
Recommend Open: Tim Shen proposes: forbid use of case-insensitive ranges with regex traits other than std::regex_traits<{char, wchar_t, char16_t, char32_t}> when regex_constants::collate is specified.
[2020-07-17; Priority set to 4 in telecon]
Proposed resolution:
Section: 22.4.9 [tuple.rel], 99 [tr.tuple.rel] Status: LEWG Submitter: David Abrahams Opened: 2005-11-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 348
Discussion:
Where possible, tuple comparison operators <,<=,=>, and > ought to be defined in terms of std::less rather than operator<, in order to support comparison of tuples of pointers.
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
If we solve this for tuple we would have to solve it for pair algorithms, etc. It is too late to do that at this time. Move to NAD Future.
Proposed resolution:
change 6.1.3.5/5 from:
Returns: The result of a lexicographical comparison between t and u. The result is defined as: (bool)(get<0>(t) < get<0>(u)) || (!(bool)(get<0>(u) < get<0>(t)) && ttail < utail), where rtail for some tuple r is a tuple containing all but the first element of r. For any two zero-length tuples e and f, e < f returns false.
to:
Returns: The result of a lexicographical comparison between t and u. For any two zero-length tuples e and f, e < f returns false. Otherwise, the result is defined as: cmp( get<0>(t), get<0>(u)) || (!cmp(get<0>(u), get<0>(t)) && ttail < utail), where rtail for some tuple r is a tuple containing all but the first element of r, and cmp(x,y) is an unspecified function template defined as follows.
Where T is the type of x and U is the type of y:
if T and U are pointer types and T is convertible to U, returns less<U>()(x,y)
otherwise, if T and U are pointer types, returns less<T>()(x,y)
otherwise, returns (bool)(x < y)
[ Berlin: This issue is much bigger than just tuple (pair, containers, algorithms). Dietmar will survey and work up proposed wording. ]
Rationale:
Recommend NAD. This will be fixed with the next revision of concepts.
[ San Francisco: ]
Solved by N2770.
Section: 24.3.7 [array] Status: Open Submitter: Bo Persson Opened: 2006-12-30 Last modified: 2022-11-12
Priority: 3
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Discussion:
The <array> header is given under 24.3 [sequences]. 24.3.7 [array]/paragraph 3 says:
"Unless otherwise specified, all array operations are as described in 24.2 [container.requirements]".
However, array isn't mentioned at all in section 24.2 [container.requirements]. In particular, Table 82 "Sequence requirements" lists several operations (insert, erase, clear) that std::array does not have in 24.3.7 [array].
Also, Table 83 "Optional sequence operations" lists several operations that std::array does have, but array isn't mentioned.
[ 2009-07 Frankfurt ]
The real issue seems to be different than what is described here. Non-normative text says that std::array is a sequence container, but there is disagreement about what that really means. There are two possible interpretations:
- a sequence container is one that satisfies all sequence container requirements
- a sequence container is one that satisfies some of the sequence container requirements. Any operation that the container supports is specified by one or more sequence container requirements, unless that operation is specifically singled out and defined alongside the description of the container itself.
Move to Tentatively NAD.
[ 2009-07-15 Loïc Joly adds: ]
The section 24.2.4 [sequence.reqmts]/1 states that array is a sequence. 24.2.4 [sequence.reqmts]/3 introduces table 83, named Sequence container requirements. This seems to me to be defining the requirements for all sequences. However, array does not follow all of this requirements (this can be read in the array specific section, for the standard is currently inconsistent).
Proposed resolution 1 (minimal change):
Say that array is a container, that in addition follows only some of the sequence requirements, as described in the array section:
The library provides
fivethree basic kinds of sequence containers:array, vector,forward_list, list, and deque. In addition, array and forward_list follows some of the requirements of sequences, as described in their respective sections.Proposed resolution 2 (most descriptive description, no full wording provided):
Introduce the notion of a Fixed Size Sequence, with it requirement table that would be a subset of the current Sequence container. array would be the only Fixed Size Sequence (but dynarray is in the queue for TR2). Sequence requirements would now be requirements in addition to Fixed Size Sequence requirements (it is currently in addition to container).
[ 2009-07 Frankfurt: ]
Move to NAD Editorial
[ 2009 Santa Cruz: ]
This will require a lot of reorganization. Editor doesn't think this is really an issue, since the description of array can be considered as overriding what's specified about sequences. Move to NAD.
[2022-10-27; Hubert Tong comments and requests to reopen]
This issue appears to be unresolved (should not be NAD).
As noted in 24.3.7.1 [array.overview] paragraph 3, array does not meet 24.2.2.2 [container.reqmts] paragraph 10. This means that array does not meet the container requirements, never mind the requirements for sequence containers or contiguous containers. However, there is wording that claims the opposite. 24.2.4 [sequence.reqmts] paragraph 1:In addition, array is provided as a sequence container which provides limited sequence operations because it has a fixed number of elements.
(Perhaps the above should be worded with "except".)
24.3.1 [sequences.general] paragraph 1:The headers <array> […] define class templates that meet the requirements for sequence containers.
24.3.7.1 [array.overview] paragraph 1:
[…] An array is a contiguous container (24.2.2.1 [container.requirements.general]).
In this comment, Casey suggests that the requirements be changed so that array does meet the requirements.
[Kona 2022-11-12; Set Priority to 3]
Proposed resolution:
Section: 33.6.4 [thread.mutex.requirements] Status: LEWG Submitter: Pete Becker Opened: 2008-12-05 Last modified: 2017-03-01
Priority: Not Prioritized
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Duplicate of: 961
Discussion:
33.6.4 [thread.mutex.requirements] describes the requirements for a type to be a "Mutex type". A Mutex type can be used as the template argument for the Lock type that's passed to condition_variable_any::wait (although Lock seems like the wrong name here, since Lock is given a different formal meaning in 33.6.5 [thread.lock]) and, although the WD doesn't quite say so, as the template argument for lock_guard and unique_lock.
The requirements for a Mutex type include:
Also, a Mutex type "shall not be copyable nor movable".
The latter requirement seems completely irrelevant, and the three requirements on return types are tighter than they need to be. For example, there's no reason that lock_guard can't be instantiated with a type that's copyable. The rule is, in fact, that lock_guard, etc. won't try to copy objects of that type. That's a constraint on locks, not on mutexes. Similarly, the requirements for void return types are unnecessary; the rule is, in fact, that lock_guard, etc. won't use any returned value. And with the return type of bool, the requirement should be that the return type is convertible to bool.
[ Summit: ]
Move to open. Related to conceptualization and should probably be tackled as part of that.
- The intention is not only to place a constraint on what types such as lock_guard may do with mutex types, but on what any code, including user code, may do with mutex types. Thus the constraints as they are apply to the mutex types themselves, not the current users of mutex types in the standard.
- This is a low priority issue; the wording as it is may be overly restrictive but this may not be a real issue.
[ Post Summit Anthony adds: ]
Section 33.6.4 [thread.mutex.requirements] conflates the requirements on a generic Mutex type (including user-supplied mutexes) with the requirements placed on the standard-supplied mutex types in an attempt to group everything together and save space.
When applying concepts to chapter 30, I suggest that the concepts Lockable and TimedLockable embody the requirements for *use* of a mutex type as required by unique_lock/lock_guard/condition_variable_any. These should be relaxed as Pete describes in the issue. The existing words in 33.6.4 [thread.mutex.requirements] are requirements on all of std::mutex, std::timed_mutex, std::recursive_mutex and std::recursive_timed_mutex, and should be rephrased as such.
[2017-03-01, Kona]
SG1: Agreement that we need a paper.
Proposed resolution:
Section: 33.6.4 [thread.mutex.requirements] Status: LEWG Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2017-03-01
Priority: Not Prioritized
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Duplicate of: 936
Discussion:
33.6.4 [thread.mutex.requirements] describes required member functions of mutex types, and requires that they throw exceptions under certain circumstances. This is overspecified. User-defined types can abort on such errors without affecting the operation of templates supplied by standard-library.
[ Summit: ]
Move to open. Related to conceptualization and should probably be tackled as part of that.
[ 2009-10 Santa Cruz: ]
Would be OK to leave it as is for time constraints, could loosen later.
Mark as NAD Future.
[2017-03-01, Kona]
SG1: Agreement that we need a paper.
Proposed resolution:
Section: 24.3.11.3 [vector.capacity] Status: Open Submitter: Daniel Krügler Opened: 2009-04-20 Last modified: 2020-07-17
Priority: 3
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Discussion:
I have the impression that even the wording of current draft N2857 does insufficiently express the intent of vector's reallocation strategy. This has produced not too old library implementations which release memory in the clear() function and even modern articles about C++ programming cultivate the belief that clear is allowed to do exactly this. A typical example is something like this:
const int buf_size = ...;
std::vector<T> buf(buf_size);
for (int i = 0; i < some_condition; ++i) {
buf.resize(buf_size);
write_or_read_data(buf.data());
buf.clear(); // Ensure that the next round get's 'zeroed' elements
}
where still the myth is ubiquitous that buf might be allowed to reallocate it's memory inside the for loop.
IMO the problem is due to the fact, that
the effects clause of std::vector's erase overloads in 24.3.11.5 [vector.modifiers]/4 is silent about capacity changes. This easily causes a misunderstanding, because the counter parting insert functions described in 24.3.11.5 [vector.modifiers]/2 explicitly say, that
Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid.
It requires a complex argumentation chain about four different places in the standard to provide the — possibly weak — proof that calling clear() also does never change the capacity of the std::vector container. Since std::vector is the de-facto replacement of C99's dynamic arrays this type is near to a built-in type and it's specification should be clear enough that usual programmers can trust their own reading.
[ Batavia (2009-05): ]
Bill believes paragraph 1 of the proposed resolution is unnecessary because it is already implied (even if tortuously) by the current wording.
Move to Review.
[ 2009-10 Santa Cruz: ]
Mark as NAD. Rationale: there is no consensus to clarify the standard, general consensus that the standard is correct as written.
[2020-05-08; Reopen after reflector discussions]
"correct as written" has been disputed.
[2020-07-17; Priority set to 3 in telecon]
Proposed resolution:
[ This is a minimum version. I also suggest that the wording explaining the allocation strategy of std::vector in 24.3.11.3 [vector.capacity]/3 and /6 is moved into a separate sub paragraph of 24.3.11.3 [vector.capacity] before any of the prototype's are discussed, but I cannot provide reasonable wording changes now. ]
Change 24.3.11.3 [vector.capacity]/6 as follows:
It is guaranteed that no reallocation takes place during insertions or erasures that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the value of capacity().
Change 24.3.11.5 [vector.modifiers]/4 as follows:
Effects: The capacity shall remain unchanged and no reallocation shall happen. Invalidates iterators and references at or after the point of the erase.
Section: 24.2.8 [unord.req] Status: Open Submitter: Pablo Halpern Opened: 2009-07-17 Last modified: 2020-09-06
Priority: 3
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Discussion:
When I look at the unordered_* constructors, I think the complexity is poorly described and does not follow the style of the rest of the standard.
The complexity for the default constructor is specified as constant. Actually, it is proportional to n, but there are no invocations of value_type constructors or other value_type operations.
For the iterator-based constructor the complexity should be:
Complexity: exactly n calls to construct value_type from InputIterator::value_type (where n = distance(f,l)). The number of calls to key_equal::operator() is proportional to n in the average case and n*n in the worst case.
[ 2010 Rapperswil: ]
Concern that the current wording may require O(1) where that cannot be delivered. We need to look at both the clause 23 requirements tables and the constructor description of each unordered container to be sure.
Howard suggests NAD Editorial as we updated the container requirement tables since this issue was written.
Daniel offers to look deeper, and hopefully produce wording addressing any outstanding concerns at the next meeting.
Move to Open.
[2011-02-26: Daniel provides wording]
I strongly suggest to clean-up the differences between requirement tables and individual specifications. In the usual way, the most specific specifications wins, which is in this case the wrong one. In regard to the concern expressed about missing DefaultConstructible requirements of the value type I disagree: The function argument n is no size-control parameter, but only some effective capacity parameter: No elements will be value-initialized by these constructors. The necessary requirement for the value type, EmplaceConstructible into *this, is already listed in Table 103 — Unordered associative container requirements. Another part of the proposed resolution is the fact that there is an inconsistency of the complexity counting when both a range and a bucket count is involved compared to constructions where only bucket counts are provided: E.g. the construction X a(n); has a complexity of n bucket allocations, but this part of the work is omitted for X a(i, j, n);, even though it is considerable larger (in the average case) for n ≫ distance(i, j).
[2011-03-24 Madrid meeting]
Move to deferred
[ 2011 Bloomington ]
The proposed wording looks good. Move to Review.
[2012, Kona]
Fix up some presentation issues with the wording, combining the big-O expressions into single expressions rather than the sum of two separate big-Os.
Strike "constant or linear", prefer "linear in the number of buckets". This allows for number of buckets being larger than requested n as well.
Default n to "unspecified" rather than "implementation-defined". It seems an un-necessary burden asking vendors to document a quantity that is easily determined through the public API of these classes.
Replace distance(f,l) with "number of elements in the range [f,l)"
Retain in Review with the updated wording
[2012, Portland: Move to Open]
The wording still does not call out Pablo's original concern, that the element constructor is called no more than N times, and that the N squared term applies to moves during rehash.
Inconsistent use of O(n)+O(N) vs. O(n+N), with a preference for the former.
AJM to update wording with a reference to "no more than N element constructor calls".
Matt concerned that calling out the O(n) requirements is noise, and dangerous noise in suggesting a precision we do not mean. The cost of constructing a bucket is very different to constructing an element of user-supplied type.
AJM notes that if there are multiple rehashes, the 'n' complexity is probably not linear.
Matt suggests back to Open, Pablo suggests potentially NAD if we keep revisitting without achieving a resolution.
Matt suggests complexity we are concerned with is the number of operations, such as constructing elements, moving nodes, and comparing/hashing keys. We are less concerned with constructing buckets, which are generally noise in this bigger picture.
[2015-01-29 Telecon]
AM: essentially correct, but do we want to complicate the spec?
HH: Pablo has given us permission to NAD it JM: when I look at the first change in the P/R I find it mildly disturbing that the existing wording says you have a constant time constructor with a single element even if your n is 10^6, so I think adding this change makes people aware there might be a large cost in initializing the hash table, even though it doesn't show up in user-visible constructions. HH: one way to avoid that problem is make the default ctor noexcept. Then the container isn't allowed to create an arbitrarily large hash table AM: but this is the constructor where the user provides n MC: happy with the changes, except I agree with the editorial recommendation to keep the two 𝒪s separate. JW: yes, the constant 'k' is different in 𝒪(n) and 𝒪(N) GR: do we want to talk about buckets at all JM: yes, good to highlight that bucket construction might be a significant cost HH: suggest we take the suggestion to split 𝒪(n+N) to 𝒪(n)+𝒪(N) and move to Tentatively Ready GR: 23.2.1p2 says all complexity requirements are stated solely in terms of the number of operations on the contained object, so we shouldn't be stating complexity in terms of the hash table initialization HH: channeling Pete, there's an implicit "unless otherwise specified" everywhere. VV: seem to be requesting modifications that render this not Tentatively Ready GR: I think it can't be T/R AM: make the editorial recommendation, consider fixing 23.2.1/3 to give us permission to state complexity in terms of bucket initialization HH: only set it to Review after we get new wording to review[2015-02 Cologne]
Update wording, revisit later.
Previous resolution [SUPERSEDED]:
Modify the following rows in Table 103 — Unordered associative container requirements to add the explicit bucket allocation overhead of some constructions. As editorial recommendation it is suggested not to shorten the sum 𝒪(n) + 𝒪(N) to 𝒪(n + N), because two different work units are involved.
Table 103 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity … X(i, j, n, hf, eq)
X a(i, j, n, hf, eq)X …
Effects: Constructs an empty container with at least n
buckets, using hf as the hash function and eq as the key
equality predicate, and inserts elements from [i, j) into it.Average case 𝒪(n + N) (N is distance(i, j)),
worst case 𝒪(n) + 𝒪(N2)X(i, j, n, hf)
X a(i, j, n, hf)X …
Effects: Constructs an empty container with at least n
buckets, using hf as the hash function and key_equal() as the key
equality predicate, and inserts elements from [i, j) into it.Average case 𝒪(n + N) (N is distance(i, j)),
worst case 𝒪(n + N2)X(i, j, n)
X a(i, j, n)X …
Effects: Constructs an empty container with at least n
buckets, using hasher() as the hash function and key_equal() as the key
equality predicate, and inserts elements from [i, j) into it.Average case 𝒪(n + N) (N is distance(i, j)),
worst case 𝒪(n + N2)… Modify 24.5.4.2 [unord.map.cnstr] p. 1-4 as indicated (The edits of p. 1 and p. 3 attempt to fix some editorial oversight.):
explicit unordered_map(size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());1 Effects: Constructs an empty unordered_map using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecified
impldefdefault number of buckets in unordered_map. max_load_factor() returns 1.0.2 Complexity:
ConstantLinear in the number of buckets.template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());3 Effects: Constructs an empty unordered_map using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecified
impldefdefault number of buckets in unordered_map. Then inserts elements from the range [f, l). max_load_factor() returns 1.0.4 Complexity:
Average case linear, worst case quadraticLinear in the number of buckets. In the average case linear in N and in the worst case quadratic in N to insert the elements, where N is equal to number of elements in the range [f,l).Modify 24.5.5.2 [unord.multimap.cnstr] p. 1-4 as indicated (The edits of p. 1 and p. 3 attempt to fix some editorial oversight.):
explicit unordered_multimap(size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());1 Effects: Constructs an empty unordered_multimap using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecified
impldefdefault number of buckets in unordered_multimap. max_load_factor() returns 1.0.2 Complexity:
ConstantLinear in the number of buckets.template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());3 Effects: Constructs an empty unordered_multimap using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecified
impldefdefault number of buckets in unordered_multimap. Then inserts elements from the range [f, l). max_load_factor() returns 1.0.4 Complexity:
Average case linear, worst case quadraticLinear in the number of buckets. In the average case linear in N and in the worst case quadratic in N to insert the elements, where N is equal to number of elements in the range [f,l).Modify 24.5.6.2 [unord.set.cnstr] p. 1-4 as indicated (The edits of p. 1 and p. 3 attempt to fix some editorial oversight.):
explicit unordered_set(size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());1 Effects: Constructs an empty unordered_set using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecified
impldefdefault number of buckets in unordered_set. max_load_factor() returns 1.0.2 Complexity:
ConstantLinear in the number of buckets.template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());3 Effects: Constructs an empty unordered_set using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecified
impldefdefault number of buckets in unordered_set. Then inserts elements from the range [f, l). max_load_factor() returns 1.0.4 Complexity:
Average case linear, worst case quadraticLinear in the number of buckets. In the average case linear in N and in the worst case quadratic in N to insert the elements, where N is equal to number of elements in the range [f,l).Modify 24.5.7.2 [unord.multiset.cnstr] p. 1-4 as indicated (The edits of p. 1 and p. 3 attempt to fix some editorial oversight.):
explicit unordered_multiset(size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());1 Effects: Constructs an empty unordered_multiset using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecified
impldefdefault number of buckets in unordered_multiset. max_load_factor() returns 1.0.2 Complexity:
ConstantLinear in the number of buckets.template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());3 Effects: Constructs an empty unordered_multiset using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecified
impldefdefault number of buckets in unordered_multiset. Then inserts elements from the range [f, l). max_load_factor() returns 1.0.4 Complexity:
Average case linear, worst case quadraticLinear in the number of buckets. In the average case linear in N and in the worst case quadratic in N to insert the elements, where N is equal to number of elements in the range [f,l).
[2019-03-17; Daniel comments and provides revised wording]
The updated wording ensures that we can now specify complexity requirements for containers even when they are not expressed in terms of the number on the contained objects by an exception of the rule. This allows us to say that 𝒪(n) describes the complexity in terms of bucket initialization instead.
Proposed resolution:
This wording is relative to N4810.
Modify 24.2.2.1 [container.requirements.general] as indicated:
-2- Unless otherwise specified,
All of thecomplexity requirements in this Clause are stated solely in terms of the number of operations on the contained objects. [Example: The copy constructor of type vector<vector<int>> has linear complexity, even though the complexity of copying each contained vector<int> is itself linear. — end example]
Modify 24.2.8 [unord.req] as indicated:
-11- In Table 70:
(11.1) — […]
[…]
(11.23) — […]
(11.?) — Notwithstanding the complexity requirements restrictions of 24.2.2.1 [container.requirements.general], the complexity form 𝒪(n) describes the number of operations on buckets.
Modify the following rows in Table 70 — "Unordered associative container requirements" to add the explicit bucket allocation overhead of some constructions.
[Drafting note: It is kindly suggested to the Project Editor not to shorten the sum 𝒪(n) + 𝒪(N) to 𝒪(n + N), because two different work units are involved. — end drafting note]
Table 70 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity … X()
X a;X Expects: […]
Effects: Constructs an empty container with an unspecified number n of
buckets, using hasher() as the hash function and key_equal() as the key
equality predicate.constant𝒪(n)X(i, j, n, hf, eq)
X a(i, j, n, hf, eq)X Expects: […]
Effects: Constructs an empty container with at least n
buckets, using hf as the hash function and eq as the key
equality predicate, and inserts elements from [i, j) into it.Average case 𝒪(n) + 𝒪(N) (N
is distance(i, j)), worst case
𝒪(n) + 𝒪(N2)X(i, j, n, hf)
X a(i, j, n, hf)X Expects: […]
Effects: Constructs an empty container with at least n
buckets, using hf as the hash function and key_equal() as the key
equality predicate, and inserts elements from [i, j) into it.Average case 𝒪(n) + 𝒪(N) (N
is distance(i, j)), worst case
𝒪(n) + 𝒪(N2)X(i, j, n)
X a(i, j, n)X Expects: […]
Effects: Constructs an empty container with at least n
buckets, using hasher() as the hash function and key_equal() as the key
equality predicate, and inserts elements from [i, j) into it.Average case 𝒪(n) + 𝒪(N) (N
is distance(i, j)), worst case
𝒪(n) + 𝒪(N2)X(i, j)
X a(i, j)X Expects: […]
Effects: Constructs an empty container with an unspecified number n of
buckets, using hasher() as the hash function and key_equal() as the key
equality predicate, and inserts elements from [i, j) into it.Average case 𝒪(n) + 𝒪(N) (N
is distance(i, j)), worst case
𝒪(n) + 𝒪(N2)…
Modify 24.5.4.1 [unord.map.overview], class template unordered_map, as indicated:
// 24.5.4.2 [unord.map.cnstr], construct/copy/destroy […] template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […] unordered_map(initializer_list<value_type> il, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […]
Modify 24.5.4.2 [unord.map.cnstr] as indicated:
unordered_map() : unordered_map(size_type(see belowunspecified)) { } explicit unordered_map(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-1- Effects: Constructs an empty unordered_map using the specified hash function, key equality predicate, and allocator, and using at least n buckets.
-?- Ensures: max_load_factor() == 1.0 -2- Complexity:For the default constructor, the number of buckets is implementation-defined.max_load_factor() returns 1.0.ConstantLinear in the number of buckets.template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_map(initializer_list<value_type> il, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-3- Effects: Constructs an empty unordered_map using the specified hash function, key equality predicate, and allocator, and using at least n buckets.
-?- Ensures: max_load_factor() == 1.0 -4- Complexity:If n is not provided, the number of buckets is implementation-defined.Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form.max_load_factor() returns 1.0.Average case linear, worst case quadraticLinear in the number of buckets, plus 𝒪(N) (average case) or 𝒪(N2) (worst case) where N is the number of insertions.
Modify 24.5.5.1 [unord.multimap.overview], class template unordered_multimap, as indicated:
// 24.5.5.2 [unord.multimap.cnstr], construct/copy/destroy […] template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […] unordered_multimap(initializer_list<value_type> il, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […]
Modify 24.5.5.2 [unord.multimap.cnstr] as indicated:
unordered_multimap() : unordered_multimap(size_type(see belowunspecified)) { } explicit unordered_multimap(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-1- Effects: Constructs an empty unordered_multimap using the specified hash function, key equality predicate, and allocator, and using at least n buckets.
-?- Ensures: max_load_factor() == 1.0 -2- Complexity:For the default constructor, the number of buckets is implementation-defined. max_load_factor() returns 1.0.ConstantLinear in the number of buckets.template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multimap(initializer_list<value_type> il, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-3- Effects: Constructs an empty unordered_multimap using the specified hash function, key equality predicate, and allocator, and using at least n buckets.
-?- Ensures: max_load_factor() == 1.0 -4- Complexity:If n is not provided, the number of buckets is implementation-defined.Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form.max_load_factor() returns 1.0.Average case linear, worst case quadraticLinear in the number of buckets, plus 𝒪(N) (average case) or 𝒪(N2) (worst case) where N is the number of insertions.
Modify 24.5.6.1 [unord.set.overview], class template unordered_set, as indicated:
// 24.5.6.2 [unord.set.cnstr], construct/copy/destroy […] template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […] unordered_set(initializer_list<value_type> il, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […]
Modify 24.5.6.2 [unord.set.cnstr] as indicated:
unordered_set() : unordered_set(size_type(see belowunspecified)) { } explicit unordered_set(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-1- Effects: Constructs an empty unordered_set using the specified hash function, key equality predicate, and allocator, and using at least n buckets.
-?- Ensures: max_load_factor() == 1.0 -2- Complexity:For the default constructor, the number of buckets is implementation-defined.max_load_factor() returns 1.0.ConstantLinear in the number of buckets.template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_set(initializer_list<value_type> il, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-3- Effects: Constructs an empty unordered_set using the specified hash function, key equality predicate, and allocator, and using at least n buckets.
-?- Ensures: max_load_factor() == 1.0 -4- Complexity:If n is not provided, the number of buckets is implementation-defined.Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form.max_load_factor() returns 1.0.Average case linear, worst case quadraticLinear in the number of buckets, plus 𝒪(N) (average case) or 𝒪(N2) (worst case) where N is the number of insertions.
Modify 24.5.6.1 [unord.set.overview], class template unordered_multiset, as indicated:
// 24.5.7.2 [unord.multiset.cnstr], construct/copy/destroy […] template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […] unordered_multiset(initializer_list<value_type> il, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […]
Modify 24.5.7.2 [unord.multiset.cnstr] as indicated:
unordered_multiset() : unordered_multiset(size_type(see belowunspecified)) { } explicit unordered_multiset(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-1- Effects: Constructs an empty unordered_multiset using the specified hash function, key equality predicate, and allocator, and using at least n buckets.
-?- Ensures: max_load_factor() == 1.0 -2- Complexity:For the default constructor, the number of buckets is implementation-defined.max_load_factor() returns 1.0.ConstantLinear in the number of buckets.template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multiset(initializer_list<value_type> il, size_type n =see belowunspecified, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-3- Effects: Constructs an empty unordered_multiset using the specified hash function, key equality predicate, and allocator, and using at least n buckets.
-?- Ensures: max_load_factor() == 1.0 -4- Complexity:If n is not provided, the number of buckets is implementation-defined.Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form.max_load_factor() returns 1.0.Average case linear, worst case quadraticLinear in the number of buckets, plus 𝒪(N) (average case) or 𝒪(N2) (worst case) where N is the number of insertions.
Section: 25.3 [iterator.requirements] Status: Open Submitter: Daniel Krügler Opened: 2009-09-19 Last modified: 2016-01-28
Priority: 4
View all other issues in [iterator.requirements].
View all issues with Open status.
Discussion:
The terms valid iterator and singular aren't properly defined. The fuzziness of those terms became even worse after the resolution of 208 (including further updates by 278). In 25.3 [iterator.requirements] as of N2723 the standard says now:
5 - These values are called past-the-end values. Values of an iterator i for which the expression *i is defined are called dereferenceable. The library never assumes that past-the-end values are dereferenceable. Iterators can also have singular values that are not associated with any container. [...] Results of most expressions are undefined for singular values; the only exceptions are destroying an iterator that holds a singular value and the assignment of a non-singular value to an iterator that holds a singular value. [...] Dereferenceable values are always non-singular.
10 - An invalid iterator is an iterator that may be singular.
First, issue 208 intentionally removed the earlier constraint that past-the-end values are always non-singular. The reason for this was to support null pointers as past-the-end iterators of e.g. empty sequences. But there seem to exist different views on what a singular (iterator) value is. E.g. according to the SGI definition a null pointer is not a singular value:
Dereferenceable iterators are always nonsingular, but the converse is not true. For example, a null pointer is nonsingular (there are well defined operations involving null pointers) even thought it is not dereferenceable.
and proceeds:
An iterator is valid if it is dereferenceable or past-the-end.
Even if the standard prefers a different meaning of singular here, the change was incomplete, because by restricting feasible expressions of singular iterators to destruction and assignment isn't sufficient for a past-the-end iterator: Of-course it must still be equality-comparable and in general be a readable value.
Second, the standard doesn't clearly say whether a past-the-end value is a valid iterator or not. E.g. 27.11 [specialized.algorithms]/1 says:
In all of the following algorithms, the formal template parameter ForwardIterator is required to satisfy the requirements of a forward iterator (24.1.3) [..], and is required to have the property that no exceptions are thrown from [..], or dereference of valid iterators.
The standard should make better clear what "singular pointer" and "valid iterator" means. The fact that the meaning of a valid value has a core language meaning doesn't imply that for an iterator concept the term "valid iterator" has the same meaning.
Let me add a final example: In 99 [allocator.concepts.members] of N2914 we find:
pointer X::allocate(size_type n);11 Returns: a pointer to the allocated memory. [Note: if n == 0, the return value is unspecified. —end note]
[..]
void X::deallocate(pointer p, size_type n);Preconditions: p shall be a non-singular pointer value obtained from a call to allocate() on this allocator or one that compares equal to it.
If singular pointer value would include null pointers this make the preconditions unclear if the pointer value is a result of allocate(0): Since the return value is unspecified, it could be a null pointer. Does that mean that programmers need to check the pointer value for a null value before calling deallocate?
[ 2010-11-09 Daniel comments: ]
A later paper is in preparation.
[ 2010 Batavia: ]
Doesn't need to be resolved for Ox
[2014-02-20 Re-open Deferred issues as Priority 4]
Consider to await the paper.
Proposed resolution:
Section: 27 [algorithms] Status: Open Submitter: Alisdair Meredith Opened: 2009-10-15 Last modified: 2020-09-06
Priority: 3
View other active issues in [algorithms].
View all other issues in [algorithms].
View all issues with Open status.
Discussion:
The library has many algorithms that take a source range represented by a pair of iterators, and the start of some second sequence given by a single iterator. Internally, these algorithms will produce undefined behaviour if the second 'range' is not as large as the input range, but none of the algorithms spell this out in Requires clauses, and there is no catch-all wording to cover this in clause 17 or the front matter of 25.
There was an attempt to provide such wording in paper n2944 but this seems incidental to the focus of the paper, and getting the wording of this issue right seems substantially more difficult than the simple approach taken in that paper. Such wording will be removed from an updated paper, and hopefully tracked via the LWG issues list instead.
It seems there are several classes of problems here and finding wording to solve all in one paragraph could be too much. I suspect we need several overlapping requirements that should cover the desired range of behaviours.
Motivating examples:
A good initial example is the swap_ranges algorithm. Here there is a clear requirement that first2 refers to the start of a valid range at least as long as the range [first1, last1). n2944 tries to solve this by positing a hypothetical last2 iterator that is implied by the signature, and requires distance(first2,last2) < distance(first1,last1). This mostly works, although I am uncomfortable assuming that last2 is clearly defined and well known without any description of how to obtain it (and I have no idea how to write that).
A second motivating example might be the copy algorithm. Specifically, let us image a call like:
copy(istream_iterator<int>(is),istream_iterator(),ostream_iterator<int>(os));
In this case, our input iterators are literally simple InputIterators, and the destination is a simple OutputIterator. In neither case am I happy referring to std::distance, in fact it is not possible for the ostream_iterator at all as it does not meet the requirements. However, any wording we provide must cover both cases. Perhaps we might deduce last2 == ostream_iterator<int>{}, but that might not always be valid for user-defined iterator types. I can well imagine an 'infinite range' that writes to /dev/null and has no meaningful last2.
The motivating example in n2944 is std::equal, and that seems to fall somewhere between the two.
Outlying examples might be partition_copy that takes two output iterators, and the _n algorithms where a range is specified by a specific number of iterations, rather than traditional iterator pair. We should also not accidentally apply inappropriate constraints to std::rotate which takes a third iterator that is not intended to be a separate range at all.
I suspect we want some wording similar to:
For algorithms that operate on ranges where the end iterator of the second range is not specified, the second range shall contain at least as many elements as the first.
I don't think this quite captures the intent yet though. I am not sure if 'range' is the right term here rather than sequence. More awkwardly, I am not convinced we can describe an Output sequence such as produce by an ostream_iterator as "containing elements", at least not as a precondition to the call before they have been written.
Another idea was to describe require that the trailing iterator support at least distance(input range) applications of operator++ and may be written through the same number of times if a mutable/output iterator.
We might also consider handling the case of an output range vs. an input range in separate paragraphs, if that simplifies how we describe some of these constraints.
[ 2009-11-03 Howard adds: ]
Moved to Tentatively NAD Future after 5 positive votes on c++std-lib.
[LEWG Kona 2017]
Recommend Open: The design is clear here; we just need wording
[2019-01-20 Reflector prioritization]
Set Priority to 3
Rationale:
Does not have sufficient support at this time. May wish to reconsider for a future standard.
Proposed resolution:
Section: 24.3.12 [vector.bool] Status: Open Submitter: BSI Opened: 2010-08-25 Last modified: 2020-09-06
Priority: 3
View other active issues in [vector.bool].
View all other issues in [vector.bool].
View all issues with Open status.
Discussion:
Addresses GB-118
vector<bool> iterators are not random access iterators because their reference type is a special class, and not bool &. All standard libary operations taking iterators should treat this iterator as if it was a random access iterator, rather than a simple input iterator.
[ Resolution proposed in ballot comment ]
Either revise the iterator requirements to support proxy iterators (restoring functionality that was lost when the Concept facility was removed) or add an extra paragraph to the vector<bool> specification requiring the library to treat vector<bool> iterators as-if they were random access iterators, despite having the wrong reference type.
[ Rapperswil Review ]
The consensus at Rapperswil is that it is too late for full support for proxy iterators, but requiring the library to respect vector<bool> iterators as-if they were random access would be preferable to flagging this container as deliberately incompatible with standard library algorithms.
Alisdair to write the note, which may become normative Remark depending on the preferences of the project editor.
[ Post-Rapperswil Alisdair provides wording ]
Initial wording is supplied, deliberately using Note in preference to Remark although the author notes his preference for Remark. The issue of whether iterator_traits<vector<bool>>::iterator_category is permitted to report random_access_iterator_tag or must report input_iterator_tag is not addressed.
[ Old Proposed Resolution: ]
Insert a new paragraph into 24.3.12 [vector.bool] between p4 and p5:
[Note All functions in the library that take a pair of iterators to denote a range shall treat vector<bool> iterators as-if they were random access iterators, even though the reference type is not a true reference.-- end note]
[ 2010-11 Batavia: ]
Closed as NAD Future, because the current iterator categories cannot correctly describe vector<bool>::iterator. But saying that they are Random Access Iterators is also incorrect, because it is not too hard to create a corresponding test that fails. We should deal with the more general proxy iterator problem in the future, and see no benefit to take a partial workaround specific to vector<bool> now.
[2017-02 in Kona, LEWG recommends NAD]
D0022 Proxy Iterators for the Ranges Extensions - as much a fix as we’re going to get for vector<bool>.
[2017-06-02 Issues Telecon]
P0022 is exploring a resolution. We consider this to be fairly important issue
Move to Open, set priority to 3
Proposed resolution:
Rationale:
No consensus to make this change at this time.
Section: 33.5.4 [atomics.order] Status: LEWG Submitter: Canada Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [atomics.order].
View all other issues in [atomics.order].
View all issues with LEWG status.
Duplicate of: 1458
Discussion:
Addresses CA-21, GB-131
33.5.5 [atomics.lockfree] p.8 states:
An atomic store shall only store a value that has been computed from constants and program input values by a finite sequence of program evaluations, such that each evaluation observes the values of variables as computed by the last prior assignment in the sequence.
... but 6.9.1 [intro.execution] p.13 states:
If A is not sequenced before B and B is not sequenced before A, then A and B are unsequenced. [ Note: The execution of unsequenced evaluations can overlap. — end note ]
Overlapping executions can make it impossible to construct the sequence described in 33.5.5 [atomics.lockfree] p.8. We are not sure of the intention here and do not offer a suggestion for change, but note that 33.5.5 [atomics.lockfree] p.8 is the condition that prevents out-of-thin-air reads.
For an example, suppose we have a function invocation f(e1,e2). The evaluations of e1 and e2 can overlap. Suppose that the evaluation of e1 writes y and reads x whereas the evaluation of e2 reads y and writes x, with reads-from edges as below (all this is within a single thread).
e1 e2
Wrlx y-- --Wrlx x
rf\ /rf
X
/ \
Rrlx x<- ->Rrlx y
This seems like it should be allowed, but there seems to be no way to produce a sequence of evaluations with the property above.
In more detail, here the two evaluations, e1 and e2, are being executed as the arguments of a function and are consequently not sequenced-before each other. In practice we'd expect that they could overlap (as allowed by 6.9.1 [intro.execution] p.13), with the two writes taking effect before the two reads. However, if we have to construct a linear order of evaluations, as in 33.5.5 [atomics.lockfree] p.8, then the execution above is not permited. Is that really intended?
[ Resolution proposed by ballot comment ]
Please clarify.
[2011-03-09 Hans comments:]
I'm not proud of 33.5.4 [atomics.order] p9 (formerly p8), and I agree with the comments that this isn't entirely satisfactory. 33.5.4 [atomics.order] p9 was designed to preclude out-of-thin-air results for races among memory_order_relaxed atomics, in spite of the fact that Java experience has shown we don't really know how to do that adequately. In the long run, we probably want to revisit this.
However, in the short term, I'm still inclined to declare this NAD, for two separate reasons:6.9.1 [intro.execution] p15 states: "If a side effect on a scalar object is unsequenced relative to either another side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined." I think the examples presented here have undefined behavior as a result. It's not completely clear to me whether examples can be constructed that exhibit this problem, and don't have undefined behavior.
This comment seems to be using a different meaning of "evaluation" from what is used elsewhere in the standard. The sequence of evaluations here doesn't have to consist of full expression evaluations. They can be evaluations of operations like lvalue to rvalue conversion, or individual assignments. In particular, the reads and writes executed by e1 and e2 in the example could be treated as separate evaluations for purposes of producing the sequence. The definition of "sequenced before" in 6.9.1 [intro.execution] makes little sense if the term "evaluation" is restricted to any notion of complete expression. Perhaps we should add yet another note to clarify this? 33.5.4 [atomics.order] p10 probably leads to the wrong impression here.
An alternative resolution would be to simply delete our flakey attempt at preventing out-of-thin-air reads, by removing 33.5.4 [atomics.order] p9-11, possibly adding a note that explains that we technically allow, but strongly discourage them. If we were starting this from scratch now, that would probably be my preference. But it seems like too drastic a resolution at this stage.[2011-03-24 Madrid]
Moved to NAD Future
Proposed resolution:
Section: 33.4.3 [thread.thread.class] Status: LEWG Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-03-01
Priority: Not Prioritized
View all issues with LEWG status.
Discussion:
Addresses US-183
There is no way to join a thread with a timeout.
[ Resolution proposed by ballot comment: ]
Add join_for and join_until. Or decide one should never join a thread with a timeout since pthread_join doesn't have a timeout version.
[ 2010 Batavia ]
The concurrency working group deemed this an extension beyond the scope of C++0x.
Rationale:
The LWG does not wish to make a change at this time.
[2017-03-01, Kona]
SG1 recommends: Close as NAD
There has not been much demand for it, and it would usually be difficult to deal with thread_local destructor races. It can be approximated with a condition variable wait followed by an unconditional join. Adding it would create implementation issues on Posix. As always, this may be revisited if we have a paper exploring the issues in detail.Proposed resolution:
Section: 33.6 [thread.mutex] Status: LEWG Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-03-01
Priority: Not Prioritized
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Discussion:
Addresses US-185
Cooperate with WG14 to improve interoperability between the C++0x and C1x threads APIs. In particular, C1x mutexes should be conveniently usable with a C++0x lock_guard. Performance overheads for this combination should be considered.
[ Resolution proposed by ballot comment: ]
Remove C++0x timed_mutex and timed_recursive_mutex if that facilitates development of more compatible APIs.
[ 2010 Batavia ]
The concurrency sub-group reviewed the options, and decided that closer harmony should wait until both standards are published.
Rationale:
The LWG does not wish to make any change at this time.
[2017-03-01, Kona]
SG1 recommends: Close as NAD
Papers about C compatibility are welcome, but there may be more pressing issues. C threads are not consistently available at this point, so there seems to be little demand to fix this particular problem.Proposed resolution:
Section: 33.6.4 [thread.mutex.requirements] Status: LEWG Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-03-01
Priority: Not Prioritized
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Discussion:
Addresses US-189
mutex and recursive_mutex should have an is_locked() member function. is_locked allows a user to test a lock without acquiring it and can be used to implement a lightweight try_try_lock.
[ Resolution proposed by ballot comment: ]
Add a member function:
bool is_locked() const;to std::mutex and std::recursive_mutex. These functions return true if the current thread would not be able to obtain a mutex. These functions do not synchronize with anything (and, thus, can avoid a memory fence).
[ 2010 Batavia ]
The Concurrency subgroup reviewed this issue and deemed it to be an extension to be handled after publishing C++0x.
Rationale:
The LWG does not wish to make a change at this time.
[2017-03-01, Kona]
SG1 recommends: Close as NAD
Several participants voiced strong objections, based on either memory model issues or lock elision. No support. It is already possible to write a wrapper that explicitly tracks ownership for testing in the owning thread, which may have been part of the intent here.Proposed resolution:
Section: 24.2.2.1 [container.requirements.general] Status: Open Submitter: Mike Spertus Opened: 2010-10-16 Last modified: 2019-01-20
Priority: 3
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Discussion:
Addresses US-104, US-141
The standard doesn't say that containers should use abstract pointer types internally. Both Howard and Pablo agree that this is the intent. Further, it is necessary for containers to be stored, for example, in shared memory with an interprocess allocator (the type of scenario that allocators are intended to support).
In spite of the (possible) agreement on intent, it is necessary to make this explicit:
An implementations may like to store the result of dereferencing the pointer (which is a raw reference) as an optimization, but that prevents the data structure from being put in shared memory, etc. In fact, a container could store raw references to the allocator, which would be a little weird but conforming as long as it has one by-value copy. Furthermore, pointers to locales, ctypes, etc. may be there, which also prevents the data structure from being put in shared memory, so we should make explicit that a container does not store raw pointers or references at all.
[ Pre-batavia ]
This issue is being opened as part of the response to NB comments US-104/141. See paper N3171 in the pre-Batavia mailing.
[2011-03-23 Madrid meeting]
Deferred
[ 2011 Batavia ]
This may be an issue, but it is not clear. We want to gain a few years experience with the C++11 allocator model to see if this is already implied by the existing specification.
[LEWG Kona 2017]
Status to Open: Acknowledged, need wording: (N4618 numbering) 23.2.1 container.requirements.general p8 first sentence. Replace non-normative note with requirement.
See discussion on LEWG Wiki
[2019-01-20 Reflector prioritization]
Set Priority to 3
Proposed resolution:
Add to the end of 24.2.2.1 [container.requirements.general] p. 8:
[..] In all container types defined in this Clause, the member get_allocator() returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement. The container may not store internal objects whose types are of the form T * or T & except insofar as they are part of the item type or members.
Section: 25.3.5.4 [output.iterators] Status: Open Submitter: Daniel Krügler Opened: 2011-02-27 Last modified: 2016-01-28
Priority: 3
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Discussion:
During the Pittsburgh meeting the proposal N3066 became accepted because it fixed several severe issues related to the iterator specification. But the current working draft (N3225) does not reflect all these changes. Since I'm unaware whether every correction can be done editorial, this issue is submitted to take care of that. To give one example: All expressions of Table 108 — "Output iterator requirements" have a post-condition that the iterator is incrementable. This is impossible, because it would exclude any finite sequence that is accessed by an output iterator, such as a pointer to a C array. The N3066 wording changes did not have these effects.
[2011-03-01: Daniel comments:]
This issue has some overlap with the issue 2038 and I would prefer if we could solve both at one location. I suggest the following approach:
The terms dereferencable and incrementable could be defined in a more general way not restricted to iterators (similar to the concepts HasDereference and HasPreincrement from working draft N2914). But on the other hand, all current usages of dereferencable and incrementable are involved with types that satisfy iterator requirements. Thus, I believe that it is sufficient for C++0x to add corresponding definitions to 25.3.1 [iterator.requirements.general] and to let all previous usages of these terms refer to this sub-clause. Since the same problem occurs with the past-the-end iterator, this proposal suggest providing similar references to usages that precede its definition as well.
We also need to ensure that all iterator expressions get either an operational semantics in terms of others or we need to add missing pre- and post-conditions. E.g. we have the following ones without semantics:
*r++ = o // output iterator *r-- // bidirectional iterator
According to the SGI specification these correspond to
{ *r = o; ++r; } // output iterator
{ reference tmp = *r; --r; return tmp; } // bidirectional iterator
respectively. Please note especially the latter expression for bidirectional iterator. It fixes a problem that we have for forward iterator as well: Both these iterator categories provide stronger guarantees than input iterator, because the result of the dereference operation is reference, and not only convertible to the value type (The exact form from the SGI documentation does not correctly refer to reference).
[2011-03-14: Daniel comments and updates the suggested wording]
In addition to the before mentioned necessary changes there is another one need, which became obvious due to issue 2042: forward_list<>::before_begin() returns an iterator value which is not dereferencable, but obviously the intention is that it should be incrementable. This leads to the conclusion that imposing dereferencable as a requirement for the expressions ++r is wrong: We only need the iterator to be incrementable. A similar conclusion applies to the expression --r of bidirectional iterators.
[ 2011 Bloomington ]
Consensus this is the correct direction, but there are (potentially) missing incrementable preconditions on some table rows, and the Remarks on when an output iterator becomes dereferencable are probably better handled outside the table, in a manner similar to the way we word for input iterators.
There was some concern about redundant pre-conditions when the operational semantic is defined in terms of operations that have preconditions, and a similar level of concern over dropping such redundancies vs. applying a consistent level of redundant specification in all the iterator tables. Wording clean-up in either direction would be welcome.
[2011-08-18: Daniel adapts the proposed resolution to honor the Bloomington request]
There is only a small number of further changes suggested to get rid of superfluous requirements and essentially non-normative assertions. Operations should not have extra pre-conditions, if defined by "in-terms-of" semantics, see e.g. a != b or a->m for Table 107. Further, some remarks, that do not impose anything or say nothing new have been removed, because I could not find anything helpful they provide. E.g. consider the remarks for Table 108 for the operations dereference-assignment and preincrement: They don't provide additional information say nothing surprising. With the new pre-conditions and post-conditions it is implied what the remarks intend to say.
[ 2011-11-03: Some observations from Alexander Stepanov via c++std-lib-31405 ]
The following sentence is dropped from the standard section on OutputIterators:
"In particular, the following two conditions should hold: first, any iterator value should be assigned through before it is incremented (this is, for an output iterator i, i++; i++; is not a valid code sequence); second, any value of an output iterator may have at most one active copy at any given time (for example, i = j; *++i = a; *j = b; is not a valid code sequence)."[ 2011-11-04: Daniel comments and improves the wording ]
In regard to the first part of the comment, the intention of the newly proposed wording was to make clear that for the expression
*r = o
we have the precondition dereferenceable and the post-condition incrementable. And for the expression
++r
we have the precondition incrementable and the post-condition dereferenceable or past-the-end. This should not allow for a sequence like i++; i++; but I agree that it doesn't exactly say that.
In regard to the second point: To make this point clearer, I suggest to add a similar additional wording as we already have for input iterator to the "Assertion/note" column of the expression ++r: "Post: any copies of the previous value of r are no longer required to be dereferenceable or incrementable." The proposed has been updated to honor the observations of Alexander Stepanov.[2015-02 Cologne]
The matter is complicated, Daniel volunteers to write a paper.
Proposed resolution:
Add a reference to 25.3.1 [iterator.requirements.general] to the following parts of the library preceding Clause 24 Iterators library: (I stopped from 24.2.8 [unord.req] on, because the remaining references are the concrete containers)
16.4.4.3 [swappable.requirements] p5:
-5- A type X satisfying any of the iterator requirements (24.2) is ValueSwappable if, for any dereferenceable (25.3.1 [iterator.requirements.general]) object x of type X, *x is swappable.
16.4.4.6 [allocator.requirements], Table 27 — "Descriptive variable definitions", row with the expression c:
a dereferenceable (25.3.1 [iterator.requirements.general]) pointer of type C*
20.2.3.3 [pointer.traits.functions]:
Returns: The first template function returns a dereferenceable (25.3.1 [iterator.requirements.general]) pointer to r obtained by calling Ptr::pointer_to(r); […]
23.4.3.4 [string.iterators] p. 2:
Returns: An iterator which is the past-the-end value (25.3.1 [iterator.requirements.general]).
30.4.6.2.3 [locale.time.get.virtuals] p. 11:
iter_type do_get(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err, tm *t, char format, char modifier) const;Requires: t shall be dereferenceable (25.3.1 [iterator.requirements.general]).
24.2.2.1 [container.requirements.general] p. 6:
[…] end() returns an iterator which is the past-the-end (25.3.1 [iterator.requirements.general]) value for the container. […]
24.2.4 [sequence.reqmts] p. 3:
[…] q denotes a valid dereferenceable (25.3.1 [iterator.requirements.general]) const iterator to a, […]
24.2.7 [associative.reqmts] p. 8 (I omit intentionally one further reference in the same sub-clause):
[…] q denotes a valid dereferenceable (25.3.1 [iterator.requirements.general]) const iterator to a, […]
24.2.8 [unord.req] p. 10 (I omit intentionally one further reference in the same sub-clause):
[…] q and q1 are valid dereferenceable (25.3.1 [iterator.requirements.general]) const iterators to a, […]
Edit 25.3.1 [iterator.requirements.general] p. 5 as indicated (The intent is to properly define incrementable and to ensure some further library guarantee related to past-the-end iterator values):
-5- Just as a regular pointer to an array guarantees that there is a pointer value pointing past the last element of the array, so for any iterator type there is an iterator value that points past the last element of a corresponding sequence. These values are called past-the-end values. Values of an iterator i for which the expression *i is defined are called dereferenceable. Values of an iterator i for which the expression ++i is defined are called incrementable. The library never assumes that past-the-end values are dereferenceable or incrementable. Iterators can also have singular values that are not associated with any sequence. […]
Modify the column contents of Table 106 — "Iterator requirements", 25.3.5.2 [iterator.iterators], as indicated:
Table 106 — Iterator requirements Expression Return type Operational semantics Assertion/note
pre-/post-condition*r reference pre: r is dereferenceable. ++r X& pre: r is incrementable.
Modify the column contents of Table 107 — "Input iterator requirements", 25.3.5.3 [input.iterators], as indicated [Rationale: The wording changes attempt to define a minimal "independent" set of operations, namely *a and ++r, and to specify the semantics of the remaining ones. This approach seems to be in agreement with the original SGI specification — end rationale]:
Table 107 — Input iterator requirements (in addition to Iterator) Expression Return type Operational semantics Assertion/note
pre-/post-conditiona != b contextually
convertible to bool!(a == b) pre: (a, b) is in the domain
of ==.*a convertible to T pre: a is dereferenceable.
The expression
(void)*a, *a is equivalent
to *a.
If a == b and (a,b) is in
the domain of == then *a is
equivalent to *b.a->m (*a).m pre: a is dereferenceable.++r X& pre: r is dereferenceableincrementable.
post: r is dereferenceable or
r is past-the-end.
post: any copies of the
previous value of r are no
longer required either to be
dereferenceable, incrementable,
or to be in the domain of ==.(void)r++ (void)++r equivalent to (void)++r*r++ convertible to T { T tmp = *r;
++r;
return tmp; }
Modify the column contents of Table 108 — "Output iterator requirements", 25.3.5.4 [output.iterators], as indicated [Rationale: The wording changes attempt to define a minimal "independent" set of operations, namely *r = o and ++r, and to specify the semantics of the remaining ones. This approach seems to be in agreement with the original SGI specification — end rationale]:
Table 108 — Output iterator requirements (in addition to Iterator) Expression Return type Operational semantics Assertion/note
pre-/post-condition*r = o result is not used pre: r is dereferenceable.
Remark: After this operation
r is not required to be
dereferenceable and any copies of
the previous value of r are no
longer required to be dereferenceable
or incrementable.
post: r is incrementable.++r X& pre: r is incrementable.
&r == &++r.
Remark: After this operationRemark: After this operation
r is not required to be
dereferenceable.
r is not required to be
incrementable and any copies of
the previous value of r are no
longer required to be dereferenceable
or incrementable.
post: r is dereferenceable
or r is past-the-endincrementable.
r++ convertible to const X& { X tmp = r;
++r;
return tmp; }Remark: After this operation
r is not required to be
dereferenceable.
post: r is incrementable.*r++ = o result is not used { *r = o; ++r; } Remark: After this operation
r is not required to be
dereferenceable.
post: r is incrementable.
Modify the column contents of Table 109 — "Forward iterator requirements", 25.3.5.5 [forward.iterators], as indicated [Rationale: Since the return type of the expression *r++ is now guaranteed to be type reference, the implied operational semantics from input iterator based on value copies is wrong — end rationale]
Table 109 — Forward iterator requirements (in addition to input iterator) Expression Return type Operational semantics Assertion/note
pre-/post-conditionr++ convertible to const X& { X tmp = r;
++r;
return tmp; }*r++ reference { reference tmp = *r;
++r;
return tmp; }
Modify the column contents of Table 110 — "Bidirectional iterator requirements", 25.3.5.6 [bidirectional.iterators], as indicated:
Table 110 — Bidirectional iterator requirements (in addition to forward iterator) Expression Return type Operational semantics Assertion/note
pre-/post-condition--r X& pre: there exists s such that
r == ++s.
post: r isdereferenceableincrementable.
--(++r) == r.
--r == --s implies r == s.
&r == &--r.r-- convertible to const X& { X tmp = r;
--r;
return tmp; }*r-- reference { reference tmp = *r;
--r;
return tmp; }
Section: 25.3.5.4 [output.iterators] Status: Open Submitter: Pete Becker Opened: 2011-02-27 Last modified: 2016-01-28
Priority: 3
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Discussion:
In comp.lang.c++, Vicente Botet raises the following questions:
"In "24.2.4 Output iterators" there are 3 uses of incrementable. I've not found the definition. Could some one point me where it is defined?
Something similar occurs with dereferenceable. While the definition is given in "24.2.1 In general" it is used several times before. Shouldn't these definitions be moved to some previous section?"
He's right: both terms are used without being properly defined.
There is no definition of "incrementable". While there is a definition of "dereferenceable", it is, in fact, a definition of "dereferenceable iterator". "dereferenceable" is used throughout Clause 23 (Containers) before its definition in Clause 24. In almost all cases it's referring to iterators, but in 16.4.4.3 [swappable.requirements] there is a mention of "dereferenceable object"; in 16.4.4.6 [allocator.requirements] the table of Descriptive variable definitions refers to a "dereferenceable pointer"; 20.2.3.3 [pointer.traits.functions] refers to a "dereferenceable pointer"; in 30.4.6.2.3 [locale.time.get.virtuals]/11 (do_get) there is a requirement that a pointer "shall be dereferenceable". In those specific cases it is not defined.[2011-03-02: Daniel comments:]
I believe that the currently proposed resolution of issue 2035 solves this issue as well.
[ 2011 Bloomington ]
Agree with Daniel, this will be handled by the resolution of 2035.
Proposed resolution:
Section: 21.3.5.4 [meta.unary.prop] Status: Open Submitter: Daniel Krügler Opened: 2011-08-20 Last modified: 2016-01-28
Priority: 3
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Discussion:
The currently agreed on proposed wording for 2015 using remove_all_extents<T>::type instead of the "an array of unknown bound" terminology in the precondition should be extended to some further entries especially in Table 49, notably the is_*constructible, is_*assignable, and is_*destructible entries. To prevent ODR violations, incomplete element types of arrays must be excluded for value-initialization and destruction for example. Construction and assignment has to be honored, when we have array-to-pointer conversions or pointer conversions of incomplete pointees in effect.
[2012, Kona]
The issue is that in three type traits, we are accidentally saying that in certain circumstances the type must give a specified answer when given an incomplete type. (Specifically: an array of unknown bound of incomplete type.) The issue asserts that there's an ODR violation, since the trait returns false in that case but might return a different version when the trait is completed.
Howard argues: no, there is no risk of an ODR violation. is_constructible<A[]> must return false regardless of whether A is complete, so there's no reason to forbid an array of unknown bound of incomplete types. Same argument applies to is_assignable. General agreement with Howard's reasoning.
There may be a real issue for is_destructible. None of us are sure what is_destructible is supposed to mean for an array of unknown bound (regardless of whether its type is complete), and the standard doesn't make it clear. The middle column doesn't say what it's supposed to do for incomplete types.
In at least one implementation, is_destructible<A[]> does return true if A is complete, which would result in ODR violation unless we forbid it for incomplete types.
Move to open. We believe there is no issue for is_constructible or is_assignable, but that there is a real issue for is_destructible.
Proposed resolution:
Section: 17.9.5 [exception.terminate] Status: Open Submitter: Daniel Krügler Opened: 2011-09-25 Last modified: 2016-01-28
Priority: 3
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Discussion:
Andrzej Krzemienski reported the following on comp.std.c++:
In N3290, which is to become the official standard, in 17.9.5.4 [terminate], paragraph 1 reads
Remarks: Called by the implementation when exception handling must be abandoned for any of several reasons (15.5.1), in effect immediately after evaluating the throw-expression (18.8.3.1). May also be called directly by the program.
It is not clear what is "in effect". It was clear in previous drafts where paragraphs 1 and 2 read:
Called by the implementation when exception handling must be abandoned for any of several reasons (15.5.1). May also be called directly by the program.
Effects: Calls the terminate_handler function in effect immediately after evaluating the throw-expression (18.8.3.1), if called by the implementation, or calls the current terminate_handler function, if called by the program.It was changed by N3189. The same applies to function unexpected (D. 11.4, paragraph 1).
Assuming the previous wording is still intended, the wording can be read "unless std::terminate is called by the program, we will use the handler that was in effect immediately after evaluating the throw-expression". This assumes that there is some throw-expression connected to every situation that triggers the call to std::terminate. But this is not the case:
- In case std::thread is assigned to or destroyed while being joinable there is no throw-expression involved.
- In case std::unexpected is called by the program, std::terminate is triggered by the implementation - no throw-expression involved.
- In case a destructor throws during stack unwinding we have two throw-expressions involved.
Which one is referred to?
In case std::nested_exception::rethrow_nested is called for an object that has captured no exception, there is no throw-expression involved directly (and may no throw be involved even indirectly). Next, 17.9.5.1 [terminate.handler], paragraph 2 saysRequired behavior: A terminate_handler shall terminate execution of the program without returning to the caller.
This seems to allow that the function may exit by throwing an exception (because word "return" implies a normal return).
One could argue that words "terminate execution of the program" are sufficient, but then why "without returning to the caller" would be mentioned. In case such handler throws, noexcept specification in function std::terminate is violated, and std::terminate would be called recursively - should std::abort not be called in case of recursive std::terminate call? On the other hand some controlled recursion could be useful, like in the following technique.
The here mentioned wording changes by N3189 in regard to 17.9.5.4 [terminate] p1 were done for a better separation of effects (Effects element) and additional normative wording explanations (Remarks element), there was no meaning change intended. Further, there was already a defect existing in the previous wording, which was not updated when further situations where defined, when std::terminate where supposed to be called by the implementation.
The part "in effect immediately after evaluating the throw-expression" should be removed and the quoted reference to 17.9.5.1 [terminate.handler] need to be part of the effects element where it refers to the current terminate_handler function, so should be moved just after "Effects: Calls the current terminate_handler function." It seems ok to allow a termination handler to exit via an exception, but the suggested idiom should better be replaced by a more simpler one based on evaluating the current exception pointer in the terminate handler, e.g.
void our_terminate (void) {
std::exception_ptr p = std::current_exception();
if (p) {
... // OK to rethrow and to determine it's nature
} else {
... // Do something else
}
}
[2011-12-09: Daniel comments]
A related issue is 2111.
[2012, Kona]
Move to Open.
There is an interaction with Core issues in this area that Jens is already supplying wording for. Review this issue again once Jens wording is available.
Alisdair to review clause 15.5 (per Jens suggestion) and recommend any changes, then integrate Jens wording into this issue.
Proposed resolution:
Section: 33.10.6 [futures.promise], 33.10.10 [futures.task] Status: LEWG Submitter: Jonathan Wakely Opened: 2011-11-01 Last modified: 2019-06-03
Priority: 4
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Discussion:
This example is ill-formed according to C++11 because uses_allocator<promise<R>, A>::value is true, but is_constructible<promise<R>, A, promise<R>&&>::value is false. Similarly for packaged_task.
#include <future>
#include <memory>
#include <tuple>
using namespace std;
typedef packaged_task<void()> task;
typedef promise<void> prom;
allocator<task> a;
tuple<task, prom> t1{ allocator_arg, a };
tuple<task, prom> t2{ allocator_arg, a, task{}, prom{} };
[2012, Portland]
This is an allocator issue, and should be dealt with directly by LWG.
[2013-03-06]
Jonathan suggests to make the new constructors non-explicit and makes some representational improvements.
[2013-09 Chicago]
Move to deferred.
This issue has much in common with similar problems with std::function that are being addressed by the polymorphic allocators proposal currently under evaluation in LEWG. Defer further discussion on this topic until the final outcome of that paper and its proposed resolution is known.
[2014-02-20 Re-open Deferred issues as Priority 4]
[2016-08 Chicago]
Fri PM: Send to LEWG - and this also applies to function in LFTS.
[2019-06-03 Jonathan Wakely provides updated wording]
Jonathan updates wording post-2976 and observes that this resolution conflicts with 3003.
Previous resolution [SUPERSEDED]:
[This wording is relative to the FDIS.]
Add to 33.10.6 [futures.promise], class template promise synopsis, as indicated:
namespace std { template <class R> class promise { public: promise(); template <class Allocator> promise(allocator_arg_t, const Allocator& a); template <class Allocator> promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept; promise(promise&& rhs) noexcept; promise(const promise& rhs) = delete; ~promise(); […] }; […] }Change 33.10.6 [futures.promise] as indicated:
promise(promise&& rhs) noexcept; template <class Allocator> promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;-5- Effects: constructs a new promise object and transfers ownership of the shared state of rhs (if any) to the newly-constructed object.
-6- Postcondition: rhs has no shared state. -?- [Note: a is not used — end note]Add to 33.10.10 [futures.task], class template packaged_task synopsis, as indicated:
namespace std { template<class> class packaged_task; // undefined template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)> { public: // construction and destruction packaged_task() noexcept; template <class Allocator> packaged_task(allocator_arg_t, const Allocator& a) noexcept; template <class F> explicit packaged_task(F&& f); template <class F, class Allocator> explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f); ~packaged_task(); // no copy packaged_task(const packaged_task&) = delete; template<class Allocator> packaged_task(allocator_arg_t, const Allocator& a, const packaged_task&) = delete; packaged_task& operator=(const packaged_task&) = delete; // move support packaged_task(packaged_task&& rhs) noexcept; template <class Allocator> packaged_task(allocator_arg_t, const Allocator& a, packaged_task&& rhs) noexcept; packaged_task& operator=(packaged_task&& rhs) noexcept; void swap(packaged_task& other) noexcept; […] }; […] }Change 33.10.10.2 [futures.task.members] as indicated:
packaged_task() noexcept; template <class Allocator> packaged_task(allocator_arg_t, const Allocator& a) noexcept;-1- Effects: constructs a packaged_task object with no shared state and no stored task.
-?- [Note: a is not used — end note][…]
packaged_task(packaged_task&& rhs) noexcept; template <class Allocator> packaged_task(allocator_arg_t, const Allocator& a, packaged_task&& rhs) noexcept;-5- Effects: constructs a new packaged_task object and transfers ownership of rhs's shared state to *this, leaving rhs with no shared state. Moves the stored task from rhs to *this.
-6- Postcondition: rhs has no shared state. -?- [Note: a is not used — end note]
Proposed resolution:
[This wording is relative to N4810.]
Add to 33.10.6 [futures.promise], class template promise synopsis, as indicated:
namespace std {
template <class R>
class promise {
public:
promise();
template <class Allocator>
promise(allocator_arg_t, const Allocator& a);
template <class Allocator>
promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;
promise(promise&& rhs) noexcept;
promise(const promise& rhs) = delete;
~promise();
[…]
};
[…]
}
Change 33.10.6 [futures.promise] as indicated:
promise(promise&& rhs) noexcept; template <class Allocator> promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;-5- Effects: constructs a new promise object and transfers ownership of the shared state of rhs (if any) to the newly-constructed object.
-6- Postcondition: rhs has no shared state. -?- [Note: a is not used — end note]
Section: 28.6.8 [template.mask.array] Status: Open Submitter: Thomas Plum Opened: 2011-12-10 Last modified: 2016-01-28
Priority: 4
View all issues with Open status.
Discussion:
Recently I received a Service Request (SR) alleging that one of our testcases causes an undefined behavior. The complaint is that 28.6.8 [template.mask.array] in C++11 (and the corresponding subclause in C++03) are interpreted by some people to require that in an assignment "a[mask] = b", the subscript mask and the rhs b must have the same number of elements.
IMHO, if that is the intended requirement, it should be stated explicitly. In any event, there is a tiny editorial cleanup that could be made: In C++11, 28.6.8.1 [template.mask.array.overview] para 2 mentions"the expression a[mask] = b;"
but the semicolon cannot be part of an expression. The correction could omit the semicolon, or change the word "expression" to "assignment" or "statement".
Here is the text of the SR, slightly modified for publication:Subject: SR01174 LVS _26322Y31 has undefined behavior [open]
[Client:]
The test case t263.dir/_26322Y31.cpp seems to be illegal as it has an undefined behaviour. I searched into the SRs but found SRs were not related to the topic explained in this mail (SR00324, SR00595, SR00838).const char vl[] = {"abcdefghijklmnopqrstuvwxyz"}; const char vu[] = {"ABCDEFGHIJKLMNOPQRSTUVWXYZ"}; const std::valarray<char> v0(vl, 27), vm5(vu, 5), vm6(vu, 6); std::valarray<char> x = v0; […] const bool vb[] = {false, false, true, true, false, true}; const std::valarray<bool> vmask(vb, 6); x = v0; x[vmask] = vm5; // ***** HERE.... steq(&x[0], "abABeCghijklmnopqrstuvwxyz"); x2 = x[vmask]; // ***** ....AND HERE […]This problem has already been discussed between [experts]: See thread http://gcc.gnu.org/ml/libstdc++/2009-11/threads.html#00051 Conclusion http://gcc.gnu.org/ml/libstdc++/2009-11/msg00099.html
[Plum Hall:]
Before I log this as an SR, I need to check one detail with you. I did read the email thread you mentioned, and I did find a citation (see INCITS ISO/IEC 14882-2003 Section 26.3.2.6 on valarray computed assignments): Quote: "If the array and the argument array do not have the same length, the behavior is undefined", But this applies to computed assignment (*=, +=, etc), not to simple assignment. Here is the C++03 citation re simple assignment: 26.3.2.2 valarray assignment [lib.valarray.assign]valarray<T>& operator=(const valarray<T>&);1 Each element of the *this array is assigned the value of the corresponding element of the argument array. The resulting behavior is undefined if the length of the argument array is not equal to the length of the *this array.
In the new C++11 (N3291), we find ...
26.6.2.3 valarray assignment [valarray.assign]valarray<T>& operator=(const valarray<T>& v);1 Each element of the *this array is assigned the value of the corresponding element of the argument array. If the length of v is not equal to the length of *this, resizes *this to make the two arrays the same length, as if by calling resize(v.size()), before performing the assignment.
So it looks like the testcase might be valid for C++11 but not for C++03; what do you think?
[Client:]
I quite agree with you but the two problems I mentioned:x[vmask] = vm5; // ***** HERE.... […] x2 = x[vmask]; // ***** ....AND HERErefer to mask_array assignment hence target the C++03 26.3.8 paragraph. Correct?
[Plum Hall:]
I mentioned the contrast between C++03 26.3.2.2 para 1 versus C++11 26.6.2.3 para 1. But in C++03 26.3.8, I don't find any corresponding restriction. Could you quote the specific requirement you're writing about? [Client:]
I do notice the difference between c++03 26.3.2.2 and c++11 26.6.2.3 about assignments between different sized valarray and I perfectly agree with you. But, as already stated, this is not a simple valarray assignment but a mask_array assignment (c++03 26.3.8 / c++11 26.6.8). See c++11 quote below: 26.6.8 Class template mask_array
26.6.8.1 Class template mask_array overview
[....]
This template is a helper template used by the mask subscript operator: mask_array<T> valarray<T>::operator[](const valarray<bool>&).
It has reference semantics to a subset of an array specified by a boolean mask. Thus, the expression a[mask] = b; has the effect of assigning the elements of b to the masked elements in a (those for which the corresponding element in mask is true.)
26.6.8.2 mask_array assignment
void operator=(const valarray<T>&) const; const mask_array& operator=(const mask_array&) const;1 These assignment operators have reference semantics, assigning the values of the argument array elements to selected elements of the valarray<T> object to which it refers.
In particular, [one of the WG21 experts] insisted on the piece "the elements of b".
That is why I reported the test t263.dir/_26322Y31.cpp having an undefined behaviour. [Plum Hall:]
OK, I can see that I will have to ask WG21; I will file an appropriate issue with the Library subgroup. In the meantime, I will mark this testcase as "DISPUTED" so that it is not required for conformance testing, until we get a definitive opinion.
[2012, Kona]
Moved to Open.
There appears to be a real need for clarification in the standard, and implementations differ in their current interpretation. This will need some research by implementers and a proposed resolution before further discussion is likely to be fruitful.
Proposed resolution:
Section: 21.3.5.4 [meta.unary.prop] Status: Open Submitter: Dave Abrahams Opened: 2011-12-09 Last modified: 2023-05-25
Priority: 3
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with Open status.
Discussion:
IMO if we specified is_[nothrow_]constructible in terms of a variable declaration whose validity requires destructibility, it is clearly a bug in our specification and a failure to realize the actual original intent. The specification should have been in terms of placement-new.
Daniel:A defaulted copy/move constructor for a class X is defined as deleted (9.5.3 [dcl.fct.def.delete]) if X has:
[…]
— any direct or virtual base class or non-static data member of a type with a destructor that is deleted or inaccessible from the defaulted constructor,
[…]
Dave:
This is about is_nothrow_constructible in particular. The fact that it is
foiled by not having a noexcept dtor is a defect.
[2012, Kona]
Move to Open.
is_nothrow_constructible is defined in terms of is_constructible, which is defined by looking at a hypothetical variable and asking whether the variable definition is known not to throw exceptions. The issue claims that this also examines the type's destructor, given the context, and thus will return false if the destructor can potentially throw. At least one implementation (Howard's) does return false if the constructor is noexcept(true) and the destructor is noexcept(false). So that's not a strained interpretation. The issue is asking for this to be defined in terms of placement new, instead of in terms of a temporary object, to make it clearer that is_nothrow_constructible looks at the noexcept status of only the constructor, and not the destructor.
Sketch of what the wording would look like:
require is_constructible, and then also require that a placement new operation does not throw. (Remembering the title of this issue... What does this imply for swap?
If we accept this resolution, do we need any changes to swap?
STL argues: no, because you are already forbidden from passing anything with a throwing destructor to swap.
Dietmar argues: no, not true. Maybe statically the destructor can conceivably throw for some values, but maybe there are some values known not to throw. In that case, it's correct to pass those values to swap.
[2017-01-27 Telecon]
Gave the issue a better title
This issue interacts with 2827
Ville would like "an evolution group" to take a look at this issue.
[2020-08; LWG reflector]
A poll was taken to close the issue as NAD, but only gained three votes in favour (and one vote against, which was subsequently withdrawn).
[2022-03; LWG reflector]
A poll was taken to close the issue as NAD, with six votes in favour. (and one vote against, subsequently withdrawn).
"Write a paper if you want something else. These traits have well established meaning now." "Minimizing requirements is not as important a concern for standard library concepts as as minimizing the number of concepts. Requirements like 'I need to construct but not destroy an object' are niche enough that we don't need to support them."
[2022-11-30; LWG telecon]
Alisdair intends to write a paper for this.
[2023-05-25; May 2023 mailing]
Alisdair provided P2842R0.
Proposed resolution:
Section: 30.4.3.3.3 [facet.num.put.virtuals], 31.5.2.2.2 [ios.fmtflags], 31.5.5.1 [fmtflags.manip] Status: Open Submitter: Benjamin Kosnik Opened: 2011-12-15 Last modified: 2023-02-07
Priority: 3
View other active issues in [facet.num.put.virtuals].
View all other issues in [facet.num.put.virtuals].
View all issues with Open status.
Discussion:
Iostreams should include a manipulator to toggle grouping on/off for locales that support grouped digits. This has come up repeatedly and been deferred. See LWG 826 for the previous attempt.
If one is using a locale that supports grouped digits, then output will always include the generated grouping characters. However, very plausible scenarios exist where one might want to output the number, un-grouped. This is similar to existing manipulators that toggle on/off the decimal point, numeric base, or positive sign. See some user commentary here.[21012, Kona]
Move to Open.
This is a feature request.
Walter is slightly uncomfortable with processing feature requests through the issues lists.
Alisdair says this is far from the first feature request that has come in from the issues list.
STL: The fact that you can turn off grouping on hex output is compelling.
Marshall: if we add this flag, we'll need to update tables 87-91 as well.
STL: If it has been implemented somewhere, and it works, we'd be glad to add it.
Howard: We need to say what the default is.
Alisdair sumarizes:
(1) We want clear wording that says what the effect is of turning the flag off;
(2) what the default values are, and
(3) how this fits into tables 87-90. (and 128)
[Issaquah 2014-02-10-12: Move to LEWG]
Since this issue was filed, we have grown a new working group that is better placed to handle feature requests.
We will track such issues with an LEWG status until we get feedback from the Library Evolution Working Group.
[Issaquah 2014-02-12: LEWG discussion]
| SF | F | N | A | SA |
| 2 | 4 | 1 | 0 | 0 |
Think about the ABI break for adding a flag. But this could be mitigated by putting the data into an iword instead of a flag.
This needs to change Stage 2 in [facet.num.put.virtuals].
Previous resolution, which needs the above corrections:
This wording is relative to the FDIS.
Insert in 30.4.3.3.3 [facet.num.put.virtuals] paragraph 5:
Stage 1: The first action of stage 1 is to determine a conversion specifier. The tables that describe this determination use the following local variables
fmtflags flags = str.flags() ; fmtflags basefield = (flags & (ios_base::basefield)); fmtflags uppercase = (flags & (ios_base::uppercase)); fmtflags floatfield = (flags & (ios_base::floatfield)); fmtflags showpos = (flags & (ios_base::showpos)); fmtflags showbase = (flags & (ios_base::showbase)); fmtflags showgrouping = (flags & (ios_base::showgrouping));Change header <ios> synopsis, [iostreams.base.overview] as indicated:
#include <iosfwd> namespace std { […] // 27.5.6, manipulators: […] ios_base& showpoint (ios_base& str); ios_base& noshowpoint (ios_base& str); ios_base& showgrouping (ios_base& str); ios_base& noshowgrouping(ios_base& str); ios_base& showpos (ios_base& str); ios_base& noshowpos (ios_base& str); […] }Change class ios_base synopsis, 31.5.2 [ios.base] as indicated:
namespace std { class ios_base { public: class failure; // 27.5.3.1.2 fmtflags typedef T1 fmtflags; […] static constexpr fmtflags showpoint = unspecified ; static constexpr fmtflags showgrouping = unspecified ; static constexpr fmtflags showpos = unspecified ; […] }; }Add a new entry to Table 122 — "fmtflags effects" as indicated:
Table 122 — fmtflags effects Element Effect(s) if set […] showpoint generates a decimal-point character unconditionally in generated floatingpoint output showgrouping generates grouping characters unconditionally in generated output […] After [ios::fmtflags] p12 insert the following:
ios_base& showgrouping(ios_base& str);-?- Effects: Calls str.setf(ios_base::showgrouping).
-?- Returns: str.ios_base& noshowgrouping(ios_base& str);-?- Effects: Calls str.unsetf(ios_base::showgrouping).
-?- Returns: str.
Proposed resolution:
Section: 16.3.2 [structure] Status: Open Submitter: Jens Maurer Opened: 2012-03-08 Last modified: 2021-06-25
Priority: 3
View all issues with Open status.
Discussion:
The front matter in clause 17 should clarify that postconditions will not hold if a standard library function exits via an exception. Postconditions or guarantees that apply when an exception is thrown (beyond the basic guarantee) are described in an "Exception safety" section.
[ 2012-10 Portland: Move to Open ]
Consensus that we do not clearly say this, and that we probably should. A likely location to describe the guarantees of postconditions could well be a new sub-clause following 99 [res.on.required] which serves the same purpose for requires clauses. However, we need such wording before we can make progress.
Also, see 2137 for a suggestion that we want to see a paper resolving both issues together.
[2015-05-06 Lenexa: EirkWF to write paper addressing 2136 and 2137]
MC: Idea is to replace all such "If no exception" postconditions with "Exception safety" sections.
[2021-06-20; Daniel comments]
An informal editorial change suggestion has recently been made whose editorial implementation would promote the idea that the default assumption is that Postconditions: are only met if the function doesn't exit with an exception.
After analyzing all current existing Postconditions: elements the following seems to hold: Affected by this issue are only non-noexcept functions and mostly non-constructor functions (unless the Postconditions: element says something about the value of its arguments). Most existing Postconditions seem to be intended to apply only in non-exceptional cases. I found some where this is presumably not intended, namely those of the expressions os << x and is >> v in Tables [tab:rand.req.eng] and [tab:rand.req.dist], maybe also 29.11.2.4 [time.zone.db.remote] p4. Nonetheless, the editorial change seems to be applicable even without having this issue resolved, because it doesn't actually change the normative state by itself.Proposed resolution:
Section: 32.7.3 [re.regex.assign] Status: Open Submitter: Jonathan Wakely Opened: 2012-03-08 Last modified: 2021-06-20
Priority: 3
View all other issues in [re.regex.assign].
View all issues with Open status.
Discussion:
The post-conditions of basic_regex<>::assign 32.7.3 [re.regex.assign] p16 say:
If no exception is thrown, flags() returns f and mark_count() returns the number of marked sub-expressions within the expression.
The default expectation in the library is that post-conditions only hold, if there is no failure (see also 2136), therefore the initial condition should be removed to prevent any misunderstanding.
[ 2012-10 Portland: Move to Open ]
A favorable resolution clearly depends on a favorable resolution to 2136. There is also a concern that this is just one example of where we would want to apply such a wording clean-up, and which is really needed to resolve both this issue and 2136 is a paper providing the clause 17 wording that gives the guarantee for postcondition paragraphs, and then reviews clauses 18-30 to apply that guarantee consistently. We do not want to pick up these issues piecemeal, as we risk opening many issues in an ongoing process.
[2015-05-06 Lenexa: EricWF to write paper addressing 2136 and 2137]
Proposed resolution:
This wording is relative to N3376.
template <class string_traits, class A> basic_regex& assign(const basic_string<charT, string_traits, A>& s, flag_type f = regex_constants::ECMAScript);[…]
-15- Effects: Assigns the regular expression contained in the string s, interpreted according the flags specified in f. If an exception is thrown, *this is unchanged. -16- Postconditions:If no exception is thrown,flags() returns f and mark_count() returns the number of marked sub-expressions within the expression.
Section: 16.4.4.2 [utility.arg.requirements] Status: Open Submitter: Nikolay Ivchenkov Opened: 2012-03-23 Last modified: 2020-10-02
Priority: 3
View all other issues in [utility.arg.requirements].
View all issues with Open status.
Discussion:
According to 16.4.4.2 [utility.arg.requirements] p1
The template definitions in the C++ standard library refer to various named requirements whose details are set out in tables 17-24. In these tables, T is an object or reference type to be supplied by a C++ program instantiating a template; a, b, and c are values of type (possibly const) T; s and t are modifiable lvalues of type T; u denotes an identifier; rv is an rvalue of type T; and v is an lvalue of type (possibly const) T or an rvalue of type const T.
Is it really intended that T may be a reference type? If so, what should a, b, c, s, t, u, rv, and v mean? For example, are "int &" and "int &&" MoveConstructible?
As far as I understand, we can explicitly specify template arguments for std::swap and std::for_each. Can we use reference types there?
#include <iostream>
#include <utility>
int main()
{
int x = 1;
int y = 2;
std::swap<int &&>(x, y); // undefined?
std::cout << x << " " << y << std::endl;
}
#include <algorithm>
#include <iostream>
#include <iterator>
#include <utility>
struct F
{
void operator()(int n)
{
std::cout << n << std::endl;
++count;
}
int count;
} f;
int main()
{
int arr[] = { 1, 2, 3 };
auto&& result = std::for_each<int *, F &&>( // undefined?
std::begin(arr),
std::end(arr),
std::move(f));
std::cout << "count: " << result.count << std::endl;
}
Are these forms of usage well-defined?
Let's also consider the following constructor of std::thread:template <class F, class ...Args> explicit thread(F&& f, Args&&... args);Requires: F and each Ti in Args shall satisfy the MoveConstructible requirements.
When the first argument of this constructor is an lvalue (e.g. a name of a global function), template argument for F is deduced to be lvalue reference type. What should "MoveConstructible" mean with regard to an lvalue reference type? Maybe the wording should say that std::decay<F>::type and each std::decay<Ti>::type (where Ti is an arbitrary item in Args) shall satisfy the MoveConstructible requirements?
[2013-03-15 Issues Teleconference]
Moved to Open.
The questions raised by the issue are real, and should have a clear answer.
[2015-10, Kona Saturday afternoon]
STL: std::thread needs to be fixed, and anything behaving like it needs to be fixed, rather than reference types. std::bind gets this right. We need to survey this. GR: That doesn't sound small to me. STL: Seach for CopyConstructible etc. It may be a long change, but not a hard one.
MC: It seems that we don't have a PR. Does anyone have one? Is anyone interested in doing a survey?
[2016-03, Jacksonville]
Casey volunteers to make a survey
[2016-06, Oulu]
During an independent survey performed by Daniel as part of the analysis of LWG 2716, some overlap was found between these two issues. Daniel suggested to take responsibility for surveying LWG 2146 and opined that the P/R of LWG 2716 should restrict to forwarding references, where the deduction to lvalue references can happen without providing an explicit template argument just by providing an lvalue function argument.
[2018-06, Rapperwsil]
Jonathan says that this will be covered by his Omnibus requirements paper.
[2019 Cologne Wednesday night]
Daniel will start working on this again; Marshall to provide rationale why some of the examples are not well-formed.
[2020-10-02; Issue processing telecon: change from P2 to P3]
For the examples given in the original report, the for_each case is now banned, because 27.2 [algorithms.requirements] p15 forbids explicit template argument lists. std::thread's constructor has also been fixed to describe the requirements on decay_t<T> instead of T.
We believe we're more careful these days about using remove_cvref or decay as needed, but there are still places where we incorrectly state requirements in terms of types that might be references. The swap case still needs solving. Still need a survey.
Proposed resolution:
Section: 16.4.4.3 [swappable.requirements], 24.2.2.1 [container.requirements.general] Status: LEWG Submitter: Robert Shearer Opened: 2012-04-13 Last modified: 2020-09-06
Priority: 3
View all other issues in [swappable.requirements].
View all issues with LEWG status.
Discussion:
Sub-clause 16.4.4.3 [swappable.requirements] defines two notions of swappability: a binary version defining when two objects are swappable with one another, and a unary notion defining whether an object is swappable (without qualification), with the latter definition requiring that the object satisfy the former with respect to all values of the same type.
Let T be a container type based on a non-propagating allocator whose instances do not necessarily compare equal. Then sub-clause 24.2.2.1 [container.requirements.general] p7 implies that no object t of type T is swappable (by the unary definition). Throughout the standard it is the unary definition of "swappable" that is listed as a requirement (with the exceptions of 22.2.2 [utility.swap] p4, 22.3.2 [pairs.pair] p31, 22.4.4.3 [tuple.swap] p2, 27.7.3 [alg.swap] p2, and 27.7.3 [alg.swap] p6, which use the binary definition). This renders many of the mutating sequence algorithms of sub-clause 27.7 [alg.modifying.operations], for example, inapplicable to sequences of standard container types, even where every element of the sequence is swappable with every other. Note that this concern extends beyond standard containers to all future allocator-based types. Resolution proposal: I see two distinct straightforward solutions:I favor the latter solution, for reasons detailed in the following issue.
[ 2012-10 Portland: Move to Open ]
The issue is broader than containers with stateful allocotors, although they are the most obvious example contained within the standard itself. The basic problem is that once you have a stateful allocator, that does not propagate_on_swap, then whether two objects of this type can be swapped with well defined behavior is a run-time property (the allocators compare equal) rather than a simple compile-time property that can be deduced from the type. Strictly speaking, any type where the nature of swap is a runtime property does not meet the swappable requirements of C++11, although typical sequences of such types are going to have elements that are all swappable with any other element in the sequence (using our other term of art for specifying requirements) as the common case is a container of elements who all share the same allocator.
The heart of the problem is that the swappable requirments demand that any two objects of the same type be swappable with each other, so if any two such objects would not be swappable with each other, then the whole type is never swappable. Many algorithms in clause 25 are specified in terms of swappable which is essentially an overspecification as all they actually need is that any element in the sequence is swappable with any other element in the sequence.
At this point Howard joins the discussion and points out that the intent of introducing the two swap-related terms was to support vector<bool>::reference types, and we are reading something into the wording that was never intended. Consuses is that regardless of the intent, that is what the words today say.
There is some support to see a paper reviewing the whole of clause 25 for this issue, and other select clauses as may be necessary.
There was some consideration to introducing a note into the front of clause 25 to indicate swappable requirements in the clause should be interpreted to allow such awkward types, but ultimately no real enthusiasm for introducing a swappable for clause 25 requirement term, especially if it confusingly had the same name as a term used with a subtly different meaning through the rest of the standard.
There was no enthusiasm for the alternate resolution of requiring containers with unequal allocators that do not propagate provide a well-defined swap behavior, as it is not believed to be possible without giving swap linear complexity for such values, and even then would require adding the constraint that the container element types are CopyConstructible.
Final conclusion: move to open pending a paper from a party with a strong interest in stateful allocators.
[2016-03 Jacksonville]
Alisdair says that his paper P0178 addresses this.
[2016-06 Oulu]
P0178 reviewed, and sent back to LEWG for confirmation.
Thursday Morning: A joint LWG/LEWG meeting declined to adopt P0178.
[2017-02 in Kona, LEWG responds]
Note in the issue that this is tracked here
[2017-06-02 Issues Telecon]
Leave as LEWG; priority 3
Proposed resolution:
Apply P0178.
Section: 22.2.2 [utility.swap], 16.4.4.3 [swappable.requirements], 24.2.2.1 [container.requirements.general] Status: LEWG Submitter: Robert Shearer Opened: 2012-04-13 Last modified: 2020-10-02
Priority: 2
View all other issues in [utility.swap].
View all issues with LEWG status.
Discussion:
Sub-clause 22.2.2 [utility.swap] defines a non-member 'swap' function with defined behavior for all MoveConstructible and MoveAssignable types. It does not guarantee constant-time complexity or noexcept in general, however this definition does render all objects of MoveConstructible and MoveAssignable type swappable (by the unary definition of sub-clause 16.4.4.3 [swappable.requirements]) in the absence of specializations or overloads.
The overload of the non-member swap function defined in Table 96, however, defines semantics incompatible with the generic non-member swap function, since it is defined to call a member swap function whose semantics are undefined for some values of MoveConstructible and MoveAssignable types. The obvious (perhaps naive) interpretation of sub-clause 16.4.4.3 [swappable.requirements] is as a guide to the "right" semantics to provide for a non-member swap function (called in the context defined by 16.4.4.3 [swappable.requirements] p3) in order to provide interoperable user-defined types for generic programming. The standard container types don't follow these guidelines. More generally, the design in the standard represents a classic example of "contract narrowing". It is entirely reasonable for the contract of a particular swap overload to provide more guarantees, such as constant-time execution and noexcept, than are provided by the swap that is provided for any MoveConstructible and MoveAssignable types, but it is not reasonable for such an overload to fail to live up to the guarantees it provides for general types when it is applied to more specific types. Such an overload or specialization in generic programming is akin to an override of an inherited virtual function in OO programming: violating a superclass contract in a subclass may be legal from the point of view of the language, but it is poor design and can easily lead to errors. While we cannot prevent user code from providing overloads that violate the more general swap contract, we can avoid doing so within the library itself. My proposed resolution is to draw a sharp distinction between member swap functions, which provide optimal performance but idiosyncratic contracts, and non-member swap functions, which should always fulfill at least the contract of 22.2.2 [utility.swap] and thus render objects swappable. The member swap for containers with non-propagating allocators, for example, would offer constant-time guarantees and noexcept but would only offer defined behavior for values with allocators that compare equal; non-member swap would test allocator equality and then dispatch to either member swap or std::swap depending on the result, providing defined behavior for all values (and rendering the type "swappable"), but offering neither the constant-time nor the noexcept guarantees.[2013-03-15 Issues Teleconference]
Moved to Open.
This topic deserves more attention than can be given in the telecon, and there is no proposed resolution.
[2016-03 Jacksonville]
Alisdair says that his paper P0178 addresses this.
[2016-08 Chicago]
Send to LEWG
[2016-06 Oulu]
P0178 reviewed, and sent back to LEWG for confirmation.
Thursday Morning: A joint LWG/LEWG meeting declined to adopt P0178.
[2020-10-02; remove P0178 as Proposed Resolution]
Proposed resolution:
Section: 24.3.7.5 [array.zero] Status: Open Submitter: Daryle Walker Opened: 2012-05-08 Last modified: 2021-03-14
Priority: 3
View all other issues in [array.zero].
View all issues with Open status.
Discussion:
Objects of std::array<T, N> are supposed to be initialized with aggregate initialization (when not the destination of a copy or move). This clearly works when N is positive. What happens when N is zero? To continue using an (inner) set of braces for initialization, a std::array<T, 0> implementation must have an array member of at least one element, and let default initialization take care of those secret elements. This cannot work when T has a set of constructors and the default constructor is deleted from that set. Solution: Add a new paragraph in 24.3.7.5 [array.zero]:
The unspecified internal structure of array for this case shall allow initializations like:
array<T, 0> a = { };and said initializations must be valid even when T is not default-constructible.
[2012, Portland: Move to Open]
Some discussion to understand the issue, which is that implementations currently have freedom to implement an empty array by holding a dummy element, and so might not support value initialization, which is surprising when trying to construct an empty container. However, this is not mandated, it is an unspecified implementation detail.
Jeffrey points out that the implication of 24.3.7.1 [array.overview] is that this initialization syntax must be supported by empty array objects already. This is a surprising inference that was not obvious to the room, but consensus is that the reading is accurate, so the proposed resolution is not necessary, although the increased clarity may be useful.
Further observation is that the same clause effectively implies that T must always be DefaultConstructible, regardless of N for the same reasons - as an initializer-list may not supply enough values, and the remaining elements must all be value initialized.
Concern that we are dancing angels on the head of pin, and that relying on such subtle implications in wording is not helpful. We need a clarification of the text in this area, and await wording.
[2015-02 Cologne]
DK: What was the outcome of Portland? AM: Initially we thought we already had the intended behaviour. We concluded that T must always be DefaultConstructible, but I'm not sure why. GR: It's p2 in std::array, "up to N". AM: That wording already implies that "{}" has to work when N is zero. But the wording of p2 needs to be fixed to make clear that it does not imply that T must be DefaultConstructible.
Conclusion: Update wording, revisit later.[2015-10, Kona Saturday afternoon]
MC: How important is this? Can you not just use default construction for empty arrays?
TK: It needs to degenerate properly from a pack. STL agrees.
JW: Yes, this is important, and we have to make it work.
MC: I hate the words "initialization like".
JW: I'll reword this.
WEB: Can I ask that once JW has reworded this we move it to Review rather than Open?
MC: We'll try to review it in a telecon and hopefully get it to tentatively ready.
STL: Double braces must also work: array<T, 0> a = {{}};.
Jonathan to reword.
[2018-03-14 Wednesday evening issues processing]
Jens suggested that we remove the requirement that begin() == end() == unique-value, specifically the unique value part.
Previous resolution [SUPERSEDED]:
This wording is relative to N3376.
Add the following new paragraph between the current 24.3.7.5 [array.zero] p1 and p2:
-1- array shall provide support for the special case N == 0.
-?- The unspecified internal structure of array for this case shall allow initializations like:array<T, 0> a = { };and said initializations must be valid even when T is not default-constructible.
-2- In the case that N == 0, begin() == end() == unique value. The return value of data() is unspecified. -3- The effect of calling front() or back() for a zero-sized array is undefined. -4- Member function swap() shall have a noexcept-specification which is equivalent to noexcept(true).
[2018-06-14, Jonathan Wakely comments and provides revised wording]
The new wording does not address the 2018-03-14 suggestion from Jens to remove the unique value. It wasn't clear to me that there was consensus to make that change, and it would be a change in behaviour not just a clarification of the existing wording.
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
Modify 24.3.7.5 [array.zero] as indicated:
-1- array
-?- A zero-sized array type is an aggregate that meets the DefaultConstructible (Table 22) and CopyConstructible (Table 24) requirements. There is a single element of the aggregate, of an unspecified DefaultConstructible type. [Note: This allows initialization of the form array<T, 0> a = {{}};. There is no requirement for T to be DefaultConstructible. — end note] -2-shallprovides support for the special case of a zero-sized array that is always empty, i.e. N == 0, with the properties described in this subclause.In the case that N == 0, begin() == end() == unique valuebegin() and end() return non-dereferenceable iterators such that begin() == end() and a.begin() != b.begin() where a and b are distinct objects of the same zero-sized array type. The return value of data() is unspecified. -3- The effect of calling front() or back() for a zero-sized array is undefined. -4- Member function swap()shall havehas constant complexity and a non-throwing exception specification.
[2018-08-30, Jonathan revises wording following feedback from Daniel Kruegler and Tim Song.]
Daniel noted that it's undefined to compare iterators from different containers,
so a.begin() != b.begin() can't be used. That means whether the iterators
from different containers are unique is unobservable anyway.
We can say they don't share the same underlying sequence, which tells users they can't compare them
and tells implementors they can't return value-initialized iterators.
Tim noted that it's not sufficient to say the unspecified type in a zero-sized array is DefaultConstructible,
it also needs to be constructible from = {}. Also, a zero-sized array should be CopyAssignable.
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Modify 24.3.7.5 [array.zero] as indicated:
-1- array
-?- A zero-sized array type is an aggregate that meets the Cpp17DefaultConstructible (Table 24) and Cpp17CopyConstructible (Table 26) and Cpp17CopyAssignable (Table 28) requirements. There is a single element of the aggregate, of an unspecified Cpp17DefaultConstructible type that is copy-list-initializable from an empty list. [Note: This allows initialization of the form array<T, 0> a = {{}};. There is no requirement for T to be Cpp17DefaultConstructible. — end note] -2-shallprovides support for the special case of a zero-sized array that is always empty, i.e. N == 0, with the properties described in this subclause.In the case that N == 0, begin() == end() == unique valuebegin() and end() return non-dereferenceable iterators such that begin() == end(). When a and b are distinct objects of the same zero-sized array type, a.begin() and b.begin() are not iterators over the same underlying sequence. [Note: Therefore begin() does not return a value-initialized iterator — end note]. The return value of data() is unspecified. -3- The effect of calling front() or back() for a zero-sized array is undefined. -4- Member function swap()shall havehas constant complexity and a non-throwing exception specification.
[2021-03-14; Johel Ernesto Guerrero Peña comments and provides improved wording]
The currently proposed wording specifies:
There is a single element of the aggregate, of an unspecified Cpp17DefaultConstructible type that is copy-list-initializable from an empty list.
This doesn't specify which expressions involving zero-sized array specializations are constant expressions. 24.3.7.1 [array.overview] p4 specifies array<T, 0> to be a structural type when T is a structural type. This requires that its single element, let's call it single-element, be a structural type. But that says nothing about which of the special member functions of single-element are constant expressions. By being a structural type, single-element is permitted to be implemented as a literal class type. To meet this requirement, single-element can be implemented to have one constexpr constructor that is not a copy or move constructor (6.8.1 [basic.types.general] p10), so its default constructor needn't be constexpr. This is unlike non-zero-sized array specializations, which inherit these properties from T. Furthermore, this permits an implementation of single-element whose default constructor stores the result of std::source_location::current() in a data member (as exemplified in the specification for current). Cpp17DefaultConstructible doesn't require the default constructor to produce equal values. The simplest way to solve these issues and any other that might arise from future changes and oversights would be to specify single-element as an empty aggregate type. Then the wording from 24.3.7.2 [array.cons] p1 makes it clear that all the special member functions are constant expressions. It would also mean that the default constructor produces template-argument-equivalent values.
Proposed resolution:
This wording is relative to N4878.
Modify 24.3.7.5 [array.zero] as indicated:
-1- array
-?- A zero-sized array type is an aggregate that meets the Cpp17DefaultConstructible (Table 29 [tab:cpp17.defaultconstructible]) and Cpp17CopyConstructible (Table 31 [tab:cpp17.copyconstructible]) and Cpp17CopyAssignable (Table 33 [tab:cpp17.copyassignable]) requirements. There is a single element of the aggregate, of an unspecified empty aggregate type. [Note: This allows initialization of the form array<T, 0> a = {{}};. There is no requirement for T to be Cpp17DefaultConstructible. — end note] -2-shallprovides support for the special case of a zero-sized array that is always empty, i.e. N == 0, with the properties described in this subclause.In the case that N == 0, begin() == end() == unique valuebegin() and end() return non-dereferenceable iterators such that begin() == end(). When a and b are distinct objects of the same zero-sized array type, a.begin() and b.begin() are not iterators over the same underlying sequence. [Note: Therefore begin() does not return a value-initialized iterator — end note].. The return value of data() is unspecified. -3- The effect of calling front() or back() for a zero-sized array is undefined. -4- Member function swap()shall havehas constant complexity and a non-throwing exception specification.
Section: 24.3.11.3 [vector.capacity] Status: Open Submitter: Nikolay Ivchenkov Opened: 2012-05-08 Last modified: 2022-11-06
Priority: 3
View other active issues in [vector.capacity].
View all other issues in [vector.capacity].
View all issues with Open status.
Discussion:
There are various operations on std::vector that can cause elements of the vector to be moved from one location to another. A move operation can use either rvalue or const lvalue as argument; the choice depends on the value of !is_nothrow_move_constructible<T>::value && is_copy_constructible<T>::value, where T is the element type. Thus, some operations on std::vector (e.g. 'resize' with single parameter, 'reserve', 'emplace_back') should have conditional requirements. For example, let's consider the requirement for 'reserve' in N3376 – 24.3.11.3 [vector.capacity]/2:
Requires: T shall be MoveInsertable into *this.
This requirement is not sufficient if an implementation is free to select copy constructor when !is_nothrow_move_constructible<T>::value && is_copy_constructible<T>::value evaluates to true. Unfortunately, is_copy_constructible cannot reliably determine whether T is really copy-constructible. A class may contain public non-deleted copy constructor whose definition does not exist or cannot be instantiated successfully (e.g., std::vector<std::unique_ptr<int>> has copy constructor, but this type is not copy-constructible). Thus, the actual requirements should be:
if !is_nothrow_move_constructible<T>::value && is_copy_constructible<T>::value then T shall be CopyInsertable into *this;
otherwise T shall be MoveInsertable into *this.
Maybe it would be useful to introduce a new name for such conditional requirement (in addition to "CopyInsertable" and "MoveInsertable").
[2016-08 Chicago]
The problem does not appear to be as severe as described. The MoveInsertable requirements are consistently correct, but an issue may arise on the exception-safety guarantees when we check for is_copy_constructible_v<T>. The problem, as described, is typically for templates that appear to have a copy constructor, but one that fails to compile once instantiated, and so gives a misleading result for the trait.
In general, users should not provide such types, and the standard would not serve users well by trying to address support for such types. However, the standard should not be providing such types either, such as vector<unique_ptr<T>>. A possible resolution would be to tighten the constraints in Table 80 — Container Requirements, so that if the Requirements for the copy constructor/assingment operator of a container are not satisfied, that operation shall be deleted.
A futher problem highlighted by this approach is that there are no constraints on the copy-assignment operator, so that vector<unique_ptr<T>> should be CopyAssignable! However, we can lift the equivalent constraints from the Allocator-aware container requirements.
[08-2016, Chicago]
Fri PM: Move to Open
[2017-11 Albuquerque Saturday issues processing]
There's a bunch of uses of "shall" here that are incorrect. Also, CopyInsertable contains some semantic requirements, which can't be checked at compile time, so 'ill-formed' is not possible for detecting that.
[2018-06 Rapperswil Wednesday issues processing]
Daniel to provide updated wording.
[2018-06-12, Daniel provides revised wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
24.2.2.1 [container.requirements.general] Table 80 — Container requirements Expression Return type Operational semantics Assertion/note/pre-/post-condition Complexity X(a) Requires: T is CopyInsertable into X (see below) ., otherwise this expression shall be ill-formed.
post: a == X(a).linear X u(a)
X u = a;Requires: T is CopyInsertable into X (see below) ., otherwise this expression shall be ill-formed.
post: u == a.linear ... ... ... ... ... r = a X& Requires: T is CopyInsertable into X and CopyAssignable, otherwise this expression shall be ill-formed.
post: r == a.linear
24.2.2.1 [container.requirements.general] Table 83 — Allocator-aware container requirements Expression Return type Operational semantics Assertion/note/pre-/post-condition Complexity a = t X& Requires: T is CopyInsertable into X and CopyAssignable ., otherwise this expression shall be ill-formed
post: r == a.linear
[2018-08-23 Batavia Issues processing. Priority to 3]
Changed CopyInsertable -> Cpp17CopyInsertable in the resolution.
Tim says that the wording is not quite right — it imposes additional requirements.
[2022-11-06; Daniel comments]
This issue has considerable overlap with LWG 3758.
Proposed resolution:
This wording is relative to N4750.
The revised wording below uses the new Mandates: element introduced by adopting P0788R3 at the Rapperswil meeting 2018 and which will become a new term of art with Jonathan's omnibus paper throughout the Standard Library.
| Expression | Return type | Operational semantics | Assertion/note/pre-/post-condition | Complexity |
| X(a) |
Mandates: Syntactic requirements of T is Cpp17CopyInsertable into X (see below). Requires: T is Cpp17CopyInsertable into X post: a == X(a). |
linear | ||
| X u(a) X u = a; |
Mandates: Syntactic requirements of T is Cpp17CopyInsertable into X (see below). Requires: T is Cpp17CopyInsertable into X post: u == a. |
linear | ||
| ... | ... | ... | ... | ... |
| r = a | X& |
Mandates: Syntactic requirements of T is Cpp17CopyInsertable into X (see below) and CopyAssignable. Requires: T is Cpp17CopyInsertable into X and CopyAssignable. post: r == a. |
linear |
| Expression | Return type | Operational semantics | Assertion/note/pre-/post-condition | Complexity |
| a = t | X& |
Mandates: Syntactic requirements of T is Cpp17CopyInsertable into X and CopyAssignable. Requires: T is Cpp17CopyInsertable into X and CopyAssignable. post: r == a. |
linear |
operator + in the description of the algorithmsSection: 27 [algorithms] Status: Open Submitter: Nikolay Ivchenkov Opened: 2012-08-01 Last modified: 2018-06-12
Priority: 4
View other active issues in [algorithms].
View all other issues in [algorithms].
View all issues with Open status.
Discussion:
According to 27.1 [algorithms.general]/12,
In the description of the algorithms operators + and - are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics of a+n is the same as that of
X tmp = a; advance(tmp, n); return tmp;
There are several places where such operator + is applied to an output iterator — for example, see the description of std::copy:
template<class InputIterator, class OutputIterator> OutputIterator copy(InputIterator first, InputIterator last, OutputIterator result);-1- Effects: Copies elements in the range [first,last) into the range [result,result + (last - first)) starting from first and proceeding to last. For each non-negative integer n < (last - first), performs *(result + n) = *(first + n).
std::advance is not supposed to be applicable to output iterators, so we need a different method of description.
See also message c++std-lib-32908.[2014-06-07 Daniel comments and provides wording]
The specification for output iterators is somewhat tricky, because here a sequence of increments is required to be combined with intervening assignments to the dereferenced iterator. I tried to respect this fact by using a conceptual assignment operation as part of the specification.
Another problem in the provided as-if-code is the question which requirements are imposed on n. Unfortunately, the corresponding function advance is completely underspecified in this regard, so I couldn't borrow wording from it. We cannot even assume here that n is the difference type of the iterator, because for output iterators there is no requirements for this associated type to be defined. The presented wording attempts to minimize assumptions, but still can be considered as controversial.[2018-06 Rapperswil Wednesday issues processing]
Status to Open
Proposed resolution:
This wording is relative to N4606.
Change 27.1 [algorithms.general] around p12 as indicated:
-12- In the description of the algorithms operators + and - are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics of a+n is the same as that of
X tmp = a; advance(tmp, n); return tmp;when X meets the input iterator requirements (25.3.5.3 [input.iterators]), otherwise it is the same as that of
X tmp = a; for (auto i = n; i; ++tmp, (void) --i) *tmp = Expr(i); return tmp;where Expr(i) denotes the (n-i)th expression that is assigned to for the corresponding algorithm; and that of b-a is the same as of
return distance(a, b);
Section: 24.2.8.2 [unord.req.except] Status: Open Submitter: Alisdair Meredith Opened: 2012-09-23 Last modified: 2019-07-22
Priority: 3
View all issues with Open status.
Discussion:
The hash functor and key-comparison functor of unordered containers are allowed to throw on swap.
24.2.8.2 [unord.req.except]p3 "For unordered associative containers, no swap function throws an exception unless that exception is thrown by the swap of the container's Hash or Pred object (if any)."
In such a case we must offer the basic exception safety guarantee, where both objects are left in valid but unspecified states, and no resources are leaked. This yields a corrupt, un-usable container if the first swap succeeds, but the second fails by throwing, as the functors form a matched pair.
So our basic scenario is first, swap the allocators if the allocators propagate on swap, according to allocator_traits. Next we swap the pointers to our internal hash table data structures, so that they match the allocators that allocated them. (Typically, this operation cannot throw). Now our containers are back in a safely destructible state if an exception follows.
Next, let's say we swap the hash functor, and that throws. We have a corrupt data structure, in that the buckets are not correctly indexed by the correct functors, lookups will give unpredicatable results etc. We can safely restore a usable state by forcibly clearing each container - which does not leak resources and leaves us with two (empty but) usable containers.
Now let us assume that the hasher swap succeeds. Next we swap the equality comparator functor, and this too could throw. The important point to bear in mind is that these two functors form an important pairing - two objects that compare equal by the equality functor must also hash to the same value. If we swap one without the other, we most likely leave the container in an unusable state, even if we clear out all elements.
1. A colleague pointed out that the solution for this is to dynamically allocate the two functors, and then we need only swap pointers, which is not a throwing operation. And if we don't want to allocate on default construction (a common QoI request), we might consider moving to a dynamically allocated functors whenever swap is called, or on first insertion. Of course, allocating memory in swap is a whole new can of worms, but this does not really sound like the design we had intended.
2. The simplest option is to say that we do not support hasher or equality functors that throw on ADL swap. Note that the requirement is simply to not throw, rather than to be explicitly marked as noexcept. Throwing functors are allowed, so long as we never use values that would actually manifest a throw when used in an unordered container.
Pablo went on to give me several more options, to be sure we have a full set to consider:
3. Disallow one or the other functor from throwing. In that case, the possibly-throwing functor must be swapped first, then the other functor, the allocator, and the data pointer(s) afterwards (in any order -- there was a TC that allocator assignment and swap may not throw if the corresponding propagation trait is true.). Of course, the question becomes: which functor is allowed to throw and which one is not?
4. Require that any successful functor swap be reliably reversible. This is very inventive. I know of no other place in the standard where such a requirement is stated, though I have occasionally wanted such a guarantee.
5. Allow a failed swap to leave the containers in a state where future insertions may fail for reasons other than is currently allowed. Specifically, if the hash and equality functors are out of sync, all insertions will fail. Presumably some "incompletely swapped" exception would be thrown. This is "slightly" inventive, although people have been discussing "radioactive" states for a while.
[2013-03-15 Issues Teleconference]
Moved to Open.
[2019 Cologne Wednesday night]
Billy to write resolution for option #2. This may require a paper.
Proposed resolution:
Section: 24.2.8 [unord.req] Status: Open Submitter: Alisdair Meredith Opened: 2012-10-09 Last modified: 2016-12-10
Priority: 3
View other active issues in [unord.req].
View all other issues in [unord.req].
View all issues with Open status.
Discussion:
The user cannot specify a max_load_factor for their unordered container at construction, it must be supplied after the event, when the container is potentially not empty. The contract for this method is deliberately vague, not guaranteeing to use the value supplied by the user, and any value actually used will be used as a ceiling that the container will attempt to respect.
The only guarantee we have is that, if user requests a max_load_factor that is less than the current load_factor, then the operation will take constant time, thus outlawing an implementation that chooses to rehash and so preserve as a class invariant that load_factor < max_load_factor.
Reasonable options conforming to the standard include ignoring the user's request if the requested value is too low, or deferring the rehash to the next insert operation and allowing the container to have a strange state (wrt max_load_factor) until then - and there is still the question of rehashing if the next insert is for a duplicate key in a unique container.
Given the deliberate vagueness of the current wording, to support a range of reasonable (but not perfect) behaviors, it is not clear why the equally reasonable rehash to restore the constraint should be outlawed. It is not thought that this is a performance critical operation, where users will be repeatedly setting low load factors on populated containers, in a tight or (less unlikely) an instant response scenario.
[2013-03-15 Issues Teleconference]
Moved to Open.
Alisdair to provide wording.
[2016-11-12, Issaquah]
Sat PM: Howard to provide wording
[2016-11-17 Howard provided wording.]
The provided wording is consistent with LWG discussion in Issaquah. An implementation of the proposed wording would be setting max_load_factor() to max(z, load_factor()). This preserves the container invariant:
load_factor() <= max_load_factor()And it preserves the existing behavior that no rehash is done by this operation.
If it is desired to change the max_load_factor() to something smaller than the current load_factor() that can be done by first reducing the current load_factor() by either increasing bucket_count() (via rehash or reserve), or decreasing size() (e.g. erase), and then changing max_load_factor().
This resolution reaffirms that load_factor() <= max_load_factor() is a container invariant which can never be violated.
[2016-11-27, Nico comments]
Current implementations behave differently.
In regard to the sentence"The only guarantee we have is that, if user requests a max_load_factor that is less than the current load_factor, then the operation will take constant time, thus outlawing an implementation that chooses to rehash and so preserve as a class invariant that load_factor < max_load_factor."Note that the current spec says that there is constant complexity without any precondition. So, rehashing to keep the invariant would violate the spec (which is probably not be the intention). This issue is related to LWG 2199.
Proposed resolution:
Modify Table 87 as follows:
| Expression | Return type | Assertion/note pre-/post-condition | Complexity |
|---|---|---|---|
| a.max_load_factor(z) | void |
Pre: z shall be positive. May change the container's maximum load factor, uing z as a hint. Post: a.load_factor() <= a.max_load_factor() Note: a.load_factor() is not modified by this operation. |
Constant |
Section: 33.10.9 [futures.async] Status: Deferred Submitter: Detlef Vollmann Opened: 2012-10-19 Last modified: 2016-01-28
Priority: 4
View other active issues in [futures.async].
View all other issues in [futures.async].
Discussion:
promise, packaged_task, and async are the only places where a shared state is actually supposed to be allocated. Accordingly, promise and packaged_task are "allocator-aware". But function template async provides no way to provide an allocator.
[2013-09 Chicago]
Matt: deprecate async
Nico: read my paper Alisdair: defer issues to wait for polymorphic allocators Alisdair: defer, active topic of research Deferred[2014-02-20 Re-open Deferred issues as Priority 4]
[2015-05 Lenexa, SG1 response]
We want whatever status approximates: "will not fix; we're working on a replacement facility and don't want to add features to a broken one"
Proposed resolution:
Section: 24.2.4 [sequence.reqmts], 24.2.7 [associative.reqmts], 24.2.8 [unord.req], 28.5.3.2 [rand.req.seedseq] Status: Open Submitter: Jeffrey Yasskin Opened: 2012-10-21 Last modified: 2020-09-06
Priority: 3
View other active issues in [sequence.reqmts].
View all other issues in [sequence.reqmts].
View all issues with Open status.
Discussion:
In 24.2.4 [sequence.reqmts] p3, we have "il designates an object of type initializer_list<value_type>", and then several functions that take 'il' as an argument. However, an expression like {1, 2, 'a'} is not an object of type initializer_list<int> unless it's used to initialize an explicitly-typed variable of that type. I believe we want:
std::vector<int> v;
v = {1, 2, 'a'};
to compile portably, so we should say something different when defining 'il'. The same phrasing happens in 24.2.7 [associative.reqmts], 24.2.8 [unord.req], and 28.5.3.2 [rand.req.seedseq].
This may just be an editorial issue because the actual class synopses declare the functions to take initializer_list<exact_type>.[2013-03-15 Issues Teleconference]
Moved to Open.
This is definitely not NAD
Should copy the suggested wording as the proposed resolution.
[2019-03-26; Daniel comments and provides wording]
The 2013-03-15 comment is confusing, since it recommends to "copy the suggested wording as the proposed resolution". I couldn't find such wording in the issue nor in the associated wiki, so I provided that wording out of myself. The tricky part is to define which kind of braced-init-list we want to allow. As Tim Song pointed out, we still need the existing support for std::initializer_list<value_type> as well, because otherwise existing semantics based on expressions such as li.begin() won't work anymore. The below suggested wording restricts supported braced-init-lists to every initializer list that can be used to copy-list-initialize an object of type std::initializer_list<value_type> by saying:
"bil designates any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list])"
As a drive-by fix, the provided wording adds another initialization "expression" that makes the construction of the form
std::vector<int> v = {1, 2, 'a'};
valid (We just miss a copy-initialization case).
Proposed resolution:
This wording is relative to N4810.
[Drafting note: We need to special-case the "expression" X u = bil; below, because for empty braced-init-list the effects are those of calling the default constructor. — end drafting note]
Modify 24.2.4 [sequence.reqmts] as indicated:
-3- In Tables 66 and 67, […] il designates a
n objectvalue of type initializer_list<value_type>, bil designates any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list]), […]
Modify Table 66 — "Sequence container requirements (in addition to container)" as indicated:
Table 66 — Sequence container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-condition[…] X(il)
X u = il;Equivalent to X(il.begin(), il.end())
or X u(il.begin(), il.end());, respectivelyX(bil) Equivalent to X(initializer_list<value_type>(bil)) X u = bil; If bil is empty, equivalent to X u;, otherwise
equivalent to X u = initializer_list<value_type>(bil);a = il X& […] a = bil X& Equivalent to a = initializer_list<value_type>(bil) […] a.insert(p, il) iterator […] a.insert(p, bil) iterator Equivalent to a.insert(p, initializer_list<value_type>(bil)) […] a.assign(il) void […] a.assign(bil) void Equivalent to a.assign(initializer_list<value_type>(bil)) […]
Modify 24.2.7 [associative.reqmts] as indicated:
-8- In Table 69, […] il designates a
n objectvalue of type initializer_list<value_type>, bil designates any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list]), […]
Modify Table 69 — "Associative container requirements (in addition to container)" as indicated:
Table 69 — Associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity […] X(il)
X u = il;same as X(il.begin(), il.end())
or X u(il.begin(), il.end());, respectivelysame as X(il.begin(), il.end())
or X u(il.begin(), il.end());, respectivelyX(bil) Equivalent to X(initializer_list<value_type>(bil)) X u = bil; If bil is empty, equivalent to X u;, otherwise
equivalent to X u = initializer_list<value_type>(bil);X(il,c) same as X(il.begin(), il.end(), c) same as X(il.begin(), il.end(), c) X(bil, c) Equivalent to X(initializer_list<value_type>(bil), c) a = il X& […] […] a = bil X& Equivalent to a = initializer_list<value_type>(bil) […] a.insert(il) void equivalent to a.insert(il.begin(), il.end()) a.insert(bil) void Equivalent to a.insert(initializer_list<value_type>(bil)) […] a.assign(il) void […] a.assign(bil) void Equivalent to a.assign(initializer_list<value_type>(bil)) […]
Modify 24.2.8 [unord.req] p11's bullet list as indicated:
-11- In Table 70:
(11.1) — […]
[…]
(11.14) — il denotes a value of type initializer_list<value_type>,
(11.?) — bil denotes any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list]),
[…]
Modify Table 70 — "Unordered associative container requirements (in addition to container)" as indicated:
[Drafting note: There is a preexisting issue with Table 70, that there is no symbol u specified ("u denotes the name of a variable being declared"), so existing initialization forms with a named variable are currently always written as "X a[…]" where a is defined as "a denotes a value of type X", the wording below follows this existing practice but the author of this wording would like to kindly ask the Project Editor to introduce said symbol u and apply it to all existing and new such named initialization forms instead. — end drafting note]
Table 70 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity […] X(il)
X a = il;X Same as X(il.begin(), il.end())
or X a(il.begin(), il.end());, respectivelySame as X(il.begin(), il.end())
or X a(il.begin(), il.end());, respectivelyX(bil) X Equivalent to X(initializer_list<value_type>(bil)) X a = bil; X If bil is empty, equivalent to X a;, otherwise
equivalent to X a = initializer_list<value_type>(bil);X(il, n) X Same as X(il.begin(), il.end(), n) Same as X(il.begin(), il.end(), n) X(bil, n) X Equivalent to X(initializer_list<value_type>(bil), n) X(il, n, hf) X Same as X(il.begin(), il.end(), n, hf) Same as X(il.begin(), il.end(), n, hf) X(bil, n, hf) X Equivalent to X(initializer_list<value_type>(bil), n, hf) X(il, n, hf, eq) X Same as X(il.begin(), il.end(), n, hf, eq) Same as X(il.begin(), il.end(), n, hf, eq) X(bil, n, hf, eq) X Equivalent to X(initializer_list<value_type>(bil), n, hf, eq) […] a = il X& […] […] a = bil X& Equivalent to a = initializer_list<value_type>(bil) […] a.insert(il) void Same as a.insert(il.begin(), il.end()). Same as a.insert(il.begin(), il.end()). a.insert(bil) void Equivalent to a.insert(initializer_list<value_type>(bil)) […]
Modify 28.5.3.2 [rand.req.seedseq] p2's bullet list as indicated:
-2- A class S satisfies the requirements of a seed sequence if the expressions shown in Table 82 are valid and have the indicated semantics, and […] In that Table and throughout this subclause:
(2.1) — […]
(2.?) — u denotes the name of a variable being declared,
[…]
(2.6) — il is a value of initializer_list<T>
.;(2.?) — bil denotes any braced-init-list suitable to copy-list-initialize an object of type initializer_list<T> (9.4.5 [dcl.init.list]).
Modify Table 82 — "Seed sequence requirements" as indicated:
Table 82 — Seed sequence requirements Expression Return type Pre/post-condition Complexity […] S(il)
S u = il;Same as S(il.begin(), il.end())
or S u(il.begin(), il.end());, respectivelysame as S(il.begin(), il.end())
or S u(il.begin(), il.end());, respectivelyS(bil) Equivalent to S(initializer_list<T>(bil)) S u = bil; If bil is empty, equivalent to S u;, otherwise
equivalent to S u = initializer_list<T>(bil);[…]
Section: 31.5.4.2 [basic.ios.cons] Status: Open Submitter: Andrey Semashev Opened: 2012-11-09 Last modified: 2021-07-31
Priority: 4
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Discussion:
There is an ambiguity in how std::basic_ios::init method (31.5.4.2 [basic.ios.cons]) can be used in the derived class. The Standard only specify the state of the basic_ios object after the call completes. However, in basic_ios default constructor description (31.5.4.2 [basic.ios.cons]) there is this sentence:
Effects: Constructs an object of class basic_ios (31.5.2.8 [ios.base.cons]) leaving its member objects uninitialized. The object shall be initialized by calling basic_ios::init before its first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.
This restriction hints that basic_ios::init should be called exactly once before the object can be used or destroyed, because basic_ios::init may not know whether it was called before or not (i.e. whether its members are actually uninitialized or are initialized by the previous call to basic_ios::init). There is no such restriction in the basic_ios::init preconditions so it is not clear whether it is allowed to call basic_ios::init multiple times or not.
This problem has already affected publicly available implementations. For example, Microsoft Visual C++ STL introduces a memory leak if basic_ios::init is called multiple times, while GCC 4.7 and STLPort reinitialize the basic_ios object correctly without memory leak or any other undesired effects. There was a discussion of this issue on Boost developers mailing list, and there is a test case that reproduces the problem. The test case is actually a bug report for my Boost.Log library, which attempts to cache basic_ostream-derived objects internally to avoid expensive construction and destruction. My stream objects allowed resetting the stream buffer pointers the stream is attached to, without requiring to destroy and construct the stream. My personal view of the problem and proposed resolution follows. While apparently the intent of basic_ios::init is to provide a way to initialize basic_ios after default construction, I see no reason to forbid it from being called multiple times to reinitialize the stream. Furthermore, it is possible to implement a conforming basic_ios that does not have this restriction. The quoted above section of the Standard that describes the effects of the default constructor is misleading. The Standard does not mandate any data members of basic_ios or ios_base (31.5.2 [ios.base]), which it derives from. This means that the implementation is allowed to use non-POD data members with default constructors that initialize the members with particular default values. For example, in the case of Microsoft Visual C++ STL the leaked memory is an std::locale instance that is dynamically allocated during basic_ios::init, a raw pointer to which is stored within ios_base. It is possible to store e.g. an unique_ptr instead of a raw pointer as a member of ios_base, the smart pointer will default initialize the underlying raw pointer on default construction and automatically destroy the allocated object upon being reset or destroyed, which would eliminate the leak and allow basic_ios::init to be called multiple times. This leads to conclusion that the default constructor of basic_ios cannot leave "its member objects uninitialized" but instead performs default initialization of the member objects, which would mean the same thing in case of POD types. However, I feel that restricting ios_base and basic_ios members to non-POD types is not acceptable. Since multiple calls to basic_ios::init are not forbidden by the Standard, I propose to correct the basic_ios default constructor description so that it is allowed to destroy basic_ios object without calling basic_ios::init. This would imply that any raw members of basic_ios and ios_base should be initialized to values suitable for destruction (essentially, this means only initializing raw pointers to NULL). The new wording could look like this:Effects: Constructs an object of class basic_ios (31.5.2.8 [ios.base.cons]) initializing its member objects to unspecified state, only suitable for basic_ios destruction. The object shall be initialized by calling basic_ios::init before its first use; otherwise the behavior is undefined.
This would remove the hint that basic_ios::init must be called exactly once. Also, this would remove the requirement for basic_ios::init to be called at all before the destruction. This is also an important issue because the derived stream constructor may throw an exception before it manages to call basic_ios::init (for example, if the streambuf constructor throws), and in this case the basic_ios destructor has undefined behavior.
To my mind, the described modification is sufficient to resolve the issue. But to emphasize the possibility to call basic_ios::init multiple times, a remark or a footnote for basic_ios::init postconditions could be added to explicitly state the semantics of calling it multiple times. The note could read as follows:The function can be called multiple times during the object lifetime. Each subsequent call reinitializes the object to the described in postconditions initial state.
[2013-04-20, Bristol]
Alisdair: The current wording is unclear but the proposed resolution is wrong
Solution: Clarify that init must be called once and only once. Move then to review.[2021-07-29 Tim comments]
The requirement that "init must be called once and only once" conflicts with the disposition of LWG 135.
Proposed resolution:
This wording is relative to N3485.
Edit 31.5.4.2 [basic.ios.cons] as indicated:
basic_ios();-2- Effects: Constructs an object of class basic_ios (31.5.2.8 [ios.base.cons])
leaving its member objects uninitializedinitializing its member objects to unspecified state, only suitable for basic_ios destruction. The object shall be initialized by calling basic_ios::init before its first useor before it is destroyed, whichever comes first; otherwise the behavior is undefined.void init(basic_streambuf<charT,traits>* sb);Postconditions: The postconditions of this function are indicated in Table 128.
-?- Remarks: The function can be called multiple times during the object lifetime. Each subsequent call reinitializes the object to the described in postconditions initial state.
Section: 24.2.7 [associative.reqmts], 24.2.8 [unord.req] Status: Open Submitter: Alisdair Meredith Opened: 2012-11-14 Last modified: 2015-10-22
Priority: 3
View other active issues in [associative.reqmts].
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Discussion:
The requirements on the functors used to arrange elements in the various associative and unordered containers are given by a set of expressions in tables 102 — Associative container requirements, and 103 — Unordered associative container requirements. In keeping with Library convention these expressions make the minimal requirements necessary on their types. For example, we have the following 3 row extracts for the unordered containers:
| Expression | Assertion/note pre-/post-condition |
X(n, hf, eq) X a(n, hf, eq) |
Requires: hasher and key_equal are CopyConstructible. |
X(n, hf) X a(n, hf) |
Requires: hasher is CopyConstructible and key_equal is DefaultConstructible. |
X(n) X a(n) |
Requires: hasher and key_equal are DefaultConstructible. |
However, the signature for each class template requires that the functors must effectively be CopyConstructible for each of these expressions:
template <class Key,
class T,
class Hash = hash<Key>,
class Pred = std::equal_to<Key>,
class Allocator = std::allocator<std::pair<const Key, T> > >
class unordered_map
{
...
// construct/destroy/copy
explicit unordered_map(size_type n = see below,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
...
}
The letter of the standard can be honored as long as implementors recognize their freedom to split this one signature into multiple overloads, so that the documented default arguments (requiring a CopyConstructible functor) are not actually passed as default arguments.
As we look into the requirements for the copy constructor and copy-assignment operator, the requirements are even more vague, as the explicit requirements on the functors are not called out, other than saying that the functors are copied.
Must the functors be CopyAssignable? Or is CopyConstructible sufficient in this case? Do we require that the functors be Swappable so that the copy-swap idiom can be deployed here? Note that a type that is both CopyConstructible and CopyAssignable is still not guaranteed to be Swappable as the user may delete the swap function for their type in their own namespace, which would be found via ADL.
Some clean-up of the requirements table looks necessary, to at least document the assignment behavior. In addition, we should have clear guidance on whether these functors should always be CopyConstructible, as suggested by the class template definitions, or if the requirement tables are correct and we should explicitly split up the constructors in the (unordered) associative containers to no longer use default (function) arguments to obtain their defaulted functors.
I recommend the simplest solution would be to always require that the functors for (unordered) associative containers be CopyConstructible, above the requirements tables themselves, so that the issue need not be addressed within the tables. I suggest that the assignment operators for these containers add the requirement that the functors be Swappable, rather than forwarding the corresponding Assignable requirement.
[2013-03-15 Issues Teleconference]
Moved to Open.
Alisdair to propose wording.
[2014-06-08, Daniel comments]
The area of this issue partially overlaps what LWG 2227 addresses.
[2015-10-20, Daniel comments]
The revised resolution of LWG 2227 should resolve this issue as well. It follows the recommendations of the submitter to require CopyConstructible requirements for the function objects owned by containers, but it does not impose any further fundamental requirements.
Proposed resolution:
See the resolution of LWG 2227.
Section: 32.10.4 [re.alg.replace] Status: New Submitter: Jeffrey Yasskin Opened: 2012-11-26 Last modified: 2016-01-28
Priority: 3
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Discussion:
template <class traits, class charT, class ST, class SA>
basic_string<charT, ST, SA>
regex_replace(const basic_string<charT, ST, SA>& s,
const basic_regex<charT, traits>& e,
const charT* fmt,
regex_constants::match_flag_type flags =
regex_constants::match_default);
and friends are documented as
Constructs an empty string result of type basic_string<charT, ST, SA> and calls regex_replace(back_inserter(result), s.begin(), s.end(), e, fmt, flags).
This appears to require the result to have a default-constructed allocator, which isn't even possible for all allocator types. I suspect the allocator should be copied from 's' instead. Possibly there should be an additional defaulted argument to override the allocator of the result.
Proposed resolution:
Section: 32.11.2.3 [re.tokiter.comp] Status: Open Submitter: Pete Becker Opened: 2012-11-21 Last modified: 2018-08-23
Priority: 3
View all issues with Open status.
Discussion:
Consider the following example:
std::string str0("x");
std::regex rg0("a");
std::regex_token_iterator it0(str0.begin(), str0.end(), rg0, -1); // points at "x" in str0
std::string str1("x");
std::regex rg1("b");
std::regex_token_iterator it1(str1.begin(), str1.end(), rg1, -1); // points at "x" in str1
32.11.2.3 [re.tokiter.comp] p1 says that it0.operator==(it1) returns true "if *this and right are both suffix iterators and suffix == right.suffix"; both conditions are satisfied in this example. It does not say that they must both be iterators into the same sequence, nor does it say (as general iterator requirements do) that they must both be in the domain of == in order for the comparison to be meaningful. It's a simple statement: they're equal if the strings they point at compare equal. Given this being a valid comparison, the obtained result of "true" looks odd.
The problem is that for iterator values prior to the suffix iterator, equality means the same regular expression and the same matched sequence (both uses of "same" refer to identity, not equality); for the suffix iterator, equality means that the matched sequences compare equal.[2014-02-10]
Priority set to 2
[2018-08-20 Casey adds a proposed resolution]
Priority changed to 3.
Marshall notes that iterator comparisons typically require the iterators to denote elements of the same sequence.Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Modify 32.11.2.3 [re.tokiter.comp] as follows:
bool operator==(const regex_token_iterator& right) const;-?- Expects: *this and right are both end-of-sequence iterators or both have the same underlying sequence.
-1- Returns: true if *this and right are both end-of-sequence iterators, or if […]
bool operator!=(const regex_token_iterator& right) const;-?- Expects: *this and right are both end-of-sequence iterators or both have the same underlying sequence.
