| Doc. no. | P2450R0 |
| Date: | 2021-09-24 |
| Audience: | WG21 |
| Reply to: | Jonathan Wakely <lwgchair@gmail.com> |
Section: 30.9.2 [re.results.const] Status: Tentatively Ready Submitter: Pete Becker Opened: 2012-10-02 Last modified: 2021-07-17
Priority: 4
View all other issues in [re.results.const].
Discussion:
30.9.2 [re.results.const]/3: "Move-constructs an object of class match_results satisfying the same postconditions as Table 141."
Table 141 lists various member functions and says that their results should be the results of the corresponding member function calls on m. But m has been moved from, so the actual requirement ought to be based on the value that m had before the move construction, not on m itself.
In addition to that, the requirements for the copy constructor should refer to Table 141.
Ganesh: Also, the requirements for move-assignment should refer to Table 141. Further it seems as if in Table 141 all phrases of "for all integers n < m.size()" should be replaced by "for all unsigned integers n < m.size()".[2019-03-26; Daniel comments and provides wording]
The previous Table 141 (Now Table 128 in N4810) has been modified to cover now the effects of move/copy constructors and move/copy assignment operators. Newly added wording now clarifies that for move operations the corresponding values refer to the values of the move source before the operation has started.
Re Ganesh's proposal: Note that no further wording is needed for the move-assignment operator, because in the current working draft the move-assignment operator's Effects: element refers already to Table 128. The suggested clarification of unsigned integers has been implemented by referring to non-negative integers instead. Upon suggestion from Casey, the wording also introduces Ensures: elements that refer to Table 128 and as drive-by fix eliminates a "Throws: Nothing." element from a noexcept function. Previous resolution [SUPERSEDED]:This wording is relative to N4810.
Add a new paragraph at the beginning of 30.9.2 [re.results.const] as indicated:
-?- Table 128 lists the postconditions of match_results copy/move constructors and copy/move assignment operators. For move operations, the results of the expressions depending on the parameter m denote the values they had before the respective function calls.
Modify 30.9.2 [re.results.const] as indicated:
match_results(const match_results& m);-3- Effects: Constructs
-?- Ensures: As indicated in Table 128.an object of class match_results, asa copy of m.match_results(match_results&& m) noexcept;-4- Effects: Move constructs
-?- Ensures: As indicated in Table 128.an object of class match_resultsfrom msatisfying the same postconditions as Table 128. Additionally, the stored Allocator value is move constructed from m.get_allocator().-5- Throws: Nothing.match_results& operator=(const match_results& m);-6- Effects: Assigns m to *this.
-?- Ensures: As indicated in Table 128.The postconditions of this function are indicated in Table 128.match_results& operator=(match_results&& m);-7- Effects: Move
-?- Ensures: As indicated in Table 128.-assigns m to *this.The postconditions of this function are indicated in Table 128.Modify 30.9.2 [re.results.const], Table 128 — "match_results assignment operator effects", as indicated:
Table 128 — match_results assignment operator effectscopy/move operation postconditionsElement Value ready() m.ready() size() m.size() str(n) m.str(n) for all non-negative integers n < m.size() prefix() m.prefix() suffix() m.suffix() (*this)[n] m[n] for all non-negative integers n < m.size() length(n) m.length(n) for all non-negative integers n < m.size() position(n) m.position(n) for all non-negative integers n < m.size()
[2021-06-25; Daniel comments and provides new wording]
The revised wording has been rebased to N4892. It replaces the now obsolete Ensures: element by the Postconditions: element and also restores the copy constructor prototype that has been eliminated by P1722R2 as anchor point for the link to Table 140.
[2021-06-30; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Add a new paragraph at the beginning of 30.9.2 [re.results.const] as indicated:
-?- Table 140 [tab:re.results.const] lists the postconditions of match_results copy/move constructors and copy/move assignment operators. For move operations, the results of the expressions depending on the parameter m denote the values they had before the respective function calls.
Modify 30.9.2 [re.results.const] as indicated:
explicit match_results(const Allocator& a);-1- Postconditions: ready() returns false. size() returns 0.
match_results(const match_results& m);-?- Postconditions: As specified in Table 140.
match_results(match_results&& m) noexcept;-2- Effects: The stored Allocator value is move constructed from m.get_allocator().
-3- Postconditions: As specified in Table 140.match_results& operator=(const match_results& m);-4- Postconditions: As specified in Table 140.
match_results& operator=(match_results&& m);-5- Postconditions: As specified in Table 140.
Modify 30.9.2 [re.results.const], Table 140 — "match_results assignment operator effects", [tab:re.results.const], as indicated:
Table 140 — match_results assignment operator effectscopy/move operation postconditionsElement Value ready() m.ready() size() m.size() str(n) m.str(n) for all non-negative integers n < m.size() prefix() m.prefix() suffix() m.suffix() (*this)[n] m[n] for all non-negative integers n < m.size() length(n) m.length(n) for all non-negative integers n < m.size() position(n) m.position(n) for all non-negative integers n < m.size()
Section: 28.4.3.2.3 [facet.num.get.virtuals] Status: Tentatively Ready Submitter: Marshall Clow Opened: 2014-04-30 Last modified: 2021-09-20
Priority: 2
View other active issues in [facet.num.get.virtuals].
View all other issues in [facet.num.get.virtuals].
Discussion:
In 28.4.3.2.3 [facet.num.get.virtuals] we have:
Stage 3: The sequence of chars accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header <cstdlib>:
For a signed integer value, the function strtoll.
For an unsigned integer value, the function strtoull.
For a floating-point value, the function strtold.
This implies that for many cases, this routine should return true:
bool is_same(const char* p)
{
std::string str{p};
double val1 = std::strtod(str.c_str(), nullptr);
std::stringstream ss(str);
double val2;
ss >> val2;
return std::isinf(val1) == std::isinf(val2) && // either they're both infinity
std::isnan(val1) == std::isnan(val2) && // or they're both NaN
(std::isinf(val1) || std::isnan(val1) || val1 == val2); // or they're equal
}
and this is indeed true, for many strings:
assert(is_same("0"));
assert(is_same("1.0"));
assert(is_same("-1.0"));
assert(is_same("100.123"));
assert(is_same("1234.456e89"));
but not for others
assert(is_same("0xABp-4")); // hex float
assert(is_same("inf"));
assert(is_same("+inf"));
assert(is_same("-inf"));
assert(is_same("nan"));
assert(is_same("+nan"));
assert(is_same("-nan"));
assert(is_same("infinity"));
assert(is_same("+infinity"));
assert(is_same("-infinity"));
These are all strings that are correctly parsed by std::strtod, but not by the stream extraction operators. They contain characters that are deemed invalid in stage 2 of parsing.
If we're going to say that we're converting by the rules of strtold, then we should accept all the things that strtold accepts.[2016-04, Issues Telecon]
People are much more interested in round-tripping hex floats than handling inf and nan. Priority changed to P2.
Marshall says he'll try to write some wording, noting that this is a very closely specified part of the standard, and has remained unchanged for a long time. Also, there will need to be a sample implementation.
[2016-08, Chicago]
Zhihao provides wording
The src array in Stage 2 does narrowing only. The actual input validation is delegated to strtold (independent from the parsing in Stage 3 which is again being delegated to strtold) by saying:
[...] If it is not discarded, then a check is made to determine if c is allowed as the next character of an input field of the conversion specifier returned by Stage 1.
So a conforming C++11 num_get is supposed to magically accept an hexfloat without an exponent
0x3.AB
because we refers to C99, and the fix to this issue should be just expanding the src array.
Support for Infs and NaNs are not proposed because of the complexity of nan(n-chars).
[2016-08, Chicago]
Tues PM: Move to Open
[2016-09-08, Zhihao Yuan comments and updates proposed wording]
Examples added.
[2018-08-23 Batavia Issues processing]
Needs an Annex C entry. Tim to write Annex C.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Change 28.4.3.2.3 [facet.num.get.virtuals]/3 Stage 2 as indicated:
static const char src[] = "0123456789abcdefpxABCDEFPX+-";
Append the following examples to 28.4.3.2.3 [facet.num.get.virtuals]/3 Stage 2 as indicated:
[Example:
Given an input sequence of "0x1a.bp+07p",
if Stage 1 returns %d, "0" is accumulated;
if Stage 1 returns %i, "0x1a" are accumulated;
if Stage 1 returns %g, "0x1a.bp+07" are accumulated.
In all cases, leaving the rest in the input.
— end example]
[2021-05-18 Tim updates wording]
Based on the git history, libc++ appears to have always included p and P in src.
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Change 28.4.3.2.3 [facet.num.get.virtuals]/3 Stage 2 as indicated:
— Stage 2:
If in == end then stage 2 terminates. Otherwise a charT is taken from in and local variables are initialized as if bychar_type ct = *in; char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atoms]; if (ct == use_facet<numpunct<charT>>(loc).decimal_point()) c = '.'; bool discard = ct == use_facet<numpunct<charT>>(loc).thousands_sep() && use_facet<numpunct<charT>>(loc).grouping().length() != 0;where the values src and atoms are defined as if by:
static const char src[] = "0123456789abcdefpxABCDEFPX+-"; char_type atoms[sizeof(src)]; use_facet<ctype<charT>>(loc).widen(src, src + sizeof(src), atoms);for this value of loc.
If discard is true, then if '.' has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if '.' has already been accumulated, the character is discarded and Stage 2 terminates. If it is not discarded, then a check is made to determine if c is allowed as the next character of an input field of the conversion specifier returned by Stage 1. If so, it is accumulated. If the character is either discarded or accumulated then in is advanced by ++in and processing returns to the beginning of stage 2. [Example:Given an input sequence of "0x1a.bp+07p",
if the conversion specifier returned by Stage 1 is %d, "0" is accumulated;
if the conversion specifier returned by Stage 1 is %i, "0x1a" are accumulated;
if the conversion specifier returned by Stage 1 is %g, "0x1a.bp+07" are accumulated.
In all cases, the remainder is left in the input.
— end example]
Add the following new subclause to C.5 [diff.cpp03]:
C.4.? [locale]: localization library [diff.cpp03.locale]
Affected subclause: 28.4.3.2.3 [facet.num.get.virtuals]
Change: The num_get facet recognizes hexadecimal floating point values.
Rationale: Required by new feature.
Effect on original feature: Valid C++2003 code may have different behavior in this revision of C++.
Section: 20.11.1.3.5 [unique.ptr.single.observers] Status: Tentatively Ready Submitter: Ville Voutilainen Opened: 2016-08-04 Last modified: 2021-06-23
Priority: 3
View all other issues in [unique.ptr.single.observers].
Discussion:
See LWG 2337. Since we aren't removing noexcept from shared_ptr's operator*, we should consider adding noexcept to unique_ptr's operator*.
[2016-08 — Chicago]
Thurs PM: P3, and status to 'LEWG'
[2016-08-05 Chicago]
Ville provides an initial proposed wording.
[LEWG Kona 2017]
->Open: Believe these should be noexcept for consistency. We like these. We agree with the proposed resolution. operator->() already has noexcept.
Also adds optional::operator*
Alisdair points out that fancy pointers might intentionally throw from operator*, and we don't want to prohibit that.
Go forward with conditional noexcept(noexcept(*decltype())).
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
[Drafting note: since this issue is all about consistency, optional's pointer-like operators are additionally included.]
In 20.11.1.3 [unique.ptr.single] synopsis, edit as follows:
add_lvalue_reference_t<T> operator*() const noexcept;Before 20.11.1.3.5 [unique.ptr.single.observers]/1, edit as follows:
add_lvalue_reference_t<T> operator*() const noexcept;In 20.6.3 [optional.optional] synopsis, edit as follows:
constexpr T const *operator->() const noexcept; constexpr T *operator->() noexcept; constexpr T const &operator*() const & noexcept; constexpr T &operator*() & noexcept; constexpr T &&operator*() && noexcept; constexpr const T &&operator*() const && noexcept;Before [optional.object.observe]/1, edit as follows:
constexpr T const* operator->() const noexcept; constexpr T* operator->() noexcept;Before [optional.object.observe]/5, edit as follows:
constexpr T const& operator*() const & noexcept; constexpr T& operator*() & noexcept;Before [optional.object.observe]/9, edit as follows:
constexpr T&& operator*() && noexcept; constexpr const T&& operator*() const && noexcept;
[2021-06-19 Tim updates wording]
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
[Drafting note: since this issue is all about consistency, optional's pointer-like operators are additionally included.]