-2- Returns: !(*this == right).
[2018-08-23 Casey revises the P/R in response to LWG feedback]
Proposed resolution:
This wording is relative to N4762.
Modify 32.11.2.3 [re.tokiter.comp] as follows:
bool operator==(const regex_token_iterator& right) const;-?- Expects: At least one of *this and right is an end-of-sequence iterator, or both *this and right have the same underlying sequence.
-1- Returns: true if *this and right are both end-of-sequence iterators, or if […]
bool operator!=(const regex_token_iterator& right) const;-?- Expects: At least one of *this and right is an end-of-sequence iterator, or both *this and right have the same underlying sequence.
-2- Returns: !(*this == right).
Section: 24.2.7 [associative.reqmts] Status: Open Submitter: Juan Soulie Opened: 2012-12-19 Last modified: 2019-04-23
Priority: 3
View other active issues in [associative.reqmts].
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Discussion:
Table 102 in 24.2.7 [associative.reqmts]/8 states on expression a.key_comp() that it "returns the comparison object out of which a was constructed". At the same time, 24.2.2.1 [container.requirements.general]/8 states (starting in the third line) that "...Any Compare, Pred, or Hash objects belonging to a and b shall be swappable and shall be exchanged by unqualified calls to non-member swap...". This is problematic for any compliant implementation, since once swapped the container cannot return the comparison object out of which it was constructed unless incurring in storing an otherwise needless object.
The simple solution is to correct that statement in Table 102, but I believe this is part of a larger problem of underspecified behavior: The new standard has made an effort in regards to allocators and now fully specifies what happens to stateful allocator objects. It has even specified what happens to stateful hasher and key_equal members of unordered containers (they propagate), but it says nothing about stateful comparison objects of (ordered) associative containers, except for the statement in 24.2.2.1 [container.requirements.general]/8 referred above and only related to swap. For example, it is unclear to me what is specified to happen on an assignment: should the comparison object be copied/moved along with the elements, or should the left-hand side object keep its own? Maybe this has been intentionally left unspecified with the purpose of compatibility with C++98, which I understand it specified that comparison objects were kept for the entire life of the container (like allocators) — an unfortunate choice. But anyway, the segment of 24.2.2.1 [container.requirements.general] quoted above seems to break any possible backwards compatibility with C++98 in this regard. Therefore, taking into consideration consistency with how this is dealed with for unordered associative containers, I propose that Table 102 is modified as follows:The row for expression a.key_comp() is changed so that its "assertion/note pre-/post-condition" reads "Returns a's comparison object."
A new row is added at the appropriate location (which I believe would be after "X(il)" row), with:
Table 102 — Associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity X(b)
X a(b)X Copy constructor. In addition to
the requirements of Table 96, copies
the comparison object.Linear in b.size() a = b X& Copy assignment operator. In addition to
the requirements of Table 96, copies the
comparison object.Linear in a.size() and b.size()
[2013-03-15 Issues Teleconference]
Moved to Review.
[2013-04-18, Bristol]
STL: can't believe we don't specify this already. this is totally necessary
Alisdair: how does it do this? copy construction? assignment? Also need it for move. STL: we already specify this for constructing from a comparator, not during copy construction though. Jonathan: don't like wording, should say "key_compare is CopyConstructible. Uses b.key_comp() as a comparison object." STL: we get it right for unordered! Jonathan: can't wordsmith this now, but I think implementations do the right thing. Alisdair: not sure what right thing is for moves. Also we say nothing about propagating allocators to functors.Moved to Open.
[2015-02 Cologne]
TK: There's no need for fine-grained propagate/not-propagate control. If you don't want to propagate the predicate, you can simply construct or insert from an iterator range.
VV: libstdc++ already implements the resolution of this issue. GR: There are a couple of other problems. We don't specify move constructor and move assignment for maps. Those are just general. TK: General container requirements already describe the semantics for {copy,move}-{construction,assignment}, so it doesn't seem that there's room for choice in std::map assignments. unordered_map is different, though. [Note: Check what general container requirements say about container equality.] DK will draft wording. The decision is to unambiguously make all {copy,move}-{construction,assignment} operations endow the LHS with the exact state of the RHS, including all predicates and hash function states. Conclusion: Update wording, revisit later.[2015-05-06 Lenexa: Waiting for updated wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N3485.
Change Table 102 as indicated:
Table 102 — Associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity … X(il) Same as X(il.begin(), il.end()). same as X(il.begin(), il.end()). X(b)
X a(b)Copy constructor. In addition to
the requirements of Table 96, copies
the comparison object.Linear in b.size() a = b X& Copy assignment operator. In addition to
the requirements of Table 96, copies the
comparison object.Linear in a.size() and b.size() … a.key_comp() X::key_compare rReturnsthea's comparison object
out of which a was constructed.constant
[2015-10-19 Daniel comments and provides alternative wording]
The current standard is especially unclear in regard to what effects move operations of unordered/associative containers should have. We have one example that is standardized exactly in this way by looking at 24.6.7.3 [priqueue.cons.alloc] p7:
template <class Alloc> priority_queue(priority_queue&& q, const Alloc& a);-7- Effects: Initializes c with std::move(q.c) as the first argument and a as the second argument, and initializes comp with std::move(q.comp)
A similarly comparable example are the move-operations of std::unique_ptr in regard to the deleter (when this is no a reference), which also respect move-capabilities of that function object.
We have wording from C++98 for associative containers (but not for unordered containers!) that was never adjusted to C++11 move-semantics in 24.2.7 [associative.reqmts] p12:When an associative container is constructed by passing a comparison object the container shall not store a pointer or reference to the passed object, even if that object is passed by reference. When an associative container is copied, either through a copy constructor or an assignment operator, the target container shall then use the comparison object from the container being copied, as if that comparison object had been passed to the target container in its constructor.
The second sentence of this wording is problematic for several reasons:
It only talks about copy operations, not about move operations, except that the term "assignment" without leading "copy" is a bit ambigious (albeit it seems clear in the complete context).
It is not really clear how to interpret "as if that comparison object had been passed to the target container in its constructor" for an assignment operation. A possible but not conclusive interpretation could be that this is wording supporting a "copy-via-swap" idiom.
There does not exist similar wording for unordered containers, except that Table 102 provides entries for copy construction and copy assignment of the containers whose wording just talks of "copies" in either case.
Existing implementations differ already:
Visual Studio 2015 uses copy construction and copy assignment for the two copy operations but uses swap operations for the move operations.
GCC's libstdc++ performs copy construction and copy assignment for the two copy operations and for the two move operations, respectively
clang++'s libc++ performs copy/move construction and copy/move assignment for the corresponding four copy/move operations
The alternative wording provided below attempts to clarify that container copy/move operations perform the corresponding copy/move operations on the owned function objects.
In addition the wording also resolves LWG 2215: I believe that the current wording should require that container function objects should meet the CopyConstructible requirements. Adding this general requirement also fixes the underspecified requirements of the accessor functions key_comp() and value_comp(). I don't think that a general requirement for Swappable is needed, only the member swap function currently requires this. Nonetheless the wording below does support stateful functors that are also moveable or move-assignable, therefore the specified semantics in terms of move operations. I should add the following warning, though: If this proposed wording would be accepted, there is a little chance of code breakage, because the current wording can be read that in general there is no requirement that the container functors are CopyConstructible. The following code example is accepted by gcc + libstd++:
#include <map>
#include <utility>
#include <iostream>
struct Cmp {
Cmp() = default;
Cmp(const Cmp&) = delete;
Cmp(Cmp&&) = delete;
Cmp& operator=(const Cmp&) = delete;
Cmp& operator=(Cmp&&) = delete;
template<class T>
bool operator()(const T& x, const T& y) const
{
return x < y;
}
};
typedef std::map<int, int, Cmp> MyMap;
int main() {
MyMap m;
std::cout << (m.find(12) == m.end()) << std::endl;
}
Previous resolution [SUPERSEDED]:
This wording is relative to N4527.
Change 24.2.7 [associative.reqmts] p8 as indicated:
-8- In Table 101, X denotes an associative container class, a denotes a value of type X, b denotes a possibly const value of type X, rv denotes a non-const rvalue of type X, u denotes the name of a variable being declared, […]
Change Table 101 as indicated:
Table 101 — Associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity … X::key_compare Compare Requires: Compare is CopyConstructible.
defaults to less<key_type>compile time X(c)
X u(c);Requires: key_compare is CopyConstructible.Effects: Constructs an empty container.
Uses a copy of c as a comparison object.[…] … X(i,j,c)
X u(i,j,c);Requires: key_compare is CopyConstructible.value_type is EmplaceConstructible into X from *i.
Effects: Constructs an empty container and inserts elements
from the range [i, j) into it; uses c as a comparison object.[…] … X(il) Same as X(il.begin(), il.end()). same as X(il.begin(), il.end()). X(b)
X a(b)(In addition to the requirements of Table 95)
Effects: Copy constructs the comparison object of a from
the comparison object of b.Linear in b.size() X(rv)
X a(rv)(In addition to the requirements of Table 95 and Table 98)
Effects: Move constructs the comparison object of a from
the comparison object of rv.constant a = b X& (In addition to the requirements of Table 95 and Table 98)
Requires: key_compare is CopyAssignable.
Effects: Copy assigns the comparison object of b
to the comparison object of a.Linear in a.size() and b.size() a = rv X& (In addition to the requirements of Table 95 and Table 98)
Requires: key_compare is MoveAssignable.
Effects: Move assigns from the comparison object of rv
to the comparison object of a.Linear … a.key_comp() X::key_compare rReturnsthea's comparison object
out of which a was constructed.constant Change 24.2.7 [associative.reqmts] p12 as indicated:
-12- When an associative container is constructed by passing a comparison object the container shall not store a pointer or reference to the passed object, even if that object is passed by reference.
When an associative container is copied, either through a copy constructor or an assignment operator, the target container shall then use the comparison object from the container being copied, as if that comparison object had been passed to the target container in its constructor.Change 24.2.8 [unord.req] p11 as indicated:
-11- In Table 102: X denotes an unordered associative container class, a denotes a value of type X, b denotes a possibly const value of type X, rv denotes a non-const rvalue of type X, […]
Change Table 102 as indicated:
Table 102 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity … X::hasher Hash Requires: Hash is CopyConstructible.
Hash shall be a unary function object type
such that the expression hf(k) has type std::size_t.compile time X::key_equal Pred Requires: Pred is CopyConstructible.
Pred shall be a binary predicate that takes
two arguments of type Key.
Pred is an equivalence relation.compile time … X(n, hf, eq)
X a(n, hf, eq)X Requires: hasher and key_equal are CopyConstructible.Effects: […][…] X(n, hf)
X a(n, hf)X Requires: hasher is CopyConstructible andkey_equal is DefaultConstructible.
Effects: […][…] … X(i, j, n, hf, eq)
X a(i, j, n, hf, eq)X Requires: hasher and key_equal are CopyConstructible.value_type is EmplaceConstructible into X from *i.
Effects: […][…] X(i, j, n, hf)
X a(i, j, n, hf)X Requires: hasher is CopyConstructible andkey_equal is DefaultConstructible.
value_type is EmplaceConstructible into X from *i.
Effects: […][…] … X(b)
X a(b)X Copy constructor. In addition(In addition to the requirements of Table 95)
to the requirements of Table 95,
copies the hash function,
predicate, and maximum load
factor.
Effects: Copy constructs the hash function, predicate, and maximum load factor
of a from the corresponding objects of b.Average case linear in
b.size(),
worst case quadratic.X(rv)
X a(rv)X (In addition to the requirements of Table 95 and Table 98)
Effects: Move constructs the hash function, predicate, and maximum load factor
of a from the corresponding objects of rv.constant a = b X& Copy assignment operator. In(In addition to the requirements of Table 95 and Table 98)
addition to the requirements of
Table 95, copies the hash
function, predicate, and
maximum load factor.
Requires: hasher and key_equal are CopyAssignable.
Effects: Copy assigns the hash function, predicate, and maximum load factor
of b to the corresponding objects of a.Average case linear in
b.size(),
worst case quadratic.a = rv X& (In addition to the requirements of Table 95 and Table 98)
Requires: hasher and key_equal are MoveAssignable.
Effects: Move assigns the hash function, predicate, and maximum load factor
from rv to the corresponding objects of a.Linear …
[2016-08-07]
Daniel removes the previously proposed wording to work on revised wording.
[2019-04-22, Billy comments]
In addition to the Cpp17CopyConstructible discussion going on there, I think we need to require that calling the comparison function when Compare itself is const needs to produce the same answer as if Compare is non-const.
Proposed resolution:
Section: 33.5.2 [atomics.syn], 33.5.4 [atomics.order] Status: SG1 Submitter: Daniel Krügler Opened: 2013-01-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The "magic" kill_dependency function is a function without any constraints on the template parameter T and is specified as
template <class T> T kill_dependency(T y) noexcept;-14- Effects: The argument does not carry a dependency to the return value (1.10).
-15- Returns: y.
I wonder whether the unconditional noexcept is really intended here: Assume we have some type U that has a potentially throwing move constructor (or it has a potentially throwing copy constructor and no move constructor), for any "normal" function template with the same signature and the same effects (modulo the dependency magic) this would mean that it cannot safely be declared noexcept because of the return statement being part of the complete function call affected by noexcept (The by-value function argument is irrelevant in this context). In other words it seems that a function call such as
struct S {
...
S(const S& r) { if(some condition) throw Something(); }
...
};
int main() {
S s1 = ...;
S s2 = std::kill_dependency(s1);
}
would be required to call std::terminate if the copy constructor of S throws during the return of std::kill_dependency.
To require copy elision for this already magic function would look like a low-hanging fruit to solve this problem, but this case is not covered by current copy elision rules see 12.8 p31 b1: "— in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) with the same cv-unqualified type as the function return type, the copy/move operation can be omitted by constructing the automatic object directly into the function's return value". Some options come into my mind:Make the exception-specification a constrained one in regard via std::is_nothrow_move_constructible:
template <class T> T kill_dependency(T y) noexcept(see below);
This is similar to the approach taken for function templates such as std::swap.
Use perfect forwarding (This needs further wording to correct the effects):
template <class T> T&& kill_dependency(T&& y) noexcept;
Impose constraints on the template arguments in regard to throwing exceptions while copying/moving.
Keep the state as it is but possibly add a note about a call of std::terminate in above scenario.
A second problem is that the current wording is not clear whether it is well-defined to call the function with types that are reference types, such as in the following example:
#include <atomic>
int main()
{
int a = 12;
int& b = std::kill_dependency<int&>(a);
}
It is unclear what kind of dependency is killed here. This is presumably a core language problem, but could affect the possible resolutions of the problem.
[2014-11 Urbana]
Recommend using a revised example:
int lookup(class D* p)
{
class E* q = p->a.load(memory_order_consume);
int y = std::kill_dependency(q->y);
}
[2015-02 Cologne]
Handed over to SG1.
Proposed resolution:
Section: 23.5 [c.strings] Status: New Submitter: Jason Merrill Opened: 2013-01-29 Last modified: 2016-01-28
Priority: 4
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Discussion:
Apparently C1X changes __STDC_UTF_16__ and __STDC_UTF_32__ from macros defined in uchar.h (and reflected in C++ by Table 79) to be predefined by the compiler. Do we want to do the same?
Proposed resolution:
Section: 23.5 [c.strings] Status: Open Submitter: Johannes Schaub Opened: 2013-02-02 Last modified: 2016-08-09
Priority: 3
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Discussion:
The non-explicit nature of the iterator-pair constructor of containers, such a
template <class InputIterator> vector(InputIterator first, InputIterator last, const Allocator& = Allocator());
can be selected in unexpected situations, leading to a hard runtime error, as demonstrated by the following example:
#include <vector>
void f(std::vector<char> v){ /* ... */}
int main() {
f({"A", "B"});
}
The actually intended initializer-list constructor isn't feasible here, so the best match is the constructor template
template <class InputIterator> vector(InputIterator first, InputIterator last, const Allocator& = Allocator());
This compiles, but will result in code running amok. The potential trap (that cannot be easily detected by the library implementation) could be reduced by making this constructor explicit. It would still have the effect to be selected here, but the code would be ill-formed, so the programmer gets a clear message here.
[2014-06 Rapperswil]
JW: can't fix this, don't want to touch this, Do The Right Thing clause has been a source of tricky issues. only really happens with string literals, that's the only way to create an array that isn't obviously an array
GR: want to see paper AM: is it only string literals, or also UDLs? STL: maybe, but we don't need to deal with that. This is only a problem in a very specific case Leave as Open.Proposed resolution:
Section: 17.3.5 [numeric.limits] Status: New Submitter: Pete Becker Opened: 2013-03-08 Last modified: 2018-11-08
Priority: 4
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Discussion:
This member should probably be named "is_ieee754". Or at least the standard should explain that IEC-559 no longer exists, and that it's been superseded by IEEE-754.
[2016-06, Oulu]
The ISO version of the standard is ISO/IEC/IEEE 60559:2011,
which C11 Annex F refers to as IEC 60559
(although C still refers to it as IEC 559 in the __STDC_IEC_559__ macro).
Proposed resolution:
Section: 20.3.1.3 [unique.ptr.single] Status: Open Submitter: Rob Desbois Opened: 2013-05-15 Last modified: 2017-03-21
Priority: 3
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Discussion:
N3337 20.3.1.3.6 [unique.ptr.single.modifiers] contains 2 non-normative notes stating:
[para 4]: "The order of these operations is significant because the call to get_deleter() may destroy *this."
[para 5]: "The postcondition does not hold if the call to get_deleter() destroys *this since this->get() is no longer a valid expression."
It seems this wording was created to resolve 998 due to the possibility that a unique_ptr may be destroyed through deletion of its stored pointer where that directly or indirectly refers to the same unique_ptr. If unique_ptr is required to support circular references then it seems this must be normative text: an implementation is currently allowed to operate on *this after the assignment and deletion specified in para 4, since this is only 'disallowed' by the non-normative note.
I propose the following draft rewording:
[para 4]: Effects: assigns p to the stored pointer, and then if the old value of the stored pointer, old_p, was not
equal to nullptr, calls get_deleter()(old_p). No operation shall be performed after the call to
get_deleter()(old_p) that requires *this to be valid, because the deletion may destroy *this if it is
referred to directly or indirectly by the stored pointer. [Note: The order of these operations is significant
because the call to get_deleter() may destroy *this. — end note]
I expect it will also be necessary to amend the requirements for a deleter, so in addition:
20.3.1.3 [unique.ptr.single] [para 1]: The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function object type (20.10), lvalue-reference to function, or lvalue-reference to function object type for which, given a value d of type D and a value ptr of type unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter. Where D is not an lvalue reference type, d(ptr) shall be valid if ptr refers directly or indirectly to the invoking unique_ptr object.
[2013-10-05, Stephan T. Lavavej comments and provides alternative wording]
In Chicago, we determined that the original proposed change to 20.3.1.3 [unique.ptr.single]/1 was insufficient, because d might be a reference to a deleter functor that's destroyed during self-destruction.
We believed that 20.3.1.3.6 [unique.ptr.single.modifiers]/4 was already sufficiently clear. The Standard occasionally prevents implementations of X from doing various things, through the principle of "nothing allows X to fail in that situation". For example, v.push_back(v[0]) is required to work for non-empty vectors because nothing allows that to fail. In this case, the intent to allow self-destruction is already clear. Additionally, we did not believe that 20.3.1.3.6 [unique.ptr.single.modifiers]/5 had to be changed. The current note is slightly squirrely but it does not lead to confusion for implementers or users.Previous resolution from Rob Desbois:
Edit 20.3.1.3 [unique.ptr.single] p1 as indicated:
The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function object type (20.10), lvalue-reference to function, or lvalue-reference to function object type for which, given a value d of type D and a value ptr of type unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter. Where D is not an lvalue reference type, d(ptr) shall be valid if ptr refers directly or indirectly to the invoking unique_ptr object.
Edit 20.3.1.3.6 [unique.ptr.single.modifiers] p4+5 as indicated:
void reset(pointer p = pointer()) noexcept;-3- Requires: The expression get_deleter()(get()) shall be well formed, shall have well-defined behavior, and shall not throw exceptions.
-4- Effects: assigns p to the stored pointer, and then if the old value of the stored pointer, old_p, was not equal to nullptr, calls get_deleter()(old_p). No operation shall be performed after the call to get_deleter()(old_p) that requires *this to be valid, because the deletion may destroy *this if it is referred to directly or indirectly by the stored pointer.[Note: The order of these operations is significant because the call to get_deleter() may destroy *this. — end note]-5- Postconditions: If the call get_deleter()(old_p) destroyed *this, none. Otherwise, get() == p.[Note: The postcondition does not hold if the call to get_deleter() destroys *this since this->get() is no longer a valid expression. — end note]
Previous resolution [SUPERSEDED]:
This wording is relative to N3691.
Edit 20.3.1.3 [unique.ptr.single] p1 as indicated:
The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function object type (20.10), lvalue-reference to function, or lvalue-reference to function object type for which, given a value d of type D and a value ptr of type unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter. d(ptr) shall be valid even if it triggers the destruction of d or (if D is an lvalue reference to function object type) the function object that d refers to.
[2015-05, Lenexa]
After some discussion in Lenexa there was some wavering on if the added sentence is necessary. Here is example code that demonstrates why the extra sentence is necessary. In this example the call to d(ptr) is valid, however the deleter references *this after destructing its element:
#include <cassert>
#include <memory>
#include <iostream>
class Deleter
{
int state_ = 0;
enum
{
destructed = -4,
self_move_assigned = -3,
move_assigned_from = -2,
move_constructed_from = -1
};
public:
~Deleter() {state_ = destructed;}
Deleter() = default;
Deleter(Deleter const&) = default;
Deleter& operator=(Deleter const&) = default;
Deleter(Deleter&& a) noexcept
: state_(a.state_)
{a.state_ = move_constructed_from;}
Deleter& operator=(Deleter&& a) noexcept
{
if (this == &a)
state_ = self_move_assigned;
else
{
state_ = a.state_;
a.state_ = move_assigned_from;
}
return *this;
}
Deleter(int state)
: state_(state)
{
assert(state >= 0);
}
template <class T>
void
operator()(T* t) const
{
std::cout << "Deleter beginning operator()(T*)\n";
std::cout << "The deleter = " << *this << '\n';
std::cout << "Deleter about to destruct the X.\n";
delete t;
std::cout << "Deleter has destructed the X.\n";
std::cout << "The deleter = " << *this << '\n';
std::cout << "Deleter ending operator()(T*)\n";
}
friend
std::ostream&
operator<<(std::ostream& os, const Deleter& a)
{
switch (a.state_)
{
case destructed:
os << "**destructed**";
break;
case self_move_assigned:
os << "self_move_assigned";
break;
case move_assigned_from:
os << "move_assigned_from";
break;
case move_constructed_from:
os << "move_constructed_from";
break;
default:
os << a.state_;
break;
}
return os;
}
};
struct X
{
Deleter deleter_{1};
};
int main()
{
auto xp = new X;
{
std::unique_ptr<X, Deleter&> p(xp, xp->deleter_);
std::cout << "unique_ptr is constructed.\n";
std::cout << "The deleter = " << p.get_deleter() << '\n';
std::cout << "Destructing unique_ptr...\n";
}
std::cout << "unique_ptr is destructed.\n";
}
Which outputs:
unique_ptr is constructed. The deleter = 1 Destructing unique_ptr... Deleter beginning operator()(T*) The deleter = 1 Deleter about to destruct the X. Deleter has destructed the X. The deleter = **destructed** Deleter ending operator()(T*) unique_ptr is destructed.
The line "The deleter = **destructed**" represents the deleter referencing itself after it has been destructed by the d(ptr) expression, but prior to that call returning.
Suggested alternative to the current proposed wording:The expression d(ptr) shall not refer to the object d after it executes ptr->~T().
[2015-07, Telecon]
Geoffrey: Deleter may or may not execute ~T().
Alisdair: After the destructor after the element has run. Say it in words instead of code.
Howard will provide updated wording. Perhaps need both normative and non-normative wording.
[2015-08-03, Howard updates P/R per telecon discussion.]
[2017-03-04, Kona]
This is related to 2751, which has been suggested NAD.
STL wants "Effects equivalent to" here - say it in code. Marshall to research.
Proposed resolution:
This wording is relative to N4431.
Edit 20.3.1.3 [unique.ptr.single] p1 as indicated:
The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function object type (20.9), lvalue-reference to function, or lvalue-reference to function object type for which, given a value d of type D and a value ptr of type unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter. The expression d(ptr), if it destructs the object referred to by ptr, shall not refer to the object d after it destructs *ptr. [Note: The object being destructed may control the lifetime of d. — end note]
Section: 33.5.4 [atomics.order] Status: Open Submitter: Brian Demsky Opened: 2013-06-17 Last modified: 2016-01-28
Priority: 4
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Discussion:
I believe that the following variation on IRIW should admit executions in which c1 = d1 = 5 and c2 = d2 = 0. If this is allowed, then what is sequence of program evaluations for 33.5.4 [atomics.order] p9 that justifies the store to z? It seems that 33.5.4 [atomics.order] p9 should not allow this execution because one of the stores to x or y has to appear earlier in the sequence, each of the fetch_adds reads the previous load in the thread (and thus must appear later in the sequence), and 33.5.4 [atomics.order] p9 states that each load must read from the last prior assignment in the sequence.
atomic_int x;
atomic_int y;
atomic_int z;
int c1, c2, d1, d2;
static void a(void* obj)
{
atomic_store_explicit(&x, 5, memory_order_relaxed);
}
static void b(void* obj)
{
atomic_store_explicit(&y, 5, memory_order_relaxed);
}
static void c(void* obj)
{
c1 = atomic_load_explicit(&x, memory_order_relaxed);
// this could also be an atomic load if the address depends on c1:
c2 = atomic_fetch_add_explicit(&y, c1, memory_order_relaxed);
}
static void d(void* obj)
{
d1 = atomic_load_explicit(&y, memory_order_relaxed);
d2 = atomic_fetch_add_explicit(&x, d1, memory_order_relaxed);
}
int user_main(int argc, char** argv)
{
thrd_t t1, t2, t3, t4;
atomic_init(&x, 0);
atomic_init(&y, 0);
printf("Main thread: creating 4 threads\n");
thrd_create(&t1, (thrd_start_t)&a, NULL);
thrd_create(&t2, (thrd_start_t)&b, NULL);
thrd_create(&t3, (thrd_start_t)&c, NULL);
thrd_create(&t4, (thrd_start_t)&d, NULL);
thrd_join(t1);
thrd_join(t2);
thrd_join(t3);
thrd_join(t4);
printf("c1=%d c2=%d\n",c1,c2);
printf("d1=%d d2=%d\n",d1,d2);
// Can this store write 1000 (i.e., c1=d1=5, c2=d2=0)?
atomic_store(&z, (c1+d1)*100+c2+d2);
printf("Main thread is finished\n");
return 0;
}
It seems that the easiest fix is to allow a load in 33.5.4 [atomics.order] p9 to read from any prior store in the evaluation order.
That said, I would personally advocate the following: It seems to me that C/C++ atomics are in a bit of different situation than Java because:People are expected to use relaxed C++ atomics in potentially racy situations, so it isn't clear that semantics as complicated as the JMM's causality would be sane.
People who use C/C++ atomics are likely to be experts and use them in a very controlled fashion. I would be really surprised if compilers would find any real wins by optimizing the use of atomics.
Why not do something like:
There is satisfaction DAG of all program evaluations. Each evaluation observes the values of variables as computed by some prior assignment in the DAG. There is an edge x->y between two evaluations x and y if:the evaluation y observes a value computed by the evaluation x or
the evaluation y is an atomic store, the evaluation x is an atomic load, and there is a condition branch c that may depend (intrathread dependence) on x and x-sb->c and c-sb->y.
This seems to allow reordering of relaxed atomics that processors do without extra fence instructions, allows most reorderings by the compiler, and gets rid of satisfaction cycles.
[2015-02 Cologne]
Handed over to SG1.
[2015-05 Lenexa, SG1 response]
This was partially addressed (weasel-worded) in C++14 (See N3786). The remainder is an open research problem. N3710 outlines a "solution" that doesn't have a consensus behind it because it costs performance. We have no better solution at the moment.
Proposed resolution:
Section: 27.8.2.4 [partial.sort.copy] Status: New Submitter: Matt Austern Opened: 2013-06-26 Last modified: 2016-01-28
Priority: 3
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Discussion:
The signature of this function is:
template<class InputIterator, class RandomAccessIterator>
RandomAccessIterator
partial_sort_copy(InputIterator first, InputIterator last,
RandomAccessIterator result_first,
RandomAccessIterator result_last);
(and the usual overload for an explicitly provided comparison function). The standard says nothing about requirements in the case where the input type (iterator_traits<InputIterator>::value_type) and the output type (iterator_traits<RandomAccessIterator>::value_type) are different.
Presumably the input type must be convertible to the output type. What's less clear is what the requirements are on the comparison operator. Does the algorithm only perform comparisons on two values of the output type, or does it also perform comparisons on values of the input type, or might it even perform heterogeneous comparisons?Proposed resolution:
Section: 24.2.2.1 [container.requirements.general] Status: New Submitter: Matt Austern Opened: 2013-06-26 Last modified: 2016-01-28
Priority: 4
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Discussion:
Consider the following code snippet:
#include <vector>
#include <algorithm>
int main() {
std::vector<int> v1(100, 3);
std::vector<int> v2(100);
copy(v1.begin(), v1.end(), v2.begin());
}
It compiles without error on my desktop. Is it required to? I can't find evidence from the standard that it is. In my test std::copy was found by argument-dependent lookup because the implementation I used made std::vector<int>::iterator a user-defined type defined in namespace std. But the standard only requires std::vector<int>::iterator to be an implementation specified random access iterator type. I can't find anything requiring it to be a user-defined type at all (and in fact there are reasonable implementation where it isn't), let alone a user defined type defined in a specific namespace.
Since the defining namespace of container iterators is visible to users, should the standard say anything about what that namespace is?
Proposed resolution:
Section: 31.8.2.5 [stringbuf.virtuals] Status: Open Submitter: Sergey Zubkov Opened: 2013-08-29 Last modified: 2018-06-12
Priority: 4
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Discussion:
In 31.8.2.5 [stringbuf.virtuals]/1, basic_stringbuf::underflow() is specified to unconditionally return traits::eof() when a read position is not available.
The semantics of basic_stringbuf require, and existing libraries implement it so that this function makes a read position available if possible to do so, e.g. if some characters were inserted into the stream since the last call to overflow(), resulting in pptr() > egptr(). Compare to the conceptually similar D.15.2.4 [depr.strstreambuf.virtuals]/15.[2018-06-06, Billy argues for NAD]
The existing "Any character in the underlying buffer which has been initialized is considered to be part of the input sequence." sentence already describes what the stringbuf is supposed to do to the get area. The specific mechanism that the stringbuf uses to alter the get area is unspecified because the mechanism by which the stringbuf remembers the "high water mark" is unspecified.
Consider the following:stringstream s; s << "Hello"; s.seekp(0); string x; s >> x;
Before this P/R, this will store Hello in x, because the characters Hello are initialized. After this P/R, the "written put area" is empty, so it will store the empty string in x.
Saying that the initialized part of the string is used already describes what needs to happen here.[2018-06 Rapperswil Wednesday issues processing]
Billy to provide rationale for closing as NAD.
Proposed resolution:
This wording is relative to N3691.
Change 31.8.2.5 [stringbuf.virtuals] as indicated:
int_type underflow();-1- Returns: If the input sequence has a read position available or the function makes a read position available (as described below), returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof(). Any character in the underlying buffer which has been initialized is considered to be part of the input sequence.
-?- The function can make a read position available only if (mode & ios_base::in) != 0 and if the write next pointer pptr() is not null and is greater than the current read end pointer egptr(). To make a read position available, the function alters the read end pointer egptr() to equal pptr().
Section: 22.3.2 [pairs.pair], 22.4.4.1 [tuple.cnstr], 29.5 [time.duration] Status: Open Submitter: Daniel Krügler Opened: 2013-09-09 Last modified: 2020-06-13
Priority: 3
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Discussion:
During the acceptance of N3471 and some similar constexpr papers, specific wording was added to pair, tuple, and other templates that were intended to impose implementation constraints that ensure that the observable constexpr "character" of a defaulted function template is solely determined by the required expressions of the user-provided types when instantiated, for example:
The defaulted move and copy constructor, respectively, of pair shall be a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function.
This wording doesn't require enough, especially since the core language via CWG 1358 does now support constexpr function template instantiations, even if such function cannot appear in a constant expression (as specified in 7.7 [expr.const]) or as a constant initializer of that object (as specified in [basic.start.init]). The wording should be improved and should require valid uses in constant expressions and as constant initializers instead.
[Lenexa 2015-05-05]
STL : notice order of move/copy and copy/move with "respectively".
General word-smithing; ask for updated wording
Are we happy with this with changes we are suggesting?
unanimous
[2016-12-14, Daniel comments]
LWG 2833 overlaps considerably and both should be resolved together.
Previous resolution from Daniel [SUPERSEDED]:This wording is relative to N3691.
Change 22.3.2 [pairs.pair] p2 as indicated:
-2-
The defaulted move and copy constructor, respectively, of pair shall be a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr functionAn invocation of the move or copy constructor of pair shall be a constant expression (7.7 [expr.const]) if all required element-wise initializations would be constant expressions. An invocation of the move or copy constructor of pair shall be a constant initializer for that pair object ( [basic.start.init]) if all required element-wise initializations would be constant initializers for the respective subobjects.Change 22.4.4.1 [tuple.cnstr] p2 as indicated:
-2-
The defaulted move and copy constructor, respectively, of tuple shall be a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function. The defaulted move and copy constructor of tuple<> shall be constexpr functionsAn invocation of the move or copy constructor of tuple shall be a constant expression (7.7 [expr.const]) if all required element-wise initializations would be constant expressions. An invocation of the move or copy constructor of tuple shall be a constant initializer for that tuple object ( [basic.start.init]) if all required element-wise initializations would be constant initializers for the respective subobjects. An invocation of the move or copy constructor of tuple<> shall be a constant expression, or a constant initializer for that tuple<> object, respectively, if the function argument would be constant expression.Change 29.5 [time.duration] p7 as indicated:
-7- Remarks:
The defaulted copy constructor of duration shall be a constexpr function if and only if the required initialization of the member rep_ for copy and move, respectively, would satisfy the requirements for a constexpr function.An invocation of the copy constructor of duration shall be a constant expression (7.7 [expr.const]) if the required initialization of the member rep_ would be a constant expression. An invocation of the copy constructor of duration shall be a constant initializer for that duration object ( [basic.start.init]) if the required initialization of the member rep_ would be constant initializers for this subobject.
[2020-06-08 Nina Dinka Ranns comments]
The revised wording provided by LWG 2833 should resolve this issue as well.
Proposed resolution:
Section: 21 [meta] Status: Open Submitter: Daniel Krügler Opened: 2013-09-02 Last modified: 2016-01-28
Priority: 3
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Discussion:
The current library specification uses at several places wording that is intended to refer to core language template deduction failure at the top-level of expressions (aka "SFINAE"), for example:
The expression declval<T>() = declval<U>() is well-formed when treated as an unevaluated operand (Clause 5). Access checking is performed as if in a context unrelated to T and U. Only the validity of the immediate context of the assignment expression is considered. [Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]
Similar wording can be found in the specification of result_of, is_constructible, and is_convertible, being added to resolve an NB comment by LWG 1390 and 1391 through N3142.
This wording is necessary to limit speculative compilations needed to implement these traits, but it is also lengthy and repetitive.[2014-05-19, Daniel suggests a descriptive term]
constrictedly well-formed expression:
An expression e depending on a set of types A1, ..., An which is well-formed when treated as an unevaluated operand (Clause 5). Access checking is performed as if in a context unrelated to A1, ..., An. Only the validity of the immediate context of e is considered. [Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note][2014-05-20, Richard and Jonathan suggest better terms]
Richard suggested "locally well-formed"
Jonathan suggested "contextually well-formed" and then "The expression ... is valid in a contrived argument deduction context"[2014-06-07, Daniel comments and improves wording]
The 2014-05-19 suggestion did only apply to expressions, but there are two important examples that are not expressions, but instead are involving an object definition (std::is_constructible) and a function definition (std::is_convertible), respectively, instead. Therefore I suggest to rephrase the usage of "expression" into "program construct" in the definition of Jonathan's suggestion of "valid in a contrived argument deduction context".
I would like to point out that given the new definition of "valid in a contrived argument deduction context", there are several other places of the Library specification that could take advantage of this wording to improve the existing specification, such as 22.10.17.3 [func.wrap.func] p2, most functions in 20.2.9.3 [allocator.traits.members], and the **Insertable, EmplaceConstructible, and Erasable definitions in 24.2.2.1 [container.requirements.general], but given that these are not fully described in terms of the aforementioned wording yet, I would recommend to fix them by a separate issue once the committee has agreed on following the suggestion presented by this issue.[2015-05-05 Lenexa: Move to Open]
...
MC: I think we like the direction but it isn't quite right: it needs some work
JW: I'm prepared to volunteer to move that further, hopefully with the help of Daniel
Roger Orr: should this be Core wording because it doesn't really have anything to do with libraries - the term could then just be used here
AM: Core has nothing to deal with that, though
HT: it seems there is nothing to imply that allows dropping out with an error - maybe that's a separate issue
MC: I'm not getting what you are getting at: could you write an issue? - any objection to move to Open?
...
Proposed resolution:
This wording is relative to N3936.
Add the following new definition to [definitions] as indicated:
valid in a contrived argument deduction context [defns.valid.contr.context]
A program construct c depending on a set of types A1, ..., An, and treated as an unevaluated operand (Clause 5) when c is an expression, which is well-formed. Access checking is performed as if in a context unrelated to A1, ..., An. Only the validity of the immediate context (13.10.3 [temp.deduct]) of c is considered. [Note: The compilation of c can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note].Change Table 49 ("Type property predicates") as indicated:
Table 49 — Type property predicates Template Condition Preconditions … template <class T, class U>
struct is_assignable;The expression declval<T>() =
declval<U>() is valid in a
contrived argument deduction context
([defns.valid.contr.context]) for types
T and U.well-formed when treated
as an unevaluated operand
(Clause 5). Access
checking is performed as if
in a context unrelated to T
and U. Only the validity of
the immediate context of
the assignment expression
is considered. [Note: The
compilation of the
expression can result in
side effects such as the
instantiation of class
template specializations
and function template
specializations, the
generation of
implicitly-defined
functions, and so on. Such
side effects are not in the
"immediate context" and
can result in the program
being ill-formed. — end
note][…] …
Change 21.3.5.4 [meta.unary.prop] p7 as indicated:
-7- Given the following function prototype:
template <class T> add_rvalue_reference_t<T> create() noexcept;the predicate condition for a template specialization is_constructible<T, Args...> shall be satisfied if and only if the following variable definition
would be well-formedfor some invented variable t would be valid in a contrived argument deduction context ([defns.valid.contr.context]) for types T and Args...:T t(create<Args>()...);[Note: These tokens are never interpreted as a function declaration. — end note]
Access checking is performed as if in a context unrelated to T and any of the Args. Only the validity of the immediate context of the variable initialization is considered. [Note: The evaluation of the initialization can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]
Change Table 57 ("Other transformations") as indicated:
Table 57 — Other transformations Template Condition Comments … template <class Fn, class... ArgTypes>
struct result_of<Fn(ArgTypes...)>;[…] If the expression
INVOKE(declval<Fn>(),
declval<ArgTypes>()...) is
valid in a contrived argument deduction
context ([defns.valid.contr.context]) for types
Fn and ArgTypes...well, the
formed when treated as an
unevaluated operand (Clause 5)
member typedef type shall name the
type
decltype(INVOKE(declval<Fn>(),
declval<ArgTypes>()...));
otherwise, there shall be no member
type.Access checking is performed as
if in a context unrelated to Fn and
ArgTypes. Only the validity of the
immediate context of the expression is
considered. [Note: The compilation of
the expression can result in side
effects such as the instantiation of
class template specializations and
function template specializations, the
generation of implicitly-defined
functions, and so on. Such side effects
are not in the "immediate context"
and can result in the program being
ill-formed. — end note]…
Change 21.3.7 [meta.rel] p4 as indicated:
-4- Given the following function prototype:
template <class T> add_rvalue_reference_t<T> create() noexcept;the predicate condition for a template specialization is_convertible<From, To> shall be satisfied if and only if the return expression in the following code would be
well-formedvalid in a contrived argument deduction context ([defns.valid.contr.context]) for types To and From, including any implicit conversions to the return type of the function:To test() { return create<From>(); }[Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]
Access checking is performed as if in a context unrelated to To and From. Only the validity of the immediate context of the expression of the return-statement (including conversions to the return type) is considered. [Note: The evaluation of the conversion can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]
Section: 17.6.3.4 [new.delete.placement] Status: New Submitter: Daniel Krügler Opened: 2013-09-18 Last modified: 2016-01-28
Priority: 3
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Discussion:
The library gives explicit permission in 16.4.5.2.1 [namespace.std] p2 that user code may explicitly instantiate a library template provided that the instantiations depend on at least one user-defined type:
A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type and the instantiation meets the standard library requirements for the original template.
But it seems that the C++11 library is not specified in a way that guarantees such an instantiation to be well-formed if the minimum requirements of the library is not satisfied.
For example, in general, the first template parameter of std::vector is not required to be DefaultConstructible in general, but due to the split of the single C++03 member function with default argumentvoid resize(size_type sz, T c = T());
into
void resize(size_type sz); void resize(size_type sz, const T& c);
the effect is now that for a type ND that is not DefaultConstructible, such as
struct NP {
NP(int);
};
the explicit instantiation of std::vector<ND> is no longer well-formed, because the attempt to instantiate the single-argument overload of resize cannot not succeed, because this function imposes the DefaultInsertable requirements and given the default allocator this effectively requires DefaultConstructible.
But DefaultConstructible is not the only point, what about CopyConstructible versus MoveConstructible alone? It turns out that currently the second resize overload would fail during an explicit instantiation for a type like
struct MO {
MO() = default;
MO(MO&&) = default;
};
because it imposes CopyInsertable requirements that end up being equivalent to the CopyConstructible requirements for the default allocator.
Technically a library can solve these issues: For special member functions by defining them in some base class, for others by transforming them effectively into a function template due to the great feature of default template arguments for function templates (At the very moment the validity of the latter approach depends on a resolution of core language issue CWG 1635, though). E.g. the here mentioned resize functions of std::vector could be prevented from instantiation by defining them like this with an implementation:
template<class = void>
void resize(size_type sz) { […] }
template<class = void>
void resize(size_type sz, const T& c) { […] }
In this case, these functions could also be defined in a base class, but the latter approach won't work in all cases.
Basically such an implementation is required to constrain all member functions that are not covered by the general requirements imposed on the actual library template parameters. I tested three different C++11 library implementations and but none could instantiate for example std::list, std::vector, or std::deque with value types that are not DefaultConstructible or only MoveConstructible. This issue is raised to clarify the current situation in regard to the actual requirements imposed on user-provided types that are used to explicitly instantiate Library-provided templates. For example, the current Container requirements impose very little requirements on the actual value type and it is unclear to which extend library implementations have to respect that. The minimum solution of this issue should be to at least realize that there is no fundamental requirement on DefaultConstructible for value types of library containers, because we have since C++03 the general statement of 16.4.4.2 [utility.arg.requirements] ("In general, a default constructor is not required."). It is unclear whether CopyConstructible should be required for an explicit instantiation request, but given the careful introduction of move operations in the library it would seem astonishing that a MoveConstructible type wouldn't suffice for value types of the container types. In any case I can envision at least two approaches to solve this issue:As indicated in LWG 2292, those function could get an explicit "Template Constraints:" element, albeit this promises more than needed to solve this issue.
The library could introduce a completely new element form, such as "Instantiation Constraints:" that would handle this situation for explicit instantiation situations. This would allow for simpler techniques to solve the issue when explicit instantiation is required compared to the first bullet, because it would not (necessarily) guarantee SFINAE-friendly expression-wellformedness, such as inspecting the expression std::declval<std::vector<ND>&>.resize(0) in an unevaluated context.
It should be noted that the 2013-08-27 comment to LWG 2193 could be resolved by a similar solution as indicated in this issue here.
Proposed resolution:
Section: 24 [containers] Status: LEWG Submitter: Zhihao Yuan Opened: 2013-09-26 Last modified: 2018-11-12
Priority: 2
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Discussion:
LWG 2193 yields explicit for default ctors to allow {}, but not for all cases of uniform initialization. For example:
explicit vector(size_type count, const Allocator& alloc = Allocator());
This prevents {n, alloc()}. Although this use is relatively rare, but the behavior is inconsistent with that of
vector(size_type count, const T& value, const Allocator& alloc = Allocator());
[Urbana 2014-11-07: Move to Open]
[2018-08 Batavia Monday issue discussion]
This really needs a paper; splitting a lot of constructors. Nevin to write paper.
[2018-11 San Diego Thursday night issue processing]
LEWG has rejected Nevin's paper, so they need to formulate a policy.
Proposed resolution:
Section: 24.2.2.1 [container.requirements.general] Status: Open Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2023-01-20
Priority: 3
View other active issues in [container.requirements.general].
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Discussion:
24.2.2.1 [container.requirements.general]/10 says that unless otherwise specified, "no swap() function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. [Note: The end() iterator does not refer to any element, so it may be invalidated. — end note]". However, move constructors and move assignment operators aren't given similar invalidation guarantees. The guarantees need several exceptions, so I do not believe that blanket language like /11 "Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container." is applicable.
[2014-02-13 Issaquah]
General agreeement on intent, several wording nits and additional paragraphs to hit.
STL to provide updated wording. Move to Open.
[2015-02 Cologne]
AM: in the proposed wording, I'd like to mention that the iterators now refer to elements of a different container. I think we're saying something like this somewhere. JY: There's some wording like that for swap I think. TK: It's also in list::splice(). DK to JY: 23.2.1p9.
VV: The issue says that STL was going to propose new wording. Has he done that? AM: I believe we're looking at that. GR: The request touches on multiple paragraphs, and this PR has only one new paragraph, so this looks like it's not up-to-date. MC: This was last updated a year ago in Issaquah. Conclusion: Skip, not up to date.[2015-06, Telecon]
Still waiting for updated wording
[2015-08 Chicago]
Still waiting for updated wording
[2018-08-23 Batavia Issues processing]
Priority to 3
[2023-01-20; std-proposals post]
Emile Cormier observed that the proposed resolution of this issue contradicts with changes made by LWG 2839. Specifially, the current draft does not require container elements to be preserved on self-move-assignment. If this issue is accepted, it would either need to allow iterator invalidation on self-move-assignment or remove the "If a and rv do not refer to the same object" changes added to the container requirements by LWG 2839.
Proposed resolution:
This wording is relative to N3691.
In 24.2.2.1 [container.requirements.general]/10 change as indicated:
-10- Unless otherwise specified (see 23.2.4.1, 23.2.5.1, 23.3.3.4, and 23.3.7.5) all container types defined in this Clause meet the following additional requirements:
[…]
no copy constructor or assignment operator of a returned iterator throws an exception.
no move constructor (or move assignment operator when allocator_traits<allocator_type>::propagate_on_container_move_assignment::value is true) of a container (except for array) invalidates any references, pointers, or iterators referring to the elements of the source container. [Note: The end() iterator does not refer to any element, so it may be invalidated. — end note]
no swap() function throws an exception.
no swap() function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. [Note: The end() iterator does not refer to any element, so it may be invalidated. — end note]
Section: 32.4.2 [re.synopt] Status: Open Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2016-01-28
Priority: 3
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Discussion:
The table in 32.4.2 [re.synopt]/1 says that regex_constants::collate "Specifies that character ranges of the form "[a-b]" shall be locale sensitive.", but 32.12 [re.grammar]/14 says that it affects individual character comparisons too.
[2012-02-12 Issaquah : recategorize as P3]
Marshall Clow: 28.13/14 only applies to ECMAScript
All: we're unsure
Jonathan Wakely: we should ask John Maddock
Move to P3
[2014-5-14, John Maddock response]
The original intention was the original wording: namely that collate only made character ranges locale sensitive. To be frank it's a feature that's probably hardly ever used (though I have no real hard data on that), and is a leftover from early POSIX standards which required locale sensitive collation for character ranges, and then later changed to implementation defined if I remember correctly (basically nobody implemented locale-dependent collation).
So I guess the question is do we gain anything by requiring all character-comparisons to go through the locale when this bit is set? Certainly it adds a great deal to the implementation effort (it's not what Boost.Regex has ever done). I guess the question is are differing code-points that collate identically an important use case? I guess there might be a few Unicode code points that do that, but I don't know how to go about verifying that. STL: If this was unintentional, then 32.4.2 [re.synopt]/1's table should be left alone, while 32.12 [re.grammar]/14 should be changed instead. Jeffrey Yasskin: This page mentions that [V] in Swedish should match "W" in a perfect world. However, the most recent version of TR18 retracts both language-specific loose matches and language-specific ranges because "for most full-featured regular expression engines, it is quite difficult to match under code point equivalences that are not 1:1" and "tailored ranges can be quite difficult to implement properly, and can have very unexpected results in practice. For example, languages may also vary whether they consider lowercase below uppercase or the reverse. This can have some surprising results: [a-Z] may not match anything if Z < a in that locale." ECMAScript doesn't include collation at all. IMO, +1 to changing 28.13 instead of 28.5.1. It seems like we'd be on fairly solid ground if we wanted to remove regex_constants::collate entirely, in favor of named character classes, but of course that's not for this issue.Proposed resolution:
This wording is relative to N3691.
In 32.4.2 [re.synopt]/1, Table 138 — "syntax_option_type effects", change as indicated:
Table 138 — syntax_option_type effects Element Effect(s) if set … collate Specifies that character ranges of the form "[a-b]"comparisons and character range comparisons shall be locale sensitive.…
Section: 32.6 [re.traits], 30.3.1.2.2 [locale.facet] Status: Open Submitter: Sergey Zubkov Opened: 2013-10-15 Last modified: 2016-02-01
Priority: 3
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Discussion:
32.6 [re.traits]/7, begins with "if typeid(use_facet<collate<charT> >) == typeid(collate_byname<charT>)", which appears to be pseudocode with the intention to convey that the collate facet has not been replaced by the user. Cf. the wording in N1429 "there is no portable way to implement transform_primary in terms of std::locale, since even if the sort key format returned by std::collate_byname<>::transform is known and can be converted into a primary sort key, the user can still install their own custom std::collate implementation into the locale object used, and that can use any sort key format they see fit.".
Taken literally, 32.6 [re.traits]/7 appears to imply that named locales are required to hold their collate facets with dynamic type std::collate_byname<charT>, which is in fact true in some implementations (e.g libc++), but not others (e.g. libstdc++). This does not follow from the description of _byname in 30.3.1.2.2 [locale.facet]/4, which is only required to provide equivalent semantics, to the named locale's facet, not to actually be one.[2015-05-06 Lenexa: Move to Open]
MC, RP: Consequence of failing to follow the rule is UB.
MC: Tightening of requirements.
RP: It should be this way, we just didn't impose it before.
MC: Second change is a bug fix, original code didn't work.
TK: Doesn't seem to make things worse.
Bring up in larger group tomorrow.
JW arrives.
JW: libstdc++ violates this due to two std::string ABIs.
JW: This prevents installing a type derived from Facet_byname, constrains the implementor from using a smarter derived class version.
JW: Can't look at facet id to detect replacement, because replacements have the same id.
RP: Can you give it multiple ids through multiple inheritance?
JW: No, the facet mechanism wouldn't like that.
JW: We should also ask Martin Sebor, he's implemented this stuff recently.
MC: Sounds like this resolution doesn't work, need a better solution.
JW: Write in words "if the facet has not been replaced by the user", the implementation knows how to detect that, but not like this.
RP: User RE traits need to detect this too.
JW: =(
Move to Open, JW will invite Martin Sebor to join LWG for discussion.
Later ...
JW: This is not needed for user specializations after all.
MC: Agree, [re.traits]/7 only applies to the stdlib traits.
NM: Effects: doesn't make sense.
JW, NM, Martin Sebor to come up with new wording.
Proposed resolution:
This wording is relative to N3691.
Modify 30.3.1.2.2 [locale.facet]/4 as indicated:
For some standard facets a standard "..._byname" class, derived from it, implements the virtual function semantics
equivalent toprovided by that facet of the locale constructed by locale(const char*) with the same name. Each such facet provides a constructor that takes a const char* argument, which names the locale, and a refs argument, which is passed to the base class constructor. Each such facet also provides a constructor that takes a string argument str and a refs argument, which has the same effect as calling the first constructor with the two arguments str.c_str() and refs. If there is no "..._byname" version of a facet, the base class implements named locale semantics itself by reference to other facets. For any locale loc constructed by locale(const char*) and facet Facet that has a corresponding standard Facet_byname class, typeid(use_facet<Facet>(loc)) == typeid(Facet_byname).
Modify 32.6 [re.traits]/7 as indicated:
template <class ForwardIterator> string_type transform_primary(ForwardIterator first, ForwardIterator last) const;-7- Effects: if typeid(use_facet<collate<charT> >(getloc())) == typeid(collate_byname<charT>) and the form of the sort key returned by collate_byname<charT>::transform(first, last) is known and can be converted into a primary sort key then returns that key, otherwise returns an empty string.
Section: 31.7.6.2 [ostream] Status: New Submitter: Alf P. Steinbach Opened: 2013-10-29 Last modified: 2016-01-28
Priority: 4
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Discussion:
For wide streams argument types wchar_t const* and wchar_t are supported only as template parameters. User defined conversions are not considered for template parameter matching. Hence inappropriate overloads of operator<< are selected when an implicit conversion is required for the argument, which is inconsistent with the behavior for char const* and char, is unexpected, and is a useless result.
Demonstration:
#include <iostream>
struct Byte_string
{
operator char const*() const { return "Hurray, it works!"; }
};
struct Wide_string
{
operator wchar_t const*() const { return L"Hurray, it works!"; }
};
struct Byte_ch
{
operator char() const { return 'X'; }
};
struct Wide_ch
{
operator wchar_t() const { return L'X'; }
};
auto main() -> int
{
using namespace std;
wcout << "'X' as char value : " << Byte_ch() << endl;
wcout << "'X' as wchar_t value: " << Wide_ch() << endl;
wcout << "Byte string pointer : " << Byte_string() << endl;
wcout << "Wide string pointer : " << Wide_string() << endl;
}
Example output:
'X' as char value : X 'X' as wchar_t value: 88 Byte string pointer : Hurray, it works! Wide string pointer : 000803C8
Proposed resolution:
This wording is relative to N3797.
Modify 31.7.6.2 [ostream], class template basic_ostream synopsis, as indicated:
namespace std {
[…]
// 27.7.3.6.4 character inserters
template<class charT, class traits>
basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
charT);
template<class charT, class traits>
basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
char);
template<class traits>
basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>&,
char);
template<class traits>
basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>&,
wchar_t);
[…]
template<class charT, class traits>
basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
const charT*);
template<class charT, class traits>
basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
const char*);
template<class traits>
basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>&,
const char*);
template<class traits>
basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>&,
const wchar_t*);
[…]
}
Modify 31.7.6.3.4 [ostream.inserters.character] as indicated: [Drafting note: The replacement of os by out in p1 and the insertion of "out." in p4 just fix two obvious typos — end drafting note]
template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, charT c); template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, char c); // specialization template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, char c); template<class traits> basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>& out, wchar_t c); // signed and unsigned template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, signed char c); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, unsigned char c);-1- Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of out. Constructs a character sequence seq. If c has type char and the character type of the stream is not char, then seq consists of out.widen(c); otherwise seq consists of c. Determines padding for seq as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Inserts seq into out. Calls
-2- Returns: out.osout.width(0).template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, const charT* s); template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, const char* s); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, const char* s); template<class traits> basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>& out, const wchar_t* s); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, const signed char* s); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, const unsigned char* s);-3- Requires: s shall not be a null pointer.
-4- Effects: Behaves like a formatted inserter (as described in 31.7.6.3.1 [ostream.formatted.reqmts]) of out. Creates a character sequence seq of n characters starting at s, each widened using out.widen() (27.5.5.3), where n is the number that would be computed as if by:
traits::length(s) for the following overloads:
where the first argument is of type basic_ostream<charT, traits>& and the second is of type const charT*,
and also for the overloadwhere the first argument is of type basic_ostream<char, traits>& and the second is of type const char*,where the first argument is of type basic_ostream<wchar_t, traits>& and the second is of type const wchar_t*,
std::char_traits<char>::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const char*,
traits::length(reinterpret_cast<const char*>(s)) for the other two overloads.
Determines padding for seq as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Inserts seq into out. Calls out.width(0).
-5- Returns: out.
Section: 22.9.2 [template.bitset], 31.7.9 [quoted.manip] Status: Open Submitter: Zhihao Yuan Opened: 2013-12-02 Last modified: 2016-01-28
Priority: 3
View all other issues in [template.bitset].
View all issues with Open status.
Discussion:
Example: char16_t('1') != u'1' is possible.
The numeric value of char16_t is defined to be Unicode code point, which is same to the ASCII value and UTF-8 for 7-bit chars. However, char is not guaranteed to have an encoding which is compatible with ASCII. For example, '1' in EBCDIC is 241. I found three places in the standard casting narrow char literals: bitset::bitset, bitset::to_string and quoted. PJ confirmed this issue and says he has a solution used in their <filesystem> implementation, and he may want to propose it to the standard. The solution in my mind, for now, is to make those default arguments magical, where the "magic" can be implemented with a C11 _Generic selection (works in clang):
#define _G(T, literal) _Generic(T{}, \
char: literal, \
wchar_t: L ## literal, \
char16_t: u ## literal, \
char32_t: U ## literal)
_G(char16_t, '1') == u'1'
[Lenexa 2015-05-05: Move to Open]
Ask for complete PR (need quoted, to string, et al.)
Will then take it up again
Expectation is that this is correct way to fix this
Proposed resolution:
This wording is relative to N3797.
[Drafting note: This is a sample wording fixing only one case; I'm just too lazy to copy-paste it before we discussed whether the solution is worth and sufficient (for example, should the other `charT`s like `unsigned char` just don't compile without supplying those arguments? I hope so). — end drafting note]Modify 22.9.2 [template.bitset] p1, class template bitset synopsis, as indicated:
namespace std {
template <size_t N> class bitset {
public:
[…]
template<class charT, class traits, class Allocator>
explicit bitset(
const basic_string<charT,traits,Allocator>& str,
typename basic_string<charT,traits,Allocator>::size_type pos = 0,
typename basic_string<charT,traits,Allocator>::size_type n =
basic_string<charT,traits,Allocator>::npos,
charT zero = charT('0')see below, charT one = charT('1')see below);
[…]
};
[…]
}
Modify 22.9.2.2 [bitset.cons] as indicated:
template<class charT, class traits, class Allocator> explicit bitset(const basic_string<charT, traits, Allocator>& str, typename basic_string<charT, traits, Allocator>::size_type pos = 0, typename basic_string<charT, traits, Allocator>::size_type n = basic_string<charT, traits, Allocator>::npos, charT zero =charT('0')see below, charT one =charT('1')see below);-?- The default values of zero and one compare equal to the character literals 0 and 1 of type charT, respectively.
-3- Requires:: pos <= str.size(). […]
Section: 21.3.5.4 [meta.unary.prop] Status: Open Submitter: Richard Smith Opened: 2014-02-01 Last modified: 2017-02-02
Priority: 3
View other active issues in [meta.unary.prop].
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Discussion:
The 'Condition' for std::is_empty is listed as:
"T is a class type, but not a union type, with no non-static data members other than bit-fields of length 0, no virtual member functions, no virtual base classes, and no base class B for which is_empty<B>::value is false."
This is incorrect: there is no such thing as a non-static data member that is a bit-field of length 0, since bit-fields of length 0 must be unnamed, and unnamed bit-fields are not members (see 11.4.10 [class.bit] p2).
It also means that classes such as:
struct S {
int : 3;
};
are empty (because they have no non-static data members). There's implementation divergence on the value of is_empty<S>::value.
I'm not sure what the purpose of is_empty is (or how it could be useful), but if it's desirable for the above type to not be treated as empty, something like this could work:"T is a class type, but not a union type, with no non-static data members
other than, no unnamed bit-fields of non-zero length0, no virtual member functions, no virtual base classes, and no base class B for which is_empty<B>::value is false."
and if the above type should be treated as empty, then this might be appropriate:
"T is a class type, but not a union type, with no (named) non-static data members
other than bit-fields of length 0, no virtual member functions, no virtual base classes, and no base class B for which is_empty<B>::value is false."
[2016-08 Chicago]
Walter says: We want is_empty_v<S> to produce false as a result. Therefore, we recommend adoption of the first of the issue's suggestions.
Tuesday AM: Moved to Tentatively Ready
Previous resolution [SUPERSEDED]:
[2016-10 by Marshall - this PR incorrectly highlighted changed portions]
Modify Table 38 — Type property predicates for is_empty as follows:
T is a non-union class type with no non-static data members
other than, no unnamed bit-fields of non-zero length0, no virtual member functions, no virtual base classes, and no base class B for which is_empty_v<B> is false.
[2016-10 Telecon]
Should probably point at section 1.8 for some of this. Status back to 'Open'
Proposed resolution:
Modify Table 38 — Type property predicates for is_empty as follows:
T
is a class type, but not a union type,is a non-union class type with no non-static data membersother than, no unnamed bit-fields of non-zero length0, no virtual member functions, no virtual base classes, and no base class B for which is_empty_v<B> is false.
Section: 24.2.7 [associative.reqmts], 24.2.8 [unord.req] Status: New Submitter: Jeffrey Yasskin Opened: 2014-02-15 Last modified: 2015-09-23
Priority: 3
View other active issues in [associative.reqmts].
View all other issues in [associative.reqmts].
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Discussion:
a_uniq.emplace(args) is specified as:
Effects: Inserts a value_type object t constructed with
std::forward<Args>(args)... if and only if there is no element in the
container with key equivalent to the key of t. The bool component of
the returned pair is true if and only if the insertion takes place,
and the iterator component of the pair points to the element with key
equivalent to the key of t.
However, we occasionally find code of the form:
std::unique_ptr<Foo> p(new Foo);
auto res = m.emplace("foo", std::move(p));
where we'd like to avoid destroying the Foo if the insertion doesn't take place (if the container already had an element with the specified key).
N3873 includes a partial solution to this in the form of a new emplace_stable member function, but LEWG's discussion strongly agreed that we'd rather have emplace() Just Work: Should map::emplace() be guaranteed not to move/copy its arguments if the insertion doesn't happen? SF: 8 F: 3 N: 0 A: 0 SA: 0 This poll was marred by the fact that we didn't notice or call out that emplace() must construct the key before doing the lookup, and it must not then move the key after it determines whether an insert is going to happen, and the mapped_type instance must live next to the key. The very similar issue 2006 was previously marked NAD, with N3178 as discussion. However, given LEWG's interest in the alternate behavior, we should reopen the question in this issue. We will need a paper that describes how to implement this before we can make more progress.Proposed resolution:
Section: 25.6.4 [istreambuf.iterator] Status: New Submitter: Hyman Rosen Opened: 2014-02-19 Last modified: 2023-04-13
Priority: 3
View other active issues in [istreambuf.iterator].
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Discussion:
Given the following code,
#include <sstream> std::stringbuf buf; std::istreambuf_iterator<char> begin(&buf); std::istreambuf_iterator<char> end;
it is not clear from the wording of the Standard whether begin.equal(end) must be true. In at least one implementation it is not (CC: Sun C++ 5.10 SunOS_sparc Patch 128228-25 2013/02/20) and in at least one implementation it is (gcc version 4.3.2 x86_64-unknown-linux-gnu).
25.6.4 [istreambuf.iterator] says that end is an end-of-stream iterator since it was default constructed. It also says that an iterator becomes equal to an end-of-stream iterator when end of stream is reached by sgetc() having returned eof(). [istreambuf.iterator::equal] says that equal() returns true iff both iterators are end of stream or not end of stream. But there seems to be no requirement that equal check for end-of-stream by calling sgetc(). Jiahan Zi at BloombergLP discovered this issue through his code failing to work correctly. Dietmar Kühl has opined in a private communication that the iterators should compare equal.[2023-03-31; Jonathan Wakely comments]
I agree that they should compare equal, but that's in conflict with the
resolution of LWG 2544, which says that begin
must not be at end-of-stream because &buf is not null.
[2023-04-12; Jonathan adds wording]
Proposed resolution:
This wording is relative to N4944.
Change 25.6.4.1 [istreambuf.iterator.general] as indicated:
constexpr istreambuf_iterator() noexcept; constexpr istreambuf_iterator(default_sentinel_t) noexcept; istreambuf_iterator(const istreambuf_iterator&) noexcept = default; ~istreambuf_iterator() = default; istreambuf_iterator(istream_type& s) noexcept;: istreambuf_iterator(s.rdbuf()) { } istreambuf_iterator(streambuf_type* s) noexcept; istreambuf_iterator(const proxy& p) noexcept; … private: streambuf_type* sbuf_; // exposition only int_type c_{}; // exposition only }; }
Change 25.6.4.3 [istreambuf.iterator.cons] as indicated:
For each istreambuf_iterator constructor in this section, an end-of-stream iterator is constructed if
and only ifthe exposition-only member sbuf_ is initialized with a null pointer value or if sbuf_->sgetc() returns traits_type::eof().constexpr istreambuf_iterator() noexcept; constexpr istreambuf_iterator(default_sentinel_t) noexcept;-1- Effects: Initializes sbuf_ with nullptr.
istreambuf_iterator(istream_type& s) noexcept;
-2- Effects: Initializes sbuf_ with s.rdbuf().istreambuf_iterator(streambuf_type* s) noexcept;[Drafting note: sgetc() can throw, but this function is noexcept. Should it swallow exceptions and create an end-of-stream iterator, to avoid weakening the exception spec of an existing function?]
-3- Effects: Initializes sbuf_ with s. If s is not null, initializes c_ with s->sgetc(). Sets sbuf_ to null if sgetc exits via an exception, or if traits_type::eq_int_type(c_, traits_type::eof()) is true.
istreambuf_iterator(const proxy& p) noexcept;-4- Effects: Initializes sbuf_ with p.sbuf_. If p.sbuf_ is not null, initializes c_ with p.keep_.
Change 25.6.4.4 [istreambuf.iterator.ops] as indicated:
charT operator*() const;-?- Preconditions: sbuf_ is not null.
-1- Returns:
The character obtained via the streambuf member sbuf_->sgetc().traits_type::to_char_type(c_).-?- Throws: Nothing.
istreambuf_iterator& operator++();-?- Preconditions: sbuf_ is not null.
-2- Effects:
As if by sbuf_->sbumpc().Performs c_ = sbuf_->snextc(), then sets sbuf_ to null if traits_type::eq_int_type(c_, traits_type::eof()) is true.-3- Returns: *this.
proxy operator++(int);-4-
Returns: proxy(sbuf_->sbumpc(), sbuf_).
Effects: Equivalent to:proxy p(**this, sbuf_); ++*this; return p;bool equal(const istreambuf_iterator& b) const;-5- Returns: bool(sbuf_) == bool(b.sbuf_).
[Note: This is true if and only if both iterators are at end-of-stream, or neither is at end-of-stream, regardless of what streambuf object they use. — end note]
template<class charT, class traits> bool operator==(const istreambuf_iterator<charT, traits>& a, const istreambuf_iterator<charT, traits>& b);-6- Returns: a.equal(b).
bool equal(const istreambuf_iterator& i, default_sentinel_t s) const;-7- Returns:
i.equal(s)i.sbuf_ == nullptr.
Section: 29.5.9 [time.duration.literals] Status: Open Submitter: Jonathan Wakely Opened: 2014-05-16 Last modified: 2014-11-08
Priority: 3
View all issues with Open status.
Discussion:
29.5.9 [time.duration.literals] p3 says:
If any of these suffixes are applied to an integer literal and the resulting chrono::duration value cannot be represented in the result type because of overflow, the program is ill-formed.
Ill-formed requires a diagnostic at compile-time, but there is no way to detect the overflow from unsigned long long to the signed duration<>::rep type.
Overflow could be detected if the duration integer literals were literal operator templates, otherwise overflow can either be undefined or a run-time error, not ill-formed.[Urbana 2014-11-07: Move to Open]
Proposed resolution:
Section: 3.36 [defns.ntcts], 30.3.1.2.1 [locale.category], 31.2.3 [iostreams.limits.pos], 31.7.6.3.1 [ostream.formatted.reqmts], 31.7.6.3.4 [ostream.inserters.character] Status: New Submitter: Jeffrey Yasskin Opened: 2014-06-01 Last modified: 2017-04-22
Priority: 3
View all issues with New status.
Discussion:
The term "character type" is used in 3.36 [defns.ntcts], 30.3.1.2.1 [locale.category], 31.2.3 [iostreams.limits.pos], 31.7.6.3.1 [ostream.formatted.reqmts], and 31.7.6.3.4 [ostream.inserters.character], but the core language only defines "narrow character types" (6.8.2 [basic.fundamental]).
"wide-character type" is used in D.26 [depr.locale.stdcvt], but the core language only defines a "wide-character set" and "wide-character literal".Proposed resolution:
Section: 17.7.3 [type.info] Status: Open Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2016-08-06
Priority: 3
View all other issues in [type.info].
View all issues with Open status.
Discussion:
type_info's destructor is depicted as being virtual, which is nearly unobservable to users (since they can't construct or copy this class, they can't usefully derive from it). However, it's technically observable (via is_polymorphic and has_virtual_destructor). It also imposes real costs on implementations, requiring them to store one vptr per type_info object, when RTTI space consumption is a significant concern.
Making this implementation-defined wouldn't affect users (who can observe this only if they're specifically looking for it) and wouldn't affect implementations who need virtual here, but it would allow other implementations to drop virtual and improve their RTTI space consumption. Richard Smith: It's observable in a few other ways.std::map<void*, something> m; m[dynamic_cast<void*>(&typeid(blah))] = stuff;
... is broken by this change, because you can't dynamic_cast a non-polymorphic class type to void*.
type_info& f(); typeid(f());
... evaluates f() at runtime without this change, and might not do so with this change.
These are probably rare things, but I can imagine at least some forms of the latter being used in SFINAE tricks.[Lenexa 2015-05-05: Move to Open]
Marshall to poll LEWG for their opinion
[2016-06]
On the reflector, STL wrote:
We'll prototype this change and report back with data in the future.
[2016-08 Chicago]
No update from STL. Set priority to P3
Proposed resolution:
This wording is relative to N3936.
Change 17.7.3 [type.info] as indicated:
namespace std { class type_info { public:virtualsee below ~type_info(); […] }; }-1- The class type_info describes type information generated by the implementation. Objects of this class effectively store a pointer to a name for the type, and an encoded value suitable for comparing two types for equality or collating order. The names, encoding rule, and collating sequence for types are all unspecified and may differ between programs. Whether ~type_info() is virtual is implementation-defined.
Section: 19.3 [assertions] Status: New Submitter: David Krauss Opened: 2014-06-25 Last modified: 2014-11-03
Priority: 4
View other active issues in [assertions].
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Discussion:
When NDEBUG is defined, assert must expand exactly to the token sequence ((void)0), with no whitespace (C99 §7.2/1 and also C11 §7.2/1). This is a lost opportunity to pass the condition along to the optimizer.
The user may observe the token sequence using the stringize operator or discriminate it by making a matching #define directive. There is little chance of practical code doing such things. It's reasonable to allow any expansion that is a void expression with no side effects or semantic requirements, for example, an extension keyword or an attribute-specifier finagled into the context. Conforming optimizations would still be limited to treating the condition as hint, not a requirement. Nonconformance on this point is quite reasonable though, given user preferences. Anyway, it shouldn't depend on preprocessor quirks. As for current practice, Darwin OS <assert.h> provides a GCC-style compiler hint __builtin_expect but only in debug mode. Shouldn't release mode preserve hints? Daniel: The corresponding resolution should take care not to conflict with the intention behind LWG 2234.Proposed resolution:
Section: 16.4.6.9 [reentrancy] Status: Open Submitter: Stephan T. Lavavej Opened: 2014-07-01 Last modified: 2021-07-31
Priority: 3
View all other issues in [reentrancy].
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Discussion:
N3936 16.4.6.9 [reentrancy]/1 talks about "functions", but that doesn't address the scenario of calling different member functions of a single object. Member functions often have to violate and then re-establish invariants. For example, vectors often have "holes" during insertion, and element constructors/destructors/etc. shouldn't be allowed to observe the vector while it's in this invariant-violating state. The [reentrancy] Standardese should be extended to cover member functions, so that implementers can either say that member function reentrancy is universally prohibited, or selectively allowed for very specific scenarios.
(For clarity, this issue has been split off from LWG 2382.)[2014-11-03 Urbana]
AJM confirmed with SG1 that they had no special concerns with this issue, and LWG should retain ownership.
AM: this is too overly broad as it also covers calling the exact same member function on a different objectMove to Open
[2015-07 Telecon Urbana]
Marshall to ping STL for updated wording.
[2016-05 email from STL]
I don't have any better suggestions than my original PR at the moment.
Previous resolution [SUPERSEDED]:
This wording is relative to N3936.
Change 16.4.6.9 [reentrancy] p1 as indicated:
-1- Except where explicitly specified in this standard, it is implementation-defined which functions (including different member functions called on a single object) in the Standard C++ library may be recursively reentered.
[2021-07-29 Tim suggests new wording]
The "this pointer" restriction is modeled on 11.9.5 [class.cdtor] p2. It allows us to continue to specify a member function f as calling some other member function g, since any such call would use something obtained from the first member function's this pointer.
In all other cases, this wording disallows such "recursion on object" unless both member functions are const (or are treated as such for the purposes of data race avoidance). Using "access" means that we also cover direct access to the object representation, such as the following pathological example from Arthur O'Dwyer, which is now undefined:std::string s = "hello world"; char *first = (char*)&s; char *last = (char*)(&s + 1); s.append(first, last);
Proposed resolution:
This wording is relative to N4892.
Add the following paragraph to 16.4.6.9 [reentrancy]:
-?- During the execution of a standard library non-static member function F on an object, if that object is accessed through a glvalue that is not obtained, directly or indirectly, from the this pointer of F, in a manner that can conflict (6.9.2.2 [intro.races]) with any access that F is permitted to perform (16.4.6.10 [res.on.data.races]), the behavior is undefined unless otherwise specified.
Section: 20.2.5 [ptr.align] Status: New Submitter: Melissa Mears Opened: 2014-08-06 Last modified: 2014-11-03
Priority: 3
View all other issues in [ptr.align].
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Discussion:
The specification of std::align does not appear to specify what happens when the value of the size parameter is 0. (The question of what happens when alignment is 0 is mentioned in another Defect Report, 2377; it would change the behavior to be undefined rather than potentially implementation-defined.)
The case of size being 0 is interesting because the result is ambiguous. Consider the following code's output:
#include <cstdio>
#include <memory>
int main()
{
alignas(8) char buffer[8];
void *ptr = &buffer[1];
std::size_t space = sizeof(buffer) - sizeof(char[1]);
void *result = std::align(8, 0, ptr, space);
std::printf("%d %td\n", !!result, result ? (static_cast<char*>(result) - buffer) : std::ptrdiff_t(-1));
}
There are four straightforward answers as to what the behavior of std::align with size 0 should be:
The behavior is undefined because the size is invalid.
The behavior is implementation-defined. This seems to be the status quo, with current implementations using #3.
Act the same as size == 1, except that if size == 1 would fail but would be defined and succeed if space were exactly 1 larger, the result is a pointer to the byte past the end of the ptr buffer. That is, the "aligned" version of a 0-byte object can be one past the end of an allocation. Such pointers are, of course, valid when not dereferenced (and a "0-byte object" shouldn't be), but whether that is desired is not specified in the Standard's definition of std::align, it appears. The output of the code sample is "1 8" in this case.
Act the same as size == 1; this means that returning "one past the end" is not a possible result. In this case, the code sample's output is "0 -1".
The two compilers I could get working with std::align, Visual Studio 2013 and Clang 3.4, implement #3. (Change %td to %Id on Visual Studio 2013 and earlier. 2014 and later will have %td.)
Proposed resolution:
Section: 28.6.5 [template.slice.array], 28.6.7 [template.gslice.array], 28.6.8 [template.mask.array], 28.6.9 [template.indirect.array] Status: New Submitter: Akira Takahashi Opened: 2014-08-12 Last modified: 2014-11-03
Priority: 4
View all other issues in [template.slice.array].
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Discussion:
I found a missing specification of the copy constructor of the following class templates:
slice_array (28.6.5 [template.slice.array])
gslice_array (28.6.7 [template.gslice.array])
mask_array (28.6.8 [template.mask.array])
indirect_array (28.6.9 [template.indirect.array])
Proposed resolution:
Before 28.6.5.2 [slice.arr.assign] insert a new sub-clause as indicated:
-?- slice_array constructors [slice.arr.cons]
slice_array(const slice_array&);-?- Effects: The constructed slice refers to the same valarray<T> object to which the argument slice refers.
Before 28.6.7.2 [gslice.array.assign] insert a new sub-clause as indicated:
-?- gslice_array constructors [gslice.array.cons]
gslice_array(const gslice_array&);-?- Effects: The constructed slice refers to the same valarray<T> object to which the argument slice refers.
Before 28.6.8.2 [mask.array.assign] insert a new sub-clause as indicated:
-?- mask_array constructors [mask.array.cons]
mask_array(const mask_array&);-?- Effects: The constructed slice refers to the same valarray<T> object to which the argument slice refers.
Before 28.6.9.2 [indirect.array.assign] insert a new sub-clause as indicated:
-?- indirect_array constructors [indirect.array.cons]
indirect_array(const indirect_array&);-?- Effects: The constructed slice refers to the same valarray<T> object to which the argument slice refers.
Section: 32.2 [re.req] Status: New Submitter: Jonathan Wakely Opened: 2014-09-30 Last modified: 2020-04-16
Priority: 3
View other active issues in [re.req].
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Discussion:
The requirements on the traits class in 32.2 [re.req] do not say whether a regular expression traits class is required to be DefaultConstructible, CopyConstructible, CopyAssignable etc.
The std::regex_traits class appears to be all of the above, but can basic_regex assume that for user-defined traits classes? Should the following statements all leave u in equivalent states?
X u{v};
X u; u = v;
X u; u.imbue(v.getloc();
Whether they are equivalent has implications for basic_regex copy construction and assignment.
[2020-04-16, Jonathan adds that 32.7.5 [re.regex.locale] requires the traits type to be default-initialized, despite no guarantee that the traits type is default constructible. ]
Proposed resolution:
Section: 21 [meta] Status: Core Submitter: Hubert Tong Opened: 2014-11-04 Last modified: 2015-10-21
Priority: 3
View other active issues in [meta].
View all other issues in [meta].
Discussion:
The BaseCharacteristic for is_constructible is defined in terms of the well-formedness of a declaration for an invented variable. The well-formedness of the described declaration itself may change for the same set of arguments because of the introduction of default arguments.
In the following program, there appears to be conflicting definitions of a specialization of std::is_constructible; however, it seems that this situation is caused without a user violation of the library requirements or the ODR. There is a similar issue with is_convertible, result_of and others. a.cc:
#include <type_traits>
struct A { A(int, int); };
const std::false_type& x1 = std::is_constructible<A, int>();
int main() { }
b.cc:
#include <type_traits>
struct A { A(int, int); };
inline A::A(int, int = 0) { }
const std::true_type& x2 = std::is_constructible<A, int>();
Presumably this program should invoke undefined behaviour, but the Library specification doesn't say that.
[2015-02 Cologne]
Core wording should say "this kind of thing is ill-formed, no diagnostic required"
Proposed resolution:
Section: 17.10 [support.initlist], 25.7 [iterator.range] Status: New Submitter: Richard Smith Opened: 2014-11-11 Last modified: 2021-06-06
Priority: 3
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Discussion:
These sections define helper functions, some of which apply to initializer_list<T>. And they're available if you include one of a long list of header files, many of which include <initializer_list>. But they are not available if you include <initializer_list>. This seems very odd.
#include <initializer_list>
auto x = {1, 2, 3};
const int *p = data(x); // error, undeclared
#include <vector>
const int *q = data(x); // ok
Proposed resolution:
Section: 16.4.4.6 [allocator.requirements], 24.3.11.3 [vector.capacity], 24.3.11.5 [vector.modifiers] Status: New Submitter: dyp Opened: 2014-12-06 Last modified: 2015-06-10
Priority: 3
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Discussion:
When resizing a vector, the accessibility and exception specification of the value type's constructors determines whether the elements are copied or moved to the new buffer. However, the copy/move is performed via the allocator's construct member function, which is assumed, but not required, to call the copy/move constructor and propagate only exceptions from the value type's copy/move constructor. The issue might also affect other classes.
The current wording in N4296 relevant here is from Table 28 — "Allocator requirements" in 16.4.4.6 [allocator.requirements]:
Table 28 — Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault … a.construct(c, args) (not used) Effect: Constructs an object of type C at c ::new ((void*)c) C(forward<Args>(args)...) …
and from 16.4.4.6 [allocator.requirements] p9:
An allocator may constrain the types on which it can be instantiated and the arguments for which its construct member may be called. If a type cannot be used with a particular allocator, the allocator class or the call to construct may fail to instantiate.
I conclude the following from the wording:
The allocator is not required to call the copy constructor if the arguments (args) is a single (potentially const) lvalue of the value type. Similarly for a non-const rvalue + move constructor. See also 24.2.2.1 [container.requirements.general] p15 which seems to try to require this, but is not sufficient: That paragraph specifies the semantics of the allocator's operations, but not which constructors of the value type are used, if any.
The allocator may throw exceptions in addition to the exceptions propagated by the constructors of the value type; it can also propagate exceptions from constructors other than a copy/move constructor.
This leads to an issue with the wording of the exception safety guarantees for vector modifiers in 24.3.11.5 [vector.modifiers] p1:
[…]
void push_back(const T& x); void push_back(T&& x);Remarks: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of T or by any InputIterator operation there are no effects. If an exception is thrown while inserting a single element at the end and T is CopyInsertable or is_nothrow_move_constructible<T>::value is true, there are no effects. Otherwise, if an exception is thrown by the move constructor of a non-CopyInsertable T, the effects are unspecified.
The wording leads to the following problem: Copy and move assignment are invoked directly from vector. For intermediary objects (see 2164), vector also directly invokes the copy and move constructor of the value type. However, construction of the actual element within the buffer is invoked via the allocator abstraction. As discussed above, the allocator currently is not required to call a copy/move constructor. If is_nothrow_move_constructible<T>::value is true for some value type T, but the allocator uses modifying operations for MoveInsertion that do throw, the implementation is required to ensure that "there are no effects", even if the source buffer has been modified.
Similarly, the vector capacity functions specify exception safety guarantees referring to the move constructor of the value type. For example, vector::resize in 24.3.11.3 [vector.capacity] p14:Remarks: If an exception is thrown other than by the move constructor of a non-CopyInsertable T there are no effects.
The wording leads to the same issue as described above.
Code example:
template<class T>
class allocator;
class pot_reg_type // a type which creates
// potentially registered instances
{
private:
friend class allocator<pot_reg_type>;
struct register_t {};
static std::set<pot_reg_type*>& get_registry()
{
static std::set<pot_reg_type*> registry;
return registry;
}
void enregister() noexcept(false)
{
get_registry().insert(this);
}
void deregister()
{
get_registry().erase(this);
}
public:
pot_reg_type(void ) noexcept(true) {}
pot_reg_type(pot_reg_type const&) noexcept(true) {}
pot_reg_type(pot_reg_type&& ) noexcept(true) {}
private:
pot_reg_type(register_t ) noexcept(false)
{ enregister(); }
pot_reg_type(register_t, pot_reg_type const&) noexcept(false)
{ enregister(); }
pot_reg_type(register_t, pot_reg_type&& ) noexcept(false)
{ enregister(); }
};
template<class T>
class allocator
{
public:
using value_type = T;
value_type* allocate(std::size_t p)
{ return (value_type*) ::operator new(p); }
void deallocate(value_type* p, std::size_t)
{ ::operator delete(p); }
void construct(pot_reg_type* pos)
{
new((void*)pos) pot_reg_type((pot_reg_type::register_t()));
}
void construct(pot_reg_type* pos, pot_reg_type const& source)
{
new((void*)pos) pot_reg_type(pot_reg_type::register_t(), source);
}
template<class... Args>
void construct(T* p, Args&&... args)
{
new((void*)p) T(std::forward<Args>(args)...);
}
};
The construct member function template is only required for rebinding, which can be required e.g. to store additional debug information in the allocated memory (e.g. VS2013).
Even though the value type has an accessible and noexcept(true) move constructor, this allocator won't call that constructor for rvalue arguments. In any case, it does not call a constructor for which vector has formulated its requirements. An exception thrown by a constructor called by this allocator is not covered by the specification in 24.3.11.5 [vector.modifiers] and therefore is guaranteed not to have any effect on the vector object when resizing. For an example how this might invalidate the exception safety guarantees, see this post on the std-discussion mailing list. Another problem arises for value types whose constructors are private, but may be called by the allocator e.g. via friendship. Those value types are not MoveConstructible (is_move_constructible is false), yet they can be MoveInsertable. It is not possible for vector to create intermediary objects (see 2164) of such a type by directly using the move constructor. Current implementations of the single-element forms of vector::insert and vector::emplace do create intermediary objects by directly calling one of the value type's constructors, probably to allow inserting objects from references that alias other elements of the container. As far as I can see, Table 100 — "Sequence container requirements" in 24.2.4 [sequence.reqmts] does not require that the creation of such intermediare objects can be performed by containers using the value type's constructor directly. It is unclear to me if the allocator's construct function could be used to create those intermediary objects, given that they have not been allocated by the allocator. Two possible solutions:Add the following requirement to the allocator_traits::construct function: If the parameter pack args consists of a single parameter of the type value_type&&, the function may only propagate exceptions if is_nothrow_move_constructible<value_type>::value is false.
Requiring alloctor_traits::construct to call a true copy/move constructor of the value type breaks std::scoped_allocator_adapter, as pointed out by Casey Carter in a post on the std-discussion mailing list.Change vector's criterion whether to move or copy when resizing:
Instead of testing the value type's constructors via is_move_constructible, check the value of noexcept( allocator_traits<Allocator>::construct(alloc, ptr, rval) ) where alloc is an lvalue of type Allocator, ptr is an expression of type allocator_traits<Allocator>::pointer and rval is a non-const rvalue of type value_type.A short discussion of the two solutions:
Solution 1 allows keeping is_nothrow_move_constructible<value_type> as the criterion for vector to decide between copying and moving when resizing. It restricts what can be done inside the construct member function of allocators, and requires implementers of allocators to pay attention to the value types used. One could conceive allocators checking the following with a static_assert: If the value type is_nothrow_move_constructible, then the constructor actually called for MoveInsertion within the construct member function is also declared as noexcept. Solution 2 requires changing both the implementation of the default allocator (add a conditional noexcept) and vector (replace is_move_constructible with an allocator-targeted check). It does not impose additional restrictions on the allocator (other than 24.2.2.1 [container.requirements.general] p15), and works nicely even if the move constructor of a MoveInsertable type is private or deleted (the allocator might be a friend of the value type). In both cases, an addition might be required to provide the basic exception safety guarantee. A short discussion on this topic can be found in the std-discussion mailing list. Essentially, if allocator_traits<Allocator>::construct throws an exception, the object may or may not have been constructed. Two solutions are mentioned in that discussion:allocator_traits<Allocator>::construct needs to tell its caller whether or not the construction was successful, in case of an exception.
If allocator_traits<Allocator>::construct propagates an exception, it shall either not have constructed an object at the specified location, or that object shall have been destroyed (or it shall ensure otherwise that no resources are leaked).
[2015-05-23, Tomasz Kamiński comments]
Solution 1 discussed in this issue also breaks support for the polymorphic_allocator proposed in the part of the Library Fundamentals TS v1, in addition to already mentioned std::scoped_allocator_adapter. Furthermore there is unknown impact on the other user-defined state-full allocators code written in the C++11.
In addition the library resolution proposed in the LWG issues 2089 and N4462, will break the relation between the std::allocator_trait::construct method and copy/move constructor even for the standard std::allocator. As example please consider following class:
struct NonCopyable
{
NonCopyable() = default;
NonCopyable(NonCopyable const&) = delete;
NonCopyable(NonCopyable&&) = delete;
};
struct InitListConstructor : NonCopyable
{
InitListConstructor() = default;
InitListConstructor(std::initializer_list<int>);
operator int() const;
};
For the above declarations following expression are ill-formed:
InitListConstructor copy(std::declval<InitListConstructor const&>()); InitListConstructor move(std::declval<InitListConstructor&&>());
So the class is not CopyConstructible nor MoveConstructible. However the following are well formed:
InitListConstructor copy{std::declval<InitListConstructor const&>()};
InitListConstructor move{std::declval<InitListConstructor&&>()};
And will be used by std::allocator<InitListConstructor>::construct in case of move-insertion and copy-insertion, after appliance of the resolution proposed in mentioned papers:
The gist of the proposed library fix is simple:
if is_constructible_v<TargetType, Args...>, use direct-nonlist-initialization
otherwise, use brace-initialization.
As consequence the requirement proposed in the Solution 1:
If the parameter pack args consists of a single parameter of the type value_type&&, the function may only propagate exceptions if is_nothrow_move_constructible<value_type>::value is false.
Will no longer hold for the std::allocator.
Proposed resolution:
Section: 27.7.1 [alg.copy] Status: Open Submitter: Jonathan Wakely Opened: 2015-01-28 Last modified: 2018-06-22
Priority: 3
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Discussion:
It's unspecified how many times copy_n increments the InputIterator. uninitialized_copy_n is specified to increment it exactly n times, which means if an istream_iterator is used then the next character after those copied is read from the stream and then discarded, losing data.
I believe all three of Dinkumware, libc++ and libstdc++ implement copy_n with n - 1 increments of the InputIterator, which avoids reading and discarding a character when used with istream_iterator, but is inconsistent with uninitialized_copy_n and causes surprising behaviour with istreambuf_iterator instead, because copy_n(in, 2, copy_n(in, 2, out)) is not equivalent to copy_n(in, 4, out)[2016-08 Chicago]
Tues PM: refer to LEWG
[LEWG Kona 2017]
This is a mess. Append to Effects: If the InputIterator is not a forward iterator, increments n-1 times. Otherwise the number of increments is not more than n. (ncm) The preceding proposition is unsatisfactory, because it is wrong for istreambuf_iterator, which is much more useful than istream_iterator. Proposing instead: Append to Effects: If InputIterator is istream_iterator for some T, increments n-1 times. Otherwise, increments n times. Want a paper exploring what the implementations actually do, and what non-uniformity is "right".
Status to Open
Proposed resolution:
Section: 22.4.9 [tuple.rel], 20.2.10.3 [allocator.globals], 20.3.1.6 [unique.ptr.special], 20.3.2.2.8 [util.smartptr.shared.cmp], 29.5.7 [time.duration.comparisons], 29.6.7 [time.point.comparisons], 20.5.5 [scoped.adaptor.operators], 25.5.1.8 [reverse.iter.cmp], 25.5.4.8 [move.iter.op.comp] Status: New Submitter: Richard Smith Opened: 2015-02-07 Last modified: 2021-06-06
Priority: 3
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Discussion:
The standard library specifies a lot of heterogeneous comparison operators. For instance:
template<class... TTypes, class... UTypes> constexpr bool operator!=(const tuple<TTypes...>&, const tuple<UTypes...>&);
This has an unfortunate consequence:
#include <tuple> #include <utility> using namespace std::rel_ops; std::tuple<int> a(0); bool b = a != a;
The last line here is ill-formed due to ambiguity: it might be rel_ops::operator!=, and it might be the heterogeneous tuple operator!=. These are not partially ordered, because they have different constraints: rel_ops requires the types to match, whereas the tuple comparison requires both types to be tuples (but not to match). The same thing happens for user code that defines its own unconstrained 'template<typename T> operator!=(const T&, const T&)' rather than using rel_ops.
One straightforward fix would be to add a homogeneous overload for each heterogeneous comparison:template<class... TTypes> constexpr bool operator!=(const tuple<TTypes...>&, const tuple<TTypes...>&);This is then unambiguously chosen over the other options in the preceding case. FWIW, libstdc++ already does this in some cases.
Proposed resolution:
Section: D.27.2 [depr.conversions.string] Status: New Submitter: Jonathan Wakely Opened: 2015-03-04 Last modified: 2017-04-22
Priority: 4
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Discussion:
How do wstring_convert::from_bytes and wstring_convert::to_bytes use the cvtstate member?
Is it passed to the codecvt member functions? Is a copy of it passed to the member functions? "Otherwise it shall be left unchanged" implies a copy is used, but if that's really what's intended there are simpler ways to say so.Proposed resolution:
Section: D.27.3 [depr.conversions.buffer] Status: New Submitter: Jonathan Wakely Opened: 2015-03-04 Last modified: 2017-04-22
Priority: 4
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Discussion:
How does wbuffer_convert use the cvtstate member?
Is the same conversion state object used for converting both the get and put areas? That means a read which runs out of bytes halfway through a multibyte character will leave some shift state in cvtstate, which would then be used by a following write, even though the shift state of the get area is unrelated to the put area.Proposed resolution:
Section: D.27.3 [depr.conversions.buffer] Status: New Submitter: Jonathan Wakely Opened: 2015-03-04 Last modified: 2017-04-22
Priority: 4
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Discussion:
If a codecvt conversion returns codecvt_base::error should that be treated as EOF? An exception? Should all the successfully converted characters before a conversion error be available to the users of the wbuffer_convert and/or the internal streambuf, or does a conversion error lose information?
Proposed resolution:
Section: D.27.2 [depr.conversions.string] Status: New Submitter: Jonathan Wakely Opened: 2015-03-04 Last modified: 2017-04-22
Priority: 4
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Discussion:
Paragraph 4 of D.27.2 [depr.conversions.string] introduces byte_err_string as "a byte string to display on errors". What does display mean? The string is returned on error, it's not displayed anywhere.
Paragraph 14 says "Otherwise, if the object was constructed with a byte-error string, the member function shall return the byte-error string." The term byte-error string is not used anywhere else. Paragraph 17 talks about storing "default values in byte_err_string". What default value? Is "Hello, world!" allowed? If it means default-construction it should say so. If paragraph 14 says it won't be used what does it matter how it's initialized? The end of the paragraph refers to storing "byte_err in byte_err_string". This should be more clearly related to the wording in paragraph 14. It might help if the constructor (and destructor) was specified before the other member functions, so it can more formally define the difference between being "constructed with a byte-error string" and not. All the same issues apply to the wide_err_string member.Proposed resolution:
Section: 32 [re] Status: New Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2015-05-05
Priority: 3
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Discussion:
Only 4 functions are marked noexcept in all of Clause 28. Many more need to be marked — for example, regex_error::code(), basic_regex::swap(), and sub_match::length().
Proposed resolution:
Section: 22.10.8 [comparisons] Status: New Submitter: Agustín K-ballo Bergé Opened: 2015-04-01 Last modified: 2021-04-10
Priority: 3
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Discussion:
It is not entirely clear if and when the specializations of std::less (and friends) for pointer types can be used in a constant expression. Consider the following code:
#include <functional>
struct foo {};
foo x, y;
constexpr bool b = std::less<foo*>{}(&x, &y); // [1]
foo z[] = {{}, {}};
constexpr bool ba = std::less<foo*>{}(&z[0], &z[1]); // [2]
Comparing the address of unrelated objects is not a constant expression since the result is unspecified, so it could be expected for [1] to fail and [2] to succeed. However, std::less specialization for pointer types is well-defined and yields a total order, so it could just as well be expected for [1] to succeed. Finally, since the implementation of such specializations is not mandated, [2] could fail as well (This could happen, if an implementation would provide such a specialization and if that would use built-in functions that would not be allowed in constant expressions, for example). In any case, the standard should be clear so as to avoid implementation-defined constexpr-ness.
[2017-01-22, Jens provides rationale and proposed wording]
std::less<T*> is required to deliver a total order on pointers. However, the layout of global objects is typically determined by the linker, not the compiler, so requiring std::less<T*> to provide an ordering at compile-time that is consistent with run-time would need results from linking to feed back to the compiler, something that C++ has traditionally not required.
Previous resolution [SUPERSEDED]:This wording is relative to N4618.
Add at the end of 22.10.8 [comparisons]:
-2- For templates less, greater, less_equal, and greater_equal, […], if the call operator calls a built-in operator comparing pointers, the call operator yields a strict total order that is consistent among those specializations and is also consistent with the partial order imposed by those built-in operators. Relational comparisons of pointer values are not required to be usable as constant expressions.
[2021-04-05; Jiang An comments and provides alternative wording]
The libc++ and MSVC STL implementations only support flat address spaces, and always use comparison operators. The libstdc++ implementation casts pointer values to uintptr_t if the direct comparison result is unusable in constant evaluation.
So, I think that we can specify that the implementation-defined strict total order (3.27 [defns.order.ptr]) generates a core constant expression if and only if the corresponding underlying comparison expression comparing pointer values is a core constant expression. No any other case should be a core constant expression, otherwise we should also make the underlying comparison expression a core constant expression. IMO the proposed resolution is already implemented in libc++, libstdc++, and MSVC STL, and implementable on compilers that either support flat address spaces only or have implemented intrinsics needed for transparent comparison operators and std::is_constant_evaluated.Proposed resolution:
This wording is relative to N4885.
Add at the end of 22.10.8 [comparisons] p2:
-2- For templates less, greater, less_equal, and greater_equal, the specializations for any pointer type yield a result consistent with the implementation-defined strict total order over pointers (3.27 [defns.order.ptr]). [Note 1: If a < b is well-defined for pointers a and b of type P, then (a < b) == less<P>()(a, b), (a > b) == greater<P>()(a, b), and so forth. — end note] For template specializations less<void>, greater<void>, less_equal<void>, and greater_equal<void>, if the call operator calls a built-in operator comparing pointers, the call operator yields a result consistent with the implementation-defined strict total order over pointers. A comparison result of pointer values is a core constant expression if and only if the corresponding built-in comparison expression is a core constant expression.
Add at the end of 22.10.9 [range.cmp] (3.1):
-3- Effects:
(3.1) — If the expression std::forward<T>(t) == std::forward<U>(u) results in a call to a built-in operator == comparing pointers: returns false if either (the converted value of) t precedes u or u precedes t in the implementation-defined strict total order over pointers (3.27 [defns.order.ptr]) and otherwise true. The result is a core constant expression if and only if std::forward<T>(t) == std::forward<U>(u) is a core constant expression.
(3.2) — Otherwise, equivalent to: return std::forward<T>(t) == std::forward<U>(u);
Add at the end of 22.10.9 [range.cmp] (7.1):
-7- Effects:
(7.1) — If the expression std::forward<T>(t) < std::forward<U>(u) results in a call to a built-in operator < comparing pointers: returns true if (the converted value of) t precedes u in the implementation-defined strict total order over pointers (3.27 [defns.order.ptr]) and otherwise false. The result is a core constant expression if and only if std::forward<T>(t) < std::forward<U>(u) is a core constant expression.
(7.2) — Otherwise, equivalent to: return std::forward<T>(t) < std::forward<U>(u);
Add at the end of 22.10.8.8 [comparisons.three.way] (3.1):
-3- Effects:
(3.1) — If the expression std::forward<T>(t) <=> std::forward<U>(u) results in a call to a built-in operator <=> comparing pointers: returns strong_ordering::less if (the converted value of) t precedes u in the implementation-defined strict total order over pointers (3.27 [defns.order.ptr]), strong_ordering::greater if u precedes t, and otherwise strong_ordering::equal. The result is a core constant expression if and only if std::forward<T>(t) <=> std::forward<U>(u) is a core constant expression.
(3.2) — Otherwise, equivalent to: return std::forward<T>(t) <=> std::forward<U>(u);
Section: 17.10 [support.initlist] Status: New Submitter: David Krauss Opened: 2015-04-27 Last modified: 2015-05-05
Priority: 4
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Discussion:
The typical use-case of std::initializer_list<T> is for a pass-by-value parameter of T's constructor. However, this contravenes 16.4.5.8 [res.on.functions]/2.5 because initializer_list doesn't specifically allow incomplete types (as do for example std::unique_ptr (20.3.1 [unique.ptr]/5) and std::enable_shared_from_this (20.3.2.5 [util.smartptr.enab]/2)).
A resolution would be to copy-paste the relevant text from such a paragraph.Proposed resolution:
Section: 21.3.5.4 [meta.unary.prop] Status: New Submitter: Hubert Tong Opened: 2015-05-07 Last modified: 2015-08-03
Priority: 3
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Discussion:
I do not believe that the wording in 21.3.5.4 [meta.unary.prop] paragraph 3 allows for the following program to be ill-formed:
#include <type_traits>
template <typename T> struct B : T { };
template <typename T> struct A { A& operator=(const B<T>&); };
std::is_assignable<A<int>, int> q;
In particular, I do not see where the wording allows for the "compilation of the expression" declval<T>() = declval<U>() to occur as a consequence of instantiating std::is_assignable<T, U> (where T and U are, respectively, A<int> and int in the example code).
Instantiating A<int> as a result of requiring it to be a complete type does not trigger the instantiation of B<int>; however, the "compilation of the expression" in question does.Proposed resolution:
Section: 31.7.6.2.4 [ostream.sentry] Status: New Submitter: Roger Orr Opened: 2015-05-08 Last modified: 2020-09-06
Priority: 3
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Discussion:
In the current 31.7.6.2.4 [ostream.sentry], p4 refers to the now deprecated std::uncaught_exception(): D.9 [depr.uncaught].
If ((os.flags() & ios_base::unitbuf) && !uncaught_exception() && os.good()) is true, calls os.rdbuf()->pubsync().
This needs to be changed, for example to use std::uncaught_exceptions() and to capture the value on entry and compare with the saved value on exit.
[2015-06, Telecon]
JW: I already added an 's' here to make it use the new function, but that doesn't resolve Roger's suggestion to capture the value on entry and check it.
[2019-03-21; Daniel comments and provides wording]
The wording below implements Roger's suggestion.
Proposed resolution:
This wording is relative to N4810.
Modify 31.7.6.2.4 [ostream.sentry], class basic_ostream::sentry synopsis, as indicated:
namespace std { template<class charT, class traits = char_traits<charT>> class basic_ostream<charT, traits>::sentry { bool ok_; // exposition only int uncaught_ = uncaught_exceptions(); // exposition only public: explicit sentry(basic_ostream<charT, traits>& os); ~sentry(); explicit operator bool() const { return ok_; } sentry(const sentry&) = delete; sentry& operator=(const sentry&) = delete; }; }[…]
~sentry();-4- If (os.flags() & ios_base::unitbuf) &&
!uncaught_exceptions() <= uncaught_ && os.good() is true, calls os.rdbuf()->pubsync(). If that function returns -1, sets badbit in os.rdstate() without propagating an exception.
Section: 31.6.3 [streambuf] Status: New Submitter: Jonathan Wakely Opened: 2015-05-28 Last modified: 2015-08-03
Priority: 3
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Discussion:
31.6.3 [streambuf] p1 says:
The class template basic_streambuf<charT, traits> serves as an abstract base class for deriving various stream buffers whose objects each control two character sequences: […]
The term "abstract base class" is not defined in the standard, but "abstract class" is (11.7.4 [class.abstract]).
According to the synopsis basic_streambuf has no pure virtual functions so is not an abstract class and none of libstdc++, libc++, or dinkumware implement it as an abstract class. I don't believe the wording was ever intended to require it to be an abstract class, but it could be read that way. I suggest the wording be changed to "polymorphic base class" or something else that can't be seen to imply a normative requirement to make it an abstract class.Proposed resolution:
Section: 6.9.2 [intro.multithread], 33.5.8 [atomics.types.generic], 17.13 [support.runtime] Status: SG1 Submitter: Geoffrey Romer Opened: 2015-05-29 Last modified: 2018-03-15
Priority: 3
View all other issues in [intro.multithread].
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Discussion:
The concurrency libraries specified in clauses 29 and 30 do not adequately specify how they relate to the concurrency model specified in 6.9.2 [intro.multithread]. In particular:
6.9.2 [intro.multithread] specifies "atomic objects" as having certain properties. I can only assume that instances of the classes defined in Clause 29 are intended to be "atomic objects" in this sense, but I can't find any wording to specify that, and it's genuinely unclear whether Clause 30 objects are atomic objects. In fact, on a literal reading the C++ Standard doesn't appear to provide any portable way to create an atomic object, or even determine whether an object is an atomic object. (It's not clear if the term "atomic object" is actually needed, given that atomic objects can have non-atomic operations, and non-atomic objects can have atomic operations. But even if the term itself goes away, there still needs to be some indication that Clause 29 objects have the properties currently attributed to atomic objects). Similarly, 6.9.2 [intro.multithread] uses "atomic operation" as a term of art, but the standard never unambiguously identifies any operation as an "atomic operation" (although in one case it unambiguously identifies an operation that is not atomic). It does come close in a few cases, but not close enough:6.9.2 [intro.multithread]/p7 could be read to imply that "synchronization operations" in Clauses 29 and 30 are also atomic operations. However, that's vague and indirect, and somewhat belied by 33.6.4.2 [thread.mutex.requirements.mutex]/p5, which specifies that mutex lock and unlock operations "behave as atomic operations", but only "for purposes of determining the existence of a data race". Furthermore, not a single operation in Clause 29 explicitly identifies itself as a "synchronization operation".
33.5.8 [atomics.types.generic]/p4 states in part that "There shall be a specialization atomic<bool> which provides the general atomic operations as specified in 29.6.1", but read in context, "general atomic operations" appears to be a loose synonym for "general operations on atomic types" as defined in [atomics.types.operations.general], rather than a use of "atomic object" as Words of Power. Incidentally, "atomic type" is never satisfactorily defined either (although the <atomic> synopsis comes close).
17.13 [support.runtime]/p10 specifies exactly which operations are "plain lock-free atomic operations", but in a standard where an "integral constant expression" isn't necessarily a "constant expression", I do not feel safe assuming that a "plain lock-free atomic operation" is an "atomic operation".
Hans Boehm tells me the operations with "atomically" in the Effects element are intended to be atomic operations, but since "atomic operation" is a term of art (e.g. in 6.9.2 [intro.multithread]/p27.4), I think this needs to be spelled out rather than assumed. Furthermore, this does not help with 33.5.11 [atomics.fences], or anything in Clause 30.
[2018-03 JAX; Geoffrey comments in behalf of SG1]
SG1 consensus is that operations outside clause 32 are not "atomic operations", and objects of types defined outside clause 32 are not "atomic objects". "Synchronization operations" are operations which act as endpoints of primitive edges of partial orders other than sequenced-before, but it may make more sense to just drop that term and inline the definition, so to speak.
We would welcome a paper to make those definitions more explicit, and revise the wording as needed to be consistent with those definitions.Proposed resolution:
Section: D.26 [depr.locale.stdcvt] Status: New Submitter: Jonathan Wakely Opened: 2015-06-08 Last modified: 2017-04-22
Priority: 3
View all other issues in [depr.locale.stdcvt].
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Discussion:
The enumeration type codecvt_mode is effectively a bitmask type (16.3.3.3.3 [bitmask.types]) with three elements, but isn't defined as such.
This harms usability because bitmask types are required to work well with bitwise operators, but codecvt_mode doesn't have overloaded operators, making it very inconvenient to combine values:std::codecvt_utf16<char32_t, 0x10FFFF, static_cast<std::codecvt_mode>(std::little_endian|std::generate_header)> cvt;
The static_cast harms readability and should not be necessary.
I suggest that either codecvt_mode is specified to be a bitmask type, or as a minimal fix we provide an overloaded operator| that returns the right type.Proposed resolution:
Section: 17.6.3.5 [new.delete.dataraces] Status: New Submitter: Hans Boehm Opened: 2015-06-09 Last modified: 2016-02-01
Priority: 3
View all other issues in [new.delete.dataraces].
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Discussion:
17.6.3.5 [new.delete.dataraces] uses obsolete wording.
It should introduce a "synchronizes with" relationship. "Happens before" is too weak, since that may not composes with sequenced before. The "shall not introduce a data race" wording is probably not technically correct either. These may race with other (non-allocation/deallocation) concurrent accesses to the object being allocated or deallocated.Proposed resolution:
Section: 30.4.6.2.3 [locale.time.get.virtuals] Status: Open Submitter: Hubert Tong Opened: 2015-06-19 Last modified: 2017-09-07
Priority: 4
View other active issues in [locale.time.get.virtuals].
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Discussion:
I recently encountered a failure related to questionable use of do_get_year. The platform where the code happened to work had an implementation which handled certain three-digit "year identifiers" as the number of years since 1900 (this article describes such an implementation).
30.4.6.2.3 [locale.time.get.virtuals] makes it implementation defined whether two-digit years are accepted, etc., but does not say anything specifically about three-digit years. The implementation freedom to not report errors in 30.4.6.2 [locale.time.get] paragraph 1 also seems to be too broad. See also the discussion following c++std-lib-38042.[2016-08 Chicago]
Wed PM: This has been this way since C++98. Don't think it's a P2.
Change to P4, and move to Open.
Proposed resolution:
Section: 23.1 [strings.general] Status: New Submitter: Jonathan Wakely Opened: 2015-06-26 Last modified: 2020-09-06
Priority: 4
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Discussion:
The allocator-aware container requirements in Table 98 impose no MoveAssignable requirements on the value_type when propagate_on_container_move_assignment is true, because typically the container's storage would be moved by just exchanging some pointers.
However for a basic_string using the small string optimization move assignment may need to assign individual characters into the small string buffer, even when the allocator propagates. The only requirement on the char-like objects stored in a basic_string are that they are non-array POD types and Destructible, which means that a POD type with a deleted move assignment operator should be usable in a basic_string, despite it being impossible to move assign:
#include <string>
struct odd_pod
{
odd_pod() = default;
odd_pod& operator=(odd_pod&&) = delete;
};
static_assert(std::is_pod<odd_pod>::value, "POD");
int main()
{
using S = std::basic_string<odd_pod>;
S s;
s = S{}; // fails
}
Using libstdc++ basic_string<odd_pod> cannot even be default-constructed because the constructor attempts to assign the null terminator to the first element of the small string buffer.
Similar problems exist with POD types with a deleted default constructor. I believe that basic_string should require its value_type to be at least DefaultConstructible and MoveAssignable.[2016-06, Oulu]
This should be resolved by P0178
Note: P0178 was sent back to LEWG in Oulu.
Proposed resolution:
Section: 28.5.9.4.2 [rand.dist.pois.exp] Status: Open Submitter: Michael Prähofer Opened: 2015-08-20 Last modified: 2018-08-22
Priority: 2
View all issues with Open status.
Discussion:
Original title was: exponential_distribution<float> sometimes returns inf.
The random number distribution class template exponential_distribution<float> may return "inf" as can be seen from the following example program:
// compiled with
// g++ -std=c++11 Error_exp_distr.cpp
#include <iostream>
#include <random>
#include <bitset>
int main(){
unsigned long long h;
std::mt19937_64 mt1(1);
std::mt19937_64 mt2(1);
mt1.discard(517517);
mt2.discard(517517);
std::exponential_distribution<float> dis(1.0);
h = mt2();
std::cout << std::bitset<64>(h) << " " << (float) -log(1 - h/pow(2, 64)) << " "
<< -log(1 - (float) h/pow(2, 64)) << " " << dis(mt1) << std::endl;
h = mt2();
std::cout << std::bitset<64>(h) << " " << (float) -log(1 - h/pow(2, 64)) << " "
<< -log(1 - (float) h/pow(2, 64)) << " " << dis(mt1) << std::endl;
}
output:
0110010110001001010011000111000101001100111110100001110011100001 0.505218 0.505218 0.505218 1111111111111111111111111101010011000110011110011000110101100110 18.4143 inf inf
The reason seems to be that converting a double x in the range [0, 1) to float may result in 1.0f if x is close enough to 1. I see two possibilities to fix that:
use internally double (or long double?) and then convert the result at the very end to float.
take only 24 random bits and convert them to a float x in the range [0, 1) and then return -log(1 - x).
I have not checked if std::exponential_distribution<double> has the same problem: For float on the average 1 out of 224 (~107) draws returns "inf", which is easily confirmed. For double on the average 1 out of 253 (~1016) draws might return "inf", which I have not tested.
Marshall:
I don't think the problem is in std::exponential_distribution; but rather in generate_canonical.
Consider:
which outputs:std::mt19937_64 mt2(1); mt2.discard(517517); std::cout << std::hexfloat << std::generate_canonical<float, std::numeric_limits<float>::digits>(mt2) << std::endl; std::cout << std::hexfloat << std::generate_canonical<float, std::numeric_limits<float>::digits>(mt2) << std::endl; std::cout << std::hexfloat << std::generate_canonical<float, std::numeric_limits<float>::digits>(mt2) << std::endl;
but generate_canonical is defined to return a result in the range [0, 1).0x1.962532p-2 0x1p+0 0x1.20d0cap-3
[2015-10, Kona Saturday afternoon]
Options:
WEB: The one thing we cannot tolerate is any output range other than [0, 1).
WEB: I believe there may be a documented algorithm for the generator, and perhaps it's possible to discover en-route that the algorithm produces the wrong result and fix it.
MC: No. I analyzed this once, and here it is: the algorithm is in [rand.util.canonical], and it's all fine until p5. The expression S/R^k is mathematically less than one, but it may round to one.
GR: Could we change the rounding mode for the computation?
HH: No, because the rounding mode is global, not thread-local.
AM: SG1 wants to get rid of the floating point environment.
STL: The problem is that the standard specifies the implementation, and the implementation doesn't work.
MC: I'm not sure if nudging it down will introduce a subtle bias.
EF: I worry about how the user's choice of floating point environment affects the behaviour.
MS offers to run the topic past colleagues.
MC: Will set the status to open. STL wants to rename the issue. WEB wants to be able to find the issue by its original name still.
Mike Spertus to run the options past his mathematical colleagues, and report back.
[2017-11 Albuquerque Wednesday issue processing]
Choice: Rerun the algorithm if it gets 1.0.
Thomas K to provide wording; Marshall and STL to review.
[2018-08 Batavia Monday issue discussion]
Davis has a paper P0952 which resolves this.
Proposed resolution:
Section: 22.4.4.1 [tuple.cnstr] Status: New Submitter: Brian Rodriguez Opened: 2015-08-25 Last modified: 2017-02-19
Priority: 3
View other active issues in [tuple.cnstr].
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Discussion:
The std::tuple order of element construction is unspecified. It is either in the same order of the type list or in reverse.
Consider the following program:
#include <iostream>
#include <tuple>
struct X
{
X(int) { std::cout << "X constructor\n"; }
};
struct Y
{
Y(int) { std::cout << "Y constructor\n"; }
};
int main()
{
std::tuple<X, Y> t(1, 2);
}
Here is a link to two sample compilations. The first uses libstdc++ and constructs in reverse order, and the second uses libc++ and constructs in in-order.
A std::tuple mimics both a struct and type-generic container and should thus follow their standards. Construction is fundamentally different from a function call, and it has been historically important for a specific order to be guaranteed; namely: whichever the developer may decide. Mandating construction order will allow developers to reference younger elements later on in the chain as well, much like a struct allows you to do with its members. There are implementation issues as well. Reversed lists will require unnecessary overhead for braced-initializer-list initialization. Since lists are evaluated from left to right, the initializers must be placed onto the stack to respect the construction order. This issue could be significant for large tuples, deeply nested tuples, or tuples with elements that require many constructor arguments. I propose that the std::tuple<A, B, ..., Y, Z>'s constructor implementation be standardized, and made to construct in the same order as its type list e.g. A{}, B{}, ..., Y{}, Z{}.Daniel:
When N3140 became accepted, wording had been added that gives at least an indication of requiring element initialization in the order of the declaration of the template parameters. This argumentation can be based on 22.4.4.1 [tuple.cnstr] p3 (emphasize mine):-3- In the constructor descriptions that follow, let i be in the range [0,sizeof...(Types)) in order, Ti be the ith type in Types, and Ui be the ith type in a template parameter pack named UTypes, where indexing is zero-based.
But the current wording needs to be improved to make that intention clearer and an issue like this one is necessary to be sure that the committee is agreeing (or disagreeing) with that intention, especially because N3140 didn't really point out the relevance of the element construction order in the discussion, and because not all constructors explicitly refer to the ordered sequence of numbers generated by the variable i (The move constructor does it right, but most other don't do that).
[2017-02-12, Alisdair comments]
Note that this issue should not be extended to cover the assignment operators, as implementations may want the freedom to re-order member-wise assignment so that, for example, all potentially-throwing assignments are performed before non-throwing assignments (as indicated by the noexcept operator).
Proposed resolution:
Section: 33.10.5 [futures.state] Status: Open Submitter: Agustín K-ballo Bergé Opened: 2015-09-03 Last modified: 2016-08-06
Priority: 3
View all other issues in [futures.state].
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Discussion:
When a shared-state is released, it may be necessary to execute user defined code for the destructor of a stored value or exception. It is unclear whether the execution of said destructor constitutes an observable side effect.
While discussing N4445 in Lenexa, Nat Goodspeed pointed out that 33.10.5 [futures.state]/5.1 does not explicitly mention the destruction of the result, so implementations should be allowed to release (or reuse) a shared state ahead of time under the "as-if" rule.
The standard should clarify whether the execution of destructors is a visible side effect of releasing a shared state.[2016-08-03 Chicago]
This is related to 2532
Fri AM: Moved to Open
Proposed resolution:
Section: 33.10.6 [futures.promise] Status: Open Submitter: Agustín K-ballo Bergé Opened: 2015-09-03 Last modified: 2016-08-06
Priority: 3
View other active issues in [futures.promise].
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Discussion:
promise::set_value_at_thread_exit and promise::set_exception_at_thread_exit operate on a shared state at thread exit, without making the thread participate in the ownership of such shared state.
Consider the following snippet:
std::promise<int>{}.set_value_at_thread_exit(42);
Arguably, since the promise abandons its shared state without actually making it ready, a broken_promise error condition should be stored in the shared state. Implementations diverge, they either crash at thread exit by dereferencing an invalid pointer, or keep the shared state around until thread exit.
[2016-08-03 Chicago]
This is related to 2530
[2016-08-03, Billy O'Neal suggests concrete wording]
Fri AM: Moved to Open
Proposed resolution:
This wording is relative to N4606.
Change 33.10.5 [futures.state] p7 as indicated:
-7- When an asynchronous provider is said to abandon its shared state, it means:
(7.1) — first, if that state is not ready or scheduled to be made ready at thread exit, the provider
(7.1.1) — stores an exception object of type future_error with an error condition of broken_promise within its shared state; and then
(7.1.2) — makes its shared state ready;
Change 33.10.5 [futures.state] p10 as indicated:
-10- Some functions (e.g., promise::set_value_at_thread_exit)
delay making the shared state ready untilschedule the shared state to be made ready when the calling thread exits. This associates a reference to the shared state with the calling thread. The destruction of each of that thread's objects with thread storage duration (6.7.5.3 [basic.stc.thread]) is sequenced before making that shared state ready. When the calling thread makes the shared state ready, if the thread holds the last reference to the shared state, the shared state is destroyed. [Note: This means that the shared state may not become ready until after the asynchronous provider has been destroyed. — end note]
Section: 99 [concurr.ts::futures.unique.future] Status: SG1 Submitter: Agustín K-ballo Bergé Opened: 2015-09-03 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with SG1 status.
Discussion:
Addresses: concurr.ts
In N4538, the continuation given to future::then can be run "on an unspecified thread of execution". This is too broad, as it allows the continuation to be run on the main thread, a UI thread, or any other thread. In comparison, functions given to async run "as if in a new thread of execution", while the Parallelism TS gives less guarantees by running "in either the invoking thread or in a thread implicitly created by the library to support parallel algorithm execution". The threads on which the continuation given to future::then can run should be similarly constrained.
[2017-03-01, Kona, SG1]
Agreement that this is a problem. Suggested addition to the issue is below. We have no immediate delivery vehicle for a fix at the moment, but we would like to make the intended direction clear.
There is SG1 consensus that .then continuations should, by default, and in the absence of executors, be run only in the following ways:If the future is not ready when .then() is called, the .then argument may be run on the execution agent that fulfills the promise.
In all cases, the .then argument may be run on an implementation-provided thread, i.e. a thread that is neither the main thread nor explicitly created by the user.
In the absence of an executor argument (which currently cannot be supplied), running of the .then() continuation will not block the thread calling .then(), even if the future is ready at the time.
Straw polls: SF | F | N | A | SA For the default behaviour: "1. Run on completed task or new execution agent" 0 | 7 | 5 | 1 | 0 "2. Run on completed task or .then caller" 0 | 0 | 5 | 5 | 3 "3. Leave as implementation defined" 1 | 2 | 4 | 3 | 3 "4. Always new execution agent" 2 | 3 | 6 | 2 | 0 The actual conclusion was to allow either (1) or (4) for now, since they are quite close, but present a very different programming mode from (2).Proposed resolution:
Section: 32.12 [re.grammar] Status: New Submitter: Hubert Tong Opened: 2015-10-08 Last modified: 2015-10-21
Priority: 4
View other active issues in [re.grammar].
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Discussion:
In 32.12 [re.grammar] paragraph 2:
basic_regex member functions shall not call any locale dependent C or C++ API, including the formatted string input functions. Instead they shall call the appropriate traits member function to achieve the required effect.
Yet, the required interface for a regular expression traits class (32.2 [re.req]) does not appear to have any reliable method for determining whether a character as encoded for the locale associated with the traits instance is the same as a character represented by a UnicodeEscapeSequence, e.g., assuming a sane ru_RU.koi8r locale:
#include <stdio.h>
#include <stdlib.h>
#include <regex>
const char data[] = "\xB3";
const char matchCyrillicCaptialLetterYo[] = R"(\u0401)";
int main(void)
{
try {
std::regex myRegex;
myRegex.imbue(std::locale("ru_RU.koi8r"));
myRegex.assign(matchCyrillicCaptialLetterYo, std::regex_constants::ECMAScript);
printf("(%s)\n", std::regex_replace(std::string(data), myRegex, std::string("E")).c_str());
myRegex.assign("[[:alpha:]]", std::regex_constants::ECMAScript);
printf("(%s)\n", std::regex_replace(std::string(data), myRegex, std::string("E")).c_str());
} catch (std::regex_error& e) {
abort();
}
return 0;
}
The implementation I tried prints:
(Ё) (E)
Which means that the character class matching worked, but not the matching to the UnicodeEscapeSequence.
Proposed resolution:
Section: 22.10.8 [comparisons], 24.2.2.1 [container.requirements.general], 33.4.3.2 [thread.thread.id] Status: New Submitter: Matt Austern Opened: 2015-10-08 Last modified: 2015-10-21
Priority: 3
View other active issues in [comparisons].
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Discussion:
A number of places in the library, including 22.10.8 [comparisons]/14, the Optional container requirements in 24.2.2.1 [container.requirements.general], and 33.4.3.2 [thread.thread.id]/8, use the phrase "total order". Unfortunately, that phrase is ambiguous. In mathematics, the most common definition is that a relation ≤ is a total order if it's total, transitive, and antisymmetric in the sense that x≤y ∧ y≤x ⇒ x=y. What we really want is a strict total order: a relation < is a strict total order if it's total, transitive, and antisymmetric in the sense that exactly one of x<y, y<x, and x=y holds.
The non-normative note in 27.8 [alg.sorting]/4 correctly uses the phrase "strict total ordering" rather than simply "total ordering".
We could address this issue by replacing "total order" with "strict total order" everywhere it appears, since I think there are no cases where we actually want a non-strict total order, or we could add something in Clause 17 saying that we always mean strict total order whenever we say total order.
Proposed resolution:
Section: 29.2 [time.syn] Status: New Submitter: Andy Giese Opened: 2016-02-05 Last modified: 2016-05-08
Priority: 4
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Discussion:
Currently 29.2 [time.syn] states
// convenience typedefs typedef duration<signed integer type of at least 64 bits, nano> nanoseconds; typedef duration<signed integer type of at least 55 bits, micro> microseconds; typedef duration<signed integer type of at least 45 bits, milli> milliseconds; typedef duration<signed integer type of at least 35 bits > seconds; typedef duration<signed integer type of at least 29 bits, ratio< 60>> minutes; typedef duration<signed integer type of at least 23 bits, ratio<3600>> hours;
However, a duration_cast<minutes>(seconds::max()) would cause overflow if the underlying signed integers only met the minimums specified.
The standard should specify that implementations guarantee that a duration_cast from any smaller duration in these "convenience typedefs" will not overflow any larger duration. That is, hours should be able to hold the maximum of minutes, which should be able to hold the maximum of seconds and so on. More formally, if the ratio typedef A and typedef B is 1:Y where Y > 1 (e.g., 1 : 60 in case of minutes : seconds), then #bitsA-1 must be at least ceil(log2(2#bitsB-1)/Y)). In the case of minutes : seconds, X = 1, Y = 60. Let #bitsseconds = 32. Therefore:2(#bitsseconds - 1) = 231 = 2147483648
ceil(log2(231 / 60) = 26
#bitsminutes - 1 = 26
#bitsminutes = 27
Therefore, a minimum of 27 bits would be needed to store minutes if 32 were used to store seconds.
I propose to change the definitions of the convenience typedefs as follows:// convenience typedefs typedef duration<signed integer type of at least 64 bits, nano> nanoseconds; typedef duration<signed integer type of at least 55 bits, micro> microseconds; typedef duration<signed integer type of at least 46 bits, milli> milliseconds; typedef duration<signed integer type of at least 37 bits > seconds; typedef duration<signed integer type of at least 32 bits, ratio< 60>> minutes; typedef duration<signed integer type of at least 27 bits, ratio<3600>> hours;
These bits were chosen to satisfy the above formula. Note that minimums only increased, so larger ranges could be held. A nice outcome of this choice is that minutes does not go above 32 bits.
[2016-04-23, Tim Song comments]
The P/R of LWG 2592 doesn't fix the issue it wants to solve, because the actual underlying type will likely have more bits than the specified minimum.
Consider seconds, which the P/R requires to have at least 37 bits. On a typical system this implies using a 64-bit integer. To ensure that casting from seconds::max() to minutes doesn't overflow in such a system, it is necessary for the latter to have at least 59 bits (which means, in practice, 64 bits too), not just 32 bits. Thus, just changing the minimum number of bits will not be able to provide the desired guarantee that casting from a smaller unit to a larger one never overflow. If such a guarantee is to be provided, it needs to be spelled out directly. Note that the difference here is 9 bits (for the 1000-fold case) and 5 bits (for the 60-fold case), which is less than the size difference between integer types on common systems, so such a requirement would effectively require those convenience typedefs to use the same underlying integer type.Proposed resolution:
This wording is relative to N4567.
Change 29.2 [time.syn], header <chrono> synopsis, as indicated
[…] // convenience typedefs typedef duration<signed integer type of at least 64 bits, nano> nanoseconds; typedef duration<signed integer type of at least 55 bits, micro> microseconds; typedef duration<signed integer type of at least 4645bits, milli> milliseconds; typedef duration<signed integer type of at least 3735bits > seconds; typedef duration<signed integer type of at least 3229bits, ratio< 60>> minutes; typedef duration<signed integer type of at least 2723bits, ratio<3600>> hours; […]
Section: 20.3.2.2 [util.smartptr.shared] Status: New Submitter: Kazutoshi Satoda Opened: 2016-02-20 Last modified: 2016-06-20
Priority: 3
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Discussion:
Latest draft (N4567) 20.3.2.2 [util.smartptr.shared] p1 says:
A shared_ptr object is empty if it does not own a pointer.
Please note that it says "own a pointer". This definition was added as the resolution for LWG defect 813.
20.3.2.2.2 [util.smartptr.shared.const] p8 says about the effect of shared_ptr(nullptr_t p, D d):Effects: Constructs a shared_ptr object that owns the object p and the deleter d.
Please note that it says "owns the object". This was intentionally changed from "the pointer" as a part of resolution for LWG defect 758, to cover nullptr_t case.
Since shared_ptr(nullptr, d) owns an object of type nullptr_t, but does not own a pointer, it is said as "empty" by a strict reading of the above mentioned definition in 20.3.2.2 [util.smartptr.shared] p1. These cause a contradiction: 20.3.2.2.2 [util.smartptr.shared.const] p9 sets a postcondition use_count() == 1 on shared_ptr(nullptr, d). But 20.3.2.2.6 [util.smartptr.shared.obs] p7 says that the return value of use_count() is "0 when *this is empty".Proposed wording changes:
Replace the last 2 words in 20.3.2.2 [util.smartptr.shared] p1 from[…] empty if it does not own a pointer.
to
[…] empty if it does not own an object.
Note that shared_ptr(nullptr_t) is defined to be empty in synopsis in 20.3.2.2 [util.smartptr.shared].
constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }
It could be less confusing if shared_ptr(nullptr, d) could be defined to be empty. But it seems too late to change that (which means changing whether the deleter is called or not, see this Stackoverflow article). Then I'm proposing just fix the contradiction.
Proposed resolution:
This wording is relative to N4594.
Change 20.3.2.2 [util.smartptr.shared] p1 as indicated:
-1- The shared_ptr class template stores a pointer, usually obtained via new. shared_ptr implements semantics of shared ownership; the last remaining owner of the pointer is responsible for destroying the object, or otherwise releasing the resources associated with the stored pointer. A shared_ptr object is empty if it does not own an object
a pointer.
Section: 25.5.1.2 [reverse.iterator], 25.5.1.6 [reverse.iter.elem] Status: New Submitter: Robert Haberlach Opened: 2016-02-28 Last modified: 2021-06-06
Priority: 3
View all other issues in [reverse.iterator].
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Discussion:
Issue 386 changed the return type of reverse_iterator::operator[] to unspecified. However, as of N3066, the return type of a random access iterator's operator[] shall be convertible to reference; thus the return type of reverse_iterator::operator[] should be reference (and it is in all common implementations).
Suggested resolution: Adjust 25.5.1.2 [reverse.iterator]'s synopsis and 25.5.1.6 [reverse.iter.elem] to use reference instead of unspecified.[2021-06-06 Tim syncs wording to current working draft]
Proposed resolution:
This wording is relative to N4885.
Edit 25.5.1.2 [reverse.iterator], class template synopsis, as indicated:
namespace std {
template <class Iterator>
class reverse_iterator {
public:
[…]
using reference = iter_reference_t<Iterator>;
[…]
constexpr referenceunspecified operator[](difference_type n) const;
[…]
};
}
Change 25.5.1.6 [reverse.iter.elem] before p3 as indicated:
constexpr referenceunspecifiedoperator[](difference_type n) const;
Section: 22.2.6 [declval], 20.3.1 [unique.ptr], 20.3.1.2.1 [unique.ptr.dltr.general], 20.3.2.2 [util.smartptr.shared], 20.3.2.3 [util.smartptr.weak], 20.3.2.5 [util.smartptr.enab] Status: New Submitter: Zhihao Yuan Opened: 2016-03-08 Last modified: 2016-04-16
Priority: 3
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Discussion:
Currently the phrase to grant this permission is:
The template parameter T of LibraryTemplate may be an incomplete type.
Two problems:
The timing is unclear. We always allow specializations like LibraryTemplate<Incomp>* p;
To the users of a template, the correct terminology should be "argument" rather than "parameter".
Suggested resolution:
In an instantiation of LibraryTemplate, an incomplete type may be used as the template argument for the template parameter T.
as shown here.
Or, to copy N4510's wording:An incomplete type T may be used when instantiating LibraryTemplate.
Proposed resolution:
Section: 31.5.2.7 [ios.base.callback] Status: New Submitter: David Krauss Opened: 2016-03-14 Last modified: 2019-06-15
Priority: 3
View all issues with New status.
Discussion:
register_callback allocates memory and so it can fail, but the case is unspecified. libc++ sets badbit, which is consistent with iword and pword. libstdc++ throws std::bad_alloc.
[2019-06-13; Billy comments]
Just as an additional data point: MSVC++ agrees with libstdc++ and also throws std::bad_alloc.
Proposed resolution:
Section: 30.4.7.4 [locale.moneypunct] Status: New Submitter: Hubert Tong Opened: 2016-04-12 Last modified: 2016-05-22
Priority: 3
View all other issues in [locale.moneypunct].
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Discussion:
The description of money_base::space is that "at least one space is required at that position." (N4582 subclause 22.4.6.3 [locale.moneypunct] paragraph 2)
When formatting for output (30.4.7.3.2 [locale.money.put.virtuals]), it is not clear that"the number of characters generated for the specified format" (excluding fill padding) includes exactly one character for money_base::space (if present), and
all characters corresponding to money_base::space (excluding fill padding) are copies of fill.
In particular, there is implementation divergence over point (b) as to whether U+0020 or fill should be used. Further, should a character other than fill be used, it is unclear when "the fill characters are placed where none or space appears in the formatting pattern", whether the fill characters are placed at the beginning or the end of the "space field".
I believe that a strict interpretation of the current wording supports U+0020; however, fill is more likely to be the pragmatic choice.Proposed resolution:
This wording is relative to N4582.
Change 30.4.7.4 [locale.moneypunct] paragraph 2 as indicated:
-2- Where none or space appears, white space is permitted in the format, except where none appears at the end, in which case no white space is permitted. For input, the value space indicates that at least one space is required at that position. For output, the value space indicates one instance of the fill character (30.4.7.3.2 [locale.money.put.virtuals]).
The value space indicates that at least one space is required at that position. Where symbol appears, the sequence of characters returned by curr_symbol() is permitted, and can be required. Where sign appears, the first (if any) of the sequence of characters returned by positive_sign() or negative_sign() (respectively as the monetary value is non-negative or negative) is required. Any remaining characters of the sign sequence are required after all other format components. Where value appears, the absolute numeric monetary value is required.
Section: 16.4.6.5 [member.functions] Status: New Submitter: Hubert Tong Opened: 2016-04-15 Last modified: 2017-02-02
Priority: 3
View all other issues in [member.functions].
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Discussion:
In N4582 subclause 17.6.5.5 [member.functions], the requirement that:
any call to the member function that would select an overload from the set of declarations described in this standard behaves as if that overload were selected
is unclear in the extent of the "as if". For example, in providing:
basic_string(const charT* s);
for a one-argument call to:
basic_string(const charT* s, const Allocator& a = Allocator());
it can be read that an implementation may be required to call the copy constructor for the allocator since the core language rules for copy elision would not allow the "a" argument to be constructed directly into the member used to store the allocator.
Clarification (even if just a note) would be appreciated.[2016-05 Issues Telecon]
This is related to issue 2563.
Proposed resolution:
Section: 30.4.3.3.3 [facet.num.put.virtuals] Status: New Submitter: Hubert Tong Opened: 2016-05-07 Last modified: 2016-05-22
Priority: 3
View other active issues in [facet.num.put.virtuals].
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Discussion:
The call to do_put(out, str, fill, (int)val) in N4582 subclause 30.4.3.3.3 [facet.num.put.virtuals] paragraph 6 cannot select a best viable function in overload resolution given the overloads listed for do_put in 30.4.3.3 [locale.nm.put].
There is implementation divergence:Some implementations call the long overload (as overriden);
some implementations call the unsigned long overload (as overriden);
some implementations call something else.
It appears that the resolution to DR 359 attempted a fix; however, the relevant portion of the change was not applied to the WP.
Proposed resolution:
Section: 30.4.3.3.3 [facet.num.put.virtuals] Status: New Submitter: Hubert Tong Opened: 2016-05-07 Last modified: 2018-02-11
Priority: 3
View other active issues in [facet.num.put.virtuals].
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Discussion:
N4582 subclause 30.4.3.3.3 [facet.num.put.virtuals] paragraph 6 makes no provision for fill-padding in its specification of the behaviour when (str.flags() & ios_base::boolalpha) != 0.
[2017-07-06, Marshall comments]
All the other cases from num_put — int, long, etc all are covered in 30.4.3.3.3 [facet.num.put.virtuals] p1 .. p5, which describe how to align and pad the output. (Specifically, stage 3) p6 does not.
With this description:cout << std::setw(15) << false;
outputs:
0 ﹎﹎﹎﹎﹎﹎﹎﹎﹎﹎﹎﹎﹎﹎// Column counter
but
cout << std::setw(15) << boolalpha << false;
outputs:
false
libc++ implements this exactly.
Dinkumware, libstdc++ and MSVC apply padding and alignment.
Proposed resolution:
Section: 24.2.4 [sequence.reqmts] Status: New Submitter: Kazutoshi Satoda Opened: 2016-05-08 Last modified: 2020-09-06
Priority: 3
View other active issues in [sequence.reqmts].
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Discussion:
Please look through the following modifications on a value v of type vector<T>:
assert(v.size() > 0); v.push_back(v[0]); v.insert(v.begin(), v[0]); v.resize(v.size() * 2, v[0]); v.assign(v.size() * 2, v[0]);
All of these use an element of itself which may be moved or destroyed by the modification.
From what I see so far, the first three are required to work. Please see library issue 526 for validity of them. But only the last one is undefined because it violates a precondition of a sequence container operation. I think this is too subtle. Should it be like that, really? The precondition is in Table 107 "Sequence container requirements" at the next of 24.2.4 [sequence.reqmts] p3.In Tables 107 and 108, X denotes a sequence container class, a denotes a value of X containing elements of type T, […] n denotes a value of X::size_type, […] t denotes an lvalue or a const rvalue of X::value_type, […]
[…]
Table 107 — Sequence container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-condition[…] a.assign(n, t) void Requires: T shall be CopyInsertable into X and CopyAssignable.
pre: t is not a reference into a.
Replaces elements in a with n copies of t.
I looked into the following implementations:
libc++ relies on the precondition.
It deallocates first on n > capacity() case, see here.libstdc++ doesn't rely on the precondition.
It creates temporary vector(n, t) and swap() on n > capacity() case, see here.MSVC relies on the precondition.
It unconditionally does clear() and then insert(begin(), n, t). I looked into my local "%PROGRAMFILES(X86)%/Microsoft Visual Studio 14.0/VC/include/vector".One drawback of libstdc++ implementation, I could find so far, is possibly increased peek memory usage (both old and new buffer exist at the same time). But, because the same can happen on the most other modifications, it seems a reasonable trade-off to remove the precondition to fill the subtle gap. Users who really needs less memory usage can do clear() and insert() by themselves.
I also found that basic_string::assign(n, c) is safe on this point. At 23.4.3.7.3 [string.assign] p17:basic_string& assign(size_type n, charT c);Effects: Equivalent to assign(basic_string(n, c)).
Returns: *this.
This can be seen as another gap.
Looking back on the history, I found that the definition of assign(n, t) was changed at C++14 for library issue 2209. There were more restricting definitions like this:void assign(size_type n, const T& t);Effects:
erase(begin(), end()); insert(begin(), n, t);
I think the precondition was probably set to accept this old definition and is not required inherently. And if the less memory usage was really intended, the standard is now underspecifying about that.
[2016-05 Issues Telecon]
Howard believes this should be NAD, but we tabled the discussion.
Previous resolution [SUPERSEDED]:This wording is relative to N4582.
In 24.2.4 [sequence.reqmts], edit Table 107 (Sequence container requirements) as indicated:
Table 107 — Sequence container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-condition[…] a.assign(n, t) void Requires: T shall be CopyInsertable into X and CopyAssignable.
pre: t is not a reference into a.
Replaces elements in a with n copies of t.
[2020-04-25, Daniel syncs current wording with recent working draft]
Proposed resolution:
This wording is relative to N4861.
In 24.2.4 [sequence.reqmts], edit Table [tab:container.seq.req] (Sequence container requirements) as indicated:
Table 77 — Sequence container requirements (in addition to container) [tab:container.seq.req] Expression Return type Assertion/note
pre-/post-condition[…] a.assign(n, t) void Preconditions: T is Cpp17CopyInsertable into X
and Cpp17CopyAssignable.t is not a reference into a.
Effects: Replaces elements in a with n copies of t.
Invalidates all references, pointers and iterators referring to the elements of a.
For vector and deque, also invalidates the past-the-end iterator.
Section: 31.12.12.2 [fs.rec.dir.itr.members] Status: Open Submitter: Eric Fiselier Opened: 2016-05-09 Last modified: 2022-12-18
Priority: 2
View all other issues in [fs.rec.dir.itr.members].
View all issues with Open status.
Discussion:
The current specification of recursion_pending() says (31.12.12.2 [fs.rec.dir.itr.members]/24):
Returns: true if disable_recursion_pending() has not been called subsequent to the prior construction or increment operation, otherwise false.
This language does not take into account cases where the prior construction was a copy construction from a iterator, it, where it.recursion_pending() == false.
[2016-08 Chicago]
Wed AM: Move to Open
[2018-1-26 issues processing telecon]
Status to 'Tentatively Ready'; Casey will explore whether making recursion_pending an exposition-only member makes this clearer.
Previous resolution from Eric [SUPERSEDED]:This wording is relative to N4582.
Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:
explicit recursive_directory_iterator(const path& p); recursive_directory_iterator(const path& p, directory_options options); recursive_directory_iterator(const path& p, directory_options options, error_code& ec) noexcept; recursive_directory_iterator(const path& p, error_code& ec) noexcept;[…]
-3- Postcondition:options() == options for the signatures with a directory_options argument, otherwise options() == directory_options::none.
options() == options for the signatures with a directory_options argument, otherwise options() == directory_options::none.
recursion_pending() == true.
[…]
[Drafting note: The following changes the specification of recursion_pending() seemingly recursive. Perhaps it would be easier to specify recursion_pending() in terms of a exposition only member in recursive_directory_iterator.]
bool recursion_pending() const;[…]
-24- Returns:true if disable_recursion_pending() has not been called subsequent to the prior construction or increment operation, otherwise falsefalse if disable_recursion_pending() has been called subsequent to the prior construction or increment operation, otherwise the value of recursion_pending() set by that operation. […]recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec) noexcept;[…]
-27- Effects: As specified by Input iterators (24.2.3), except that: […] -?- Postcondition: recursion_pending() == true.
[2018-01-29: Casey provides a PR with an exposition-only member]
Status to 'Review'.
Previous resolution from Casey [SUPERSEDED]:This wording is relative to N4713.
Change [fs.rec.dir.itr] as indicated:
[…] // other members as required by 25.3.5.3 [input.iterators], input iterators private: bool recurse_; // exposition-only }; }Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:
explicit recursive_directory_iterator(const path& p); recursive_directory_iterator(const path& p, directory_options options); recursive_directory_iterator(const path& p, directory_options options, error_code& ec) noexcept; recursive_directory_iterator(const path& p, error_code& ec) noexcept;[…]
-3- Postconditions:
options() == options for the signatures with a directory_options argument, otherwise options() == directory_options::none.
options() == options for the signatures with a directory_options argument, otherwise options() == directory_options::none.
recurse_ == true.
[…]
recursive_directory_iterator(const recursive_directory_iterator& rhs);[…]
-8- Postconditions:
[…]
(8.3) —
recursion_pending() == rhs.recursion_pending()recurse_ == rhs.recurse_recursive_directory_iterator(recursive_directory_iterator&& rhs) noexcept;[…]
-10- Postconditions: options(), depth(), and
recursion_pending()recurse_ have the values that rhs.options(), rhs.depth(), andrhs.recursion_pending()rhs.recurse_, respectively, had before the function call.recursive_directory_iterator& operator=(const recursive_directory_iterator& rhs);[…]
-12- Postconditions:
[…]
(12.3) —
recursion_pending() == rhs.recursion_pending()recurse_ == rhs.recurse_[…]
recursive_directory_iterator& operator=(recursive_directory_iterator&& rhs) noexcept;[…]
-15- Postconditions: options(), depth(), and
recursion_pending()recurse_ have the values that rhs.options(), rhs.depth(), andrhs.recursion_pending()rhs.recurse_, respectively, had before the function call.[…]
bool recursion_pending() const;-21- Returns:
true if disable_recursion_pending() has not been called subsequent to the prior construction or increment operation, otherwise falserecurse_.[…]
recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec) noexcept;-23- Effects: As specified for the prefix increment operation of Input iterators ( [iterators.input]), except that:
[…]
-?- Postcondition: recurse_ == true.
void disable_recursion_pending();-28- Postcondition
s:recursion_pending()recurse_ == false.[…]
[2018-05-23: Casey restores the intended design with an expansion of the original PR]
The intended design is that all copies of a single recursive_directory_iterator share a common block of state which includes the values returned by options, depth, and recursion_pending - hence the mandate that those functions not be called on a non-dereferenceable iterator in 31.12.12 [fs.class.rec.dir.itr] para 2. To allow an implementation with such shared state, it's necessary to make changes to the value returned by recursion_pending() visible to all copies of the same dereferenceable iterator.
Also:
pop notionally calls increment repeatedly until the current directory is exhausted, pop should affect the value of recursion_pending similarly to increment.
options is not valid for all constructor signatures described by 31.12.12.2 [fs.rec.dir.itr.members] para 2.
the copies and moves don't specify what they actually do
it's not quite kosher for the copies and moves to have postconditions on the value of expressions that have UB if the iterator copied/moved from is not dereferenceable.
[2018-06, Rapperswil, Wednesday evening]
JW: p21 currently can just say "unspecified"
BO: if we are OK with only remote implementations we can remove the unspecifiedness
BO: the problematic business is the "recursion pending" bit
JW: I want time to work on this
[2018-08-23 Batavia Issues processing]
General agreement that flag should be shared; Casey to reword.
[2022-12-18; Daniel comments]
Note that this proposed wording has some overlap with LWG 3668 for recursive_directory_iterator's constructors without options argument. If we would like a different wording form for this textual location in one issue we should resync the other issue to reduce the chance of a merge conflict.
Proposed resolution:
This wording is relative to N4750.
Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:
explicit recursive_directory_iterator(const path& p); recursive_directory_iterator(const path& p, directory_options options); recursive_directory_iterator(const path& p, directory_options options, error_code& ec) noexcept; recursive_directory_iterator(const path& p, error_code& ec) noexcept;-?- For the signatures with no parameter options, let options be directory_options::none.
-2- Effects: […]
-3- Postconditions:
options() == options for the signatures with a directory_options argument, otherwise options() == directory_options::none.
this->options() == options
recursion_pending() == true
[…]
recursive_directory_iterator(const recursive_directory_iterator& rhs);-7- Effects: Constructs an
object of class recursive_directory_iteratoriterator that denotes the same directory entry as rhs, if any..-8- Postconditions: If rhs is dereferenceable,
[…]
recursive_directory_iterator(recursive_directory_iterator&& rhs) noexcept;-9- Effects: Constructs an
object of class recursive_directory_iteratoriterator that denotes the directory entry denoted by rhs before the function call, if any..-10- Postconditions: If rhs is dereferenceable, […]
recursive_directory_iterator& operator=(const recursive_directory_iterator& rhs);-11- Effects:
If *this and rhs are the same object, the member has no effect.Causes *this to denote the same directory entry denoted by rhs, if any.-12- Postconditions: If rhs is dereferenceable,
[…]
recursive_directory_iterator& operator=(recursive_directory_iterator&& rhs) noexcept;-14- Effects:
If *this and rhs are the same object, the member has no effect.Causes *this to denote the directory entry denoted by rhs before the function call, if any.-15- Postconditions: If rhs was dereferenceable before the function call, […]
-16- Returns: *this.
-x- Remarks: If *this and rhs do not refer to the same object, the resulting state of rhs is unspecified (16.4.6.15 [lib.types.movedfrom]).
directory_options options() const;-17- Returns: The value
of the argument passed to the constructor for the options parameter, if present, otherwise directory_options::noneestablished by the most recently called member that has a postcondition for options().[…]
bool recursion_pending() const;-21- Returns:
true if disable_recursion_pending() has not been called subsequent to the prior construction or increment operation, otherwise false.If disable_recursion_pending() has been called on a copy of *this, an unspecified value. Otherwise, the value established for recursion_pending() by the postcondition of the most recent construction, assignment, increment, or disable_recursion_pending operation.[…]
recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec);-23- Effects: As specified for the prefix increment operation of Input iterators (25.3.5.3 [input.iterators]), except that: […]
-?- Postconditions: If *this is dereferenceable, recursion_pending() == true.
[…]
void pop(); void pop(error_code& ec);-26- Effects: If depth() == 0, set *this to recursive_directory_iterator(). […]
-?- Postconditions: If *this is dereferenceable, recursion_pending() == true.
[…]
Section: 24.5 [unord] Status: New Submitter: Billy Robert O'Neal III Opened: 2016-05-20 Last modified: 2022-07-16
Priority: 3
View all other issues in [unord].
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Discussion:
The resolution of LWG 2210 missed constructors accepting a range or initializer list and allocator.
Previous resolution [SUPERSEDED]:
This wording is relative to N4582.
Add to the synopsis in 24.5.4.1 [unord.map.overview] p3:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > { class unordered_map { public: […] unordered_map(size_type n, const hasher& hf, const allocator_type& a) : unordered_map(n, hf, key_equal(), a) { } template <class InputIterator> unordered_map(InputIterator f, InputIterator l, const allocator_type& a) : unordered_map(f, l, see below, hasher(), key_equal(), a) { } template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_map(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_map(f, l, n, hf, key_equal(), a) { } unordered_map(initializer_list<value_type> il, const allocator_type& a) : unordered_map(il, see below, hasher(), key_equal(), a) { } unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_map(il, n, hasher(), key_equal(), a) { } […] }; }Add to the synopsis in 24.5.5.1 [unord.multimap.overview] p3:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > { class unordered_multimap { public: […] unordered_multimap(size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(n, hf, key_equal(), a) { } template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a) : unordered_multimap(f, l, see below, hasher(), key_equal(), a) { } template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multimap(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(f, l, n, hf, key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, const allocator_type& a) : unordered_multimap(il, see below, hasher(), key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multimap(il, n, hasher(), key_equal(), a) { } […] }; }Add to the synopsis in 24.5.6.1 [unord.set.overview] p3:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > { class unordered_set { public: […] unordered_set(size_type n, const hasher& hf, const allocator_type& a) : unordered_set(n, hf, key_equal(), a) { } template <class InputIterator> unordered_set(InputIterator f, InputIterator l, const allocator_type& a) : unordered_set(f, l, see below, hasher(), key_equal(), a) { } template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_set(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_set(f, l, n, hf, key_equal(), a) { } unordered_set(initializer_list<value_type> il, const allocator_type& a) : unordered_set(il, see below, hasher(), key_equal(), a) { } unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_set(il, n, hasher(), key_equal(), a) { } […] }; }Add to the synopsis in 24.5.7.1 [unord.multiset.overview] p3:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > { class unordered_multiset { public: […] unordered_multiset(size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(n, hf, key_equal(), a) { } template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a) : unordered_multiset(f, l, see below, hasher(), key_equal(), a) { } template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multiset(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(f, l, n, hf, key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, const allocator_type& a) : unordered_multiset(il, see below, hasher(), key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multiset(il, n, hasher(), key_equal(), a) { } […] }; }
[2016-06, Oulu — Daniel comments and provides new wording]
During the LWG discussion of this issue it has been observed, that the interpretation of the embedded see below is not really clear and that we should split declaration and definition of the new overloads, so that we have a place that allows us to specify what "see below" stands for. In addition, the new wording wraps the "see below" as "size_type(see below)" to clarify the provided expression type, similar as we did for the default constructor of unordered_map.
[Oulu, 2016-06]
Alisdair to review wording.
Previous resolution [SUPERSEDED]:
This wording is relative to N4594.
Add to the synopsis in 24.5.4.1 [unord.map.overview] p3:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > { class unordered_map { public: […] unordered_map(size_type n, const hasher& hf, const allocator_type& a) : unordered_map(n, hf, key_equal(), a) { } template <class InputIterator> unordered_map(InputIterator f, InputIterator l, const allocator_type& a); template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_map(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_map(f, l, n, hf, key_equal(), a) { } unordered_map(initializer_list<value_type> il, const allocator_type& a); unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_map(il, n, hasher(), key_equal(), a) { } […] }; }Insert the following new prototype specification just after 24.5.4.2 [unord.map.cnstr] p2
template <class InputIterator> unordered_map(InputIterator f, InputIterator l, const allocator_type& a) : unordered_map(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_map(initializer_list<value_type> il, const allocator_type& a) : unordered_map(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The number of buckets is implementation-defined.
Add to the synopsis in 24.5.5.1 [unord.multimap.overview] p3:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > { class unordered_multimap { public: […] unordered_multimap(size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(n, hf, key_equal(), a) { } template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a); template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multimap(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(f, l, n, hf, key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, const allocator_type& a); unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multimap(il, n, hasher(), key_equal(), a) { } […] }; }Insert the following new prototype specification just after 24.5.5.2 [unord.multimap.cnstr] p2
template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a) : unordered_multimap(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, const allocator_type& a) : unordered_multimap(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The number of buckets is implementation-defined.
Add to the synopsis in 24.5.6.1 [unord.set.overview] p3:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > { class unordered_set { public: […] unordered_set(size_type n, const hasher& hf, const allocator_type& a) : unordered_set(n, hf, key_equal(), a) { } template <class InputIterator> unordered_set(InputIterator f, InputIterator l, const allocator_type& a); template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_set(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_set(f, l, n, hf, key_equal(), a) { } unordered_set(initializer_list<value_type> il, const allocator_type& a); unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_set(il, n, hasher(), key_equal(), a) { } […] }; }Insert the following new prototype specification just after 24.5.6.2 [unord.set.cnstr] p2
template <class InputIterator> unordered_set(InputIterator f, InputIterator l, const allocator_type& a) : unordered_set(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_set(initializer_list<value_type> il, const allocator_type& a) : unordered_set(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The number of buckets is implementation-defined.
Add to the synopsis in 24.5.7.1 [unord.multiset.overview] p3:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > { class unordered_multiset { public: […] unordered_multiset(size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(n, hf, key_equal(), a) { } template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a); template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multiset(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(f, l, n, hf, key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, const allocator_type& a); unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multiset(il, n, hasher(), key_equal(), a) { } […] }; }Insert the following new prototype specification just after 24.5.7.2 [unord.multiset.cnstr] p2
template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a) : unordered_multiset(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, const allocator_type& a) : unordered_multiset(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The number of buckets is implementation-defined.
[2017-08-04, Daniel and Alisdair finetune wording]
We decided to improve the added Remarks: elements by changing from the previous form:
Remarks: The number of buckets is implementation-defined.
to the more elaborate form:
Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
[2020-11-29; Reflector discussions]
It has been pointed out that this issue is related to LWG 1199, LWG 2210, and LWG 3506.
Previous resolution [SUPERSEDED]:
This resolution is relative to N4687.
Add to the synopsis in 24.5.4.1 [unord.map.overview] p3:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > { class unordered_map { public: […] unordered_map(size_type n, const hasher& hf, const allocator_type& a) : unordered_map(n, hf, key_equal(), a) { } template <class InputIterator> unordered_map(InputIterator f, InputIterator l, const allocator_type& a); template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_map(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_map(f, l, n, hf, key_equal(), a) { } unordered_map(initializer_list<value_type> il, const allocator_type& a); unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_map(il, n, hasher(), key_equal(), a) { } […] }; }Insert the following new prototype specification just after 24.5.4.2 [unord.map.cnstr] p2
template <class InputIterator> unordered_map(InputIterator f, InputIterator l, const allocator_type& a) : unordered_map(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_map(initializer_list<value_type> il, const allocator_type& a) : unordered_map(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
Add to the synopsis in 24.5.5.1 [unord.multimap.overview] p3:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > { class unordered_multimap { public: […] unordered_multimap(size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(n, hf, key_equal(), a) { } template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a); template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multimap(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(f, l, n, hf, key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, const allocator_type& a); unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multimap(il, n, hasher(), key_equal(), a) { } […] }; }Insert the following new prototype specification just after 24.5.5.2 [unord.multimap.cnstr] p2
template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a) : unordered_multimap(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, const allocator_type& a) : unordered_multimap(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
Add to the synopsis in 24.5.6.1 [unord.set.overview] p3:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > { class unordered_set { public: […] unordered_set(size_type n, const hasher& hf, const allocator_type& a) : unordered_set(n, hf, key_equal(), a) { } template <class InputIterator> unordered_set(InputIterator f, InputIterator l, const allocator_type& a); template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_set(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_set(f, l, n, hf, key_equal(), a) { } unordered_set(initializer_list<value_type> il, const allocator_type& a); unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_set(il, n, hasher(), key_equal(), a) { } […] }; }Insert the following new prototype specification just after 24.5.6.2 [unord.set.cnstr] p2
template <class InputIterator> unordered_set(InputIterator f, InputIterator l, const allocator_type& a) : unordered_set(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_set(initializer_list<value_type> il, const allocator_type& a) : unordered_set(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
Add to the synopsis in 24.5.7.1 [unord.multiset.overview] p3:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > { class unordered_multiset { public: […] unordered_multiset(size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(n, hf, key_equal(), a) { } template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a); template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multiset(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(f, l, n, hf, key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, const allocator_type& a); unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multiset(il, n, hasher(), key_equal(), a) { } […] }; }Insert the following new prototype specification just after 24.5.7.2 [unord.multiset.cnstr] p2
template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a) : unordered_multiset(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, const allocator_type& a) : unordered_multiset(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
[2022-07-10; Daniel comments]
It is has been noticed by Daniel Eiband on [std-discussion] that the following deduction guides for the following constructors of the set types std::unordered_set and std::unordered_multiset are missing:
unordered_set(InputIterator, InputIterator, Allocator); unordered_set(initializer_list<T>, Allocator); unordered_multiset(InputIterator, InputIterator, Allocator); unordered_multiset(initializer_list<T>, Allocator);
Since this issue is adding these missing constructors it should also add the associated deduction guides. The proposed wording has been updated to this effect and also rebased to N4910.
Previous resolution [SUPERSEDED]:
This resolution is relative to N4910.
Add to the synopsis in 24.5.4.1 [unord.map.overview] p3:
namespace std { template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>> { class unordered_map { public: […] unordered_map(size_type n, const hasher& hf, const allocator_type& a) : unordered_map(n, hf, key_equal(), a) { } template<class InputIterator> unordered_map(InputIterator f, InputIterator l, const allocator_type& a); template<class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_map(f, l, n, hasher(), key_equal(), a) { } template<class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_map(f, l, n, hf, key_equal(), a) { } […] unordered_map(initializer_list<value_type> il, const allocator_type& a); unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_map(il, n, hasher(), key_equal(), a) { } […] }; }Insert the following new prototype specification just after 24.5.4.2 [unord.map.cnstr] p2
template<class InputIterator> unordered_map(InputIterator f, InputIterator l, const allocator_type& a) : unordered_map(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_map(initializer_list<value_type> il, const allocator_type& a) : unordered_map(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
Add to the synopsis in 24.5.5.1 [unord.multimap.overview] p3:
namespace std { template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>> { class unordered_multimap { public: […] unordered_multimap(size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(n, hf, key_equal(), a) { } template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a); template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multimap(f, l, n, hasher(), key_equal(), a) { } template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(f, l, n, hf, key_equal(), a) { } […] unordered_multimap(initializer_list<value_type> il, const allocator_type& a); unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multimap(il, n, hasher(), key_equal(), a) { } […] }; }Insert the following new prototype specification just after 24.5.5.2 [unord.multimap.cnstr] p2
template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a) : unordered_multimap(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, const allocator_type& a) : unordered_multimap(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
Add to the synopsis in 24.5.6.1 [unord.set.overview] p3:
namespace std { template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<Key>> { class unordered_set { public: […] unordered_set(size_type n, const hasher& hf, const allocator_type& a) : unordered_set(n, hf, key_equal(), a) { } template<class InputIterator> unordered_set(InputIterator f, InputIterator l, const allocator_type& a); template<class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_set(f, l, n, hasher(), key_equal(), a) { } template<class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_set(f, l, n, hf, key_equal(), a) { } unordered_set(initializer_list<value_type> il, const allocator_type& a); unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_set(il, n, hasher(), key_equal(), a) { } […] }; […] template<class T, class Allocator> unordered_set(initializer_list<T>, typename see below::size_type, Allocator) -> unordered_set<T, hash<T>, equal_to<T>, Allocator>; template<class T, class Hash, class Allocator> unordered_set(initializer_list<T>, typename see below::size_type, Hash, Allocator) -> unordered_set<T, Hash, equal_to<T>, Allocator>; template<class InputIterator, class Allocator> unordered_set(InputIterator, InputIterator, Allocator) -> unordered_set<iter-value-type<InputIterator>, hash<iter-value-type<InputIterator>>, equal_to<iter-value-type<InputIterator>>, Allocator>; template<class T, class Allocator> unordered_set(initializer_list<T>, Allocator) -> unordered_set<T, hash<T>, equal_to<T>, Allocator>; }Insert the following new prototype specification just after 24.5.6.2 [unord.set.cnstr] p2
template<class InputIterator> unordered_set(InputIterator f, InputIterator l, const allocator_type& a) : unordered_set(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_set(initializer_list<value_type> il, const allocator_type& a) : unordered_set(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
Add to the synopsis in 24.5.7.1 [unord.multiset.overview] p3:
namespace std { template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<Key>> { class unordered_multiset { public: […] unordered_multiset(size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(n, hf, key_equal(), a) { } template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a); template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multiset(f, l, n, hasher(), key_equal(), a) { } template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(f, l, n, hf, key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, const allocator_type& a); unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multiset(il, n, hasher(), key_equal(), a) { } […] }; […] template<class T, class Allocator> unordered_multiset(initializer_list<T>, typename see below ::size_type, Allocator) -> unordered_multiset<T, hash<T>, equal_to<T>, Allocator>; template<class T, class Hash, class Allocator> unordered_multiset(initializer_list<T>, typename see below ::size_type, Hash, Allocator) -> unordered_multiset<T, Hash, equal_to<T>, Allocator>; template<class InputIterator, class Allocator> unordered_multiset(InputIterator, InputIterator, Allocator) -> unordered_multiset<iter-value-type<InputIterator>, hash<iter-value-type<InputIterator>>, equal_to<iter-value-type<InputIterator>>, Allocator>; template<class T, class Allocator> unordered_multiset(initializer_list<T>, Allocator) -> unordered_multiset<T, hash<T>, equal_to<T>, Allocator>; }Insert the following new prototype specification just after 24.5.7.2 [unord.multiset.cnstr] p2
template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a) : unordered_multiset(f, l, size_type(see below), hasher(), key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, const allocator_type& a) : unordered_multiset(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
[2022-07-15; Casey comments]
P1206R7 added from_range_t constructors corresponding to existing iterator pair constructors for the standard containers. For consistency, this issue should add from_range_t constructors corresponding to each new iterator pair constructor.
[2022-07-16; Daniel comments and updates wording]
The new from_range_t constructors have been added for each added new iterator pair constructor. Note that the corresponding deduction guides already exist.
Proposed resolution:
This resolution is relative to N4910.
Add to the synopsis in 24.5.4.1 [unord.map.overview] p3:
namespace std {
template<class Key,
class T,
class Hash = hash<Key>,
class Pred = equal_to<Key>,
class Allocator = allocator<pair<const Key, T>>> {
class unordered_map {
public:
[…]
unordered_map(size_type n, const hasher& hf, const allocator_type& a)
: unordered_map(n, hf, key_equal(), a) { }
template<class InputIterator>
unordered_map(InputIterator f, InputIterator l, const allocator_type& a);
template<class InputIterator>
unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
: unordered_map(f, l, n, hasher(), key_equal(), a) { }
template<class InputIterator>
unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
const allocator_type& a)
: unordered_map(f, l, n, hf, key_equal(), a) { }
template<container-compatible-range<value_type> R>
unordered_map(from_range_t, R&& rg, const allocator_type& a);
template<container-compatible-range<value_type> R>
unordered_map(from_range_t, R&& rg, size_type n, const allocator_type& a)
: unordered_map(from_range, std::forward<R>(rg), n, hasher(), key_equal(), a) { }
template<container-compatible-range<value_type> R>
unordered_map(from_range_t, R&& rg, size_type n, const hasher& hf, const allocator_type& a)
: unordered_map(from_range, std::forward<R>(rg), n, hf, key_equal(), a) { }
unordered_map(initializer_list<value_type> il, const allocator_type& a);
unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a)
: unordered_map(il, n, hasher(), key_equal(), a) { }
[…]
};
}
Insert the following new prototype specification just after 24.5.4.2 [unord.map.cnstr] p2
template<class InputIterator> unordered_map(InputIterator f, InputIterator l, const allocator_type& a) : unordered_map(f, l, size_type(see below), hasher(), key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_map(from_range_t, R&& rg, const allocator_type& a) : unordered_map(from_range, std::forward<R>(rg), size_type(see below), hasher(), key_equal(), a) { } unordered_map(initializer_list<value_type> il, const allocator_type& a) : unordered_map(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
Add to the synopsis in 24.5.5.1 [unord.multimap.overview] p3:
namespace std {
template<class Key,
class T,
class Hash = hash<Key>,
class Pred = equal_to<Key>,
class Allocator = allocator<pair<const Key, T>>> {
class unordered_multimap {
public:
[…]
unordered_multimap(size_type n, const hasher& hf, const allocator_type& a)
: unordered_multimap(n, hf, key_equal(), a) { }
template<class InputIterator>
unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a);
template<class InputIterator>
unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
: unordered_multimap(f, l, n, hasher(), key_equal(), a) { }
template<class InputIterator>
unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf,
const allocator_type& a)
: unordered_multimap(f, l, n, hf, key_equal(), a) { }
template<container-compatible-range<value_type> R>
unordered_multimap(from_range_t, R&& rg, const allocator_type& a);
template<container-compatible-range<value_type> R>
unordered_multimap(from_range_t, R&& rg, size_type n, const allocator_type& a)
: unordered_multimap(from_range, std::forward<R>(rg),
n, hasher(), key_equal(), a) { }
[…]
unordered_multimap(initializer_list<value_type> il, const allocator_type& a);
unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a)
: unordered_multimap(il, n, hasher(), key_equal(), a) { }
[…]
};
}
Insert the following new prototype specification just after 24.5.5.2 [unord.multimap.cnstr] p2
template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a) : unordered_multimap(f, l, size_type(see below), hasher(), key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_multimap(from_range_t, R&& rg, const allocator_type& a) : unordered_multimap(from_range, std::forward<R>(rg), size_type(see below), hasher(), key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, const allocator_type& a) : unordered_multimap(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
Add to the synopsis in 24.5.6.1 [unord.set.overview] p3:
namespace std {
template<class Key,
class Hash = hash<Key>,
class Pred = equal_to<Key>,
class Allocator = allocator<Key>> {
class unordered_set {
public:
[…]
unordered_set(size_type n, const hasher& hf, const allocator_type& a)
: unordered_set(n, hf, key_equal(), a) { }
template<class InputIterator>
unordered_set(InputIterator f, InputIterator l, const allocator_type& a);
template<class InputIterator>
unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
: unordered_set(f, l, n, hasher(), key_equal(), a) { }
template<class InputIterator>
unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
const allocator_type& a)
: unordered_set(f, l, n, hf, key_equal(), a) { }
unordered_set(initializer_list<value_type> il, const allocator_type& a);
unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a)
: unordered_set(il, n, hasher(), key_equal(), a) { }
template<container-compatible-range<value_type> R>
unordered_set(from_range_t, R&& rg, const allocator_type& a);
template<container-compatible-range<value_type> R>
unordered_set(from_range_t, R&& rg, size_type n, const allocator_type& a)
: unordered_set(from_range, std::forward<R>(rg), n, hasher(), key_equal(), a) { }
[…]
};
[…]
template<class T, class Allocator>
unordered_set(initializer_list<T>, typename see below::size_type, Allocator)
-> unordered_set<T, hash<T>, equal_to<T>, Allocator>;
template<class T, class Hash, class Allocator>
unordered_set(initializer_list<T>, typename see below::size_type, Hash, Allocator)
-> unordered_set<T, Hash, equal_to<T>, Allocator>;
template<class InputIterator, class Allocator>
unordered_set(InputIterator, InputIterator, Allocator)
-> unordered_set<iter-value-type<InputIterator>,
hash<iter-value-type<InputIterator>>,
equal_to<iter-value-type<InputIterator>>,
Allocator>;
template<class T, class Allocator>
unordered_set(initializer_list<T>, Allocator)
-> unordered_set<T, hash<T>, equal_to<T>, Allocator>;
}
Insert the following new prototype specification just after 24.5.6.2 [unord.set.cnstr] p2
template<class InputIterator> unordered_set(InputIterator f, InputIterator l, const allocator_type& a) : unordered_set(f, l, size_type(see below), hasher(), key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_set(from_range_t, R&& rg, const allocator_type& a) : unordered_set(from_range, std::forward<R>(rg), size_type(see below), hasher(), key_equal(), a) { } unordered_set(initializer_list<value_type> il, const allocator_type& a) : unordered_set(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
Add to the synopsis in 24.5.7.1 [unord.multiset.overview] p3:
namespace std {
template<class Key,
class Hash = hash<Key>,
class Pred = equal_to<Key>,
class Allocator = allocator<Key>> {
class unordered_multiset {
public:
[…]
unordered_multiset(size_type n, const hasher& hf, const allocator_type& a)
: unordered_multiset(n, hf, key_equal(), a) { }
template<class InputIterator>
unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a);
template<class InputIterator>
unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
: unordered_multiset(f, l, n, hasher(), key_equal(), a) { }
template<class InputIterator>
unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf,
const allocator_type& a)
: unordered_multiset(f, l, n, hf, key_equal(), a) { }
template<container-compatible-range<value_type> R>
unordered_multiset(from_range_t, R&& rg, const allocator_type& a);
template<container-compatible-range<value_type> R>
unordered_multiset(from_range_t, R&& rg, size_type n, const allocator_type& a)
: unordered_multiset(from_range, std::forward<R>(rg),
n, hasher(), key_equal(), a) { }
[…]
unordered_multiset(initializer_list<value_type> il, const allocator_type& a);
unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a)
: unordered_multiset(il, n, hasher(), key_equal(), a) { }
[…]
};
[…]
template<class T, class Allocator>
unordered_multiset(initializer_list<T>, typename see below ::size_type, Allocator)
-> unordered_multiset<T, hash<T>, equal_to<T>, Allocator>;
template<class T, class Hash, class Allocator>
unordered_multiset(initializer_list<T>, typename see below ::size_type, Hash, Allocator)
-> unordered_multiset<T, Hash, equal_to<T>, Allocator>;
template<class InputIterator, class Allocator>
unordered_multiset(InputIterator, InputIterator, Allocator)
-> unordered_multiset<iter-value-type<InputIterator>,
hash<iter-value-type<InputIterator>>,
equal_to<iter-value-type<InputIterator>>,
Allocator>;
template<class T, class Allocator>
unordered_multiset(initializer_list<T>, Allocator)
-> unordered_multiset<T, hash<T>, equal_to<T>, Allocator>;
}
Insert the following new prototype specification just after 24.5.7.2 [unord.multiset.cnstr] p2
template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a) : unordered_multiset(f, l, size_type(see below), hasher(), key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_multiset(from_range_t, R&& rg, const allocator_type& a)) : unordered_multiset(from_range, std::forward<R>(rg), size_type(see below), hasher(), key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, const allocator_type& a) : unordered_multiset(il, size_type(see below), hasher(), key_equal(), a) { }-?- Remarks: The initial number of buckets supplied by the size_type argument is implementation-defined.
Section: 28.4.6 [complex.ops] Status: New Submitter: Tim Song Opened: 2016-05-23 Last modified: 2018-10-16
Priority: 3
View all other issues in [complex.ops].
View all issues with New status.
Discussion:
The specification of operator>>(istream&, complex<T>&) is extremely short on details. It currently reads, in its entirety (28.4.6 [complex.ops]/12-15):
template<class T, class charT, class traits> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, complex<T>& x);Effects: Extracts a complex number x of the form: u, (u), or (u,v), where u is the real part and v is the imaginary part (31.7.5.3 [istream.formatted]).
Requires: The input values shall be convertible to T. If bad input is encountered, calls is.setstate(ios_base::failbit) (which may throw ios::failure (31.5.4.4 [iostate.flags])). Returns: is. Remarks: This extraction is performed as a series of simpler extractions. Therefore, the skipping of whitespace is specified to be the same for each of the simpler extractions.
It is completely unclear:
Previous resolution [SUPERSEDED]:
Drafting note: the following wording is based on:
- Characters are extracted using operator>> and compared using traits::eq.
- Mismatched characters are returned to the stream.
This wording is relative to N4582.
Replace 28.4.6 [complex.ops]/12-15 with the following paragraphs:
template<class T, class charT, class traits> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, complex<T>& x);-?- Effects: First, extracts a character from is.
In the description above, characters are extracted from is as if by operator>> (31.7.5.3.3 [istream.extractors]), and returned to the stream as if by basic_istream::putback (31.7.5.4 [istream.unformatted]). Character equality is determined using traits::eq. An object t of type T is extracted from is as if by is >> t. If any extraction operation fails, no further operation is performed and the whole extraction fails. On failure, calls is.setstate(ios_base::failbit) (which may throw ios::failure (31.5.4.4 [iostate.flags])). -?- Returns: is. -?- [Note: This extraction is performed as a series of simpler extractions. Therefore, the skipping of whitespace is specified to be the same for each of the simpler extractions. — end note]
- If the character extracted is equal to is.widen('('), extracts an object u of type T from is, then extracts a character from is.
- If this character is equal to is.widen(')'), then assigns complex<T>(u) to x.
- Otherwise, if this character is equal to is.widen(','), extracts an object v of type T from is, then extracts a character from is. If this character is equal to is.widen(')'), then assigns complex<T>(u, v) to x; otherwise returns the character to is and the extraction fails.
- Otherwise, returns the character to is and the extraction fails.
- Otherwise, returns the character to is, extracts an object u of type T from is, and assigns complex<T>(u) to x.
[2017-12-13 Tim Song adjusts the P/R to avoid relying on putback.]
Proposed resolution:
Drafting note: the following wording assumes that:
- Characters are extracted using operator>> and compared using traits::eq.
- Mismatched characters are not extracted.
- x is assigned a value-initialized complex on failure for consistency with the arithmetic extractors (compare LWG 696).
This wording is relative to N4778.
Replace 28.4.6 [complex.ops]/12-16 with the following paragraphs:
template<class T, class charT, class traits> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, complex<T>& x);-?- Effects: Let PEEK(is) be a formatted input function (31.7.5.3.1 [istream.formatted.reqmts]) of is that returns the next character that would be extracted from is by operator>>. [Note: The sentry object is constructed and destroyed, but the returned character is not extracted from the stream. — end note]
In the description above, characters are extracted from is as if by operator>> (31.7.5.3.3 [istream.extractors]), character equality is determined using traits::eq, and an object t of type T is extracted from is as if by is >> t. If any extraction operation fails, no further operation is performed and the whole extraction fails. On failure, assigns complex<T>() to x and calls is.setstate(ios_base::failbit) (which may throw ios::failure (31.5.4.4 [iostate.flags])). -?- Returns: is. -?- [Note: This extraction is performed as a series of simpler extractions. Therefore, the skipping of whitespace is specified to be the same for each of the simpler extractions. — end note]
- If PEEK(is) is not equal to is.widen('('), extracts an object u of type T from is, and assigns complex<T>(u) to x.
- Otherwise, extracts that character from is, then extracts an object u of type T from is, then:
- If PEEK(is) is equal to is.widen(')'), then extracts that character from is and assigns complex<T>(u) to x.
- Otherwise, if it is equal to is.widen(','), then extracts that character from is and then extracts an object v of type T from is, then:
- If PEEK(is) is equal to is.widen(')'), then extracts that character from is and assigns complex<T>(u, v) to x.
- Otherwise, the extraction fails.
- Otherwise, the extraction fails.
Section: 17.3.5 [numeric.limits] Status: Open Submitter: Richard Smith Opened: 2016-06-09 Last modified: 2018-11-12
Priority: 3
View other active issues in [numeric.limits].
View all other issues in [numeric.limits].
View all issues with Open status.
Discussion:
I've received this report at the project editor mail alias, and it seems like it may be worthy of a LWG issue:
I recently had this problem:
- I was storing data in a vector of __uint128_ts
- I used a sorting library which used numeric_limits<T>::max() as a sentinel value
- GCC's libstdc++ provides a numeric_limits specialisation for that type, but
- Clang's libc++ does not.
This broke the sorting for me on different platforms, and it was quite difficult to determine why. If the default numeric_limits didn't default to 0s and false values (18.3.2.4 of N4582), and instead static_asserted, causing my code to not compile, I would have found the solution immediately.
I know that __uint128_t is non-standard, so neither GCC nor Clang is doing the wrong thing nor the right thing here. I could just submit a patch to libc++ providing the specialisations, but it doesn't fix the problem at its core. I am wondering, what is the rationale behind the defaults being 0 and false? It seems like it is inviting a problem for any future numeric types, whether part of a library, compiler extension, and possibly even future updates to C++'s numeric types. I think it would be much better to prevent code that tries to use unspecified numeric_limits from compiling.
An alternative to this suggestion would be to still define the primary template, but not provide any of the members except is_specialized. Either way, this would make numeric_limits members SFINAEable.
Along the same lines, one might wonder why the members that only make sense for floating-point types are required to be defined to nonsense values for integer types.[2016-11-12, Issaquah]
Sat PM: This looks like a good idea. Jonathan and Marshall will do post C++17 implementations and report back.
[2018-11 San Diego Thursday night issue processing]
See Walter's paper P0437 for ideas and/or future directions.
[2023-04]
See Walter's paper P1841 for the preferred direction.
Proposed resolution:
Section: 17.6.3.2 [new.delete.single] Status: New Submitter: Clark Nelson Opened: 2016-06-21 Last modified: 2016-08-01
Priority: 3
View other active issues in [new.delete.single].
View all other issues in [new.delete.single].
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Discussion:
It should be considered whether the description of the single-object allocation functions should say "or smaller", like the array allocation functions. For example, according to 17.6.3.2 [new.delete.single] p1 (emphasis mine):
The allocation function (3.7.4.1) called by a new-expression (5.3.4) to allocate size bytes of storage suitably aligned to represent any object of that size.
In contrast to this, 17.6.3.3 [new.delete.array] p1 says (emphasis mine):
The allocation function (3.7.4.1) called by the array form of a new-expression (5.3.4) to allocate size bytes of storage suitably aligned to represent any array object of that size or smaller. (footnote: It is not the direct responsibility of operator new[](std::size_t) or operator delete[](void*) to note the repetition count or element size of the array. Those operations are performed elsewhere in the array new and delete expressions. The array new expression, may, however, increase the size argument to operator new[](std::size_t) to obtain space to store supplemental information.)
Proposed resolution:
Section: 22.5.3.4 [optional.assign], 22.6.3.5 [variant.mod], 22.7.4.4 [any.modifiers] Status: New Submitter: Richard Smith Opened: 2016-07-13 Last modified: 2020-05-10
Priority: 3
View all issues with New status.
Discussion:
Referring to N4604:
In [optional.object.assign]: emplace (normal form) has a Requires that the construction works.
Requires: is_constructible_v<T, Args&&...> is true.
emplace (initializer_list form) has a SFINAE condition:
Remarks: […] unless is_constructible_v<T, initializer_list<U>&, Args&&...> is true.
In 22.7.4.4 [any.modifiers]: emplace (normal form) has a Requires that the construction works:
Requires: is_constructible_v<T, Args...> is true.
emplace (initializer_list form) has a SFINAE condition:
Remarks: […] unless is_constructible_v<T, initializer_list<U>&, Args...> is true.
In 22.6.3.5 [variant.mod]: emplace (T, normal form) has a SFINAE condition:
Remarks: […] unless is_constructible_v<T, Args...> is true, and T occurs exactly once in Types....
emplace (Idx, normal form) has a both a Requires and a SFINAE condition:
Requires: I < sizeof...(Types)
Remarks: […] unless is_constructible_v<T, Args...> is true, and T occurs exactly once in Types....
emplace (T, initializer_list form) has a SFINAE condition:
Remarks: […] unless is_constructible_v<T, initializer_list<U>&, Args...> is true, and T occurs exactly once in Types....
emplace (Idx, initializer_list form) has a both a Requires and a SFINAE condition:
Requires: I < sizeof...(Types)
Remarks: […] unless is_constructible_v<T, Args...> is true, and T occurs exactly once in Types....
Why the inconsistency? Should all the cases have a SFINAE requirement?
I see that variant has an additional requirement (T occurs exactly once in Types...), but that only agues that it must be a SFINAE condition — doesn't say that the other cases (any/variant) should not. map/multimap/unordered_map/unordered_multimap have SFINAE'd versions of emplace that don't take initializer_lists, but they don't have any emplace versions that take ILs. Suggested resolution: Add SFINAE requirements to optional::emplace(Args&&... args) and any::emplace(Args&&... args);[2016-08 Chicago]
During issue prioritization, people suggested that this might apply to any as well.
Ville notes that 2746, 2754 and 2756 all go together.
[2020-05-10; Daniel comments and provides wording]
The inconsistency between the two any::emplace overloads have been removed by resolving LWG 2754 to use Constraints: elements. The last Mandating paper (P1460R1), adopted in Prague, changed the Requires: elements for variant::emplace, "I < sizeof...(Types)" to Mandates:, but that paper was focused on fixing inappropriate preconditions, not searching for consistency here. Given that the in_place_index_t constructors of variant uses SFINAE-conditions for this form of static precondition violation, I recommend that its emplace functions use the same style, which would bring them also in consistency with their corresponding type-based emplace forms that are Mandates:-free but delegate to the index-based forms.
Proposed resolution:
This wording is relative to N4861.
Modify 22.5.3.4 [optional.assign], as indicated:
template<class... Args> T& emplace(Args&&... args);-29-
[…]MandatesConstraints: is_constructible_v<T, Args...> is true.template<class U, class... Args> T& emplace(initializer_list<U> il, Args&&... args);-35- Constraints: is_constructible_v<T, initializer_list<U>&, Args...> is true.
[…]
Modify 22.6.3.5 [variant.mod], as indicated:
template<class T, class... Args> T& emplace(Args&&... args);-1- Constraints: is_constructible_v<T, Args...> is true, and T occurs exactly once in Types.
[…]template<class T, class U, class... Args> T& emplace(initializer_list<U> il, Args&&... args);-3- Constraints: is_constructible_v<T, initializer_list<U>&, Args...> is true, and T occurs exactly once in Types.
[…]template<size_t I, class... Args> variant_alternative_t<I, variant<Types...>>& emplace(Args&&... args);-6- Constraints: is_constructible_v<TI, Args...> is true and I < sizeof...(Types) is true. […]
-5- Mandates: I < sizeof...(Types).template<size_t I, class U, class... Args> variant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U> il, Args&&... args);-13- Constraints: is_constructible_v<TI, initializer_list<U>&, Args...> is true and I < sizeof...(Types) is true. […]
-12- Mandates: I < sizeof...(Types).
Section: 20.3.2.2.3 [util.smartptr.shared.dest] Status: New Submitter: Aaron Jacobs Opened: 2016-07-21 Last modified: 2017-10-08
Priority: 4
View all other issues in [util.smartptr.shared.dest].
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Discussion:
The C++14 standard contains no language that guarantees the deleter run by a shared_ptr will see all associated weak_ptr instances as expired. For example, the standard doesn't appear to guarantee that the assertion in the following snippet won't fire:
std::weak_ptr<Foo> weak;
std::shared_ptr<Foo> strong{
new Foo,
[&weak] (Foo* f) {
assert(weak.expired());
delete f;
},
};
weak = strong;
strong.reset();
It seems clear that the intent is that associated weak_ptrs are expired, because otherwise shared_ptr deleters could resurrect a reference to an object that is being deleted.
Suggested fix: 20.3.2.2.3 [util.smartptr.shared.dest] should specify that the decrease in use_count() caused by the destructor is sequenced before the call to the deleter or the call to delete p.[2016-11-08, Jonathan and STL suggest NAD]
STL and Jonathan feel that the example has unspecified behaviour, and the assertion is allowed to fire, and that's OK (the program's expectation is not reasonable). Otherwise it's necessary to move-construct a copy of the deleter and use that copy to destroy the owned pointer. We do not want to be required to do that.
See also 2262.
[2017-09-20, Jonathan comments]
I'd like to withdraw my NAD suggestion. The value of use_count() is already observable during the destructor via shared_ptr and weak_ptr objects that share ownership, so specifying when it changes ensures correct behaviour.
Proposed resolution:
Section: 22.3.3 [pairs.spec], 22.4.12 [tuple.special], 22.5.9 [optional.specalg], 22.6.10 [variant.specalg], 20.3.1.6 [unique.ptr.special], 24.3.7.4 [array.special], 24.6.6.6 [queue.special], 24.6.7.5 [priqueue.special], 24.6.8.7 [stack.special] Status: New Submitter: Agustín K-ballo Bergé Opened: 2016-08-15 Last modified: 2020-09-06
Priority: 3
View all other issues in [pairs.spec].
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Discussion:
Related: 2748 swappable traits for optionals, 2749 swappable traits for variants.
The adoption of P0185R1 "Adding [nothrow-]swappable traits" makes certain non-swappable types indirectly swappable. Consider a type defined as follows:
struct non_swappable {
friend void swap(non_swappable&, non_swappable&) = delete;
};
non_swappable ns1, ns2;
using std::swap;
swap(ns1, ns2); // ill-formed
static_assert(std::is_swappable_v<non_swappable> == false); // holds
Lvalues of type non_swappable are not swappable, as defined by 16.4.4.3 [swappable.requirements], overload resolution selects the deleted function. Consistently, is_swappable_v<non_swappable> yields false. It should be noted that since non_swappable is move constructible and move assignable, a qualified call to std::swap would be well-formed, even under P0185. Now consider the following snippet:
std::tuple<non_swappable> tns1, tns2; using std::swap; swap(tns1, tns2); // previously ill-formed, now well-formed static_assert(std::is_swappable_v<std::tuple<non_swappable>> == false); // fires
Before P0185, this snippet would violate the implicit requirement of specialized swap for tuples that each tuple element be swappable. After P0185, this specialized swap overload for tuples would be SFINAEd away, resulting in overload resolution selecting the base swap overload, and performing the exchange via move construction and move assignment of tuples.
This issue affects all of pair, tuple, unique_ptr, array, queue, priority_queue, stack, and should eventually also apply to optional and variant. Previous resolution [SUPERSEDED]:This wording is relative to N4606, except when otherwise noted.
Modify 22.3.3 [pairs.spec] as indicated:
template<class T1, class T2> void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));-7- Effects: As if by x.swap(y).
-8- Remarks: This function shallnot participate in overload resolutionbe defined as deleted unless is_swappable_v<T1> is true and is_swappable_v<T2> is true.Modify 22.4.12 [tuple.special] as indicated:
template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);-1- Remarks: This function shall
not participate in overload resolutionbe defined as deleted unless is_swappable_v<Ti> is true for all i, where 0 <= i and i < sizeof...(Types). The expression inside noexcept is equivalent to:noexcept(x.swap(y))-2- Effects: As if by x.swap(y).
Modify 20.3.1.6 [unique.ptr.special] as indicated:
template <class T, class D> void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;-1- Remarks: This function shall
-2- Effects: Calls x.swap(y).not participate in overload resolutionbe defined as deleted unless is_swappable_v<D> is true.Modify 24.3.7.4 [array.special] as indicated:
template <class T, size_t N> void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y)));-1- Remarks: This function shall
-2- Effects: As if by x.swap(y). […]not participate in overload resolutionbe defined as deleted unless N == 0 or is_swappable_v<T> is true.Modify 24.6.6.6 [queue.special] as indicated:
template <class T, class Container> void swap(queue<T, Container>& x, queue<T, Container>& y) noexcept(noexcept(x.swap(y)));-1- Remarks: This function shall
-2- Effects: As if by x.swap(y).not participate in overload resolutionbe defined as deleted unless is_swappable_v<Container> is true.Modify 24.6.7.5 [priqueue.special] as indicated:
template <class T, class Container, class Compare> void swap(priority_queue<T, Container, Compare>& x, priority_queue<T, Container, Compare>& y) noexcept(noexcept(x.swap(y)));-1- Remarks: This function shall
-2- Effects: As if by x.swap(y).not participate in overload resolutionbe defined as deleted unless is_swappable_v<Container> is true and is_swappable_v<Compare> is true.Modify 24.6.8.7 [stack.special] as indicated:
template <class T, class Container> void swap(stack<T, Container>& x, stack<T, Container>& y) noexcept(noexcept(x.swap(y)));-1- Remarks: This function shall
-2- Effects: As if by x.swap(y).not participate in overload resolutionbe defined as deleted unless is_swappable_v<Container> is true.Modify 22.5.9 [optional.specalg] as indicated:
This change should be performed if and only if LWG 2748 is accepted and is against the wording of 2748:
template <class T> void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)));-1- Effects: Calls x.swap(y).
-2- Remarks: This function shallnot participate in overload resolutionbe defined as deleted unless is_move_constructible_v<T> is true and is_swappable_v<T> is true.Modify 22.6.10 [variant.specalg] as indicated:
This change should be performed if and only if LWG 2749 is accepted and is against the wording of 2749:
template <class... Types> void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);-1- Effects: Equivalent to v.swap(w).
-2- Remarks: This function shallnot participate in overload resolutionbe defined as deleted unless is_move_constructible_v<Ti> && is_swappable_v<Ti> is true for all i. The expression inside noexcept is equivalent to noexcept(v.swap(w)).
[2019-04-17 Jonathan updates proposed resolution based on Ville's 2016-11-17 observation that the container adaptors always require swappable sequences anyway. The new proposed resolution is based on the latest WP, "de-shalled", and Remarks elements are repositioned after the Effects.]
Proposed resolution:
This wording is relative to N4810.
Modify 22.3.3 [pairs.spec] as indicated:
template<class T1, class T2> constexpr void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));-7- Effects: As if by x.swap(y).
-8- Remarks: This functionshall not participate in overload resolutionis defined as deleted unless is_swappable_v<T1> is true and is_swappable_v<T2> is true.
Modify 22.4.12 [tuple.special] as indicated:
template <class... Types> constexpr void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);-?- Effects: As if by x.swap(y).
-1- Remarks: This function
shall not participate in overload resolutionis defined as deleted unless is_swappable_v<Ti> is true for all i, where 0 <= i and i < sizeof...(Types). The expression inside noexcept is equivalent to:noexcept(x.swap(y))-2-
Effects: As if by x.swap(y).
Modify 22.5.9 [optional.specalg] as indicated:
template <class T> void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)));-1- Effects: Calls x.swap(y).
-2- Remarks: This functionshall not participate in overload resolutionis defined as deleted unless is_move_constructible_v<T> is true and is_swappable_v<T> is true.
Modify 22.6.10 [variant.specalg] as indicated:
template <class... Types> void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);-1- Effects: Equivalent to v.swap(w).
-2- Remarks: This functionshall not participate in overload resolutionis defined as deleted unless is_move_constructible_v<Ti> && is_swappable_v<Ti> is true for all i. The expression inside noexcept is equivalent to noexcept(v.swap(w)).
Modify 20.3.1.6 [unique.ptr.special] as indicated:
template <class T, class D> void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;-?- Effects: Calls x.swap(y).
-1- Remarks: This function
shall not participate in overload resolutionis defined as deleted unless is_swappable_v<D> is true.-2-
Effects: Calls x.swap(y).
Modify 24.3.7.4 [array.special] as indicated:
template <class T, size_t N> void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y)));-1-
-2- Effects: As if by x.swap(y). -3- Complexity: Linear in N. -?- Remarks: This function is defined as deleted unless N == 0 or is_swappable_v<T> is true.Constraints: N == 0 or is_swappable_v<T> is true.
Section: 22.5.3.2 [optional.ctor], 22.5.3.4 [optional.assign], 22.6.3.2 [variant.ctor], 22.7.4.2 [any.cons], 22.7.4.4 [any.modifiers] Status: New Submitter: Tim Song Opened: 2016-10-29 Last modified: 2020-06-13
Priority: 3
View all other issues in [optional.ctor].
View all issues with New status.
Discussion:
Throughout optional/variant/any's specification references are made to "the selected constructor of T". For example, 22.5.3.2 [optional.ctor]/16 says of the constructor from const T&:
-16- Remarks: If T's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor.
Similarly, the in-place constructor has this wording (22.5.3.2 [optional.ctor]/25-26):
-25- Throws: Any exception thrown by the selected constructor of T.
-26- Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor shall be a constexpr constructor.
If T is a scalar type, it has no constructor at all. Moreover, even for class types, the in-place constructor wording ignores any implicit conversion done on the argument before the selected constructor is called, which 1) may not be valid in constant expressions and 2) may throw an exception; such exceptions aren't thrown "by the selected constructor of T" but outside it.
The wording should probably be recast to refer to the entire initialization.[Issues Telecon 16-Dec-2016]
Priority 3; Jonathan to provide wording.
[2020-06-11; Nina Dinka Ranns comments and provides initial wording]
This wording depends on the current resolution for LWG 2833, which covers the constexpr portion of this issue.
Proposed resolution:
This wording is relative to N4861.
Modify 22.5.3.2 [optional.ctor] as indicated:
constexpr optional(const optional& rhs);-3- […]
-4- […] -5- Throws: Any exception thrown by theselected constructor of Tinitialization of the contained value.constexpr optional(optional&& rhs) noexcept(see below);-7- […]
[…] -10- Throws: Any exception thrown by theselected constructor of Tinitialization of the contained value.template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);-12- […]
[…] -15- Throws: Any exception thrown by theselected constructor of Tinitialization of the contained value.template<class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);-17- […]
[…] -20- Throws: Any exception thrown by theselected constructor of Tinitialization of the contained value.template<class U = T> constexpr explicit(see below) optional(U&& v);-22- […]
[…] -25- Throws: Any exception thrown by theselected constructor of Tinitialization of the contained value.template<class U> explicit(see below) optional(const optional<U>& rhs);-27- […]
[…] -30- Throws: Any exception thrown by theselected constructor of Tinitialization of the contained value.template<class U> explicit(see below) optional(optional<U>&& rhs);-32- […]
[…] -35- Throws: Any exception thrown by theselected constructor of Tinitialization of the contained value.
Modify 22.6.3.2 [variant.ctor] as indicated:
template<class T, class... Args> constexpr explicit variant(in_place_type_t<T>, Args&&... args);-20- […]
[…] -23- Throws: Any exception thrown bycalling the selected constructor of Tthe initialization of the contained value.template<class T, class U, class... Args> constexpr explicit variant(in_place_type_t<T>, initializer_list<U> il, Args&&... args);-25- […]
[…] -28- Throws: Any exception thrown bycalling the selected constructor of Tthe initialization of the contained value.template<size_t I, class... Args> constexpr explicit variant(in_place_index_t<I>, Args&&... args);-30 […]
[…] -33- Throws: Any exception thrown bycalling the selected constructor of TIthe initialization of the contained value.
Modify 22.7.4.2 [any.cons] as indicated:
any(const any& other);[…]-2- Effects: […]
-3- Throws: Any exceptionsarising from calling the selected constructor forthrown by the initialization of the contained value.template<class T> any(T&& value);-5- […]
[…] -9- Throws: Any exception thrown by theselected constructor of VTinitialization of the contained value.template<class T, class... Args> explicit any(in_place_type_t<T>, Args&&... args);-10- […]
[…] -15- Throws: Any exception thrown by theselected constructor of VTinitialization of the contained value.template<class T, class U, class... Args> explicit any(in_place_type_t<T>, initializer_list<U> il, Args&&... args);-16- […]
[…] -21- Throws: Any exception thrown by theselected constructor of VTinitialization of the contained value.
Modify 22.7.4.4 [any.modifiers] as indicated:
template<class T, class... Args> decay_t<T>& emplace(Args&&... args);-1- […]
[…] -7- Throws: Any exception thrown by theselected constructor of VTinitialization of the contained value. -8- […]template<class T, class U, class... Args> decay_t<T>& emplace(initializer_list<U> il, Args&&... args);-9- […]
[…] -15- Throws: Any exception thrown by theselected constructor of VTinitialization of the contained value.
Section: 17.5 [support.start.term] Status: New Submitter: Jean-François Bastien Opened: 2016-11-07 Last modified: 2020-09-06
Priority: 3
View other active issues in [support.start.term].
View all other issues in [support.start.term].
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Discussion:
While SG1 was processing NB comments CA1 and LATE2 regarding P0270R1, we decided to remove the proposed guarantee that quick_exit be made signal safe.
Our reasoning is that functions registered with at_quick_exit aren't forbidden from calling quick_exit, but quick_exit implementations likely acquire some form of a lock before processing all registered functions (because a note forbids the implementation from introducing data races). The following code can therefore deadlock:
#include <cstdlib>
int main()
{
std::at_quick_exit([] () { std::quick_exit(0); });
std::quick_exit(1);
return 0;
}
The same applies if a function registered in at_quick_exit handles a signal, and that signal calls quick_exit. SG1 believes that both issues (same thread deadlock, and signal deadlock) can be resolved in the same manner. Either:
Option 2. seems preferable, and can be implemented along the lines of:
#include <array>
#include <atomic>
#include <cstddef>
namespace {
typedef void (*func)();
std::array<func, 32> quick_exit_functions;
const auto* quick_exit_functions_ptr = &quick_exit_functions;
std::atomic_flag lock = ATOMIC_FLAG_INIT;
struct scope
{
scope() { while (lock.test_and_set(std::memory_order_acquire)) ; }
~scope() { lock.clear(std::memory_order_release); }
};
}
namespace std {
extern "C" void quick_exit(int status) noexcept
{
decltype(quick_exit_functions_ptr) f;
{
scope s;
f = quick_exit_functions_ptr;
quick_exit_functions_ptr = nullptr;
}
if (f) {
size_t pos = f->size();
while (pos > 0)
(*f)[--pos]();
}
_Exit(status);
}
extern "C++" int at_quick_exit(func f) noexcept
{
scope s;
if (!quick_exit_functions_ptr || quick_exit_functions.size() == quick_exit_functions.max_size())
return -1;
quick_exit_functions[quick_exit_functions.size()] = f;
return 0;
}
}
Ideally, the resolution would also add back the wording which SG1 dropped from P0270R1:
Add at new element to the end of 17.5 [support.start.term] p13 (quick_exit()):
Remarks: The function quick_exit() is signal-safe (17.13.4 [csignal.syn]). [Note: It might still be unsafe to call quick_exit() from a handler, because the functions registered with at_quick_exit() might not be signal-safe. — end note]
[Issues Telecon 16-Dec-2016]
Priority 3
Proposed resolution:
This wording is relative to N4606.
Add at new element to the end of 17.5 [support.start.term] p13 (quick_exit()):
[[noreturn]] void quick_exit(int status) noexcept;-13- Effects: Functions registered by calls to at_quick_exit are called in the reverse order of their registration, except that a function shall be called after any previously registered functions that had already been called at the time it was registered. Objects shall not be destroyed as a result of calling quick_exit. If control leaves a registered function called by quick_exit because the function does not provide a handler for a thrown exception, std::terminate() shall be called. [Note: at_quick_exit may call a registered function from a different thread than the one that registered it, so registered functions should not rely on the identity of objects with thread storage duration. — end note] After calling registered functions, quick_exit shall call _Exit(status). [Note: The standard file buffers are not flushed. See: ISO C 7.22.4.5. — end note]
-?- Remarks: The function quick_exit() is signal-safe (17.13.4 [csignal.syn]). [Note: It might still be unsafe to call quick_exit() from a handler, because the functions registered with at_quick_exit() might not be signal-safe. — end note]
Section: 33.2.5 [thread.req.lockable], 33.6.4 [thread.mutex.requirements] Status: New Submitter: Agustín K-ballo Bergé Opened: 2016-11-12 Last modified: 2022-11-06
Priority: 3
View all issues with New status.
Discussion:
The current draft contains 14 occurrences of a Return type: clause. That clause is not covered by 16.3.2.4 [structure.specifications] p3. This was reported as editorial request #266.
[Issues Telecon 16-Dec-2016]
Priority 3; Jonathan to provide wording.
[12-May-2020, Jonathan provides wording to correct the 13 occurrences.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4681.
Modify 33.2.5.3 [thread.req.lockable.req] as indicated:
-1- A type
Lmeets the Cpp17Lockable requirements if it meets the Cpp17BasicLockable requirements and the following expressions are well-formed, have typebool, and have the specified semantics (mdenotes a value of typeL).
m.try_lock()-2- Effects: [...]
-3- Return type:bool.-4- Returns:
trueif the lock was acquired,falseotherwise.Modify 33.2.5.4 [thread.req.lockable.timed] as indicated:
-1- A type
Lmeets the Cpp17TimedLockable requirements if it meets the Cpp17BasicLockable requirements and the following expressions are well-formed, have typebool, and have the specified semantics (mdenotes a value of typeL,rel_timedenotes a value of an instantiation ofduration(29.5 [time.duration]), andabs_timedenotes a value of an instantiation oftime_point(29.6 [time.point])).
m.try_lock_for(rel_time)-2- Effects: [...]
-3- Return type:bool.-4- Returns:
trueif the lock was acquired,falseotherwise.
m.try_lock_until(abs_time)-2- Effects: [...]
-3- Return type:bool.-4- Returns:
trueif the lock was acquired,falseotherwise.Modify 33.6.4.2 [thread.mutex.requirements.mutex] as indicated:
-6- The expression
m.lock()is well-formed, has typevoid, and has the following semantics:-7- Preconditions: [...]
-8- Effects: [...]
-9- Postconditions: [...]
-10- Return type:void.-11- Synchronization: [...]
-12- Throws: [...]
-13- Error conditions: [...]
-14- The expression
m.try_lock()is well-formed, has typebool, and has the following semantics:-15- Preconditions: [...]
-16- Effects: [...]
-17- Return type:bool.-18- Returns:
trueif ownership of the mutex was obtained for the calling thread, otherwisefalse.-19- Synchronization: [...]
-20- Throws: Nothing.
-21- The expression
m.unlock()is well-formed, has typevoid, and has the following semantics:-22- Preconditions: [...]
-23- Effects: [...]
-24- Return type:void.-25- Synchronization: [...]
-26- Throws: Nothing.
Modify 33.6.4.3 [thread.timedmutex.requirements] as indicated:
-1- The timed mutex types are the standard library types [...]
-2- The timed mutex types meet the Cpp17TimedLockable requirements (33.2.5.4 [thread.req.lockable.timed]).
-3- The expression
m.try_lock_for(rel_time)is well-formed, has typebool, and has the following semantics:-4- Preconditions: [...]
-5- Effects: [...]
-6- Return type:bool.-7- Returns:
trueif the shared lock was acquired,falseotherwise.-8- Synchronization: [...]
-9- Throws: [...]
-10- The expression
m.try_lock_until(abs_time)is well-formed, has typebool, and has the following semantics:-11- Preconditions: [...]
-12- Effects: [...]
-13- Return type:bool.-14- Returns:
trueif ownership was obtained, otherwisefalse.-15- Synchronization: [...]
-16- Throws: [...]
Modify 33.6.4.4 [thread.sharedmutex.requirements] as indicated:
-1- The standard library types
shared_mutexandshared_timed_mutexare shared mutex types. [...]-2- In addition to the exclusive lock ownership mode [...]
-3- The expression
m.lock_shared()is well-formed, has typevoid, and has the following semantics:-4- Preconditions: [...]
-5- Effects: [...]
-6- Postconditions: [...]
-7- Return type:void.-8- Synchronization: [...]
-9- Throws: [...]
-10- Error conditions: [...]
-11- The expression
m.unlock_shared()is well-formed, has typevoid, and has the following semantics:-12- Preconditions: [...]
-13- Effects: [...]
-14- Return type:void.-15- Synchronization: [...]
-16- Throws: [...]
-17- The expression
m.try_lock_shared()is well-formed, has typebool, and has the following semantics:-18- Preconditions: [...]
-19- Effects: [...]
-20- Return type:bool.-21- Returns:
trueif the shared ownership lock was acquired,falseotherwise.-22- Synchronization: [...]
-23- Throws: [...]
Modify 33.6.4.5 [thread.sharedtimedmutex.requirements] as indicated:
-1- The standard library type
shared_timed_mutexis a shared timed mutex type. [...]-2- The expression
m.try_lock_shared_for(rel_time)is well-formed, has typebool, and has the following semantics:-3- Preconditions: [...]
-4- Effects: [...]
-5- Return type:bool.-6- Returns:
trueif the shared lock was acquired,falseotherwise.-7- Synchronization: [...]
-8- Throws: [...]
-9- The expression
m.try_lock_shared_until(abs_time)is well-formed, has typebool, and has the following semantics:-10- Preconditions: [...]
-11- Effects: [...]
-12- Return type:bool.-13- Returns:
trueif the shared lock was acquired,falseotherwise.-14- Synchronization: [...]
-15- Throws: [...]
[2022-11-06; Daniel comments and provides alternative wording]
Now that we have the new element Result: specified in 16.3.2.4 [structure.specifications], we can simply replace all occurrences of the Return type: by this element.
Proposed resolution:
This wording is relative to N4917.
Modify 33.2.5.3 [thread.req.lockable.req] as indicated:
-1- A type L meets the Cpp17Lockable requirements if it meets the Cpp17BasicLockable requirements and the following expressions are well-formed and have the specified semantics (m denotes a value of type L).
m.try_lock()
-2- Effects: [...]
-3-
Return typeResult: bool.-4- Returns: true if the lock was acquired, otherwise false.
Modify 33.2.5.4 [thread.req.lockable.timed] as indicated:
-1- A type L meets the Cpp17TimedLockable requirements if it meets the Cpp17Lockable requirements and the following expressions are well-formed and have the specified semantics (m denotes a value of type L, rel_time denotes a value of an instantiation of duration (29.5 [time.duration]), and abs_time denotes a value of an instantiation of time_point (29.6 [time.point])).
m.try_lock_for(rel_time)
-2- Effects: [...]
-3-
Return typeResult: bool.-4- Returns: true if the lock was acquired, otherwise false.
m.try_lock_until(abs_time)
-5- Effects: [...]
-6-
Return typeResult: bool.-7- Returns: true if the lock was acquired, otherwise false.
Modify 33.6.4.2.1 [thread.mutex.requirements.mutex.general] as indicated:
-5- The expression m.lock() is well-formed and has the following semantics:
-6- Preconditions: [...]
-7- Effects: [...]
-8- Synchronization: [...]
-9- Postconditions: [...]
-10-
Return typeResult: void.-11- Throws: [...]
-12- Error conditions: [...]
-13- The expression m.try_lock() is well-formed and has the following semantics:
-14- Preconditions: [...]
-15- Effects: [...]
-16- Synchronization: [...]
-17-
Return typeResult: bool.-18- Returns: true if ownership was obtained, otherwise false.
-19- Throws: Nothing.
-20- The expression m.unlock() is well-formed and has the following semantics:
-21- Preconditions: [...]
-22- Effects: [...]
-23-
Return typeResult: void.-24- Synchronization: [...]
-25- Throws: Nothing.
Modify 33.6.4.3.1 [thread.timedmutex.requirements.general] as indicated:
-1- The timed mutex types are the standard library types […]
-2- The expression m.try_lock_for(rel_time) is well-formed and has the following semantics:
-3- Preconditions: [...]
-4- Effects: [...]
-5- Synchronization: [...]
-6-
Return typeResult: bool.-7- Returns: true if ownership was obtained, otherwise false.
-8- Throws: [...]
-9- The expression m.try_lock_until(abs_time) is well-formed and has the following semantics:
-10- Preconditions: [...]
-11- Effects: [...]
-12- Synchronization: [...]
-13-
Return typeResult: bool.-14- Returns: true if ownership was obtained, otherwise false.
-15- Throws: [...]
Modify 33.6.4.4.1 [thread.sharedmutex.requirements.general] as indicated:
-1- The standard library types shared_mutex and shared_timed_mutex are shared mutex types. [...]
-2- In addition to the exclusive lock ownership mode [...]
-3- The expression m.lock_shared() is well-formed and has the following semantics:
-4- Preconditions: [...]
-5- Effects: [...]
-6- Synchronization: [...]
-7- Postconditions: [...]
-8-
Return typeResult: void.-9- Throws: [...]
-10- Error conditions: [...]
-11- The expression m.unlock_shared() is well-formed and has the following semantics:
-12- Preconditions: [...]
-13- Effects: [...]
-14-
Return typeResult: void.-15- Synchronization: [...]
-16- Throws: [...]
-17- The expression m.try_lock_shared() is well-formed and has the following semantics:
-18- Preconditions: [...]
-19- Effects: [...]
-20- Synchronization: [...]
-21-
Return typeResult: bool.-22- Returns: true if the shared lock was acquired, otherwise false.
-23- Throws: [...]
Modify 33.6.4.5.1 [thread.sharedtimedmutex.requirements.general] as indicated:
-1- The standard library type shared_timed_mutex is a shared timed mutex type. [...]
-2- The expression m.try_lock_shared_for(rel_time) is well-formed and has the following semantics:
-3- Preconditions: [...]
-4- Effects: [...]
-5- Synchronization: [...]
-6-
Return typeResult: bool.-7- Returns: true if the shared lock was acquired, otherwise false.
-8- Throws: [...]
-9- The expression m.try_lock_shared_until(abs_time) is well-formed and has the following semantics:
-10- Preconditions: [...]
-11- Effects: [...]
-12- Synchronization: [...]
-13-
Return typeResult: bool.-14- Returns: true if the shared lock was acquired, otherwise false.
-15- Throws: [...]
Section: 24.3.7.1 [array.overview] Status: Open Submitter: Robert Haberlach Opened: 2016-11-16 Last modified: 2018-03-19
Priority: 3
View other active issues in [array.overview].
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Discussion:
LWG 2590's resolution is incomplete:
std::array<int, 1> arr{{0}};
should be fine, but isn't guaranteed, since {0} has no type. We should rather go for implicit conversion:
An array is an aggregate (9.4.2 [dcl.init.aggr]) that can be list-initialized with up to N elements
whose types are convertible to Tthat can be implicitly converted to T.
[2016-11-26, Tim Song comments]
This is not possible as written, because due to the brace elision rules for aggregate initialization, std::array<int, 2> arr{{0}, {1}}; will never work: the {0} is taken as initializing the inner array, and the {1} causes an error.
[2017-01-27 Telecon]
Priority 2; consensus is that the P/R is not quite right.
[2018-3-14 Wednesday evening issues processing; priority to 3; move to Open]
Jens: There's nothing you can do about the double braces in std::array. That's a core thing.
STL to write paper to resolve this.
Proposed resolution:
This wording is relative to N4606.
Change 24.3.7.1 [array.overview] p2 as indicated:
-2- An array is an aggregate (9.4.2 [dcl.init.aggr]) that can be list-initialized with up to N elements
whose types are convertiblethat can be implicitly converted to T.
Section: 21.3.5.4 [meta.unary.prop] Status: New Submitter: Richard Smith Opened: 2016-11-17 Last modified: 2023-05-25
Priority: 3
View other active issues in [meta.unary.prop].
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Discussion:
struct S
{
~S(); // non-trivial
};
static_assert(std::is_trivially_constructible<S>::value, "");
Should the assert pass? Implementations disagree.
Per 21.3.5.4 [meta.unary.prop]'s Table 38, this trait looks at whether the following variable definition is known to call no operation that is not trivial:S t(create<Args>()...);
... where Args is an empty pack in this case. That variable definition results in a call to the S destructor. Should that call be considered by the trait?
[2017-01-27 Telecon]
Priority 3
This issue interacts with 2116
[2020-01-24; Peter Dimov comments]
std::is_trivially_copy_constructible_v<D>, where D is
struct D
{
~D() {}
};
reports false. This is because the definition of is_trivially_copy_constructible requires the invented variable definition T t(declval<Args>()...);, which in our case is D t(declval<D>());, to not call any nontrivial operations.
This is interpreted by implementations to include the destructor call, presumably for consistency with is_nothrow_copy_constructible. But that's wrong; the copy constructor is trivial. As a consequence, variant<D> also doesn't have a trivial copy constructor, which causes (completely unnecessary) inefficiencies when said variant is copied.[2023-05-25; May 2023 mailing]
Alisdair provided P2842R0.
Proposed resolution:
Section: 22.5.3.6 [optional.observe] Status: Open Submitter: Richard Smith Opened: 2016-11-24 Last modified: 2020-06-13
Priority: 2
View other active issues in [optional.observe].
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Discussion:
After applying LWG 2740, we have:
constexpr const T* operator->() const; constexpr T* operator->();-1- Requires: *this contains a value.
-2- Returns: val. -3- Throws: Nothing. -4- Remarks: These functions shall be constexpr functions.
Paragraph 4 is completely superfluous. We already said these functions were constexpr in the synopsis. Can it be removed?
[Issues Telecon 16-Dec-2016]
Priority 2
Jonathan notes: Although Richard is correct, I suggest we don't strike the paragraph, so that we remember to fix it as part of 2833, when we know how to say this properly.
[2018-06 Rapperswil Thursday issues processing]
Status to Open; also see 7.7 [expr.const]/6 and 2289.
[2020-06-08 Nina Dinka Ranns comments]
The revised wording provided by LWG 2833 should resolve this issue as well.
Proposed resolution:
Section: 22.6.3.2 [variant.ctor] Status: Open Submitter: Richard Smith Opened: 2016-11-28 Last modified: 2020-12-19
Priority: 2
View other active issues in [variant.ctor].
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Discussion:
The library has lots of functions declared constexpr, but it's not clear what that means. The constexpr keyword implies that there needs to be some invocation of the function, for some set of template arguments and function arguments, that is valid in a constant expression (otherwise the program would be ill-formed, with no diagnostic required), along with a few side conditions. I suspect the library intends to require something a lot stronger than that from implementations (something along the lines of "all calls that could reasonably be constant subexpressions are in fact constant subexpressions, unless otherwise stated").
[variant.ctor]/1 contains this, which should also be fixed:"This function shall be constexpr if and only if the value-initialization of the alternative type T0 would satisfy the requirements for a constexpr function."
This is the wrong constraint: instead of constraining whether the function is constexpr, we should constrain whether a call to it is a constant subexpression.
Daniel:
This is has some considerable overlap with LWG 2289 but is phrased in a more general way.[2016-12-16, Issues Telecon]
Priority 2; this is also the general case of 2829.
[2017-02-20, Alisdair comments and suggests concrete wording]
Below is is draft wording I was working on at Issaquah to try to address both issues.
[2017-11 Albuquerque Wednesday issue processing]
Status to Open; really needs a paper.
STL says "What about plus<T>?" plus<int> needs to be usable in a constexpr context, but plus<string> can't be.
[2017-11 Albuquerque Saturday issues processing]
Geoffrey to write a paper resolving this.
[2018-06 Rapperswil Thursday issues processing]
Geoffrey has been unable to write this paper due to time constraints. He wrote up his progress here. Daniel has offered to help someone to write this paper; he's willing to be a co-author.
[2018-08-23 Batavia Issues processing]
Michael Wong to investigate.
Previous resolution from Daniel [SUPERSEDED]:This wording is relative to N4640.
Modify 16.4.6.7 [constexpr.functions] as indicated:
17.6.5.6 constexpr functions and constructors [constexpr.functions]
-1- This International Standard explicitly requires that certain standard library functions are constexpr (9.2.6 [dcl.constexpr]). If the specification for a templated entity requires that it shall be a constexpr templated entity, then that templated entity shall be usable in a constant expression.. An implementation
shall notmay declareanyadditional standard library function signature as constexprexcept for those where it is explicitly required. Within any header that provides any non-defining declarations of constexpr functions or constructors an implementation shall provide corresponding definitions.
[2020-06-08 Nina Dinka Ranns comments and provides alternative wording]
The revised wording draft also resolves LWG 2289, LWG 2829, and LWG 3215.
Previous resolution [SUPERSEDED]:This wording is relative to N4861.
1. Modify 16.4.6.7 [constexpr.functions] as indicated:
-1- This document explicitly requires that certain standard library functions are constexpr (9.2.6 [dcl.constexpr]). An implementation shall not declare any standard library function signature as constexpr except for those where it is explicitly required. Within any header that provides any non-defining declarations of constexpr functions or constructors an implementation shall provide corresponding definitions.
-?- Let F denote a standard library function template or member function of a class template. If the specification of F declares it to be constexpr, unless otherwise specified, then F can be used in a constant expression if and only if all the expressions that are evaluated as specified in the description of F's semantics can be used in a constant expression.2. - 10. […] // Remainder of Nina's update
[2020-10-02 Jens Maurer improves wording]
Specifically the wording for 16.4.6.7 [constexpr.functions] needs improvement and is updated below.
[2020-10-02 Tim Song comments]
The new wording doesn't cover the following example:
// global scope int x; int y; constexpr int j = (std::swap(x, y), 0); // error
swap is a "standard library function template...declared constexpr"
x and y are (lvalue) constant expressions
std::swap(x, y) is plainly not a constant expression
[2020-10-04 Jens Maurer comments]
Yes, we're still lacking text for that (and maybe Nina's old text helps for that).
[2020-12-14; Jiang An comments]
The item "constexpr functions" is also used in 24.2.2.1 [container.requirements.general]/14 and 25.3.1 [iterator.requirements.general]/16, and such usage should also be modified by this issue here.
Proposed resolution:
This wording is relative to N4861.
Modify 16.4.6.7 [constexpr.functions] as indicated:
-1- This document explicitly requires that certain standard library functions are constexpr (9.2.6 [dcl.constexpr]). An implementation shall not declare any standard library function signature as constexpr except for those where it is explicitly required. Within any header that provides any non-defining declarations of constexpr functions or constructors an implementation shall provide corresponding definitions.
-?- Let F denote a standard library function template or member function of a class template declared constexpr. Unless otherwise specified, a function call expression (7.6.1.3 [expr.call]) whose postfix-expression names F is a constant expression if all of the argument subexpressions are constant expressions.
Modify 22.3.2 [pairs.pair] as indicated:
-2- The defaulted
move and copyconstructors, respectively,of pairis a constexpr functioncan be used in a constant expression if and only if all required element-wise initializationsfor move and copy, respectively, would satisfy the requirements for a constexpr functioncan be used in a constant expression.
Modify 22.4.4.1 [tuple.cnstr] as indicated:
-3- The defaulted
move and copyconstructors, respectively,of tupleis a constexpr functioncan be used in a constant expression if and only if all required element-wise initializationsfor move and copy, respectively, would satisfy the requirements for a constexpr functioncan be used in a constant expression. The defaultedmove and copyconstructors of tuple<>are constexpr functionscan be used in a constant expression.
Modify 22.5.3.2 [optional.ctor] as indicated:
constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept;[…]-1- […]
-2- Remarks: No contained value is initialized.For every object type T these constructors are constexpr constructors (9.2.6 [dcl.constexpr]).template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);-12- […]
-13- […] -14- […] -15- […]-16- Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor.template<class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);-17- […]
-18- […] -19- […] -20- […]-21- Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor.template<class U = T> constexpr explicit(see below) optional(U&& v);-22- […]
-23- […] -24- […] -25- […] -26- Remarks:If T's constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor.The expression inside explicit is equivalent to:!is_convertible_v<U, T>
Modify 22.5.3.6 [optional.observe] as indicated:
constexpr const T* operator->() const; constexpr T* operator->();-1- […]
-2- […] -3- […]-4- Remarks: These functions are constexpr functions.constexpr const T& operator*() const&; constexpr T& operator*() &;[…]-5- […]
-6- […] -7- […]-8- Remarks: These functions are constexpr functions.constexpr explicit operator bool() const noexcept;-11- Returns: true if and only if *this contains a value.
-12- Remarks: This function is a constexpr function.constexpr bool has_value() const noexcept;-13- Returns: true if and only if *this contains a value.
-14- Remarks: This function is a constexpr function.
Modify 22.5.6 [optional.relops] as indicated:
template<class T, class U> constexpr bool operator==(const optional<T>& x, const optional<U>& y);-1- […]
-2- […]-3- Remarks: Specializations of this function template for which *x == *y is a core constant expression are constexpr functions.template<class T, class U> constexpr bool operator!=(const optional<T>& x, const optional<U>& y);-4- […]
-5- […]-6- Remarks: Specializations of this function template for which *x != *y is a core constant expression are constexpr functions.template<class T, class U> constexpr bool operator<(const optional<T>& x, const optional<U>& y);-7- […]
-8- […]-9- Remarks: Specializations of this function template for which *x < *y is a core constant expression are constexpr functions.template<class T, class U> constexpr bool operator>(const optional<T>& x, const optional<U>& y);-10- […]
-11- […]-12- Remarks: Specializations of this function template for which *x > *y is a core constant expression are constexpr functions.template<class T, class U> constexpr bool operator<=(const optional<T>& x, const optional<U>& y);-13- […]
-14- […]-15- Remarks: Specializations of this function template for which *x <= *y is a core constant expression are constexpr functions.template<class T, class U> constexpr bool operator>=(const optional<T>& x, const optional<U>& y);-16- […]
-17- […]-18- Remarks: Specializations of this function template for which *x >= *y is a core constant expression are constexpr functions.template<class T, three_way_comparable_with<T> U> constexpr compare_three_way_result_t<T,U> operator<=>(const optional<T>& x, const optional<U>& y);-19- Returns: If x && y, *x <=> *y; otherwise bool(x) <=> bool(y).
-20- Remarks: Specializations of this function template for which *x <=> *y is a core constant expression are constexpr functions.
Modify 22.6.3.2 [variant.ctor] as indicated:
constexpr variant() noexcept(see below);[…]-1- […]
-2- […] -3- […] -4- […] -5- […] -6- Remarks:This function is constexpr if and only if the value-initialization of the alternative type T0 would satisfy the requirements for a constexpr function.The expression inside noexcept is equivalent to is_nothrow_default_constructible_v<T0>. [Note: See also class monostate. — end note]template<class T> constexpr variant(T&& t) noexcept(see below);-14- […]
[…] -19- Remarks: The expression inside noexcept is equivalent to is_nothrow_constructible_v<Tj, T>.If Tj's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.template<class T, class... Args> constexpr explicit variant(in_place_type_t<T>, Args&&... args);-20- […]
[…]-24- Remarks: If T's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.template<class T, class U, class... Args> constexpr explicit variant(in_place_type_t<T>, initializer_list<U> il, Args&&... args);-25- […]
[…]-29- Remarks: If T's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.template<size_t I, class... Args> constexpr explicit variant(in_place_index_t<I>, Args&&... args);-30- […]
[…]-34- Remarks: If TI's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.template<size_t I, class U, class... Args> constexpr explicit variant(in_place_index_t<I>, initializer_list<U> il, Args&&... args);-35- […]
[…]-38- Remarks: If TI's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.
Modify 25.5.4.11 [move.sent.ops] as indicated:
constexpr move_sentinel();-1- Effects: Value-initializes last.
If is_trivially_default_constructible_v<S> is true, then this constructor is a constexpr constructor.
Modify 22.15.3 [bit.cast] as indicated:
template<class To, class From> constexpr To bit_cast(const From& from) noexcept;-1- […]
-3- Remarks: This functionis constexprcan be used in a constant expression if and only if To, From, and the types of all subobjects of To and From are types T such that:
(3.1) — is_union_v<T> is false;
(3.2) — is_pointer_v<T> is false;
(3.3) — is_member_pointer_v<T> is false;
(3.4) — is_volatile_v<T> is false; and
(3.5) — T has no non-static data members of reference type.
Modify 29.5 [time.duration] as indicated:
-5- The defaulted
copyconstructors of durationshall be a constexpr functioncan be used in a constant expression if and only if the required initialization of the member rep_for copy and move, respectively, would satisfy the requirements for a constexpr functioncan be used in a constant expression.
Section: 24.2.7 [associative.reqmts], 24.2.8 [unord.req] Status: Open Submitter: Matt Austern Opened: 2016-12-14 Last modified: 2020-02-14
Priority: 3
View other active issues in [associative.reqmts].
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Discussion:
If we write a_uniq.insert(i, j) and [i, j) has multiple elements with keys that compare equivalent, which ones get inserted? Consider, for example, inserting into a map<string, int> with
m.insert({{"red", 5}, {"green", 3}, {"red", 7}, {"blue", 2}, {"pink", 6}});
Which value for "red" will the map have?
On my implementation we got "red" -> 5, and I suspect that's true on most or all implementations, but I don't believe that's guaranteed by anything in the requirements. The wording in Table 90 just says that it "inserts each element from the range [i, j) if and only if there is no element with key equivalent to the key of that element", but that doesn't tell us what happens if [i, j) contains duplicate keys because it doesn't say what order the insertions are performed in. The standard should either guarantee that the first value is the one that gets inserted, or explicitly say that this is unspecified. The same issue applies to the range constructor, and to the unordered associative containers.[2017-01-27 Telecon]
Priority 3; Nico to provide wording.
[2020-02-14, Prague]
LWG discussion. Suggestion to specify that we like the direction of the wording for insert of unordered containers, but would also like to clarify that the loop is meant to be "in order" of the sequence elements.
Daniel K. volunteered to provide such wording.Proposed resolution:
Section: 21.3.2 [meta.rqmts] Status: New Submitter: Tim Song Opened: 2016-12-14 Last modified: 2017-02-02
Priority: 3
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Discussion:
[meta.rqmts]/3 defines TransformationTrait as follows:
A TransformationTrait modifies a property of a type. It shall be a class template that takes one template type argument and, optionally, additional arguments that help define the modification. It shall define a publicly accessible nested type named type, which shall be a synonym for the modified type.
enable_if, result_of and common_type do not necessarily "define a publicly accessible nested type named type". aligned_storage takes no template type argument (it only has two non-type parameters). Yet [meta.trans]/2 says that they are all TransformationTraits.
Incidentally, with the exception of decay, it's not clear that any of the traits in [meta.trans.other] could really be described as "modify[ing] a property of a type".[2017-01-27 Telecon]
Priority 3
Proposed resolution:
Section: 28.4.9 [cmplx.over], 28.7.1 [cmath.syn] Status: New Submitter: Jens Maurer Opened: 2016-12-15 Last modified: 2019-03-18
Priority: 3
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Discussion:
In [cmplx.over] and [cmath.syn], when talking about "sufficient additional overloads", we use the phrase "effectively cast", but that is not a defined term.
A hostile interpretation could read "reinterpret_cast" here. Likely we mean "apply floating-point promotions, floating-integral conversions, and floating-point conversions", but that should be spelled out somewhere, e.g. in the library definitions section. (Source: Editorial issue #1248)[2017-01-27 Telecon]
Priority 3
[2019-03-16; Daniel comments and provides wording]
I decided to use the form "implicitly converted" and to refer to 7.3 [conv] for 28.7.1 [cmath.syn] and 28.4.9 [cmplx.over], because those conversions can all be done implicitly. This also holds for the pow specification 28.4.9 [cmplx.over] p3, because the described conversions of complex<T> to complex<U> involve only the need of non-explicit constructors.
Proposed resolution:
This wording is relative to N4810.
Change 28.4.9 [cmplx.over], as indicated:
-2- The additional overloads shall be sufficient to ensure:
-3 Function template pow shall have additional overloads sufficient to ensure, for a call with at least one argument of type complex<T>:
(2.1) — If the argument has type long double, then it is
effectively castimplicitly converted (7.3 [conv]) to complex<long double>.(2.2) — Otherwise, if the argument has type double or an integer type, then it is
effectively castimplicitly converted to complex<double>.(2.3) — Otherwise, if the argument has type float, then it is
effectively castimplicitly converted to complex<float>.
(3.1) — If either argument has type complex<long double> or type long double, then both arguments are
effectively castimplicitly converted (7.3 [conv]) to complex<long double>.(3.2) — Otherwise, if either argument has type complex<double>, double, or an integer type, then both arguments are
effectively castimplicitly converted to complex<double>.(3.3) — Otherwise, if either argument has type complex<float> or float, then both arguments are
effectively castimplicitly converted to complex<float>.
Change 28.7.1 [cmath.syn], as indicated:
-2- For each set of overloaded functions within <cmath>, with the exception of abs, there shall be additional overloads sufficient to ensure:
If any argument of arithmetic type corresponding to a double parameter has type long double, then all arguments of arithmetic type (6.8.2 [basic.fundamental]) corresponding to double parameters are
effectively castimplicitly converted (7.3 [conv]) to long double.Otherwise, if any argument of arithmetic type corresponding to a double parameter has type double or an integer type, then all arguments of arithmetic type corresponding to double parameters are
effectively castimplicitly converted to double.Otherwise, all arguments of arithmetic type corresponding to double parameters have type float.
Section: 28.7.1 [cmath.syn] Status: New Submitter: Jens Maurer Opened: 2016-12-15 Last modified: 2020-09-06
Priority: 3
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Discussion:
With P0175R1, we now show in [cmath.syn] three overloads for the sin function: One taking a float, one taking a double, and one taking a long double. However, there is no statement that sin(long double) should actually invoke sinl, presumably delivering extra precision.
An implementation like
inline long double sin(long double x)
{ return sinf(x); }
seems to satisfy the "effectively cast" requirement, but is certainly unintentional.
The same issue arises for all math functions inherited from C. (Source: Editorial issue #1247)[2017-01-27 Telecon]
Priority 3
Proposed resolution:
Section: 20.4.5.2 [mem.res.pool.options] Status: New Submitter: Jens Maurer Opened: 2016-12-15 Last modified: 2017-02-02
Priority: 3
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Discussion:
20.4.5.2 [mem.res.pool.options] p3 talks about a "pass-through-threshold".
First, the phrase is not defined and it seems it could be easily avoided given the context. Second, given the phrasing here, it seems the implementation is essentially allowed to ignore the value largest_required_pool_block as it sees fit. It is unclear whether that is the intention.[2017-01-27 Telecon]
Priority 3; Jonathan will ask Alisdair for wording.
Proposed resolution:
Section: 25.5.1 [reverse.iterators] Status: New Submitter: Hubert Tong Opened: 2017-01-28 Last modified: 2020-09-06
Priority: 4
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Discussion:
Further to LWG 2472, the case of reverse_iterator comparisons is a regression introduced by LWG 280.
Consider the following program:
#include <utility>
#include <iterator>
using namespace std::rel_ops;
bool f(std::reverse_iterator<int *> it) { return it != it; }
Under C++03, the operator!= in lib.reverse.iterator is more specialized than the operator!= in std::rel_ops.
Following LWG 280, neither operator!= candidate is more specialized than the other. The program is observed to fail with libc++. Online compiler example, see here. Suggested resolution: Reintroduce the homogeneous comparison operators from C++03 alongside the new ones.[2017-03-04, Kona]
Set priority to 4. STL to write a paper deprecating relops Alisdair to provide an example for Annex C.
Proposed resolution:
This wording is relative to N4618.
Modify 25.5.1 [reverse.iterators], class template reverse_iterator synopsis, as indicated:
template <class Iterator1, class Iterator2>
constexpr bool operator==(
const reverse_iterator<Iterator1>& x,
const reverse_iterator<Iterator2>& y);
template <class Iterator1, class Iterator2>
constexpr bool operator<(
const reverse_iterator<Iterator1>& x,
const reverse_iterator<Iterator2>& y);
template <class Iterator1, class Iterator2>
constexpr bool operator!=(
const reverse_iterator<Iterator1>& x,
const reverse_iterator<Iterator2>& y);
template <class Iterator1, class Iterator2>
constexpr bool operator>(
const reverse_iterator<Iterator1>& x,
const reverse_iterator<Iterator2>& y);
template <class Iterator1, class Iterator2>
constexpr bool operator>=(
const reverse_iterator<Iterator1>& x,
const reverse_iterator<Iterator2>& y);
template <class Iterator1, class Iterator2>
constexpr bool operator<=(
const reverse_iterator<Iterator1>& x,
const reverse_iterator<Iterator2>& y);
template <class Iterator>
constexpr bool operator==(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
constexpr bool operator<(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
constexpr bool operator!=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
constexpr bool operator>(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
constexpr bool operator>=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
constexpr bool operator<=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
Modify [reverse.iter.op==] as indicated:
template <class Iterator1, class Iterator2> constexpr bool operator==( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y); template <class Iterator> constexpr bool operator==( const reverse_iterator<Iterator>& x, const reverse_iterator<Iterator>& y);-1- Returns: x.current == y.current.
Modify [reverse.iter.op<] as indicated:
template <class Iterator1, class Iterator2> constexpr bool operator<( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y); template <class Iterator> constexpr bool operator<( const reverse_iterator<Iterator>& x, const reverse_iterator<Iterator>& y);-1- Returns: x.current > y.current.
Modify [reverse.iter.op!=] as indicated:
template <class Iterator1, class Iterator2> constexpr bool operator!=( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y); template <class Iterator> constexpr bool operator!=( const reverse_iterator<Iterator>& x, const reverse_iterator<Iterator>& y);-1- Returns: x.current != y.current.
Modify [reverse.iter.op>] as indicated:
template <class Iterator1, class Iterator2> constexpr bool operator>( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y); template <class Iterator> constexpr bool operator>( const reverse_iterator<Iterator>& x, const reverse_iterator<Iterator>& y);-1- Returns: x.current < y.current.
Modify [reverse.iter.op>=] as indicated:
template <class Iterator1, class Iterator2> constexpr bool operator>=( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y); template <class Iterator> constexpr bool operator>=( const reverse_iterator<Iterator>& x, const reverse_iterator<Iterator>& y);-1- Returns: x.current <= y.current.
Modify [reverse.iter.op<=] as indicated:
template <class Iterator1, class Iterator2> constexpr bool operator<=( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y); template <class Iterator> constexpr bool operator<=( const reverse_iterator<Iterator>& x, const reverse_iterator<Iterator>& y);-1- Returns: x.current >= y.current.
Section: 22.6.3 [variant.variant] Status: New Submitter: Switzerland Opened: 2017-02-03 Last modified: 2020-09-06
Priority: 3
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Discussion:
Addresses CH 7Consider making the variant statically !valueless_by_exception() for cases where is_nothrow_move_constructible_v<T_i> for all alternative types Ti
Proposed change: Adopt section III of P0308R0.
[2017-07 Toronto Thurs Issue Prioritization]
Priority 3. This is similar to 2904, Casey to investigate
Proposed resolution:
Section: 23.3 [string.view] Status: LEWG Submitter: Switzerland Opened: 2017-02-03 Last modified: 2017-07-15
Priority: Not Prioritized
View all other issues in [string.view].
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Discussion:
Addresses CH 9The standard library should provide string_view parameters instead or in addition for functions defined with char const * or string const & as parameter types. Most notably in cases where both such overloads exist or where an internal copy is expected anyway.
It might be doubted that the non-null termination of string_view could be an issue with functions that pass the char * down to OS functions, such as fstream_buf::open() etc. and those shouldn't provide it and favour generating a std::string temporary instead in that case. However, std::path demonstrates it is usable to have string_view overloads and there might be many places where it can be handy, or even better.
Proposed change: Provide the overloads for std::regex, the exception classes, std::bitset, std::locale and more.
[Post-Kona 2017]
Most (all?) of these changes were proposed in P0506r1, which was discussed by LEWG in Kona.
[2017-07 Toronto Thurs Issue Prioritization]
Status LEWG - they're already looking at this.
Proposed resolution:
Section: 24 [containers], 22 [utilities] Status: LEWG Submitter: Finland Opened: 2017-02-03 Last modified: 2017-06-27
Priority: Not Prioritized
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Discussion:
Addresses FI 16Relational operators for containers should sfinae; if the underlying type is not comparable, neither should the container be. Same applies to tuple and pair.
Proposed change: Make the relational operators of containers and utility components reflect the validity of the underlying element types.
[ 2017-06-27 Moved to LEWG after 5 positive votes on c++std-lib. ]
Proposed resolution:
Section: 24 [containers], 22 [utilities] Status: LEWG Submitter: Finland Opened: 2017-02-03 Last modified: 2017-07-15
Priority: Not Prioritized
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Discussion:
Addresses FI 17The relational operators of optional and variant completely reflect the semantics of the element types; this is inconsistent with other types in the library, like pair, tuple and containers. If we believe it's important that we don't synthesize relational operators for wrapper types, we should believe it's important for other types as well. Otherwise comparing containers of floating-point types and tuples/pairs etc. of floating point types will give incorrect answers.
Proposed change: Make the relational operators of containers and utility components reflect the semantics of the operators for the underlying element types.
[2017-07 Toronto Thurs Issue Prioritization]
Move to LEWG
Proposed resolution:
Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: New Submitter: United States Opened: 2017-02-03 Last modified: 2017-07-15
Priority: 3
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Discussion:
Addresses US 130There is no ability to supply an allocator for the control block when constructing a shared_ptr from a unique_ptr. Note that no further shared_ptr constructors need an allocator, as they all have pre-existing control blocks that are shared, or already have the allocator overload.
Proposed change: Add an additional shared_ptr constructor, template <class Y, class D, class A> shared_ptr(unique_ptr<Y, D>&& r, A alloc), with the same semantics as the existing constructor taking a unique_ptr, but using the alloc argument to supply memory as required.
[2017-07 Toronto Thurs Issue Prioritization]
Priority 3; Alisdair to provide wording
Proposed resolution:
Section: 21.3.3 [meta.type.synop] Status: LEWG Submitter: United States Opened: 2017-02-03 Last modified: 2017-07-15
Priority: Not Prioritized
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Discussion:
Addresses US 171The *_constant<> templates (including the proposed addition, bool_constant<>) do not make use of the new template<auto> feature.
Proposed change: Add a constant<> (subject to bikeshedding) template which uses template<auto>.
Define integral_constant<> as using integral_constant<T, V> = constant<T(V)> or integral_constant<T, V> = constant<V>.
Either remove bool_constant, define it as using bool_constant = constant<bool(B)> or using bool_constant = constant<B>.
[2017-03-03, Kona, LEWG]
Straw polls:
| constant | 3 |
| numeric_constant | 8 |
| static_constant | 1 |
| scalar_constant | 7 |
| integer_constant (Over LWG's dead body) | 1 |
| auto_constant | 4 |
| integral_c | 7 |
| int_ | 0 |
| | |
| scalar_constant | 6 |
| numeric_constant | 3 |
| integral_c | 5 |
Accept P0377 with "scalar_constant" for C++17 to address LWG 2922 and US 171:
SF | F | N | A | SA 0 | 1 | 3 | 7 | 5[2017-07 Toronto Thurs Issue Prioritization]
Status LEWG with P0377
Proposed resolution:
Section: 28.7.1 [cmath.syn] Status: New Submitter: United States Opened: 2017-02-03 Last modified: 2017-07-15
Priority: 4
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Discussion:
Addresses US 172noexcept is inconsistently applied across headers which import components of the C standard library into the C++ library; some functions (std::abort(), std::_Exit(), etc) are defined as noexcept in some places, but not in others. Some functions which seem like they should be noexcept (std::abs(), std::div(), etc) are not defined as noexcept.
Proposed change: Make the majority of the C library functions (with exceptions such as std::qsort() and std::bsearch(), which can call user code) noexcept.
[2017-07 Toronto Thurs Issue Prioritization]
Priority 4
Proposed resolution:
Section: 25.4.3 [iterator.operations] Status: Open Submitter: Morwenn Opened: 2017-02-04 Last modified: 2018-12-03
Priority: 3
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Discussion:
It seems that std::next is missing an optimization opportunity when taking a single parameter. The standard mandates that std::next shall call std::advance on the passed iterator and return it. For random-access iterators, it means that operator+= will be called on the iterator. However, if a single-argument overload was added to std::next, it could call ++it directly instead of std::advance(it, 1), which means that operator++ would be called instead of operator+=. This might make a small performance difference for complicated iterators such as std::deque's ones, where operator++ has a simpler logic than operator+=.
An equivalent optimization could be allowed by adding a single-argument overload to std::prev too.[2017-03-04, Kona]
Set priority to 3. Alisdair to provide wording.
[2018-11-30, Jonathan comments, recommending NAD]
Jonathan suggested NAD, because the proposed "just use increment when n==1" optimization can be done in std::next (and/or std::advance, and/or complicated iterators like deque::iterator) without adding an overload. Billy said the overload would avoid metaprogramming costs for dispatching to the right std::advance, and help in non-optimized builds. Zhihao said the overload would make it clear to users that the n==1 case is optimized (Jonathan thinks this is irrelevant as there's no requirement that we tell users what we optimize).
Proposed resolution:
Section: 22.4.5 [tuple.creation] Status: New Submitter: Eric Fiselier Opened: 2017-02-06 Last modified: 2020-09-06
Priority: 3
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Discussion:
The current PR for LWG 2773 changes std::ignore to be a constexpr variable. However it says nothing about whether using std::ignore in std::tie is a constant expression. I think the intent was clearly to allow this. Therefore I suggest we update the resolution to explicitly call this out in a note. (I don't think new normative wording is needed).
I propose we update the current PR as follows:Keep the current changes proposed by the PR.
Add a note after [tuple.creation]/p7 (std::tie):
[Note: The constructors and assignment operators provided by ignore shall be constexpr]
Perhaps LWG feels the existing wording is clear enough, but if not I think the above changes sufficiently clarify it.
The ability to constexpr assign to std::ignore can be important: Here is an extremely contrived example:
constexpr bool foo() {
auto res = std::tie(std::ignore);
std::get<0>(res) =42;
return true;
}
static_assert(foo());
[2017-03-04, Kona]
Set priority to 3. P/R is incorrect; it should NOT be a note. Marshall to work with Eric to get better wording. STL says "use an exposition-only class".
Proposed resolution:
This wording is relative to N4640.
Modify 22.4.5 [tuple.creation] as indicated:
template<class... TTypes> constexpr tuple<TTypes&...> tie(TTypes&... t) noexcept;-7- Returns: […]
-?- [Note: The constructors and assignment operators provided by ignore shall be constexpr. — end note] -8- [Example: […] — end example]
Section: 21.3.3 [meta.type.synop] Status: New Submitter: Daniel Krügler Opened: 2017-03-02 Last modified: 2020-02-13
Priority: 2
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Discussion:
LWG 2797 (RU 2) suggests that certain type-traits should be required to diagnose violations of their pre-conditions. The basic idea is founded and I see no problems for requiring this for the mentioned traits alignment_of or is_base_of, for example. But if we want to require this diagnostics for some other traits, such as is_convertible, is_constructible (and friends), or is_callable (and possibly some others), we really should be sure that our current requirements are OK.
Unfortunately, there exists some cases, where we currently overspecify imposing complete type requirements where they are not actually required. For example, for the following situation the answer of the trait could be given without ever needing the complete type of X:struct X; // Never defined static_assert(std::is_convertible_v<X, const X&>);
Unfortunately we cannot always allow incomplete types, because most type constructions or conversions indeed require a complete type, so generally relaxing the current restrictions is also not an option.
The core language has a solution for this "small" gap of situations, where the response of the compiler might depend on type completeness: Undefined behaviour. So, I believe we need a somewhat more detailled form to express the intend here. Informally, I would suggest that the program should only be ill-formed in the situation described by LWG 2797, if there exists the possibility that the compiler would require complete types for the considered operation. The example shown above, std::is_convertible_v<X, const X&>, would never require the need to complete X, so here no violation should exist. The presented example might seem a tiny one, but the Standard Library type traits are extreme fundamental tools and we should try to not give the impression that an approximate rule of the current type constraints breaks reasonable code. It is correct, that above example has currently undefined behaviour due to the breakage of pre-conditions, therefore this issue suggests to fix the current situation before enforcing a diagnostic for such valid situations.[2017-03-04, Kona]
Set priority to 2. Is related to 2797, but really needs an audit of the type traits.
[2018-08 Batavia Monday issue discussion]
Issues 2797, 2939, 3022, and 3099 are all closely related. Walter to write a paper resolving them.
[2020-02 Prague Thursday issue discussion]
Two of the issues (2797 and 3022) had been resolved by the acceptance of P1285R0.
Proposed resolution:
Section: 31.12.8.1 [fs.enum.path.format], 31.12.11 [fs.class.directory.iterator], 31.12.13.4 [fs.op.copy] Status: New Submitter: Thomas Köppe Opened: 2017-03-14 Last modified: 2023-02-07
Priority: 3
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Discussion:
During the application of P0430R2, we came across several terms that seem insufficiently clear and lacking proper definitions.
We would like clarification on what those terms mean, and we would welcome wording suggestions, or alternatively a verbose explanation and dispensation to change the presentation editorially. The items in question are:What does it mean to 'always interpret [the character sequence] in the same way'? ([fs.enum.path.format], which is [path.format] in the paper)
What is a "directory-like" file type? ([fs.class.directory_iterator])
What is an "implementation-defined file type"? (e.g. [fs.op.copy])
[2017-07 Toronto Wed Issue Prioritization]
Priority 3
[2018-01-16, Jonathan comments]
In 31.12.8.1 [fs.enum.path.format] "always interpreted in the same way" means irrespective of the path::format value, or the content of the string. Maybe add ", rather than depending on the path::format value or the content of the character sequence".
In [fs.class.directory_iterator] an "implementation-defined directory-like file type" is an implementation-defined file type (see [fs.enum.file_type] and Table 115) that is treated like a directory by the special rules that the OS has for non-regular files (see 31.12.6.2 [fs.path.generic]). In 31.12.13.4 [fs.op.copy], an "implementation-defined file type" is exactly that, see [fs.enum.file_type] and Table 115 again. I don't see what isn't clear about that. Maybe add a cross-reference to [fs.enum.file_type].Proposed resolution:
Section: 16 [library] Status: New Submitter: Jens Maurer Opened: 2017-03-20 Last modified: 2017-07-15
Priority: 4
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Discussion:
This is from editorial issue #1088:
It is not always made explicit whether the requirement is referring to time or space complexity, or both."Linear time." vs. "Linear."
"Constant time." vs. "Constant."
16.3.2.4 [structure.specifications] says that the Complexity element specifies "the time and/or space complexity of the function", so being explicit about this would be good.
Examples:[sequence.reqmts] a.clear()
[deque.cons]
[vector.cons]
[map.cons]
[multimap.cons]
[set.cons]
[multiset.cons]
[unord.map.cnstr]
[unord.multimap.cnstr]
etc.
[2017-07 Toronto Wed Issue Prioritization]
Priority 4; Robert to look at
Proposed resolution:
Section: 23.2.4.4 [char.traits.specializations.char16.t] Status: New Submitter: Jonathan Wakely Opened: 2017-05-05 Last modified: 2019-04-02
Priority: 3
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Discussion:
The standard requires that char_traits<char16_t>::int_type is uint_least16_t, so when that has the same representation as char16_t there are no bits left to represent the eof value.
23.2.4.4 [char.traits.specializations.char16.t] says:— The member eof() shall return an implementation-defined constant that cannot appear as a valid UTF-16 code unit.
Existing practice is to use the "noncharacter" u'\uffff' for this value, but the Unicode spec is clear that U+FFFF and other noncharacters are valid, and their appearance in a UTF-16 string does not make it ill-formed. See here and here:
The fact that they are called "noncharacters" and are not intended for open interchange does not mean that they are somehow illegal or invalid code points which make strings containing them invalid.
In practice this means there's no way to tell if basic_streambuf<char16_t>::sputc(u'\uffff') succeeded or not. If it can insert the character it returns to_int_type(u'\uffff') and otherwise it returns eof(), which is the same value.
I believe that char_traits<char16_t>::to_int_type(char_type c) can be defined to transform U+FFFF into U+FFFD, so that the invariant eq_int_type(eof(), to_int_type(c)) == false holds for any c (and the return value of sputc will be distinct from eof). I don't think any implementation currently meets that invariant. I think at the very least we need to correct the statement "The member eof() shall return an implementation-defined constant that cannot appear as a valid UTF-16 code unit", because there are no such constants if sizeof(uint_least16_t) == sizeof(char16_t). This issue is closely related to LWG 1200, but there it's a slightly different statement of the problem, and neither the submitter's recommendation nor the proposed resolution solves this issue here. It seems that was closed as NAD before the Unicode corrigendum existed, so at the time our standard just gave "surprising results" but wasn't strictly wrong. Now it makes a normative statement that conflicts with Unicode.[2017-07 Toronto Wed Issue Prioritization]
Priority 3
Proposed resolution:
Section: 25.3.5.3 [input.iterators] Status: Open Submitter: Gašper Ažman Opened: 2017-05-10 Last modified: 2022-04-25
Priority: 2
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Discussion:
In Table 95 in 25.3.5.3 [input.iterators], it is specified that the expression *a returns reference, which must be convertible to value_type. This is not true for move-only types, which incidentally means that std::vector<std::unique_ptr<int>> does not possess even a lowly InputIterator, which is, of course, absurd.
With the advent of concepts as first-class citizens in the language, getting this right as soon as possible is a priority. This issue seems to be similar to both LWG 448 and LWG 484, but not the same. The proposed resolution stems from two considerations outlined below:Convertibility is too strong for all algorithms
No algorithm in the standard library requires convertibility to value_type. If algorithms require things that smell of that, they specify the assignment or constructibility flavor they need directly. I checked this by going through the specification of each and every one of them in <algorithm> and <numeric>, which highlighted several issues unrelated to this one. These issues are presented in Algorithms with underspecified iterator requirements (LWG 2963). reference needs to be related to value_type Algorithms need this for the following reasons:
lifetime-extension: served as adequately by T const& as by T. Also works for iterators that return by value. T&& also correctly binds to T const&.
passing to predicates: again, served adequately by T const&
writing to *result: not provided by the requirement anyway.
capture-by-copy: currently implicitly guaranteed, but unused in the standard library (always specified separately). A separate specification can always be made for algorithms that need to capture-by-copy.
We must give due consideration to code that so far required its inputs to be CopyConstructible implicitly by requiring convertibility to T. This is done in the issue LWG 2963, which presents the results of a comb-through of <algorithm> and <numeric> to find algorithms that have this requirement, but where it is not specified. While related issues have been identified, no algorithms seems to require more than T const& convertibility without separately requiring convertibility to T.
Since such code is already compiling today, relaxing this requirement does not break code. The only code this could possibly break is if, in a concept checking library, the InputIterator concept requirement on reference being convertible to value_type gets relaxed. Such a library, if it offered overloading based on most-specific modeled concept, could now, once fixed, resolve the call to a different algorithm, which could break user code that uses a hypothetical algorithm with a move-only container and was relying to select some other overload for move-only types based on the implicit CopyConstructible assertion provided by the iterator. In our internal concepts-checking library, we have had this issue "fixed" since the very beginning — move-only types were too important for our internal algorithms library, and also no algorithm in it seems to require something like Iterator::value_type x = *first without also requiring copy-constructibility anyway.
[2017-07 Toronto Monday issue prioritization]
Priority 2; also could affect the ranges TS
Previous resolution [SUPERSEDED]:
This wording is relative to N4659.
Change Table 95 — "Input iterator requirements", 25.3.5.3 [input.iterators] as indicated:
Table 107 — Input iterator requirements (in addition to Iterator) Expression Return type Operational
semanticsAssertion/note pre-/post-condition … *a reference,
convertible to T
that binds to const T&[…] … *r++ convertible to T
that binds to const T&{ Tauto&& tmp = *r;
++r;
return tmp; }
[2018-04-20; Eric Niebler provides improved wording]
The revised wording makes it clear that you can only rely on those operational semantics when the value type is constructible from the reference type and is movable. When those conditions aren't met, we can make no guarantees about the operational semantics of *r++ (which is why *r++ is no longer a required expression of the InputIterator concept in the Ranges TS).
Really, no generic code should be doing *r++ on input iterators. Another option would be to simply deprecate this requirement for input iterators, but that might need a paper. (For forward iterators, *r++ is already required to return reference exactly, and the multi-pass guarantee gives it the proper semantics.) I also now have a question about the proposed return type of *a and *r++, which says they must be something that "binds to const T&". Does this mean that an iterator with a reference type reference-to-[const?]-volatile-T is no longer considered an iterator? I don't think that's what we want to say. Perhaps these should read "binds to const volatile T& instead, except that has the problem for InputIterators that return prvalues that a prvalue is not bindable to a volatile reference.[2018-11 San Diego Thursday night issue processing]
Look at Ranges; EricWF to investigate. Status to Open
Previous resolution [SUPERSEDED]:
This wording is relative to N4741.
Change Table 89 — "Input iterator requirements", 25.3.5.3 [input.iterators] as indicated:
Table 89 — Input iterator requirements (in addition to Iterator) Expression Return type Operational
semanticsAssertion/note pre-/post-condition … *a reference,
convertible to T
that binds to const T&[…] … *r++ convertible to T
that binds to const T&When T tmp = *r is well-formed and
T is MoveConstructible, then
{ T tmp = *r;
++r;
return tmp; }
[2022-04-25; Daniel rebases wording on N4910]
Proposed resolution:
This wording is relative to N4910.
Change 25.3.5.3 [input.iterators], Table 83 — "Cpp17InputIterator requirements (in addition to Cpp17Iterator) [tab:inputiterator]" as indicated:
Table 83 — Cpp17InputIterator requirements (in addition to Cpp17Iterator) [tab:inputiterator] Expression Return type Operational
semanticsAssertion/note pre-/post-condition … *a reference,
convertible to T
that binds to const T&[…] … *r++ convertible to T
that binds to const T&When T tmp = *r is well-formed and
T is Cpp17MoveConstructible, then
{ T tmp = *r;
++r;
return tmp; }
Section: 27 [algorithms], 28 [numerics] Status: New Submitter: Gašper Ažman Opened: 2017-05-10 Last modified: 2017-07-12
Priority: 3
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Discussion:
While researching whether the proposed resolution of Iterators of Containers of move-only types do not model InputIterator (LWG 2962), I came across several algorithms that underspecify their requirements, mostly with regard to some associated type of the iterator type they operate on. A list can be found below.
The list of algorithms with underspecified requirements from <algorithm> and <numeric> follows. With the advent of concepts, these algorithms will need better specifications if we are ever hoping to be allowed to overload based on them. I want this issue to bring the standard algorithms closer to having their concept requirements directly transcribable to library annotations. Suggested resolution:copy, copy_if, copy_n, copy_backward
Add to description: *result shall be assignable from *first.
move, move_backward
Add to description: *result shall be move-assignable from *first.
transform
Add to description: The result of the expression op(*first) or binary_op(*first1, *first2) shall be writable to result.
rotate_copy
Add to description: *first shall be writable to result.
merge
Add to description: *first1 and *first2 shall be writable to result..
set_union, set_intersection, set_difference, set_symmetric_difference
Add to description: *first1 and *first2 shall be writable to result.
partial_sum
acc is not defined.
Change description: acc, a variable of InputIterator's value type, shall be constructibleadjacent_difference
acc is not defined.
Change description: acc, a variable of InputIterator's value type, shall be MoveAssignable and shall be constructible from the type of *first.iota
iota is mis-specified. Since the expression we need to support is *first = value: *first is required to be of type InputIterator::reference, and value is an lvalue of type T. The current specification allows calling iota with a const output iterator!
[2017-07 Toronto Monday issue prioritization]
Priority 3; Marshall to work with Gaspar to improve wording.
Proposed resolution:
Section: 27.8.6 [alg.merge] Status: LEWG Submitter: Billy Robert O'Neal III Opened: 2017-06-08 Last modified: 2020-09-06
Priority: Not Prioritized
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Discussion:
At the moment, inplace_merge requires exactly N - 1 comparisons, if enough additional memory is available (and in practice enough additional memory is always available). However, this prohibits implementing the merge operation using forms of binary search, as in Timsort's 'Galloping Mode', a useful optimization for non-uniform input data. It's not really useful to prohibit standard libraries from trying a few extra speculative compares like this, given that users must be prepared for the fallback "not enough memory" 𝒪(N lg N) algorithm.
[2017-07 Toronto Monday issue prioritization]
Status to LEWG
Proposed resolution:
This wording is relative to N4659.
Edit 27.8.6 [alg.merge] as indicated:
template<class BidirectionalIterator> void inplace_merge(BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last); template<class ExecutionPolicy, class BidirectionalIterator> void inplace_merge(ExecutionPolicy&& exec, BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> void inplace_merge(BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last, Compare comp); template<class ExecutionPolicy, class BidirectionalIterator, class Compare> void inplace_merge(ExecutionPolicy&& exec, BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last, Compare comp);[…]
-8- Complexity: Let N = last - first:
(8.1) — For the overloads with no ExecutionPolicy, if enough additional memory is available,
exactlyN - 1 comparisons on average, 𝒪(N) comparisons in the worst case.(8.2) — For the overloads with no ExecutionPolicy if no additional memory is available, 𝒪(N log N) comparisons.
(8.3) — For the overloads with an ExecutionPolicy, 𝒪(N log N) comparisons.
-9- Remarks: Stable (16.4.6.8 [algorithm.stable]).
Section: 30.4.7.3.2 [locale.money.put.virtuals] Status: New Submitter: Jonathan Wakely Opened: 2017-06-21 Last modified: 2017-06-27
Priority: 3
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Discussion:
Whether you get ".99" or "0.99" for the following depends on the implementation:
std::cout.imbue(std::locale("en_US"));
std::cout << std::put_money(99.L);
I don't see any justification in [locale.money.put.virtuals] for the leading 0, although that seems more useful.
If we want the leading zero, we should say so.[2017-06-27, Jonathan comments and provides wording]
I suggest that we require a leading zero. The wording below is similar to how C specifies the %f format specifier for fprintf.
Proposed resolution:
This wording is relative to N4659.
Edit 30.4.7.3.2 [locale.money.put.virtuals] as indicated:
iter_type do_put(iter_type s, bool intl, ios_base& str, char_type fill, long double units) const; iter_type do_put(iter_type s, bool intl, ios_base& str, char_type fill, const string_type& digits) const;[…]
-2- Remarks: The currency symbol is generated if and only if (str.flags() & str.showbase) is nonzero. If the format specifies a decimal point, at least one digit character appears before it. If the number of characters generated for the specified format is less than the value returned by str.width() on entry to the function, then copies of fill are inserted as necessary to pad to the specified width. For the value af equal to (str.flags() & str.adjustfield), if (af == str.internal) is true, the fill characters are placed where none or space appears in the formatting pattern; otherwise if (af == str.left) is true, they are placed after the other characters; otherwise, they are placed before the other characters.
Section: 31.7.8 [ext.manip] Status: New Submitter: Jonathan Wakely Opened: 2017-06-22 Last modified: 2017-06-26
Priority: 3
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Discussion:
[ext.manip] p5 says:
Requires: The type moneyT shall be either long double or a specialization of the basic_string template (Clause 24).
This means that put_money(99), put_money(99.), put_money("99"), and put_money(string_view{"99"}) are all undefined, when in practice they will compile fine and do the right thing, converting the argument to long double or std::string as needed.
We could change it to be "otherwise the program is ill-formed", or to remove the function templates from overload resolution when the argument is not long double or a std::basic_string, but that will unnecessarily break code that works fine today. We should accept types convertible to long double or the relevant money_put facet's string_type (which is not known until we attempt to write the unspecified type to an ostream). The requirement is also insufficient, because cout << put_money(wstring(L"99")) won't compile on any implementation, despite the argument type being a specialization of basic_string. This same problem exists for std::get_money.[2017-06-24, Daniel comments and provides wording]
The wording changes below are supposed to support all moneyT types that are convertible to either long double or to money_put/get<Ch, o/istreambuf_iterator<Ch, Tr>>::string_type (but not to both), where Ch and Tr are determined by the concrete instantiated specialization of the exposition-only function template f that is used to specify the semantics of put_money and get_money, respectively. XOR-ing the requirements outlaws types that are convertible to both, which would cause an ambiguity unless we would provide wording that would introduce an ordered application of these convertibility constraints. This is the rationale for the seemingly odd new Remarks formulation. Note also, that the wording provided below intentionally attempts to distinguish between the statically testable conditions based on the is_convertible_v expressions within the Remarks: element and the well-defined runtime behaviour requirement of the actually provided argument of deduced type moneyT within the pre-existing Requires: element. Another point worth pointing out is that the wording attempts to fix an currently existing ambiguity of the meaning of the type moneyT (and to a lesser extend for charT and traits) as either the template parameter of put/get_money or that of the corresponding template argument of the exposition-only f templates. The revised form makes it clearer that it refers to the latter.
It should be emphasized that this extension of the current wording would provide support for put_money(99), put_money(99.), and put_money("99"), but not yet for put_money(string_view{"99"}), because string_view is not convertible to string. To realize support for the latter, this wording approach could be extended by referring to is_constructible instead of is_convertible, though.Proposed resolution:
This wording is relative to N4659.
Edit 31.7.8 [ext.manip] as indicated:
template <class moneyT> unspecified get_money(moneyT& mon, bool intl = false);-?- For an expression in >> get_money(mon, intl) described below, let Mo, Ch, and Tr be the deduced template argument types of the template parameters moneyT, charT, and traits, respectively, of the instantiated specialization of the template f.