Edit 20.11.1.3.1 [unique.ptr.single.general], class template unique_ptr synopsis, as indicated:
namespace std {
template<class T, class D = default_delete<T>> class unique_ptr {
public:
[…]
// 20.11.1.3.5 [unique.ptr.single.observers], observers
add_lvalue_reference_t<T> operator*() const noexcept(see below);
[…]
};
}
Edit 20.11.1.3.5 [unique.ptr.single.observers] as indicated:
add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>()));-1- Preconditions: get() != nullptr.
-2- Returns: *get().
Edit 20.6.3.1 [optional.optional.general], class template optional synopsis, as indicated:
namespace std {
template<class T>
class optional {
public:
[…]
// 20.6.3.6 [optional.observe], observers
constexpr const T* operator->() const noexcept;
constexpr T* operator->() noexcept;
constexpr const T& operator*() const & noexcept;
constexpr T& operator*() & noexcept;
constexpr T&& operator*() && noexcept;
constexpr const T&& operator*() const && noexcept;
[…]
};
}
Edit 20.6.3.6 [optional.observe] as indicated:
constexpr const T* operator->() const noexcept; constexpr T* operator->() noexcept;-1- Preconditions: *this contains a value.
-2- Returns: val.-3- Throws: Nothing.-4- Remarks: These functions are constexpr functions.constexpr const T& operator*() const & noexcept; constexpr T& operator*() & noexcept;-5- Preconditions: *this contains a value.
-6- Returns: *val.-7- Throws: Nothing.-8- Remarks: These functions are constexpr functions.constexpr T&& operator*() && noexcept; constexpr const T&& operator*() const && noexcept;-9- Preconditions: *this contains a value.
-10- Effects: Equivalent to: return std::move(*val);
Section: 20.5.3.1 [tuple.cnstr] Status: Tentatively Ready Submitter: Matt Calabrese Opened: 2018-06-12 Last modified: 2021-05-21
Priority: 2
View other active issues in [tuple.cnstr].
View all other issues in [tuple.cnstr].
Discussion:
Currently the tuple constructors of the form:
template<class... UTypes> EXPLICIT constexpr tuple(UTypes&&...);
are not properly constrained in that in the 1-element tuple case, the constraints do no short-circuit when the constructor would be (incorrectly) considered as a possible copy/move constructor candidate. libc++ has a workaround for this, but the additional short-circuiting does not actually appear in the working draft.
As an example of why this lack of short circuiting is a problem in practice, consider the following line:bool a = std::is_copy_constructible_v<std::tuple<any>>;
The above code will cause a compile error because of a recursive trait definition. The copy constructibility check implies doing substitution into the UTypes&&... constructor overloads, which in turn will check if tuple<any> is convertible to any, which in turn will check if tuple<any> is copy constructible (and so the trait is dependent on itself).
I do not provide wording for the proposed fix in anticipation of requires clauses potentially changing how we do the specification, however, the basic solution should be similar to what we've done for other standard library types, which is to say that the very first constraint should be to check that if sizeof...(UTypes) == 1 and the type, after applying remove_cvref_t, is the tuple type itself, then we should force substitution failure rather than checking any further constraints.[2018-06-23 after reflector discussion]
Priority set to 3
[2018-08-20, Jonathan provides wording]
[2018-08-20, Daniel comments]
The wording changes by this issue are very near to those suggested for LWG 3155.
[2018-11 San Diego Thursday night issue processing]
Jonathan to update wording - using conjunction. Priority set to 2
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Modify 20.5.3.1 [tuple.cnstr] as indicated:
template<class... UTypes> explicit(see below) constexpr tuple(UTypes&&... u);-9- Effects: Initializes the elements in the tuple with the corresponding value in std::forward<UTypes>(u).
-10- Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) == sizeof...(UTypes) and sizeof...(Types) >= 1 and (sizeof...(Types) > 1 || !is_same_v<remove_cvref_t<U0>, tuple>) and is_constructible_v<Ti, Ui&&> is true for all i. The expression inside explicit is equivalent to: !conjunction_v<is_convertible<UTypes, Types>...>
[2021-05-20 Tim updates wording]
The new wording below also resolves LWG 3155, relating to an allocator_arg_t tag argument being treated by this constructor template as converting to the first tuple element instead of as a tag. To minimize collateral damage, this wording takes this constructor out of overload resolution only if the tuple is of size 2 or 3, the first argument is an allocator_arg_t, but the first tuple element isn't of type allocator_arg_t (in both cases after removing cv/ref qualifiers). This avoids damaging tuples that actually contain an allocator_arg_t as the first element (which can be formed during uses-allocator construction, thanks to uses_allocator_construction_args).
The proposed wording has been implemented and tested on top of libstdc++.[2021-08-20; LWG telecon]
Set status to Tentatively Ready after telecon review.
Proposed resolution:
This wording is relative to N4885, and also resolves LWG 3155.
Modify 20.5.3.1 [tuple.cnstr] as indicated:
template<class... UTypes> explicit(see below) constexpr tuple(UTypes&&... u);-?- Let disambiguating-constraint be:
(?.1) — negation<is_same<remove_cvref_t<U0>, tuple>> if sizeof...(Types) is 1;
(?.2) — otherwise, bool_constant<!is_same_v<remove_cvref_t<U0>, allocator_arg_t> || is_same_v<remove_cvref_t<T0>, allocator_arg_t>> if sizeof...(Types) is 2 or 3;
(?.3) — otherwise, true_type.
-12- Constraints:
(12.1) — sizeof...(Types) equals sizeof...(UTypes),
and(12.2) — sizeof...(Types) ≥ 1, and
(12.3) — conjunction_v<disambiguating-constraint, is_constructible<Types, UTypes>...> is true
is_constructible_v<Ti, Ui> is true for all i.-13- Effects: Initializes the elements in the tuple with the corresponding value in std::forward<UTypes>(u).
-14- Remarks: The expression inside explicit is equivalent to: !conjunction_v<is_convertible<UTypes, Types>...>
Section: 27.5 [time.duration] Status: Tentatively Ready Submitter: Johel Ernesto Guerrero Peña Opened: 2018-06-22 Last modified: 2021-09-20
Priority: 3
View all other issues in [time.duration].
Discussion:
[time.duration]/4 states:
Members of duration shall not throw exceptions other than those thrown by the indicated operations on their representations.
Where representation is defined in the non-normative, brief description at [time.duration]/1:
[…] A duration has a representation which holds a count of ticks and a tick period. […]
[time.duration.cons]/2 doesn't indicate the operation undergone by its representation, merely stating a postcondition in [time.duration.cons]/3:
Effects: Constructs an object of type duration.
Postconditions: count() == static_cast<rep>(r).
I suggest this reformulation that follows the format of [time.duration.cons]/5.
Effects: Constructs an object of type duration, constructing rep_ from r.
Now it is clear why the constructor would throw.
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
Change 27.5.2 [time.duration.cons] as indicated:
template<class Rep2> constexpr explicit duration(const Rep2& r);-1- Remarks: This constructor shall not participate in overload resolution unless […]
-2- Effects: Constructs an object of type duration, constructing rep_ from r. -3- Postconditions: count() == static_cast<rep>(r).
[2018-06-27 after reflector discussion]
Priority set to 3. Improved wording as result of that discussion.
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
Change 27.5.2 [time.duration.cons] as indicated:
template<class Rep2> constexpr explicit duration(const Rep2& r);-1- Remarks: This constructor shall not participate in overload resolution unless […]
-2- Effects:Constructs an object of type durationInitializes rep_ with r.-3- Postconditions: count() == static_cast<rep>(r).
[2020-05-02; Daniel resyncs wording with recent working draft]
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4861.
Change 27.5.2 [time.duration.cons] as indicated:
template<class Rep2> constexpr explicit duration(const Rep2& r);-1- Constraints: is_convertible_v<const Rep2&, rep> is true and
(1.1) — treat_as_floating_point_v<rep> is true or
(1.2) — treat_as_floating_point_v<Rep2> is false.
[Example: […] end example]
-?- Effects: Initializes rep_ with r.-2- Postconditions: count() == static_cast<rep>(r).
Section: 20.14.6.6 [refwrap.helpers] Status: Tentatively Ready Submitter: Agustín K-ballo Bergé Opened: 2018-07-10 Last modified: 2021-05-22
Priority: 3
Discussion:
The overloads of std::ref/cref that take a reference_wrapper as argument are defined as calling std::ref/cref recursively, whereas the return type is defined as unwrapping just one level. Calling these functions with arguments of multiple level of wrapping leads to ill-formed programs:
int i = 0; std::reference_wrapper<int> ri(i); std::reference_wrapper<std::reference_wrapper<int>> rri(ri); std::ref(rri); // error within 'std::ref'
[Note: these overloads were added by issue resolution 10.29 for TR1, which can be found at N1688, at Redmond 2004]
[2018-08-20 Priority set to 3 after reflector discussion]
[2021-05-22 Tim syncs wording to the current working draft]
[2021-08-20; LWG telecon]
Set status to Tentatively Ready after telecon review.
Proposed resolution:
This wording is relative to N4885.
Change 20.14.6.6 [refwrap.helpers] as indicated:
template<class T> constexpr reference_wrapper<T> ref(reference_wrapper<T> t) noexcept;[…]-3- Returns:
ref(t.get())t.template<class T> constexpr reference_wrapper<const T> cref(reference_wrapper<T> t) noexcept;-5- Returns:
cref(t.get())t.
Section: 20.15.8.7 [meta.trans.other] Status: Tentatively Ready Submitter: Casey Carter Opened: 2018-08-10 Last modified: 2021-06-23
Priority: 3
View other active issues in [meta.trans.other].
View all other issues in [meta.trans.other].
Discussion:
20.15.8.7 [meta.trans.other] p5 characterizes the requirements for program-defined specializations of common_type with the sentence:
Such a specialization need not have a member named type, but if it does, that member shall be a typedef-name for an accessible and unambiguous cv-unqualified non-reference type C to which each of the types T1 and T2 is explicitly convertible.
This sentence - which 20.15.8.7 [meta.trans.other] p7 largely duplicates to specify requirements on program-defined specializations of basic_common_reference - has two problems:
The grammar term "typedef-name" is overconstraining; there's no reason to prefer a typedef-name here to an actual type, and
"accessible" and "unambiguous" are not properties of types, they are properties of names and base classes.
While we're here, we may as well strike the unused name C which both Note B and Note D define for the type denoted by type.
[2018-08 Batavia Monday issue prioritization]
Priority set to 3
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4762.
Modify 20.15.8.7 [meta.trans.other] p5 as follows:
-5- Note B: Notwithstanding the provisions of 20.15.3 [meta.type.synop], and pursuant to 16.4.5.2.1 [namespace.std], a program may specialize common_type<T1, T2> for types T1 and T2 such that is_same_v<T1, decay_t<T1>> and is_same_v<T2, decay_t<T2>> are each true. [Note: …] Such a specialization need not have a member named type, but if it does,
that member shall be a typedef-name for an accessible and unambiguousthe qualified-id common_type<T1, T2>::type shall denote a cv-unqualified non-reference typeCto which each of the types T1 and T2 is explicitly convertible. Moreover, […]
Modify 20.15.8.7 [meta.trans.other] p7 similarly:
-7- Note D: Notwithstanding the provisions of 20.15.3 [meta.type.synop], and pursuant to 16.4.5.2.1 [namespace.std], a program may partially specialize basic_common_reference<T, U, TQual, UQual> for types T and U such that is_same_v<T, decay_t<T>> and is_same_v<U, decay_t<U>> are each true. [Note: …] Such a specialization need not have a member named type, but if it does,
that member shall be a typedef-name for an accessible and unambiguousthe qualified-id basic_common_reference<T, U, TQual, UQual>::type shall denote a cv-unqualified non-reference typeCto which each of the types TQual<T> and UQual<U> is convertible. Moreover, […]
Section: 23.5.3.9 [move.iter.nonmember] Status: Tentatively Ready Submitter: Bo Persson Opened: 2019-09-13 Last modified: 2021-06-23
Priority: 3
View all other issues in [move.iter.nonmember].