-2- Requires:The type moneyT shall be either long double or a specialization of the basic_string template (Clause 23 [strings])Mo shall be either convertible to long double or shall be convertible to money_get<Ch, istreambuf_iterator<Ch, Tr>>::string_type. -?- Remarks: If is_convertible_v<Mo, long double> == is_convertible_v<Mo, money_get<Ch, istreambuf_iterator<Ch, Tr>>::string_type>, the program is ill-formed. -3- Effects: The expression in >> get_money(mon, intl) described below behaves as a formatted input function (31.7.5.3.1 [istream.formatted.reqmts]). -4- Returns: An object of unspecified type such that if in is an object of type basic_istream<charTCh,traitsTr> then the expression in >> get_money(mon, intl) behaves as if it called f(in, mon, intl), where the function f is defined as:template <class charT, class traits, class moneyT> void f(basic_ios<charT, traits>& str, moneyT& mon, bool intl) { using Iter = istreambuf_iterator<charT, traits>; using MoneyGet = money_get<charT, Iter>; ios_base::iostate err = ios_base::goodbit; const MoneyGet& mg = use_facet<MoneyGet>(str.getloc()); mg.get(Iter(str.rdbuf()), Iter(), intl, str, err, mon); if (ios_base::goodbit != err) str.setstate(err); }The expression in >> get_money(mon, intl) shall have type basic_istream<
charTCh,traitsTr>& and value in.template <class moneyT> unspecified put_money(const moneyT& mon, bool intl = false);-?- For an expression out << put_money(mon, intl) described below, let Mo, Ch, and Tr be the deduced template argument types of the template parameters moneyT, charT, and traits, respectively, of the instantiated specialization of the template f.
-5- Requires:The type moneyT shall be either long double or a specialization of the basic_string template (Clause 23 [strings])Mo shall be either convertible to long double or shall be convertible to money_put<Ch, ostreambuf_iterator<Ch, Tr>>::string_type. -?- Remarks: If is_convertible_v<Mo, long double> == is_convertible_v<Mo, money_put<Ch, ostreambuf_iterator<Ch, Tr>>::string_type>, the program is ill-formed. -6- Returns: An object of unspecified type such that if out is an object of type basic_ostream<charTCh,traitsTr> then the expression out << put_money(mon, intl) behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) that calls f(out, mon, intl), where the function f is defined as:template <class charT, class traits, class moneyT> void f(basic_ios<charT, traits>& str, const moneyT& mon, bool intl) { using Iter = ostreambuf_iterator<charT, traits>; using MoneyPut = money_put<charT, Iter>; const MoneyPut& mp = use_facet<MoneyPut>(str.getloc()); const Iter end = mp.put(Iter(str.rdbuf()), intl, str, str.fill(), mon); if (end.failed()) str.setstate(ios::badbit); }The expression out << put_money(mon, intl) shall have type basic_ostream<
charTCh,traitsTr>& and value out.
Section: 27.7.10 [alg.reverse] Status: LEWG Submitter: Billy O'Neal III Opened: 2017-06-24 Last modified: 2018-04-03
Priority: Not Prioritized
View all other issues in [alg.reverse].
View all issues with LEWG status.
Discussion:
The fine folks on our backend team suggested that we special case std::reverse of 1/2/4/8 to take advantage of vector units. Unfortunately, at present std::reverse says it does N/2 iter_swaps, which doesn't permit our vector implementation even if the iterator inputs are pointers to trivially copyable Ts.
The vectorized version for pointers to shorts is ~8x faster on Skylake than the serial version, and about 7x faster for unsigned long longs; and users don't actually care whether or not we call swap here.[2017-07 Toronto Monday issue prioritization]
Status to LEWG; this is similar to 2973
[2018-04-02, Billy comments]
This issue should be resolved by P0551, because it prohibits user specialization of std::swap and std::iter_swap, which means the proposed vectorization optimization for pointers-to-trivially-copyable is now implementable without changes to reverse's specification (We can detect if the user has provided an alternate swap in their own namespace, but not if they explicitly specialized swap or iter_swap).
Proposed resolution:
This wording is relative to N4659.
Edit 27.7.10 [alg.reverse] as indicated:
template<class BidirectionalIterator> void reverse(BidirectionalIterator first, BidirectionalIterator last); template<class ExecutionPolicy, class BidirectionalIterator> void reverse(ExecutionPolicy&& exec, BidirectionalIterator first, BidirectionalIterator last);-1- Requires: *first shall be swappable (16.4.4.3 [swappable.requirements]).
-2- Effects: For each non-negative integer i < (last - first) / 2, applies iter_swap to all pairs of iterators first + i, (last - i) - 1. If is_trivially_copyable_v<typename iterator_traits<BidirectionalIterator>::value_type> is true, an implementation may permute the elements by making temporary copies, rather than by calling iter_swap. [Note: this allows the implementation to be vectorized. — end note]
Section: 32.12 [re.grammar] Status: New Submitter: Hubert Tong Opened: 2017-06-25 Last modified: 2017-07-12
Priority: 4
View other active issues in [re.grammar].
View all other issues in [re.grammar].
View all issues with New status.
Discussion:
In N4660 subclause 31.13 [re.grammar] paragraph 5:
The productions ClassAtomExClass, ClassAtomCollatingElement and ClassAtomEquivalence provide functionality equivalent to that of the same features in regular expressions in POSIX.
The broadness of the above statement makes it sound like it is merely a statement of intent; however, this appears to be a necessary normative statement insofar as identifying the general semantics to be associated with the syntactic forms identified. In any case, if it is meant for ClassAtomCollatingElement to provide functionality equivalent to a collating symbol in a POSIX bracket expression, multi-character collating elements need to be considered.
In [re.grammar] paragraph 14:The behavior of the internal finite state machine representation when used to match a sequence of characters is as described in ECMA-262. The behavior is modified according to any match_flag_type flags specified when using the regular expression object in one of the regular expression algorithms. The behavior is also localized by interaction with the traits class template parameter as follows: [bullets 14.1 to 14.4]
In none of the bullets does the wording handle multi-character collating elements in a clear manner:
14.1 deals in characters.
14.2 deals in characters (traits_inst.translate accepts only a single character).
14.3 might handle a multi-character collating element; however, there is no specification of how such a collating element is to be identified from the sequence of characters. Additionally, the definition of primary equivalence class specifies that it is a set of characters (not of collating elements).
14.4 deals in characters.
The ECMA-262 specification for ClassRanges also deals in characters.
[2017-07 Toronto Monday issue prioritization]
Priority 4
Proposed resolution:
Section: 32.12 [re.grammar] Status: New Submitter: Hubert Tong Opened: 2017-06-25 Last modified: 2017-07-12
Priority: 3
View other active issues in [re.grammar].
View all other issues in [re.grammar].
View all issues with New status.
Discussion:
For a user to implement a regular expression traits class meaningfully, the relationship between the return value of traits_inst.lookup_collatename to the behaviour of the finite state machine corresponding to a regular expression needs to be better specified.
From N4660 subclause 31.13 [re.grammar], traits_inst.lookup_collatename only feeds clearly into two operations:a test if the returned string is empty ([re.grammar]/8), and
a test if the result of traits_inst.transform_primary, with the returned string, is empty ([re.grammar]/10).
Note: It is unclear if bullet 14.3 in [re.grammar]/14 refers to the result of traits_inst.lookup_collatename when it refers to a "collating element"; and if it does, it is unclear what input is to be used.
It is therefore unclear what the effect is if traits_inst.lookup_collatename substitutes another member of the equivalence class as its output. For example, when processing "[[.AA.]]" as a pattern under a locale da_DK.utf8, what is the expected behaviour difference (if any) should traits_inst.lookup_collatename return, for "AA", "\u00C5" (where U+00C5 is A with ring, which sorts the same as "AA")?[2017-07 Toronto Monday issue prioritization]
Priority 3
Proposed resolution:
Section: 22.5.3 [optional.optional] Status: Open Submitter: Casey Carter Opened: 2017-06-27 Last modified: 2018-01-28
Priority: 3
View all other issues in [optional.optional].
View all issues with Open status.
Discussion:
optional<T>::value_type is T, which can be a cv-qualified object type. This is inconsistent with the uses of the name value_type elsewhere in the standard. We should either require optional<T>::value_type to be remove_cv_t<T> — a true value type — or rename the type alias to element_type.
[2017-07 Toronto Tuesday PM issue prioritization]
Priority 3; may also affect array
[2018-1-26 issues processing telecon]
Status to 'Open'
Proposed resolution:
This wording is relative to N4659.
Edit 22.5.3 [optional.optional], class template optional synopsis, as indicated:
template <class T>
class optional {
public:
using value_type = remove_cv_t<T>;
[…]
};
Section: 22.6.3.2 [variant.ctor] Status: LEWG Submitter: Peter Dimov Opened: 2017-06-27 Last modified: 2017-07-12
Priority: Not Prioritized
View other active issues in [variant.ctor].
View all other issues in [variant.ctor].
View all issues with LEWG status.
Discussion:
The copy constructor of std::variant is not conditionally noexcept (I think it was in the original proposal.)
It should be, for two reasons: first, this would be consistent with the other three constructorsconstexpr variant() noexcept(see below); variant(variant&&) noexcept(see below); template <class T> constexpr variant(T&&) noexcept(see below);
and second, variant itself makes use of is_nothrow_copy_constructible, so it's inconsistent for it to take a stance against it.
[2017-07 Toronto Tuesday PM issue prioritization]
Status to LEWG
Proposed resolution:
This wording is relative to N4659.
Edit 22.6.3 [variant.variant], class template variant synopsis, as indicated:
template <class... Types>
class variant {
public:
// 23.7.3.1, constructors
constexpr variant() noexcept(see below);
variant(const variant&) noexcept(see below);
variant(variant&&) noexcept(see below);
[…]
};
Edit 22.6.3.2 [variant.ctor] as indicated:
variant(const variant& w) noexcept(see below);[…]
-8- Remarks: This function shall not participate in overload resolution unless is_copy_constructible_v<Ti> is true for all i. The expression inside noexcept is equivalent to the logical AND of is_nothrow_copy_constructible_v<Ti> for all i.
Section: 33.10.6 [futures.promise] Status: Open Submitter: Billy O'Neal III Opened: 2017-07-16 Last modified: 2020-09-06
Priority: 2
View other active issues in [futures.promise].
View all other issues in [futures.promise].
View all issues with Open status.
Discussion:
In Toronto Saturday afternoon LWG discussed LWG 2976 which finishes the job of removing allocator support from packaged_task. LWG confirmed that, despite the removal of packaged_task allocators "because it looks like std::function" was incorrect, they wanted to keep the allocator removals anyway, in large part due to this resolution being a response to an NB comment.
If we don't want the type erased allocator situation at all, then we should remove them from the remaining place they exist in <future>, namely, in promise. This change also resolves potential implementation divergence on whether allocator::construct is intended to be used on elements constructed in the shared state, and allows the emplace-construction-in-future paper, P0319, to be implemented without potential problems there.[28-Nov-2017 Mailing list discussion - set priority to P2]
Lots of people on the ML feel strongly about this; the suggestion was made that a paper would be welcomed laying out the rationale for removing allocator support here (and in other places).
[2018-1-26 issues processing telecon]
Status to 'Open'; Billy to write a paper.
[2019-06-03]
Jonathan observes that this resolution conflicts with 2095.
Proposed resolution:
This resolution is relative to N4659.
Edit 33.10.6 [futures.promise], class template promise synopsis, as indicated:
template<class R> class promise { public: promise();[…]template <class Allocator> promise(allocator_arg_t, const Allocator& a);[…] }; template <class R> void swap(promise<R>& x, promise<R>& y) noexcept;template <class R, class Alloc> struct uses_allocator<promise<R>, Alloc>;template <class R, class Alloc> struct uses_allocator<promise<R>, Alloc> : true_type { };
-3- Requires: Alloc shall be an Allocator (16.4.4.6 [allocator.requirements]).promise();template <class Allocator> promise(allocator_arg_t, const Allocator& a);-4- Effects: constructs a promise object and a shared state.
The second constructor uses the allocator a to allocate memory for the shared state.
Section: 19.3 [assertions] Status: Open Submitter: Jonathan Wakely Opened: 2017-08-18 Last modified: 2018-08-20
Priority: 2
View other active issues in [assertions].
View all other issues in [assertions].
View all issues with Open status.
Discussion:
The C standard says that the expression in an assert must have a scalar type, and implies (or at least allows) that the condition is tested by comparison to zero. C++ says that the expression is a constant subexpression if it can be contextually converted to bool. Those ways to test the condition are not equivalent.
It's possible to have expressions that meet the C++ requirements for a constant subexpression, but fail to meet the C requirements, and so don't compile.
#include <stdlib.h>
// A toy implementation of assert:
#define assert(E) (void)(((E) != 0) || (abort(), 0))
struct X {
constexpr explicit operator bool() const { return true; }
};
constexpr bool f(const X& x) {
assert(x);
return true;
}
C++ says that assert(x) is a constant subexpression, but as it doesn't have scalar type it's not even a valid expression.
I think either 19.3.2 [cassert.syn] or 19.3.3 [assertions.assert] should repeat the requirement from C that E has scalar type, either normatively or in a note. We should also consider whether "contextually converted to bool" is the right condition, or if we should use comparison to zero instead.[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 2; status to Open
Jonathan is discussing this with WG14
[2018-08-20, Jonathan comments]
This was reported to WG14 as N2207.
Proposed resolution:
Section: 19.5.3.4 [syserr.errcat.derived] Status: New Submitter: Thomas Köppe Opened: 2017-09-20 Last modified: 2017-11-09
Priority: 3
View all issues with New status.
Discussion:
The presentation of section [syserr.errcat.derived] is currently somewhat problematic:
It is not clear why this section exists and what it is specifying. Presumably, it is the subject of the phrase "in this subclause" of the sibling section [syserr.errcat.overview], but that's confusing (because this would require the interpretation of "this subclause" as the containing superclause). It would be an improvement to say "shall behave as specified in [syserr.errcat.derived]" in the introduction.
The current wording of [syserr.errcat.derived] requires that derived classes keep the name member function pure-virtual, making it impossible to have non-abstract derived classes. This appears to be an editorial error. Surely name should just not be required to be pure-virtual.
There seems to be no requirement concerning the message virtual member function.
We should use override rather than virtual.
I would welcome a short, introductory paragraph in [syserr.errcat.derived] that states (non-redundantly) that the following are requirements on users' derived classes.
Partial wording proposal:
In 19.5.3.1 [syserr.errcat.overview] p1, change:
-1- The class error_category serves as a base class for types used to identify the source and encoding of a particular category of error code. Classes may be derived from error_category to support categories of errors in addition to those defined in this International Standard. Such classes shall behave as specified in
this subclause19.5.3.4 [syserr.errcat.derived]. [Note: error_category objects are passed by reference, and two such objects are equal if they have the same address. This means that applications using custom error_category types should create a single object of each such type. — end note]
In 19.5.3.4 [syserr.errcat.derived], change:
virtualconst char* name() const noexcept override= 0;-1- Returns: A string naming the error category.
virtualerror_condition default_error_condition(int ev) const noexcept override;-2- Returns: An object of type error_condition that corresponds to ev.
virtualbool equivalent(int code, const error_condition& condition) const noexcept override;-3- Returns: true if, for the category of error represented by *this, code is considered equivalent to condition; otherwise, false.
virtualbool equivalent(const error_code& code, int condition) const noexcept override;-4- Returns: true if, for the category of error represented by *this, code is considered equivalent to condition; otherwise, false.
[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 3.
Jonathan to talk to Chris K and Walter about writing a paper describing the use of error_code, error_condition and defining your own.
Proposed resolution:
Section: 16.2.2 [networking.ts::buffer.reqmts.constbuffersequence] Status: New Submitter: Vinnie Falco Opened: 2017-09-20 Last modified: 2020-09-06
Priority: 3
View all issues with New status.
Discussion:
Addresses: networking.ts
The post-condition buffer sequence requirements mandate pointer equivalence. This means that a copies of buffer sequences must point to the same pieces of underlying memory. While this is appropriate for MutableBufferSequence, it is unnecessary for ConstBufferSequence and can actually prevent useful implementation strategies such as the following constant buffer sequence which avoids dynamic allocations:
/// A buffer sequence containing a chunk-encoding header
class chunk_size
{
public:
// Storage for the longest hex string we might need
class value_type
{
friend class chunk_size;
// First byte holds the length
char buf_[1 + 2 * sizeof(std::size_t)];
template<class = void>
void prepare(std::size_t n);
template<class OutIter>
static OutIter to_hex(OutIter last, std::size_t n)
{
if (n == 0)
{
*--last = '0';
return last;
}
while (n)
{
*--last = "0123456789abcdef"[n & 0xf];
n >>= 4;
}
return last;
}
public:
operator boost::asio::const_buffer() const
{
return {
buf_ + sizeof(buf_) - buf_[0],
static_cast(buf_[0])
};
}
};
using const_iterator = value_type const*;
chunk_size(chunk_size const& other) = default;
/** Construct a chunk header
@param n The number of octets in this chunk.
*/
chunk_size(std::size_t n)
{
value_.prepare(n);
}
const_iterator begin() const
{
return &value_;
}
const_iterator end() const
{
return begin() + 1;
}
private:
value_type value_;
};
Proposed resolution:
This wording is relative to N4588.
Modify 16.2.2 [networking.ts::buffer.reqmts.constbuffersequence] Table 13 "ConstBufferSequence requirements" as indicated:
Table 13 — ConstBufferSequence requirements expression return type assertion/note
pre/post-condition[…] X u(x); post:
equal( net::buffer_sequence_begin(x), net::buffer_sequence_end(x), net::buffer_sequence_begin(u), net::buffer_sequence_end(u), [](const typename X::value_type& v1, const typename X::value_type& v2) { const_buffer b1(v1); const_buffer b2(v2);return b1.data() == b2.data() && b1.size() == b2.size()return b1.size() == b2.size() && memcmp(b1.data(), b2.data(), b1.size()) == 0; })
Section: 16.2.4 [networking.ts::buffer.reqmts.dynamicbuffer] Status: New Submitter: Vinnie Falco Opened: 2017-10-16 Last modified: 2020-09-06
Priority: 3
View other active issues in [networking.ts::buffer.reqmts.dynamicbuffer].
View all other issues in [networking.ts::buffer.reqmts.dynamicbuffer].
View all issues with New status.
Discussion:
Addresses: networking.ts
The current wording for the DynamicBuffer prepare member function implies that std::length_error is the only allowable thrown exception. This should be changed to reflect that any exception may be thrown, with std::length_error thrown in particular when size() + n exceeds max_size().
[2017-11-08]
Priority set to 3 after five votes on mailing list
Proposed resolution:
This wording is relative to N4588.
Change 16.2.4 [networking.ts::buffer.reqmts.dynamicbuffer], Table 14 "DynamicBuffer requirements", as indicated:
Table 14 — DynamicBuffer requirements expression return type assertion/note pre/post-condition […] x.prepare(n) X::mutable_buffers_type Returns a mutable buffer sequence u
representing the writable bytes, and where
buffer_size(u) == n . The dynamic buffer
reallocates memory as required. All constant or
mutable buffer sequences previously obtained using
data() or prepare() are invalidated.
Throws: length_error if size() + n
exceeds max_size() or any other exception
if the request cannot otherwise be satisfied.
Section: 27.8.8.3 [pop.heap] Status: Open Submitter: Mathias Stearn Opened: 2017-11-04 Last modified: 2020-09-06
Priority: 3
View all issues with Open status.
Discussion:
The spec for <algorithms> pop_heap includes
-1- Requires: The range [first, last) shall be a valid non-empty heap.
This has the unfortunate consequence that to pop a value and push a new value is substantially less efficient than necessary. The popped value must be extracted by pop_heap (using up to 2 log N compares and swaps), and then, in push_heap, the new value must be inserted (for up to N compares and swaps, but more usually something like log N).
Simply relaxing the requirement to-1- Requires: The range [first, last - 1) shall be a valid heap.
enables use of pop_heap in an integrated push-and-pop operation, with less than half the number of expected compare and swap operations. Furthermore, if, as is often the case, the newly pushed value would have ended up at position first, the push/pop operation could complete in time 𝒪(1), instead of (3 log N).
The effect of the proposed relaxation on existing library implementations would be minimal in the extreme, and on existing user code nil. The base algorithm code remains exactly identical. The only changes needed would be to any instrumentation in a debugging version of the library, which would just need to relax its check, and to test suites that should exercise the newly tolerated input. Users today are tempted to get the improved performance by relying on existing implementations' tacit tolerance of input that only satisfies the proposed, relaxed requirements. In fact, the cppreference.com page on pop_heap offers no hint that this usage is not already allowed. This change would bless such reliance as formally permitted. After this change, minor extensions to std::priority_queue would enable it to take advantage of the newly efficient operation, perhaps:void pop_push(const Type&); void pop_push(Type&&); template <class... Args> void pop_emplace(Args&&... args);
These will appear in a formal proposal if the resolution is accepted.
[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 3
[2017-11 Albuquerque Saturday issues processing]
status to Open; Marshall to review
Proposed resolution:
This wording is relative to N4700.
Change 27.8.8.3 [pop.heap] as indicated:
template<class RandomAccessIterator> void pop_heap(RandomAccessIterator first, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void pop_heap(RandomAccessIterator first, RandomAccessIterator last, Compare comp);-1- Requires: The range [first, last - 1) shall be a valid
non-emptyheap. RandomAccessIterator shall satisfy the requirements of ValueSwappable (16.4.4.3 [swappable.requirements]). The type of *first shall satisfy the requirements of MoveConstructible (Table 23) and of MoveAssignable (Table 25).
Section: 16.4.4.6 [allocator.requirements] Status: New Submitter: Jon Cohen Opened: 2017-12-06 Last modified: 2022-04-25
Priority: 3
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with New status.
Discussion:
Table 31 in the C++17 standard specifies X::max_size() (where X is an allocator type) as "The largest value that can meaningfully be passed to X::allocate()". Noticeably missing is the statement "Throws: Nothing".
As an example of why this is an issue, note that vector::max_size() and allocator_traits::max_size() are both marked noexcept. We must then interpret max_size() as being allowed to sometimes call std::terminate, or else {vector, allocator_traits, ...}::max_size() must be allowed to directly calculate numeric_limits<size_type>::max() / sizeof(value_type) instead of querying the allocator, even if Alloc::max_size() exists. This seems like a bug in the wording for the requirements of max_size() in an allocator type. I think an issue should be opened on this subject to add Throws: Nothing or similar to the requirements of max_size() for an allocator. As an example consider writing up a framework to test the exception-safety of types in a given framework, since they were all written in an exception-free environment. One of the types in the framework is an allocator which, in a controlled way, can throw an exception at any point where it is allowed by the standard. It's important that the test framework be as pedantic as possible, so the allocator type throws on max_size(), since it is currently allowed to by the standard. When a reasonable vector implementation (at least those in libstdc++ and msvc) is, for example, asked to construct a vector from an initializer_list, it will call allocator_traits<Alloc>::max_size(), which will terminate the program because the exception thrown in Alloc::max_size() propagated through the noexcept traits function. Although this is conformant behavior, I think it's a bug in the standard that a function as benign as max_size() can terminate the program in this manner, and I think the fix is that a conformant allocator should be required to supply a non-throwing max_size() member function. Daniel: This problem was shortly discussed during review of LWG 2162 (see comment 2012-08-05). At that time the more drastic but also more consistent requirement that an allocator's max_size function shall not throw exceptions has not been added. IMO this position should be reconsidered to follow the spirit of the new issue LWG 3044.[2018-01; Priority set to 3 after mailing list discussion]
[2018-08-21, Jonathan comments and provides wording]
The phrase "the largest value that can meaningfully be passed to X::allocate()" is meaningless. Is it a requirement on the caller, so that larger values must not be passed? Or a hint from the allocator implementor that larger values will produce a bad_alloc exception? Can the return value change dynamically, based on the free memory available to the allocator?! — LWG 197 says it can't change.
As noted in the LWG 2162 comments, we don't currently guarantee it can be called on a const object (so allocator_traits will not use the allocator's max_size() if it's non-const, although that was unclear before DR 2284). In addition to adding "Throws: nothing" we should ensure it's callable on const lvalues, and clarify what "meaningfully" means and who is supposed to care about it. My proposed resolution doesn't achieve all of this, but is a start.Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Change 16.4.4.6 [allocator.requirements], Table 32 — "Descriptive variable definitions", as indicated:
Table 32 — Descriptive variable definitions Variable Definition T, U, C any cv-unqualified object type (3.9) … a, a1, a2 lvalues of type X a3 an lvalue of type const X … Change 16.4.4.6 [allocator.requirements], Table 33 — "Cpp17Allocator requirements", as indicated:
Table 33 — Cpp17Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault … a3.max_size() X::size_type the largest value that can
meaningfully be passed to
X::allocate().
[Note: Larger values might cause
an exception to be thrown. — end note]
Throws: Nothing.numeric_limits<size_type>::max()
/ sizeof(value_type)…
[2022-04-25; Daniel rebases wording on N4910]
Proposed resolution:
This wording is relative to N4910.
Change 16.4.4.6.1 [allocator.requirements.general] as indicated:
-2- In subclause 16.4.4.6 [allocator.requirements],
(2.1) — […]
[…]
(2.6) — a, a1, a2 denote lvalues of type X,
(?.?) — a3 denotes an lvalue of type const X,
[…]
[…]
a3.max_size()-50- Result: X::size_type
-51- Returns: The largest value that can meaningfully be passed to X::allocate(). [Note: Larger values might cause an exception to be thrown. — end note] -?- Throws: Nothing. -52- Remarks: Default: numeric_limits<size_type>::max() / sizeof(value_type)
Section: 22.10.6 [refwrap] Status: New Submitter: Jonathan Wakely Opened: 2017-12-14 Last modified: 2020-09-06
Priority: 3
View all other issues in [refwrap].
View all issues with New status.
Discussion:
[refwrap] says that reference_wrapper<T> is a "wrapper around a reference to an object or function of type T" but this doesn't actually constrain it, and doesn't forbid non-referenceable function types like int() const.
There is no way to construct a reference_wrapper<int() const> but implementations are required to provide partial specializations for functions with cv-qualifiers and ref-qualifiers in order to define a nested result_type. It should be undefined to instantiate reference_wrapper<T> with a non-referenceable type, or with a reference type (since references to references are not possible). Making it undefined (rather than ill-formed or unspecified) means implementations are not required to diagnose such invalid specializations, but also don't have to go to the effort of supporting weak result types etc.[2018-01; Priority set to 3 after mailing list discussion]
Previous resolution [SUPERSEDED]:
This wording is relative to N4713.
Modify 22.10.6 [refwrap] as indicated:
-1- reference_wrapper<T> is a CopyConstructible and CopyAssignable wrapper around a reference to an object or function of type T. T shall be a referenceable type (3.45 [defns.referenceable]) that is not a reference type.
-2- reference_wrapper<T> shall be a trivially copyable type (6.8 [basic.types]).
[2019-03-15; Daniel comments and provides revised wording]
The current wording is now far behind the working draft and a synchronization is therefore recommended. In particular, with the acceptance of P0357R1, the specification of reference_wrapper has no longer any weak result type. Second, I would like to concur with a remark from Tomasz to change the wording to replace the undefined behavior by an ill-formed program instead, because every attempt to instantiate the definition of reference_wrapper will instantiate its member declarations, and this would cause the program to become ill-formed anyway because of the illegal formation of references to non-referenceable function types for member functions such as T& get() const noexcept.
As concrete wording suggestion I would recommend wording that ensures that an ill-formed program is only required when a specialization of reference_wrapper is instantiated, because in the absence of a constrained template parameter we shouldn't require implementations to diagnose even forming the name of a reference_wrapper specialization such as in the following example:using X = reference_wrapper<int() const>;
The wording below does not take advantage of a Mandates: element to prevent a dependency on LWG 3193 and because such an element is rarely used to specify class templates. If the committee wishes to use such an element, the equivalent wording would be:
Mandates: reference_wrapper is instantiated with a referenceable type (3.45 [defns.referenceable]) as the argument for the template parameter T.
Proposed resolution:
This wording is relative to N4800.
Modify 22.10.6 [refwrap] as indicated:
-1- reference_wrapper<T> is a Cpp17CopyConstructible and Cpp17CopyAssignable wrapper around a reference to an object or function of type T. If reference_wrapper is instantiated with a non-referenceable type (3.45 [defns.referenceable]) as the argument for the template parameter T, the program is ill-formed.
-2- reference_wrapper<T> is a trivially copyable type (6.8 [basic.types]). -3- The template parameter T of reference_wrapper may be an incomplete type.
Section: 33.5.8.3 [atomics.types.int], 33.5.8.5 [atomics.types.pointer], 33.5.8.6 [atomics.types.memop] Status: New Submitter: Tim Song Opened: 2017-12-15 Last modified: 2020-09-06
Priority: 3
View all issues with New status.
Discussion:
Given atomic<int> meow{INT_MAX};, meow.fetch_add(1) has well-defined behavior because 33.5.8.3 [atomics.types.int] p7 says that
but meow += 1 and ++meow have undefined behavior, because these operator functions are defined (by, respectively, 33.5.8.3 [atomics.types.int] p8 and 33.5.8.6 [atomics.types.memop]) to be equivalent to return fetch_add(1) + 1;, and so the addition of 1 to the result of fetch_add — which causes an integer overflow in this case — occurs outside the protection of fetch_add magic. Additionally, the return value might differ from what fetch_add actually wrote since that addition isn't required to use two's complement. This seems like a trap for the unwary. Is it intended? A similar issue affects the atomic<T*> partial specialization for pointers.Remarks: For signed integer types, arithmetic is defined to use two's complement representation. There are no undefined results.
[2018-01; Priority set to 3 after mailing list discussion]
[2019-04-15; JF Bastien comments and provides wording]
As discussed by LWG during the San Diego 2018 meeting, Jens removed LWG 3047 from "P1236R1: Alternative Wording for P 0907R4 Signed Integers are Two's Complement".
Proposed resolution:
This wording is relative to N4810.
Modify 33.5.7.3 [atomics.ref.int] as indicated:
integral operator op=(integral operand) const noexcept;-7- Effects: Equivalent to: return static_cast<integral>(static_cast<make_unsigned_t<integral>>(fetch_key(operand)) op static_cast<make_unsigned_t<integral>>(operand));
Modify 33.5.7.6 [atomics.ref.memop] as indicated:
T* operator++() const noexcept;-3- Effects: Equivalent to: return static_cast<T>(static_cast<make_unsigned_t<T>>(fetch_add(1)) + static_cast<make_unsigned_t<T>>(1));
T* operator--(int) const noexcept;-4- Effects: Equivalent to: return static_cast<T>(static_cast<make_unsigned_t<T>>(fetch_sub(1)) - static_cast<make_unsigned_t<T>>(1));
Modify 33.5.8.3 [atomics.types.int] as indicated:
T operator op=(T operand) volatile noexcept; T operator op=(T operand) noexcept;-8- Effects: Equivalent to: return static_cast<T>(static_cast<make_unsigned_t<T>>(fetch_key(operand)) op static_cast<make_unsigned_t<T>>(operand));
[Drafting note: atomic<integral>'s working for operator++/operator-- is shared with atomic<T*>. — end drafting note]
[Drafting note: atomic<floating-point> seems to be correct, LWG should confirm that it is. — end drafting note]
Modify 33.5.8.5 [atomics.types.pointer] as indicated:
T* operator op=(ptrdiff_t operand) volatile noexcept; T* operator op=(ptrdiff_t operand) noexcept;-8- Effects: Equivalent to: return reinterpret_cast<T*>(reinterpret_cast<ptrdiff_t>(fetch_key(operand)) op operand);
Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.
Modify 33.5.8.6 [atomics.types.memop] as indicated:
T operator++() volatile noexcept; T operator++() noexcept;-3- Effects: Equivalent to: return static_cast<T>(static_cast<make_unsigned_t<T>>(fetch_add(1)) + static_cast<make_unsigned_t<T>>(1));
T operator--() volatile noexcept; T operator--() noexcept;-4- Effects: Equivalent to: return static_cast<T>(static_cast<make_unsigned_t<T>>(fetch_sub(1)) - static_cast<make_unsigned_t<T>>(1));
[Drafting note: Alternatively, LWG may want to separate the integral overload of operator++/operator-- from that of atomic<T*>. end drafting note]
Section: 27.2 [algorithms.requirements] Status: Open Submitter: Jared Hoberock Opened: 2017-12-04 Last modified: 2022-04-25
Priority: 3
View all other issues in [algorithms.requirements].
View all issues with Open status.
Discussion:
When designing the parallel algorithms library, we intended for parallel algorithms to copy their function objects parameters when it is possible and useful to do so, but there doesn't appear to be any wording to enable that latitude. To the contrary, algorithm specifications refer to their function object parameters by name, implying that a copy of the parameter may not be used as a substitute.
This was noticed when Billy O'Neal observed that parallel generate() did not share parallel for_each() and for_each_n()'s special requirement for a CopyConstructible user-provided function object. This CopyConstructible Function requirement was added to relax legacy for_each()'s MoveConstructible Function requirement to allow parallel implementations to make copies as necessary. All parallel algorithms need similar permissions, but a strong requirement for CopyConstructible in all algorithms is too restrictive. What we require is to allow algorithm implementations to use copies of function objects as substitutes for their original parameters, while not requiring that all function object parameters be copyable. Casey Carter noted that 27.2 [algorithms.requirements] p8 grants permission to all algorithms to copy their function object parameters. However, this paragraph is not normative and does not indicate how the algorithm is allowed to use such copies. Additionally, it does not specify which algorithm parameters are the ones called out as function objects. For example, 27.7.7 [alg.generate] refers to gen as a function object, but 27.6.5 [alg.foreach] does not refer to f as a function object. All the other types of callable algorithm parameters (i.e. Predicate, BinaryPredicate, Compare, UnaryOperation, BinaryOperation, BinaryOperation1, and BinaryOperation2) are defined to be function objects in 27.2 [algorithms.requirements] and 27.3.2 [algorithms.parallel.user]. This list intentionally omits Function and Generator by design. A potential resolution would introduce normative wording to explicitly allow algorithms to use copies of function object parameters as substitutes for their function object parameters, and remove ambiguity in algorithm specifications about which parameters are function objects.[2018-01; Priority set to 3 after mailing list discussion]
[2018-3-14 Wednesday evening issues processing; move to Open]
We thought that the notes in [alg.foreach]/1 and /11 should be unwrapped as well. Bryce to work with Jared on updated wording.
Previous resolution [SUPERSEDED]:
This wording is relative to N4713.
Modify 27.2 [algorithms.requirements] as indicated:
-8-
[Note:Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely. When an algorithm's specification requires the invocation of a function object parameter, such a copy may be invoked as a substitute for the original function object parameter. [Note: This implies that copyable user-supplied function objects should not rely on their identity. Programmers for whom object identity is important should consider using a wrapper class that points to a noncopied implementation object such as reference_wrapper<T> (22.10.6 [refwrap]), or some equivalent solution. — end note]Modify 27.6.5 [alg.foreach] as indicated:
template<class InputIterator, class Function> constexpr Function for_each(InputIterator first, InputIterator last, Function f);[…]
-2- Effects: Applies the function object f to the result of dereferencing every iterator in the range [first, last), […] […]template<class ExecutionPolicy, class ForwardIterator, class Function> void for_each(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last, Function f);-7- Effects: Applies the function object f to the result of dereferencing every iterator in the range [first, last). […] […]
-6- Requires: Function shall meet the requirements of CopyConstructible.template<class InputIterator, class Size, class Function> constexpr InputIterator for_each_n(InputIterator first, Size n, Function f);[…]
-13- Effects: Applies the function object f to the result of dereferencing every iterator in the range [first, first + n) in order. […] […]template<class ExecutionPolicy, class ForwardIterator, class Size, class Function> ForwardIterator for_each_n(ExecutionPolicy&& exec, ForwardIterator first, Size n, Function f);[…] -18- Effects: Applies the function object f to the result of dereferencing every iterator in the range [first, first + n). […] […]
-16- Requires: Function shall meet the requirements of CopyConstructible.
[2022-04-25; Daniel rebases wording on N4910]
The previously refactored note term "can" in 27.2 [algorithms.requirements] p10 has been reverted to "permitted" to specify a normative implementation freedom.
[2022-04-25; Daniel comments]
Bryce and Jared have unassigned from this issue.
Proposed resolution:
This wording is relative to N4910.
Modify 27.2 [algorithms.requirements] as indicated:
-10-
[Note 2:Unless otherwise specified, algorithms that take function objects as argumentscanare permitted to copy those function objects freely. When an algorithm's specification requires the invocation of a function object parameter, such a copy may be invoked as a substitute for the original function object parameter. [Note: This implies that copyable user-supplied function objects should not rely on their identity. If object identity is important, a wrapper class that points to a noncopied implementation object such as reference_wrapper<T> (22.10.6 [refwrap]), or some equivalent solution, can be used. — end note]
Modify 27.6.5 [alg.foreach] as indicated:
template<class InputIterator, class Function> constexpr Function for_each(InputIterator first, InputIterator last, Function f);[…]
-2- Effects: Applies the function object f to the result of dereferencing every iterator in the range [first, last), […] […]template<class ExecutionPolicy, class ForwardIterator, class Function> void for_each(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last, Function f);-7- Effects: Applies the function object f to the result of dereferencing every iterator in the range [first, last). […] […]
-6- Preconditions: Function meets the Cpp17CopyConstructible requirements.template<class InputIterator, class Size, class Function> constexpr InputIterator for_each_n(InputIterator first, Size n, Function f);[…]
-18- Effects: Applies the function object f to the result of dereferencing every iterator in the range [first, first + n) in order. […] […]template<class ExecutionPolicy, class ForwardIterator, class Size, class Function> ForwardIterator for_each_n(ExecutionPolicy&& exec, ForwardIterator first, Size n, Function f);[…]
-22- Preconditions: n >= 0 is true.Function meets the Cpp17CopyConstructible requirements.-23- Effects: Applies the function object f to the result of dereferencing every iterator in the range [first, first + n). […] […]
Section: 19.5.4.1 [syserr.errcode.overview] Status: New Submitter: Antony Polukhin Opened: 2018-01-24 Last modified: 2020-09-06
Priority: 3
View all other issues in [syserr.errcode.overview].
View all issues with New status.
Discussion:
Constructor error_code(int val, const error_category& cat) and member function void assign(int val, const error_category& cat) could be misused if a custom error_category is provided:
error_code ec{1, test_category{}}; // ec holds a pointer/reference to a temporary
[2018-06-18 after reflector discussion]
Priority set to 3
Proposed resolution:
This wording is relative to N4713.
Modify 19.5.4.1 [syserr.errcode.overview] as indicated:
namespace std {
class error_code {
public:
// 19.5.4.2 [syserr.errcode.constructors], constructors
[…]
error_code(int val, const error_category& cat) noexcept;
error_code(int val, const error_category&& cat) = delete;
[…]
// 19.5.4.3 [syserr.errcode.modifiers], modifiers
void assign(int val, const error_category& cat) noexcept;
void assign(int val, const error_category&& cat) = delete;
[…]
};
[…]
}
Section: 31.12.13.5 [fs.op.copy.file] Status: New Submitter: Davis Herring Opened: 2018-01-26 Last modified: 2020-09-06
Priority: 3
View all other issues in [fs.op.copy.file].
View all issues with New status.
Discussion:
(To resolve C++17 CD comment Late 25.) It is not stated which attributes are copied by copy_file().
[2018-1-26 issues processing telecon]
Priority 3
Proposed resolution:
This wording is relative to N4713.
Modify 31.12.13.5 [fs.op.copy.file] as indicated:
Rationale:
The attributes specified are the useful subset of the attributes that can be queried in C++17. Existing practice is complicated: POSIX "cp -p" attempts to preserve user/group IDs, for instance, but cannot in general do so, and setuid/setgid permissions may be stripped.
bool copy_file(const path& from, const path& to, copy_options options); bool copy_file(const path& from, const path& to, copy_options options, error_code& ec) noexcept;[…]
[…]
[…]
(4.2) — Otherwise, copy the contents, permissions, and data modification time
and attributesof the file from resolves to, to the file to resolves to, if:[…]
(4.2.3) — (options & copy_options::update_existing) != copy_options::none and from is more recent than to, determined as if by use of the last_write_time function (31.12.13.26 [fs.op.last.write.time]).
Other implementation-defined attributes may be copied. Failure (or partial failure) to copy attributes is not an error.
[…]
Section: 31.12.13.4 [fs.op.copy] Status: Open Submitter: Davis Herring Opened: 2018-01-29 Last modified: 2020-09-06
Priority: 2
View all other issues in [fs.op.copy].
View all issues with Open status.
Discussion:
(The resolution of #3 resolves part of C++17 NB comment US 36.)
The handling of several options for filesystem::copy() is wrong:Single-level directory copying is silently suppressed by any flag other than copy_options::recursive (even copy_options::directories_only). Single-level directory copying operates via using some unspecified flag to trigger this misfeature.
copy_options::create_symlinks and copy_options::skip_symlinks affect the interpretation of the destination name; the latter shouldn't ever, and the former should affect only broken symlinks (since it would want to replace them).
The copy_options groups for existing target files and the form of copying are consulted only for creating regular files.
copy("file", "dir") creates dir/file, but copy("symlink", "dir", copy_options::copy_symlinks) fails.
If a symlink is encountered with copy_options::copy_symlinks and copy_options::create_symlinks, the latter flag is ignored (but its otherwise sensible restriction to absolute paths applies) rather than the former.
copy_options::create_symlinks without copy_options::copy_symlinks fails if it encounters a symlink. (This is particularly damaging for recursive operation.)
This issue, since it replaces so much text, also addresses two error-handling concerns in passing:
The significance of equivalent(from, to) failing is unspecified. (Ignoring such failures entirely would make dangerous those operations that replace the target with a link.)
Copying a directory involves several operations. When an error_code is being used, the process continues past errors and (because successful functions call ec.clear()) may suppress them.
This expands on the resolution for LWG 2681.
This also addresses the same issue as LWG 2682, but has a different result (based on the fact that the Example successfully copies directories to new, non-existent names).[2018-06; Rapperswil Wednesday evening, discussing LWG 2682]
JW: can we use the words we are shipping already since two years?
BO: what we got is better than what we had before
no objection to moving to Ready
ACTION move to Ready
ACTION link LWG 2682 and LWG 3057 and set a priority 2 and look at 3057 in San Diego
[2018-11 San Diego Thursday night issue processing]
Need to gather implementation experience; revisit in Kona. Status to Open.
[2018-11-13; Billy O'Neal comments]
I (Billy O'Neal) prefer Davis' solution to LWG 3057, as I think the wording follows the meaning of the individual enum values more closely, and enables more scenarios to function correctly instead of reporting such cases as errors.
However, I don't want to adopt that wording as is because it requires my implementation to detect errors in places that force us to do a bunch of extra system calls, and I don't believe those specific ways error handling happens is relevant to what the copy API wants to do. Ideally, the wording would be structured such that it said "here's a list of error conditions, if they happen we aren't going to tell you when exactly they are detected" and then listed the behavior irrespective of when errors happen. That way implementations can do the error checks when it makes sense according to what their system APIs report. For example, anything that requires symlink resolution is very expensive on my platform so I'd want to be able to defer anything related to status (rather than symlink_status) to after I've detected that there's actually a symlink (or junction) involved.Proposed resolution:
This wording is relative to N4750.
Modify Table 115 — "Enum class copy_options" as indicated:
Option group controlling copy and copy_file function effects for existing target files Constant Meaning […] […]
Modify 31.12.13.4 [fs.op.copy] as indicated:
Rationale:
POSIX.1-2008 allows the implementation to create hard links "to" symbolic links, and provides linkat() to choose between the symlink and its target. 31.12.13.4 [fs.op.copy]/4.9 is redundant given 31.12.5 [fs.err.report]/3.1.
void copy(const path& from, const path& to, copy_options options); void copy(const path& from, const path& to, copy_options options, error_code& ec) noexcept;-3- Requires: At most one element from each option group (31.12.8.3 [fs.enum.copy.opts]) is set in options.
-4- Effects:Before the first use of f and t:If each is needed below,
(4.1) — If […]
[…]
(4.10) — Otherwise, no effects.auto linkf = (options & (copy_options::copy_symlinks | copy_options::skip_symlinks)) != copy_options::none; auto f = linkf ? symlink_status(from) : status(from), t = status(to); auto to2 = !is_directory(f) && is_directory(t) ? to/from.filename() : to. bool linkt = (options & (copy_options::create_symlinks | copy_options::create_hard_links)) != copy_options::none || is_symlink(f); auto t2 = linkt ? symlink_status(to2) : status(to2);Effects are then as follows:[Drafting note: copy_options::create_symlinks is intentionally omitted for linkf; it may simply have been a typo for copy_options::copy_symlinks (which was added by LWG 2681) since at least N3940.]
-5- Throws: As specified in 31.12.5 [fs.err.report]. -6- Remarks: For the signature with argument ec, any library functions called by the implementation shall have an error_code argument if applicable. If any such function fails, copy returns immediately without (further) modifying ec.
(?.?) — If f.type() or t.type() is an implementation-defined file type [fs.enum.file_type], then the effects are implementation-defined.
[Drafting note: the text between the previous drafting note and this one is the only unchanged text under /4.]
(?.?) — Otherwise, if exists(f) is false, report an error as specified in 31.12.5 [fs.err.report].
(?.?) — Otherwise, do nothing if
(?.?.?) — (options & copy_options::directories_only) != copy_options::none and is_directory(f) is false, or
(?.?.?) — (options & copy_options::skip_symlinks) != copy_options::none and is_symlink(f) is true, or
(?.?.?) — (options & copy_options::skip_existing) != copy_options::none and exists(t2) is true.
(?.?) — Otherwise, report an error as specified in 31.12.5 [fs.err.report] if:
(?.?.?) — is_other(f) || is_other(t2) is true, or
(?.?.?) — exists(t2) && exists(from) == exists(to2) && equivalent(from, to) is true.
(?.?) — Otherwise, if is_directory(f) is true, then:
(?.?.?) — create_directory(to, from).
(?.?.?) — If (options & copy_options::recursive) != copy_options::none or if (options & copy_options::directories_only) == copy_options::none, iterate over the files in from, as if by
for (const directory_entry& x : directory_iterator(from)) if ((options & copy_options::recursive) != copy_options::none || !is_directory(linkf ? symlink_status(x.path()) : status(x.path()))) copy(x.path(), to/x.path().filename(), options);(?.?) — Otherwise, do nothing if (options & copy_options::update_existing) != copy_options::none, exists(to2) is true, and from is not more recent than to2, determined as if by use of the last_write_time function ( [fs.op.last_write_time]).
(?.?) — Otherwise, report an error as specified in 31.12.5 [fs.err.report] if:
(?.?.?) — is_directory(t2) is true, or
(?.?.?) — (options & (copy_options::overwrite_existing | copy_options::update_existing)) == copy_options::none and exists(t2) is true.
(?.?) — Otherwise, if linkt is true, then:
(?.?.?) — remove(to2) if an existing to2 would prevent the following link creation.
(?.?.?) — If (options & copy_options::create_symlinks) != copy_options::none, create_symlink(from, to2). [Note: If from is a symbolic link, it is not followed. — end note]
(?.?.?) — Otherwise, if (options & copy_options::create_hard_links) != copy_options::none, then create a hard link to from, if linkf is true, or else to the file that from resolves to. [Note: Not all file systems that support hard links and symbolic links support creating hard links to symbolic links. — end note]
(?.?.?) — Otherwise, copy_symlink(from, to2).
(?.?) — Otherwise, copy_file(from, to2, options).
Section: 24.2 [container.requirements] Status: New Submitter: Richard Smith Opened: 2018-02-05 Last modified: 2018-02-14
Priority: 3
View all other issues in [container.requirements].
View all issues with New status.
Discussion:
What are the requirements for
a = b;
... where a and b are of map-like associative container type (map, multimap, unordered_map, unordered_multimap)?
The general container requirements say just:r = a // Postconditions: r == a
(Incidentally, earlier in the table, there is a clear error: the general container requirements permit "a = rv" for assignment from an rvalue, but "a" here is a potentially-const container. Oops.) Oddly. there are no requirements at all on T here.
The allocator-aware container requirements add:a = t // Requires: T is CopyInsertable into X and CopyAssignable.
... where T is the container's value_type, that is, pair<const key_type, mapped_type>. Note that such a pair is not CopyAssignable for "normal" key types that disallow assignment to const objects. They also add:
a = rv // Requires: if !POCMA, T is MoveInsertable into X and MoveAssignable.
... which has the same problem in the !POCMA case.
The associative container requirements and unordered associative container requirements have a similar problem for assignment from an initializer list:a = il // Requires: value_type is CopyInsertable into X and CopyAssignable.
Presumably these assignments are intended to actually work, but what are the intended constraints? Do we wish to allow implementations to perform node reuse for these map-like containers? Presumably yes, and if so, the key_type portion of the node must be assigned as well as the value_type portion (for instance, with whatever implementation technique is used to power node_handle) as we cannot assume that key equivalence (or, for unordered_*map, even key equality) implies substitutability.
I think, then, that the associative container requirements and unordered associative container requirements should specify different requirements for the "a = t", "a = rv", and "a = il" for the map-like containers; specifically:for "a = t" and "a = il", we should require that value_type is CopyInsertable into X, and key_type and mapped_type are CopyAssignable
for "a = rv", if !POCMA, we should require that value_type is MoveInsertable into X and key_type and mapped_type are MoveAssignable
(And we should fix the general container requirements to constrain "r = rv", not "a = rv".)
Daniel: The "a = rv" problematic is already handled by LWG 3028.[2018-02-13, Priority set to 3 after mailing list discussion]
Proposed resolution:
Section: 27.10.8 [exclusive.scan], 27.10.9 [inclusive.scan], 27.10.10 [transform.exclusive.scan], 27.10.11 [transform.inclusive.scan] Status: New Submitter: Billy O'Neal III Opened: 2018-02-06 Last modified: 2019-01-20
Priority: 2
View all other issues in [exclusive.scan].
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Discussion:
All of the scan algorithms ([exclusive.scan], [inclusive.scan], [transform.exclusive.scan], [transform.inclusive.scan]) have language like "If init is provided, T shall be MoveConstructible (Table 23); otherwise, ForwardIterator1's value type shall be MoveConstructible.". However, the algorithms operational semantics require that that type be written "by copy" to the destination range, making support for move only types impossible.
We probably need to examine real implementations of these things and see what requirements are actually necessary, as in general GENERALIZED_SUM and GENERALIZED_NONCOMMUTATIVE_SUM need to specify the type used to store intermediate calculations.[2019-01-20 Reflector prioritization]
Set Priority to 2
Proposed resolution:
Section: 27.11 [specialized.algorithms] Status: New Submitter: Alisdair Meredith Opened: 2018-02-12 Last modified: 2020-09-06
Priority: 3
View other active issues in [specialized.algorithms].
View all other issues in [specialized.algorithms].
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Discussion:
The parallel forms of the uninitialized memory algorithms in <memory> are underspecified in two ways. First, they missed the change that all parallel algorithms require at least Forward Iterators, even for input ranges. See P0467R2 for more details.
The second problem is that they do not have a separate specification to the serial forms. This is a problem in two ways. First, there is no more blanket wording saying a parallel policy algorithm has the same semantics as the serial form unless otherwise specified, so in principle these algorithms are totally unspecified. However, assuming that intent, all of the existing specifications use an as-if formulation with code that is explicitly serial in nature, so need a new specification that talks about the effects on each element without including the iteration order.[2018-02-20, Priority set to 3 after mailing list discussion]
Proposed resolution:
Section: 27.11 [specialized.algorithms] Status: New Submitter: Alisdair Meredith Opened: 2018-02-12 Last modified: 2020-05-02
Priority: 3
View other active issues in [specialized.algorithms].
View all other issues in [specialized.algorithms].
View all issues with New status.
Discussion:
A typical specification of the algorithms for initializing raw memory in <memory> looks like:
Effects: Equivalent to:
for (; first != last; ++first) ::new (static_cast<void*>(addressof(*first))) typename iterator_traits<ForwardIterator>::value_type;
However, this hides a nasty question:
How do we bind a reference to an uninitialized object when dereferencing our iterator, so that static_cast<void*>(addressof(*first)) does not trigger undefined behavior on the call to *first? When pointers are the only iterators we cared about, we could simply cast the iterator value to void* without dereferencing. I don't see how to implement this spec safely without introducing another customization point for iterators that performs the same function as casting a pointer to void* in order to get the address of the element.[2018-02-20, Priority set to 3 after mailing list discussion]
Proposed resolution:
Section: 28.7.6 [sf.cmath] Status: New Submitter: Casey Carter Opened: 2018-02-17 Last modified: 2018-06-19
Priority: 3
View all issues with New status.
Discussion:
28.7.6 [sf.cmath]/1 uses the phrase "report a domain error" (emphasis mine):
If any argument value to any of the functions specified in this subclause is a NaN (Not a Number), the function shall return a NaN but it shall not report a domain error. Otherwise, the function shall report a domain error for just those argument values for which:
the function description's Returns: clause explicitly specifies a domain and those argument values fall outside the specified domain, or
the corresponding mathematical function value has a nonzero imaginary component, or
the corresponding mathematical function is not mathematically defined.
The behavior this phrase is attempting to convey is unclear. A quick search through the text of the standard for "domain error" finds only the domain_error exception type defined in 19.2.4 [domain.error]. Is the intent of "report a domain error" that the special math functions throw an exception of type domain_error, or is it that they behave as specified in C11 7.12.1 "Treatment of error conditions" para 2 which defines the term "domain error"?
[2018-06-18 after reflector discussion]
Priority set to 3
Proposed resolution:
Section: 22.6.3.4 [variant.assign] Status: New Submitter: Antony Polukhin Opened: 2018-02-20 Last modified: 2020-09-06
Priority: 3
View all other issues in [variant.assign].
View all issues with New status.
Discussion:
variant::emplace functions in 22.6.3.5 [variant.mod] destroy the currently contained value before initializing it to a new value. Assignments in 22.6.3.4 [variant.assign] are described in terms on emplace.
This leads to situation, when move/copy assigning subobject from variant into the same variant corrupts data:
#include <variant>
#include <memory>
#include <iostream>
using str_t = std::string;
using str_ptr_t = std::unique_ptr<str_t>;
using var_t = std::variant<str_t, str_ptr_t>;
int main()
{
var_t v = str_ptr_t{
new str_t{"Long string that does not fit into SS buffer and forces dynamic allocation"}
};
// Any of the following lines corrupt the variant's content:
v = *std::get<str_ptr_t>(v);
//v = std::move(*std::get<str_ptr_t>(v));
std::cout << std::get<str_t>(v) << std::endl; // SEGV - 'str_t' inside 'v' is invalid
}
Such behavior confuses users, especially those users who are used to boost::variant's behavior. Consider making variant assignments safer by defining them close to copy-and-swap.
[2018-06-18 after reflector discussion]
Priority set to 3; Antony volunteered to write a paper for Rapperswil.
Proposed resolution:
This wording is relative to N4727.
Change 22.6.3.4 [variant.assign] as indicated:
variant& operator=(const variant& rhs);-1- Let j be rhs.index().
-2- Effects:[…]
(2.1) — If neither *this nor rhs holds a value, there is no effect.
(2.2) — Otherwise, if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
(2.3) — Otherwise, if index() == j, assigns the value contained in rhs to the value contained in *this.
(2.4) — Otherwise, if either is_nothrow_copy_constructible_v<Tj> is true or is_nothrow_move_constructible_v<Tj> is false, equivalent to emplace<j>(get<j>(rhs)).(2.5) — Otherwise, equivalent to emplace<j>(Tj{get<j>(rhs)})
operator=(variant(rhs)).variant& operator=(variant&& rhs) noexcept(see below);-6- Let j be rhs.index().
-7- Effects:[…]
(7.1) — If neither *this nor rhs holds a value, there is no effect.
(7.2) — Otherwise, if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
(7.3) — Otherwise, if index() == j, assigns get<j>(std::move(rhs)) to the value contained in *this.
(7.4) — Otherwise, equivalent to emplace<j>(Tj{get<j>(std::move(rhs))}).
-10- Let Tj be a type that is determined as follows: build an imaginary function FUN(Ti) for each alternative type Ti. The overload FUN(Tj) selected by overload resolution for the expression FUN(std::forward<T>(t)) defines the alternative Tj which is the type of the contained value after assignment.
-11- Effects:[…]
(11.1) — If *this holds a Tj, assigns std::forward<T>(t) to the value contained in *this.
(11.2) — Otherwise, if is_nothrow_constructible_v<Tj, T> || !is_nothrow_move_constructible_v<Tj> is true, equivalent to emplace<j>(std::forward<T>(t)).(11.3) — Otherwise, equivalent to emplace<j>(Tj{std::forward<T>(t)})
operator=(variant(std::forward<T>(t))).
Section: 16.2.4 [networking.ts::buffer.reqmts.dynamicbuffer], 17.6 [networking.ts::buffer.async.read], 17.8 [networking.ts::buffer.async.write], 17.10 [networking.ts::buffer.async.read.until] Status: New Submitter: Christopher Kohlhoff Opened: 2018-02-26 Last modified: 2020-09-06
Priority: 3
View other active issues in [networking.ts::buffer.reqmts.dynamicbuffer].
View all other issues in [networking.ts::buffer.reqmts.dynamicbuffer].
View all issues with New status.
Discussion:
Addresses: networking.tsThe DynamicBuffer overloads of async_read and async_write, and
async_read_until, are underspecified with respect to the lifetime of the dynamic
buffer argument b.
Asio's implementation (and the intended specification) performs DECAY_COPY(b)
in the async_read, async_write, and async_read_until
initiating functions. All operations performed on b are actually performed on that
decay-copy, or on a move-constructed descendant of it. The copy is intended to refer to the same
underlying storage and be otherwise interchangeable with the original in every way.
Most initiating functions' argument lifetimes are covered by [async.reqmts.async.lifetime]. As an rvalue reference it falls under the second bullet, which specifies that the object is copied (but doesn't say decay-copied).
The proposed resolution adds a postcondition for DynamicBuffer move construction, and
specifies that DECAY_COPY(b) be used for each of these functions. The following
two alternative resolutions may also be considered:
Add an extra bullet to [async.reqmts.async.lifetime] to cover rvalue parameters (but specifically exclude CompletionTokens).
Change the DynamicBuffer arguments to be by-value. (And also change the corresponding synchronous operations to be consistent.)
However, the proposed resolution below is presented as a change that minimizes the scope of the impact.
[2018-06-18 after reflector discussion]
Priority set to 3
Proposed resolution:
This wording is relative to N4711.
Edit 16.2.4 [networking.ts::buffer.reqmts.dynamicbuffer] as indicated:
-3- In Table 14,
xdenotes a value of typeX,x1denotes a (possibly const) value of typeX,andmx1denotes an xvalue of typeX,ndenotes a (possibly const) value of typesize_t, andudenotes an identifier.
Table 14 — DynamicBuffer requirements expression type assertion/note pre/post-conditions X u(mx1);post:
u.size()is equal to the prior value ofmx1.size().u.max_size()is equal to the prior value ofmx1.max_size().u.capacity()is equal to the prior value ofmx1.capacity().u.data()satisfies the ConstBufferSequence requirements (16.2.2 [buffer.reqmts.constbuffersequence]) as if copy constructed from the prior value ofmx1.data().- All valid const or mutable buffer sequences that were previously obtained using
mx1.data()ormx1.prepare()remain valid.
Edit 17.6 [networking.ts::buffer.async.read] as indicated:
-11- Let
bdbe the result ofDECAY_COPY(b). Data is placed into the dynamic buffer (16.2.4 [buffer.reqmts.dynamicbuffer]) object. A mutable buffer sequence (16.2.1 [buffer.reqmts.mutablebuffersequence]) is obtained prior to eachbbdread_somecall usingbd.prepare(N), whereNis an unspecified value less than or equal tobd.max_size() - bd.size(). [Note: Implementations are encouraged to usebd.capacity()when determiningN, to minimize the number ofread_somecalls performed on the stream. -- end note] After eachread_somecall, the implementation performsbd.commit(n), wherenis the return value fromread_some.[…]
-13- The synchronous read operation continues until:
bd.size() == bd.max_size(); orthe completion condition returns
0.
Edit 17.8 [networking.ts::buffer.async.write] as indicated:
-11- Let
bdbe the result ofDECAY_COPY(b). Data is written from the dynamic buffer (16.2.4 [buffer.reqmts.dynamicbuffer]) objectbd. A constant buffer sequence (16.2.2 [buffer.reqmts.constbuffersequence]) is obtained usingbd.data(). After the data has been written to the stream, the implementation performsbd.consume(n), wherenis the number of bytes successfully written.[…]
-13- The asynchronous write operation continues until:
bd.size() == 0; orthe completion condition returns
0.
Edit 17.10 [networking.ts::buffer.async.read.until] as indicated:
-3- Effects: Let
bdbe the result ofDECAY_COPY(b). Initiates an asynchronous operation to read data from the buffer-oriented asynchronous read stream (17.1.2 [buffer.stream.reqmts.asyncreadstream]) objectstreamby performing zero or more asynchronous read_some operations on the stream, until the readable bytes of the dynamic buffer (16.2.4 [buffer.reqmts.dynamicbuffer]) objectbdcontain the specified delimiterdelim.-4- Data is placed into the dynamic buffer object
bd. A mutable buffer sequence (16.2.1 [buffer.reqmts.mutablebuffersequence]) is obtained prior to eachasync_read_somecall usingbd.prepare(N), whereNis an unspecified value such thatN <= max_size() - size(). [Note: Implementations are encouraged to usebd.capacity()when determiningN, to minimize the number of asynchronous read_some operations performed on the stream. — end note] After the completion of each asynchronous read_some operation, the implementation performsbd.commit(n), wherenis the value passed to the asynchronous read_some operation's completion handler.-5- The asynchronous read_until operation continues until:
the readable bytes of
bdcontain the delimiterdelim; or
bd.size() == bd.max_size(); oran asynchronous read_some operation fails.
[…]
-8- On completion of the asynchronous operation, if the readable bytes of
bdcontain the delimiter,ecis set such that!ecistrue. Otherwise, ifbd.size() == bd.max_size(),ecis set such thatec == stream_errc::not_found. Ifbd.size() < bd.max_size(),ecis theerror_codefrom the most recent asynchronous read_some operation.nis the number of readable bytes inbdup to and including the delimiter, if present, otherwise0.
Section: 17 [networking.ts::buffer.stream] Status: New Submitter: Christopher Kohlhoff Opened: 2018-02-27 Last modified: 2020-09-06
Priority: 3
View all issues with New status.
Discussion:
Addresses: networking.tsSuppose that we have a user-defined class dyn_buf that satisfies the DynamicBuffer
requirements ([buffer.reqmts.dynamicbuffer]), and is additionally copy-constructible. The following snippet compiles, as expected:
dyn_buf b; net::read_until(my_socket, b, "\n");
However, this next snippet will not compile, when it should:
dyn_buf b; net::read(my_socket, b);
This is due to:
DynamicBuffer template parameter being deduced as dyn_buf&, andread overload being specified to not participate in overload resolution unless is_dynamic_buffer_v<DynamicBuffer> is true.This can fixed by changing the test to is_dynamic_buffer_v<decay_t<DynamicBuffer>>.
[2019-01-20 Reflector prioritization]
Set Priority to 3
Proposed resolution:
This wording is relative to N4711.
Edit 17.5 [networking.ts::buffer.read] as indicated:
-14- Remarks: This function shall not participate in overload resolution unless
is_dynamic_buffer_v<decay_t<DynamicBuffer>>istrue.
Edit 17.6 [networking.ts::buffer.async.read] as indicated:
-16- Remarks: This function shall not participate in overload resolution unless
is_dynamic_buffer_v<decay_t<DynamicBuffer>>istrue.
Edit 17.7 [networking.ts::buffer.write] as indicated:
-14- Remarks: This function shall not participate in overload resolution unless
is_dynamic_buffer_v<decay_t<DynamicBuffer>>istrue.
Edit 17.8 [networking.ts::buffer.async.write] as indicated:
-16- Remarks: This function shall not participate in overload resolution unless
is_dynamic_buffer_v<decay_t<DynamicBuffer>>istrue.
Section: 31.12.10 [fs.class.directory.entry], 31.12.11 [fs.class.directory.iterator], 31.12.12 [fs.class.rec.dir.itr] Status: New Submitter: Gor Nishanov Opened: 2018-03-05 Last modified: 2019-04-02
Priority: 3
View all other issues in [fs.class.directory.entry].
View all issues with New status.
Discussion:
An implementation of directory_entry class is likely to store a filesystem::path as a member. Constructors and assign member functions take filesystem::path by const& thus forcing creation of a copy.
An implementation of directory_iterator class is likely to store a directory_entry or a path as a part of its state. Constructors take filesystem::path by const& thus forcing creation of a copy. An implementation of recursive_directory_iterator class is likely to store a directory_entry or a path as a part of its state. Constructors take filesystem::path by const& thus forcing creation of a copy. Suggested resolution: Add overloads to directory_entry, directory_iterator, and recursive_directory_iterator that take filesystem::path by &&. Make it unspecified in case an exception is thrown from those new members where an argument was moved from or not.explicit directory_entry(const filesystem::path& p); explicit directory_entry(filesystem::path&& p); directory_entry(const filesystem::path& p, error_code& ec); directory_entry(filesystem::path&& p, error_code& ec); void directory_entry::assign(const filesystem::path& p); void directory_entry::assign(filesystem::path&& p); void directory_entry::assign(const filesystem::path& p, error_code& ec); void directory_entry::assign(filesystem::path&& p, error_code& ec); explicit directory_iterator(const path& p); explicit directory_iterator(path&& p); directory_iterator(const path& p, directory_options options); directory_iterator(path&& p, directory_options options); directory_iterator(const path& p, error_code& ec) noexcept; directory_iterator(path&& p, error_code& ec) noexcept; directory_iterator(const path& p, directory_options options, error_code& ec) noexcept; directory_iterator(path&& p, directory_options options, error_code& ec) noexcept; explicit recursive_directory_iterator(const path& p); explicit recursive_directory_iterator(path&& p); recursive_directory_iterator(const path& p, directory_options options); recursive_directory_iterator(path&& p, directory_options options); recursive_directory_iterator(const path& p, directory_options options, error_code& ec) noexcept; recursive_directory_iterator(path&& p, directory_options options, error_code& ec) noexcept; recursive_directory_iterator(const path& p, error_code& ec) noexcept; recursive_directory_iterator(path&& p, error_code& ec) noexcept;
[2018-03-20 Priority set to 3 after discussion on the reflector.]
Proposed resolution:
Section: 22.13.3 [charconv.from.chars] Status: Open Submitter: Greg Falcon Opened: 2018-03-12 Last modified: 2023-03-29
Priority: 2
View other active issues in [charconv.from.chars].
View all other issues in [charconv.from.chars].
View all issues with Open status.
Discussion:
strtod() distinguishes between overflow and underflow by returning a value that is either very large or very small. Floating point from_chars does not currently offer any way for callers to distinguish these two cases.
It would be beneficial if users could migrate from strtod() to from_chars without loss of functionality. I recommend that floating point from_chars use value as an overflow-vs-underflow reporting channel, in the same manner as strtod(). My proposed wording gives from_chars the same wide latitude that strtod() enjoys for handling underflow. A high-quality implementation would likely set ec == result_out_of_range for underflow only when the nearest representable float to the parsed value is a zero and the parsed mantissa was nonzero. In this case value would be set to (an appropriately-signed) zero. It is worth considering giving from_chars this more predictable behavior, if library writers feel they can provide this guarantee for all platforms. (I have a proof-of-concept integer-based implementation for IEEE doubles with this property.)[2018-06 Rapperswil Wednesday issues processing]
Marshall to provide updated wording and propose Tentatively Ready on the reflector.
Priority set to 2
[2018-08-23 Batavia Issues processing]
Status to Open; Marshall to reword
[2023-03-29; Jonathan adds further discussion]
There are conflicting interpretations of "not in the range representable"
for floating-point types. One view is that 1e-10000 and 1e+10000 are outside
the representable range for a 64-bit double-precision double
(which has min/max exponents of -1022 and 1023). Another view is that the
representable range for floating-point types is [-inf,+inf], which means
that there are values that cannot be accurately represented,
but there are no values "not in the range representable". And 1e-10000 is
clearly within the range [0,numeric_limits<double>::max()],
even if we don't use infinity as the upper bound of the range.
Under the second interpretation, the result will be ±0.0 for underflow
and ±inf for overflow, but ec will not be set.
The current proposed resolution does address this, by making it clear
that value should be set to a very small or very large value
(with appropriate sign), but that ec should also be set.
The use of the word "overflow" for the integer overloads is a problem though,
because the result cannot "overflow" an unsigned integer type,
but can certainly be outside its range.
Proposed resolution:
This wording is relative to N4727.
Edit 22.13.3 [charconv.from.chars] as indicated:
[…] Otherwise, the characters matching the pattern are interpreted as a representation of a value of the type of value. The member ptr of the return value points to the first character not matching the pattern, or has the value last if all characters match. If the parsed value is not in the range representable by the type of value,
value is unmodified andthe member ec of the return value is equal to errc::result_out_of_range. Otherwise, value is set to the parsed value, after rounding according to round_to_nearest (17.3.4 [round.style]), and the member ec is value-initialized.from_chars_result from_chars(const char* first, const char* last, see below& value, int base = 10);-2- Requires: base has a value between 2 and 36 (inclusive).
-3- Effects: The pattern is the expected form of the subject sequence in the "C" locale for the given nonzero base, as described for strtol, except that no "0x" or "0X" prefix shall appear if the value of base is 16, and except that '-' is the only sign that may appear, and only if value has a signed type. On overflow, value is unmodified. […]from_chars_result from_chars(const char* first, const char* last, float& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, double& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, long double& value, chars_format fmt = chars_format::general);-6- Requires: fmt has the value of one of the enumerators of chars_format.
-7- Effects: The pattern is the expected form of the subject sequence in the "C" locale, as described for strtod, except that
(7.1) […]
(7.2) […]
(7.3) […]
(7.4) […]
In any case, the resulting value is one of at most two floating-point values closest to the value of the string matching the pattern. On overflow, value is set to plus or minus std::numeric_limits<T>::max() of the appropriate type. On underflow, value is set to a value with magnitude no greater than std::numeric_limits<T>::min().
[…]
Section: 22.13.3 [charconv.from.chars] Status: Open Submitter: Greg Falcon Opened: 2018-03-12 Last modified: 2020-09-06
Priority: 2
View other active issues in [charconv.from.chars].
View all other issues in [charconv.from.chars].
View all issues with Open status.
Discussion:
P0682R1 added the requirement that from_chars use round_to_nearest when converting from string, but later text in the section suggests that the implementation has latitude in its choice of rounding logic.
If the intent is merely that the floating point environment should not affect from_chars behavior, the rounding-mode text should be weakened. If the intent is to always require round_to_nearest, the text suggesting a latitude in rounding logic should be removed.[2018-03-27 Priority set to 2 after discussion on the reflector.]
[2018-06 Rapperswil Wednesday issues processing]
Status to open; also this needs to say that the intent is to be independent of the floating point environment.
[2018-08-23 Batavia Issues processing]
Marshall to talk to Jens about this
Proposed resolution:
This wording is relative to N4727.
Edit 22.13.3 [charconv.from.chars] as indicated:
from_chars_result from_chars(const char* first, const char* last, float& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, double& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, long double& value, chars_format fmt = chars_format::general);-6- Requires: fmt has the value of one of the enumerators of chars_format.
-7- Effects: The pattern is the expected form of the subject sequence in the "C" locale, as described for strtod, except that
(7.1) […]
(7.2) […]
(7.3) […]
(7.4) […]
In any case, the resulting value is
[…]one of at most twothe floating-point valuesclosest to the value of the string matching the pattern, with ties broken according to round_to_nearest.
Section: 17.5 [support.start.term], 17.9.5 [exception.terminate] Status: New Submitter: JF Bastien Opened: 2018-03-15 Last modified: 2018-04-03
Priority: 3
View other active issues in [support.start.term].
View all other issues in [support.start.term].
View all issues with New status.
Discussion:
It's unclear how different termination facilities in C++ interact (and how they interact with the C termination facilities). Individually some of these functions try to handle corner cases, but hilarity ensues when combined with each other. As a simple example, can an atexit handler call exit? If not, can it call quick_exit, and can then at_quick_exit handler call exit? Is it possible to install an atexit handler from an at_quick_exit, without strongly happens before, while handling a separate atexit handler (and what happens then)?
The termination handlers and termination conditions I collected:returning from main calls atexit handlers.
atexit / exit
at_quick_exit / quick_exit
set_terminate
violating noexcept and other things that call std::terminate (see [except.terminate])
violating exception specification
parallel algorithms leaving with uncaught exception
some std::signal such as SIGTERM, SIGSEGV, SIGINT, SIGILL, SIGABRT, and (maybe?) SIGFPE.
set_unexpected (now a zombie)
unexpected_handler (now a zombie)
What's unclear is:
Is termination handling a DAG?
Which thread(s) are termination handlers called on?
Is program termination Turing complete?
I've written a sample program which exercises some of this, see here.
[2018-04-02, Jens comments]
Any potential wording should carefully take [basic.start] into account, and maybe should actually be integrated into the core wording, not the library wording.
[2018-04-02 Priority set to 3 after discussion on the reflector.]
Proposed resolution:
Section: 17.6.3.2 [new.delete.single] Status: New Submitter: William M. Miller Opened: 2018-03-16 Last modified: 2020-09-06
Priority: 3
View other active issues in [new.delete.single].
View all other issues in [new.delete.single].
View all issues with New status.
Discussion:
In general requirements on a whole program, as opposed to a single translation unit, generally specify "no diagnostic required", since we don't want to require implementations to do multi-translation-unit analysis. However, 17.6.3.2 [new.delete.single] paragraph 11 says,
If a function with a size parameter is defined, the program shall also define the corresponding version without the size parameter.
This is clearly not restricted to a single translation unit; should "no diagnostic required" be added?
[2018-04-03; Thomas Köppe and Tim Song suggest wording]
[2018-06-18 after reflector discussion]
Priority set to 3
Proposed resolution:
This wording is relative to N4727.
Edit 17.6.3.2 [new.delete.single] as indicated:
void operator delete(void* ptr) noexcept; void operator delete(void* ptr, std::size_t size) noexcept; void operator delete(void* ptr, std::align_val_t alignment) noexcept; void operator delete(void* ptr, std::size_t size, std::align_val_t alignment) noexcept;-10- Effects: […]
-11- Replaceable: A C++ program may define functions with any of these function signatures, and thereby displace the default versions defined by the C++ standard library. If a function without a size parameter is defined, the program should also define the corresponding function with a size parameter. If a function with a size parameter is defined, the program shall also define the corresponding version without the size parameter; no diagnostic is required. [Note: The default behavior below may change in the future, which will require replacing both deallocation functions when replacing the allocation function. — end note]
Edit 17.6.3.3 [new.delete.array] as indicated:
void operator delete[](void* ptr) noexcept; void operator delete[](void* ptr, std::size_t size) noexcept; void operator delete[](void* ptr, std::align_val_t alignment) noexcept; void operator delete[](void* ptr, std::size_t size, std::align_val_t alignment) noexcept;-9- Effects: […]
-10- Replaceable: A C++ program may define functions with any of these function signatures, and thereby displace the default versions defined by the C++ standard library. If a function without a size parameter is defined, the program should also define the corresponding function with a size parameter. If a function with a size parameter is defined, the program shall also define the corresponding version without the size parameter; no diagnostic is required. [Note: The default behavior below may change in the future, which will require replacing both deallocation functions when replacing the allocation function. — end note]
Section: 27.7.1 [alg.copy] Status: New Submitter: Marshall Clow Opened: 2018-03-21 Last modified: 2022-11-06
Priority: 3
View other active issues in [alg.copy].
View all other issues in [alg.copy].
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Discussion:
All the copy algorithms have some kind of prohibition on having the input and output ranges overlap.
The serial version of copy says:Requires: result shall not be in the range [first, last).
The parallel version of copy says:
Requires: The ranges [first, last) and [result, result + (last - first)) shall not overlap.
copy_if says:
Requires: The ranges [first, last) and [result, result + (last - first)) shall not overlap.
copy_backwards says:
Requires: result shall not be in the range [first, last).
But copy_n has no such requirement.
I think it should. I checked the minutes of the LWG discussion from 2008 when this was added, and there was no discussion of overlapping ranges.What formulation do we want here? Is it sufficient to say "... shall not be in the range ..." or should we use the stronger "... shall not overlap ..."? Some copy variants use one, some use the other. Should we be consistent? Issue 3085 is a similar issue for char_traits::copy.
[2018-06-18 after reflector discussion]
Priority set to 3
Previous resolution [SUPERSEDED]:
This wording is relative to N4727.
Edit 27.7.1 [alg.copy] as indicated:
[Drafting note: I'm using the permission in 27.2 [algorithms.requirements]/10 to do random-access arithmetic on (possibly) input iterators.]
template<class InputIterator, class Size, class OutputIterator> constexpr OutputIterator copy_n(InputIterator first, Size n, OutputIterator result); template<class ExecutionPolicy, class ForwardIterator1, class Size, class ForwardIterator2> ForwardIterator2 copy_n(ExecutionPolicy&& exec, ForwardIterator1 first, Size n, ForwardIterator2 result);-?- Requires: result shall not be in the range [first, first + n).
-9- Effects: For each non-negative integer i < n, performs *(result + i) = *(first + i). -10- Returns: result + n. -11- Complexity: Exactly n assignments.
[2022-11-06; Daniel syncs wording with recent working draft]
Proposed resolution:
This wording is relative to N4917.
Edit 27.7.1 [alg.copy] as indicated:
[Drafting note: I'm using the permission in 27.2 [algorithms.requirements]/10 to do random-access arithmetic on (possibly) input iterators.]
template<class InputIterator, class Size, class OutputIterator> constexpr OutputIterator copy_n(InputIterator first, Size n, OutputIterator result); template<class ExecutionPolicy, class ForwardIterator1, class Size, class ForwardIterator2> ForwardIterator2 copy_n(ExecutionPolicy&& exec, ForwardIterator1 first, Size n, ForwardIterator2 result); template<input_iterator I, weakly_incrementable O> requires indirectly_copyable<I, O> constexpr ranges::copy_n_result<I, O> ranges::copy_n(I first, iter_difference_t<I> n, O result);-10- Let N be max(0, n).
-11- Mandates: The type Size is convertible to an integral type (7.3.9, 11.4.8). -?- Preconditions: result is not in the range [first, first + n). -12- Effects: For each non-negative integer i < N, performs *(result + i) = *(first + i). -13- Returns:
(13.1) — result + N for the overloads in namespace std.
(13.2) — {first + N, result + N} for the overload in namespace ranges.
-14- Complexity: Exactly N assignments.
Section: 29.5.2 [time.duration.cons] Status: New Submitter: Richard Smith Opened: 2018-03-22 Last modified: 2020-09-12
Priority: 3
View all other issues in [time.duration.cons].
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Discussion:
29.5.2 [time.duration.cons] p4 says:
template<class Rep2, class Period2> constexpr duration(const duration<Rep2, Period2>& d);Remarks: This constructor shall not participate in overload resolution unless no overflow is induced in the conversion and treat_as_floating_point_v<rep> is true or both ratio_divide<Period2, period>::den is 1 and treat_as_floating_point_v<Rep2> is false.
with this example:
duration<int, milli> ms(3); duration<int, micro> us = ms; // OK duration<int, milli> ms2 = us; // error
It's unclear to me what "no overflow is induced in the conversion" means in the above. What happens here:
duration<int, milli> ms(INT_MAX); duration<int, micro> us = ms; // ???
An overflow is clearly induced in the conversion here: internally, we'll multiply INT_MAX by 1000. But that cannot be determined statically (in general), and so can't affect the result of overload resolution.
So what's actually supposed to happen? Are we actually just supposed to check that Rep2 is no larger than Rep? (If so, what happens on overflow? Undefined behavior?) It has been pointed out by Howard Hinnant:This refers to the compile-time conversion factor to convert Period2 to Period. If that conversion factor is not representable as a (reduced) ratio<N, D>, then the constructor is SFINAE'd out. This might happen (for example) converting years to picoseconds.
I would not have guessed that from the wording. Maybe replacing "no overflow is induced in the conversion" with "the result of ratio_divide<Period2, Period> is representable as a ratio" or similar would help?
[2018-06-18 after reflector discussion]
Priority set to 3
[2020-09-12 Jonathan adds a proposed resolution]
Since the result of the ratio_divide has to be a ratio, if it's not representable then the result simply isn't a valid type. Implementations are not required to make ratio_divide SFINAE-friendly to implement this constraint. They can perform the equivalent calculations to check if they would overflow, without actually using ratio_divide.
Proposed resolution:
This wording is relative to N4861.
Modify 29.5.2 [time.duration.cons] as indicated:
template<class Rep2, class Period2> constexpr duration(const duration<Rep2, Period2>& d);
-3- Constraints: is_convertible_v<const Rep2&, rep> is true. ratio_divide<typename Period2::type, period> is a valid ratio specialization. Either:
No overflow is induced in the conversion and
treat_as_floating_point_v<rep> is true
or both ratio_divide<Period2, period>::den is 1
and treat_as_floating_point_v<Rep2> is false.
[Note: This requirement prevents implicit truncation errors
when converting between integral-based duration types.
Such a construction could easily lead to confusion about the value of the
duration.
— end note]
Section: 16.3.3.3.3 [bitmask.types] Status: Open Submitter: Geoffrey Romer Opened: 2018-03-26 Last modified: 2020-09-06
Priority: 3
View all other issues in [bitmask.types].
View all issues with Open status.
Discussion:
[bitmask.types] specifies the semantics of a bitmask type in terms of an "exposition only" enum definition, together with some constants and overloads. Notably, it is depicted as an unscoped enum, which implies among other things that it is implicitly convertible to int_type. At least some sources treat that as normative (as of this writing, cppreference.com's documentation for BitmaskType says the expression (X & Y) != 0 is guaranteed to be well-formed), and it's hard to argue that they're wrong on the basis of the existing wording.
On the other hand, many standard library types are depicted as scoped enums, but still specified to be "bitmask types". As far as I can tell, it's impossible in library code to make a scoped enum implicitly convertible to its underlying type, and even if you could, what would be the point? Presumably the specification of those types as scoped enums is intended to have some sort of observable consequences. In addition, some library types (notably in clause 31) are specified to be bitmask types, without indicating whether they are scoped or unscoped. It's not clear what the standard guarantees about e.g. whether they can be implicitly converted. I assume the intent is that "bitmask type" doesn't specify an implicit conversion, or any of the other ways unscoped and scoped enums differ, but the standard doesn't actually say that. We really ought to rewrite [bitmask.types] as a requirements table, but here's a quick hack to the existing wording as a stopgap.[2018-04-23 Priority set to 2 after discussion on the reflector.]
[2018-08-23 Batavia Issues processing]
N3110 also touches on this.
Nico to survey the enums in the library and report back on which ones should be class.
[2019 Cologne Wednesday night]
Changing existing enums to class enums is an ABI break on some platforms; current wording does not require the use of enums. See N3110.
Daniel to provide requirements tables, Jonathan to assist. Reduce priority to 3
Proposed resolution:
This wording is relative to N4727.
Edit 16.3.3.3.3 [bitmask.types] as indicated:
-2- The bitmask type bitmask can be written:
// For exposition only. // int_type is an integral type capable of representing all values of the bitmask type.enumE bitmask : int_type { V0 = 1 << 0, V1 = 1 << 1, V2 = 1 << 2, V3 = 1 << 3, ..... }; […]-3- Here, E may represent either enum or enum class (the choice is implementation-defined unless otherwise specified), and the names C0, C1, etc. represent bitmask elements for this particular bitmask type. The zero value bitmask{} is used to represent an empty bitmask, in which no bitmask elements are set. All
suchbitmask elements have distinct, nonzero values such that, for any pair Ci and Cj where i ≠ j, Ci & Ci is nonzero and Ci & Cj is zero.Additionally, the value 0 is used to represent an empty bitmask, in which no bitmask elements are set.
Section: 28.7.2 [c.math.abs], 17.2.2 [cstdlib.syn], 31.13.2 [cinttypes.syn] Status: New Submitter: Richard Smith Opened: 2018-03-30 Last modified: 2018-06-19
Priority: 3
View all issues with New status.
Discussion:
LWG 2192 changed the library specification so that you always get all the std::abs overloads for fundamental types if you get any of them.
Except that it missed one: <cinttypes> provides a std::abs (and std::div) overload for intmax_t if it's not long long (since LWG 1449). Presumably that one should also follow the <cstdlib>/<cmath> pattern and we should make the complete abs overload set available whenever any of the three headers is included? (And likewise we should make the complete std::div overload set available whenever either <cstdlib> or <cinttypes> is included.)[2018-06-18 after reflector discussion]
Priority set to 3
Proposed resolution:
Section: D.15.2.4 [depr.strstreambuf.virtuals] Status: New Submitter: Billy O'Neal III Opened: 2018-04-04 Last modified: 2020-09-06
Priority: 4
View all other issues in [depr.strstreambuf.virtuals].
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Discussion:
strstreambuf refers to a nonexistent member function of fpos in the specification of the member function seekpos, D.15.2.4 [depr.strstreambuf.virtuals]/18 (emphasize mine):
For a sequence to be positioned, if its next pointer is a null pointer, the positioning operation fails. Otherwise, the function determines newoff from sp.offset():
The intent is clearly to get the corresponding streamoff from the fpos, as p19 says "the resultant offset newoff (of type off_type)". The mechanism to make that conversion is a normal explicit conversion, as indicated in the last row of the table in [fpos.operations].
[2018-06-18 after reflector discussion]
Priority set to 4
Proposed resolution:
This wording is relative to N4727.
Edit D.15.2.4 [depr.strstreambuf.virtuals] as indicated:
pos_type seekpos(pos_type sp, ios_base::openmode which = ios_base::in | ios_base::out) override;-17- Effects: […]
-18- For a sequence to be positioned, if its next pointer is a null pointer, the positioning operation fails. Otherwise, the function determines newoff from static_cast<off_type>(sp).offset(): […]
Section: 31.8.2.5 [stringbuf.virtuals] Status: New Submitter: Billy O'Neal III Opened: 2018-04-07 Last modified: 2020-09-06
Priority: 3
View other active issues in [stringbuf.virtuals].
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Discussion:
Paragraph citations relative to N4727.
[stringbuf.virtuals]/10 says that newoff might be calculated from xnext - xbegin, or from high_mark - xbegin. After newoff is calculated, it does the null pointer check against and zero offset check. However, that means the effects may have already done nullptr - non-nullptr, or non-nullptr - nullptr, which [expr.add]/5 says is undefined behavior.
Moreover, the attempt at avoiding this problem only tests newoff, not the value actually used which is newoff + off. For example, buf.seekoff(100, ios_base::beg, ios_base::out) on a read-only streambuf would try to assign pptr() + newoff + off to pptr(), but pptr() may have been nullptr, giving nullptr + 0 + 100 which triggers UB. (Perhaps the "refers to an uninitialized character" bit protects that though). Last, the Returns: element says that it returns newoff, but then also says it returns the resulting stream position, which should be something like newoff + off. (I checked libc++ and MSVC++ and we both return newoff + off) We probably want to resolve that by renaming the value that comes out of Table 108 to something like "basis" and make "newoff" actually be the new offset instead of the starting offset.[2018-04-16 Priority set to 3 after discussion on the reflector.]
Proposed resolution:
Section: 31.12.6.5.9 [fs.path.decompose] Status: New Submitter: Jonathan Wakely Opened: 2018-04-06 Last modified: 2020-09-06
Priority: 3
View all other issues in [fs.path.decompose].
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Discussion:
The example in [fs.path.decompose] p7 includes:
path("//host").filename(); // yields ""
This result isn't guaranteed, it depends whether the implementation interprets "//host" as a root-name or as a root-directory (with a redundant directory-separator) followed by the filename "host".
The example should make it clear that this interpretation is allowed.Previous resolution [SUPERSEDED]:
This wording is relative to N4727.
Edit 31.12.6.5.9 [fs.path.decompose] as indicated:
path filename() const;-6- Returns: relative_path().empty() ? path() : *--end().
[Example:path("/foo/bar.txt").filename(); // yields "bar.txt" path("/foo/bar").filename(); // yields "bar" path("/foo/bar/").filename(); // yields "" path("/").filename(); // yields "" path("//host").filename(); // yields "" or "host" path(".").filename(); // yields "." path("..").filename(); // yields ".."— end example]
[2018-04-10, Jonathan comments and provides revised wording]
Based on the reflector discussion I'd like to change the P/R to Billy's suggestion of simply removing that line from the example.
[2018-06-18 after reflector discussion]
Priority set to 3
Proposed resolution:
This wording is relative to N4741.
Edit 31.12.6.5.9 [fs.path.decompose] as indicated:
path filename() const;-6- Returns: relative_path().empty() ? path() : *--end().
[Example:path("/foo/bar.txt").filename(); // yields "bar.txt" path("/foo/bar").filename(); // yields "bar" path("/foo/bar/").filename(); // yields "" path("/").filename(); // yields ""path("//host").filename(); // yields ""path(".").filename(); // yields "." path("..").filename(); // yields ".."— end example]
Section: 21.3.3 [meta.type.synop] Status: New Submitter: Casey Carter Opened: 2018-04-10 Last modified: 2020-09-06
Priority: 2
View other active issues in [meta.type.synop].
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Discussion:
LWG 2939 suggests that the the preconditions of the type traits need reevaluation. This issue focuses specifically on is_assignable and, by extension, its variants:
We note a discrepancy: is_copy_assignable<T> requires T to be a complete type, but the equivalent form is_assignable<T&, const T&> does not. The requirement for is_copy_assignable<T> seems sensible, since there's no way to determine whether or not the assignment declval<T&>() = declval<const T&>() is well-formed when T is incomplete. It seems that the same argument should apply to all of the above "assignable" traits, and that they must require that the referent type is complete when given a reference type parameter to be implementable.
[2018-08 Batavia Monday issue discussion]
Issues 2797, 2939, 3022, and 3099 are all closely related. Walter to write a paper resolving them.
[2020-02-14, Prague]
LWG discussions. Set priority to 2.
Proposed resolution:
This wording is relative to N4741.
- In 21.3.5.4 [meta.unary.prop] Table 42, change the Precondition text for is_assignable, is_trivially_assignable, and is_nothrow_assignable as follows:
remove_cvref_t<T> and remove_cvref_t<U> shall be complete types,cvvoid, or arrays of unknown bound.- In 21.3.5.4 [meta.unary.prop] Table 42, change the Precondition text for is_copy_assignable, is_move_assignable, is_trivially_copy_assignable, is_trivially_move_assignable, is_nothrow_copy_assignable, and is_nothrow_move_assignable as follows:
remove_cvref_t<T> shall be a complete type,cvvoid, or an array of unknown bound.
Section: 16 [library] Status: New Submitter: Jens Maurer Opened: 2018-04-24 Last modified: 2018-06-19
Priority: 3
View other active issues in [library].
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Discussion:
The library wording frequently uses the construction "type T1 is convertible to type T2", but this is an undefined phrase.
For requirements on user code (e.g. [tuple.rel]), it is unclear whether all expressions of type T1 must satisfy the convertibility requirement, regardless of value category, or whether a single value category is in view only. Consider:
struct C
{
operator int() &&;
};
int main()
{
int x = C(); // prvalue can be implicitly converted to int
C c;
int y = c; // lvalue can't
}
The library has an "is_convertible<T1, T2>" trait, but that checks convertibility only for a single value category, not all possible ones.
[2018-06-18 after reflector discussion]
Priority set to 3
Proposed resolution:
Section: 25.6.4 [istreambuf.iterator] Status: New Submitter: Billy O'Neal III Opened: 2018-04-26 Last modified: 2020-09-06
Priority: 4
View other active issues in [istreambuf.iterator].
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Discussion:
LWG has said recently that we don't want public exposition-only things, as that may encourage users to try to use those names (and some implementers to actually use those names).
[2018-06-18 after reflector discussion]
Priority set to 4
Proposed resolution:
This wording is relative to N4741.
Edit 25.6.4 [istreambuf.iterator] as indicated:
namespace std {
template<class charT, class traits = char_traits<charT>>
class istreambuf_iterator {
public:
[…]
using istream_type = basic_istream<charT,traits>;
class proxy; // exposition only
constexpr istreambuf_iterator() noexcept;
[…]
private:
class proxy; // exposition only
streambuf_type* sbuf_; // exposition only
};
[…]
}
Section: 25.6.4.2 [istreambuf.iterator.proxy] Status: New Submitter: Billy O'Neal III Opened: 2018-04-26 Last modified: 2020-09-06
Priority: 3
View all issues with New status.
Discussion:
operator* on iterators is usually intended to be const; see 25.3.5.3 [input.iterators] Table 87, *a, where a is of type X or const X. (Technically, proxy is implementing the *r++ requirement in this table, and r doesn't imply a const iterator, but there's no reason for the iterator's operator* to differ from the proxy)
[2018-06-18 after reflector discussion]
Priority set to 3
Proposed resolution:
This wording is relative to N4741.
Edit 25.6.4.2 [istreambuf.iterator.proxy] as indicated:
namespace std {
template<class charT, class traits = char_traits<charT>>
class istreambuf_iterator<charT, traits>::proxy { // exposition only
charT keep_;
basic_streambuf<charT,traits>* sbuf_;
proxy(charT c, basic_streambuf<charT,traits>* sbuf)
: keep_(c), sbuf_(sbuf) { }
public:
charT operator*() const { return keep_; }
};
}
Section: D.15.2 [depr.strstreambuf] Status: New Submitter: Jonathan Wakely Opened: 2018-05-02 Last modified: 2018-06-19
Priority: 4
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Discussion:
In C++03 strstreambuf was not copyable, because basic_streambuf wasn't copyable. In C++11 we made basic_streambuf copyable by derived classes, and strstreambuf doesn't define any special members, so it (unintentionally?) became copyable, with completely unspecified semantics.
VC++ and libc++ make it movable not copyable, and libstdc++ still follows C++03, so it's neither movable nor copyable. Making it movable seems to be the sane option, and consistent with filebuf and stringbuf.[2018-06-18 after reflector discussion]
Priority set to 4
Proposed resolution:
Section: 16 [networking.ts::buffer], 17 [networking.ts::buffer.stream] Status: LEWG Submitter: Vinnie Falco Opened: 2018-05-18 Last modified: 2020-09-06
Priority: Not Prioritized
View all issues with LEWG status.
Discussion:
Addresses: networking.tsAsynchronous algorithms are started by a call to an initiating function. When these algorithms are constructed from calls to other initiating functions, the result is called a composed operation. For example, async_read may be implemented in terms of zero or more calls to a stream's async_read_some algorithm. For operations where the caller cannot easily determine ahead of time the storage requirements needed for an algorithm to meet its post-conditions, [networking.ts] introduces the DynamicBuffer concept:
A dynamic buffer encapsulates memory storage that may be automatically resized as required, where the memory is divided into two regions: readable bytes followed by writable bytes. [buffer.reqmts.dynamicbuffer]
Signatures for algorithms in the technical specification which accept dynamic buffers use forwarding references:
// 17.10 [networking.ts::buffer.async.read.until], asynchronous delimited read operations: template< class AsyncReadStream, class DynamicBuffer, class CompletionToken> DEDUCED async_read_until( AsyncReadStream& s, DynamicBuffer&& b, char delim, CompletionToken&& token);
Because the initiating function returns immediately, and the associated composed operation executes later, it is necessary for the algorithm to manage the lifetime of the dynamic buffer. Guidance for doing so is given in the TS:
13.2.7.5 Lifetime of initiating function arguments [async.reqmts.async.lifetime]1. Unless otherwise specified, the lifetime of arguments to initiating functions shall be treated as follows: […] the implementation does not assume the validity of the argument after the initiating function completes […] The implementation may make copies of the argument, and all copies shall be destroyed no later than immediately after invocation of the completion handler.
Given the guidance above, the most sensible approach is for the implementation to move or make a decay-copy of the argument. An implementation of the TS, authored by the principal architect of the TS itself, does precisely that:
template <
typename AsyncReadStream,
typename DynamicBuffer,
typename ReadHandler>
class read_until_delim_op
{
public:
template <typename DeducedBuffers>
read_until_delim_op(
AsyncReadStream& stream,
DeducedBuffers&& buffers,
char delim, ReadHandler& handler)
: […]
buffers_(std::forward<DeducedBuffers>(buffers))
[…]
{
}
[…]
DynamicBuffer buffers_;
[…]
};
template <
typename AsyncReadStream,
typename DynamicBuffer,
typename ReadHandler>
NET_TS_INITFN_RESULT_TYPE(ReadHandler,
void (std::error_code, std::size_t))
async_read_until(
AsyncReadStream& s,
DynamicBuffer&& buffers,
char delim,
ReadHandler&& handler)
{
// If you get an error on the following line it means that your handler does
// not meet the documented type requirements for a ReadHandler.
NET_TS_READ_HANDLER_CHECK(ReadHandler, handler) type_check;
async_completion<ReadHandler,
void (std::error_code, std::size_t)> init(handler);
detail::read_until_delim_op<
AsyncReadStream,
typename decay<DynamicBuffer>::type,
NET_TS_HANDLER_TYPE(ReadHandler, void (std::error_code, std::size_t))>(
s,
DynamicBuffer&&buffers,
delim,
init.completion_handler)(std::error_code(), 0, 1);
return init.result.get();
}
Given the semantics of dynamic buffers implied by the wording, instances of dynamic buffers behave more like references to storage types rather than storage types, as copies refer to the same underlying storage. This can be seen in the declaration of dynamic_string_buffer which meets the requirements of DynamicBuffer:
template <typename Elem, typename Traits, typename Allocator>
class dynamic_string_buffer
{
[…]
private:
std::basic_string<Elem, Traits, Allocator>& string_;
std::size_t size_;
const std::size_t max_size_;
};
A dynamic string buffer contains a reference to the underlying string. Copies of a dynamic string buffer refer to the same string. Note that the dynamic string buffer also contains some state: the size_ and max_size_ data members. This additional metadata informs the dynamic string buffer of the boundaries between the readable and writable bytes, as well as the maximum allowed size of the total of the readable and writable bytes.
When only one composed operation handles the dynamic buffer, things seem to work. However, if a composed operation wishes to invoke another composed operation and pass that dynamic buffer, a problem arises. Consider two composed operations f and g, which both operate on an instance of dynamic buffer. When f is invoked, it makes a copy of the dynamic buffer and then calls g with the copy. At this point, g must also make a copy. Copies share the underlying storage, but in the case of dynamic string buffers each copy maintains its own distinct metadata. When g has finished mutating the dynamic buffer and returns control back to f by invoking the completion handler, the metadata in the copy of the dynamic buffer held by f will not have the changes made by g. Another design problem caused by adding metadata to the dynamic buffer concept is illustrated in the following example code:
template<class MutableBufferSequence>
std::size_t read(const MutableBufferSequence&)
{
throw std::exception{};
}
int main()
{
std::string s;
assert(s.empty());
try
{
auto b = boost::asio::dynamic_buffer(s);
b.commit(read(b.prepare(32)));
}
catch(const std::exception&)
{
assert(s.empty()); // fails
}
}
While not technically incorrect, it may be surprising to the user that the string contains additional value-initialized data which was not part of the original readable bytes (which in this case was empty). The wording of the dynamic buffer concept does not address this case.
The solution we propose is to change the semantics of DynamicBuffer to represent a true storage type rather than a hybrid reference with metadata. Instances of dynamic buffers will be passed by reference, and callers will be required to manage the lifetime of dynamic buffer objects for the duration of any asynchronous operations. An additional benefit of this change is that it also solves the problem of exceptions described above.[2018-06-18 after reflector discussion]
Status to LEWG; there will be a paper P1100R0 in the post-Rapperswil mailing addressing this.
[2020-05-28; Billy Baker comments]
From Cologne 2019 paper discussion: P1100 has been superseded by P1790.
Proposed resolution:
This wording is relative to N4734.
[Drafting note: The project editor is kindly asked to replace all occurrences of DynamicBuffer&& with DynamicBuffer& as indicated by the provided wording changes below. — end drafting note]
Modify 16.1 [networking.ts::buffer.synop], header <experimental/buffer> synopsis, as indicated:
[…] // 16.11 [networking.ts::buffer.creation], buffer creation: […] template<class T, class Allocator> class dynamic_vector_buffer; template<class CharT, class Traits, class Allocator> class basic_dynamic_string_buffer;// 16.14 [networking.ts::buffer.dynamic.creation], dynamic buffer creation:template<class T, class Allocator> dynamic_vector_buffer<T, Allocator> dynamic_buffer(vector<T, Allocator>& vec) noexcept; template<class T, class Allocator> dynamic_vector_buffer<T, Allocator> dynamic_buffer(vector<T, Allocator>& vec, size_t n) noexcept;template<class CharT, class Traits, class Allocator> dynamic_string_buffer<CharT, Traits, Allocator> dynamic_buffer(basic_string<CharT, Traits, Allocator>& str) noexcept; template<class CharT, class Traits, class Allocator> dynamic_string_buffer<CharT, Traits, Allocator> dynamic_buffer(basic_string<CharT, Traits, Allocator>& str, size_t n) noexcept;[…] // 17.5 [networking.ts::buffer.read], synchronous read operations: […] template<class SyncReadStream, class DynamicBuffer> size_t read(SyncReadStream& stream, DynamicBuffer&&b); template<class SyncReadStream, class DynamicBuffer> size_t read(SyncReadStream& stream, DynamicBuffer&&b, error_code& ec); template<class SyncReadStream, class DynamicBuffer, class CompletionCondition> size_t read(SyncReadStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition); template<class SyncReadStream, class DynamicBuffer, class CompletionCondition> size_t read(SyncReadStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition, error_code& ec); // 17.6 [networking.ts::buffer.async.read], asynchronous read operations: […] template<class AsyncReadStream, class DynamicBuffer, class CompletionToken> DEDUCED async_read(AsyncReadStream& stream, DynamicBuffer&&b, CompletionToken&& token); template<class AsyncReadStream, class DynamicBuffer, class CompletionCondition, class CompletionToken> DEDUCED async_read(AsyncReadStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition, CompletionToken&& token); // 17.7 [networking.ts::buffer.write], synchronous write operations: […] template<class SyncWriteStream, class DynamicBuffer> size_t write(SyncWriteStream& stream, DynamicBuffer&&; b); template<class SyncWriteStream, class DynamicBuffer> size_t write(SyncWriteStream& stream, DynamicBuffer&&b, error_code& ec); template<class SyncWriteStream, class DynamicBuffer, class CompletionCondition> size_t write(SyncWriteStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition); template<class SyncWriteStream, class DynamicBuffer, class CompletionCondition> size_t write(SyncWriteStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition, error_code& ec); // 17.8 [networking.ts::buffer.async.write], asynchronous write operations: […] template<class AsyncWriteStream, class DynamicBuffer, class CompletionToken> DEDUCED async_write(AsyncWriteStream& stream, DynamicBuffer&&b, CompletionToken&& token); template<class AsyncWriteStream, class DynamicBuffer, class CompletionCondition, class CompletionToken> DEDUCED async_write(AsyncWriteStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition, CompletionToken&& token); // 17.9 [networking.ts::buffer.read.until], synchronous delimited read operations: template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&&b, char delim); template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&&b, char delim, error_code& ec); template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&&b, string_view delim); template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&&b, string_view delim, error_code& ec); // 17.10 [networking.ts::buffer.async.read.until], asynchronous delimited read operations: template<class AsyncReadStream, class DynamicBuffer, class CompletionToken> DEDUCED async_read_until(AsyncReadStream& s, DynamicBuffer&&b, char delim, CompletionToken&& token); template<class AsyncReadStream, class DynamicBuffer, class CompletionToken> DEDUCED async_read_until(AsyncReadStream& s, DynamicBuffer&&b, string_view delim, CompletionToken&& token); […]
Modify 16.2.4 [networking.ts::buffer.reqmts.dynamicbuffer], as indicated:
-1- […]
-2- A type X meets the DynamicBuffer requirements if it satisfies the requirements of Destructible (C++ 2014 [destructible])and MoveConstructible (C++ 2014 [moveconstructible]),as well as the additional requirements listed in Table 14.
Modify 16.12 [networking.ts::buffer.dynamic.vector], as indicated:
[…] template<class T, class Allocator = allocator<T>> class dynamic_vector_buffer { public: // types: using value_type = vector<T, Allocator>; using const_buffers_type = const_buffer; using mutable_buffers_type = mutable_buffer; // constructors: dynamic_vector_buffer() = default; explicit dynamic_vector_buffer(size_t maximum_size); explicit dynamic_vector_buffer(vector<T, Allocator>&vec)noexcept;dynamic_vector_buffer(vector<T, Allocator>&vec, size_t maximum_size)noexcept;dynamic_vector_buffer(dynamic_vector_buffer&&) = default; // members: size_t size() const noexcept; size_t max_size() const noexcept; void max_size(size_t maximum_size); size_t capacity() const noexcept; const_buffers_type data() const noexcept; mutable_buffers_type prepare(size_t n); void commit(size_t n); void consume(size_t n); span<const T> get() const noexcept value_type release(); private: vector<T, Allocator>&vec_; // exposition only size_t size_; // exposition onlyconstsize_t max_size_; // exposition only }; […]-2- […]
-3- […]explicit dynamic_vector_buffer(size_t maximum_size)-?- Effects: Default-constructs vec_. Initializes size_ with 0, and max_size_ with maximum_size.
explicit dynamic_vector_buffer(vector<T, Allocator>&vec)noexcept-4- Effects: Initializes vec_ with move(vec), size_ with vec_.size(), and max_size_ with vec_.max_size()
dynamic_vector_buffer(vector<T, Allocator>&vec, size_t maximum_size)noexcept;-5- Requires: vec.size() <= maximum_size
-6- Effects: Initializes vec_ with move(vec), size_ with vec_.size(), and max_size_ with maximum_size. […]size_t max_size() const noexcept;-8- Returns: max_size_.
void max_size(size_t maximum_size)[…]-?- Effects: Performs max_size_ = maximum_size.
void consume(size_t n);-15- Effects: […]
span<const T> get() const noexcept-?- Returns: span<const T>(vec_.data(), size_).
value_type release()-?- Returns: move(vec_).
Modify 16.13 [networking.ts::buffer.dynamic.string], as indicated:
template<class CharT, class Traits, class Allocator> class basic_dynamic_string_buffer { public: // types: using value_type = basic_string<CharT, Traits, Allocator>; using const_buffers_type = const_buffer; using mutable_buffers_type = mutable_buffer; // constructors: basic_dynamic_string_buffer() = default; explicit basic_dynamic_string_buffer(size_t maximum_size); explicit basic_dynamic_string_buffer(basic_string<CharT, Traits, Allocator>&str)noexcept;basic_dynamic_string_buffer(basic_string<CharT, Traits, Allocator>&str, size_t maximum_size)noexcept;basic_dynamic_string_buffer(basic_dynamic_string_buffer&&) = default; // members: size_t size() const noexcept; size_t max_size() const noexcept; void max_size(size_t maximum_size) size_t capacity() const noexcept; const_buffers_type data() const noexcept; mutable_buffers_type prepare(size_t n); void commit(size_t n) noexcept; void consume(size_t n); basic_string_view<CharT, Traits> get() const noexcept value_type release(); private: basic_string<CharT, Traits, Allocator>&str_; // exposition only size_t size_; // exposition onlyconstsize_t max_size_; // exposition only }; using dynamic_string_buffer = basic_dynamic_string_buffer<char, char_traits<char>, allocator<char>>-2- […]
-3- […]explicit basic_dynamic_string_buffer(size_t maximum_size)-?- Effects: Default-constructs str_. Initializes size_ with 0, and max_size_ with maximum_size.
[…]
explicit basic_dynamic_string_buffer(basic_string<CharT, Traits, Allocator>&str)noexcept-4- Effects: Initializes str_ with move(str), size_ with str_.size(), and max_size_ with str_.max_size()
basic_dynamic_string_buffer(string<CharT, Traits, Allocator>&str, size_t maximum_size)noexcept;-5- Requires: str.size() <= maximum_size.
-6- Effects: Initializes str_ with move(str), size_ with str_.size(), and max_size_ with maximum_size.
[…]
size_t max_size() const noexcept;-8- Returns: max_size_.
void max_size(size_t maximum_size)-?- Effects: Performs max_size_ = maximum_size.
[…]
void consume(size_t n);-15- Effects: […]
basic_string_view<CharT, Traits> get() const noexcept-?- Returns: basic_string_view<CharT, Traits>(str_).
value_type release()-?- Returns: move(str_).
Remove 16.14 [networking.ts::buffer.dynamic.creation] entirely
Modify 17.5 [networking.ts::buffer.read], as indicated:
[…] template<class SyncReadStream, class DynamicBuffer> size_t read(SyncReadStream& stream, DynamicBuffer&&b); template<class SyncReadStream, class DynamicBuffer> size_t read(SyncReadStream& stream, DynamicBuffer&&b, error_code& ec); template<class SyncReadStream, class DynamicBuffer, class CompletionCondition> size_t read(SyncReadStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition); template<class SyncReadStream, class DynamicBuffer, class CompletionCondition> size_t read(SyncReadStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition, error_code& ec); […]
Modify 17.6 [networking.ts::buffer.async.read], as indicated:
[…] template<class AsyncReadStream, class DynamicBuffer, class CompletionToken> DEDUCED async_read(AsyncReadStream& stream, DynamicBuffer&&b, CompletionToken&& token); template<class AsyncReadStream, class DynamicBuffer, class CompletionCondition, class CompletionToken> DEDUCED async_read(AsyncReadStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition, CompletionToken&& token); […]-14- The program shall ensure both the AsyncReadStream object stream and the DynamicBuffer object b are
isvalid until the completion handler for the asynchronous operation is invoked.
Modify 17.7 [networking.ts::buffer.write], as indicated:
[…] template<class SyncWriteStream, class DynamicBuffer> size_t write(SyncWriteStream& stream, DynamicBuffer&&b); template<class SyncWriteStream, class DynamicBuffer> size_t write(SyncWriteStream& stream, DynamicBuffer&&b, error_code& ec); template<class SyncWriteStream, class DynamicBuffer, class CompletionCondition> size_t write(SyncWriteStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition); template<class SyncWriteStream, class DynamicBuffer, class CompletionCondition> size_t write(SyncWriteStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition, error_code& ec); […]
Modify 17.8 [networking.ts::buffer.async.write], as indicated:
[…] template<class AsyncWriteStream, class DynamicBuffer, class CompletionToken> DEDUCED async_write(AsyncWriteStream& stream, DynamicBuffer&&b, CompletionToken&& token); template<class AsyncWriteStream, class DynamicBuffer, class CompletionCondition, class CompletionToken> DEDUCED async_write(AsyncWriteStream& stream, DynamicBuffer&&b, CompletionCondition completion_condition, CompletionToken&& token); […]-14- The program shall ensure both the AsyncWriteStream object stream and the DynamicBuffer object b
memory associated with the dynamic buffer bare valid until the completion handler for the asynchronous operation is invoked.
Modify 17.9 [networking.ts::buffer.read.until], as indicated:
template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&&b, char delim); template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&&b, char delim, error_code& ec); template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&&b, string_view delim); template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&&b, string_view delim, error_code& ec); […]
Modify 17.10 [networking.ts::buffer.async.read.until], as indicated:
template<class AsyncReadStream, class DynamicBuffer, class CompletionToken> DEDUCED async_read_until(AsyncReadStream& s, DynamicBuffer&&b, char delim, CompletionToken&& token); template<class AsyncReadStream, class DynamicBuffer, class CompletionToken> DEDUCED async_read_until(AsyncReadStream& s, DynamicBuffer&&b, string_view delim, CompletionToken&& token); […]-6- The program shall ensure both the AsyncReadStream object stream and the DynamicBuffer object b are
isvalid until the completion handler for the asynchronous operation is invoked.
Section: 13.7.5 [networking.ts::async.exec.ctx.globals] Status: New Submitter: Billy O'Neal III Opened: 2018-06-23 Last modified: 2020-09-06
Priority: 3
View all issues with New status.
Discussion:
Addresses: networking.tsmake_service and use_service create arbitrary numbers of type Service, a type provided by the user, similar to how locale's use_facet works. As a result there's no amount of storage that could be reserved inside execution_context to avoid allocating memory. However, there's no allocator support here, and make_service is forbidden from throwing allocation related exceptions by N4734 [async.exec.ctx.globals]/7.
If the intent is for execution_context to allocate memory, are user overloads of operator new on type Service intended to be used?[2018-07-20 Priority set to 3 after reflector discussion]
[Jonathan provides wording.]
Proposed resolution:
This wording is relative to the N4762.
Modify 13.7.5 [networking.ts::async.exec.ctx.globals] p7:
-7- Throws: service_already_exists if a corresponding service object of type Service::key_type is already present in the set , bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained. Any exception thrown by the constructor of Service.
Section: 32.8 [re.submatch] Status: New Submitter: Jonathan Wakely Opened: 2018-06-26 Last modified: 2020-09-06
Priority: 3
View all other issues in [re.submatch].
View all issues with New status.
Discussion:
std::sub_match::compare can be called with a basic_string or a pointer to a null-terminated character sequence, but can't be called with a basic_string_view. To compare to a string_view requires either conversion to basic_string (with a potential allocation) or a redundant call to traits_type::length to calculate a length that is already known.
[2018-07-02, Jonathan comments and completes proposed wording]
To make the relational and equality operators for sub_match support string views I propose specifying the semantics, not adding another 12 overloaded operators to namespace std, in addition to the 42 already there. This allows them to be implemented as "hidden friends" if the implementation so desires, or to retain namespace-scope declaration if backwards compatibility with C++11 - C++17 is preferred.
[2018-07-20 Priority set to 3 after reflector discussion]
Proposed resolution:
This wording is relative to N4750.
Change 32.3 [re.syn], header <regex> synopsis, as indicated:
#include <initializer_list>
namespace std {
[…]
using csub_match = sub_match<const char*>;
using wcsub_match = sub_match<const wchar_t*>;
using ssub_match = sub_match<string::const_iterator>;
using wssub_match = sub_match<wstring::const_iterator>;
// 32.8.3 [re.submatch.op], sub_match non-member operators
template<class BiIter>
bool operator==(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs);
[…]
template<class BiIter>
bool operator>=(const sub_match<BiIter>& lhs,
const typename iterator_traits<BiIter>::value_type& rhs);
template<class charT, class ST, class BiIter>
basic_ostream<charT, ST>&
operator<<(basic_ostream<charT, ST>& os, const sub_match<BiIter>& m);
[…]
}
Change 32.8 [re.submatch], class template sub_match synopsis, as indicated:
namespace std {
template<class BidirectionalIterator>
class sub_match : public pair<BidirectionalIterator, BidirectionalIterator> {
public:
using value_type =
typename iterator_traits<BidirectionalIterator>::value_type;
[…]
int compare(const sub_match& s) const;
int compare(const string_type& s) const;
int compare(const value_type* s) const;
int compare(basic_string_view<value_type> s) const;
};
}
Change 32.8.2 [re.submatch.members] as indicated:
int compare(const value_type* s) const;-7- Returns: str().compare(s).
int compare(basic_string_view<value_type> s) const;-?- Returns: str().compare(s).
Change sub-clause 32.8.3 [re.submatch.op] as indicated:
31.9.2 sub_match non-member operators [re.submatch.op]
template<class BiIter> bool operator==(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs);
-1- Returns: lhs.compare(rhs) == 0.[…]template<class BiIter> bool operator>=(const sub_match<BiIter>& lhs, const typename iterator_traits<BiIter>::value_type& rhs);
-42- Returns: !(lhs < rhs).template<class charT, class ST, class BiIter> basic_ostream<charT, ST>& operator<<(basic_ostream<charT, ST>& os, const sub_match<BiIter>& m);-43- Returns: os << m.str().
Class template sub_match provides overloaded relational operators (7.6.9 [expr.rel]) and equality operators (7.6.10 [expr.eq]) for comparisons with another sub_match, with a string, or with a single character. The expressions shown in Table ?? are valid when one of the operands is a type S, that is a specialization of sub_match, and the other expression is one of:
(?.?) — a value x of a type S, in which case STR(x) is x.str();
(?.?) — a value x of type basic_string<S::value_type, T, A> for any types T and A, in which case STR(x) is basic_string_view<S::value_type>(x.data(), x.length());
(?.?) — a value x of type basic_string_view<S::value_type, T> for any type T, in which case STR(x) is basic_string_view<S::value_type>(x.data(), x.length());
(?.?) — a value x of a type convertible to const S::value_type*, in which case STR(x) is basic_string_view<S::value_type>(x);
(?.?) — a value x of type convertible to S::value_type, in which case STR(x) is basic_string_view<S::value_type>(&x, 1).
Table ?? — sub_match comparisons Expression Return type Operational
semanticss == t bool STR(s).compare(STR(t)) == 0 s != t bool STR(s).compare(STR(t)) != 0 s < t bool STR(s).compare(STR(t)) < 0 s > t bool STR(s).compare(STR(t)) > 0 s <= t bool STR(s).compare(STR(t)) <= 0 s >= t bool STR(s).compare(STR(t)) >= 0
Section: 16.4.5.8 [res.on.functions] Status: New Submitter: Casey Carter Opened: 2018-07-07 Last modified: 2018-11-27
Priority: 3
View all other issues in [res.on.functions].
View all issues with New status.
Discussion:
16.4.5.8 [res.on.functions]/2 states:
While undefined behavior is appropriate for the other cases specified in the earlier bullets, which describe failure to meet (runtime) semantic requirements, "instantiating a template component or evaluating a concept" with an incomplete type happens at compile-time, and could potentially be diagnosed. It would therefore be more appropriate to specify that programs which do so are ill-formed with no diagnostic required.-2- In particular, the effects are undefined in the following cases:
[…]
(2.5) — if an incomplete type (6.8 [basic.types]) is used as a template argument when instantiating a template component or evaluating a concept, unless specifically allowed for that component.
[2018-11 Reflector prioritization]
Set Priority to 3
Proposed resolution:
This wording is relative to N4762.
Change 16.4.5.8 [res.on.functions] as indicated:
(2.5) — if an incomplete type (6.8 [basic.types]) is used as a template argument when instantiating a template component or evaluating a concept, unless specifically allowed for that component.-?- Unless explicitly stated otherwise, a program that instantiates a template component or evaluates a concept with an incomplete type (6.8 [basic.types]) as a template argument is ill-formed with no diagnostic required.
Section: 16.4.4.6 [allocator.requirements] Status: New Submitter: Billy O'Neal III Opened: 2018-09-07 Last modified: 2018-12-16
Priority: 3
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with New status.
Discussion:
In annotating things required to be called by ~vector, Casey pointed out that several operations I guarded with noexcept aren't actually mandated by the standard to be noexcept. However, the STL, and more specifically here, containers, consider inability to destroy an element an unrecoverable condition. This is evidenced for the whole STL by 16.4.6.13 [res.on.exception.handling]/3 "Every destructor in the C ++ standard library shall behave as if it had a non-throwing exception specification.".
As a result, allocator::destroy and fancy pointer operations must be non-throwing for valid input, or the containers don't make any sense. This is obvious for things like vector::~vector, but less obviously the containers rely on these guarantees whenever inserting more than one element, etc. Moreover, we too narrowly specify the domain of the pointer_traits::pointer_to requirement in the Cpp17Allocator requirements, because any node-based container that uses container-internal sentinel nodes needs to be able to form pointers to said sentinel nodes; that operation must also be non-throwing.[2018-09 Reflector prioritization]
Set Priority to 3
Proposed resolution:
This wording is relative to N4762.
Modify 16.4.4.6 [allocator.requirements], Table 32 "Descriptive variable definitions" as indicated:
Table 32 — Descriptive variable definitions Variable Definition … YY the type allocator_traits<Y> Z an allocator-aware container type (24.2.2.1 [container.requirements.general]) … y a value of type XX::const_void_pointer obtained by
conversion from a result value of YY::allocate, or else a
value of type (possibly const) std::nullptr_t.z an lvalue of type Z such that z.get_allocator() == a r1 a reference to any member subobject of z n a value of type XX::size_type. …
Modify 16.4.4.6 [allocator.requirements], Table 33 "Cpp17Allocator requirements" as indicated:
Table 33 — Cpp17Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault … pointer_traits<
X::pointer
>::pointer_to(r)X::pointer S same as p.
Throws: Nothing.pointer_traits<
X::pointer
>::pointer_to(r1)A value of type YY::pointer or
YY::const_pointer k such that
*k is r1.
Throws: Nothing.… a.destroy(c) (not used) Effects: Destroys the object at c.
Throws: Nothing.c->~C() …
Modify 16.4.4.6 [allocator.requirements], p5, as indicated:
-5- An allocator type X shall satisfy the Cpp17CopyConstructible requirements (Table 26). The X::pointer, X::const_pointer, X::void_pointer, and X::const_void_pointer types shall satisfy the Cpp17NullablePointer requirements (Table 30). No constructor, comparison function, copy operation, move operation, or swap operation on these pointer types shall exit via an exception. X::pointer and X::const_pointer shall also satisfy the requirements for a random access iterator (25.3.5.7 [random.access.iterators]) and of a contiguous iterator (25.3.1 [iterator.requirements.general]) and operations in those requirements shall not exit via an exception so long as resulting iterators are dereferencable or past-the-end.
Modify 20.2.9.3 [allocator.traits.members], as indicated:
template<class T> static void destroy(Alloc& a, T* p);-6- Effects: Calls a.destroy(p) if that call is well-formed; otherwise, invokes p->~T().
-?- Throws: Nothing.
Section: 20.3.1.3 [unique.ptr.single] Status: New Submitter: Jonathan Wakely Opened: 2018-09-17 Last modified: 2018-10-06
Priority: 3
View other active issues in [unique.ptr.single].
View all other issues in [unique.ptr.single].
View all issues with New status.
Discussion:
20.3.1.3 [unique.ptr.single] p1 says:
The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function object type (19.14), lvalue reference to function, or lvalue reference to function object type for which, given a value d of type D and a value ptr of type unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter.
That means this is undefined:
#include <memory>
struct IncompleteBase;
struct Deleter {
void operator()(IncompleteBase*) const;
};
struct IncompleteDerived;
struct X {
std::unique_ptr<IncompleteDerived, Deleter> p;
~X();
};
unique_ptr::pointer is IncompleteDerived*, but is_invocable<Deleter, IncompleteDerived*> is unknowable until the type is complete (see LWG 3099 etc).
The intention is that IncompleteDerived only needs to be complete when the deleter is invoked, which is in the definition of X::~X() for this example. But the requirement for d(ptr) to be valid requires a complete type. If the unique_ptr implementation adds static_assert(is_invocable_v<D, pointer>) to enforce the requirement, the example above fails to compile. GCC recently added that assertion. Do we want to relax that requirement, or do we want to force the code above to define Deleter::pointer as IncompleteBase* so that the is_invocable condition can be checked? The destructor and reset member function already require that the deleter can be invoked (and that Requires: element will be turned into a Mandates: one soon). We can just remove that requirement from the preamble for the class template, or say that the expression only needs to be valid when the destructor and reset member are instantiated. We could also rephrase it in terms of is_invocable_v<D, unique_ptr<T, D>::pointer>.[2018-10 Reflector prioritization]
Set Priority to 3
Proposed resolution:
Section: 24.6.8 [stack], 24.6.6 [queue] Status: Open Submitter: Marshall Clow Opened: 2018-10-02 Last modified: 2020-05-09
Priority: 3
View all other issues in [stack].
View all issues with Open status.
Discussion:
24.6.8 [stack] p1 says:
Any sequence container supporting operations back(), push_back() and pop_back() can be used to instantiate stack.
but then in 24.6.8.2 [stack.defn] we have the following code:
template<class... Args>
decltype(auto) emplace(Args&&... args)
{ return c.emplace_back(std::forward<Args>(args)...); }
The same pattern appears in 24.6.6 [queue].
I see two ways to resolve this:
The first is to add emplace_back() to the list of requirements for underlying containers for stack and queue
The second is to replace the calls to c.emplace_back(std::forward<Args>(args)...) with c.emplace(c.end(), std::forward<Args>(args)...). We can do this w/o messing with the list above because emplace is part of the sequence container requirements, while emplace_back is not. I checked the libc++ implementation of vector, deque, and list, and they all do the same thing for emplace(end(), ...) and emplace_back(...).
[2019-02; Kona Wednesday night issue processing]
Status to Open; Casey to provide updated wording, and re-vote on reflector.
Polls were: NAD - 5-1-3; "Option B" - 2-5-2 and "Probe the container" - 7-2-0
Previous resolution [SUPERSEDED]:This wording is relative to N4762.
I have prepared two mutually exclusive options.
Option A a requirement for emplace_back to the underlying container.
Option B one replaces the calls to emplace_back with calls to emplace.Option A
Edit 24.6.8 [stack], as indicated:
Any sequence container supporting operations back(), push_back(), emplace_back() and pop_back() can be used to instantiate stack.
Edit 24.6.6.1 [queue.defn], as indicated:
Any sequence container supporting operations front(), back(), push_back(), emplace_back() and pop_front() can be used to instantiate queue.
Option B
Edit 24.6.8.2 [stack.defn], class template stack definition, as indicated:
template<class... Args> decltype(auto) emplace(Args&&... args) { return c.emplace_back(c.end(), std::forward<Args>(args)...); }Edit 24.6.6.1 [queue.defn], class template queue definition, as indicated:
template<class... Args> decltype(auto) emplace(Args&&... args) { return c.emplace_back(c.end(), std::forward<Args>(args)...); }
[2020-05 Casey provides new wording]
This is the "probe for emplace_back with fallback to emplace" approach that LWG wanted to see wording for in Kona.
[2020-05-09; Reflector prioritization]
Set priority to 3 after reflector discussions.
Proposed resolution:
This wording is relative to N4861.
Edit 24.6.8.2 [stack.defn], class template stack definition, as indicated:
template<class... Args>
decltype(auto) emplace(Args&&... args) {
if constexpr (requires { c.emplace_back(std::forward<Args>(args)...); }) {
return c.emplace_back(std::forward<Args>(args)...);
} else {
return c.emplace(c.end(), std::forward<Args>(args)...);
}
}
Edit 24.6.6.1 [queue.defn], class template queue definition, as indicated:
template<class... Args>
decltype(auto) emplace(Args&&... args) {
if constexpr (requires { c.emplace_back(std::forward<Args>(args)...); }) {
return c.emplace_back(std::forward<Args>(args)...);
} else {
return c.emplace(c.end(), std::forward<Args>(args)...);
}
}
Section: 19.5.8.2 [syserr.syserr.members] Status: New Submitter: Peter Dimov Opened: 2018-10-02 Last modified: 2020-04-07
Priority: 3
View all other issues in [syserr.syserr.members].
View all issues with New status.
Discussion:
The constructor for std::system_error taking a single argument of type std::error_code is not marked explicit, which allows implicit conversions from error_code to system_error. I think that this is an oversight and not intentional, and that we should make this constructor explicit.
[2020-04-07 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
This wording is relative to N4762.
Change 19.5.8.1 [syserr.syserr.overview] p2, class system_error synopsis, as indicated
namespace std {
class system_error : public runtime_error {
public:
system_error(error_code ec, const string& what_arg);
system_error(error_code ec, const char* what_arg);
explicit system_error(error_code ec);
system_error(int ev, const error_category& ecat, const string& what_arg);
system_error(int ev, const error_category& ecat, const char* what_arg);
system_error(int ev, const error_category& ecat);
const error_code& code() const noexcept;
const char* what() const noexcept override;
};
}
Change 19.5.8.2 [syserr.syserr.members] as indicated
explicit system_error(error_code ec);-5- Effects: […]
-6- Ensures: […]
Section: 22.3.4 [pair.astuple], 22.6.4 [variant.helper], 24.3.7.7 [array.tuple], 20.2.3.2 [pointer.traits.types], 20.2.9.2 [allocator.traits.types], 20.5.2 [allocator.adaptor.types] Status: New Submitter: Walter Brown Opened: 2018-11-08 Last modified: 2020-09-06
Priority: 3
View all other issues in [pair.astuple].
View all issues with New status.
Discussion:
In N4778, 22.3.4 [pair.astuple], 22.6.4 [variant.helper], and 24.3.7.7 [array.tuple], are partly specified via a fictitious descriptive element Value: Moreover, 99 [span.tuple] is on track to do likewise in the near future.
Let's invent such a Value: element and properly document it within 16.3.2.4 [structure.specifications], or else let's respecify the offending uses.[2018-11 Reflector prioritization]
Set Priority to 3
[2020-05-01; Daniel comments]
It should be pointed out that the originally referred to Value: element has since then be renamed to Type: but the reported problem (its lack of definition) still exists in N4861.
Proposed resolution:
Section: 5.2 [fund.ts.v3::memory.observer.ptr] Status: Open Submitter: Alisdair Meredith Opened: 2018-11-14 Last modified: 2022-10-12
Priority: 3
View all issues with Open status.
Discussion:
Addresses: fund.ts.v3
From the wording, function pointers are never clearly considered. P1 infers support for only objects, but it is not clear that wording is intended to be a deliberate restriction, or is casual wording.
Paragraph 2 mandates that we cannot instantiate for reference types, and do support incomplete types, but still does not consider function types. Calling out references specifically, as they are not object types, suggests the inferred support for only objects in p1 is more a case of casual phrasing, than deliberate intent. However, if we did intend to support function pointers, we may want to consider adding a function-call operator, constrained to supporting pointer-to function types. One other possibility is that the explicit conversion to pointer already serves that purpose, although I need to double-check the core language that contextual conversions to function pointer are considered when invoking the function call operator. If this is the case, then I suggest a note to that effect in [memory.observer.ptr.conv] instead.[2018-11 Reflector prioritization]
Set Priority to 3
[2022-10-12 LWG telecon]
Discussed on reflector in July 2022, no consensus on how observer_ptr is even meant to be used.
shared_ptr supports function types, no good reason to disallow them here.
No desire to make any change for LFTSv3, but keep the issue open until/unless the TS is withdrawn. That way we will be less likely to forget about this if observer_ptr is propsed for the IS.
Proposed resolution:
Section: 28.7.3 [c.math.hypot3] Status: New Submitter: Matthias Kretz Opened: 2018-12-06 Last modified: 2018-12-21
Priority: 3
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Discussion:
The 2-arg hypot function specified in the C standard is required to avoid overflow and underflow (7.12.7.3 p2). Furthermore C's Annex F (IEC 60559 floating-point arithmetic) defines special cases for inputs of ±0 and ±inf (F. 10.4.3). The 3-arg hypot function 28.7.3 [c.math.hypot3] is only specified as "Returns: .". This is inconsistent with the 2-arg overload.
It is not clear whether C's Annex F is "imported" into the C++ standard. [cmath.syn] p1 suggests that it is: "The contents and meaning of the header <cmath> are the same as the C standard library header <math.h>, […]".[2018-12-21 Reflector prioritization]
Set Priority to 3
Proposed resolution:
This wording is relative to N4778.
[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
Option A
Modify 28.7.3 [c.math.hypot3] as indicated:
float hypot(float x, float y, float z); double hypot(double x, double y, double z); long double hypot(long double x, long double y, long double z);-?- Effects: The hypot functions compute the square root of the sum of the squares of x, y, and z, without undue overflow or underflow. A range error may occur.
-1- Returns: . -?- Remarks: If __STDC_IEC_559__ is defined, the following guarantees hold:
hypot(x, y, z), hypot(x, z, y), hypot(z, y, x), and hypot(x, y, -z) are equivalent.
if y2 + z2 == ±0, hypot(x, y, z) is equivalent to fabs(x).
hypot(±∞, y, z) returns +∞, even if y and/or z is a NaN.
Option B
Add a note that clarifies that the behavior of the 3-arg hypot function differs from the C specification. I.e. that no special guarantees wrt. over-/underflow or special values are given.
Section: 21.3.7 [meta.rel] Status: New Submitter: Casey Carter Opened: 2018-12-09 Last modified: 2019-03-16
Priority: 3
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Discussion:
Per Table 49 in 21.3.7 [meta.rel], the preconditions for both is_convertible<From, To> and is_nothrow_convertible<From, To> are:
From and To shall be complete types, arrays of unknown bound, or cv void types.
Consequently, this program fragment:
struct S; static_assert(is_convertible_v<S, const S&>);
has undefined behavior despite that the actual behavior of is_convertible specified in [meta.rel]/5:
-5- The predicate condition for a template specialization is_convertible<From, To> shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:
To test() { return declval<From>(); }[ Note: …]
is well-formed: declval<S>() is an xvalue of type S, which certainly does implicitly convert to const S&. We should relax the precondition to allow this perfectly valid case (and similar cases like is_convertible<S, S&&>), letting the cases that would in fact be invalid fall through to the blanket "incompletely-defined object type" wording in [meta.rqmts]/5.
[2018-12-21 Reflector prioritization]
Set Priority to 3
Proposed resolution:
This wording is relative to N4791.
Modify Table 49 in 21.3.7 [meta.rel] as follows:
| Template | Condition | Comments |
|---|---|---|
| […] | […] | […] |
template<class From, class To> struct is_convertible; |
see below |
complete type array or cv void |
template<class From, class To> struct is_nothrow_convertible; |
is_convertible_v<From, To> is true and the conversion, as defined by is_convertible, is known not to throw any exceptions (7.6.2.7 [expr.unary.noexcept]) |
complete type array or cv void |
| […] | […] | […] |
Section: 25.6.4 [istreambuf.iterator] Status: New Submitter: Jonathan Wakely Opened: 2019-02-21 Last modified: 2019-10-30
Priority: 3
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Discussion:
The current working paper requires iterator_traits<Iter>::pointer to be void if Iter doesn't define operator-> (25.3.2.3 [iterator.traits]). We recently removed operator-> from istreambuf_iterator via LWG 2790, therefore either its pointer member should be void, or there should be a partial specialization of iterator_traits. Do we want to change unspecified to void in the class synopsis in [istreambuf.iterator]?
[2019-03-03, Daniel provides concrete wording]
[2019-03-05 Priority set to 3 after reflector discussion]
[2019-03-05, Daniel comments]
With the acceptance of P1252R2 the committee decided to deprecate operator-> of move_iterator, interestingly without mentioning what should happen with its current pointer typedef (which is equal to the template parameter Iterator and thus never void). Effectively this is a very similar situation as for the here discussed istreambuf_iterator case and it seems attractive to me to solve both cases similarly.
[2019-10-30, Jonathan comments]
Also, reverse_iterator::operator->() is now constrained and so not always defined, but reverse_iterator::pointer is defined unconditionally.
Proposed resolution:
This wording is relative to N4800.
Change class template istreambuf_iterator synopsis, 25.6.4 [istreambuf.iterator], as indicated:
template<class charT, class traits = char_traits<charT>>
class istreambuf_iterator {
public:
using iterator_category = input_iterator_tag;
using value_type = charT;
using difference_type = typename traits::off_type;
using pointer = voidunspecified;
using reference = charT;
[…]
};
Section: 24.6.7 [priority.queue] Status: New Submitter: Jonathan Wakely Opened: 2019-02-21 Last modified: 2019-03-05
Priority: 3
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Discussion:
We don't require that the Compare template parameter can be invoked with arguments of the queue's value type. It seems like something we can use Mandates: for, since it'll be ill-formed otherwise. Something like is_invocable_r_v<bool, Compare&, value_type&, value_type&>.
This might also apply to the Compare parameter for the merge and sort functions of forward_list and list.[2019-03-05 Priority set to 3 after reflector discussion]
Proposed resolution:
Section: 20.2.8.2 [allocator.uses.construction] Status: New Submitter: Jonathan Wakely Opened: 2019-02-28 Last modified: 2020-05-01
Priority: 3
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Discussion:
The new functions added by P0591R4 misbehave for cv-qualified types. A specialization std::uses_allocator<X, Alloc> will not match const X, so std::uses_allocator_construction_args<const X> will return a different result from std::uses_allocator_construction_args<X>. It makes no sense to construct X and const X differently, either the type wants to use an allocator or it doesn't. I think std::uses_allocator_construction_args<T> should remove cv-qualifiers before checking uses_allocator, so that it works consistently.
We could consider changing std::make_obj_using_allocator to also strip cv-qualifiers, but it's not necessary as C++17 guaranteed elision works even for prvalues of const types. We only need to make the construction args ignore cv-qualifiers. We don't want to make cv-qualified types ill-formed, because that would require users of uses-allocator construction to strip cv-qualifiers before using these functions, e.g. in cases like std::tuple<const int> t(allocator_arg, alloc, 1);[2019-03-15 Priority set to 3 after reflector discussion]
Previous resolution [SUPERSEDED]:This wording is relative to N4800.
Change 20.2.8.2 [allocator.uses.construction] as indicated:
template <class T, class Alloc, class... Args> auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) -> see below;[…]-4- Constraints: T is not a specialization of pair.
-5- Returns: A tuple value determined as follows, where U denotes the type remove_cv_t<T>:
(5.1) — If uses_allocator_v<
TU, Alloc> is false and is_constructible_v<T, Args...> is true, return forward_as_tuple(std::forward<Args>(args)...).(5.2) — Otherwise, if uses_allocator_v<
TU, Alloc> is true and is_constructible_v<T, allocator_arg_t, Alloc, Args...> is true, returntuple<allocator_arg_t, const Alloc&, Args&&...>( allocator_arg, alloc, std::forward<Args>(args)...)(5.3) — Otherwise, if uses_allocator_v<
TU, Alloc> is true and is_constructible_v<T, Args..., Alloc> is true, return forward_as_tuple(std::forward<Args>(args)..., alloc).(5.4) — Otherwise, the program is ill-formed.
template <class T, class Alloc, class Tuple1, class Tuple2> auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t, Tuple1&& x, Tuple2&& y) -> see below;-6- Constraints: T is a specialization of pair.
-7- Effects: For T specified as (possibly const) pair<T1, T2>, equivalent to: […]template <class T, class Alloc, class... Args> T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc, Args&&... args);-17- Effects: Equivalent to:
return ::new(static_cast<void*>voidify(*p)) T(make_obj_using_allocator<T>(alloc, std::forward<Args>(args)...));
[2020-05-01; Daniel syncs wording with recent working draft]
The previously needed change for uninitialized_construct_using_allocator is no longer required, because the reworded call to construct_at does do the right thing now.
Proposed resolution:
This wording is relative to N4861.
Change 20.2.8.2 [allocator.uses.construction] as indicated:
template <class T, class Alloc, class... Args> constexpr auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) noexcept -> see below;[…]-4- Constraints: T is not a specialization of pair.
-5- Returns: A tuple value determined as follows, where U denotes the type remove_cv_t<T>:
(5.1) — If uses_allocator_v<
TU, Alloc> is false and is_constructible_v<T, Args...> is true, return forward_as_tuple(std::forward<Args>(args)...).(5.2) — Otherwise, if uses_allocator_v<
TU, Alloc> is true and is_constructible_v<T, allocator_arg_t, const Alloc&, Args...> is true, returntuple<allocator_arg_t, const Alloc&, Args&&...>( allocator_arg, alloc, std::forward<Args>(args)...)(5.3) — Otherwise, if uses_allocator_v<
TU, Alloc> is true and is_constructible_v<T, Args..., const Alloc&> is true, return forward_as_tuple(std::forward<Args>(args)..., alloc).(5.4) — Otherwise, the program is ill-formed.
template <class T, class Alloc, class Tuple1, class Tuple2> constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t, Tuple1&& x, Tuple2&& y) noexcept -> see below;-6- Constraints: T is a specialization of pair.
-7- Effects: For T specified as (possibly const) pair<T1, T2>, equivalent to: […]
Section: 16.3.2.4 [structure.specifications] Status: New Submitter: Daniel Krügler Opened: 2019-03-04 Last modified: 2020-06-11
Priority: 3
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Discussion:
The working paper uses the special elements Mandates:, Expects: as well as Requires: to types, albeit 16.3.2.4 [structure.specifications] defines them only for functions, for example 16.3.2.4 [structure.specifications] sub-bullet (3.4):
Expects: the conditions (sometimes termed preconditions) that the function assumes to hold whenever it is called.
Examples for such usages on types are (from N4800):
23.2.3 [char.traits.typedefs] for types int_type and state_type
22.3.4 [pair.astuple] for tuple_element<I, pair<T1, T2>>::type
22.4.7 [tuple.helper] for tuple_element<I, tuple<Types...>>::type
22.4.11 [tuple.traits] for uses_allocator<tuple<Types...>, Alloc>
Table 62 — "Container requirements" for type XX::value_type
Table 65 — "Allocator-aware container requirements" for type allocator_type
Table 69 — "Associative container requirements" for types X::value_type and X::key_compare
Table 70 — "Unordered associative container requirements" for types X::value_type and X::key_equal
Instead of replacing these elements usages for these places by extra wording to reach the same effects I recommend to update instead 16.3.2.4 [structure.specifications] to ensure that requirement-expressing elements are defined in a way that it also allows to express requirements imposed on types by these elements to standardize "existing practice".
Considering details, it seems obvious that Mandates:, Expects: as well as Requires: are "suitable" to be defined for types (With the acceptance of P1463R1 there are now also Mandates: for types such as Table 65 — "Allocator-aware container requirements" for type allocator_type). For Constraints: the meaning would not be so clear: Should it mean that there is conditionally a type defined or not? According to the submitters knowledge there are currently no known examples for Constraints: to specify constraint on types, therefore I'm suggesting to restrict this extension to Mandates:, Expects:, and Requires: alone.[2019-03-15 Priority set to 3 after reflector discussion]
[2019-03-15; Daniel comments and provides wording]
During the preparation of the wording for this issue it was found that we should allow Remarks: elements to be used for other things than functions. One example of imposed restrictions can be found in 17.11.3 [cmp.common]:
template<class... Ts> struct common_comparison_category { using type = see below; };-2- Remarks: The member typedef-name type denotes the common comparison type (11.10.3 [class.spaceship]) of Ts..., the expanded parameter pack. […]
The discussion of this issue speaks of "type" restrictions (versus the specified restrictions on functions), because even the non-type template argument restrictions of 22.3.4 [pair.astuple] appear in the context of a member type specification, but there are examples where not really a single (member) type is involved, e.g. in the 22.4.7 [tuple.helper] example mentioned above.
Another example is when such elements are used for the specification of template specializations, e.g. in 22.4.7 [tuple.helper]:template<class T> struct tuple_size;-1- Remarks: All specializations of tuple_size shall satisfy the Cpp17UnaryTypeTrait requirements (21.3.2 [meta.rqmts]) with a base characteristic of integral_constant<size_t, N> for some N.
Besides class template specializations, a second relevant use-case is the specification of member types (Which are not necessarily part of a template), typically within the requirement tables, e.g. in Table 62 — "Container requirements"'s entry X::value_type:
Requires: T is Cpp17Erasable from X
The suggested wording tries to cover the generalization by means of the term "non-function entities" in addition to the existing functions to prevent being enforced to enumerate all entities to which the extended rules apply.
Previous resolution [SUPERSEDED]:This wording is relative to N4810.
Change 16.3.2.4 [structure.specifications], as indicated:
-3- Descriptions of function semantics contain the following elements (as appropriate); some of these elements may also appear in the description of non-function entities as denoted below: (footnote […])
(3.1) — Requires: the preconditions imposed on a non-function entity, or for calling the function.
(3.2) — Constraints: […]
(3.3) — Mandates: the conditions that, if not met, render the program ill-formed. [Example: An implementation might express such a condition via the constant-expression in a static_assert-declaration (Clause 9). If the diagnostic is to be emitted only after the function has been selected by overload resolution, an implementation might express such a condition via a constraint-expression (13.5.3 [temp.constr.decl]) and also define the function as deleted. — end example]
(3.4) — Expects: the conditions (sometimes termed preconditions) imposed on a non-function entity, or that the function assumes to hold whenever it is called. [Example: An implementation might express such conditions via an attribute such as [[expects]] ( [dcl.attr.contract]) on a function declaration. However, some such conditions might not lend themselves to expression via code. — end example]
[…]
(3.11) — Remarks: additional semantic constraints
on the function.[…]
Change 99 [res.on.required], as indicated:
-1- Violation of any preconditions specified in a
-2- Violation of any preconditions specified in anfunction'sRequires: element results in undefined behavior unless the function's Throws: element specifies throwing an exception whenthea function's precondition is violated.function'sExpects: element results in undefined behavior.
[2020-05-01; Daniel comments and adjusts wording to recent working draft]
It should be pointed out that the originally referred to Expects: element has since then be renamed to Preconditions: and that the Requires: element does now only occur in annex D.
[2020-06-11; Jonathan comments]
This issue also affects some type traits such as alignment_of and make_signed/make_unsigned. In addition to clarifying what Mandates: means on a non-function we need to decide exactly what is being mandated in the type traits. Is instantiating the class template ill-formed, or just odr-using the nested type or value member?
Proposed resolution:
This wording is relative to N4861.
Change 16.3.2.4 [structure.specifications], as indicated:
-3- Descriptions of function semantics contain the following elements (as appropriate); some of these elements may also appear in the description of non-function entities as denoted below: (footnote […])
(3.1) — Constraints: […]
(3.2) — Mandates: the conditions that, if not met, render the program ill-formed. [Example: An implementation might express such a condition via the constant-expression in a static_assert-declaration (9.1 [dcl.pre]). If the diagnostic is to be emitted only after the function has been selected by overload resolution, an implementation might express such a condition via a constraint-expression (13.5.3 [temp.constr.decl]) and also define the function as deleted. — end example]
(3.3) — Preconditions: the conditions imposed on a non-function entity, or that the function assumes to hold whenever it is called.
[…]
(3.10) — Remarks: additional semantic constraints
on the function.[…]
Change [res.on.expects], as indicated:
-1- Violation of any preconditions specified in a
function'sPreconditions: element results in undefined behavior.
Change D.11 [depr.res.on.required], as indicated:
[Drafting note: Interestingly, albey the Requires: element has nearly vanished, the issue is still relevant, see D.19 [depr.meta.types]]
-1- Violation of any preconditions specified in a
function'sRequires: element results in undefined behavior unless the function's Throws: element specifies throwing an exception whenthea function's precondition is violated.
Section: 25.4.3 [iterator.operations] Status: New Submitter: Billy O'Neal III Opened: 2019-04-03 Last modified: 2022-04-22
Priority: 3
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Discussion:
MSVC++ (and apparently libc++) have asserts that std::prev only accepts BidirectionalIterators, because it's declared in the standard as accepting only BidirectionalIterator. libc++ changed their tests (in this commit), apparently from a bug report from Ville and Jonathan, saying that one could theoretically call std::prev with a negative number.
The standardese in [iterator.operations] strongly indicates that prev requires a BidirectionalIterator, but I don't see the usual wording that connects template type parameters of that name to the <algorithm> requirements or similar. So perhaps one could argue that the name Bidirectional there has no meaning. Even if that is the case, that's a defect in the other direction.[2019-06-12 Priority set to 3 after reflector discussion]
[2022-04-22; Jonathan adds a comment]
P2408 changes the requirements for types substituting BidirectionalIterator etc. in the Algorithms clause. We should consider whether that is appropriate here, especially as algorithms might make use of std::prev internally. An algorithm that was changed by P2408 to accept types that model bidirectional_iterator instead of requiring Cpp17BidirectionalIterator might have to stop using std::prev if we don't resolve this issue to allow it.
We should consider whether distance, advance and next need the same treatment.
Proposed resolution:
This wording is relative to N4810.
[Drafting Note: Three mutually exclusive options are prepared, depicted below by Option A, Option B, and Option C, respectively.]
Option A
NAD, the name BidirectionalIterator actually means that prev requires bidirectional iterators, in which case this change to libcxx is incorrect.
Option B
Modify 25.2 [iterator.synopsis], header <iterator> synopsis, as indicated:
// 25.4.3 [iterator.operations], iterator operations […] template<classBidirectionalInputIterator> constexprBidirectionalInputIterator prev(BidirectionalInputIterator x, typename iterator_traits<BidirectionalInputIterator>::difference_type n = 1);
Modify 25.4.3 [iterator.operations] as indicated:
template<classBidirectionalInputIterator> constexprBidirectionalInputIterator prev(BidirectionalInputIterator x, typename iterator_traits<BidirectionalInputIterator>::difference_type n = 1);-7- Effects: Equivalent to: advance(x, -n); return x;
Option C
The intent of the wording is that the template parameters apply requirements, and the defect is that they do not. We should add a requirement in 25.4.3 [iterator.operations]/1 to the effect that the template parameter names impose said requirements.
Section: 24.7.2.2.1 [span.overview] Status: New Submitter: Johel Ernesto Guerrero Peña Opened: 2019-05-04 Last modified: 2020-09-06
Priority: 2
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Discussion:
span doesn't explicitly point out when its accessed elements are invalidated like string_view does in 23.3.3.4 [string.view.iterators] p2.
[2019-06-12 Priority set to 2 after reflector discussion]
Proposed resolution:
This wording is relative to N4810.
Modify 24.7.2.2.1 [span.overview] as indicated:
-4- ElementType is required to be a complete object type that is not an abstract class type.
-?- For a span s, any operation that invalidates a pointer in the range [s.data(), s.data() + s.size()) invalidates pointers, iterators, and references other than *this returned from s's member functions.
Section: 21.3.8.7 [meta.trans.other] Status: New Submitter: Casey Carter Opened: 2019-05-12 Last modified: 2022-04-25
Priority: 3
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Discussion:
P0898R4 "The One Ranges Proposal" added a new fallback case to the definition of common_type in 21.3.8.7 [meta.trans.other], bullet 3.3.4:
Otherwise, if COND_RES(CREF(D1), CREF(D2)) denotes a type, let C denote the type decay_t<COND_RES(CREF(D1), CREF(D2))>.
Per para 3.3, D1 and D2 are decayed types. If both are void, bullet 3.3.4 is not reached. If either is an abominable function type or void, the COND_RES type expression above is ill-formed and bullet 3.3.4 does not apply. In all cases in which the COND_RES expression is well-formed, D1 and D2 denote cv-unqualified non-array object types. Given that fact, (1) CREF(D1) and CREF(D2) are equivalent to const D1& and const D2&, respectively, and (2) the COND_RES expression is equivalent to decltype(false ? declval<const D1&>() : declval<const D1&>()), i.e., the second and third operands of the conditional operator are lvalues of type const D1 and const D2, respectively.
[expr.cond]/3 cannot apply since the operands are not glvalue bit-fields. If D1 and D2 are the same type, [expr.cond]/4 does not apply. If D1 and D2 are different types, there are a few cases to consider:If [expr.cond]/4.1 applies, one operand is converted into an lvalue reference to the type of the other, i.e., both resulting operands are lvalues of type either const D1 or const D2.
[expr.cond]/4.2 cannot apply since neither operand is an xvalue.
[expr.cond]/4.3.1 cannot apply since it would imply that the operands have the same type.
If [expr.cond]/4.3.2 applies — if either D1 or D2 is a base class of the other — again the resulting operands are lvalues of type either const D1 or const D2.
If [expr.cond]/4.3.3 applies, the either the const D1& operand converts to const D2 or the const D2& operand converts to const D1.
If none of the sub-bullets in [expr.cond]/4 applies, the operands are left unchanged.
[expr.cond]/5 applies if the operands initially had the same type, or in cases 1 and 4 above. The conditional expression is an lvalue of type const D1 or const D2, and the COND_RES expression yields const D1& or const D2&.
Only cases 5 and 6 reach [expr.cond]/6. This paragraph performs overload resolution, which may result in converting both operands to the same non-class type to invoke a builtin conditional operator "overload". [expr.cond]/7 applies standard conversions including array-to-pointer and function-to-pointer conversion to the operands. Consequently, the operands are once more "decayed" if [expr.cond]/6 converted them to an array or function type. Again case-by-case:[expr.cond]/7.1 applies if the operands now have the same type, which is the type of the conditional expression.
[expr.cond]/7.2 applies if the operands have arithmetic or enumeration type; the conditional expression yields the result of applying the usual arithmetic conversions.
[expr.cond]/7.3 applies if the operands have pointer type; the conditional expression yields their composite pointer type.
[expr.cond]/7.4 applies if the operands have pointer-to-member type; the conditional expression applies some more standard conversions and yields their composite pointer type.
[expr.cond]/7.5 applies if one operand has type nullptr_t and the other is either a null pointer constant or has type nullptr_t; the conditional expression yields nullptr_t.
In every case above, the conditional expression is either ill-formed, an lvalue of type const D1 or const D2, or a prvalue of a non-array non-function type. Consequently the COND_RES type expression always yields a non-array non-function type, for which decay_t and remove_cvref_t are equivalent. We can therefore replace COND_RES(CREF(D1), CREF(D2)) in [meta.trans.other]/3.3.4 with decltype(false ? declval<const D1&>() : declval<const D2&>()), and replace the usage of decay_t with remove_cvref_t.
Furthermore, there are now quite a few different cases describing the behavior of common_type. It's not clear that common_type<T...>::type is always a decayed type without in-depth analysis. We should non-normatively clarify that fact.[2019-06-12 Priority set to 3 after reflector discussion]
[2020-05-01; Daniel adjusts wording to recent working draft]
[2022-04-25; Daniel adjusts wording to recent working draft]
Proposed resolution:
This wording is relative to N4910.
Modify 21.3.8.7 [meta.trans.other] as indicated:
-2- Let:
(2.1) — CREF(A) be add_lvalue_reference_t<const remove_reference_t<A>>,(2.2) — […]
[…]
(2.9) — […]
If any of the types computed above is ill-formed, then COMMON-REF(A, B) is ill-formed.
-3- Note A: For the common_type trait applied to a template parameter pack T of types, the member type shall be either defined or not present as follows:
(3.1) — […]
(3.2) — […]
(3.3) — If sizeof...(T) is two, let the first and second types constituting T be denoted by T1 and T2, respectively, and let D1 and D2 denote the same types as decay_t<T1> and decay_t<T2>, respectively.
(3.3.1) — […]
(3.3.2) — […]
(3.3.3) — Otherwise, if
decay_t<decltype(false ? declval<D1>() : declval<D2>())>denotes a valid type, let C denote that type.
(3.3.4) — Otherwise, if
COND-RES(CREF(D1), CREF(D2))remove_cvref_t<decltype(false ? declval<const D1&>() : declval<const D2&>())>denotes a type, let C denote th
eat typedecay_t<COND-RES(CREF(D1), CREF(D2))>.(3.4) — […]
[Note: Whenever the qualified-id common_type<T...>::type is valid, it denotes the same type as decay_t<common_type<T...>::type>. — end note]
-4- Note B: […]
Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: New Submitter: Billy O'Neal III Opened: 2019-05-29 Last modified: 2020-09-06
Priority: 3
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Discussion:
I implemented the fix for LWG 3008 and Stephan pointed out there's an inconsistency here for allocate_shared<const T>.
20.3.2.2.7 [util.smartptr.shared.create] p3 says that the allocator construct call is done without removing cv qualifiers, but 20.3.2.2.7 [util.smartptr.shared.create] p7.12 says that the destroy call is done with removed cv qualifiers. The fallback for allocator_traits::construct rejects const T* (since it static_casts to void*), so the most likely outcome of attempting to do this today is to fail to compile, which is a break with C++17. Our options are:Fix the allocator model to deal with const elements somehow, which breaks compatibility with existing allocators unprepared for const elements here. We would need to extend the allocator requirements to allow const T* to be passed here, and fix our default to const_cast.
Fix allocate_shared to remove const before calling construct, which changes the experience for C++17 customers because allocate_shared constructs a T instead of a const T, but not in a way substantially different to edits P0674 already made here.
Back out allocate_shared's interaction with this part of the allocator model (reverting this part of P0674 and reopening LWG 3008).
Go around the problem by prohibiting allocate_shared<const T>, which breaks existing C++17 customers.
Billy O'Neal argues that only (2) preserves the design intent P0674 while maintaining compatibility for most allocators and most C++17 customers.
Peter Dimov argues that (1) isn't likely to break enough to matter.[2019-06-16 Priority set to 3 based on reflector discussion]
Proposed resolution:
This wording is relative to N4810.
[Drafting note: As the issue submitter prefers option (2), this is wording for that.]
Modify 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
template<class T, ...> shared_ptr<T> make_shared(args); template<class T, class A, ...> shared_ptr<T> allocate_shared(const A& a, args); template<class T, ...> shared_ptr<T> make_shared_default_init(args); template<class T, class A, ...> shared_ptr<T> allocate_shared_default_init(const A& a, args);-2- Requires: […]
[…] -7- Remarks:
(7.1) — […]
[…]
(7.5) — When a (sub)object of a non-array type U is specified to have an initial value of v, or U(l...), where l... is a list of constructor arguments, allocate_shared shall initialize this (sub)object via the expression
(7.5.1) — allocator_traits<A2>::construct(a2, pv, v) or
(7.5.2) — allocator_traits<A2>::construct(a2, pv, l...)
respectively, where pv points to storage suitable to hold an object of type remove_cv_t<U> and a2 of type A2 is a rebound copy of the allocator a passed to allocate_shared such that its value_type is remove_cv_t<U>.
Section: 30.4.3.2.3 [facet.num.get.virtuals] Status: New Submitter: Jonathan Wakely Opened: 2019-06-03 Last modified: 2019-08-23
Priority: 4
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Discussion:
30.4.3.2.3 [facet.num.get.virtuals] paragraph 4 says:
"Digit grouping is checked. That is, the positions of discarded separators is examined for consistency with use_facet<numpunct<charT>>(loc).grouping(). If they are not consistent then ios_base::failbit is assigned to err."
It's unclear what is considered consistent or not.
Obviously if the expected grouping is "1,234,567" then an input of "1,234,567" is consistent. Libstdc++, MSVC and Boost all consider "1234567" to be consistent with an expected grouping "1,234,567" (and it looks like libc++ is going to agree soon). That can be justified by saying that there are no discarded separators to examine, so no inconsistency. But what about "1234,567"? There is only one discarded separator here, and its position is consistent with the expected format. The wording should clarify that if there are no separators at all, that is OK. If there are one or more separators then they must be at the expected positions, and there must not be any missing.[2019-07 Issue Prioritization]
Priority to 4 after discussion on the reflector.
Proposed resolution:
Section: 22.6.3.2 [variant.ctor] Status: New Submitter: Louis Dionne Opened: 2019-06-04 Last modified: 2020-09-06
Priority: 2
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Discussion:
In 22.6.3.2 [variant.ctor] p5, we say:
Remarks: This function shall be constexpr if and only if the value-initialization of the alternative type T0 would satisfy the requirements for a constexpr function. […]
First of all, I find it confusing that we say "This function shall be constexpr if […]", when the declaration of the function clearly has the constexpr keyword on it unconditionally. Instead, I would use the wording "This function shall be usable in a constexpr context if […]".
Secondly, I think we shouldn't be using if-and-only-if since it restricts whether implementations can be constexpr-friendly as an extension. Instead, it seems better to just say "if". Finally, I think the condition under which the function must be constexpr-friendly is not something we can test for because it says "value-initialization of the alternative type T0 would satisfy the requirements for a constexpr function", which doesn't imply the value initialization can actually be be performed inside a constexpr context (for example the default constructor could be constexpr friendly but not marked with the constexpr keyword).[2017-06-17, Tim Song comments]
This issue is related to LWG 2833.
[2019-07 Issue Prioritization]
Priority to 2 after discussion on the reflector.
Previous resolution from Daniel [SUPERSEDED]:This wording is relative to N4810.
Modify 22.6.3.2 [variant.ctor] as indicated:
constexpr variant() noexcept(see below);-2- Effects: […]
-3- Ensures: […] -4- Throws: […] -5- Remarks: This function shall be usable in a context that requires constant evaluation if the alternative type T0 can be value-initialized in a context that requires constant evaluationconstexpr if and only if the value-initialization of the alternative type T0 would satisfy the requirements for a constexpr function. […]
[2020-06-08 Nina Dinka Ranns comments]
The revised wording provided by LWG 2833 should resolve this issue as well.
Proposed resolution:
Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: New Submitter: Billy O'Neal III Opened: 2019-06-11 Last modified: 2020-09-06
Priority: 3
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Discussion:
The new allocate_shared wording says we need to rebind the allocator back to T's type before we can call construct or destroy, but this is suboptimal (might make extra unnecessary allocator copies), and is inconsistent with the containers' behavior, which call allocator construct on whatever T they want. (For example, std::list<T, alloc<T>> rebinds to alloc<_ListNode<T>>, but calls construct(T*) without rebinding back)
It seems like we should be consistent with the containers and not require a rebind here. PR would look something like this, relative to N4810; I'm still not super happy with this wording because it looks like it might be saying a copy of the allocator must be made we would like to avoid…[2019-07 Issue Prioritization]
Priority to 3 after discussion on the reflector.
Proposed resolution:
This wording is relative to N4810.
Modify 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
[Drafting note: The edits to change pv to pu were suggested by Jonathan Wakely (thanks!). This wording also has the remove_cv_t fixes specified by LWG 3210 — if that change is rejected some of those have to be stripped here.]
template<class T, ...> shared_ptr<T> make_shared(args); template<class T, class A, ...> shared_ptr<T> allocate_shared(const A& a, args); template<class T, ...> shared_ptr<T> make_shared_default_init(args); template<class T, class A, ...> shared_ptr<T> allocate_shared_default_init(const A& a, args);-2- Requires: […]
[…] -7- Remarks:
(7.1) — […]
[…]
(7.5) — When a (sub)object of a non-array type U is specified to have an initial value of v, or U(l...), where l... is a list of constructor arguments, allocate_shared shall initialize this (sub)object via the expression
(7.5.1) — allocator_traits<A2>::construct(a2, p
vu, v) or(7.5.2) — allocator_traits<A2>::construct(a2, p
vu, l...)respectively, where p
vu is a pointer of type remove_cv_t<U>* pointsing to storage suitable to hold an object of type remove_cv_t<U> and a2 of type A2 is a potentially rebound copy of the allocator a passed to allocate_sharedsuch that its value_type is remove_cv_t<U>.(7.6) — […]
(7.7) — When a (sub)object of non-array type U is specified to have a default initial value, allocate_shared
shallinitializes this (sub)object via the expression allocator_traits<A2>::construct(a2, pvu), where pvu is a pointer of type remove_cv_t<U>* pointsing to storage suitable to hold an object of type remove_cv_t<U> and a2 of type A2 is a potentially rebound copy of the allocator a passed to allocate_sharedsuch that its value_type is remove_cv_t<U>.[…]
(7.12) — When a (sub)object of non-array type U that was initialized by allocate_shared is to be destroyed, it is destroyed via the expression allocator_traits<A2>::destroy(a2, p
vu) where pvu is a pointer of type remove_cv_t<U>* pointsing to that object of type remove_cv_t<U> and a2 of type A2 is a potentially rebound copy of the allocator a passed to allocate_sharedsuch that its value_type is remove_cv_t<U>.
Section: 17.3.1 [support.limits.general] Status: New Submitter: Jonathan Wakely Opened: 2019-06-12 Last modified: 2020-09-06
Priority: 3
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Discussion:
There are parallel overloads of algorithms in <memory>, so it should define the macro. Also, <execution> defines the exec policies for use with the algos, so that should define the macro too.
[2019-07 Issue Prioritization]
Priority to 3 after discussion on the reflector.
Proposed resolution:
This wording is relative to N4810.
Modify 17.3.1 [support.limits.general], Table 36 — "Standard library feature-test macros", as indicated:
Table 36 — Standard library feature-test macros Macro name Value Header(s) […] __cpp_lib_parallel_algorithm 201603L <algorithm> <execution> <memory> <numeric> […]
Section: 24.3.7.1 [array.overview], 24.2.2.1 [container.requirements.general] Status: New Submitter: Nevin Liber & Christian Trott Opened: 2019-06-13 Last modified: 2022-04-24
Priority: 3
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Discussion:
The requirements specified in 24.3.7.1 [array.overview] p3 are incorrect; namely:
A default constructed array<T, N> where 0 < N has linear, not constant complexity.
A default constructed array<T, 0> is empty and has constant complexity.
[2019-07 Issue Prioritization]
Priority to 3 after discussion on the reflector.
Previous resolution [SUPERSEDED]:
This wording is relative to N4810.
Modify 24.2.2.1 [container.requirements.general], Table 62 — "Container requirements", as indicated (This table can be identified by the "section" identifier [tab:container.req] in the next working draft):
Table 62 — Container requirements Expression Return type Operational
semanticsAssertion/note
pre/post-conditionComplexity […] X u; Ensures: !u.empty() for array<T, N> where 0 < N, and
Ensures: u.empty() for all other standard containers.constant(Note A)X() Ensures: !X().empty() for array<T, N> where 0 < N, and
Ensures: X().empty() for all other standard containers.constant(Note A)[…] Those entries marked "(Note A)" or "(Note B)" have linear complexity for array<T, N> where 0 < N and have constant complexity for all other standard containers.
Modify 24.3.7.1 [array.overview] as indicated:
-2- An array is an aggregate (9.4.2 [dcl.init.aggr]) that can be list-initialized with up to N elements whose types are convertible to T.
-3- An array<T, 0> satisfies all of the requirements of a container and of a reversible container (24.2 [container.requirements]). An array<T, N> where 0 < N satisfies all of the requirements of a container and of a reversible container (24.2 [container.requirements]), except that a default constructed array<T, N> object is not empty andthatboth default construction and swapdoes nothaveconstantlinear complexity. An array satisfies some of the requirements of a sequence container (24.2.4 [sequence.reqmts]). Descriptions are provided here only for operations on array that are not described in one of these tables and for operations where there is additional semantic information.
[2022-04-24; Daniel rebases wording on N4910]
Proposed resolution:
This wording is relative to N4910.
Modify 24.2.2.2 [container.reqmts] as indicated:
X u; X u = X();[…]-10- Postconditions: !u.empty() for array<T, N> where 0 < N, and u.empty() for all other standard containers.
-11- Complexity:ConstantLinear for array<T, N> where 0 < N and constant for all other standard containers.X u(rv); X u = rv;[…]-15- Postconditions: u is equal to the value that rv had before this construction.
-11- Complexity: Linear for array<T, N> where 0 < N and constant for all other standard containers.a.swap(b)-45- Result: void
-46- Effects: Exchanges the contents of a and b. -47- Complexity: Linear for array<T, N> where 0 < N and constant for all other standard containers.
Modify 24.3.7.1 [array.overview] as indicated:
-2- An array is an aggregate (9.4.2 [dcl.init.aggr]) that can be list-initialized with up to N elements whose types are convertible to T.
-3- An array<T, 0> meets all of the requirements of a container (24.2.2.2 [container.reqmts]) and of a reversible container (24.2.2.3 [container.rev.reqmts]). An array<T, N> where 0 < N meets all of the requirements of a container (24.2.2.2 [container.reqmts]) and of a reversible container (24.2.2.3 [container.rev.reqmts]), except that a default constructed array<T, N> object is not empty if N > 0 and default construction, move construction, and swap have linear complexity if N > 0. An array meets some of the requirements of a sequence container (24.2.4 [sequence.reqmts]). Descriptions are provided here only for operations on array that are not described in one of these tables and for operations where there is additional semantic information.
Section: 33.5.2 [atomics.syn] Status: New Submitter: Casey Carter Opened: 2019-06-11 Last modified: 2020-09-06
Priority: 3
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Discussion:
This well-formed C++14 program:
#include <atomic>
#include <memory>
struct Abstract { virtual void test() = 0; };
struct Concrete : Abstract { virtual void test() override {} };
int main() {
std::shared_ptr<Abstract> ptr;
std::atomic_store<Abstract>(&ptr, std::make_shared<Concrete>());
}
is ill-formed in C++17. P0558 changed the non-member non-shared_ptr atomic functions to avoid deducing from their second argument, e.g. C++14 atomic_store:
template<class T> void atomic_store(atomic<T>*, T); // #1
became C++17 atomic_store:
template<class T> void atomic_store(atomic<T>*, typename atomic<T>::value_type); // #2
The program intends to call the "other" atomic_store from D.24 [depr.util.smartptr.shared.atomic]:
template<class T> void atomic_store(shared_ptr<T>*, shared_ptr<T>); // #3
In C++14, the call expression in the sample program — std::atomic_store<Abstract>(&ptr, std::make_shared<Concrete>()) — selects overload #3; overload #1 fails to be viable due to the lack of conversions from shared_ptr<Abstract>* to atomic<Abstract>* and from shared_ptr<Concrete> to Abstract. In C++17, overload #2 doesn't get to the point of considering argument conversions: when we try to generate the declaration of the specialization for T = Abstract we must instantiate atomic<Abstract> in order to substitute typename atomic<Abstract>::value_type, but doing so violates the requirement in [atomics.types.generic] p1 that "The type of the template argument T shall be trivially copyable"
The fix is fairly straightforward since atomic<T>::value_type is always an alias for T: for those non-member atomic functions with overloads defined in D.24 [depr.util.smartptr.shared.atomic], use a different form to require that T in the type of the second parameter is non-deduced.[2019-07 Issue Prioritization]
Priority to 3 after discussion on the reflector.
Proposed resolution:
This wording is relative to N4810.
Modify 33.5.2 [atomics.syn], header <atomic> synopsis, as indicated:
[…] // 33.5.9 [atomics.nonmembers], non-member functions […] template<class T> void atomic_store(volatile atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>) noexcept; template<class T> void atomic_store(atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>) noexcept; template<class T> void atomic_store_explicit(volatile atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>, memory_order) noexcept; template<class T> void atomic_store_explicit(atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>, memory_order) noexcept; […] template<class T> T atomic_exchange(volatile atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>) noexcept; template<class T> T atomic_exchange(atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>) noexcept; template<class T> T atomic_exchange_explicit(volatile atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>, memory_order) noexcept; template<class T> T atomic_exchange_explicit(atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>, memory_order) noexcept; template<class T> bool atomic_compare_exchange_weak(volatile atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>*,typename atomic<T>::value_typetype_identity_t<T>) noexcept; template<class T> bool atomic_compare_exchange_weak(atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>*,typename atomic<T>::value_typetype_identity_t<T>) noexcept; template<class T> bool atomic_compare_exchange_strong(volatile atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>*,typename atomic<T>::value_typetype_identity_t<T>) noexcept; template<class T> bool atomic_compare_exchange_strong(atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>*,typename atomic<T>::value_typetype_identity_t<T>) noexcept; template<class T> bool atomic_compare_exchange_weak_explicit(volatile atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>*,typename atomic<T>::value_typetype_identity_t<T>, memory_order, memory_order) noexcept; template<class T> bool atomic_compare_exchange_weak_explicit(atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>*,typename atomic<T>::value_typetype_identity_t<T>, memory_order, memory_order) noexcept; template<class T> bool atomic_compare_exchange_strong_explicit(volatile atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>*,typename atomic<T>::value_typetype_identity_t<T>, memory_order, memory_order) noexcept; template<class T> bool atomic_compare_exchange_strong_explicit(atomic<T>*,typename atomic<T>::value_typetype_identity_t<T>*,typename atomic<T>::value_typetype_identity_t<T>, memory_order, memory_order) noexcept;
Modify 33.5.9 [atomics.nonmembers] as indicated:
-1- A non-member function template whose name matches the pattern atomic_f or the pattern atomic_f_explicit invokes the member function f, with the value of the first parameter as the object expression and the values of the remaining parameters (if any) as the arguments of the member function call, in order. An argument for a parameter of type atomic<T>::value_type* or type_identity_t<T>* is dereferenced when passed to the member function call. If no such member function exists, the program is ill-formed.
Section: 24.2.7 [associative.reqmts], 24.2.8 [unord.req] Status: New Submitter: Konstantin Boyarinov Opened: 2019-06-25 Last modified: 2022-04-24
Priority: 3
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Discussion:
Ordered and unordered associative containers in C++14 contained an issue, which caused an ambiguity while invoking std::map::erase when key_type of the map can be constructed from the iterator. In this case both overloads erase(const key_type&) and erase(const_iterator) could be chosen.
The issue LWG 2059 was reported and resolved in C++17 by adding an extra overload for erase in ordered and unordered associative containers which accepts iterator as an argument. C++17 also introduced new functionality for splicing ordered and unordered maps and sets. One of the extensions allows to extract a node from the container by passing either key_type& or const_iterator to the extract() member function:node_type extract(const key_type& x); node_type extract(const_iterator position);
Providing these two extract overloads causes the same problem as for erase. Consider the following example:
#include <map>
#include <string>
struct Key
{
template <typename T>
Key(const T&) {}
};
bool operator<(const Key&, const Key&) { return false; }
int main()
{
using map_type = std::map<Key, std::string>;
map_type m({ {Key(1), "a"}, {Key(2), "b"} });
map_type::iterator it = m.begin();
auto nh = m.extract(it);
}
In this case, call to extract() is ambiguous, because the overloads which accept const_iterator and key_type are equally good matches for the argument it.
Consequently, this issue can be resolved in the same way as for std::map::erase by adding an overload for extract which accepts iterator as an argument.[2019-07 Issue Prioritization]
Priority to 3 after discussion on the reflector.
Previous resolution [SUPERSEDED]:
This wording is relative to N4820.
Modify [tab:container.assoc.req], Table 69 — "Associative container requirements", as indicated:
Table 69 — Associative container requirements (in addition to container) [tab:container.assoc.req] Expression Return type Assertion/note pre-/post-condition Complexity … a.extract(q) node_type Effects: Removes the element
pointed to by q.
Returns: A node_type owning
that element.amortized constant a.extract(r) node_type Effects: Removes the element
pointed to by r.
Returns: A node_type owning
that element.amortized constant … Modify [tab:container.assoc.req], Table 70 — "Unordered associative container requirements", as indicated:
Table 70 — Unordered associative container requirements (in addition to container) [tab:container.hash.req] Expression Return type Assertion/note pre-/post-condition Complexity … a.extract(q) node_type Effects: Removes the element
pointed to by q.
Returns: A node_type owning
that element.Average case 𝒪(1), worst case 𝒪(a.size()). a.extract(r) node_type Effects: Removes the element
pointed to by r.
Returns: A node_type owning
that element.Average case 𝒪(1), worst case 𝒪(a.size()). … Modify 24.4.4.1 [map.overview], class template map synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]Modify 24.4.5.1 [multimap.overview], class template multimap synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]Modify 24.4.6.1 [set.overview], class template set synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]Modify 24.4.7.1 [multiset.overview], class template multiset synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]Modify 24.5.4.1 [unord.map.overview], class template unordered_map synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]Modify 24.5.5.1 [unord.multimap.overview], class template unordered_multimap synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]Modify 24.5.6.1 [unord.set.overview], class template unordered_set synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]Modify 24.5.7.1 [unord.multiset.overview], class template unordered_multiset synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]
[2022-04-24; Daniel rebases wording on N4910]
Proposed resolution:
This wording is relative to N4910.
Modify 24.2.7.1 [associative.reqmts.general] as indicated:
a.extract(q)-108- Result: node_type
-109- Effects: Removes the element pointed to by q. -110- Returns: A node_type owning that element. -111- Complexity: Amortized constant.a.extract(r)-?- Result: node_type
-?- Effects: Removes the element pointed to by r. -?- Returns: A node_type owning that element. -?- Complexity: Amortized constant.
Modify 24.2.8.1 [unord.req.general] as indicated:
a.extract(q)-141- Result: node_type
-142- Effects: Removes the element pointed to by q. -143- Returns: A node_type owning that element. -144- Complexity: Average case 𝒪(1), worst case 𝒪(a.size()).a.extract(r)-?- Result: node_type
-?- Effects: Removes the element pointed to by r. -?- Returns: A node_type owning that element. -?- Complexity: Average case 𝒪(1), worst case 𝒪(a.size()).
Modify 24.4.4.1 [map.overview], class template map synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]
Modify 24.4.5.1 [multimap.overview], class template multimap synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]
Modify 24.4.6.1 [set.overview], class template set synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]
Modify 24.4.7.1 [multiset.overview], class template multiset synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]
Modify 24.5.4.1 [unord.map.overview], class template unordered_map synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]
Modify 24.5.5.1 [unord.multimap.overview], class template unordered_multimap synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]
Modify 24.5.6.1 [unord.set.overview], class template unordered_set synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]
Modify 24.5.7.1 [unord.multiset.overview], class template unordered_multiset synopsis, as indicated:
[…] node_type extract(iterator position); node_type extract(const_iterator position); node_type extract(const key_type& x); […]
Section: 16.4.6.13 [res.on.exception.handling] Status: New Submitter: Nevin Liber Opened: 2019-06-28 Last modified: 2023-01-29
Priority: 3
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Discussion:
16.4.6.13 [res.on.exception.handling]#3 says:
Destructor operations defined in the C++ standard library shall not throw exceptions. Every destructor in the C++ standard library shall behave as if it had a non-throwing exception specification.
However, types like pair and array have implicitly declared destructors, where 14.5 [except.spec]#8 applies:
The exception specification for an implicitly-declared destructor, or a destructor without a noexcept-specifier, is potentially-throwing if and only if any of the destructors for any of its potentially constructed subobjects is potentially-throwing or the destructor is virtual and the destructor of any virtual base class is potentially throwing.
We can relax 16.4.6.13 [res.on.exception.handling] to only cover explicitly declared destructors because if they hold a user type where the destructor throws then we get UB from 16.4.5.8 [res.on.functions]#2:
In particular, the effects are undefined in the following cases: […]
[…]
(2.4) — if any replacement function or handler function or destructor operation exits via an exception, unless specifically allowed in the applicable Required behavior: paragraph.
[…]
and the referred to UB happens before [res.on.exception.handling] could apply.
[2019-07 Issue Prioritization]
Priority to 3 after discussion on the reflector.
[2023-01-29; Daniel comments]
This issue has very much overlap with LWG 3854.
Proposed resolution:
This wording is relative to N4820.
Modify 16.4.6.13 [res.on.exception.handling] as indicated:
-3- Destructor operations defined in the C++ standard library shall not throw exceptions. Every explicitly declared destructor in the C++ standard library shall behave as if it had a non-throwing exception specification.
Section: 16.4.3.2 [using.headers] Status: New Submitter: Alisdair Meredith Opened: 2019-07-24 Last modified: 2020-04-07
Priority: 3
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Discussion:
Quoting 16.4.3.2 [using.headers] p3:
"[…] and shall include the header lexically before the first reference in that translation unit to any of the entities declared in that header."
This suggests we may be able to use macros and typedefs (like size_t) declared in standard headers without the corresponding #include. Clearly that is not the intended behavior!
I thought about replacing 'entity' with 'name', but the same name may occur with different contexts throughout the standard library. I thought about "contents of that header", but run afoul of the same function overloads (std::begin etc.) being declared in multiple headers. It may be simpler to turn this sentence around, along the lines of:"No part of the standard library shall be used in a translation unit prior to a including or importing a header that provides that feature."
Even here, 'used' may be a problematic term of art. Perhaps "named"?
[2020-04-07 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
Section: 32.7 [re.regex], 32.9 [re.results] Status: New Submitter: Billy O'Neal III Opened: 2019-08-17 Last modified: 2019-10-07
Priority: 3
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Discussion:
std::basic_regex and std::match_results have noexcept move construction, and std::basic_regex has noexcept move assignment, but both of them have throwing swaps. We probably need an Expects: or something to say that BidirectionalIterator doesn't throw through these operations. We probably also need match_results::operator= to respect propagate_on_container_move_assignment (and maybe the copy ctor respect propagate_on_container_copy_assignment).
[2019-09-02; Tim Song comments]
The issue is related to LWG 2490.
[2019-10 Priority set to 3 after reflector discussion]
Proposed resolution:
Section: 33.5.6 [atomics.wait] Status: New Submitter: Geoffrey Romer Opened: 2019-08-19 Last modified: 2020-09-06
Priority: 3
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Discussion:
It appears that in a conforming implementation, all but one wait() call on a given atomic object may block forever, regardless of any notify_one() calls, because in principle every notify_one() call could be considered to unblock the same single wait() call. Common sense suggests (and David Olsen confirms) that the intent is for each waiting function call to be (non-spuriously) unblocked by at most one notifying function call, but as far as I can tell the words never say that.
[2019-09-14 Priority set to 3 based on reflector discussion]
Proposed resolution:
This wording is relative to N4830.
Modify 33.5.6 [atomics.wait] as indicated:
-?- All blocking and unblocking events on a single atomic object occur in a single total order that is consistent with the "happens before" partial order.
-4- A call to an atomic waiting operation on an atomic object M is eligible to be unblocked by a call to an atomic notifying operation on M if it has not been unblocked, and there exist side effects X and Y on M such that:
(4.1) — the atomic waiting operation has blocked after observing the result of X,
(4.2) — X precedes Y in the modification order of M, and
(4.3) — Y happens before the call to the atomic notifying operation.
Section: 16.4.4.6 [allocator.requirements] Status: New Submitter: FrankHB1989 Opened: 2019-08-27 Last modified: 2023-01-14
Priority: 4
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Discussion:
[allocator.requirements] does not mention the interaction between is_always_equal and allocator rebinding. As the result, a rebound allocator may have different is_always_equal::value to the original allocator.
Further, for an allocator type X satisfying std::allocator_type<X>::is_always_equal::value == true, rebound allocators of X with same type are not guaranteed equal. Consider:X is used as an allocator for value_type used in a node-based container;
Y is the rebound allocator type for the node type used in the implementation;
b1 and b2 are values of Y from different allocator objects.
Then, std::allocator_type<X>::is_always_equal::value == true does not necessarily imply b1 == b2.
Since some of containers in the standard have already explicitly relied on is_always_equal of allocators for their value_type (notably, in the exception specification of the move assignment), this can cause subtle problems. In general, the implementation of the move assignment operator of such a container can not avoid allocation for new nodes when !std::allocator_traits<Y>::propagate_on_container_move_assignment::value && b1 != b2. This can throw, and it can clash with the required exception specification based on std::allocator_traits<value_type>::is_always_equal:
#include <utility>
#include <memory>
#include <new>
#include <map>
#include <functional>
#include <type_traits>
using K = int;
using V = int;
using P = std::pair<const K, V>;
bool stop_alloc;
template<typename T>
struct AT
{
using value_type = T;
std::shared_ptr<void> sp = {};
template<typename U>
struct rebind
{
using other = AT<U>;
};
using is_always_equal = std::is_same<T, P>;
AT() : sp(is_always_equal::value ? nullptr : new T*()) {}
AT(const AT& a) = default;
template<typename U>
AT(const AT<U>& a) noexcept : sp(a.sp) {}
T* allocate(std::size_t size)
{
if (stop_alloc)
throw std::bad_alloc();
return static_cast<T*>(::operator new(size * sizeof(T)));
}
void deallocate(T* p, std::size_t)
{
::operator delete(p);
}
friend bool operator==(const AT& x, const AT& y) noexcept
{
return !x.sp.owner_before(y.sp) && !y.sp.owner_before(x.sp);
}
friend bool operator!=(const AT& x, const AT& y) noexcept
{
return !(x == y);
}
};
using A = AT<P>;
int main()
{
// Some sanity checks:
static_assert(std::is_same_v<A::template rebind<A::value_type>::other, A>);
// For any U:
using U = int;
static_assert(std::is_same_v<A::template rebind<U>::other::template rebind<A::value_type>::other, A>);
using C = std::less<>;
using M = std::map<K, V, C, A>;
// As required by the current wording of the container move operator:
using always_equal = std::allocator_traits<A>::is_always_equal;
constexpr bool std_nothrow = always_equal::value && std::is_nothrow_move_assignable_v<C>;
static_assert(std_nothrow);
// For conforming implementations:
// static_assert(!(std_nothrow && !std::is_nothrow_move_assignable<M>::value));
M m{{K(), V()}}, m2;
auto a = m.get_allocator();
a.sp = std::make_shared<int>(42);
stop_alloc = true;
try
{
// Call terminate with conforming implementations. This does not work on libstdc++.
m2 = std::move(m);
// For libstdc++, terminate on allocator-extended move constructor call.
// M m3(std::move(m), a);
}
catch(...)
{}
}
[2019-10 Priority set to 4 after reflector discussion]
Previous resolution [SUPERSEDED]:
This wording is relative to N4830.
[Drafting note: Additional questions: Is it necessary to ensure that
XX::propagate_on_container_copy_assignment::value == YY::propagate_on_container_copy_assignment::value is true as well?]
Modify 16.4.4.6 [allocator.requirements], Table [tab:cpp17.allocator] "Cpp17Allocator requirements" as indicated:
Table 34 — Cpp17Allocator requirements [tab:cpp17.allocator] Expression Return type Assertion/note
pre-/post-conditionDefault … typename
X::template
rebind<U>::otherY For all U (including T),
Y::template
rebind<T>::other is X.
XX::is_always_equal::value == YY::is_always_equal::value
is true.See Note A,
below.…
[2022-04-24; Daniel rebases wording on N4910]
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
[Drafting note: Additional questions: Is it necessary to ensure that
XX::propagate_on_container_copy_assignment::value == YY::propagate_on_container_copy_assignment::value is true as well?]
Modify 16.4.4.6 [allocator.requirements] as indicated:
typename X::template rebind<U>::other-16- Result: Y
-17- Postconditions: For all U (including T), Y::template rebind<T>::other is X. XX::is_always_equal::value == YY::is_always_equal::value is true. -18- Remarks: If Allocator is a class template instantiation of the form SomeAllocator<T, Args>, where Args is zero or more type arguments, and Allocator does not supply a rebind member template, the standard allocator_traits template uses SomeAllocator<U, Args> in place of Allocator::rebind<U>::other by default. For allocator types that are not template instantiations of the above form, no default is provided. -19- [Note 1: The member class template rebind of X is effectively a typedef template. In general, if the name Allocator is bound to SomeAllocator<T>, then Allocator::rebind<U>::other is the same type as SomeAllocator<U>, where SomeAllocator<T>::value_type is T and SomeAllocator<U>::value_type is U. — end note]
[2023-01-08; Jiang An comments and provides improved wording]
Exception specifications of some container operations (added by N4258 and LWG 3778) are specified with the propagation properties of template parameter Allocator. However, for node-based containers and std::deque (and common implementations of std::vector<bool, A>), rebound allocators are needed to be propagated, and common implementations are currently detecting the propagation properties of rebound allocators.
I think if the allocator provided as template argument and the rebound have different propagation properties and behaves differently on propagation, then it is difficult or impossible for implementations to follow the current exception specifications.Proposed resolution:
This wording is relative to N4917.
Modify 16.4.4.6 [allocator.requirements] as indicated:
typename X::template rebind<U>::other-16- Result: Y
-17- Postconditions: For all U (including T), Y::template rebind<T>::other is X. All of XX::is_always_equal::value == YY::is_always_equal::value, XX::propagate_on_container_copy_assignment::value == YY::propagate_on_container_copy_assignment::value, XX::propagate_on_container_move_assignment::value == YY::propagate_on_container_move_assignment::value, and XX::propagate_on_container_swap::value == YY::propagate_on_container_swap::value are true. -18- Remarks: If Allocator is a class template instantiation of the form SomeAllocator<T, Args>, where Args is zero or more type arguments, and Allocator does not supply a rebind member template, the standard allocator_traits template uses SomeAllocator<U, Args> in place of Allocator::rebind<U>::other by default. For allocator types that are not template instantiations of the above form, no default is provided. -19- [Note 1: The member class template rebind of X is effectively a typedef template. In general, if the name Allocator is bound to SomeAllocator<T>, then Allocator::rebind<U>::other is the same type as SomeAllocator<U>, where SomeAllocator<T>::value_type is T and SomeAllocator<U>::value_type is U. — end note]
Section: 33.5.4 [atomics.order] Status: New Submitter: Eric Fiselier Opened: 2019-08-31 Last modified: 2020-09-06
Priority: 4
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Discussion:
P0439R0 renamed the std::memory_order enumerators when making it a scoped enumeration. The paper makes the old unscoped names available in the global namespace, but not within the scope of the enumeration.
For example: std::memory_order::memory_order_consume is no longer well-formed but std::memory_order::consume and std::memory_order_consume are. In order to prevent unnecessary breakage, we should re-add the memory_order_foo names to the enumeration.[2019-10 Priority set to 4 after reflector discussion]
Proposed resolution:
This wording is relative to N4830.
Add a new sub-clause at the end of Clause D [depr] as indicated:
D.?? Deprecated memory_order enumerators
-?- The following enumerators are declared in addition to those specified in 33.5.4 [atomics.order]:namespace std { enum class memory_order : unspecified { memory_order_relaxed = relaxed, memory_order_consume = consume, memory_order_acquire = acquire, memory_order_release = release, memory_order_acq_rel = acq_rel, memory_order_seq_cst = seq_cst }; }
Section: 30.4.6.2.3 [locale.time.get.virtuals] Status: New Submitter: Marshall Clow Opened: 2019-09-09 Last modified: 2020-09-06
Priority: 3
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Discussion:
According to 30.4.6.2.3 [locale.time.get.virtuals] p11:
Requires: t shall point to an object
[Note: In my "Mandates" paper, I changed this to "Expects: t points to an object"]
Nevertheless, it's odd, and inconsistent. time_get::get does not have any such stated requirement, and it calls do_get. None of the other "time" calls in time_get have such a (stated) requirement. I believe that this requirement is redundant, that it is implied by the wording in P12 and P14. P12: "or until it has extracted and assigned those struct tm members" P14: "It is unspecified whether multiple calls to do_get() with the address of the same struct tm object will update the current contents of the object or simply overwrite its members." If the pointer is invalid (null, or points to unmapped memory, say), you've got UB anyway. All the other calls in [locale.time.get.virtuals] were from C++98. do_get_time was added in C++11, and p11 originally said "t shall be dereferenceable". This was changed to "t shall point to an object" as part of the resolution of CWG issue 342[2019-10 Priority set to 3 after reflector discussion]
Proposed resolution:
This wording is relative to N4830.
Modify 30.4.6.2.3 [locale.time.get.virtuals] as indicated:
iter_type do_get(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err, tm* t, char format, char modifier) const;-12- Effects: […]
-11- Requires: t shall point to an object.
Section: 25.3.2.3 [iterator.traits] Status: New Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2020-09-06
Priority: 3
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Discussion:
The new C++20 iterator concepts use common_reference to constrain the value, reference, and rvalue_reference associated types in order to support proxy references (see 25.3.4.2 [iterator.concept.readable]).
However, the C++17 iterators did not support proxy references, so the use of common_reference in 25.3.2.3 [iterator.traits]/p2 is needlessly complex. The common_reference constraints can be replaced with simple convertibility requirements to a const lvalue reference to the value type. This fix has been implemented in range-v3.[2019-10-14 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
This wording is relative to N4830.
Modify 25.3.2.3 [iterator.traits] as indicated:
-2- The definitions in this subclause make use of the following exposition-only concepts:
template<class I> concept cpp17-iterator = copyable<I> && requires(I i) { { *i } -> can-reference; { ++i } -> same_as<I&>; { *i++ } -> can-reference; }; template<class I> concept cpp17-input-iterator = cpp17-iterator<I> && equality_comparable<I> && requires(I i) { typename incrementable_traits<I>::difference_type; typename readable_traits<I>::value_type;typename common_reference_t<iter_reference_t<I>&&, typename readable_traits<I>::value_type&>; typename common_reference_t<decltype(*i++)&&, typename readable_traits<I>::value_type&>;{ *i } -> convertible_to<const typename readable_traits<I>::value_type&>; { *i++ } -> convertible_to<const typename readable_traits<I>::value_type&>; requires signed_integral<typename incrementable_traits<I>::difference_type>; }; […]
Section: 33.5.6 [atomics.wait] Status: New Submitter: Anthony Williams Opened: 2019-09-11 Last modified: 2020-09-06
Priority: 2
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Discussion:
I am concerned by the wording around atomic<T>::wait()/atomic<T>::notify_one().
33.5.6 [atomics.wait] p4 requires that the thread that calls wait() observed a value X prior to the value Y which results from a store that happens-before the notify in order to be eligible to be unlocked. I am not sure how to implement that.atomic<int> a = 0; T1: int ra=a, read 0 T1: a.wait(0) T2: a=42 T3: int ra=a, read 42 T3: a.wait(42) T2: a.notify_one()
The wording requires that T1 is eligible to be unlocked, but not T3, as there is not a write after the value read by T3 that happens-before the notify.
However, both T1 and T3 are waiting, so T3 may be woken by the OS. Waking T3 is allowed (wait() says it may wake spuriously), but waking T1 is currently required as it is the only thread "eligible to be unblocked". This requires notify_one() to wake all waiters, which defeats the purpose. I suspect we need to change 33.5.6 [atomics.wait] p4. How about:"A call to an atomic waiting operation W on an atomic object M is eligible to be unlocked by a call to an atomic notifying operation N on M if
"
N does not happen-before W
There are no side effects X and Y in the modification order of M such that N happens-before X, X precedes Y in the modification order of M and an atomic operation that observes the effects of Y happens-before W.
This would allow T3 to be woken in the preceding example, but prevent it being woken in the following case:
T1: int ra=a, read 0 T1: a.wait(0) T2: a=42 T2: a.notify_one() T2: a=69 T3: int ra=a, read 69 T3: a.wait(69)
[2020-07-17; Priority set to 2 in telecon]
Proposed resolution:
This wording is relative to N4830.
Modify 33.5.6 [atomics.wait] as indicated:
-4- A call to an atomic waiting operation W on an atomic object M is eligible to be unblocked by a call to an atomic notifying operation N on M if
there exist side effects X and Y on M such that:
(4.1) — N does not happen before W
the atomic waiting operation has blocked after observing the result of X,(4.2) — There are no side effects X and
precedesY in the modification order of M, andsuch that N happens before X, X precedes Y in the modification order of M and an atomic operation that observes the effects of Y happens before W.
(4.3) — Y happens before the call to the atomic notifying operation.
Section: 24.2.4 [sequence.reqmts] Status: New Submitter: Casey Carter Opened: 2019-09-17 Last modified: 2022-04-24
Priority: 3
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Discussion:
24.2.4 [sequence.reqmts] paragraph 3 says that the names i and j denote "iterators that meet the Cpp17InputIterator requirements and refer to elements implicitly convertible to value_type". Ignoring for the moment that this is an occurrence of LWG 3105 — we really mean that *i and *j must be implicitly convertible to value_type — this requirement seems to be completely extraneous.
The names i and j are used in three places in the requirements table:The range constructors X(i, j) and X u(i, j), which require that the container's value type is Cpp17EmplaceConstructible into the container from *i; implicit conversion is neither necessary nor sufficient.
The range insert overload a.insert(p, i, j) which also requires Cpp17EmplaceConstructible, as well as the capability to move elements around for vector and deque; again, implicit conversion is neither necessary nor sufficient. It would be useful / performant here to require that the container's value type is assignable from *i, which may have been the intent of the implicit conversion requirement — would doing so be too breaking?
The range assign overload a.assign(i, j) which requires both Cpp17EmplaceConstructible as above and that it can assign the result of dereferencing an iterator directly to the container's value type; again, implicit conversion is not useful here.
We should strike the implicit conversion requirement since it is not useful and only serves to confuse readers of the Standard (see e.g. here).
[2019-10-31 Issue Prioritization]
Priority to 3 after reflector discussion.
Previous resolution [SUPERSEDED]:
This wording is relative to N4830.
Modify 24.2.4 [sequence.reqmts] as indicated:
-3- In Tables 76 and 77, X denotes a sequence container class, a denotes a value of type X containing elements of type T, u denotes the name of a variable being declared, A denotes X::allocator_type if the qualified-id X::allocator_type is valid and denotes a type (13.10.3 [temp.deduct]) and allocator<T> if it doesn't, i and j denote iterators that meet the Cpp17InputIterator requirements
and refer to elements implicitly convertible to value_type, [i, j) denotes a valid range, […]
[2022-04-24; Daniel rebases wording on N4910]
Proposed resolution:
This wording is relative to N4910.
Modify 24.2.4 [sequence.reqmts] as indicated:
-3- In this subclause,
(3.1) — […]
[…]
(3.5) — i and j denote iterators that meet the Cpp17InputIterator requirements
and refer to elements implicitly convertible to value_type,[…]
Section: 22.7.5 [any.nonmembers] Status: Open Submitter: John Shaw Opened: 2019-10-16 Last modified: 2022-02-03
Priority: 2
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Discussion:
any foo; void* p = any_cast<void>(&foo);
Per 22.7.5 [any.nonmembers]/9, since the operand isn't nullptr and operand->type() == typeid(T) (because T = void in this case), we should return a pointer to the object contained by operand. But there is no such object.
We need to handle the T = void case, probably by just explicitly returning nullptr.[2019-11 Priority to 2 during Monday issue prioritization in Belfast. There is implementation divergence here.]
[2020-02 LWG discussion in Prague did not reach consensus. Status to Open.]
There was discussion about whether or not any_cast<void>(a) should be ill-formed, or return nullptr.
Poll "should it return nullptr" was 0-4-5-5-1.
[2022-02 Currently ill-formed in MSVC ("error C2338: std::any cannot contain void") and returns null pointer in libstdc++ and libc++.]
Proposed resolution:
This wording is relative to N4835.
Modify 22.7.5 [any.nonmembers] as indicated:
template<class T> const T* any_cast(const any* operand) noexcept; template<class T> T* any_cast(any* operand) noexcept;-9- Returns: If operand != nullptr && operand->type() == typeid(T) && is_object_v<T>, a pointer to the object contained by operand; otherwise, nullptr.
[…]
Section: 24.3.8.4 [deque.modifiers], 24.3.11.5 [vector.modifiers] Status: New Submitter: Billy O'Neal III Opened: 2019-10-29 Last modified: 2019-11-04
Priority: 3
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Discussion:
It seems incorrect that a container would invalidate anything as a result of being asked to erase 0 elements. This came up in a recent customer bug report against Visual Studio, where given a vector v, v.erase(v.begin(), v.begin()) triggered a self-assignment of all the elements in the vector.
deque has language enumerating erasures of the first and last element which invalidate fewer iterators, and a fallback that says all iterators are invalidated, which seems to intend to be talking about middle-of-container erasures. However, erasing 0 elements isn't really a middle of container erasure. vector says that iterators and references are invalidated after the 'point of the erase', but when 0 elements are erased it's unclear what that even means. We should say that erasures that erase 0 elements are no ops and be clearer about which elements are invalidated for vector.[2019-11 Priority to 3 during Monday issue prioritization in Belfast]
Proposed resolution:
This wording is relative to N4835.
Modify 24.3.8.4 [deque.modifiers] as indicated:
iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last); void pop_front(); void pop_back();-4- Effects: Erases elements as indicated in Table 75 [tab:container.seq.req]. An erase operation that erases the last element of a deque invalidates only the past-the-end iterator and all iterators and references to the erased elements. An erase operation that erases the first element of a deque but not the last element invalidates only iterators and references to the erased elements. An erase operation that erases any elements, but neither the first element nor the last element of a deque invalidates the past-the-end iterator and all iterators and references to all the elements of the deque.
[…][Note: pop_front and pop_back are erase operations. — end note]
Modify 24.3.11.5 [vector.modifiers] as indicated:
constexpr iterator erase(const_iterator position); constexpr iterator erase(const_iterator first, const_iterator last); constexpr void pop_back();-3- Effects: Erases elements as indicated in Table 75 [tab:container.seq.req]. Invalidates iterators and references at or after the
[…]point of the erase.first erased element. [Note: For the second overload of erase, if first == last, no elements are erased, and no iterators or references are invalidated. — end note]
Section: 31.8 [string.streams], 31.10 [file.streams] Status: New Submitter: Jens Maurer Opened: 2019-11-01 Last modified: 2019-11-04
Priority: 3
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Discussion:
It is unclear whether the streams headers implicitly #include <ios> and make (for example) the name std::basic_ios available after including <sstream>.
This question becomes more important as header units arrive with modules, where there is an explicit choice whether to re-export names from subordinate headers.[2019-11 Priority to 3 during Monday issue prioritization in Belfast]
Proposed resolution:
Section: 30.4.2.5.3 [locale.codecvt.virtuals] Status: New Submitter: Richard Smith Opened: 2019-11-15 Last modified: 2019-11-30
Priority: 3
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Discussion:
In 30.4.2.5.3 [locale.codecvt.virtuals] paragraphs 6 and 11, we find:
Preconditions: (to <= to_end) is well-defined and true; state is initialized, if at the beginning of a sequence, or else is equal to the result of converting the preceding characters in the sequence."
This doesn't make sense. What is the value of state if we're at the beginning of a sequence? Is the fact that we say that it's initialized in that case supposed to imply that it need not be initialized otherwise?
Perhaps this means "value-initialized" or "default-initialized" instead of merely "initialized"?[2019-11-30 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
Section: 23.4.3 [basic.string], 24.3.11 [vector], 24.5.4 [unord.map], 24.5.6 [unord.set], 24.2.2.1 [container.requirements.general] Status: New Submitter: Nathan Myers Opened: 2019-11-17 Last modified: 2020-09-06
Priority: 3
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Discussion:
The Standard leaves unspecified the capacity() of a string or vector, and bucket_count() of an unordered_(multi)set or unordered_(multi)map, constructed by move from an empty other.
For a non-empty other, normative language in the Standard constrains the new object to use (mostly) the same storage as the other, by way of lifetime of iterators and pointers to elements. For an empty other, there can be no such pointers or iterators. However, the empty container may have a non-zero capacity() or bucket_count(), and having reserved storage there, one naturally expects that storage to be delivered to the new object in the same way as if it had elements. Existing implementations, in fact, do move storage to the new container, provided it can be deallocated using the new object's allocator. It is likely that existing programs have come to depend on this behavior. The resolution proposed is to add language to the Standard specifying that, if the allocators of the existing and new container objects are compatible, the storage of the new object is the same as of the old, so that no allocations or deallocations are performed in the process, as existing implementations in fact do. This appears to affect only string, vector, unordered_set, unordered_multiset, unordered_map, and unordered_multimap, but any new container types may also need similar attention. Note that in the case of the hashed containers, the array of buckets appears not to be required to be moved, even when elements contained are. This seems to be a similar oversight; extant implementations do move the bucket array. The resolution should cover this case as well. It is expected and intended that the proposed resolution does not require changes to the behavior of implementations. See also LWG 2321 and P0966R1.[2019-11-30 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
Section: 32.7.2 [re.regex.construct] Status: New Submitter: Денис Захаров Opened: 2019-11-17 Last modified: 2022-04-22
Priority: 3
View other active issues in [re.regex.construct].
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Discussion:
This is description of a basic_regex range constructor from N4835, 32.7.2 [re.regex.construct]:
template<class ForwardIterator> basic_regex(ForwardIterator first, ForwardIterator last, flag_type f = regex_constants::ECMAScript);-17- Throws: regex_error if the sequence [first, last) is not a valid regular expression.
-18- Effects: Constructs an object of class basic_regex; the object's internal finite state machine is constructed from the regular expression contained in the sequence of characters [first, last), and interpreted according to the flags specified in f. -19- Ensures: flags() returns f. mark_count() returns the number of marked sub-expressions within the expression.
It seems that there are no requirements about dereferenced iterator's element type, that, apparently, must be implicitly convertible to the basic_regex::value_type. For example, containers having range constructor satisfy a SequenceContainer requirements, where implicit converting to its elements is specified.
[2019-11-30 Issue Prioritization]
Priority to 3 after reflector discussion.
[2021-10-04; Jonathan adds a comment]
There isn't even a requirement that the arguments are iterators. And assign(InputIterator, InputIterator, flag_type) is not constrained to only accept iterators either, meaning you can call it with two integers and call the basic_string(size_type, char_type) constructor.
[2022-04-22; Jonathan adds a comment]
As well as requiring the reference type to be convertible to charT, we might want to consider constraining these with forward_iterator as per P2408.
Proposed resolution:
Section: 22.3.2 [pairs.pair], 22.4.4.1 [tuple.cnstr], 22.6.3.2 [variant.ctor], 33.5.8.7.2 [util.smartptr.atomic.shared], 20.5.3 [allocator.adaptor.cnstr], 22.14.6.5 [format.parse.ctx], 22.14.8.1 [format.arg], 24.6 [container.adaptors], 25.5 [predef.iterators], 26.5.4.2 [range.subrange.ctor], 26.6 [range.factories], 26.7 [range.adaptors], 27.10 [numeric.ops], 29.9 [time.hms], 32.11 [re.iter], 33.5.8 [atomics.types.generic], 33.5.9 [atomics.nonmembers], 33.3 [thread.stoptoken], 33.4 [thread.threads], 33.6 [thread.mutex], 33.8 [thread.sema], 33.9 [thread.coord], 33.10 [futures] Status: New Submitter: Richard Smith Opened: 2019-11-21 Last modified: 2019-12-08
Priority: 3
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Discussion:
The problem was discussed here:
It seems to me that this is just one instance of a systemic problem in the library wording. This phrasing "initializes x with y" is common, but underspecified (and formally meaningless) — the library wording either needs to say what kind of initialization is performed, or specify an initializer (not an expression) with which to initialize. We should ask LWG to think about this; for each "initializes x with y" utterance, the reader should know what kind of initialization we mean.
Looking at random through the library wording, the first case I found: 24.6.6.2 [queue.cons]/1:Effects: Initializes c with cont.
The meaning of this depends on whether this is direct- or copy-initialization. (It's obscure, but if T is not Cpp17CopyInsertable into the container, it could be the case that one form of initialization works and the other does not, or that they both work and do different things.)
Another random sample: 27.10.7 [partial.sum]/2:Effects: For a non-empty range, the function creates an accumulator acc whose type is InputIterator's value type, initializes it with *first, and assigns the result to *result.
Again the difference between direct- and copy-initialization is observable here.
Perhaps the library should have blanket wording that when it says "initializes", it means by direct- or copy-initialization, and that it's unspecified which one you get (or something like that) — and someone should go through all the instances and check if any of them mean something else (I doubt this is the only case that does).
Suggestion: either
add blanket wording defining what you mean when you say "initializes x with y" (e.g., it's unspecified whether copy-initialization or direct-initialization is performed) and make sure that that's what's intended for all uses, or
stop using the "initializes x with y" formulation entirely, and specify the kind of initialization on each use, or
for each such use, ensure that y is an initializer (that is, of the form "= expr" or "(expr, expr, …)" or "= { … }" or "{ … }", and not merely an expression)
[2019-12-08 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
Section: 33.7.3 [thread.condition.nonmember] Status: New Submitter: Lewis Baker Opened: 2019-11-21 Last modified: 2020-02-24
Priority: 3
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Discussion:
33.7.3 [thread.condition.nonmember] p2 states:
Effects: Transfers ownership of the lock associated with lk into internal storage and schedules cond to be notified when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed. This notification shall be as if:
lk.unlock(); cond.notify_all();
One common use-cases for the notify_all_at_thread_exit() is in conjunction with thread::detach() to allow detached threads to signal when they complete and to allow another thread to wait for them to complete using the condition_variable/mutex pair.
However, the current wording for notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk) makes it impossible to know when it is safe to destroy the condition_variable in the presence of spurious wake-ups and detached threads. For example: Consider the following code-snippet:
#include <condition_variable>
#include <mutex>
#include <thread>
int main() {
std::condition_variable cv;
std::mutex mut;
bool complete = false;
std::thread{[&] {
// do work here
// Signal thread completion
std::unique_lock lk{mut};
complete = true;
std::notify_all_at_thread_exit(cv, std::move(lk));
}}.detach();
// Wait until thread completes
std::unique_lock lk{mut};
cv.wait(lk, [&] { return complete; });
// condition_variable destroyed on scope exit
return 0;
}
This seems to an intended usage of thread::detach() and std::notify_all_at_thread_exit() and yet this code contains a race involving the call to cv.notify_all() on the created thread, and the destructor of the condition_variable.
To highlight the issue, consider the following case: Let T0 be the thread that executes main() and T1 be the thread created by the std::thread construction.T0: creates thread T1
T1: acquires mutex lock
T0: context-switched out by OS
T1: starts running
T1: sets complete = true
T1: calls notify_all_at_thread_exit()
T1: returns from thread-main function and runs all thread-local destructors
T1: calls lk.unlock()
T1: context-switched out by OS
T0: resumes execution
T0: acquires mutex lock
T0: calls cv.wait() which returns immediately as complete is true
T0: returns from main(), destroying condition_variable
T1: resumes execution
T1: calls cv.notify_all() on a dangling cv reference (undefined behaviour)
Other sequencings are possible involving spurious wake-ups of the cv.wait() call.
A proof-of-concept showing this issue can be found here. The current wording requires releasing the mutex lock before calling cv.notify_all(). In the presence of spurious wake-ups of a condition_variable::wait(), there is no way to know whether or not a detached thread that called std::notify_all_at_thread_exit() has finished calling cv.notify_all(). This means there is no portable way to know when it will be safe for the waiting thread to destroy that condition_variable. However, if we were to reverse the order of the calls to lk.unlock() and cond.notify_all() then the thread waiting for the detached thread to exit would not be able to observe the completion of the thread (in the above case, this would be observing the assignment of true to the complete variable) until the mutex lock was released by that thread and subsequently acquired by the waiting thread which would only happen after the completion of the call to cv.notify_all(). This would allow the above code example to eliminate the race between a subsequent destruction of the condition-variable and the call to cv.notify_all().[2019-12-08 Issue Prioritization]
Priority to 3 after reflector discussion.
[2019-12-15; Daniel synchronizes wording with N4842]
[2020-02, Prague]
Response from SG1: "We discussed it in Prague. We agree it’s an error and SG1 agreed with the PR."
Proposed resolution:
This wording is relative to N4842.
Change 33.7.3 [thread.condition.nonmember] as indicated:
void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);[…]
-2- Effects: Transfers ownership of the lock associated with lk into internal storage and schedules cond to be notified when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed. This notification is equivalent to:lk.unlock();cond.notify_all(); lk.unlock();
Section: 25.4.3 [iterator.operations], 25.4.4.2 [range.iter.op.advance] Status: New Submitter: Casey Carter Opened: 2019-11-22 Last modified: 2019-12-07
Priority: 3
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Discussion:
ranges::advance (25.4.4.2 [range.iter.op.advance]) and std::advance (25.4.3 [iterator.operations]) can be called with a negative count n when the iterator argument i models bidirectional_iterator (respectively, meets the Cpp17BidirectionalIterator requirements). In this case, they are specified to "decrement i by -n". If n is the most-negative value of a signed integral type, the expression -n has undefined behavior. This UB is unfortunate given that typical implementations never actually form the expression -n. It's nonsensical to describe the effects of a function in terms of an expression with undefined behavior, so we should either define the behavior or exclude this case via precondition.
ranges::prev() and std::prev (25.4.3 [iterator.operations]) have a similar problem: prev(i, n) is equivalent to:advance(i, -n); return i;
which has undefined behavior when n is numeric_limits<T>::min() where T is iter_difference_t<decltype(i)> (for ranges::prev) or some signed integral type (for std::prev). There is an implicit precondition here thanks to "Effects: Equivalent to" since the equivalent code has a precondition that n is not a most-negative value, so this wording is not defective. We could, however, define behavior for prev regardless of the value of n by duplicating the specification of advance and inverting the "direction" of the operations. We should consider doing so.
[2019-12-07 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
This wording is relative to N4835.
[Drafting note: I've chosen to provide wording for the conservative "define behavior for advance and leave prev as status quo" middle ground.
The occurrences of "|" in the below are math-font vertical bars (indicating absolute value). I've changed both positive and negative cases for consistency of presentation. ]
Modify 25.4.3 [iterator.operations] as indicated:
template<class InputIterator, class Distance> constexpr void advance(InputIterator& i, Distance n);-2- Expects: n is negative only for bidirectional iterators.
-3- Effects: Increments i by |n| if n is non-negative, and decrements i by-|n| otherwise.
Modify 25.4.4.2 [range.iter.op.advance] as indicated:
template<input_or_output_iterator I> constexpr void ranges::advance(I& i, iter_difference_t<I> n);[…]-1- Expects: If I does not model bidirectional_iterator, n is not negative.
-2- Effects:
(2.1) — If I models random_access_iterator, equivalent to i += n.
(2.2) — Otherwise, if n is non-negative, increments i by |n|.
(2.3) — Otherwise, decrements i by
-|n|.template<input_or_output_iterator I, sentinel_for<I> S> constexpr iter_difference_t<I> ranges::advance(I& i, iter_difference_t<I> n, S bound);-5- Expects: […]
-6- Effects:
(6.1) — If S and I model sized_sentinel_for<S, I>:
(6.1.1) — If |n| ≥ |bound - i|, equivalent to ranges::advance(i, bound).:
(6.1.2) — Otherwise, equivalent to ranges::advance(i, n).
(6.2) — Otherwise,
(6.2.1) — if n is non-negative, while bool(i != bound) is true, increments i but at most |n| times.:
(6.2.2) — Otherwise, while bool(i != bound) is true, decrements i but at most
-|n| times.
Section: 30.3.1 [locale] Status: New Submitter: Stephan T. Lavavej Opened: 2019-12-06 Last modified: 2019-12-21
Priority: 3
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Discussion:
Curiously, locale's copy assignment operator currently returns const locale&. As Casey Carter noted in microsoft/STL#268, this is:
Weird!
The only occurrence in the entire Standard Library.
Preventing locale from satisfying std::copyable.
We aren't aware of any reason for this to be const. (I observe that this hasn't changed since N1804 on 2005-04-27 and probably goes back to C++98; I suspect that when this was originally specified, copy assignment operators were relatively new, and conventions for them weren't rigorously followed.)
[2019-12-21 Issue Prioritization]
Priority to 3 after reflector discussion based on the observation that we have implementation divergence.
Proposed resolution:
This wording is relative to N4842.
Modify 30.3.1 [locale] as indicated:
[…] ~locale(); // not virtualconstlocale& operator=(const locale& other) noexcept; template<class Facet> locale combine(const locale& other) const; […]
Modify 30.3.1.3 [locale.cons] as indicated:
constlocale& operator=(const locale& other) noexcept;-14- Effects: Creates a copy of other, replacing the current value.
-15- Returns: *this.
Section: 99 [fund.ts.v3::rand.util.randint] Status: Open Submitter: Zhihao Yuan Opened: 2019-12-10 Last modified: 2022-11-30
Priority: 3
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Discussion:
Addresses: fund.ts.v3
Although "implementation may select this type on the basis of performance, size, quality, or any combination of such factors," but changing this typedef is an ABI-break for implementations. Specifying per-thread engine to use this typedef results in losses of performance, size, and/or quality.
Since this type is not involved in randint facilities' interface (other than its member typedef), the current specification should be relaxed.[2020-01 Priority set to 3 and assigned to LEWG after review on the reflector.]
[2020-05-28; LEWG issue reviewing]
LEWG issue processing voted to reject 3357 as NAD. Status change to Open.
Reject LWG3357 as NAD SF F N A SA 1 10 4 2 1
[2022-10-19; Reflector poll]
Set status to "Tentatively NAD" based on LEWG recommendation and reflector poll.
[2022-11-30; LWG telecon]
Prefer to keep an open issue for the TS than to possibly forget to address it if this feature is proposed for the IS some day.
Proposed resolution:
This wording is relative to N4840.
Modify 9.1.1 [fund.ts.v3::rand.syn], header <experimental/random> synopsis, as indicated:
#include <random>
namespace std::experimental {
inline namespace fundamentals_v3 {
// 10.1.2.1, Function template randint
template <class IntType>
IntType randint(IntType a, IntType b);
void reseed();
void reseed(default_random_engine::result_typeuint_fast32_t value);
} // inline namespace fundamentals_v3
} // namespace std::experimental
Modify 99 [fund.ts.v3::rand.util.randint] as indicated:
-1- A separate per-thread engine of
[…]type default_random_engine (C++17 §29.6.5)unspecified type that meets the requirements of random number engine (C++17 [rand.req.eng]), initialized to an unpredictable state, shall be maintained for each thread. [Note: The implementation may choose the engine type on the basis of performance, size, quality, or any combination of such factors, so as to provide at least acceptable engine behavior for relatively casual, inexpert, and/or lightweight use. — end note]void reseed(); void reseed(default_random_engine::result_typeuint_fast32_t value);-7- Effects: Let g be the per-thread engine. The first form sets g to an unpredictable state. The second form invokes g.seed(value).
-8- Postconditions: Subsequent calls to randint do not depend on values produced by g before calling reseed. [Note: reseed also resets any instances of uniform_int_distribution used by randint. — end note]
Section: 17.4.1 [cstdint.syn], 16.4.2.3 [headers] Status: New Submitter: Dawid Pilarski Opened: 2020-01-14 Last modified: 2020-01-25
Priority: 3
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Discussion:
This issue has been submitted, because the editorial change requests c++-draft-issue 3521 and c++-draft-pull request 3528 has been rejected as not being editorial changes:
Currently given wording of 17.4.1 [cstdint.syn]p2:
The header defines all types and macros the same as the C standard library header <stdint.h>.
might be understood as intended: typedefs inside stdint.h and inside cstdint in namespace std:: refer to the same types, but another interpretation could be, that it's understood as:
Definitions in both files are done the same way (cstdint provides typedefs not in namespace std, because it would be a different definition than one in stdint.h). Also 16.4.2.3 [headers]p5 is non sufficiently clear:[…] the contents of each header cname is the same as that of the corresponding header name.h […]
As it doesn't say what does "same content" mean. For example is an implementation allowed to do following:
// __impl.h
typedef int __my_int;
namespace std { typedef long __my_int; }
// cname header
#include "__impl.h"
namespace std {
typedef __my_int uint32_t;
}
// name.h header
#include "__impl.h"
typedef __my_int uint32_t;
?
In this case typedef from namespace std and from global namespace refer to different types? Proposed change: Apply wording, that will unambiguously make typedefs from namespace std refer to the same types as typedefs from global namespace for all headers name.h and their corresponding headers cname.[2020-01-25 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
Section: 22.4.2 [tuple.syn], 22.4.7 [tuple.helper] Status: New Submitter: Casey Carter Opened: 2020-01-17 Last modified: 2021-11-04
Priority: 3
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Discussion:
22.4.7 [tuple.helper]/6 makes the const/volatile/const volatile partial specializations of tuple_size available when any of <array>, <ranges>, <span>, or <utility> is included. 22.4.7 [tuple.helper]/8 makes the const/volatile/const volatile partial specializations of tuple_element available when any of those same headers is included. This leads to a couple of problems:
For users of the Standard Library, it's not helpful to have these partial specializations of class templates available when the preferred interface — the variable template tuple_size_v and alias template tuple_element_t — are not.
For specifiers of the Standard Library, we must update two distinct yet identical lists of headers that make this same set of templates available when adding another header.
We could solve both of these problems by coalescing the two paragraphs into one and including the variable and alias template in the set of declarations made available by the pertinent (now single) list of headers.
[2020-02-08 Issue Prioritization]
Priority to 3 after reflector discussion. Tim Song said: It’s not clear that entities only mentioned in the synopsis are "defined in this subclause".
[2021-11-04; Jonathan Wakely adds note]
The __cpp_lib_tuple_element_t macro in
17.3.2 [version.syn] should be updated too.
Proposed resolution:
This wording is relative to N4842.
Modify 22.4.2 [tuple.syn], header <tuple> synopsis, as indicated:
namespace std {
[…]
// 22.4.7 [tuple.helper], tuple helper classes
template<class T> struct tuple_size; // not defined
template<class T> struct tuple_size<const T>;
template<class T> struct tuple_size<volatile T>;
template<class T> struct tuple_size<const volatile T>;
template<class T>
inline constexpr size_t tuple_size_v = tuple_size<T>::value;
template<class... Types> struct tuple_size<tuple<Types...>>;
template<size_t I, class T> struct tuple_element; // not defined
template<size_t I, class T> struct tuple_element<I, const T>;
template<size_t I, class T> struct tuple_element<I, volatile T>;
template<size_t I, class T> struct tuple_element<I, const volatile T>;
template<size_t I, class... Types>
struct tuple_element<I, tuple<Types...>>;
template<size_t I, class T>
using tuple_element_t = typename tuple_element<I, T>::type;
// 22.4.8 [tuple.elem], element access
template<class... Types> struct tuple_size<tuple<Types...>>;
template<size_t I, class... Types>
struct tuple_element<I, tuple<Types...>>;
template<size_t I, class... Types>
constexpr tuple_element_t<I, tuple<Types...>>& get(tuple<Types...>&) noexcept;
template<size_t I, class... Types>
constexpr tuple_element_t<I, tuple<Types...>>&& get(tuple<Types...>&&) noexcept;
[…]
// 22.4.7 [tuple.helper], tuple helper classes
template<class T>
inline constexpr size_t tuple_size_v = tuple_size<T>::value;
}
Modify 22.4.7 [tuple.helper] as indicated:
20.5.6 Tuple helper classes [tuple.helper]
-?- In addition to being available via inclusion of the <tuple> header, the entities defined in this subclause [tuple.helper] are available when any of the headers <array> (24.3.2 [array.syn]), <ranges> (26.2 [ranges.syn]), <span> (24.7.2.1 [span.syn]), or <utility> (22.2.1 [utility.syn]) are included.template<class T> struct tuple_size;-1- Remarks: All specializations of tuple_size shall meet the Cpp17UnaryTypeTrait requirements (21.3.2 [meta.rqmts]) with a base characteristic of integral_constant<size_t, N> for some N.
template<class... Types> struct tuple_size<tuple<Types...>> : public integral_constant<size_t, sizeof...(Types)> { }; template<size_t I, class... Types> struct tuple_element<I, tuple<Types...>> { using type = TI; };
-2- Requires: I < sizeof...(Types). The program is ill-formed if I is out of bounds.-3- Type: TI is the type of the Ith element of Types, where indexing is zero-based.template<class T> struct tuple_size<const T>; template<class T> struct tuple_size<volatile T>; template<class T> struct tuple_size<const volatile T>;-4- Let TS denote tuple_size<T> of the cv-unqualified type T. If the expression TS::value is well-formed when treated as an unevaluated operand, then each of the three templates shall meet the Cpp17UnaryTypeTrait requirements (21.3.2 [meta.rqmts]) with a base characteristic of
Otherwise, they shall have no member value. -5- Access checking is performed as if in a context unrelated to TS and T. Only the validity of the immediate context of the expression is considered. [Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]integral_constant<size_t, TS::value>-6- In addition to being available via inclusion of the <tuple> header, the three templates are available when any of the headers <array> (24.3.2 [array.syn]), <ranges> (26.2 [ranges.syn]), <span> (24.7.2.1 [span.syn]), or <utility> (22.2.1 [utility.syn]) are included.template<size_t I, class T> struct tuple_element<I, const T>; template<size_t I, class T> struct tuple_element<I, volatile T>; template<size_t I, class T> struct tuple_element<I, const volatile T>;-7- Let TE denote tuple_element_t<I, T> of the cv-unqualified type T. Then each of the three templates shall meet the Cpp17TransformationTrait requirements (21.3.2 [meta.rqmts]) with a member typedef type that names the following type:
(7.1) — for the first specialization, add_const_t<TE>,
(7.2) — for the second specialization, add_volatile_t<TE>, and
(7.3) — for the third specialization, add_cv_t<TE>.
-8- In addition to being available via inclusion of the <tuple> header, the three templates are available when any of the headers <array> (24.3.2 [array.syn]), <ranges> (26.2 [ranges.syn]), <span> (24.7.2.1 [span.syn]), or <utility> (22.2.1 [utility.syn]) are included.
Modify 22.4.8 [tuple.elem] as indicated:
[Drafting note: Since this issue performs colliding text changes with P1460R0, we perform similar wording changes as suggested on page 19 [tuple.helper] p2.]
20.5.7 Element access [tuple.elem]
template<class... Types> struct tuple_size<tuple<Types...>> : public integral_constant<size_t, sizeof...(Types)> { }; template<size_t I, class... Types> struct tuple_element<I, tuple<Types...>> { using type = TI; };-?- Mandates: I < sizeof...(Types).
-?- Type: TI is the type of the Ith element of Types, where indexing is zero-based.template<size_t I, class... Types> constexpr tuple_element_t<I, tuple<Types...>>& get(tuple<Types...>& t) noexcept; […][…]
Section: 21.3.7 [meta.rel] Status: New Submitter: Jiang An Opened: 2020-02-10 Last modified: 2020-09-06
Priority: 3
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Discussion:
This issue was submitted after a previous editorial change request had been rejected by the project editors.
I find that all known implementations (at least msvcstl, libc++, libstdc++ and the sample in P0758R1) of std::is_nothrow_convertible may be not clear enough to indicate that whether destruction of the destination type is considered (or not). For example, given a type Weird defined as
struct Weird
{
Weird(int) noexcept {}
~Weird() noexcept(false) {}
};
Then std::is_nothrow_convertible_v<int, Weird> is false in every known implementation. However, it seems that the conversion itself is noexcept.
[2020-02-22, Daniel comments]
This seems to be quite related to the existing issue LWG 2116.
[2020-03-11 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
Section: 16.3.2.4 [structure.specifications] Status: New Submitter: Casey Carter Opened: 2020-02-14 Last modified: 2020-03-11
Priority: 3
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Discussion:
We have quite a few occurrences of the phrase "as if by" in Effects: elements in the library specification. Is the meaning of this phrase the same as "equivalent to"? If so, we should replace occurrences of "as if by" with "equivalent to" to make it clear that the magic "Effects: Equivalent to" wording in 16.3.2.4 [structure.specifications] para 4 is intended to apply.
[2020-03-11 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
Section: 28.5.9.3.4 [rand.dist.bern.negbin] Status: New Submitter: Ahti Leppänen Opened: 2020-02-17 Last modified: 2020-03-11
Priority: 3
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Discussion:
This issue has been created because a corresponding editorial change request had been rejected.
The resolution of LWG 2406 added a note to the definition of negative_binomial_distribution:[Note: This implies that is undefined when p == 1. — end note]
This issue argues that the note is invalid as are the premises on which LWG 2406 was based on. It's also argued that current normative standard text allowing p == 1 is valid both conceptually and mathematically, and that it follows existing conventions in other software.
Problems with the added note:Why does p == 1 imply that is undefined? The only questionable factor in the definition of seems to be that in case of p == 1, the factor (1 - p)i leads to 00 when i == 0. While it is true that there's no generally accepted convention what this means, std::binomial_distribution already uses the common convention 00 == 1 (e.g. with p == 1 && t == i, leads to 00)
Even if the term was undefined mathematically, does a non-normative note of mathematical term being undefined mean that the behaviour of the program is undefined (instead of e.g. resulting to NaN) even when no preconditions are violated?
The note has lead to unclear situation of being able to construct a distribution object, but calling operator() might lead to undefined behaviour even though no preconditions are violated: for example the cppreference.com notes that
If p == 1, subsequent calls to the operator() overload that does not accept a param_type object will cause undefined behavior.
Invalidity of premises of LWG 2406:
For p == 1, this is "* 1^k * 0^i", so every integer i >= 0 is produced with zero probability. (Let's avoid thinking about 0^0.)
This is contradictory: first assuming that 0^i == 0 for all i >= 0 (implying that 0^0 == 0), but then comments not to think about 0^0. The very essence of the issue is interpretation of 0^0 and given the definition of binomial_distribution, where 0^0 == 1, the claim "so every integer i >= 0 is produced with zero probability" can be considered faulty.
Wikipedia states that p must be within (0, 1), exclusive on both sides.
I cannot find any mention of this in the Wikipedia's version as of 2014-06-02 (i.e. around the time when LWG 2406 was opened). Note that the Wikipedia's version is not the same as in C++ standard; in Wikipedia, p parameter is the same — i.e. the probability of success — but the integer parameter (> 0) is number of failures, while in C++ it is the number of successes. In the failure formulation p == 1 is indeed invalid for essentially the same reason why p == 0 is invalid for the C++ definition (i.e. leads to == 0 for all i).
Validity of p == 1:
[…] distribution of the number of failures in a sequence of trials with success probability p before n successes occur.
(from Wolfram documentation). When p == 1, this means that trial always succeeds, so it's obvious that the probability to get 0 failures is 1, and the probability for i > 0 failures is 0. This is exactly what the mathematical definition in 28.5.9.3.4 [rand.dist.bern.negbin] gives with convention 00 = 1 when p == 1.
Software such as Mathematica, Matlab and R all accept p == 1 for negative binomial distribution and they use the integer parameter as number of successes like the C++ standard.
What comes to the reasons why p == 1 could have been considered invalid, it seems that major implementations — namely libstd++, libc++ and MSVC standard library — are using std::gamma_distribution in std::negative_binomial_distribution and passing (1 - p)/p as the second argument of std::gamma_distribution. Case p == 1 is not checked leading to violation of precondition of std::gamma_distribution, which requires argument to be > 0.
For these reasons the note added by resolution of LWG 2406 seems invalid and could be considered for removal. However given the current status and history regarding handling of case p == 1, removing the note might not be the only option to consider.[2020-03-11 Issue Prioritization]
Priority to 3 and hand over to SG6 after reflector discussion.
Proposed resolution:
Section: 33.5.7.2 [atomics.ref.ops] Status: New Submitter: Andrey Semashev Opened: 2020-02-27 Last modified: 2020-09-06
Priority: 3
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Discussion:
N4849 33.5.7.2 [atomics.ref.ops]/1 describes atomic_ref<T>::required_alignment constant as follows:
The alignment required for an object to be referenced by an atomic reference, which is at least alignof(T).
This wording allows for an implementation to always define required_alignment to be equal to alignof(T) and implement atomic operations using locking, even if a lock-free implementation is possible at a higher alignment. For example, on x86-64, atomic_ref<complex<double>> could be lock-free only when the referred object is aligned to 16 bytes, but the above definition allows an implementation to define required_alignment to 8 and use locking.
The note in 33.5.7.2 [atomics.ref.ops]/2 does mention that lock-free operations may require higher alignment, but it does not provide guidance to the implementations so that required_alignment reflects alignment required for lock-free operations, if possible, and not just minimum alignment required for any kind of implementation. The suggested resolution is to change the wording so that it is clear that required_alignment indicates the alignment required for lock-free implementation, if one is possible, or alignof(T) otherwise. Further, the note in 33.5.7.2 [atomics.ref.ops]/2 contains this sentence:Further, whether operations on an atomic_ref are lock-free could depend on the alignment of the referenced object.
This sentence is misleading, because according to is_lock_free() definition in 33.5.7.2 [atomics.ref.ops]/4, the lock-free property is not allowed to depend on the alignment of a particular referenced object (is_lock_free() must return true or false if operations on all objects of the given type T are lock-free or not). In other words, atomic_ref can only refer to an object aligned at least to required_alignment and its lock-free capability cannot depend on the actual runtime alignment of the object.
To avoid the confusion, I propose to remove the sentence. The rest of the note can stay intact. However, this second edit is less important than the first one and can be omitted in case of disagreement.[2020-04-04 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
This wording is relative to N4849.
Modify 33.5.7.2 [atomics.ref.ops] as indicated:
static constexpr size_t required_alignment;-1- Let A be t
-2- [Note: Hardware could require an object referenced by an atomic_ref to have stricter alignment (6.7.6 [basic.align]) than other objects of type T.The alignment required for an object to be referenced by an atomic reference,which is at least alignof(T)so that is_always_lock_free is true. If there is no such alignment or A is less than alignof(T), required_alignment equals alignof(T). Otherwise, required_alignment equals A.Further, whether operations on an atomic_ref are lock-free could depend on the alignment of the referenced object.For example, lock-free operations on std::complex<double> could be supported only if aligned to 2*alignof(double). — end note]
Section: 22.7.4 [any.class] Status: New Submitter: Thomas Köppe Opened: 2020-03-04 Last modified: 2020-04-04
Priority: 3
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Discussion:
Several of the function specifications in 22.7.4 [any.class] have Throws: elements, but those only mention "exceptions thrown by a constructor". It seems necessary for std::any to perform dynamic allocation in general, and so in general there should be a possibility of an exception raised by such dynamic allocation. (This may come from a user-provided T::operator new, as far as I can tell.)
We should revise the specifications to add relevant sources of exceptions. The functions that should probably mention allocations are:any(const any& other)
template<class T> any(T&& value)
both any(in_place_t<T>, …) overloads
any& operator=(const any& rhs)
template<class T> any& operator=(T&& rhs)
all emplace overloads
[2020-04-04 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
Section: 33.5.8.2 [atomics.types.operations] Status: SG1 Submitter: Alisdair Meredith Opened: 2020-03-19 Last modified: 2020-09-06
Priority: 3
View all other issues in [atomics.types.operations].
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Discussion:
Paper P1831R1 aimed to deprecate all the atomic overloads for volatile qualified member function of std::atomic unless is_always_lock_free is true. There are a few omissions in the wording.
First, operator++ and operator-- are correctly constrained, but the deprecated overloads are not restored in Annex D, unlike the other member functions. I confirmed with the paper author this is an accidental oversight, and not an intended change of behavior for C++20. Secondly, the wait/notify APIs were added after the initial wording For this paper was drafted, and the paper did not catch up. Again, I confirmed with the paper author that these functions should be similarly constrained and deprecated.[2020-04-04 Issue Prioritization]
Priority to 3 after reflector discussion. The suggested wording was generally accepted, but there were considerable doubts that it is necessary to add deprecated functions of the new wait/notify functions instead of declaring only the non-volatile overloads. The wish was expressed that both SG1 and LEWG should express their opinion here.
Proposed resolution:
This wording is relative to N4861.
Modify 33.5.8.2 [atomics.types.operations] as indicated:
void wait(T old, memory_order order = memory_order::seq_cst) const volatile noexcept; void wait(T old, memory_order order = memory_order::seq_cst) const noexcept;Constraints: For the volatile overload of this function, is_always_lock_free is true.
-29- Preconditions: […] […]void notify_one() volatile noexcept; void notify_one() noexcept;Constraints: For the volatile overload of this function, is_always_lock_free is true.
-32- Effects: […] […]void notify_all() volatile noexcept; void notify_all() noexcept;Constraints: For the volatile overload of this function, is_always_lock_free is true.
-34- Effects: […] […]
Modify D.30.2 [depr.atomics.volatile], annex D, as indicated:
If an atomic specialization has one of the following overloads, then that overload participates in overload resolution even if atomic<T>::is_always_lock_free is false:
void store(T desired, memory_order order = memory_order::seq_cst) volatile noexcept; […] T* fetch_key(ptrdiff_t operand, memory_order order = memory_order::seq_cst) volatile noexcept; value_type operator++(int) volatile noexcept; value_type operator--(int) volatile noexcept; value_type operator++() volatile noexcept; value_type operator--() volatile noexcept; void wait(T old, memory_order order = memory_order::seq_cst) const volatile noexcept; void notify_one() volatile noexcept; void notify_all() volatile noexcept;
Section: 33.5.9 [atomics.nonmembers] Status: New Submitter: Alisdair Meredith Opened: 2020-03-19 Last modified: 2020-09-06
Priority: 3
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Discussion:
Paper P1831R1 deprecated the volatile-qualified member functions of std::atomic unless is_always_lock_free is true. 33.5.9 [atomics.nonmembers] maps free functions calls, declared in the <atomic> header, to those member functions, but does not deprecate them under the same circumstances.
I have confirmed with the paper author that the intended design was to deprecate these too, but currently we have no wording.[2020-03-29; Daniel provides wording]
The suggested wording changes for 33.5.9 [atomics.nonmembers] attempts to make clear that any of the specification elements of the member function (including but not restricted to Constraints: elements) are also imposed on the corresponding non-member function template. According to 16.3.2.4 [structure.specifications], the wording "the semantics of the code sequence are determined by the Constraints,[…], and Error conditions specified for the function invocations contained in the code sequence." should realize the wanted effect. The advantage of this more general wording form is that we don't need to to worry in case that in the future Constraints: elements of the member functions are modified.
[2020-03-30; Tim improves wording]
[2020-04-25 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
This wording is relative to N4861.
Modify 33.5.9 [atomics.nonmembers] as indicated:
-1- A non-member function template whose name matches the pattern atomic_f or the pattern atomic_f_explicit invokes the member function f, with the value of the first parameter as the object expression and the values of the remaining parameters (if any) as the arguments of the member function call, in order. An argument for a parameter of type atomic<T>::value_type* is dereferenced when passed to the member function call. If no such member function exists, the program is ill-formed. Otherwise, a call to such a function template has effects equivalent to (16.3.2.4 [structure.specifications]) the effective code sequence containing the f invocation specified in this subclause.
Modify D.30.2 [depr.atomics.volatile], annex D, as indicated:
If an atomic specialization has one of the following overloads, then that overload participates in overload resolution even if atomic<T>::is_always_lock_free is false:
void store(T desired, memory_order order = memory_order::seq_cst) volatile noexcept; […] T* fetch_key(ptrdiff_t operand, memory_order order = memory_order::seq_cst) volatile noexcept;In addition, the following non-member function templates participate in overload resolution even if atomic<T>::is_always_lock_free is false:
template<class T> void atomic_store(volatile atomic<T>*, typename atomic<T>::value_type) noexcept; template<class T> T atomic_load(const volatile atomic<T>*) noexcept; template<class T> T atomic_load_explicit(const volatile atomic<T>*, memory_order) noexcept; template<class T> T atomic_exchange(volatile atomic<T>*, typename atomic<T>::value_type) noexcept; template<class T> T atomic_exchange_explicit(volatile atomic<T>*, typename atomic<T>::value_type, memory_order) noexcept; template<class T> bool atomic_compare_exchange_weak(volatile atomic<T>*, typename atomic<T>::value_type*, typename atomic<T>::value_type) noexcept; template<class T> bool atomic_compare_exchange_strong(volatile atomic<T>*, typename atomic<T>::value_type*, typename atomic<T>::value_type) noexcept; template<class T> bool atomic_compare_exchange_weak_explicit(volatile atomic<T>*, typename atomic<T>::value_type*, typename atomic<T>::value_type, memory_order, memory_order) noexcept; template<class T> bool atomic_compare_exchange_strong_explicit(volatile atomic<T>*, typename atomic<T>::value_type*, typename atomic<T>::value_type, memory_order, memory_order) noexcept; template<class T> T atomic_fetch_key(volatile atomic<T>*, typename atomic<T>::difference_type) noexcept; template<class T> T atomic_fetch_key_explicit(volatile atomic<T>*, typename atomic<T>::difference_type, memory_order) noexcept; template<class T> T atomic_fetch_key(volatile atomic<T>*, typename atomic<T>::value_type) noexcept; template<class T> T atomic_fetch_key_explicit(volatile atomic<T>*, typename atomic<T>::value_type, memory_order) noexcept; template<class T> void atomic_wait(const volatile atomic<T>*, typename atomic<T>::value_type); template<class T> void atomic_wait_explicit(const volatile atomic<T>*, typename atomic<T>::value_type, memory_order); template<class T> void atomic_notify_one(volatile atomic<T>*); template<class T> void atomic_notify_all(volatile atomic<T>*);
Section: 22.7.5 [any.nonmembers] Status: New Submitter: Casey Carter Opened: 2020-04-02 Last modified: 2020-09-06
Priority: 3
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Discussion:
The return type of the non-pointer overloads of std::any_cast<T> is T. This is silly when T is cv-qualified (and, since merging P1152 "Deprecating volatile" into the working draft, deprecated when T is volatile-qualified). We should strip cv-qualifiers to determine the type returned.
[2020-04-18 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
This wording is relative to N4861.
Modify 22.7.2 [any.synop] as indicated:
[…] template<class T> remove_cv_t<T> any_cast(const any& operand); template<class T> remove_cv_t<T> any_cast(any& operand); template<class T> remove_cv_t<T> any_cast(any&& operand); […]
Modify 22.7.5 [any.nonmembers] as indicated:
template<class T> remove_cv_t<T> any_cast(const any& operand); template<class T> remove_cv_t<T> any_cast(any& operand); template<class T> remove_cv_t<T> any_cast(any&& operand);-4- Let R be the type remove_cv_t<T>, and U be the type remove_cvref_t<T>.
-5- Mandates: For the first overload, is_constructible_v<TR, const U&> is true. For the second overload, is_constructible_v<TR, U&> is true. For the third overload, is_constructible_v<TR, U> is true. -6- Returns: For the first and second overload, static_cast<TR>(*any_cast<U>(&operand)). For the third overload, static_cast<TR>(std::move(*any_cast<U>(&operand))). -7- Throws: bad_any_cast if operand.type() != typeid(remove_reference_t<T>).
Section: 22.5.3.6 [optional.observe] Status: New Submitter: Casey Carter Opened: 2020-04-02 Last modified: 2023-02-10
Priority: 3
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Discussion:
The optional<T>::value_or overloads are specified to return T. This seems silly when T is const or volatile qualified — return types should never be cv-qualified. (In the volatile case, it is even deprecated since merging P1152R4 "Deprecating volatile" into the working draft.) We should strip cv-qualifiers from these return types.
[2020-04-18 Issue Prioritization]
Priority to 3 after reflector discussion.
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Modify 22.5.3 [optional.optional] as indicated:
[…] template<class U> constexpr remove_cv_t<T> value_or(U&&) const&; template<class U> constexpr remove_cv_t<T> value_or(U&&) &&; […]Modify 22.5.3.6 [optional.observe] as indicated:
template<class U> constexpr remove_cv_t<T> value_or(U&& v) const&;-?- Let R be remove_cv_t<T>.
-17- Mandates:is_copy_constructible_v<T>is_convertible_v<const T&, R> && is_convertible_v<U&&, T> is true. -18- Effects: Equivalent to:return bool(*this) ? **this : static_cast<TR>(std::forward<U>(v));template<class U> constexpr remove_cv_t<T> value_or(U&& v) &&;-?- Let R be remove_cv_t<T>.
-19- Mandates:is_move_constructible_v<T>is_convertible_v<T, R> && is_convertible_v<U&&, T> is true. -20- Effects: Equivalent to:return bool(*this) ? std::move(**this) : static_cast<TR>(std::forward<U>(v));
[2023-02-09 Casey improves wording and expands to cover expected::value_or]
Since expected was modeled on optional, it has the same issue.
Proposed resolution:
This wording is relative to N4928.
Modify 22.5.3.1 [optional.optional.general] as indicated:
[…] template<class U> constexpr remove_cv_t<T> value_or(U&&) const&; template<class U> constexpr remove_cv_t<T> value_or(U&&) &&; […]
Modify 22.5.3.6 [optional.observe] as indicated:
[Drafting note: The two removals of the && in is_convertible_v<U&&, T> below is a simplification to restore consistency with the wording for expected::value_or.]
template<class U> constexpr remove_cv_t<T> value_or(U&& v) const &;-?- Let R be remove_cv_t<T>.
-15- Mandates:is_copy_constructible_v<T>is_convertible_v<const T&, R> && is_convertible_v<U&&,TR> is true. -16- Effects: Equivalent to:return bool(*this) ? **this : static_cast<TR>(std::forward<U>(v));template<class U> constexpr remove_cv_t<T> value_or(U&& v) &&;-?- Let R be remove_cv_t<T>.
-17- Mandates:is_move_constructible_v<T>is_convertible_v<T, R> && is_convertible_v<U&&,TR> is true. -18- Effects: Equivalent to:return bool(*this) ? std::move(**this) : static_cast<TR>(std::forward<U>(v));
Modify 22.8.6.1 [expected.object.general] as indicated:
[…] template<class U> constexpr remove_cv_t<T> value_or(U&&) const &; template<class U> constexpr remove_cv_t<T> value_or(U&&) &&; […]
Modify 22.8.6.6 [expected.object.obs] as indicated:
template<class U> constexpr remove_cv_t<T> value_or(U&& v) const &;-?- Let R be remove_cv_t<T>.
-16- Mandates:is_copy_constructible_v<T>is_convertible_v<const T&, R> && is_convertible_v<U,TR> is true. -17- Returns: has_value() ? **this : static_cast<TR>(std::forward<U>(v)).template<class U> constexpr remove_cv_t<T> value_or(U&& v) &&;-?- Let R be remove_cv_t<T>.
-18- Mandates:is_move_constructible_v<T>is_convertible_v<T, R> && is_convertible_v<U,TR> is true. -19- Returns: has_value() ? std::move(**this) : static_cast<TR>(std::forward<U>(v)).
Section: 16.4.5.11 [res.on.requirements] Status: New Submitter: Tim Song Opened: 2020-04-07 Last modified: 2020-09-06
Priority: 3
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Discussion:
It has been pointed out both on the LWG reflector and as editorial issue 3912 that the definition of "models" added in P2101R0 is not a model of clarity when a concept is defined in terms of other concepts.
[2020-04-25 Issue Prioritization]
Priority to 3 after reflector discussion. There was a temptation to resolve this as P0, but concerns were expressed that the "satisfied as part of the satisfaction" part was a bit confusing.
Proposed resolution:
This wording is relative to N4861.
Modify 16.4.5.11 [res.on.requirements] as indicated:
-1- A sequence Args of template arguments is said to model a concept C if:
(1.1) — Args satisfies C (13.5.3 [temp.constr.decl]);
(1.2) —
andArgs meets all semantic requirements (if any) given in the specification of C; and(1.3) — every concept found to be satisfied as part of the satisfaction determination above is also modeled.
Section: 24.2.2.4 [container.opt.reqmts] Status: New Submitter: Jonathan Wakely Opened: 2020-04-17 Last modified: 2022-04-24
Priority: 2
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Discussion:
The precondition for <=> on containers is:
"Either <=> is defined for values of type (possibly const) T, or < is defined for values of type (possibly const) T and < is a total ordering relationship." I don't think <=> is sufficient, because synth-three-way won't use <=> unless three_way_comparable<T> is satisfied, which requires weakly-equality-comparable-with<T, T> as well as <=>. So to use <=> I think the type also requires ==, or more precisely, it must satisfy three_way_comparable. The problem becomes clearer with the following example:
#include <compare>
#include <vector>
struct X
{
friend std::strong_ordering operator<=>(X, X) { return std::strong_ordering::equal; }
};
std::vector<X> v(1);
std::strong_ordering c = v <=> v;
This doesn't compile, because despite X meeting the preconditions for <=> in [tab:container.opt], synth-three-way will return std::weak_ordering.
Here is another example:
#include <compare>
#include <vector>
struct X
{
friend bool operator<(X, X) { return true; } // The return value is intentional, see below
friend std::strong_ordering operator<=>(X, X) { return std::strong_ordering::equal; }
};
std::vector<X> v(1);
std::weak_ordering c = v <=> v;
This meets the precondition because it defines <=>, but the result of <=> on vector<X> will be nonsense, because synth-three-way will use operator< not operator<=> and that defines a broken ordering.
So we're stating a precondition which implies "if you do this, you don't get garbage results" and then we give garbage results anyway. The proposed resolution is one way to fix that, by tightening the precondition so that it matches what synth-three-way actually does.[2020-04-25 Issue Prioritization]
Priority to 2 after reflector discussion.
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Modify 24.2.2.1 [container.requirements.general], Table [tab:container.opt], as indicated:
Table 75: Optional container operations [tab:container.opt] Expression Return type Operational
semanticsAssertion/note
pre-/post-conditionComplexity … a <=> b synth-three-way-result<value_type> lexicographical_compare_three_way(
a.begin(), a.end(), b.begin(), b.end(),
synth-three-way)Preconditions: Either <=> is defined for
values of type (possibly const)
T satisfies three_way_comparable,
or < is defined for values of type
(possibly const) T and
< is a total ordering relationship.linear …
[2022-04-24; Daniel rebases wording on N4910]
Proposed resolution:
This wording is relative to N4910.
Modify 24.2.2.4 [container.opt.reqmts] as indicated:
a <=> b-2- Result: synth-three-way-result<X::value_type>.
-3- Preconditions: Either<=> is defined for values of type (possibly const)T satisfies three_way_comparable, or < is defined for values of type (possibly const) T and < is a total ordering relationship. -4- Returns: lexicographical_compare_three_way(a.begin(), a.end(), b.begin(), b.end(), synth-three-way) [Note 1: The algorithm lexicographical_compare_three_way is defined in Clause 27. — end note] -5- Complexity: Linear.
Section: 27.11.8 [specialized.construct] Status: New Submitter: Jonathan Wakely Opened: 2020-04-29 Last modified: 2021-12-11
Priority: 2
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Discussion:
std::construct_at is ill-formed for array types, because the type of the new-expression is T not T* so it cannot be converted to the return type.
In C++17 allocator_traits::construct did work for arrays, because it returns void so there is no ill-formed conversion. On the other hand, in C++17 allocator_traits::destroy didn't work for arrays, because p->~T() isn't valid. In C++20 allocator_traits::destroy does work, because std::destroy_at treats arrays specially, but allocator_traits::construct no longer works because it uses std::construct_at. It seems unnecessary and/or confusing to remove support for arrays in construct when we're adding it in destroy. I suggest that std::construct_at should also handle arrays. It might be reasonable to restrict that support to the case where sizeof...(Args) == 0, if supporting parenthesized aggregate-initialization is not desirable in std::construct_at.[2020-05-09; Reflector prioritization]
Set priority to 2 after reflector discussions.
[2021-01-16; Zhihao Yuan provides wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4878.
Modify 27.11.8 [specialized.construct] as indicated:
template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); namespace ranges { template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); }-1- Constraints: The expression ::new (declval<void*>()) T(declval<Args>()...) is well-formed when treated as an unevaluated operand.
-2- Effects: Equivalent to:returnauto ptr = ::new (voidify(*location)) T(std::forward<Args>(args)...); if constexpr (is_array_v<T>) return launder(location); else return ptr;
[2021-12-07; Zhihao Yuan comments and provides improved wording]
The previous PR allows constructing arbitrary number of elements when T is an array of unknown bound:
extern int a[]; std::construct_at(&a, 0, 1, 2);
and leads to a UB.
Proposed resolution:
This wording is relative to N4901.
Modify 27.11.8 [specialized.construct] as indicated:
template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); namespace ranges { template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); }-1- Constraints: The expression ::new (declval<void*>()) T(declval<Args>()...) is well-formed when treated as an unevaluated operand (7.2.3 [expr.context]) and is_unbounded_array_v<T> is false.
-2- Effects: Equivalent to:returnauto ptr = ::new (voidify(*location)) T(std::forward<Args>(args)...); if constexpr (is_array_v<T>) return launder(location); else return ptr;
Section: 24.2.5.1 [container.node.overview] Status: New Submitter: Jens Maurer Opened: 2020-04-30 Last modified: 2020-05-09
Priority: 3
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Discussion:
This issue resulted out of this editorial change request.
In 24.2.5.1 [container.node.overview], there is Table 79 [tab:container.node.compat] which indicates which containers have compatible nodes. It appears that rows alongmultimap<K, T, C1, A> multimap<K, T, C2, A>
(i.e. multimaps with differing Compare functions) are missing from the table.
(Same for multiset and unordered_multiset/map.) However, the introductory sentences in 24.2.5.1 [container.node.overview] do not relate "compatible nodes" with Table 79 and do not clearly state transitivity:A node handle is an object that accepts ownership of a single element from an associative container (24.2.7 [associative.reqmts]) or an unordered associative container (24.2.8 [unord.req]). It may be used to transfer that ownership to another container with compatible nodes. Containers with compatible nodes have the same node handle type. Elements may be transferred in either direction between container types in the same row of Table 79 [tab:container.node.compat].
[2020-05-09; Reflector prioritization]
Set priority to 3 after reflector discussions.
Proposed resolution:
Section: 25.4.3 [iterator.operations], 27.7.12 [alg.random.sample] Status: New Submitter: Jens Maurer Opened: 2020-05-01 Last modified: 2020-05-09
Priority: 3
View other active issues in [iterator.operations].
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Discussion:
The library specification conveys pre-concept requirements on template parameters by naming, e.g. "InputIterator".
There is no general specification for template parameters named "Distance", as used by std::advance (see 25.4.3 [iterator.operations]). When addressing this omission, the local restriction in 27.7.12 [alg.random.sample] could possibly be removed. See the related issue 3213 for the Size template parameter.[2020-05-09; Reflector prioritization]
Set priority to 3 after reflector discussions.
Proposed resolution:
Section: 19.1.2 [networking.ts::socket.streambuf.members] Status: New Submitter: Jonathan Wakely Opened: 2020-05-14 Last modified: 2020-07-17
Priority: 2
View all issues with New status.
Discussion:
Addresses: networking.ts
The effects in 19.1.2 [networking.ts::socket.streambuf.members] p3 say that the function loops through every endpoint in the sequence, attempting to establish a connection. It needs to say that as soon as a connection is successfully established it returns. Otherwise even if a connection is made, it closes the socket and tries the next endpoint in the sequence. That means it will always be left in whatever state resulted from trying the last endpoint in the sequence (or from timing out if the expiry time was reached before iterating through all endpoints).
[2020-07-17; Priority set to 2 in telecon]
Jonathan to provide wording.
Proposed resolution:
Section: 19.2.1 [networking.ts::socket.iostream.cons], 19.2.2 [networking.ts::socket.iostream.members] Status: LEWG Submitter: Jonathan Wakely Opened: 2020-05-14 Last modified: 2021-05-10
Priority: 3
View all other issues in [networking.ts::socket.iostream.cons].
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Discussion:
Addresses: networking.ts
basic_socket_streambuf<P, C, W>::connect(Args&&...) is constrained to only exist when P meets the InternetProtocol requirements, but basic_socket_iostream<P, C, W>::connect(Args&&...) is not constrained. Since it just passes those arguments straight to the streambuf, the outer connect(Args&&...) should be constrained too.
In addition to that, the basic_socket_iostream(Args&&...) constructor should be constrained, so that is_constructible gives the right answer.[2020-07-17; Priority set to 3 in telecon]
Proposed resolution:
This wording is relative to N4771.
Modify 19.2.1 [networking.ts::socket.iostream.cons] as indicated:
[Drafting note: As a drive-by fix, a missing std:: qualification in front of forward has been added]
template<class... Args> explicit basic_socket_iostream(Args&&... args);-4- Effects: Initializes the base class as basic_iostream<char>(&sb_)), value-initializes sb_, and performs setf(std::ios_base::unitbuf). Then calls rdbuf()->connect(std::forward<Args>(args)...). If that function returns a null pointer, calls setstate(failbit).
-?- Remarks: This function shall not participate in overload resolution unless the expression rdbuf()->connect(std::forward<Args>(args)...) is well-formed.
Modify 19.2.2 [networking.ts::socket.iostream.members] as indicated:
[Drafting note: As a drive-by fix, a missing std:: qualification in front of forward has been added]
template<class... Args> void connect(Args&&... args);-1- Effects: Calls rdbuf()->connect(std::forward<Args>(args)...). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure).
-?- Remarks: This function shall not participate in overload resolution unless the expression rdbuf()->connect(std::forward<Args>(args)...) is well-formed.
Section: 23.4.3 [basic.string], 26.7.14.2 [range.join.view] Status: New Submitter: Johel Ernesto Guerrero Peña Opened: 2020-06-11 Last modified: 2020-07-17
Priority: 3
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Discussion:
The library inconsistently marks deduction guides as explicit. join_view and basic_string account for the only two occurrences of unconditionally explicit deduction guides. All other deduction guides have no explicit-specifier. Following is a list of unconditionally explicit constructors with their deduction guides.
template<class Y>
explicit shared_ptr(const weak_ptr<Y>& r);
template<class T>
shared_ptr(weak_ptr<T>) -> shared_ptr<T>;
template<class T>
constexpr explicit basic_string(const T& t, const Allocator& a = Allocator());
template<class charT,
class traits,
class Allocator = allocator<charT>>
explicit basic_string(basic_string_view<charT, traits>, const Allocator& = Allocator())
-> basic_string<charT, traits, Allocator>;
explicit queue(const Container&);
explicit queue(Container&&);
template<class Container>
queue(Container) -> queue<typename Container::value_type, Container>;
explicit stack(const Container&);
explicit stack(Container&&);
template<class Container>
stack(Container) -> stack<typename Container::value_type, Container>;
constexpr explicit join_view(V base);
template<class R>
explicit join_view(R&&) -> join_view<views::all_t<R>>;
constexpr explicit common_view(V r);
template<class R>
common_view(R&&) -> common_view<views::all_t<R>>;
constexpr explicit reverse_view(V r);
template<class R>
reverse_view(R&&) -> reverse_view<views::all_t<R>>;
explicit zoned_time(TimeZonePtr z);
explicit zoned_time(string_view name);
template<class TimeZonePtrOrName>
zoned_time(TimeZonePtrOrName&&)
-> zoned_time<seconds, time-zone-representation<TimeZonePtrOrName>>;
template<class C>
explicit stop_callback(const stop_token& st, C&& cb)
noexcept(is_nothrow_constructible_v<Callback, C>);
template<class C>
explicit stop_callback(stop_token&& st, C&& cb)
noexcept(is_nothrow_constructible_v<Callback, C>);
template<class Callback>
stop_callback(stop_token, Callback) -> stop_callback<Callback>;
[2020-07-17; Priority set to 3 in telecon]
Proposed resolution:
This wording is relative to N4861.
Modify 23.4.3 [basic.string], class template basic_string synopsis, as indicated:
[…]
template<class charT,
class traits,
class Allocator = allocator<charT>>
explicit basic_string(basic_string_view<charT, traits>, const Allocator& = Allocator())
-> basic_string<charT, traits, Allocator>;
[…]
Modify 23.4.3.3 [string.cons] as indicated:
[…] template<class charT, class traits, class Allocator = allocator<charT>>explicitbasic_string(basic_string_view<charT, traits>, const Allocator& = Allocator()) -> basic_string<charT, traits, Allocator>; […]-22- Constraints: Allocator is a type that qualifies as an allocator (24.2.2.1 [container.requirements.general]).
Modify 26.7.14.2 [range.join.view], class template join_view synopsis, as indicated:
[…] template<class R>explicitjoin_view(R&&) -> join_view<views::all_t<R>>; […]
Section: 20.2.3 [pointer.traits] Status: LEWG Submitter: Alisdair Meredith Opened: 2020-06-21 Last modified: 2022-07-21
Priority: Not Prioritized
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Discussion:
Trying to implement a constexpr std::list (inspired by Tim Song's note on using variant members in the node) as part of evaluating the constexpr container and adapters proposals, I hit problems I could not code around in pointer_traits, as only the specialization for native pointers has a constexpr pointer_to function.
This means that containers of my custom allocator, that delegates all allocation behavior to std::allocator<T> but adds extra telemetry and uses a fancy pointer, does not work with the approach I tried for implementing list (common link type, shared between nodes, and stored as end sentinel directly in the list object).[2020-07-17; Forwarded to LEWG after review in telecon]
[2022-07-19; Casey Carter comments]
This is no longer simply a theoretical problem that impedes implementing constexpr std::list, but an actual defect affecting current implementations of constexpr std::string. More specifically, it makes it impossible to support so-called "fancy pointers" in a constexpr basic_string that performs the small string optimization (SSO). (pointer_traits::pointer_to is critically necessary to get a pointer that designates the SSO buffer.) As things currently stand, constexpr basic_string can support fancy pointers or SSO, but not both.
Proposed resolution:
This wording is relative to N4861.
Modify 20.2.3 [pointer.traits] as indicated:
-1- The class template pointer_traits supplies a uniform interface to certain attributes of pointer-like types.
namespace std { template<class Ptr> struct pointer_traits { using pointer = Ptr; using element_type = see below; using difference_type = see below; template<class U> using rebind = see below; static constexpr pointer pointer_to(see below r); }; […] }
Modify 20.2.3.3 [pointer.traits.functions] as indicated:
static constexpr pointer pointer_traits::pointer_to(see below r); static constexpr pointer pointer_traits<T*>::pointer_to(see below r) noexcept;-1- Mandates: For the first member function, Ptr::pointer_to(r) is well-formed.
-2- Preconditions: For the first member function, Ptr::pointer_to(r) returns a pointer to r through which indirection is valid. -3- Returns: The first member function returns Ptr::pointer_to(r). The second member function returns addressof(r). -4- Remarks: If element_type is cv void, the type of r is unspecified; otherwise, it is element_type&.
Section: 22.13.3 [charconv.from.chars] Status: New Submitter: Jonathan Wakely Opened: 2020-06-23 Last modified: 2020-09-06
Priority: 3
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Discussion:
The intention of 22.13.3 [charconv.from.chars] p7 is that the fmt argument modifies the expected pattern, so that only a specific subset of valid strtod patterns are recognized for each format. This is not clear from the wording.
When fmt == chars_format::fixed no exponent is to be used, so any trailing characters that match the form of a strtod exponent are ignored. For example, "1.23e4" should produce the result 1.23 for the fixed format. The current wording says "the optional exponent part shall not appear" which can be interpreted to mean that "1.23e4" violates a precondition and so has undefined behaviour! When fmt != chars_format::hex only decimal numbers should be recognized. This means that for any format except scientific, "0x123" produces 0.0 (it's invalid when fmt == chars_format::scientific because there's no exponent). The current wording only says that when hex is used the string has an assumed "0x" prefix, so is interpreted as a hexadecimal float, it doesn't say that when fmt != hex that the string is not interpreted as a hexadecimal float. Two alternative resolutions are provided, one is a minimal fix and the other attempts to make it clearer by not referring to a modified version of the C rules.[2020-07-14; Jonathan fixes the strtod call in Option B]
[2020-07-17; Priority set to 3 in telecon]
Proposed resolution:
This wording is relative to N4861.
Option A:Modify 22.13.3 [charconv.from.chars] as indicated:
from_chars_result from_chars(const char* first, const char* last, float& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, double& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, long double& value, chars_format fmt = chars_format::general);-6- Preconditions: fmt has the value of one of the enumerators of chars_format.
-7- Effects: The pattern is the expected form of the subject sequence in the "C" locale, as described for strtod, except that
(7.1) — the sign '+' may only appear in the exponent part;
(7.2) — if fmt has chars_format::scientific set but not chars_format::fixed, the
otherwise optional exponent part shall appearexponent part is not optional;(7.3) — if fmt has chars_format::fixed set but not chars_format::scientific,
the optional exponent part shall not appear; andthere is no exponent part;(?.?) — if fmt is not chars_format::hex, only decimal digits and an optional '.' appear before the exponent part (if any); and
(7.4) — if fmt is chars_format::hex, the prefix "0x" or "0X" is assumed. [Example: The string 0x123 is parsed to have the value 0 with remaining characters x123. — end example]
In any case, the resulting value is one of at most two floating-point values closest to the value of the string matching the pattern.