Discussion:
Section 23.5.3.9 [move.iter.nonmember]/2-3 says:
template<class Iterator> constexpr move_iterator<Iterator> operator+(iter_difference_t<Iterator> n, const move_iterator<Iterator>& x);Constraints: x + n is well-formed and has type Iterator.
Returns: x + n.
However, the return type of this operator is move_iterator<Iterator>, so the expression x + n ought to have that type. Also, there is no operator+ that matches the constraints, so it effectively disables the addition.
[2019-10-31 Issue Prioritization]
Priority to 3 after reflector discussion.
Previous resolution [SUPERSEDED]:This wording is relative to N4830.
Modify 23.5.3.9 [move.iter.nonmember] as indicated:
template<class Iterator> constexpr move_iterator<Iterator> operator+(iter_difference_t<Iterator> n, const move_iterator<Iterator>& x);-2- Constraints: x + n is well-formed and has type move_iterator<Iterator>.
-3- Returns: x + n.
[2019-11-04; Casey comments and provides revised wording]
After applying the P/R the Constraint element requires x + n to be well-formed (it always is, since that operation is unconstrained) and requires x + n to have type move_iterator<Iterator> (which it always does). Consequently, this Constraint is always satisfied and it has no normative effect. The intent of the change in P0896R4 was that this operator be constrained to require addition on the base iterator to be well-formed and have type Iterator, which ensures that the other semantics of this operation are implementable.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4835.
Modify 23.5.3.9 [move.iter.nonmember] as indicated:
template<class Iterator> constexpr move_iterator<Iterator> operator+(iter_difference_t<Iterator> n, const move_iterator<Iterator>& x);-2- Constraints: x.base() + n is well-formed and has type Iterator.
-3- Returns: x + n.
Section: 24.4.2 [range.range] Status: Tentatively Ready Submitter: Johel Ernesto Guerrero Peña Opened: 2019-12-19 Last modified: 2021-09-20
Priority: 3
Discussion:
24.5.5 [range.dangling] p2 hints at how safe_range should allow lvalue ranges to model it. However, its wording doesn't take into account that case.
[2020-01 Priority set to 3 after review on the reflector.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4842.
Modify 24.4.2 [range.range] as indicated:
template<class T> concept safe_range = range<T> && (is_lvalue_reference_v<T> || enable_safe_range<remove_cvref_t<T>>);-5-
Given an expression E such that decltype((E)) is T,A type T models safe_rangeonlyif:-6- [Note:
(5.1) — is_lvalue_reference_v<T> is true, or
(5.2) — given an expression E such that decltype((E)) is T,
ifthe validity of iterators obtained from the object denoted by E is not tied to the lifetime of that object.Since the validity of iterators is not tied to the lifetime of an object whose type models safe_range, aA function can accept arguments ofsucha type that models safe_rangeby valueand return iterators obtained from it without danger of dangling. — end note]
[2021-05-19 Tim updates wording]
The new wording below attempts to keep the "borrowed" property generally applicable to all models of borrowed_range, instead of bluntly carving out lvalue reference types.
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll in June.
Proposed resolution:
This wording is relative to N4885.
Modify 24.4.2 [range.range] as indicated:
template<class T> concept borrowed_range = range<T> && (is_lvalue_reference_v<T> || enable_borrowed_range<remove_cvref_t<T>>);-5- Let U be remove_reference_t<T> if T is an rvalue reference type, and T otherwise. Given
-6- [Note: Since the validity of iterators is not tied to the lifetime ofan expression E such that decltype((E)) is Ta variable u of type U, T models borrowed_range only if the validity of iterators obtained fromthe object denoted by Eu is not tied to the lifetime of thatobjectvariable.an objecta variable whose type models borrowed_range, a functioncan accept arguments ofwith a parameter of such a typeby value andcan return iterators obtained from it without danger of dangling. — end note]
Section: 23.4.4.3 [range.iter.op.distance] Status: Tentatively Ready Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2021-06-23
Priority: 3
Discussion:
One cannot use ranges::distance(I, S) to compute the distance between a move-only counted_iterator and the default_sentinel. In other words, the following is invalid
// iter is a counted_iterator with an move-only underlying iterator ranges::distance(iter, default_sentinel);
and yet
(default_sentinel - iter);
is valid. The first example is invalid because ranges::distance() takes its first argument by value so when invoking it with an iterator lvalue argument the iterator must be copyable, which a move-only iterator is not. The second example is valid because counted_iterator::operator-() takes its iterator argument by const reference, so it doesn't require copyability of the counted_iterator.
This incongruency poses an inconvenience in generic code which uses ranges::distance() to efficiently compute the distance between two iterators or between an iterator-sentinel pair. Although it's a bit of an edge case, it would be good if ranges::distance() does the right thing when the iterator is a move-only lvalue with a sized sentinel. If this is worth fixing, one solution might be to define a separate overload of ranges::distance(I, S) that takes its arguments by const reference, as follows.[2020-02 Prioritized as P3 and LEWG Monday morning in Prague]
[2020-05-28; LEWG issue reviewing]
LEWG issue processing voted to accept the direction of 3392. Status change to Open.
Accept the direction of LWG3392 SF F N A SA 14 6 0 0 0
Previous resolution [SUPERSEDED]:
Modify 23.2 [iterator.synopsis], header <iterator> synopsis, as indicated:
#include <concepts> namespace std { […] // 23.4.4 [range.iter.ops], range iterator operations namespace ranges { […] // 23.4.4.3 [range.iter.op.distance], ranges::distance template<input_or_output_iterator I, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> distance(I first, S last); template<input_or_output_iterator I, sized_sentinel_for<I> S> constexpr iter_difference_t<I> distance(const I& first, const S& last); template<range R> constexpr range_difference_t<R> distance(R&& r); […] } […] }Modify 23.4.4.3 [range.iter.op.distance] as indicated:
template<input_or_output_iterator I, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> ranges::distance(I first, S last);-1- Preconditions: [first, last) denotes a range
-2- Effects:, or [last, first) denotes a range and S and I model same_as<S, I> && sized_sentinel_for<S, I>.If S and I model sized_sentinel_for<S, I>, returns (last - first); otherwise, rReturns the number of increments needed to get from first to last.template<input_or_output_iterator I, sized_sentinel_for<I> S> constexpr iter_difference_t<I> ranges::distance(const I& first, const S& last);-?- Preconditions: S and I model sized_sentinel_for<S, I> and either:
-? Effects: Returns (last - first);
(?.1) — [first, last) denotes a range, or
(?.2) — [last, first) denotes a range and S and I model same_as<S, I>.
[2021-05-19 Tim updates wording]
The wording below removes the explicit precondition on the sized_sentinel_for overload of distance, relying instead on the semantic requirements of that concept and the "Effects: Equivalent to:" word of power. This also removes the potentially surprising inconsistency that given a non-empty std::vector<int> v, ranges::distance(v.begin(), v.cend()) is well-defined but ranges::distance(v.cend(), v.begin()) is currently undefined.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 23.2 [iterator.synopsis], header <iterator> synopsis, as indicated:
[…]
namespace std {
[…]
// 23.4.4 [range.iter.ops], range iterator operations
namespace ranges {
[…]
// 23.4.4.3 [range.iter.op.distance], ranges::distance
template<input_or_output_iterator I, sentinel_for<I> S>
requires (!sized_sentinel_for<S, I>)
constexpr iter_difference_t<I> distance(I first, S last);
template<input_or_output_iterator I, sized_sentinel_for<I> S>
constexpr iter_difference_t<I> distance(const I& first, const S& last);
template<range R>
constexpr range_difference_t<R> distance(R&& r);
[…]
}
[…]
}
Modify 23.4.4.3 [range.iter.op.distance] as indicated:
template<input_or_output_iterator I, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> ranges::distance(I first, S last);-1- Preconditions: [first, last) denotes a range
-2-, or [last, first) denotes a range and S and I model same_as<S, I> && sized_sentinel_for<S, I>.Effects: If S and I model sized_sentinel_for<S, I>, returns (last - first); otherwise, returns theReturns: The number of increments needed to get from first to last.template<input_or_output_iterator I, sized_sentinel_for<I> S> constexpr iter_difference_t<I> ranges::distance(const I& first, const S& last);-?- Effects: Equivalent to return last - first;
Section: 24.7.8.1 [range.take.overview], 24.6.4 [range.iota] Status: Tentatively Ready Submitter: Patrick Palka Opened: 2020-02-21 Last modified: 2021-06-19
Priority: 2
Discussion:
Section 6.1 of P1739R4 changes the specification of views::take as follows:
-2- The name views::take denotes a range adaptor object (24.7.2 [range.adaptor.object]).
Given subexpressions E and F, the expression views::take(E, F) is expression-equivalent to take_view{E, F}.Let E and F be expressions, let T be remove_cvref_t<decltype((E))>, and let D be range_difference_t<decltype((E))>. If decltype((F)) does not model convertible_to<D>, views::take(E, F) is ill-formed. Otherwise, the expression views::take(E, F) is expression-equivalent to:
— if T is a specialization of ranges::empty_view (24.6.2.2 [range.empty.view]), then ((void) F, decay-copy(E));
— otherwise, if T models random_access_range and sized_range and is
— a specialization of span (22.7.3 [views.span]) where T::extent == dynamic_extent,
— a specialization of basic_string_view (21.4 [string.view]),
— a specialization of ranges::iota_view (24.6.4.2 [range.iota.view]), or
— a specialization of ranges::subrange (24.5.4 [range.subrange]),
then T{ranges::begin(E), ranges::begin(E) + min<D>(ranges::size(E), F)}, except that E is evaluated only once;
— otherwise, ranges::take_view{E, F}.
Consider the case when T = subrange<counted_iterator<int>, default_sentinel_t>. Then according to the above wording, views::take(E, F) is expression-equivalent to
T{ranges::begin(E), ranges:begin(E) + min<D>(ranges::size(E), F)}; (*)
But this expression is ill-formed for the T we chose because subrange<counted_iterator<int>, default_sentinel_t> has no matching constructor that takes an iterator-iterator pair.
More generally the above issue applies anytime T is a specialization of subrange that does not model common_range. But a similar issue also exists when T is a specialization of iota_view whose value type differs from its bound type. In this case yet another issue arises: In order for the expression (*) to be well-formed when T is a specialization of iota_view, we need to be able to construct an iota_view out of an iterator-iterator pair, and for that it seems we need to add another constructor to iota_view.[2020-02-24, Casey comments]
Furthermore, the pertinent subrange constructor is only available when subrange::StoreSize is false (i.e., when either the subrange specialization's third template argument is not subrange_kind::sized or its iterator and sentinel types I and S model sized_sentinel_for<S, I>).
[2020-03-16, Tomasz comments]
A similar problem occurs for the views::drop for the subrange<I, S, subrange_kind::sized>, that explicitly stores size (i.e. sized_sentinel_for<I, S> is false). In such case, the (iterator, sentinel) constructor that views::drop will be expression-equivalent is not available.
[2020-03-29 Issue Prioritization]
Priority to 2 after reflector discussion.
[2021-05-18 Tim adds wording]
The proposed resolution below is based on the MSVC implementation, with one caveat: the MSVC implementation uses a SCARY iterator type for iota_view and therefore its iota_view case for take is able to directly construct the new view from the iterator type of the original. This is not required to work, so the wording below constructs the iota_view from the result of dereferencing the iterators instead in this case.