Modify 22.13.3 [charconv.from.chars] as indicated:
from_chars_result from_chars(const char* first, const char* last, float& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, double& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, long double& value, chars_format fmt = chars_format::general);-6- Preconditions: fmt has the value of one of the enumerators of chars_format.
-7- Effects:The pattern is the expected form of the subject sequence in the "C" locale, as described for strtod, except thatThe pattern is an optional '-' sign followed by one of:
(7.1) —
the sign '+' may only appear in the exponent partINF or INFINITY, ignoring case;(7.2) —
if fmt has chars_format::scientific set but not chars_format::fixed, the otherwise optional exponent part shall appearif numeric_limits<T>::has_quiet_NaN is true, NAN or NAN(n-char-sequenceopt), ignoring case in the NAN part, where:n-char-sequence: digit nondigit n-char-sequence digit n-char-sequence nondigit;
(7.3) —
if fmt has chars_format::fixed set but not chars_format::scientific, the optional exponent part shall not appear; andif fmt is equal to chars_format::scientific, a sequence of characters matching chars-format-dec exponent-part, where:chars-format-dec: fractional-constant digit-sequence;
(7.4) —
if fmt is chars_format::hex, the prefix "0x" or "0X" is assumed. [Example: The string 0x123 is parsed to have the value 0 with remaining characters x123. — end example]if fmt is equal to chars_format::fixed, a sequence of characters matching chars-format-dec;(?.?) — if fmt is equal to chars_format::general, a sequence of characters matching chars-format-dec exponent-partopt; or
(?.?) — if fmt is equal to chars_format::hex, a sequence of characters matching chars-format-hex binary-exponent-partopt, where:
chars-format-hex: hexadecimal-fractional-constant hexadecimal-digit-sequence[Note: The pattern is derived from the subject sequence in the "C" locale for strtod, with the value of fmt limiting which forms of the subject sequence are recognized, and with no 0x or 0X prefix recognized. — end note]
For a character sequence INF, INFINITY, NAN, or NAN(n-char-sequenceopt) the resulting value is obtained as if by evaluating strtod(string(first, last).c_str(), nullptr) in the "C" locale. In all other cases
In any case, the resulting value is one of at most two floating-point values closest to the value of the string matching the pattern.
Section: 23.3.3 [string.view.template] Status: New Submitter: Johel Ernesto Guerrero Peña Opened: 2020-06-26 Last modified: 2020-09-06
Priority: 3
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Discussion:
23.3.3 [string.view.template] states:
For a basic_string_view str, any operation that invalidates a pointer in the range [str.data(), str.data() + str.size()) invalidates pointers, iterators, and references returned from str's member functions.
The assignment operators return a reference to *this, but *this is not invalidated.
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Modify 23.3.3 [string.view.template] as indicated:
-2- For a basic_string_view str, any operation that invalidates a pointer in the range [str.data(), str.data() + str.size()) invalidates pointers, iterators, and references to the elements in that range and its past-the-end iterator returned from str's member functions.
[2020-06-29; Casey comments and provides alternative proposed wording]
I think we should additionally strike "returned from str's member functions" from the end of the sentence. Provenance shouldn't affect invalidation; ranges::next(some_string_view.begin(), 42), for example, returns an iterator that denotes an element of some_string_view, but is not a member function of basic_string_view.
[2020-07-05; Reflector prioritization]
Set priority to 3 after reflector discussions.
Proposed resolution:
This wording is relative to N4861.
Modify 23.3.3 [string.view.template] as indicated:
-2- For a basic_string_view str, any operation that invalidates a pointer in the range [str.data(), str.data() + str.size()) invalidates pointers, iterators, and references to the elements in that range and its past-the-end iterator
returned from str's member functions.
Section: 18.4.4 [concept.convertible] Status: New Submitter: S. B. Tam Opened: 2020-06-30 Last modified: 2020-07-12
Priority: 3
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Discussion:
18.4.4 [concept.convertible] p2:
Types From and To model convertible_to<From, To> only if:
(2.1) — To is not an object or reference-to-object type, or static_cast<To>(f()) is equal to test(f).
[…]
This requires the implicit and explicit conversions to produce equal results.
However, it seems that when To is a reference-to-function type, this restriction does not apply. This makes it possible to create a class that models convertible_to, but produces different results depending on the kind of conversion:
#include <concepts>
int foo() { return 0; }
int bar() { return 42; }
using FT = int();
struct A
{
operator FT&() const { return foo; }
explicit operator FT&() { return bar; }
};
static_assert(std::convertible_to<A, FT&>);
A a;
FT& x = a; // x == foo
auto y = static_cast<FT&>(a); // y == bar
[2020-07-12; Reflector prioritization]
Set priority to 3 after reflector discussions.
Proposed resolution:
Section: 27.10.11 [transform.inclusive.scan] Status: New Submitter: Agustín K-ballo Bergé Opened: 2020-07-07 Last modified: 2020-09-06
Priority: 3
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Discussion:
The requirements for the overloads of std::transform_inclusive_scan without an initial value incorrectly assume that the internal accumulator uses the iterator's value type, as it does for std::inclusive_scan, rather than the transformed type of the iterator's value type, as it was intended.
According to the standard, the following program is ill-formed as it requires std::string to be convertible to int:
auto vs = {0, 1, 2};
std::transform_inclusive_scan(
vs.begin(), vs.end(),
std::ostream_iterator<std::string>(std::cout, ";"),
[](std::string x, std::string y) { return x + y; },
[](int x) { return std::to_string(x); });
libstdc++ and Microsoft's STL accept the snippet, producing 0;01;012; as expected, libc++ strictly conforms to the standard and rejects it.
These constrains were introduced by P0574R1.[2020-07-17; Priority set to 3 in telecon]
Proposed resolution:
This wording is relative to N4861.
[Drafting note: Current implementations that accept the code, do some form of auto acc = unary_op(*first);, therefore the following proposed wording uses decay_t instead of e.g. remove_cvref_t.]
Modify 27.10.11 [transform.inclusive.scan] as indicated:
template<class InputIterator, class OutputIterator, class BinaryOperation, class UnaryOperation> constexpr OutputIterator transform_inclusive_scan(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op, UnaryOperation unary_op); template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2, class BinaryOperation, class UnaryOperation> ForwardIterator2 transform_inclusive_scan(ExecutionPolicy&& exec, ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result, BinaryOperation binary_op, UnaryOperation unary_op); template<class InputIterator, class OutputIterator, class BinaryOperation, class UnaryOperation, class T> constexpr OutputIterator transform_inclusive_scan(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op, UnaryOperation unary_op, T init); template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2, class BinaryOperation, class UnaryOperation, class T> ForwardIterator2 transform_inclusive_scan(ExecutionPolicy&& exec, ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result, BinaryOperation binary_op, UnaryOperation unary_op, T init);-1- Let U be
-2- […]the value type of decltype(first)decay_t<decltype(unary_op(*first))>.
Section: 33.4.3.3 [thread.thread.constr], 33.4.4.2 [thread.jthread.cons] Status: New Submitter: Billy O'Neal III Opened: 2020-08-14 Last modified: 2020-09-06
Priority: 3
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Discussion:
(j)thread's constructor needs to decay-copy the supplied parameters and callable over to the started thread through an operating system API that generally only accepts a single void*. The MSVC++ and libc++ implementations do this by putting the parameters in a std::tuple allocated from the heap, passing a pointer to that tuple through the operating system API, and leaving ownership of the parameters to the other thread.
It might be theoretically possible to introduce an additional copy and synchronization where the starting thread blocks for the started thread to make a move constructed copy of that tuple from the parameters, but that would introduce unreasonable synchronization overhead since the starting thread would have to block for all TLS initializers and similar in the started thread. It is technically possible to implement the current design by transforming this allocation failure into resource_unavailable_try_again, but the description for this error in the standard is that some thread-based limitation has been reached, not a general memory limit, so that doesn't seem to meet the spirit of the requirement.[2020-08-21; Issue processing telecon: set priority to 3]
Jonathan: I prefer Option A, but I think we need something like: "any exceptions thrown by the decay-copy calls, or ...".
Proposed resolution:
This wording is relative to N4861.
[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
Option A: The memory allocation failure results in bad_alloc.
Modify 33.4.3.3 [thread.thread.constr] as indicated:
template<class F, class... Args> explicit thread(F&& f, Args&&... args);-3- Constraints: […]
[…] -8- Postconditions: get_id() != id(). *this represents the newly started thread. -9- Throws: bad_alloc if memory to transfer parameters to the new thread cannot be obtained. system_error if unable to start the new thread. -10- Error conditions:
(10.1) — resource_unavailable_try_again — the system lacked the necessary resources to create another thread, or the system-imposed limit on the number of threads in a process would be exceeded.
Modify 33.4.4.2 [thread.jthread.cons] as indicated:
template<class F, class... Args> explicit jthread(F&& f, Args&&... args);-3- Constraints: […]
[…] -8- Postconditions: get_id() != id() is true and ssource.stop_possible() is true and *this represents the newly started thread. [Note: The calling thread can make a stop request only once, because it cannot replace this stop token. — end note] -9- Throws: bad_alloc if memory to transfer parameters to the new thread cannot be obtained. system_error if unable to start the new thread. -10- Error conditions:
(10.1) — resource_unavailable_try_again — the system lacked the necessary resources to create another thread, or the system-imposed limit on the number of threads in a process would be exceeded.
Option B: The memory allocation failure results in a system_error with the error condition out_of_memory.
Modify 33.4.3.3 [thread.thread.constr] as indicated:
template<class F, class... Args> explicit thread(F&& f, Args&&... args);-3- Constraints: […]
[…] -8- Postconditions: get_id() != id(). *this represents the newly started thread. -9- Throws: system_error if unable to start the new thread. -10- Error conditions:
(10.?) — not_enough_memory — the system lacked memory resources to transfer parameters to the new thread.
(10.1) — resource_unavailable_try_again — the system lacked the necessary resources to create another thread, or the system-imposed limit on the number of threads in a process would be exceeded.
Modify 33.4.4.2 [thread.jthread.cons] as indicated:
template<class F, class... Args> explicit jthread(F&& f, Args&&... args);-3- Constraints: […]
[…] -8- Postconditions: get_id() != id() is true and ssource.stop_possible() is true and *this represents the newly started thread. [Note: The calling thread can make a stop request only once, because it cannot replace this stop token. — end note] -9- Throws: system_error if unable to start the new thread. -10- Error conditions:
(10.?) — not_enough_memory — the system lacked memory resources to transfer parameters to the new thread.
(10.1) — resource_unavailable_try_again — the system lacked the necessary resources to create another thread, or the system-imposed limit on the number of threads in a process would be exceeded.
Section: 17.14.7 [support.c.headers.other] Status: New Submitter: Thomas Köppe Opened: 2020-09-06 Last modified: 2022-09-18
Priority: 3
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Discussion:
From this editorial issue request:
The header <stddef.h> is currently specified in 17.14.7 [support.c.headers.other] to not declare a global name corresponding to std::byte, but no similar exclusion exists for std::nullptr_t. Is an oversight or intentional? There does not seem to be an interoperability reason to provide a global namespace name ::nullptr_t, since this construction would be meaningless in C and thus the name would not be encountered in code that is both valid C and C++. For lack of justification, I would like to propose to require normatively that no name ::nullptr_t be declared by <stddef.h>. Additional notes: The proposing paper N2431 mentions only an addition of "nullptr_t" to <cstddef> and does not discuss the impact on <stddef.h>. By omission this means the default rules for <stddef.h> apply and the global name should exist, but this does not provide us with a positive signal of intention. I also realize that this is a rather obscure point, and that vendors are already shipping ::nullptr_t, so I am also happy to drop this issue as not being worth the churn and the increase in implementation complexity (since removals don't generally simplify implementations). I would welcome a bit of discussion, though.[2020-09-29; Priority to P3 after reflector discussions]
[2022-09-18; Daniel comments]
See also 3782, which points out that nullptr_t has become a part of the C standard library as of C23 (see WG14-N3042 and WG14-N3048).
Proposed resolution:
This wording is relative to N4910.
Modify 17.14.7 [support.c.headers.other] as indicated:
-1- Every C header other than <complex.h> (17.14.2 [complex.h.syn]), <iso646.h> (17.14.3 [iso646.h.syn]), <stdalign.h> (17.14.4 [stdalign.h.syn]), <stdatomic.h> (33.5.12 [stdatomic.h.syn]), <stdbool.h> (17.14.5 [stdbool.h.syn]), and <tgmath.h> (17.14.6 [tgmath.h.syn]), each of which has a name of the form <name.h>, behaves as if each name placed in the standard library namespace by the corresponding <cname> header is placed within the global namespace scope, except for the functions described in 28.7.6 [sf.cmath], the declaration of std::byte and std::nullptr_t (17.2.1 [cstddef.syn]), and the functions and function templates described in 17.2.5 [support.types.byteops]. It is unspecified whether these names are first declared or defined within namespace scope (6.4.6 [basic.scope.namespace]) of the namespace std and are then injected into the global namespace scope by explicit using-declarations (9.9 [namespace.udecl]).
In [diff.cpp??] [Editorial note: new compatibility section to be created for C++20], add a new entry:
Affected subclause: 17.14.7 [support.c.headers.other]
Change: Removal of name ::nullptr_t from header <stddef.h>.
Rationale: std::nullptr_t is a C++-only feature that is not useful for interoperability with ISO C.
Effect on original feature: Valid C++20 code may fail to compile in this International Standard. Occurrences of #include <stddef.h> and nullptr_t should be replaced with #include <cstddef> and std::nullptr_t.
Section: 33.5.7 [atomics.ref.generic] Status: SG1 Submitter: Billy O'Neal III Opened: 2020-09-12 Last modified: 2020-09-29
Priority: 3
View all other issues in [atomics.ref.generic].
View all issues with SG1 status.
Discussion:
Consider the following program:
#include <atomic>
#include <iostream>
#include <thread>
using namespace std;
int main() {
int i{500};
atomic_ref atom{i};
i += 500;
thread t1{[&atom] { for (int val{0}, x{0}; x < 70;) {
if (atom.compare_exchange_weak(val, val + 10)) { ++x; }}}};
thread t2{[&atom] { for (int val{0}, y{0}; y < 29;) {
if (atom.compare_exchange_weak(val, val + 1)) { ++y; }}}};
t1.join(); t2.join();
cout << i << endl; // 1729
}
Technically this program has undefined behavior. 33.5.7 [atomics.ref.generic] p3 says that, during the lifetime of any atomic_ref referring to an object, that the object may only be accessed through the atomic_ref instances. However, in this example the atomic_ref is constructed before the i+=500 and is not destroyed before the print, even though we have a happens-before relationship between the atomic and non-atomic 'phases' of access of the value.
The user would instead have to write:
#include <atomic>
#include <iostream>
#include <thread>
using namespace std;
int main() {
int i{500};
i += 500;
{
atomic_ref atom{i};
thread t1{[&atom] { for (int val{0}, x{0}; x < 70;) {
if (atom.compare_exchange_weak(val, val + 10)) { ++x; }}}};
thread t2{[&atom] { for (int val{0}, y{0}; y < 29;) {
if (atom.compare_exchange_weak(val, val + 1)) { ++y; }}}};
t1.join(); t2.join();
} // destroy atom
cout << i << endl; // 1729
}
We should probably get SG1 on record clarifying whether they intend the first program to be acceptable. I can think of a reason to for atomic_ref's ctor to do something (zeroing out padding), but in our implementation it does nothing. I can't think of any reason for atomic_ref's dtor to do anything.
[2020-09-29; Priority to P3 after reflector discussions; Status set to "SG1"]
Proposed resolution:
Section: 21.3.5.4 [meta.unary.prop] Status: LEWG Submitter: Jonathan Wakely Opened: 2020-10-01 Last modified: 2020-10-01
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with LEWG status.
Discussion:
According to the current wording, std::is_constructible<int[], int> should be true, because the preconditions are met (all types are complete types or unbounded arrays) and the variable definition is well-formed since C++20:
using T = int[];
T t(declval<int>()); // equiv. to int t[] = {1};
However, this doesn't construct an object of type int[] because it deduces the array bound from the initializers, and so constructs int[1], which is not the type being asked about. It seems more logical for the trait to give a false result for an unbounded array, because it's an incomplete type, and no int[] can ever be constructed.
On the reflector Tim Song noted:
On the other hand, the result is something to which an int(&)[] can be bound directly thanks to another C++20 change, so a lot of things might Just Work (for some definition of "Work") despite the type difference.
This seems to me a reasonable rationale for is_constructible<int(&&)[], int> to be true (which it is today), but not for int[].
Peter Dimov replied:
Placement new, which is often the way to construct we're interested in, is not going to work even for T[2].
For example:
using T2 = int[2]; T2 x; new(x) T2(1, 2); // ill-formed
We need to decide what behaviour we want here. Do we just want is_constructible to reflect the T(declval<Args...>); construct as currently specified in 21.3.5.4 [meta.unary.prop] p8, or do we want to give a more useful/meaningful answer here?
Should we revisit 21.3.5.4 [meta.unary.prop] p8 in the light of parenthesized aggregate init, so that is_constructible<T[], T> and is_constructible<T[1], T> are false?
There may be some interaction with LWG 3436.
Proposed resolution:
Section: 27.10 [numeric.ops] Status: New Submitter: Matthias Kretz Opened: 2020-10-01 Last modified: 2020-10-02
Priority: 3
View all other issues in [numeric.ops].
View all issues with New status.
Discussion:
The algorithms partial_sum, exclusive_scan, inclusive_scan, transform_exclusive_scan, transform_inclusive_scan, and adjacent_difference with no ExecutionPolicy do not have a precondition "result is not in the range [first, last)". But they explicitly allow "result may be equal to first". This suggests the precondition got lost, because otherwise the permission is redundant.
Suggested fix: Add "result is not in the range [first + 1, last)." to the Preconditions paragraphs of the affected generalized numeric operations.
[2020-10-02; Issue processing telecon: Priority set to P3.]
Proposed resolution:
Section: 24.3.7.4 [array.special] Status: Open Submitter: Casey Carter Opened: 2020-10-01 Last modified: 2020-10-04
Priority: 3
View all issues with Open status.
Discussion:
Per 24.3.7.4 [array.special]/1, std::array's non-member swap participates in overload resolution when the array has size 0 or swappable elements. The effects of non-member swap are "As if by [member swap]", but member swap's effects are simply "Equivalent to swap_ranges(begin(), end(), y.begin())" per 24.3.7.3 [array.members]/4. In effect, we've gone out of our way to ensure that is_swappable_v<array<T, 0>> and swappable<array<T, 0>> are always true despite that actually swapping such an array may be ill-formed.
It seems that the wording stops half-way to making array<T, 0> swappable regardless of T. I personally find that design distasteful - it seems a gratuitous difference between array<T, N> and array<T, 0> - but I'd prefer a consistent design over the status quo even if it's the "wrong" design.
[2020-10-02; Issue processing telecon]
Preference for Option B, and successful vote to move to Tentatively Ready. But on the reflector Tim Song pointed out a conflict with 2157 and question the decision. Status to Open instead. Priority set to P3 in line with 2157.
Proposed resolution:
Wording relative to N4861.
This resolution proposes two wording alternatives: Option A makes array<T, 0> swappable regardless of T, and the clearly superior Option B makes array<T, N> swappable only if T is swappable (i.e., regardless of N) removing gratuitous special-case behavior for the N == 0 case.Option A:
Change 24.3.7.3 [array.members] as follows:
constexpr void swap(array& y) noexcept(N == 0 || is_nothrow_swappable_v<T>);
-4- Effects: If N == 0, no effects. Otherwise, equivalent Equivalent to swap_ranges(begin(), end(), y.begin()).
-5- […]
Also remove the now-redundant paragraph four from 24.3.7.5 [array.zero] as follows:
-4- Member function swap() shall have a non-throwing exception specification.
Option B:
Change 24.3.7.4 [array.special] as follows:
template<class T, size_t N> constexpr void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y)));
-1- Constraints: N == 0 or is_swappable_v<T> is true.
Also remove paragraph four from 24.3.7.5 [array.zero] as follows:
-4- Member function swap() shall have a non-throwing exception specification.
Section: 26.6.6.3 [range.istream.iterator] Status: New Submitter: Michael Schellenberger Costa Opened: 2020-10-09 Last modified: 2021-09-02
Priority: 3
View all other issues in [range.istream.iterator].
View all issues with New status.
Discussion:
While implementing iranges::stream_view we found some issues with the Preconditions on the member functions of istream_view::iterator, which are superfluous or incorrect.
26.6.6.3 [range.istream.iterator] p2 reads as:
Preconditions: parent_->stream_ != nullptr is true.
However in the Effects element 26.6.6.3 [range.istream.iterator] p3 it reads:
Effects: Equivalent to:
*parent_->stream_ >> parent_->object_; return *this;
For the Effects element to be valid, we implicitly require that parent_ != nullptr, parent_->stream_ != nullptr and — because we are reading from the underlying stream — !*x.parent_->stream_.
Given that the Preconditions element only mentions one of the three preconditions and essentially means that we are not at the end of the stream, we should replace 26.6.6.3 [range.istream.iterator] p2 by:Preconditions: *this != default_sentinel.
We should use the same precondition for 26.6.6.3 [range.istream.iterator] p4, even if it is implicit via the Effects element in 26.6.6.3 [range.istream.iterator] p5, as that requires experts knowledge of the standard.
The Precondition in 26.6.6.3 [range.istream.iterator] p6 is completely bogus, as accessing the cached object has no dependency on the stream. We assume it is meant that we are not at the end of the stream. Again we should change this to:
Preconditions: *this != default_sentinel.
[2020-10-14; Priority to P3 after reflector discusssion]
[2021-09-02; Jonathan comments:]
The preconditions were removed by P2325R3 approved in June 2021.
Although the pointers now cannot be null, it's unclear if we want to require
fail() to be false for the stream.
Proposed resolution:
This wording is relative to N4861.
Modify 26.6.6.3 [range.istream.iterator] as indicated:
iterator& operator++();-2- Preconditions:
-3- Effects: Equivalent to:parent_->stream_ != nullptr*this != default_sentinel is true.*parent_->stream_ >> parent_->object_; return *this;void operator++(int);-4- Preconditions:
-5- Effects: Equivalent to ++*this.parent_->stream_ != nullptr*this != default_sentinel is true.Val& operator*() const;-6- Preconditions:
-7- Effects: Equivalent to: return parent_->object_;parent_->stream_ != nullptr*this != default_sentinel is true.friend bool operator==(const iterator& x, default_sentinel_t);-8- Effects: Equivalent to: return x.parent_ == nullptr || !*x.parent_->stream_;
Section: 17.11.6 [cmp.alg], 26.7.6.1 [range.all.general], 17.11.6 [cmp.alg] Status: New Submitter: Alisdair Meredith Opened: 2020-10-27 Last modified: 2021-10-30
Priority: 3
View other active issues in [cmp.alg].
View all other issues in [cmp.alg].
View all issues with New status.
Discussion:
Some of our newer wording for C++20 uses the term "decayed type" as if it were a defined term of art. While I have intuition for what may be intended in these cases, it turns out the "function to function pointer decay" and "array to array pointer decay" were never actually Core terms of art — having searched all standards going back as far as C++03.
We should either define this term for library use, or find a way to state our intent using existing well-defined terms of art. Affected clauses:17.11.6 [cmp.alg]: Most heavily used here.
26.7.6.1 [range.all.general]
17.11.6 [cmp.alg]
[2021-01-15; Telecon prioritization]
Set priority to 3 following reflector and telecon discussions.
Proposed resolution:
Section: 22.10.17.3.2 [func.wrap.func.con] Status: New Submitter: Ville Voutilainen Opened: 2020-10-31 Last modified: 2021-08-20
Priority: 3
View all other issues in [func.wrap.func.con].
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Discussion:
In P0288, any_invocable is (correctly) constraining its constructor that takes an F:
template<class F> any_invocable(F&& f);Let VT be decay_t<F>.
Constraints:
— […]
— is_constructible_v<VT, F> is true, and
— […]
std::function doesn't do that. According to N4868, 22.10.17.3.2 [func.wrap.func.con] p8 has a constraint for Lvalue-Callable, but not for copy-constructibility. There is a precondition in p9, but that's not enough for portable well/ill-formedness.
Since this is a constructor, and we want to give the right answer to is_constructible/constructible_from queries, we should add the relevant constraint.[2020-11-01; Daniel comments]
This issue has some overlap with LWG 2774.
[2021-01-15; Telecon prioritization]
Set priority to 3 following reflector and telecon discussions.
[2021-05-17; Tim comments]
The new constraint causes constraint recursion in an example like:
struct C {
explicit C(std::function<void()>); // #1
void operator()() {}
};
static_assert(std::is_constructible_v<C, const C&>);
Here, to determine whether a C can be constructed from a const C lvalue, the overload resolution will attempt to determine whether the constructor marked #1 is a viable candidate, which involves a determination of whether that lvalue can be implicitly converted to a std::function<void()>, which, with the new constraint, requires a determination whether C is copy-constructible — in other words, whether it can be constructed from a C lvalue.
This is similar to LWG 3420: in both cases we have a class (filesystem::path there, function here) that is convertible from every type that are, inter alia, copy constructible, and this then results in constraint recursion when we ask whether a different type that is constructible from such a class is copy constructible. The C above is reduced from an internal helper type in libstdc++. Given the ubiquity of call wrappers — types that are callable in their own right and therefore may not be able to be ruled out by the Lvalue-Callable constraint, and can also naturally have a constructor that take the wrapped function object as the argument, triggering the recursion scenario — it is not clear that there is a good way to add this constraint without causing undue breakage.[2021-08-20; LWG telecon]
LWG requested that the constraint cited above for
move_only_function (né any_invocable)
be moved to a Mandates: element instead, to avoid the same
constraint recursion.
Proposed resolution:
This wording is relative to N4868.
Modify 22.10.17.3.2 [func.wrap.func.con] as indicated:
template<class F> function(F f);-8- Constraints: F is Lvalue-Callable (22.10.17.3.1 [func.wrap.func.general]) for argument types ArgTypes... and return type R, and is_copy_constructible_v<F> is true.
-9- Preconditions: F meets the Cpp17CopyConstructible requirements. […]
Section: 31.11.2.4 [syncstream.syncbuf.members] Status: New Submitter: Jonathan Wakely Opened: 2020-11-10 Last modified: 2020-11-21
Priority: 3
View other active issues in [syncstream.syncbuf.members].
View all other issues in [syncstream.syncbuf.members].
View all issues with New status.
Discussion:
31.11.2.4 [syncstream.syncbuf.members] p5 says "May call member functions of wrapped while holding a lock uniquely associated with wrapped."
It's unclear what "uniquely associated" means. Is it required to be a one-to-one mapping, so that every streambuf* that is wrapped is associated with a different lock? I believe the intention is only that for a given streambuf* every syncbuf that wraps it uses the same lock. The intention was that it's a valid implementation for the same lock to be used for more than one streambuf* (e.g. using a table of N locks which are indexed by a hash of the streambuf* value). The current wording can be interpreted to forbid that implementation.[2020-11-21; Reflector prioritization]
Set priority to 3 during reflector discussions.
Proposed resolution:
Section: 31.11.2.4 [syncstream.syncbuf.members] Status: New Submitter: Jonathan Wakely Opened: 2020-11-10 Last modified: 2020-11-21
Priority: 3
View other active issues in [syncstream.syncbuf.members].
View all other issues in [syncstream.syncbuf.members].
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Discussion:
31.11.2.4 [syncstream.syncbuf.members] p2 says:
Postconditions: On success, the associated output is empty.
Are there any postconditions on failure? If part of the associated output was written, is that part still in the associated output? Will another call to emit() duplicate that part?
[2020-11-21; Reflector prioritization]
Set priority to 3 during reflector discussions.
Proposed resolution:
Section: 33.2.5.4 [thread.req.lockable.timed], 33.6.4.3 [thread.timedmutex.requirements] Status: New Submitter: Tim Song Opened: 2020-11-14 Last modified: 2020-11-21
Priority: 3
View all issues with New status.
Discussion:
The timed lockable and mutex requirements currently use "rel_time denotes an object of an instantiation of duration, and abs_time denotes an object of an instantiation of time_point" to define the variables used to specify the timed lock functions. During LWG review of P2160R0, it was noted that this definition is deficient in two aspects:
It doesn't allow for cv-qualification of rel_time and abs_time
For time_points, it should require that the clock is a real Cpp17Clock. We impose that requirement via 33.2.1 [thread.req.paramname], but there are no template parameters named Clock here.
[2020-11-21; Reflector prioritization]
Set priority to 3 during reflector discussions.
Proposed resolution:
Section: 31.7.6.5 [ostream.manip] Status: New Submitter: Jonathan Wakely Opened: 2020-11-15 Last modified: 2020-11-21
Priority: 3
View all other issues in [ostream.manip].
View all issues with New status.
Discussion:
From this editorial issue request:
The three basic_syncbuf-related manipulators emit_on_flush, noemit_on_flush, and flush_emit use in their Effects: elements the following wording:
"If os.rdbuf() is a basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, calls […]
There are two problems with that wording (even when considering the helpful note following p8): First, the type Allocator is not defined elsewhere (e.g. it is not part of the function signature) and second, os.rdbuf() has type basic_streambuf<charT, traits>* and not any other type.
By introducing an expository SYNCBUF to detect basic_syncbuf during the work on the above mentioned editorial issue to solve these problems it turned out that the suggested wording fix would introduce an apparently normative change that the syncbuf type must not use a program-defined specialization.[2020-11-21; Reflector prioritization]
Set priority to 3 during reflector discussions.
Proposed resolution:
This wording is relative to N4868.
This proposed wording is known to be incorrect, but is nonetheless depicted to present the overall idea.
Modify 31.7.6.5 [ostream.manip] as indicated:
-1- Each instantiation of any of the function templates specified in this subclause is a designated addressable function (16.4.5.2.1 [namespace.std]).
-?- In this subclause, SYNCBUF(p) for a pointer p of type B* is determined as follows. If *p is a base class subobject of an object of type S, where S is a specialization generated from the basic_syncbuf primary template, and is_convertible_v<S*, B*> is true, then SYNCBUF(p) is dynamic_cast<S*>(p). Otherwise, SYNCBUF(p) is static_cast<void*>(nullptr). [Note ?: To work around the issue that the Allocator template argument of S cannot be deduced, implementations can introduce an intermediate base class to basic_syncbuf that manages its emit_on_sync flag. — end note] […]template<class charT, class traits> basic_ostream<charT, traits>& emit_on_flush(basic_ostream<charT, traits>& os);Let p be SYNCBUF(os.rdbuf()).
-8- Effects: If pos.rdbuf()is not nulla basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, calls pbuf->set_emit_on_sync(true). Otherwise this manipulator has no effect.[Note 1: To work around the issue that the Allocator template argument cannot be deduced, implementations can introduce an intermediate base class to basic_syncbuf that manages its emit_on_sync flag. — end note]-9- Returns: os.template<class charT, class traits> basic_ostream<charT, traits>& noemit_on_flush(basic_ostream<charT, traits>& os);Let p be SYNCBUF(os.rdbuf()).
-10- Effects: If pos.rdbuf()is not nulla basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, calls pbuf->set_emit_on_sync(false). Otherwise this manipulator has no effect. -11- Returns: os.template<class charT, class traits> basic_ostream<charT, traits>& flush_emit(basic_ostream<charT, traits>& os);Let p be SYNCBUF(os.rdbuf()).
-12- Effects: Calls os.flush(). Then, if pos.rdbuf()is not nulla basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, calls pbuf->emit(). -13- Returns: os.
Section: 29.5.8 [time.duration.cast] Status: New Submitter: Jonathan Wakely Opened: 2020-11-18 Last modified: 2020-11-29
Priority: 3
View all other issues in [time.duration.cast].
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Discussion:
29.5.8 [time.duration.cast] p7 requires that the return value is "The least result t representable in ToDuration for which t >= d".
This means that chrono::ceil<chrono::microseconds>(chrono::duration<float, milli>(m)).count() is required to be the smallest integer n such that (float)n == m*1000.0f, which might be less than the mathematically correct value of m x 1000. (The specific values below assume float uses the IEEE binary32 format and default rounding, but similar problems will exist for other formats, even if the specific values are different.) For example, if m == 13421772.0f then the naively expected result is n == 13421772000, but the standard requires n == 13421771265, a significantly lower value. This surprising result is a consequence of how the chrono::ceil spec interacts with floating-point arithmetic, due to the fact that for the integers in the range [13421770753, 13421772799], only one can be exactly represented as 32-bit float. All but that one will be rounded to a different value when converted to float. A straightforward implementation of chrono::ceil will produce (long long)(13421772.0f * 1000) which is 13421771776, which is less than the expected result, but compares equal using the t >= d expression. That expression converts both operands to their common_type, which is chrono::duration<float, micro>. That means we compare (float)13421771776 >= (13421772.0f * 1000) which is true. But the spec requires an even worse result. All integers in [13421771265, 13421771776) are also rounded to that value when converted to float. That means chrono::microseconds(13421771265) is "the least result representable in ToDuration for which t >= d". Meeting the "least result" requirement is impractical, and unhelpful. The straightforward result 13421771776 is already lower than the naively expected result (which is surprising for a "ceil" function). To meet the standard's requirements the implementation would have to do extra work, just to produce an even lower (and even more surprising) result. It might be impractical to require the naively expected value to be returned (the additional work might have unacceptable performance implications), but the standard should at least permit the straightforward result instead of requiring an even worse one. The same problem almost certainly exists for chrono::floor in reverse.[2020-11-29; Reflector prioritization]
Set priority to 3 during reflector discussions.
Proposed resolution:
Section: 33.7.4 [thread.condition.condvar] Status: New Submitter: Jonathan Wakely Opened: 2020-11-18 Last modified: 2020-11-29
Priority: 3
View all other issues in [thread.condition.condvar].
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Discussion:
33.7.4 [thread.condition.condvar] p24 says:
Effects: Equivalent to: return wait_until(lock, chrono::steady_clock::now() + rel_time);
This is overspecification, removing implementer freedom to make cv.wait_for(duration<float>(1)) work accurately.
The type of steady_clock::now() + duration<float>(n) is time_point<steady_clock, duration<float, steady_clock::period>>. If the steady clock's period is std::nano and its epoch is the time the system booted, then in under a second a 32-bit float becomes unable to exactly represent those time_points! Every second after boot makes duration<float, nano> less precise. This means that adding a duration<float> to a time_point (or duration) measured in nanoseconds is unlikely to produce an accurate value. Either it will round down to a value less than now(), or round up to one greater than now() + 1s. Either way, the wait_for(rel_time) doesn't wait for the specified time, and users think the implementation is faulty. A possible solution is to use steady_clock::now() + ceil<steady_clock::duration>(rel_time) instead. This converts the relative time to a suitably large integer, and then the addition is not affected by floating-point rounding errors due to the limited precision of 32-bit float. Libstdc++ has been doing this for nearly three years. Alternatively, the standard could just say that the relative timeout is converted to an absolute timeout measured against the steady clock, and leave the details to the implementation. Some implementations might not be affected by the problem (e.g. if the steady clock measures milliseconds, or processes only run for a few seconds and use the process start as the steady clock's epoch). This also affects the other overload of condition_variable::wait_for, and both overloads of condition_variable_any::wait_for.[2020-11-29; Reflector prioritization]
Set priority to 3 during reflector discussions.
Proposed resolution:
Section: 19.6.3.4 [stacktrace.entry.query] Status: Open Submitter: Thomas Köppe Opened: 2020-12-02 Last modified: 2023-01-11
Priority: 2
View all issues with Open status.
Discussion:
The specification of 19.6.3.4 [stacktrace.entry.query] uses the terms "presumed or actual name of the source file" and "actual line number". It makes reference to 15.11 [cpp.predefined], which introduces the term "presumed". It does not clearly define the term, but it describes how the presumed values can be modified with preprocessor directives. However, there is no definition whatsoever of "actual".
The term should either be defined, or we should strike the "actual" parts of the stacktrace wording. We should consult implementers about this. I don't have a proposed resolution, but if we want to keep "actual", then perhaps we should define both "presumed" and "actual" in 15.11 [cpp.predefined].[2021-01-15; Telecon prioritization]
Set priority to 2 following reflector and telecon discussions.
[2023-01-11; LWG telecon]
We want to know the original intended meaning of "actual line number" here. Presumably debuginfo stored in binaries uses a real line number in the source, not one that might have been set by a #line directive. The "presumed or actual" wording was added in R2 of the paper, possibly as a result of LWG review.
Proposed resolution:
Section: 33.5.7.1 [atomics.ref.generic.general] Status: New Submitter: Casey Carter Opened: 2020-12-02 Last modified: 2020-12-19
Priority: 2
View all issues with New status.
Discussion:
atomic_ref<T> requires only that its template parameter T is trivially copyable, which is not sufficient to implement many of the class template's member functions. Consider, for example:
int main() {
static const int x = 42;
std::atomic_ref<const int> meow{x};
meow.store(0);
return meow.load();
}
Since const int is trivially copyable, this is a well-formed C++20 program that returns 0. That said, the store call that mutates the value of a constant object is (a) not implementable with library technology, and (b) pure madness that violates the language semantics. atomic_ref::store should presumably require is_copy_assignable_v<T>, and other operations need to have their requirements audited as well. (Related: LWG 3012 made similar changes to atomic.)
As a related issue, volatile atomic<T> recently had its member functions deprecated when they are not lock-free. Presumably atomic_ref<volatile T> should require that atomic operations on T be lock-free for consistency. Jonathan: The last point is related to LWG 3417 (another case where we screwed up the volatile deprecations).[2020-12-19; Reflector prioritization]
Set priority to 2 during reflector discussions.
Proposed resolution:
Section: 16.4.5.9 [res.on.arguments] Status: New Submitter: Gonzalo Brito Gadeschi Opened: 2020-12-08 Last modified: 2021-01-15
Priority: 3
View all other issues in [res.on.arguments].
View all issues with New status.
Discussion:
The intent of LWG 1204 is to allow standard library APIs accepting rvalue arguments:
to move from their arguments, e.g., without having to specify that they might do this as part of their Effects clause, and
to assume that rvalue arguments do not alias any pointer in the scope of the standard library API, e.g., to allow vector's push_back(T&& t) to assume that t is not an element of the vector.
The current wording in 16.4.5.9 [res.on.arguments]/1.3 states:
If a function argument binds to an rvalue reference parameter, the implementation may assume that this parameter is a unique reference to this argument.
This sentence is not clear about the scope in which the reference can be assumed to be unique, and it does not explicitly state that the function can modify the argument, e.g., to move from it.
If the scope of the "unique reference" is "whole program scope", this example:
void example(vector<int>& a, int* b)
{
int* c = b; // reference to object pointed at by b
a.push_back(move(*b)); // UB: rvalue reference aliases c: not unique in whole-program scope
assert(c == b); // FAILS: if rvalue reference to *b is unique, b is unique, and c == b is false
}
exhibits UB because the implementation may assume that the reference to b is unique, which does not hold since c is also a reference to b.
If the scope of the "unique reference" is the "function scope" of the standard library API, then the semantics of the rvalue reference argument are very similar to those of C's restrict. This allows aliasing optimizations, for example:void std_api(int&& a, int&& b); // allowed to assume that a and b do not alias int a, b, c; std_api(move(a), move(b)); // OK: two unique references in std_api std_api(move(c), move(c)); // UB: a and b alias
See llvm Bug 48238 for a bug tracking the implementation of these optimizations in clang.
This also allows optimizing vector::push_back(T&& t) since if t does not alias any pointer in vector::push_back's scope, it also does not alias this, this->data(), (*this)[0], etc.[2021-01-15; Telecon prioritization]
Set priority to 3 following reflector and telecon discussions.
Proposed resolution:
This wording is relative to N4868.
Modify 16.4.5.9 [res.on.arguments] as indicated:
-1- Each of the following applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.
(1.1) — If an argument to a function has an invalid value (such as a value outside the domain of the function or a pointer invalid for its intended use), the behavior is undefined.
(1.2) — If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid.
(1.3) — If a function argument binds to an rvalue reference parameter, the implementation may assume that this parameter is a unique reference to
[Example ?:this argumentthe value within the function's scope and may move from it.void std_api(int&& a); int a; std_api(move(a)); // a is in an unspecified but valid state— end example]
[Example ?:void std_api(int&& a, int&& b); int a, b, c; std_api(move(a), move(b)); // OK: int&& a and int&& b do not alias std_api(move(c), move(c)); // UB: int&& a and int&& b alias— end example]
[Example ?:std::vector<int> a = {...}; a.push_back(move(42)); // OK: unique reference a.push_back(move(a[0])); // UB: (*this)[0] and rvalue argument alias— end example]
[Note 1: If the parameter is a generic parameter of the form T&& and an lvalue of type A is bound, the argument binds to an lvalue reference (13.10.3.2 [temp.deduct.call]) and thus is not covered bythe previous sentencethis item. — end note][Note 2: If a program casts an lvalue to an xvalue while passing that lvalue to a library function (e.g., by calling the function with the argument std::move(x)), the program is effectively asking that function to treat that lvalue as a temporary object. The implementation is free to optimize away aliasing checks which might be needed if the argument was an lvalue. — end note]
Section: 22.10.19 [unord.hash] Status: New Submitter: Jonathan Wakely Opened: 2020-12-28 Last modified: 2021-01-29
Priority: 3
View all other issues in [unord.hash].
View all issues with New status.
Discussion:
See this editorial issue.
22.10.19 [unord.hash] p5.4 ends with "shall not throw an exception unless hash<Key> is a program-defined specialization that depends on at least one program-defined type." This seems wrong, because hash<optional<T>> is not a program-defined specialization, but it might throw if hash<T> can throw. There are also other partial specializations of std::hash defined in the standard library but that depend on program-defined specializations and so can throw.[2021-01-29; reflector prioritization]
Set priority to 3 following reflector discussions. It was pointed out that this wording could be simplified if 3513 changes the definition of program-defined type.Proposed resolution:
Section: 3.43 [defns.prog.def.type] Status: New Submitter: Johel Ernesto Guerrero Peña Opened: 2020-12-29 Last modified: 2021-01-30
Priority: 3
View all issues with New status.
Discussion:
Consider the following definitions:
3.42 [defns.prog.def.spec]
program-defined specialization
<library> explicit template specialization or partial specialization that is not part of the C++ standard library and not defined by the implementation
and
3.43 [defns.prog.def.type]
program-defined type
<library> non-closure class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or a closure type of a non-implementation-provided lambda expression, or an instantiation of a program-defined specialization [Note 1 to entry: Types defined by the implementation include extensions (4.1) and internal types used by the library. — end note]
A program-defined type is either a type or an instantiation. "program-defined type" is used in 16.4.5.2.1 [namespace.std] p2 to give permission to specialize standard class templates:
Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace std provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.
ISO requires that the terms in Clause 3 be substitutable with their definitions. If this were done for "program-defined type", we'd end up with "or an instantiation of a program-defined specialization". It's fine to depend on a type, but not an instantiated type, because all you need is the name of its specialization (its type) as a template argument to explicitly or partially specialize a template.
[2021-01-29; reflector prioritization]
Set priority to 3 following reflector discussions. It was pointed out that it might be easier to resolve 3512 if this issue changes the definition of program-defined type.Proposed resolution:
This wording is relative to N4878.
Modify 3.43 [defns.prog.def.type] as indicated:
3.43 [defns.prog.def.type]
program-defined type
<library> non-closure class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or a closure type of a non-implementation-provided lambda expression, oran instantiationa name of a program-defined specialization or instantiation thereof [Note 1 to entry: Types defined by the implementation include extensions (4.1 [intro.compliance]) and internal types used by the library. — end note]
Section: 33.4.3.2 [thread.thread.id] Status: New Submitter: Casey Carter Opened: 2021-01-26 Last modified: 2021-03-12
Priority: 3
View all other issues in [thread.thread.id].
View all issues with New status.
Discussion:
33.4.3.2 [thread.thread.id]/5-7 specify the behavior of == and <=> for std::thread::id:
bool operator==(thread::id x, thread::id y) noexcept;-5- Returns: true only if x and y represent the same thread of execution or neither x nor y represents a thread of execution.
strong_ordering operator<=>(thread::id x, thread::id y) noexcept;-6- Let P(x, y) be an unspecified total ordering over thread::id as described in 25.8.
-7- Returns: strong_ordering::less if P(x, y) is true. Otherwise, strong_ordering::greater if P(y, x) is true. Otherwise, strong_ordering::equal.
"Unspecified total ordering" provides too much freedom, since it does not require that !P(x, y) holds when x and y both represent the same thread of execution or both represent no thread of execution. A conforming implementation could return strong_ordering::equal from <=> for a pair of thread::id values for which == returns false. We should guarantee consistency of == and <=> for thread::id to preserve sanity of the programming model.
[2021-03-12; Reflector poll]
Set priority to 3 following reflector poll.
Proposed resolution:
This wording is relative to N4878.
Modify 33.4.3.2 [thread.thread.id] as indicated:
strong_ordering operator<=>(thread::id x, thread::id y) noexcept;-6- Let P(x, y) be
-7- Returns: strong_ordering::less if P(x, y) is true. Otherwise, strong_ordering::greater if P(y, x) is true. Otherwise, strong_ordering::equal.an unspecifieda total ordering over thread::id as described in 27.8 [alg.sorting], unspecified except that P(x, y) is false when x and y both represent the same thread of execution, or when neither represents a thread of execution.
Section: 24.4.4.1 [map.overview], 24.4.5.1 [multimap.overview], 24.5.4.1 [unord.map.overview], 24.5.5.1 [unord.multimap.overview] Status: New Submitter: Mike Spertus Opened: 2021-03-09 Last modified: 2021-04-20
Priority: 3
View all other issues in [map.overview].
View all issues with New status.
Discussion:
The resolution for LWG 3025 enabled code like the following to be accepted:
map m1{{pair{1, 2}, {3, 4}}, less<int>()};
but breaks code that had been previously working like the following
using value_type = pair<const int, int>;
map m2{{value_type{1, 2}, {3, 4}}, less<int>()};
as shown on godbolt.
[Acknowledgment to Tim Song and Arthur O'Dwyer for independently pointing out this case on the LWG mailing list][2021-04-20; Reflector poll]
Priority set to 3. Three preferences expressed for Option B, none for A.
Proposed resolution:
This wording is relative to N4878.
We present two partial wording options for std::map, denoted by (A) and (B) below. If the committee accepts one of them, we will complete them to all key-value containers.
(A) Wording option 1: In this option, we restore the deduction guide that was removed in LWG 3025 while maintaining the one that was added, demonstrates this working.
Modify 24.4.4.1 [map.overview] as indicated:
[…]
template<class Key, class T, class Compare = less<Key>,
class Allocator = allocator<pair<const Key, T>>>
map(initializer_list<pair<Key, T>>, Compare = Compare(), Allocator = Allocator())
-> map<Key, T, Compare, Allocator>;
template<class Key, class T, class Compare = less<Key>,
class Allocator = allocator<pair<const Key, T>>>
map(initializer_list<pair<const Key, T>>, Compare = Compare(), Allocator = Allocator())
-> map<Key, T, Compare, Allocator>;
template<class InputIterator, class Allocator>
map(InputIterator, InputIterator, Allocator)
-> map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
less<iter-key-type<InputIterator>>, Allocator>;
template<class Key, class T, class Allocator>
map(initializer_list<pair<Key, T>>, Allocator) -> map<Key, T, less<Key>, Allocator>;
template<class Key, class T, class Allocator>
map(initializer_list<pair<const Key, T>>, Allocator) -> map<Key, T, less<Key>, Allocator>;
[…]
(B) Wording option 2: This one follows Tim Song's suggestion: "It seems that the cleanest fix is to 1) disallow the initializer_list<value_type> constructors from being used for CTAD, and 2) change the guides to use remove_const_t<Key>." This change has been tested locally with g++ similar to the above godbolt.
Modify 24.4.4.1 [map.overview] as indicated:
[…]
// types
using key_type = Key;
using mapped_type = T;
using value_type = type_identity_t<pair<const Key, T>>;
[…]
template<class Key, class T, class Compare = less<remove_const_t<Key>>,
class Allocator = allocator<pair<const Key, T>>>
map(initializer_list<pair<Key, T>>, Compare = Compare(), Allocator = Allocator())
-> map<remove_const_t<Key>, T, Compare, Allocator>;
template<class InputIterator, class Allocator>
map(InputIterator, InputIterator, Allocator)
-> map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
less<iter-key-type<InputIterator>>, Allocator>;
template<class Key, class T, class Allocator>
map(initializer_list<pair<Key, T>>, Allocator)
-> map<remove_const_t<Key>, T, less<remove_const_t<Key>>, Allocator>;
[…]
Section: 27.8.7.4 [set.intersection], 27.8.7.5 [set.difference] Status: LEWG Submitter: Alexander Bessonov Opened: 2021-03-16 Last modified: 2021-04-20
Priority: 3
View all issues with LEWG status.
Discussion:
The std::mergeable concept requires elements of both source ranges to be copyable to the output iterator, while the standard specifically tells that both algorithms ranges::set_intersection and ranges::set_difference only use the first range as the source of elements to be copied into output. The following code snippet illustrates the problem:
#include <vector>
#include <ranges>
#include <algorithm>
#include <cassert>
int main()
{
std::vector<std::pair<int, int>> v1;
std::vector<int> v2;
assert(std::ranges::is_sorted(v1));
assert(std::ranges::is_sorted(v2));
std::vector<std::pair<int, int>> v3;
// Compilation error on the following line:
std::ranges::set_intersection(v1, v2, std::back_inserter(v3),
std::less{}, [](const auto& p) { return p.first; });
}
The proposed solution is to introduce a new concept. It could be declared "exposition-only" and is worded half-mergeable below:
template<class I1, class I2, class Out, class R = ranges::less,
class P1 = identity, class P2 = identity>
concept half-mergeable =
input_iterator<I1> &&
input_iterator<I2> &&
weakly_incrementable<Out> &&
indirectly_copyable<I1, Out> &&
// indirectly_copyable<I2, Out> && <— this line removed
indirect_strict_weak_order<R, projected<I1, P1>, projected<I2, P2>>;
After such template is introduced, std::mergeable may be defined based on it:
template<class I1, class I2, class Out, class R = ranges::less,
class P1 = identity, class P2 = identity>
concept mergeable = half-mergeable<I1, I2, Out, R, P1, P2> &&
indirectly_copyable<I2, Out>;
See also the related discussion on reddit.
[2021-04-20; Reflector poll]
Priority set to 3. Send to LEWG.
Proposed resolution:
This wording is relative to N4878.
Modify 25.2 [iterator.synopsis], header <iterator> synopsis, as indicated:
[…] // 25.3.7.7 [alg.req.mergeable], concept mergeable template<class I1, class I2, class Out, class R = ranges::less, class P1 = identity, class P2 = identity> concept half-mergeable = see below; // exposition only template<class I1, class I2, class Out, class R = ranges::less, class P1 = identity, class P2 = identity> concept mergeable = see below; […]
Modify 25.3.7.7 [alg.req.mergeable] as indicated:
23.3.7.7 Concept mergeable [alg.req.mergeable]
-1- The mergeable concept specifies the requirements of algorithms that merge sorted sequences into an output sequence by copying elements.template<class I1, class I2, class Out, class R = ranges::less, class P1 = identity, class P2 = identity> concept half-mergeable = // exposition only input_iterator<I1> && input_iterator<I2> && weakly_incrementable<Out> && indirectly_copyable<I1, Out> && indirect_strict_weak_order<R, projected<I1, P1>, projected<I2, P2>>; template<class I1, class I2, class Out, class R = ranges::less, class P1 = identity, class P2 = identity> concept mergeable = half-mergeable<I1, I2, Out, R, P1, P2> &&input_iterator<I1> && input_iterator<I2> && weakly_incrementable<Out> && indirectly_copyable<I1, Out> &&indirectly_copyable<I2, Out>&& indirect_strict_weak_order<R, projected<I1, P1>, projected<I2, P2>>;
Modify 27.8.7.4 [set.intersection] as indicated:
[…] template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2, weakly_incrementable O, class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity> requires half-mergeablemergeable<I1, I2, O, Comp, Proj1, Proj2> constexpr ranges::set_intersection_result<I1, I2, O> ranges::set_intersection(I1 first1, S1 last1, I2 first2, S2 last2, O result, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<input_range R1, input_range R2, weakly_incrementable O, class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity> requires half-mergeablemergeable<iterator_t<R1>>, iterator_t<R2>, O, Comp, Proj1, Proj2> constexpr ranges::set_intersection_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, O> ranges::set_intersection(R1&& r1, R2&& r2, O result, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});[…]
-6- Remarks: Stable (16.4.6.8 [algorithm.stable]). If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the first min(m, n) elements are copied from the first range to the output range, in order.
Modify 27.8.7.5 [set.difference] as indicated:
[…] template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2, weakly_incrementable O, class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity> requires half-mergeablemergeable<I1, I2, O, Comp, Proj1, Proj2> constexpr ranges::set_difference_result<I1, O> ranges::set_difference(I1 first1, S1 last1, I2 first2, S2 last2, O result, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<input_range R1, input_range R2, weakly_incrementable O, class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity> requires half-mergeablemergeable<iterator_t<R1>>, iterator_t<R2>, O, Comp, Proj1, Proj2> constexpr ranges::set_difference_result<borrowed_iterator_t<R1>, O> ranges::set_difference(R1&& r1, R2&& r2, O result, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});[…]
-6- Remarks: If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the last max(m - n, 0) elements from [first1, last1) is copied to the output range, in order.
Section: 25.7 [iterator.range] Status: New Submitter: Jiang An Opened: 2021-03-21 Last modified: 2021-04-20
Priority: 3
View other active issues in [iterator.range].
View all other issues in [iterator.range].
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Discussion:
Overloads for std::rbegin/rend for built-in arrays and std::initializer_list's has no preconditions and never throw exceptions, thus should be noexcept. LWG 2280 addressed a similar issue for std::begin/end.
Suggestion: change these declarations in 25.7 [iterator.range] (p10, 11, 12, 13) as indicated:template<class T, size_t N> constexpr reverse_iterator<T*> rbegin(T (&array)[N]) noexcept; template<class T, size_t N> constexpr reverse_iterator<T*> rend(T (&array)[N]) noexcept; template<class E> constexpr reverse_iterator<const E*> rbegin(initializer_list<E> il) noexcept; template<class E> constexpr reverse_iterator<const E*> rend(initializer_list<E> il) noexcept;
If this change is accepted, we may also specify conditional noexcept for std::crbegin/crend (in 25.7 [iterator.range] p14, 15), by adding noexcept(noexcept(std::rbegin/crend(c))), like in LWG 2280.
Conditional noexcept for other functions in 25.7 [iterator.range] (p2, 3, 7, 8, 16, 18, 20 22) may also be added for consistency.[2021-03-21; Daniel comments]
There is intentionally no P/R provided at this point, but I'm volunteering to provide it if we got feedback whether adding conditional noexcept specifiers similar to those provided by LWG 2280 would be preferred or not.
[2021-04-20; Reflector poll]
Priority set to 3.
Jonathan: This would create a strange situation where std::rbegin and std::crbegin on an initializer_list are noexcept but std::begin and std::cbegin aren't guaranteed to be (because an initializer_list uses the generic std::begin and std::cbegin overloads, which have no conditional noexcept).
Casey: I don't think we should mark these rbegin/rend overloads noexcept without making the pertinent reverse_iterator constructors conditionally noexcept.
Proposed resolution:
Section: 16.2 [library.c] Status: New Submitter: Jiang An Opened: 2021-03-29 Last modified: 2021-04-20
Priority: 2
View all issues with New status.
Discussion:
P0551R3 has made almost all standard library functions non-addressable, including all functions from the C standard library. However, C17 (and the latest C23 working draft) explicitly allows taking address of a C standard library function in 7.1.4/1.
Should we require something like "every function from the C standard library is addressable unless it is overloaded", in order to minimize the incompatibility with C? Or explicitly say something in C.7 [diff.library] if such requirement is not wanted?[2021-04-20; Reflector poll]
Priority set to 2.
Proposed resolution:
Section: 16.4.5.3 [reserved.names] Status: New Submitter: Hubert Tong Opened: 2021-05-10 Last modified: 2021-05-20
Priority: 3
View all other issues in [reserved.names].
View all issues with New status.
Discussion:
The C standard has a section called "Future library directions". This is subclause 7.31 in C17, which is the version that the C++ working draft references. C reserves the names described in that subclause for its standard library, allowing C library vendors to introduce some names from future standards as conforming, "orthogonal" extensions. ISO C++ does not appear to reserve these names in a clear way. In particular, 16.4.5.3 [reserved.names] has wording about names and function signatures "from the C standard library declared with external linkage", but C's "Future library directions" describes names that "may be added to the declarations in [some] header[s]".
Extra clarity regarding whether these names are actually intended to be reserved by C++ would be appreciated.[2021-05-20; Reflector poll]
Priority set to 3.
Proposed resolution:
Section: 16.3.2.3 [structure.requirements] Status: New Submitter: Tim Song Opened: 2021-05-23 Last modified: 2021-05-26
Priority: 3
View all other issues in [structure.requirements].
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Discussion:
16.3.2.3 [structure.requirements] p9 says:
A declaration may explicitly impose requirements through its associated constraints (13.5.3 [temp.constr.decl]). When the associated constraints refer to a concept (13.7.9 [temp.concept]), the semantic constraints specified for that concept are additionally imposed on the use of the declaration.
There are at least two issues with this wording:
First, "associated constraints" is a Core term that refers to the constraints on a declaration after normalization, at which point direct uses of concepts have been decomposed into the constituent atomic constraints and are no longer visible.
Second, "refers to" is too vague. Does !C<T> "refer to" C and impose its semantic constraints? Does C1<T> || C2<T> "refer to" both C1 and C2 and therefore impose the semantic constraints of both?
[2021-05-26; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 24.2.7.1 [associative.reqmts.general], 24.2.8.1 [unord.req.general] Status: New Submitter: Joaquín M López Muñoz Opened: 2021-08-04 Last modified: 2021-08-20
Priority: 3
View other active issues in [associative.reqmts.general].
View all other issues in [associative.reqmts.general].
View all issues with New status.
Discussion:
For the expression a.merge(a2), it is not explicitly stated whether a2 can be the same object as a. libstdc++-v3 and libc++ seemingly assume this is not allowed, as the following code produces an infinite loop with both standard library implementations:
#include <set>
int main()
{
std::multiset<int> c={0, 0};
c.merge(c);
}
A strict reading of postconditions seems to ban the case where a and a2 are the same:
24.2.7.1 [associative.reqmts.general]: "Iterators referring to the transferred elements […] now behave as iterators into a, not into a2": if a and a2 are the same, a transferred iterator can't be both an iterator to a and not an iterator to a2.
24.2.8.1 [unord.req.general]: "Iterators referring to the transferred elements and all iterators referring to a will be invalidated, but iterators to elements remaining in a2 will remain valid": if a and a2 are the same, an iterator can't both be invalidated and remain valid.
Even if a provision is made that, when a and a2 are the same, no elements are transferred by convention, 24.2.8.1 [unord.req.general] would still implicitly ban the case, as all iterators would be invalidated but the iterators to the remaining elements (again, all iterators) would remain valid, which is contradictory.
For context, analogous operations for std::list take inconsistent approaches:splice(const_iterator position, list& x) requires that source and destination be not the same.
splice(const_iterator position, list& x, const_iterator i) implicitly allows addressof(x) == this, as the case position == i is taken care of.
std::list::merge explicitly allows the case addressof(x) == this (resulting in a no-op).
[2021-08-20; Reflector poll]
Set priority to 3 after reflector poll.
Tim Song commented:
"I think the current PR of LWG2414 bans this code,
but we might want to have consistency with list::merge instead."
Proposed resolution:
Section: 24.2.7.1 [associative.reqmts.general] Status: New Submitter: Joaquín M López Muñoz Opened: 2021-08-04 Last modified: 2021-08-20
Priority: 3
View other active issues in [associative.reqmts.general].
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Discussion:
For the expression a.merge(a2), postconditions say that "iterators referring to the transferred elements […] now behave as iterators into a […]". When a and a2 are of different types, this seems to imply, under the widest interpretation for "behaving as", that a-iterators and a2-iterators are actually of the same type, that is, that associative containers have SCARY iterators, which is, to the best of my knowledge, not currently mandated by the standard.
There are (at least) three possible resolutions to this ambiguity, ordered by intrusiveness:Indicate that "behaving as" only applies to the case where a and a2 are of the same type.
Clarify what "behaving as" means. A non-SCARY interpretation is that an a2-iterator to a transferred element can still be dereferenced, incremented (if not past the last element of a) and decremented (if not pointing to the first element of a), while comparison with a-iterators and use in the interface of a is not guaranteed.
Mandate SCARY iterators by, for instance, requiring that associative containers with compatible nodes (24.2.5.1 [container.node.overview]/1) have the same iterator types.
Note that this issue does not extend to unordered associative containers, as there (24.2.8.1 [unord.req.general]) iterators to transferred elements are invalidated, which makes the point of SCARYness irrelevant. That said, if SCARY iterators are finally required for associative containers, it makes much sense to extend the requirement to unordered associative containers as well.
[2021-08-20; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 33.10.9 [futures.async] Status: New Submitter: Jonathan Wakely Opened: 2021-08-23 Last modified: 2021-09-30
Priority: 3
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Discussion:
33.10.9 [futures.async] (3.1) says:
Any exception propagated from the execution of invoke(decay-copy(std::forward<F>(f)), decay-copy(std::forward<Args>(args))...) is stored as the exceptional result in the shared state.
It's not clear whether this includes the evaluation of the decay-copy calls in the calling thread, or only the invocation of invoke with the results of those decay-copy calls.
A literal reading suggests that any exceptions from any part of that expression should be stored in the shared state. All of libstdc++, libc++ and MSVC only store exceptions from the call to invoke, not the calls to decay-copy. Exceptions from the decay-copy calls are propagated to the caller of std::async. We should clarify that that's what the standard means.[2021-09-20; Reflector poll]
Set priority to 3 after reflector poll.
[2021-09-20; Jonathan updates wording to change the Throws: and attempt to align the Effects: with the deferred function case. ]
Previous resolution [SUPERSEDED]:
This wording is relative to N4892.
Modify 33.10.9 [futures.async] as indicated:
template<class F, class... Args> [[nodiscard]] future<invoke_result_t<decay_t<F>, decay_t<Args>...>> async(F&& f, Args&&... args); template<class F, class... Args> [[nodiscard]] future<invoke_result_t<decay_t<F>, decay_t<Args>...>> async(launch policy, F&& f, Args&&... args);-2- Mandates: […]
-3- Effects: The first function behaves the same as a call to the second function with a policy argument of launch::async | launch::deferred and the same arguments for F and Args. The second function creates a shared state that is associated with the returned future object. The further behavior of the second function depends on the policy argument as follows (if more than one of these conditions applies, the implementation may choose any of the corresponding policies):
(3.1) — If launch::async is set in policy, calls invoke(decay-copy(std::forward<F>(f)), decay-copy(std::forward<Args>(args))...) (22.10.4 [func.require], 33.4.3.3 [thread.thread.constr]) as if in a new thread of execution represented by a thread object with the calls to decay-copy being evaluated in the thread that called async. Any return value is stored as the result in the shared state. Any exception propagated from the
execution of invoke(decay-copy(std::forward<F>(f)), decay-copy(std::forward<Args>(args))...)call to invoke is stored as the exceptional result in the shared state. [Note ?: Exceptions from the decay-copy calls are propagated to the caller. — end note] The thread object is stored in the shared state and affects the behavior of any asynchronous return objects that reference that state.[…]
Proposed resolution:
This wording is relative to N4892.
Modify 33.10.9 [futures.async] as indicated:
template<class F, class... Args> [[nodiscard]] future<invoke_result_t<decay_t<F>, decay_t<Args>...>> async(F&& f, Args&&... args); template<class F, class... Args> [[nodiscard]] future<invoke_result_t<decay_t<F>, decay_t<Args>...>> async(launch policy, F&& f, Args&&... args);-2- Mandates: […]
-3- Effects: The first function behaves the same as a call to the second function with a policy argument of launch::async | launch::deferred and the same arguments for F and Args. The second function creates a shared state that is associated with the returned future object. The further behavior of the second function depends on the policy argument as follows (if more than one of these conditions applies, the implementation may choose any of the corresponding policies):
(3.1) — If launch::async is set in policy, calls invoke(decay-copy(std::forward<F>(f)), decay-copy(std::forward<Args>(args))...) (22.10.4 [func.require], 33.4.3.3 [thread.thread.constr]) as if in a new thread of execution represented by a thread object with the calls to decay-copy being evaluated in the thread that called async. Any return value is stored as the result in the shared state. Any exception propagated from the execution of invoke(
decay-copy(std::forward<F>(f)), decay-copy(std::forward<Args>(args)...)std::move(g), std::move(xyz)) is stored as the exceptional result in the shared state, where g is the result of decay-copy(std::forward<F>(f)) and xyz is the result of decay-copy(std::forward<Args>(args))... . [Note ?: Exceptions from the decay-copy calls are propagated to the caller. — end note] The thread object is stored in the shared state and affects the behavior of any asynchronous return objects that reference that state.[…]
[…]
-6- Throws: system_error if policy == launch::async and the implementation is unable to start a new thread;
std::bad_alloc if memory for the internal data structures cannot be allocated; or any exception thrown by the initialization of the objects returned by the decay-copy calls.
Section: 22.4.4.1 [tuple.cnstr] Status: New Submitter: Jonathan Wakely Opened: 2021-08-23 Last modified: 2021-09-20
Priority: 3
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Discussion:
We do not specify whether or not short-circuiting is expected to happen for individual conditions stated in a Constraints: element. For example, 22.4.4.1 [tuple.cnstr] p12 says:
Constraints: sizeof...(Types) equals sizeof...(UTypes) and sizeof...(Types) ≤ 1 and is_constructible_v<Ti , Ui> is true for all i.
It's not even possible to test the is_constructible part unless the first part is true, so presumably it is expected that the sizeof... expressions are tested first, and so the is_constructible traits don't even need to be instantiated.
In some cases it might be user observable whether short circuiting happens, as whether later conditions are tested might affect whether errors outside the immediate context are permitted or not. We should clarify what is intended, and refactor any Constraints: elements that are special cases where something different is required.[2021-09-20; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 17.11.3 [cmp.common] Status: New Submitter: Peter Brett Opened: 2021-08-23 Last modified: 2021-09-20
Priority: 3
View all issues with New status.
Discussion:
17.11.3 [cmp.common]/1 says:
The type common_comparison_category provides an alias for the strongest comparison category to which all of the template arguments can be converted.
A naive reader like me might interpret this as meaning that (1) you attempt to convert the template arguments to comparison categories and then (2) obtain the strongest among them.
However, the intent is in fact to realize the common comparison type notion from 11.10.3 [class.spaceship]/4. To obtain a non-void result, all the template arguments must be comparison categories, rather than convertible to comparison categories. 17.11.3 [cmp.common]/2 mildly contradicts the first paragraph:Remarks: The member typedef-name type denotes the common comparison type (11.10.3 [class.spaceship]) of Ts..., the expanded parameter pack, or void if any element of Ts is not a comparison category type.
It more precisely states the behaviour, cross-references 11.10.3 [class.spaceship], and uses the correct core terminology for the metafunction that the template represents.
Suggested resolution; Delete 17.11.3 [cmp.common]/1, because it does not provide any information not already more precisely included in 17.11.3 [cmp.common]/2.[2021-09-20; Reflector poll]
Set priority to 3 after reflector poll.
[2021-09-20; Reflector poll]
Jens suggests alternative wording.
Previous resolution [SUPERSEDED]:
This wording is relative to N4892.
Modify 17.11.3 [cmp.common] as indicated:
-1- The type common_comparison_category provides an alias for the strongest comparison category to which all of the template arguments can be converted. [Note 1: A comparison category type is stronger than another if they are distinct types and an instance of the former can be converted to an instance of the latter. — end note]template<class... Ts> struct common_comparison_category { using type = see below; };-2- Remarks: The member typedef-name type denotes the common comparison type (11.10.3 [class.spaceship]) of Ts..., the expanded parameter pack, or void if any element of Ts is not a comparison category type.
Proposed resolution:
This wording is relative to N4892.
Modify 17.11.3 [cmp.common] as indicated:
-1- The type common_comparison_category provides an alias for the strongest comparison category
to which all of the template arguments can be convertedamong all the template arguments. [Note 1: A comparison category type is stronger than another if they are distinct types and an instance of the former can be converted to an instance of the latter. — end note]template<class... Ts> struct common_comparison_category { using type = see below; };-2- Remarks: The member typedef-name type denotes the common comparison type (11.10.3 [class.spaceship]) of Ts..., the expanded parameter pack, or void if any element of Ts is not a comparison category type.
Section: 22.14.2.2 [format.string.std] Status: New Submitter: Mark de Wever Opened: 2021-09-05 Last modified: 2021-12-04
Priority: 2
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Discussion:
The alignment options specified in 22.14.2.2 [format.string.std], Table [tab:format.align] causes an inconsistency when formatting characters. The output differs depending on whether an integer is formatted using a character presentation type or when using a character directly:
format("{:3}", '*'); -> "* " // aligned at the start of the available space
format("{:3c}", 42); -> " *" // aligned at the end of the available space
I expect both calls to return the same value: "* ". The current wording mixes the type and the presentation type. To me, it seems clearer to adjust to wording to only use the presentation type. Another approach would be adjusting the wording to add an exception when an integer type uses the character presentation.
[2021-09-20; Reflector poll]
Set priority to 2 after reflector poll.
Victor said "It mostly looks correct but I think the wording needs a bit more work because we don't mention arithmetic presentation types anywhere."
[2021-11-29; Daniel comments]
This issue touches the same wording area as LWG 3644 does.
Proposed resolution:
This wording is relative to N4892.
Modify 22.14.2.2 [format.string.std], Table [tab:format.align], as indicated:
Table 59 — Meaning of align options [tab:format.align] Option Meaning < Forces the field to be aligned to the start of the available space. This is the default when the presentation type is a non-arithmetic type for non-arithmetic types, charT, and bool, unless an integer presentation type is specified.> Forces the field to be aligned to the end of the available space. This is the default when the presentation type is an arithmetic type for arithmetic types other than charT and bool or when an integer presentation type is specified.[…]
Section: 17.11.4 [cmp.concept] Status: New Submitter: Jiang An Opened: 2021-09-06 Last modified: 2021-10-23
Priority: 3
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Discussion:
Due to the current specification of std::common_comparison_category, compare-as<void, void> is satisfied. And thus given two types T and U with regular ==, !=, <, >, <=, and >= but irregular <=> that has return type void, std::three_way_comparable_with<T, U, void> is satisfied.
However, in this situation std::three_way_comparable_with<T, U, void> can't be modeled, because Cat(t <=> u) == Cat(C(t) <=> C(u)) is ill-formed if Cat is void (see this godbolt link). I don't think this issue has any impact on meaningful codes, but it seems to be a hole in the current specification. It seems that compares-as<void, void> should be changed to false. compares-as<NonComparisonCategoryType, void> might need refinement. (Note: The same problem does not exists for std::three_way_comparable, because std::three_way_comparable<T, void> can be modeled if the <=> on T has "regular" definition or its return type is not a comparison category type, but the return value has "regular" semantics)[2021-09-20; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 26.7.16.2 [range.lazy.split.view] Status: New Submitter: Hewill Kang Opened: 2021-09-23 Last modified: 2021-10-14
Priority: 3
View other active issues in [range.lazy.split.view].
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Discussion:
Consider the following code snippet:
#include <ranges>
int main() {
auto p = std::views::iota(0)
| std::views::take(1)
| std::views::reverse;
auto r = std::views::single(42)
| std::views::lazy_split(p);
auto f = r.front();
}
r.front() is ill-formed even if r is a forward_range.
This is because the const overload of lazy_split_view::begin is not constrained by the const Pattern, which makes it still well-formed in such cases. When the const overload of view_interface<lazy_split_view<V, Pattern>>::front is instantiated, the subrange{parent_->pattern_} inside lazy_split_view::outer-iterator<true>::operator++() will cause a hard error since const Pattern is not a range.
[2021-09-24; Daniel comments]
This issue is related to LWG 3592.
[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 26.7.16.2 [range.lazy.split.view], class template lazy_split_view synopsis, as indicated:
namespace std::ranges { […] template<input_range V, forward_range Pattern> requires view<V> && view<Pattern> && indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> && (forward_range<V> || tiny-range<Pattern>) class lazy_split_view : public view_interface<lazy_split_view<V, Pattern>> { private: […] public: […] constexpr auto begin() { […] } constexpr auto begin() const requires forward_range<V> && forward_range<const V> && forward_range<const Pattern>{ return outer-iterator<true>{*this, ranges::begin(base_)}; } constexpr auto end() requires forward_range<V> && common_range<V> { […] } constexpr auto end() const { if constexpr (forward_range<V> && forward_range<const V> && common_range<const V> && forward_range<const Pattern>) return outer-iterator<true>{*this, ranges::end(base_)}; else return default_sentinel; } }; […] }
Section: 25.5.1.4 [reverse.iter.cons], 25.5.4.4 [move.iter.cons] Status: New Submitter: Hewill Kang Opened: 2021-09-26 Last modified: 2021-10-14
Priority: 3
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Discussion:
In order to remove the incorrect bi-convertibility of reverse_iterator<int*> and reverse_iterator<const int*>, LWG 3435 adds two constraints to reverse_iterator's converting assignment, namely convertible_to<const U&, Iterator> and assignable_from<Iterator&, const U&>, but since this function only assigns u.current to current, there is no need to require convertible_to<const U&, Iterator> — the latter is sufficient.
We should remove this constraint and be consistent with the move_sentinel and counted_iterator' converting assignment.[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
[Tim Song commented:]
This was
intentional,
but I think we missed the fact that counted_iterator did something
else already. These should probably be made consistent one way or another.
[Tomasz Kamiński commented]
The move_iterator/reverse_iterator were present before C++20,
and this change restores their compatibility with C++17 code,
where only assignment was required.
They are materially different from adapters introduced with C++20,
and I believe we should put more weight into backward compatibility
than consistency with newer iterator wrappers.
Proposed resolution:
This wording is relative to N4892.
Modify 25.5.1.4 [reverse.iter.cons] as indicated:
template<class U> constexpr reverse_iterator& operator=(const reverse_iterator<U>& u);-5- Constraints: is_same_v<U, Iterator> is false
-6- Effects: Assigns u.current to current. -7- Returns: *this., const U& models convertible_to<Iterator>,and assignable_from<Iterator&, const U&> is modeled.
Modify 25.5.4.4 [move.iter.cons] as indicated:
[Drafting note: As drive-by fix a missing "Returns: *this" has been added as well.]
template<class U> constexpr move_iterator& operator=(const move_iterator<U>& u);-5- Constraints: is_same_v<U, Iterator> is false
-6- Effects: Assigns u.current to current. -?- Returns: *this., const U& models convertible_to<Iterator>,and assignable_from<Iterator&, const U&> is modeled.
Section: 32.7.2 [re.regex.construct], 32.10 [re.alg] Status: New Submitter: Jonathan Wakely Opened: 2021-09-27 Last modified: 2021-10-14
Priority: 3
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Discussion:
ECMAScript says that \0 is an ordinary character and can be matched. POSIX says the opposite:
"The interfaces specified in POSIX.1-2017 do not permit the inclusion of a NUL character in an RE or in the string to be matched. If during the operation of a standard utility a NUL is included in the text designated to be matched, that NUL may designate the end of the text string for the purposes of matching." So does that mean std::regex{"", 1, regex::basic} should throw an exception? And std::regex_match(string{"a\0b", 3}, regex{"a.b", regex::basic}) should fail? The POSIX rule is because those interfaces are specified with NTBS arguments, so there's no way to distinguish "a\0b" and "a". The C++ interfaces could allow it, but we never specify any divergence from POSIX, so presumably the rule still applies. Is that what was intended and is it what we want?[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 32.7.2 [re.regex.construct], 32.4.2 [re.synopt] Status: New Submitter: Jonathan Wakely Opened: 2021-09-27 Last modified: 2021-10-14
Priority: 3
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Discussion:
32.4.2 [re.synopt] says:
A valid value of type syntax_option_type shall have at most one of the grammar elements ECMAScript, basic, extended, awk, grep, egrep, set.
But then we never say what happens if an invalid value is used.
What does std::regex(".", std::regex::grep|std::regex::awk) do? Is it undefined? Does it throw? It seems reasonable for basic_regex constructors to throw if f is not a valid value, i.e. for each non-default constructor:Throws: regex_error if f is not a valid value, or if […] is not a valid regular expression according to the grammar specified by f.
However, there is no regex_constants::error_code value suitable for this error condition.
Also, the constructors say "Postconditions: flags() returns f." This prevents an implementation from storing f|ECMAScript in flags() if no grammar element is present in f. This seems like an unnecessary restriction, and forces implementations to do extra work to check if the ECMAScript grammar is in use. Arguably, it would even be better to require implementations to set ECMAScript in flags() if no grammar element was set in the flags passed to the constructor. This problem was introduced by LWG 2330.[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 32.4.3 [re.matchflag] Status: New Submitter: Jonathan Wakely Opened: 2021-09-27 Last modified: 2021-10-14
Priority: 3
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Discussion:
The standard doesn't say what it means if match_prev_avail is set. Table [tab:re.matchflag] says:
--first is a valid iterator position. When this flag is set the flags match_not_bol and match_not_bow shall be ignored by the regular expression algorithms (32.10 [re.alg]) and iterators (32.11 [re.iter]).
What difference does it make whether --first is a valid iterator position or not?
What behaviour is changed when it's a valid iterator position? The standard doesn't say anything else about that. When do the regex algorithms care about --first?
Examples like regex_match("xa"+1, regex("^a"), match_prev_avail)
and regex_match("xa"+1, regex("\\ba"), match_prev_avail)
are presumably supposed to inspect the character at --first
to determine if there is a match.
The standard doesn't specify that *--first is ever inspected,
only that it's a valid character
(which is a useless guarantee if nothing looks at it).
[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 32.2 [re.req] Status: New Submitter: Jonathan Wakely Opened: 2021-09-28 Last modified: 2021-10-14
Priority: 3
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Discussion:
Why is locale_type part of the regular expression traits requirements in 32.2 [re.req]? When would locale_type not be std::locale? What are the requirements on the type? Does it have to provide exactly the same interface as std::locale, or just some unspecified interface that a custom regex traits type needs from it? Why is none of this specified?
Currently the only requirement on locale_type in the standard is that it's copy constructible. Clearly it needs to be default constructible as well, otherwise you can't construct a basic_regex, since none of them allows passing in a locale, so they have to default construct it (see also LWG 2431). The other requirements on locale_type are a mystery. Why do we allow custom locale types, but not say anything about what they should do? Can we just require locale_type to be std::locale? Is anybody really going to use boost::locale with std::basic_regex, when they could just use boost::basic_regex instead? Why does the regular expression traits requirements table say that imbue and getloc talk about the locale used, "if any". How would there not be one already? Why is imbuing a locale into a basic_regex a separate operation from compiling the regular expression pattern? Is the following supposed to change the compiled regex?
std::regex r("[a-z]");
r.imbue(std::locale("en_GB.UTF-8"));
Hasn't the regex constructor already made use of the locale to compile the "[a-z]" pattern, and so changing the locale is too late? So do we need to do the following to compile the regex with a specific locale?
std::regex r;
r.imbue(std::locale("en_GB.UTF-8"));
r.assign("[a-z]");
Why require two-stage initialization like this, is it just so that we appear consistent with the imbue/getloc API of std::ios_base? It works for ios_base, because the new locale is effective after imbuing it, but for basic_regex the pattern has already been compiled using the old locale and imbuing a new one can't change that. Is the basic_regex supposed to store the pattern and recompile it after imbue, or is this just an inappropriate API for basic_regex?
[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 18.4.4 [concept.convertible] Status: New Submitter: Tim Song Opened: 2021-09-28 Last modified: 2021-10-23
Priority: 3
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Discussion:
The semantic requirements of convertible_to express implicit conversion by means of a function:
To test(FromR (&f)()) {
return f();
}
and it requires that static_cast<To>(f()) be equal to test(f) for some equality-preserving function f. However, when To is a reference type but FromR is such that the conversion binds the reference to a temporary, the static_cast is valid but test(f) would produce a dangling reference.
We should rephrase this requirement to just perform the implicit conversion in words. Using a function handles the convert-void-to-void case, but the semantic requirements wording already excluded that case anyway.[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 26.6.4.4 [range.iota.sentinel] Status: New Submitter: Jiang An Opened: 2021-09-25 Last modified: 2021-10-14
Priority: 3
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Discussion:
Currently std::ranges::iota_view<int, long> uses its special sentinel type, as its W and Bound are different types and Bound is not std::unreachable_sentinel_t. However, as W (int) is not an iterator type, the iterator and sentinel types don't satisfy std::sized_sentinel_for, and thus not subtractable.
IMO we should handle operator- overloads for iota_view::iterator and iota_view::sentinel like iota_view::size and operator- for iota_view::iterator.[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
[Tim Song commented:]
We might be able to simplify the (y_value > x.value_) conditional
since we know that we are working with an iterator and its sentinel.
Proposed resolution:
This wording is relative to N4892.
Modify 26.6.4.4 [range.iota.sentinel] as indicated:
namespace std::ranges { template<weakly_incrementable W, semiregular Bound> requires weakly-equality-comparable-with<W, Bound> && copyable<W> struct iota_view<W, Bound>::sentinel { private: Bound bound_ = Bound(); // exposition only public: sentinel() = default; constexpr explicit sentinel(Bound bound); friend constexpr bool operator==(const iterator& x, const sentinel& y); friend constexpriter_difference_t<W>IOTA-DIFF-T(W) operator-(const iterator& x, const sentinel& y) requires (is-integer-like<W> && is-integer-like<Bound>) || sized_sentinel_for<Bound, W>; friend constexpriter_difference_t<W>IOTA-DIFF-T(W) operator-(const sentinel& x, const iterator& y) requires (is-integer-like<W> && is-integer-like<Bound>) || sized_sentinel_for<Bound, W>; }; }[…]
friend constexpriter_difference_t<W>IOTA-DIFF-T(W) operator-(const iterator& x, const sentinel& y) requires (is-integer-like<W> && is-integer-like<Bound>) || sized_sentinel_for<Bound, W>;-3- Effects: Equivalent to:
return x.value_ - y.bound_;using D = IOTA-DIFF-T(W); if constexpr (is-integer-like<W>) { auto y_value = W(y.bound_); if constexpr (is-signed-integer-like<W>) { return D(D(x.value_) - D(y_value)); } else { return (y_value > x.value_) ? D(-D(y_value - x.value_)) : D(x.value_ - y_value); } } else { return x.value_ - y.bound_; }friend constexpriter_difference_t<W>IOTA-DIFF-T(W) operator-(const sentinel& x, const iterator& y) requires (is-integer-like<W> && is-integer-like<Bound>) || sized_sentinel_for<Bound, W>;-4- Effects: Equivalent to: return -(y - x);
Section: 22.5.4 [optional.nullopt] Status: New Submitter: Frank Birbacher Opened: 2021-10-01 Last modified: 2021-10-14
Priority: 3
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Discussion:
The Standard defines a number of types that are used to create overload disambiguators for constructors, like nullopt_t and allocator_tag_t. Values of such types are passed by value to such constructors to give it particular meaning. For pass-by-value these types need to be copy-constructible and for consistency should also be copy-assignable. Of those types the specification of nullopt_t doesn't clearly state that the type is copyable, 22.5.4 [optional.nullopt].
The reason that nullopt_t is defined differently from other such types is to avoid ambiguity in expressions that assign an empty brace initializer to an optional.[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
[Daniel commented:]
I would prefer to see the wording use
is_trivially_copy_constructible_v
and trivially_copy_assignable_v,
which is consistent with similar usage of trivial-requirements in
std::optional.
[Tim commented:]
We need to say that it models copyable and is trivially copyable
(not sure if we need the latter but might as well - does anyone do it differently?).
"has a copy constructor" isn't enough - T(T&) is a copy constructor.
Proposed resolution:
This wording is relative to N4892.
Modify 22.5.4 [optional.nullopt] as indicated:
struct nullopt_t{see below}; inline constexpr nullopt_t nullopt(unspecified);-1- The struct nullopt_t is an empty class type used as a unique type to indicate the state of not containing a value for optional objects. In particular, optional<T> has a constructor with nullopt_t as a single argument; this indicates that an optional object not containing a value shall be constructed.
-2- Type nullopt_t shall not have a default constructor or an initializer-list constructor,andshall not be an aggregate, and shall have a copy constructor and a copy assignment operator, both shall be public and trivial.
Section: 26.6.4.2 [range.iota.view] Status: New Submitter: Jiang An Opened: 2021-10-01 Last modified: 2021-10-14
Priority: 3
View all other issues in [range.iota.view].
View all issues with New status.
Discussion:
According to 26.6.4.2 [range.iota.view]/15, when both W and Bound are integer-like, the expression in the return statement uses -value_ and -bound_. These operations result in undefined behavior when - is applied to the most negative integer value of a promoted type.
I believe that we can simply convert value_ and bound_ to the return type (make-unsigned-like-t<common_type_t<W, Bound>>) and then perform the subtraction. Such method should give the same results with UB eliminated. Additionally, if we decide that iota_view<uint8_t, uint8_t>(uint8_t(1)).size() is well-defined (LWG 3597), it should give the correct result. We can truncate the result to fit the type W.[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4892.
[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
Option A: Just fixes the most negative values
Modify 26.6.4.2 [range.iota.view] as indicated:
constexpr auto size() const requires see below;-15- Effects: Equivalent to:
if constexpr (is-integer-like<W> && is-integer-like<Bound>) {return (value_ < 0) ? ((bound_ < 0) ? to-unsigned-like(-value_) - to-unsigned-like(-bound_) : to-unsigned-like(bound_) + to-unsigned-like(-value_)) : to-unsigned-like(bound_) - to-unsigned-like(value_);using UC = make-unsigned-like-t<common_type_t<W, Bound>>; return UC(bound_) - UC(value_); } else return to-unsigned-like(bound_ - value_);-16- Remarks: […]
Option B: Also fixes pathological cases involving unsigned-integer-like types
Modify 26.6.4.2 [range.iota.view] as indicated:
constexpr auto size() const requires see below;-15- Effects: Equivalent to:
if constexpr (is-integer-like<W> && is-integer-like<Bound>) {return (value_ < 0) ? ((bound_ < 0) ? to-unsigned-like(-value_) - to-unsigned-like(-bound_) : to-unsigned-like(bound_) + to-unsigned-like(-value_)) : to-unsigned-like(bound_) - to-unsigned-like(value_);using UC = make-unsigned-like-t<common_type_t<W, Bound>>; if constexpr (is-signed-integer-like<W>) return UC(bound_) - UC(value_); else return UC(W(UC(bound_) - UC(value_))); } else return to-unsigned-like(bound_ - value_);-16- Remarks: […]
Section: 25.3.2.1 [incrementable.traits] Status: New Submitter: Hewill Kang Opened: 2021-09-30 Last modified: 2022-01-31
Priority: 3
View other active issues in [incrementable.traits].
View all other issues in [incrementable.traits].
View all issues with New status.
Discussion:
The last specialization of incrementable_traits requires a - b to be well-formed, where the types of both operands are const lvalue reference of type T. However inside the struct, it uses decltype(declval<T>() - declval<T>()) to define the difference_type, that is, non-const rvalue reference of type T.
[2022-01-29; Reflector poll]
Set priority to 3 after reflector poll that failed to reach consensus. Some suggested NAD: "Implicit expression variations ([concepts.equality]/6) apply here."
Proposed resolution:
This wording is relative to N4892.
Modify 25.3.2.1 [incrementable.traits] as indicated:
namespace std {
[…]
template<class T>
requires (!requires { typename T::difference_type; } &&
requires(const T& a, const T& b) { { a - b } -> integral; })
struct incrementable_traits<T> {
using difference_type = make_signed_t<decltype(declval<const T&>() - declval<const T&>())>;
};
[…]
}
Section: 16.3.3.3.4.1 [character.seq.general] Status: New Submitter: Dawn Perchik Opened: 2021-10-17 Last modified: 2022-01-29
Priority: 3
View all issues with New status.
Discussion:
The definitions for execution character set and execution wide-character set were reworded and moved to 16.3.3.3.4.1 [character.seq.general]/p1.2 after applying CWG motion 10 "P2314R4 Character sets and encodings", but I can't figure out what these terms mean from the wording, which now reads:
"The execution character set and the execution wide-character set are supersets of the basic literal character set (5.3 [lex.charset]). The encodings of the execution character sets and the sets of additional elements (if any) are locale-specific."
Would it be possible to provide complete definitions for these and give examples of each?
[2022-01-29; Reflector poll]
Set priority to 3 after reflector poll.
Should say something, even if just "unspecified".
P1885R9 will expose the possible encoding programmatically,
so could say it's implementationd-defined and implementations can document
that you should ask std::text_encoding.
Proposed resolution:
Section: 25.5.1.1 [reverse.iterators.general] Status: New Submitter: Jiang An Opened: 2021-10-23 Last modified: 2022-01-29
Priority: 3
View all issues with New status.
Discussion:
Currently it is unspecified whether the definitions of std::reverse_iterator and its related operators are available in <vector>, <array>, etc. So, it's unspecified now whether the following program is well-formed because it's unspecified whether the equality operator is available:
#include <vector>
int main()
{
auto v = std::vector<int>(42);
for (auto it = v.rbegin(); it != v.rend(); ++it);
for (auto it = std::rbegin(v); it != std::rend(v); ++it);
}Such underspecification also leaves the guarantee that std::rbegin, std::rend, std::crbegin, and std::crend are available in some other headers seems not so meaningful. In order to guarantee these function templates can be used meaningfully with containers, users are still required to include <iterator> manually.
I think the standard should guarantee that wherever the member rbegin (that returns std::reverse_iterator) or std::rbegin is provided, the definitions of std::reverse_iterator and its related operators are also provided. This strategy is already implemented by libc++, libstdc++, and MSVC STL, and thus I believe we should standardize it to reduce uncertainty for users. Note that the situation for std::reverse_iterator is different from LWG 1361, because every operation on std::size_t is still valid when the typedef-name itself is absent, but == and != on std::reverse_iterator fail if the corresponding declarations are unavailable.[2022-01-29; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4901.
Modify 25.5.1.1 [reverse.iterators.general] as indicated:
-1- Class template reverse_iterator is an iterator adaptor that iterates from the end of the sequence defined by its underlying iterator to the beginning of that sequence.
-?- In addition to being available via inclusion of the <iterator> header, class template reverse_iterator and function templates in 25.5.1.8 [reverse.iter.cmp] and 25.5.1.9 [reverse.iter.nonmember] are available when any of the following headers are included: <array> (24.3.2 [array.syn]), <deque> (24.3.3 [deque.syn]), <forward_list> (24.3.4 [forward.list.syn]), <list> (24.3.5 [list.syn]), <map> (24.4.2 [associative.map.syn]), <regex> (32.3 [re.syn]), <set> (24.4.3 [associative.set.syn]), <span> (24.7.2.1 [span.syn]), <stacktrace> (19.6.2 [stacktrace.syn]), <string> (23.4.2 [string.syn]), <string_view> (23.3.2 [string.view.synop]), <unordered_map> (24.5.2 [unord.map.syn]), <unordered_set> (24.5.3 [unord.set.syn]), and <vector> (24.3.6 [vector.syn]).
Section: 17.7 [support.rtti], 17.10 [support.initlist], 17.11 [cmp] Status: New Submitter: Jiang An Opened: 2021-10-23 Last modified: 2022-01-29
Priority: 3
View all issues with New status.
Discussion:
Standard library headers <typeinfo>, <initializer_list>, and <compare> are required for some core language features, as specified in 7.6.1.8 [expr.typeid]/7, 9.4.5 [dcl.init.list]/2, and 7.6.8 [expr.spaceship]/8. In C++11 (via N2930), every header that has dependency on std::initializer_list is required to include <initializer_list>. The similar requirements are added for operator<=> and <compare> in C++20 (via LWG 3330).
As N2930 and LWG3330 have been adpoted, IMO there are some inconsistencies in the standard library now:No operation is done for <typeinfo>, although <typeindex> (std::type_index), <functional> (std::function, since C++11), and <any> (std::any) have dependency on std::type_info;
<iterator> has dependency on std::initializer_list since C++14/LWG 2128 (the std::rbegin overload and its friends), but it is not required to include <initializer_list>;
<stacktrace> is not required to include <compare> while it provides operator <=> overloads.
The situation may be quite serious for std::type_index. Perhaps no expected operation on std::type_index is guaranteed to work when only <typeindex> but not <typeinfo> is included.
libc++, libstdc++, and MSVC STL include <typeinfo> and <initializer_list> when the required standard interface depends on them. I think we should standardize the existing practice (except that <stackstrace> has not been implemented now) to reduce uncertainty for users.[2021-10-24; Daniel comments]
This issue is related to and depending on LWG 3625.
[2022-01-29; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4901.
[Drafting note: The proposed wording below contains one conditional change, it is therefore depending upon LWG 3625.]
Add #include <typeinfo> to 22.7.2 [any.synop], 22.10.2 [functional.syn], and 22.11.1 [type.index.synopsis].
Add #include <initializer_list> to 25.2 [iterator.synopsis].
Add #include <compare> to 19.6.2 [stacktrace.syn].
If we decide to add range access function templates (25.7 [iterator.range]) to <stacktrace>, we should also add #include <initializer_list> to 19.6.2 [stacktrace.syn].
Section: 19.6.2 [stacktrace.syn], 25.7 [iterator.range] Status: New Submitter: Jiang An Opened: 2021-10-23 Last modified: 2022-01-29
Priority: 3
View all other issues in [stacktrace.syn].
View all issues with New status.
Discussion:
Range access function templates (25.7 [iterator.range]) are available in every standard header for containers. As std::basic_stacktrace provides some container-like interfaces (member functions begin, end, size, etc.), should we add these free function templates to <stacktrace> for consistency?
[2021-10-24; Daniel comments]
This issue is related to LWG 3624.
[2022-01-29; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4901.
[Drafting note: The proposed wording below contains one conditional change, it is therefore depending upon a decision]
Modify 25.7 [iterator.range] as indicated:
-1- In addition to being available via inclusion of the <iterator> header, the function templates in 25.7 [iterator.range] are available when any of the following headers are included: <array> (24.3.2 [array.syn]), <deque> (24.3.3 [deque.syn]), <forward_list> (24.3.4 [forward.list.syn]), <list> (24.3.5 [list.syn]), <map> (24.4.2 [associative.map.syn]), <regex> (32.3 [re.syn]), <set> (24.4.3 [associative.set.syn]), <span> (24.7.2.1 [span.syn]), <stacktrace> (19.6.2 [stacktrace.syn]), <string> (23.4.2 [string.syn]), <string_view> (23.3.2 [string.view.synop]), <unordered_map> (24.5.2 [unord.map.syn]), <unordered_set> (24.5.3 [unord.set.syn]), and <vector> (24.3.6 [vector.syn]). […]
If we decide that <initializer_list> should be included if the header has dependency on std::initializer_list (it may be introduce by std::rbegin, std::data, etc.), #include <initializer_list> should also be added to 19.6.2 [stacktrace.syn].
Section: 19.6.4.1 [stacktrace.basic.overview] Status: New Submitter: Jiang An Opened: 2021-10-23 Last modified: 2022-01-29
Priority: 3
View all issues with New status.
Discussion:
Currently std::basic_stacktrace has an exposition-only std::vector member for storing its elements. According to 16.3.3.5 [objects.within.classes]/3, it seems that it is effectively required that elements of a std::basic_stacktrace are contiguously stored. However, the implication seems not used otherwhere. The iterator type of a std::basic_stacktrace is only required to be random access iterator.
IMO if it is required that std::basic_stacktrace uses contiguous storage, we should explicitly strengthen some requirements, perhaps a the member function data should be added.[2022-01-29; Reflector poll]
Set priority to 3 after reflector poll.
"The problem here is that a handful of member functions
(operator[], at, perhaps begin)
expose references to the vector elements directly,
which can be read to require contiguity.
We should rephrase the members at issue to not do that."
Proposed resolution:
This wording is relative to N4901.
[Drafting note: The proposed wording below contains also conditional changes, it is therefore depending upon a decision]
Modify 19.6.4.1 [stacktrace.basic.overview] as indicated:
-1- The class template basic_stacktrace satisfies the requirements of an allocator-aware container (Table 80 [tab:container.alloc.req]), a sequence container (24.2.4 [sequence.reqmts]), a contiguous container, and a reversible container (24.2.2.1 [container.requirements.general]) except that […]
Modify 19.6.4.3 [stacktrace.basic.obs] as indicated:
using const_iterator = implementation-defined;-1- The type models
random_access_iteratorcontiguous_iterator (25.3.4.13 [iterator.concept.random.access]25.3.4.14 [iterator.concept.contiguous]) and meets the Cpp17RandomAccessIterator requirements (25.3.5.7 [random.access.iterators]).
Optional additional changes (the following parts are proposed only if data() is wanted)
Modify 19.6.4.1 [stacktrace.basic.overview], class template basic_stacktrace synopsis, as indicated:
[…] const_reference operator[](size_type) const; const_reference at(size_type) const; const stacktrace_entry* data() const noexcept; // 19.6.4.4 [stacktrace.basic.cmp], comparisons […]
Modify 19.6.4.3 [stacktrace.basic.obs] as indicated:
const_reference at(size_type frame_no) const;-13- […]
-14- […]const stacktrace_entry* data() const noexcept;-?- Returns: frames_.data().
Section: 22.5.9 [optional.specalg] Status: New Submitter: Jiang An Opened: 2021-10-23 Last modified: 2022-01-29
Priority: 3
View all issues with New status.
Discussion:
Three std::make_optional overloads are specified in 22.5.9 [optional.specalg]. The first one is specified by "Returns:" and the other two are specified by "Effects: Equivalent to:". According to 16.3.2.4 [structure.specifications]/4, such uses of "Effects: Equivalent to:" propagate the Constraints specified for constructors. As the selected constructor for the first overload has "Constraints:" (22.5.3.2 [optional.ctor]/22), it seems that inconsistency is introduced here.
Existing implementations are inconsistent: libstdc++ constrains all three overloads, while libc++ and MSVC STL do not constrain any of them. IMO all three overloads should be constrained.[2022-01-29; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4901.
Modify 22.5.9 [optional.specalg] as indicated:
template<class T> constexpr optional<decay_t<T>> make_optional(T&& v);-3-
ReturnsEffects: Equivalent to: return optional<decay_t<T>>(std::forward<T>(v));.
Section: 27.11 [specialized.algorithms] Status: New Submitter: Jiang An Opened: 2021-10-23 Last modified: 2022-01-29
Priority: 3
View other active issues in [specialized.algorithms].
View all other issues in [specialized.algorithms].
View all issues with New status.
Discussion:
Most uninitialized memory algorithms (27.11 [specialized.algorithms]) are specified by plain "Effects: Equivalent to:". According to 16.3.2.4 [structure.specifications]/4, such wording requires propagation of "Constraints" of selected constructors. The first two overloads of std::reduce (27.10.4 [reduce]) are specified similarly.
I feel the wording for uninitialized memory algorithms is incorrect, because it means that the constraints, especially for algorithms in the std, depend on "Constraints" in the specifications of many standard library types (but not any user-defined type), which is implementable but brings serious inconsistency. Perhaps we should add "Mandates:" to these algorithms (except for algorithms in std::ranges).[2022-01-29; Reflector poll]
Set priority to 3 after reflector poll.
Tim:
"P2. Not for this particular case (I'm pretty sure there'll be agreement
that this shouldn't induce any constraint), but for the more general
issue of "Effects: Equivalent to" propagating Constraints:;
I'm not sure that's the right approach in general (unlike the other elements,
Constraints: requires special handling beyond "use this code" and is
pretty hard to work through if we have a lengthy code block) - and it
certainly doesn't really make a lot of sense to propagate Constraints:
but not actual core-language constraints."
Proposed resolution:
basic_regex construction and assignment from iterator rangeSection: 32.7.2 [re.regex.construct] Status: New Submitter: Jonathan Wakely Opened: 2021-10-31 Last modified: 2022-01-29
Priority: 4
View other active issues in [re.regex.construct].
View all other issues in [re.regex.construct].
View all issues with New status.
Discussion:
We have:
template<class ForwardIterator>
basic_regex(ForwardIterator first, ForwardIterator last,
flag_type f = regex_constants::ECMAScript);
and:
template<class InputIterator>
basic_regex& assign(InputIterator first, InputIterator last,
flag_type f = regex_constants::ECMAScript);
Ignoring the lack of proper requirements (which is LWG 3341),
why does the constructor take forward iterators,
but the assign function takes input iterators?
Why could construction from input iterators not be implemented as simply
assign(first, last, f)?
The current constructor signature is the result of N2409
which was resolving LWG 682.
It looks like the assign function should have been changed
at the same time, to keep them consistent.
I see no reason why they can't both take input iterators.
The meta-programming needed to avoid an additional string copy for the
input iterator case is trivial with if constexpr
and C++20 iterator concepts.
[2022-01-29; Reflector poll]
Set priority to 4 after reflector poll.
Proposed resolution:
Section: 33.5 [atomics] Status: New Submitter: Wesley Maxey Opened: 2021-11-05 Last modified: 2022-01-29
Priority: 3
View all other issues in [atomics].
View all issues with New status.
Discussion:
The specification of atomic and atomic<T*> (in 33.5.8.1 [atomics.types.generic.general] and 33.5.8.5 [atomics.types.pointer]) explicitly deletes the following functions:
atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete;
The intent is to make atomic objects not copyable, so that initializing an atomic object from another atomic, or assigning an atomic object with a value from another atomic, must be an explicit operation.
We also explicitly support volatile objects of types that are specializations of std::atomic; some of the functions that are vital for the support of volatile atomics are the following conversion operators:operator T() const volatile noexcept; // for non-pointers operator T*() const volatile noexcept; // for pointers
The presence of this conversion operator means that all the statements in the following piece of code compile successfully today, despite the deleted functions mentioned earlier:
volatile std::atomic<int> a; volatile std::atomic<int> b(a); std::atomic<int> c(a); b = a; c = a;
However, if a is not a volatile object, none of the last four lines compile.
[2022-01-29; Reflector poll]
Set priority to 3 after reflector poll.
This PR would allow
atomic<int> x, y = std::move(x);because const volatile& doesn't bind to rvalues. It sounds like we'll need to delete both const volatile& and const volatile&&.
Proposed resolution:
This wording is relative to N4901.
Modify 33.5.8.1 [atomics.types.generic.general], class template atomic synopsis, as indicated:
[…] atomic(const volatile atomic&) = delete; atomic& operator=(const volatile atomic&) = delete; atomic& operator=(const volatile atomic&) volatile = delete; […]
Modify 33.5.8.3 [atomics.types.int], class template atomic<integral> specialization synopsis, as indicated:
[…] atomic(const volatile atomic&) = delete; atomic& operator=(const volatile atomic&) = delete; atomic& operator=(const volatile atomic&) volatile = delete; […]
Modify 33.5.8.4 [atomics.types.float], class template atomic<floating-point> specialization synopsis, as indicated:
[…] atomic(const volatile atomic&) = delete; atomic& operator=(const volatile atomic&) = delete; atomic& operator=(const volatile atomic&) volatile = delete; […]
Modify 33.5.8.5 [atomics.types.pointer], class template atomic<T*> partial specialization synopsis, as indicated:
[…] atomic(const volatile atomic&) = delete; atomic& operator=(const volatile atomic&) = delete; atomic& operator=(const volatile atomic&) volatile = delete; […]
Section: 20.4.4 [mem.res.global], 19.5.3.5 [syserr.errcat.objects] Status: New Submitter: Jiang An Opened: 2021-11-07 Last modified: 2022-01-29
Priority: 3
View all other issues in [mem.res.global].
View all issues with New status.
Discussion:
Both std::pmr::new_delete_resource and std::pmr::null_memory_resource return pointers to static-duration objects. It seems unspecified when the pointed-to objects are destroyed, so users can't reliably use these objects during destructions of their static-duration objects.
std::generic_category and std::system_category have the same issue, except that the referred-to objects may have different storage duration. Should we specify in which case can these objects be used in the termination of a program?[2022-01-29; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 20.4.2 [mem.res.class] Status: New Submitter: Jonathan Wakely Opened: 2021-11-12 Last modified: 2022-01-30
Priority: 3
View all other issues in [mem.res.class].
View all issues with New status.
Discussion:
20.4.2.3 [mem.res.private] says that pmr::memory_resource::do_allocate returns "a pointer to allocated storage" and references 6.7.5.5.2 [basic.stc.dynamic.allocation]. But it's not really clear which parts of 6.7.5.5.2 [basic.stc.dynamic.allocation] define "allocated storage". pmr::memory_resource::allocate is not "an allocation function" and not a "replaceable allocation function", so "the value returned by a replaceable allocation function is a non-null pointer value" doesn't apply here, and neither does "different from any previously returned value".
Is pmr::memory_resource::allocate allowed to return a null pointer on success? Is it allowed to return the same address twice, without an intervening deallocation? What about if you call pmr::monotonic_buffer_resource::release(), is that a deallocation? When discussed on the reflector the consensus was that returning null should not be allowed, it should throw an exception or return a valid dereferenceable pointer. The reference to 6.7.5.5.2 [basic.stc.dynamic.allocation] doesn't work to specify this though, so we should restate the requirements without directly using the core wording for operator new. It was also suggested that returning the same value should not be allowed without an intervening deallocation, but that "deallocation" should not only mean a call to deallocate on the resource, but include things like pmr::monotonic_buffer_resource::release(), and when a memory resource's destructor returns memory to an upstream resource.[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 24.3.12 [vector.bool] Status: New Submitter: Jonathan Wakely Opened: 2021-11-12 Last modified: 2022-01-30
Priority: 3
View other active issues in [vector.bool].
View all other issues in [vector.bool].
View all issues with New status.
Discussion:
vector<bool> provides a static member function that can be used to swap rvalues of type vector<bool>::reference like so:
vector<bool> v{true, false};
vector<bool>::swap(v[0], v[1]);
This is not useful. Nobody calls swap like that. This fails to make v[0] swappable with v[1] as per 16.4.4.3 [swappable.requirements]. The similar SGI STL bit_vector class that vector<bool> is partially inspired by has a "global function" with the same signature, described as:
"Swaps the bits referred to by x and y. This is a global function, not a member function. It is necessary because the ordinary version of swap takes arguments of type T&, and bit_vector::reference is a class, not a built-in C++ reference."
For some reason this became a static member function of vector<bool> in the C++ standard.
We should restore the intended functionality, and deprecate the useless function.Previous resolution [SUPERSEDED]:
This wording is relative to N4901.
Create a new subclause [vector.bool.general] below 24.3.12 [vector.bool] and move paragraphs p1-p3 (including the class template vector<bool, Allocator> partial specialization synopsis) into that subclause.
Add to the synopsis in [vector.bool.general] p1 (née 24.3.12 [vector.bool] p1):
[…] // bit reference class reference { friend class vector; constexpr reference() noexcept; public: constexpr reference(const reference&) = default; constexpr ~reference(); constexpr operator bool() const noexcept; constexpr reference& operator=(bool x) noexcept; constexpr reference& operator=(const reference& x) noexcept; constexpr const reference& operator=(bool x) const noexcept; constexpr void flip() noexcept; // flips the bit friend constexpr void swap(reference x, reference y) noexcept; friend constexpr void swap(reference x, bool& y) noexcept; friend constexpr void swap(bool& x, reference y) noexcept; }; […]Remove the static swap function from the class template vector<bool, Allocator> partial specialization synopsis:
[…] constexpr void swap(vector&);constexpr static void swap(reference x, reference y) noexcept;constexpr void flip() noexcept; // flips all bits […]Create a new subclause [vector.bool.ref] after p3, with p4 as its first paragraph, and add after it:
22.3.12.? Class vector<bool, Allocator>::reference [vector.bool.ref]
-1- reference is a class that simulates the behavior of references of a single bit in vector<bool>. The conversion function returns true when the bit is set, and false otherwise. The assignment operators set the bit when the argument is (convertible to) true and clear it otherwise. flip reverses the state of the bit.constexpr void flip() noexcept;-?- Effects: *this = !*this;
friend constexpr void swap(reference x, reference y) noexcept; friend constexpr void swap(reference x, bool& y) noexcept; friend constexpr void swap(bool& x, reference y) noexcept;-?- Effects: Exchanges the contents of x and y as if by:
bool b = x; x = y; y = b;Create a new subclause [vector.bool.mem] after that, containing the paragraphs describing flip() and the hash specialization:
22.3.12.? Class vector<bool, Allocator> members [vector.bool.mem]
constexpr void flip() noexcept;-1- Effects: Replaces each element in the container with its complement.
constexpr static void swap(reference x, reference y) noexcept;
-6- Effects: Exchanges the contents of x and y as if by:bool b = x; x = y; y = b;template<class Allocator> struct hash<vector<bool, Allocator>>;-7- The specialization is enabled (22.10.19 [unord.hash]).
Create a new subclause [depr.vector.bool.swap] after D.25 [depr.string.capacity]
D.? Deprecated vector<bool, Allocator> swap [depr.vector.bool.swap]
-?- The following member is declared in addition to those members specified in 24.3.12 [vector.bool]:namespace std { template<class Allocator> class vector<bool, Allocator> { public: constexpr static void swap(reference x, reference y) noexcept; }; }constexpr static void swap(reference x, reference y) noexcept;-?- Effects: swap(x, y).
[2022-01-22; Jonathan replaces swap(x, y) in the Annex D
wording, following reflector discussion about lookup for swap
finding itself in that context.
]
[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4901.
Create a new subclause [vector.bool.general] below 24.3.12 [vector.bool] and move paragraphs p1-p3 (including the class template vector<bool, Allocator> partial specialization synopsis) into that subclause.
Add to the synopsis in [vector.bool.general] p1 (née 24.3.12 [vector.bool] p1):
[…]
// bit reference
class reference {
friend class vector;
constexpr reference() noexcept;
public:
constexpr reference(const reference&) = default;
constexpr ~reference();
constexpr operator bool() const noexcept;
constexpr reference& operator=(bool x) noexcept;
constexpr reference& operator=(const reference& x) noexcept;
constexpr const reference& operator=(bool x) const noexcept;
constexpr void flip() noexcept; // flips the bit
friend constexpr void swap(reference x, reference y) noexcept;
friend constexpr void swap(reference x, bool& y) noexcept;
friend constexpr void swap(bool& x, reference y) noexcept;
};
[…]
Remove the static swap function from the class template vector<bool, Allocator> partial specialization synopsis:
[…] constexpr void swap(vector&);constexpr static void swap(reference x, reference y) noexcept;constexpr void flip() noexcept; // flips all bits […]
Create a new subclause [vector.bool.ref] after p3, with p4 as its first paragraph, and add after it:
22.3.12.? Class vector<bool, Allocator>::reference [vector.bool.ref]
-1- reference is a class that simulates the behavior of references of a single bit in vector<bool>. The conversion function returns true when the bit is set, and false otherwise. The assignment operators set the bit when the argument is (convertible to) true and clear it otherwise. flip reverses the state of the bit.constexpr void flip() noexcept;-?- Effects: *this = !*this;
friend constexpr void swap(reference x, reference y) noexcept; friend constexpr void swap(reference x, bool& y) noexcept; friend constexpr void swap(bool& x, reference y) noexcept;-?- Effects: Exchanges the contents of x and y as if by:
bool b = x; x = y; y = b;
Create a new subclause [vector.bool.mem] after that, containing the paragraphs describing flip() and the hash specialization:
22.3.12.? Class vector<bool, Allocator> members [vector.bool.mem]
constexpr void flip() noexcept;-1- Effects: Replaces each element in the container with its complement.
constexpr static void swap(reference x, reference y) noexcept;
-6- Effects: Exchanges the contents of x and y as if by:bool b = x; x = y; y = b;template<class Allocator> struct hash<vector<bool, Allocator>>;-7- The specialization is enabled (22.10.19 [unord.hash]).
Create a new subclause [depr.vector.bool.swap] after D.25 [depr.string.capacity]
D.? Deprecated vector<bool, Allocator> swap [depr.vector.bool.swap]
-?- The following member is declared in addition to those members specified in 24.3.12 [vector.bool]:namespace std { template<class Allocator> class vector<bool, Allocator> { public: constexpr static void swap(reference x, reference y) noexcept; }; }constexpr static void swap(reference x, reference y) noexcept;-?- Effects: Exchanges the contents of x and y as if by:
bool b = x; x = y; y = b;
Section: 16.4.4 [utility.requirements] Status: New Submitter: Johel Ernesto Guerrero Peña Opened: 2021-11-01 Last modified: 2022-10-01
Priority: 3
View all issues with New status.
Discussion:
This originated from the editorial issues #4863 and #4869.
Some Throws: elements are specified to throw the exceptions an evaluation of E exits with. This wording excludes exceptions thrown involving the initialization and destruction of parameters of E, temporaries of E, and the destruction of the result of E. The proposed wording below fixes this with front matter. As if affects more than just Throws: elements, it talks about requirements and guarantees when E exits via an exception. As noted in the originating editorial issues, some LWG members prefer fixing each individual case of wording used to describe exception propagation rather than patching them up with front matter.[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll. "Any throwing destructor is library UB already, so there's no need to contort the wording to accommodate those."
Previous resolution [SUPERSEDED]:
This wording is relative to N4901.
Add a new subclause [exception.propagation] at the end of 16.4.4 [utility.requirements] (after 16.4.4.6.2 [allocator.requirements.completeness]):
16.4.4.? Exception propagation requirements [exception.propagation]
-?- Some functions defined in the C++ standard library impose requirements and guarantees R-G when a described evaluation E exits via an exception. Let F be an evaluation that is implied by evaluating E up to the complete evaluation of its enclosing full-expression. Unless stated otherwise, an execution of F that exits via an exception also has R-G imposed. [Note ?: This includes when initializing and destroying parameters, evaluating default arguments, and destroying temporaries (including discarded-value expressions) (7.6.1.3 [expr.call]) exit via an exception. — end note]
[2022-09-28; Johel provides revised wording]
Proposed resolution:
This wording is relative to N4917.
Add a new subclause [exception.propagation] at the end of 16.4.4 [utility.requirements] (after 16.4.4.6.2 [allocator.requirements.completeness]):
16.4.4.? Exception propagation requirements [exception.propagation]
-?- Some functions defined in the C++ standard library impose requirements and guarantees R-G when a described evaluation E of a constructor or construction exits via an exception. Let F be the initialization denoted by E. Unless stated otherwise, F also has R-G imposed. [Note ?: This includes the initialization of parameters and the evaluation of default arguments as part of F. — end note]
Section: 22.14.1 [format.syn] Status: New Submitter: Mark de Wever Opened: 2021-11-14 Last modified: 2022-01-30
Priority: 3
View all issues with New status.
Discussion:
During the 2019 Cologne meeting the papers P1614R2 "The Mothership has Landed" and P0645R10 "Text Formatting" have been accepted. P1614R2 adds operator== to to_chars_result and from_chars_result. P0645R10 adds a similar type format_to_n_result without an operator==. LWG 3373 reaffirms these three types are similar by ensuring they can be used in structured bindings.
It seems due to accepting P1614R2 and P0645R10 during the same meeting the addition of operator== wasn't applied to format_to_n_result. I propose to add operator== to format_to_n_result to keep these types similar. The Out template argument of format_to_n_result is unconstrained. Since it's returned from format_to_n it's indirectly constrained to an output_iterator. An output_iterator doesn't require equality_comparable, thus the defaulted operator== can be defined deleted.[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll.
Several votes for NAD. Unclear why to_chars_result is equality
comparable, but whatever the reason, this is unrelated to them and doesn't
need to be.
Proposed resolution:
This wording is relative to N4901.
Modify 22.14.1 [format.syn], header <format> synopsis, as indicated:
[…]
template<class Out> struct format_to_n_result {
Out out;
iter_difference_t<Out> size;
friend bool operator==(const format_to_n_result&, const format_to_n_result&) = default;
};
[…]
Section: 22.10.17.4.3 [func.wrap.move.ctor] Status: New Submitter: Alexander Guteniev Opened: 2021-11-20 Last modified: 2022-01-30
Priority: 3
View other active issues in [func.wrap.move.ctor].
View all other issues in [func.wrap.move.ctor].
View all issues with New status.
Discussion:
22.10.17.4.3 [func.wrap.move.ctor]/22 and 22.10.17.4.3 [func.wrap.move.ctor]/25 define the effects of assignment as following:
move_only_function& operator=(move_only_function&& f);[…]Effects: Equivalent to: move_only_function(std::move(f)).swap(*this);
template<class F> move_only_function& operator=(F&& f);Effects: Equivalent to: move_only_function(std::forward<F>(f)).swap(*this);
The assignment via swap with temporary makes the implementation to do the following:
move out the previous target to a temporary location
move in the new target
finally destroy the previous target.
As everything is noexcept here, I think it can be short cut to just:
destroy the previous target.
move in the new target
Looks like the implementation cannot do such optimization in a generic case with small functor optimization enabled and non-trivial move constructor for the new target and with non-trivial destruction of the previous target, since the difference is observable.
Apparently the optimization is precluded for no reason.[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll. Some suggestions for NAD, but others disagreed.
Proposed resolution:
This wording is relative to N4901.
Modify 22.10.17.4.3 [func.wrap.move.ctor] as indicated:
move_only_function& operator=(move_only_function&& f);[…]-22- Effects: Sets the target object of this to the target object of f before the assignment and leaves f in a valid state with an unspecified value.
Equivalent to: move_only_function(std::move(f)).swap(*this);template<class F> move_only_function& operator=(F&& f);-25- Effects: Equivalent to: *this = move_only_function(std::forward<F>(f))
.swap(*this);
Section: 22.14.2.2 [format.string.std] Status: New Submitter: Charlie Barto Opened: 2021-11-23 Last modified: 2022-11-01
Priority: 2
View other active issues in [format.string.std].
View all other issues in [format.string.std].
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Discussion:
22.14.2.2 [format.string.std] specifies the behavior of several format specifiers in terms of "integer presentation types"; for example 22.14.2.2 [format.string.std]/4 states:
"The sign option is only valid for arithmetic types other than charT and bool or when an integer presentation type is specified".
Unfortunately nowhere does the standard actually define the term "integer presentation type". The closest it comes is in 22.14.2.2 [format.string.std]/19 and [tab:format.type.int], but that explicitly excludes charT and bool. [tab:format.type.char] and [tab:format.type.bool] then refer to [tab:format.type.int].
I can come up with many interpretations for what could happen when 'c' is used with charT or bool, but the following table is what msvc does right now (throws is the same as does not compile after P2216 in all these cases, although not in general for 'c'):
Argument type Specifiers Throws? bool # Yes bool #c No bool :+ Yes bool +c Yes bool ^ No bool ^c No bool 0 Yes bool 0c Yes bool c No charT # Yes charT #c Yes charT + Yes charT +c Yes charT ^ No charT ^c No charT 0 Yes charT 0c Yes
As you can see we don't interpret 'c' as an "integer type specifier", except when explicitly specified for bool with #. I think this is because for # the standard states
"This option is valid for arithmetic types other than charT and bool or when an integer presentation type is specified, and not otherwise",
and [tab:format.type.bool] puts 'c' in the same category as all the other "integer type specifiers", whereas [tab:format.type.char] separates it out into the char-specific types. If this issue's proposed resolution is adopted our behavior would become non-conforming (arguably it already is) and "#c" with bools would become invalid.
[2021-11-29; Tim comments]
This issue touches the same wording area as LWG 3586 does.
[2022-01-30; Reflector poll]
Set priority to 2 after reflector poll.
[2021-11-29; Jonathan comments]
LWG 3648 removed 'c' as a valid presentation type for bool. The last change in the resolution below (and the drafting note) can be dropped.
LWG 3586 could be resolved as part of this issue by using "this is the default unless formatting a floating-point type or using an integer presentation type" for '<' and by using "this is the default when formatting a floating-point type or using an integer presentation type" for '>'.
Proposed resolution:
This wording is relative to N4901.
Modify 22.14.2.2 [format.string.std] as indicated:
-6- The # option causes the alternate form to be used for the conversion. This option is only valid for arithmetic types other than charT and bool or when an integer presentation type is specified
[…], and not otherwise. For integral types, the alternate form inserts the base prefix (if any) specified in Table 65 into the output after the sign character (possibly space) if there is one, or before the output of to_chars otherwise. For floating-point types, the alternate form causes the result of the conversion of finite values to always contain a decimal-point character, even if no digits follow it. Normally, a decimal-point character appears in the result of these conversions only if a digit follows it. In addition, for g and G conversions, trailing zeros are not removed from the result.[…][Drafting note: This modification is a simple cleanup given the other changes further below, to bring the wording for # in line with the wording for the other modifiers, in the interest of preventing confusion.]
-16- The type determines how the data should be presented.
-?- An integer presentation type is one of the following type specifiers in Table [tab:format.type.integer_presentation], or none, if none is defined to have the same behavior as one of the type specifiers in Table [tab:format.type.integer_presentation].
Table ? — Meaning of type options for integer representations [tab:format.type.integer_presentation] Type Meaning b to_chars(first, last, value, 2); the base prefix is 0b. B The same as b, except that the base prefix is 0B. d to_chars(first, last, value). o to_chars(first, last, value, 8); the base prefix is 0 if value is nonzero and is empty otherwise. x to_chars(first, last, value, 16); the base prefix is 0x. X The same as x, except that it uses uppercase letters for digits above 9 and the base prefix is 0X. [Drafting note: This is the same as [tab:format.type.int] with "none" and 'c' removed]
-17- The available string presentation types are specified in Table 64 ([tab:format.type.string]).
[…]
Table 65 — Meaning of type options for integer types [tab:format.type.int] Type Meaning b, B, d, o, x, X As specified in Table [tab:format.type.integer_presentation] to_chars(first, last, value, 2); the base prefix is 0b.BThe same as b, except that the base prefix is 0B.c Copies the character static_cast<charT>(value) to the output. Throws format_error if value is not in the range of representable values for charT. dto_chars(first, last, value).oto_chars(first, last, value, 8); the base prefix is 0 if value is nonzero and is empty otherwise.xto_chars(first, last, value, 16); the base prefix is 0x.XThe same as x, except that it uses uppercase letters for digits above 9 and the base prefix is 0X.none The same as d. [Note 8: If the formatting argument type is charT or bool, the default is instead c or s, respectively. — end note]
Table 66 — Meaning of type options for charT [tab:format.type.char] Type Meaning none, c Copies the character to the output. b, B, d, o, x, X As specified in Table [tab:format.type.int][tab:format.type.integer_presentation].
Table 67 — Meaning of type options for bool [tab:format.type.bool] Type Meaning none, s Copies textual representation, either true or false, to the output. b, B, c,d, o, x, XAs specified in Table [tab:format.type.int][tab:format.type.integer_presentation] for the value static_cast<unsigned char>(value).c Copies the character static_cast<unsigned char>(value) to the output. [Drafting note: allowing the 'c' specifier for bool is pretty bizarre behavior, but that's very clearly what the standard says now, so I'm preserving it. I would suggest keeping discussion of changing that behavior to a separate issue or defect report (the reworking of the tables in this issue makes addressing that easier anyway).
The inconsistency with respect to using static_cast<unsigned char> here and static_cast<charT> in [tab:format.type.int] is pre-existing and should be addressed in a separate issue if needed. ]
Section: 27.11.2 [special.mem.concepts] Status: New Submitter: Konstantin Varlamov Opened: 2021-11-30 Last modified: 2022-01-30
Priority: 3
View all issues with New status.
Discussion:
27.11.2 [special.mem.concepts] states that a type qualifies as a nothrow-input-iterator only if no exceptions are thrown from, among other things, copy construction and copy assignment. However, being copyable isn't part of the requirements for an input_iterator on which nothrow-input-iterator is based (indeed, one of the things forward_iterator adds to input_iterator is copyability), and the nothrow-input-iterator concept doesn't add any new constraints related to copyability.
[2021-12-19; Daniel comments]
During LWG discussion of the issue one argument brought forward against the proposed wording was that it might be incomplete, because it doesn't adjust the nothrow-forward-iterator concept, which adds among other things the copyable requirements. But nothrow-forward-iterator also requires nothrow-sentinel-for<I, I>, which already extends this necessary no-throw requirement for copy operations by p. 4:
Types S and I model nothrow-sentinel-for only if no exceptions are thrown from copy construction, move construction, copy assignment, move assignment, or comparisons between valid values of type I and S.
It should also be emphasized that the definitions of move construction (3.34 [defns.move.constr]) and move assignment (3.33 [defns.move.assign]) are compatible even for copyable input iterator types, because these definitions refer just to expression conditions, and not to concrete operator overloads. So as long as an implementation applies these expression conditions, we are safe.
[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4901.
Modify 27.11.2 [special.mem.concepts] as indicated:
template<class I> concept nothrow-input-iterator = // exposition only input_iterator<I> && is_lvalue_reference_v<iter_reference_t<I>> && same_as<remove_cvref_t<iter_reference_t<I>>, iter_value_t<I>>;-2- A type I models nothrow-input-iterator only if no exceptions are thrown from increment,
copy construction,move construction,copy assignment,move assignment, or indirection through valid iterators.
Section: 22.14 [format] Status: New Submitter: Barry Revzin Opened: 2021-12-08 Last modified: 2022-01-30
Priority: 3
View all issues with New status.
Discussion:
Is this program guaranteed to be valid:
#include <format>
#include <algorithm>
struct Thing { };
template <>
struct std::formatter<Thing> {
std::string_view spec;
constexpr auto parse(std::format_parse_context& ctx) {
auto end = std::find(ctx.begin(), ctx.end(), '}');
spec = std::string_view(ctx.begin(), end);
return end;
}
auto format(Thing, std::format_context& ctx) {
return std::ranges::copy(spec, ctx.out()).out;
}
};
int main() {
std::print("{:lwg issue}\n", Thing{});
}
In parse(), the formatter for Thing holds onto a string view of its specifiers. And then in format(), it just prints them. I don't think we say anywhere that this works. Does this code print "lwg issue" because there's no issue or does it print some garbage memory somewhere because there is one?
libfmt's implementation internally stores string_view's into the format string (for named argument support), which implies that it should work. But it'd be nice to come out and say that.[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll.
"Presumably we need to say in [formatter.requirements] that
[pc.begin(), pc.end())
is guaranteed to be a valid range until the
next call to parse() or f is destroyed,
whichever comes first."
Proposed resolution:
Section: 17.12.2 [coroutine.syn] Status: New Submitter: Jonathan Wakely Opened: 2021-12-17 Last modified: 2022-01-30
Priority: 3
View all other issues in [coroutine.syn].
View all issues with New status.
Discussion:
The <coroutine> header is required for freestanding implementations, but it defines a specialization of std::hash, which is not required for freestanding implementations.
Presumably we should make the std::hash specialization conditionally present.[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll. Would be resolved by P1642 for C++23, but it's still a defect in C++20.
Proposed resolution:
Section: 31.6.3.3.5 [streambuf.pub.put] Status: New Submitter: Jonathan Wakely Opened: 2022-01-17 Last modified: 2022-01-30
Priority: 3
View all issues with New status.
Discussion:
The specification for basic_streambuf::sputn is:
Returns: xsputn(s, n).
One interpretation of this implies that sputn can't insert characters directly into the put area if there is space for them, it has to make a virtual call. That has significant overhead for repeated calls, such as inserting many small strings/string_views in a loop.
But another interpretation is that it doesn't use "Effects: Equivalent to" or anything like that, so doesn't say an actual call to xsputn happens. Strictly speaking, it only says it returns the value that xsputn would return, and doesn't even have to produce any of its effects! We should describe the effects, not the return value, and we should do so in a way that does not overconstrain the implementation. It should not be necessary to make a virtual call to xsputn if the put area has capacity for the characters. On the other hand, if it doesn't have capacity, then calling xsputn is the best option; it allows the derived streambuf to decide how best to handle large writes. The proposed resolution replaces the Returns: element with an Effects: element, so that we specify that those effects actually occur. A normative remark is added to give the implementation leeway to avoid the virtual call when it isn't needed.[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll.
Jonathan to revise P/R to also cover sgetn.
Proposed resolution:
This wording is relative to N4901.
Modify 31.6.3.3.5 [streambuf.pub.put] as indicated:
streamsize sputn(const char_type* s, streamsize n);-?- Preconditions: [s, s+n) is a valid range.
-?- Effects: return xsputn(s, n). -?- Remarks: When (epptr() - pptr()) >= n, it is unspecified whether the characters are written directly to the output sequence without calling xsputn.-2- Returns: xsputn(s, n).
Section: 23.4.3.7.2 [string.append], 23.4.3.7.3 [string.assign] Status: New Submitter: Jonathan Wakely Opened: 2022-01-21 Last modified: 2022-01-30
Priority: 3
View all other issues in [string.append].
View all issues with New status.
Discussion:
This will allocate temporary string, then copy one byte from it:
std::string s;
s.append("a very very long string that doesn't fit in SSO buffer", 13, 1);
The problem is that there is no overload accepting an NTBS there, so it converts to a temporary string using:
append(const basic_string&, size_t, size_t = npos);
C++17 added a new overload for a string_view:
append(const T&, size_t, size_t = npos);
But that overload is constrained to not allow const char* arguments, so we still create a temporary string.
If we're going to accept those arguments, we should do so efficiently, without forcing an unnecessary allocation. We could do better if we split the append(const T&, ...) overload into:append(const T&, size_t); append(const T&, size_t, size_t);
and then remove the !is_convertible_v<const T&, const charT*> constraint from the second overload (Option A in the proposed resolution).
Alternatively, we can leave the append(const T&, size_t, size_t) overload alone and just add one taking exactly the arguments used above. That seems simpler to me, and I prefer that (Option B in the Proposed Resolution). The same problem applies to assign("...", pos, n).[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4901.
[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
Option A:
Modify 23.4.3.1 [basic.string.general], class template basic_string synopsis, as indicated:
[…] // 23.4.3.7 [string.modifiers], modifiers […] template<class T> constexpr basic_string& append(const T& t); template<class T> constexpr basic_string& append(const T& t, size_type pos); template<class T> constexpr basic_string& append(const T& t, size_type pos, size_type n= npos); constexpr basic_string& append(const charT* s, size_type n); […] template<class T> constexpr basic_string& assign(const T& t); template<class T> constexpr basic_string& assign(const T& t, size_type pos); template<class T> constexpr basic_string& assign(const T& t, size_type pos, size_type n= npos); constexpr basic_string& assign(const charT* s, size_type n); […]
Modify 23.4.3.7.2 [string.append] as indicated:
template<class T> constexpr basic_string& append(const T& t);-3- Constraints:
(3.1) — is_convertible_v<const T&, basic_string_view<charT, traits>> is true and
(3.2) — is_convertible_v<const T&, const charT*> is false.
-4- Effects: Equivalent to:
basic_string_view<charT, traits> sv = t; return append(sv.data(), sv.size());template<class T> constexpr basic_string& append(const T& t, size_type pos);-?- Constraints:
(?.1) — is_convertible_v<const T&, basic_string_view<charT, traits>> is true and
(?.2) — is_convertible_v<const T&, const charT*> is false.
-?- Effects: Equivalent to:
basic_string_view<charT, traits> sv = t; return append(sv.substr(pos));template<class T> constexpr basic_string& append(const T& t, size_type pos, size_type n= npos);-5- Constraints:
(5.1) —is_convertible_v<const T&, basic_string_view<charT, traits>> is trueand
(5.2) — is_convertible_v<const T&, const charT*> is false.-6- Effects: Equivalent to:
basic_string_view<charT, traits> sv = t; return append(sv.substr(pos, n));
Modify 23.4.3.7.3 [string.assign] as indicated:
template<class T> constexpr basic_string& assign(const T& t);-4- Constraints:
(4.1) — is_convertible_v<const T&, basic_string_view<charT, traits>> is true and
(4.2) — is_convertible_v<const T&, const charT*> is false.
-5- Effects: Equivalent to:
basic_string_view<charT, traits> sv = t; return assign(sv.data(), sv.size());template<class T> constexpr basic_string& assign(const T& t, size_type pos);-?- Constraints:
(?.1) — is_convertible_v<const T&, basic_string_view<charT, traits>> is true and
(?.2) — is_convertible_v<const T&, const charT*> is false.
-?- Effects: Equivalent to:
basic_string_view<charT, traits> sv = t; return assign(sv.substr(pos));template<class T> constexpr basic_string& assign(const T& t, size_type pos, size_type n= npos);-6- Constraints:
(6.1) —is_convertible_v<const T&, basic_string_view<charT, traits>> is trueand
(6.2) — is_convertible_v<const T&, const charT*> is false.-7- Effects: Equivalent to:
basic_string_view<charT, traits> sv = t; return assign(sv.substr(pos, n));
Option B:
Modify 23.4.3.1 [basic.string.general], class template basic_string synopsis, as indicated:
[…] // 23.4.3.7 [string.modifiers], modifiers […] constexpr basic_string& append(const charT* s, size_type n); constexpr basic_string& append(const charT* s); constexpr basic_string& append(const charT* s, size_type pos, size_type n); constexpr basic_string& append(size_type n, charT c); […] constexpr basic_string& assign(const charT* s, size_type n); constexpr basic_string& assign(const charT* s); constexpr basic_string& assign(const charT* s, size_type pos, size_type n); constexpr basic_string& assign(size_type n, charT c); […]
Modify 23.4.3.7.2 [string.append] as indicated:
constexpr basic_string& append(const charT* s, size_type n);-7- Preconditions: [s, s + n) is a valid range.
-8- Effects: Appends a copy of the range [s, s + n) to the string. -9- Returns: *this.constexpr basic_string& append(const charT* s);-10- Effects: Equivalent to: return append(s, traits::length(s));
constexpr basic_string& append(const charT* s, size_type pos, size_type n);-?- Effects: Equivalent to: return append(basic_string_view<charT, traits>(s).substr(pos, n));
Modify 23.4.3.7.3 [string.assign] as indicated:
constexpr basic_string& assign(const charT* s, size_type n);-8- Preconditions: [s, s + n) is a valid range.
-9- Effects: Replaces the string controlled by *this with a copy of the range [s, s + n). -10- Returns: *this.constexpr basic_string& assign(const charT* s);-11- Effects: Equivalent to: return assign(s, traits::length(s));
constexpr basic_string& assign(const charT* s, size_type pos, size_type n);-?- Effects: Equivalent to: return assign(basic_string_view<charT, traits>(s).substr(pos, n));
Section: 23.4.3.3 [string.cons], 23.4.3.7.3 [string.assign] Status: New Submitter: Jonathan Wakely Opened: 2022-01-21 Last modified: 2022-01-30
Priority: 3
View all other issues in [string.cons].
View all issues with New status.
Discussion:
This will do a copy not a move:
struct Str : std::string {
Str() = default;
Str(Str&& s) : std::string(std::move(s)) { }
};
Str s;
Str s2(std::move(s));
The problem is that the new C++17 constructor:
basic_string(const T&, const Alloc& = Alloc());
is an exact match, but the basic_string move constructor requires a derived-to-base conversion.
This is a regression since C++14, because the move constructor was called in C++14. This problem also exists for assign(const T&) and operator=(const T&). We can fix this by constraining those functions with !derived_from<T, basic_string>, so that the move constructor is the only viable function. Libstdc++ has done something very similar since 2017, but using !is_convertible<const T*, const basic_string*> instead of derived_from.[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll.
Jonathan to revise P/R to use is_base_of or
is_convertible instead of derived_from.
Proposed resolution:
This wording is relative to N4901.
Modify 23.4.3.3 [string.cons] as indicated:
template<class T> constexpr explicit basic_string(const T& t, const Allocator& a = Allocator());-7- Constraints:
(7.1) — is_convertible_v<const T&, basic_string_view<charT, traits>> is true and
(7.?) — derived_from<T, basic_string> is false and
(7.2) — is_convertible_v<const T&, const charT*> is false.
-8- Effects: Creates a variable, sv, as if by basic_string_view<charT, traits> sv = t; and then behaves the same as basic_string(sv.data(), sv.size(), a).
[…]
template<class T> constexpr basic_string& operator=(const T& t);-27- Constraints:
(27.1) — is_convertible_v<const T&, basic_string_view<charT, traits>> is true and
(27.?) — derived_from<T, basic_string> is false and
(27.2) — is_convertible_v<const T&, const charT*> is false.
-28- Effects: Equivalent to:
basic_string_view<charT, traits> sv = t; return assign(sv);
Modify 23.4.3.7.3 [string.assign] as indicated:
template<class T> constexpr basic_string& assign(const T& t);-4- Constraints:
(4.1) — is_convertible_v<const T&, basic_string_view<charT, traits>> is true and
(4.?) — derived_from<T, basic_string> is false and
(4.2) — is_convertible_v<const T&, const charT*> is false.
-5- Effects: Equivalent to:
basic_string_view<charT, traits> sv = t; return assign(sv.data(), sv.size());
Section: 20.2.9.2 [allocator.traits.types] Status: New Submitter: Jiang An Opened: 2022-01-24 Last modified: 2022-03-04
Priority: 3
View all other issues in [allocator.traits.types].
View all issues with New status.
Discussion:
20.2.9.2 [allocator.traits.types]/11 says that the instantiation of rebind_alloc is sometimes ill-formed, however, it's unclear such ill-formedness results in substitution failure or hard error. It seems that current implementations (libc++, libstd++, MSVC STL) give substitution errors, and we should reword 20.2.9.2 [allocator.traits.types]/11 with "Constraints:".
[2022-01-29; Daniel comments]
This issue has some overlap with LWG 3545 in regard to the question how we should handle the rebind member template of the pointer_traits primary template as specified by 20.2.3.2 [pointer.traits.types] p3. It would seem preferable to decide for the same approach in both cases.
[2022-03-04; Reflector poll]
Set priority to 3 after reflector poll. Probably NAD, since [allocator.requirements.general]/11 allows any allocator instantiation to fail with a hard error outside the immediate context, making it impossible to guarantee a SFINAE-friendly result. Also unclear what motivation there is for it being SFINAE friendly.
Proposed resolution:
Section: 26.7.14 [range.join] Status: New Submitter: Tomasz Kamiński Opened: 2022-01-30 Last modified: 2022-03-04
Priority: 2
View all other issues in [range.join].
View all issues with New status.
Discussion:
Despite the fact that join_view may produce more elements than each joined view(s) contains, we require implementations to use the common_type of their difference types (per 26.7.14.3 [range.join.iterator] p3), which may lead to UB. As we already have provided implementation freedom to define extended integer-like types we should make provision for them to be used here as well. The same issue applies to the join_with_view as proposed by P2441.
[2022-03-04; Reflector poll]
Set priority to 2 after reflector poll. Might be a design issue.
Proposed resolution:
Section: 31.12.11.2 [fs.dir.itr.members], 31.12.12.2 [fs.rec.dir.itr.members] Status: New Submitter: Jonathan Wakely Opened: 2022-01-31 Last modified: 2022-12-18
Priority: 3
View all other issues in [fs.dir.itr.members].
View all issues with New status.
Discussion:
31.12.11.2 [fs.dir.itr.members] p2 and 31.12.12.2 [fs.rec.dir.itr.members] p3 refer to the options parameter, but not all overloads have a parameter of that name.
We need some "for the overloads with …" wording.[2022-03-04; Reflector poll]
Set priority to 3 after reflector poll.
[2022-12-18; Daniel comments and provides wording]
It seems to me that we should differentiate the problems of directory_iterator and recursive_directory_iterator regarding the constructors that don't provide any options arguments at least from a logical point of view:
From the existing Postconditions: element of recursive_directory_iterator we can at least guess that the intention of the existing wording is that these suppose to reflect that the effective options within the Effects: element are also equal to directory_options::none. But for directory_iterator we don't have any options "getter" nor Postconditions from deducing the intent. Fortunately, existing implementations (I looked at the current V2 2022 library and libstdc++ implementation) seem to behave already as if directory_options::none is set for the affected constructors for both iterator types. Note that this issue here has some wording overlap with the the proposed wording of LWG 2708, I'm therefore using the exact wording form currently suggested there. If LWG would like to change this form (e.g. by using the word "overload" instead of "signature") this should also be done in that other issue to reduce possible merge conflicts. Note that the here suggested form using "signature" has existing practice in other places of Clause 31.12 [filesystems], so I recommend keeping that form.Proposed resolution:
This wording is relative to N4917.
Modify 31.12.11.2 [fs.dir.itr.members] as indicated:
[Drafting note:
(1) The proposed wording form takes care to use the same wording form used in LWG 2708 for recursive_directory_iterator at the time of writing.
(2) We don't need similar wording for the default constructor because this creates an end iterator and there exist no way for the user to observe the effects of directory_options for such an iterator value. ]
directory_iterator() noexcept;-1- Effects: Constructs the end iterator.
explicit directory_iterator(const path& p); directory_iterator(const path& p, directory_options options); directory_iterator(const path& p, error_code& ec); directory_iterator(const path& p, directory_options options, error_code& ec);-?- For the signatures with no parameter options, let options be directory_options::none.
-2- Effects: For the directory that p resolves to, constructs an iterator for the first element in a sequence of directory_entry elements representing the files in the directory, if any; otherwise the end iterator. However, if(options & directory_options::skip_permission_denied) != directory_options::noneand construction encounters an error indicating that permission to access p is denied, constructs the end iterator and does not report an error.
[…]
Modify 31.12.12.2 [fs.rec.dir.itr.members] as indicated:
[Drafting note:
(1) The proposed wording form takes care to be in sync with the wording form used in LWG 2708 at the time of writing. This issue here doesn't touch the wording for the options function though, and leaves any changes to LWG 2708, because these changes don't seem essential to solve the issue here.
(2) We don't need similar wording for the default constructor because this creates an end iterator and from 31.12.12.1 [fs.class.rec.dir.itr.general] p2 we can conclude that user code has no way to determine the effective directory_options value of this iterator value (interestingly p15 quoted below can be read to apply here as well, but 31.12.12.1 [fs.class.rec.dir.itr.general] p2 seems to be very prohibitive and LWG 2708 is going to solve this special case as well).
(3) We easily simplify the wording of p3 as shown below, because the "scope" of the introductory new paragraph includes the Postconditions: element. But similar to the arguments provided in (1) we don't wish to introduce merge conflicts with the LWG 2708 wording and therefore intentionally leave any changes of p3 to that issue. ]
recursive_directory_iterator() noexcept;-1- Effects: Constructs the end iterator.
explicit recursive_directory_iterator(const path& p); recursive_directory_iterator(const path& p, directory_options options); recursive_directory_iterator(const path& p, directory_options options, error_code& ec); recursive_directory_iterator(const path& p, error_code& ec);[…]-?- For the signatures with no parameter options, let options be directory_options::none.
-2- Effects: Constructs an iterator representing the first entry in the directory to which p resolves, if any; otherwise, the end iterator. However, if(options & directory_options::skip_permission_denied) != directory_options::noneand construction encounters an error indicating that permission to access p is denied, constructs the end iterator and does not report an error.
-3- Postconditions: options() == options for the signatures with a directory_options argument, otherwise options() == directory_options::none. […]directory_options options() const;-15- Returns: The value of the argument passed to the constructor for the options parameter, if present, otherwise directory_options::none.
-16- Throws: Nothing.
Section: 4.1.2 [intro.abstract] Status: New Submitter: Jens Maurer Opened: 2022-02-03 Last modified: 2022-03-04
Priority: 3
View all issues with New status.
Discussion:
4.1.2 [intro.abstract] p6.2 should be amended to say that filesystem operations such as mkdir are observable behaviour. Any resolution would need CWG review.
[2022-03-04; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 30.3.1.4 [locale.members] Status: New Submitter: Hubert Tong Opened: 2022-02-12 Last modified: 2022-03-04
Priority: 2
View all other issues in [locale.members].
View all issues with New status.
Discussion:
LWG 2394 removed the only text in the wording that requires that the name of a locale is usable for constructing further locales.
The relevant notes from the wiki appear to make it seem like LWG thought the change was editorial. Perhaps the resolution was motivated by a different defect than the one that led to the change? Namely,explicit locale(const char* std_name);
is said to use "standard C locale names".
There is no LC_MESSAGES in standard C. Thus, it is a question whether the aforementioned constructor should be able to consume names produced by the C++ implementation. See also the use of the name of the C++ locale in locale::global() for use with setlocale. The following would restore the equivalence of locales that have the same name. It also addresses the suitability of the name for use with setlocale and the locale(const char*) constructor as a matter of QoI.[2022-03-04; Reflector poll]
Set priority to 2 after reflector poll.
Proposed resolution:
This wording is relative to N4901.
Modify 30.3.1.4 [locale.members] as indicated:
string name() const;-5- Returns: The name of *this, if it has one; otherwise, the string "*".
-?- Remarks: Two locales have identical names only if their facets have identical virtual function semantics. -?- Recommended practice: The name of a locale that has a name should be such that setlocale(LC_ALL, name().c_str()) returns a non-null pointer. [Note 1: With such a name locale(name().c_str()) succeeds and does not throw runtime_error. — end note]
Section: 31.5.2.6 [ios.base.storage] Status: New Submitter: Jiang An Opened: 2022-02-14 Last modified: 2022-03-04
Priority: 4
View all other issues in [ios.base.storage].
View all issues with New status.
Discussion:
Currently 31.5.2.6 [ios.base.storage] p5 and p8 say "On failure, a valid long&/void*& initialized to 0". Such wording seems wrong, because a long&/void*& variable or return value can't be initialized with 0. And the values of referenced objects may be underspecified, because an implementation may reuse the same long/void* objects on failure, and thus it's insufficient to specify the initial values of these objects only.
[2022-03-04; Reflector poll]
Set priority to 4 after reflector poll.
Proposed resolution:
This wording is relative to N4901.
Modify 31.5.2.6 [ios.base.storage] as indicated:
long& iword(int idx);-3- […]
-4- […] -5- Returns: On success iarray[idx]. On failure, anvalidlvalue of type long&with value 0Linitialized to 0.void*& pword(int idx);-6- […]
-7- […] -8- Returns: On success parray[idx]. On failure, anvalidlvalue of type void*&with a null pointer valueinitialized to 0. -9- Remarks: After a subsequent call to pword(int) for the same object, the earlier return value may no longer be valid.
Section: 29.11.5.1 [time.zone.overview] Status: New Submitter: Jiang An Opened: 2022-02-23 Last modified: 2022-03-04
Priority: 4
View all issues with New status.
Discussion:
In 29.11.5.1 [time.zone.overview], only defaulted move constructor and move assignment operator for std::chrono::time_zone are shown, other constructors are said to be "unspecified additional constructors". Presumably the intent is that the default constructor is not declared (suppressed) and the copy constructor is implicitly deleted, but it is not clear if they are not "unspecified additional constructors" and hence implicitly specified.
On the other hand, the defaulted definitions of move functions bring almost no specification, as no exposition only member is shown. So it is unspecified whether these functions are deleted, trivial, constexpr, or noexcept. Perhaps we want these functions to be non-deleted and noexcept, while triviality and constexpr-ness should be left unspecified.[2022-03-04; Reflector poll]
Set priority to 4 after reflector poll.
Proposed resolution:
Section: 26.6.6 [range.istream] Status: LEWG Submitter: Barry Revzin Opened: 2022-02-24 Last modified: 2022-11-10
Priority: 3
View all issues with LEWG status.
Discussion:
The following is rejected by libstdc++:
#include <ranges> void call(std::ranges::istream_view<int>& v);
The error is quite cryptic, but ultimately the issue is not including <istream>. I think this currently isn't required to work, so the fact that it does not work is conforming. But should it be required to work?
I think either this should work or we should add a note to 26.6.6 [range.istream] about the include.[2022-03-04; Reflector poll]
Set priority to 3 after reflector poll.
[Kona 2022-11-08; discussed at joint LWG/SG9 session. Send to LEWG (with suggested options)]
Proposed resolution:
Section: 22.10.17.4.3 [func.wrap.move.ctor] Status: New Submitter: Zhihao Yuan Opened: 2022-02-27 Last modified: 2022-03-04
Priority: 2
View other active issues in [func.wrap.move.ctor].
View all other issues in [func.wrap.move.ctor].
View all issues with New status.
Discussion:
The following test compiles and holds:
struct X
{
int operator()() & { return 1; } // #1
int operator()() && { return 2; } // #2
};
using T = move_only_function<int()>;
assert(T(X{})() == 1);
In other words, #2 is never used. But if you really remove #2, the code doesn't compile.
The change was introduced between P0288R5 and P0288R6, with an intention (assumed) to require move_only_function<R(Args...) cv> erasing callable objects with operator() without ref-qualifiers. But the actual outcome outlawed programs that were valid when using std::function. It also outlaws future programs such as
T x = [captures](this auto&) { return ...; }
where declaring this auto&& without forwarding only to satisfy move_only_function's constraints would be strange.
[2022-03-04; Reflector poll]
Set priority to 2 after reflector poll. Probably needs a paper for LEWG.
Proposed resolution:
This wording is relative to N4901.
Modify 22.10.17.4.3 [func.wrap.move.ctor] as indicated:
template<class VT> static constexpr bool is-callable-from = see below;-1- If noex is true, is-callable-from<VT> is equal to:
is_nothrow_invocable_r_v<R, VT cv ref, ArgTypes...> &&is_nothrow_invocable_r_v<R, VT inv-quals, ArgTypes...>Otherwise, is-callable-from<VT> is equal to:
is_invocable_r_v<R, VT cv ref, ArgTypes...> &&is_invocable_r_v<R, VT inv-quals, ArgTypes...>
Section: 20.4.1 [mem.res.syn], 20.2.2 [memory.syn] Status: New Submitter: Jiang An Opened: 2022-02-28 Last modified: 2022-05-17
Priority: 4
View all issues with New status.
Discussion:
The issue reflected in LWG 3679 is not limited to <ranges> and std::ranges::istream_view.
Example:
#include <vector>
// some standard headers other than <memory> and <memory_resource>
template<class T> my_ator {/*definition, meeting the requirements of Cpp17Allocator*/};
int main()
{
std::vector<int> v1; // Generally works. Is this guaranteed?
std::pmr::vector<int> v2; // Usually fails to work on libstdc++. Is this intendedly permitted??
std::vector<int, my_ator<int>> v3; // Generally works. Is this guaranteed?
}
Currently libstdc++ only provides forward declarations of std::pmr::polymorphic_allocator in headers of standard allocator-aware containers, which means that users are required to include both <memory_resource> and <vector> in order to create a std::pmr::vector<T> object. If libstdc++ is technically conforming here, one may say the definition of std::allocator is also not guaranteed to be available in these headers, so <memory> is required to be included together with such a header to make standard-container<T> work.
Furthermore, the specification of allocator-aware containers are heavily dependent on std::allocator_traits. If these containers are not guaranteed to work when the definition of std::allocator_traits is not available, and the definition of std::allocator_traits is not guaranteed to be provided in headers of these containers, then users are effectively always required to include <memory> in order to create a container object, even if they are using their own allocators.[2022-05-17; Reflector poll]
Set priority to 4 after reflector poll.
Proposed resolution:
Section: 16.4.4.6.1 [allocator.requirements.general] Status: New Submitter: Jiang An Opened: 2022-03-18 Last modified: 2022-05-17
Priority: 3
View all other issues in [allocator.requirements.general].
View all issues with New status.
Discussion:
Currently (at least since P0593R6), the allocate function of an allocator is required to create an array of elements (16.4.4.6.1 [allocator.requirements.general]), which means the allocated storage must be properly aligned (otherwise the array of requested size can't be created). However, according to paragraph 12 it is also allowed that "the allocator also may silently ignore the requested alignment".
IMO the allowance is contradictory and should be removed.[2022-05-17; Reflector poll]
Set priority to 3 after reflector poll.
"You can allocate from such an allocator but you can't construct objects in the allocated memory without manually checking the alignment of the returned pointer. That doesn't seem useful in practice."
"maybe even NAD/LEWG? I can't see us declaring most in-the-wild allocators to no longer be allocators. If anything, a design change is necessary to have a protocol for allocators to declare proper support for overalignment and for allocator_traits to implement such support "manually" for older allocators."
Proposed resolution:
Section: 20.2.10.2 [allocator.members] Status: New Submitter: Jiang An Opened: 2022-03-22 Last modified: 2022-05-17
Priority: 3
View all other issues in [allocator.members].
View all issues with New status.
Discussion:
std::allocator<T>::allocate_at_least is a constexpr function that allocates memory during constant evaluation, but its restrictions is not clear. Presumably the restrictions are same as those of std::allocator<T>::allocate, and we should specify allocate_at_least in term of allocate.
The MSVC STL implementation returns allocation_result<T*>{allocate(n), n} now. Perhaps we should adopt this strategy for constant evaluation to avoid additional mechanism in the compiler.[2022-05-17; Reflector poll]
Set priority to 3 after reflector poll. Suggestion to fix this in Core instead.
Proposed resolution:
This wording is relative to N4910.
Modify 20.2.10.2 [allocator.members] as indicated:
[[nodiscard]] constexpr allocation_result<T*> allocate_at_least(size_t n);-6- Mandates: T is not an incomplete type (6.8.1 [basic.types.general]).
-7- Returns: allocation_result<T*>{ptr, count}, where ptr is a pointer to the initial element of an array of count T and count ≥ n. -8- Throws: bad_array_new_length if numeric_limits<size_t>::max() / sizeof(T) < n, or bad_alloc if the storage cannot be obtained. -9- Remarks: The storage for the array is obtained by calling ::operator new, but it is unspecified when or how often this function is called. This function starts the lifetime of the array object, but not that of any of the array elements. This function returns allocation_result<T*>{allocate(n), n} within the evaluation of a core constant expression.
Section: 26.7.16.2 [range.lazy.split.view] Status: New Submitter: Konstantin Varlamov Opened: 2022-03-23 Last modified: 2022-05-17
Priority: 3
View other active issues in [range.lazy.split.view].
View all other issues in [range.lazy.split.view].
View all issues with New status.
Discussion:
In lazy_split_view, the deduction guide that accepts two arbitrary types wraps the arguments in views::all_t (26.7.16.2 [range.lazy.split.view]):
template<class R, class P> lazy_split_view(R&&, P&&) -> lazy_split_view<views::all_t<R>, views::all_t<P>>;
When trying to use an input_range as the input, lazy_split_view requires the pattern type to satisfy the exposition-only concept tiny-range. Trying to use CTAD with an input_range and a tiny-range as arguments results in a compiler error, as demonstrated in the demo link:
// Assuming `InputRange` and `TinyRange` are valid types satisfying the
// corresponding concepts.
std::ranges::lazy_split_view view{InputRange(), TinyRange()}; // Compiler error
The underlying reason is that tiny-range requires the given type to contain a static member function size() that returns a number <=1 (26.7.16.2 [range.lazy.split.view]):
template<class R>
concept tiny-range = // exposition only
sized_range<R> &&
requires { typename require-constant<remove_reference_t<R>::size()>; } &&
(remove_reference_t<R>::size() <= 1);
However, when given a range, views::all_t wraps the type in a ranges::owning_view. owning_view doesn't satisfy tiny-range for any template parameter because it never contains the static size() function required by the concept.
A general resolution might be modifying owning_view so that it satisfies tiny-range when the given type is a tiny-range (that would require moving the tiny-range concept from 26.7.16.2 [range.lazy.split.view] to 26.5.2 [range.utility.helpers]). A more localized solution can be to change the deduction guide in lazy_split_view to avoid wrapping a type satisfying tiny-range in views::all_t.[2022-05-17; Reflector poll]
Set priority to 3 after reflector poll. One vote for NAD.
Proposed resolution:
This wording is relative to N4910.
Modify 26.5.2 [range.utility.helpers] as indicated:
[Drafting note: This change effectively just moves the definitions of require-constant and tiny-range from 26.7.16.2 [range.lazy.split.view] to 26.5.2 [range.utility.helpers].]
[…]
template<class T, class U>
concept different-from = // exposition only
!same_as<remove_cvref_t<T>, remove_cvref_t<U>>;
template<auto> struct require-constant; // exposition only
template<class R>
concept tiny-range = // exposition only
sized_range<R> &&
requires { typename require-constant<remove_reference_t<R>::size()>; } &&
(remove_reference_t<R>::size() <= 1);
Modify 26.7.6.3 [range.owning.view], class template owning_view synopsis, as indicated:
[…]
constexpr static auto size() requires tiny-range<R>
{ return R::size(); }
constexpr auto size() requires sized_range<R>
{ return ranges::size(r_); }
constexpr auto size() const requires sized_range<const R>
{ return ranges::size(r_); }
[…]
Modify 26.7.16.2 [range.lazy.split.view], class template lazy_split_view synopsis, as indicated:
[Drafting note: This change effectively just moves the definitions of require-constant and tiny-range from 26.7.16.2 [range.lazy.split.view] to 26.5.2 [range.utility.helpers].]
namespace std::ranges {
template<auto> struct require-constant; // exposition only
template<class R>
concept tiny-range = // exposition only
sized_range<R> &&
requires { typename require-constant<remove_reference_t<R>::size()>; } &&
(remove_reference_t<R>::size() <= 1);
template<input_range V, forward_range Pattern>
requires view<V> && view<Pattern> &&
indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
(forward_range<V> || tiny-range<Pattern>)
class lazy_split_view : public view_interface<lazy_split_view<V, Pattern>> {
[…]
};
[…]
}
Section: 26.7.16.3 [range.lazy.split.outer], 26.7.16.5 [range.lazy.split.inner] Status: New Submitter: Konstantin Varlamov Opened: 2022-03-23 Last modified: 2022-05-17
Priority: 3
View other active issues in [range.lazy.split.outer].
View all other issues in [range.lazy.split.outer].
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Discussion:
The internal iterator types outer-iterator and inner-iterator of lazy_split_view are default-constructible, but trying to compare a default-constructed instance of either of these classes to std::default_sentinel results in null pointer dereference (and, in all likelihood, a crash), as demonstrated in this demo link:
// Assuming `OuterIter` is an alias for `outer-iterator` of // some `lazy_split_view` instantiation. OuterIter o; o == std::default_sentinel; // Null pointer dereference InnerIter i; // Similar to `OuterIter` above. i == std::default_sentinel; // Null pointer dereference
This is due to unchecked pointer access in the implementation of outer-iterator (26.7.16.3 [range.lazy.split.outer] p8):
return x.current == ranges::end(x.parent_->base_) && !x.trailing_empty_;
(parent_ is null for a default-constructed iterator x, making the access to base_ invalid)
And similarly for inner-iterator (26.7.16.5 [range.lazy.split.inner] p7):
auto [pcur, pend] = subrange{x.i_.parent_->pattern_};
(For a default-constructed inner-iterator x, i_ is a default-constructed outer-iterator member variable and i_.parent_ is null, making the access to pattern_ invalid)
It seems a reasonable expectation for users to expect comparing a default-constructed iterator to std::default_sentinel to be a well-defined operation that returns true. Alternatively, the corresponding operator== functions should add a non-normative note stating that the iterator cannot be default-constructed.[2022-05-17; Reflector poll]
Set priority to 3 after reflector poll. Three votes for NAD.
Proposed resolution:
This wording is relative to N4910.
Modify 26.7.16.3 [range.lazy.split.outer] as indicated:
friend constexpr bool operator==(const outer-iterator& x, default_sentinel_t);-8- Effects: Equivalent to:
if (!x.parent_) return true; return x.current == ranges::end(x.parent_->base_) && !x.trailing_empty_;
Modify 26.7.16.5 [range.lazy.split.inner], as indicated:
friend constexpr bool operator==(const inner-iterator& x, default_sentinel_t);-7- Effects: Equivalent to:
if (!x.i_.parent_) return true; auto [pcur, pend] = subrange{x.i_.parent_->pattern_}; […]
Section: 22.8.4 [expected.bad], 22.8.5 [expected.bad.void], 22.8.6.6 [expected.object.obs], 22.8.7.6 [expected.void.obs] Status: New Submitter: Jiang An Opened: 2022-03-24 Last modified: 2023-05-25
Priority: 2
View all issues with New status.
Discussion:
The move constructor and the move assignment operator of standard exception types are not covered by 17.9.3 [exception]/2, and thus it is currently effectively unspecified whether these move functions of std::bad_expected_access<void> are noexcept. Furthermore, std::bad_expected_access<void> has protected special member functions, which overrides (or conflicts with?) the general rule in 17.9.3 [exception]/2.
The primary template std::bad_expected_access<E> stores an E object, and copying the stored object may throw an exception. Is it intended that the copy functions of std::bad_expected_access<E> are noexcept while those of E are not? When the copy happens because a std::bad_expected_access<E> is caught by value, if the copy constructor of E throws, std::terminate is called no matter whether that of std::bad_expected_access<E> is noexcept. But std::bad_expected_access<E> may be copied/moved in other circumstances. I think the move constructor and the move assignment operator of a standard exception type should be specified to be public and noexcept when they exist, although sometimes whether they exist may be unspecified. And the move functions should also propagate the result of what() when the source and target have the same dynamic type, except that they can leave the result of what() from the source valid but unspecified. Related to this, std::expected<T, E>::value overloads are specified to throw std::bad_expected_access<std::decay_t<E>> when an E is contained. However, it seems that the copy constructor of std::bad_expected_access is implicitly deleted if E is move-only, so the throw-expression is ill-formed. Is it intended that std::expected<T, E>::value is ill-formed in such cases?[2022-05-17; Reflector poll]
Set priority to 2 after reflector poll.
[2023-05-25; Jonathan comments]
The last part was clarified by LWG 3843, confirming that
value() is ill-formed for move-only E.
Proposed resolution:
Section: 30.4.3.2.3 [facet.num.get.virtuals] Status: New Submitter: Hubert Tong Opened: 2022-03-28 Last modified: 2022-05-17
Priority: 3
View other active issues in [facet.num.get.virtuals].
View all other issues in [facet.num.get.virtuals].
View all issues with New status.
Discussion:
30.4.3.2.3 [facet.num.get.virtuals] stage 3 specifies that "by the rules of" various strto* functions, sequences of chars are converted to a numeric value (producing the "converted value" that is referred to).
It then goes on to specify various error cases when the "field represents a value" that is outside the range of representable values. Clearly, the value that would be returned from the appropriate strto* function is not outside the range of representable values for long long, unsigned long long, float, double, and long double; therefore (unless if we expect no range-related errors for those types), the field "represents a value" other than the "converted value". Issue 1: It is too subtle to have two distinct values without calling more attention to them by giving them names aside from the prose descriptions. If the field "represents" a value other than the value that would be returned from the appropriate strto* function, then what value does the field "represent"? Note that, strictly speaking, it is the process that results in the converted value that the wording says is obtained "by the rules of" the strto* functions, which is not the same thing as saying that the value represented is interpreted "by the rules of" the strto* functions. If the field "represents" the mathematical value, then for unsigned integer types, all negative values cannot be represented. This does not match existing practice. If negative integer values are interpreted using the rules of the strto* functions by obtaining the magnitude and then having it "negated (in the return type)", then the return type of strtoull is unsigned long long, meaning (where unsigned long long is 64-bit) that "-18446744073709551615" is 1 (even when converting to unsigned integer types of less width). That does not match existing practice. It is also worth noting that negating in the return type does not work well if the magnitude is not representable as a positive value in the return type (e.g., for signed integer types and their most negative representable values). Issue 2: The effect of the minus sign with respect to unsigned integer types can reasonably be interpreted in ways that do not match existing practice (and are presumably unintended). The interpretation that works better for unsigned long long does not work as well for signed long long. If the field does not "represent" the mathematical value, then for floating-point types, it is unclear whether the minus sign takes effect before or after any possible rounding. For literals, the minus sign takes effect after rounding. Issue 3: The effect of the minus sign with respect to floating-point types is unclear. Lastly, for floating-point types with signed infinities, there are no finite values outside the range of representable values; therefore, conversions of all finite values to such types are specified to "succeed". That does not match existing implementation practice. Issue 4: The conditions for identifying range-related errors for conversions to floating-point types do not match the conditions that constitute overflow for floating-point types. There is implementation divergence: libc++ appears to check for floating-point overflow; libstdc++ appears to check for infinities.[2022-05-17; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 16.4.2.2 [contents], 22.4.6 [tuple.apply] Status: New Submitter: Jiang An Opened: 2022-04-06 Last modified: 2022-05-17
Priority: 3
View all other issues in [contents].
View all issues with New status.
Discussion:
Currently it is not clear in 16.4.2.2 [contents]/3 whether all possible overloads in the standard library are considered to be found "in the context of D". As a result, it seems underspecified whether a certain std::get overload is found by std::tuple_cat, std::make_from_tuple, std::apply, or exposition-only concept pair-like or has-tuple-element.
There is implementation divergence: MSVC STL's std::make_from_tuple accepts std::ranges::subrange, but libstdc++'s doesn't, which is originally discussed in GCC bug #102301. IMO std::get overloads need some special rules: when referred by tuple-like facilities, overloads for std::variant should be excluded (or at least leave whether it's found unspecified), and all other overloads should be found; and the opposite rule should be used when referred in 22.6 [variant].[2022-04-25; Jiang An comments and provides wording]
Currently this program is accepted when using MSVC STL and libstdc++, although the acception seems unintended and problematic.
#include <variant>
#include <span>
#include <ranges>
struct Foo : std::variant<int, long> {};
template<>
struct std::tuple_element<0, Foo> { using type = int; };
template<>
struct std::tuple_element<1, Foo> { using type = long; };
template<>
struct std::tuple_size<Foo> : std::integral_constant<std::size_t, 2> {};
constexpr auto bad_keys = std::span<Foo>{} | std::views::values;
int main() {} // COMPILE-ONLY
[2022-05-17; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4910.
Modify 16.4.2.2 [contents] as indicated:
[…]
-3- Whenever an unqualified name other than swap is used in the specification of a declaration D in Clause 17 [support] through Clause 33 [thread] or Annex D [depr], its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context of D. [Note 1: Argument-dependent lookup is not performed. — end note] Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of D. [Example 1: The reference to is_array_v in the specification of std::to_array (24.3.7.6 [array.creation]) refers to ::std::is_array_v. — end example] [Note 2: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note] The meaning of the unqualified name swap is established in an overload resolution context for swappable values (16.4.4.3 [swappable.requirements]). Certain entities in the standard library are specified to select tuple-like get function templates. An implementation shall behave as if every tuple-like get function template is found in the definition of such an entity. Furthermore, an implementation shall ensure that no get function template that is not tuple-like is found in the definition of such an entity.
Add to the end of 22.3.4 [pair.astuple], 22.4.8 [tuple.elem], 24.3.7.7 [array.tuple], and 26.5.4.3 [range.subrange.access] as indicated:
The get function templates specified in this section are tuple-like (16.4.2.2 [contents]).
Modify 22.4.6 [tuple.apply] as indicated:
template<class F, class Tuple> constexpr decltype(auto) apply(F&& f, Tuple&& t);-1- Effects: Given the exposition-only function:
namespace std { template<class F, class Tuple, size_t... I> constexpr decltype(auto) apply-impl(F&& f, Tuple&& t, index_sequence<I...>) { // exposition only return INVOKE(std::forward<F>(f), get<I>(std::forward<Tuple>(t))...); // see 22.10.4 [func.require] } }Equivalent to:
return apply-impl(std::forward<F>(f), std::forward<Tuple>(t), make_index_sequence<tuple_size_v<remove_reference_t<Tuple>>>{});-?- Remarks: apply-impl selects tuple-like get function templates.
template<class T, class Tuple> constexpr T make_from_tuple(Tuple&& t);-2- Mandates: If tuple_size_v<remove_reference_t<Tuple>> is 1, then reference_constructs_from_temporary_v<T, decltype(get<0>(declval<Tuple>()))> is false.
-3- Effects: Given the exposition-only function:namespace std { template<class T, class Tuple, size_t... I> requires is_constructible_v<T, decltype(get<I>(declval<Tuple>()))...> constexpr T make-from-tuple-impl(Tuple&& t, index_sequence<I...>) { // exposition only return T(get<I>(std::forward<Tuple>(t))...); } }Equivalent to:
return make-from-tuple-impl<T>( std::forward<Tuple>(t), make_index_sequence<tuple_size_v<remove_reference_t<Tuple>>>{});[…]
-?- Remarks: make-from-tuple-impl selects tuple-like get function templates.
Add at the end of 26.5.4.1 [range.subrange.general] (after the synopsis) as indicated:
[Drafting note: Although IIUC pair-like is not needed to handle array and subrange.]
[…]
-?- Remarks: pair-like selects tuple-like get function templates.
Add after the synopsis of 26.7.22.2 [range.elements.view] as indicated:
[…]
-?- Remarks: has-tuple-element selects tuple-like get function templates. […]
Section: 24.2.7.1 [associative.reqmts.general], 24.2.8.1 [unord.req.general] Status: New Submitter: Jens Maurer Opened: 2022-04-19 Last modified: 2022-05-17
Priority: 3
View other active issues in [associative.reqmts.general].
View all other issues in [associative.reqmts.general].
View all issues with New status.
Discussion:
Keys for elements of associative containers are presented as const subobjects, preventing their modification by user code according to 9.2.9.2 [dcl.type.cv] p4.
However, that does not prevent those keys to be transparently replaced, for example viastd::map<int, int> map; map.emplace(1, 2); using KT = std::map<int, int>::key_type; auto it = map.begin(); it->first.~KT(); new (const_cast<int*>(&it->first)) KT(3);
This, of course, breaks the ordering of the keys, and should be undefined behavior.
Related issue: CWG 2514.[2022-05-17; Reflector poll]
Set priority to 3 after reflector poll. One vote for NAD.
Proposed resolution:
This wording is relative to N4910.
Modify 24.2.7.1 [associative.reqmts.general] as indicated:
-5- For set and multiset the value type is the same as the key type. For map and multimap it is equal to pair<const Key, T>. Ending the lifetime of the key subobject of a container element by means other than invoking a member function of the container results in undefined behavior.
Modify 24.2.8.1 [unord.req.general] as indicated:
-7- For unordered_set and unordered_multiset the value type is the same as the key type. For unordered_map and unordered_multimap it is pair<const Key, T>. Ending the lifetime of the key subobject of a container element by means other than invoking a member function of the container results in undefined behavior.
Section: 28.7 [c.math] Status: New Submitter: Jiang An Opened: 2022-04-28 Last modified: 2022-05-17
Priority: 2
View all other issues in [c.math].
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Discussion:
IIUC LWG 3234 will be resolved by the recently approved paper P1467R9. While considering adding the newly required overloads of math special functions to MSVC STL, I found that it may be more convenient to implement the whole overload set as a single function template.
However, it's unclear for me whether the every "overload for each cv-unqualified floating-point type", or every currently separately shown overloads in the synopsis of <cmath>, is required to be a separated function. As discussed in microsoft/STL#1335, if there were only a separated double overload (usually comes from the C standard library) and a fused template overload, calling the overload set with {} would be accepted, which is definitely ambiguous when there are separated float/double/long double overloads. I think it may be better to allow implementations to arbitrarily fuse the required overloads.[2022-05-17; Reflector poll]
Set priority to 2 after reflector poll. One vote for NAD.
Proposed resolution:
Section: 23.2.2 [char.traits.require] Status: New Submitter: Jiang An Opened: 2022-05-04 Last modified: 2022-05-17
Priority: 4
View all other issues in [char.traits.require].
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Discussion:
MSVC STL's implementation of the std::quoted overload for const charT* calculates the length of the NTCTS and stores the result within the return value. Because the returned value may be output by std::basic_ostream specializations with different traits_types, this strategy can be conforming only if all possible traits_type::length functions for the same char_type have equivalent return values.
It seems not clear whether traits_type::length should be customizable. In a related PR, Stephan T. Lavavej said :I argue that you've found a defect in the char_traits specification — it should say that whatever eq() does, it should consider charT() to be distinct from all other values, which aligns with the common understanding of how null-terminated strings behave.
The original implementation and the char_traits::length change both handle arbitrary character types — the only difference would be for custom char_traits that consider null terminators to be equal to other values, which I have never seen used in practice (e.g. case-insensitive traits don't do this).
If it is decided that traits_type::length is customizable, then the implementation in MSVC STL should be fixed. Otherwise, we should explicitly require in 23.2.2 [char.traits.require] that whenever c is not "equal" to char_type(), traits_type::eq(c, char_type()) is false.
However, equivalence of two charT values seems not easy to specify, as there may be no operator== for charT, or the operator== behaves pathologically. IMO possible way may beusing the equivalence relation of value representations, or
using == and only imposing the requirement when char_type is an encoded character type (or more generally, a scalar type, while treating an enum type as its underlying type).
[2022-05-17; Reflector poll]
Set priority to 4 after reflector poll. One vote for NAD.
Proposed resolution:
Section: 31.2.2 [stream.types] Status: New Submitter: Jiang An Opened: 2022-05-07 Last modified: 2022-05-17
Priority: 3
View all issues with New status.
Discussion:
Raised from editorial issue #5240.
The phrase "signed basic integral types" in 31.2.2 [stream.types] has been present since C++98 but never defined. It is unclear whether "basic integral types" are "standard integer types" or "integer types" (including extended integer types). As std::streamoff should be wide enough to represent the largest possible file size, and std::uintmax_t is used as the return type of std::filesystem::file_size, we should not disallow std::streamoff to be an extended integer type which may be wider than long long. On the other hand, as std::size_t and std::ptrdiff_t have already been allowed to be extended integer types, std::streamsize should also be allowed to be an extended integer type for consistency. So I think we should just use "signed integer types" instead of "signed basic integral types" in 31.2.2 [stream.types].[2022-05-17; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 21.3.5.4 [meta.unary.prop] Status: New Submitter: Jiang An Opened: 2022-05-10 Last modified: 2022-05-17
Priority: 3
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
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Discussion:
std::reference_constructs_from_temporary and std::reference_converts_from_temporary are only useful when T is a reference type, and a reference type is always complete. Whenever T is an incomplete object type, it's clear that these traits inherit from std::false_type, without any further detection which may result in UB. However, when T is X& or X&& where X is an incomplete object type, UB may be needed because of the potentially problematic detection in std::is_constructible or std::is_convertible.
I'm not sure whether the general rule in 21.3.2 [meta.rqmts]/5 affects these cases.[2022-05-17; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4910.
Modify 21.3.3 [meta.type.synop], Table 46 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:
Table 46: Type property predicates [tab:meta.unary.prop] Template Condition Preconditions … template<class T, class U>
struct reference_constructs_from_temporary;conjunction_v<is_reference<T>, is_constructible<T, U>> is
true, and the initialization T t(VAL<U>); binds t to a
temporary object whose lifetime is extended (6.7.7 [class.temporary]).If T is a reference type, remove_reference_t<T>
and UTshall beacomplete types,
cv void, oranarrays of unknown bound.template<class T, class U>
struct reference_converts_from_temporary;conjunction_v<is_reference<T>, is_convertible<U, T>> is
true, and the initialization T t = VAL<U>; binds t to a
temporary object whose lifetime is extended (6.7.7 [class.temporary]).If T is a reference type, remove_reference_t<T>
and U shall be complete types,
cv void, or arrays of unknown bound.
Section: 31.12.6.5.11 [fs.path.gen] Status: New Submitter: Nicole Mazzuca Opened: 2022-05-12 Last modified: 2022-05-17
Priority: 3
View all other issues in [fs.path.gen].
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Discussion:
As a resolution to LWG 3070, in path lexically_relative(const path& base) const, bullet 3.4 was added:
If: […] any filename in relative_path() or base.relative_path() can be interpreted as a root-name, […] returns path().
This resolution was correct when we have really weird paths like abc\X:\c, but the MSVC standard library implementation treats UNC drive-relative paths as:
\\?\C:\foo\bar = { root-name = \\?, root-directory = \, relative-path = C:\foo\bar }
If we were able to go back in time, we might have root-name = \\?\C:, but we can't make that change at that point without silently breaking users; therefore, we believe it would be best to instead change lexically_relative() to work around this issue.
There exists a related github issue. I don't yet have standard wording, but I think it would be reasonable to do something like:If relative_path().has_root_path() && base.relative_path().has_root_path(), and relative_path().root_path() == base.relative_path().root_path(), then return relative_path().lexically_relative(base.relative_path()).
[2022-05-17; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 22.14.6.3 [format.formatter.spec] Status: New Submitter: S. B. Tam Opened: 2022-05-31 Last modified: 2022-06-21
Priority: 3
View other active issues in [format.formatter.spec].
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Discussion:
Consider
#include <format>
#include <iostream>
extern char str[];
auto result = std::format("{}", str);
char str[] = "hello";
int main()
{
std::cout << result << std::endl;
}
Currently MSVC STL (as well as fmtlib when fmt::format is used instead of std::format) accepts the initializer of result, while libc++ produces an error (apparently because there's no formatter for char[]).
Should this be valid?
Daniel:
This issue is similar to LWG 3701, but not the same, because the latter wants template<size_t N> struct formatter<charT[N], charT>, while this one needs template<>struct formatter<charT[], charT> (that is, without the array bound).[2022-06-21; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 25.3.4.11 [iterator.concept.forward], 25.3.5.5 [forward.iterators] Status: New Submitter: Jiang An Opened: 2022-06-15 Last modified: 2022-07-06
Priority: 3
View all issues with New status.
Discussion:
There are two different definitions of multi-pass guarantee since P0896R4. The old one (perhaps introduced by N3066) seems less reasonable because it requires increment on rvalue iterators of class types, and the semantics of such increment is largely unspecified.
Perhaps only the new definition should be used.[2022-07-06; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 26.5.7.2 [range.utility.conv.to] Status: New Submitter: Hewill Kang Opened: 2022-06-20 Last modified: 2022-07-08
Priority: 4
View other active issues in [range.utility.conv.to].
View all other issues in [range.utility.conv.to].
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Discussion:
In bullet 1.1.4 of ranges::to, if the Container satisfies container-insertable and R models sized_range, it will first construct the Container with args... and then preallocate memory by calling c.reserve().
However, this only makes sense when c is default-initialized. If instead the size of the Container created by args... is not 0, the value passed into c.reserve() will be wrong, for example:
ranges::to<std::string>(std::views::single('o'), "hell");
The size of the string created by "hell" is already 4, whereas the size of R is only 1, which makes c.reserve(1) useless.
[2022-07-08; Reflector poll]
Set priority to 4 after reflector poll. Some suggestions for NAD.
Proposed resolution:
This wording is relative to N4910.
Modify 26.5.7.2 [range.utility.conv.to] as indicated:
template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args);-1- Returns: An object of type C constructed from the elements of r in the following manner:
(1.1) — If convertible_to<range_reference_t<R>, range_value_t<C>> is true:
(1.1.1) — If constructible_from<C, R, Args...> is true:
C(std::forward<R>(r), std::forward<Args>(args)...)(1.1.2) — Otherwise, if constructible_from<C, from_range_t, R, Args...> is true:
C(from_range, std::forward<R>(r), std::forward<Args>(args)...)(1.1.3) — Otherwise, if
(1.1.3.1) — common_range<R> is true,
(1.1.3.2) — cpp17-input-iterator<iterator_t<R>> is true, and
(1.1.3.3) — constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...> is true:
C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)(1.1.4) — Otherwise, if
(1.1.4.1) — constructible_from<C, Args...> is true, and
(1.1.4.2) — container-insertable<C, range_reference_t<R>> is true:
C c(std::forward<Args>(args)...); if constexpr (sized_range<R> && reservable-container<C>) { using ST = range_size_t<C>; using CT = common_type_t<ST, range_size_t<R>>; auto sz = ST(CT(ranges::size(c)) + CT(ranges::size(r))); c.reserve(szranges::size(r)); } ranges::copy(r, container-inserter<range_reference_t<R>>(c));(1.2) — Otherwise, if input_range<range_reference_t<R>> is true:
to<C>(r | views::transform([](auto&& elem) { return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); }), std::forward<Args>(args)...);(1.3) — Otherwise, the program is ill-formed.
Section: 25.5.1.6 [reverse.iter.elem] Status: New Submitter: Hewill Kang Opened: 2022-06-26 Last modified: 2022-07-09
Priority: 3
View all other issues in [reverse.iter.elem].
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Discussion:
When the underlying iterator is not a pointer type, reverse_iterator::operator-> returns prev(current).operator->(). However, prev only works with Cpp17BidirectionalIterator, given that C++20 bidirectional_iterator may just be Cpp17InputIterator, we shouldn't use prev here.
[2022-07-08; Reflector poll]
Set priority to 3 after reflector poll.
Suggested to use ranges::prev instead.
Proposed resolution:
This wording is relative to N4910.
Modify 25.5.1.6 [reverse.iter.elem] as indicated:
constexpr pointer operator->() const requires (is_pointer_v<Iterator> || requires(const Iterator i) { i.operator->(); });-2- Effects:
(2.1) — If Iterator is a pointer type, equivalent to: return prev(current);
(2.2) — Otherwise, equivalent to:
return prev(current).operator->();Iterator tmp = current; --tmp; return tmp.operator->();
Section: 22.4.9 [tuple.rel] Status: New Submitter: Corentin Jabot Opened: 2022-06-28 Last modified: 2022-07-08
Priority: 4
View other active issues in [tuple.rel].
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Discussion:
The specification of operator<=>(tuple, tuple) (22.4.9 [tuple.rel]) is described in terms of imaginary tuples (ttail, utail, rtail) which is a bit confusing. Indeed, It is not clear that these imaginary tuples need to respect the order of elements of u and t, nor whether the value category of the elements in these imaginary tuples can or should be conserved. It is possible to reformulate and simplify that description so that no imaginary tuple is involved.
The remark is copied from the similar wording of operator==[2022-07-08; Reflector poll]
Set priority to 4 after reflector poll.
Some votes for NAD and preference for the current wording, adding "in order"
to clarify the order of elements in rtail.
Proposed resolution:
This wording is relative to N4910.
Modify 22.4.9 [tuple.rel] as indicated:
template<class... TTypes, class... UTypes> constexpr common_comparison_category_t<synth-three-way-result<TTypes, UTypes>...> operator<=>(const tuple<TTypes...>& t, const tuple<UTypes...>& u);-4-
EffectsReturns: synth-three-way(get<i>(t), get<i>(u)) for the first i for which the result of that expression does not compare equal to 0. If no such i exists, strong_ordering::equal.Performs a lexicographical comparison between t and u. For any two zero-length tuples t and u, t <=> u returns strong_ordering::equal. Otherwise, equivalent to:if (auto c = synth-three-way(get<0>(t), get<0>(u)); c != 0) return c; return ttail <=> utail;-?- Remarks: The elementary synth-three-way(get<i>(t), get<i>(u)) expressions are evaluated in order from the zeroth index upwards. No element accesses are performed after the first invocation that results in a value that does not compare equal to 0.
where rtail for some tuple r is a tuple containing all but the first element of r.-5- [Note 1: The above definition does not require ttail (or utail) to be constructed. It might not even be possible, as t and u are not required to be copy constructible. Also, all comparison operator functions are short circuited; they do not perform element accesses beyond what is required to determine the result of the comparison. — end note]
Section: 26.2 [ranges.syn] Status: New Submitter: Jiang An Opened: 2022-06-30 Last modified: 2022-07-08
Priority: 4
View other active issues in [ranges.syn].
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Discussion:
std::ranges::subrange<int * volatile>> is weird but valid. The return type (deduced type for auto) of std::ranges::get for this type is int*, because the replacing of that (cv-unqualified) auto drops top-level cv-qualifiers. I think get is doing the right thing, and std::tuple_element_t should be consistent with the return types.
[2022-07-08; Reflector poll]
Set priority to 4 after reflector poll. "This is just contrived, especially since lots of the iterator requirements on such a type involve deprecated operations."
Proposed resolution:
This wording is relative to N4910.
Modify 26.2 [ranges.syn], header <ranges> synopsis, as indicated:
[…]
namespace std {
[…]
template<class I, class S, ranges::subrange_kind K>
struct tuple_element<0, ranges::subrange<I, S, K>> {
using type = remove_cv_t<I>;
};
template<class I, class S, ranges::subrange_kind K>
struct tuple_element<1, ranges::subrange<I, S, K>> {
using type = remove_cv_t<S>;
};
template<class I, class S, ranges::subrange_kind K>
struct tuple_element<0, const ranges::subrange<I, S, K>> {
using type = remove_cv_t<I>;
};
template<class I, class S, ranges::subrange_kind K>
struct tuple_element<1, const ranges::subrange<I, S, K>> {
using type = remove_cv_t<S>;
};
}
[…]
Section: 26.7.12.2 [range.drop.view] Status: New Submitter: Jiang An Opened: 2022-07-03 Last modified: 2022-07-17
Priority: 3
View all other issues in [range.drop.view].
View all issues with New status.
Discussion:
The bodies of both overloads of drop_view<V>::size are specified as:
const auto s = ranges::size(base_); const auto c = static_cast<decltype(s)>(count_); return s < c ? 0 : s - c;
Given the return type is specified with auto, the actual return type is the promoted type of the size type of the underlying view, which may be different from the underlying size type (e.g. if the underlying size is unsigned short).
Note that take_view always has the same size type as its underlying view. So I think the difference on the size types is an oversight. On the other hand, the const used here seems redundant and inconsistent with other parts of the standard wording, although implementations may tend to use it.[2022-07-08; Reflector poll]
Set priority to 3 after reflector poll.
"The PR is incorrect - integer-class types do not support mixed-signedess operations, so you have to cast one of the two first."
[2022-07-17; Daniel comments]
This issue should be resolved by keeping LWG 3739 and 3740 in mind.
Proposed resolution:
This wording is relative to N4910.
Modify 26.7.12.2 [range.drop.view], class template drop_view synopsis, as indicated:
[Drafting note: s and count_ usually have different types, but I think it's safe to perform comparison and subtraction, as count_ is non-negative as long as the behavior is well-defined.]
[…]
constexpr auto size() requires sized_range<V> {
const auto s = ranges::size(base_);
const auto c = static_cast<decltype(s)>(count_);
return static_cast<decltype(s)>(s < ccount_ ? 0 : s - ccount_);
}
constexpr auto size() const requires sized_range<const V> {
const auto s = ranges::size(base_);
const auto c = static_cast<decltype(s)>(count_);
return static_cast<decltype(s)>(s < ccount_ ? 0 : s - ccount_);
}
[…]
Section: 26.7.24.2 [range.zip.view], 26.7.26.2 [range.adjacent.view], 26.7.32.2 [range.cartesian.view] Status: New Submitter: Hewill Kang Opened: 2022-07-04 Last modified: 2023-02-07
Priority: 3
View all other issues in [range.zip.view].
View all issues with New status.
Discussion:
Both zip_view::iterator's (26.7.24.3 [range.zip.iterator]) and adjacent_view::iterator's (26.7.26.3 [range.adjacent.iterator]) operator* have similar Effects: elements:
return tuple-transform([](auto& i) -> decltype(auto) { return *i; }, current_);
where tuple-transform is defined as:
template<class F, class Tuple>
constexpr auto tuple-transform(F&& f, Tuple&& tuple) { // exposition only
return apply([&]<class... Ts>(Ts&&... elements) {
return tuple-or-pair<invoke_result_t<F&, Ts>...>(
invoke(f, std::forward<Ts>(elements))...
);
}, std::forward<Tuple>(tuple));
}
That is, zip_view::iterator will invoke the operator* of each iterator of Views and return a tuple containing its reference.
This is not a problem when the reference of iterators is actually the reference type. However, when the operator* returns a prvalue of non-movable type, tuple-transform will be ill-formed since there are no suitable constructors for tuple:
#include <ranges>
struct NonMovable {
NonMovable() = default;
NonMovable(NonMovable&&) = delete;
};
auto r = std::views::iota(0, 5)
| std::views::transform([](int) { return NonMovable{}; });
auto z = std::views::zip(r);
auto f = *z.begin(); // hard error
We should constrain the range_reference_t of the underlying range to be move_constructible when it is not a reference type, which also solves similar issues in zip_view::iterator and adjacent_view::iterator's operator[] and iter_move.
[2022-08-23; Reflector poll]
Set priority to 3 after reflector poll.
"The constraint should just be move_constructible."
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 26.2 [ranges.syn], header <ranges> synopsis, as indicated:
namespace std::ranges { […] // 26.7.24 [range.zip], zip view template<class Ref> concept tuple-constructible-reference = see below; // exposition only template<input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) && (tuple-constructible-reference<range_reference_t<Views>> && ...) class zip_view; […] // 26.7.26 [range.adjacent], adjacent view template<forward_range V, size_t N> requires view<V> && (N > 0) && tuple-constructible-reference<range_reference_t<V>> class adjacent_view; } […]Modify 26.7.24.2 [range.zip.view] as indicated:
namespace std::ranges { template<class Ref> concept tuple-constructible-reference = // exposition only is_reference_v<Ref> || move_constructible<Ref>; […] template<input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) && (tuple-constructible-reference<range_reference_t<Views>> && ...) class zip_view : public view_interface<zip_view<Views...>> { […] }; […] }Modify 26.7.26.2 [range.adjacent.view] as indicated:
namespace std::ranges { template<forward_range V, size_t N> requires view<V> && (N > 0) && tuple-constructible-reference<range_reference_t<V>> class adjacent_view : public view_interface<adjacent_view<V, N>> { […] }; […] }
[2022-09-25; Hewill provides improved wording]
Proposed resolution:
This wording is relative to N4917.
Modify 26.2 [ranges.syn], header <ranges> synopsis, as indicated:
namespace std::ranges {
[…]
template<class R>
concept has-tuplable-ref = // exposition only
move_constructible<range_reference_t<R>>;
// 26.7.24 [range.zip], zip view
template<input_range... Views>
requires (view<Views> && ...) && (sizeof...(Views) > 0) &&
(has-tuplable-ref<Views> && ...)
class zip_view;
[…]
// 26.7.26 [range.adjacent], adjacent view
template<forward_range V, size_t N>
requires view<V> && (N > 0) && has-tuplable-ref<V>
class adjacent_view;
[…]
// 26.7.32 [range.cartesian], cartesian product view
template<input_range First, forward_range... Vs>
requires (view<First> && ... && view<Vs>) &&
(has-tuplable-ref<First> && ... && has-tuplable-ref<Vs>)
class cartesian_product_view;
[…]
}
Modify 26.7.24.2 [range.zip.view] as indicated:
namespace std::ranges {
[…]
template<input_range... Views>
requires (view<Views> && ...) && (sizeof...(Views) > 0) &&
(has-tuplable-ref<Views> && ...)
class zip_view : public view_interface<zip_view<Views...>> {
[…]
};
}
Modify 26.7.24.3 [range.zip.iterator] as indicated:
namespace std::ranges {
[…]
template<input_range... Views>
requires (view<Views> && ...) && (sizeof...(Views) > 0) &&
(has-tuplable-ref<Views> && ...)
template<bool Const>
class zip_view<Views...>::iterator {
[…]
};
}
Modify 26.7.24.4 [range.zip.sentinel] as indicated:
namespace std::ranges {
template<input_range... Views>
requires (view<Views> && ...) && (sizeof...(Views) > 0) &&
(has-tuplable-ref<Views> && ...)
template<bool Const>
class zip_view<Views...>::sentinel {
[…]
};
}
Modify 26.7.26.2 [range.adjacent.view] as indicated:
namespace std::ranges {
template<forward_range V, size_t N>
requires view<V> && (N > 0) && has-tuplable-ref<V>
class adjacent_view : public view_interface<adjacent_view<V, N>> {
[…]
};
}
Modify 26.7.26.3 [range.adjacent.iterator] as indicated:
namespace std::ranges {
template<forward_range V, size_t N>
requires view<V> && (N > 0) && has-tuplable-ref<V>
template<bool Const>
class adjacent_view<V, N>::iterator {
[…]
};
}
Modify 26.7.26.4 [range.adjacent.sentinel] as indicated:
namespace std::ranges {
template<forward_range V, size_t N>
requires view<V> && (N > 0) && has-tuplable-ref<V>
template<bool Const>
class adjacent_view<V, N>::sentinel {
[…]
};
}
Modify 26.7.32.2 [range.cartesian.view] as indicated:
namespace std::ranges {
[…]
template<input_range First, forward_range... Vs>
requires (view<First> && ... && view<Vs>) &&
(has-tuplable-ref<First> && ... && has-tuplable-ref<Vs>)
class cartesian_product_view : public view_interface<cartesian_product_view<First, Vs...>> {
[…]
};
}
Modify 26.7.32.3 [range.cartesian.iterator] as indicated:
namespace std::ranges {
template<input_range First, forward_range... Vs>
requires (view<First> && ... && view<Vs>) &&
(has-tuplable-ref<First> && ... && has-tuplable-ref<Vs>)
template<bool Const>
class cartesian_product_view<First, Vs...>::iterator {
[…]
};
}
Section: 31.12.13.6 [fs.op.copy.symlink] Status: New Submitter: Nicole Mazzuca Opened: 2022-07-25 Last modified: 2022-08-23
Priority: 3
View all issues with New status.
Discussion:
The specification for copy_symlink is (31.12.13.6 [fs.op.copy.symlink]):
Effects: Equivalent to function(read_symlink(existing_symlink), new_symlink) or function(read_symlink(existing_symlink, ec), new_symlink, ec), respectively, where in each case function is create_symlink or create_directory_symlink as appropriate.
The specification for read_symlink is (31.12.13.29 [fs.op.read.symlink]):
Returns: If p resolves to a symbolic link, a path object containing the contents of that symbolic link.
And finally, the definition of a "symbolic link" is (31.12.1 [fs.general]):
A symbolic link is a type of file with the property that when the file is encountered during pathname resolution (31.12.6 [fs.class.path]), a string stored by the file is used to modify the pathname resolution.
On Unix, symlink is the only kind of symbolic link. However, on Windows, there are symbolic link files which are not symlinks (app execution aliases and junctions) — this means that read_symlink should almost certainly get the target of these files if possible. However, copy_symlink specifically requires creating a symlink, not whatever type of file was there originally. IMO, copy_symlink should require its target to be a symlink. I think the original assumption was that read_symlink would take care of that for copy_symlink; this is clearly not the case on Windows, though.
[2022-08-23; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 25.5.5.6 [common.iter.cmp], 25.5.7.5 [counted.iter.nav] Status: New Submitter: Hewill Kang Opened: 2022-08-01 Last modified: 2022-08-23
Priority: 3
View all issues with New status.
Discussion:
Both common_iterator and counted_iterator explicitly specify that the return type of their operator- is iter_difference_t<I2>, however, given that the calculated type may be iter_difference_t<I>, we should do an explicit conversion here since the latter is not necessarily implicitly convertible to the former:
#include <ranges>
struct Y;
struct X {
X(Y);
using difference_type =
#ifdef __GNUC__
std::ranges::__detail::__max_diff_type;
#elif defined(_MSC_VER)
std::_Signed128;
#endif
int& operator*() const;
X& operator++();
void operator++(int);
};
struct Y {
using difference_type = std::ptrdiff_t;
int& operator*() const;
Y& operator++();
void operator++(int);
};
int main() {
std::counted_iterator<Y> y;
return std::counted_iterator<X>(y) - y; // hard error in stdlibc++ and MSVC-STL
}
Daniel:
This issue shouldn't we voted until a decision for LWG 3749 has been made, because the first part of it overlaps with LWG 3749's second part.
[2022-08-23; Reflector poll]
Set priority to 3 after reflector poll.
"I think common_iterator should reject iterators with
integer-class difference types since it can't possibly achieve the design intent
of adapting them to Cpp17Iterators, so this issue should only affect
counted_iterator."
"If the difference types of I and I2 are different
then the operator- can't be used to model
sized_sentinel_for,
since i - i2 and i2 - i would have different types.
Providing operator- under such circumstances seems
to be of dubious value."
Proposed resolution:
This wording is relative to N4910.
Modify 25.5.5.6 [common.iter.cmp] as indicated:
template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2> requires sized_sentinel_for<S, I2> friend constexpr iter_difference_t<I2> operator-( const common_iterator& x, const common_iterator<I2, S2>& y);-5- Preconditions: x.v_.valueless_by_exception() and y.v_.valueless_by_exception() are each false.
-6- Returns: 0 if i and j are each 1, and otherwise static_cast<iter_difference_t<I2>>(get<i>(x.v_) - get<j>(y.v_)), where i is x.v_.index() and j is y.v_.index().
Modify 25.5.7.5 [counted.iter.nav] as indicated:
template<common_with<I> I2> friend constexpr iter_difference_t<I2> operator-( const counted_iterator& x, const counted_iterator<I2>& y);-13- Preconditions: x and y refer to elements of the same sequence (25.5.7.1 [counted.iterator]).
-14- Effects: Equivalent to: return static_cast<iter_difference_t<I2>>(y.length - x.length);
Section: 25.5.5 [iterators.common] Status: New Submitter: Hewill Kang Opened: 2022-08-01 Last modified: 2022-08-23
Priority: 2
View all issues with New status.
Discussion:
The partial specialization of iterator_traits for common_iterator is defined in 25.5.5.1 [common.iterator] as
template<input_iterator I, class S>
struct iterator_traits<common_iterator<I, S>> {
using iterator_concept = see below;
using iterator_category = see below;
using value_type = iter_value_t<I>;
using difference_type = iter_difference_t<I>;
using pointer = see below;
using reference = iter_reference_t<I>;
};
where difference_type is defined as iter_difference_t<I> and iterator_category is defined as at least input_iterator_tag. However, when difference_type is an integer-class type, common_iterator does not satisfy Cpp17InputIterator, which makes iterator_category incorrectly defined as input_iterator_tag.
Since the main purpose of common_iterator is to be compatible with the legacy iterator system, which is reflected in its efforts to try to provide the operations required by C++17 iterators even if the underlying iterator does not support it. We should handle this case of difference type incompatibility as well.
The proposed solution is to provide a C++17 conforming difference type by clamping the integer-class type to ptrdiff_t. Daniel:The second part of this issue provides an alternative resolution for the first part of LWG 3748 and solves the casting problem mentioned in LWG 3748 as well.
[2022-08-23; Reflector poll]
Set priority to 2 after reflector poll.
"I think common_iterator should reject iterators with
integer-class difference types since it can't possibly achieve the design intent
of adapting them to Cpp17Iterators."
"I'm not yet convinced that we need to outright reject such uses, but I'm pretty sure that we shouldn't mess with the difference type and that the PR is in the wrong direction."
Proposed resolution:
This wording is relative to N4910.
Modify 25.5.5.1 [common.iterator] as indicated:
[Drafting note: common_iterator requires iterator type I must model input_or_output_iterator which ensures that iter_difference_t<I> is a signed-integer-like type. The modification of common_iterator::operator- is to ensure that the pair of common_iterator<I, S> models sized_sentinel_for when sized_sentinel_for<I, S> is modeled for iterator type I with an integer-class difference type and its sentinel type S.]
namespace std {
template<class D>
requires is-signed-integer-like<D>
using make-cpp17-diff-t = conditional_t<signed_integral<D>, D, ptrdiff_t>; // exposition only
template<input_or_output_iterator I, sentinel_for<I> S>
requires (!same_as<I, S> && copyable<I>)
class common_iterator {
public:
[…]
template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
requires sized_sentinel_for<S, I2>
friend constexpr make-cpp17-diff-t<iter_difference_t<I2>> operator-(
const common_iterator& x, const common_iterator<I2, S2>& y);
[…]
};
template<class I, class S>
struct incrementable_traits<common_iterator<I, S>> {
using difference_type = make-cpp17-diff-t<iter_difference_t<I>>;
};
template<input_iterator I, class S>
struct iterator_traits<common_iterator<I, S>> {
using iterator_concept = see below;
using iterator_category = see below;
using value_type = iter_value_t<I>;
using difference_type = make-cpp17-diff-t<iter_difference_t<I>>;
using pointer = see below;
using reference = iter_reference_t<I>;
};
}
Modify 25.5.5.6 [common.iter.cmp] as indicated:
[Drafting note: If this issue is voted in at the same time as LWG 3748, the editor is kindly informed that the changes indicated below supersede those of the before mentioned issues first part.]
template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2> requires sized_sentinel_for<S, I2> friend constexpr make-cpp17-diff-t<iter_difference_t<I2>> operator-( const common_iterator& x, const common_iterator<I2, S2>& y);-5- Preconditions: x.v_.valueless_by_exception() and y.v_.valueless_by_exception() are each false.
-6- Returns: 0 if i and j are each 1, and otherwise static_cast<make-cpp17-diff-t<iter_difference_t<I2>>>(get<i>(x.v_) - get<j>(y.v_)), where i is x.v_.index() and j is y.v_.index().
Section: 24.3.11.3 [vector.capacity], 24.3.11.5 [vector.modifiers], 24.3.8.3 [deque.capacity], 24.3.8.4 [deque.modifiers] Status: New Submitter: Jiang An Opened: 2022-08-24 Last modified: 2022-11-06
Priority: 3
View other active issues in [vector.capacity].
View all other issues in [vector.capacity].
View all issues with New status.
Discussion:
This issue is raised from editorial issue #5776.
In order to achieve strong exception safety, some operations of std:vector and std::deque may use copy insertion for relocation of old elements, if move construction of its element type is potentially throwing and copy insertion is available. However, currently only Cpp17MoveInsertable is mentioned in many of their Preconditions (e.g. those of insert for rvalues), which seemly fails to cover the cases in which copy insertion is formally invalid but the semantic requirements of Cpp17CopyInsertable are not met. Perhaps we should create a new named requirement for these operations, which is equivalent to Cpp17CopyInsertable when !is_nothrow_move_constructible_v<T> && is_copy_constructible_v<T> is true, and equivalent to Cpp17MoveInsertable otherwise.[2022-09-23; Reflector poll]
Set priority to 3 after reflector poll.
Jonathan: I think the point (which LWG 3758 fails to explain clearly) is that today's implementations
sometimes use copy insertion when move insertion is syntactically valid, but is potentially-throwing.
But the preconditions don't require copy insertion. If vector::resize(size_type) decides to
use copy construction, because move construction might throw and the type is copy constructible (which
is implied to be permitted by the Remarks), do we require Cpp17CopyInsertable's semantic
requirement that the new value is equivalent to the one we copied? We don't say so.
tl;dr The user is trying to resize a vector and the value type is Cpp17MoveInsertable into the vector,
but the implementation decides to copy not move. What are the preconditions on the user's type?
[2022-11-06; Daniel comments]
This issue has considerable overlap with LWG 2158.
Proposed resolution:
Section: 26.6.5.3 [range.repeat.iterator], 26.7.32.3 [range.cartesian.iterator] Status: New Submitter: Hewill Kang Opened: 2022-08-27 Last modified: 2023-02-07
Priority: 3
View all issues with New status.
Discussion:
After P2259, the range adaptor' iterators only provide iterator_category member when the underlying range models forward_range, which is mainly based on the premise that all valid C++20 forward iterators meet the C++17 input iterator requirements.
However, this is not strictly correct. When the underlying range's difference_type is an integer-class type, its iterator does not conform Cpp17InputIterator. Although iterator_traits<I>::iterator_category will still deduce the correct category in this case since these iterators have no reference member, it might be misleading to provide these incorrect member types. Do we need to aggressively prohibit these iterators from providing iterator_category when their difference type is an integer-class type? The proposed resolution makes repeat_view::iterator conditionally provide iterator_category, because it explicitly mentions IOTA-DIFF-T(index-type) in the definition of difference_type, which makes it consistent with LWG 3670. It also removes the reference member type of cartesian_product_view::iterator, which prevents iterator_traits<I>::iterator_category from being aliased to its member iterator_category, so that iterator_traits<I>::iterator_category will not always be an input_iterator_tag when its difference_type is an integer-class type. This is also consistent with other range adaptors, as none of them have a reference member type.[2022-09-23; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4917.
Modify 26.6.5.3 [range.repeat.iterator] as indicated:
namespace std::ranges {
template<move_constructible W, semiregular Bound = unreachable_sentinel_t>
requires (is_object_v<W> && same_as<W, remove_cv_t<W>> &&
(is-integer-like<Bound> || same_as<Bound, unreachable_sentinel_t>))
class repeat_view<W, Bound>::iterator {
private:
using index-type = // exposition only
conditional_t<same_as<Bound, unreachable_sentinel_t>, ptrdiff_t, Bound>;
const W* value_ = nullptr; // exposition only
index-type current_ = index-type(); // exposition only
constexpr explicit iterator(const W* value, index-type b = index-type()); // exposition only
public:
using iterator_concept = random_access_iterator_tag;
using iterator_category = random_access_iterator_tag; // present only if difference_type
// is an integral type
using value_type = W;
using difference_type = conditional_t<is-signed-integer-like<index-type>,
index-type,
IOTA-DIFF-T(index-type)>;
[…]
};
}
Modify 26.7.32.3 [range.cartesian.iterator] as indicated:
namespace std::ranges { template<input_range First, forward_range... Vs> requires (view<First> && ... && view<Vs>) template<bool Const> class cartesian_product_view<First, Vs...>::iterator { public: using iterator_category = input_iterator_tag; using iterator_concept = see below; using value_type = tuple<range_value_t<maybe-const<Const, First>>, range_value_t<maybe-const<Const, Vs>>...>;using reference = tuple<range_reference_t<maybe-const<Const, First>>, range_reference_t<maybe-const<Const, Vs>>...>;using difference_type = see below; […] constexpr autoreferenceoperator[](difference_type n) const requires cartesian-product-is-random-access<Const, First, Vs...>; […] }; }[…]
constexpr autoreferenceoperator[](difference_type n) const requires cartesian-product-is-random-access<Const, First, Vs...>;-24- Effects: return *((*this) + n);
Section: 30.3.1.2.1 [locale.category], 30.4.2.5.1 [locale.codecvt.general] Status: Open Submitter: Victor Zverovich Opened: 2022-09-05 Last modified: 2023-02-19
Priority: 3
View all other issues in [locale.category].
View all issues with Open status.
Discussion:
Table [tab:locale.category.facets] includes the following two facets:
codecvt<char16_t, char8_t, mbstate_t>
codecvt<char32_t, char8_t, mbstate_t>
However, neither of those actually has anything to do with a locale and therefore it doesn't make sense to dynamically register them with std::locale. Instead they provide conversions between fixed encodings (UTF-8, UTF-16, UTF-32) that are unrelated to locale encodings other than they may happen to coincide with encodings of some locales by accident.
The issue was introduced when adding codecvt<char[16|32]_t, char, mbstate_t> in N2035 which gave no design rationale for using codecvt in the first place. Likely it was trying to do a minimal amount of changes and copied the wording for codecvt<wchar_t, char, mbstate_t> but unfortunately didn't consider encoding implications. P0482 changed char to char8_t in these facets which made the issue more glaring but unfortunately, despite the breaking change, it failed to address it. Apart from an obvious design mistake this also adds a small overhead for every locale construction because the implementation has to copy these pseudo-facets for no good reason violating "don't pay for what you don't use" principle. A simple fix is to remove the two facets from table [tab:locale.category.facets] and make them directly constructible.[2022-09-23; Reflector poll]
Set priority to 3 after reflector poll. Send to SG16 (then maybe LEWG).
[2022-09-28; SG16 responds]
SG16 agrees that the codecvt facets mentioned in LWG3767 "codecvt<charN_t, char8_t, mbstate_t> incorrectly added to locale" are intended to be invariant with respect to locale. Unanimously in favor.
[Issaquah 2023-02-10; LWG issue processing]
Removing these breaks most code using them today, because the most obvious way to use them is via use_facet on a locale, which would throw if they're removed (and because they were guaranteed to be present, code using them might have not bothered to check for them using has_facet). Instead of removing them, deprecate the guarantee that they're always present (so move them to D.28 [depr.locale.category]). Don't bother changing the destructor. Victor to update wording.
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 30.3.1.2.1 [locale.category], Table 105 ([tab:locale.category.facets]) — "Locale category facets" — as indicated:
Table 105: Locale category facets [tab:locale.category.facets] Category Includes facets … ctype ctype<char>, ctype<wchar_t>
codecvt<char, char, mbstate_t>
codecvt<char16_t, char8_t, mbstate_t>
codecvt<char32_t, char8_t, mbstate_t>
codecvt<wchar_t, char, mbstate_t>… Modify 30.4.2.5.1 [locale.codecvt.general] as indicated:
namespace std { […] template<class internT, class externT, class stateT> class codecvt : public locale::facet, public codecvt_base { public: using intern_type = internT; using extern_type = externT; using state_type = stateT; explicit codecvt(size_t refs = 0); ~codecvt(); […] protected:~codecvt();[…] }; }[…]
-3- The specializations required in Table105 [tab:locale.category.facets]106 [tab:locale.spec] (30.3.1.2.1 [locale.category]) convert the implementation-defined native character set. codecvt<char, char, mbstate_t> implements a degenerate conversion; it does not convert at all. The specialization codecvt<char16_t, char8_t, mbstate_t> converts between the UTF-16 and UTF-8 encoding forms, and the specialization codecvt<char32_t, char8_t, mbstate_t> converts between the UTF-32 and UTF-8 encoding forms. codecvt<wchar_t, char, mbstate_t> converts between the native character sets for ordinary and wide characters. Specializations on mbstate_t perform conversion between encodings known to the library implementer. Other encodings can be converted by specializing on a program-defined stateT type. Objects of type stateT can contain any state that is useful to communicate to or from the specialized do_in or do_out members.
[2023-02-10; Victor Zverovich comments and provides improved wording]
Per today's LWG discussion the following changes have been implemented in revised wording:
Deprecated the facets instead of removing them (also _byname variants which were previously missed).
Removed the changes to facet dtor since with deprecation it's no longer critical to provide other ways to access them.
Proposed resolution:
This wording is relative to N4928.
Modify 30.3.1.2.1 [locale.category], Table 105 ([tab:locale.category.facets]) — "Locale category facets" — and Table 106 ([tab:locale.spec]) "Required specializations" as indicated:
[…]
Table 105: Locale category facets [tab:locale.category.facets] Category Includes facets … ctype ctype<char>, ctype<wchar_t>
codecvt<char, char, mbstate_t>
codecvt<char16_t, char8_t, mbstate_t>
codecvt<char32_t, char8_t, mbstate_t>
codecvt<wchar_t, char, mbstate_t>…
Table 106: Required specializations [tab:locale.spec] Category Includes facets … ctype ctype_byname<char>, ctype_byname<wchar_t>
codecvt_byname<char, char, mbstate_t>
codecvt_byname<char16_t, char8_t, mbstate_t>
codecvt_byname<char32_t, char8_t, mbstate_t>
codecvt_byname<wchar_t, char, mbstate_t>…
Modify 30.4.2.5.1 [locale.codecvt.general] as indicated:
[…]
-3- The specializations required in Table 105 (30.3.1.2.1 [locale.category]) convert the implementation-defined native character set. codecvt<char, char, mbstate_t> implements a degenerate conversion; it does not convert at all.The specialization codecvt<char16_t, char8_t, mbstate_t> converts between the UTF-16 and UTF-8 encoding forms, and the specialization codecvt<char32_t, char8_t, mbstate_t> converts between the UTF-32 and UTF-8 encoding forms.codecvt<wchar_t, char, mbstate_t> converts between the native character sets for ordinary and wide characters. Specializations on mbstate_t perform conversion between encodings known to the library implementer. Other encodings can be converted by specializing on a program-defined stateT type. Objects of type stateT can contain any state that is useful to communicate to or from the specialized do_in or do_out members.
Modify D.28 [depr.locale.category] (Deprecated locale category facets) in Annex D as indicated:
-1- The ctype locale category includes the following facets as if they were specified in table Table 105 [tab:locale.category.facets] of 30.4.2.5.1 [locale.codecvt.general].
codecvt<char16_t, char, mbstate_t> codecvt<char32_t, char, mbstate_t> codecvt<char16_t, char8_t, mbstate_t> codecvt<char32_t, char8_t, mbstate_t>-1- The ctype locale category includes the following facets as if they were specified in table Table 106 [tab:locale.spec] of 30.4.2.5.1 [locale.codecvt.general].
codecvt_byname<char16_t, char, mbstate_t> codecvt_byname<char32_t, char, mbstate_t> codecvt_byname<char16_t, char8_t, mbstate_t> codecvt_byname<char32_t, char8_t, mbstate_t>-3- The following class template specializations are required in addition to those specified in 30.4.2.5 [locale.codecvt]. The specializations codecvt<char16_t, char, mbstate_t> and codecvt<char16_t, char8_t, mbstate_t> convert
sbetween the UTF-16 and UTF-8 encoding forms, and the specializations codecvt<char32_t, char, mbstate_t> and codecvt<char32_t, char8_t, mbstate_t> convertsbetween the UTF-32 and UTF-8 encoding forms.
Section: 22.14.6.1 [formatter.requirements] Status: LEWG Submitter: Mark de Wever Opened: 2022-08-28 Last modified: 2023-02-07
Priority: 3
View all other issues in [formatter.requirements].
View all issues with LEWG status.
Discussion:
A format string 22.14.2.1 [format.string.general]/1 has an optional format-specifier replacement field. If this field is not present, the current wording makes it clear the intention is to call parse when a formatting argument use a handle class 22.14.8.1 [format.arg]/19
Effects: Initializes ptr_ with addressof(val) and format_ with
[](basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx, const void* ptr) { typename Context::template formatter_type<TD> f; parse_ctx.advance_to(f.parse(parse_ctx)); format_ctx.advance_to(f.format(*const_cast<TQ*>(static_cast<const TD*>(ptr)), format_ctx)); }
For other types it is implied by [tab:formatter.basic]
Formats u according to the specifiers stored in *this, writes the output to fc.out(), and returns an iterator past the end of the output range. The output shall only depend on u, fc.locale(), fc.arg(n) for any value n of type size_t, and the range [pc.begin(), pc.end()) from the last call to f.parse(pc).
(Similar wording is used in [tab:formatter])
Before the parse function is called it is known whether or not a format-spec is present. It seems wasteful to call a function that is known not to parse anything. Therefore I propose to make the call optional. This change is only observable for formatter specializations using the handle class, for the formatter specializations in 22.14.6.3 [format.formatter.spec] the change has no observable effect.[2022-10-12; Reflector poll]
Set status to "LEWG" and priority to 3 after reflector poll.
Several votes for NAD as this would be a design change requiring a paper. Dissenting opinions: "Different handling of empty and default specs is entirely incidental and not a design feature. Moreover, built-in formatters treat these cases identically so it's a good idea to make this explicit." "Don't need a full paper to clarify the meaning of "don't call member parse if there's no format spec". If LEWG agree the direction PR can be polished."
[Issaquah 2023-02-07; LWG]
Will be resolved differently by P2733.
Proposed resolution:
This wording is relative to N4917.
Modify 22.14.6.1 [formatter.requirements] as indicated:
-3- Given character type charT, output iterator type Out, and formatting argument type T, in Table 74 and Table 75:
[…] pc.begin() points to the beginning of the format-spec (22.14.2 [format.string]) of the replacement field being formatted in the format string. If format-spec is not present or empty then either pc.begin() == pc.end() or *pc.begin() == '}'.
Modify BasicFormatter requirements [tab:formatter.basic] as indicated:
Table 74: BasicFormatter requirements [tab:formatter.basic] Expression Return type Requirement … f.format(u, fc) FC::iterator Formats u according to the specifiers stored in
*this, writes the output to fc.out(), and returns
an iterator past the end of the output range.
The output shall only depend on u, fc.locale(),
fc.arg(n) for any value n of type size_t, and
the range [pc.begin(), pc.end()) from the last
call to f.parse(pc). When the format-spec
(22.14.2 [format.string]) is not present or empty
the call to f.parse(pc) is omitted.
Modify Formatter requirements [tab:formatter] as indicated:
Table 75: Formatter requirements [tab:formatter] Expression Return type Requirement f.format(t, fc) FC::iterator Formats t according to the specifiers stored in
*this, writes the output to fc.out(), and returns
an iterator past the end of the output range.
The output shall only depend on t, fc.locale(),
fc.arg(n) for any value n of type size_t, and
the range [pc.begin(), pc.end()) from the last
call to f.parse(pc). When the format-spec
(22.14.2 [format.string]) is not present or empty
the call to f.parse(pc) is omitted.…
Modify 22.14.8.1 [format.arg] as indicated:
[…]
-16- The class handle allows formatting an object of a user-defined type.namespace std { template<class Context> class basic_format_arg<Context>::handle { const void* ptr_; // exposition only void (*format_)(basic_format_parse_context<char_type>&*, Context&, const void*); // exposition only template<class T> explicit handle(T&& val) noexcept; // exposition only friend class basic_format_arg<Context>; // exposition only public: void format(basic_format_parse_context<char_type>&, Context& ctx) const; }; }template<class T> explicit handle(T&& val) noexcept;-17- […]
-18- […] -19- Effects: Initializes ptr_ with addressof(val) and format_ with[](basic_format_parse_context<char_type>&* parse_ctx, Context& format_ctx, const void* ptr) { typename Context::template formatter_type<TD> f; if (parse_ctx) parse_ctx.->advance_to(f.parse(*parse_ctx)); format_ctx.advance_to(f.format(*const_cast<TQ*>(static_cast<const TD*>(ptr)), format_ctx)); }void format(basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx) const;-20- Effects: If the format-spec (22.14.2 [format.string]) is not present or empty, equivalent to:
format_(nullptr, format_ctx, ptr_);otherwise, e
Equivalent to:format_(addressof(parse_ctx), format_ctx, ptr_);
Section: 26.7.32.2 [range.cartesian.view] Status: Open Submitter: Tomasz Kamiński Opened: 2022-09-12 Last modified: 2023-02-07
Priority: 2
View all issues with Open status.
Discussion:
In case when cartesian_product_view is common and one of the inner ranges is empty, it needs to produce equal iterators from begin/end. We currently create a sequence of begin iterators as both begin and end iterators. This assumes that begin iterator is copyable, which may not be the case with the input range, even in the case if that range is common — in such case, we require that only sentinel is semantically copy-constructible, not begin even if they are the same type.
To illustrate, C++98 input iterators (like directory_iterator) are syntactically copy-constructible, but only default constructed object, that corresponds to sentinels are semantically copyable — the copy produces an equivalent result. As a consequence for directory_iterator d, and empty std::string_view sv, the view::cartesian_product(d, sv) produces an invalid range. To fix the problem, we need to move the logic of adjusting the first range iterator to return [end, begin, ..., begin] for begin. This is safe, as we require the end to be always semantically copy-constructible. This again can be done only if computing the end can be done in 𝒪(1) i.e. the first range is common.[2022-09-28; Reflector poll]
Set priority to 2 after reflector poll.
[2022-09-28; LWG telecon]
Discussed issue. Tim suggested to add a new semantic requirement to sentinel_for that when S and I are the same type then i == i is true for any non-singular i of type I.
Proposed resolution:
This wording is relative to N4910.
Modify 26.7.32.2 [range.cartesian.view] as indicated:
[Drafting note: We can optimize the comparison with default_sentinel_t to compare only the iterator to the first range if the range is common. This is observable, as we call comparison of user-provided iterators.]
constexpr iterator<false> begin() requires (!simple-view<First> || ... || !simple-view<Vs>);
-2- Effects: Equivalent to: return iterator<false>(tuple-transform(ranges::begin, bases_));constexpr iterator<true> begin() const requires (range<const First> && ... && range<const Vs>);
-3- Effects: Equivalent to: return iterator<true>(tuple-transform(ranges::begin, bases_));constexpr iterator<false> end() requires ((!simple-view<First> || ... || !simple-view<Vs>) && cartesian-product-is-common<First, Vs...>); constexpr iterator<true> end() const requires cartesian-product-is-common<const First, const Vs...>;-4- Let:
(4.1) — is-const be true for the const-qualified overloads, and false otherwise;
(4.?) — is-end be true for the end overloads, and false otherwise;
(4.2) — is-empty be true if the expression ranges::empty(rng) is true for any rng among the underlying ranges except the first one and false otherwise; and
(4.3) — begin-or-first-end(rng) be expression-equivalent to is-end || is-empty ? cartesian-common-arg-end(rng) : ranges::begin(rng)
is-empty ? ranges::begin(rng) : cartesian-common-arg-end(rng)if cartesian-product-common-arg<maybe-const<is-const, First>> is true and rng is the first underlying range, and ranges::begin(rng) otherwise.-5- Effects: Equivalent to:
iterator<is-const> it(tuple-transform( [](auto& rng){ return begin-or-first-end(rng); }, bases_)); return it;
Modify 26.7.32.3 [range.cartesian.iterator] as indicated:
friend constexpr bool operator==(const iterator& x, default_sentinel_t);-26- Returns:
(?.1) — If cartesian-product-common-arg<maybe-const<Const, First>> is true, returns std::get<0>(x.current_) == ranges::end(std::get<0>(x.parent_->bases_)).
(?.2) — Otherwise,
trueif std::get<i>(x.current_) == ranges::end(std::get<i>(x.parent_->bases_)) is true for any integer 0 ≤ i ≤ sizeof...(Vs),; otherwise, falsereturns true.(?.3) — Otherwise, returns false.
Section: 25.5.5.1 [common.iterator], 25.5.5.6 [common.iter.cmp] Status: New Submitter: Hewill Kang Opened: 2022-09-18 Last modified: 2022-10-12
Priority: 3
View all other issues in [common.iterator].
View all issues with New status.
Discussion:
P2325R3 makes input_or_output_iterator no longer require default_initializable, which means that the template parameter I of common_iterator no longer requires default_initializable.
In this case, since common_iterator itself cannot be default-constructed, it can never be a valid sentinel even if it can be compared to itself. Furthermore, this also makes views::common return a non-range even if it is well-formed (online example):
#include <ranges>
#include <vector>
int main() {
std::vector<int> v;
auto r = std::views::counted(std::back_inserter(v), 3);
auto cr = r | std::views::common;
static_assert(std::ranges::range<decltype(cr)>); // failed
}
which causes views::common to be unable to convert a range into a view, making it not a valid range adaptor.
I think common_iterator should always be default_initializable, which makes it eligible to be a legitimate sentinel. The proposed resolution provides a default constructor for common_iterator when I is not default_initializable, in which case constructs the variant with an alternative type of S.[2022-09-28; Reflector poll]
Set priority to 3 after reflector poll.
"The P/R means that sometimes the variant containers an iterator and sometimes contains a sentinel, depending on whether the iterator is default constructible. Always constructing a sentinel would be more consistent."
Proposed resolution:
This wording is relative to N4917.
Modify 25.5.5.1 [common.iterator], class template common_iterator synopsis, as indicated:
namespace std {
template<input_or_output_iterator I, sentinel_for<I> S>
requires (!same_as<I, S> && copyable<I>)
class common_iterator {
public:
constexpr common_iterator() requires default_initializable<I> = default;
constexpr common_iterator();
[…]
};
[…]
}
Modify 25.5.5.3 [common.iter.const] as indicated:
constexpr common_iterator();-?- Effects: Initializes v_ as if by v_{in_place_type<S>}.
constexpr common_iterator(I i);-1- Effects: Initializes v_ as if by v_{in_place_type<I>, std::move(i)}.
Section: 27.6.5 [alg.foreach], 27.6.15 [alg.search], 27.7.1 [alg.copy], 27.7.6 [alg.fill], 27.7.7 [alg.generate] Status: New Submitter: Jiang An Opened: 2022-10-05 Last modified: 2022-10-12
Priority: 3
View all other issues in [alg.foreach].
View all issues with New status.
Discussion:
Algorithms std::for_each_n, std::search_n, std::copy_n, std::fill_n, and std::generate_n have similar requirements for the Size template parameter in Mandates, requiring Size to be convertible to an integral type.
However, it is currently underspecified to which integral type Size is converted before further operations. There is implementation divergence:It is also notable that when the conversion from the source type to integral types is sufficiently ambiguous, none of these implementations accepts such a source type.
For example, currently the following program is rejected by all mainstream implementations.
#include <algorithm>
#include <cstdio>
struct BadFrom {
operator short() const { return 1; }
operator unsigned short() const { return 1; }
};
int main()
{
int arr[42]{};
std::for_each_n(arr, BadFrom{}, [&arr](int i)
{
std::printf("%d\n", i);
});
}
I think libc++'s strategy make the most sense. But is it really intended to support using a floating-point type or a class type as Size?
Daniel: The conversion from class type was indeed intended, see the original wording for LWG 3213, which was transferred to P1718R2. See also LWG 3439 for a similar underspecified situation for template parameter Distance and for the underspecified Size template parameter in various uninitialized_*_n and destroy_n algorithms in 27.11.2 [special.mem.concepts]. ranges::destroy_n has the luxury to simply require iter_difference_t<I>.[2022-10-12; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 31.12.6.6 [fs.path.itr] Status: New Submitter: Jiang An Opened: 2022-10-17 Last modified: 2022-11-01
Priority: 3
View all other issues in [fs.path.itr].
View all issues with New status.
Discussion:
Currently, 31.12.6.6 [fs.path.itr]/2 effectively requires std::filesystem::path::iterator::reference to be a reference type, due to the requirements for (legacy) bidirectional iterators. However, it's reasonable for the operator* to return path by value, which can make the iterator model std::bidirectional_iterator, be compatible with std::reverse_iterator, and avoid complicated data structures (e.g. those in libstdc++) for achieving such purpose.
libc++ is already doing so. I think we should make such a strategy conforming.[2022-11-01; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4917.
Modify 31.12.6.6 [fs.path.itr] as indicated:
-2- A path::iterator is a constant iterator meeting all the requirements of a bidirectional iterator (25.3.5.6 [bidirectional.iterators]) except that, for dereferenceable iterators a and b of type path::iterator with a == b, there is no requirement that *a and *b are bound to the same object, and its reference may be path. Its value_type is path.
Section: 26.7.22.2 [range.elements.view] Status: New Submitter: Hui Xie Opened: 2022-10-21 Last modified: 2022-11-01
Priority: 2
View all other issues in [range.elements.view].
View all issues with New status.
Discussion:
This issue came up when I tried to integrate the C++23 changes to tuple-like into ranges::elements_view in libc++. Given the following test:
Using SubRange = ranges::subrange<MoveOnlyIter, Sent>;
std::vector<SubRange> srs = ...; // a vector of subranges
for(auto&& iter : srs | views::elements<0>){
}
The above code results in a hard error in deciding the iterator_category (The base is a random access range so it should exist). The immediate hard error complains that the following expression is invalid.
std::get<N>(*current_);
Note that even if iterator_category does not complain, it will complain later when we dereference the iterator.
Here are the declarations of the "get" overloads for subrange:template<size_t N, class I, class S, subrange_kind K> requires ((N == 0 && copyable<I>) || N == 1) constexpr auto get(const subrange<I, S, K>& r); template<size_t N, class I, class S, subrange_kind K> requires (N < 2) constexpr auto get(subrange<I, S, K>&& r);
Note that the first overload requires copyable<I> which is false and the second overload requires an rvalue, which is also not the case. So we don't have a valid "get" in this case.
But why does elements_view allow the instantiation in the first place? Let's look at its requirements:
template<class T, size_t N>
concept returnable-element = // exposition only
is_reference_v<T> || move_constructible<tuple_element_t<N, T>>;
template<input_range V, size_t N>
requires view<V> && has-tuple-element<range_value_t<V>, N> &&
has-tuple-element<remove_reference_t<range_reference_t<V>>, N> &&
returnable-element<range_reference_t<V>, N>
class elements_view;
It passed the "is_reference_v<range_reference_t<V>>" requirement, because it is "subrange&". Here the logic has an assumption: if the tuple-like is a reference, then we can always "get" and return a reference. This is not the case for subrange. subrange's get always return by value.
[2022-11-01; Reflector poll]
Set priority to 2 after reflector poll.
"The actual issue is that P2165 broke has-tuple-element for this case. We should unbreak it."
Proposed resolution:
This wording is relative to N4917.
[Drafting Note: Three mutually exclusive options are prepared, depicted below by Option A, Option B, and Option C, respectively.]
Option A: Properly disallow this case (preferred solution)
Modify 26.7.22.2 [range.elements.view] as indicated:
namespace std::ranges {
[…]
template<class T, size_t N>
concept returnable-element = // exposition only
requires { std::get<N>(declval<T>()); } &&
is_reference_v<T> || move_constructible<tuple_element_t<N, T>>;
[…]
}
Option B: Relax subrange's get to have more overloads. Since subrange's non-const begin unconditionally moves the iterator (even for lvalue-reference),
[[nodiscard]] constexpr I begin() requires (!copyable<I>); Effects: Equivalent to: return std::move(begin_);
if we add more get overloads, it would work. The non-const lvalue-ref overload would work (and it also moves because non-const lvalue begin moves). This solution would make another way to let subrange's iterator in moved-from state, which is not good.
Modify 26.2 [ranges.syn] as indicated:
[…]
namespace std::ranges {
[…]
template<size_t N, class I, class S, subrange_kind K>
requires ((N == 0 && copyable<I>) || N == 1)
constexpr auto get(const subrange<I, S, K>& r);
template<size_t N, class I, class S, subrange_kind K>
requires (N < 2)
constexpr auto get(subrange<I, S, K>&& r);
template<size_t N, class I, class S, subrange_kind K>
requires ((N == 0 && constructible_from<I, const I&&>) || N == 1)
constexpr auto get(const subrange<I, S, K>&& r);
template<size_t N, class I, class S, subrange_kind K>
requires (N < 2)
constexpr auto get(subrange<I, S, K>& r);
}
[…]
Option C: Make subrange's get to return by reference. This seems to significantly change the subrange's tuple protocol, which is not ideal.
Section: 17.14.7 [support.c.headers.other] Status: New Submitter: Jiang An Opened: 2022-10-22 Last modified: 2022-11-01
Priority: 3
View other active issues in [support.c.headers.other].
View all other issues in [support.c.headers.other].
View all issues with New status.
Discussion:
See also LWG 3782. Like lerp, neither <math.h> nor C compatibility is mentioned in P0030R1, and MSVC STL decides not to declare 3-argument hypot overloads in the global namespace (perhaps so does libc++).
Perhaps we should also avoid providing these overloads in the global namespace. However, such change seems a bit difficult for libstdc++'s <math.h>.[2022-11-01; Reflector poll]
Set priority to 3 after reflector poll. "This affects the exports of std.compat".
Proposed resolution:
Section: 24.6.9 [flat.map], 24.6.10 [flat.multimap], 24.6.11 [flat.set], 24.6.12 [flat.multiset] Status: New Submitter: Arthur O'Dwyer Opened: 2022-10-25 Last modified: 2022-11-04
Priority: 2
View other active issues in [flat.map].
View all other issues in [flat.map].
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Discussion:
Compare 24.6.7.2 [priqueue.cons]'s overload set
priority_queue(const Compare&, const Container&); priority_queue(const Compare&, Container&&); template<class Alloc> priority_queue(const Compare&, const Container&, const Alloc&); template<class Alloc> priority_queue(const Compare&, Container&&, const Alloc&);
against 24.6.9 [flat.map]'s overload set
flat_map(key_container_type, mapped_container_type); template<class Allocator> flat_map(const key_container_type&, const mapped_container_type&, const Allocator& a);
I see two issues here:
(A) The allocator-extended ctor of flat_map always copies the key_container and value_container, when it should be move-enabled.
(B) Almost certainly the Allocator parameter should be named Alloc instead, and there should be a separate "Constructors with allocators" section with wording similar to 24.6.7.3 [priqueue.cons.alloc] explaining that these ctors don't participate in overload resolution unless uses_allocator_v<KeyContainer, Alloc> && uses_allocator_v<MappedContainer, Alloc>.
I suggest this overload set to replace the two overloads above:
flat_map(key_container_type, mapped_container_type); template<class Alloc> flat_map(const key_container_type&, const mapped_container_type&, const Alloc& a); template<class Alloc> flat_map(const key_container_type&, mapped_container_type&&, const Alloc& a); template<class Alloc> flat_map(key_container_type&&, const mapped_container_type&, const Alloc& a); template<class Alloc> flat_map(key_container_type&&, mapped_container_type&&, const Alloc& a);
This preserves the apparent assumption that KeyContainer(std::move(kc)) is always efficient but KeyContainer(std::move(kc), otheralloc) might not be. Similar wording changes would have to be made to all the flat_foo containers.
Tony Table:
template<class T, class Comp = std::less<T>, class Container = std::pmr::vector<T>>
using pmr_flat_set = std::flat_set<T, Comp, Container>;
std::pmr::vector<pmr_flat_set<int>> vs;
std::pmr::vector<int> data = {1,2,3};
vs.reserve(1);
vs.emplace_back(std::move(data));
// constructs-in-place with the argument list (std::move(data), get_allocator())
// BEFORE: copies (causes heap traffic)
// AFTER: moves (no heap traffic)
[2022-11-04; Reflector poll]
Set priority to 2 after reflector poll.
Proposed resolution:
Section: 24.6.9 [flat.map], 24.6.10 [flat.multimap], 24.6.11 [flat.set], 24.6.12 [flat.multiset] Status: New Submitter: Arthur O'Dwyer Opened: 2022-10-25 Last modified: 2022-11-04
Priority: 2
View other active issues in [flat.map].
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Discussion:
Tony Table:
std::vector<int> v; std::flat_set s = std::flat_set(v, MyAllocator<int>()); std::flat_set s = std::flat_set(v, std::less(), MyAllocator<int>()); std::flat_set s = std::flat_set(v.begin(), v.end(), MyAllocator<int>()); std::flat_set s = std::flat_set(v.begin(), v.end(), std::less(), MyAllocator<int>()); // BEFORE: all fail to compile // AFTER: all compile successfully
Contrast 24.6.11.3 [flat.set.cons] with 24.6.7.2 [priqueue.cons], where most of these are okay:
std::vector<int, MyAllocator<int>> v; std::priority_queue pq1 = std::priority_queue(v, std::less(), MyAllocator<int>()); std::priority_queue pq2 = std::priority_queue(v.begin(), v.end(), MyAllocator<int>()); std::priority_queue pq3 = std::priority_queue(v.begin(), v.end(), std::less(), MyAllocator<int>()); // BEFORE AND AFTER: pq1 compiles successfully // BEFORE AND AFTER: pq2 and pq3 also compile successfully thanks to LWG 3506
[2022-11-04; Reflector poll]
Set priority to 2 after reflector poll.
Proposed resolution:
Section: 22.10 [function.objects], 26.5.7.3 [range.utility.conv.adaptors], 26.7.2 [range.adaptor.object] Status: New Submitter: Johel Ernesto Guerrero Peña Opened: 2022-10-26 Last modified: 2022-11-01
Priority: 4
View all other issues in [function.objects].
View all issues with New status.
Discussion:
22.10.3 [func.def] defines
-6- A call wrapper is an object of a call wrapper type.
Most importantly, a call wrapper is an object.
A number of functions in 22.10 [function.objects] and expressions in 26.5.7.3 [range.utility.conv.adaptors] and 26.7.2 [range.adaptor.object] are specified to result in a call wrapper. Most notably, the return type of ranges::to is auto, so its result is definitely a prvalue and not a "call wrapper" object. Where a prvalue result is meant, the wording should be clarified to mean "a prvalue whose object it initializes is a call wrapper".[2022-11-01; Reflector poll]
Set priority to 4 after reflector poll.
Proposed resolution:
Section: 28.5.4.4 [rand.eng.sub] Status: New Submitter: Jonathan Wakely Opened: 2022-11-02 Last modified: 2022-11-25
Priority: 3
View all other issues in [rand.eng.sub].
View all issues with New status.
Discussion:
The standard requires subtract_with_carry_engine<T> to use:
linear_congruential_engine<T, 40014u, 0u, 2147483563u>
where each of those values is converted to T.
This appears to mean subtract_with_carry_engine cannot be used with uint16_t, because 2147483563u cannot be converted to uint16_t without narrowing. What is the intention here? Should it be ill-formed? Should the seed engine be linear_congruential_engine<uint_least32_t, …> instead? The values from the linear_congruential_engine are used modulo 2^32 so getting 64-bit values from it is pointless, and getting 16-bit values from it doesn't compile.[Kona 2022-11-12; Set priority to 3]
[Kona 2022-11-25; Jonathan provides wording]
Proposed resolution:
This wording is relative to N4917.
Modify the class synopsis in 28.5.4.4 [rand.eng.sub] as indicated:
namespace std {
template<class UIntType, size_t w, size_t s, size_t r>
class subtract_with_carry_engine {
public:
// types
using result_type = UIntType;
// engine characteristics
static constexpr size_t word_size = w;
static constexpr size_t short_lag = s;
static constexpr size_t long_lag = r;
static constexpr result_type min() { return 0; }
static constexpr result_type max() { return m − 1; }
static constexpr result_typeuint_least32_t default_seed = 19780503u;
// constructors and seeding functions
subtract_with_carry_engine() : subtract_with_carry_engine(default_seed0u) {}
explicit subtract_with_carry_engine(result_type value);
template<class Sseq> explicit subtract_with_carry_engine(Sseq& q);
void seed(result_type value = default_seed0u);
template<class Sseq> void seed(Sseq& q);
Modify 28.5.4.4 [rand.eng.sub] p7 as indicated:
explicit subtract_with_carry_engine(result_type value);-7- Effects: Sets the values of , in that order, as specified below. If is then , sets to ; otherwise sets to .
To set the values , first construct e, a linear_congruential_engine object, as if by the following definition:
linear_congruential_engine<result_typeuint_least32_t, 40014u,0u,2147483563u> e(value == 0u ? default_seed : value);Then, to set each , obtain new values from successive invocations of e. Set to .
Section: 3.2.2.5 [fund.ts.v3::propagate_const.const_observers] Status: New Submitter: Giuseppe D'Angelo Opened: 2022-11-04 Last modified: 2022-11-12
Priority: 3
View all issues with New status.
Discussion:
Addresses: fund.ts.v3
This issue has its origin in the discussion of gcc issue 107525.
The current draft of LFTSv3 specifies this conversion function for propagate_const in 3.2.2.5 [fund.ts.v3::propagate_const.const_observers]:constexpr operator const element_type*() const;-7- Returns: get().
-8- Remarks: This function shall not participate in overload resolution unless T is an object pointer type or has an implicit conversion to const element_type*.
The constraint should however specify that const T (and not T) needs to have an implicit conversion to const element_type *.
Basically: if const T cannot do the conversion, then neither const propagate_const<T> should be able to. One can design a type X such as a const X cannot convert to const element_type * (for instance, by =deleteing the corresponding conversion function). If now one asks whether const propagate_const<X> is convertible to const element_type *, the answer is (surprisingly) "yes".[Kona 2022-11-12; Set priority to 3]
Proposed resolution:
This wording is relative to N4840.
Modify 3.2.2.5 [fund.ts.v3::propagate_const.const_observers] as indicated:
constexpr operator const element_type*() const;-7- Returns: get().
-8- Remarks: This function shall not participate in overload resolution unless T is an object pointer type or const T has an implicit conversion to const element_type*.
Section: 24.7.2.2.1 [span.overview] Status: New Submitter: Jiang An Opened: 2022-11-06 Last modified: 2022-11-12
Priority: 2
View other active issues in [span.overview].
View all other issues in [span.overview].
View all issues with New status.
Discussion:
This issue is discovered when implementing the span part of P2278R4 in MSVC STL.
P2278R4 added the const_iterator member type to std::span, which required its iterator type to model std::input_iterator (same for const_reverse_iterator and reverse_iterator). However, when element_type is volatile T and T is a class type, the iterator type generally fails to satisfy input_iterator, because:input_iterator<iterator> requires indirectly_readable<iterator>;
indirectly_readable<iterator> requires common_reference_with<iterator_reference_t<iterator>&&, iterator_rvalue_reference_t<iterator>&&>, that is common_reference_with<volatile T&, volatile T&&>;
common_reference_t<volatile T&, volatile T&&> is T (which is problematic), and thus common_reference_with<volatile T&, volatile T&&> requires both convertible_to<volatile T&, T> and convertible_to<volatile T&&, T>;
However, the class type T generally doesn't have constructors from volatile T or const volatile T glvalues, and thus neither convertible_to<volatile T&, T> and convertible_to<volatile T&&, T> is satisfied.
Ideally, span should not require any form of construction of element_type. Although usages of class types provided by the standard library via volatile glvalues are generally not supported, I think span<volatile T, E> is still useful for some user-defined class type T, and thus shouldn't be forbidden.
[Kona 2022-11-12; Set priority to 2]
Proposed resolution:
Section: 26.7.7.2 [range.as.rvalue.view] Status: New Submitter: Hewill Kang Opened: 2022-11-13 Last modified: 2022-11-30
Priority: 3
View all issues with New status.
Discussion:
Currently, when the underlying range is not a common range, as_rvalue_view::begin and end return move_iterator and move_sentinel respectively, this seems reasonable since move_sentinel is a sentinel adaptor introduced by C++20 specifically for comparison with move_iterator.
However, in the case where the sentinel type of the underlying type is an input iterator, this may lead to some performance issues, consider:
#include <list>
#include <ranges>
int main() {
std::list<std::tuple<int&>> l;
auto k = std::move(l) | std::views::keys;
auto s = std::ranges::subrange(std::cbegin(k), std::end(k));
(void) std::ranges::next(s.begin(), s.end()); // constant time
auto r = s | std::views::as_rvalue;
(void) std::ranges::next(r.begin(), r.end()); // linear time
}
The above subrange is constructed by the elements_view::iterator<true> and elements_view::iterator<false> pair, and since the former can be assigned by the latter, when we use ranges::next to increment the begin of s to its end, the assignable_from branch will be executed, so we get a constant-time complexity.
However, when we apply views::as_rvalue to s, the as_rvalue_view::end will go into the non-common branch and return move_sentinel. And because move_iterator cannot be assigned by move_sentinel, ranges::next will successively increment the begin of s until its end, we get the linear-time complexity this time. I think it is more appropriate to return move_iterator for the above case, as this preserves the assignability, but also preserves the iterator operability that the original sentinel type has. Another benefit of doing this is that when the sentinel type of underlying range can be subtracted from its iterator type but does not model sized_sentinel_for, returning different move_iterator makes them still subtractable, because its operator- only constrain the x.base() - y.base() being well-formed. This also solves the issue of as_rvalue_view being a valid type but does not model a range in some cases, for example:
#include <ranges>
int main() {
std::move_iterator<int*> i;
std::move_iterator<const int*> ci;
std::ranges::subrange s(i, ci);
std::ranges::as_rvalue_view r(s); // not failed
static_assert(std::ranges::range<decltype(r)>); // failed
}
This is because currently, as_rvalue_view does not explicitly specify the template parameters of the returned move_iterator and move_sentinel, so based on CTAD, its begin will return move_iterator(move_iterator(...)) which is still move_iterator<int*>, and its end will return move_sentinel<move_iterator<const int*>>. Those two types are not comparable, so r does not constitute a valid range.
The proposed resolution is to return move_iterator when the sentinel type of the underlying range models input_iterator.[2022-11-30; Reflector poll]
Set priority to 3 after reflector poll.
"NAD, these examples seem entirely contrived. If not NAD, don't need the common_range check if we are checking that the sentinel models input_iterator."
Proposed resolution:
This wording is relative to N4917.
Modify 26.7.7.2 [range.as.rvalue.view] as indicated:
namespace std::ranges {
template<view V>
requires input_range<V>
class as_rvalue_view : public view_interface<as_rvalue_view<V>> {
[…]
constexpr auto begin() requires (!simple-view<V>)
{ return move_iterator(ranges::begin(base_)); }
constexpr auto begin() const requires range<const V>
{ return move_iterator(ranges::begin(base_)); }
constexpr auto end() requires (!simple-view<V>) {
if constexpr (common_range<V> || input_iterator<sentinel_t<V>>) {
return move_iterator(ranges::end(base_));
} else {
return move_sentinel(ranges::end(base_));
}
}
constexpr auto end() const requires range<const V> {
if constexpr (common_range<const V> || input_iterator<sentinel_t<const V>>) {
return move_iterator(ranges::end(base_));
} else {
return move_sentinel(ranges::end(base_));
}
}
[…]
};
[…]
}
Section: 26.7.20.2 [range.reverse.view] Status: New Submitter: Hewill Kang Opened: 2022-11-14 Last modified: 2022-11-30
Priority: 3
View all other issues in [range.reverse.view].
View all issues with New status.
Discussion:
In order to ensure that begin has an amortized constant time, when the underlying range is not a common range, reverse_view always caches the result of ranges::next.
However, for some non-common ranges, incrementing its begin to end still guarantees constant time, for example:
#include <ranges>
#include <vector>
#include <list>
int main() {
std::vector v{42};
auto x = std::ranges::subrange(std::counted_iterator(v.begin(), 1), std::default_sentinel)
| std::views::reverse;
(void) x.begin(); // still caches end iterator in MSVC-STL
std::list l{42};
auto y = std::ranges::subrange(l.cbegin(), l.end())
| std::views::reverse;
(void) y.begin(); // still caches end iterator in both libstdc++ and MSVC-STL
}
In the above example, although neither subrange is a common range, applying ranges::next to their iterator-sentinel pairs is still constant time, in this case, there's no need to introduce a cache for reverse_view to store the results. We shouldn't pay for things we don't need to use.
[2022-11-30; Reflector poll]
Set priority to 3 after reflector poll.
"NAD as specified, the Remarks don't need to be precise, they already allow caching to be omitted if not needed. But we could still enable const overloads of begin for cases like these."
"NAD, no need to complicate constraints for such contrived examples. We don't care about random access, sized, non-common ranges like the first case. Any change here should be a paper covering all adaptors, not a piecemeal issue."
Proposed resolution:
This wording is relative to N4917.
Modify 26.7.20.2 [range.reverse.view] as indicated:
constexpr reverse_iterator<iterator_t<V>> begin();-2- Returns:
make_reverse_iterator(ranges::next(ranges::begin(base_), ranges::end(base_)))-3- Remarks: In order to provide the amortized constant time complexity required by the range concept, this function caches the result within the reverse_view for use on subsequent calls when both assignable_from<I&, S> and random_access_iterator<I> && sized_sentinel_for<S, I> are false, where I is iterator_t<V> and S is sentinel_t<V>.
Section: 29.12 [time.format], 29.13 [time.parse] Status: New Submitter: Matt Stephanson Opened: 2022-11-18 Last modified: 2022-11-30
Priority: 3
View other active issues in [time.format].
View all other issues in [time.format].
View all issues with New status.
Discussion:
An issue has been identified regarding the two-digit formatting of negative years according to Table [tab:time.format.spec] (29.12 [time.format]):
cout << format("{:%y} ", 1976y) // "76"
<< format("{:%y}", -1976y); // also "76"?
The relevant wording is
The last two decimal digits of the year. If the result is a single digit it is prefixed by 0. The modified command %Oy produces the locale's alternative representation. The modified command %Ey produces the locale's alternative representation of offset from %EC (year only).
MSVC STL treats the regular modified form symmetrically. Just as %Ey is the offset from %EC, so %y is the offset from %C, which is itself "[t]he year divided by 100 using floored division." (emphasis added). Because -1976 is the 24th year of the -20th century, the above code will print "76 24" using MSVC STL. However, many users expect, and libc++ gives, a result based on the literal wording, "76 76".
IEEE 1003.1-2008 strftime expects the century to be nonnegative, but the glibc implementation prints 24 for -1976. My own opinion is that this is the better result, because it consistently interprets %C and %y as the quotient and remainder of floored division by 100. Howard Hinnant, coauthor of the original 29.12 [time.format] wording in P0355 adds:On the motivation for this design it is important to remember a few things:
POSIX strftime/strptime doesn't handle negative years in this department, so this is an opportunity for an extension in functionality.
This is a formatting/parsing issue, as opposed to a computational issue. This means that human readability of the string syntax is the most important aspect. Computational simplicity takes a back seat (within reason).
%C can't be truncated division, otherwise the years [-99, -1] would map to the same century as the years [0, 99]. So floored division is a pretty easy and obvious solution.
%y is obvious for non-negative years: The last two decimal digits, or y % 100.
This leaves how to represent negative years with %y. I can think of 3 options:
Use the last two digits without negating: -1976 → 76.
Use the last two digits and negate it: -1976 → -76.
Use floored modulus arithmetic: -1976 → 24.
The algorithm to convert %C and %y into a year is not important to the client because these are both strings, not integers. The client will do it with parse, not 100*C + y.
I discounted solution 3 as not sufficiently obvious. If the output for -1976 was 23, the human reader wouldn't immediately know that this is off by 1. The reader is expecting the POSIX spec:the last two digits of the year as a decimal number [00,99].
24 just doesn't cut it.
That leaves solution 1 or 2. I discounted solution 2 because having the negative in 2 places (the %C and %y) seemed overly complicated and more error prone. The negative sign need only be in one place, and it has to be in %C to prevent ambiguity. That leaves solution 1. I believe this is the solution for an extension of the POSIX spec to negative years with the property of least surprise to the client. The only surprise is in %C, not %y, and the surprise in %C seems unavoidable.
[2022-11-30; Reflector poll]
Set priority to 3 after reflector poll.
A few votes for priority 2. Might need to go to LEWG.
Proposed resolution:
This wording is relative to N4917.
[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
Option A: This is Howard Hinnant's choice (3)
Modify 29.12 [time.format], Table [tab:time.format.spec] as indicated:
Table 102 — Meaning of conversion specifiers [tab:time.format.spec] Specifier Replacement […] %y The last two decimal digits of the yearremainder after dividing the year by 100 using floored division.
If the result is a single digit it is prefixed by 0.
The modified command %Oy produces the locale's alternative representation. The
modified command %Ey produces the locale's alternative representation of offset from
%EC (year only).[…]
Modify 29.13 [time.parse], Table [tab:time.parse.spec] as indicated:
Table 103 — Meaning of parse flags [tab:time.parse.spec] Flag Parsed value […] %y The last two decimal digits of the yearremainder after dividing the year by 100 using floored division.
If the century is not otherwise specified (e.g.
with %C), values in the range [69, 99] are presumed to refer to the years 1969 to 1999,
and values in the range [00, 68] are presumed to refer to the years 2000 to 2068. The
modified command %N y specifies the maximum number of characters to read. If N is
not specified, the default is 2. Leading zeroes are permitted but not required. The
modified commands %Ey and %Oy interpret the locale's alternative representation.[…]
Option B: This is Howard Hinnant's choice (1)
Modify 29.12 [time.format], Table [tab:time.format.spec] as indicated:
Table 102 — Meaning of conversion specifiers [tab:time.format.spec] Specifier Replacement […] %y The last two decimal digits of the year, regardless of the sign of the year.
If the result is a single digit it is prefixed by 0.
The modified command %Oy produces the locale's alternative representation. The
modified command %Ey produces the locale's alternative representation of offset from
%EC (year only).
[Example ?: cout << format("{:%C %y}", -1976y); prints -20 76. — end example][…]
Modify 29.13 [time.parse], Table [tab:time.parse.spec] as indicated:
Table 103 — Meaning of parse flags [tab:time.parse.spec] Flag Parsed value […] %y The last two decimal digits of the year, regardless of the sign of the year.
If the century is not otherwise specified (e.g.
with %C), values in the range [69, 99] are presumed to refer to the years 1969 to 1999,
and values in the range [00, 68] are presumed to refer to the years 2000 to 2068. The
modified command %N y specifies the maximum number of characters to read. If N is
not specified, the default is 2. Leading zeroes are permitted but not required. The
modified commands %Ey and %Oy interpret the locale's alternative representation.
[Example ?: year y; istringstream{"-20 76"} >> parse("%3C %y", y); results in
y == -1976y. — end example][…]
Section: 26.7.22.3 [range.elements.iterator] Status: New Submitter: Jiang An Opened: 2022-11-23 Last modified: 2023-01-28
Priority: 3
View all other issues in [range.elements.iterator].
View all issues with New status.
Discussion:
In LWG 3798 (voted into WP in November 2022), iterator types of several range adaptors may have forward_iterator_tag or stronger iterator tag types as their iterator_category type when their operator* returns rvalue references. However, the proposed resolution missed the similar change for element_view::iterator.
[2022-11-30; Reflector poll]
Set priority to 3 after reflector poll.
"The proposed resolution is incorrect - just because the get expression is an xvalue doesn't mean operator* returns by reference."
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 26.7.22.3 [range.elements.iterator] as indicated:
-2- The member typedef-name iterator_category is defined if and only if Base models forward_range. In that case, iterator_category is defined as follows: Let C denote the type iterator_traits<iterator_t<Base>>::iterator_category.
(2.1) — If std::get<N>(*current_) is a
nprvalue, iterator_category denotes input_iterator_tag.(2.2) — Otherwise, if C models derived_from<random_access_iterator_tag>, iterator_category denotes random_access_iterator_tag.
(2.3) — Otherwise, iterator_category denotes C.
[2023-01-22; Jiang An comments and provides improved wording]
The old proposed resolution was incorrect. I think the correct criteria could be that determined from the return type of get-element.
Proposed resolution:
This wording is relative to N4928.
Modify 26.7.22.3 [range.elements.iterator] as indicated:
-2- The member typedef-name iterator_category is defined if and only if Base models forward_range. In that case, iterator_category is defined as follows: Let C denote the type iterator_traits<iterator_t<Base>>::iterator_category.
(2.1) — If range_reference_t<Base> is a reference type and std::get<N>(*current_) is a
nprvalue or neither range_reference_t<Base> nor tuple_element_t<N, range_reference_t<Base> is a reference type, iterator_category denotes input_iterator_tag.(2.2) — Otherwise, if C models derived_from<random_access_iterator_tag>, iterator_category denotes random_access_iterator_tag.
(2.3) — Otherwise, iterator_category denotes C.
Section: 32.1 [re.general] Status: New Submitter: Xie He Opened: 2022-11-28 Last modified: 2023-01-06
Priority: 4
View all issues with New status.
Discussion:
In 32.1 [re.general], the character type used in the regex library (CharT), is only required to be "char-like". This means "struct A { int m; };" satisfies the requirements for CharT. Clearly there have to be more requirements for CharT.
Comparison operators such as "==" or "<=" must be defined between CharT objects, because these operators are used at various places of 32 [re], including 32.2 [re.req] paragraph 14 and 16, 32.6 [re.traits] paragraph 12, and 32.12 [re.grammar] paragraph 14. Similarly, "==" between a CharT object and integer 0 must also be defined, as it is used at 32.2 [re.req] paragraph 11.
Alternatively, we can also interpret these operators in the text, as code implemented using "std::char_traits<CharT>" comparison functions. This interpretation has the advantage of keeping 32 [re] consistent with the strings library and string comparisons, which are also used in 32 [re], including 32.2 [re.req] paragraph 7, 18, 20, and 32.12 [re.grammar] paragraph 14.2. Also, only with this interpretation, can 32.6 [re.traits] paragraph 3 be consistent with 32.2 [re.req] paragraph 11.There must be a way to convert between CharT and char, otherwise there is no way to recognize regex syntactical characters, such as '*' or '+'. One way is to simply do a type conversion from char to CharT (which requires char to be convertible to CharT). But this doesn't allow us to convert between character encodings.
Alternatively, if we have an object of the Traits class (32.12 [re.grammar] paragraph 2) denoted v, we can use "use_facet<ctype<CharT>>(v.getloc()).widen" to convert from char to CharT. This allows us to convert between character encodings. This requires that we can actually get this facet from this locale object.[2023-01-06; Reflector poll]
Set priority to 4 after reflector poll.
Proposed resolution:
Section: 23.4.4.5 [string.erasure], 24.3.8.5 [deque.erasure], 24.3.9.7 [forward.list.erasure], 24.3.10.6 [list.erasure], 24.3.11.6 [vector.erasure] Status: New Submitter: Giuseppe D'Angelo Opened: 2022-12-04 Last modified: 2023-01-06
Priority: 3
View all issues with New status.
Discussion:
Consistent uniform erasure added several overloads of std::erase_if. All these overloads have fully specified effects, and are either implemented through the erase/remove_if idiom (for string, vector and deque), through calls to the container's own remove_if non-static member function (for list and forward_list), or through a hand-rolled loop (for the associative containers — map, set, and their unordered and multi variants).
For the overloads that deal with non-associative containers the predicate passed to std::erase_if is always copied into the inner call to std::remove_if or the container's remove_if (see 23.4.4.5 [string.erasure], 24.3.11.6 [vector.erasure], 24.3.10.6 [list.erasure], 24.3.9.7 [forward.list.erasure], and 24.3.8.5 [deque.erasure]). Now, algorithms are generally allowed to take as many copies of predicates as they want — this is 27.2 [algorithms.requirements]/9, but cf. LWG 3049. However it still feels strange/sloppy that a copy here is mandated by the Standard. An implementation that would otherwise make no copies of a predicate in an algorithm must make a copy in the erase_if overloads. The copy of the predicate could be instead replaced by a move without any change in functionality; after being passed to the underlying algorithm, the predicate object is never used again by erase_if. I am therefore proposing to add a std::move call for the predicate object. One could argue that erase_if should be re-specified so that it is not necessarily implemented in terms of underlying calls to other facilities, and could therefore even remove the need of a move on a high-quality implementation. This is a design change, and so I consider it out of scope for a library issue. Of course, moving instead of copying is a detectable change, and "pathological" predicate types that are copyable but have disabled moves will get broken, but I don't think those deserve to be supported.[2023-01-06; Reflector poll]
Set priority to 3 after reflector poll.
"An alternative resolution would be to wrap the predicate in reference_wrapper."
"I'd prefer blanket wording that says the actual number of copies and/or moves is unspecified when the wording depicts a copy of a function object."
Related to LWG 3049.
Proposed resolution:
This wording is relative to N4917.
Modify 23.4.4.5 [string.erasure] as indicated:
template<class charT, class traits, class Allocator, class Predicate> constexpr typename basic_string<charT, traits, Allocator>::size_type erase_if(basic_string<charT, traits, Allocator>& c, Predicate pred);-2- Effects: Equivalent to:
auto it = remove_if(c.begin(), c.end(), std::move(pred)); auto r = distance(it, c.end()); c.erase(it, c.end()); return r;
Modify 24.3.8.5 [deque.erasure] as indicated:
template<class T, class Allocator, class Predicate> typename deque<T, Allocator>::size_type erase_if(deque<T, Allocator>& c, Predicate pred);-2- Effects: Equivalent to:
auto it = remove_if(c.begin(), c.end(), std::move(pred)); auto r = distance(it, c.end()); c.erase(it, c.end()); return r;
Modify 24.3.9.7 [forward.list.erasure] as indicated:
template<class T, class Allocator, class Predicate> typename forward_list<T, Allocator>::size_type erase_if(forward_list<T, Allocator>& c, Predicate pred);-2- Effects: Equivalent to: return c.remove_if(std::move(pred));
Modify 24.3.10.6 [list.erasure] as indicated:
template<class T, class Allocator, class Predicate> typename list<T, Allocator>::size_type erase_if(list<T, Allocator>& c, Predicate pred);-2- Effects: Equivalent to: return c.remove_if(std::move(pred));
Modify 24.3.11.6 [vector.erasure] as indicated:
template<class T, class Allocator, class Predicate> constexpr typename vector<T, Allocator>::size_type erase_if(vector<T, Allocator>& c, Predicate pred);-2- Effects: Equivalent to:
auto it = remove_if(c.begin(), c.end(), std::move(pred)); auto r = distance(it, c.end()); c.erase(it, c.end()); return r;
Section: 25.3.2.1 [incrementable.traits] Status: New Submitter: Hewill Kang Opened: 2022-12-07 Last modified: 2023-01-06
Priority: 3
View other active issues in [incrementable.traits].
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Discussion:
The last specialization of incremental_traits requires that the result obtained by subtracting two objects of type const T must model integral, then apply make_signed_t to it as the difference type of type T.
However, since bool also models integral, but is not a valid template argument for make_signed_t, we should ban such cases to avoid unnecessary hard errors (online example):
#include <ranges>
struct Bool {
bool operator-(Bool) const;
};
template<class T>
concept can_iota_view = requires(T t) { std::ranges::iota_view(t); };
static_assert(!can_iota_view<Bool>); // hard error
[2022-12-13; Minor wording improvements after LWG reflector discussion]
Remove remove_cv_t within nonbool-integral, because bool prvalues cannot be cv-qualified.
[2023-01-06; Reflector poll]
Set priority to 3 after reflector poll.
"I would prefer to place the new checks directly in the requires-clause instead of introducing nonbool-integral."
Proposed resolution:
This wording is relative to N4917.
Modify 25.3.2.1 [incrementable.traits] as indicated:
namespace std {
template<class T>
concept nonbool-integral = integral<T> && !same_as<T, bool>; // exposition only
template<class T> struct incrementable_traits { };
[…]
template<class T>
requires (!requires { typename T::difference_type; } &&
requires(const T& a, const T& b) { { a - b } -> nonbool-integralintegral; })
struct incrementable_traits<T> {
using difference_type = make_signed_t<decltype(declval<T>() - declval<T>())>;
};
[…]
}
Section: D.29 [depr.fs.path.factory] Status: Open Submitter: Daniel Krügler Opened: 2022-12-10 Last modified: 2023-01-28
Priority: 3
View all other issues in [depr.fs.path.factory].