[2021-06-18 Tim syncs wording to the current working draft]
The wording below also corrects the size calculation in the presence of integer-class types.
[2021-08-20; LWG telecon]
Set status to Tentatively Ready after telecon review.
Proposed resolution:
This wording is relative to N4892.
Edit 24.7.8.1 [range.take.overview] as indicated:
-2- The name views::take denotes a range adaptor object (24.7.2 [range.adaptor.object]). Let E and F be expressions, let T be remove_cvref_t<decltype((E))>, and let D be range_difference_t<decltype((E))>. If decltype((F)) does not model convertible_to<D>, views::take(E, F) is ill-formed. Otherwise, the expression views::take(E, F) is expression-equivalent to:
(2.1) — if T is a specialization of ranges::empty_view (24.6.2.2 [range.empty.view]), then ((void) F, decay-copy(E)), except that the evaluations of E and F are indeterminately sequenced;
(2.2) — otherwise, if T models random_access_range and sized_range, and is a specialization of span (22.7.3 [views.span]), basic_string_view (21.4 [string.view]), or ranges::subrange (24.5.4 [range.subrange]), then U(ranges::begin(E), ranges::begin(E) + std::min<D>(ranges::distance(E), F)), except that E is evaluated only once, where U is a type determined as follows:
(2.2.1) — if T is a specialization of span
(22.7.3 [views.span]) where T::extent == dynamic_extent, then U is span<typename T::element_type>;(2.2.2) — otherwise, if T is a specialization of basic_string_view
(21.4 [string.view]), then U is T;
(2.2.3) — a specialization of ranges::iota_view (24.6.4.2 [range.iota.view]), or(2.2.4) — otherwise, T is a specialization of ranges::subrange
(24.5.4 [range.subrange]), and U is ranges::subrange<iterator_t<T>>;
then T{ranges::begin(E), ranges::begin(E) + min<D>(ranges::size(E), F)}, except that E is evaluated only once;(2.?) — otherwise, if T is a specialization of ranges::iota_view (24.6.4.2 [range.iota.view]) that models random_access_range and sized_range, then ranges::iota_view(*ranges::begin(E), *(ranges::begin(E) + std::min<D>(ranges::distance(E), F))), except that E and F are each evaluated only once;
(2.3) — otherwise, ranges::take_view(E, F).
Edit 24.7.10.1 [range.drop.overview] as indicated:
-2- The name views::drop denotes a range adaptor object (24.7.2 [range.adaptor.object]). Let E and F be expressions, let T be remove_cvref_t<decltype((E))>, and let D be range_difference_t<decltype((E))>. If decltype((F)) does not model convertible_to<D>, views::drop(E, F) is ill-formed. Otherwise, the expression views::drop(E, F) is expression-equivalent to:
(2.1) — if T is a specialization of ranges::empty_view (24.6.2.2 [range.empty.view]), then ((void) F, decay-copy(E)), except that the evaluations of E and F are indeterminately sequenced;
(2.2) — otherwise, if T models random_access_range and sized_range, and is
(2.2.1) —a specialization of span (22.7.3 [views.span])
where T::extent == dynamic_extent,(2.2.2) — a specialization of basic_string_view (21.4 [string.view]),
(2.2.3) — a specialization of ranges::iota_view (24.6.4.2 [range.iota.view]), or
(2.2.4) — a specialization of ranges::subrange (24.5.4 [range.subrange]) where T::StoreSize is false,
then
TU(ranges::begin(E) + std::min<D>(ranges::sizedistance(E), F), ranges::end(E)), except that E is evaluated only once, where U is span<typename T::element_type> if T is a specialization of span and T otherwise;(2.?) — otherwise, if T is a specialization of ranges::subrange (24.5.4 [range.subrange]) that models random_access_range and sized_range, then T(ranges::begin(E) + std::min<D>(ranges::distance(E), F), ranges::end(E), to-unsigned-like(ranges::distance(E) - std::min<D>(ranges::distance(E), F))), except that E and F are each evaluated only once;
(2.3) — otherwise, ranges::drop_view(E, F).
Section: 26.6.8.1 [rand.util.seedseq] Status: Tentatively Ready Submitter: Jiang An Opened: 2020-03-25 Last modified: 2021-06-23
Priority: 3
View other active issues in [rand.util.seedseq].
View all other issues in [rand.util.seedseq].
Discussion:
26.6.8.1 [rand.util.seedseq] says that std::seed_seq has following 3 constructors:
#1: seed_seq()
#2: template<class T> seed_seq(initializer_list<T> il)
#3: template<class InputIterator> seed_seq(InputIterator begin, InputIterator end)
#include <random>
#include <vector>
int main()
{
std::vector<int> v(32);
std::seed_seq{std::begin(v), std::end(v)}; // error: #2 matched and T is not an integer type
std::seed_seq(std::begin(v), std::end(v)); // OK
}
#3 should be made available in list-initialization by changing Mandates in 26.6.8.1 [rand.util.seedseq]/3 to Constraints IMO.
[2020-04-18 Issue Prioritization]
Priority to 3 after reflector discussion.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4861.
Modify 26.6.8.1 [rand.util.seedseq] as indicated:
class seed_seq {
public:
// types
using result_type = uint_least32_t;
// constructors
seed_seq() noexcept;
[…]
};
seed_seq() noexcept;-1- Postconditions: v.empty() is true.
-2- Throws: Nothing.template<class T> seed_seq(initializer_list<T> il);-3- Constraints
-4- Effects: Same as seed_seq(il.begin(), il.end()).Mandates: T is an integer type.
Section: 24.5.4 [range.subrange] Status: Tentatively Ready Submitter: S. B. Tam Opened: 2020-07-26 Last modified: 2021-06-23
Priority: 3
View other active issues in [range.subrange].
View all other issues in [range.subrange].
Discussion:
Consider
#include <ranges>
int main()
{
int a[3] = { 1, 2, 3 };
int* b[3] = { &a[2], &a[0], &a[1] };
auto c = std::ranges::subrange<const int*const*>(b);
}
The construction of c is ill-formed because convertible-to-non-slicing<int**, const int*const*> is false, although the conversion does not involve object slicing.
I think subrange should allow such qualification conversion, just like unique_ptr<T[]> already does. (Given that this constraint is useful in more than one context, maybe it deserves a named type trait?)[2020-08-21; Reflector prioritization]
Set priority to 3 after reflector discussions.
[2021-05-19 Tim adds wording]
The wording below, which has been implemented and tested on top of libstdc++, uses the same technique we use for unique_ptr, shared_ptr, and span. It seems especially appropriate to have feature parity between subrange and span in this respect.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 24.5.4 [range.subrange] as indicated:
namespace std::ranges {
template<class From, class To>
concept uses-nonqualification-pointer-conversion = // exposition only
is_pointer_v<From> && is_pointer_v<To> &&
!convertible_to<remove_pointer_t<From>(*)[], remove_pointer_t<To>(*)[]>;
template<class From, class To>
concept convertible-to-non-slicing = // exposition only
convertible_to<From, To> &&
!uses-nonqualification-pointer-conversion<decay_t<From>, decay_t<To>>;
!(is_pointer_v<decay_t<From>> &&
is_pointer_v<decay_t<To>> &&
not-same-as<remove_pointer_t<decay_t<From>>, remove_pointer_t<decay_t<To>>>
);
[…]
}
[…]
Section: 29.12.11 [fs.class.directory.iterator], 29.12.12 [fs.class.rec.dir.itr] Status: Tentatively Ready Submitter: Barry Revzin Opened: 2020-08-27 Last modified: 2021-06-23
Priority: 3
View all other issues in [fs.class.directory.iterator].
Discussion:
std::filesystem::directory_iterator and std::filesystem::recursive_directory_iterator are intended to be ranges, but both fail to satisfy the concept std::ranges::range.
They both opt in to being a range the same way, via non-member functions:directory_iterator begin(directory_iterator iter) noexcept; directory_iterator end(const directory_iterator&) noexcept; recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept; recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;
This is good enough for a range-based for statement, but for the range concept, non-member end is looked up in a context that includes (24.3.3 [range.access.end]/2.6) the declarations:
void end(auto&) = delete; void end(const auto&) = delete;
Which means that non-const directory_iterator and non-const recursive_directory_iterator, the void end(auto&) overload ends up being a better match and thus the CPO ranges::end doesn't find a candidate. Which means that {recursive_,}directory_iterator is not a range, even though const {recursive_,}directory_iterator is a range.
This could be fixed by having the non-member end for both of these types just take by value (as libstdc++ currently does anyway) or by adding member functions begin() const and end() const. A broader direction would be to consider removing the poison pill overloads. Their motivation from P0970 was to support what are now called borrowed ranges — but that design now is based on specializing a variable template instead of providing a non-member begin that takes an rvalue, so the initial motivation simply no longer exists. And, in this particular case, causes harm.[2020-09-06; Reflector prioritization]
Set priority to 3 during reflector discussions.
[2021-02-22, Barry Revzin comments]
When we do make whichever of the alternative adjustments necessary such that range<directory_iterator> is true, we should also remember to specialize enable_borrowed_range for both types to be true (since the iterator is the range, this is kind of trivially true).
[2021-05-17, Tim provides wording]
Both MSVC and libstdc++'s end already take its argument by value, so the wording below just does that. Any discussion about changing or removing the poison pills is probably better suited for a paper.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Edit 29.12.4 [fs.filesystem.syn], header <filesystem> synopsis, as indicated:
[…]
namespace std::filesystem {
[…]
// 29.12.11.3 [fs.dir.itr.nonmembers], range access for directory iterators
directory_iterator begin(directory_iterator iter) noexcept;
directory_iterator end(const directory_iterator&) noexcept;
[…]
// 29.12.12.3 [fs.rec.dir.itr.nonmembers], range access for recursive directory iterators
recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept;
recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;
[…]
}
namespace std::ranges {
template<>
inline constexpr bool enable_borrowed_range<filesystem::directory_iterator> = true;
template<>
inline constexpr bool enable_borrowed_range<filesystem::recursive_directory_iterator> = true;
template<>
inline constexpr bool enable_view<filesystem::directory_iterator> = true;
template<>
inline constexpr bool enable_view<filesystem::recursive_directory_iterator> = true;
}
Edit 29.12.11.3 [fs.dir.itr.nonmembers] as indicated:
-1- These functions enable range access for directory_iterator.
directory_iterator begin(directory_iterator iter) noexcept;-2- Returns: iter.
directory_iterator end(constdirectory_iterator&) noexcept;-3- Returns: directory_iterator().
Edit 29.12.12.3 [fs.rec.dir.itr.nonmembers] as indicated:
-1- These functions enable use of recursive_directory_iterator with range-based for statements.
recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept;-2- Returns: iter.
recursive_directory_iterator end(constrecursive_directory_iterator&) noexcept;-3- Returns: recursive_directory_iterator().
Section: 29.11.2.1 [syncstream.syncbuf.overview], 29.11.2.3 [syncstream.syncbuf.assign] Status: Tentatively Ready Submitter: Jonathan Wakely Opened: 2020-11-10 Last modified: 2021-06-23
Priority: 3
View all other issues in [syncstream.syncbuf.overview].
Discussion:
The synopsis in 29.11.2.1 [syncstream.syncbuf.overview] shows the move assignment operator and swap member as potentially throwing. The detailed descriptions in 29.11.2.3 [syncstream.syncbuf.assign] are noexcept.
Daniel: This mismatch is already present in the originally accepted paper P0053R7, so this is nothing that could be resolved editorially.[2020-11-21; Reflector prioritization]
Set priority to 3 during reflector discussions.
[2021-05-22 Tim adds PR]
The move assignment is specified to call emit() which can throw, and there's nothing in the wording providing for catching/ignoring the exception, so it can't be noexcept. The swap needs to call basic_streambuf::swap, which isn't noexcept, so it shouldn't be noexcept either.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 29.11.2.3 [syncstream.syncbuf.assign] as indicated:
basic_syncbuf& operator=(basic_syncbuf&& rhs)noexcept;-1- Effects: […]
-2- Postconditions: […] -3- Returns: […] -4- Remarks: […]void swap(basic_syncbuf& other)noexcept;-5- Preconditions: […]
-6- Effects: […]
Section: 24.7.12.3 [range.join.iterator] Status: Tentatively Ready Submitter: Casey Carter Opened: 2021-03-16 Last modified: 2021-06-23
Priority: 2
View other active issues in [range.join.iterator].
View all other issues in [range.join.iterator].
Discussion:
Per 24.7.12.3 [range.join.iterator]/1, join_view::iterator::iterator_concept denotes bidirectional_iterator_tag if ref-is-glvalue is true and Base and range_reference_t<Base> each model bidirectional_range. Similarly, paragraph 2 says that join_view::iterator::iterator_category is present if ref-is-glvalue is true, and denotes bidirectional_iterator_tag if the categories of the iterators of both Base and range_reference_t<Base> derive from bidirectional_iterator_tag.
However, the constraints on the declarations of operator-- and operator--(int) in the synopsis that immediately precedes paragraph 1 disagree. Certainly they also consistently require ref-is-glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>>, but they additionally require common_range<range_reference_t<Base>>. So as currently specified, this iterator sometimes declares itself to be bidirectional despite not implementing --. This is not incorrect for iterator_concept — recall that iterator_concept is effectively an upper bound since the concepts require substantial syntax — but slightly misleading. It is, however, very much incorrect for iterator_category which must not denote a type derived from a tag that corresponds to a stronger category than that iterator implements. It's worth pointing out, that LWG 3313 fixed the constraints on operator--() and operator--(int) by adding the common_range requirements, but failed to make a consistent change to the definitions of iterator_concept and iterator_category.[2021-04-04; Daniel comments]
The below proposed wording can be compared as being based on N4885.
[2021-04-20; Reflector poll]
Priority set to 2.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to the post-2021-February-virtual-meeting working draft.
Modify 24.7.12.3 [range.join.iterator] as indicated:
-1- iterator::iterator_concept is defined as follows:
(1.1) — If ref-is-glvalue is true,
andBaseand range_reference_t<Base> eachmodels bidirectional_range, and range_reference_t<Base> models both bidirectional_range and common_range, then iterator_concept denotes bidirectional_iterator_tag.[…]
[…]
-2- The member typedef-name iterator_category is defined if and only if ref-is-glvalue is true, Base models forward_range, and range_reference_t<Base> models forward_range. In that case, iterator::iterator_category is defined as follows:
(2.1) — Let OUTERC denote iterator_traits<iterator_t<Base>>::iterator_category, and let INNERC denote iterator_traits<iterator_t<range_reference_t<Base>>>::iterator_category.
(2.2) — If OUTERC and INNERC each model derived_from<bidirectional_iterator_tag>, and range_reference_t<Base> models common_range, iterator_category denotes bidirectional_iterator_tag.
[…]
Section: 27.13 [time.parse] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2021-05-22 Last modified: 2021-06-23
Priority: 2
View all other issues in [time.parse].
Discussion:
The chrono::parse functions take const basic_string<charT, traits, Alloc>& parameters to specify the format strings for the parse. Due to an oversight on my part in the proposal, overloads taking const charT* and basic_string_view<charT, traits> were omitted. These are necessary when the supplied arguments is a string literal or string_view respectively:
in >> parse("%F %T", tp);
These overloads have been implemented in the example implementation.
[2021-05-26; Reflector poll]
Set priority to 2 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4885.
Modify 27.2 [time.syn], header <chrono> synopsis, as indicated:
[…] namespace chrono { // 27.13 [time.parse], parsing template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp); template<class charT, class traits, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev); template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, minutes& offset); template<class charT, class traits, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset); […] } […]Modify 27.13 [time.parse] as indicated:
template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp);-?- Constraints: The expression
from_stream(declval<basic_istream<charT>&>(), fmt, tp)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent to return parse(basic_string<charT>{fmt}, tp);template<class charT, class traits, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent to return parse(basic_string<charT, traits>{fmt}, tp);template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt, tp, addressof(abbrev))is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent to return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev);template<class charT, class traits, class Alloc, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp, addressof(abbrev))is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent to return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev);template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, minutes& offset);-?- Constraints: The expression
from_stream(declval<basic_istream<charT>&>(), fmt, tp, declval<basic_string<charT>*>(), &offset)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent to return parse(basic_string<charT>{fmt}, tp, offset);template<class charT, class traits, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, minutes& offset);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp, declval<basic_string<charT, traits>*>(), &offset)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent to return parse(basic_string<charT, traits>{fmt}, tp, offset);template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt, tp, addressof(abbrev), &offset)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent to return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev, offset);template<class charT, class traits, class Alloc, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp, addressof(abbrev), &offset)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent to return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev, offset);template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp);-2- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp)is well-formed when treated as an unevaluated operand.
-3- Returns: A manipulator such that the expression is >> parse(fmt, tp) has type I, has value is, and calls from_stream(is, fmt.c_str(), tp).
[2021-06-02 Tim comments and provides updated wording]
The current specification suggests that parse takes a reference to the format string (stream extraction on the resulting manipulator is specified to call from_stream on fmt.c_str(), not some copy), so we can't call it with a temporary string.
Additionally, since the underlying API (from_stream) requires a null-terminated string, the usual practice is to avoid providing a string_view overload (see, e.g., LWG 3430). The wording below therefore only provides const charT* overloads. It does not use "Equivalent to" to avoid requiring that the basic_string overload and the const charT* overload return the same type. As the manipulator is intended to be immediately consumed, the wording also adds normative encouragement to make misuse more difficult.[2021-06-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 27.2 [time.syn], header <chrono> synopsis, as indicated:
[…]
namespace chrono {
// 27.13 [time.parse], parsing
template<class charT, class Parsable>
unspecified
parse(const charT* fmt, Parsable& tp);
template<class charT, class traits, class Alloc, class Parsable>
unspecified
parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp);
template<class charT, class traits, class Alloc, class Parsable>
unspecified
parse(const charT* fmt, Parsable& tp,
basic_string<charT, traits, Alloc>& abbrev);
template<class charT, class traits, class Alloc, class Parsable>
unspecified
parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp,
basic_string<charT, traits, Alloc>& abbrev);
template<class charT, class Parsable>
unspecified
parse(const charT* fmt, Parsable& tp, minutes& offset);
template<class charT, class traits, class Alloc, class Parsable>
unspecified
parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp,
minutes& offset);
template<class charT, class traits, class Alloc, class Parsable>
unspecified
parse(const charT* fmt, Parsable& tp,
basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
template<class charT, class traits, class Alloc, class Parsable>
unspecified
parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp,
basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
[…]
}
[…]
Modify 27.13 [time.parse] as indicated:
-1- Each parse overload specified in this subclause calls from_Âstream unqualified, so as to enable argument dependent lookup (6.5.4 [basic.lookup.argdep]). In the following paragraphs, let is denote an object of type basic_Âistream<charT, traits> and let I be basic_Âistream<charT, traits>&, where charT and traits are template parameters in that context.
-?- Recommended practice: Implementations should make it difficult to accidentally store or use a manipulator that may contain a dangling reference to a format string, for example by making the manipulators produced by parse immovable and preventing stream extraction into an lvalue of such a manipulator type.template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp);-?- Let F be fmt for the first overload and fmt.c_str() for the second overload. Let traits be char_traits<charT> for the first overload.
-2- Constraints: The expressionfrom_stream(declval<basic_istream<charT, traits>&>(),fmt.c_str()F, tp)is well-formed when treated as an unevaluated operand.
-3- Returns: A manipulator such that the expression is >> parse(fmt, tp) has type I, has value is, and calls from_Âstream(is,fmt.c_str()F, tp).template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev);-?- Let F be fmt for the first overload and fmt.c_str() for the second overload.
-4- Constraints: The expressionfrom_stream(declval<basic_istream<charT, traits>&>(),fmt.c_str()F, tp, addressof(abbrev))is well-formed when treated as an unevaluated operand.
-5- Returns: A manipulator such that the expression is >> parse(fmt, tp, abbrev) has type I, has value is, and calls from_Âstream(is,fmt.c_str()F, tp, addressof(abbrev)).template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, minutes& offset);-?- Let F be fmt for the first overload and fmt.c_str() for the second overload. Let traits be char_traits<charT> and Alloc be allocator<charT> for the first overload.
-6- Constraints: The expressionfrom_stream(declval<basic_istream<charT, traits>&>(),fmt.c_str()F, tp, declval<basic_string<charT, traits, Alloc>*>(), &offset)is well-formed when treated as an unevaluated operand.
-7- Returns: A manipulator such that the expression is >> parse(fmt, tp, offset) has type I, has value is, and calls:from_stream(is,fmt.c_str()F, tp, static_cast<basic_string<charT, traits, Alloc>*>(nullptr), &offset)template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset);-?- Let F be fmt for the first overload and fmt.c_str() for the second overload.
-8- Constraints: The expressionfrom_stream(declval<basic_istream<charT, traits>&>(),fmt.c_str()F, tp, addressof(abbrev), &offset)is well-formed when treated as an unevaluated operand.
-9- Returns: A manipulator such that the expression is >> parse(fmt, tp, abbrev, offset) has type I, has value is, and calls from_Âstream(is,fmt.c_str()F, tp, addressof(abbrev), &offset).
Section: 18.4.4 [concept.convertible] Status: Tentatively Ready Submitter: Tim Song Opened: 2021-05-26 Last modified: 2021-06-07
Priority: Not Prioritized
View other active issues in [concept.convertible].
View all other issues in [concept.convertible].
Discussion:
The specification of convertible_to implicitly requires static_cast<To>(f()) to be equality-preserving. Under 18.2 [concepts.equality] p1, the operand of this expression is f, but what we really want is for f() to be treated as the operand. We should just use declval (which is treated specially by the definition of "operand" for this purpose) instead of reinventing the wheel.
[2021-06-07; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 18.4.4 [concept.convertible] as indicated:
template<class From, class To>
concept convertible_to =
is_convertible_v<From, To> &&
requires(add_rvalue_reference_t<From> (&f)()) {
static_cast<To>(fdeclval<From>());
};
Section: 24.4.3 [range.sized] Status: Tentatively Ready Submitter: Tim Song Opened: 2021-06-03 Last modified: 2021-06-14
Priority: Not Prioritized
View all other issues in [range.sized].
Discussion:
24.4.3 [range.sized] p2.1 requires that for a sized_range t, ranges::size(t) is equal to ranges::distance(t). But for a sized_range, ranges::distance(t) is simply ranges::size(t) cast to the difference type.
Presumably what we meant is that distance(begin, end) — the actual distance you get from traversing the range — is equal to what ranges::size produces, and not merely that casting from the size type to difference type is value-preserving. Otherwise, sized_range would be more or less meaningless.[2021-06-14; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 24.4.3 [range.sized] as indicated:
template<class T> concept sized_range = range<T> && requires(T& t) { ranges::size(t); };-2- Given an lvalue t of type remove_reference_t<T>, T models sized_range only if
(2.1) — ranges::size(t) is amortized 𝒪(1), does not modify t, and is equal to ranges::distance(ranges::begin(t), ranges::end(t)
t), and(2.2) — if iterator_t<T> models forward_iterator, ranges::size(t) is well-defined regardless of the evaluation of ranges::begin(t).
Section: 25.6.11 [alg.equal], 25.6.12 [alg.is.permutation] Status: Tentatively Ready Submitter: Tim Song Opened: 2021-06-03 Last modified: 2021-06-23
Priority: Not Prioritized
View all other issues in [alg.equal].
Discussion:
ranges::equal and ranges::is_permutation are currently only required to short-circuit on different sizes if the iterator and sentinel of the two ranges pairwise model sized_sentinel_for. They should also short-circuit if the ranges are sized.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 25.6.11 [alg.equal] as indicated:
template<class InputIterator1, class InputIterator2> constexpr bool equal(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2); […] template<class InputIterator1, class InputIterator2, class BinaryPredicate> constexpr bool equal(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, BinaryPredicate pred); […] template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity> requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2> constexpr bool ranges::equal(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<input_range R1, input_range R2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity> requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2> constexpr bool ranges::equal(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});[…]
-3- Complexity: Ifthe types of first1, last1, first2, and last2:
(3.1) — the types of first1, last1, first2, and last2 meet the Cpp17RandomAccessIterator requirements (23.3.5.7 [random.access.iterators]) and last1 - first1 != last2 - first2 for the overloads in namespace std;
(3.2) — the types of first1, last1, first2, and last2 pairwise model sized_sentinel_for (23.3.4.8 [iterator.concept.sizedsentinel]) and last1 - first1 != last2 - first2 for the first overload
sin namespace ranges,(3.3) — R1 and R2 each model sized_range and ranges::distance(r1) != ranges::distance(r2) for the second overload in namespace ranges,
and last1 - first1 != last2 - first2,then no applications of the corresponding predicate and each projection; otherwise, […]
Modify 25.6.12 [alg.is.permutation] as indicated:
template<forward_iterator I1, sentinel_for<I1> S1, forward_iterator I2, sentinel_for<I2> S2, class Proj1 = identity, class Proj2 = identity, indirect_equivalence_relation<projected<I1, Proj1>, projected<I2, Proj2>> Pred = ranges::equal_to> constexpr bool ranges::is_permutation(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<forward_range R1, forward_range R2, class Proj1 = identity, class Proj2 = identity, indirect_equivalence_relation<projected<iterator_t<R1>, Proj1>, projected<iterator_t<R2>, Proj2>> Pred = ranges::equal_to> constexpr bool ranges::is_permutation(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});[…]
-7- Complexity: No applications of the corresponding predicate and projections if:
(7.1) — for the first overload,
(7.1.1) — S1 and I1 model sized_sentinel_for<S1, I1>,
(7.1.2) — S2 and I2 model sized_sentinel_for<S2, I2>, and
(7.1.3) — last1 - first1 != last2 - first2
.;(7.2) — for the second overload, R1 and R2 each model sized_range, and ranges::distance(r1) != ranges::distance(r2).
Otherwise, exactly last1 - first1 applications of the corresponding predicate and projections if ranges::equal(first1, last1, first2, last2, pred, proj1, proj2) would return true; otherwise, at worst 𝒪(N2), where N has the value last1 - first1.
Section: 26.6.5.2 [rand.adapt.disc] Status: Tentatively Ready Submitter: Ilya Burylov Opened: 2021-06-03 Last modified: 2021-06-14
Priority: Not Prioritized
View all other issues in [rand.adapt.disc].
Discussion:
A discard_block_engine engine adaptor is described in 26.6.5.2 [rand.adapt.disc]. It produces random numbers from the underlying engine discarding (p - r) last values of p - size block, where r and p are template parameters of std::size_t type:
template<class Engine, size_t p, size_t r> class discard_block_engine;
The transition algorithm for discard_block_engine is described as follows (paragraph 2):
The transition algorithm discards all but r > 0 values from each block of p = r values delivered by ℯ. The state transition is performed as follows: If n = r, advance the state of e from ei to ei+p-r and set n to 0. In any case, then increment n and advance e's then-current state ej to ej+1.
Where n is of integer type. In the API of discard block engine, n is represented in the following way:
[…] int n; // exposition only
In cases where int is equal to int32_t, overflow is possible for n that leads to undefined behavior. Such situation can happen when the p and r template parameters exceed INT_MAX.
This misleading exposition block leads to differences in implementations:GNU Libstdc++ uses size_t for n — in this case no overflow happened even if template parameters exceed INT_MAX
LLVM Libcxx uses int for n together with a static_assert that is checking that p and r values are <= INT_MAX
Such difference in implementation makes code not portable and may potentially breaks random number sequence consistency between different implementors of C++ std lib.
The problematic use case is the following one:
#include <iostream>
#include <random>
#include <limits>
int main() {
std::minstd_rand0 generator;
constexpr std::size_t skipped_size = static_cast<std::size_t>(std::numeric_limits<int>::max());
constexpr std::size_t block_size = skipped_size + 5;
constexpr std::size_t used_block = skipped_size;
std::cout << "block_size = " << block_size << std::endl;
std::cout << "used_block = " << used_block << "\n" << std::endl;
std::discard_block_engine<std::minstd_rand0, block_size, used_block> discard_generator;
// Call discard procedures
discard_generator.discard(used_block);
generator.discard(block_size);
// Call generation. Results should be equal
for (std::int32_t i = 0; i < 10; ++i)
{
std::cout << discard_generator() << " should be equal " << generator() << std::endl;
}
}
We see no solid reason for n to be an int, given that the relevant template parameters are std::size_t. It seems like a perfectly fine use case to generate random numbers in amounts larger than INT_MAX.
The proposal is to change exposition text to std::size_t:size_t n; // exposition only
It will not mandate the existing libraries to break ABI, but at least guide for better implementation.
[2021-06-14; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 26.6.5.2 [rand.adapt.disc], class template discard_block_engine synopsis, as indicated:
[…]
// generating functions
result_type operator()();
void discard(unsigned long long z);
// property functions
const Engine& base() const noexcept { return e; };
private:
Engine e; // exposition only
intsize_t n; // exposition only
};
Section: 24.7.18.1 [range.elements.overview] Status: Tentatively Ready Submitter: Barry Revzin Opened: 2021-05-29 Last modified: 2021-09-20
Priority: 3
Discussion:
In 24.7.18.1 [range.elements.overview] we have:
keys_view is an alias for elements_view<views::all_t<R>, 0>, and is useful for extracting keys from associative containers.
[Example 2:auto names = keys_view{historical_figures}; for (auto&& name : names) { cout << name << ' '; // prints Babbage Hamilton Lovelace Turing }— end example]
Where keys_view is defined as:
template<class R> using keys_view = elements_view<views::all_t<R>, 0>;
There's a similar example for values_view.
But class template argument deduction cannot work here, keys_view(r) cannot deduce R — it's a non-deduced context. At the very least, the example is wrong and should be rewritten as views::keys(historical_figures). Or, I guess, as keys_view<decltype(historical_figures)>(historical_figures)> (but really, not).[2021-05-29 Tim comments and provides wording]
I have some ideas about making CTAD work for keys_view and values_view, but current implementations of alias template CTAD are not yet in a shape to permit those ideas to be tested. What is clear, though, is that the current definitions of keys_view and values_view can't possibly ever work for CTAD, because R is only used in a non-deduced context and so they can never meet the "deducible" constraint in 12.2.2.9 [over.match.class.deduct] p2.3.
The proposed resolution below removes the views::all_t transformation in those alias templates. This makes these aliases transparent enough to support CTAD out of the box when a view is used, and any questions about deduction guides on elements_view can be addressed later once the implementations become more mature. This also makes them consistent with all the other views: you can't write elements_view<map<int, int>&, 0> or reverse_view<map<int, int>&>, so why do we allow keys_view<map<int, int>&>? It is, however, a breaking change, so it is important that we get this in sooner rather than later. The proposed resolution has been implemented and tested.[2021-06-14; Reflector poll]
Set priority to 3 after reflector poll in August. Partially addressed by an editorial change.
Previous resolution [SUPERSEDED]:
This wording is relative to N4885.
Modify 24.2 [ranges.syn], header <ranges> synopsis, as indicated:
[…] namespace std::ranges { […] // 24.7.18 [range.elements], elements view template<input_range V, size_t N> requires see below class elements_view; template<class T, size_t N> inline constexpr bool enable_borrowed_range<elements_view<T, N>> = enable_borrowed_range<T>; template<class R> using keys_view = elements_view<views::all_t<R>, 0>; template<class R> using values_view = elements_view<views::all_t<R>, 1>; […] }Modify 24.7.18.1 [range.elements.overview] as indicated:
-3- keys_view is an alias for elements_view<
[Example 2:views::all_t<R>, 0>, and is useful for extracting keys from associative containers.auto names = keys_view{views::all(historical_figures)}; for (auto&& name : names) { cout << name << ' '; // prints Babbage Hamilton Lovelace Turing }— end example]
-4- values_view is an alias for elements_view<views::all_t<R>, 1>, and is useful for extracting values from associative containers. [Example 3:auto is_even = [](const auto x) { return x % 2 == 0; }; cout << ranges::count_if(values_view{views::all(historical_figures)}, is_even); // prints 2— end example]
[2021-06-18 Tim syncs wording to latest working draft]
The examples have been editorially corrected, so the new wording simply changes the definition of keys_view and values_view.
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 24.2 [ranges.syn], header <ranges> synopsis, as indicated:
[…]
namespace std::ranges {
[…]
// 24.7.18 [range.elements], elements view
template<input_range V, size_t N>
requires see below
class elements_view;
template<class T, size_t N>
inline constexpr bool enable_borrowed_range<elements_view<T, N>> = enable_borrowed_range<T>;
template<class R>
using keys_view = elements_view<views::all_t<R>, 0>;
template<class R>
using values_view = elements_view<views::all_t<R>, 1>;
[…]
}
Modify 24.7.18.1 [range.elements.overview] as indicated:
-3- keys_view is an alias for elements_view<
[Example 2: … — end example] -4- values_view is an alias for elements_view<views::all_t<R>, 0>, and is useful for extracting keys from associative containers.views::all_t<R>, 1>, and is useful for extracting values from associative containers. [Example 3: … — end example]
Section: 20.6.8 [optional.comp.with.t] Status: Tentatively Ready Submitter: Casey Carter Opened: 2021-06-07 Last modified: 2021-06-23
Priority: Not Prioritized
View all other issues in [optional.comp.with.t].
Discussion:
The three-way-comparison operator for optionals and non-optionals is defined in [optional.comp.with.t] paragraph 25:
template<class T, three_way_comparable_with<T> U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>& x, const U& v);-25- Effects: Equivalent to: return bool(x) ? *x <=> v : strong_ordering::less;
Checking three_way_comparable_with in particular requires checking that x < v is well-formed for an lvalue const optional<T> and lvalue const U. x < v can be rewritten as (v <=> x) > 0. If U is a specialization of optional, this overload could be used for v <=> x, but first we must check the constraints…
The straightforward fix for this recursion seems to be to refuse to check three_way_comparable_with<T> when U is a specialization of optional. MSVC has been shipping such a fix; our initial tests for this new operator triggered the recursion.
Previous resolution [SUPERSEDED]:
This wording is relative to N4885.
Modify 20.6.2 [optional.syn], header <optional> synopsis, as indicated:
[…] // 20.6.8 [optional.comp.with.t], comparison with T […] template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&); template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&); template<class T,three_way_comparable_with<T>class U> requires see below constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const U&); […]Modify 20.6.8 [optional.comp.with.t] as indicated:
template<class T,three_way_comparable_with<T>class U> requires (!is-specialization-of<U, optional>) && three_way_comparable_with<T, U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const U&);-?- The exposition-only trait template is-specialization-of<A, B> is a constant expression with value true when A denotes a specialization of the class template B, and false otherwise.
-25- Effects: Equivalent to: return bool(x) ? *x <=> v : strong_ordering::less;
[2021-06-14; Improved proposed wording based on reflector discussion]
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 20.6.2 [optional.syn], header <optional> synopsis, as indicated:
[…] // 20.6.3 [optional.optional], class template optional template<class T> class optional; template<class T> constexpr bool is-optional = false; // exposition only template<class T> constexpr bool is-optional<optional<T>> = true; // exposition only […] // 20.6.8 [optional.comp.with.t], comparison with T […] template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&); template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&); template<class T,three_way_comparable_with<T>class U> requires (!is-optional<U>) && three_way_comparable_with<T, U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const U&); […]
Modify 20.6.8 [optional.comp.with.t] as indicated:
template<class T,three_way_comparable_with<T>class U> requires (!is-optional<U>) && three_way_comparable_with<T, U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>& x, const U& v);-25- Effects: Equivalent to: return bool(x) ? *x <=> v : strong_ordering::less;
Section: 20.20.6.4 [format.context] Status: Tentatively Ready Submitter: Casey Carter Opened: 2021-06-08 Last modified: 2021-06-23
Priority: Not Prioritized
Discussion:
LWG 3539 fixed copies of potentially-move-only iterators in the format_to and vformat_to overloads, but missed the fact that member functions of basic_format_context are specified to copy iterators as well. In particular, 20.20.6.4 [format.context] states:
iterator out();-7- Returns: out_.
void advance_to(iterator it);-8- Effects: Equivalent to: out_ = it;
both of which appear to require copyability.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 20.20.6.4 [format.context] as indicated:
iterator out();-7-
Returns: out_.Effects: Equivalent to: return std::move(out_);void advance_to(iterator it);-8- Effects: Equivalent to: out_ = std::move(it);
Section: 24.6.5.2 [range.istream.view] Status: Tentatively Ready Submitter: Casey Carter Opened: 2021-06-15 Last modified: 2021-06-23
Priority: Not Prioritized
Discussion:
P2325R3 removes the default member initializers for basic_istream_view's exposition-only stream_ and value_ members. Consequently, copying a basic_istream_view before the first call to begin may produce undefined behavior since doing so necessarily copies the uninitialized value_. We should restore value_'s default member initializer and remove this particular footgun.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
Proposed resolution:
Wording relative to the post-202106 working draft (as near as possible). This PR is currently being implemented in MSVC.
Modify 24.6.5.2 [range.istream.view] as indicated:
namespace std::ranges {
[…]
template<movable Val, class CharT, class Traits>
requires default_initializable<Val> &&
stream-extractable<Val, CharT, Traits>
class basic_istream_view : public view_interface<basic_istream_view<Val, CharT, Traits>> {
[…]
Val value_ = Val(); // exposition only
};
}
Section: 29.11.3.3 [syncstream.osyncstream.members] Status: Tentatively Ready Submitter: Tim Song Opened: 2021-06-19 Last modified: 2021-06-23
Priority: Not Prioritized
Discussion:
basic_osyncstream::emit seems rather similar to basic_ostream::flush — both are "flushing" operations that forward to member functions of the streambuf and set badbit if those functions fail. But the former isn't currently specified as an unformatted output function, so it a) doesn't construct or check a sentry and b) doesn't set badbit if emit exits via an exception. At least the latter appears to be clearly desirable — a streambuf operation that throws ordinarily results in badbit being set.
The reference implementation in P0053R7 constructs a sentry and only calls emit on the streambuf if the sentry converts to true, so this seems to be an accidental omission in the wording.[2021-06-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 29.11.3.3 [syncstream.osyncstream.members] as indicated:
void emit();-1- Effects: Behaves as an unformatted output function (29.7.5.4 [ostream.unformatted]). After constructing a sentry object, c
Calls sb.emit(). If that call returns false, calls setstate(ios_base::badbit).
Section: 29.7.5.5 [ostream.manip] Status: Tentatively Ready Submitter: Tim Song Opened: 2021-06-19 Last modified: 2021-06-23
Priority: Not Prioritized
View other active issues in [ostream.manip].
View all other issues in [ostream.manip].
Discussion:
basic_osyncstream::emit is specified to set badbit if it fails (29.11.3.3 [syncstream.osyncstream.members] p1), but the emit part of the flush_emit manipulator isn't, even though the flush part does set badbit if it fails.
More generally, given an osyncstream s, s << flush_emit; should probably have the same behavior as s.flush(); s.emit(). The reference implementation linked in P0753R2 does set badbit on failure, so at least this part appears to be an oversight. As discussed in LWG 3570, basic_osyncstream::emit should probably be an unformatted output function, so the emit part of flush_emit should do so too.[2021-06-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4885.
Modify 29.7.5.5 [ostream.manip] as indicated:
template<class charT, class traits> basic_ostream<charT, traits>& flush_emit(basic_ostream<charT, traits>& os);-12- Effects: Calls os.flush(). Then, if os.rdbuf() is a basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, behaves as an unformatted output function (29.7.5.4 [ostream.unformatted]) of os. After constructing a sentry object, calls buf->emit(). If that call returns false, calls os.setstate(ios_base::badbit).
Section: 24.7.3 [range.copy.wrap] Status: Tentatively Ready Submitter: Tim Song Opened: 2021-06-19 Last modified: 2021-07-17
Priority: Not Prioritized
Discussion:
P2231R1 made optional fully constexpr, but missed its cousin semiregular-box (which was renamed to copyable-box by P2325R3). Most operations of copyable-box are already constexpr simply because copyable-box is specified in terms of optional; the only missing ones are the assignment operators layered on top when the wrapped type isn't assignable itself.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 24.7.3 [range.copy.wrap] as indicated:
-1- Many types in this subclause are specified in terms of an exposition-only class template copyable-box. copyable-box<T> behaves exactly like optional<T> with the following differences:
(1.1) — […]
(1.2) — […]
(1.3) — If copyable<T> is not modeled, the copy assignment operator is equivalent to:
constexpr copyable-box& operator=(const copyable-box& that) noexcept(is_nothrow_copy_constructible_v<T>) { if (this != addressof(that)) { if (that) emplace(*that); else reset(); } return *this; }(1.4) — If movable<T> is not modeled, the move assignment operator is equivalent to:
constexpr copyable-box& operator=(copyable-box&& that) noexcept(is_nothrow_move_constructible_v<T>) { if (this != addressof(that)) { if (that) emplace(std::move(*that)); else reset(); } return *this; }
Section: 21.4.3.2 [string.view.cons] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2021-07-13 Last modified: 2021-07-17
Priority: Not Prioritized
View other active issues in [string.view.cons].
View all other issues in [string.view.cons].
Discussion:
The standard does not specify the exceptions of this constructor, but since std::to_address is a noexcept function, this constructor throws if and only when end - begin throws, we should add a Throws element for it.
[2021-08-20; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 21.4.3.2 [string.view.cons] as indicated:
template<class It, class End> constexpr basic_string_view(It begin, End end);-7- Constraints:
[…] -8- Preconditions: […] -9- Effects: Initializes data_ with to_address(begin) and initializes size_ with end - begin. -?- Throws: When and what end - begin throws.
Section: 23.5.4.1 [common.iterator] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2021-07-21 Last modified: 2021-08-23
Priority: 3
View all other issues in [common.iterator].
Discussion:
After P2231R1, variant becomes completely constexpr-able, which means that common_iterator can also be completely constexpr-able.
[2021-08-20; Reflector poll]
Set priority to 3 after reflector poll.
[2021-08-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 23.5.4.1 [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(I i);
constexpr common_iterator(S s);
template<class I2, class S2>
requires convertible_to<const I2&, I> && convertible_to<const S2&, S>
constexpr common_iterator(const common_iterator<I2, S2>& x);
template<class I2, class S2>
requires convertible_to<const I2&, I> && convertible_to<const S2&, S> &&
assignable_from<I&, const I2&> && assignable_from<S&, const S2&>
constexpr common_iterator& operator=(const common_iterator<I2, S2>& x);
constexpr decltype(auto) operator*();
constexpr decltype(auto) operator*() const
requires dereferenceable<const I>;
constexpr decltype(auto) operator->() const
requires see below;
constexpr common_iterator& operator++();
constexpr decltype(auto) operator++(int);
template<class I2, sentinel_for<I> S2>
requires sentinel_for<S, I2>
friend constexpr bool operator==(
const common_iterator& x, const common_iterator<I2, S2>& y);
template<class I2, sentinel_for<I> S2>
requires sentinel_for<S, I2> && equality_comparable_with<I, I2>
friend constexpr bool operator==(
const common_iterator& x, const common_iterator<I2, S2>& y);
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);
friend constexpr iter_rvalue_reference_t<I> iter_move(const common_iterator& i)
noexcept(noexcept(ranges::iter_move(declval<const I&>())))
requires input_iterator<I>;
template<indirectly_swappable<I> I2, class S2>
friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y)
noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));
private:
variant<I, S> v_; // exposition only
};
[…]
}
Modify 23.5.4.3 [common.iter.const] as indicated:
template<class I2, class S2> requires convertible_to<const I2&, I> && convertible_to<const S2&, S> && assignable_from<I&, const I2&> && assignable_from<S&, const S2&> constexpr common_iterator& operator=(const common_iterator<I2, S2>& x);-5- Preconditions: x.v_.valueless_by_exception() is false.
[…]
Modify 23.5.4.4 [common.iter.access] as indicated:
constexpr decltype(auto) operator*(); constexpr decltype(auto) operator*() const requires dereferenceable<const I>;-1- Preconditions: holds_alternative<I>(v_) is true.
[…]constexpr decltype(auto) operator->() const requires see below;-3- The expression in the requires-clause is equivalent to:
[…]
Modify 23.5.4.5 [common.iter.nav] as indicated:
constexpr common_iterator& operator++();-1- Preconditions: holds_alternative<I>(v_) is true.
[…]constexpr decltype(auto) operator++(int);-4- Preconditions: holds_alternative<I>(v_) is true.
[…]
Modify 23.5.4.6 [common.iter.cmp] as indicated:
template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y);-1- Preconditions: […]
template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> && equality_comparable_with<I, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y);-3- Preconditions: […]
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: […]
Modify 23.5.4.7 [common.iter.cust] as indicated:
friend constexpr iter_rvalue_reference_t<I> iter_move(const common_iterator& i) noexcept(noexcept(ranges::iter_move(declval<const I&>()))) requires input_iterator<I>;-1- Preconditions: […]
template<indirectly_swappable<I> I2, class S2> friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y) noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));-3- Preconditions: […]
Section: 24.6.4.3 [range.iota.iterator] Status: Tentatively Ready Submitter: Zoe Carver Opened: 2021-08-14 Last modified: 2021-08-20
Priority: Not Prioritized
View all other issues in [range.iota.iterator].
Discussion:
iota_view's iterator's operator+ could avoid a copy construct by doing "i += n; return i" rather than "return i += n;" as seen in 24.6.4.3 [range.iota.iterator].
This is what libc++ has implemented and shipped, even though it may not technically be conforming (if a program asserted the number of copies, for example). It might be good to update this operator and the minus operator accordingly.[2021-08-20; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 24.6.4.3 [range.iota.iterator] as indicated:
friend constexpr iterator operator+(iterator i, difference_type n) requires advanceable<W>;-20- Effects: Equivalent to:
returni += n; return i;friend constexpr iterator operator+(difference_type n, iterator i) requires advanceable<W>;-21- Effects: Equivalent to: return i + n;
friend constexpr iterator operator-(iterator i, difference_type n) requires advanceable<W>;-22- Effects: Equivalent to:
returni -= n; return i;
Section: 21.4.3.2 [string.view.cons] Status: Tentatively Ready Submitter: Jiang An, Casey Carter Opened: 2021-08-16 Last modified: 2021-09-20
Priority: Not Prioritized
View other active issues in [string.view.cons].
View all other issues in [string.view.cons].
Discussion:
Jiang An:
The issue is found in this Microsoft STL pull request.
I think an additional constraint should be added to 21.4.3.2 [string.view.cons]/11 (a similar constraint is already present for reference_wrapper):(11.?) — is_same_v<remove_cvref_t<R>, basic_string_view> is false.
Casey Carter:
P1989R2 "Range constructor for std::string_view 2: Constrain Harder" added a converting constructor to basic_string_view that accepts (by forwarding reference) any sized contiguous range with a compatible element type. This constructor unfortunately intercepts attempts at move construction which previously would have resulted in a call to the copy constructor. (I suspect the authors of P1989 were under the mistaken impression that basic_string_view had a defaulted move constructor, which would sidestep this issue by being chosen by overload resolution via the "non-template vs. template" tiebreaker.)
The resulting inefficiency could be corrected by adding a defaulted move constructor and move assignment operator to basic_string_view, but it would be awkward to add text to specify that these moves always leave the source intact. Presumably this is why the move operations were omitted in the first place. We therefore recommend constraining the conversion constructor template to reject arguments whose decayed type is basic_string_view (which we should probably do for all conversion constructor templates in the Standard Library). Implementation experience: MSVC STL is in the process of implementing this fix. Per Jonathan Wakely, libstdc++ has a similar constraint.[2021-09-20; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 21.4.3.2 [string.view.cons] as indicated:
template<class R> constexpr basic_string_view(R&& r);-10- Let d be an lvalue of type remove_cvref_t<R>.
-11- Constraints:
(11.?) — remove_cvref_t<R> is not the same type as basic_string_view,
(11.1) — R models ranges::contiguous_range and ranges::sized_range,
(11.2) — is_same_v<ranges::range_value_t<R>, charT> is true,
[…]
Section: 20.7.3.4 [variant.assign] Status: Tentatively Ready Submitter: Barry Revzin Opened: 2021-09-01 Last modified: 2021-09-20
Priority: Not Prioritized
View other active issues in [variant.assign].
View all other issues in [variant.assign].
Discussion:
Example originally from StackOverflow but with a more reasonable example from Tim Song:
#include <variant>
#include <string>
struct A {
A() = default;
A(A&&) = delete;
};
int main() {
std::variant<A, std::string> v;
v = "hello";
}
There is implementation divergence here: libstdc++ rejects, libc++ and msvc accept.
20.7.3.4 [variant.assign] bullet (13.3) says that if we're changing the alternative in assignment and it is not the case that the converting construction won't throw (as in the above), then "Otherwise, equivalent to operator=(variant(std::forward<T>(t)))." That is, we defer to move assignment. variant<A, string> isn't move-assignable (because A isn't move constructible). Per the wording in the standard, we have to reject this. libstdc++ follows the wording of the standard. But we don't actually have to do a full move assignment here, since we know the situation we're in is changing the alternative, so the fact that A isn't move-assignable shouldn't matter. libc++ and msvc instead do something more direct, allowing the above program.[2021-09-20; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 20.7.3.4 [variant.assign] as indicated:
[Drafting note: This should cover the case that we want to cover: that if construction of Tj from T throws, we haven't yet changed any state in the variant.]
template<class T> constexpr variant& operator=(T&& t) noexcept(see below);-11- Let Tj be a type that is determined as follows: build an imaginary function FUN(Ti) for each alternative type Ti for which Ti x[] = {std::forward<T>(t)}; is well-formed for some invented variable x. 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.
-12- Constraints: […] -13- Effects:
(13.1) — If *this holds a Tj, assigns std::forward<T>(t) to the value contained in *this.
(13.2) — Otherwise, if is_nothrow_constructible_v<Tj, T> || !is_nothrow_move_constructible_v<Tj> is true, equivalent to emplace<j>(std::forward<T>(t)).
(13.3) — Otherwise, equivalent to
operator=(variant(std::forward<T>(t)))emplace<j>(Tj(std::forward<T>(t))).
Section: 24.5.4 [range.subrange] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2021-09-08 Last modified: 2021-09-20
Priority: 3
View other active issues in [range.subrange].
View all other issues in [range.subrange].
Discussion:
The const lvalue reference overload of get used for subrange only constraint that N < 2, this will cause the "discards qualifiers" error inside the function body when applying get to the lvalue subrange that stores a non-copyable iterator since its begin() is non-const qualified, we probably need to add a constraint for it.
[2021-09-20; Reflector poll]
Set priority to 3 after reflector poll.
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 24.5.4 [range.subrange] as indicated:
namespace std::ranges {
[…]
template<size_t N, class I, class S, subrange_kind K>
requires ((N < 2== 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);
}
Modify 24.5.4.3 [range.subrange.access] as indicated:
[…] template<size_t N, class I, class S, subrange_kind K> requires ((N< 2== 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);-10- Effects: Equivalent to:
if constexpr (N == 0) return r.begin(); else return r.end();
Section: 24.7.14.2 [range.split.view] Status: Tentatively Ready Submitter: Tim Song Opened: 2021-09-13 Last modified: 2021-09-24
Priority: Not Prioritized
Discussion:
Unlike every other range adaptor, split_view::base() const & is constrained on copyable<V> instead of copy_constructible<V>.
Since this function just performs a copy construction, there is no reason to require all of copyable — copy_constructible is sufficient.[2021-09-24; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 24.7.14.2 [range.split.view], class template split_view synopsis, as indicated:
namespace std::ranges {
template<forward_range V, forward_range Pattern>
requires view<V> && view<Pattern> &&
indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to>
class split_view : public view_interface<split_view<V, Pattern>> {
private:
[…]
public:
[…]
constexpr V base() const& requires copyablecopy_constructible<V> { return base_; }
constexpr V base() && { return std::move(base_); }
[…]
};
Section: 24.7.13.5 [range.lazy.split.inner] Status: Tentatively Ready Submitter: Tim Song Opened: 2021-09-13 Last modified: 2021-09-24
Priority: Not Prioritized
Discussion:
The base() && members of iterator adaptors (and iterators of range adaptors) invalidate the adaptor itself by moving from the contained iterator. This is generally unobjectionable — since you are calling base() on an rvalue, it is expected that you won't be using it afterwards.
But lazy_split_view<input_view>::inner-iterator::base() && is special: the iterator being moved from is stored in the lazy_split_view itself and shared between the inner and outer iterators, so the operation invalidates not just the inner-iterator on which it is called, but also the outer-iterator from which the inner-iterator was obtained. This spooky-action-at-a-distance behavior can be surprising, and the value category of the inner iterator seems to be too subtle to base it upon. The PR below constrains this overload to forward ranges. Forward iterators are copyable anyway, but moving could potentially be more efficient.[2021-09-24; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 24.7.13.5 [range.lazy.split.inner] as indicated:
[Drafting note: The constraint uses forward_range<V> since that's the condition for caching in lazy_split_view.]
[…]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>) template<bool Const> struct lazy_split_view<V, Pattern>::inner-iterator { private: […] public: […] constexpr const iterator_t<Base>& base() const &; constexpr iterator_t<Base> base() && requires forward_range<V>; […] };constexpr iterator_t<Base> base() && requires forward_range<V>;-4- Effects: Equivalent to: return std::move(i_.current);
Section: 24.7.13.2 [range.lazy.split.view] Status: Tentatively Ready Submitter: Tim Song Opened: 2021-09-15 Last modified: 2021-09-24
Priority: Not Prioritized
View other active issues in [range.lazy.split.view].
View all other issues in [range.lazy.split.view].
Discussion:
For forward ranges, the non-const versions of lazy_split_view::begin() and lazy_split_view::end() returns an outer-iterator<simple-view<V>>, promoting to const if V models simple-view. However, this failed to take Pattern into account, even though that will also be accessed as const if const-promotion takes place. There is no requirement that const Pattern is a range, however.
[2021-09-24; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 24.7.13.2 [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() { if constexpr (forward_range<V>) return outer-iterator<simple-view<V> && simple-view<Pattern>>{*this, ranges::begin(base_)}; else { current_ = ranges::begin(base_); return outer-iterator<false>{*this}; } } […] constexpr auto end() requires forward_range<V> && common_range<V> { return outer-iterator<simple-view<V> && simple-view<Pattern>>{*this, ranges::end(base_)}; } […] }; […] }
Section: 23.5.3 [move.iterators], 23.5.6 [iterators.counted], 24.7.6.3 [range.filter.iterator], 24.7.7.3 [range.transform.iterator], 24.7.13.4 [range.lazy.split.outer.value], 24.7.13.5 [range.lazy.split.inner], 24.7.18.3 [range.elements.iterator] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2021-09-14 Last modified: 2021-09-24
Priority: Not Prioritized
View all other issues in [move.iterators].
Discussion:
LWG 3391 and 3533 changed some iterators' base() const & from returning value to returning const reference, which also prevents them from throwing exceptions, we should add noexcept for them. Also, lazy_split_view::outer-iterator::value_type::end() can be noexcept since it only returns default_sentinel.
[2021-09-24; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 23.5.3 [move.iterators], class template move_iterator synopsis, as indicated:
[…] constexpr const iterator_type& base() const & noexcept; constexpr iterator_type base() &&; […]
Modify 23.5.3.5 [move.iter.op.conv] as indicated:
constexpr const Iterator& base() const & noexcept;-1- Returns: current.
Modify 23.5.6.1 [counted.iterator], class template counted_iterator synopsis, as indicated:
[…] constexpr const I& base() const & noexcept; constexpr I base() &&; […]
Modify 23.5.6.3 [counted.iter.access] as indicated:
constexpr const I& base() const & noexcept;-1- Effects: Equivalent to: return current;
Modify 24.7.6.3 [range.filter.iterator] as indicated:
[…][…] constexpr const iterator_t<V>& base() const & noexcept; constexpr iterator_t<V> base() &&; […]constexpr const iterator_t<V>& base() const & noexcept;-5- Effects: Equivalent to: return current_;
Modify 24.7.7.3 [range.transform.iterator] as indicated:
[…][…] constexpr const iterator_t<Base>& base() const & noexcept; constexpr iterator_t<Base> base() &&; […]constexpr const iterator_t<Base>& base() const & noexcept;-5- Effects: Equivalent to: return current_;
Modify 24.7.13.4 [range.lazy.split.outer.value] as indicated:
[…][…] constexpr inner-iterator<Const> begin() const; constexpr default_sentinel_t end() const noexcept; […]constexpr default_sentinel_t end() const noexcept;-3- Effects: Equivalent to: return default_sentinel;
Modify 24.7.13.5 [range.lazy.split.inner] as indicated:
[…][…] constexpr const iterator_t<Base>& base() const & noexcept; constexpr iterator_t<Base> base() &&; […]constexpr const iterator_t<Base>& base() const & noexcept;-3- Effects: Equivalent to: return i_.current;
Modify 24.7.18.3 [range.elements.iterator] as indicated:
[…][…] constexpr const iterator_t<Base>& base() const& noexcept; constexpr iterator_t<Base> base() &&; […]constexpr const iterator_t<Base>& base() const& noexcept;-6- Effects: Equivalent to: return current_;
Section: 23.5.4.4 [common.iter.access], 23.5.4.5 [common.iter.nav] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2021-09-18 Last modified: 2021-09-24
Priority: Not Prioritized
Discussion:
LWG 3574 added constexpr to all member functions and friends of common_iterator to make it fully constexpr, but accidentally omitted the exposition-only classes proxy and postfix-proxy defined for some member functions. We should make these two classes fully constexpr, too.
[2021-09-24; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4892.
Modify 23.5.4.4 [common.iter.access] as indicated:
decltype(auto) operator->() const requires see below;-3- […]
-4- […] -5- Effects:
(5.1) — […]
(5.2) — […]
(5.3) — Otherwise, equivalent to: return proxy(*get<I>(v_)); where proxy is the exposition-only class:
class proxy { iter_value_t<I> keep_; constexpr proxy(iter_reference_t<I>&& x) : keep_(std::move(x)) {} public: constexpr const iter_value_t<I>* operator->() const noexcept { return addressof(keep_); } };
Modify 23.5.4.5 [common.iter.nav] as indicated:
decltype(auto) operator++(int);-4- […]
-5- Effects: If I models forward_iterator, equivalent to: […] Otherwise, if […] […] Otherwise, equivalent to:postfix-proxy p(**this); ++*this; return p;where postfix-proxy is the exposition-only class:
class postfix-proxy { iter_value_t<I> keep_; constexpr postfix-proxy(iter_reference_t<I>&& x) : keep_(std::forward<iter_reference_t<I>>(x)) {} public: constexpr const iter_value_t<I>& operator*() const noexcept { return keep_; } };