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Discussion:
The filesystem::u8path function became deprecated with the adoption of P0482R6, but the rationale for that change is rather thin:
"The C++ standard must improve support for UTF-8 by removing the existing barriers that result in redundant tagging of character encodings, non-generic UTF-8 specific workarounds like u8path."
The u8path function is still useful if my original string source is a char sequence and I do know that the encoding of this sequence is UTF-8.
The deprecation note suggests that one should use std::u8string instead, which costs me an additional transformation and doesn't work without reinterpret_cast. Even in the presence of char8_t, legacy code bases often are still ABI-bound to char. In the future we may solve this problem using the tools provided by P2626 instead, but right now this is not part of the standard and it wasn't at the time when u8path became deprecated. This is in my opinion a good reason to undeprecate u8path now and decide later on the appropriate time to deprecate it again (if it really turns out to be obsolete by alternative functionality). Billy O'Neal provides a concrete example where the current deprecation status causes pain:Example: vcpkg-tool files.cpp#L21-L45
Before p0482, we could just call std::u8path and it would do the right thing on both POSIX and Windows. After compilers started implementing '20, we have to make assumptions about the correct 'internal' std::path encoding because there is no longer a way to arrive to std::path with a char buffer that we know is UTF-8 encoded and get the correct results. It's one of the reasons we completely ripped out use of std::filesystem on most platforms from vcpkg, so you won't see this in current sources.
[2023-01-06; Reflector poll]
Set priority to 3 after reflector poll. Set status to LEWG.
[2023-05-30; status to "Open"]
LEWG discussed this in January and had no consensus for undeprecation.
Proposed resolution:
This wording is relative to N4917.
Restore the u8path declarations to 31.12.4 [fs.filesystem.syn], header <filesystem> synopsis, as indicated:
namespace std::filesystem {
// 31.12.6 [fs.class.path], paths
class path;
// 31.12.6.8 [fs.path.nonmember], path non-member functions
void swap(path& lhs, path& rhs) noexcept;
size_t hash_value(const path& p) noexcept;
// [fs.path.factory], path factory functions
template<class Source>
path u8path(const Source& source);
template<class InputIterator>
path u8path(InputIterator first, InputIterator last);
// 31.12.7 [fs.class.filesystem.error], filesystem errors
class filesystem_error;
[…]
}
Restore the previous sub-clause [fs.path.factory] by copying the contents of D.29 [depr.fs.path.factory] to a new sub-clause [fs.path.factory] between 31.12.6.8 [fs.path.nonmember] and 31.12.6.9 [fs.path.hash] and without Note 1 as indicated:
[Drafting note: As additional stylistic adaption we replace the obsolete Requires element by a Preconditions element plus a Mandates element (similar to that of 31.12.6.5.1 [fs.path.construct] p5).
As a second stylistic improvement we convert the now more unusual "if […]; otherwise" construction in bullets by "Otherwise, if […]" constructions.]
? Factory functions [fs.path.factory]
template<class Source> path u8path(const Source& source); template<class InputIterator> path u8path(InputIterator first, InputIterator last);-?- Mandates: The value type of Source and InputIterator is char or char8_t.
-?- Preconditions: The source and [first, last) sequences are UTF-8 encoded. -?- Returns:
(?.1) — If value_type is char and the current native narrow encoding (31.12.6.3.2 [fs.path.type.cvt]) is UTF-8, return path(source) or path(first, last).
(?.2) — Otherwise, if value_type is wchar_t and the native wide encoding is UTF-16, or if value_type is char16_t or char32_t, convert source or [first, last) to a temporary, tmp, of type string_type and return path(tmp).
(?.3) — Otherwise, convert source or [first, last) to a temporary, tmp, of type u32string and return path(tmp).
-?- Remarks: Argument format conversion (31.12.6.3.1 [fs.path.fmt.cvt]) applies to the arguments for these functions. How Unicode encoding conversions are performed is unspecified.
-?- [Example 1: A string is to be read from a database that is encoded in UTF-8, and used to create a directory using the native encoding for filenames:namespace fs = std::filesystem; std::string utf8_string = read_utf8_data(); fs::create_directory(fs::u8path(utf8_string));For POSIX-based operating systems with the native narrow encoding set to UTF-8, no encoding or type conversion occurs.
For POSIX-based operating systems with the native narrow encoding not set to UTF-8, a conversion to UTF-32 occurs, followed by a conversion to the current native narrow encoding. Some Unicode characters may have no native character set representation. For Windows-based operating systems a conversion from UTF-8 to UTF-16 occurs. — end example]
Delete sub-clause D.29 [depr.fs.path.factory] in its entirety.
Section: 29.12 [time.format] Status: Open Submitter: Jonathan Wakely Opened: 2022-12-20 Last modified: 2023-02-10
Priority: 3
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Discussion:
The wording of 29.12 [time.format] p4 seems to have some unwanted consequences. It says:
The result of formatting a std::chrono::duration instance holding a negative value, or an hh_mm_ss object h for which h.is_negative() is true, is equivalent to the output of the corresponding positive value, with a STATICALLY-WIDEN<charT>("-") character sequence placed before the replacement of the initial conversion specifier.
Taken literally, I think that means:
format("{:%q}", -1s) == "-s"
format("{:%t%j}", -25h) == "-\t1"
format("{:%p%I}", -11h) == "-am11"
The last one probably doesn't matter (what does -11am mean anyway?) but some of them do matter, for example something like "{:(%q)%t%Q}" intends to put the unit suffix first in parens but will print "(-s)\t1" which probably isn't what the user wanted.
I think we want to place the sign before the first numeric conversion specifier, not "the initial conversion specifier". That is what Howard's date::format and <fmt> both do.
[Issaquah 2023-02-10; LWG issue processing]
Set priority to 3. Proposed a hybrid resolution.
Previous resolution [SUPERSEDED]:
Two alternatives are presented, choose one of Option A or Option B.
Modify 29.12 [time.format] as indicated:
Option A
-4- The result of formatting a
std::chrono::duration instance holding a negative value, or anchrono::hh_mm_ss object h for which h.is_negative() is true, is equivalent to the output of the corresponding positive value, with a STATICALLY-WIDEN<charT>("-") character sequence placed before the replacement of theinitialfirst conversion specifier that is not one of %n, %p, %q, %t, or %%.Option B
-4- Among the specifiers, %H, %I, %M, %S, and %T, the
Theresult of formatting astd::chrono::duration instance holding a negative value, or anchrono::hh_mm_ss object h for which h.is_negative() is true, is equivalent to the output of the corresponding positive value, with a STATICALLY-WIDEN<charT>("-") character sequence placed before the replacement of the initial conversion specifier.
Proposed resolution:
This wording is relative to N4928.
Modify 29.12 [time.format] as indicated:
-4- The result of formatting a
std::chrono::duration instance holding a negative value, or anchrono::hh_mm_ss object h for which h.is_negative() is true, is equivalent to the output of the corresponding positive value, with a STATICALLY-WIDEN<charT>("-") character sequence placed before the replacement of theinitialfirst conversion specifier that is one of %H, %I, %M, %S, or %T.
Section: 26.5.7.2 [range.utility.conv.to] Status: New Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-02-02
Priority: 4
View other active issues in [range.utility.conv.to].
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Discussion:
In bullet (1.1.2), ranges::to checks whether the container type C models constructible_from<from_range_t, ...> and constructs it via C(from_range, ...).
Since from_range is a constexpr variable here, it would be more accurate to constrain C to be constructible from a const lvalue tag rather than an rvalue tag.
[2023-02-02; Reflector poll]
Set priority to 4 after reflector poll. Several votes for "Tentatively Ready", but also two objections, preferring NAD. The proposed change would appear to bless unconventional uses of from_range, and we don't want to support or encourage that. The current wording is simpler, and works for the intended cases.
Proposed resolution:
This wording is relative to N4917.
Modify 26.5.7.2 [range.utility.conv.to] as indicated:
template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args);-1- Returns: An object of type C constructed from the elements of r in the following manner:
(1.1) — If convertible_to<range_reference_t<R>, range_value_t<C>> is true:
(1.1.1) — If constructible_from<C, R, Args...> is true:
C(std::forward<R>(r), std::forward<Args>(args)...)(1.1.2) — Otherwise, if constructible_from<C, const from_range_t&, R, Args...> is true:
C(from_range, std::forward<R>(r), std::forward<Args>(args)...)[…]
Section: 26.6.4.3 [range.iota.iterator] Status: New Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-02-01
Priority: 3
View all other issues in [range.iota.iterator].
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Discussion:
Currently, two iota_view::iterators can be subtracted only when the underlying W type models advanceable, where advanceable consists of a series of syntactic and semantic requirements similar to the random_access_iterator concept.
However, when W is an C++20 iterator type, whether it provides subtraction is irrelevant to its iterator category. In such cases, still requiring W to support a series of random access iterator-like operations seems too restrictive. Consider:
#include <list>
#include <ranges>
int main() {
std::list l{1, 2, 3, 4, 5};
auto it = std::counted_iterator(l.begin(), l.size());
auto r = std::views::iota(it, std::next(it, 3));
auto sz = r.size(); // 3 as expected
auto d = r.end() - r.begin(); // error: no match for 'operator-'
}
We can get the correct size of iota_view by subtracting two counted_iterators, but we cannot subtract two iota_view::iterators to get their difference, even though the underlying counted_iterator already models sized_sentinel_for for itself, which is not satisfactory.
I think we should relax the constraints of iota_view::iterator::operator- to allow the above case, which also makes it compatible with iota_view::sentinel::operator-.
[2023-02-01; Reflector poll]
Set priority to 3 after reflector poll. Several P0 votes, but an objection to P0 on the basis that we don't define what it means to use sized_sentinel_for on non-iterators. Others responded that we don't need to, as we only use it with iterators, and do not intend it to be usable with anything else.
Proposed resolution:
This wording is relative to N4917.
Modify 26.6.4.3 [range.iota.iterator] as indicated:
[…]namespace std::ranges { template<weakly_incrementable W, semiregular Bound> requires weakly-equality-comparable-with<W, Bound> && copyable<W> struct iota_view<W, Bound>::iterator { private: W value_ = W(); // exposition only public: […] friend constexpr iterator operator-(iterator i, difference_type n) requires advanceable<W>; friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires advanceable<W> || sized_sentinel_for<W, W>; }; }friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires advanceable<W> || sized_sentinel_for<W, W>;-23- Effects: Equivalent to:
using D = difference_type; if constexpr (is-integer-like<W>) { if constexpr (is-signed-integer-like<W>) return D(D(x.value_) - D(y.value_)); else return (y.value_ > x.value_) ? D(-D(y.value_ - x.value_)) : D(x.value_ - y.value_); } else { return x.value_ - y.value_; }
Section: 26.7.15.3 [range.join.with.iterator] Status: New Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-02-01
Priority: 3
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Discussion:
In order to preserve room for optimization, the standard always tries to propagate the noexcept specification of custom iter_move/iter_swap for different iterators.
But for join_with_view::iterator, these two specializations are the only ones in the standard that do not have a noexcept specification. This is because both invoke visit in the function body, and visit may throw an exception when the variant does not hold a value.
However, implementors are not required to follow the standard practice. Since the join_with_view::iterator's variant member only contains two alternative types, both libstdc++ and MSVC-STL avoid heavyweight visit calls by simply using multiple if statements. This means that it is still possible to add a conditional noexcept specification to these overloads, and there is already a precedent in the standard, namely common_iterator. All we need to do is add a Preconditions.
[2023-02-01; Reflector poll]
Set priority to 3 after reflector poll. "The iter_swap specification is wrong since we can swap Pattern and Inner. And this is something implementations can strengthen."
Proposed resolution:
This wording is relative to N4917.
Modify 26.7.15.3 [range.join.with.iterator] as indicated:
[…]namespace std::ranges { template<input_range V, forward_range Pattern> requires view<V> && input_range<range_reference_t<V>> && view<Pattern> && compatible-joinable-ranges<range_reference_t<V>, Pattern> template<bool Const> class join_with_view<V, Pattern>::iterator { […] Parent* parent_ = nullptr; // exposition only OuterIter outer_it_ = OuterIter(); // exposition only variant<PatternIter, InnerIter> inner_it_; // exposition only […] public: […] friend constexpr decltype(auto) iter_move(const iterator& x) noexcept(see below);{ using rvalue_reference = common_reference_t< iter_rvalue_reference_t<InnerIter>, iter_rvalue_reference_t<PatternIter>>; return visit<rvalue_reference>(ranges::iter_move, x.inner_it_); }friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(see below) requires indirectly_swappable<InnerIter, PatternIter>;{ visit(ranges::iter_swap, x.inner_it_, y.inner_it_); }}; }friend constexpr decltype(auto) iter_move(const iterator& x) noexcept(see below);-?- Let rvalue_reference be:
common_reference_t<iter_rvalue_reference_t<InnerIter>, iter_rvalue_reference_t<PatternIter>>-?- Preconditions: x.inner_it_.valueless_by_exception() is false.
-?- Effects: Equivalent to: return visit<rvalue_reference>(ranges::iter_move, x.inner_it_);
-?- Remarks: The exception specification is equivalent to:
noexcept(ranges::iter_move(declval<const InnerIter&>())) && noexcept(ranges::iter_move(declval<const PatternIter&>())) && is_nothrow_convertible_v<iter_rvalue_reference_t<InnerIter>, rvalue_reference> && is_nothrow_convertible_v<iter_rvalue_reference_t<PatternIter>, rvalue_reference>friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(see below) requires indirectly_swappable<InnerIter, PatternIter>;-?- Preconditions: x.inner_it_.valueless_by_exception() and y.inner_it_.valueless_by_exception() are each false.
-?- Effects: Equivalent to: visit(ranges::iter_swap, x.inner_it_, y.inner_it_).
-?- Remarks: The exception specification is equivalent to:
noexcept(ranges::iter_swap(declval<const InnerIter&>(), declval<const InnerIter&>())) && noexcept(ranges::iter_swap(declval<const PatternIter&>(), declval<const PatternIter&>()))
Section: 16.4.6.13 [res.on.exception.handling] Status: New Submitter: Jiang An Opened: 2023-01-07 Last modified: 2023-02-01
Priority: 3
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Discussion:
Some standard library types, such as std::pair, std::tuple, and std::array, are currently allowed to have a member or element type with a destructor that is noexcept(false). In order to conform to 16.4.6.13 [res.on.exception.handling]/3, these types can't always have implicitly declared destructors because the implicit exception specification may be noexcept(false).
However, it may be intentional to implement these types with implicitly declared destructors, and mainstream implementations (libc++, libstdc++, and MSVC STL) are already doing so. Furtherly, explicitly declared destructors are not allowed for algorithm result types (27.5 [algorithms.results]), while currently these class templates are allowed to be instantiated with classes with potentially throwing destructors, which is contradictory to 16.4.6.13 [res.on.exception.handling]/3.[2023-01-29; Daniel comments]
This issue has very much overlap with LWG 3229.
[2023-02-01; Reflector poll]
Set priority to 3 after reflector poll. Unclear who the "shall" imposes requirements on. Maybe split into constraint on the library to not put noexcept(false) on destructors, and constraint on users that the library can assume destructors don't throw. Should also make it clear which parts of the subclause are normative and which are not.
Proposed resolution:
Section: 26.7.16.2 [range.lazy.split.view] Status: New Submitter: Hewill Kang Opened: 2023-01-07 Last modified: 2023-02-01
Priority: 4
View other active issues in [range.lazy.split.view].
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Discussion:
Currently, lazy_split_view supports input range when the element of the pattern is less than or equal to 1. In order to ensure this condition at compile time, tiny-range constrains the type R to model sized_range and requires (remove_reference_t<R>::size() <= 1) to be a constant expression.
However, modeling a sized_range does not guarantee that ranges::size will be evaluated by R::size(). For example, when disable_sized_range<R> is specialized to true or R::size() returns a non-integer-like type, ranges::size can still compute the size by subtracting the iterator-sentinel pair when both satisfy sized_sentinel_for. Since the lazy_split_view's iterator uses R::size() to get the constant value of the pattern, we must ensure that this is indeed how ranges::size is calculated. Also, I think we can simplify tiny-range with bool_constant in a way similar to LWG 3150, which removes the introduction of require-constant.[2023-02-01; Reflector poll]
Set priority to 4 after reflector poll. Only matters for pathological types. Maybe use requires bool_constant<ranges::size(r) <= 1>.
Proposed resolution:
This wording is relative to N4917.
Modify 26.7.16.2 [range.lazy.split.view] as indicated:
namespace std::ranges {template<auto> struct require-constant; // exposition onlytemplate<class R> concept tiny-range = // exposition only sized_range<R> && requires { typename require-constant<remove_reference_t<R>::size()>; } && (remove_reference_t<R>::size() <= 1);template<input_range V, forward_range Pattern> requires view<V> && view<Pattern> && indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> && (forward_range<V> || tiny-range<Pattern>) class lazy_split_view : public view_interface<lazy_split_view<V, Pattern>> { […] }; […] }template<class R> concept tiny-range = // exposition only sized_range<R> && requires { requires bool_constant<(remove_reference_t<R>::size() <= 1)>::value; };-?- Given an lvalue r of type remove_reference_t<R>, R models tiny-range only if ranges::size(r) is evaluated by remove_reference_t<R>::size().
constexpr lazy_split_view(V base, Pattern pattern);-1- Effects: : Initializes base_ with std::move(base), and pattern_ with std::move(pattern).
Section: 29.12 [time.format] Status: New Submitter: Tam S. B. Opened: 2023-01-14 Last modified: 2023-05-30
Priority: 3
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Discussion:
29.12 [time.format]/3:
If the formatted object does not contain the information the conversion specifier refers to, an exception of type format_error is thrown.29.12 [time.format]/6:
If the type being formatted does not contain the information that the format flag needs, an exception of type format_error is thrown.
It's unclear how to determine if a type contain the needed information, and implementations diverge.
For example, consider
#include <chrono>
#include <format>
auto f(std::chrono::month_day_last mdl) {
return std::format("{:%j}", mdl);
}
Both libstdc++ and libc++ produce a compile-time error, claiming that the argument does not contain the information, while MSVC STL throws format_error at run time unless mdl is `January/last`, in which case the function returns "031".
Another interesting case is format("{:%d}", mdl) where the value
can be printed for all months except February, which requires a year
to know how many days it has.
A related example from Jonathan Wakely:
std::chrono::weekday_indexed wdi(Monday, 7); // 7th Monday in the month
assert( ! wdi.ok() );
assert( wdi.weekday().ok() );
std::format("{:%a}", wdi);
For %a the required information is "a valid weekday", and arguably this does contain a valid weekday. On the other hand, there's no 7th Monday, so this isn't valid. Should this throw or not?
This was discussed by LWG and Howard Hinnant summarized the intended behaviour as:
"The intention of 29.12 [time.format]/6 is to address things like formatting a duration with %F. A duration doesn't contain the calendrical information that %F requires (year, month, day). Ditto for using %a (weekday name) with a year. It is meant to address mismatching types and flags, and not meant to address values."
The type chrono::weekday does contain the information needed to print a weekday. A specific invalid value doesn't change that. The type chrono::month_day_last does not contain the information needed to print the day of the year. A specific value where the day can be known doesn't change that. The day of month is more interesting, and might need more discussion.
Jonathan proposed adding more examples to clarify the intention that only the type matters, and not the value. There is some redundancy between p3 and p6. Referring to "the formatted object" in p3 seems unclear. Saying "type" as in p6 is better. But p6 refers to "format flag" which is not defined, whereas p3 uses "conversion specifier" (defined at the start of that paragraph). The two uses of "flag" in p6 look like remnants from the earlier chrono::format feature that was replaced by integration with std::format.
[2023-02-01; Reflector poll]
Set priority to 3 after reflector poll.
[2023-05-30; Jonathan adds wording]
Proposed resolution:
This wording is relative to N4950.
Modify 29.12 [time.format] as indicated:
-3- Each conversion specifier conversion-spec is replaced by appropriate characters as described in Table 101 ([tab:time.format.spec]); the formats specified in ISO 8601:2004 shall be used where so described. Some of the conversion specifiers depend on the formatting locale. If the string literal encoding is a Unicode encoding form and the locale is among an implementation-defined set of locales, each replacement that depends on the locale is performed as if the replacement character sequence is converted to the string literal encoding. If the
formatted objecttype being formatted does not contain the information the conversion specifier refers to, an exception of typeformat_erroris thrown.[Example ?: A
durationdoes not contain enough information to format as a weekday using%w. Aweekday_indexeddoes contain enough information to format using%wandMonday[7]can be formatted as"1"even thoughMonday[7].ok()isfalse. Amonth_daydoes not contain enough information to format as the day of the year using%j, even when themonth()part isJanuary. — end example]However, if a flag refers to a "time of day" (e.g.,
%H,%I,%p, etc.), then a specialization ofdurationis interpreted as the time of day elapsed since midnight.-4- The result of formatting a
std::chrono::durationinstance holding a negative value, or anhh_mm_ssobjecthfor whichh.is_negative()istrue, is equivalent to the output of the corresponding positive value, with aSTATICALLY-WIDEN<charT>("-")character sequence placed before the replacement of the initial conversion specifier.[Example 1:
— end example]cout << format("{:%T}", -10'000s); // prints: -02:46:40 cout << format("{:%H:%M:%S}", -10'000s); // prints: -02:46:40 cout << format("minutes {:%M, hours %H, seconds %S}", -10'000s); // prints: minutes -46, hours 02, seconds 40-5- Unless explicitly requested, the result of formatting a chrono type does not contain time zone abbreviation and time zone offset information. If the information is available, the conversion specifiers
%Zand%zwill format this information (respectively).[Note 1: If the information is not available and a
%Zor%zconversion specifier appears in the chrono-format-spec, an exception of typeformat_erroris thrown, as described above. — end note]
-6- If the type being formatted does not contain the information that the format flag needs, an exception of typeformat_erroris thrown, as described above.
[Example 2: Adurationdoes not contain enough information to format as aweekday. — end example]
However, if a flag refers to a "time of day" (e.g.,%H,%I,%p, etc.), then a specialization ofdurationis interpreted as the time of day elapsed since midnight.
Section: 24.7.3.4.7 [mdspan.layout.stride] Status: New Submitter: Christian Robert Trott Opened: 2023-01-25 Last modified: 2023-02-06
Priority: 1
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Discussion:
During work on some follow on proposals for C++26 (like the padded layouts and submdspan) we stumbled over an issue in the C++23 mdspan definition.
If you have a layout_stride::mapping with just static extents, its default constructor will produce an inconsistent mapping. All its strides will be 0 but its extents are not. Thus the is_unique property is violated. We wrote a paper to fix this: P2763R0. We describe two options to fix it and state why we prefer one over the other, with proposed wording changes.[2023-02-06; Reflector poll]
Set priority to 1 after reflector poll.
Proposed resolution:
Section: 25.5.3.2 [const.iterators.alias] Status: New Submitter: Hewill Kang Opened: 2023-01-26 Last modified: 2023-02-06
Priority: 2
View all issues with New status.
Discussion:
In the C++20 iterator system, input_iterator is guaranteed to have a common reference, which reflects the indirectly_readable requires common_reference_t<iter_reference_t<I>&&, iter_value_t<I>&> to be a valid type.
However, for iter_const_reference_t which with a similar form:
template<indirectly_readable It>
using iter_const_reference_t =
common_reference_t<const iter_value_t<It>&&, iter_reference_t<It>>;
it is still theoretically possible to create an input_iterator that does not have a valid iter_const_reference_t, for example:
#include <iterator>
struct ref {
ref(int&);
};
struct rref {
rref(const int&);
rref(const ref&);
};
struct I {
using value_type = int;
using difference_type = std::ptrdiff_t;
ref operator*() const;
I& operator++();
I operator++(int);
friend rref iter_move(const I&);
};
static_assert(std::input_iterator<I>); // pass
using CR = std::iter_const_reference_t<I>; // error: no type named 'type' in 'struct std::common_reference<const int&&, ref>'
which causes basic_const_iterator<I> to produce a hard error internally when it is instantiated.
[2023-02-06; Reflector poll]
Set priority to 2 after reflector poll. Seems contrived, should probably constrain not give a hard error. The question of whether each range needs to have const reference to elems is important.
Proposed resolution:
Section: 26.7.24 [range.zip] Status: New Submitter: Barry Revzin Opened: 2023-01-28 Last modified: 2023-02-06
Priority: 4
View all issues with New status.
Discussion:
Consider:
#include <ranges>
struct Abstract {
virtual ~Abstract() = default;
virtual int f() = 0;
};
struct Concrete : Abstract {
int f() override { return 42; }
};
int main() {
Concrete c[10];
auto xformed = c | std::views::transform([](Concrete& c) -> Abstract& {
return c;
});
for (Abstract& a : xformed) { } // ok
auto zipped = std::views::zip(xformed);
for (auto&& [a] : zipped) { } // error
}
Here, xformed is a range whose reference type is Abstract& and whose value_type is Abstract. Even though you can't actually create a value of that value_type, that's okay here, because no code is actually trying to do so.
On the other hand, zipped is a range whose reference type is std::tuple<Abstract&> and whose value_type is std::tuple<Abstract>. No code here is actually trying to construct a value_type either, but this code fails because simply instantiating std::tuple<Abstract> is an error. There's no other possible value_type for zipped to have, std::tuple<Abstract> is correct — it's just that it happens to be an ill-formed type in this context. There are workarounds for this case — you would have to make xformed be a range of Abstract* or, probably better, a range of reference_wrapper<Abstract> instead. This is unfortunate because many (most?) algorithms don't actually make any use of a range's value_type. The ones that do (like ranges::min) obviously could not work, but currently we end up rejecting all uses. Probably the only possible way to make this work is to allow value_type to be void (or absent), but it is currently a fairly fundamental type due to its use in indirectly_readable to identify input iterators.[2023-02-06; Reflector poll]
Set priority to 4 after reflector poll. Several votes for NAD. Maybe tuple<Abstract> should be explicitly made ill-formed (currently seems underspecified) or should be "disabled" like invalid hash specializations and formatters.
Proposed resolution:
Section: 27.7.5 [alg.replace], 27.7.6 [alg.fill] Status: LEWG Submitter: Hewill Kang Opened: 2023-01-29 Last modified: 2023-02-06
Priority: 4
View all other issues in [alg.replace].
View all issues with LEWG status.
Discussion:
In C++20, output_iterator is required to support *o++ = t mainly for backward compatibility, that is to say, in addition to avoiding needless breakage, this semantic is actually not very useful, as it can be equivalently replaced by *o = t; ++o;.
This is reflected in the current implementation for constrained algorithms in libstdc++ and MSVC-STL. Even if the algorithm explicitly requires output_iterator, there is no code of the form *o++ = t in practice, and the latter of a more generic form is used instead. It seems to me that constrained algorithms should never require output_iterator, since there really isn't any desirable reason to use *o++ = t in the new iterator system. It would be more appropriate to relax output_iterator to weakly_incrementable (or input_or_output_iterator) and indirectly_writable, given that many constrained algorithms already do that.[2023-02-06; Reflector poll]
Set priority to 4 after reflector poll. Send to LEWG. Several votes for NAD.
Proposed resolution:
This wording is relative to N4928.
Modify 27.4 [algorithm.syn], header <algorithm> synopsis, as indicated:
#include <initializer_list> // see 17.10.2 [initializer.list.syn] namespace std { […] namespace ranges { template<class I, class O> using replace_copy_result = in_out_result<I, O>; template<input_iterator I, sentinel_for<I> S, class T1, class T2, weakly_incrementableoutput_iterator<const T2&>O, class Proj = identity> requires indirectly_writable<O, const T2&> && indirectly_copyable<I, O> && indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T1*> constexpr replace_copy_result<I, O> replace_copy(I first, S last, O result, const T1& old_value, const T2& new_value, Proj proj = {}); template<input_range R, class T1, class T2, weakly_incrementableoutput_iterator<const T2&>O, class Proj = identity> requires indirectly_writable<O, const T2&> && indirectly_copyable<iterator_t<R>, O> && indirect_binary_predicate<ranges::equal_to, projected<iterator_t<R>, Proj>, const T1*> constexpr replace_copy_result<borrowed_iterator_t<R>, O> replace_copy(R&& r, O result, const T1& old_value, const T2& new_value, Proj proj = {}); template<class I, class O> using replace_copy_if_result = in_out_result<I, O>; template<input_iterator I, sentinel_for<I> S, class T, weakly_incrementableoutput_iterator<const T&>O, class Proj = identity, indirect_unary_predicate<projected<I, Proj>> Pred> requires indirectly_writable<O, const T&> && indirectly_copyable<I, O> constexpr replace_copy_if_result<I, O> replace_copy_if(I first, S last, O result, Pred pred, const T& new_value, Proj proj = {}); template<input_range R, class T, weakly_incrementableoutput_iterator<const T&>O, class Proj = identity, indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred> requires indirectly_writable<O, const T&> && indirectly_copyable<iterator_t<R>, O> constexpr replace_copy_if_result<borrowed_iterator_t<R>, O> replace_copy_if(R&& r, O result, Pred pred, const T& new_value, Proj proj = {}); } […] namespace ranges { template<class T, input_or_output_iteratoroutput_iterator<const T&>O, sentinel_for<O> S> requires indirectly_writable<O, const T&> constexpr O fill(O first, S last, const T& value); template<class T, output_range<const T&> R> constexpr borrowed_iterator_t<R> fill(R&& r, const T& value); template<class T, input_or_output_iteratoroutput_iterator<const T&>O> requires indirectly_writable<O, const T&> constexpr O fill_n(O first, iter_difference_t<O> n, const T& value); } […] }
Modify 27.7.5 [alg.replace] as indicated:
[…] template<input_iterator I, sentinel_for<I> S, class T1, class T2, weakly_incrementableoutput_iterator<const T2&>O, class Proj = identity> requires indirectly_writable<O, const T2&> && indirectly_copyable<I, O> && indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T1*> constexpr ranges::replace_copy_result<I, O> ranges::replace_copy(I first, S last, O result, const T1& old_value, const T2& new_value, Proj proj = {}); template<input_range R, class T1, class T2, weakly_incrementableoutput_iterator<const T2&>O, class Proj = identity> requires indirectly_writable<O, const T2&> && indirectly_copyable<iterator_t<R>, O> && indirect_binary_predicate<ranges::equal_to, projected<iterator_t<R>, Proj>, const T1*> constexpr ranges::replace_copy_result<borrowed_iterator_t<R>, O> ranges::replace_copy(R&& r, O result, const T1& old_value, const T2& new_value, Proj proj = {}); template<input_iterator I, sentinel_for<I> S, class T, weakly_incrementableoutput_iterator<const T&>O, class Proj = identity, indirect_unary_predicate<projected<I, Proj>> Pred> requires indirectly_writable<O, const T&> && indirectly_copyable<I, O> constexpr ranges::replace_copy_if_result<I, O> ranges::replace_copy_if(I first, S last, O result, Pred pred, const T& new_value, Proj proj = {}); template<input_range R, class T, weakly_incrementableoutput_iterator<const T&>O, class Proj = identity, indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred> requires indirectly_writable<O, const T&> && indirectly_copyable<iterator_t<R>, O> constexpr ranges::replace_copy_if_result<borrowed_iterator_t<R>, O> ranges::replace_copy_if(R&& r, O result, Pred pred, const T& new_value, Proj proj = {});-6- Let E be
[…]
Modify 27.7.6 [alg.fill] as indicated:
[…] template<class T, input_or_output_iteratoroutput_iterator<const T&>O, sentinel_for<O> S> requires indirectly_writable<O, const T&> constexpr O ranges::fill(O first, S last, const T& value); template<class T, output_range<const T&> R> constexpr borrowed_iterator_t<R> ranges::fill(R&& r, const T& value); template<class T, input_or_output_iteratoroutput_iterator<const T&>O> requires indirectly_writable<O, const T&> constexpr O ranges::fill_n(O first, iter_difference_t<O> n, const T& value);-1- Let N be max(0, n) for the fill_n algorithms, and last - first for the fill algorithms.
[…]
Section: 26.7.15.2 [range.join.with.view] Status: New Submitter: Hewill Kang Opened: 2023-02-04 Last modified: 2023-02-10
Priority: 3
View all issues with New status.
Discussion:
In order to ensure that the pattern range is compatible with the inner range, join_with_view requires that the two range types must satisfy compatible-joinable-ranges, which requires that the value type, reference type, and rvalue reference type of the two range types share a common type.
However, when the two range types are const-qualified, there is no guarantee that their common reference type still exists, in which case a hard error may occur since join_with_view's const begin does not check for this (online example):
#include <ranges>
struct S {
S(const int&);
S(int&&);
S(const int&&) = delete;
};
int main() {
const auto r = std::views::single(std::views::single(0))
| std::views::join_with(std::views::single(S{0}));
auto e = std::ranges::iter_move(r.begin()); // hard error
}
[Issaquah 2023-02-10; LWG issue processing]
Set priority to 3.
Proposed resolution:
Section: 22.4.9 [tuple.rel] Status: New Submitter: Corentin Jabot Opened: 2023-02-08 Last modified: 2023-03-22
Priority: 3
View other active issues in [tuple.rel].
View all other issues in [tuple.rel].
View all issues with New status.
Discussion:
In 22.4.9 [tuple.rel]:
template<class... TTypes, tuple-like UTuple> constexpr bool operator==(const tuple<TTypes...>& t, const UTuple& u);
Is defined as a non-member non-friend function that "is to be found via argument-dependent lookup only."
The intent is that it should be defined as a hidden friend in tuple. The current specification is confusing as to which class should contain that hidden friend, or how to otherwise implement that adl only restriction. (An hostile reading may consider it to be a hidden friend of UTuple), and does not follow the guidance of P1601 "Recommendations for Specifying ``Hidden Friends''". We should consider making these operator== and operator<=> overloads hidden friends of tuple, i.e.
std::tuple {
template<class... TTypes, tuple-like UTuple>
friend constexpr bool operator==(const tuple& t, const UTuple& u);
template<class... TTypes, tuple-like UTuple>
friend constexpr see below operator<=>(const tuple&, const UTuple&);
};
[2023-02-18; Daniel provides wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4928.
Modify 22.4.2 [tuple.syn], header <tuple> synopsis, as indicated:
namespace std { […] // 22.4.9 [tuple.rel], relational operators template<class... TTypes, class... UTypes> constexpr bool operator==(const tuple<TTypes...>&, const tuple<UTypes...>&);template<class... TTypes, tuple-like UTuple> constexpr bool operator==(const tuple<TTypes...>&, const UTuple&);template<class... TTypes, class... UTypes> constexpr common_comparison_category_t<synth-three-way-result<TTypes, UTypes>...> operator<=>(const tuple<TTypes...>&, const tuple<UTypes...>&);template<class... TTypes, tuple-like UTuple> constexpr see below operator<=>(const tuple<TTypes...>&, const UTuple&);[…] }Modify 22.4.4 [tuple.tuple], class template tuple synopsis, as indicated:
namespace std { template<class... Types> class tuple { public: […] template<tuple-like UTuple> constexpr tuple& operator=(UTuple&&); template<tuple-like UTuple> constexpr const tuple& operator=(UTuple&&) const; // 22.4.9 [tuple.rel], relational operators template<tuple-like UTuple> friend constexpr bool operator==(const tuple&, const UTuple&); template<tuple-like UTuple> friend constexpr see below operator<=>(const tuple&, const UTuple&); // 22.4.4.3 [tuple.swap], tuple swap constexpr void swap(tuple&) noexcept(see below); constexpr void swap(const tuple&) const noexcept(see below); }; }Modify 22.4.9 [tuple.rel] as indicated:
template<class... TTypes, class... UTypes> constexpr bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u); template<class... TTypes,tuple-like UTuple> friend constexpr bool operator==(const tuple<TTypes...>& t, const UTuple& u);-1- For the first overload let UTuple be tuple<UTypes...>. For the second overload let TTypes denote the pack Types.
[…] -5- Remarks:
(5.1) — The elementary comparisons are performed in order from the zeroth index upwards. No comparisons or element accesses are performed after the first equality comparison that evaluates to false.
(5.2) — The second overload is to be found via argument-dependent lookup (6.5.4 [basic.lookup.argdep]) only.
template<class... TTypes, class... UTypes> constexpr common_comparison_category_t<synth-three-way-result<TTypes, UTypes>...> operator<=>(const tuple<TTypes...>& t, const tuple<UTypes...>& u); template<class... TTypes,tuple-like UTuple> friend constexpr common_comparison_category_t<synth-three-way-result<TTypes, Elems>...> operator<=>(const tuple<TTypes...>& t, const UTuple& u);-6- For the second overload, let TTypes denote the pack Types and Elems denotes the pack of types tuple_element_t<0, UTuple>, tuple_element_t<1, UTuple>, ... , tuple_element_t<tuple_size_v<UTuple> - 1, UTuple>.
[…] -8- Remarks: The second overload is to be found via argument-dependent lookup (6.5.4 [basic.lookup.argdep]) only.
[2023-03-05; Daniel comments and provides improved wording]
The revised wording ensures that no ambiguity exists between the overload candidates, furthermore the additional wording about being found via argument-dependent lookup only has been eliminated, because the general conventions of 16.4.6.6 [hidden.friends] apply.
The reusage of the exposition-only concept different-from is already existing practice in various places of 22.4 [tuple]. We have also existing wording practice where we have an extra Constraints: element added even though a prototype has already a language constraint as part of its signature.[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4928.
Modify 22.4.2 [tuple.syn], header <tuple> synopsis, as indicated:
namespace std {
[…]
// 22.4.9 [tuple.rel], relational operators
template<class... TTypes, class... UTypes>
constexpr bool operator==(const tuple<TTypes...>&, const tuple<UTypes...>&);
template<class... TTypes, tuple-like UTuple>
constexpr bool operator==(const tuple<TTypes...>&, const UTuple&);
template<class... TTypes, class... UTypes>
constexpr common_comparison_category_t<synth-three-way-result<TTypes, UTypes>...>
operator<=>(const tuple<TTypes...>&, const tuple<UTypes...>&);
template<class... TTypes, tuple-like UTuple>
constexpr see below operator<=>(const tuple<TTypes...>&, const UTuple&);
[…]
}
Modify 22.4.4 [tuple.tuple], class template tuple synopsis, as indicated:
namespace std {
template<class... Types>
class tuple {
public:
[…]
template<tuple-like UTuple>
constexpr tuple& operator=(UTuple&&);
template<tuple-like UTuple>
constexpr const tuple& operator=(UTuple&&) const;
// 22.4.9 [tuple.rel], relational operators
template<tuple-like UTuple>
friend constexpr bool operator==(const tuple&, const UTuple&);
template<tuple-like UTuple>
friend constexpr see below operator<=>(const tuple&, const UTuple&);
// 22.4.4.3 [tuple.swap], tuple swap
constexpr void swap(tuple&) noexcept(see below);
constexpr void swap(const tuple&) const noexcept(see below);
};
}
Modify 22.4.9 [tuple.rel] as indicated:
template<class... TTypes, class... UTypes> constexpr bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u); template<class... TTypes,tuple-like UTuple> friend constexpr bool operator==(const tuple<TTypes...>& t, const UTuple& u);-1- For the first overload let UTuple be tuple<UTypes...>. For the second overload let TTypes denote the pack Types.
-?- Constraints: For the second overload, different-from<UTuple, tuple> (26.5.2 [range.utility.helpers]) is true. -2- Mandates: […] […] -5- Remarks:
(5.1) —The elementary comparisons are performed in order from the zeroth index upwards. No comparisons or element accesses are performed after the first equality comparison that evaluates to false.
(5.2) — The second overload is to be found via argument-dependent lookup (6.5.4 [basic.lookup.argdep]) only.template<class... TTypes, class... UTypes> constexpr common_comparison_category_t<synth-three-way-result<TTypes, UTypes>...> operator<=>(const tuple<TTypes...>& t, const tuple<UTypes...>& u); template<class... TTypes,tuple-like UTuple> friend constexpr common_comparison_category_t<synth-three-way-result<TTypes, Elems>...> operator<=>(const tuple<TTypes...>& t, const UTuple& u);-6- For the second overload, let TTypes denote the pack Types and Elems denotes the pack of types tuple_element_t<0, UTuple>, tuple_element_t<1, UTuple>, ... , tuple_element_t<tuple_size_v<UTuple> - 1, UTuple>.
-?- Constraints: For the second overload, different-from<UTuple, tuple> (26.5.2 [range.utility.helpers]) is true. -7- Effects: […]-8- Remarks: The second overload is to be found via argument-dependent lookup (6.5.4 [basic.lookup.argdep]) only.
Section: 17.14.7 [support.c.headers.other] Status: New Submitter: Alex Mills Opened: 2023-02-09 Last modified: 2023-03-22
Priority: 4
View other active issues in [support.c.headers.other].
View all other issues in [support.c.headers.other].
View all issues with New status.
Discussion:
In 17.14.7 [support.c.headers.other], it states that the <name.h> headers behave "as if each name placed in the standard library namespace by the corresponding header is placed within the global namespace scope". There are exceptions for several different definitions in the headers, but it's left ambiguous as to whether their inclusion in the <name.h> headers is required or unspecified.
While it is clear that, for example, ::byte is prohibited, it isn't clear whether std::byte is required to be included at all (this applies to the other listed exceptions as well). Though example 1 makes it appear that their inclusion requirements are meant to be left unspecified, paragraph 1 does not provide clarification. Adding specific wording will ensure this will not be misinterpreted in the future. To do this, we should append a sentence to [support.c.headers.other] p1:Whether the listed exceptions are included in their corresponding <name.h> headers under the standard library namespace is unspecified.
[2023-03-22; Reflector poll]
Set priority to 4 after reflector poll.
"The new wording is confusingly inconsistent with the existing wording
in that paragraph. Would prefer '... Whether the listed exceptions are declared
in the namespace std by their corresponding <name.h>
headers is unspecified.'."
Proposed resolution:
This wording is relative to N4928.
Modify 17.14.7 [support.c.headers.other] as indicated:
-1- Every C header other than <complex.h> (17.14.2 [complex.h.syn]), <iso646.h> (17.14.3 [iso646.h.syn]), <stdalign.h> (17.14.4 [stdalign.h.syn]), <stdatomic.h> (33.5.12 [stdatomic.h.syn]), <stdbool.h> (17.14.5 [stdbool.h.syn]), and <tgmath.h> (17.14.6 [tgmath.h.syn]), each of which has a name of the form <name.h>, behaves as if each name placed in the standard library namespace by the corresponding <cname> header is placed within the global namespace scope, except for the functions described in 28.7.6 [sf.cmath], the std::lerp function overloads (28.7.4 [c.math.lerp]), the declaration of std::byte (17.2.1 [cstddef.syn]), and the functions and function templates described in 17.2.5 [support.types.byteops]. It is unspecified whether these names are first declared or defined within namespace scope (6.4.6 [basic.scope.namespace]) of the namespace std and are then injected into the global namespace scope by explicit using-declarations (9.9 [namespace.udecl]). Whether the listed exceptions are included in their corresponding <name.h> headers under the standard library namespace is unspecified.
Section: 22.5.3.1 [optional.optional.general], 22.8.6.1 [expected.object.general] Status: New Submitter: Casey Carter Opened: 2023-02-13 Last modified: 2023-03-22
Priority: 3
View all other issues in [optional.optional.general].
View all issues with New status.
Discussion:
While implementing P2505R5 "Monadic Functions for std::expected" we found it odd that the template type parameter for the assignment operator that accepts an argument by forwarding reference is defaulted, but the template type parameter for value_or is not. For consistency, it would seem that meow.value_or(woof) should accept the same arguments woof as does meow = woof, even when those arguments are braced-initializers.
That said, it would be peculiar to default the template type parameter of value_or to T instead of remove_cv_t<T>. For expected<const vector<int>, int> meow{unexpect, 42};, for example, meow.value_or({1, 2, 3}) would create a temporary const vector<int> for the argument and return a copy of that argument. Were the default template argument instead remove_cv_t<T>, meow.value_or({1, 2, 3}) could move construct its return value from the argument vector<int>. For the same reason, the constructor that accepts a forwarding reference with a default template argument of T should default that argument to remove_cv_t<T>. For consistency, it would be best to default the template argument of the perfect-forwarding construct, perfect-forwarding assignment operator, and value_or to remove_cv_t<T>. Since all of the arguments presented apply equally to optional, we believe optional should be changed consistently with expected. MSVCSTL has prototyped these changes successfully.[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4928.
Modify 22.5.3.1 [optional.optional.general] as indicated:
namespace std {
template<class T>
class optional {
public:
[…]
template<class U = remove_cv_t<T>>
constexpr explicit(see below) optional(U&&);
[…]
template<class U = remove_cv_t<T>> constexpr optional& operator=(U&&);
[…]
template<class U = remove_cv_t<T>> constexpr T value_or(U&&) const &;
template<class U = remove_cv_t<T>> constexpr T value_or(U&&) &&;
[…]
};
[…]
}
Modify 22.5.3.2 [optional.ctor] as indicated:
template<class U = remove_cv_t<T>> constexpr explicit(see below) optional(U&& v);-23- Constraints: […]
Modify 22.5.3.4 [optional.assign] as indicated:
template<class U = remove_cv_t<T>> constexpr optional& operator=(U&& v);-12- Constraints: […]
Modify 22.5.3.6 [optional.observe] as indicated:
template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) const &;-15- Mandates: […]
[…]template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) &&;-17- Mandates: […]
Modify 22.8.6.1 [expected.object.general] as indicated:
namespace std {
template<class T, class E>
class expected {
public:
[…]
template<class U = remove_cv_t<T>>
constexpr explicit(see below) expected(U&& v);
[…]
template<class U = remove_cv_t<T>> constexpr expected& operator=(U&&);
[…]
template<class U = remove_cv_t<T>> constexpr T value_or(U&&) const &;
template<class U = remove_cv_t<T>> constexpr T value_or(U&&) &&;
[…]
};
[…]
}
Modify 22.8.6.2 [expected.object.cons] as indicated:
template<class U = remove_cv_t<T>> constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);-23- Constraints: […]
Modify 22.8.6.4 [expected.object.assign] as indicated:
template<class U = remove_cv_t<T>> constexpr expected& operator=(U&& v);-9- Constraints: […]
Modify 22.8.6.6 [expected.object.obs] as indicated:
template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) const &;-16- Mandates: […]
[…]template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) &&;-18- Mandates: […]
Section: 27.11.8 [specialized.construct] Status: New Submitter: Jiang An Opened: 2023-02-17 Last modified: 2023-03-22
Priority: 3
View other active issues in [specialized.construct].
View all other issues in [specialized.construct].
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Discussion:
LWG 3870 removed the support for constructing objects via cv-qualified lvalues. However, it had not changed anything in the immediate context, which means some previously permitted usages become hard errors instead of substitution failures.
Note that ranges::uninitialized_default_construct and ranges::uninitialized_default_construct_n are underconstrained even before LWG 3870, because remove_reference_t<iter_reference_t<I>> may be a const type, while only it's cv-unqualified version (iter_value_t<I>) is required to be default_initializable. construct_at and ranges::construct_at are also made underconstrained because the function body is no longer valid when T is cv-qualified, which is not reflected in Constraints:.[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4928.
Modify 27.11.2 [special.mem.concepts] as indicated:
template<class I> concept nothrow-input-iterator = // exposition only input_iterator<I> && is_lvalue_reference_v<iter_reference_t<I>> && same_as<remove_cvref_t<iter_reference_t<I>>, remove_reference_t<iter_reference_t<I>> && same_as<remove_cvref_t<iter_reference_t<I>>, iter_value_t<I>>;
Modify 27.11.8 [specialized.construct] as indicated:
template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); namespace ranges { template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); }-1- Constraints: T is a cv-unqualified type, and t
-2- Effects: Equivalent to:The expression ::new (declval<void*>()) T(declval<Args>()...) is well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).return ::new (voidify(*location)) T(std::forward<Args>(args)...);
Section: 27.11.9 [specialized.destroy] Status: New Submitter: Jiang An Opened: 2023-02-17 Last modified: 2023-03-22
Priority: 3
View all issues with New status.
Discussion:
Currently, std::(ranges::)destroy_at is specified to destroy array elements in the increasing index order (27.11.9 [specialized.destroy]/1.1), which is inconsistent with the decreasing order specified in the core language (11.4.7 [class.dtor]/13) and the order for arrays created by std::make_shared and std::allocate_shared (mandated by LWG 3005).
Previous resolution [SUPERSEDED]:
This wording is relative to N4928.
Modify 27.11.9 [specialized.destroy] as indicated:
template<class T> constexpr void destroy_at(T* location); namespace ranges { template<destructible T> constexpr void destroy_at(T* location) noexcept; }-1- Effects:
(1.1) — If T is an array type, equivalent to destroy(rbegin
begin(*location), rendend(*location)).(1.2) — Otherwise, equivalent to location->~T().
[2023-02-26; Daniel comments and provides alternative wording]
The suggested fix indeed corrects an inconsistency, but also implies a silent behaviour change at runtime, since at least MSVC STL and libstdc++ implement the array destruction order as specified (others not tested). The below wording therefore suggests to introduce a specific feature macro for this, so that user code can potentially react on this, regardless of potential vendor API breakage hesitations. The natural feature macro to increase would be that which introduced the specific array destruction behavior of destroy_at, which was P0896R4, and which introduced __cpp_lib_ranges, on the other hand the specification change affects both the std::ranges and the std forms of destroy_at, so it seems plausible to suggest a new, specific feature macro for both destroy_at function templates. This is what the proposed wording does.
LWG should clarify whether an entry to C.1 [diff.cpp20] should be added as well.[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4928.
Modify 17.3.2 [version.syn], header <version> synopsis, as indicated and replace the placeholder YYYYMML by the year and month of adoption of this issue:
[…] #define __cpp_lib_coroutine 201902L // also in <coroutine> #define __cpp_lib_destroy_at YYYYMML // also in <memory> #define __cpp_lib_destroying_delete 201806L // also in <new> […]
Modify 27.11.9 [specialized.destroy] as indicated:
template<class T> constexpr void destroy_at(T* location); namespace ranges { template<destructible T> constexpr void destroy_at(T* location) noexcept; }-1- Effects:
(1.1) — If T is an array type, equivalent to destroy(rbegin
begin(*location), rendend(*location)).(1.2) — Otherwise, equivalent to location->~T().
Section: 25.3.4.4 [iterator.concept.winc] Status: New Submitter: Jiang An Opened: 2023-02-19 Last modified: 2023-03-22
Priority: 3
View all other issues in [iterator.concept.winc].
View all issues with New status.
Discussion:
Currently integer-class types are required to be wider than every integer type (25.3.4.4 [iterator.concept.winc]/3). As a result, if compiler provides extended integer types later whose width are not less than that of an integer-class type, then integer-class types with that width must be abandoned, which seems ABI-breaking (see also LWG 3828).
I think we should allow some extended integer types to be wider than some integer-class types.[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
"Proposed resolution is insufficient. A lot of the wording depends on the fact
that the range of widths of integer-class types and that of integer types are
disjoint. And it also disallows signed integer -> unsigned integer-class
conversion and by extension mixed ops, which is seriously breaking (e.g., it
disallows x += 1)."
Proposed resolution:
This wording is relative to N4928.
Modify 25.3.4.4 [iterator.concept.winc] as indicated:
-3- The range of representable values of an integer-class type is the continuous set of values over which it is defined. For any integer-class type, its range of representable values is either -2N-1 to 2N-1 - 1 (inclusive) for some integer N, in which case it is a signed-integer-class type, or 0 to 2N - 1 (inclusive) for some integer N, in which case it is an unsigned-integer-class type. In both cases, N is called the width of the integer-class type. The width of an integer-class type is greater than that of every standard integral type of the same signedness.
[…] -6- Expressions of integer-class type are explicitly convertible to any integer-like type, and implicitly convertible to any integral type or integer-class type of equal or greater width and the same signedness. Expressions of integral type areboth implicitly andexplicitly convertible to any integer-class type, and implicitly convertible to any integer-class type that can represent all values of the source type. Conversions between integral and integer-class types and between two integer-class types do not exit via an exception. The result of such a conversion is the unique value of the destination type that is congruent to the source modulo 2N, where N is the width of the destination type.
Section: 22.8.6.1 [expected.object.general] Status: New Submitter: Jiang An Opened: 2023-02-19 Last modified: 2023-03-22
Priority: 2
View all other issues in [expected.object.general].
View all issues with New status.
Discussion:
Currently the value_type of std::expected can be a cv-qualified type, which is possibly intented. However, LWG 3870 disallows std::construct_at to construct objects via cv T*, which breaks std::expected<cv T, E> because some operations are specified with std::construct_at (22.8.6.4 [expected.object.assign], 22.8.6.5 [expected.object.swap]).
I think when T is cv-qualified, it would be better to store std::remove_cv_t<T> subobject while sometimes (other than construction/destruction) access it via a cv-qualified glvalue, which can also avoid UB associated with const/volatile objects.[2023-03-22; Reflector poll]
Set priority to 2 after reflector poll.
"Not clear if all these wording changes are needed or desired."
"Unconvinced that the mixed-value-error swap should use value(),
source is destroyed immediately anyway. The else branch should use
remove_cv_t too."
Proposed resolution:
This wording is relative to N4928.
[Drafting note: When assignment and swap need to backup the old value by move construction, the source should be considered cv-unqualified, as the backup mechanism is only used internally.]
Modify 22.8.6.1 [expected.object.general] as indicated:
[…]
bool has_val; // exposition only
union {
remove_cv_t<T> val; // exposition only
E unex; // exposition only
};
[…]
Modify 22.8.6.4 [expected.object.assign] as indicated:
constexpr expected& operator=(const expected& rhs);[…]-2- Effects:
(2.1) — If this->has_value() && rhs.has_value() is true, equivalent to value()
val= *rhs.[…]
constexpr expected& operator=(expected&& rhs) noexcept(see below);[…][…]
-6- Effects:
(6.1) — If this->has_value() && rhs.has_value() is true, equivalent to value()
val= std::move(*rhs).[…]
template<class U = T> constexpr expected& operator=(U&& v);[…]
-10- Effects:
(10.1) — If has_value() is true, equivalent to value()
val= std::forward<U>(v).[…]
Modify Table 64: swap(expected&) effects [tab:expected.object.swap] as indicated:
Table 64 — swap(expected&) effects [tab:expected.object.swap] this->has_value() !this->has_value() rhs.has_value() equivalent to: using std::swap;
swap(value()val, rhs.value()val);calls rhs.swap(*this) […]
Modify 22.8.6.5 [expected.object.swap] as indicated:
constexpr void swap(expected& rhs) noexcept(see below);-1- Constraints: […]
-2- Effects: See Table 64 [tab:expected.object.swap]. For the case where rhs.value() is false and this->has_value() is true, equivalent to:if constexpr (is_nothrow_move_constructible_v<E>) { E tmp(std::move(rhs.unex)); destroy_at(addressof(rhs.unex)); try { construct_at(addressof(rhs.val), std::move(value()val)); destroy_at(addressof(val)); construct_at(addressof(unex), std::move(tmp)); } catch(...) { construct_at(addressof(rhs.unex), std::move(tmp)); throw; } } else { T tmp(std::move(val)); destroy_at(addressof(val)); try { construct_at(addressof(unex), std::move(rhs.unex)); destroy_at(addressof(rhs.unex)); construct_at(addressof(rhs.val), std::move(tmp)); } catch (...) { construct_at(addressof(val), std::move(tmp)); throw; } } has_val = false; rhs.has_val = true;
Section: 22.14.7.2 [format.range.formatter], 22.14.9 [format.tuple] Status: New Submitter: Victor Zverovich Opened: 2023-02-20 Last modified: 2023-03-22
Priority: 2
View all other issues in [format.range.formatter].
View all issues with New status.
Discussion:
formatter specializations for ranges and tuples set debug format for underlying element formatters in their parse functions e.g. 22.14.7.2 [format.range.formatter] p9:
template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx);Effects: Parses the format specifier as a range-format-spec and stores the parsed specifiers in *this. The values of opening-bracket_, closing-bracket_, and separator_ are modified if and only if required by the range-type or the n option, if present. If:
— the range-type is neither s nor ?s,
— underlying_.set_debug_format() is a valid expression, and
— there is no range-underlying-spec,
then calls underlying_.set_debug_format().
However, they don't say anything about calling parse functions of those formatters. As as result, formatting of nested ranges can be incorrect, e.g.
std::string s = std::format("{}", std::vector<std::vector<std::string>>{{"a, b", "c"}});
With the current specification s is [[a, b, c]] instead of [["a, b", "c"]], i.e. strings in the output are not correctly escaped. The same is true for nested tuples and combinations of tuples and ranges.
The fix approved by LEWG as part of P2733 (which was trying to address a different issue) is to always call parse for underlying formatter. Additionally the standard should clarify that format-spec cannot start with '}' because that's the implicit assumption in range formatting and what happens when format-spec is not present.[2023-03-22; Reflector poll]
Set priority to 2 after reflector poll.
Proposed resolution:
This wording is relative to N4928.
Modify 22.14.2.1 [format.string.general] p1 as indicated:
- […]
- format-specifier:
- : format-spec
- format-spec:
- as specified by the formatter specialization for the argument type; cannot start with }
Modify 22.14.6.1 [formatter.requirements] as indicated:
-3- Given character type charT, output iterator type Out, and formatting argument type T, in Table 74 [tab:formatter.basic] and Table 75 [tab:formatter]:
[…] pc.begin() points to the beginning of the format-spec (22.14.2 [format.string]) of the replacement field being formatted in the format string. If format-spec is not present or empty then either pc.begin() == pc.end() or *pc.begin() == '}'.
Modify 22.14.7.2 [format.range.formatter] as indicated:
template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx);-9- Effects: Parses the format specifiers as a range-format-spec and stores the parsed specifiers in *this. Calls underlying_.parse(ctx) to parse format-spec in range-format-spec or, if the latter is not present, empty format-spec. The values of opening-bracket_, closing-bracket_, and separator_ are modified if and only if required by the range-type or the n option, if present. If:
(9.1) — the range-type is neither s nor ?s,
(9.2) — underlying_.set_debug_format() is a valid expression, and
(9.3) — there is no range-underlying-spec,
then calls underlying_.set_debug_format().
Modify 22.14.9 [format.tuple] as indicated:
template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx);-7- Effects: Parses the format specifiers as a tuple-format-spec and stores the parsed specifiers in *this. The values of opening-bracket_, closing-bracket_, and separator_ are modified if and only if required by the tuple-type, if present. For each element e in underlying_, calls e.parse(ctx) to parse empty format-spec and, if e.set_debug_format() is a valid expression, calls e.set_debug_format().
Section: 18.3 [concepts.syn], 25.2 [iterator.synopsis] Status: New Submitter: blacktea hamburger Opened: 2023-02-25 Last modified: 2023-03-22
Priority: 3
View all issues with New status.
Discussion:
std::indirect_equivalence_relation and std::indirect_strict_weak_order have default values for the second template parameters:
template<class F, class I1, class I2 = I1> concept indirect_equivalence_relation = see below; template<class F, class I1, class I2 = I1> concept indirect_strict_weak_order = see below;
But std::relation, std::equivalence_relation, std::strict_weak_order, and std::indirect_binary_predicate are missing such default values:
template<class R, class T, class U> concept relation = see below; template<class R, class T, class U> concept equivalence_relation = see below; template<class R, class T, class U> concept strict_weak_order = see below; template<class F, class I1, class I2> concept indirect_binary_predicate = see below;
That makes them inconsistent and it should not be expected.
[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
"Borderline design change."
"Should not change indirect_binary_predicate."
"NAD, write a paper."
"NAD, default argument would make typos compile. Explicit is good."
Proposed resolution:
This wording is relative to N4928.
Modify 18.3 [concepts.syn], header <concepts> synopsis, as indicated:
// all freestanding
namespace std {
[…]
// 18.7.5 [concept.relation], concept relation
template<class R, class T, class U = T>
concept relation = see below;
// 18.7.6 [concept.equiv], concept equivalence_relation
template<class R, class T, class U = T>
concept equivalence_relation = see below;
// 18.7.7 [concept.strictweakorder], concept strict_weak_order
template<class R, class T, class U = T>
concept strict_weak_order = see below;
}
Modify 18.7.5 [concept.relation] as indicated:
template<class R, class T, class U = T>
concept relation =
predicate<R, T, T> && predicate<R, U, U> &&
predicate<R, T, U> && predicate<R, U, T>;
Modify 18.7.6 [concept.equiv] as indicated:
template<class R, class T, class U = T> concept equivalence_relation = relation<R, T, U>;
Modify 18.7.7 [concept.strictweakorder] as indicated:
template<class R, class T, class U = T> concept strict_weak_order = relation<R, T, U>;
Modify 25.2 [iterator.synopsis], header <iterator> synopsis, as indicated:
[…]
namespace std {
[…]
// 25.3.6.3 [indirectcallable.indirectinvocable], indirect callables
[…]
template<class F, class I1, class I2 = I1>
concept indirect_binary_predicate = see below; // freestanding
template<class F, class I1, class I2 = I1>
concept indirect_equivalence_relation = see below; // freestanding
[…]
}
Modify 25.3.6.3 [indirectcallable.indirectinvocable] as indicated:
-1- The indirect callable concepts are used to constrain those algorithms that accept callable objects (22.10.4 [func.require]) as arguments.
[…] template<class F, class I1, class I2 = I1> concept indirect_binary_predicate = indirectly_readable<I1> && indirectly_readable<I2> && copy_constructible<F> && predicate<F&, iter_value_t<I1>&, iter_value_t<I2>&> && predicate<F&, iter_value_t<I1>&, iter_reference_t<I2>> && predicate<F&, iter_reference_t<I1>, iter_value_t<I2>&> && predicate<F&, iter_reference_t<I1>, iter_reference_t<I2>> && predicate<F&, iter_common_reference_t<I1>, iter_common_reference_t<I2>>; […]
Section: 26.4.5 [range.refinements] Status: New Submitter: Hewill Kang Opened: 2023-02-27 Last modified: 2023-03-22
Priority: 4
View all other issues in [range.refinements].
View all issues with New status.
Discussion:
The requirements of viewable_range for view type is view<remove_cvref_t<T>> && constructible_from<remove_cvref_t<T>, T>, that is, when the decayed type of T models view, it must be constructible from T.
This part of the constraint corresponds to first bullet of views::all (26.7.6.1 [range.all.general]), which returns decay-copy(E) if the decayed type of E models view.
However, decay-copy(E) constraints convertible_to<T, decay_t<T>> which is a stronger requirement than constructible_from, which is reflected in its rejection of types with explicit copy constructors.
This inconsistency is such that the following causes the range adapter to produce a hard error when invoked (online example):
#include <ranges>
struct View : std::ranges::view_base
{
View();
explicit View(const View&); // explicit copy constructor
View& operator=(const View&);
View(View&&);
int* begin();
int* end();
};
int main()
{
View v;
auto r = std::views::take(v, 5); // hard error
}
[2023-03-22; Reflector poll]
Set priority to 4 after reflector poll. "About as contrived as it gets." "Add generic front matter telling users the library doesn't support types with explicit copy constructors."
Proposed resolution:
This wording is relative to N4928.
Modify 26.4.5 [range.refinements] as indicated:
template<class T>
concept viewable_range =
range<T> &&
((view<remove_cvref_t<T>> && convertible_to<T, remove_cvref_t<T>>constructible_from<remove_cvref_t<T>, T>) ||
(!view<remove_cvref_t<T>> &&
(is_lvalue_reference_v<T> || (movable<remove_reference_t<T>> && !is-initializer-list<T>))));
Section: 20.3.4.3 [inout.ptr.t] Status: New Submitter: Doug Cook Opened: 2023-02-27 Last modified: 2023-03-22
Priority: 2
View all other issues in [inout.ptr.t].
View all issues with New status.
Discussion:
inout_ptr seems useful for two purposes:
Using smart pointers with C-style APIs.
Annotating raw pointers for use with C-style APIs.
Unfortunately, as presently specified, it is not safe for developers to use inout_ptr for the second purpose. It is not safe to change code from
void* raw_ptr1; InitSomething(&raw_ptr1); UpdateSomething(&raw_ptr1); // In some cases may set raw_ptr1 = nullptr. CleanupSomething(raw_ptr1);
to
void* raw_ptr2; InitSomething(std::out_ptr(raw_ptr2)); UpdateSomething(std::inout_ptr(raw_ptr2)); // May leave dangling pointer CleanupSomething(raw_ptr2); // Possible double-delete
In the case where UpdateSomething would set raw_ptr1 = nullptr, the currently-specified inout_ptr implementation will leave raw_ptr2 at its old value. This would likely lead to a double-delete in CleanupSomething.
The issue occurs because inout_ptr is specified as follows:Constructor: If the user's pointer is a smart pointer, perform a "release" operation.
(C-style API executes)
If the C-style API returns a non-NULL pointer, propagate the returned value to the user's pointer.
If the user's pointer is not a smart pointer, no "release" operation occurs, and if the C-style API returns a NULL pointer, no propagation of the NULL occurs. We're left with a dangling raw pointer which is different from the original behavior using &.
I see two potential solutions:Make the "release" operation unconditional (i.e. it applies to both smart and raw pointers). For raw pointers, define the "release" operation as setting the raw pointer to nullptr.
Make the return value propagation unconditional for raw pointers.
Solution #2 seems likely to lead to more optimal code as it avoids an unnecessary branch.
[2023-03-22; Reflector poll]
Set priority to 2 after reflector poll.
Proposed resolution:
This wording is relative to N4928.
Modify 20.3.4.3 [inout.ptr.t] as indicated:
~inout_ptr_t();-9- Let SP be POINTER_OF_OR(Smart, Pointer) (20.2.1 [memory.general]).
-10- Let release-statement be s.release(); if an implementation does not call s.release() in the constructor. Otherwise, it is empty. -11- Effects: Equivalent to:
(11.1) —
if (p) {apply([&](auto&&... args) { s = Smart( static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));}if is_pointer_v<Smart> is true;
(11.2) — otherwise,
release-statement; if (p) { apply([&](auto&&... args) { s.reset(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a)); }if the expression s.reset(static_cast<SP>(p), std::forward<Args>(args)...) is well-formed;
(11.3) — otherwise,
release-statement; if (p) { apply([&](auto&&... args) { s = Smart(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a)); }if is_constructible_v<Smart, SP, Args...> is true;
(11.4) — otherwise, the program is ill-formed.
Section: 33.9.3.3 [thread.barrier.class] Status: New Submitter: Jiang An Opened: 2023-03-02 Last modified: 2023-03-22
Priority: 3
View all issues with New status.
Discussion:
33.9.3.3 [thread.barrier.class]/5 currently says:
[…] is_nothrow_invocable_v<CompletionFunction&> shall be true.
This requirement introduces a kind of undefined behavior and permits implementation divergence. Currently MSVC STL enforces the requirement, while libstdc++ and libc++ don't.
If implementation divergence is not intended, I don't think it makes much sense to introduce UB in this way. I guess we should either strengthen the requirement to require well-formedness affection or relax it.[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4928.
[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
Option A: Effectively impose a Mandates: requirement.
Modify 33.9.3.3 [thread.barrier.class] as indicated:
-5- CompletionFunction shall meet the Cpp17MoveConstructible (Table 32) and Cpp17Destructible (Table 36) requirements. Instantiation of barrier<CompletionFunction> is ill-formed if is_nothrow_invocable_v<CompletionFunction&> is not true
is_nothrow_invocable_v<CompletionFunction&> shall be true.Option B: Clarify that we impose a no-throw precondition here, whose violation causes UB.
Modify 33.9.3.3 [thread.barrier.class] as indicated:
-3- The phase completion step that is executed at the end of each phase has the following effects:
(3.1) — Invokes the completion function, equivalent to completion(). If any invocation to the completion function throws an exception, the behavior is undefined.
(3.2) — Unblocks all threads that are blocked on the phase synchronization point.
[…]
-5- CompletionFunction shall meet the Cpp17MoveConstructible (Table 32) and Cpp17Destructible (Table 36) requirements.is_nothrow_invocable_v<CompletionFunction&> shall be true.
[2023-03-22; Jonathan provides improved wording]
Proposed resolution:
This wording is relative to N4928.
Modify 33.9.3.3 [thread.barrier.class] as indicated:
-3- The phase completion step that is executed at the end of each phase has the following effects:
(3.1) — Invokes the completion function, equivalent to
completion(); if that invocation exits via an exception, the functionstd::terminateis invoked.(3.2) — Unblocks all threads that are blocked on the phase synchronization point.
[…]
-5- CompletionFunction shall meet the Cpp17MoveConstructible (Table 32) and Cpp17Destructible (Table 36) requirements.
A program that instantiatesis_nothrow_invocable_v<CompletionFunction&>shall betrue.barrier<CompletionFunction>is ill-formed ifis_invocable_v<CompletionFunction&>isfalse.
Section: 26.8.5 [coro.generator.promise] Status: New Submitter: Tim Song Opened: 2023-03-04 Last modified: 2023-03-22
Priority: 3
View other active issues in [coro.generator.promise].
View all other issues in [coro.generator.promise].
View all issues with New status.
Discussion:
Consider:
std::generator<int> f();
std::generator<int> g() {
auto gen = f();
auto gen2 = f();
co_yield std::ranges::elements_of(std::move(gen)); // #1
co_yield std::ranges::elements_of(gen2); // #2
// other stuff
}
Both #1 and #2 compile. The differences are:
#2 is significantly less efficient (it uses the general overload of yield_value, so it creates a new coroutine frame and doesn't do symmetric transfer into gen2's coroutine)
the coroutine frame of gen and gen2 are destroyed at different times: gen's frame is destroyed at the end of #1, but gen2's is not destroyed until the closing brace.
But as far as the user is concerned, neither gen nor gen2 is usable after the co_yield. In both cases the only things you can do with the objects are:
destroying them;
assigning to them;
call end() on them to get a copy of default_sentinel.
We could make #2 ill-formed, but that seems unnecessary: there is no meaningful difference between generator and any other single-pass input range (or a generator with a different yielded type that has to go through the general overload) in this regard. We should just make #2 do the efficient thing too.
[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4928.
Modify 26.8.5 [coro.generator.promise] as indicated:
[…]namespace std { template<class Ref, class V, class Allocator> class generator<Ref, V, Allocator>::promise_type { public: […] auto yield_value(const remove_reference_t<yielded>& lval) requires is_rvalue_reference_v<yielded> && constructible_from<remove_cvref_t<yielded>, const remove_reference_t<yielded>&>; template<class R2, class V2, class Alloc2, class Unused> requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded> auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&&, Unused> g) noexcept; template<class R2, class V2, class Alloc2, class Unused> requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded> auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&, Unused> g) noexcept; template<ranges::input_range R, class Alloc> requires convertible_to<ranges::range_reference_t<R>, yielded> auto yield_value(ranges::elements_of<R, Alloc> r) noexcept; […] }; }template<class R2, class V2, class Alloc2, class Unused> requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded> auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&&, Unused> g) noexcept; template<class R2, class V2, class Alloc2, class Unused> requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded> auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&, Unused> g) noexcept;-10- Preconditions: A handle referring to the coroutine whose promise object is *this is at the top of *active_ of some generator object x. The coroutine referred to by g.range.coroutine_ is suspended at its initial suspend point.
-11- Returns: An awaitable object of an unspecified type (7.6.2.4 [expr.await]) into which g.range is moved, whose member await_ready returns false, whose member await_suspend pushes g.range.coroutine_ into *x.active_ and resumes execution of the coroutine referred to by g.range.coroutine_, and whose member await_resume evaluates rethrow_exception(except_) if bool(except_) is true. If bool(except_) is false, the await_resume member has no effects. -12- Remarks: A yield-expression that callsthis functionone of these functions has type void (7.6.17 [expr.yield]).
Section: 26.8.5 [coro.generator.promise] Status: New Submitter: Tim Song Opened: 2023-03-04 Last modified: 2023-03-22
Priority: 3
View other active issues in [coro.generator.promise].
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Discussion:
When the allocator is not type-erased, the allocator_arg_t overloads of generator::promise_type::operator new are constrained on convertible_to<const Alloc&, Allocator>. As a result, if the the allocator is default-constructible (like polymorphic_allocator is) but the user accidentally provided a wrong type (say, memory_resource& instead of memory_resource*), their code will silently fall back to using a default-constructed allocator. It would seem better to take the tag as definitive evidence of the user's intent to supply an allocator for the coroutine, and error out if the supplied allocator cannot be used.
This change does mean that the user cannot deliberately pass an incompatible allocator (preceded by an std::allocator_arg_t tag) for their own use inside the coroutine, but that sort of API seems fragile and confusing at best, since the usual case is that allocators so passed will be used by generator.[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4928.
Modify 26.8.5 [coro.generator.promise] as indicated:
[…]namespace std { template<class Ref, class V, class Allocator> class generator<Ref, V, Allocator>::promise_type { public: […] void* operator new(size_t size) requires same_as<Allocator, void> || default_initializable<Allocator>; template<class Alloc, class... Args>requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>void* operator new(size_t size, allocator_arg_t, const Alloc& alloc, const Args&...); template<class This, class Alloc, class... Args>requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>void* operator new(size_t size, const This&, allocator_arg_t, const Alloc& alloc, const Args&...); […] }; }void* operator new(size_t size) requires same_as<Allocator, void> || default_initializable<Allocator>; template<class Alloc, class... Args>requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>void* operator new(size_t size, allocator_arg_t, const Alloc& alloc, const Args&...); template<class This, class Alloc, class... Args>requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>void* operator new(size_t size, const This&, allocator_arg_t, const Alloc& alloc, const Args&...);-17- Let A be
(17.1) — Allocator, if it is not void,
(17.2) — Alloc for the overloads with a template parameter Alloc, or
(17.3) — allocator<void> otherwise.
Let B be allocator_traits<A>::template rebind_alloc<U> where U is an unspecified type whose size and alignment are both __STDCPP_DEFAULT_NEW_ALIGNMENT__.
-18- Mandates: allocator_traits<B>::pointer is a pointer type. For the overloads with a template parameter Alloc, same_as<Allocator, void> || convertible_to<const Alloc&, Allocator> is modeled. -19- Effects: Initializes an allocator b of type B with A(alloc), for the overloads with a function parameter alloc, and with A() otherwise. Uses b to allocate storage for the smallest array of U sufficient to provide storage for a coroutine state of size size, and unspecified additional state necessary to ensure that operator delete can later deallocate this memory block with an allocator equal to b. -20- Returns: A pointer to the allocated storage.
Section: 22.2.6 [declval] Status: New Submitter: Jiang An Opened: 2023-03-07 Last modified: 2023-03-22
Priority: 4
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Discussion:
Currently libc++ and libstdc++ determine the return type of std::declval like this:
template<class _Tp> _Tp&& __declval_ret(int); // selected when _Tp is a referenceable type template<class _Tp> _Tp __declval_ret(long); // selected when _Tp is cv void template<class _Tp> decltype(__declval_ret<_Tp>(0)) declval() noexcept;
This strategy avoids instantiation of class templates. But it also drops cv-qualifiers of the return type when the type is cv void, which is different from the standard requirements. Such difference has no impact in normal use of std::declval, but is observable via decltype(std::declval<const void>) and its friends.
Given maintainers may think it's reasonable to keep the current implementation (see GCC Bugzilla #109049), it may be worthwhile to legitimate such strategy. Should we make such construction ill-formed?[2023-03-22; Reflector poll]
Set priority to 4 after reflector poll.
"The testcase isn't even valid with the previous 'conforming' libstdc++ implementation."
"declval isn't an addressable function, so would prefer if this
was ill-formed rather than complicating the definition for this case."
Proposed resolution:
This wording is relative to N4928.
Modify 22.2.1 [utility.syn], header <utility> synopsis, and 22.2.6 [declval] as indicated:
template<class T> remove_cv_t<add_rvalue_reference_t<T>> declval() noexcept; // as unevaluated operand
Section: 33.5.8.5 [atomics.types.pointer], 33.5.7.5 [atomics.ref.pointer] Status: New Submitter: Jiang An Opened: 2023-03-18 Last modified: 2023-03-18
Priority: 3
View all issues with New status.
Discussion:
This was originally editorial issue #6185.
The term "undefined address" is used in 33.5.8.5 [atomics.types.pointer]/8 and 33.5.7.5 [atomics.ref.pointer]/6, however, it seems not properly defined and the intended meaning is unclear. The term originally appeared in the paper N2393 and was reused by P0019R8, but no explanation can be found in these papers. Perhaps "undefined address" is related to the undefined behavior specified in 7.6.6 [expr.add].[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 26.6 [range.factories], 26.7 [range.adaptors] Status: New Submitter: Jiang An Opened: 2023-03-18 Last modified: 2023-03-18
Priority: 3
View all issues with New status.
Discussion:
[For the term SCARY see N2911 and N2913.]
In 26.6 [range.factories] and 26.7 [range.adaptors], many iterator types are specified as exposition-only nested classes or nested class templates. This has some observable differences from specifying them as class templates declared in the namespace scope:value types are generally not associated entities of iterators or sentinels during ADL, which prevents unintended overloads to be found;
two different range adaptor/factory types, e.g. iota_view<I, S1> and iota_view<I, S2>, have different iterator types.
The ADL reduction seems preferable. However, is it intended to required to any two different range adaptor/factory types to have two different iterator types, even when the same iterator type can work as expected?
Sentinel types don't seem able to be SCARY as they are dependent on iterator types.[2023-06-01; Reflector poll]
Set priority to 3 after reflector poll.
"The ADL hiding stuff is a red herring - SCARY-ness doesn't prevent ADL hiding. Shouldn't require them to be different types (and similarly should not require const/non-const iterators to be implemented as class templates with a single bool template parameter)."
Proposed resolution:
<iostream> on initialization are not yet precisely specifiedSection: 31.4.2 [iostream.objects.overview] Status: New Submitter: Jiang An Opened: 2023-03-27 Last modified: 2023-03-27
Priority: 4
View all other issues in [iostream.objects.overview].
View all issues with New status.
Discussion:
LWG 3878 specified the effects of importing standard library modules on initialization.
However, the effects of including <iostream> are not yet precisely specified.
A hostile reading may consider this to allow the ios_base::Init variable to be
a function-local static variable or an inline variable, which defeats the purpose.
[2023-05-24; Reflector poll]
Set priority to 4 after reflector poll.
Proposed resolution:
This wording is relative to N4944.
Change 31.4.2 [iostream.objects.overview]/5 as indicated:
-5- The results of including
<iostream>in a translation unit shall be as if<iostream>defined aninstance ofios_base::Initwithstatic storage durationordered initialization (6.9.3.3 [basic.start.dynamic]). Each C++ library module (16.4.2.4 [std.modules]) in a hosted implementation shall behave as if it contains an interface unit that defines an unexportedios_base::Initvariable with ordered initialization(6.9.3.3 [basic.start.dynamic]).
unique_ptr's operator* is missing a mandateSection: 20.3.1.3.5 [unique.ptr.single.observers], 20.3.1.4.4 [unique.ptr.runtime.observers] Status: New Submitter: Brian Bi Opened: 2023-03-27 Last modified: 2023-06-01
Priority: 3
View all other issues in [unique.ptr.single.observers].
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Discussion:
The return type of std::unique_ptr<T>::operator* is
std::add_lvalue_reference_t<T>,
but there is no mandate stating that *get() is convertible to that type.
There also does not appear to be a mandate that *get() is a valid expression;
dereferenceability is not part of the Cpp17NullablePointer requirements.
A similar issue appears to exist for std::unique_ptr<T[]>::operator[].
[2023-03-28; Reflector poll]
Set priority to 3 after reflector poll.
"It would be nice to Mandate !reference_converts_from_temporary_v<add_lvalue_reference_t<T>, decltype(*get())>."
"noexcept-specifier isn't quite right, conversion from *get() to T& can throw."
Previous resolution [SUPERSEDED]:
This wording is relative to N4944.
Add the following bullet before 20.3.1.3.5 [unique.ptr.single.observers] paragraph 1:
constexpr add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>()));-?- Mandates:
*get()is a valid expression that is convertible toadd_lvalue_reference_t<T>.-1- Preconditions:
get() != nullptr.-2- Returns:
*get().Add the following bullet before 20.3.1.4.4 [unique.ptr.runtime.observers] paragraph 1:
constexpr T& operator[](size_t i) const;-?- Mandates:
get()[i]is a valid expression that is convertible toT&.-1- Preconditions:
i< the number of elements in the array to which the stored pointer points.-2- Returns:
get()[i].
[2023-04-03; Jonathan provides new wording as requested by LWG]
Proposed resolution:
This wording is relative to N4944.
Add the following bullet before 20.3.1.3.5 [unique.ptr.single.observers] paragraph 1:
constexpr add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>()));-1- Preconditions:
get() != nullptr.-2-
ReturnsEffects: Equivalent to:return*get();.
Add the following bullet before 20.3.1.4.4 [unique.ptr.runtime.observers] paragraph 1:
constexpr T& operator[](size_t i) const;-1- Preconditions:
get() != nullptr.i< the number of elements in the array to which the stored pointer points-2-
ReturnsEffects: Equivalent to:returnget()[i];.
ranges::const_iterator_t<range R> fails to accept arrays of unknown boundSection: 26.2 [ranges.syn] Status: SG9 Submitter: Arthur O'Dwyer Opened: 2023-03-28 Last modified: 2023-06-01
Priority: 3
View other active issues in [ranges.syn].
View all other issues in [ranges.syn].
Discussion:
ranges::iterator_t<T> is unconstrained; but in contrast,
ranges::const_iterator_t<T> constrains T to be a range,
i.e., it must have an end.
static_assert(std::is_same_v<std::ranges::iterator_t<int(&)[2]>, int*>);
static_assert(std::is_same_v<std::ranges::iterator_t<int(&)[]>, int*>); // OK
static_assert(std::is_same_v<std::ranges::const_iterator_t<int(&)[2]>, const int*>);
static_assert(std::is_same_v<std::ranges::const_iterator_t<int(&)[]>, const int*>); // hard error
[2023-06-01; Reflector poll]
Set priority to 3 after reflector poll. Several votes for NAD. Send to SG9 to be looked at with P2836.
Proposed resolution:
This wording is relative to N4944.
Modify 26.2 [ranges.syn] as indicated:
template<class T> using iterator_t = decltype(ranges::begin(declval<T&>())); // freestanding template<range R> using sentinel_t = decltype(ranges::end(declval<R&>())); // freestanding template<rangeclass R> using const_iterator_t = const_iterator<iterator_t<R>>; // freestanding template<range R> using const_sentinel_t = const_sentinel<sentinel_t<R>>; // freestanding
Section: 20.2.10 [default.allocator], 20.4.3.1 [mem.poly.allocator.class.general], 24.2 [container.requirements] Status: New Submitter: Stephan T. Lavavej Opened: 2023-04-02 Last modified: 2023-04-11
Priority: 3
View other active issues in [default.allocator].
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Discussion:
LWG 2447 adjusted what is now N4944 16.4.4.6.1 [allocator.requirements.general]/2.1 so that the Cpp17Allocator requirements are specified for cv-unqualified object types only.
However, nothing in 20.2.10 [default.allocator] restricts what T in allocator<T> can be, except where 20.2.10.2 [allocator.members]/2 and /6 require it to be complete for allocate() and allocate_at_least() (and by implication, for deallocate()). (Long ago, allocator had member functions whose signatures implied that const-qualified types were forbidden, but those signatures were deprecated and removed. This explains the phrasing of an MSVC static assertion.) 20.4.3.1 [mem.poly.allocator.class.general] says a bit more about Tp in polymorphic_allocator<Tp> but doesn't forbid anything. It says that if Tp is a cv-unqualified object type, then polymorphic_allocator<Tp> meets the Cpp17Allocator requirements and allocator completeness requirements, but that's all. There's some implementation variation here. libstdc++ and MSVC reject allocator<const int>. libc++ (as of LLVM 16) accepts it but this appears to have been an extension that they're trying to get rid of (their maintainers may be able to comment further; see llvm-project/commit/a54d028895c91da356a4aaf30e27a5a5b90dd313). These 3 implementations all reject allocator<volatile int>, polymorphic_allocator<const int>, and polymorphic_allocator<volatile int> with varying messages. The Standard should provide clarity here, by mandating that only cv-unqualified object types be given to allocator, polymorphic_allocator, and containers. (allocator<void> must also be allowed, of course. I forget if polymorphic_allocator is supposed to accept void.) This would simply Standardize existing/desired practice. While it may seem arcane, attempting to form vector<const T> is a common novice mistake — so common that MSVC had to add a static_assert to emit a clear error message.[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 20.2.10 [default.allocator], 20.4.3 [mem.poly.allocator.class] Status: New Submitter: Daniel Krügler Opened: 2023-04-08 Last modified: 2023-04-11
Priority: 3
View other active issues in [default.allocator].
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Discussion:
Before P0174 had been approved for the working paper, the validity of using void as template argument for std::allocator was obvious due to the existing specification of the explicit specialization allocator<void>.
This specialization was first moved to Annex D ([depr.default.allocator]) and later completely eradicated from the working paper via adoption of P0619. The problem is that, since then, we have no explicit wording for std::allocator to support void as template argument any more. We fall now under the constraints for template "components" specified in 16.4.5.8 [res.on.functions] bullet 2.5.-2- In particular, the behavior is undefined in the following cases:
[…]
(2.5) — If an incomplete type (6.8.1 [basic.types.general]) is used as a template argument when instantiating a template component or evaluating a concept, unless specifically allowed for that component.
But no such allowance wording exists for allocator<void> nor for polymorphic_allocator<void>, more to the contrary, 16.4.4.6.1 [allocator.requirements.general] only refers to cv-unqualified object types as value types and void is not an object type.
Now we could argue that the wording is clear that instantiations of these are invalid, but there exists at least some indication that the instantiations are intended to be allowed. As of 26.8.5 [coro.generator.promise] bullet (17.3) the static operator new members of std::generator mention a fall-back of using allocator<void>. 20.2.10.1 [default.allocator.general] says that all specializations of the default allocator meet the allocator completeness requirements (16.4.4.6.2 [allocator.requirements.completeness]), but albeit this specification does not specifically exclude the existence of an incomplete value type, the wording here does also not provide a definite statement, that it is valid (The wording originally was provided when we started adding support for (yet) incomplete value types that at some point later will become complete, but void can never be completed), since it is mostly focused on the completeness requirement for the allocator type itself. The situation is similar (albeit maybe not that strong) for polymorphic_allocator<void>, since 20.4.3 [mem.poly.allocator.class] p1 has some unusual wording form that says-1- A specialization of class template pmr::polymorphic_allocator meets the Cpp17Allocator requirements (16.4.4.6.1 [allocator.requirements.general]) if its template argument is a cv-unqualified object type.
and says then in p2:
-2- A specialization of class template pmr::polymorphic_allocator meets the allocator completeness requirements (16.4.4.6.2 [allocator.requirements.completeness]) if its template argument is a cv-unqualified object type.
Again, this wording is not conclusive, whether void is intended to be supported, it is certainly not completely ruled out, but that is not strong enough to counterpart 16.4.5.8 [res.on.functions] (2.5). It is maybe worth pointing out that for a while we were considering to use void as default template argument for pmr::polymorphic_allocator, see e.g. P0339R0, but that thought was later replaced by deciding for std::byte instead, which is a complete object type.
I assume that at least the intention exists that std::allocator is intended to support incomplete types, maybe also for polymorphic_allocator. If polymorphic_allocator is intended to support incomplete types as well, we should also amend 20.4.3.3 [mem.poly.allocator.mem] p1 and p8 with a Mandates: element similarly as we did for std::allocator via LWG 3307.[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4944.
Add the following new paragraph at the end of 20.2.10.1 [default.allocator.general] as indicated:
-2- allocator_traits<allocator<T>>::is_always_equal::value is true for any T.
-?- The template parameter T of allocator may be an incomplete type.
Add the following new paragraph at the end of 20.4.3.1 [mem.poly.allocator.class.general] (possibly just after the class template synopsis) as indicated:
-?- The template parameter Tp of polymorphic_allocator may be an incomplete type.
Modify 20.4.3.3 [mem.poly.allocator.mem] as indicated:
[Drafting note: The reference to sizeof(Tp) gives indirect evidence that we want to exclude incomplete types here, but we cannot rely on the "equivalent to" magic formula, because that is defined conditionally]
[[nodiscard]] Tp* allocate(size_t n);-?- Mandates: Tp is not an incomplete type (6.8.1 [basic.types.general]).
-1- Effects: If numeric_limits<size_t>::max() / sizeof(Tp) < n, throws bad_array_new_length. Otherwise equivalent to:return static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));[Drafting note: We don't need extra wording for the member templates allocate_object, deallocate_object, new_object, delete_object, or construct, because their semantics does not depend on template parameter Tp and the general wording of 16.4.5.8 [res.on.functions] (2.5) again requires the completeness of T here.
For deallocate we also omit the completeness requirement (as we did so for allocator::deallocate), because this is indirectly implied by the existing precondition. ]
Section: 27.11.6 [uninitialized.move] Status: LEWG Submitter: Jiang An Opened: 2023-04-04 Last modified: 2023-06-01
Priority: 3
View all issues with LEWG status.
Discussion:
Currently std::move is unconditionally used in std::uninitialized_move and std::uninitialized_move_n, which may involve unnecessary move construction if dereferencing the input iterator yields a prvalue.
The status quo was mentioned in paper issue #975, but no further process is done since then.[2023-06-01; Reflector poll]
Set priority to 3 after reflector poll. Send to LEWG.
"P2283 wants to remove guaranteed elision here."
"Poorly motivated, not clear anybody is using these algos with proxy iterators."
"Consider using iter_move in the move algos."
Proposed resolution:
This wording is relative to N4944.
Modify 27.11.1 [specialized.algorithms.general] as indicated:
-3- Some algorithms specified in 27.11 [specialized.algorithms] make use of the following exposition-only functions
voidify:template<class T> constexpr void* voidify(T& obj) noexcept { return addressof(obj); } template<class I> decltype(auto) deref-move(const I& it) { if constexpr (is_lvalue_reference_v<decltype(*it)>) return std::move(*it); else return *it; }
Modify 27.11.6 [uninitialized.move] as indicated:
template<class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last, NoThrowForwardIterator result);[…]-1- Preconditions: result + [0, (last - first)) does not overlap with [first, last).
-2- Effects: Equivalent to:for (; first != last; (void)++result, ++first) ::new (voidify(*result)) typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first)); return result;template<class InputIterator, class Size, class NoThrowForwardIterator> pair<InputIterator, NoThrowForwardIterator> uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result);-6- Preconditions: result + [0, n) does not overlap with first + [0, n).
-7- Effects: Equivalent to:for (; n > 0; ++result,(void) ++first, --n) ::new (voidify(*result)) typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first)); return {first, result};
Section: 26.7.23 [range.enumerate] Status: New Submitter: Patrick Palka Opened: 2023-04-07 Last modified: 2023-04-15
Priority: 3
View all issues with New status.
Discussion:
For a sized common range, enumerate_view::end() is specified to call ranges::distance. But ranges::distance is not necessarily well-defined for a sized non-forward range after calling ranges::begin (according to 26.4.3 [range.sized]).
So for a sized common non-forward underlying range, it seems calling enumerate_view::begin() followed by enumerate_view::end() may invoke UB and thus make enumerate_view potentially unusable for such ranges. I suppose we might need to instead call and cache the result of ranges::distance from enumerate_view::begin() for such ranges.[2022-04-12; Patrick Palka provides wording]
The proposed wording follows the suggestion provided by Tim Song, to simply make enumerate non-common for this case.
[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4944.
Modify 26.7.23.2 [range.enumerate.view], class template class enumerate_view synopsis, as indicated:
[…] constexpr auto end() requires (!simple-view<V>) { if constexpr (forward_range<V> && common_range<V> && sized_range<V>) return iterator<false>(ranges::end(base_), ranges::distance(base_)); else return sentinel<false>(ranges::end(base_)); } constexpr auto end() const requires range-with-movable-references<const V> { if constexpr (forward_range<const V> && common_range<const V> && sized_range<const V>) return iterator<true>(ranges::end(base_), ranges::distance(base_)); else return sentinel<true>(ranges::end(base_)); } […]
Section: 16.4.5.3.4 [extern.names] Status: New Submitter: Alisdair Meredith Opened: 2023-04-12 Last modified: 2023-04-15
Priority: 3
View all issues with New status.
Discussion:
16.4.5.3.4 [extern.names] p1 and p2 reserve certain names with external linkage to the implementation.
Each paragraph has a footnote, stating that such names include errno, setjmp(jmpbuf), and va_end(va_list). These names are all defines as macros, not entities with that name and external linkage. Are these footnotes intended by a normative extension to the list of reserved names with external linkage? If so, they should be promoted to the main text, as they are no longer a note. If they intend to serve as examples, it is not clear to me what principle is being shown, as there are many other macros that might be in a similar position, and I do not see how to follow the a principle that is being noted, and presumably follows from the normative text. I think this is intended to be a normative extension to the set of reserved names with external linkage, is limited to exactly those three named macros, so should be promoted as a non-note into the main text, but want to hear what LWG consensus is before drafting wording.[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 29.12 [time.format] Status: New Submitter: Jiang An Opened: 2023-04-14 Last modified: 2023-04-23
Priority: 3
View other active issues in [time.format].
View all other issues in [time.format].
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Discussion:
Currently none of MSVC STL, libc++, and libstdc++ correctly formats duration<int64_t, ratio<INT64_MAX -1, INT64_MAX>>{40} (Godbolt link). For MSVC and libstdc++, hh_mm_ss is used when formatting duration and thus duration_cast is also involved. And it's extremely easy for duration_cast to cause UB when the source type is duration<int64_t, ratio<INT64_MAX -1, INT64_MAX>>.
However, 29.12 [time.format] doesn't say hh_mm_ss is equivalently used when formatting duration. And it seems that the current wording require duration (whose rep type is an arithmetic type) to be correctly formatted without UB, even if it is of a weird type or extremely large (e.g. when it is duration<int64_t, ratio<INT64_MAX, 1>>{INT64_MAX}). So, if it is intended that hh_mm_ss is used for formatting duration, perhaps we should explicitly so specify in 29.12 [time.format]. And we may need to fix initialization of hh_mm_ss to reduce UB whenever suitable.[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
"NAD. A compile-time error is required when ratio arithmetic overflows, 21.4.4 [ratio.arithmetic] p2"
Proposed resolution:
Section: 17.3.5.1 [numeric.limits.general] Status: New Submitter: Christopher Di Bella Opened: 2023-04-14 Last modified: 2023-04-18
Priority: 3
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Discussion:
16.4.5.2.1 [namespace.std]/p2 notes that "unless explicitly prohibited", a user may conditionally specialize any class template for program-defined types. 17.3.5.1 [numeric.limits.general]/p1 doesn't explicitly prohibit this, but it does create a bit of a grey area with its wording because it describes numeric_limits as a class template describing "the implementation's representation" of "the arithmetic types".
Since this type is about what the implementation provides, and about arithmetic types only, it's not clear whether a user can specialize it for their own library type such as ns::int256. ns::int256 isn't an arithmetic type (despite resembling one), so one can interpret 16.4.5.2.1 [namespace.std]/p2's (b) condition to mean that numeric_limits<ns::int256> needs to be explicitly specialized as if it were the primary template. Daniel: This issue has overlap with LWG 3923.[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
Several votes for Tentatively Ready with Option A.
Proposed resolution:
This wording is relative to N4944.
[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
Option A: (This assumes that the above described scenario is intended to be supported)
Add the following new paragraph at the end of 17.3.5.1 [numeric.limits.general] as indicated:
[Drafting Note: The particular wording form "emulating an arithmetic type" has been borrowed from Table 99 — Cpp17Clock requirements [tab:time.clock] and 29.5.1 [time.duration.general] p2, respectively.]
-?- The numeric_limits template may be specialized for program-defined types emulating arithmetic types.
Option B: (This assumes that the above described scenario is not intended to be supported)
Add the following new paragraph at the end of 17.3.5.1 [numeric.limits.general] as indicated:
[Drafting Note: The particular wording form has been borrowed from 17.12.4.1 [coroutine.handle.general] p2 and 20.2.9.1 [allocator.traits.general] p1, respectively.]
-?- If a program declares an explicit or partial specialization of numeric_limits, the program is ill-formed, no diagnostic required.
Section: 17.3.5.1 [numeric.limits.general] Status: New Submitter: Daniel Krügler Opened: 2023-04-15 Last modified: 2023-04-24
Priority: 3
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Discussion:
The wording of 17.3.5.1 [numeric.limits.general] seemingly has not been gone through a similar thorough rewording review which we performed in the past to clean-up the working draft as we did via the series of "Mandating" papers by Marshall Clow (P1458 - P1465 and even more).
17.3.5.1 [numeric.limits.general] contains several nowadays inappropriate wording forms, which don't distinguish well enough between requirements imposed on implementations (Where we shouldn't use "shall" wording in the ambiguous form of "Specializations shall be provided for each arithmetic type") and requirements imposed on user types, this has also caused confusion as expressed in LWG 3922. It is "obvious" that a program is intended to be allowed to provide program-defined specializations, but as LWG 3922 points out, it is unclear how such a specialization is able to meet the requirement "(b) the specialization meets the standard library requirements for the original template" specified in 16.4.5.2.1 [namespace.std] p2. Another problem is the usage of the unclear wording "Non-arithmetic standard types", which should be replaced by a more precise wording form. An additional problem is that we actually already do require an implementation to provide specializations for the (library-provided) integer-class types (25.3.4.4 [iterator.concept.winc]), so contrary to what p6 says, we already have at least one exception, where the library is required to specialize numeric_limits for a non-arithmetic type. We should make that a bit clearer here as well. This issue is related to LWG 3922 and paper P1841.[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4944.
[Drafting Note: This wording would also solve LWG issue 3922 under the assumption that option A is intended]
Modify 17.3.3 [limits.syn], header <limits> synopsis, as indicated:
[…] // 17.3.5.1 [numeric.limits.general], class template numeric_limits template<class T> class numeric_limits; // 17.3.5.3 [numeric.special], numeric_limits specializations template<class T> class numeric_limits<const T>; template<class T> class numeric_limits<volatile T>; template<class T> class numeric_limits<const volatile T>; template<> class numeric_limits<bool>; […]
Modify 17.3.5.1 [numeric.limits.general] as indicated:
-1- The numeric_limits class template provides a C++ program with information about various properties of the implementation's representation of the arithmetic types.
[…][Drafting Note: It is unclear whether the requirement in the following paragraph 2 is intended to apply to program-defined specializations as well. Consider as an example a user-defined arithmetic-like type that provides arbitrary precision arithmetic which may require dynamic memory for certain object constructions. Is it invalid to specialize numeric_limits for such a type or may the program-defined specialization deviate from this requirement for at least some of its members?
If we want to make this restriction relaxed for program-defined specializations, further wording would be needed to give that permission]-2- For all members declared static constexpr in the numeric_limits template, specializations shall define these values in such a way that they are usable as constant expressions.
-3- For the numeric_limits primary template, all data members are value-initialized and all member functions return a value-initialized object. [Note 1: This means all members have zero or false values unless numeric_limits is specialized for a type. — end note] -4- An implementation shall provide sSpecializationsshall be providedfor each arithmetic type, both floating-point and integer, including bool. The member is_specializedshall beis true for all such specializations of numeric_limits.-5- The value of each member of a specialization of numeric_limits on a cv-qualified type cv T shall be equal to the value of the corresponding member of the specialization on the unqualified type T.[Drafting Note: If we had introduced numeric_limits today we would likely have only allowed to provide specializations for cv-unqualified program-defined types, but that ship has sailed long ago.
Interestingly currently there doesn't exist a specification that defines under which situations the static member is_specialized should be defined true or false (This touches LWG 205). The wording below does not attempt to improve that situation, but at least clarifies that its value may be different from that of the primary template. Note that this proposed wording does — opposed to the approach of LWG 3922 — not restrict that specializations can only be provided by program-defined types "emulating an arithmetic type", because that would break existing specializations and is also problematic in the light of the non-existing definition of that term. The below wording strategy gives permission to specialize numeric_limits only for non-array object types. An alternative approach could make it a precondition instead to instantiate the template for non-array object types, for example.]-?- A program may specialize the numeric_limits template for a program-defined non-array object type. Such a specialization is permitted to define a value for any static member that differs from what the primary template would have defined, as appropriate for that type.
[Note: It still has to meet the general requirements specified in subclause 17.3.5.1 [numeric.limits.general] and subclause 17.3.5.3 [numeric.special] — end note].[Drafting Note: The following restriction is carefully drafted to ensure that a library has the freedom to provide such specializations for "extended" types (That are not necessarily integer-class types). The restriction is intended to apply only to "official" (strict) C++ standard library types]
-6- An implementation shall not provide specializations for n
Non-arithmeticstandardtypes of the C++ standard library, such as complex<T> (28.4.3 [complex]), unless specified otherwise (e.g. for integer-class types, 25.3.4.4 [iterator.concept.winc]), shall not have specializations.
Modify 17.3.5.3 [numeric.special] as indicated:
[Drafting Note: I have left the "shall" usage in p1, because this seems to be a requirement for program-defined specializations as well. The second sentence of p1 is one of the funny ones which partially look like introductory wording, but also seems to indicate requirements, albeit specified in an unusual way ("meaningful").
The extra wording added after p2 below attempts to improve the wording situation caused by LWG 559 and does that by following a similar approach as done in 22.4.7 [tuple.helper]. ]
-1- All members shall be provided for all specializations. However, many values are only required to be meaningful under certain conditions (for example, epsilon() is only meaningful if is_integer is false). Any value that is not "meaningful" shall be set to 0 or false.
-2- [Example 1: […] — end note]template<class T> class numeric_limits<const T>; template<class T> class numeric_limits<volatile T>; template<class T> class numeric_limits<const volatile T>;-?- Let NL denote numeric_limits<T> of the cv-unqualified type T. Then the value of each member of these specializations of numeric_limits is equal to the value of the corresponding member of the specialization NL.
-3- The specialization for boolshall beis provided as follows: […]
Section: 33.3.1 [thread.stoptoken.intro] Status: New Submitter: Brian Bi Opened: 2023-04-16 Last modified: 2023-04-16
Priority: 3
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Discussion:
The first sentence of 33.3.1 [thread.stoptoken.intro]/5 says:
Calls to the functions request_stop, stop_requested, and stop_possible do not introduce data races.
This could be read as saying that if you have a program that doesn't contain data races, and you change it by adding a call to any of these three functions, such a change does not "introduce" data races into the program. In other words, it could be read as saying that these three functions don't race with any other member functions on the same stop_token or stop_source object.
I'm guessing the actual intended meaning is that calls to these three functions do not race with each other, because it would be more expensive if concurrent calls with stop_token::operator= were also required to not race. (Also, none of these functions can avoid racing with the destructor.)[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
"What about get_token? operator==?"
"Other functions like get_stop_token can also be called
concurrently. I think the text is actually correct."
Proposed resolution:
This wording is relative to N4944.
Modify 33.3.1 [thread.stoptoken.intro] as indicated:
-5- Calls to the functions request_stop, stop_requested, and stop_possible do not introduce data races with each other. A call to request_stop that returns true synchronizes with a call to stop_requested on an associated stop_token or stop_source object that returns true. Registration of a callback synchronizes with the invocation of that callback.
Section: 16.4.5.2.1 [namespace.std] Status: New Submitter: jim x Opened: 2023-04-19 Last modified: 2023-04-29
Priority: 4
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Discussion:
16.4.5.2.1 [namespace.std] p1 says
Unless otherwise specified, the behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std.
Which std namespace does the rule intend to refer to? Does the text refer to any namespace named std? Consider this case:
namespace A{
namespace std{ // UB or not?
void show(){}
namespace B{
void fun(){} // UB or not?
}
}
}
int main(){
}
A resemble usage like the above example can be seen in a libcudacxx github code, for example.
Suggested resolution: We may want to say the namespace std only refers to the namespace whose declaration inhabits the global scope. 16.4.5.2.1 [namespace.std] p2, p7 that refers to namespace std have a similar issue.[2023-04-24; Ville provides wording]
[2023-05-24; Reflector poll]
Set priority to 4 after reflector poll.
Proposed resolution:
This wording is relative to N4944.
Modify 16.4.5.2.1 [namespace.std] as indicated:
-?- The restrictions on the use of namespace std apply only to a top-level namespace named std. Namespaces named std nested in program-defined namespaces are not restricted this way or otherwise reserved.
-1- Unless otherwise specified, the behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std.
Section: 16.4.5.2.2 [namespace.posix] Status: New Submitter: Jiang An Opened: 2023-04-29 Last modified: 2023-04-30
Priority: 3
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Discussion:
The namespace posix is made reserved by N2667.
It is clear in the paper that only ::posix is intendedly reserved, but the current normative wording is not clear enough on this (see also LWG 3926).[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4944.
Modify 16.4.5.2.2 [namespace.posix] as indicated:
-1- The behavior of a C++ program is undefined if it adds declarations or definitions to top-level namespace posix or to a namespace within that namespace
posixunless otherwise specified. The top-level namespace posix is reserved for use by ISO/IEC/IEEE 9945 and other POSIX standards.
Section: 21.3.5.4 [meta.unary.prop], 21.3.7 [meta.rel] Status: New Submitter: Alisdair Meredith Opened: 2023-05-01 Last modified: 2023-05-06
Priority: 3
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Discussion:
Since we have adopted the Constraints/Mandates/Preconditions form of wording, "preconditions" refer to runtime requirements, not compile-time. As such, the column labeled "preconditions" in Table 47: Type property predicates [tab:meta.unary.prop] would be better labeled as "Mandates".
This is an LWG issue and not editorial, and "Mandates" would require the library to diagnose violations, but after reviewing all traits in this table, I believe that is reasonable. Table 48: Type property queries [tab:meta.unary.prop.query] shows how Mandates: elements can be integrated into the "Value" column if we preferred that approach, but for the number of entries in the first table seems like an aggressive change for consistency. Similarly, for Table 49: Type relationship predicates [tab:meta.rel] the "Comments" column serves as a "Mandates" feature without using that term, so I suggest changing that column title too. The other tables and type traits wording already appear to be adapted to the Mandates wording style.[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
See also issue 2939. We should not turn the preconditions into Mandates without fixing them first.
Proposed resolution:
This wording is relative to N4944.
Change in 21.3.5.4 [meta.unary.prop], Table 47: Type property predicates [tab:meta.unary.prop] the column title "Preconditions" to "Mandates".
Change in 21.3.7 [meta.rel], Table 49: Type relationship predicates [tab:meta.rel] the column title "Comments" to "Mandates".
Section: 17.3.2 [version.syn] Status: New Submitter: Jiang An Opened: 2023-05-03 Last modified: 2023-05-06
Priority: 3
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Discussion:
Currently MSVC STL implements P2602R2 and P2609R3 in C++20 mode as if they were defect reports. However, since P2387R3 and P2494R2, which are possibly considered pure functionality extensions in C++23, also bump __cpp_lib_ranges, it's impossible to detect the status of P2602R2 and P2609R3 in C++20 mode (see the discussion in MSVC STL repo).
This may be a non-defect as P2602R2 and P2609R3 are not officially DRs. However, if these papers (including upcoming P2538R1) are expected to be implemented in C++20 modes, it seems better to establish another feature-test macro (e.g., __cpp_lib_ranges_mechanism) for them.[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
"Needs a more descriptive name than mechanism."
Proposed resolution:
This wording is relative to N4944.
Modify 17.3.2 [version.syn] as indicated:
[Drafting note: It is proposed to restore __cpp_lib_ranges to the value denoting P2494R2.]
[…] #define __cpp_lib_ranges202302L202207L […] #define __cpp_lib_ranges_join_with 202202L // also in <ranges> #define __cpp_lib_ranges_mechanism 202302L // also in <algorithm>, <functional>, <iterator>, <memory>, <ranges> #define __cpp_lib_ranges_repeat 202207L // also in <ranges> […]
Section: 17.11.6 [cmp.alg] Status: New Submitter: Jiang An Opened: 2023-05-04 Last modified: 2023-05-06
Priority: 3
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Discussion:
Currently, comparison CPOs may call ADL-found strong_order, weak_order, and partial_order functions. It might be impossible to meet the expression-equivalent requirement if one passed argument is a prvalue and an ADL-found function is selected, because temporary materialization must take place before entering the CPO's operator().
Perhaps we should say when E or F is a prvalue expression of an object type, temporary materialization takes place first, and then an xvalue referring to the temporary object is used instead of the original E or F. There is currently implementation divergence in these CPOs: when performing internal comparison, libstdc++ and libc++ perfect-forward arguments, while MSVC STL always treats arguments as lvalues.[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 28.4.3 [complex] Status: New Submitter: Jiang An Opened: 2023-05-16 Last modified: 2023-06-01
Priority: 4
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Discussion:
In C++20 and earlier revisions, there are constructors taking two floating-point numbers by value in explicit specializations of std::complex for standard floating-point types. Since P1467R9 has removed these explicit specializations, the corresponding constructor in the primary template that takes arguments by const T& are used instead. As a result, the following program becomes ill-formed after the changes.
#include <complex>
int main()
{
volatile double x = 0.0;
std::complex<double> z{x, x}; // ill-formed due to P1467R9 because const double& cannot be bound to a volatile double lvalue
}
Currently, libstdc++ has implemented complex specializations for extended floating-point types, but the corresponding constructors of these specializations takes two arguments by value, which is consistent with old specializations.
It seems that it's unintended to change the signatures of these constructors. Perhaps we should restore the signatures for required specializations.Daniel:
Not only constructors are affected, but also all assignment operators taking the value_type as parameter and I suggest that LEWG should have a look at this issue.[2023-05-20; Daniel comments and suggests wording]
The wording below attempts to restore the exact previous behaviour: For all floating-point types the function parameter types are "by value" and for other types are "by const reference". The wording adds for specification purposes a dependency to the concept std::floating_point, but that doesn't mean that an implementation couldn't realize the required effects without usage of concepts or the <type_traits> header.
Note that we have already precedence for similar cases where we later reintroduced concept requirements to library parts where no concept dependencies had exist before, such as the iterator_traits specialization for pointers (to object) or the additional constraints of set's member function iterator erase(const_iterator position) or the constraint for reverse_iterator::operator->(), just to name a few. One alternative approach could be to switch to "by-value" signatures only for the affected signatures. This could affect user-defined floating-point-like types such as those with an arbitrary precision, therefore I started with the most conservative approach restoring the original effects that was present in the working draft N4910 and older ones. It might we worth pointing out that the existing "setter" functions imag and real have always been using "by-value" signatures for all specializations. There exists also the possible argument to close this issue as NAD based on the argument that all existing non-member operators taking a value_type argument had always been defined to use const T& as parameter (such as the operator@(const T& lhs, const complex<T>& rhs) forms). My main argument to solve this issue as shown below is based on the ground that the refactoring done by P1467R9 was mainly inspired to simplify the existing wording and to make it more easy to integrate the addition of the extended floating-point types here, as quoted from P1467R9 section 6.6. <complex>:[…] The explicit specializations of std::complex<T> are removed. The only differences between the explicit specializations was the explicit-ness of the constructors that take a complex number of a different type.
This issue has some overlap with LWG 3934, which suggests a yet missing specification for the assignment operator taking the value_type as parameter.
[2023-06-01; Reflector poll]
Set priority to 4 after reflector poll.
Several votes for NAD, as this only affects volatile arguments,
so this might even be an accidental improvement.
Proposed resolution:
This wording is relative to N4950.
Modify 28.4.3 [complex], class template complex synopsis, as indicated:
namespace std {
template<class T> class complex {
public:
using value_type = T;
constexpr complex(T re = T(), T im = T()) requires floating_point<T>;
constexpr complex(const T& re = T(), const T& im = T()) requires (!floating_point<T>);
[…]
constexpr complex& operator= (T) requires floating_point<T>;
constexpr complex& operator= (const T&) requires (!floating_point<T>);
constexpr complex& operator+=(T) requires floating_point<T>;
constexpr complex& operator+=(const T&) requires (!floating_point<T>);
constexpr complex& operator-=(T) requires floating_point<T>;
constexpr complex& operator-=(const T&) requires (!floating_point<T>);
constexpr complex& operator*=(T) requires floating_point<T>;
constexpr complex& operator*=(const T&) requires (!floating_point<T>);
constexpr complex& operator/=(T) requires floating_point<T>;
constexpr complex& operator/=(const T&) requires (!floating_point<T>);
[…]
};
}
Modify 28.4.4 [complex.members] as indicated:
constexpr complex(T re = T(), T im = T()) requires floating_point<T>; constexpr complex(const T& re = T(), const T& im = T()) requires (!floating_point<T>);-1- Postconditions: real() == re && imag() == im is true.
Modify 28.4.5 [complex.member.ops] as indicated:
[Drafting note: We have an pre-existing specification hole that the effects of the non-compound assignment operator taking the value_type as parameter are nowhere specified. This is going to be submitted as a separate issue, see LWG 3934.]
constexpr complex& operator+=(T rhs) requires floating_point<T>; constexpr complex& operator+=(const T& rhs) requires (!floating_point<T>);-1- Effects: Adds the scalar value rhs to the real part of the complex value *this and stores the result in the real part of *this, leaving the imaginary part unchanged.
-2- Returns: *this.constexpr complex& operator-=(T rhs) requires floating_point<T>; constexpr complex& operator-=(const T& rhs) requires (!floating_point<T>);-3- Effects: Subtracts the scalar value rhs from the real part of the complex value *this and stores the result in the real part of *this, leaving the imaginary part unchanged.
-4- Returns: *this.constexpr complex& operator*=(T rhs) requires floating_point<T>; constexpr complex& operator*=(const T& rhs) requires (!floating_point<T>);-5- Effects: Multiplies the scalar value rhs by the complex value *this and stores the result in *this.
-6- Returns: *this.constexpr complex& operator/=(T rhs) requires floating_point<T>; constexpr complex& operator/=(const T& rhs) requires (!floating_point<T>);-7- Effects: Divides the scalar value rhs into the complex value *this and stores the result in *this.
-8- Returns: *this.
Section: 28.4.3 [complex] Status: New Submitter: Daniel Krügler Opened: 2023-05-20 Last modified: 2023-06-01
Priority: 3
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Discussion:
The class template complex synopsis in 28.4.3 [complex] shows the following member function:
constexpr complex& operator= (const T&);
but does not specify its semantics. This affects a code example such as the following one:
#include <complex>
#include <iostream>
int main()
{
std::complex<double> z(1, 1);
z = 2;
std::cout << z << std::endl;
}
This problem exists since the 1998 version of the standard (at that time this was declared in subclause [lib.complex]), but fortunately the three major implementations all behave consistently by assigning the provided value to the real part and nullifying the imaginary part, causing the output (2, 0), which is consistent with the expected behaviour of usual mathematical convention and that of C's built-in complex types. We should specify this.
The lack of this member specification was observed while a proposed wording for LWG 3933 was prepared.[2023-06-01; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4950.
[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
[Drafting note: The wording forms used below intentionally deviate from the rest of the [complex.member.ops] wording forms, because it seems much simpler and clearer to follow the wording forms used that specify the effects of imag and real functions. I decided to use "part" instead of "component", which is shorter and more often used in the rest of the specification]
Option A: This assumes that LWG 3933 is considered as NAD and just adds the missing prototype specification assuming that the parameter style of the current working draft is intended.
Add a new prototype specification at the very beginning of 28.4.5 [complex.member.ops] as indicated:
constexpr complex& operator=(const T& rhs);-?- Effects: Assigns the value rhs to the real part and the value T() to the imaginary part of the complex value *this.
-?- Returns: *this.constexpr complex& operator+=(const T& rhs);[…]
Option b: This assumes that LWG 3933 will be resolved as initially presented and just adds the missing prototype specification assuming that the parameter style suggesting two mutually excluded overloads is intended. The wording delta is presented against the proposed wording of LWG 3933.
Add a new prototype specification at the very beginning of 28.4.5 [complex.member.ops] as indicated:
constexpr complex& operator=(T rhs) requires floating_point<T>; constexpr complex& operator=(const T& rhs) requires (!floating_point<T>);-?- Effects: Assigns the value rhs to the real part and the value T() to the imaginary part of the complex value *this.
-?- Returns: *this.constexpr complex& operator+=(T) requires floating_point<T>; constexpr complex& operator+=(const T&) requires (!floating_point<T>);[…]
Section: 31.7.7 [std.manip], 31.7.8 [ext.manip], 31.7.9 [quoted.manip] Status: New Submitter: Jonathan Wakely Opened: 2023-05-25 Last modified: 2023-06-01
Priority: 3
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Discussion:
All the manipulators in 31.7.7 [std.manip] and 31.7.8 [ext.manip] use language of the form:
An object of unspecified type such that ifoutis an object of typebasic_ostream<charT, traits>then the expressionout << resetiosflags(mask)behaves as if [...]
This needs to work for any object with a public and unambiguous base class
of type basic_ostream<charT, traits>, not only objects
of exactly that type.
In C++98 and C++03, setfill did actually get this right
("if out is (or is derived from)
basic_ostream<charT,traits>"),
but that was "fixed" for C++11 to be consistent with the other manipulators.
31.7.9 [quoted.manip] is slightly different, talking about
"an instance of basic_ostream", but that's not right either.
[2023-06-01; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 22.14.2.2 [format.string.std] Status: New Submitter: S. B. Tam Opened: 2023-05-26 Last modified: 2023-06-01
Priority: 3
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Discussion:
(See discussion at microsoft/STL/pull/3723)
22.14.2.2 [format.string.std] p21 says:The available integer presentation types for integral types other than bool and charT are specified in Table 68.
When charT is wchar_t, the ordinary character type char falls into this category, and thus a char gets formatted as an integer by default, not as a character.
This arguably doesn't affect the std::format family, because they are specified in terms of make_format_args, which calls the basic_format_arg constructor, which converts char to wchar_t (22.14.8.1 [format.arg] (6.2)). But it does affect the std::formatter<char, wchar_t> specialization, which isn't specified to use basic_format_arg. This is especially problematic after P2286R8, which makes std::formatter<char, wchar_t> debug-enabled, but there's no debug format for integral types other than charT. Perhaps [format.string.std] should say that the formatting arguments are converted as if through the basic_format_arg constructor. At the time of writing, on libstdc++ and libc++, std::formatter<char, wchar_t> formats the argument as a character when no specifier is given (godbolt.org/z/nnsEcvna3), while MSVC STL's std::formatter<char, wchar_t> outputs the integer value. But I'm about to change MSVC STL to match the other implementations.[2023-06-01; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
Section: 33.5.4 [atomics.order] Status: SG1 Submitter: Hans Boehm Opened: 2023-05-27 Last modified: 2023-06-01
Priority: 3
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Discussion:
[Discovered by Ori Lahav, lead author of , which P0668 more or less tried to implement. This description also includes observations from Olivier Giroux and David Goldblatt.]
Currently, we require that the sequential consistency order S is consistent with the extended coherence order, which includes "reads before" and "reads from". This may require sc reads to be consistently ordered even if they see non-sc writes. This is not enforced by some "fence-after-store"/"trailing fence" sc implementations, notably on X86. It may also be violated by compiler transformations that eliminate a sequentially consistent load after a store into the same location, e.g. on sequentially consistent hardware. As Ori pointed out, this is exhibited by their (SB + rfis) example on x86 reproduced below. I would expect this also happens on other platforms. The (SB + rfis) example:Thread 1: x.store(1, mo_release); r1 = x.load() /* sees 1 */; r2 = y.load() /* sees 0 */ Thread 2: y.store(1, mo_release); r1 = y.load() /* sees 1 */; r2 = x.load() /* sees 0 */
This is unintentionally and unexpectedly disallowed by the current standard. We can see this as follows:
We have the following coherence-ordered-before relationships:By 3.1 in 33.5.4 [atomics.order] (reads-from): The store in each thread is coherence-ordered-before the immediately following load.
By 3.3 (reads-before): The final load in each thread is coherence-ordered before the store in the other thread.
By 3.4 (transitivity): The final load in each thread is coherence-ordered-before the initial load in the other thread.
On the other hand, the initial load in each thread is sequenced-before and thus strongly-happens-before 6.9.2.2 [intro.races] p12.1 the final load in that thread. Thus coherence-ordered-before union strongly-happens-before is cyclic. But the S relation 33.5.4 [atomics.order] p4 must order all the loads, consistent with both strongly-happens-before (p4) and coherence-ordered-before (p4.1), which is clearly impossible.
With the standard x86 implementation, no fences are inserted. Thread 1 and Thread 2 see the stores to x and y in inconsistent orders, which is allowed by TSO, since each thread can see its own write early. Thus the implementation allows the outcome in question, as expected. The problem here is an error during the attempt to simplify and translate plv.mpi-sws.org/scfix/paper.pdf to standardese. There is no known issue with the underlying paper. I have not yet wrapped my head around this sufficiently to be able to suggest a solution, or to understand how difficult that will be. This happened as a consequence of attempting to strengthen SC fences, and not fixing the Power compilation problem (P0668). So we could probably improve matters by reverting the strengthening of sc fences. Clearly, we'd prefer to avoid that. We could just adopt the formulation in the paper, though that seems even harder to motivate informally. Fundamentally, it feels like we inadvertently strengthened (and broke) the semantics of ordinary sc operations, when we intended to only affect fences. The original paper makes more of a distinction there. The question is how we can reformulate that suitably here. [ Meta-observation from a small off-line discussion that may be relevant to how we phrase the resolution here: The fact that nobody noticed this for a very long time, and implementers were not bothered by it, suggests that the audience for this part of the standard is nearly empty. We conjecture that implementers actually rely on atomics mappings generated by memory model experts, who are more interested in formal models than standardese. A more formal description is likely to increase the size of the audience, and would definitely ease verification and reduce the probability of mistakes like this.][2023-06-01; Reflector poll]
Set priority to 3 after reflector poll. Send to SG1.
Proposed resolution:
Section: 23.2.4 [char.traits.specializations] Status: New Submitter: Jiang An Opened: 2023-05-27 Last modified: 2023-06-01
Priority: 3
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Discussion:
In the standard specializations of std::char_traits (std::char_traits<char> etc.), there are a few member functions take a single character via a const char_type& parameter, while other functions take a single character by value.
In C++98, there were more functions taking const char_type&. N2349 changed this in C++11 by making some, but not all of them take char_type. It is unclear whether the inconsistency is intended, and it seems better for these standard specializations to take a character by value. However, libstdc++ hasn't implemented the signature changes, perhaps due to ABI concerns. So it might be better to loose the restrictions for the purpose of standardization.[2023-06-01; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution:
This wording is relative to N4950.
[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
Option A: This assumes that "by-value" arguments are intended.
Modify the class template char_traits specialization synopses in 23.2.4.2 [char.traits.specializations.char], 23.2.4.3 [char.traits.specializations.char8.t], 23.2.4.4 [char.traits.specializations.char16.t], 23.2.4.5 [char.traits.specializations.char32.t], and 23.2.4.6 [char.traits.specializations.wchar.t] as indicated:
[…]
static constexpr void assign(char_type& c1, const char_type& c2) noexcept;
[…]
static constexpr const char_type* find(const char_type* s, size_t n,
const char_type& a);
[…]
Option B: This assumes that implementation-freedom to keep ABI stability is intended.
[Drafting Note: It is intended to keep the assign(s, n, a) taking the character by value, because the argument may be a character in [s, s + n).]
Add a paragraph at the end of 23.2.4.1 [char.traits.specializations.general] as indicated:
-?- For each occurrence of the placeholder const-char-t in the synopsis of each of these specializations, it is unspecified whether it denotes char_type or const char_type&. Likewise, for each occurrence of the placeholder const-int-t in the synopsis of each of these specializations, it is unspecified whether it denotes int_type or const int_type&
Modify the class template char_traits specialization synopses in 23.2.4.2 [char.traits.specializations.char], 23.2.4.3 [char.traits.specializations.char8.t], 23.2.4.4 [char.traits.specializations.char16.t], 23.2.4.5 [char.traits.specializations.char32.t], and 23.2.4.6 [char.traits.specializations.wchar.t] as indicated:
[…]
static constexpr void assign(char_type& c1, const char_type&const-char-t c2) noexcept;
static constexpr bool eq(char_typeconst-char-t c1, char_typeconst-char-t c2) noexcept;
static constexpr bool lt(char_typeconst-char-t c1, char_typeconst-char-t c2) noexcept;
[…]
static constexpr const char_type* find(const char_type* s, size_t n,
const char_type&const-char-t a);
[…]
static constexpr int_type not_eof(int_typeconst-int-t c) noexcept;
static constexpr char_type to_char_type(int_typeconst-int-t c) noexcept;
static constexpr int_type to_int_type(char_typeconst-char-t c) noexcept;
static constexpr bool eq_int_type(int_typeconst-int-t c1, int_typeconst-int-t c2) noexcept;
[…]
Section: 22.14.6.2 [format.formattable] Status: New Submitter: Mark de Wever Opened: 2023-06-01 Last modified: 2023-06-03
Priority: 3
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Discussion:
A bug has been filed against libc++'s format implementation. The question arose whether the parsed chrono-specs should still be available during executing the formatter's format member function. Libc++'s implementation requires this MSVC STL's implementation does not.
It turns out there is a lifetime a requirement in the Standard, but it would be good to clarify the intention of the wording. Prosed a small wording improvement and a note to clarify the intention. Currently the format function may use elements in the range [pc.begin(), pc.end()) that have not been parsed. This does not seem to be in the spirit of the formatting library. The wording change proposes to restrict this range to the parsed range.[2023-06-08; Reflector poll]
Set priority to 3 after reflector poll.
The proposed resolution is missing a guarantee that the parsed portion of the
range is unchanged between the calls to parse and
format. It must remain valid and unchanged.
Proposed resolution:
This wording is relative to N4950.
Modify BasicFormatter requirements [tab:formatter.basic] as indicated:
Table 73: BasicFormatter requirements [tab:formatter.basic] Expression Return type Requirement … f.format(u, fc) FC::iterator Formats u according to the specifiers stored in
*this, writes the output to fc.out(), and returns
an iterator past the end of the output range.
The output shall only depend on u, fc.locale(),
fc.arg(n) for any value n of type size_t, and
the elements in the parsed range of [pc.begin(), pc.end())
from the last call to f.parse(pc). [Note ?: Using elements in the parsed range of [pc.begin(),
pc.end()) allows the formatter to store references, pointers, or
iterators to elements in the parsed range. For example,
formatter<chrono::day> might store the parsed chrono-specs
(29.12 [time.format]) in a basic_string_view. — end note]
Modify Formatter requirements [tab:formatter] as indicated:
Table 74: Formatter requirements [tab:formatter] Expression Return type Requirement f.format(t, fc) FC::iterator Formats t according to the specifiers stored in
*this, writes the output to fc.out(), and returns
an iterator past the end of the output range.
The output shall only depend on t, fc.locale(),
fc.arg(n) for any value n of type size_t, and
the elements in the parsed range of [pc.begin(), pc.end())
from the last call to f.parse(pc). [Note ?: Using elements in the parsed range of [pc.begin(),
pc.end()) allows the formatter to store references, pointers, or
iterators to elements in the parsed range. For example,
formatter<chrono::day> might store the parsed chrono-specs
(29.12 [time.format]) in a basic_string_view. — end note]…
Section: 22.14.6.3 [format.formatter.spec] Status: SG16 Submitter: Mark de Wever Opened: 2023-06-01 Last modified: 2023-06-08
Priority: 3
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Discussion:
I noticed some interesting features introduced by the range based formatters in C++23
// Ill-formed in C++20 and C++23
const char* cstr = "hello";
char* str = const_cast<char*>(cstr);
std::format(L"{}", str);
std::format(L"{}",cstr);
// Ill-formed in C++20
// In C++23 they give L"['h', 'e', 'l', 'l', 'o']"
std::format(L"{}", "hello"); // A libc++ bug prevents this from working.
std::format(L"{}", std::string_view("hello"));
std::format(L"{}", std::string("hello"));
std::format(L"{}", std::vector{'h', 'e', 'l', 'l', 'o'});
An example is shown here. This only shows libc++ since libstdc++ and MSVC STL have not implemented the formatting ranges papers (P2286R8 and P2585R0) yet.
The difference between C++20 and C++23 is the existence of range formatters. These formatters use the formatter specialization formatter<char, wchar_t> which converts the sequence of chars to a sequence of wchar_ts. In this conversion same_as<char, charT> is false, thus the requirements of the range-type s and ?s ([tab:formatter.range.type]) aren't met. So the following is ill-formed:
std::format(L"{:s}", std::string("hello")); // Not L"hello"
It is surprising that some string types can be formatted as a sequence of wide-characters, but others not. A sequence of characters can be a sequence UTF-8 code units. This is explicitly supported in the width estimation of string types. The conversion of char to wchar_t will convert the individual code units, which will give incorrect results for multi-byte code points. It will not transcode UTF-8 to UTF-16/32. The current behavior is not in line with the note in 22.14.6.3 [format.formatter.spec]/2
[Note 1: Specializations such as formatter<wchar_t, char> and formatter<const char*, wchar_t> that would require implicit multibyte / wide string or character conversion are disabled. — end note]
Disabling this could be done by explicitly disabling the char to wchar_t sequence formatter. Something along the lines of
template<ranges::input_range R>
requires(format_kind<R> == range_format::sequence &&
same_as<remove_cvref_t<ranges::range_reference_t<R>>, char>)
struct formatter<R, wchar_t> : __disabled_formatter {};
where __disabled_formatter satisfies 22.14.6.3 [format.formatter.spec]/5, would do the trick. This disables the conversion for all sequences not only the string types. So vector, array, span, etc. would be disabled.
This does not disable the conversion in the range_formatter. This allows users to explicitly opt in to this formatter for their own specializations. An alternative would be to only disable this conversion for string type specializations (22.14.6.3 [format.formatter.spec]/2.2) where char to wchar_t is used:template<size_t N> struct formatter<charT[N], charT>; template<class traits, class Allocator> struct formatter<basic_string<charT, traits, Allocator>, charT>; template<class traits> struct formatter<basic_string_view<charT, traits>, charT>;
Disabling following the following two is not strictly required:
template<> struct formatter<char*, wchar_t>; template<> struct formatter<const char*, wchar_t>;
However, if (const) char* becomes an input_range in a future version C++, these formatters would become enabled. Disabling all five instead of the three required specializations seems like a future proof solution.
Since there is no enabled narrowing formatter specializationtemplate<> struct formatter<wchar_t, char>;
there are no issues for wchar_t to char conversions.
Before proceeding with a proposed resolution the following design questions need to be addressed:Do we want to allow string types of chars to be formatted as sequences of wchar_ts?
Do we want to allow non string type sequences of chars to be formatted as sequences of wchar_ts?
Should we disable char to wchar_t conversion in the range_formatter?
SG16 has indicated they would like to discuss this issue during a telecon.
[2023-06-08; Reflector poll]
Set status to SG16 and priority to 3 after reflector poll.
Proposed resolution: