| Doc. no. | N3135=10-0125 |
| Date: | 2010-08-25 |
| Project: | Programming Language C++ |
| Reply to: | Alisdair Meredith <wg21@alisdairm.net> |
Reference ISO/IEC IS 14882:2003(E)
Also see:
This document contains only library issues which have been closed by the Library Working Group as duplicates or not defects. That is, issues which have a status of Dup or NAD. See the Library Active Issues List active issues and more information. See the Library Defect Reports List for issues considered defects. The introductory material in that document also applies to this document.
Section: D.10.1.3 [auto.ptr.conv] Status: NAD Submitter: Nathan Myers Opened: 1997-12-04 Last modified: 2006-12-29
View all issues with NAD status.
Discussion:
Paragraph 1 in "Effects", says "Calls p->release()" where it clearly must be "Calls p.release()". (As it is, it seems to require using auto_ptr<>::operator-> to refer to X::release, assuming that exists.)
Proposed resolution:
Change 20.7.4.3 [meta.unary.prop] paragraph 1 Effects from "Calls p->release()" to "Calls p.release()".
Rationale:
Not a defect: the proposed change is already found in the standard. [Originally classified as a defect, later reclassified.]
Section: 21.4 [basic.string] Status: NAD Submitter: Beman Dawes Opened: 1997-11-16 Last modified: 2006-12-27
View all other issues in [basic.string].
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Discussion:
In Morristown we changed the size_type and difference_type typedefs for all the other containers to implementation defined with a reference to 23.2 [container.requirements]. This should probably also have been done for strings.
Rationale:
Not a defect. [Originally classified as a defect, later reclassified.] basic_string, unlike the other standard library template containers, is severely constrained by its use of char_traits. Those types are dictated by the traits class, and are far from implementation defined.
Section: 27.5.3 [fpos] Status: NAD Submitter: Matt Austern Opened: 1997-12-15 Last modified: 2006-12-27
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Discussion:
Table 88, in I/O, is too strict; it's unimplementable on systems where a file position isn't just an offset. It also never says just what fpos<> is really supposed to be. [Here's my summary, which Jerry agrees is more or less accurate. "I think I now know what the class really is, at this point: it's a magic cookie that encapsulates an mbstate_t and a file position (possibly represented as an fpos_t), it has syntactic support for pointer-like arithmetic, and implementors are required to have real, not just syntactic, support for arithmetic." This isn't standardese, of course.]
Rationale:
Not a defect. The LWG believes that the Standard is already clear, and that the above summary is what the Standard in effect says.
Section: 22.4.1.5 [locale.codecvt.byname] Status: Dup Submitter: Matt Austern Opened: 1998-01-14 Last modified: 2006-12-30
View all other issues in [locale.codecvt.byname].
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Duplicate of: 19
Discussion:
Section 22.2.1.5.2 says that codecvt<>::do_in and do_out should return the value noconv if "no conversion was needed". However, I don't see anything anywhere that defines what it means for a conversion to be needed or not needed. I can think of several circumstances where one might plausibly think that a conversion is not "needed", but I don't know which one is intended here.
Rationale:
Section: 20.2.5 [allocator.requirements] Status: NAD Submitter: Angelika Langer Opened: 1998-02-23 Last modified: 2006-12-30
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Discussion:
I couldn't find a statement in the standard saying whether the allocator object held by a container is held as a copy of the constructor argument or whether a pointer of reference is maintained internal. There is an according statement for compare objects and how they are maintained by the associative containers, but I couldn't find anything regarding allocators.
Did I overlook it? Is it an open issue or known defect? Or is it deliberately left unspecified?
Rationale:
Not a defect. The LWG believes that the Standard is already clear. See 23.2 [container.requirements], paragraph 8.
Section: 22.4.1.5 [locale.codecvt.byname] Status: Dup Submitter: Brendan Kehoe Opened: 1998-06-01 Last modified: 2006-12-30
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Duplicate of: 33
Discussion:
Rationale:
Section: 27.8.3 [ostringstream] Status: NAD Submitter: Matthias Mueller Opened: 1998-05-27 Last modified: 2006-12-27
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Discussion:
In a comp.lang.c++.moderated Matthias Mueller wrote:
"We are not sure how to interpret the CD2 (see 27.3 [iostream.forward], 27.8.3.1 [ostringstream.cons], 27.8.1.1 [stringbuf.cons]) with respect to the question as to what the correct initial positions of the write and read pointers of a stringstream should be."
"Is it the same to output two strings or to initialize the stringstream with the first and to output the second?"
[PJ Plauger, Bjarne Stroustrup, Randy Smithey, Sean Corfield, and Jerry Schwarz have all offered opinions; see reflector messages lib-6518, 6519, 6520, 6521, 6523, 6524.]
Rationale:
The LWG believes the Standard is correct as written. The behavior of stringstreams is consistent with fstreams, and there is a constructor which can be used to obtain the desired effect. This behavior is known to be different from strstreams.
Section: 27.7.1.2.3 [istream::extractors] Status: NAD Submitter: Matt Austern Opened: 1998-07-01 Last modified: 2006-12-27
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Discussion:
27.6.1.2.3 has member functions for extraction of signed char and unsigned char, both singly and as strings. However, it doesn't say what it means to extract a char from a basic_streambuf<charT, Traits>.
basic_streambuf, after all, has no members to extract a char, so basic_istream must somehow convert from charT to signed char or unsigned char. The standard doesn't say how it is to perform that conversion.
Rationale:
The Standard is correct as written. There is no such extractor and this is the intent of the LWG.
Section: D.8.1.3 [depr.strstreambuf.virtuals] Status: NAD Submitter: Matt Austern Opened: 1998-08-18 Last modified: 2006-12-27
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Discussion:
The standard says how this member function affects the current stream position. (gptr or pptr) However, it does not say how this member function affects the beginning and end of the get/put area.
This is an issue when seekoff is used to position the get pointer beyond the end of the current read area. (Which is legal. This is implicit in the definition of seekhigh in D.7.1, paragraph 4.)
Rationale:
The LWG agrees that seekoff() is underspecified, but does not wish to invest effort in this deprecated feature.
Section: 21.4.8.9 [string.io] Status: Dup Submitter: Steve Clamage Opened: 1998-07-09 Last modified: 2006-12-30
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Duplicate of: 25
Discussion:
In a comp.std.c++ posting Michel Michaud wrote: What should be output by:
string text("Hello");
cout << '[' << setw(10) << right << text << ']';
Shouldn't it be:
[ Hello]
Another person replied: Actually, according to the FDIS, the width of the field should be the minimum of width and the length of the string, so the output shouldn't have any padding. I think that this is a typo, however, and that what is wanted is the maximum of the two. (As written, setw is useless for strings. If that had been the intent, one wouldn't expect them to have mentioned using its value.)
It's worth pointing out that this is a recent correction anyway; IIRC, earlier versions of the draft forgot to mention formatting parameters whatsoever.
Rationale:
Section: 22.4.1.4 [locale.codecvt] Status: Dup Submitter: Nathan Myers Opened: 1998-08-24 Last modified: 2006-12-30
View all other issues in [locale.codecvt].
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Duplicate of: 24
Discussion:
In 22.4.1.4 [locale.codecvt] par 3, and in 22.4.1.4.2 [locale.codecvt.virtuals] par 8, a nonexistent member function "do_convert" is mentioned. This member was replaced with "do_in" and "do_out", the proper referents in the contexts above.
Rationale:
Section: 27.9.1 [fstreams] Status: NAD Submitter: Matt Austern Opened: 1998-08-27 Last modified: 2006-12-27
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Discussion:
Classes basic_ifstream, basic_ofstream, and basic_fstream all have a member function is_open. It should be a const member function, since it does nothing but call one of basic_filebuf's const member functions.
Rationale:
Not a defect. This is a deliberate feature; const streams would be meaningless.
Section: 26.6.2.3 [valarray.access] Status: Dup Submitter: Levente Farkas Opened: 1998-09-09 Last modified: 2007-10-11
View all other issues in [valarray.access].
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Duplicate of: 389
Discussion:
valarray:
T operator[] (size_t) const;
why not
const T& operator[] (size_t) const;
as in vector ???
One can't copy even from a const valarray eg:
memcpy(ptr, &v[0], v.size() * sizeof(double));
[I] find this bug in valarray is very difficult.
Rationale:
The LWG believes that the interface was deliberately designed that way. That is what valarray was designed to do; that's where the "value array" name comes from. LWG members further comment that "we don't want valarray to be a full STL container." 26.6.2.3 [valarray.access] specifies properties that indicate "an absence of aliasing" for non-constant arrays; this allows optimizations, including special hardware optimizations, that are not otherwise possible.
Section: 26.6.5 [template.slice.array], 26.6.7 [template.gslice.array], 26.6.8 [template.mask.array], 26.6.9 [template.indirect.array] Status: NAD Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2006-12-29
View all other issues in [template.slice.array].
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Discussion:
Isn't the definition of copy constructor and assignment operators wrong? Instead of
slice_array(const slice_array&); slice_array& operator=(const slice_array&);
IMHO they have to be
slice_array(const slice_array<T>&); slice_array& operator=(const slice_array<T>&);
Same for gslice_array.
Rationale:
Not a defect. The Standard is correct as written.
Section: 23.2.4 [associative.reqmts] Status: NAD Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2007-01-15
View other active issues in [associative.reqmts].
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Discussion:
Paragraph 5 specifies:
For set and multiset the value type is the same as the key type. For map and multimap it is equal to pair<const Key, T>.
Strictly speaking, this is not correct because for set and multiset the value type is the same as the constant key type.
Rationale:
Not a defect. The Standard is correct as written; it uses a different mechanism (const &) for set and multiset. See issue 103 for a related issue.
Section: 21.4.5 [string.access] Status: NAD Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2006-12-27
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Discussion:
If I try
s.insert(0,1,' ');
I get an nasty ambiguity. It might be
s.insert((size_type)0,(size_type)1,(charT)' ');
which inserts 1 space character at position 0, or
s.insert((char*)0,(size_type)1,(charT)' ')
which inserts 1 space character at iterator/address 0 (bingo!), or
s.insert((char*)0, (InputIterator)1, (InputIterator)' ')
which normally inserts characters from iterator 1 to iterator ' '. But according to 23.1.1.9 (the "do the right thing" fix) it is equivalent to the second. However, it is still ambiguous, because of course I mean the first!
Rationale:
Not a defect. The LWG believes this is a "genetic misfortune" inherent in the design of string and thus not a defect in the Standard as such .
Section: 21 [strings] Status: NAD Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2006-12-27
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Discussion:
The standard seems not to require that charT is equivalent to traits::char_type. So, what happens if charT is not equivalent to traits::char_type?
Rationale:
There is already wording in 21.2 [char.traits] paragraph 3 that requires them to be the same.
Section: 21.4.6.8 [string::swap] Status: Dup Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2006-12-30
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Duplicate of: 5
Discussion:
The following compare() description is obviously a bug:
int compare(size_type pos, size_type n1,
charT *s, size_type n2 = npos) const;
because without passing n2 it should compare up to the end of the string instead of comparing npos characters (which throws an exception)
Rationale:
Section: 21.4.6.4 [string::insert], 21.4.6.2 [string::append] Status: NAD Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2006-12-27
View all other issues in [string::insert].
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Discussion:
Why does
template<class InputIterator>
basic_string& append(InputIterator first, InputIterator last);
return a string, while
template<class InputIterator>
void insert(iterator p, InputIterator first, InputIterator last);
returns nothing ?
Rationale:
The LWG believes this stylistic inconsistency is not sufficiently serious to constitute a defect.
Section: 21.4.6.4 [string::insert], 21.4.6.6 [string::replace] Status: Dup Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2006-12-30
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Duplicate of: 83
Discussion:
All insert() and replace() members for strings with an iterator as first argument lack a throw specification. The throw specification should probably be: length_error if size exceeds maximum.
Rationale:
Considered a duplicate because it will be solved by the resolution of issue 83.
Section: 26.6 [numarray] Status: NAD Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2006-12-29
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Discussion:
You can easily create subsets, but you can't easily combine them with other subsets. Unfortunately, you almost always needs an explicit type conversion to valarray. This is because the standard does not specify that valarray subsets provide the same operations as valarrays.
For example, to multiply two subsets and assign the result to a third subset, you can't write the following:
va[slice(0,4,3)] = va[slice(1,4,3)] * va[slice(2,4,3)];
Instead, you have to code as follows:
va[slice(0,4,3)] = static_cast<valarray<double> >(va[slice(1,4,3)]) *
static_cast<valarray<double> >(va[slice(2,4,3)]);
This is tedious and error-prone. Even worse, it costs performance because each cast creates a temporary objects, which could be avoided without the cast.
Proposed resolution:
Extend all valarray subset types so that they offer all valarray operations.
Rationale:
This is not a defect in the Standard; it is a request for an extension.
Section: 17.6.4 [conforming] Status: NAD Submitter: Matt Austern Opened: 1998-01-22 Last modified: 2006-12-27
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Discussion:
Is it a permitted extension for library implementors to add template parameters to standard library classes, provided that those extra parameters have defaults? For example, instead of defining template <class T, class Alloc = allocator<T> > class vector; defining it as template <class T, class Alloc = allocator<T>, int N = 1> class vector;
The standard may well already allow this (I can't think of any way that this extension could break a conforming program, considering that users are not permitted to forward-declare standard library components), but it ought to be explicitly permitted or forbidden.
comment from Steve Cleary via comp.std.c++:
I disagree [with the proposed resolution] for the following reason: consider user library code with template template parameters. For example, a user library object may be templated on the type of underlying sequence storage to use (deque/list/vector), since these classes all take the same number and type of template parameters; this would allow the user to determine the performance tradeoffs of the user library object. A similar example is a user library object templated on the type of underlying set storage (set/multiset) or map storage (map/multimap), which would allow users to change (within reason) the semantic meanings of operations on that object.
I think that additional template parameters should be forbidden in the Standard classes. Library writers don't lose any expressive power, and can still offer extensions because additional template parameters may be provided by a non-Standard implementation class:
template <class T, class Allocator = allocator<T>, int N = 1> class __vector { ... }; template <class T, class Allocator = allocator<T> > class vector: public __vector<T, Allocator> { ... };
Proposed resolution:
Add a new subclause [presumably 17.4.4.9] following 17.6.4.11 [res.on.exception.handling]:
17.4.4.9 Template Parameters
A specialization of a template class described in the C++ Standard Library behaves the same as if the implementation declares no additional template parameters.
Footnote: Additional template parameters with default values are thus permitted.
Add "template parameters" to the list of subclauses at the end of 17.6.4 [conforming] paragraph 1.
[Kona: The LWG agreed the standard needs clarification. After discussion with John Spicer, it seems added template parameters can be detected by a program using template-template parameters. A straw vote - "should implementors be allowed to add template parameters?" found no consensus ; 5 - yes, 7 - no.]
Rationale:
There is no ambiguity; the standard is clear as written. Library implementors are not permitted to add template parameters to standard library classes. This does not fall under the "as if" rule, so it would be permitted only if the standard gave explicit license for implementors to do this. This would require a change in the standard.
The LWG decided against making this change, because it would break user code involving template template parameters or specializations of standard library class templates.
Section: 17.6.4.5 [member.functions] Status: NAD Submitter: AFNOR Opened: 1998-10-07 Last modified: 2006-12-27
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Discussion:
In 17.3.4.4/2 vs 17.3.4.7/0 there is a hole; an implementation could add virtual members a base class and break user derived classes.
Example:
// implementation code:
struct _Base { // _Base is in the implementer namespace
virtual void foo ();
};
class vector : _Base // deriving from a class is allowed
{ ... };
// user code:
class vector_checking : public vector
{
void foo (); // don't want to override _Base::foo () as the
// user doesn't know about _Base::foo ()
};
Proposed resolution:
Clarify the wording to make the example illegal.
Rationale:
This is not a defect in the Standard. The example is already illegal. See 17.6.4.5 [member.functions] paragraph 2.
Section: 23.3.6 [vector] Status: NAD Submitter: AFNOR Opened: 1998-10-07 Last modified: 2009-10-26
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Discussion:
vector<bool> is not a container as its reference and pointer types are not references and pointers.
Also it forces everyone to have a space optimization instead of a speed one.
See also: 99-0008 == N1185 Vector<bool> is Nonconforming, Forces Optimization Choice.
[In Santa Cruz the LWG felt that this was Not A Defect.]
[In Dublin many present felt that failure to meet Container requirements was a defect. There was disagreement as to whether or not the optimization requirements constituted a defect.]
[The LWG looked at the following resolutions in some detail:
* Not A Defect.
* Add a note explaining that vector<bool> does not meet
Container requirements.
* Remove vector<bool>.
* Add a new category of container requirements which
vector<bool> would meet.
* Rename vector<bool>.
No alternative had strong, wide-spread, support and every alternative
had at least one "over my dead body" response.
There was also mention of a transition scheme something like (1) add
vector_bool and deprecate vector<bool> in the next standard. (2)
Remove vector<bool> in the following standard.]
[Modifying container requirements to permit returning proxies (thus allowing container requirements conforming vector<bool>) was also discussed.]
[It was also noted that there is a partial but ugly workaround in that vector<bool> may be further specialized with a customer allocator.]
[Kona: Herb Sutter presented his paper J16/99-0035==WG21/N1211, vector<bool>: More Problems, Better Solutions. Much discussion of a two step approach: a) deprecate, b) provide replacement under a new name. LWG straw vote on that: 1-favor, 11-could live with, 2-over my dead body. This resolution was mentioned in the LWG report to the full committee, where several additional committee members indicated over-my-dead-body positions.]
Discussed at Lillehammer. General agreement that we should deprecate vector<bool> and introduce this functionality under a different name, e.g. bit_vector. This might make it possible to remove the vector<bool> specialization in the standard that comes after C++0x. There was also a suggestion that in C++0x we could additional say that it's implementation defined whether vector<bool> refers to the specialization or to the primary template, but there wasn't general agreement that this was a good idea.
We need a paper for the new bit_vector class.
[ Batavia: ]
The LWG feels we need something closer to SGI's bitvector to ease migration from vector<bool>. Although some of the funcitonality from N2050 could well be used in such a template. The concern is easing the API migration for those users who want to continue using a bit-packed container. Alan and Beman to work.
[ Post Summit Alisdair adds: ]
vector<bool> is now a conforming container under the revised terms of C++0x, which supports containers of proxies.
Recommend NAD.
Two issues remain:
i/ premature optimization in the specification. There is still some sentiment that deprecation is the correct way to go, although it is still not clear what it would mean to deprecate a single specialization of a template.
Recommend: Create a new issue for the discussion, leave as Open.
ii/ Request for a new bitvector class to guarantee the optimization, perhaps with a better tuned interface.
This is a clear extension request that may be handled via a future TR.
[ Batavia (2009-05): ]
We note that most of this issue has become moot over time, and agree with Alisdair's recommendations. Move to NAD Future for reconsideration of part (ii).
[ 2009-07-29 Alisdair reopens: ]
This infamous issue was closed as NAD Future when concepts introduced support for proxy iterators, so the only remaining requirement was to provide a better type to support bitsets of dynamic length. I fear we must re-open this issue until the post-concept form of iterators is available, and hopefully will support the necessary proxy functionality to allow us to close this issue as NAD.
I recommend we spawn a separate issue (1184) requesting a dynamic length bitset and pre-emptively file it as NAD Future. It is difficult to resolve #96 when it effectively contains two separate sub-issues.
[ 2009-10 Santa Cruz: ]
Mark as NAD, and give rationale.
Proposed resolution:
Rationale:
We want to support proxy iterators but that is going to be separate work. Don't want to see this issue come back in these kinds of terms. We're interested in a separate container, and proxy iterators, but both of those are separate issues.
We've looked at a lot of ways to fix this that would be close to this, but those things would break existing code. Attempts to fix this directly have not been tractable, and removing it has not been tractable. Therefore we are closing.
Section: 23 [containers] Status: NAD Submitter: AFNOR Opened: 1998-10-07 Last modified: 2006-12-27
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Discussion:
insert(iterator, const value_type&) is defined both on sequences and on set, with unrelated semantics: insert here (in sequences), and insert with hint (in associative containers). They should have different names (B.S. says: do not abuse overloading).
Rationale:
This is not a defect in the Standard. It is a genetic misfortune of the design, for better or for worse.
Section: 24.5.1.3.13 [reverse.iter.op==] Status: NAD Submitter: AFNOR Opened: 1998-10-07 Last modified: 2006-12-27
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Discussion:
The <, >, <=, >= comparison operator are wrong: they return the opposite of what they should.
Note: same problem in CD2, these were not even defined in CD1. SGI STL code is correct; this problem is known since the Morristown meeting but there it was too late
Rationale:
This is not a defect in the Standard. A careful reading shows the Standard is correct as written. A review of several implementations show that they implement exactly what the Standard says.
Section: 24.5.2 [insert.iterators], 24.6.4 [ostreambuf.iterator] Status: NAD Submitter: AFNOR Opened: 1998-10-07 Last modified: 2006-12-27
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Discussion:
Overspecified For an insert iterator it, the expression *it is required to return a reference to it. This is a simple possible implementation, but as the SGI STL documentation says, not the only one, and the user should not assume that this is the case.
Rationale:
The LWG believes this causes no harm and is not a defect in the standard. The only example anyone could come up with caused some incorrect code to work, rather than the other way around.
Section: 23.3.6 [vector], 23.3.1 [array] Status: NAD Editorial Submitter: AFNOR Opened: 1998-10-07 Last modified: 2010-02-13
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Discussion:
Reserve can not free storage, unlike string::reserve
[ 2010-02-13 Alisdair adds: ]
This issue has been revisited and addressed (755, 850). This issues should be reclassified to NAD Editorial to reflect this action.
Rationale:
This is not a defect in the Standard. The LWG has considered this issue in the past and sees no need to change the Standard. Deque has no reserve() member function. For vector, shrink-to-fit can be expressed in a single line of code (where v is vector<T>):
vector<T>(v).swap(v); // shrink-to-fit v
Section: 23.2.4 [associative.reqmts] Status: Dup Submitter: AFNOR Opened: 1998-10-07 Last modified: 2006-12-30
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Duplicate of: 264
Discussion:
Table 69 of Containers say that a.insert(i,j) is linear if [i, j) is ordered. It seems impossible to implement, as it means that if [i, j) = [x], insert in an associative container is O(1)!
Proposed resolution:
N+log (size()) if [i,j) is sorted according to value_comp()
Rationale:
Subsumed by issue 264.
Section: 21.4.4 [string.capacity] Status: NAD Submitter: AFNOR Opened: 1998-10-07 Last modified: 2006-12-27
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Discussion:
It is not clear that undefined behavior applies when pos == size () for the non const version.
Proposed resolution:
Rewrite as: Otherwise, if pos > size () or pos == size () and the non-const version is used, then the behavior is undefined.
Rationale:
The Standard is correct. The proposed resolution already appears in the Standard.
Section: 27.9 [file.streams] Status: Dup Submitter: AFNOR Opened: 1998-10-07 Last modified: 2008-01-05
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Duplicate of: 454
Discussion:
fstream ctors take a const char* instead of string.
fstream ctors can't take wchar_t
An extension to add a const wchar_t* to fstream would make the implementation non conforming.
Rationale:
This is not a defect in the Standard. It might be an interesting extension for the next Standard.
Section: 26.6.2 [template.valarray] Status: NAD Submitter: AFNOR Opened: 1998-10-07 Last modified: 2006-12-29
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Discussion:
The order of the arguments is (elem, size) instead of the normal (size, elem) in the rest of the library. Since elem often has an integral or floating point type, both types are convertible to each other and reversing them leads to a well formed program.
Proposed resolution:
Inverting the arguments could silently break programs. Introduce the two signatures (const T&, size_t) and (size_t, const T&), but make the one we do not want private so errors result in a diagnosed access violation. This technique can also be applied to STL containers.
Rationale:
The LWG believes that while the order of arguments is unfortunate, it does not constitute a defect in the standard. The LWG believes that the proposed solution will not work for valarray<size_t> and perhaps other cases.
Section: 24.6.3.5 [istreambuf.iterator::equal] Status: NAD Submitter: Nathan Myers Opened: 1998-10-15 Last modified: 2009-07-14
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Discussion:
The member istreambuf_iterator<>::equal is specified to be unnecessarily inefficient. While this does not affect the efficiency of conforming implementations of iostreams, because they can "reach into" the iterators and bypass this function, it does affect users who use istreambuf_iterators.
The inefficiency results from a too-scrupulous definition, which requires a "true" result if neither iterator is at eof. In practice these iterators can only usefully be compared with the "eof" value, so the extra test implied provides no benefit, but slows down users' code.
The solution is to weaken the requirement on the function to return true only if both iterators are at eof.
[ Summit: ]
Reopened by Alisdair.
[ Post Summit Daniel adds: ]
Recommend NAD. The proposed wording would violate the axioms of concept requirement EqualityComparable axioms as part of concept InputIterator and more specifically it would violate the explicit wording of 24.2.3 [input.iterators]/7:
If two iterators a and b of the same type are equal, then either a and b are both dereferenceable or else neither is dereferenceable.[ 2009-07 Frankfurt ]
Agree NAD.
Proposed resolution:
Replace 24.6.3.5 [istreambuf.iterator::equal], paragraph 1,
-1- Returns: true if and only if both iterators are at end-of-stream, or neither is at end-of-stream, regardless of what streambuf object they use.
with
-1- Returns: true if and only if both iterators are at end-of-stream, regardless of what streambuf object they use.
Rationale:
It is not clear that this is a genuine defect. Additionally, the LWG was reluctant to make a change that would result in operator== not being a equivalence relation. One consequence of this change is that an algorithm that's passed the range [i, i) would no longer treat it as an empty range.
Section: 27.7.1.1 [istream], 27.7.1.3 [istream.unformatted] Status: NAD Submitter: Steve Clamage Opened: 1998-10-13 Last modified: 2006-12-27
View all other issues in [istream].
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Discussion:
In 27.6.1.1, class basic_istream has a member function sync, described in 27.6.1.3, paragraph 36.
Following the chain of definitions, I find that the various sync functions have defined semantics for output streams, but no semantics for input streams. On the other hand, basic_ostream has no sync function.
The sync function should at minimum be added to basic_ostream, for internal consistency.
A larger question is whether sync should have assigned semantics for input streams.
Classic iostreams said streambuf::sync flushes pending output and attempts to return unread input characters to the source. It is a protected member function. The filebuf version (which is public) has that behavior (it backs up the read pointer). Class strstreambuf does not override streambuf::sync, and so sync can't be called on a strstream.
If we can add corresponding semantics to the various sync functions, we should. If not, we should remove sync from basic_istream.
Rationale:
A sync function is not needed in basic_ostream because the flush function provides the desired functionality.
As for the other points, the LWG finds the standard correct as written.
Section: 20.5 [template.bitset] Status: Dup Submitter: Judy Ward Opened: 1998-11-06 Last modified: 2008-03-14
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Duplicate of: 778
Discussion:
The following code does not compile with the EDG compiler:
#include <bitset>
using namespace std;
bitset<32> b("111111111");
If you cast the ctor argument to a string, i.e.:
bitset<32> b(string("111111111"));
then it will compile. The reason is that bitset has the following templatized constructor:
template <class charT, class traits, class Allocator> explicit bitset (const basic_string<charT, traits, Allocator>& str, ...);
According to the compiler vendor, Steve Adamcyk at EDG, the user cannot pass this template constructor a const char* and expect a conversion to basic_string. The reason is "When you have a template constructor, it can get used in contexts where type deduction can be done. Type deduction basically comes up with exact matches, not ones involving conversions."
I don't think the intention when this constructor became templatized was for construction from a const char* to no longer work.
Proposed resolution:
Add to 20.5 [template.bitset] a bitset constructor declaration
explicit bitset(const char*);
and in Section 20.5.1 [bitset.cons] add:
explicit bitset(const char* str);Effects:
Calls bitset((string) str, 0, string::npos);
Rationale:
Although the problem is real, the standard is designed that way so it is not a defect. Education is the immediate workaround. A future standard may wish to consider the Proposed Resolution as an extension.
Section: 22.3.1.1.1 [locale.category] Status: NAD Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2006-12-27
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Discussion:
Section 22.1.1.1.1 has the following listed in Table 51: ctype<char> , ctype<wchar_t>.
Also Section 22.4.1.1 [locale.ctype] says:
The instantiations required in Table 51 (22.1.1.1.1) namely ctype<char> and ctype<wchar_t> , implement character classing appropriate to the implementation's native character set.
However, Section 22.4.1.3 [facet.ctype.special] only has a detailed description of the ctype<char> specialization, not the ctype<wchar_t> specialization.
Proposed resolution:
Add the ctype<wchar_t> detailed class description to Section 22.4.1.3 [facet.ctype.special].
Rationale:
Specialization for wchar_t is not needed since the default is acceptable.
Section: 27.8 [string.streams], 27.9 [file.streams] Status: NAD Submitter: Angelika Langer Opened: 1999-02-22 Last modified: 2009-07-14
View all other issues in [string.streams].
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Discussion:
The following question came from Thorsten Herlemann:
You can set a mode when constructing or opening a file-stream or filebuf, e.g. ios::in, ios::out, ios::binary, ... But how can I get that mode later on, e.g. in my own operator << or operator >> or when I want to check whether a file-stream or file-buffer object passed as parameter is opened for input or output or binary? Is there no possibility? Is this a design-error in the standard C++ library?
It is indeed impossible to find out what a stream's or stream buffer's open mode is, and without that knowledge you don't know how certain operations behave. Just think of the append mode.
Both streams and stream buffers should have a mode() function that returns the current open mode setting.
[ post Bellevue: Alisdair requested to re-Open. ]
[ 2009-07 Frankfurt ]
Neither Howard nor Bill has received a customer request for this.
No consensus for change. The programmer can save this information to the side.
Moved to NAD.
Proposed resolution:
For stream buffers, add a function to the base class as a non-virtual function qualified as const to 27.6.2 [streambuf]:
openmode mode() const;
Returns the current open mode.
With streams, I'm not sure what to suggest. In principle, the mode could already be returned by ios_base, but the mode is only initialized for file and string stream objects, unless I'm overlooking anything. For this reason it should be added to the most derived stream classes. Alternatively, it could be added to basic_ios and would be default initialized in basic_ios<>::init().
Rationale:
This might be an interesting extension for some future, but it is not a defect in the current standard. The Proposed Resolution is retained for future reference.
Section: 23.3.4.4 [list.ops] Status: NAD Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2007-02-19
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Discussion:
What happens if a splice operation causes the size() of a list to grow beyond max_size()?
Rationale:
Size() cannot grow beyond max_size().
Section: 27.7.1.5.1 [iostream.cons] Status: NAD Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2006-12-27
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Discussion:
-1- Effects Constructs an object of class basic_iostream, assigning initial values to the base classes by calling basic_istream<charT,traits>(sb) (lib.istream) and basic_ostream<charT,traits>(sb) (lib.ostream)
The called for basic_istream and basic_ostream constructors call init(sb). This means that the basic_iostream's virtual base class is initialized twice.
Proposed resolution:
Change 27.6.1.5.1, paragraph 1 to:
-1- Effects Constructs an object of class basic_iostream, assigning initial values to the base classes by calling basic_istream<charT,traits>(sb) (lib.istream).
Rationale:
The LWG agreed that the init() function is called twice, but said that this is harmless and so not a defect in the standard.
Section: 22.4.1.4 [locale.codecvt] Status: NAD Submitter: Angelika Langer Opened: 1999-03-18 Last modified: 2009-07-14
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Discussion:
Section 22.4.1.4 [locale.codecvt] specifies that ctype_byname<char> must be a specialization of the ctype_byname template.
It is common practice in the standard that specializations of class templates are only mentioned where the interface of the specialization deviates from the interface of the template that it is a specialization of. Otherwise, the fact whether or not a required instantiation is an actual instantiation or a specialization is left open as an implementation detail.
Clause 22.2.1.4 deviates from that practice and for that reason is misleading. The fact, that ctype_byname<char> is specified as a specialization suggests that there must be something "special" about it, but it has the exact same interface as the ctype_byname template. Clause 22.2.1.4 does not have any explanatory value, is at best redundant, at worst misleading - unless I am missing anything.
Naturally, an implementation will most likely implement ctype_byname<char> as a specialization, because the base class ctype<char> is a specialization with an interface different from the ctype template, but that's an implementation detail and need not be mentioned in the standard.
[ Summit: ]
Reopened by Alisdair.
[ 2009-07 Frankfurt ]
Moved to NAD.
Rationale:
The standard as written is mildly misleading, but the correct fix is to deal with the underlying problem in the ctype_byname base class, not in the specialization. See issue 228.
Section: 23.4.1 [map] Status: NAD Editorial Submitter: Mark Mitchell Opened: 1999-04-14 Last modified: 2008-03-14
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Discussion:
23.2 [container.requirements]
expression return type pre/post-condition
------------- ----------- -------------------
X::value_type T T is assignable
23.4.1 [map]
A map satisfies all the requirements of a container.
For a map<Key, T> ... the value_type is pair<const Key, T>.
There's a contradiction here. In particular, `pair<const Key, T>' is not assignable; the `const Key' cannot be assigned to. So, map<Key, T>::value_type does not satisfy the assignable requirement imposed by a container.
[See issue 103 for the slightly related issue of modification of set keys.]
Rationale:
The LWG believes that the standard is inconsistent, but that this is a design problem rather than a strict defect. May wish to reconsider for the next standard.
Section: D.6 [depr.c.headers] Status: NAD Submitter: Christophe de Dinechin Opened: 1999-05-04 Last modified: 2006-12-27
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Discussion:
[depr.c.headers] paragraph 2 reads:
Each C header, whose name has the form name.h, behaves as if each name placed in the Standard library namespace by the corresponding cname header is also placed within the namespace scope of the namespace std and is followed by an explicit using-declaration (_namespace.udecl_)
I think it should mention the global name space somewhere... Currently, it indicates that name placed in std is also placed in std...
I don't know what is the correct wording. For instance, if struct tm is defined in time.h, ctime declares std::tm. However, the current wording seems ambiguous regarding which of the following would occur for use of both ctime and time.h:
// version 1:
namespace std {
struct tm { ... };
}
using std::tm;
// version 2:
struct tm { ... };
namespace std {
using ::tm;
}
// version 3:
struct tm { ... };
namespace std {
struct tm { ... };
}
I think version 1 is intended.
[Kona: The LWG agreed that the wording is not clear. It also agreed that version 1 is intended, version 2 is not equivalent to version 1, and version 3 is clearly not intended. The example below was constructed by Nathan Myers to illustrate why version 2 is not equivalent to version 1.
Although not equivalent, the LWG is unsure if (2) is enough of a problem to be prohibited. Points discussed in favor of allowing (2):
- It may be a convenience to implementors.
- The only cases that fail are structs, of which the C library contains only a few.
]
Example:
#include <time.h>
#include <utility>
int main() {
std::tm * t;
make_pair( t, t ); // okay with version 1 due to Koenig lookup
// fails with version 2; make_pair not found
return 0;
}
Proposed resolution:
Replace D.6 [depr.c.headers] paragraph 2 with:
Each C header, whose name has the form name.h, behaves as if each name placed in the Standard library namespace by the corresponding cname header is also placed within the namespace scope of the namespace std by name.h and is followed by an explicit using-declaration (_namespace.udecl_) in global scope.
Rationale:
The current wording in the standard is the result of a difficult compromise that averted delay of the standard. Based on discussions in Tokyo it is clear that there is no still no consensus on stricter wording, so the issue has been closed. It is suggested that users not write code that depends on Koenig lookup of C library functions.
Section: 27.5.4.1 [basic.ios.cons] Status: NAD Submitter: Angelika Langer Opened: 1999-05-12 Last modified: 2006-12-27
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Discussion:
There is no initial value for the adjustfield defined, although many people believe that the default adjustment were right. This is a common misunderstanding. The standard only defines that, if no adjustment is specified, all the predefined inserters must add fill characters before the actual value, which is "as if" the right flag were set. The flag itself need not be set.
When you implement a user-defined inserter you cannot rely on right being the default setting for the adjustfield. Instead, you must be prepared to find none of the flags set and must keep in mind that in this case you should make your inserter behave "as if" the right flag were set. This is surprising to many people and complicates matters more than necessary.
Unless there is a good reason why the adjustfield should not be initialized I would suggest to give it the default value that everybody expects anyway.
Rationale:
This is not a defect. It is deliberate that the default is no bits set. Consider Arabic or Hebrew, for example. See 22.4.2.2.2 [facet.num.put.virtuals] paragraph 19, Table 61 - Fill padding.
Section: 27.5.2.5 [ios.base.storage] Status: Dup Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2007-01-15
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Duplicate of: 41
Discussion:
According to paragraphs 2 and 4 of 27.5.2.5 [ios.base.storage], the functions iword() and pword() "set the badbit (which might throw an exception)" on failure. ... but what does it mean for ios_base to set the badbit? The state facilities of the IOStream library are defined in basic_ios, a derived class! It would be possible to attempt a down cast but then it would be necessary to know the character type used...
Rationale:
Section: 27.7.1.2.3 [istream::extractors] Status: Dup Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2007-01-15
View all other issues in [istream::extractors].
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Duplicate of: 60
Discussion:
It appears to be somewhat nonsensical to consider the functions defined in the paragraphs 1 to 5 to be "Formatted input function" but since these functions are defined in a section labeled "Formatted input functions" it is unclear to me whether these operators are considered formatted input functions which have to conform to the "common requirements" from 27.7.1.2.1 [istream.formatted.reqmts]: If this is the case, all manipulators, not just ws, would skip whitespace unless noskipws is set (... but setting noskipws using the manipulator syntax would also skip whitespace :-)
See also issue 166 for the same problem in formatted output
Rationale:
Section: 27.7.1.3 [istream.unformatted] Status: Dup Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2007-01-15
View all other issues in [istream.unformatted].
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Duplicate of: 60
Discussion:
It is not clear which functions are to be considered unformatted input functions. As written, it seems that all functions in 27.7.1.3 [istream.unformatted] are unformatted input functions. However, it does not really make much sense to construct a sentry object for gcount(), sync(), ... Also it is unclear what happens to the gcount() if eg. gcount(), putback(), unget(), or sync() is called: These functions don't extract characters, some of them even "unextract" a character. Should this still be reflected in gcount()? Of course, it could be read as if after a call to gcount() gcount() return 0 (the last unformatted input function, gcount(), didn't extract any character) and after a call to putback() gcount() returns -1 (the last unformatted input function putback() did "extract" back into the stream). Correspondingly for unget(). Is this what is intended? If so, this should be clarified. Otherwise, a corresponding clarification should be used.
Rationale:
Section: 27.7.2.6.3 [ostream.inserters] Status: Dup Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2007-01-15
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Duplicate of: 60
Discussion:
From 27.7.2.6.1 [ostream.formatted.reqmts] it appears that all the functions defined in 27.7.2.6.3 [ostream.inserters] have to construct a sentry object. Is this really intended?
This is basically the same problem as issue 162 but for output instead of input.
Rationale:
Section: 26.4.6 [complex.ops] Status: NAD Submitter: Judy Ward Opened: 1999-07-02 Last modified: 2006-12-27
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Discussion:
A user who tries to explicitly instantiate a complex non-member operator will get compilation errors. Below is a simplified example of the reason why. The problem is that iterator_traits cannot be instantiated on a non-pointer type like float, yet when the compiler is trying to decide which operator+ needs to be instantiated it must instantiate the declaration to figure out the first argument type of a reverse_iterator operator.
namespace std {
template <class Iterator>
struct iterator_traits
{
typedef typename Iterator::value_type value_type;
};
template <class T> class reverse_iterator;
// reverse_iterator operator+
template <class T>
reverse_iterator<T> operator+
(typename iterator_traits<T>::difference_type, const reverse_iterator<T>&);
template <class T> struct complex {};
// complex operator +
template <class T>
complex<T> operator+ (const T& lhs, const complex<T>& rhs)
{ return complex<T>();}
}
// request for explicit instantiation
template std::complex<float> std::operator+<float>(const float&,
const std::complex<float>&);
See also c++-stdlib reflector messages: lib-6814, 6815, 6816.
Rationale:
Implementors can make minor changes and the example will work. Users are not affected in any case.
According to John Spicer, It is possible to explicitly instantiate these operators using different syntax: change "std::operator+<float>" to "std::operator+".
The proposed resolution of issue 120 is that users will not be able to explicitly instantiate standard library templates. If that resolution is accepted then library implementors will be the only ones that will be affected by this problem, and they must use the indicated syntax.
Section: 27.4.1 [narrow.stream.objects] Status: NAD Submitter: Judy Ward Opened: 1999-07-02 Last modified: 2006-12-27
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Discussion:
Section 27.3.1 says "After the object cerr is initialized, cerr.flags() & unitbuf is nonzero. Its state is otherwise the same as required for ios_base::init (lib.basic.ios.cons). It doesn't say anything about the the state of clog. So this means that calling cerr.tie() and clog.tie() should return 0 (see Table 89 for ios_base::init effects).
Neither of the popular standard library implementations that I tried does this, they both tie cerr and clog to &cout. I would think that would be what users expect.
Rationale:
The standard is clear as written.
27.3.1/5 says that "After the object cerr is initialized, cerr.flags() & unitbuf is nonzero. Its state is otherwise the same as required for ios_base::init (27.4.4.1)." Table 89 in 27.4.4.1, which gives the postconditions of basic_ios::init(), says that tie() is 0. (Other issues correct ios_base::init to basic_ios::init().)
Section: 26.6.2.6 [valarray.cassign] Status: NAD Submitter: Gabriel Dos Reis Opened: 1999-08-15 Last modified: 2008-03-11
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Discussion:
26.5.2.6 defines augmented assignment operators valarray<T>::op=(const T&), but fails to provide corresponding versions for the helper classes. Thus making the following illegal:
#include <valarray>
int main()
{
std::valarray<double> v(3.14, 1999);
v[99] *= 2.0; // Ok
std::slice s(0, 50, 2);
v[s] *= 2.0; // ERROR
}
I can't understand the intent of that omission. It makes the valarray library less intuitive and less useful.
Rationale:
Although perhaps an unfortunate design decision, the omission is not a defect in the current standard. A future standard may wish to add the missing operators.
Section: 25.4.7 [alg.min.max] Status: NAD Submitter: Mark Rintoul Opened: 1999-08-26 Last modified: 2009-07-14
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Discussion:
Both std::min and std::max are defined as template functions. This
is very different than the definition of std::plus (and similar
structs) which are defined as function objects which inherit
std::binary_function.
This lack of inheritance leaves std::min and std::max somewhat useless in standard library algorithms which require
a function object that inherits std::binary_function.
[ post Bellevue: Alisdair requested to re-Open. ]
[ 2009-07 Frankfurt ]
C++0x has lambdas to address this problem now.
Moved to NAD.
Rationale:
Although perhaps an unfortunate design decision, the omission is not a defect in the current standard. A future standard may wish to consider additional function objects.
Section: 25.4.3 [alg.binary.search] Status: NAD Submitter: Nico Josuttis Opened: 1999-10-10 Last modified: 2006-12-27
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Discussion:
The complexity of binary_search() is stated as "At most
log(last-first) + 2 comparisons", which seems to say that the
algorithm has logarithmic complexity. However, this algorithms is
defined for forward iterators. And for forward iterators, the need to
step element-by-element results into linear complexity. But such a
statement is missing in the standard. The same applies to
lower_bound(), upper_bound(), and equal_range().
However, strictly speaking the standard contains no bug here. So this
might considered to be a clarification or improvement.
Rationale:
The complexity is expressed in terms of comparisons, and that complexity can be met even if the number of iterators accessed is linear. Paragraph 1 already says exactly what happens to iterators.
Section: 23.2.4 [associative.reqmts] Status: NAD Submitter: Ed Brey Opened: 1999-06-06 Last modified: 2006-12-30
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Duplicate of: 233
Discussion:
As defined in 23.1.2, paragraph 7 (table 69), a.insert(p,t) suffers from several problems:
| expression | return type | pre/post-condition | complexity |
| a.insert(p,t) | iterator | inserts t if and only if there is no element with key equivalent to the key of t in containers with unique keys; always inserts t in containers with equivalent keys. always returns the iterator pointing to the element with key equivalent to the key of t . iterator p is a hint pointing to where the insert should start to search. | logarithmic in general, but amortized constant if t is inserted right after p . |
1. For a container with unique keys, only logarithmic complexity is guaranteed if no element is inserted, even though constant complexity is always possible if p points to an element equivalent to t.
2. For a container with equivalent keys, the amortized constant complexity guarantee is only useful if no key equivalent to t exists in the container. Otherwise, the insertion could occur in one of multiple locations, at least one of which would not be right after p.
3. By guaranteeing amortized constant complexity only when t is inserted after p, it is impossible to guarantee constant complexity if t is inserted at the beginning of the container. Such a problem would not exist if amortized constant complexity was guaranteed if t is inserted before p, since there is always some p immediately before which an insert can take place.
4. For a container with equivalent keys, p does not allow specification of where to insert the element, but rather only acts as a hint for improving performance. This negates the added functionality that p would provide if it specified where within a sequence of equivalent keys the insertion should occur. Specifying the insert location provides more control to the user, while providing no disadvantage to the container implementation.
Proposed resolution:
In 23.2.4 [associative.reqmts] paragraph 7, replace the row in table 69 for a.insert(p,t) with the following two rows:
| expression | return type | pre/post-condition | complexity |
| a_uniq.insert(p,t) | iterator | inserts t if and only if there is no element with key equivalent to the key of t. returns the iterator pointing to the element with key equivalent to the key of t. | logarithmic in general, but amortized constant if t is inserted right before p or p points to an element with key equivalent to t. |
| a_eq.insert(p,t) | iterator | inserts t and returns the iterator pointing to the newly inserted element. t is inserted right before p if doing so preserves the container ordering. | logarithmic in general, but amortized constant if t is inserted right before p. |
Rationale:
Too big a change. Furthermore, implementors report checking both before p and after p, and don't want to change this behavior.
Section: 27.5.4 [ios] Status: NAD Submitter: Steve Clamage Opened: 1999-09-07 Last modified: 2006-12-27
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Discussion:
In classic iostreams, base class ios had an rdbuf function that returned a pointer to the associated streambuf. Each derived class had its own rdbuf function that returned a pointer of a type reflecting the actual type derived from streambuf. Because in ARM C++, virtual function overrides had to have the same return type, rdbuf could not be virtual.
In standard iostreams, we retain the non-virtual rdbuf function design, and in addition have an overloaded rdbuf function that sets the buffer pointer. There is no need for the second function to be virtual nor to be implemented in derived classes.
Minor question: Was there a specific reason not to make the original rdbuf function virtual?
Major problem: Friendly compilers warn about functions in derived classes that hide base-class overloads. Any standard implementation of iostreams will result in such a warning on each of the iostream classes, because of the ill-considered decision to overload rdbuf only in a base class.
In addition, users of the second rdbuf function must use explicit qualification or a cast to call it from derived classes. An explicit qualification or cast to basic_ios would prevent access to any later overriding version if there was one.
What I'd like to do in an implementation is add a using- declaration for the second rdbuf function in each derived class. It would eliminate warnings about hiding functions, and would enable access without using explicit qualification. Such a change I don't think would change the behavior of any valid program, but would allow invalid programs to compile:
filebuf mybuf; fstream f; f.rdbuf(mybuf); // should be an error, no visible rdbuf
I'd like to suggest this problem as a defect, with the proposed resolution to require the equivalent of a using-declaration for the rdbuf function that is not replaced in a later derived class. We could discuss whether replacing the function should be allowed.
Rationale:
For historical reasons, the standard is correct as written. There is a subtle difference between the base class rdbuf() and derived class rdbuf(). The derived class rdbuf() always returns the original streambuf, whereas the base class rdbuf() will return the "current streambuf" if that has been changed by the variant you mention.
Permission is not required to add such an extension. See 17.6.4.5 [member.functions].
Section: 18.6.1.3 [new.delete.placement] Status: Dup Submitter: Herb Sutter Opened: 1998-12-15 Last modified: 2006-12-30
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Duplicate of: 114
Discussion:
The example in 18.6.1.3 [new.delete.placement] paragraph 4 reads:
[Example: This can be useful for constructing an object at a known address:
char place[sizeof(Something)];
Something* p = new (place) Something();
end example]
This example has potential alignment problems.
Rationale:
Section: 20.2.5 [allocator.requirements], 23.2 [container.requirements] Status: NAD Submitter: Andy Sawyer Opened: 1999-10-21 Last modified: 2006-12-30
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Discussion:
Must the value returned by max_size() be unchanged from call to call?
Must the value returned from max_size() be meaningful?
Possible meanings identified in lib-6827:
1) The largest container the implementation can support given "best
case" conditions - i.e. assume the run-time platform is "configured to
the max", and no overhead from the program itself. This may possibly
be determined at the point the library is written, but certainly no
later than compile time.
2) The largest container the program could create, given "best case"
conditions - i.e. same platform assumptions as (1), but take into
account any overhead for executing the program itself. (or, roughly
"storage=storage-sizeof(program)"). This does NOT include any resource
allocated by the program. This may (or may not) be determinable at
compile time.
3) The largest container the current execution of the program could
create, given knowledge of the actual run-time platform, but again,
not taking into account any currently allocated resource. This is
probably best determined at program start-up.
4) The largest container the current execution program could create at
the point max_size() is called (or more correctly at the point
max_size() returns :-), given it's current environment (i.e. taking
into account the actual currently available resources). This,
obviously, has to be determined dynamically each time max_size() is
called.
Proposed resolution:
Rationale:
max_size() isn't useful for very many things, and the existing wording is sufficiently clear for the few cases that max_size() can be used for. None of the attempts to change the existing wording were an improvement.
It is clear to the LWG that the value returned by max_size() can't change from call to call.
Section: 27.7.1.1.3 [istream::sentry] Status: NAD Submitter: Matt McClure and Dietmar Kühl Opened: 2000-01-01 Last modified: 2007-01-15
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Discussion:
27.6.1.1.2 Paragraph 4 states:
To decide if the character c is a whitespace character, the constructor performs ''as if'' it executes the following code fragment:
const ctype<charT>& ctype = use_facet<ctype<charT> >(is.getloc()); if (ctype.is(ctype.space,c)!=0) // c is a whitespace character.
But Table 51 in 22.1.1.1.1 only requires an implementation to provide specializations for ctype<char> and ctype<wchar_t>. If sentry's constructor is implemented using ctype, it will be uninstantiable for a user-defined character type charT, unless the implementation has provided non-working (since it would be impossible to define a correct ctype<charT> specialization for an arbitrary charT) definitions of ctype's virtual member functions.
It seems the intent the standard is that sentry should behave, in every respect, not just during execution, as if it were implemented using ctype, with the burden of providing a ctype specialization falling on the user. But as it is written, nothing requires the translation of sentry's constructor to behave as if it used the above code, and it would seem therefore, that sentry's constructor should be instantiable for all character types.
Note: If I have misinterpreted the intent of the standard with respect to sentry's constructor's instantiability, then a note should be added to the following effect:
An implementation is forbidden from using the above code if it renders the constructor uninstantiable for an otherwise valid character type.
In any event, some clarification is needed.
Rationale:
It is possible but not easy to instantiate on types other than char or wchar_t; many things have to be done first. That is by intention and is not a defect.
Section: 24.4.4 [iterator.operations] Status: NAD Submitter: Rintala Matti Opened: 2000-01-28 Last modified: 2008-09-30
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Discussion:
Section 24.3.4 describes the function distance(first, last) (where first and last are iterators) which calculates "the number of increments or decrements needed to get from 'first' to 'last'".
The function should work for forward, bidirectional and random access iterators, and there is a requirement 24.3.4.5 which states that "'last' must be reachable from 'first'".
With random access iterators the function is easy to implement as "last - first".
With forward iterators it's clear that 'first' must point to a place before 'last', because otherwise 'last' would not be reachable from 'first'.
But what about bidirectional iterators? There 'last' is reachable from 'first' with the -- operator even if 'last' points to an earlier position than 'first'. However, I cannot see how the distance() function could be implemented if the implementation does not know which of the iterators points to an earlier position (you cannot use ++ or -- on either iterator if you don't know which direction is the "safe way to travel").
The paragraph 24.3.4.1 states that "for ... bidirectional iterators they use ++ to provide linear time implementations". However, the ++ operator is not mentioned in the reachability requirement. Furthermore 24.3.4.4 explicitly mentions that distance() returns the number of increments _or decrements_, suggesting that it could return a negative number also for bidirectional iterators when 'last' points to a position before 'first'.
Is a further requirement is needed to state that for forward and bidirectional iterators "'last' must be reachable from 'first' using the ++ operator". Maybe this requirement might also apply to random access iterators so that distance() would work the same way for every iterator category?
Rationale:
"Reachable" is defined in the standard in X [iterator.concepts] paragraph 6. The definition is only in terms of operator++(). The LWG sees no defect in the standard.
Section: 18.3.1.2 [numeric.limits.members] Status: NAD Submitter: Steve Cleary Opened: 2000-01-28 Last modified: 2006-12-27
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Discussion:
In several places in 18.3.1.2 [numeric.limits.members], a member is described as "Meaningful for all floating point types." However, no clear method of determining a floating point type is provided.
In 18.3.1.5 [numeric.special], paragraph 1 states ". . . (for example, epsilon() is only meaningful if is_integer is false). . ." which suggests that a type is a floating point type if is_specialized is true and is_integer is false; however, this is unclear.
When clarifying this, please keep in mind this need of users: what exactly is the definition of floating point? Would a fixed point or rational representation be considered one? I guess my statement here is that there could also be types that are neither integer or (strictly) floating point.
Rationale:
It is up to the implementor of a user define type to decide if it is a floating point type.
Section: 22.4.1.3.2 [facet.ctype.char.members] Status: Dup Submitter: Robert Klarer Opened: 1999-11-02 Last modified: 2006-12-30
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Duplicate of: 153
Discussion:
The widen and narrow member functions are described in 22.2.1.3.2, paragraphs 9-11. In each case we have two overloaded signatures followed by a Returns clause. The Returns clause only describes one of the overloads.
Proposed resolution:
Change the returns clause in 22.4.1.3.2 [facet.ctype.char.members] paragraph 10 from:
Returns: do_widen(low, high, to).
to:
Returns: do_widen(c) or do_widen(low, high, to), respectively.
Change the returns clause in 22.4.1.3.2 [facet.ctype.char.members] paragraph 11 from:
Returns: do_narrow(low, high, to).
to:
Returns: do_narrow(c) or do_narrow(low, high, to), respectively.
Rationale:
Subsumed by issue 153, which addresses the same paragraphs.
Section: 26.8 [c.math] Status: NAD Submitter: Nico Josuttis Opened: 2000-02-26 Last modified: 2006-12-29
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Discussion:
Due to the additional overloaded versions of numeric functions for float and long double according to Section 26.5, calls such as int x; std::pow (x, 4) are ambiguous now in a standard conforming implementation. Current implementations solve this problem very different (overload for all types, don't overload for float and long double, use preprocessor, follow the standard and get ambiguities).
This behavior should be standardized or at least identified as implementation defined.
Rationale:
These math issues are an understood and accepted consequence of the design. They have been discussed several times in the past. Users must write casts or write floating point expressions as arguments.
Section: 23.2.4 [associative.reqmts] Status: NAD Submitter: Judy Ward Opened: 2000-02-29 Last modified: 2006-12-27
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Discussion:
A user noticed that this doesn't compile with the Rogue Wave library because the rb_tree class declares a key_allocator, and allocator<const int> is not legal, I think:
map < const int, ... > // legal?
which made me wonder whether it is legal for a map's key_type to be const. In email from Matt Austern he said:
I'm not sure whether it's legal to declare a map with a const key type. I hadn't thought about that question until a couple weeks ago. My intuitive feeling is that it ought not to be allowed, and that the standard ought to say so. It does turn out to work in SGI's library, though, and someone in the compiler group even used it. Perhaps this deserves to be written up as an issue too.
Rationale:
The "key is assignable" requirement from table 69 in 23.2.4 [associative.reqmts] already implies the key cannot be const.
Section: 27.7.3 [std.manip] Status: Dup Submitter: Hyman Rosen Opened: 2000-02-29 Last modified: 2006-12-30
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Duplicate of: 193
Discussion:
27.7.3 [std.manip] paragraph 5 says:
smanip setbase(int base);Returns: An object s of unspecified type such that if out is an (instance of) basic_ostream then the expression out<<s behaves as if f(s) were called, in is an (instance of) basic_istream then the expression in>>s behaves as if f(s) were called. Where f can be defined as:
ios_base& f(ios_base& str, int base) { // set basefield str.setf(n == 8 ? ios_base::oct : n == 10 ? ios_base::dec : n == 16 ? ios_base::hex : ios_base::fmtflags(0), ios_base::basefield); return str; }
There are two problems here. First, f takes two parameters, so the description needs to say that out<<s and in>>s behave as if f(s,base) had been called. Second, f is has a parameter named base, but is written as if the parameter was named n.
Actually, there's a third problem. The paragraph has grammatical errors. There needs to be an "and" after the first comma, and the "Where f" sentence fragment needs to be merged into its preceding sentence. You may also want to format the function a little better. The formatting above is more-or-less what the Standard contains.
Rationale:
The resolution of this defect is subsumed by the proposed resolution for issue 193.
[Tokyo: The LWG agrees that this is a defect and notes that it occurs additional places in the section, all requiring fixes.]
Section: 25.4 [alg.sorting] Status: NAD Submitter: Pablo Halpern Opened: 2000-03-06 Last modified: 2006-12-27
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Discussion:
Many of the algorithms take an argument, pred, of template parameter type BinaryPredicate or an argument comp of template parameter type Compare. These algorithms usually have an overloaded version that does not take the predicate argument. In these cases pred is usually replaced by the use of operator== and comp is replaced by the use of operator<.
This use of hard-coded operators is inconsistent with other parts of the library, particularly the containers library, where equality is established using equal_to<> and ordering is established using less<>. Worse, the use of operator<, would cause the following innocent-looking code to have undefined behavior:
vector<string*> vec; sort(vec.begin(), vec.end());
The use of operator< is not defined for pointers to unrelated objects. If std::sort used less<> to compare elements, then the above code would be well-defined, since less<> is explicitly specialized to produce a total ordering of pointers.
Rationale:
This use of operator== and operator< was a very deliberate, conscious, and explicitly made design decision; these operators are often more efficient. The predicate forms are available for users who don't want to rely on operator== and operator<.
Section: 25.2.5 [alg.find] Status: NAD Submitter: Pablo Halpern Opened: 2000-03-06 Last modified: 2009-07-14
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Discussion:
The find function always searches for a value using operator== to compare the value argument to each element in the input iterator range. This is inconsistent with other find-related functions such as find_end and find_first_of, which allow the caller to specify a binary predicate object to be used for determining equality. The fact that this can be accomplished using a combination of find_if and bind_1st or bind_2nd does not negate the desirability of a consistent, simple, alternative interface to find.
[ Summit: ]
Reopened by Alisdair.
[ 2009-07 Frankfurt ]
The same thing can be achieved using find_if (as noted in the issue).
Moved to NAD.
Proposed resolution:
In section 25.2.5 [alg.find], add a second prototype for find (between the existing prototype and the prototype for find_if), as follows:
template<class InputIterator, class T, class BinaryPredicate> InputIterator find(InputIterator first, InputIterator last, const T& value, BinaryPredicate bin_pred);Change the description of the return from:
Returns: The first iterator i in the range [first, last) for which the following corresponding conditions hold: *i == value, pred(*i) != false. Returns last if no such iterator is found.
to:
Returns: The first iterator i in the range [first, last) for which the following corresponding condition holds: *i == value, bin_pred(*i,value) != false, pred(*) != false. Return last if no such iterator is found.
Rationale:
This is request for a pure extension, so it is not a defect in the current standard. As the submitter pointed out, "this can be accomplished using a combination of find_if and bind_1st or bind_2nd".
Section: 22.4.1.3.2 [facet.ctype.char.members] Status: Dup Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2007-01-15
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Duplicate of: 28
Discussion:
The description of the is() member in paragraph 4 of 22.4.1.3.2 [facet.ctype.char.members] is broken: According to this description, the second form of the is() method modifies the masks in the ctype object. The correct semantics if, of course, to obtain an array of masks. The corresponding method in the general case, ie. the do_is() method as described in 22.4.1.1.2 [locale.ctype.virtuals] paragraph 1 does the right thing.
Proposed resolution:
Change paragraph 4 from
The second form, for all *p in the range [low, high), assigns vec[p-low] to table()[(unsigned char)*p].
to become
The second form, for all *p in the range [low, high), assigns table()[(unsigned char)*p] to vec[p-low].
Rationale:
Section: 25.2.5 [alg.find] Status: NAD Submitter: Andrew Koenig Opened: 2000-05-02 Last modified: 2006-12-27
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Discussion:
Is the following implementation of find acceptable?
template<class Iter, class X>
Iter find(Iter begin, Iter end, const X& x)
{
X x1 = x; // this is the crucial statement
while (begin != end && *begin != x1)
++begin;
return begin;
}
If the answer is yes, then it is implementation-dependent as to whether the following fragment is well formed:
vector<string> v;
find(v.begin(), v.end(), "foo");
At issue is whether there is a requirement that the third argument of find be CopyConstructible. There may be no problem here, but analysis is necessary.
Rationale:
There is no indication in the standard that find's third argument is required to be Copy Constructible. The LWG believes that no such requirement was intended. As noted above, there are times when a user might reasonably pass an argument that is not Copy Constructible.
Section: 24.6.1 [istream.iterator] Status: NAD Submitter: Andrew Koenig Opened: 2000-05-02 Last modified: 2006-12-27
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Discussion:
I do not think the standard specifies what operation(s) on istream iterators trigger input operations. So, for example:
istream_iterator<int> i(cin);
int n = *i++;
I do not think it is specified how many integers have been read from cin. The number must be at least 1, of course, but can it be 2? More?
Rationale:
The standard is clear as written: the stream is read every time operator++ is called, and it is also read either when the iterator is constructed or when operator* is called for the first time. In the example above, exactly two integers are read from cin.
There may be a problem with the interaction between istream_iterator and some STL algorithms, such as find. There are no guarantees about how many times find may invoke operator++.
Section: 23.2.4 [associative.reqmts] Status: Dup Submitter: Mark Rodgers Opened: 2000-05-19 Last modified: 2007-01-15
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Duplicate of: 233
Discussion:
Closed issue 192 raised several problems with the specification of this function, but was rejected as Not A Defect because it was too big a change with unacceptable impacts on existing implementations. However, issues remain that could be addressed with a smaller change and with little or no consequent impact.
The specification is inconsistent with the original proposal and with several implementations.
The initial implementation by Hewlett Packard only ever looked immediately before p, and I do not believe there was any intention to standardize anything other than this behavior. Consequently, current implementations by several leading implementors also look immediately before p, and will only insert after p in logarithmic time. I am only aware of one implementation that does actually look after p, and it looks before p as well. It is therefore doubtful that existing code would be relying on the behavior defined in the standard, and it would seem that fixing this defect as proposed below would standardize existing practice.
The specification is inconsistent with insertion for sequence containers.
This is difficult and confusing to teach to newcomers. All insert operations that specify an iterator as an insertion location should have a consistent meaning for the location represented by that iterator.
As specified, there is no way to hint that the insertion should occur at the beginning of the container, and the way to hint that it should occur at the end is long winded and unnatural.
For a container containing n elements, there are n+1 possible insertion locations and n+1 valid iterators. For there to be a one-to-one mapping between iterators and insertion locations, the iterator must represent an insertion location immediately before the iterator.
When appending sorted ranges using insert_iterators, insertions are guaranteed to be sub-optimal.
In such a situation, the optimum location for insertion is always immediately after the element previously inserted. The mechanics of the insert iterator guarantee that it will try and insert after the element after that, which will never be correct. However, if the container first tried to insert before the hint, all insertions would be performed in amortized constant time.
Proposed resolution:
In 23.1.2 [lib.associative.reqmts] paragraph 7, table 69, make the following changes in the row for a.insert(p,t):
assertion/note pre/post condition:
Change the last sentence from
"iterator p is a hint pointing to where the insert should start to search."
to
"iterator p is a hint indicating that immediately before p may be a correct location where the insertion could occur."
complexity:
Change the words "right after" to "immediately before".
Rationale:
Section: D.10.1 [auto.ptr] Status: NAD Submitter: Joseph Gottman Opened: 2000-06-30 Last modified: 2006-12-29
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Discussion:
According to section 20.4.5, the function auto_ptr::operator=() returns a reference to an auto_ptr. The reason that operator=() usually returns a reference is to facilitate code like
int x,y,z;
x = y = z = 1;
However, given analogous code for auto_ptrs,
auto_ptr<int> x, y, z;
z.reset(new int(1));
x = y = z;
the result would be that z and y would both be set to NULL, instead of all the auto_ptrs being set to the same value. This makes such cascading assignments useless and counterintuitive for auto_ptrs.
Proposed resolution:
Change auto_ptr::operator=() to return void instead of an auto_ptr reference.
Rationale:
The return value has uses other than cascaded assignments: a user can call an auto_ptr member function, pass the auto_ptr to a function, etc. Removing the return value could break working user code.
Section: 27.6.2 [streambuf] Status: NAD Future Submitter: Martin Sebor Opened: 2000-08-12 Last modified: 2009-07-14
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Discussion:
The basic_streambuf members gbump() and pbump() are specified to take an int argument. This requirement prevents the functions from effectively manipulating buffers larger than std::numeric_limits<int>::max() characters. It also makes the common use case for these functions somewhat difficult as many compilers will issue a warning when an argument of type larger than int (such as ptrdiff_t on LLP64 architectures) is passed to either of the function. Since it's often the result of the subtraction of two pointers that is passed to the functions, a cast is necessary to silence such warnings. Finally, the usage of a native type in the functions signatures is inconsistent with other member functions (such as sgetn() and sputn()) that manipulate the underlying character buffer. Those functions take a streamsize argument.
[ 2009-07 Frankfurt ]
This is part of a bigger problem. If anyone cares enough, they should write a paper solving the bigger problem of offset types in iostreams.
This is related to the paper about large file sizes. Beman has already agreed to drop the section of that paper that deals with this.
int is big enough for reasonable buffers.
Move to NAD Future.
This is related to LWG 423.
Proposed resolution:
Change the signatures of these functions in the synopsis of template class basic_streambuf (27.5.2) and in their descriptions (27.5.2.3.1, p4 and 27.5.2.3.2, p4) to take a streamsize argument.
Although this change has the potential of changing the ABI of the library, the change will affect only platforms where int is different than the definition of streamsize. However, since both functions are typically inline (they are on all known implementations), even on such platforms the change will not affect any user code unless it explicitly relies on the existing type of the functions (e.g., by taking their address). Such a possibility is IMO quite remote.
Alternate Suggestion from Howard Hinnant, c++std-lib-7780:
This is something of a nit, but I'm wondering if streamoff wouldn't be a better choice than streamsize. The argument to pbump and gbump MUST be signed. But the standard has this to say about streamsize (27.4.1/2/Footnote):
[Footnote: streamsize is used in most places where ISO C would use size_t. Most of the uses of streamsize could use size_t, except for the strstreambuf constructors, which require negative values. It should probably be the signed type corresponding to size_t (which is what Posix.2 calls ssize_t). --- end footnote]
This seems a little weak for the argument to pbump and gbump. Should we ever really get rid of strstream, this footnote might go with it, along with the reason to make streamsize signed.
Rationale:
The LWG believes this change is too big for now. We may wish to reconsider this for a future revision of the standard. One possibility is overloading pbump, rather than changing the signature.
[ [2006-05-04: Reopened at the request of Chris (Krzysztof ?elechowski)] ]
Section: 20.8.3 [base], 24.4.2 [iterator.basic] Status: NAD Submitter: Robert Dick Opened: 2000-08-17 Last modified: 2006-12-29
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Discussion:
According to the November 1997 Draft Standard, the results of deleting an object of a derived class through a pointer to an object of its base class are undefined if the base class has a non-virtual destructor. Therefore, it is potentially dangerous to publicly inherit from such base classes.
Defect:
The STL design encourages users to publicly inherit from a number of classes
which do nothing but specify interfaces, and which contain non-virtual
destructors.
Attribution:
Wil Evers and William E. Kempf suggested this modification for functional
objects.
Proposed resolution:
When a base class in the standard library is useful only as an interface specifier, i.e., when an object of the class will never be directly instantiated, specify that the class contains a protected destructor. This will prevent deletion through a pointer to the base class without performance, or space penalties (on any implementation I'm aware of).
As an example, replace...
template <class Arg, class Result>
struct unary_function {
typedef Arg argument_type;
typedef Result result_type;
};
... with...
template <class Arg, class Result>
struct unary_function {
typedef Arg argument_type;
typedef Result result_type;
protected:
~unary_function() {}
};
Affected definitions:
20.3.1 [lib.function.objects] -- unary_function, binary_function
24.3.2 [lib.iterator.basic] -- iterator
Rationale:
The standard is clear as written; this is a request for change, not a defect in the strict sense. The LWG had several different objections to the proposed change. One is that it would prevent users from creating objects of type unary_function and binary_function. Doing so can sometimes be legitimate, if users want to pass temporaries as traits or tag types in generic code.
Section: D.8.1.3 [depr.strstreambuf.virtuals] Status: NAD Submitter: Martin Sebor Opened: 2000-10-05 Last modified: 2007-01-15
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Discussion:
It appears that the interaction of the strstreambuf members overflow() and seekoff() can lead to undefined behavior in cases where defined behavior could reasonably be expected. The following program demonstrates this behavior:
#include <strstream>
int main ()
{
std::strstreambuf sb;
sb.sputc ('c');
sb.pubseekoff (-1, std::ios::end, std::ios::in);
return !('c' == sb.sgetc ());
}
D.7.1.1, p1 initializes strstreambuf with a call to basic_streambuf<>(), which in turn sets all pointers to 0 in 27.5.2.1, p1.
27.5.2.2.5, p1 says that basic_streambuf<>::sputc(c) calls overflow(traits::to_int_type(c)) if a write position isn't available (it isn't due to the above).
D.7.1.3, p3 says that strstreambuf::overflow(off, ..., ios::in) makes at least one write position available (i.e., it allows the function to make any positive number of write positions available).
D.7.1.3, p13 computes newoff = seekhigh - eback(). In D.7.1, p4 we see seekhigh = epptr() ? epptr() : egptr(), or seekhigh = epptr() in this case. newoff is then epptr() - eback().
D.7.1.4, p14 sets gptr() so that gptr() == eback() + newoff + off, or gptr() == epptr() + off holds.
If strstreambuf::overflow() made exactly one write position available then gptr() will be set to just before epptr(), and the program will return 0. Buf if the function made more than one write position available, epptr() and gptr() will both point past pptr() and the behavior of the program is undefined.
Proposed resolution:
Change the last sentence of D.8.1 [depr.strstreambuf] paragraph 4 from
Otherwise, seeklow equals gbeg and seekhigh is either pend, if pend is not a null pointer, or gend.
to become
Otherwise, seeklow equals gbeg and seekhigh is either gend if 0 == pptr(), or pbase() + max where max is the maximum value of pptr() - pbase() ever reached for this stream.
[ pre-Copenhagen: Dietmar provided wording for proposed resolution. ]
[ post-Copenhagen: Fixed a typo: proposed resolution said to fix 4.7.1, not D.7.1. ]
Rationale:
This is related to issue 65: it's not clear what it means to seek beyond the current area. Without resolving issue 65 we can't resolve this. As with issue 65, the library working group does not wish to invest time nailing down corner cases in a deprecated feature.
Section: 18.8 [support.exception] Status: NAD Submitter: J. Stephen Adamczyk Opened: 2000-10-10 Last modified: 2006-12-27
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Discussion:
One of our customers asks whether this is valid C++:
#include <cstdarg>
void bar(const char *, va_list);
void
foo(const char *file, const char *, ...)
{
va_list ap;
va_start(ap, file);
bar(file, ap);
va_end(ap);
}
The issue being whether it is valid to use cstdarg when the final parameter before the "..." is unnamed. cstdarg is, as far as I can tell, inherited verbatim from the C standard. and the definition there (7.8.1.1 in the ISO C89 standard) refers to "the identifier of the rightmost parameter". What happens when there is no such identifier?
My personal opinion is that this should be allowed, but some tweak might be required in the C++ standard.
Rationale:
Not a defect, the C and C++ standards are clear. It is impossible to use varargs if the parameter immediately before "..." has no name, because that is the parameter that must be passed to va_start. The example given above is broken, because va_start is being passed the wrong parameter.
There is no support for extending varargs to provide additional functionality beyond what's currently there. For reasons of C/C++ compatibility, it is especially important not to make gratuitous changes in this part of the C++ standard. The C committee has already been requested not to touch this part of the C standard unless necessary.
Section: 20.2.5 [allocator.requirements] Status: NAD Submitter: Matt Austern Opened: 2000-11-07 Last modified: 2006-12-30
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Discussion:
In 20.1.5, paragraph 5, the standard says that "Implementors are encouraged to supply libraries that can accept allocators that encapsulate more general memory models and that support non-equal instances." This is intended as normative encouragement to standard library implementors. However, it is possible to interpret this sentence as applying to nonstandard third-party libraries.
Proposed resolution:
In 20.1.5, paragraph 5, change "Implementors" to "Implementors of the library described in this International Standard".
Rationale:
The LWG believes the normative encouragement is already sufficiently clear, and that there are no important consequences even if it is misunderstood.
Section: 23.2 [container.requirements] Status: NAD Submitter: Steve Cleary Opened: 2000-11-27 Last modified: 2006-12-27
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Discussion:
This came from an email from Steve Cleary to Fergus in reference to issue 179. The library working group briefly discussed this in Toronto and believes it should be a separate issue.
Steve said: "We may want to state that the const/non-const iterators must have the same difference type, size_type, and category."
(Comment from Judy) I'm not sure if the above sentence should be true for all const and non-const iterators in a particular container, or if it means the container's iterator can't be compared with the container's const_iterator unless the above it true. I suspect the former.
Proposed resolution:
In Section: 23.2 [container.requirements], table 65, in the assertion/note pre/post condition for X::const_iterator, add the following:
typeid(X::const_iterator::difference_type) == typeid(X::iterator::difference_type)
typeid(X::const_iterator::size_type) == typeid(X::iterator::size_type)
typeid(X::const_iterator::category) == typeid(X::iterator::category)
Rationale:
Going through the types one by one: Iterators don't have a size_type. We already know that the difference types are identical, because the container requirements already say that the difference types of both X::iterator and X::const_iterator are both X::difference_type. The standard does not require that X::iterator and X::const_iterator have the same iterator category, but the LWG does not see this as a defect: it's possible to imagine cases in which it would be useful for the categories to be different.
It may be desirable to require X::iterator and X::const_iterator to have the same value type, but that is a new issue. (Issue 322.)
Section: 27.5.2.2 [fmtflags.state] Status: NAD Submitter: Judy Ward Opened: 2000-12-30 Last modified: 2006-12-27
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Discussion:
The Effects clause for ios_base::setf(fmtflags fmtfl) says "Sets fmtfl in flags()". What happens if the user first calls ios_base::scientific and then calls ios_base::fixed or vice-versa? This is an issue for all of the conflicting flags, i.e. ios_base::left and ios_base::right or ios_base::dec, ios_base::hex and ios_base::oct.
I see three possible solutions:
Most existing implementations that I tried seem to conform to resolution #3, except that when using the iomanip manipulator hex or oct then that always overrides dec, but calling setf(ios_base::hex) doesn't.
There is a sort of related issue, which is that although the ios_base constructor says that each ios_base member has an indeterminate value after construction, all the existing implementations I tried explicitly set ios_base::dec.
Proposed resolution:
Rationale:
adjustfield, basefield, and floatfield are each multi-bit fields. It is possible to set multiple bits within each of those fields. (For example, dec and oct). These fields are used by locale facets. The LWG reviewed the way in which each of those three fields is used, and believes that in each case the behavior is well defined for any possible combination of bits. See for example Table 58, in 22.4.2.2.2 [facet.num.put.virtuals], noting the requirement in paragraph 6 of that section.
Users are advised to use manipulators, or else use the two-argument version of setf, to avoid unexpected behavior.
Section: 26.8 [c.math] Status: NAD Submitter: Judy Ward Opened: 2000-12-30 Last modified: 2007-01-15
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Discussion:
In ISO/IEC 9899:1990 Programming Languages C we find the following concerning <math.h>:
7.13.4 Mathematics <math.h>
The names of all existing functions declared in the <math.h> header, suffixed with f or l, are reserved respectively for corresponding functions with float and long double arguments are return values.
For example, float sinf(float) is reserved.
In the C99 standard, <math.h> must contain declarations for these functions.
So, is it acceptable for an implementor to add these prototypes to the C++ versions of the math headers? Are they required?
Proposed resolution:
Add these Functions to Table 80, section 26.5 and to Table 99, section C.2:
acosf asinf atanf atan2f ceilf cosf coshf
expf fabsf floorf fmodf frexpf ldexpf
logf log10f modff powf sinf sinhf sqrtf
tanf tanhf
acosl asinl atanl atan2l ceill cosl coshl
expl fabsl floorl fmodl frexpl ldexpl
logl log10l modfl powl sinl sinhl sqrtl
tanl tanhl
There should probably be a note saying that these functions are optional and, if supplied, should match the description in the 1999 version of the C standard. In the next round of C++ standardization they can then become mandatory.
Rationale:
The C90 standard, as amended, already permits (but does not require) these functions, and the C++ standard incorporates the C90 standard by reference. C99 is not an issue, because it is never referred to by the C++ standard.
Section: 25.2.4 [alg.foreach] Status: NAD Submitter: Angelika Langer Opened: 2001-01-03 Last modified: 2009-07-14
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Discussion:
The specification of the for_each algorithm does not have a "Requires" section, which means that there are no restrictions imposed on the function object whatsoever. In essence it means that I can provide any function object with arbitrary side effects and I can still expect a predictable result. In particular I can expect that the function object is applied exactly last - first times, which is promised in the "Complexity" section.
I don't see how any implementation can give such a guarantee without imposing requirements on the function object.
Just as an example: consider a function object that removes elements from the input sequence. In that case, what does the complexity guarantee (applies f exactly last - first times) mean?
One can argue that this is obviously a nonsensical application and a theoretical case, which unfortunately it isn't. I have seen programmers shooting themselves in the foot this way, and they did not understand that there are restrictions even if the description of the algorithm does not say so.
[Lillehammer: This is more general than for_each. We don't want the function object in transform invalidiating iterators either. There should be a note somewhere in clause 17 (17, not 25) saying that user code operating on a range may not invalidate iterators unless otherwise specified. Bill will provide wording.]
[ 2009-07 Frankfurt ]
Moved to NAD.
It was felt that the current description is adequate, and that there are limits to what the standard can reasonably say to prohibit perverse uses of the library.
Proposed resolution:
Section: 25.3.4 [alg.transform] Status: NAD Submitter: Angelika Langer Opened: 2001-01-04 Last modified: 2007-01-15
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Discussion:
This issue is related to issue 242. In case that the resolution proposed for issue 242 is accepted, we have have the following situation: The 4 numeric algorithms (accumulate and consorts) as well as transform would allow a certain category of side effects. The numeric algorithms specify that they invoke the functor "for every iterator i in the range [first, last) in order". transform, in contrast, would not give any guarantee regarding order of invocation of the functor, which means that the functor can be invoked in any arbitrary order.
Why would that be a problem? Consider an example: say the transformator that is a simple enumerator ( or more generally speaking, "is order-sensitive" ). Since a standard compliant implementation of transform is free to invoke the enumerator in no definite order, the result could be a garbled enumeration. Strictly speaking this is not a problem, but it is certainly at odds with the prevalent understanding of transform as an algorithms that assigns "a new _corresponding_ value" to the output elements.
All implementations that I know of invoke the transformator in definite order, namely starting from first and proceeding to last - 1. Unless there is an optimization conceivable that takes advantage of the indefinite order I would suggest to specify the order, because it eliminate the uncertainty that users would otherwise have regarding the order of execution of their potentially order-sensitive function objects.
Proposed resolution:
In section 25.2.3 - Transform [lib.alg.transform] change:
-1- Effects: Assigns through every iterator i in the range [result, result + (last1 - first1)) a new corresponding value equal to op(*(first1 + (i - result)) or binary_op(*(first1 + (i - result), *(first2 + (i - result))).
to:
-1- Effects: Computes values by invoking the operation op or binary_op for every iterator in the range [first1, last1) in order. Assigns through every iterator i in the range [result, result + (last1 - first1)) a new corresponding value equal to op(*(first1 + (i - result)) or binary_op(*(first1 + (i - result), *(first2 + (i - result))).
Rationale:
For Input Iterators an order is already guaranteed, because only one order is possible. If a user who passes a Forward Iterator to one of these algorithms really needs a specific order of execution, it's possible to achieve that effect by wrapping it in an Input Iterator adaptor.
Section: 24.2.6 [bidirectional.iterators], 24.2.7 [random.access.iterators] Status: NAD Editorial Submitter: John Potter Opened: 2001-01-22 Last modified: 2010-03-14
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Discussion:
In section 24.2.6 [bidirectional.iterators], Table 75 gives the return type of *r-- as convertible to T. This is not consistent with Table 74 which gives the return type of *r++ as T&. *r++ = t is valid while *r-- = t is invalid.
In section 24.2.7 [random.access.iterators], Table 76 gives the return type of a[n] as convertible to T. This is not consistent with the semantics of *(a + n) which returns T& by Table 74. *(a + n) = t is valid while a[n] = t is invalid.
Discussion from the Copenhagen meeting: the first part is uncontroversial. The second part, operator[] for Random Access Iterators, requires more thought. There are reasonable arguments on both sides. Return by value from operator[] enables some potentially useful iterators, e.g. a random access "iota iterator" (a.k.a "counting iterator" or "int iterator"). There isn't any obvious way to do this with return-by-reference, since the reference would be to a temporary. On the other hand, reverse_iterator takes an arbitrary Random Access Iterator as template argument, and its operator[] returns by reference. If we decided that the return type in Table 76 was correct, we would have to change reverse_iterator. This change would probably affect user code.
History: the contradiction between reverse_iterator and the Random Access Iterator requirements has been present from an early stage. In both the STL proposal adopted by the committee (N0527==94-0140) and the STL technical report (HPL-95-11 (R.1), by Stepanov and Lee), the Random Access Iterator requirements say that operator[]'s return value is "convertible to T". In N0527 reverse_iterator's operator[] returns by value, but in HPL-95-11 (R.1), and in the STL implementation that HP released to the public, reverse_iterator's operator[] returns by reference. In 1995, the standard was amended to reflect the contents of HPL-95-11 (R.1). The original intent for operator[] is unclear.
In the long term it may be desirable to add more fine-grained iterator requirements, so that access method and traversal strategy can be decoupled. (See "Improved Iterator Categories and Requirements", N1297 = 01-0011, by Jeremy Siek.) Any decisions about issue 299 should keep this possibility in mind.
Further discussion: I propose a compromise between John Potter's resolution, which requires T& as the return type of a[n], and the current wording, which requires convertible to T. The compromise is to keep the convertible to T for the return type of the expression a[n], but to also add a[n] = t as a valid expression. This compromise "saves" the common case uses of random access iterators, while at the same time allowing iterators such as counting iterator and caching file iterators to remain random access iterators (iterators where the lifetime of the object returned by operator*() is tied to the lifetime of the iterator).
Note that the compromise resolution necessitates a change to reverse_iterator. It would need to use a proxy to support a[n] = t.
Note also there is one kind of mutable random access iterator that will no longer meet the new requirements. Currently, iterators that return an r-value from operator[] meet the requirements for a mutable random access iterartor, even though the expression a[n] = t will only modify a temporary that goes away. With this proposed resolution, a[n] = t will be required to have the same operational semantics as *(a + n) = t.
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-09-18 Alisdair adds: ]
Why can't we write through the reference returned from operator[] on a random access iterator?
Recommended solution:
In table Table 104 -- Random access iterator requirements, replace
a[n] : convertible toconst T &T& if X is mutable, otherwise convertible to const T&
[ 2009-10 Santa Cruz: ]
Leave Open. Alisdair to spearhead a paper on revivification.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3066.
Proposed resolution:
In section 24.1.4 [lib.bidirectdional.iterators], change the return type in table 75 from "convertible to T" to T&.
In section 24.1.5 [lib.random.access.iterators], change the operational semantics for a[n] to " the r-value of a[n] is equivalent to the r-value of *(a + n)". Add a new row in the table for the expression a[n] = t with a return type of convertible to T and operational semantics of *(a + n) = t.
[Lillehammer: Real problem, but should be addressed as part of iterator redesign]
Rationale:
[ San Francisco: ]
Solved by N2758.
Section: 22.4.1.5 [locale.codecvt.byname] Status: NAD Submitter: Gregory Bumgardner Opened: 2001-01-25 Last modified: 2006-12-27
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Discussion:
The effects of codecvt<>::do_length() are described in 22.2.1.5.2, paragraph 10. As implied by that paragraph, and clarified in issue 75, codecvt<>::do_length() must process the source data and update the stateT argument just as if the data had been processed by codecvt<>::in(). However, the standard does not specify how do_length() would report a translation failure, should the source sequence contain untranslatable or illegal character sequences.
The other conversion methods return an "error" result value to indicate that an untranslatable character has been encountered, but do_length() already has a return value (the number of source characters that have been processed by the method).
Proposed resolution:
This issue cannot be resolved without modifying the interface. An exception cannot be used, as there would be no way to determine how many characters have been processed and the state object would be left in an indeterminate state.
A source compatible solution involves adding a fifth argument to length() and do_length() that could be used to return position of the offending character sequence. This argument would have a default value that would allow it to be ignored:
int length(stateT& state,
const externT* from,
const externT* from_end,
size_t max,
const externT** from_next = 0);
virtual
int do_length(stateT& state,
const externT* from,
const externT* from_end,
size_t max,
const externT** from_next);
Then an exception could be used to report any translation errors and the from_next argument, if used, could then be used to retrieve the location of the offending character sequence.
Rationale:
The standard is already clear: the return value is the number of "valid complete characters". If it encounters an invalid sequence of external characters, it stops.
Section: X [iterator.concepts] Status: NAD Submitter: Dave Abrahams Opened: 2001-02-05 Last modified: 2008-09-30
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Discussion:
We all "know" that input iterators are allowed to produce values when dereferenced of which there is no other in-memory copy.
But: Table 72, with a careful reading, seems to imply that this can only be the case if the value_type has no members (e.g. is a built-in type).
The problem occurs in the following entry:
a->m pre: (*a).m is well-defined
Equivalent to (*a).m
*a.m can be well-defined if *a is not a reference type, but since operator->() must return a pointer for a->m to be well-formed, it needs something to return a pointer to. This seems to indicate that *a must be buffered somewhere to make a legal input iterator.
I don't think this was intentional.
Rationale:
The current standard is clear and consistent. Input iterators that return rvalues are in fact implementable. They may in some cases require extra work, but it is still possible to define an operator-> in such cases: it doesn't have to return a T*, but may return a proxy type. No change to the standard is justified.
Section: 27.7 [iostream.format] Status: NAD Submitter: Martin Sebor Opened: 2001-03-19 Last modified: 2009-07-14
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Discussion:
The descriptions of the constructors of basic_istream<>::sentry (27.7.1.1.3 [istream::sentry]) and basic_ostream<>::sentry (27.7.2.4 [ostream::sentry]) do not explain what the functions do in case an exception is thrown while they execute. Some current implementations allow all exceptions to propagate, others catch them and set ios_base::badbit instead, still others catch some but let others propagate.
The text also mentions that the functions may call setstate(failbit) (without actually saying on what object, but presumably the stream argument is meant). That may have been fine for basic_istream<>::sentry prior to issue 195, since the function performs an input operation which may fail. However, issue 195 amends 27.7.1.1.3 [istream::sentry], p2 to clarify that the function should actually call setstate(failbit | eofbit), so the sentence in p3 is redundant or even somewhat contradictory.
The same sentence that appears in 27.7.2.4 [ostream::sentry], p3 doesn't seem to be very meaningful for basic_istream<>::sentry which performs no input. It is actually rather misleading since it would appear to guide library implementers to calling setstate(failbit) when os.tie()->flush(), the only called function, throws an exception (typically, it's badbit that's set in response to such an event).
Additional comments from Martin, who isn't comfortable with the current proposed resolution (see c++std-lib-11530)
The istream::sentry ctor says nothing about how the function deals with exemptions (27.6.1.1.2, p1 says that the class is responsible for doing "exception safe"(*) prefix and suffix operations but it doesn't explain what level of exception safety the class promises to provide). The mockup example of a "typical implementation of the sentry ctor" given in 27.6.1.1.2, p6, removed in ISO/IEC 14882:2003, doesn't show exception handling, either. Since the ctor is not classified as a formatted or unformatted input function, the text in 27.6.1.1, p1 through p4 does not apply. All this would seem to suggest that the sentry ctor should not catch or in any way handle exceptions thrown from any functions it may call. Thus, the typical implementation of an istream extractor may look something like [1].
The problem with [1] is that while it correctly sets ios::badbit if an exception is thrown from one of the functions called from the sentry ctor, if the sentry ctor reaches EOF while extracting whitespace from a stream that has eofbit or failbit set in exceptions(), it will cause an ios::failure to be thrown, which will in turn cause the extractor to set ios::badbit.
The only straightforward way to prevent this behavior is to move the definition of the sentry object in the extractor above the try block (as suggested by the example in 22.2.8, p9 and also indirectly supported by 27.6.1.3, p1). See [2]. But such an implementation will allow exceptions thrown from functions called from the ctor to freely propagate to the caller regardless of the setting of ios::badbit in the stream object's exceptions().
So since neither [1] nor [2] behaves as expected, the only possible solution is to have the sentry ctor catch exceptions thrown from called functions, set badbit, and propagate those exceptions if badbit is also set in exceptions(). (Another solution exists that deals with both kinds of sentries, but the code is non-obvious and cumbersome -- see [3].)
Please note that, as the issue points out, current libraries do not behave consistently, suggesting that implementors are not quite clear on the exception handling in istream::sentry, despite the fact that some LWG members might feel otherwise. (As documented by the parenthetical comment here: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1480.html#309)
Also please note that those LWG members who in Copenhagen felt that "a sentry's constructor should not catch exceptions, because sentries should only be used within (un)formatted input functions and that exception handling is the responsibility of those functions, not of the sentries," as noted here http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2001/n1310.html#309 would in effect be either arguing for the behavior described in [1] or for extractors implemented along the lines of [3].
The original proposed resolution (Revision 25 of the issues list) clarifies the role of the sentry ctor WRT exception handling by making it clear that extractors (both library or user-defined) should be implemented along the lines of [2] (as opposed to [1]) and that no exception thrown from the callees should propagate out of either function unless badbit is also set in exceptions().
[1] Extractor that catches exceptions thrown from sentry:
struct S { long i; };
istream& operator>> (istream &strm, S &s)
{
ios::iostate err = ios::goodbit;
try {
const istream::sentry guard (strm, false);
if (guard) {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
}
catch (...) {
bool rethrow;
try {
strm.setstate (ios::badbit);
rethrow = false;
}
catch (...) {
rethrow = true;
}
if (rethrow)
throw;
}
if (err)
strm.setstate (err);
return strm;
}
[2] Extractor that propagates exceptions thrown from sentry:
istream& operator>> (istream &strm, S &s)
{
istream::sentry guard (strm, false);
if (guard) {
ios::iostate err = ios::goodbit;
try {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
catch (...) {
bool rethrow;
try {
strm.setstate (ios::badbit);
rethrow = false;
}
catch (...) {
rethrow = true;
}
if (rethrow)
throw;
}
if (err)
strm.setstate (err);
}
return strm;
}
[3] Extractor that catches exceptions thrown from sentry but doesn't set badbit if the exception was thrown as a result of a call to strm.clear().
istream& operator>> (istream &strm, S &s)
{
const ios::iostate state = strm.rdstate ();
const ios::iostate except = strm.exceptions ();
ios::iostate err = std::ios::goodbit;
bool thrown = true;
try {
const istream::sentry guard (strm, false);
thrown = false;
if (guard) {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
}
catch (...) {
if (thrown && state & except)
throw;
try {
strm.setstate (ios::badbit);
thrown = false;
}
catch (...) {
thrown = true;
}
if (thrown)
throw;
}
if (err)
strm.setstate (err);
return strm;
}
[Pre-Berlin] Reopened at the request of Paolo Carlini and Steve Clamage.
[Pre-Portland] A relevant newsgroup post:
The current proposed resolution of issue #309 (http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#309) is unacceptable. I write commerical software and coding around this makes my code ugly, non-intuitive, and requires comments referring people to this very issue. Following is the full explanation of my experience.
In the course of writing software for commercial use, I constructed std::ifstream's based on user-supplied pathnames on typical POSIX systems.
It was expected that some files that opened successfully might not read successfully -- such as a pathname which actually refered to a directory. Intuitively, I expected the streambuffer underflow() code to throw an exception in this situation, and recent implementations of libstdc++'s basic_filebuf do just that (as well as many of my own custom streambufs).
I also intuitively expected that the istream code would convert these exceptions to the "badbit' set on the stream object, because I had not requested exceptions. I refer to 27.6.1.1. P4.
However, this was not the case on at least two implementations -- if the first thing I did with an istream was call operator>>( T& ) for T among the basic arithmetic types and std::string. Looking further I found that the sentry's constructor was invoking the exception when it pre-scanned for whitespace, and the extractor function (operator>>()) was not catching exceptions in this situation.
So, I was in a situation where setting 'noskipws' would change the istream's behavior even though no characters (whitespace or not) could ever be successfully read.
Also, calling .peek() on the istream before calling the extractor() changed the behavior (.peek() had the effect of setting the badbit ahead of time).
I found this all to be so inconsistent and inconvenient for me and my code design, that I filed a bugzilla entry for libstdc++. I was then told that the bug cannot be fixed until issue #309 is resolved by the committee.
[ 2009-07 Frankfurt ]
Moved to NAD.
See the rationale in the issue. Paolo, who requested that the issue be reopened, agreed with the rationale.
Proposed resolution:
Rationale:
The LWG agrees there is minor variation between implementations, but believes that it doesn't matter. This is a rarely used corner case. There is no evidence that this has any commercial importance or that it causes actual portability problems for customers trying to write code that runs on multiple implementations.
Section: 18.8.3.3 [terminate] Status: NAD Submitter: Judy Ward Opened: 2001-04-03 Last modified: 2006-12-27
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Discussion:
According to section 18.7.3.3 of the standard, std::terminate() is supposed to call the terminate_handler in effect immediately after evaluating the throw expression.
Question: what if the terminate_handler in effect is itself std::terminate?
For example:
#include <exception>
int main () {
std::set_terminate(std::terminate);
throw 5;
return 0;
}
Is the implementation allowed to go into an infinite loop?
I think the same issue applies to std::set_unexpected.
Proposed resolution:
Rationale:
Infinite recursion is to be expected: users who set the terminate handler to terminate are explicitly asking for terminate to call itself.
Section: 18.8.3.3 [terminate] Status: NAD Submitter: Detlef Vollmann Opened: 2001-04-11 Last modified: 2007-01-15
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Discussion:
The standard appears to contradict itself about whether the stack is unwound when the implementation calls terminate().
From 18.7.3.3p2:
Calls the terminate_handler function in effect immediately after evaluating the throw-expression (lib.terminate.handler), if called by the implementation [...]
So the stack is guaranteed not to be unwound.
But from 15.3p9:
[...]whether or not the stack is unwound before this call to terminate() is implementation-defined (except.terminate).
And 15.5.1 actually defines that in most cases the stack is unwound.
Proposed resolution:
Rationale:
There is definitely no contradiction between the core and library clauses; nothing in the core clauses says that stack unwinding happens after terminate is called. 18.7.3.3p2 does not say anything about when terminate() is called; it merely specifies which terminate_handler is used.
Section: 26.8 [c.math] Status: NAD Submitter: Dave Abrahams Opened: 2001-06-04 Last modified: 2008-03-12
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Discussion:
Currently the standard mandates the following overloads of abs():
abs(long), abs(int) in <cstdlib>
abs(float), abs(double), abs(long double) in <cmath>
template<class T> T abs(const complex<T>&) in <complex>
template<class T> valarray<T> abs(const valarray<T>&); in <valarray>
The problem is that having only some overloads visible of a function that works on "implicitly inter-convertible" types is dangerous in practice. The headers that get included at any point in a translation unit can change unpredictably during program development/maintenance. The wrong overload might be unintentionally selected.
Currently, there is nothing that mandates the simultaneous visibility of these overloads. Indeed, some vendors have begun fastidiously reducing dependencies among their (public) headers as a QOI issue: it helps people to write portable code by refusing to compile unless all the correct headers are #included.
The same issue may exist for other functions in the library.
Redmond: PJP reports that C99 adds two new kinds of abs: complex, and int_max_abs.
Related issue: 343.
[ Bellevue: ]
The situation is not sufficiently severe to warrant a change.
Rationale:
The programs that could potentially be broken by this situation are already fragile, and somewhat contrived: For example, a user-defined class that has conversion overloads both to long and to float. If x is a value of such a class, then abs(x) would give the long version if the user included <cstdlib>, the float version if the user included <cmath>, and would be diagnosed as ambiguous at compile time if the user included both headers. The LWG couldn't find an example of a program whose meaning would be changed (as opposed to changing it from well-formed to ill-formed) simply by adding another standard header.
Since the harm seems minimal, and there don't seem to be any simple and noninvasive solutions, this is being closed as NAD. It is marked as "Future" for two reasons. First, it might be useful to define an <all> header that would include all Standard Library headers. Second, we should at least make sure that future library extensions don't make this problem worse.
Section: 22.4.6.4 [locale.moneypunct.byname] Status: NAD Submitter: Martin Sebor Opened: 2001-07-05 Last modified: 2006-12-27
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Discussion:
The definition of the moneypunct facet contains the typedefs char_type and string_type. Only one of these names, string_type, is defined in the derived facet, moneypunct_byname.
Proposed resolution:
For consistency with the numpunct facet, add a typedef for char_type to the definition of the moneypunct_byname facet in 22.4.6.4 [locale.moneypunct.byname].
Rationale:
The absence of the typedef is irrelevant. Users can still access the typedef, because it is inherited from the base class.
Section: 22.3.1 [locale] Status: NAD Submitter: Martin Sebor Opened: 2001-07-15 Last modified: 2006-12-27
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Discussion:
The "exposition only" value of the std::locale::none constant shown in the definition of class locale is misleading in that it on many systems conflicts with the value assigned to one if the LC_XXX constants (specifically, LC_COLLATE on AIX, LC_ALL on HP-UX, LC_CTYPE on Linux and SunOS). This causes incorrect behavior when such a constant is passed to one of the locale member functions that accept a locale::category argument and interpret it as either the C LC_XXX constant or a bitmap of locale::category values. At least three major implementations adopt the suggested value without a change and consequently suffer from this problem.
For instance, the following code will (presumably) incorrectly copy facets belonging to the collate category from the German locale on AIX:
std::locale l (std::locale ("C"), "de_DE", std::locale::none);
Rationale:
The LWG agrees that it may be difficult to implement locale member functions in such a way that they can take either category arguments or the LC_ constants defined in <cctype>. In light of this requirement (22.3.1.1.1 [locale.category], paragraph 2), and in light of the requirement in the preceding paragraph that it is possible to combine category bitmask elements with bitwise operations, defining the category elements is delicate, particularly if an implementor is constrained to work with a preexisting C library. (Just using the existing LC_ constants would not work in general.) There's no set of "exposition only" values that could give library implementors proper guidance in such a delicate matter. The non-normative example we're giving is no worse than any other choice would be.
See issue 347.
Section: 27.5.3 [fpos] Status: NAD Submitter: PremAnand M. Rao Opened: 2001-08-27 Last modified: 2006-12-27
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Discussion:
Increment and decrement operators are missing from Table 88 -- Position type requirements in 27.5.3 [fpos].
Proposed resolution:
Table 88 (section 27.4.3) -- Position type requirements be updated to include increment and decrement operators.
expression return type operational note
++p fpos& p += O(1)
p++ fpos { P tmp = p;
++p;
return tmp; }
--p fpos& p -= O(1)
p-- fpos { P tmp = p;
--p;
return tmp; }
Rationale:
The LWG believes this is a request for extension, not a defect report. Additionally, nobody saw a clear need for this extension; fpos is used only in very limited ways.
Section: 27.7.1.3 [istream.unformatted] Status: NAD Submitter: Howard Hinnant Opened: 2001-10-09 Last modified: 2009-07-14
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Discussion:
I think we have a defect.
According to lwg issue 60 which is now a dr, the description of seekg in 27.7.1.3 [istream.unformatted] paragraph 38 now looks like:
Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail() != true, executes rdbuf()->pubseekpos( pos).
And according to lwg issue 243 which is also now a dr, 27.6.1.3, paragraph 1 looks like:
Each unformatted input function begins execution by constructing an object of class sentry with the default argument noskipws (second) argument true. If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input. Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of type bool, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to 0; unformatted input functions taking a character array of non-zero size as an argument shall also store a null character (using charT()) in the first location of the array. If an exception is thrown during input then ios::badbit is turned on in *this'ss error state. If (exception()&badbit)!= 0 then the exception is rethrown. It also counts the number of characters extracted. If no exception has been thrown it ends by storing the count in a member object and returning the value specified. In any event the sentry object is destroyed before leaving the unformatted input function.
And finally 27.6.1.1.2/5 says this about sentry:
If, after any preparation is completed, is.good() is true, ok_ != false otherwise, ok_ == false.
So although the seekg paragraph says that the operation proceeds if !fail(), the behavior of unformatted functions says the operation proceeds only if good(). The two statements are contradictory when only eofbit is set. I don't think the current text is clear which condition should be respected.
Further discussion from Redmond:
PJP: It doesn't seem quite right to say that seekg is "unformatted". That makes specific claims about sentry that aren't quite appropriate for seeking, which has less fragile failure modes than actual input. If we do really mean that it's unformatted input, it should behave the same way as other unformatted input. On the other hand, "principle of least surprise" is that seeking from EOF ought to be OK.
Pre-Berlin: Paolo points out several problems with the proposed resolution in Ready state:
[ 2009-07 Frankfurt ]
Moved to NAD. Will reopen if proposed resolution is supplied.
Proposed resolution:
Change 27.7.1.3 [istream.unformatted] to:
Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted, does not affect the value returned by subsequent calls to gcount(), and does not examine the value returned by the sentry object. After constructing a sentry object, if fail() != true, executes rdbuf()->pubseekpos(pos). In case of success, the function calls clear(). In case of failure, the function calls setstate(failbit) (which may throw ios_base::failure).
[Lillehammer: Matt provided wording.]
Rationale:
In C, fseek does clear EOF. This is probably what most users would expect. We agree that having eofbit set should not deter a seek, and that a successful seek should clear eofbit. Note that fail() is true only if failbit or badbit is set, so using !fail(), rather than good(), satisfies this goal.
Section: 17 [library] Status: NAD Submitter: Martin Sebor Opened: 2001-10-09 Last modified: 2009-07-28
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Discussion:
The synopses of the C++ library headers clearly show which names are required to be defined in each header. Since in order to implement the classes and templates defined in these headers declarations of other templates (but not necessarily their definitions) are typically necessary the standard in 17.4.4, p1 permits library implementers to include any headers needed to implement the definitions in each header.
For instance, although it is not explicitly specified in the synopsis of <string>, at the point of definition of the std::basic_string template the declaration of the std::allocator template must be in scope. All current implementations simply include <memory> from within <string>, either directly or indirectly, to bring the declaration of std::allocator into scope.
Additionally, however, some implementation also include <istream> and <ostream> at the top of <string> to bring the declarations of std::basic_istream and std::basic_ostream into scope (which are needed in order to implement the string inserter and extractor operators (21.3.7.9 [lib.string.io])). Other implementations only include <iosfwd>, since strictly speaking, only the declarations and not the full definitions are necessary.
Obviously, it is possible to implement <string> without actually providing the full definitions of all the templates std::basic_string uses (std::allocator, std::basic_istream, and std::basic_ostream). Furthermore, not only is it possible, doing so is likely to have a positive effect on compile-time efficiency.
But while it may seem perfectly reasonable to expect a program that uses the std::basic_string insertion and extraction operators to also explicitly include <istream> or <ostream>, respectively, it doesn't seem reasonable to also expect it to explicitly include <memory>. Since what's reasonable and what isn't is highly subjective one would expect the standard to specify what can and what cannot be assumed. Unfortunately, that isn't the case.
The examples below demonstrate the issue.
Example 1:
It is not clear whether the following program is complete:
#include <string>
extern std::basic_ostream<char> &strm;
int main () {
strm << std::string ("Hello, World!\n");
}
or whether one must explicitly include <memory> or <ostream> (or both) in addition to <string> in order for the program to compile.
Example 2:
Similarly, it is unclear whether the following program is complete:
#include <istream>
extern std::basic_iostream<char> &strm;
int main () {
strm << "Hello, World!\n";
}
or whether one needs to explicitly include <ostream>, and perhaps even other headers containing the definitions of other required templates:
#include <ios>
#include <istream>
#include <ostream>
#include <streambuf>
extern std::basic_iostream<char> &strm;
int main () {
strm << "Hello, World!\n";
}
Example 3:
Likewise, it seems unclear whether the program below is complete:
#include <iterator>
bool foo (std::istream_iterator<int> a, std::istream_iterator<int> b)
{
return a == b;
}
int main () { }
or whether one should be required to include <istream>.
There are many more examples that demonstrate this lack of a requirement. I believe that in a good number of cases it would be unreasonable to require that a program explicitly include all the headers necessary for a particular template to be specialized, but I think that there are cases such as some of those above where it would be desirable to allow implementations to include only as much as necessary and not more.
[ post Bellevue: ]
Position taken in prior reviews is that the idea of a table of header dependencies is a good one. Our view is that a full paper is needed to do justice to this, and we've made that recommendation to the issue author.
[ 2009-07 Frankfurt ]
NAD. Handled by LWG 1178.
Proposed resolution:
For every C++ library header, supply a minimum set of other C++ library headers that are required to be included by that header. The proposed list is below (C++ headers for C Library Facilities, table 12 in 17.4.1.2, p3, are omitted):
+------------+--------------------+ | C++ header |required to include | +============+====================+ |<algorithm> | | +------------+--------------------+ |<bitset> | | +------------+--------------------+ |<complex> | | +------------+--------------------+ |<deque> |<memory> | +------------+--------------------+ |<exception> | | +------------+--------------------+ |<fstream> |<ios> | +------------+--------------------+ |<functional>| | +------------+--------------------+ |<iomanip> |<ios> | +------------+--------------------+ |<ios> |<streambuf> | +------------+--------------------+ |<iosfwd> | | +------------+--------------------+ |<iostream> |<istream>, <ostream>| +------------+--------------------+ |<istream> |<ios> | +------------+--------------------+ |<iterator> | | +------------+--------------------+ |<limits> | | +------------+--------------------+ |<list> |<memory> | +------------+--------------------+ |<locale> | | +------------+--------------------+ |<map> |<memory> | +------------+--------------------+ |<memory> | | +------------+--------------------+ |<new> |<exception> | +------------+--------------------+ |<numeric> | | +------------+--------------------+ |<ostream> |<ios> | +------------+--------------------+ |<queue> |<deque> | +------------+--------------------+ |<set> |<memory> | +------------+--------------------+ |<sstream> |<ios>, <string> | +------------+--------------------+ |<stack> |<deque> | +------------+--------------------+ |<stdexcept> | | +------------+--------------------+ |<streambuf> |<ios> | +------------+--------------------+ |<string> |<memory> | +------------+--------------------+ |<strstream> | | +------------+--------------------+ |<typeinfo> |<exception> | +------------+--------------------+ |<utility> | | +------------+--------------------+ |<valarray> | | +------------+--------------------+ |<vector> |<memory> | +------------+--------------------+
Rationale:
The portability problem is real. A program that works correctly on one implementation might fail on another, because of different header dependencies. This problem was understood before the standard was completed, and it was a conscious design choice.
One possible way to deal with this, as a library extension, would be an <all> header.
Hinnant: It's time we dealt with this issue for C++0X. Reopened.
Section: 22.4.2 [category.numeric] Status: NAD Submitter: Howard Hinnant Opened: 2001-10-13 Last modified: 2007-01-15
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Discussion:
When both grouping and showbase are active and the basefield is octal, does the leading 0 participate in the grouping or not? For example, should one format as: 0,123,456 or 0123,456?
An analogy can be drawn with hexadecimal. It appears that 0x123,456 is preferred over 0x,123,456. However, this analogy is not universally accepted to apply to the octal base. The standard is not clear on how to format (or parse) in this manner.
Proposed resolution:
Insert into 22.4.3.1.2 [facet.numpunct.virtuals] paragraph 3, just before the last sentence:
The leading hexadecimal base specifier "0x" does not participate in grouping. The leading '0' octal base specifier may participate in grouping. It is unspecified if the leading '0' participates in formatting octal numbers. In parsing octal numbers, the implementation is encouraged to accept both the leading '0' participating in the grouping, and not participating (e.g. 0123,456 or 0,123,456).
Rationale:
The current behavior may be unspecified, but it's not clear that it matters. This is an obscure corner case, since grouping is usually intended for the benefit of humans and oct/hex prefixes are usually intended for the benefit of machines. There is not a strong enough consensus in the LWG for action.
Section: 20.3.5 [pairs] Status: Dup Submitter: Andy Sawyer Opened: 2001-10-23 Last modified: 2008-01-05
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Duplicate of: 532
Discussion:
The current wording of 20.2.2 [lib.pairs] p6 precludes the use of operator< on any pair type which contains a pointer.
Proposed resolution:
In 20.3.5 [pairs] paragraph 6, replace:
Returns: x.first < y.first || (!(y.first < x.first) && x.second <
y.second).
With:
Returns: std::less<T1>()( x.first, y.first ) ||
(!std::less<T1>()( y.first, x.first) &&
std::less<T2>()( x.second, y.second ) )
Rationale:
This is an instance of a much more general problem. If we want operator< to translate to std::less for pairs of pointers, where do we draw the line? The same issue applies to individual pointers, smart pointer wrappers, std::vector<T*>, and so on.
Andy Koenig suggests that the real issue here is that we aren't distinguishing adequately between two different orderings, a "useful ordering" and a "canonical ordering" that's used just because we sometimes need some ordering without caring much which ordering it is. Another example of the later is typeinfo's before.
Section: 20.9.5.1 [allocator.members], 20.2.5 [allocator.requirements], 17.6.1.1 [contents] Status: Dup Submitter: Nathan Myers Opened: 2001-10-25 Last modified: 2007-10-11
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Duplicate of: 634
Discussion:
See c++std-lib-9006 and c++std-lib-9007. This issue is taken verbatim from -9007.
The core language feature allowing definition of operator&() applied to any non-builtin type makes that operator often unsafe to use in implementing libraries, including the Standard Library. The result is that many library facilities fail for legal user code, such as the fragment
class A { private: A* operator&(); };
std::vector<A> aa;
class B { };
B* operator&(B&) { return 0; }
std::vector<B> ba;
In particular, the requirements table for Allocator (Table 32) specifies no semantics at all for member address(), and allocator<>::address is defined in terms of unadorned operator &.
Proposed resolution:
In 20.6.1.1, Change the definition of allocator<>::address from:
Returns: &x
to:
Returns: The value that the built in operator&(x) would return if not overloaded.
In 20.1.6, Table 32, add to the Notes column of the a.address(r) and a.address(s) lines, respectively:
allocator<T>::address(r) allocator<T>::address(s)
In addition, in clause 17.4.1.1, add a statement:
The Standard Library does not apply operator& to any type for which operator& may be overloaded.
Rationale:
The LWG believes both examples are ill-formed. The contained type is required to be CopyConstructible (20.2.1 [utility.arg.requirements]), and that includes the requirement that &t return the usual types and values. Since allocators are intended to be used in conjunction with containers, and since the CopyConstructible requirements appear to have been written to deal with the concerns of this issue, the LWG feels it is NAD unless someone can come up with a well-formed example exhibiting a problem.
It may well be that the CopyConstructible requirements are too restrictive and that either the container requirements or the CopyConstructive requirements should be relaxed, but that's a far larger issue. Marking this issue as "future" as a pointer to that larger issue.
Section: 20.8 [function.objects] Status: NAD Editorial Submitter: Dale Riley Opened: 2001-11-12 Last modified: 2007-04-24
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Discussion:
In 20.8 [function.objects] the header <functional> synopsis declares the unary_negate and binary_negate function objects as struct. However in 20.8.9 [negators] the unary_negate and binary_negate function objects are defined as class. Given the context, they are not "basic function objects" like negate, so this is either a typo or an editorial oversight.
[Taken from comp.std.c++]
Proposed resolution:
Change the synopsis to reflect the useage in 20.8.9 [negators]
[Curaçao: Since the language permits "struct", the LWG views this as NAD. They suggest, however, that the Project Editor might wish to make the change as editorial.]
Section: 20.3.5 [pairs] Status: NAD Editorial Submitter: Martin Sebor Opened: 2001-12-02 Last modified: 2008-01-05
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Discussion:
The class template std::pair defines a template ctor (20.2.2, p4) but no template assignment operator. This may lead to inefficient code since assigning an object of pair<C, D> to pair<A, B> where the types C and D are distinct from but convertible to A and B, respectively, results in a call to the template copy ctor to construct an unnamed temporary of type pair<A, B> followed by an ordinary (perhaps implicitly defined) assignment operator, instead of just a straight assignment.
Proposed resolution:
Add the following declaration to the definition of std::pair:
template<class U, class V>
pair& operator=(const pair<U, V> &p);
And also add a paragraph describing the effects of the function template to the end of 20.2.2:
template<class U, class V>
pair& operator=(const pair<U, V> &p);
Effects: first = p.first; second = p.second; Returns: *this
[Curaçao: There is no indication this is was anything other than a design decision, and thus NAD. May be appropriate for a future standard.]
[ Pre Bellevue: It was recognized that this was taken care of by N1856, and thus moved from NAD Future to NAD Editorial. ]
Section: 22.4.1 [category.ctype] Status: NAD Submitter: Matt Austern Opened: 2002-01-23 Last modified: 2006-12-27
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Discussion:
What should the following program print?
#include <locale>
#include <iostream>
class my_ctype : public std::ctype<char>
{
typedef std::ctype<char> base;
public:
my_ctype(std::size_t refs = 0) : base(my_table, false, refs)
{
std::copy(base::classic_table(), base::classic_table() + base::table_size,
my_table);
my_table[(unsigned char) '_'] = (base::mask) (base::print | base::space);
}
private:
mask my_table[base::table_size];
};
int main()
{
my_ctype ct;
std::cout << "isspace: " << ct.is(std::ctype_base::space, '_') << " "
<< "isalpha: " << ct.is(std::ctype_base::alpha, '_') << std::endl;
}
The goal is to create a facet where '_' is treated as whitespace.
On gcc 3.0, this program prints "isspace: 1 isalpha: 0". On Microsoft C++ it prints "isspace: 1 isalpha: 1".
I believe that both implementations are legal, and the standard does not give enough guidance for users to be able to use std::ctype's protected interface portably.
The above program assumes that ctype_base::mask enumerators like space and print are disjoint, and that the way to say that a character is both a space and a printing character is to or those two enumerators together. This is suggested by the "exposition only" values in 22.4.1 [category.ctype], but it is nowhere specified in normative text. An alternative interpretation is that the more specific categories subsume the less specific. The above program gives the results it does on the Microsoft compiler because, on that compiler, print has all the bits set for each specific printing character class.
From the point of view of std::ctype's public interface, there's no important difference between these two techniques. From the point of view of the protected interface, there is. If I'm defining a facet that inherits from std::ctype<char>, I'm the one who defines the value that table()['a'] returns. I need to know what combination of mask values I should use. This isn't so very esoteric: it's exactly why std::ctype has a protected interface. If we care about users being able to write their own ctype facets, we have to give them a portable way to do it.
Related reflector messages: lib-9224, lib-9226, lib-9229, lib-9270, lib-9272, lib-9273, lib-9274, lib-9277, lib-9279.
Issue 339 is related, but not identical. The proposed resolution if issue 339 says that ctype_base::mask must be a bitmask type. It does not say that the ctype_base::mask elements are bitmask elements, so it doesn't directly affect this issue.
More comments from Benjamin Kosnik, who believes that that C99 compatibility essentially requires what we're calling option 1 below.
I think the C99 standard is clear, that isspace -> !isalpha.
--------
#include <locale>
#include <iostream>
class my_ctype : public std::ctype<char>
{
private:
typedef std::ctype<char> base;
mask my_table[base::table_size];
public:
my_ctype(std::size_t refs = 0) : base(my_table, false, refs)
{
std::copy(base::classic_table(), base::classic_table() + base::table_size,
my_table);
mask both = base::print | base::space;
my_table[static_cast<mask>('_')] = both;
}
};
int main()
{
using namespace std;
my_ctype ct;
cout << "isspace: " << ct.is(ctype_base::space, '_') << endl;
cout << "isprint: " << ct.is(ctype_base::print, '_') << endl;
// ISO C99, isalpha iff upper | lower set, and !space.
// 7.5, p 193
// -> looks like g++ behavior is correct.
// 356 -> bitmask elements are required for ctype_base
// 339 -> bitmask type required for mask
cout << "isalpha: " << ct.is(ctype_base::alpha, '_') << endl;
}
Proposed resolution:
Informally, we have three choices:
Either of the first two options is just as good from the standpoint of portability. Either one will require some implementations to change.
Rationale:
The LWG agrees that this is a real ambiguity, and that both interpretations are conforming under the existing standard. However, there's no evidence that it's causing problems for real users. Users who want to define ctype facets portably can test the ctype_base masks to see which interpretation is being used.
Section: 26.8 [c.math] Status: NAD Editorial Submitter: Ray Lischner Opened: 2002-02-26 Last modified: 2007-04-24
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Discussion:
The float versions of the math functions have no meaningful value to return for a range error. The long double versions have a value they can return, but it isn't necessarily the most reasonable value.
Section 26.5 [lib.c.math], paragraph 5, says that C++ "adds float and long double overloaded versions of these functions, with the same semantics," referring to the math functions from the C90 standard.
The C90 standard, in section 7.5.1, paragraph 3, says that functions return "the value of the macro HUGE_VAL" when they encounter a range error. Section 7.5, paragraph 2, defines HUGE_VAL as a macro that "expands to a positive double expression, not necessarily representable as a float."
Therefore, the float versions of the math functions have no way to signal a range error. [Curaçao: The LWG notes that this isn't strictly correct, since errno is set.] The semantics require that they return HUGE_VAL, but they cannot because HUGE_VAL might not be representable as a float.
The problem with long double functions is less severe because HUGE_VAL is representable as a long double. On the other hand, it might not be a "huge" long double value, and might fall well within the range of normal return values for a long double function. Therefore, it does not make sense for a long double function to return a double (HUGE_VAL) for a range error.
Proposed resolution:
Curaçao: C99 was faced with a similar problem, which they fixed by adding HUGE_VALF and HUGE_VALL in addition to HUGE_VAL.
C++ must also fix, but it should be done in the context of the general C99 based changes to C++, not via DR. Thus the LWG in Curaçao felt the resolution should be NAD, FUTURE, but the issue is being held open for one more meeting to ensure LWG members not present during the discussion concur.
Rationale:
Will be fixed as part of more general work in the TR.
Section: 22.4.2.2.2 [facet.num.put.virtuals] Status: NAD Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2007-01-15
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Discussion:
22.2.2.2.2, p12 specifies that thousands_sep is to be inserted only for integral types (issue 282 suggests that this should be done for all arithmetic types).
22.2.2.1.2, p12 requires that grouping be checked for all extractors including that for void*.
I don't think that's right. void* values should not be checked for grouping, should they? (Although if they should, then num_put needs to write them out, otherwise their extraction will fail.)
Proposed resolution:
Change the first sentence of 22.2.2.2.2, p12 from
Digit grouping is checked. That is, the positions of discarded separators is examined for consistency with use_facet<numpunct<charT> >(loc).grouping(). If they are not consistent then ios_base::failbit is assigned to err.
to
Except for conversions to void*, digit grouping is checked...
Rationale:
This would be a change: as it stands, the standard clearly specifies that grouping applies to void*. A survey of existing practice shows that most existing implementations do that, as they should.
Section: 27 [input.output] Status: NAD Submitter: Walter Brown, Marc Paterno Opened: 2002-05-10 Last modified: 2006-12-27
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Discussion:
The following member functions are declared const, yet return non-const pointers. We believe they are should be changed, because they allow code that may surprise the user. See document N1360 for details and rationale.
[Santa Cruz: the real issue is that we've got const member functions that return pointers to non-const, and N1360 proposes replacing them by overloaded pairs. There isn't a consensus about whether this is a real issue, since we've never said what our constness policy is for iostreams. N1360 relies on a distinction between physical constness and logical constness; that distinction, or those terms, does not appear in the standard.]
Proposed resolution:
In 27.4.4 and 27.4.4.2
Replace
basic_ostream<charT,traits>* tie() const;
with
basic_ostream<charT,traits>* tie(); const basic_ostream<charT,traits>* tie() const;
and replace
basic_streambuf<charT,traits>* rdbuf() const;
with
basic_streambuf<charT,traits>* rdbuf(); const basic_streambuf<charT,traits>* rdbuf() const;
In 27.5.2 and 27.5.2.3.1
Replace
char_type* eback() const;
with
char_type* eback(); const char_type* eback() const;
Replace
char_type gptr() const;
with
char_type* gptr(); const char_type* gptr() const;
Replace
char_type* egptr() const;
with
char_type* egptr(); const char_type* egptr() const;
In 27.5.2 and 27.5.2.3.2
Replace
char_type* pbase() const;
with
char_type* pbase(); const char_type* pbase() const;
Replace
char_type* pptr() const;
with
char_type* pptr(); const char_type* pptr() const;
Replace
char_type* epptr() const;
with
char_type* epptr(); const char_type* epptr() const;
In 27.7.2, 27.7.2.2, 27.7.3 27.7.3.2, 27.7.4, and 27.7.6
Replace
basic_stringbuf<charT,traits,Allocator>* rdbuf() const;
with
basic_stringbuf<charT,traits,Allocator>* rdbuf(); const basic_stringbuf<charT,traits,Allocator>* rdbuf() const;
In 27.8.1.5, 27.8.1.7, 27.8.1.8, 27.8.1.10, 27.8.1.11, and 27.8.1.13
Replace
basic_filebuf<charT,traits>* rdbuf() const;
with
basic_filebuf<charT,traits>* rdbuf(); const basic_filebuf<charT,traits>* rdbuf() const;
Rationale:
The existing specification is a bit sloppy, but there's no particular reason to change this other than tidiness, and there are a number of ways in which streams might have been designed differently if we were starting today. There's no evidence that the existing constness policy is harming users. We might consider a different constness policy as part of a full stream redesign.
Section: 25.3.8 [alg.remove] Status: NAD Submitter: Anthony Williams Opened: 2002-05-13 Last modified: 2006-12-27
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Discussion:
remove_copy and remove_copy_if (25.3.8 [alg.remove]) permit their input range to be marked with Input Iterators. However, since two operations are required against the elements to copy (comparison and assigment), when the input range uses Input Iterators, a temporary copy must be taken to avoid dereferencing the iterator twice. This therefore requires the value type of the InputIterator to be CopyConstructible. If the iterators are at least Forward Iterators, then the iterator can be dereferenced twice, or a reference to the result maintained, so the temporary is not required.
Proposed resolution:
Add "If InputIterator does not meet the requirements of forward iterator, then the value type of InputIterator must be copy constructible. Otherwise copy constructible is not required." to 25.3.8 [alg.remove] paragraph 6.
Rationale:
The assumption is that an input iterator can't be dereferenced twice. There's no basis for that assumption in the Standard.
Section: 21.4.6.6 [string::replace] Status: NAD Editorial Submitter: Beman Dawes Opened: 2002-06-03 Last modified: 2007-04-24
View all other issues in [string::replace].
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Discussion:
21.4.6.6 [string::replace] basic_string::replace, second signature, given in paragraph 1, has two "Throws" paragraphs (3 and 5).
In addition, the second "Throws" paragraph (5) includes specification (beginning with "Otherwise, the function replaces ...") that should be part of the "Effects" paragraph.
Proposed resolution:
Rationale:
This is editorial. Both "throws" statements are true. The bug is just that the second one should be a sentence, part of the "Effects" clause, not a separate "Throws". The project editor has been notified.
Section: 17.6.4.11 [res.on.exception.handling], 18.7.1 [type.info] Status: NAD Submitter: Randy Maddox Opened: 2002-07-22 Last modified: 2006-12-27
View all other issues in [res.on.exception.handling].
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Discussion:
Paragraph 3 under clause 17.6.4.11 [res.on.exception.handling], Restrictions on Exception Handling, states that "Any other functions defined in the C++ Standard Library that do not have an exception-specification may throw implementation-defined exceptions unless otherwise specified." This statement is followed by a reference to footnote 178 at the bottom of that page which states, apparently in reference to the C++ Standard Library, that "Library implementations are encouraged (but not required) to report errors by throwing exceptions from (or derived from) the standard exceptions."
These statements appear to be in direct contradiction to clause 18.7.1 [type.info], which states "The class exception defines the base class for the types of objects thrown as exceptions by the C++ Standard library components ...".
Is this inconsistent?
Proposed resolution:
Rationale:
Clause 17 is setting the overall library requirements, and it's clear and consistent. This sentence from Clause 18 is descriptive, not setting a requirement on any other class.
Section: 22.4.6.3.1 [locale.moneypunct.members], 22.4.6.3.2 [locale.moneypunct.virtuals] Status: NAD Submitter: Ray Lischner Opened: 2002-08-08 Last modified: 2006-12-27
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Discussion:
In section 22.4.6.3.1 [locale.moneypunct.members], frac_digits() returns type "int". This implies that frac_digits() might return a negative value, but a negative value is nonsensical. It should return "unsigned".
Similarly, in section 22.4.6.3.2 [locale.moneypunct.virtuals], do_frac_digits() should return "unsigned".
Proposed resolution:
Rationale:
Regardless of whether the return value is int or unsigned, it's always conceivable that frac_digits might return a nonsensical value. (Is 4294967295 really any better than -1?) The clients of moneypunct, the get and put facets, can and do perform range checks.
Section: 21.4.6.4 [string::insert] Status: NAD Submitter: Ray Lischner Opened: 2002-08-16 Last modified: 2006-12-27
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Discussion:
Section 21.4.6.4 [string::insert], paragraph 4, contains the following, "Then throws length_error if size() >= npos - rlen."
Related to DR 83, this sentence should probably be removed.
Proposed resolution:
Rationale:
This requirement is redundant but correct. No change is needed.
Section: 22.3.1 [locale] Status: Dup Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2006-12-30
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Duplicate of: 31
Discussion:
I think there is a problem with 22.1.1, p6 which says that
-6- An instance of locale is immutable; once a facet reference
is obtained from it, that reference remains usable as long
as the locale value itself exists.
and 22.1.1.2, p4:
const locale& operator=(const locale& other) throw();
-4- Effects: Creates a copy of other, replacing the current value.
How can a reference to a facet obtained from a locale object remain valid after an assignment that clearly must replace all the facets in the locale object? Imagine a program such as this
std::locale loc ("de_DE");
const std::ctype<char> &r0 = std::use_facet<std::ctype<char> >(loc);
loc = std::locale ("en_US");
const std::ctype<char> &r1 = std::use_facet<std::ctype<char> >(loc);
Is r0 really supposed to be preserved and destroyed only when loc goes out of scope?
Proposed resolution:
[Summer '04 mid-meeting mailing: Martin and Dietmar believe this is a duplicate of issue 31 and recommend that it be closed. ]
Section: 22.4.1.4 [locale.codecvt] Status: NAD Submitter: Martin Sebor Opened: 2002-08-30 Last modified: 2009-07-14
View all other issues in [locale.codecvt].
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Discussion:
It seems that the descriptions of codecvt do_in() and do_out() leave sufficient room for interpretation so that two implementations of codecvt may not work correctly with the same filebuf. Specifically, the following seems less than adequately specified:
Finally, the conditions described at the end of 22.4.1.4.2 [locale.codecvt.virtuals], p4 don't seem to be possible:
"A return value of partial, if (from_next == from_end), indicates that either the destination sequence has not absorbed all the available destination elements, or that additional source elements are needed before another destination element can be produced."
If the value is partial, it's not clear to me that (from_next ==from_end) could ever hold if there isn't enough room in the destination buffer. In order for (from_next==from_end) to hold, all characters in that range must have been successfully converted (according to 22.4.1.4.2 [locale.codecvt.virtuals], p2) and since there are no further source characters to convert, no more room in the destination buffer can be needed.
It's also not clear to me that (from_next==from_end) could ever hold if additional source elements are needed to produce another destination character (not element as incorrectly stated in the text). partial is returned if "not all source characters have been converted" according to Table 53, which also implies that (from_next==from) does NOT hold.
Could it be that the intended qualifying condition was actually (from_next != from_end), i.e., that the sentence was supposed to read
"A return value of partial, if (from_next != from_end),..."
which would make perfect sense, since, as far as I understand it, partial can only occur if (from_next != from_end)?
[Lillehammer: Defer for the moment, but this really needs to be fixed. Right now, the description of codecvt is too vague for it to be a useful contract between providers and clients of codecvt facets. (Note that both vendors and users can be both providers and clients of codecvt facets.) The major philosophical issue is whether the standard should only describe mappings that take a single wide character to multiple narrow characters (and vice versa), or whether it should describe fully general N-to-M conversions. When the original standard was written only the former was contemplated, but today, in light of the popularity of utf8 and utf16, that doesn't seem sufficient for C++0x. Bill supports general N-to-M conversions; we need to make sure Martin and Howard agree.]
[ 2009-07 Frankfurt ]
codecvt is meant to be a 1-to-N to N-to-1 conversion. It does not work well for N-to-M conversions. wbuffer_convert now exists, and handles N-to-M cases. Also, there is a new specialization of codecvt that permits UTF-16 <-> UTF-8 conversions.
NAD without prejudice. Will reopen if proposed resolution is supplied.
Proposed resolution:
Section: 17 [library] Status: NAD Submitter: Matt Austern Opened: 2002-10-23 Last modified: 2007-04-18
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Discussion:
Many function templates have parameters that are passed by value; a typical example is find_if's pred parameter in 25.2.5 [alg.find]. Are the corresponding template parameters (Predicate in this case) implicitly required to be CopyConstructible, or does that need to be spelled out explicitly?
This isn't quite as silly a question as it might seem to be at first sight. If you call find_if in such a way that template argument deduction applies, then of course you'll get call by value and you need to provide a copy constructor. If you explicitly provide the template arguments, however, you can force call by reference by writing something like find_if<my_iterator, my_predicate&>. The question is whether implementation are required to accept this, or whether this is ill-formed because my_predicate& is not CopyConstructible.
The scope of this problem, if it is a problem, is unknown. Function object arguments to generic algorithms in clauses 25 [algorithms] and 26 [numerics] are obvious examples. A review of the whole library is necessary.
[ This is really two issues. First, predicates are typically passed by value but we don't say they must be Copy Constructible. They should be. Second: is specialization allowed to transform value arguments into references? References aren't copy constructible, so this should not be allowed. ]
[ 2007-01-12, Howard: First, despite the note above, references are copy constructible. They just aren't assignable. Second, this is very closely related to 92 and should be consistent with that. That issue already says that implementations are allowed to copy function objects. If one passes in a reference, it is copyable, but susceptible to slicing if one passes in a reference to a base. Third, with rvalue reference in the language one only needs to satisfy MoveConstructible to pass an rvalue "by value". Though the function might still copy the function object internally (requiring CopyConstructible). Finally (and fwiw), if we wanted to, it is easy to code all of the std::algorithms such that they do not copy function objects internally. One merely passes them by reference internally if desired (this has been fully implemented and shipped for several years). If this were mandated, it would reverse 92, allowing function objects to reliably maintain state. E.g. the example in 92 would reliably remove only the third element. ]
Proposed resolution:
Recommend NAD.
Rationale:
Generic algorithms will be marked with concepts and these will imply a requirement of MoveConstructible (not CopyConstructible). The signature of the function will then precisely describe and enforce the precise requirements.
Section: 26.4 [complex.numbers] Status: NAD Submitter: Gabriel Dos Reis Opened: 2002-11-08 Last modified: 2008-02-27
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Discussion:
Practice with std::complex<> and the associative containers occasionally reveals artificial and distracting issues with constructs resembling: std::set<std::complex<double> > s;
The main reason for the above to fail is the absence of an approriate definition for std::less<std::complex<T> >. That in turn comes from the definition of the primary template std::less<> in terms of operator<.
The usual argument goes as follows: Since there is no ordering over the complex field compatible with field operations it makes little sense to define a function operator< operating on the datatype std::complex<T>. That is fine. However, that reasoning does not carry over to std::less<T> which is used, among other things, by associative containers as an ordering useful to meet complexity requirements.
Related issue: 348.
[ Pre Bellevue: Reopened at the request of Alisdair. ]
[ Bellevue: ]
This is a request for a design change, and not a defect in the standard. It is in scope to consider, but the group feels that it is not a change that we need to do. Is there a total ordering for floating point values, including NaN? There is not a clear enough solution or big enough problem for us to solve. Solving this problem would require solving the problem for floating point, which is equally unclear. The LWG noted that users who want to put objects into an associative container for which operator< isn't defined can simply provide their own comparison function object. NAD
Proposed resolution:
Informally: Add a specialization of std::less for std::complex.
Rationale:
Discussed in Santa Cruz. An overwhelming majority of the LWG believes this should not be treated a DR: it's a request for a design change, not a defect in the existing standard. Most people (10-3) believed that we probably don't want this change, period: as with issue 348, it's hard to know where to draw the line. The LWG noted that users who want to put objects into an associative container for which operator< isn't defined can simply provide their own comparison function object.
Section: 20.2.1 [utility.arg.requirements] Status: NAD Editorial Submitter: Doug Gregor Opened: 2002-10-24 Last modified: 2008-03-14
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Discussion:
The CopyConstructible requirements in Table 30 state that for an object t of type T (where T is CopyConstructible), the expression &t returns the address of t (with type T*). This requirement is overly strict, in that it disallows types that overload operator& to not return a value of type T*. This occurs, for instance, in the Boost.Lambda library, where operator& is overloaded for a Boost.Lambda function object to return another function object.
Example:
std::vector<int> u, v; int x; // ... std::transform(u.begin(), u.end(), std::back_inserter(v), _1 * x);
_1 * x returns an unnamed function object with operator& overloaded to not return T* , therefore rendering the std::transform call ill-formed. However, most standard library implementations will compile this code properly, and the viability of such binder libraries is severely hindered by the unnecessary restriction in the CopyConstructible requirements.
For reference, the address of an object can be retrieved without using the address-of operator with the following function template:
template <typename T> T* addressof(T& v)
{
return reinterpret_cast<T*>(
&const_cast<char&>(reinterpret_cast<const volatile char &>(v)));
}
Note: this relates directly to library issue 350, which will need to be reexamined if the CopyConstructible requirements change.
Proposed resolution:
Remove the last two rows of Table 30, eliminating the requirements that &t and &u return the address of t and u, respectively.
Rationale:
This was a deliberate design decision. Perhaps it should be reconsidered for C++0x.
Section: 24.2.3 [input.iterators] Status: NAD Submitter: Corwin Joy Opened: 2002-12-11 Last modified: 2006-12-27
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Discussion:
In section 24.2.3 [input.iterators] table 72 - 'Input Iterator Requirements' we have as a postcondition of *a: "If a==b and (a, b) is in the domain of == then *a is equivalent to *b".
In section 24.6.3.5 [istreambuf.iterator::equal] it states that "istreambuf_iterator::equal returns true if and only if both iterators are at end-of-stream, or neither is at end-of-stream, regardless of what streambuf object they use." (My emphasis).
The defect is that either 'equivalent' needs to be more precisely defined or the conditions for equality in 24.6.3.5 [istreambuf.iterator::equal] are incorrect. (Or both).
Consider the following example:
#include <iostream>
#include <fstream>
#include <iterator>
using namespace std;
int main() {
ifstream file1("file1.txt"), file2("file2.txt");
istreambuf_iterator<char> f1(file1), f2(file2);
cout << "f1 == f2 : " << boolalpha << (f1 == f2) << endl;
cout << "f1 = " << *f1 << endl;
cout << "f2 = " << *f2 << endl;
return 0;
}
Now assuming that neither f1 or f2 are at the end-of-stream then f1 == f2 by 24.6.3.5 [istreambuf.iterator::equal].
However, it is unlikely that *f1 will give the same value as *f2 except by accident.
So what does *f1 'equivalent' to *f2 mean? I think the standard should be clearer on this point, or at least be explicit that this does not mean that *f1 and *f2 are required to have the same value in the case of input iterators.
Proposed resolution:
Rationale:
The two iterators aer not in the domain of ==
Section: 22.4.1.4.2 [locale.codecvt.virtuals] Status: NAD Editorial Submitter: Alberto Barbati Opened: 2002-12-24 Last modified: 2008-07-02
View all other issues in [locale.codecvt.virtuals].
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Discussion:
this DR follows the discussion on the previous thread "codecvt::do_in not consuming external characters". It's just a clarification issue and not a request for a change.
Can do_in()/do_out() produce output characters without consuming input characters as a result of operation on state?
Proposed resolution:
Add a note at the end of 22.4.1.4.2 [locale.codecvt.virtuals], paragraph 3:
[Note: As a result of operations on state, it can return ok or partial and set from_next == from and to_next != to. --end note]
Rationale:
The submitter believes that standard already provides an affirmative answer to the question. However, the current wording has induced a few library implementors to make the incorrect assumption that do_in()/do_out() always consume at least one internal character when they succeed.
The submitter also believes that the proposed resolution is not in conflict with the related issue 76. Moreover, by explicitly allowing operations on state to produce characters, a codecvt implementation may effectively implement N-to-M translations without violating the "one character at a time" principle described in such issue. On a side note, the footnote in the proposed resolution of issue 76 that informally rules out N-to-M translations for basic_filebuf should be removed if this issue is accepted as valid.
[ Kona (2007): The proposed resolution is to add a note. Since this is non-normative, the issue is editorial, but we believe that the note is correct. Proposed Disposition: NAD, Editorial ]
Section: 27.7.2.6.1 [ostream.formatted.reqmts] Status: NAD Submitter: Martin Sebor Opened: 2002-12-27 Last modified: 2009-07-14
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Discussion:
There is a contradiction in Formatted output about what bit is supposed to be set if the formatting fails. On sentence says it's badbit and another that it's failbit.
27.6.2.5.1, p1 says in the Common Requirements on Formatted output functions:
... If the generation fails, then the formatted output function
does setstate(ios::failbit), which might throw an exception.
27.6.2.5.2, p1 goes on to say this about Arithmetic Inserters:
... The formatting conversion occurs as if it performed the following code fragment:
bool failed =
use_facet<num_put<charT,ostreambuf_iterator<charT,traits>
> >
(getloc()).put(*this, *this, fill(), val). failed();
... If failed is true then does setstate(badbit) ...
The original intent of the text, according to Jerry Schwarz (see c++std-lib-10500), is captured in the following paragraph:
In general "badbit" should mean that the stream is unusable because of some underlying failure, such as disk full or socket closure; "failbit" should mean that the requested formatting wasn't possible because of some inconsistency such as negative widths. So typically if you clear badbit and try to output something else you'll fail again, but if you clear failbit and try to output something else you'll succeed.
In the case of the arithmetic inserters, since num_put cannot report failure by any means other than exceptions (in response to which the stream must set badbit, which prevents the kind of recoverable error reporting mentioned above), the only other detectable failure is if the iterator returned from num_put returns true from failed().
Since that can only happen (at least with the required iostream specializations) under such conditions as the underlying failure referred to above (e.g., disk full), setting badbit would seem to be the appropriate response (indeed, it is required in 27.6.2.5.2, p1). It follows that failbit can never be directly set by the arithmetic (it can only be set by the sentry object under some unspecified conditions).
The situation is different for other formatted output functions which can fail as a result of the streambuf functions failing (they may do so by means other than exceptions), and which are then required to set failbit.
The contradiction, then, is that ostream::operator<<(int) will set badbit if the disk is full, while operator<<(ostream&, char) will set failbit under the same conditions. To make the behavior consistent, the Common requirements sections for the Formatted output functions should be changed as proposed below.
[Kona: There's agreement that this is a real issue. What we decided at Kona: 1. An error from the buffer (which can be detected either directly from streambuf's member functions or by examining a streambuf_iterator) should always result in badbit getting set. 2. There should never be a circumstance where failbit gets set. That represents a formatting error, and there are no circumstances under which the output facets are specified as signaling a formatting error. (Even more so for string output that for numeric because there's nothing to format.) If we ever decide to make it possible for formatting errors to exist then the facets can signal the error directly, and that should go in clause 22, not clause 27. 3. The phrase "if generation fails" is unclear and should be eliminated. It's not clear whether it's intended to mean a buffer error (e.g. a full disk), a formatting error, or something else. Most people thought it was supposed to refer to buffer errors; if so, we should say so. Martin will provide wording.]
[ 2009-07 Frankfurt ]
NAD. This issue is already fixed.
Proposed resolution:
Rationale:
Section: 27.7.2.4 [ostream::sentry] Status: NAD Editorial Submitter: Martin Sebor Opened: 2003-01-05 Last modified: 2010-03-09
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Discussion:
17.4.4.8, p3 prohibits library dtors from throwing exceptions.
27.6.2.3, p4 says this about the ostream::sentry dtor:
-4- If ((os.flags() & ios_base::unitbuf) && !uncaught_exception())
is true, calls os.flush().
27.6.2.6, p7 that describes ostream::flush() says:
-7- If rdbuf() is not a null pointer, calls rdbuf()->pubsync().
If that function returns ?-1 calls setstate(badbit) (which
may throw ios_base::failure (27.4.4.3)).
That seems like a defect, since both pubsync() and setstate() can throw an exception.
[ The contradiction is real. Clause 17 says destructors may never throw exceptions, and clause 27 specifies a destructor that does throw. In principle we might change either one. We're leaning toward changing clause 17: putting in an "unless otherwise specified" clause, and then putting in a footnote saying the sentry destructor is the only one that can throw. PJP suggests specifying that sentry::~sentry() should internally catch any exceptions it might cause. ]
[ See 418 and 622 for related issues. ]
[ 2009-07 Frankfurt ]
Move to Review. Add "Throws: nothing" to the specification of ostream::sentry::~sentry().
[ 2009-10-13 Daniel adds: ]
The proposed resolution of 835 is written to match the outcome of this issue.
[ 2009 Santa Cruz: ]
Move to Open. Our intent is to solve this issue with 835.
[ 2010-03-06 Martin updates wording. ]
[ 2010 Pittsburgh: ]
Moved to NAD Editorial.
Rationale:
Solved by 835.
Proposed resolution:
Add after 27.7.2.4 [ostream::sentry] p17:
~sentry();-17- If (os.flags() & ios_base::unitbuf) is true, calls os.flush().
Throws: Nothing.
Section: 27.7.2.4 [ostream::sentry] Status: NAD Submitter: Martin Sebor Opened: 2003-01-05 Last modified: 2009-07-14
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Discussion:
While reviewing unformatted input member functions of istream for their behavior when they encounter end-of-file during input I found that the requirements vary, sometimes unexpectedly, and in more than one case even contradict established practice (GNU libstdc++ 3.2, IBM VAC++ 6.0, STLPort 4.5, SunPro 5.3, HP aCC 5.38, Rogue Wave libstd 3.1, and Classic Iostreams).
The following unformatted input member functions set eofbit if they encounter an end-of-file (this is the expected behavior, and also the behavior of all major implementations):
basic_istream<charT, traits>&
get (char_type*, streamsize, char_type);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
get (char_type*, streamsize);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
getline (char_type*, streamsize, char_type);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
getline (char_type*, streamsize);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
ignore (int, int_type);
basic_istream<charT, traits>&
read (char_type*, streamsize);
Also sets failbit if it encounters end-of-file.
streamsize readsome (char_type*, streamsize);
The following unformated input member functions set failbit but not eofbit if they encounter an end-of-file (I find this odd since the functions make it impossible to distinguish a general failure from a failure due to end-of-file; the requirement is also in conflict with all major implementation which set both eofbit and failbit):
int_type get();
basic_istream<charT, traits>&
get (char_type&);
These functions only set failbit of they extract no characters, otherwise they don't set any bits, even on failure (I find this inconsistency quite unexpected; the requirement is also in conflict with all major implementations which set eofbit whenever they encounter end-of-file):
basic_istream<charT, traits>&
get (basic_streambuf<charT, traits>&, char_type);
basic_istream<charT, traits>&
get (basic_streambuf<charT, traits>&);
This function sets no bits (all implementations except for STLport and Classic Iostreams set eofbit when they encounter end-of-file):
int_type peek ();
Informally, what we want is a global statement of intent saying that eofbit gets set if we trip across EOF, and then we can take away the specific wording for individual functions. A full review is necessary. The wording currently in the standard is a mishmash, and changing it on an individual basis wouldn't make things better. Dietmar will do this work.
[ 2009-07 Frankfurt ]
Moved to NAD. See 27.7.1.1 [istream] p3.
Proposed resolution:
Section: 27.7.1.3 [istream.unformatted] Status: NAD Submitter: Martin Sebor Opened: 2003-01-05 Last modified: 2006-12-27
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Discussion:
The Effects clauses for the two functions below violate the general requirements on unformatted input functions outlined in 27.6.1.3: they do not begin by constructing a sentry object. Instead, they begin by calling widen ('\n'), which may throw an exception. The exception is then allowed to propagate from the unformatted input function irrespective of the setting of exceptions().
Note that in light of 27.6.1.1, p3 and p4, the fact that the functions allow exceptions thrown from widen() to propagate may not strictly speaking be a defect (but the fact that the functions do not start by constructing a sentry object still is). However, since an exception thrown from ctype<charT> ::widen() during any other input operation (say, from within a call to num_get<charT>::get()) will be caught and cause badbit to be set, these two functions should not be treated differently for the sake of consistency.
Proposed resolution:
Rationale:
Not a defect. The standard is consistent, and the behavior required by the standard is unambiguous. Yes, it's theoretically possible for widen to throw. (Not that this will happen for the default ctype facet or for most real-world replacement ctype facets.) Users who define ctype facets that can throw, and who care about this behavior, can use alternative signatures that don't call widen.
Section: 24.2 [iterator.requirements] Status: NAD Editorial Submitter: Nathan Myers Opened: 2003-06-03 Last modified: 2010-03-14
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Discussion:
I've been discussing iterator semantics with Dave Abrahams, and a surprise has popped up. I don't think this has been discussed before.
X [iterator.concepts] says that the only operation that can be performed on "singular" iterator values is to assign a non-singular value to them. (It doesn't say they can be destroyed, and that's probably a defect.) Some implementations have taken this to imply that there is no need to initialize the data member of a reverse_iterator<> in the default constructor. As a result, code like
std::vector<std::reverse_iterator<char*> > v(7); v.reserve(1000);
invokes undefined behavior, because it must default-initialize the vector elements, and then copy them to other storage. Of course many other vector operations on these adapters are also left undefined, and which those are is not reliably deducible from the standard.
I don't think that 24.1 was meant to make standard-library iterator types unsafe. Rather, it was meant to restrict what operations may be performed by functions which take general user- and standard iterators as arguments, so that raw pointers would qualify as iterators. However, this is not clear in the text, others have come to the opposite conclusion.
One question is whether the standard iterator adaptors have defined copy semantics. Another is whether they have defined destructor semantics: is
{ std::vector<std::reverse_iterator<char*> > v(7); }
undefined too?
Note this is not a question of whether algorithms are allowed to rely on copy semantics for arbitrary iterators, just whether the types we actually supply support those operations. I believe the resolution must be expressed in terms of the semantics of the adapter's argument type. It should make clear that, e.g., the reverse_iterator<T> constructor is actually required to execute T(), and so copying is defined if the result of T() is copyable.
Issue 235, which defines reverse_iterator's default constructor more precisely, has some relevance to this issue. However, it is not the whole story.
The issue was whether
reverse_iterator() { }
is allowed, vs.
reverse_iterator() : current() { }
The difference is when T is char*, where the first leaves the member uninitialized, and possibly equal to an existing pointer value, or (on some targets) may result in a hardware trap when copied.
8.5 paragraph 5 seems to make clear that the second is required to satisfy DR 235, at least for non-class Iterator argument types.
But that only takes care of reverse_iterator, and doesn't establish a policy for all iterators. (The reverse iterator adapter was just an example.) In particular, does my function
template <typename Iterator>
void f() { std::vector<Iterator> v(7); }
evoke undefined behavior for some conforming iterator definitions? I think it does, now, because vector<> will destroy those singular iterator values, and that's explicitly disallowed.
24.1 shouldn't give blanket permission to copy all singular iterators, because then pointers wouldn't qualify as iterators. However, it should allow copying of that subset of singular iterator values that are default-initialized, and it should explicitly allow destroying any iterator value, singular or not, default-initialized or not.
[ We don't want to require all singular iterators to be copyable, because that is not the case for pointers. However, default construction may be a special case. Issue: is it really default construction we want to talk about, or is it something like value initialization? We need to check with core to see whether default constructed pointers are required to be copyable; if not, it would be wrong to impose so strict a requirement for iterators. ]
[ 2009-05-10 Alisdair provided wording. ]
The comments regarding destroying singular iterators have already been resolved. That just leaves copying (with moving implied).
[ 2009-07 Frankfurt ]
This is related to LWG 1012.
Note that there is a bug in the proposed resolution to LWG 1012. The change to [reverse.iter.con] should be modified so that the word "default" in the second sentence of the Effects clause is replaced by "value."
We believe that the proposed fix to LWG 1012 (now corrected) is sufficient to solve the problem for reverse_iterator. However, Alisdair pointed out that LWG 1012 does not solve the general problem for authors of iterator adaptors.
There are some problems with the proposed resolution. The phrase "safely copyable" is not a term of art. Also, it mentions a DefaultConstructible? concept.
Move to Review after Alisdair updates the wording.
[ 2009-07-31 Alisdair revised wording: ]
[ 2009-08-17 Alisdair and Daniel collaborate on slightly revised wording. This issue depends upon 724 ]
[ 2009-10-14 Daniel adds: ]
There is a clear dependency on 1213, because the term "singular", which is used as part of the resolution, is not properly defined yet.
[ 2009-10 Santa Cruz: ]
Moved to Open. Alisdair will provide improved wording to make this have "value semantics" and otherwise behave like a valid iterator.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3066.
Proposed resolution:
Add a new paragrpah to Iterator concepts 24.2 [iterator.requirements] after para 5 (the one describing singular iterators)
Just as a regular pointer to an array guarantees that there is a pointer value pointing past the last element of the array, so for any iterator type there is an iterator value that points past the last element of a corresponding container. These values are called past-the-end values. Values of an iterator i for which the expression *i is defined are called dereferenceable. The library never assumes that past-the-end values are dereferenceable. Iterators can also have singular values that are not associated with any container. [Example: After the declaration of an uninitialized pointer x (as with int* x;), x must always be assumed to have a singular value of a pointer. — end example] Results of most expressions are undefined for singular values; the only exceptions are destroying an iterator that holds a singular value and the assignment of a non-singular value to an iterator that holds a singular value. In this case the singular value is overwritten the same way as any other value. Dereferenceable values are always non-singular.
After value-initialization, any iterator that satisfies the DefaultConstructible requirements ([defaultconstructible]) shall not introduce undefined behaviour when used as the source of a copy or move operation, even if it would otherwise be singular. [Note: This guarantee is not offered for default-initialization (8.5 [dcl.init]), although the distinction only matters for types with trivial default constructors such as pointers. — end note]
Section: 22.4.1.1.2 [locale.ctype.virtuals] Status: NAD Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2009-07-14
View all other issues in [locale.ctype.virtuals].
View all issues with NAD status.
Discussion:
The Effects and Returns clauses of the do_widen() member function of the ctype facet fail to specify the behavior of the function on failure. That the function may not be able to simply cast the narrow character argument to the type of the result since doing so may yield the wrong value for some wchar_t encodings. Popular implementations of ctype<wchar_t> that use mbtowc() and UTF-8 as the native encoding (e.g., GNU glibc) will fail when the argument's MSB is set. There is no way for the the rest of locale and iostream to reliably detect this failure.
[Kona: This is a real problem. Widening can fail. It's unclear what the solution should be. Returning WEOF works for the wchar_t specialization, but not in general. One option might be to add a default, like narrow. But that's an incompatible change. Using traits::eof might seem like a good idea, but facets don't have access to traits (a recurring problem). We could have widen throw an exception, but that's a scary option; existing library components aren't written with the assumption that widen can throw.]
[ 2009-07 Frankfurt ]
NAD. The behavior is specified for all of the facets that an implementation is required to provide, for the basic character set.
Proposed resolution:
Section: 27.5.2.1.6 [ios::Init] Status: NAD Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2009-07-14
View all other issues in [ios::Init].
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Discussion:
The dtor of the ios_base::Init object is supposed to call flush() on the 6 standard iostream objects cout, cerr, clog, wcout, wcerr, and wclog. This call may cause an exception to be thrown.
17.4.4.8, p3 prohibits all library destructors from throwing exceptions.
The question is: What should this dtor do if one or more of these calls to flush() ends up throwing an exception? This can happen quite easily if one of the facets installed in the locale imbued in the iostream object throws.
[Kona: We probably can't do much better than what we've got, so the LWG is leaning toward NAD. At the point where the standard stream objects are being cleaned up, the usual error reporting mechanism are all unavailable. And exception from flush at this point will definitely cause problems. A quality implementation might reasonably swallow the exception, or call abort, or do something even more drastic.]
[ See 397 and 622 for related issues. ]
[ 2009-07 Frankfurt ]
Moved to NAD, no consensus for change.
Proposed resolution:
Section: 27.6.2.1 [streambuf.cons] Status: NAD Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2009-07-14
View all other issues in [streambuf.cons].
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Discussion:
The reflector thread starting with c++std-lib-11346 notes that the class template basic_streambuf, along with basic_stringbuf and basic_filebuf, is copy-constructible but that the semantics of the copy constructors are not defined anywhere. Further, different implementations behave differently in this respect: some prevent copy construction of objects of these types by declaring their copy ctors and assignment operators private, others exhibit undefined behavior, while others still give these operations well-defined semantics.
Note that this problem doesn't seem to be isolated to just the three types mentioned above. A number of other types in the library section of the standard provide a compiler-generated copy ctor and assignment operator yet fail to specify their semantics. It's believed that the only types for which this is actually a problem (i.e. types where the compiler-generated default may be inappropriate and may not have been intended) are locale facets. See issue 439.
[ 2009-07 Frankfurt ]
NAD. Option B is already in the Working Draft.
Proposed resolution:
27.5.2 [lib.streambuf]: Add into the synopsis, public section, just above the destructor declaration:
basic_streambuf(const basic_streambuf& sb); basic_streambuf& operator=(const basic_streambuf& sb);
Insert after 27.5.2.1, paragraph 2:
basic_streambuf(const basic_streambuf& sb);Constructs a copy of sb.
Postcondtions:
eback() == sb.eback() gptr() == sb.gptr() egptr() == sb.egptr() pbase() == sb.pbase() pptr() == sb.pptr() epptr() == sb.epptr() getloc() == sb.getloc()basic_streambuf& operator=(const basic_streambuf& sb);Assigns the data members of sb to this.
Postcondtions:
eback() == sb.eback() gptr() == sb.gptr() egptr() == sb.egptr() pbase() == sb.pbase() pptr() == sb.pptr() epptr() == sb.epptr() getloc() == sb.getloc()Returns: *this.
27.7.1 [lib.stringbuf]:
Option A:
Insert into the basic_stringbuf synopsis in the private section:
basic_stringbuf(const basic_stringbuf&); // not defined basic_stringbuf& operator=(const basic_stringbuf&); // not defined
Option B:
Insert into the basic_stringbuf synopsis in the public section:
basic_stringbuf(const basic_stringbuf& sb); basic_stringbuf& operator=(const basic_stringbuf& sb);27.7.1.1, insert after paragraph 4:
basic_stringbuf(const basic_stringbuf& sb);Constructs an independent copy of sb as if with sb.str(), and with the openmode that sb was constructed with.
Postcondtions:
str() == sb.str() gptr() - eback() == sb.gptr() - sb.eback() egptr() - eback() == sb.egptr() - sb.eback() pptr() - pbase() == sb.pptr() - sb.pbase() getloc() == sb.getloc()Note: The only requirement on epptr() is that it point beyond the initialized range if an output sequence exists. There is no requirement that epptr() - pbase() == sb.epptr() - sb.pbase().
basic_stringbuf& operator=(const basic_stringbuf& sb);After assignment the basic_stringbuf has the same state as if it were initially copy constructed from sb, except that the basic_stringbuf is allowed to retain any excess capacity it might have, which may in turn effect the value of epptr().
27.8.1.1 [lib.filebuf]
Insert at the bottom of the basic_filebuf synopsis:
private: basic_filebuf(const basic_filebuf&); // not defined basic_filebuf& operator=(const basic_filebuf&); // not defined
[Kona: this is an issue for basic_streambuf itself and for its derived classes. We are leaning toward allowing basic_streambuf to be copyable, and specifying its precise semantics. (Probably the obvious: copying the buffer pointers.) We are less sure whether the streambuf derived classes should be copyable. Howard will write up a proposal.]
[Sydney: Dietmar presented a new argument against basic_streambuf being copyable: it can lead to an encapsulation violation. Filebuf inherits from streambuf. Now suppose you inhert a my_hijacking_buf from streambuf. You can copy the streambuf portion of a filebuf to a my_hijacking_buf, giving you access to the pointers into the filebuf's internal buffer. Perhaps not a very strong argument, but it was strong enough to make people nervous. There was weak preference for having streambuf not be copyable. There was weak preference for having stringbuf not be copyable even if streambuf is. Move this issue to open for now. ]
[ 2007-01-12, Howard: Rvalue Reference Recommendations for Chapter 27 recommends protected copy constructor and assignment for basic_streambuf with the same semantics as would be generated by the compiler. These members aid in derived classes implementing move semantics. A protected copy constructor and copy assignment operator do not expose encapsulation more so than it is today as each data member of a basic_streambuf is already both readable and writable by derived classes via various get/set protected member functions (eback(), setp(), etc.). Rather a protected copy constructor and copy assignment operator simply make the job of derived classes implementing move semantics less tedious and error prone. ]
Rationale:
27.5.2 [lib.streambuf]: The proposed basic_streambuf copy constructor and assignment operator are the same as currently implied by the lack of declarations: public and simply copies the data members. This resolution is not a change but a clarification of the current standard.
27.7.1 [lib.stringbuf]: There are two reasonable options: A) Make basic_stringbuf not copyable. This is likely the status-quo of current implementations. B) Reasonable copy semantics of basic_stringbuf can be defined and implemented. A copyable basic_streambuf is arguably more useful than a non-copyable one. This should be considered as new functionality and not the fixing of a defect. If option B is chosen, ramifications from issue 432 are taken into account.
27.8.1.1 [lib.filebuf]: There are no reasonable copy semantics for basic_filebuf.
Section: 27 [input.output] Status: NAD Future Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2009-07-14
View all other issues in [input.output].
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Discussion:
A third party test suite tries to exercise istream::ignore(N) with a negative value of N and expects that the implementation will treat N as if it were 0. Our implementation asserts that (N >= 0) holds and aborts the test.
I can't find anything in section 27 that prohibits such values but I don't see what the effects of such calls should be, either (this applies to a number of unformatted input functions as well as some member functions of the basic_streambuf template).
[ 2009-07 Frankfurt ]
This is related to LWG 255.
Move to NAD Future.
Proposed resolution:
I propose that we add to each function in clause 27 that takes an argument, say N, of type streamsize a Requires clause saying that "N >= 0." The intent is to allow negative streamsize values in calls to precision() and width() but disallow it in calls to streambuf::sgetn(), istream::ignore(), or ostream::write().
[Kona: The LWG agreed that this is probably what we want. However, we need a review to find all places where functions in clause 27 take arguments of type streamsize that shouldn't be allowed to go negative. Martin will do that review.]
Section: 17.5.1.2 [structure.summary] Status: NAD Editorial Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2009-07-13
View all issues with NAD Editorial status.
Discussion:
The text in 17.3.1.1, p1 says:
"Paragraphs labelled "Note(s):" or "Example(s):" are informative, other
paragraphs are normative."
The library section makes heavy use of paragraphs labeled "Notes(s),"
some of which are clearly intended to be normative (see list 1), while
some others are not (see list 2). There are also those where the intent
is not so clear (see list 3).
List 1 -- Examples of (presumably) normative Notes:
20.9.5.1 [allocator.members], p3,
20.9.5.1 [allocator.members], p10,
21.4.2 [string.cons], p11,
22.3.1.2 [locale.cons], p11,
23.3.2.3 [deque.modifiers], p2,
25.4.7 [alg.min.max], p3,
26.4.6 [complex.ops], p15,
27.6.2.4.3 [streambuf.virt.get], p7.
List 2 -- Examples of (presumably) informative Notes:
18.6.1.3 [new.delete.placement], p3,
21.4.6.6 [string::replace], p14,
22.4.1.4.2 [locale.codecvt.virtuals], p3,
25.2.4 [alg.foreach], p4,
26.4.5 [complex.member.ops], p1,
27.5.2.5 [ios.base.storage], p6.
List 3 -- Examples of Notes that are not clearly either normative
or informative:
22.3.1.2 [locale.cons], p8,
22.3.1.5 [locale.statics], p6,
27.6.2.4.5 [streambuf.virt.put], p4.
None of these lists is meant to be exhaustive.
[Definitely a real problem. The big problem is there's material that doesn't quite fit any of the named paragraph categories (e.g. Effects). Either we need a new kind of named paragraph, or we need to put more material in unnamed paragraphs jsut after the signature. We need to talk to the Project Editor about how to do this. ]
[ Bellevue: Specifics of list 3: First 2 items correct in std (22.1.1.2, 22.1.1.5) Third item should be non-normative (27.5.2.4.5), which Pete will handle editorially. ]
[ post San Francisco: Howard: reopened, needs attention. ]
[Pete: I changed the paragraphs marked "Note" and "Notes" to use "Remark" and "Remarks". Fixed as editorial. This change has been in the WD since the post-Redmond mailing, in 2004. Recommend NAD.]
[ Batavia: We feel that the references in List 2 above should be changed from Remarks to Notes. We also feel that those items in List 3 need to be double checked for the same change. Alan and Pete to review. ]
[ Batavia (2009-05): ]
A spot-check of List 2 suggests the issue is still relevant, and a review of List 3 still seems called-for.
Move to NAD Editorial.
Proposed resolution:
Section: 27.5.4.3 [iostate.flags] Status: Dup Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2006-12-30
View all other issues in [iostate.flags].
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Duplicate of: 412
Discussion:
The Effects clause in 27.4.4.3, p5 describing the effects of a call to the ios_base member function clear(iostate state) says that the function only throws if the respective bits are already set prior to the function call. That's obviously not the intent. If it was, a call to clear(badbit) on an object for which (rdstate() == goodbit && exceptions() == badbit) holds would not result in an exception being thrown.
Proposed resolution:
The text ought to be changed from
"If (rdstate() & exceptions()) == 0, returns. ..."
to
"If (state & exceptions()) == 0, returns. ..."
Rationale:
Section: 20.2.5 [allocator.requirements], 25 [algorithms] Status: NAD Editorial Submitter: Matt Austern Opened: 2003-09-20 Last modified: 2009-10-26
View all other issues in [allocator.requirements].
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Discussion:
Clause 20.2.5 [allocator.requirements] paragraph 4 says that implementations are permitted to supply containers that are unable to cope with allocator instances and that container implementations may assume that all instances of an allocator type compare equal. We gave implementers this latitude as a temporary hack, and eventually we want to get rid of it. What happens when we're dealing with allocators that don't compare equal?
In particular: suppose that v1 and v2 are both objects of type vector<int, my_alloc> and that v1.get_allocator() != v2.get_allocator(). What happens if we write v1.swap(v2)? Informally, three possibilities:
1. This operation is illegal. Perhaps we could say that an implementation is required to check and to throw an exception, or perhaps we could say it's undefined behavior.
2. The operation performs a slow swap (i.e. using three invocations of operator=, leaving each allocator with its original container. This would be an O(N) operation.
3. The operation swaps both the vectors' contents and their allocators. This would be an O(1) operation. That is:
my_alloc a1(...);
my_alloc a2(...);
assert(a1 != a2);
vector<int, my_alloc> v1(a1);
vector<int, my_alloc> v2(a2);
assert(a1 == v1.get_allocator());
assert(a2 == v2.get_allocator());
v1.swap(v2);
assert(a1 == v2.get_allocator());
assert(a2 == v1.get_allocator());
[Kona: This is part of a general problem. We need a paper saying how to deal with unequal allocators in general.]
[pre-Sydney: Howard argues for option 3 in N1599. ]
[ 2007-01-12, Howard: This issue will now tend to come up more often with move constructors and move assignment operators. For containers, these members transfer resources (i.e. the allocated memory) just like swap. ]
[ Batavia: There is agreement to overload the container swap on the allocator's Swappable requirement using concepts. If the allocator supports Swappable, then container's swap will swap allocators, else it will perform a "slow swap" using copy construction and copy assignment. ]
[ 2009-04-28 Pablo adds: ]
Fixed in N2525. I argued for marking this Tentatively-Ready right after Bellevue, but there was a concern that N2525 would break in the presence of the RVO. (That breakage had nothing to do with swap, but never-the-less). I addressed that breakage in in N2840 (Summit) by means of a non-normative reference:[Note: in situations where the copy constructor for a container is elided, this function is not called. The behavior in these cases is as if select_on_container_copy_construction returned x — end note]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Addressed by N2982.
Proposed resolution:
Section: 18.8.2.4 [unexpected] Status: NAD Submitter: Vyatcheslav Sysoltsev Opened: 2003-09-29 Last modified: 2006-12-27
View all issues with NAD status.
Discussion:
Clause 15.5.2 [except.unexpected] paragraph 1 says that "void unexpected(); is called (18.7.2) immediately after completing the stack unwinding for the former function", but 18.7.2.4 (Effects) says that "void unexpected(); . . . Calls the unexpected_handler function in effect immediately after evaluating the throwexpression (18.7.2.2),". Isn't here a contradiction: 15.5.2 requires stack have been unwound when in void unexpected() and therefore in unexpected_handler but 18.7.2.4 claims that unexpected_handler is called "in effect immediately" after evaluation of throw expression is finished, so there is no space left for stack to be unwound therefore? I think the phrase "in effect immediately" should be removed from the standard because it brings ambiguity in understanding.
Proposed resolution:
Rationale:
There is no contradiction. The phrase "in effect immediately" is just to clarify which handler is to be called.
Section: 27.7.2.6.2 [ostream.inserters.arithmetic] Status: NAD Submitter: Ivan Godard Opened: 2003-10-24 Last modified: 2006-12-27
View all other issues in [ostream.inserters.arithmetic].
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Discussion:
Given:
void f(int) {}
void(*g)(int) = f;
cout << g;
(with the expected #include and usings), the value printed is a rather surprising "true". Rather useless too.
The standard defines:
ostream& operator<<(ostream&, void*);
which picks up all data pointers and prints their hex value, but does not pick up function pointers because there is no default conversion from function pointer to void*. Absent that, we fall back to legacy conversions from C and the function pointer is converted to bool.
There should be an analogous inserter that prints the address of a function pointer.
Proposed resolution:
Rationale:
This is indeed a wart, but there is no good way to solve it. C doesn't provide a portable way of outputting the address of a function point either.
Section: 22.4 [locale.categories] Status: NAD Submitter: Matt Austern Opened: 2003-11-02 Last modified: 2006-12-27
View all other issues in [locale.categories].
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Discussion:
The following facets classes have no copy constructors described in the standard, which, according to the standard, means that they are supposed to use the compiler-generated defaults. Default copy behavior is probably inappropriate. We should either make these classes uncopyable or else specify exactly what their constructors do.
Related issue: 421.
ctype_base
ctype
ctype_byname
ctype<char>
ctype_byname<char>
codecvt_base
codecvt
codecvt_byname
num_get
num_put
numpunct
numpunct_byname
collate
collate_byname
time_base
time_get
time_get_byname
time_put
time_put_byname
money_get
money_put
money_base
moneypunct
moneypunct_byname
messages_base
messages
messages_byname
Proposed resolution:
Rationale:
The copy constructor in the base class is private.
Section: 26.4.8 [complex.transcendentals] Status: NAD Submitter: Matt Austern Opened: 2003-11-05 Last modified: 2009-03-21
View all issues with NAD status.
Discussion:
Operations like pow and exp on complex<T> are typically implemented in terms of operations like sin and cos on T. Should implementations write this as std::sin, or as plain unqualified sin?
The issue, of course, is whether we want to use argument-dependent lookup in the case where T is a user-defined type. This is similar to the issue of valarray transcendentals, as discussed in issue 226.
This issue differs from valarray transcendentals in two important ways. First, "the effect of instantiating the template complex for types other than float, double or long double is unspecified." (26.4.1 [complex.syn]) Second, the standard does not dictate implementation, so there is no guarantee that a particular real math function is used in the implementation of a particular complex function.
Proposed resolution:
Rationale:
If you instantiate std::complex for user-defined types, all bets are off.
Section: 24.2 [iterator.requirements], 23.2 [container.requirements] Status: NAD Editorial Submitter: Andy Koenig Opened: 2003-12-16 Last modified: 2010-03-14
View all other issues in [iterator.requirements].
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Discussion:
What requirements does the standard place on equality comparisons between iterators that refer to elements of different containers. For example, if v1 and v2 are empty vectors, is v1.end() == v2.end() allowed to yield true? Is it allowed to throw an exception?
The standard appears to be silent on both questions.
[Sydney: The intention is that comparing two iterators from different containers is undefined, but it's not clear if we say that, or even whether it's something we should be saying in clause 23 or in clause 24. Intuitively we might want to say that equality is defined only if one iterator is reachable from another, but figuring out how to say it in any sensible way is a bit tricky: reachability is defined in terms of equality, so we can't also define equality in terms of reachability. ]
[ 2009-07 Frankfurt ]
Daniel volunteered to work on this.
[ 2009-09-20 Daniel provided wording. ]
[ 2009-10 Santa Cruz: ]
Leave as Open. Alisdair has volunteered to refine the wording.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3066.
Proposed resolution:
Insert a new paragraph between 24.2 [iterator.requirements]/7+8:
[..] The result of the application of functions in the library to invalid ranges is undefined.
The result of directly or indirectly evaluating any comparison function or the binary - operator with two iterator values as arguments that were obtained from two different ranges r1 and r2 (including their past-the-end values) which are not subranges of one common range is undefined, unless explicitly described otherwise.
Section: 22.3.1.1.1 [locale.category] Status: Dup Submitter: Pete Becker Opened: 2003-12-26 Last modified: 2007-01-15
View all other issues in [locale.category].
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Duplicate of: 327
Discussion:
22.1.1.1.1/4, table 52, "Required Instantiations", lists, among others:
time_get<char,InputIterator>
time_get_byname<char,InputIterator>
time_get<wchar_t,OutputIterator>
time_get_byname<wchar_t,OutputIterator>
The second argument to the last two should be InputIterator, not OutputIterator.
Proposed resolution:
Change the second template argument to InputIterator.
Rationale:
Section: 23.4.3 [set] Status: Dup Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2009-05-01
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Duplicate of: 214
Discussion:
map/multimap have:
iterator find(const key_type& x) const;
const_iterator find(const key_type& x) const;
which is consistent with the table of associative container requirements. But set/multiset have:
iterator find(const key_type&) const;
set/multiset should look like map/multimap, and honor the requirements table, in this regard.
Proposed resolution:
Rationale:
Section: 23.2.4 [associative.reqmts], 23.4 [associative] Status: Dup Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2009-05-01
View other active issues in [associative.reqmts].
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Duplicate of: 130
Discussion:
map/multimap/set/multiset have:
void erase(iterator);
void erase(iterator, iterator);
But there's no good reason why these can't return an iterator, as for vector/deque/list:
iterator erase(iterator);
iterator erase(iterator, iterator);
Proposed resolution:
Informally: The table of associative container requirements, and the relevant template classes, should return an iterator designating the first element beyond the erased subrange.
Rationale:
Section: 22.3.1.3 [locale.members] Status: NAD Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2009-05-01
View all other issues in [locale.members].
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Discussion:
template<class Facet>
locale::combine(const locale&) const;
is obliged to create a locale that has no name. This is overspecification and overkill. The resulting locale should follow the usual rules -- it has a name if the locale argument has a name and Facet is one of the standard facets.
[ Sydney and post-Sydney (see c++std-lib-13439, c++std-lib-13440, c++std-lib-13443): agreed that it's overkill to say that the locale is obligated to be nameless. However, we also can't require it to have a name. At the moment, locale names are based on categories and not on individual facets. If a locale contains two different facets of different names from the same category, then this would not fit into existing naming schemes. We need to give implementations more freedom. Bill will provide wording. ]
Rationale:
After further discussion the LWG decided to close this as NAD. The fundamental problem is that names right now are per-category, not per-facet. The combine member function works at the wrong level of granularity.
Section: 27.9.1.4 [filebuf.members] Status: NAD Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2009-05-01
View all other issues in [filebuf.members].
View all issues with NAD status.
Duplicate of: 105
Discussion:
basic_filebuf *basic_filebuf::open(const char *, ios_base::open_mode);
should be supplemented with the overload:
basic_filebuf *basic_filebuf::open(const wchar_t *, ios_base::open_mode);
Depending on the operating system, one of these forms is fundamental and the other requires an implementation-defined mapping to determine the actual filename.
[Sydney: Yes, we want to allow wchar_t filenames. Bill will provide wording.]
[ In Toronto we noted that this is issue 5 from N1569. ]
How does this interact with the newly-defined character types, and how do we avoid interface explosion considering std::string overloads that were added? Propose another solution that is different than the suggestion proposed by PJP.
Suggestion is to make a member template function for basic_string (for char, wchar_t, u16char, u32char instantiations), and then just keep a const char* member.
Goal is to do implicit conversion between character string literals to appropriate basic_string type. Not quite sure if this is possible.
Implementors are free to add specific overloads for non-char character types.
[ Martin adds pre-Sophia Antipolis: ]
Please see issue 454: problems and solutions.
[ Sophia Antipolis: ]
Beman is concerned that making these changes to basic_filebuf is not usefully changed unless fstream is also changed; this also only handles wchar_t and not other character types.
The TR2 filesystem library is a more complete solution, but is not available soon.
[ Martin adds: please reference N2683 for problems and solutions. ]
Proposed resolution:
Change from:
basic_filebuf<charT,traits>* open( const char* s, ios_base::openmode mode );Effects: If is_open() != false, returns a null pointer. Otherwise, initializes the filebuf as required. It then opens a file, if possible, whose name is the NTBS s ("as if" by calling std::fopen(s,modstr)).
to:
basic_filebuf<charT,traits>* open( const char* s, ios_base::openmode mode ); basic_filebuf<charT,traits>* open( const wchar_t* ws, ios_base::openmode mode );Effects: If is_open() != false, returns a null pointer. Otherwise, initializes the filebuf as required. It then opens a file, if possible, whose name is the NTBS s ("as if" by calling std::fopen(s,modstr)). For the second signature, the NTBS s is determined from the WCBS ws in an implementation-defined manner.
(NOTE: For a system that "naturally" represents a filename as a WCBS, the NTBS s in the first signature may instead be mapped to a WCBS; if so, it follows the same mapping rules as the first argument to open.)
Rationale:
Slightly controversial, but by a 7-1 straw poll the LWG agreed to move this to Ready. The controversy was because the mapping between wide names and files in a filesystem is implementation defined. The counterargument, which most but not all LWG members accepted, is that the mapping between narrow files names and files is also implemenation defined.
[Lillehammer: Moved back to "open" status, at Beman's urging. (1) Why just basic_filebuf, instead of also basic_fstream (and possibly other things too). (2) Why not also constructors that take std::basic_string? (3) We might want to wait until we see Beman's filesystem library; we might decide that it obviates this.]
[ post Bellevue: ]
Move again to Ready.
There is a timing issue here. Since the filesystem library will not be in C++0x, this should be brought forward. This solution would remain valid in the context of the proposed filesystem.
This issue has been kicking around for a while, and the wchar_t addition alone would help many users. Thus, we suggest putting this on the reflector list with an invitation for someone to produce proposed wording that covers basic_fstream. In the meantime, we suggest that the proposed wording be adopted as-is.
If more of the Lillehammer questions come back, they should be introduced as separate issues.
[ San Francisco: ]
Some existing implementations provide overload already. Expected filesystem "path" object overloads neatly, without surprises; implying NAD.
Section: 24.2.7 [random.access.iterators] Status: NAD Submitter: Daniel Frey Opened: 2004-02-27 Last modified: 2009-10-26
View all other issues in [random.access.iterators].
View all issues with NAD status.
Discussion:
In 24.1.5 [lib.random.access.iterators], table 76 the operational semantics for the expression "r -= n" are defined as "return r += -n". This means, that the expression -n must be valid, which is not the case for unsigned types.
[ Sydney: Possibly not a real problem, since difference type is required to be a signed integer type. However, the wording in the standard may be less clear than we would like. ]
[ Post Summit Alisdair adds: ]
This issue refers to a requirements table we have removed.
The issue might now relate to 24.2.7 [random.access.iterators] p5. However, the rationale in the issue already recognises that the difference_type must be signed, so this really looks NAD.
[ Batavia (2009-05): ]
We agree with Alisdair's observations.
Move to NAD.
[ 2009-07 Frankfurt: ]
Need to look at again without concepts.
There was a question about this phrase in the discussion: "the expression -n must be valid, which is not the case for unsigned types." If n is an object ofthe iterator difference_type (eg ptrdiff_t), then it is never unsigned.
[ 2009-10 Santa Cruz: ]
The group reviewed the wording in the draft and agreed that n is of difference type, the difference type is signed, and the current wording is correct. Moved to NAD.
Proposed resolution:
To remove this limitation, I suggest to change the operational semantics for this column to:
{ Distance m = n;
if (m >= 0)
while (m--) --r;
else
while (m++) ++r;
return r; }
Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: NAD Submitter: Martin Sebor Opened: 2004-03-16 Last modified: 2009-07-14
View all other issues in [facet.num.get.virtuals].
View all issues with NAD status.
Discussion:
When parsing strings of wide-character digits, the standard requires the library to widen narrow-character "atoms" and compare the widened atoms against the characters that are being parsed. Simply narrowing the wide characters would be far simpler, and probably more efficient. The two choices are equivalent except in convoluted test cases, and many implementations already ignore the standard and use narrow instead of widen.
First, I disagree that using narrow() instead of widen() would necessarily have unfortunate performance implications. A possible implementation of narrow() that allows num_get to be implemented in a much simpler and arguably comparably efficient way as calling widen() allows, i.e. without making a virtual call to do_narrow every time, is as follows:
inline char ctype<wchar_t>::narrow (wchar_t wc, char dflt) const
{
const unsigned wi = unsigned (wc);
if (wi > UCHAR_MAX)
return typeid (*this) == typeid (ctype<wchar_t>) ?
dflt : do_narrow (wc, dflt);
if (narrow_ [wi] < 0) {
const char nc = do_narrow (wc, dflt);
if (nc == dflt)
return dflt;
narrow_ [wi] = nc;
}
return char (narrow_ [wi]);
}
Second, I don't think the change proposed in the issue (i.e., to use narrow() instead of widen() during Stage 2) would be at all drastic. Existing implementations with the exception of libstdc++ currently already use narrow() so the impact of the change on programs would presumably be isolated to just a single implementation. Further, since narrow() is not required to translate alternate wide digit representations such as those mentioned in issue 303 to their narrow equivalents (i.e., the portable source characters '0' through '9'), the change does not necessarily imply that these alternate digits would be treated as ordinary digits and accepted as part of numbers during parsing. In fact, the requirement in 22.4.1.1.2 [locale.ctype.virtuals], p13 forbids narrow() to translate an alternate digit character, wc, to an ordinary digit in the basic source character set unless the expression (ctype<charT>::is(ctype_base::digit, wc) == true) holds. This in turn is prohibited by the C standard (7.25.2.1.5, 7.25.2.1.5, and 5.2.1, respectively) for charT of either char or wchar_t.
[Sydney: To a large extent this is a nonproblem. As long as you're only trafficking in char and wchar_t we're only dealing with a stable character set, so you don't really need either 'widen' or 'narrow': can just use literals. Finally, it's not even clear whether widen-vs-narrow is the right question; arguably we should be using codecvt instead.]
[ 2009-07 Frankfurt ]
NAD. The standard is clear enough as written.
Proposed resolution:
Change stage 2 so that implementations are permitted to use either technique to perform the comparison:
Section: 3.6.3 [basic.start.term], 18.4 [cstdint] Status: NAD Submitter: Bill Plauger Opened: 2004-03-23 Last modified: 2008-02-25
View all issues with NAD status.
Discussion:
3.6.3 Termination spells out in detail the interleaving of static destructor calls and calls to functions registered with atexit. To match this behavior requires intimate cooperation between the code that calls destructors and the exit/atexit machinery. The former is tied tightly to the compiler; the latter is a primitive mechanism inherited from C that traditionally has nothing to do with static construction and destruction. The benefits of intermixing destructor calls with atexit handler calls is questionable at best, and very difficult to get right, particularly when mixing third-party C++ libraries with different third-party C++ compilers and C libraries supplied by still other parties.
I believe the right thing to do is defer all static destruction until after all atexit handlers are called. This is a change in behavior, but one that is likely visible only to perverse test suites. At the very least, we should permit deferred destruction even if we don't require it.
[If this is to be changed, it should probably be changed by CWG. At this point, however, the LWG is leaning toward NAD. Implementing what the standard says is hard work, but it's not impossible and most vendors went through that pain years ago. Changing this behavior would be a user-visible change, and would break at least one real application.]
[ Batavia: Send to core with our recommendation that we should permit deferred destruction but not require it. ]
[ Howard: The course of action recommended in Batavia would undo LWG issue 3 and break current code implementing the "phoenix singleton". Search the net for "phoenix singleton atexit" to get a feel for the size of the adverse impact this change would have. Below is sample code which implements the phoenix singleton and would break if atexit is changed in this way: ]
#include <cstdlib>
#include <iostream>
#include <type_traits>
#include <new>
class A
{
bool alive_;
A(const A&);
A& operator=(const A&);
public:
A() : alive_(true) {std::cout << "A()\n";}
~A() {alive_ = false; std::cout << "~A()\n";}
void use()
{
if (alive_)
std::cout << "A is alive\n";
else
std::cout << "A is dead\n";
}
};
void deallocate_resource();
// This is the phoenix singleton pattern
A& get_resource(bool create = true)
{
static std::aligned_storage<sizeof(A), std::alignment_of<A>::value>::type buf;
static A* a;
if (create)
{
if (a != (A*)&buf)
{
a = ::new (&buf) A;
std::atexit(deallocate_resource);
}
}
else
{
a->~A();
a = (A*)&buf + 1;
}
return *a;
}
void deallocate_resource()
{
get_resource(false);
}
void use_A(const char* message)
{
A& a = get_resource();
std::cout << "Using A " << message << "\n";
a.use();
}
struct B
{
~B() {use_A("from ~B()");}
};
B b;
int main()
{
use_A("from main()");
}
The correct output is:
A() Using A from main() A is alive ~A() A() Using A from ~B() A is alive ~A()
[ Bellevue: Confirmed no interaction with quick_exit. Strong feeling against mandating the change. Leaning towards NAD rather than permitting the change, as this would make common implementations of pheonix-singleton pattern implementation defined, as noted by Howard. Bill agrees issue is no longer serious, and accepts NAD. ]
Proposed resolution:
Section: D.10.1 [auto.ptr] Status: NAD Submitter: Rani Sharoni Opened: 2003-12-07 Last modified: 2009-10-26
View all other issues in [auto.ptr].
View all issues with NAD status.
Discussion:
TC1 CWG DR #84 effectively made the template<class Y> operator auto_ptr<Y>() member of auto_ptr (20.4.5.3/4) obsolete.
The sole purpose of this obsolete conversion member is to enable copy initialization base from r-value derived (or any convertible types like cv-types) case:
#include <memory>
using std::auto_ptr;
struct B {};
struct D : B {};
auto_ptr<D> source();
int sink(auto_ptr<B>);
int x1 = sink( source() ); // #1 EDG - no suitable copy constructor
The excellent analysis of conversion operations that was given in the final auto_ptr proposal (http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/1997/N1128.pdf) explicitly specifies this case analysis (case 4). DR #84 makes the analysis wrong and actually comes to forbid the loophole that was exploited by the auto_ptr designers.
I didn't encounter any compliant compiler (e.g. EDG, GCC, BCC and VC) that ever allowed this case. This is probably because it requires 3 user defined conversions and in fact current compilers conform to DR #84.
I was surprised to discover that the obsolete conversion member actually has negative impact of the copy initialization base from l-value derived case:
auto_ptr<D> dp; int x2 = sink(dp); // #2 EDG - more than one user-defined conversion applies
I'm sure that the original intention was allowing this initialization using the template<class Y> auto_ptr(auto_ptr<Y>& a) constructor (20.4.5.1/4) but since in this copy initialization it's merely user defined conversion (UDC) and the obsolete conversion member is UDC with the same rank (for the early overloading stage) there is an ambiguity between them.
Removing the obsolete member will have impact on code that explicitly invokes it:
int y = sink(source().operator auto_ptr<B>());
IMHO no one ever wrote such awkward code and the reasonable workaround for #1 is:
int y = sink( auto_ptr<B>(source()) );
I was even more surprised to find out that after removing the obsolete conversion member the initialization was still ill-formed: int x3 = sink(dp); // #3 EDG - no suitable copy constructor
This copy initialization semantically requires copy constructor which means that both template conversion constructor and the auto_ptr_ref conversion member (20.4.5.3/3) are required which is what was explicitly forbidden in DR #84. This is a bit amusing case in which removing ambiguity results with no candidates.
I also found exception safety issue with auto_ptr related to auto_ptr_ref:
int f(auto_ptr<B>, std::string); auto_ptr<B> source2(); // string constructor throws while auto_ptr_ref // "holds" the pointer int x4 = f(source2(), "xyz"); // #4
The theoretic execution sequence that will cause a leak:
According to 20.4.5.3/3 and 20.4.5/2 the auto_ptr_ref conversion member returns auto_ptr_ref<Y> that holds *this and this is another defect since the type of *this is auto_ptr<X> where X might be different from Y. Several library vendors (e.g. SGI) implement auto_ptr_ref<Y> with Y* as member which is much more reasonable. Other vendor implemented auto_ptr_ref as defectively required and it results with awkward and catastrophic code: int oops = sink(auto_ptr<B>(source())); // warning recursive on all control paths
Dave Abrahams noticed that there is no specification saying that auto_ptr_ref copy constructor can't throw.
My proposal comes to solve all the above issues and significantly simplify auto_ptr implementation. One of the fundamental requirements from auto_ptr is that it can be constructed in an intuitive manner (i.e. like ordinary pointers) but with strict ownership semantics which yield that source auto_ptr in initialization must be non-const. My idea is to add additional constructor template with sole propose to generate ill-formed, diagnostic required, instance for const auto_ptr arguments during instantiation of declaration. This special constructor will not be instantiated for other types which is achievable using 14.8.2/2 (SFINAE). Having this constructor in hand makes the constructor template<class Y> auto_ptr(auto_ptr<Y> const&) legitimate since the actual argument can't be const yet non const r-value are acceptable.
This implementation technique makes the "private auxiliary class" auto_ptr_ref obsolete and I found out that modern C++ compilers (e.g. EDG, GCC and VC) consume the new implementation as expected and allow all intuitive initialization and assignment cases while rejecting illegal cases that involve const auto_ptr arguments.
The proposed auto_ptr interface:
namespace std {
template<class X> class auto_ptr {
public:
typedef X element_type;
// 20.4.5.1 construct/copy/destroy:
explicit auto_ptr(X* p=0) throw();
auto_ptr(auto_ptr&) throw();
template<class Y> auto_ptr(auto_ptr<Y> const&) throw();
auto_ptr& operator=(auto_ptr&) throw();
template<class Y> auto_ptr& operator=(auto_ptr<Y>) throw();
~auto_ptr() throw();
// 20.4.5.2 members:
X& operator*() const throw();
X* operator->() const throw();
X* get() const throw();
X* release() throw();
void reset(X* p=0) throw();
private:
template<class U>
auto_ptr(U& rhs, typename
unspecified_error_on_const_auto_ptr<U>::type = 0);
};
}
One compliant technique to implement the unspecified_error_on_const_auto_ptr helper class is using additional private auto_ptr member class template like the following:
template<typename T> struct unspecified_error_on_const_auto_ptr;
template<typename T>
struct unspecified_error_on_const_auto_ptr<auto_ptr<T> const>
{ typedef typename auto_ptr<T>::const_auto_ptr_is_not_allowed type; };
There are other techniques to implement this helper class that might work better for different compliers (i.e. better diagnostics) and therefore I suggest defining its semantic behavior without mandating any specific implementation. IMO, and I didn't found any compiler that thinks otherwise, 14.7.1/5 doesn't theoretically defeat the suggested technique but I suggest verifying this with core language experts.
Further changes in standard text:
Remove section 20.4.5.3
Change 20.4.5/2 to read something like: Initializing auto_ptr<X> from const auto_ptr<Y> will result with unspecified ill-formed declaration that will require unspecified diagnostic.
Change 20.4.5.1/4,5,6 to read:
template<class Y> auto_ptr(auto_ptr<Y> const& a) throw();
4 Requires: Y* can be implicitly converted to X*.
5 Effects: Calls const_cast<auto_ptr<Y>&>(a).release().
6 Postconditions: *this holds the pointer returned from a.release().
Change 20.4.5.1/10
template<class Y> auto_ptr& operator=(auto_ptr<Y> a) throw();
10 Requires: Y* can be implicitly converted to X*. The expression delete get() is well formed.
LWG TC DR #127 is obsolete.
Notice that the copy constructor and copy assignment operator should remain as before and accept non-const auto_ptr& since they have effect on the form of the implicitly declared copy constructor and copy assignment operator of class that contains auto_ptr as member per 12.8/5,10:
struct X {
// implicit X(X&)
// implicit X& operator=(X&)
auto_ptr<D> aptr_;
};
In most cases this indicates about sloppy programming but preserves the current auto_ptr behavior.
Dave Abrahams encouraged me to suggest fallback implementation in case that my suggestion that involves removing of auto_ptr_ref will not be accepted. In this case removing the obsolete conversion member to auto_ptr<Y> and 20.4.5.3/4,5 is still required in order to eliminate ambiguity in legal cases. The two constructors that I suggested will co exist with the current members but will make auto_ptr_ref obsolete in initialization contexts. auto_ptr_ref will be effective in assignment contexts as suggested in DR #127 and I can't see any serious exception safety issues in those cases (although it's possible to synthesize such). auto_ptr_ref<X> semantics will have to be revised to say that it strictly holds pointer of type X and not reference to an auto_ptr for the favor of cases in which auto_ptr_ref<Y> is constructed from auto_ptr<X> in which X is different from Y (i.e. assignment from r-value derived to base).
[Redmond: punt for the moment. We haven't decided yet whether we want to fix auto_ptr for C++-0x, or remove it and replace it with move_ptr and unique_ptr.]
[ Oxford 2007: Recommend NAD. We're just going to deprecate it. It still works for simple use cases and people know how to deal with it. Going forward unique_ptr is the recommended tool. ]
[ 2007-11-09: Reopened at the request of David Abrahams, Alisdair Meredith and Gabriel Dos Reis. ]
[ 2009-07 Frankfurt ]
This is a complicated issue, so we agreed to defer discussion until later in the week so that interested parties can read up on it.
[ 209-10-04 Daniel adds: ]
I suggest to close this issue as NAD. The reasons are two-fold: First, the suggested proposed resolution uses no longer appropriate language means to solve this issue, which has the effect that the recommended resolution is another - but better - form of hack. Second, either following the suggested resolution or the now more natural alternative via the added member set
template<class Y> auto_ptr(auto_ptr<Y>&&) throw(); template<class Y> auto_ptr& operator=(auto_ptr<Y>&&) throw();would still have a non-zero probability to break user-code that actively references auto_ptr_ref. This risk seems to indicate that a decision which would not touch the current spec of auto_ptr at all (but deprecating it) and instead recommending to use unique_ptr for new code instead might have the best cost-benefit ratio. IMO the current solution of 1100 can be considered as an active user-support for this transition.
[ 2009-10 Santa Cruz: ]
Mark as NAD. Alisdair will open a new issue (1247) with proposed wording to handle auto_ptr_ref.
Proposed resolution:
Change the synopsis in D.10.1 [auto.ptr]:
namespace std {
template <class Y> struct auto_ptr_ref {};
// exposition only
template <class T> struct constant_object;
// exposition only
template <class T>
struct cannot_transfer_ownership_from
: constant_object<T> {};
template <class X> class auto_ptr {
public:
typedef X element_type;
// D.9.1.1 construct/copy/destroy:
explicit auto_ptr(X* p =0) throw();
auto_ptr(auto_ptr&) throw();
template<class Y> auto_ptr(auto_ptr<Y> const&) throw();
auto_ptr& operator=(auto_ptr&) throw();
template<class Y> auto_ptr& operator=(auto_ptr<Y>&) throw();
auto_ptr& operator=(auto_ptr_ref<X> r) throw();
~auto_ptr() throw();
// D.9.1.2 members:
X& operator*() const throw();
X* operator->() const throw();
X* get() const throw();
X* release() throw();
void reset(X* p =0) throw();
// D.9.1.3 conversions:
auto_ptr(auto_ptr_ref<X>) throw();
template<class Y> operator auto_ptr_ref<Y>() throw();
template<class Y> operator auto_ptr<Y>() throw();
// exposition only
template<class U>
auto_ptr(U& rhs, typename cannot_transfer_ownership_from<U>::error = 0);
};
template <> class auto_ptr<void>
{
public:
typedef void element_type;
};
}
Remove D.10.1.3 [auto.ptr.conv].
Change D.10.1 [auto.ptr], p3:
The auto_ptr provides a semantics of strict ownership. An auto_ptr owns the object it holds a pointer to. Copying an auto_ptr copies the pointer and transfers ownership to the destination. If more than one auto_ptr owns the same object at the same time the behavior of the program is undefined. Templates constant_object and cannot_transfer_ownership_from, and the final constructor of auto_ptr are for exposition only. For any types X and Y, initializing auto_ptr<X> from const auto_ptr<Y> is ill-formed, diagnostic required. [Note: The uses of auto_ptr include providing temporary exception-safety for dynamically allocated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function. auto_ptr does not meet the CopyConstructible and Assignable requirements for Standard Library container elements and thus instantiating a Standard Library container with an auto_ptr results in undefined behavior. -- end note]
Change D.10.1.1 [auto.ptr.cons], p5:
template<class Y> auto_ptr(auto_ptr<Y> const& a) throw();Requires: Y* can be implicitly converted to X*.
Effects: Calls const_cast<auto_ptr<Y>&>(a).release().
Postconditions: *this holds the pointer returned from a.release().
Change D.10.1.1 [auto.ptr.cons], p10:
template<class Y> auto_ptr& operator=(auto_ptr<Y>&a) throw();Requires: Y* can be implicitly converted to X*. The expression delete get() is well formed.
Effects: Calls reset(a.release()).
Returns: *this.
Section: 21.4.1 [string.require] Status: NAD Submitter: Daniel Frey Opened: 2004-06-10 Last modified: 2009-07-14
View all other issues in [string.require].
View all issues with NAD status.
Discussion:
Today, my colleagues and me wasted a lot of time. After some time, I found the problem. It could be reduced to the following short example:
#include <string>
int main() { std::string( 0 ); }
The problem is that the tested compilers (GCC 2.95.2, GCC 3.3.1 and Comeau online) compile the above without errors or warnings! The programs (at least for the GCC) resulted in a SEGV.
I know that the standard explicitly states that the ctor of string requires a char* which is not zero. STLs could easily detect the above case with a private ctor for basic_string which takes a single 'int' argument. This would catch the above code at compile time and would not ambiguate any other legal ctors.
[Redmond: No great enthusiasm for doing this. If we do, however, we want to do it for all places that take charT* pointers, not just the single-argument constructor. The other question is whether we want to catch this at compile time (in which case we catch the error of a literal 0, but not an expression whose value is a null pointer), at run time, or both. Recommend NAD. Relegate this functionality to debugging implementations.]
[ Post Summit: Alisdair requests this be re-opened as several new language facilities are designed to solve exactly this kind of problem. ]
[ Batavia (2009-05): ]
We are unable to achieve consensus on an approach to a resolution. There is some sentiment for treating this as a QOI matter. It is also possible that when string is brought into the concepts world, this issue might be addressed in that context.
[ 2009-07 Frankfurt ]
We considered three options:
- The proposed resolution.
- NAD
- Interpret a null pointer as the empty string.
The consensus was NAD.
Proposed resolution:
Add to the synopsis in 21.4 [basic.string]
basic_string( nullptr_t ) = delete;
Section: 23 [containers] Status: NAD Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2007-04-18
View all other issues in [containers].
View all issues with NAD status.
Discussion:
The standard doesn't prohibit the destructors (or any other special functions) of containers' elements invoked from a member function of the container from "recursively" calling the same (or any other) member function on the same container object, potentially while the container is in an intermediate state, or even changing the state of the container object while it is being modified. This may result in some surprising (i.e., undefined) behavior.
Read email thread starting with c++std-lib-13637 for more.
Proposed resolution:
Add to Container Requirements the following new paragraph:
Unless otherwise specified, the behavior of a program that
invokes a container member function f from a member function
g of the container's value_type on a container object c that
called g from its mutating member function h, is undefined.
I.e., if v is an element of c, directly or indirectly calling
c.h() from v.g() called from c.f(), is undefined.
[Redmond: This is a real issue, but it's probably a clause 17 issue, not clause 23. We get the same issue, for example, if we try to destroy a stream from one of the stream's callback functions.]
Rationale:
Recommend NAD. We agree this is an issue, but not a defect. We believe that there is no wording we can put in the standard that will cover all cases without introducing unfortunate corner cases.
Section: 25.4.3.3 [equal.range] Status: Dup Submitter: Prateek R Karandikar Opened: 2004-06-30 Last modified: 2006-12-30
View all other issues in [equal.range].
View all issues with Dup status.
Duplicate of: 270
Discussion:
There is no "Returns:" clause for std::equal_range, which returns non-void.
Proposed resolution:
Rationale:
Fixed as part of issue 270.
Section: 24.2.5 [forward.iterators] Status: NAD Submitter: Dave Abrahams Opened: 2004-07-09 Last modified: 2007-01-15
View all other issues in [forward.iterators].
View all issues with NAD status.
Discussion:
24.1/3 says:
Forward iterators satisfy all the requirements of the input and output iterators and can be used whenever either kind is specified
The problem is that satisfying the requirements of output iterator means that you can always assign *something* into the result of dereferencing it. That makes almost all non-mutable forward iterators non-conforming. I think we need to sever the refinement relationship between forward iterator and output iterator.
Related issue: 200. But this is not a dup.
Proposed resolution:
Rationale:
Yes, 24.1/3 does say that. But it's introductory material. The precise specification is in 24.1.3, and the requrements table there is right. We don't need to fine-tune introductory wording. (Especially since this wording is likely to be changed as part of the iterator overhaul.)
Section: 24.2.5 [forward.iterators] Status: Dup Submitter: Dave Abrahams Opened: 2004-07-11 Last modified: 2007-01-15
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Duplicate of: 478
Discussion:
The Forward Iterator requirements table contains the following:
expression return type operational precondition
semantics
========== ================== =========== ==========================
a->m U& if X is mutable, (*a).m pre: (*a).m is well-defined.
otherwise const U&
r->m U& (*r).m pre: (*r).m is well-defined.
The first line is exactly right. The second line is wrong. Basically it implies that the const-ness of the iterator affects the const-ness of referenced members. But Paragraph 11 of [lib.iterator.requirements] says:
In the following sections, a and b denote values of type const X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T, o denotes a value of some type that is writable to the output iterator.
AFAICT if we need the second line at all, it should read the same as the first line.
Related issue: 478
Proposed resolution:
Rationale:
The LWG agrees that this is a real problem. Marked as a DUP because the LWG chose to adopt the solution proposed in 478.
Section: 23.2 [container.requirements] Status: Dup Submitter: Herb Sutter Opened: 2004-08-01 Last modified: 2007-04-18
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Duplicate of: 580
Discussion:
Nothing in the standard appears to make this program ill-formed:
struct C {
void* operator new( size_t s ) { return ::operator new( s ); }
// NOTE: this hides in-place and nothrow new
};
int main() {
vector<C> v;
v.push_back( C() );
}
Is that intentional? We should clarify whether or not we intended to require containers to support types that define their own special versions of operator new.
[ Lillehammer: A container will definitely never use this overridden operator new, but whether it will fail to compile is unclear from the standard. Are containers supposed to use qualified or unqualified placement new? 20.4.1.1 is somewhat relevant, but the standard doesn't make it completely clear whether containers have to use Allocator::construct(). If containers don't use it, the details of how containers use placement new are unspecified. That is the real bug, but it needs to be fixed as part of the allocator overhaul. Weak support that the eventual solution should make this code well formed. ]
Proposed resolution:
Section: 20.8.3 [base] Status: NAD Submitter: Joe Gottman Opened: 2004-08-19 Last modified: 2006-12-29
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Discussion:
The classes std::unary_function and std::binary_function are both designed to be inherited from but contain no virtual functions. This makes it too easy for a novice programmer to write code like binary_function<int, int, int> *p = new plus<int>; delete p;
There are two common ways to prevent this source of undefined behavior: give the base class a public virtual destructor, or give it a protected nonvirtual destructor. Since unary_function and binary_function have no other virtual functions, (note in particular the absence of an operator()() ), it would cost too much to give them public virtual destructors. Therefore, they should be given protected nonvirtual destructors.
Proposed resolution:
Change Paragraph 20.3.1 of the Standard from
template <class Arg, class Result>
struct unary_function {
typedef Arg argument_type;
typedef Result result_type;
};
template <class Arg1, class Arg2, class Result>
struct binary_function {
typedef Arg1 first_argument_type;
typedef Arg2 second_argument_type;
typedef Result result_type;
};
to
template <class Arg, class Result>
struct unary_function {
typedef Arg argument_type;
typedef Result result_type;
protected:
~unary_function() {}
};
template <class Arg1, class Arg2, class Result>
struct binary_function {
typedef Arg1 first_argument_type;
typedef Arg2 second_argument_type;
typedef Result result_type;
protected:
~binary_function() {}
};
Rationale:
The LWG doesn't believe the existing definition causes anybody any concrete harm.
Section: 25.3.9 [alg.unique] Status: NAD Submitter: Andrew Koenig Opened: 2004-08-30 Last modified: 2006-12-27
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Discussion:
The standard says that unique(first, last) "eliminates all but the first element from every consecutive group of equal elements" in [first, last) and returns "the end of the resulting range". So a postcondition is that [first, result) is the same as the old [first, last) except that duplicates have been eliminated.
What postconditions are there on the range [result, last)? One might argue that the standard says nothing about those values, so they can be anything. One might also argue that the standard doesn't permit those values to be changed, so they must not be. Should the standard say something explicit one way or the other?
Proposed resolution:
Rationale:
We don't want to make many guarantees about what's in [result, end). Maybe we aren't being quite explicit enough about not being explicit, but it's hard to think that's a major problem.
Section: 20.3.5 [pairs], 20.4 [tuple] Status: NAD Editorial Submitter: Andrew Koenig Opened: 2004-09-14 Last modified: 2007-05-06
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Discussion:
(Based on recent comp.std.c++ discussion)
Pair (and tuple) should specialize std::swap to work in terms of std::swap on their components. For example, there's no obvious reason why swapping two objects of type pair<vector<int>, list<double> > should not take O(1).
[Lillehammer: We agree it should be swappable. Howard will provide wording.]
[ Post Oxford: We got swap for pair but accidently missed tuple. tuple::swap is being tracked by 522. ]
Proposed resolution:
Wording provided in N1856.
Rationale:
Recommend NAD, fixed by N1856.
Section: 25.2 [alg.nonmodifying], 25.3 [alg.modifying.operations] Status: Dup Submitter: Peter Dimov Opened: 2004-09-20 Last modified: 2007-01-15
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Duplicate of: 283
Discussion:
c++std-lib-14262
[lib.alg.find] requires T to be EqualityComparable:
template <class InputIterator, class T>
InputIterator find(InputIterator first, InputIterator last,
const T& value);
However the condition being tested, as specified in the Effects clause, is actually *i == value, where i is an InputIterator.
The two clauses are in agreement only if the type of *i is T, but this isn't necessarily the case. *i may have a heterogeneous comparison operator that takes a T, or a T may be convertible to the type of *i.
Further discussion (c++std-lib-14264): this problem affects a number of algorithsm in clause 25, not just find. We should try to resolve this problem everywhere it appears.
Proposed resolution:
[lib.alg.find]:
Remove [lib.alg.find]/1.
[lib.alg.count]:
Remove [lib.alg.count]/1.
[lib.alg.search]:
Remove "Type T is EqualityComparable (20.1.1), " from [lib.alg.search]/4.
[lib.alg.replace]:
Remove [lib.alg.replace]/1. Replace [lb.alg.replace]/2 with:
For every iterator i in the range [first, last) for which *i == value or pred(*i) holds perform *i = new_value.
Remove the first sentence of /4. Replace the beginning of /5 with:
For every iterator i in the range [result, result + (last - first)), assign to *i either...
(Note the defect here, current text says assign to i, not *i).
[lib.alg.fill]:
Remove "Type T is Assignable (23.1), " from /1. Replace /2 with:
For every iterator i in the range [first, last) or [first, first + n), perform *i = value.
[lib.alg.remove]:
Remove /1. Remove the first sentence of /6.
Rationale:
Duplicate of (a subset of) issue 283.
Section: 24.2.3 [input.iterators] Status: NAD Future Submitter: Chris Jefferson Opened: 2004-09-16 Last modified: 2009-10-26
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Discussion:
From comp.std.c++:
I note that given an input iterator a for type T, then *a only has to be "convertable to T", not actually of type T.
Firstly, I can't seem to find an exact definition of "convertable to T". While I assume it is the obvious definition (an implicit conversion), I can't find an exact definition. Is there one?
Slightly more worryingly, there doesn't seem to be any restriction on the this type, other than it is "convertable to T". Consider two input iterators a and b. I would personally assume that most people would expect *a==*b would perform T(*a)==T(*b), however it doesn't seem that the standard requires that, and that whatever type *a is (call it U) could have == defined on it with totally different symantics and still be a valid inputer iterator.
Is this a correct reading? When using input iterators should I write T(*a) all over the place to be sure that the object i'm using is the class I expect?
This is especially a nuisance for operations that are defined to be "convertible to bool". (This is probably allowed so that implementations could return say an int and avoid an unnessary conversion. However all implementations I have seen simply return a bool anyway. Typical implemtations of STL algorithms just write things like while(a!=b && *a!=0). But strictly speaking, there are lots of types that are convertible to T but that also overload the appropriate operators so this doesn't behave as expected.
If we want to make code like this legal (which most people seem to expect), then we'll need to tighten up what we mean by "convertible to T".
[Lillehammer: The first part is NAD, since "convertible" is well-defined in core. The second part is basically about pathological overloads. It's a minor problem but a real one. So leave open for now, hope we solve it as part of iterator redesign.]
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Mark as NAD Future. We agree there's an issue, but there is no proposed solution at this time and this will be solved by concepts in the future.
Proposed resolution:
Rationale:
[ San Francisco: ]
Solved by N2758.
Section: 24.2.4 [output.iterators] Status: NAD Editorial Submitter: Chris Jefferson Opened: 2004-10-13 Last modified: 2010-03-11
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Discussion:
The note on 24.1.2 Output iterators insufficiently limits what can be performed on output iterators. While it requires that each iterator is progressed through only once and that each iterator is written to only once, it does not require the following things:
Note: Here it is assumed that x is an output iterator of type X which has not yet been assigned to.
a) That each value of the output iterator is written to: The standard allows: ++x; ++x; ++x;
b) That assignments to the output iterator are made in order X a(x); ++a; *a=1; *x=2; is allowed
c) Chains of output iterators cannot be constructed: X a(x); ++a; X b(a); ++b; X c(b); ++c; is allowed, and under the current wording (I believe) x,a,b,c could be written to in any order.
I do not believe this was the intension of the standard?
[Lillehammer: Real issue. There are lots of constraints we intended but didn't specify. Should be solved as part of iterator redesign.]
[ 2009-07 Frankfurt ]
Bill provided wording according to consensus.
[ 2009-07-21 Alisdair requests change from Review to Open. See thread starting with c++std-lib-24459 for discussion. ]
[ 2009-10 Santa Cruz: ]
Modified wording. Set to Review.
[ 2009-10 Santa Cruz: ]
Move to Ready after looking at again in a larger group in Santa Cruz.
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added below.
Rationale:
Solved by N3066.
Proposed resolution:
Change Table 101 — Output iterator requirements in 24.2.4 [output.iterators]:
Table 101 — Output iterator requirements Expression Return type Operational semantics Assertion/note pre-/post-condition X(a) a = t is equivalent to X(a) = t. note: a destructor is assumed. X u(a);
X u = a;*r = o result is not used Post: r is not required to be dereferenceable. r is incrementable. ++r X& &r == &++r Post: r is dereferenceable, unless otherwise specified. r is not required to be incrementable. r++ convertible to const X& {X tmp = r;
++r;
return tmp;}Post: r is dereferenceable, unless otherwise specified. r is not required to be incrementable. *r++ = o; result is not used
Section: 25.4.7 [alg.min.max] Status: Dup Submitter: Dave Abrahams Opened: 2004-10-13 Last modified: 2006-12-30
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Duplicate of: 281
Discussion:
A straightforward implementation of these algorithms does not need to copy T.
Proposed resolution:
drop the the words "and CopyConstructible" from paragraphs 1 and 4
Rationale:
Section: 20.2.5 [allocator.requirements] Status: NAD Submitter: Dhruv Matani Opened: 2004-10-17 Last modified: 2006-12-30
View all other issues in [allocator.requirements].
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Discussion:
The standard's version of allocator::construct(pointer, const_reference) severely limits what you can construct using this function. Say you can construct a socket from a file descriptor. Now, using this syntax, I first have to manually construct a socket from the fd, and then pass the constructed socket to the construct() function so it will just to an uninitialized copy of the socket I manually constructed. Now it may not always be possible to copy construct a socket eh! So, I feel that the changes should go in the allocator::construct(), making it:
template<typename T>
struct allocator{
template<typename T1>
void construct(pointer T1 const& rt1);
};
Now, the ctor of the class T which matches the one that takes a T1 can be called! Doesn't that sound great?
Proposed resolution:
Rationale:
NAD. STL uses copying all the time, and making it possible for allocators to construct noncopyable objects is useless in the absence of corresponding container changes. We might consider this as part of a larger redesign of STL.
Section: 25.3.8 [alg.remove] Status: NAD Submitter: Thomas Mang Opened: 2004-12-12 Last modified: 2006-12-27
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Discussion:
In Section 25.2.7 [lib.alg.remove], paragraphs 1 to 5 describe the behavior of the mutating sequence operations std::remove and std::remove_if. However, the wording does not reflect the intended behavior [Note: See definition of intended behavior below] of these algorithms, as it is known to the C++ community [1].
1) Analysis of current wording:
25.2.7 [lib.alg.remove], paragraph 2:
Current wording says: "Effects: Eliminates all the elements referred to by iterator i in the range [first, last) for which the following corresponding conditions hold: *i == value, pred(*i) != false."
This sentences expresses specifically that all elements denoted by the (original) range [first, last) for which the corresponding condition hold will be eliminated. Since there is no formal definition of the term "eliminate" provided, the meaning of "eliminate" in everyday language implies that as postcondition, no element in the range denoted by [first, last) will hold the corresponding condition on reiteration over the range [first, last).
However, this is neither the intent [Note: See definition of intended behavior below] nor a general possible approach. It can be easily proven that if all elements of the original range[first, last) will hold the condition, it is not possible to substitute them by an element for which the condition will not hold.
25.2.7 [lib.alg.remove], paragraph 3:
Current wording says: "Returns: The end of the resulting range."
The resulting range is not specified. In combination with 25.2.7 [lib.alg.remove], paragraph 2, the only reasonable interpretation of this so-called resulting range is the range [first,last) - thus returning always the ForwardIterator 'last' parameter.
25.2.7 [lib.alg.remove], paragraph 4:
Current wording says: "Notes: Stable: the relative order of the elements that are not removed is the same as their relative order in the original range"
This sentences makes use of the term "removed", which is neither specified, nor used in a previous paragraph (which uses the term "eliminate"), nor unamgiuously separated from the name of the algorithm.
2) Description of intended behavior:
For the rest of this Defect Report, it is assumed that the intended behavior was that all elements of the range [first, last) which do not hold the condition *i == value (std::remove) or pred(*i) != false (std::remove_if)], call them s-elements [Note: s...stay], will be placed into a contiguous subrange of [first, last), denoted by the iterators [first, return value). The number of elements in the resulting range [first, return value) shall be equal to the number of s-elements in the original range [first, last). The relative order of the elements in the resulting subrange[first, return value) shall be the same as the relative order of the corresponding elements in the original range. It is undefined whether any elements in the resulting subrange [return value, last) will hold the corresponding condition, or not.
All implementations known to the author of this Defect Report comply with this intent. Since the intent of the behavior (contrary to the current wording) is also described in various utility references serving the C++ community [1], it is not expected that fixing the paragraphs will influence current code - unless the code relies on the behavior as it is described by current wording and the implementation indeed reflects the current wording, and not the intent.
3) Proposed fixes:
Change 25.2.7 [lib.alg.remove], paragraph 2 to:
"Effect: Places all the elements referred to by iterator i in the range [first, last) for which the following corresponding conditions hold : !(*i == value), pred(*i) == false into the subrange [first, k) of the original range, where k shall denote a value of type ForwardIterator. It is undefined whether any elements in the resulting subrange [k, last) will hold the corresponding condition, or not."
Comments to the new wording:
a) "Places" has no special meaning, and the everyday language meaning should fit. b) The corresponding conditions were negated compared to the current wording, becaue the new wording requires it. c) The wording "of the original range" might be redundant, since any subrange starting at 'first' and containing no more elements than the original range is implicitly a subrange of the original range [first, last). d) The iterator k was introduced instead of "return value" in order to avoid a cyclic dependency on 25.2.7/3. The wording ", where k shall denote a value of type ForwardIterator" might be redundant, because it follows implicitly by 25.2.7/3. e) "Places" does, in the author's opinion, explicitly forbid duplicating any element holding the corresponding condition in the original range [first, last) within the resulting range [first, k). If there is doubt this term might be not unambiguous regarding this, it is suggested that k is specified more closely by the following wording: "k shall denote a value of type ForwardIterator [Note: see d)] so that k - first is equal to the number of elements in the original range [first, last) for which the corresponding condition did hold". This could also be expressed as a separate paragraph "Postcondition:" f) The senctence "It is undefined whether any elements in the resulting subrange [k, last) will hold the corresponding condition, or not." was added consciously so the term "Places" does not imply if the original range [first, last) contains n elements holding the corresponding condition, the identical range[first, last) will also contain exactly n elements holding the corresponding condition after application of the algorithm.
Change 25.2.7 [lib.alg.remove], paragraph 3 to: "Returns: The iterator k."
Change 25.2.7 [lib.alg.remove], paragraph 4 to: "Notes: Stable: the relative order of the elements that are placed into the subrange [first, return value) shall be the same as their relative order was in the original range [first, last) prior to application of the algorithm."
Comments to the new wording:
a) the wording "was ... prior to application of the algorithm" is used to explicitly distinguish the original range not only by means of iterators, but also by a 'chronological' factor from the resulting range [first, return value). It might be redundant.
[1]: The wording of these references is not always unambiguous, and provided examples partially contradict verbal description of the algorithms, because the verbal description resembles the problematic wording of ISO/IEC 14882:2003.
Proposed resolution:
Rationale:
The LWG believes that the standard is sufficiently clear, and that there is no evidence of any real-world confusion about this point.
Section: 25.3.9 [alg.unique] Status: NAD Submitter: Thomas Mang Opened: 2004-12-12 Last modified: 2006-12-27
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Discussion:
In Section 25.2.8 [lib.alg.unique], paragraphs 1 to 3 describe the behavior of the mutating sequence operation std::unique. However, the wording does not reflect the intended behavior [Note: See definition of intended behavior below] of these algorithms, as it is known to the C++ community [1].
1) Analysis of current wording:
25.2.8 [lib.alg.unique], paragraph 1:
Current wording says: "Effects: Eliminates all but the first element from every consecutive group of equal elements referred to by the iterator i in the range [first, last) for which the following corresponding conditions hold: *i == *(i - 1) or pred(*i, *(i -1)) != false"
This sentences expresses specifically that all elements denoted by the (original) range [first, last) which are not but the first element from a consecutive group of equal elements (where equality is defined as *i == *(i - 1) or pred(*i, *(i - 1)) ! = false) [Note: See DR 202], call them r-elements [Note: r...remove], will be eliminated. Since there is no formal definition of the term "eliminate" provided, it is undefined how this "elimination" takes place. But the meaning of "eliminate" in everyday language seems to disallow explicitly that after application of the algorithm, any r-element will remain at any position of the range [first, last) [2].
Another defect in the current wording concerns the iterators used to compare two elements for equality: The current wording contains the expression "(i - 1)", which is not covered by 25/9 [Note: See DR submitted by Thomas Mang regarding invalid iterator arithmetic expressions].
25.2.8 [lib.alg.unique], paragraph 2:
Current wording says: "Returns: The end of the resulting range."
The resulting range is not specified. In combination with 25.2.8 [lib.alg.unique], paragraph 1, one reasonable interpretation (in the author's opinion even the only possible interpretation) of this so-called resulting range is the range [first, last) - thus returning always the ForwardIterator 'last' parameter.
2) Description of intended behavior:
For the rest of this Defect Report, it is assumed that the intended behavior was that all elements denoted by the original range [first, last) which are the first element from a consecutive group of elements for which the corresponding conditions: *(i-1) == *i (for the version of unique without a predicate argument) or pred(*(i-1), *i) ! = false (for the version of unique with a predicate argument) [Note: If such a group of elements consists of only a single element, this is also considered the first element] [Note: See resolutions of DR 202], call them s-elements [Note: s...stay], will be placed into a contiguous subrange of [first, last), denoted by the iterators [first, return value). The number of elements in the resulting range [first, return value) shall be equal to the number of s-elements in the original range [first, last). Invalid iterator arithmetic expressions are expected to be resolved as proposed in DR submitted by Thomas Mang regarding invalid iterator arithmetic expressions. It is also assumed by the author that the relative order of the elements in the resulting subrange [first, return value) shall be the same as the relative order of the corresponding elements (the s-elements) in the original range [Note: If this was not intended behavior, the additional proposed paragraph about stable order will certainly become obsolete]. Furthermore, the resolutions of DR 202 are partially considered.
All implementations known to the author of this Defect Report comply with this intent [Note: Except possible effects of DR 202]. Since this intent of the behavior (contrary to the current wording) is also described in various utility references serving the C++ community [1], it is not expected that fixing the paragraphs will influence current code [Note: Except possible effects of DR 202] - unless the code relies on the behavior as it is described by current wording and the implementation indeed reflects the current wording, and not the intent.
3) Proposed fixes:
Change 25.2.8 [lib.alg.unique], paragraph 1 to:
"Effect: Places the first element from every consecutive group of elements, referred to by the iterator i in the range [first, last), for which the following conditions hold: *(i-1) == *i (for the version of unique without a predicate argument) or pred(*(i -1), *i) != false (for the version of unique with a predicate argument), into the subrange [first, k) of the original range, where k shall denote a value of type ForwardIterator."
Comments to the new wording:
a) The new wording was influenced by the resolutions of DR 202. If DR 202 is resolved in another way, the proposed wording need also additional review. b) "Places" has no special meaning, and the everyday language meaning should fit. c) The expression "(i - 1)" was left, but is expected that DR submitted by Thomas Mang regarding invalid iterator arithmetic expressions will take this into account. d) The wording "(for the version of unique without a predicate argument)" and "(for the version of unique with a predicate argument)" was added consciously for clarity and is in resemblence with current 23.2.2.4 [lib.list.ops], paragraph 19. It might be considered redundant. e) The wording "of the original range" might be redundant, since any subrange starting at first and containing no more elements than the original range is implicitly a subrange of the original range [first, last). f) The iterator k was introduced instead of "return value" in order to avoid a cyclic dependency on 25.2.8 [lib.alg.unique], paragraph 2. The wording ", where k shall denote a value of type ForwardIterator" might be redundant, because it follows implicitly by 25.2.8 [lib.alg.unique], paragraph 2. g) "Places" does, in the author's opinion, explicitly forbid duplicating any s-element in the original range [first, last) within the resulting range [first, k). If there is doubt this term might be not unambiguous regarding this, it is suggested that k is specified more closely by the following wording: "k shall denote a value of type ForwardIterator [Note: See f)] so that k - first is equal to the number of elements in the original range [first, last) being the first element from every consecutive group of elements for which the corresponding condition did hold". This could also be expressed as a separate paragraph "Postcondition:". h) If it is considered that the wording is unclear whether it declares the element of a group which consists of only a single element implicitly to be the first element of this group [Note: Such an interpretation could eventually arise especially in case last - first == 1] , the following additional sentence is proposed: "If such a group of elements consists of only a single element, this element is also considered the first element."
Change 25.2.8 [lib.alg.unique], paragraph 2 to: "Returns: The iterator k."
Add a separate paragraph "Notes:" as 25.2.8 [lib.alg.unique], paragraph 2a or 3a, or a separate paragraph "Postcondition:" before 25.2.8 [lib.alg.unique], paragraph 2 (wording inside {} shall be eliminated if the preceding expressions are used, or the preceding expressions shall be eliminated if wording inside {} is used):
"Notes:{Postcondition:} Stable: the relative order of the elements that are placed into the subrange [first, return value {k}) shall be the same as their relative order was in the original range [first, last) prior to application of the algorithm."
Comments to the new wording:
a) It is assumed by the author that the algorithm was intended to be stable. In case this was not the intent, this paragraph becomes certainly obsolete. b) The wording "was ... prior to application of the algorithm" is used to explicitly distinguish the original range not only by means of iterators, but also by a 'chronological' factor from the resulting range [first, return value). It might be redundant.
25.2.8 [lib.alg.unique], paragraph 3:
See DR 239.
4) References to other DRs:
See DR 202, but which does not address any of the problems described in this Defect Report [Note: This DR is supposed to complement DR 202]. See DR 239. See DR submitted by Thomas Mang regarding invalid iterator arithmetic expressions.
[1]: The wording of these references is not always unambiguous, and provided examples partially contradict verbal description of the algorithms, because the verbal description resembles the problematic wording of ISO/IEC 14882:2003.
[2]: Illustration of conforming implementations according to current wording:
One way the author of this DR considers how this "elimination" could be achieved by a conforming implementation according to current wording is by substituting each r-element by _any_ s-element [Note: s...stay; any non-r-element], since all r-elements are "eliminated".
In case of a sequence consisting of elements being all 'equal' [Note: See DR 202], substituting each r-element by the single s-element is the only possible solution according to current wording.
Proposed resolution:
Rationale:
The LWG believes the standard is sufficiently clear. No implementers get it wrong, and changing it wouldn't cause any code to change, so there is no real-world harm here.
Section: 23.3.4.4 [list.ops] Status: NAD Submitter: Thomas Mang Opened: 2004-12-12 Last modified: 2007-02-19
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Discussion:
In Section 23.3.4.4 [list.ops], paragraphs 19 to 21 describe the behavior of the std::list<T, Allocator>::unique operation. However, the current wording is defective for various reasons.
1) Analysis of current wording:
23.3.4.4 [list.ops], paragraph 19:
Current wording says: "Effects: Eliminates all but the first element from every consecutive group of equal elements referred to by the iterator i in the range [first + 1, last) for which *i == *(i - 1) (for the version of unique with no argument) or pred(*i, *(i -1)) (for the version of unique with a predicate argument) holds."
This sentences makes use of the undefined term "Eliminates". Although it is, to a certain degree, reasonable to consider the term "eliminate" synonymous with "erase", using "Erase" in the first place, as the wording of 23.3.4.4 [list.ops], paragraph 15 does, would be clearer.
The range of the elements referred to by iterator i is "[first + 1, last)". However, neither "first" nor "last" is defined.
The sentence makes three times use of iterator arithmetic expressions ( "first + 1", "*i == *(i - 1)", "pred(*i, *(i -1))" ) which is not defined for bidirectional iterator [see DR submitted by Thomas Mang regarding invalid iterator arithmetic expressions].
The same problems as pointed out in DR 202 (equivalence relation / order of arguments for pred()) apply to this paragraph.
23.3.4.4 [list.ops], paragraph 20:
Current wording says: "Throws: Nothing unless an exception in thrown by *i == *(i-1) or pred(*i, *(i - 1))"
The sentence makes two times use of invalid iterator arithmetic expressions ( "*i == *(i - 1)", "pred(*i, *(i -1))" ).
[Note: Minor typos: "in" / missing dot at end of sentence.]
23.3.4.4 [list.ops], paragraph 21:
Current wording says: "Complexity: If the range (last - first) is not empty, exactly (last - first) - 1 applications of the corresponding predicate, otherwise no application of the predicate.
See DR 315 regarding "(last - first)" not yielding a range.
Invalid iterator arithmetic expression "(last - first) - 1" left .
2) Description of intended behavior:
For the rest of this Defect Report, it is assumed that "eliminate" is supposed to be synonymous to "erase", that "first" is equivalent to an iterator obtained by a call to begin(), "last" is equivalent to an iterator obtained by a call to end(), and that all invalid iterator arithmetic expressions are resolved as described in DR submitted by Thomas Mang regarding invalid iterator arithmetic expressions.
Furthermore, the resolutions of DR 202 are considered regarding equivalence relation and order of arguments for a call to pred.
All implementations known to the author of this Defect Report comply with these assumptions, apart from the impact of the alternative resolution of DR 202. Except for the changes implied by the resolutions of DR 202, no impact on current code is expected.
3) Proposed fixes:
Change 23.3.4.4 [list.ops], paragraph 19 to:
"Effect: Erases all but the first element from every consecutive group of elements, referred to by the iterator i in the range [begin(), end()), for which the following conditions hold: *(i-1) == *i (for the version of unique with no argument) or pred(*(i-1), *i) != false (for the version of unique with a predicate argument)."
Comments to the new wording:
a) The new wording was influenced by DR 202 and the resolutions presented there. If DR 202 is resolved in another way, the proposed wording need also additional review. b) "Erases" refers in the author's opinion unambiguously to the member function "erase". In case there is doubt this might not be unamgibuous, a direct reference to the member function "erase" is suggested [Note: This would also imply a change of 23.3.4.4 [list.ops], paragraph 15.]. c) The expression "(i - 1)" was left, but is expected that DR submitted by Thomas Mang regarding invalid iterator arithmetic expressions will take this into account. d) The wording "(for the version of unique with no argument)" and "(for the version of unique with a predicate argument)" was kept consciously for clarity. e) "begin()" substitutes "first", and "end()" substitutes "last". The range need adjustment from "[first + 1, last)" to "[begin(), end())" to ensure a valid range in case of an empty list. f) If it is considered that the wording is unclear whether it declares the element of a group which consists of only a single element implicitly to be the first element of this group [Note: Such an interpretation could eventually arise especially in case size() == 1] , the following additional sentence is proposed: "If such a group of elements consists of only a single element, this element is also considered the first element."
Change 23.3.4.4 [list.ops], paragraph 20 to:
"Throws: Nothing unless an exception is thrown by *(i-1) == *i or pred(*(i-1), *i)."
Comments to the new wording:
a) The wording regarding the conditions is identical to proposed 23.3.4.4 [list.ops], paragraph 19. If 23.3.4.4 [list.ops], paragraph 19 is resolved in another way, the proposed wording need also additional review. b) The expression "(i - 1)" was left, but is expected that DR submitted by Thomas Mang regarding invalid iterator arithmetic expressions will take this into account. c) Typos fixed.
Change 23.3.4.4 [list.ops], paragraph 21 to:
"Complexity: If empty() == false, exactly size() - 1 applications of the corresponding predicate, otherwise no applications of the corresponding predicate."
Comments to the new wording:
a) The new wording is supposed to also replace the proposed resolution of DR 315, which suffers from the problem of undefined "first" / "last".
5) References to other DRs:
See DR 202. See DR 239. See DR 315. See DR submitted by Thomas Mang regarding invalid iterator arithmetic expressions.
Proposed resolution:
Rationale:
"All implementations known to the author of this Defect Report comply with these assumption", and "no impact on current code is expected", i.e. there is no evidence of real-world confusion or harm.
Section: 17.5.1.4 [structure.specifications] Status: NAD Submitter: Thomas Mang Opened: 2004-12-12 Last modified: 2009-07-15
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Discussion:
Various clauses other than clause 25 make use of iterator arithmetic not supported by the iterator category in question. Algorithms in clause 25 are exceptional because of 25 [lib.algorithms], paragraph 9, but this paragraph does not provide semantics to the expression "iterator - n", where n denotes a value of a distance type between iterators.
1) Examples of current wording:
Current wording outside clause 25:
23.2.2.4 [lib.list.ops], paragraphs 19-21: "first + 1", "(i - 1)", "(last - first)" 23.3.1.1 [lib.map.cons], paragraph 4: "last - first" 23.3.2.1 [lib.multimap.cons], paragraph 4: "last - first" 23.3.3.1 [lib.set.cons], paragraph 4: "last - first" 23.3.4.1 [lib.multiset.cons], paragraph 4: "last - first" 24.4.1 [lib.reverse.iterators], paragraph 1: "(i - 1)"
[Important note: The list is not complete, just an illustration. The same issue might well apply to other paragraphs not listed here.]
None of these expressions is valid for the corresponding iterator category.
Current wording in clause 25:
25.1.1 [lib.alg.foreach], paragraph 1: "last - 1" 25.1.3 [lib.alg.find.end], paragraph 2: "[first1, last1 - (last2-first2))" 25.2.8 [lib.alg.unique], paragraph 1: "(i - 1)" 25.2.8 [lib.alg.unique], paragraph 5: "(i - 1)"
However, current wording of 25 [lib.algorithms], paragraph 9 covers neither of these four cases:
Current wording of 25 [lib.algorithms], paragraph 9:
"In the description of the algorithms operator + and - are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics of a+n is the same as that of
{X tmp = a;
advance(tmp, n);
return tmp;
}
and that of b-a is the same as of return distance(a, b)"
This paragrpah does not take the expression "iterator - n" into account, where n denotes a value of a distance type between two iterators [Note: According to current wording, the expression "iterator - n" would be resolved as equivalent to "return distance(n, iterator)"]. Even if the expression "iterator - n" were to be reinterpreted as equivalent to "iterator + -n" [Note: This would imply that "a" and "b" were interpreted implicitly as values of iterator types, and "n" as value of a distance type], then 24.3.4/2 interfers because it says: "Requires: n may be negative only for random access and bidirectional iterators.", and none of the paragraphs quoted above requires the iterators on which the algorithms operate to be of random access or bidirectional category.
2) Description of intended behavior:
For the rest of this Defect Report, it is assumed that the expression "iterator1 + n" and "iterator1 - iterator2" has the semantics as described in current 25 [lib.algorithms], paragraph 9, but applying to all clauses. The expression "iterator1 - n" is equivalent to an result-iterator for which the expression "result-iterator + n" yields an iterator denoting the same position as iterator1 does. The terms "iterator1", "iterator2" and "result-iterator" shall denote the value of an iterator type, and the term "n" shall denote a value of a distance type between two iterators.
All implementations known to the author of this Defect Report comply with these assumptions. No impact on current code is expected.
3) Proposed fixes:
Change 25 [lib.algorithms], paragraph 9 to:
"In the description of the algorithms operator + and - are used for some of the iterator categories for which they do not have to be defined. In this paragraph, a and b denote values of an iterator type, and n denotes a value of a distance type between two iterators. In these cases the semantics of a+n is the same as that of
{X tmp = a;
advance(tmp, n);
return tmp;
}
,the semantics of a-n denotes the value of an iterator i for which the following condition holds: advance(i, n) == a, and that of b-a is the same as of return distance(a, b)".
Comments to the new wording:
a) The wording " In this paragraph, a and b denote values of an iterator type, and n denotes a value of a distance type between two iterators." was added so the expressions "b-a" and "a-n" are distinguished regarding the types of the values on which they operate. b) The wording ",the semantics of a-n denotes the value of an iterator i for which the following condition holds: advance(i, n) == a" was added to cover the expression 'iterator - n'. The wording "advance(i, n) == a" was used to avoid a dependency on the semantics of a+n, as the wording "i + n == a" would have implied. However, such a dependency might well be deserved. c) DR 225 is not considered in the new wording.
Proposed fixes regarding invalid iterator arithmetic expressions outside clause 25:
Either a) Move modified 25 [lib.algorithms], paragraph 9 (as proposed above) before any current invalid iterator arithmetic expression. In that case, the first sentence of 25 [lib.algorithms], paragraph 9, need also to be modified and could read: "For the rest of this International Standard, ...." / "In the description of the following clauses including this ...." / "In the description of the text below ..." etc. - anyways substituting the wording "algorithms", which is a straight reference to clause 25. In that case, 25 [lib.algorithms] paragraph 9 will certainly become obsolete. Alternatively, b) Add an appropiate paragraph similar to resolved 25 [lib.algorithms], paragraph 9, to the beginning of each clause containing invalid iterator arithmetic expressions. Alternatively, c) Fix each paragraph (both current wording and possible resolutions of DRs) containing invalid iterator arithmetic expressions separately.
5) References to other DRs:
See DR 225. See DR 237. The resolution could then also read "Linear in last - first".
[ Bellevue: ]
Keep open and ask Bill to provide wording.
[ 2009-05-09 Alisdair adds: ]
This issue is related to 997.
[ 2009-07 Frankfurt ]
Hinnant: this isn't going to change any user's code or any vendor's implementation.
No objection to "NAD without prejudice." If anyone proposes a resolution, the LWG will consider it.
Move to NAD.
Proposed resolution:
[Lillehammer: Minor issue, but real. We have a blanket statement about this in 25/11. But (a) it should be in 17, not 25; and (b) it's not quite broad enough, because there are some arithmetic expressions it doesn't cover. Bill will provide wording.]
Section: 24.2.3 [input.iterators] Status: NAD Submitter: Chris Jefferson Opened: 2004-12-13 Last modified: 2006-12-27
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Discussion:
1) In 24.1.1/3, the following text is currently present.
"Note: For input iterators, a==b does not imply ++a=++b (Equality does not guarantee the substitution property or referential transparency)."
However, when in Table 72, part of the definition of ++r is given as:
"pre: r is dereferenceable. post: any copies of the previous value of r are no longer required either to be dereferenceable ..."
While a==b does not imply that b is a copy of a, this statement should perhaps still be made more clear.
2) There are no changes to intended behaviour
3) This Note should be altered to say "Note: For input iterators a==b, when its behaviour is defined ++a==++b may still be false (Equality does not guarantee the substitution property or referential transparency).
Proposed resolution:
Rationale:
This is descriptive text, not normative, and the meaning is clear.
Section: 23.2.4 [associative.reqmts] Status: NAD Submitter: Hans B os Opened: 2004-12-19 Last modified: 2006-12-27
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Discussion:
According to [lib.associative.reqmts] table 69, the runtime comlexity of insert(p, t) and erase(q) can be done in amortized constant time.
It was my understanding that an associative container could be implemented as a balanced binary tree.
For inser(p, t), you 'll have to iterate to p's next node to see if t can be placed next to p. Furthermore, the insertion usually takes place at leaf nodes. An insert next to the root node will be done at the left of the root next node
So when p is the root node you 'll have to iterate from the root to its next node, which takes O(log(size)) time in a balanced tree.
If you insert all values with insert(root, t) (where root is the root of the tree before insertion) then each insert takes O(log(size)) time. The amortized complexity per insertion will be O(log(size)) also.
For erase(q), the normal algorithm for deleting a node that has no empty left or right subtree, is to iterate to the next (or previous), which is a leaf node. Then exchange the node with the next and delete the leaf node. Furthermore according to DR 130, erase should return the next node of the node erased. Thus erasing the root node, requires iterating to the next node.
Now if you empty a map by deleting the root node until the map is empty, each operation will take O(log(size)), and the amortized complexity is still O(log(size)).
The operations can be done in amortized constant time if iterating to the next node can be done in (non amortized) constant time. This can be done by putting all nodes in a double linked list. This requires two extra links per node. To me this is a bit overkill since you can already efficiently insert or erase ranges with erase(first, last) and insert(first, last).
Proposed resolution:
Rationale:
Only "amortized constant" in special circumstances, and we believe that's implementable. That is: doing this N times will be O(N), not O(log N).
Section: 25.4.1.2 [stable.sort] Status: NAD Editorial Submitter: Prateek Karandikar Opened: 2005-04-12 Last modified: 2008-03-13
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Discussion:
17.3.1.1 Summary
1 The Summary provides a synopsis of the category, and introduces the first-level subclauses. Each subclause also provides a summary, listing the headers specified in the subclause and the library entities provided in each header.
2 Paragraphs labelled "Note(s):" or "Example(s):" are informative, other paragraphs are normative.
So this means that a "Notes" paragraph wouldn't be normative.
25.3.1.2 stable_sort
template<class RandomAccessIterator> void stable_sort(RandomAccessIterat or first, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void stable_sort(RandomAccessIterat or first, RandomAccessIterator last, Compare comp);1 Effects: Sorts the elements in the range [first, last).
2 Complexity: It does at most N(log N)^2 (where N == last - first) comparisons; if enough extra memory is available, it is N log N.
3 Notes: Stable: the relative order of the equivalent elements is preserved.
The Notes para is informative, and nowhere else is stability mentioned above.
Also, I just searched for the word "stable" in my copy of the Standard. and the phrase "Notes: Stable: the relative order of the elements..." is repeated several times in the Standard library clauses for describing various functions. How is it that stability is talked about in the informative paragraph? Or am I missing something obvious?
Proposed resolution:
Rationale:
This change has already been made.
Section: 22.4.1.5 [locale.codecvt.byname] Status: NAD Submitter: Krzysztof Żelechowski Opened: 2005-05-24 Last modified: 2008-03-13
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Discussion:
Contradiction.
Proposed resolution:
Section: 20.8.3 [base] Status: NAD Submitter: Me <anti_spam_email2003@yahoo.com> Opened: 2005-06-07 Last modified: 2008-03-13
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Discussion:
"For templates greater, less, greater_equal, and less_equal, the specializations for any pointer type yield a total order, even if the built-in operators <, >, <=, >= do not."
The standard should do much better than guarantee that these provide a total order, it should guarantee that it can be used to test if memory overlaps, i.e. write a portable memmove. You can imagine a platform where the built-in operators use a uint32_t comparison (this tests for overlap on this platform) but the less<T*> functor is allowed to be defined to use a int32_t comparison. On this platform, if you use std::less with the intent of making a portable memmove, comparison on an array that straddles the 0x7FFFFFFF/0x8000000 boundary can give incorrect results.
Proposed resolution:
Add a footnote to 20.5.3/8 saying:
Given a p1 and p2 such that p1 points to N objects of type T and p2 points to M objects of type T. If [p1,p1+N) does not overlap [p2,p2+M), less returns the same value when comparing all pointers in [p1,p1+N) to all pointers in [p2,p2+M). Otherwise, there is a value Q and a value R such that less returns the same value when comparing all pointers in [p1,p1+Q) to all pointers in [p2,p2+R) and an opposite value when comparing all pointers in [p1+Q,p1+N) to all pointers in [p2+R,p2+M). For the sake of completeness, the null pointer value (4.10) for T is considered to be an array of 1 object that doesn't overlap with any non-null pointer to T. less_equal, greater, greater_equal, equal_to, and not_equal_to give the expected results based on the total ordering semantics of less. For T of void, treat it as having similar semantics as T of char i.e. less<cv T*>(a, b) gives the same results as less<cv void*>(a, b) which gives the same results as less<cv char*>((cv char*)(cv void*)a, (cv char*)(cv void*)b).
I'm also thinking there should be a footnote to 20.5.3/1 saying that if A and B are similar types (4.4/4), comp<A>(a,b) returns the same value as comp<B>(a,b) (where comp is less, less_equal, etc.). But this might be problematic if there is some really funky operator overloading going on that does different things based on cv (that should be undefined behavior if somebody does that though). This at least should be guaranteed for all POD types (especially pointers) that use the built-in comparison operators.
Rationale:
less is already required to provide a strict weak ordering which is good enough to detect overlapping memory situations.
Section: 22.3.1.1.1 [locale.category] Status: NAD Submitter: Christopher Conrade Zseleghovski Opened: 2005-06-07 Last modified: 2009-07-15
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Discussion:
Motivation:
This requirement seems obvious to me, it is the essence of code modularity. I have complained to Mr. Plauger that the Dinkumware library does not observe this principle but he objected that this behaviour is not covered in the standard.
[ 2009-07 Frankfurt ]
No objection to NAD, Fixed.
Move to NAD.
Proposed resolution:
Append the following point to 22.1.1.1.1:
6. The implementation of a facet of Table 52 parametrized with an InputIterator/OutputIterator should use that iterator only as character source/sink respectively. For a *_get facet, it means that the value received depends only on the sequence of input characters and not on how they are accessed. For a *_put facet, it means that the sequence of characters output depends only on the value to be formatted and not of how the characters are stored.
[ Berlin: Moved to Open, Need to clean up this area to make it clear locales don't have to contain open ended sets of facets. Jack, Howard, Bill. ]
Section: 22.4 [locale.categories] Status: NAD Submitter: P.J. Plauger Opened: 2005-06-20 Last modified: 2009-07-15
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Discussion:
a) In 22.2.1.1 para. 2 we refer to "the instantiations required in Table 51" to refer to the facet *objects* associated with a locale. And we almost certainly mean just those associated with the default or "C" locale. Otherwise, you can't switch to a locale that enforces a different mapping between narrow and wide characters, or that defines additional uppercase characters.
b) 22.2.1.5 para. 3 (codecvt) has the same issues.
c) 22.2.1.5.2 (do_unshift) is even worse. It *forbids* the generation of a homing sequence for the basic character set, which might very well need one.
d) 22.2.1.5.2 (do_length) likewise dictates that the default mapping between wide and narrow characters be taken as one-for-one.
e) 22.2.2 para. 2 (num_get/put) is both muddled and vacuous, as far as I can tell. The muddle is, as before, calling Table 51 a list of instantiations. But the constraint it applies seems to me to cover *all* defined uses of num_get/put, so why bother to say so?
f) 22.2.3.1.2 para. 1(do_decimal_point) says "The required instantiations return '.' or L'.'.) Presumably this means "as appropriate for the character type. But given the vague definition of "required" earlier, this overrules *any* change of decimal point for non "C" locales. Surely we don't want to do that.
g) 22.2.3.1.2 para. 2 (do_thousands_sep) says "The required instantiations return ',' or L','.) As above, this probably means "as appropriate for the character type. But this overrules the "C" locale, which requires *no* character ('\0') for the thousands separator. Even if we agree that we don't mean to block changes in decimal point or thousands separator, we should also eliminate this clear incompatibility with C.
h) 22.2.3.1.2 para. 2 (do_grouping) says "The required instantiations return the empty string, indicating no grouping." Same considerations as for do_decimal_point.
i) 22.2.4.1 para. 1 (collate) refers to "instantiations required in Table 51". Same bad jargon.
j) 22.2.4.1.2 para. 1 (do_compare) refers to "instantiations required in Table 51". Same bad jargon.
k) 22.2.5 para. 1 (time_get/put) uses the same muddled and vacuous as num_get/put.
l) 22.2.6 para. 2 (money_get/put) uses the same muddled and vacuous as num_get/put.
m) 22.2.6.3.2 (do_pos/neg_format) says "The instantiations required in Table 51 ... return an object of type pattern initialized to {symbol, sign, none, value}." This once again *overrides* the "C" locale, as well as any other locale."
3) We constrain the use_facet calls that can be made by num_get/put, so why don't we do the same for money_get/put? Or for any of the other facets, for that matter?
4) As an almost aside, we spell out when a facet needs to use the ctype facet, but several also need to use a codecvt facet and we don't say so.
[ Berlin: Bill to provide wording. ]
[ 2009-07 Frankfurt ]
No objection to NAD.
Move to NAD.
Proposed resolution:
Section: 26.5.1 [rand.req], TR1 5.1.1 [tr.rand.req] Status: NAD Editorial Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2008-03-13
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Discussion:
In [tr.rand.req], Paragraph 2 states that "... s is a value of integral type, g is an ... object returning values of unsigned integral type ..."
Proposed resolution:
In 5.1.1 [tr.rand.req], Paragraph 2 replace
... s is a value of integral type, g is an lvalue of a type other than X that defines a zero-argument function object returning values of
unsigned integraltype unsigned long int, ...
In 5.1.1 [tr.rand.seq], Table 16, replace in the line for X(s)
creates an engine with the initial internal state determined by static_cast<unsigned long>(s)
[ Mont Tremblant: Both s and g should be unsigned long. This should refer to the constructor signatures. Jens provided wording post Mont Tremblant. ]
[ Berlin: N1932 adopts the proposed resolution: see 26.3.1.3/1e and Table 3 row 2. Moved to Ready. ]
Rationale:
Jens: Just requiring X(unsigned long) still makes it possible for an evil library writer to also supply a X(int) that does something unexpected. The wording above requires that X(s) always performs as if X(unsigned long) would have been called. I believe that is sufficient and implements our intentions from Mont Tremblant. I see no additional use in actually requiring a X(unsigned long) signature. u.seed(s) is covered by its reference to X(s), same arguments.
[ Portland: Subsumed by N2111. ]
Section: 26.5 [rand], TR1 5.1.3 [tr.rand.var] Status: NAD Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2008-03-13
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Discussion:
Paragraph 3 requires that template argument U (which corresponds to template parameter Engine) satisfy all uniform random number generator requirements. However, there is no analogous requirement regarding the template argument that corresponds to template parameter Distribution. We believe there should be, and that it should require that this template argument satisfy all random distribution requirements.
Proposed resolution:
Consequence 1: Remove the precondition clauses [tr.rand.var]/16 and /18.
Consequence 2: Add max() and min() functions to those distributions that do not already have them.
[ Mont Tremblant: Jens reccommends NAD, min/max not needed everywhere. Marc supports having min and max to satisfy generic programming interface. ]
Rationale:
Berlin: N1932 makes this moot: variate_generator has been eliminated.
Section: 26.5.8.1 [rand.dist.uni], TR1 5.1.7.1 [tr.rand.dist.iunif] Status: NAD Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2008-03-13
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Discussion:
In [tr.rand.dist.iunif] the uniform_int distribution currently has a single template parameter, IntType, used as the input_type and as the result_type of the distribution. We believe there is no reason to conflate these types in this way.
Proposed resolution:
We recommend that there be a second template parameter to reflect the distribution's input_type, and that the existing first template parameter continue to reflect (solely) the result_type:
template< class IntType = int, UIntType = unsigned int >
class uniform_int
{
public:
// types
typedef UIntType input_type;
typedef IntType result_type;
[ Berlin: Moved to NAD. N1932 makes this moot: the input_type template parameter has been eliminated. ]
Section: 26.5.8.2 [rand.dist.bern], TR1 5.1.7.2 [tr.rand.dist.bern] Status: NAD Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2008-03-13
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Discussion:
In [tr.rand.dist.bern] the distribution currently requires;
typedef int input_type;
Proposed resolution:
We believe this is an unfortunate choice, and recommend instead:
typedef unsigned int input_type;
[ Berlin: Moved to NAD. N1932 makes this moot: the input_type template parameter has been eliminated. ]
Section: 26.5.8 [rand.dist], TR1 5.1.7.5 [tr.rand.dist.bin] Status: NAD Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2008-03-13
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Discussion:
Unlike all other distributions in TR1, this binomial_distribution has an implementation-defined input_type. We believe this is an unfortunate choice, because it hinders users from writing portable code. It also hinders the writing of compliance tests. We recommend instead:
typedef RealType input_type;
While this choice is somewhat arbitrary (as it was for some of the other distributions), we make this particular choice because (unlike all other distributions) otherwise this template would not publish its RealType argument and so users could not write generic code that accessed this second template parameter. In this respect, the choice is consistent with the other distributions in TR1.
We have two reasons for recommending that a real type be specified instead. One reason is based specifically on characteristics of binomial distribution implementations, while the other is based on mathematical characteristics of probability distribution functions in general.
Implementations of binomial distributions commonly use Stirling approximations for values in certain ranges. It is far more natural to use real values to represent these approximations than it would be to use integral values to do so. In other ranges, implementations reply on the Bernoulli distribution to obtain values. While TR1's bernoulli_distribution::input_type is specified as int, we believe this would be better specified as double.
This brings us to our main point: The notion of a random distribution rests on the notion of a cumulative distribution function, which in turn mathematically depends on a continuous dependent variable. Indeed, such a distribution function would be meaningless if it depended on discrete values such as integers - and this remains true even if the distribution function were to take discrete steps.
Although this note is specifically about binomial_distribution::input_type, we intend to recommend that all of the random distributions input_types be specified as a real type (either a RealType template parameter, or double, as appropriate).
Of the nine distributions in TR1, four already have this characteristic (uniform_real, exponential_distribution, normal_distribution, and gamma_distribution). We have already argued the case for the binomial the remaining four distributions.
In the case of uniform_int, we believe that the calculations to produce an integer result in a specified range from an integer in a different specified range is best done using real arithmetic. This is because it involves a product, one of whose terms is the ratio of the extents of the two ranges. Without real arithmetic, the results become less uniform: some numbers become more (or less) probable that they should be. This is, of course, undesireable behavior in a uniform distribution.
Finally, we believe that in the case of the bernoulli_distribution (briefly mentioned earlier), as well as the cases of the geometric_distribution and the poisson_distribution, it would be far more natural to have a real input_type. This is because the most natural computation involves the random number delivered and the distribution's parameter p (in the case of bernoulli_distribution, for example, the computation is a comparison against p), and p is already specified in each case as having some real type.
Proposed resolution:
typedef RealType input_type;
[ Berlin: Moved to NAD. N1932 makes this moot: the input_type template parameter has been eliminated. ]
Section: 26.5.3 [rand.eng], TR1 5.1.4.4 [tr.rand.eng.sub1] Status: NAD Editorial Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2008-03-13
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Discussion:
Paragraph 8 specifies the algorithm by which a subtract_with_carry_01 engine is to be seeded given a single unsigned long. This algorithm is seriously flawed in the case where the engine parameter w (also known as word_size) exceeds 31 [bits]. The key part of the paragraph reads:
sets x(-r) ... x(-1) to (lcg(1)*2**(-w)) mod 1
and so forth.
Since the specified linear congruential engine, lcg, delivers numbers with a maximum of 2147483563 (just a shade under 31 bits), then when w is, for example, 48, each of the x(i) will be less than 2**-17. The consequence is that roughly the first 400 numbers delivered will be conspicuously close to either zero or one.
Unfortunately, this is not an innocuous flaw: One of the predefined engines in [tr.rand.predef], namely ranlux64_base_01, has w = 48 and would exhibit this poor behavior, while the original N1378 proposal states that these pre-defined engines are intended to be of "known good properties."
Proposed resolution:
In 5.1.4.4 [tr.rand.eng.sub1], replace the "effects" clause for void seed(unsigned long value = 19780503) by
Effects: If value == 0, sets value to 19780503. In any case,
with a linear congruential generator lcg(i) having parameters mlcg = 2147483563, alcg = 40014, clcg = 0, and lcg(0) = value,sets carry(-1) and x(-r) … x(-1) as if executinglinear_congruential<unsigned long, 40014, 0, 2147483563> lcg(value); seed(lcg);
to (lcg(1) · 2-w) mod 1 … (lcg(r) · 2-w) mod 1, respectively. If x(-1) == 0, sets carry(-1) = 2-w, else sets carry(-1) = 0.
[ Jens provided revised wording post Mont Tremblant. ]
[ Berlin: N1932 adopts the originally-proposed resolution of the issue. Jens's supplied wording is a clearer description of what is intended. Moved to Ready. ]
Rationale:
Jens: I'm using an explicit type here, because fixing the prose would probably not qualify for the (with issue 504 even stricter) requirements we have for seed(Gen&).
[ Portland: Subsumed by N2111. ]
Section: 26.5.3 [rand.eng], TR1 5.1.4.4 [tr.rand.eng.sub1] Status: NAD Editorial Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2008-03-13
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Discussion:
Paragraph 3 begins:
The size of the state is r.
However, this is not quite consistent with the remainder of the paragraph which specifies a total of nr+1 items in the textual representation of the state. We recommend the sentence be corrected to match:
The size of the state is nr+1.
To give meaning to the coefficient n, it may be also desirable to move n's definition from later in the paragraph. Either of the following seem reasonable formulations:
With n=..., the size of the state is nr+1.
The size of the state is nr+1, where n=... .
Proposed resolution:
[ Jens: I plead for "NAD" on the grounds that "size of state" is only used as an argument for big-O complexity notation, thus constant factors and additions don't count. ]
[ Berlin: N1932 adopts the proposed NAD. ]
Section: 26.5.3.3 [rand.eng.sub], TR1 5.1.4.3 [tr.rand.eng.sub] Status: NAD Editorial Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2008-03-13
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Discussion:
Paragraph 2 begins:
The size of the state is r.
However, the next sentence specifies a total of r+1 items in the textual representation of the state, r specific x's as well as a specific carry. This makes a total of r+1 items that constitute the size of the state, rather than r.
Proposed resolution:
We recommend the sentence be corrected to match:
The size of the state is r+1.
[ Jens: I plead for "NAD" on the grounds that "size of state" is only used as an argument for big-O complexity notation, thus constant factors and additions don't count. ]
[ Berlin: N1932 adopts the proposed NAD. ]
Section: 26.5.2 [rand.synopsis], TR1 5.1.2 [tr.rand.synopsis] Status: NAD Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2008-03-13
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Discussion:
To accompany the concept of a pseudo-random number engine as defined in Table 17, we propose and recommend an adjunct template, engine_traits, to be declared in [tr.rand.synopsis] as:
template< class PSRE > class engine_traits;
This template's primary purpose would be as an aid to generic programming involving pseudo-random number engines. Given only the facilities described in tr1, it would be very difficult to produce any algorithms involving the notion of a generic engine. The intent of this proposal is to provide, via engine_traits<>, sufficient descriptive information to allow an algorithm to employ a pseudo-random number engine without regard to its exact type, i.e., as a template parameter.
For example, today it is not possible to write an efficient generic function that requires any specific number of random bits. More specifically, consider a cryptographic application that internally needs 256 bits of randomness per call:
template< class Eng, class InIter, class OutIter > void crypto( Eng& e, InIter in, OutIter out );
Without knowning the number of bits of randomness produced per call to a provided engine, the algorithm has no means of determining how many times to call the engine.
In a new section [tr.rand.eng.traits], we proposed to define the engine_traits template as:
template< class PSRE >
class engine_traits
{
static std::size_t bits_of_randomness = 0u;
static std::string name() { return "unknown_engine"; }
// TODO: other traits here
};
Further, each engine described in [tr.rand.engine] would be accompanied by a complete specialization of this new engine_traits template.
Proposed resolution:
[ Berlin: Walter: While useful for implementation per TR1, N1932 has no need for this feature. Recommend close as NAD. ]
Rationale:
Recommend NAD, N1932, N2111 covers this. Already in WP.
Section: 26.5.3 [rand.eng], TR1 5.1.4.4 [tr.rand.eng.sub1] Status: NAD Editorial Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2008-03-13
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Discussion:
Paragraph 6 says:
... obtained by successive invocations of g, ...
We recommend instead:
... obtained by taking successive invocations of g mod 2**32, ...
as the context seems to require only 32-bit quantities be used here.
Proposed resolution:
Berlin: N1932 adopts the proposed resultion: see 26.3.3.4/7. Moved to Ready.
[ Portland: Subsumed by N2111. ]
Section: 26.5.1 [rand.req], TR1 5.1.1 [tr.rand.req] Status: NAD Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2008-03-13
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Discussion:
The last two rows of Table 16 deal with the i/o requirements of an engine, specifying that the textual representation of an engine's state, appropriately formatted, constitute the engine's external representation.
This seems adequate when an engine's type is known. However, it seems inadequate in the context of generic code, where it becomes useful and perhaps even necessary to determine an engine's type via input.
Proposed resolution:
We therefore recommend that, in each of these two rows of Table 16, the text "textual representation" be expanded so as to read "engine name followed by the textual representation."
[ Berlin: N1932 considers this NAD. This is a QOI issue. ]
Section: 28 [re] Status: NAD Future Submitter: Eric Niebler Opened: 2005-07-01 Last modified: 2009-07-15
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Discussion:
A problem with TR1 regex is currently being discussed on the Boost developers list. It involves the handling of case-insensitive matching of character ranges such as [Z-a]. The proper behavior (according to the ECMAScript standard) is unimplementable given the current specification of the TR1 regex_traits<> class template. John Maddock, the author of the TR1 regex proposal, agrees there is a problem. The full discussion can be found at http://lists.boost.org/boost/2005/06/28850.php (first message copied below). We don't have any recommendations as yet.
-- Begin original message --
The situation of interest is described in the ECMAScript specification (ECMA-262), section 15.10.2.15:
"Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case ASCII letters as well as the symbols [, \, ], ^, _, and `."
A more interesting case is what should happen when doing a case-insentitive match on a range such as [Z-a]. It should match z, Z, a, A and the symbols [, \, ], ^, _, and `. This is not what happens with Boost.Regex (it throws an exception from the regex constructor).
The tough pill to swallow is that, given the specification in TR1, I don't think there is any effective way to handle this situation. According to the spec, case-insensitivity is handled with regex_traits<>::translate_nocase(CharT) -- two characters are equivalent if they compare equal after both are sent through the translate_nocase function. But I don't see any way of using this translation function to make character ranges case-insensitive. Consider the difficulty of detecting whether "z" is in the range [Z-a]. Applying the transformation to "z" has no effect (it is essentially std::tolower). And we're not allowed to apply the transformation to the ends of the range, because as ECMA-262 says, "the case of the two ends of a range is significant."
So AFAICT, TR1 regex is just broken, as is Boost.Regex. One possible fix is to redefine translate_nocase to return a string_type containing all the characters that should compare equal to the specified character. But this function is hard to implement for Unicode, and it doesn't play nice with the existing ctype facet. What a mess!
-- End original message --
[ John Maddock adds: ]
One small correction, I have since found that ICU's regex package does implement this correctly, using a similar mechanism to the current TR1.Regex.
Given an expression [c1-c2] that is compiled as case insensitive it:
Enumerates every character in the range c1 to c2 and converts it to it's case folded equivalent. That case folded character is then used a key to a table of equivalence classes, and each member of the class is added to the list of possible matches supported by the character-class. This second step isn't possible with our current traits class design, but isn't necessary if the input text is also converted to a case-folded equivalent on the fly.
ICU applies similar brute force mechanisms to character classes such as [[:lower:]] and [[:word:]], however these are at least cached, so the impact is less noticeable in this case.
Quick and dirty performance comparisons show that expressions such as "[X-\\x{fff0}]+" are indeed very slow to compile with ICU (about 200 times slower than a "normal" expression). For an application that uses a lot of regexes this could have a noticeable performance impact. ICU also has an advantage in that it knows the range of valid characters codes: code points outside that range are assumed not to require enumeration, as they can not be part of any equivalence class. I presume that if we want the TR1.Regex to work with arbitrarily large character sets enumeration really does become impractical.
Finally note that Unicode has:
Three cases (upper, lower and title). One to many, and many to one case transformations. Character that have context sensitive case translations - for example an uppercase sigma has two different lowercase forms - the form chosen depends on context(is it end of a word or not), a caseless match for an upper case sigma should match either of the lower case forms, which is why case folding is often approximated by tolower(toupper(c)).
Probably we need some way to enumerate character equivalence classes, including digraphs (either as a result or an input), and some way to tell whether the next character pair is a valid digraph in the current locale.
Hoping this doesn't make this even more complex that it was already,
[ Portland: Alisdair: Detect as invalid, throw an exception. Pete: Possible general problem with case insensitive ranges. ]
[ 2009-07 Frankfurt ]
We agree that this is a problem, but we do not know the answer.
We are going to declare this NAD until existing practice leads us in some direction.
No objection to NAD Future.
Move to NAD Future.
Proposed resolution:
Section: 20.7.4 [meta.unary], TR1 4.5 [tr.meta.unary] Status: NAD Editorial Submitter: Robert Klarer Opened: 2005-07-11 Last modified: 2008-03-13
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Discussion:
It is not completely clear how the primary type traits deal with cv-qualified types. And several of the secondary type traits seem to be lacking a definition.
[ Berlin: Howard to provide wording. ]
Proposed resolution:
Wording provided in N2028. A revision (N2157) provides more detail for motivation.
Rationale:
Solved by revision (N2157) in the WP.Section: 23.2.3 [sequence.reqmts] Status: NAD Submitter: Chris Jefferson Opened: 2005-09-14 Last modified: 2008-03-13
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Discussion:
Problem: There are a number of places in the C++ standard library where it is possible to write what appear to be sensible ways of calling functions, but which can cause problems in some (or all) implementations, as they cause the values given to the function to be changed in a way not specified in standard (and therefore not coded to correctly work). These fall into two similar categories.
1) Parameters taken by const reference can be changed during execution of the function
Examples:
Given std::vector<int> v:
v.insert(v.begin(), v[2]);
v[2] can be changed by moving elements of vector
Given std::list<int> l:
l.remove(*l.begin());
Will delete the first element, and then continue trying to access it. This is particularily vicious, as it will appear to work in almost all cases.
2) A range is given which changes during the execution of the function: Similarly,
v.insert(v.begin(), v.begin()+4, v.begin()+6);
This kind of problem has been partly covered in some cases. For example std::copy(first, last, result) states that result cannot be in the range [first, last). However, does this cover the case where result is a reverse_iterator built from some iterator in the range [first, last)? Also, std::copy would still break if result was reverse_iterator(last + 1), yet this is not forbidden by the standard
Solution:
One option would be to try to more carefully limit the requirements of each function. There are many functions which would have to be checked. However as has been shown in the std::copy case, this may be difficult. A simpler, more global option would be to somewhere insert text similar to:
If the execution of any function would change either any values passed by reference or any value in any range passed to a function in a way not defined in the definition of that function, the result is undefined.
Such code would have to at least cover chapters 23 and 25 (the sections I read through carefully). I can see no harm on applying it to much of the rest of the standard.
Some existing parts of the standard could be improved to fit with this, for example the requires for 25.2.1 (Copy) could be adjusted to:
Requires: For each non-negative integer n < (last - first), assigning to *(result + n) must not alter any value in the range [first + n, last).
However, this may add excessive complication.
One other benefit of clearly introducing this text is that it would allow a number of small optimisations, such as caching values passed by const reference.
Matt Austern adds that this issue also exists for the insert and erase members of the ordered and unordered associative containers.
[ Berlin: Lots of controversey over how this should be solved. Lots of confusion as to whether we're talking about self referencing iterators or references. Needs a good survey as to the cases where this matters, for which implementations, and how expensive it is to fix each case. ]
Proposed resolution:
Rationale:
Recommend NAD.
Section: 23.5 [unord], TR1 6.3.4 [tr.unord.unord] Status: NAD Submitter: Paolo Carlini Opened: 2005-10-12 Last modified: 2008-03-13
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Discussion:
while implementing the resolution of issue 6.19 I'm noticing the following: according to 6.3.4.3/2 (and 6.3.4.5/2), for unordered_set and unordered_multiset:
"The iterator and const_iterator types are both const types. It is unspecified whether they are the same type"
Now, according to the resolution of 6.19, we have overloads of insert with hint and erase (single and range) both for iterator and const_iterator, which, AFAICS, can be meaningful at the same time *only* if iterator and const_iterator *are* in fact different types.
Then, iterator and const_iterator are *required* to be different types? Or that is an unintended consequence? Maybe the overloads for plain iterators should be added only to unordered_map and unordered_multimap? Or, of course, I'm missing something?
Proposed resolution:
Add to 6.3.4.3p2 (and 6.3.4.5p2):
2 ... The iterator and const_iterator types are both const
constant iterator types.
It is unspecified whether they are the same type.
Add a new subsection to 17.4.4 [lib.conforming]:
An implementation shall not supply an overloaded function signature specified in any library clause if such a signature would be inherently ambiguous during overload resolution due to two library types referring to the same type.
[Note: For example, this occurs when a container's iterator and const_iterator types are the same. -- end note]
[ Post-Berlin: Beman supplied wording. ]
Rationale:
Toronto: The first issue has been fixed by N2350 (the insert and erase members are collapsed into one signature). Alisdair to open a separate issue on the chapter 17 wording.Section: 17.6.3.11 [res.on.required] Status: NAD Editorial Submitter: David Abrahams Opened: 2005-10-25 Last modified: 2008-03-13
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Discussion:
17.4.3.8/1 says:
Violation of the preconditions specified in a function's Required behavior: paragraph results in undefined behavior unless the function's Throws: paragraph specifies throwing an exception when the precondition is violated.
This implies that a precondition violation can lead to defined behavior. That conflicts with the only reasonable definition of precondition: that a violation leads to undefined behavior. Any other definition muddies the waters when it comes to analyzing program correctness, because precondition violations may be routinely done in correct code (e.g. you can use std::vector::at with the full expectation that you'll get an exception when your index is out of range, catch the exception, and continue). Not only is it a bad example to set, but it encourages needless complication and redundancy in the standard. For example:
21 Strings library 21.3.3 basic_string capacity void resize(size_type n, charT c); 5 Requires: n <= max_size() 6 Throws: length_error if n > max_size(). 7 Effects: Alters the length of the string designated by *this as follows:
The Requires clause is entirely redundant and can be dropped. We could make that simplifying change (and many others like it) even without changing 17.4.3.8/1; the wording there just seems to encourage the redundant and error-prone Requires: clause.
[ Batavia: Alan and Pete to work. ]
[ Bellevue: NAD Editorial, this group likes N2121, Pete agrees, accepting it is Pete's business. General agreement that precondition violations are synonymous with UB. ]
Proposed resolution:
1. Change 17.4.3.8/1 to read:
Violation of the preconditions specified in a function's Required behavior: paragraph results in undefined behavior
unless the function's Throws: paragraph specifies throwing an exception when the precondition is violated.
2. Go through and remove redundant Requires: clauses. Specifics to be provided by Dave A.
[ Berlin: The LWG requests a detailed survey of part 2 of the proposed resolution. ]
[ Alan provided the survey N2121. ]
Section: 20.4.2.7 [tuple.rel], TR1 6.1.3.5 [tr.tuple.rel] Status: NAD Future Submitter: David Abrahams Opened: 2005-11-29 Last modified: 2009-10-24
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Duplicate of: 348
Discussion:
Where possible, tuple comparison operators <,<=,=>, and > ought to be defined in terms of std::less rather than operator<, in order to support comparison of tuples of pointers.
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
If we solve this for tuple we would have to solve it for pair algorithms, etc. It is too late to do that at this time. Move to NAD Future.
Proposed resolution:
change 6.1.3.5/5 from:
Returns: The result of a lexicographical comparison between t and u. The result is defined as: (bool)(get<0>(t) < get<0>(u)) || (!(bool)(get<0>(u) < get<0>(t)) && ttail < utail), where rtail for some tuple r is a tuple containing all but the first element of r. For any two zero-length tuples e and f, e < f returns false.
to:
Returns: The result of a lexicographical comparison between t and u. For any two zero-length tuples e and f, e < f returns false. Otherwise, the result is defined as: cmp( get<0>(t), get<0>(u)) || (!cmp(get<0>(u), get<0>(t)) && ttail < utail), where rtail for some tuple r is a tuple containing all but the first element of r, and cmp(x,y) is an unspecified function template defined as follows.
Where T is the type of x and U is the type of y:
if T and U are pointer types and T is convertible to U, returns less<U>()(x,y)
otherwise, if T and U are pointer types, returns less<T>()(x,y)
otherwise, returns (bool)(x < y)
[ Berlin: This issue is much bigger than just tuple (pair, containers, algorithms). Dietmar will survey and work up proposed wording. ]
Rationale:
Recommend NAD. This will be fixed with the next revision of concepts.
[ San Francisco: ]
Solved by N2770.
Section: 23.2 [container.requirements] Status: Dup Submitter: Joaquín M López Muñoz Opened: 2005-12-17 Last modified: 2007-04-18
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Duplicate of: 589
Discussion:
The iterator constructor X(i,j) for containers as defined in 23.1.1 and 23.2.2 does only require that i and j be input iterators but nothing is said about their associated value_type. There are three sensible options:
The issue has practical implications, and stdlib vendors have taken divergent approaches to it: Dinkumware follows 2, libstdc++ follows 3.
The same problem applies to the definition of insert(p,i,j) for sequences and insert(i,j) for associative contianers, as well as assign.
[ The following added by Howard and the example code was originally written by Dietmar. ]
Valid code below?
#include <vector>
#include <iterator>
#include <iostream>
struct foo
{
explicit foo(int) {}
};
int main()
{
std::vector<int> v_int;
std::vector<foo> v_foo1(v_int.begin(), v_int.end());
std::vector<foo> v_foo2((std::istream_iterator<int>(std::cin)),
std::istream_iterator<int>());
}
[ Berlin: Some support, not universal, for respecting the explicit qualifier. ]
Proposed resolution:
Section: C.2 [diff.library] Status: NAD Editorial Submitter: Martin Sebor Opened: 2005-11-25 Last modified: 2007-04-24
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Discussion:
According to C.2.2.3, p1, "the macro NULL, defined in any of <clocale>, <cstddef>, <cstdio>, <cstdlib>, <cstring>, <ctime>, or <cwchar>." This is consistent with the C standard.
However, Table 95 in C.2 fails to mention <clocale> and <cstdlib>.
In addition, C.2, p2 claims that "The C++ Standard library provides 54 standard macros from the C library, as shown in Table 95." While table 95 does have 54 entries, since a couple of them (including the NULL macro) are listed more than once, the actual number of macros defined by the C++ Standard Library may not be 54.
Proposed resolution:
I propose we add <clocale> and <cstdlib> to Table 96 and remove the number of macros from C.2, p2 and reword the sentence as follows:
The C++ Standard library
provides 54 standard macros fromdefines a number macros corresponding to those defined by the C Standard library, as shown in Table 96.
[ Portland: Resolution is considered editorial. It will be incorporated into the WD. ]
Section: TR1 5.1.1 [tr.rand.req] Status: NAD Submitter: Matt Austern Opened: 2006-01-10 Last modified: 2009-07-15
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Discussion:
The TR sneaks in two new integer types, _Longlong and _Ulonglong, in [tr.c99]. The rest of the TR should use that type. I believe this affects two places. First, the random number requirements, 5.1.1/10-11, lists all of the types with which template parameters named IntType and UIntType may be instantiated. _Longlong (or "long long", assuming it is added to C++0x) should be added to the IntType list, and UIntType (again, or "unsigned long long") should be added to the UIntType list. Second, 6.3.2 lists the types for which hash<> is required to be instantiable. _Longlong and _Ulonglong should be added to that list, so that people may use long long as a hash key.
[ 2009-07 Frankfurt ]
We are not going to fix TR1.
The paper "long long goes to the library" addresses the integration of long long into the C++0x library.
Move to NAD.
Proposed resolution:
Section: 26.5 [rand], TR1 5.1 [tr.rand] Status: NAD Submitter: Matt Austern Opened: 2006-01-10 Last modified: 2007-04-18
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Discussion:
Paragraph 10 describes how a variate generator uses numbers produced by an engine to pass to a generator. The sentence that concerns me is: "Otherwise, if the value for engine_value_type::result_type is true and the value for Distribution::input_type is false [i.e. if the engine produces integers and the engine wants floating-point values], then the numbers in s_eng are divided by engine().max() - engine().min() + 1 to obtain the numbers in s_e." Since the engine is producing integers, both the numerator and the denominator are integers and we'll be doing integer division, which I don't think is what we want. Shouldn't we be performing a conversion to a floating-point type first?
Proposed resolution:
Rationale:
Recommend NAD as the affected section is now gone and so the issue is moot. N2111.
Section: 26.5.6 [rand.device], TR1 5.1.6 [tr.rand.device] Status: NAD Submitter: Matt Austern Opened: 2006-01-10 Last modified: 2007-10-18
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Discussion:
Class random_device "produces non-deterministic random numbers", using some external source of entropy. In most real-world systems, the amount of available entropy is limited. Suppose that entropy has been exhausted. What is an implementation permitted to do? In particular, is it permitted to block indefinitely until more random bits are available, or is the implementation required to detect failure immediately? This is not an academic question. On Linux a straightforward implementation would read from /dev/random, and "When the entropy pool is empty, reads to /dev/random will block until additional environmental noise is gathered." Programmers need to know whether random_device is permitted to (or possibly even required to?) behave the same way.
[ Berlin: Walter: N1932 considers this NAD. Does the standard specify whether std::cin may block? ]
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423 (NAD).
Section: 26.5.8 [rand.dist], TR1 5.1.7.5 [tr.rand.dist.bin] Status: NAD Editorial Submitter: Matt Austern Opened: 2006-01-10 Last modified: 2007-04-24
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Discussion:
Paragraph 1 says that "A binomial distributon random distribution produces integer values i>0 with p(i) = (n choose i) * p*i * (1-p)^(t-i), where t and p are the parameters of the distribution. OK, that tells us what t, p, and i are. What's n?
Proposed resolution:
Berlin: Typo: "n" replaced by "t" in N1932: see 26.3.7.2.2/1.
[ Portland: Subsumed by N2111. ]
Section: 18.4.1 [cstdint.syn], TR1 8.22.1 [tr.c99.cstdint.syn] Status: NAD Editorial Submitter: Paolo Carlini Opened: 2006-01-30 Last modified: 2007-07-02
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Discussion:
In the synopsis, some types are identified as optional: int8_t, int16_t, and so on, consistently with C99, indeed.
On the other hand, intptr_t and uintptr_t, are not marked as such and probably should, consistently with C99, 7.18.1.4.
Proposed resolution:
Change 18.4.1 [cstdint.syn]:
... typedef signed integer type intptr_t; // optional ... typedef unsigned integer type uintptr_t; // optional ...
Rationale:
Recommend NAD and fix as editorial with the proposed resolution.Section: 18.3.1.5 [numeric.special] Status: NAD Submitter: Howard Hinnant Opened: 2006-01-29 Last modified: 2007-01-15
View all other issues in [numeric.special].
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Discussion:
I believe we have a bug in the resolution of: lwg 184 (WP status).
The resolution spells out each member of numeric_limits<bool>. The part I'm having a little trouble with is:
static const bool traps = false;
Should this not be implementation defined? Given:
int main()
{
bool b1 = true;
bool b2 = false;
bool b3 = b1/b2;
}
If this causes a trap, shouldn't numeric_limits<bool>::traps be true?
Proposed resolution:
Change 18.2.1.5p3:
-3- The specialization for bool shall be provided as follows:
namespace std { template <> class numeric_limits<bool> { ... static const bool traps =falseimplementation-defined; ... }; }
[ Redmond: NAD because traps refers to values, not operations. There is no bool value that will trap. ]
Section: TR1 8.21 [tr.c99.boolh] Status: NAD Editorial Submitter: Paolo Carlini Opened: 2006-02-02 Last modified: 2007-04-24
View all issues with NAD Editorial status.
Discussion:
This one, if nobody noticed it yet, seems really editorial: s/cstbool/cstdbool/
Proposed resolution:
Change 8.21p1:
-1- The header behaves as if it defines the additional macro defined in <cstdbool> by including the header <cstdbool>.
[ Redmond: Editorial. ]
Section: 18.4 [cstdint], TR1 8.22 [tr.c99.cstdint] Status: NAD Editorial Submitter: Paolo Carlini Opened: 2006-02-06 Last modified: 2008-07-02
View all other issues in [cstdint].
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Discussion:
I'm seeing a problem with such overloads: when, _Longlong == intmax_t == long long we end up, essentially, with the same arguments and different return types (lldiv_t and imaxdiv_t, respectively). Similar issue with abs(_Longlong) and abs(intmax_t), of course.
Comparing sections 8.25 and 8.11, I see an important difference, however: 8.25.3 and 8.25.4 carefully describe div and abs for _Longlong types (rightfully, because not moved over directly from C99), whereas there is no equivalent in 8.11: the abs and div overloads for intmax_t types appear only in the synopsis and are not described anywhere, in particular no mention in 8.11.2 (at variance with 8.25.2).
I'm wondering whether we really, really, want div and abs for intmax_t...
Proposed resolution:
[ Portland: no consensus. ]
Rationale:
[ Batavia, Bill: The <cstdint> synopsis in TR1 8.11.1 [tr.c99.cinttypes.syn] contains: ]
intmax_t imaxabs(intmax_t i); intmax_t abs(intmax_t i); imaxdiv_t imaxdiv(intmax_t numer, intmax_t denom); imaxdiv_t div(intmax_t numer, intmax_t denom);
[ and in TR1 8.11.2 [tr.c99.cinttypes.def]: ]
The header defines all functions, types, and macros the same as C99 subclause 7.8.
[ This is as much definition as we give for most other C99 functions, so nothing need change. We might, however, choose to add the footnote: ]
[Note: These overloads for abs and div may well be equivalent to those that take long long arguments. If so, the implementation is responsible for avoiding conflicting declarations. -- end note]
[ Bellevue: NAD Editorial. Pete must add a footnote, as described below. ]
[ Looks like a real problem. Dietmar suggests div() return a template type. Matt: looks like imaxdiv_t is loosly defined. Can it be a typedef for lldiv_t when _Longlong == intmax_t? PJP seems to agree. We would need a non-normative note declaring that the types lldiv_t and imaxdiv_t may not be unique if intmax_t==_longlong. ]
Section: 24.2.3 [input.iterators] Status: NAD Editorial Submitter: David Abrahams Opened: 2006-02-09 Last modified: 2007-04-24
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Discussion:
24.1.1 Input iterators [lib.input.iterators]
1 A class or a built-in type X satisfies the requirements of an input iterator for the value type T if the following expressions are valid, where U is the type of any specified member of type T, as shown in Table 73.
There is no capital U used in table 73. There is a lowercase u, but that is clearly not meant to denote a member of type T. Also, there's no description in 24.1.1 of what lowercase a means. IMO the above should have been...Hah, a and b are already covered in 24.1/11, so maybe it should have just been:
Proposed resolution:
Change 24.1.1p1:
-1- A class or a built-in type X satisfies the requirements of an input iterator for the value type T if the following expressions are valid
, where U is the type of any specified member of type T,as shown in Table 73.
[ Portland: Editorial. ]
Section: 20.2.5 [allocator.requirements] Status: NAD Submitter: Sergey P. Derevyago Opened: 2006-02-17 Last modified: 2007-04-18
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Discussion:
User-defined allocators without default constructor are not explicitly supported by the standard but they can be supported just like std::vector supports elements without default constructor.
As a result, there exist implementations that work well with such allocators and implementations that don't.
1) The standard doesn't explicitly state this intent but it should. In particular, 20.1.5p5 explicitly state the intent w.r.t. the allocator instances that compare non-equal. So it can similarly state the intent w.r.t. the user-defined allocators without default constructor.
2) Some container operations are obviously underspecified. In particular, 21.3.7.1p2 tells:
template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator> operator+( const charT* lhs, const basic_string<charT,traits,Allocator>& rhs );Returns: basic_string<charT,traits,Allocator>(lhs) + rhs.
That leads to the basic_string<charT,traits,Allocator>(lhs, Allocator()) call. Obviously, the right requirement is:
Returns: basic_string<charT,traits,Allocator>(lhs, rhs.get_allocator()) + rhs.
It seems like a lot of DRs can be submitted on this "Absent call to get_allocator()" topic.
1) Explicitly state the intent to allow for user-defined allocators without default constructor in 20.1.5 Allocator requirements.
2) Correct all the places, where a correct allocator object is available through the get_allocator() call but default Allocator() gets passed instead.
Let's suppose that the following memory pool is available:
class mem_pool {
// ...
void* allocate(size_t size);
void deallocate(void* ptr, size_t size);
};
So the following allocator can be implemented via this pool:
class stl_allocator {
mem_pool& pool;
public:
explicit stl_allocator(mem_pool& mp) : pool(mp) {}
stl_allocator(const stl_allocator& sa) : pool(sa.pool) {}
template <class U>
stl_allocator(const stl_allocator<U>& sa) : pool(sa.get_pool()) {}
~stl_allocator() {}
pointer allocate(size_type n, std::allocator<void>::const_pointer = 0)
{
return (n!=0) ? static_cast<pointer>(pool.allocate(n*sizeof(T))) : 0;
}
void deallocate(pointer p, size_type n)
{
if (n!=0) pool.deallocate(p, n*sizeof(T));
}
// ...
};
Then the following code works well on some implementations and doesn't work on another:
typedef basic_string<char, char_traits<char>, stl_allocator<char> >
tl_string;
mem_pool mp;
tl_string s1("abc", stl_allocator<int>(mp));
printf("(%s)\n", ("def"+s1).c_str());
In particular, on some implementations the code can't be compiled without default stl_allocator() constructor.
The obvious way to solve the compile-time problems is to intentionally define a NULL pointer dereferencing default constructor
stl_allocator() : pool(*static_cast<mem_pool*>(0)) {}
in a hope that it will not be called. The problem is that it really gets called by operator+(const char*, const string&) under the current 21.3.7.1p2 wording.
Proposed resolution:
Rationale:
Recommend NAD. operator+() with string already requires the desired semantics of copying the allocator from one of the strings (lhs when there is a choice).
Section: TR1 8.16.4 [tr.c99.cmath.over] Status: NAD Submitter: Paolo Carlini Opened: 2006-03-07 Last modified: 2009-07-13
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Discussion:
log2 is missing from the list of "additional overloads" in TR1 8.16.4 [tr.c99.cmath.over] p1.
Hinnant: This is a TR1 issue only. It is fixed in the current (N2135) WD.
[ Batavia (2009-05): ]
We agree this has been fixed in the Working Draft. Move to NAD.
Proposed resolution:
Add log2 to the list of functions in TR1 8.16.4 [tr.c99.cmath.over] p1.
Section: 27.5.4.3 [iostate.flags] Status: Dup Submitter: Seungbeom Kim Opened: 2006-03-10 Last modified: 2006-12-30
View all other issues in [iostate.flags].
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Duplicate of: 272
Discussion:
Section: 27.4.4.3 [lib.iostate.flags]
Paragraph 4 says:
void clear(iostate state = goodbit);Postcondition: If rdbuf()!=0 then state == rdstate(); otherwise rdstate()==state|ios_base::badbit.
The postcondition "rdstate()==state|ios_base::badbit" is parsed as "(rdstate()==state)|ios_base::badbit", which is probably what the committee meant.
Rationale:
Section: 21.2 [char.traits] Status: NAD Submitter: Jack Reeves Opened: 2006-04-06 Last modified: 2008-06-18
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Discussion:
Currently, the Standard Library specifies only a declaration for template class char_traits<> and requires the implementation provide two explicit specializations: char_traits<char> and char_traits<wchar_t>. I feel the Standard should require explicit specializations for all built-in character types, i.e. char, wchar_t, unsigned char, and signed char.
I have put together a paper (N1985) that describes this in more detail and includes all the necessary wording.
[ Portland: Jack will rewrite N1985 to propose a primary template that will work with other integral types. ]
[ Toronto: issue has grown with addition of char16_t and char32_t. ]
[ post Bellevue: ]
We suggest that Jack be asked about the status of his paper, and if it is not forthcoming, the work-item be assigned to someone else. If no one steps forward to do the paper before the next meeting, we propose to make this NAD without further discussion. We leave this Open for now, but our recommendation is NAD.
Note: the issue statement should be updated, as the Toronto comment has already been resolved. E.g., char_traits specializations for char16_t and char32_t are now in the working paper.
[ Sophia Antipolis: ]
Nobody has submitted the requested paper, so we move to NAD, as suggested by the decision at the last meeting.
Proposed resolution:
Section: 1.2 [intro.refs] Status: NAD Editorial Submitter: Beman Dawes Opened: 2006-04-08 Last modified: 2007-07-02
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Discussion:
1.2 Normative references [intro.refs] of the WP currently refers to ISO/IEC 9899:1990, Programming languages - C. Should that be changed to ISO/IEC 9899:1999?
What impact does this have on the library?
Proposed resolution:
In 1.2/1 [intro.refs] of the WP, change:
- ISO/IEC 9899:
19901999 + TC1 + TC2, Programming languages - C
Rationale:
Recommend NAD, fixed editorially.Section: 26.5 [rand], TR1 5.1 [tr.rand] Status: NAD Submitter: Howard Hinnant Opened: 2006-04-11 Last modified: 2007-04-18
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Discussion:
In Berlin, as a working group, we voted in favor of N1932 which makes issue 507 moot: variate_generator has been eliminated. Then in full committee we voted to give this issue WP status (mistakenly).
Proposed resolution:
Strike the proposed resolution of issue 507.
[ post-Portland: Walter and Howard recommend NAD. The proposed resolution of 507 no longer exists in the current WD. ]
Rationale:
NAD. Will be moot once N2135 is adopted.
Section: 27.5.3 [fpos] Status: NAD Submitter: Beman Dawes Opened: 2006-04-12 Last modified: 2009-07-15
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Discussion:
There are two deficiencies related to file sizes:
The Dinkumware implementation of the Standard Library as shipped with the Microsoft compiler copes with these issues by:
fpos_t seekpos() const;
Because there are so many types relating to file positions and offsets (fpos_t, fpos, pos_type, off_type, streamoff, streamsize, streampos, wstreampos, and perhaps more), it is difficult to know if the Dinkumware extensions are sufficient. But they seem a useful starting place for discussions, and they do represent existing practice.
[ Kona (2007): We need a paper. It would be nice if someone proposed clarifications to the definitions of pos_type and off_type. Currently these definitions are horrible. Proposed Disposition: Open ]
[ 2009-07 Frankfurt ]
This is the subject of paper N2926.
If we choose to take any action, we will move the paper, so the issue can be closed.
Move to NAD.
Proposed resolution:
Section: 23.2.1 [container.requirements.general] Status: NAD Editorial Submitter: Martin Sebor Opened: 2006-06-14 Last modified: 2009-10-26
View other active issues in [container.requirements.general].
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Duplicate of: 479
Discussion:
C++ Standard Library templates that take an allocator as an argument
are required to call the allocate() and
deallocate() members of the allocator object to obtain
storage. However, they do not appear to be required to call any other
allocator members such as construct(),
destroy(), address(), and
max_size(). This makes these allocator members less than
useful in portable programs.
It's unclear to me whether the absence of the requirement to use these allocator members is an unintentional omission or a deliberate choice. However, since the functions exist in the standard allocator and since they are required to be provided by any user-defined allocator I believe the standard ought to be clarified to explictly specify whether programs should or should not be able to rely on standard containers calling the functions.
I propose that all containers be required to make use of these functions.
[ Batavia: We support this resolution. Martin to provide wording. ]
[ pre-Oxford: Martin provided wording. ]
[ 2009-04-28 Pablo adds: ]
N2554 (scoped allocators), N2768 (allocator concepts), and N2810 (allocator defects), address all of these points EXCEPT max_size(). So, I would add a note to that affect and re-class the defect as belonging to section 23.2.1 [container.requirements.general].
[ 2009-07 Frankfurt ]
The comment in the description of this issue that this "would be" rendered editorial by the adoption of N2257 is confusing. It appears that N2257 was never adopted.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Addressed by N2982.
Proposed resolution:
Specifically, I propose to change 23.2 [container.requirements], p9 as follows:
-9- Copy constructors for all container types defined in this clause that are parametrized on
Allocatorcopyanthe allocator argument from their respective first parameters. All other constructors for these container types take anconstAllocator&argument (20.1.6), an allocator whosevalue_typeis the same as the container'svalue_type. A copy of this argumentisshall be used for any memory allocation and deallocation performed,by these constructors and by all member functions,during the lifetime of each container object. Allocation shall be performed "as if" by calling theallocate()member function on a copy of the allocator object of the appropriate type New Footnote), and deallocation "as if" by callingdeallocate()on a copy of the same allocator object of the corresponding type. A copy of this argument shall also be used to construct and destroy objects whose lifetime is managed by the container, including but not limited to those of the container'svalue_type, and to obtain their address. All objects residing in storage allocated by a container's allocator shall be constructed "as if" by calling theconstruct()member function on a copy of the allocator object of the appropriate type. The same objects shall be destroyed "as if" by callingdestroy()on a copy of the same allocator object of the same type. The address of such objects shall be obtained "as if" by calling theaddress()member function on a copy of the allocator object of the appropriate type. Finally, a copy of this argument shall be used by its container object to determine the maximum number of objects of the container'svalue_typethe container may store at the same time. The container member functionmax_size()obtains this number from the value returned by a call toget_allocator().max_size(). In all container types defined in this clause that are parametrized onAllocator, the memberget_allocator()returns a copy of theAllocatorobject used to construct the container.258)New Footnote: This type may be different from
Allocator: it may be derived fromAllocatorviaAllocator::rebind<U>::otherfor the appropriate typeU.
The proposed wording seems cumbersome but I couldn't think of a better
way to describe the requirement that containers use their
Allocator to manage only objects (regardless of their
type) that persist over their lifetimes and not, for example,
temporaries created on the stack. That is, containers shouldn't be
required to call Allocator::construct(Allocator::allocate(1),
elem) just to construct a temporary copy of an element, or
Allocator::destroy(Allocator::address(temp), 1) to
destroy temporaries.
[ Howard: This same paragraph will need some work to accommodate 431. ]
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
Section: 20.9.9.2 [uninitialized.copy] Status: NAD Submitter: Martin Sebor Opened: 2006-06-14 Last modified: 2009-07-15
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Discussion:
Related to 1029
The specialized algorithms [lib.specialized.algorithms] are specified as having the general effect of invoking the following expression:
new (static_cast<void*>(&*i))
typename iterator_traits<ForwardIterator>::value_type (x)
This expression is ill-formed when the type of the subexpression
&*i is some volatile-qualified T.
[ Batavia: Lack of support for proposed resolution but agree there is a defect. Howard to look at wording. Concern that move semantics properly expressed if iterator returns rvalue. ]
[ 2009-06-17 Pablo adds: ]
Propose that Issue 582 be closed NAD.
Issue 582 asks that uninitialized_copy, uninitialized_fill, and uninitialized_fill_n should be well-formed if the result type is volatile. My feeling is that the standard does not, and should not, guarantee any useful behavior when constructors are invoked on volatile storage, so making it syntactically legal to call uninitialized_copy on volatile storage is not useful. A possible editorial change would be to put my previous sentence into a non-normative note.
Note that the three sections starting with 20.9.9.2 [uninitialized.copy] do not yet have concepts. Here's a first crack at the first one:
template <InputIterator InIter, OutputIterator OutIter> requires ExplicitConvertible<HasDereference<OutIter::reference>::result, OutIter::value_type&> && Convertible<OutIter::value_type*, void*> && ExplicitConvertible<OutIter::value_type, InIter::reference> OutIter uninitialized_copy(InIter first, InIter last, OutIter result);Effects:
while (first != last) { typedef OutIter::value_type value_type; value_type& outRef = static_cast<value_type&>(*result++); ::new (static_cast<void*>(addressof(outRef))) value_type(*first++); }Notes:
- This definition is actually LESS constrained than in C++03 because there is no requirement that the result be a forward iterator.
- If OutIter returns a proxy type with an overloaded operator&, this definition probably won't compile. Lifting this limitation while allowing value_type to have an overloaded operator& would be hard, but is probably possible with careful overloading. I'm not sure it's worth it.
- This definition retains the prohibition on the use of volatile types for the result.
[ 2009-07 Frankfurt ]
We don't deal with volatile in the library.
Jim: should we state that explicitly somewhere?
Beman: you might argue that clause 17 should say something about volatile. However, if you want to raise we argument, we should open it as a separate issue and consult with experts on concurrency.
Hinnant: actually, some library components do handle volatile, so we'd need to be very careful about what we say in clause 17.
No objection to NAD.
Move to NAD.
Proposed resolution:
In order to allow these algorithms to operate on volatile storage I propose to change the expression so as to make it well-formed even for pointers to volatile types. Specifically, I propose the following changes to clauses 20 and 24. Change 20.6.4.1, p1 to read:
Effects:
typedef typename iterator_traits<ForwardIterator>::pointer pointer;
typedef typename iterator_traits<ForwardIterator>::value_type value_type;
for (; first != last; ++result, ++first)
new (static_cast<void*>(const_cast<pointer>(&*result))
value_type (*first);
change 20.6.4.2, p1 to read
Effects:
typedef typename iterator_traits<ForwardIterator>::pointer pointer;
typedef typename iterator_traits<ForwardIterator>::value_type value_type;
for (; first != last; ++result, ++first)
new (static_cast<void*>(const_cast<pointer>(&*first))
value_type (*x);
and change 20.6.4.3, p1 to read
Effects:
typedef typename iterator_traits<ForwardIterator>::pointer pointer;
typedef typename iterator_traits<ForwardIterator>::value_type value_type;
for (; n--; ++first)
new (static_cast<void*>(const_cast<pointer>(&*first))
value_type (*x);
In addition, since there is no partial specialization for
iterator_traits<volatile T*> I propose to add one
to parallel such specialization for <const T*>. Specifically, I
propose to add the following text to the end of 24.3.1, p3:
and for pointers to volatile as
namespace std {
template<class T> struct iterator_traits<volatile T*> {
typedef ptrdiff_t difference_type;
typedef T value_type;
typedef volatile T* pointer;
typedef volatile T& reference;
typedef random_access_iterator_tag iterator_category;
};
}
Note that the change to iterator_traits isn't necessary
in order to implement the specialized algorithms in a way that allows
them to operate on volatile strorage. It is only necesassary in order
to specify their effects in terms of iterator_traits as
is done here. Implementations can (and some do) achieve the same
effect by means of function template overloading.
Section: 26.8 [c.math] Status: NAD Submitter: Beman Dawes Opened: 2006-06-15 Last modified: 2007-07-25
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Discussion:
There is no div() function for unsigned integer types.
There are several possible resolutions. The simplest one is noted below. Other possibilities include a templated solution.
Proposed resolution:
Add to 26.7 [lib.c.math] paragraph 8:
struct udiv_t div(unsigned, unsigned); struct uldiv_t div(unsigned long, unsigned long); struct ulldiv_t div(unsigned long long, unsigned long long);
Rationale:
Toronto: C99 does not have these unsigned versions because the signed version exist just to define the implementation-defined behavior of signed integer division. Unsigned integer division has no implementation-defined behavior and thus does not need this treatment.Section: 26.8 [c.math] Status: NAD Submitter: Beman Dawes Opened: 2006-06-15 Last modified: 2007-07-25
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Discussion:
There is no pow() function for any integral type.
Proposed resolution:
Add something like:
template< typename T> T power( T x, int n ); // requires: n >=0
Rationale:
Toronto: We already have double pow(integral, integral) from 26.8 [c.math] p11.Section: 22.4 [locale.categories] Status: NAD Submitter: Martin Sebor, Paolo Carlini Opened: 2006-06-22 Last modified: 2009-07-15
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Discussion:
Section 22.2, paragraph 2 requires facet get() members
that take an ios_base::iostate& argument,
err, to ignore the (initial) value of the
argument, but to set it to ios_base::failbit in case of a
parse error.
We believe there are a few minor problems with this blanket
requirement in conjunction with the wording specific to each
get() member function.
First, besides get() there are other member functions
with a slightly different name (for example,
get_date()). It's not completely clear that the intent of
the paragraph is to include those as well, and at least one
implementation has interpreted the requirement literally.
Second, the requirement to "set the argument to
ios_base::failbit suggests that the functions are not
permitted to set it to any other value (such as
ios_base::eofbit, or even ios_base::eofbit |
ios_base::failbit).
However, 22.2.2.1.2, p5 (Stage 3 of num_get parsing) and
p6 (bool parsing) specifies that the do_get
functions perform err |= ios_base::eofbit, which
contradicts the earlier requirement to ignore err's initial
value.
22.2.6.1.2, p1 (the Effects clause of the money_get
facet's do_get member functions) also specifies that
err's initial value be used to compute the final
value by ORing it with either ios_base::failbit or
withios_base::eofbit | ios_base::failbit.
[ 2009-07 Frankfurt ]
Move to NAD.
Proposed resolution:
We believe the intent is for all facet member functions that take an
ios_base::iostate& argument to:
err argument,
err to ios_base::goodbit prior
to any further processing,
ios_base::eofbit, or
ios_base::failbit, or both in err, as
appropriate, in response to reaching the end-of-file or on parse
error, or both.
To that effect we propose to change 22.2, p2 as follows:
The put() members make no provision for error
reporting. (Any failures of the OutputIterator argument must be
extracted from the returned iterator.) Unless otherwise
specified, the get() members that
take an ios_base::iostate& argument whose value
they ignore, but set to ios_base::failbit in case of a parse
error., err, start by evaluating
err = ios_base::goodbit, and may subsequently set
err to either ios_base::eofbit, or
ios_base::failbit, or ios_base::eofbit |
ios_base::failbit in response to reaching the end-of-file or in
case of a parse error, or both, respectively.
[ Kona (2007): We need to change the proposed wording to clarify that the phrase "the get members" actually denotes get(), get_date(), etc. Proposed Disposition: Open ]
Section: D.8.2.1 [depr.istrstream.cons] Status: NAD Editorial Submitter: Martin Sebor Opened: 2006-06-22 Last modified: 2007-05-11
View all issues with NAD Editorial status.
Discussion:
The iststream(char*, streamsize) ctor is in the class
synopsis in D.7.2 but its signature is missing in the description
below (in D.7.2.1).
Proposed resolution:
This seems like a simple editorial issue and the missing signature can
be added to the one for const char* in paragraph 2.
[ post Oxford: Noted that it is already fixed in N2284 ]
Section: 23.3.1 [array] Status: NAD Submitter: Gennaro Prota Opened: 2006-07-18 Last modified: 2009-10-20
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Discussion:
The wording used for section 23.2.1 [lib.array] seems to be subtly ambiguous about zero sized arrays (N==0). Specifically:
* "An instance of array<T, N> stores N elements of type T, so that [...]"
Does this imply that a zero sized array object stores 0 elements, i.e. that it cannot store any element of type T? The next point clarifies the rationale behind this question, basically how to implement begin() and end():
* 23.2.1.5 [lib.array.zero], p2: "In the case that N == 0, begin() == end() == unique value."
What does "unique" mean in this context? Let's consider the following possible implementations, all relying on a partial specialization:
a)
template< typename T >
class array< T, 0 > {
....
iterator begin()
{ return iterator( reinterpret_cast< T * >( this ) ); }
....
};
This has been used in boost, probably intending that the return value had to be unique to the specific array object and that array couldn't store any T. Note that, besides relying on a reinterpret_cast, has (more than potential) alignment problems.
b)
template< typename T >
class array< T, 0 > {
T t;
iterator begin()
{ return iterator( &t ); }
....
};
This provides a value which is unique to the object and to the type of the array, but requires storing a T. Also, it would allow the user to mistakenly provide an initializer list with one element.
A slight variant could be returning *the* null pointer of type T
return static_cast<T*>(0);
In this case the value would be unique to the type array<T, 0> but not to the objects (all objects of type array<T, 0> with the same value for T would yield the same pointer value).
Furthermore this is inconsistent with what the standard requires from allocation functions (see library issue 9).
c) same as above but with t being a static data member; again, the value would be unique to the type, not to the object.
d) to avoid storing a T *directly* while disallowing the possibility to use a one-element initializer list a non-aggregate nested class could be defined
struct holder { holder() {} T t; } h;
and then begin be defined as
iterator begin() { return &h.t; }
But then, it's arguable whether the array stores a T or not. Indirectly it does.
-----------------------------------------------------
Now, on different issues:
* what's the effect of calling assign(T&) on a zero-sized array? There seems to be only mention of front() and back(), in 23.2.1 [lib.array] p4 (I would also suggest to move that bullet to section 23.2.1.5 [lib.array.zero], for locality of reference)
* (minor) the opening paragraph of 23.2.1 [lib.array] wording is a bit inconsistent with that of other sequences: that's not a problem in itself, but compare it for instance with "A vector is a kind of sequence that supports random access iterators"; though the intent is obvious one might argue that the wording used for arrays doesn't tell what an array is, and relies on the reader to infer that it is what the <array> header defines.
* it would be desiderable to have a static const data member of type std::size_t, with value N, for usage as integral constant expression
* section 23.1 [lib.container.requirements] seem not to consider fixed-size containers at all, as it says: "[containers] control allocation and deallocation of these objects [the contained objects] through constructors, destructors, *insert and erase* operations"
* max_size() isn't specified: the result is obvious but, technically, it relies on table 80: "size() of the largest possible container" which, again, doesn't seem to consider fixed size containers
[ 2009-05-29 Daniel adds: ]
star bullet 1 ("what's the effect of calling assign(T&) on a zero-sized array?[..]");
assign has been renamed to fill and the semantic of fill is now defined in terms of the free algorithm fill_n, which is well-defined for this situation.star bullet 3 ("it would be desiderable to have a static const data member..."):
It seems that tuple_size<array<T, N> >::value as of 23.3.1.8 [array.tuple] does provide this functionality now.
[ 2009-07 Frankfurt ]
Alisdair to address by the next meeting, or declare NAD.
Moved to Tentatively NAD.
[ 2009 Santa Cruz: ]
Moved to NAD.
Proposed resolution:
[ Kona (2007): requirements on zero sized tr1::arrays and other details Issue 617: std::array is a sequence that doesn't satisfy the sequence requirements? Alisdair will prepare a paper. Proposed Disposition: Open ]
Section: 20.7 [meta], TR1 4.9 [tr.meta.req] Status: NAD Editorial Submitter: Beman Dawes Opened: 2006-08-10 Last modified: 2007-05-11
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Discussion:
20.4.9 [lib.meta.req], Implementation requirements, provides latitude for type traits implementers that is not needed in C++0x. It includes the wording:
[Note: the latitude granted to implementers in this clause is temporary, and is expected to be removed in future revisions of this document. -- end note]
Note: N2157: Minor Modifications to the type traits Wording also has the intent of removing this wording from the WP.
Proposed resolution:
Remove 20.4.9 [lib.meta.req] in its entirety from the WP.
[ post-Oxford: Recommend NAD Editorial. This resolution is now in the current working draft. ]
Section: 18.3.1.2 [numeric.limits.members] Status: NAD Editorial Submitter: whyglinux Opened: 2006-08-08 Last modified: 2007-07-02
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Discussion:
18.2.1.2 numeric_limits members [lib.numeric.limits.members] Paragraph 7:
"For built-in integer types, the number of non-sign bits in the representation."
26.1 Numeric type requirements [lib.numeric.requirements] Footnote:
"In other words, value types. These include built-in arithmetic types, pointers, the library class complex, and instantiations of valarray for value types."
Integer types (which are bool, char, wchar_t, and the signed and unsigned integer types) and arithmetic types (which are integer and floating types) are all built-in types and thus there are no non-built-in (that is, user-defined) integer or arithmetic types. Since the redundant "built-in" in the above 2 sentences can mislead that there may be built-in or user-defined integer and arithmetic types (which is not correct), the "built-in" should be removed.
Proposed resolution:
18.2.1.2 numeric_limits members [lib.numeric.limits.members] Paragraph 7:
"For
built-ininteger types, the number of non-sign bits in the representation."
26.1 Numeric type requirements [lib.numeric.requirements] Footnote:
"In other words, value types. These include
built-inarithmetic types, pointers, the library class complex, and instantiations of valarray for value types."
Rationale:
Recommend NAD / Editorial. The proposed resolution is accepted as editorial.
Section: 27.9.1.9 [ifstream.members] Status: NAD Editorial Submitter: Christopher Kohlhoff Opened: 2006-08-17 Last modified: 2008-07-02
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Discussion:
I just spotted a minor problem in 27.8.1.7 [lib.ifstream.members] para 4 and also 27.8.1.13 [lib.fstream.members] para 4. In both places it says:
void close();Effects: Calls rdbuf()->close() and, if that function returns false, ...
However, basic_filebuf::close() (27.8.1.2) returns a pointer to the filebuf on success, null on failure, so I think it is meant to say "if that function returns a null pointer". Oddly, it is correct for basic_ofstream.
Proposed resolution:
Change 27.9.1.9 [ifstream.members], p5:
Effects: Calls rdbuf()->close() and, if that function fails (returns
falsea null pointer), calls setstate(failbit) (which may throw ios_base::failure (27.4.4.3)).
Change 27.9.1.17 [fstream.members], p5:
Effects: Calls rdbuf()->close() and, if that function fails (returns
falsea null pointer), calls setstate(failbit) (which may throw ios_base::failure (27.4.4.3)).
[ Kona (2007): Proposed Disposition: NAD, Editorial ]
Section: 20.2.1 [utility.arg.requirements] Status: NAD Editorial Submitter: Niels Dekker Opened: 2006-11-02 Last modified: 2010-03-11
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Discussion:
It seems undesirable to define the Swappable requirement in terms of CopyConstructible and Assignable requirements. And likewise, once the MoveConstructible and MoveAssignable requirements (N1860) have made it into the Working Draft, it seems undesirable to define the Swappable requirement in terms of those requirements. Instead, it appears preferable to have the Swappable requirement defined exclusively in terms of the existence of an appropriate swap function.
Section 20.1.4 [lib.swappable] of the current Working Draft (N2009) says:
The Swappable requirement is met by satisfying one or more of the following conditions:
- T is Swappable if T satisfies the CopyConstructible requirements (20.1.3) and the Assignable requirements (23.1);
- T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in Table 33.
I can think of three disadvantages of this definition:
If a client's type T satisfies the first condition (T is both CopyConstructible and Assignable), the client cannot stop T from satisfying the Swappable requirement without stopping T from satisfying the first condition.
A client might want to stop T from satisfying the Swappable requirement, because swapping by means of copy construction and assignment might throw an exception, and she might find a throwing swap unacceptable for her type. On the other hand, she might not feel the need to fully implement her own swap function for this type. In this case she would want to be able to simply prevent algorithms that would swap objects of type T from being used, e.g., by declaring a swap function for T, and leaving this function purposely undefined. This would trigger a link error, if an attempt would be made to use such an algorithm for this type. For most standard library implementations, this practice would indeed have the effect of stopping T from satisfying the Swappable requirement.
A client's type T that does not satisfy the first condition can not be made Swappable by providing a specialization of std::swap for T.
While I'm aware about the fact that people have mixed feelings about providing a specialization of std::swap, it is well-defined to do so. It sounds rather counter-intuitive to say that T is not Swappable, if it has a valid and semantically correct specialization of std::swap. Also in practice, providing such a specialization will have the same effect as satisfying the Swappable requirement.
For a client's type T that satisfies both conditions of the Swappable requirement, it is not specified which of the two conditions prevails. After reading section 20.1.4 [lib.swappable], one might wonder whether objects of T will be swapped by doing copy construction and assignments, or by calling the swap function of T.
I'm aware that the intention of the Draft is to prefer calling the swap function of T over doing copy construction and assignments. Still in my opinion, it would be better to make this clear in the wording of the definition of Swappable.
I would like to have the Swappable requirement defined in such a way that the following code fragment will correctly swap two objects of a type T, if and only if T is Swappable:
using std::swap; swap(t, u); // t and u are of type T.
This is also the way Scott Meyers recommends calling a swap function, in Effective C++, Third Edition, item 25.
Most aspects of this issue have been dealt with in a discussion on comp.std.c++ about the Swappable requirement, from 13 September to 4 October 2006, including valuable input by David Abrahams, Pete Becker, Greg Herlihy, Howard Hinnant and others.
[ San Francisco: ]
Recommend NAD. Solved by N2774.
[ 2009-07 Frankfurt ]
Moved to Open. Waiting for non-concepts draft.
[ 2009-11-08 Howard adds: ]
This issue is very closely related to 742.
[ 2010-02-03 Sean Hunt adds: ]
While reading N3000, I independently came across Issue 594. Having seen that it's an issue under discussion, I think the proposed wording needs fixing to something more like "...function call swap(t,u) that includes std::swap in its overload set is valid...", because "...is valid within the namespace std..." does not allow other libraries to simply use the Swappable requirement by referring to the standard's definition, since they cannot actually perform any calls within std.
This wording I suggested would also make overloads visible in the same scope as the `using std::swap` valid for Swappable requirements; a more complex wording limiting the non-ADL overload set to std::swap might be required.
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added.
Rationale:
Solved by N3048.
Proposed resolution:
Change section 20.1.4 [lib.swappable] as follows:
The Swappable requirement is met by satisfying
one or more of the following conditions:the following condition:
T is Swappable if T satisfies the CopyConstructible requirements (20.1.3) and the Assignable requirements (23.1);T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in Table 33.T is Swappable if an unqualified function call swap(t,u) is valid within the namespace std, and has the semantics described in Table 33.
Section: TRDecimal 3.2 [trdec.types.types] Status: NAD Submitter: Daveed Vandevoorde Opened: 2006-04-05 Last modified: 2009-07-15
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Discussion:
In a private email, Daveed writes:
I am not familiar with the C TR, but my guess is that the class type approach still won't match a built-in type approach because the notion of "promotion" cannot be emulated by user-defined types.
Here is an example:
struct S {
S(_Decimal32 const&); // Converting constructor
};
void f(S);
void f(_Decimal64);
void g(_Decimal32 d) {
f(d);
}
If _Decimal32 is a built-in type, the call f(d) will likely resolve to f(_Decimal64) because that requires only a promotion, whereas f(S) requires a user-defined conversion.
If _Decimal32 is a class type, I think the call f(d) will be ambiguous because both the conversion to _Decimal64 and the conversion to S will be user-defined conversions with neither better than the other.
Robert comments:
In general, a library of arithmetic types cannot exactly emulate the behavior of the intrinsic numeric types. There are several ways to tell whether an implementation of the decimal types uses compiler intrinisics or a library. For example:
_Decimal32 d1;
d1.operator+=(5); // If d1 is a builtin type, this won't compile.
In preparing the decimal TR, we have three options:
We decided as a group to pursue option #3, but that approach implies that implementations may not agree on the semantics of certain use cases (first example, above), or on whether certain other cases are well-formed (second example). Another potentially important problem is that, under the present definition of POD, the decimal classes are not POD types, but builtins will be.
Note that neither example above implies any problems with respect to C-to-C++ compatibility, since neither example can be expressed in C.
[ 2009-07 Frankfurt ]
Decimal numeric types may either be builtin types or library types. We only intend to specify the common subset of behaviors of the two implementation approaches. The front matter of the Decimal TR says this explicitly.
Move to NAD.
Proposed resolution:
Section: TRDecimal 3.2 [trdec.types.types] Status: NAD Submitter: Martin Sebor Opened: 2006-06-15 Last modified: 2009-07-15
View all other issues in [trdec.types.types].
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Discussion:
In c++std-lib-17205, Martin writes:
...was it a deliberate design choice to make narrowing assignments ill-formed while permitting narrowing compound assignments? For instance:
decimal32 d32;
decimal64 d64;
d32 = 64; // error
d32 += 64; // okay
In c++std-lib-17229, Robert responds:
It is a vestige of an old idea that I forgot to remove from the paper. Narrowing assignments should be permitted. The bug is that the converting constructors that cause narrowing should not be explicit. Thanks for pointing this out.
[ 2009-07 Frankfurt ]
The current state of the Decimal TR is the result of a deliberate design decision that has been examined many times.
Move to NAD.
Proposed resolution:
1. In "3.2.2 Class decimal32" synopsis, remove the explicit specifier from the narrowing conversions:
// 3.2.2.2 conversion from floating-point type:
explicit decimal32(decimal64 d64);
explicit decimal32(decimal128 d128);
2. Do the same thing in "3.2.2.2. Conversion from floating-point type."
3. In "3.2.3 Class decimal64" synopsis, remove the explicit specifier from the narrowing conversion:
// 3.2.3.2 conversion from floating-point type:
explicit decimal64(decimal128 d128);
4. Do the same thing in "3.2.3.2. Conversion from floating-point type."
[ Redmond: We prefer explicit conversions for narrowing and implicit for widening. ]
Section: 21.4 [basic.string] Status: NAD Submitter: Bo Persson Opened: 2006-12-05 Last modified: 2009-07-15
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Discussion:
This is based on N2134, where 21.3.1/2 states: "... The Allocator object used shall be a copy of the Allocator object passed to the basic_string object's constructor or, if the constructor does not take an Allocator argument, a copy of a default-constructed Allocator object."
Section 21.3.2/1 lists two constructors:
basic_string(const basic_string<charT,traits,Allocator>& str );
basic_string(const basic_string<charT,traits,Allocator>& str ,
size_type pos , size_type n = npos,
const Allocator& a = Allocator());
and then says "In the first form, the Allocator value used is copied from str.get_allocator().", which isn't an option according to 21.3.1.
[ Batavia: We need blanket statement to the effect of: ]
[ Review constructors and functions that return a string; make sure we follow these rules (substr, operator+, etc.). Howard to supply wording. ]
[
Bo adds: The new container constructor which takes only a size_type is not
consistent with 23.2 [container.requirements], p9 which says in part:
All other constructors for these container types take an
Allocator& argument (20.1.2), an allocator whose value type
is the same as the container's value type. A copy of this argument is
used for any memory allocation performed, by these constructors and by
all member functions, during the lifetime of each container object.
]
[ post Bellevue: We re-confirm that the issue is real. Pablo will provide wording. ]
[ 2009-07 Frankfurt ]
Move to NAD.
Proposed resolution:
Section: 21.7 [c.strings] Status: NAD Editorial Submitter: Bo Persson Opened: 2006-12-11 Last modified: 2007-04-24
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Discussion:
In the current draft N2134, 21.4/1 says
"Tables 59,228) 60, 61, 62,and 63 229) 230) describe headers <cctype>, <cwctype>, <cstring>, <cwchar>, and <cstdlib> (character conversions), respectively."
Here footnote 229 applies to table 62, not table 63.
Also, footnote 230 lists the new functions in table 63, "atoll, strtoll, strtoull, strtof, and strtold added by TR1". However, strtof is not present in table 63.
Proposed resolution:
Rationale:
Recommend NAD, editorial. Send to Pete.
Section: 23.3.1 [array] Status: NAD Submitter: Bo Persson Opened: 2006-12-30 Last modified: 2009-10-20
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Discussion:
The <array> header is given under 23.3 [sequences]. 23.3.1 [array]/paragraph 3 says:
"Unless otherwise specified, all array operations are as described in 23.2 [container.requirements]".
However, array isn't mentioned at all in section 23.2 [container.requirements]. In particular, Table 82 "Sequence requirements" lists several operations (insert, erase, clear) that std::array does not have in 23.3.1 [array].
Also, Table 83 "Optional sequence operations" lists several operations that std::array does have, but array isn't mentioned.
[ 2009-07 Frankfurt ]
The real issue seems to be different than what is described here. Non-normative text says that std::array is a sequence container, but there is disagreement about what that really means. There are two possible interpretations:
- a sequence container is one that satisfies all sequence container requirements
- a sequence container is one that satisfies some of the sequence container requirements. Any operation that the container supports is specified by one or more sequence container requirements, unless that operation is specifically singled out and defined alongside the description of the container itself.
Move to Tentatively NAD.
[ 2009-07-15 Loïc Joly adds: ]
The section 23.2.3 [sequence.reqmts]/1 states that array is a sequence. 23.2.3 [sequence.reqmts]/3 introduces table 83, named Sequence container requirements. This seems to me to be defining the requirements for all sequences. However, array does not follow all of this requirements (this can be read in the array specific section, for the standard is currently inconsistent).
Proposed resolution 1 (minimal change):
Say that array is a container, that in addition follows only some of the sequence requirements, as described in the array section:
The library providesfivethree basic kinds of sequence containers:array, vector,forward_list, list, and deque. In addition, array and forward_list follows some of the requirements of sequences, as described in their respective sections.Proposed resolution 2 (most descriptive description, no full wording provided):
Introduce the notion of a Fixed Size Sequence, with it requirement table that would be a subset of the current Sequence container. array would be the only Fixed Size Sequence (but dynarray is in the queue for TR2). Sequence requirements would now be requirements in addition to Fixed Size Sequence requirements (it is currently in addition to container).
[ 2009-07 Frankfurt: ]
Move to NAD Editorial
[ 2009 Santa Cruz: ]
This will require a lot of reorganization. Editor doesn't think this is really an issue, since the description of array can be considered as overriding what's specified about sequences. Move to NAD.
Proposed resolution:
Section: 17 [library] Status: NAD Editorial Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2010-03-10
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Duplicate of: 895
Discussion:
Many member functions of basic_string are overloaded,
with some of the overloads taking a string argument,
others value_type*, others size_type, and
others still iterators. Often, the requirements on one of
the overloads are expressed in the form of Effects,
Throws, and in the Working Paper
(N2134)
also Remark clauses, while those on the rest of the overloads
via a reference to this overload and using a Returns clause.
The difference between the two forms of specification is that per 17.5.1.4 [structure.specifications], p3, an Effects clause specifies "actions performed by the functions," i.e., its observable effects, while a Returns clause is "a description of the return value(s) of a function" that does not impose any requirements on the function's observable effects.
Since only Notes are explicitly defined to be informative and all other paragraphs are explicitly defined to be normative, like Effects and Returns, the new Remark clauses also impose normative requirements.
So by this strict reading of the standard there are some member
functions of basic_string that are required to throw an
exception under some conditions or use specific traits members while
many other otherwise equivalent overloads, while obliged to return the
same values, aren't required to follow the exact same requirements
with regards to the observable effects.
Here's an example of this problem that was precipitated by the change from informative Notes to normative Remarks (presumably made to address 424):
In the Working Paper, find(string, size_type) contains a
Remark clause (which is just a Note in the current
standard) requiring it to use traits::eq().
find(const charT *s, size_type pos) is specified to
return find(string(s), pos) by a Returns clause
and so it is not required to use traits::eq(). However,
the Working Paper has replaced the original informative Note
about the function using traits::length() with a
normative requirement in the form of a Remark. Calling
traits::length() may be suboptimal, for example when the
argument is a very long array whose initial substring doesn't appear
anywhere in *this.
Here's another similar example, one that existed even prior to the introduction of Remarks:
insert(size_type pos, string, size_type, size_type) is
required to throw out_of_range if pos >
size().
insert(size_type pos, string str) is specified to return
insert(pos, str, 0, npos) by a Returns clause and
so its effects when pos > size() are strictly speaking
unspecified.
I believe a careful review of the current Effects and Returns clauses is needed in order to identify all such problematic cases. In addition, a review of the Working Paper should be done to make sure that the newly introduced normative Remark clauses do not impose any undesirable normative requirements in place of the original informative Notes.
[ Batavia: Alan and Pete to work. ]
[ Bellevue: Marked as NAD Editorial. ]
[ Post-Sophia Antipolis: Martin indicates there is still work to be done on this issue. Reopened. ]
[ Batavia (2009-05): ]
Tom proposes we say that, unless specified otherwise, it is always the caller's responsibility to verify that supplied arguments meet the called function's requirements. If further semantics are specified (e.g., that the function throws under certain conditions), then it is up to the implementer to check those conditions. Alan feels strongly that our current use of Requires in this context is confusing, especially now that requires is a new keyword.
[ 2009-07 Frankfurt ]
Move to Tentatively NAD.
[ 2009 Santa Cruz: ]
Move to Open. Martin will work on proposed wording.
[ 2010 Pittsburgh: ]
Moved to NAD Editorial, solved by revision to N3021.
Rationale:
Solved by revision to N3021.
Proposed resolution:
Section: 17.5.1.4 [structure.specifications] Status: NAD Editorial Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2008-02-25
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Discussion:
The Remark clauses newly introduced into the Working Paper (N2134) are not mentioned in 17.5.1.4 [structure.specifications] where we list the meaning of Effects, Requires, and other clauses (with the exception of Notes which are documented as informative in 17.5.1.2 [structure.summary], p2, and which they replace in many cases).
Propose add a bullet for Remarks along with a brief description.
[ Batavia: Alan and Pete to work. ]
[ Bellevue: Already resolved in current working paper. ]
Proposed resolution:
Section: 18.6.1.1 [new.delete.single] Status: NAD Submitter: P.J. Plauger Opened: 2007-01-23 Last modified: 2008-02-25
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Discussion:
I recognize the need for nothrow guarantees in the exception reporting mechanism, but I strongly believe that implementors also need an escape hatch when memory gets really low. (Like, there's not enough heap to construct and copy exception objects, or not enough stack to process the throw.) I'd like to think we can put this escape hatch in 18.6.1.1 [new.delete.single], operator new, but I'm not sure how to do it. We need more than a footnote, but the wording has to be a bit vague. The idea is that if new can't allocate something sufficiently small, it has the right to abort/call terminate/call unexpected.
[ Bellevue: NAD. 1.4p2 specifies a program must behave correctly "within its resource limits", so no further escape hatch is necessary. ]
Proposed resolution:
Section: 25 [algorithms] Status: NAD Submitter: James Kanze Opened: 2007-01-31 Last modified: 2010-03-08
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Discussion:
The general requirements for BinaryPredicate (in 25 [algorithms]/8) contradict the implied specific requirements for some functions. In particular, it says that:
[...] if an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments, it should work correctly in the construct if (binary_pred (*first1 , *first2 )){...}. BinaryPredicate always takes the first iterator type as its first argument, that is, in those cases when T value is part of the signature, it should work correctly in the context of if (binary_pred (*first1 , value)){...}.
In the description of upper_bound (25.4.3.2 [upper.bound]/2), however, the use is described as "!comp(value, e)", where e is an element of the sequence (a result of dereferencing *first).
In the description of lexicographical_compare, we have both "*first1 < *first2" and "*first2 < *first1" (which presumably implies "comp( *first1, *first2 )" and "comp( *first2, *first1 )".
Logically, the BinaryPredicate is used as an ordering relationship, with the semantics of "less than". Depending on the function, it may be used to determine equality, or any of the inequality relationships; doing this requires being able to use it with either parameter first. I would thus suggest that the requirement be:
Alternatively, one could specify an order for each function. IMHO, this would be more work for the committee, more work for the implementors, and of no real advantage for the user: some functions, such as lexicographical_compare or equal_range, will still require both functions, and it seems like a much easier rule to teach that both functions are always required, rather than to have a complicated list of when you only need one, and which one.
[ Toronto: Moved to Open. ConceptGCC seems to get lower_bound and upper_bound to work withoutt these changes. ]
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Move to Review. The small problem with the "iterator type" will be fixed. The cited functions (lower_bound, uppwer_bound, equal_range) don't actually use BinaryPredicate , and where it is used, it is consistent with [algorithm]/8, so the main complaint of the issue is moot.
[ 2010-01-16 Beman clarified wording. ]
[ 2010-01-31: Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
[ post San Francisco: ]
Solved by N2759.
2010-01-31: The draft standard is well specified as is, and this specification is desired. Issues 556 and 870 solve the remaining unclearness regarding the meaning of BinaryPredicate.
Proposed resolution:
Change 25 [algorithms] paragraph 8 as indicated:
8 The BinaryPredicate parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and type T when T is part of the signature returns a value testable as true. BinaryPredicate always takes the first iterator value_type as one of its arguments; which argument is unspecified.
In other words, ifIf an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments, it should work correctly both in the construct if (binary_pred(*first1, *first2)){...} and if (binary_pred (*first2, *first1)){...}.BinaryPredicate always takes the first iterator type as its first argument, that is, inIn those cases when T value is part of the signature, it should work correctly in the context of if (binary_pred(*first1, value)){...} and of if (binary_pred (value, *first1)){...}.binary_pred shall not apply any non-constant function through the dereferenced iterators.[Note: if the two types are not identical, and neither is convertable to the other, this may require that the BinaryPredicate be a functional object with two overloaded operator()() functions. — end note]
Section: 23.2 [container.requirements] Status: NAD Submitter: Lionel B Opened: 2007-02-01 Last modified: 2009-07-15
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Discussion:
A recent news group discussion:
Anyone know if the Standard has anything to say about the time complexity of size() for std::set? I need to access a set's size (/not/ to know if it is empty!) heavily during an algorithm and was thus wondering whether I'd be better off tracking the size "manually" or whether that'd be pointless.
That would be pointless. size() is O(1).
Nit: the standard says "should" have constant time. Implementations may take license to do worse. I know that some do this for std::list<> as a part of some trade-off with other operation.
I was aware of that, hence my reluctance to use size() for std::set.
However, this reason would not apply to std::set<> as far as I can see.
Ok, I guess the only option is to try it and see...
If I have any recommendation to the C++ Standards Committee it is that implementations must (not "should"!) document clearly[1], where known, the time complexity of *all* container access operations.
[1] In my case (gcc 4.1.1) I can't swear that the time complexity of size() for std::set is not documented... but if it is it's certainly well hidden away.
[ Kona (2007): This issue affects all the containers. We'd love to see a paper dealing with the broad issue. We think that the complexity of the size() member of every container -- except possibly list -- should be O(1). Alan has volunteered to provide wording. ]
[ Bellevue: ]
Mandating O(1) size will not fly, too many implementations would be invalidated. Alan to provide wording that toughens wording, but that does not absolutely mandate O(1).
[ Batavia (2009-05): ]
We observed that the wording "should" (in note a) has no effect. Howard prefers that O(1) size be mandated. It is not clear that this issue can be resolved to everyone's satisfaction, but Alan will provide wording nonetheless.
[ 2009-07 Frankfurt ]
Fixed by paper N2923.
Proposed resolution:
Section: 20.8.14.2.5 [func.wrap.func.targ] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-02-03 Last modified: 2007-07-02
View all issues with NAD Editorial status.
Discussion:
20.8.14.2.5 [func.wrap.func.targ], p4 says:
Returns: If type() == typeid(T), a pointer to the stored function target; otherwise a null pointer.
Proposed resolution:
Change 20.8.14.2.5 [func.wrap.func.targ], p4:
Returns: If
type()target_type() == typeid(T) && typeid(T) != typeid(void), a pointer to the stored function target; otherwise a null pointer.
[ Pete: Agreed. It's editorial, so I'll fix it. ]
Section: 20.2.5 [allocator.requirements] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2007-02-08 Last modified: 2009-10-26
View all other issues in [allocator.requirements].
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Discussion:
The table of allocator requirements in 20.2.5 [allocator.requirements] describes allocator::address as:
a.address(r) a.address(s)
where r and s are described as:
a value of type X::reference obtained by the expression *p.
and p is
a value of type X::pointer, obtained by calling a1.allocate, where a1 == a
This all implies that to get the address of some value of type T that value must have been allocated by this allocator or a copy of it.
However sometimes container code needs to compare the address of an external value of type T with an internal value. For example list::remove(const T& t) may want to compare the address of the external value t with that of a value stored within the list. Similarly vector or deque insert may want to make similar comparisons (to check for self-referencing calls).
Mandating that allocator::address can only be called for values which the allocator allocated seems overly restrictive.
[ post San Francisco: ]
Pablo recommends NAD Editorial, solved by N2768.
[ 2009-04-28 Pablo adds: ]
Tentatively-ready NAD Editorial as fixed by N2768.
[ 2009-07 Frankfurt ]
Fixed by N2768.
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Addressed by N2982.
Proposed resolution:
Change 20.2.5 [allocator.requirements]:
r : a value of type X::reference
obtained by the expression *p.s : a value of type X::const_reference
obtained by the expression *q or by conversion from a value r.
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
[ Kona (2007): This issue is section 8 of N2387. There was some discussion of it but no resolution to this issue was recorded. Moved to Open. ]
Section: 26.6.2.3 [valarray.access] Status: NAD Editorial Submitter: Bo Persson Opened: 2007-02-11 Last modified: 2007-07-02
View all other issues in [valarray.access].
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Discussion:
The signature of the const operator[] has been changed to return a const reference.
The description in paragraph 1 still says that the operator returns by value.
[ Pete recommends editorial fix. ]
Proposed resolution:
Section: 26.8 [c.math] Status: NAD Editorial Submitter: Bo Persson Opened: 2007-02-13 Last modified: 2007-07-26
View all other issues in [c.math].
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Discussion:
26.8 [c.math], paragraph 10 has long lists of added signatures for float and long double functions. All the signatures have float/long double return values, which is inconsistent with some of the double functions they are supposed to overload.
Proposed resolution:
Change 26.8 [c.math], paragraph 10,
floatint ilogb(float);floatlong lrint(float);floatlong lround(float);floatlong long llrint(float);floatlong long llround(float);long doubleint ilogb(long double);long doublelong lrint(long double);long doublelong lround(long double);long doublelong long llrint(long double);long doublelong long llround(long double);
Section: 27.7.1.2.3 [istream::extractors], 27.7.2.6.3 [ostream.inserters] Status: NAD Submitter: Daniel Krügler Opened: 2007-02-17 Last modified: 2007-10-10
View all other issues in [istream::extractors].
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Discussion:
There already exist two active DR's for the wording of 27.7.1.2.3 [istream::extractors]/13 from 14882:2003(E), namely 64 and 413.
Even with these proposed corrections, already maintained in N2134, I have the feeling, that the current wording does still not properly handle the "exceptional" situation. The combination of para 14
"[..] Characters are extracted and inserted until any of the following occurs:
[..]
- an exception occurs (in which case the exception is caught)."
and 15
"If the function inserts no characters, it calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). If it inserted no characters because it caught an exception thrown while extracting characters from *this and failbit is on in exceptions() (27.4.4.3), then the caught exception is rethrown."
both in N2134 seems to imply that any exception, which occurs *after* at least one character has been inserted is caught and lost for ever. It seems that even if failbit is on in exceptions() rethrow is not allowed due to the wording "If it inserted no characters because it caught an exception thrown while extracting".
Is this behaviour by design?
I would like to add that its output counterpart in 27.7.2.6.3 [ostream.inserters]/7-9 (also N2134) does not demonstrate such an exception-loss-behaviour. On the other side, I wonder concerning several subtle differences compared to input::
1) Paragraph 8 says at its end:
"- an exception occurs while getting a character from sb."
Note that there is nothing mentioned which would imply that such an exception will be caught compared to 27.7.1.2.3 [istream::extractors]/14.
2) Paragraph 9 says:
"If the function inserts no characters, it calls setstate(failbit) (which may throw ios_base::failure (27.4.4.3)). If an exception was thrown while extracting a character, the function sets failbit in error state, and if failbit is on in exceptions() the caught exception is rethrown."
The sentence starting with "If an exception was thrown" seems to imply that such an exception *should* be caught before.
Proposed resolution:
(a) In 27.7.1.2.3 [istream::extractors]/15 (N2134) change the sentence
If the function inserts no characters, it calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). If
it inserted no characters because it caught an exception thrown while extracting characters from *thisan exception was thrown while extracting a character from *this, the function sets failbit in error state, and failbit is on in exceptions() (27.4.4.3), then the caught exception is rethrown.
(b) In 27.7.2.6.3 [ostream.inserters]/8 (N2134) change the sentence:
Gets characters from sb and inserts them in *this. Characters are read from sb and inserted until any of the following occurs:
- end-of-file occurs on the input sequence;
- inserting in the output sequence fails (in which case the character to be inserted is not extracted);
- an exception occurs while getting a character from sb (in which case the exception is caught).
Rationale:
This extractor is described as a formatted input function so the exception behavior is already specified. There is additional behavior described in this section that applies to the case in which failbit is set. This doesn't contradict the usual exception behavior for formatted input functions because that applies to the case in which badbit is set.Section: 27.7.4 [ext.manip] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-02-18 Last modified: 2007-07-02
View all other issues in [ext.manip].
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Discussion:
The function f in para 4 (27.7.4 [ext.manip]) references an unknown strm in the following line:
mg.get(Iter(str.rdbuf()), Iter(), intl, strm, err, mon);
Proposed resolution:
Change 27.7.4 [ext.manip], p4:
mg.get(Iter(str.rdbuf()), Iter(), intl, strm, err, mon);
[ Oxford: Editorial. ]
Section: 27.9.1.9 [ifstream.members], 27.9.1.13 [ofstream.members] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-02-20 Last modified: 2007-07-02
View all other issues in [ifstream.members].
View all issues with NAD Editorial status.
Discussion:
The standard wording of N2134 has extended the 14882:2003(E) wording for the ifstream/ofstream/fstream open function to fix a long standing problem, see 409.
Now it's properly written as
"If that function does not return a null pointer calls clear(), otherwise calls setstate(failbit)[..]"
instead of the previous
"If that function returns a null pointer, calls setstate(failbit)[..]
While the old footnotes saying
"A successful open does not change the error state."
where correct and important, they are invalid now for ifstream and ofstream (because clear *does* indeed modify the error state) and should be removed (Interestingly fstream itself never had these, although they where needed for that time).
Proposed resolution:
In 27.9.1.9 [ifstream.members], remove footnote:
334) A successful open does not change the error state.
In 27.9.1.13 [ofstream.members], remove footnote:
335) A successful open does not change the error state.
Section: 20.8.14.2 [func.wrap.func] Status: NAD Submitter: Bo Persson Opened: 2007-02-25 Last modified: 2009-07-13
View all other issues in [func.wrap.func].
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Discussion:
20.8.14.2 [func.wrap.func]
The note in paragraph 2 refers to 'undefined void operators', while the section declares a pair of operators returning bool.
[ Post-Sophia Antipolis: ]
Changed from Pending WP to Open. This issue was voted to WP at the same time the operators were changed from private to deleted. The two issues stepped on each other. What do we want the return type of these deleted functions to be?
[ 2009-05-02 Daniel adds: ]
I suggest harmonizing this issue with similar classes. E.g. in 20.9.11.3 [util.smartptr.weak] bool return values for
template <class Y> bool operator<(weak_ptr<Y> const&) const = delete; template <class Y> bool operator<=(weak_ptr<Y> const&) const = delete; template <class Y> bool operator>(weak_ptr<Y> const&) const = delete; template <class Y> bool operator>=(weak_ptr<Y> const&) const = delete;are used and basically all newer provided deleted copy assignment operators of type X use the canonical return type X& instead of void. Since the note mentioned in the issue description has now already been changed to
deleted overloads close possible hole in the type systemit seems to be of even lesser need to perform the change. Therefore I recommend declaring the issue as NAD.
[ Batavia (2009-05): ]
We agree with Daniel's recommendation.
Move to NAD.
Proposed resolution:
Change 20.8.14.2 [func.wrap.func]
... private: // 20.8.14.2 [func.wrap.func], undefined operators: template<class Function2>boolvoid operator==(const function<Function2>&); template<class Function2>boolvoid operator!=(const function<Function2>&); };
Change 20.8.14.2 [func.wrap.func]
template<class Function2>boolvoid operator==(const function<Function2>&); template<class Function2>boolvoid operator!=(const function<Function2>&);
Section: 28.10 [re.results] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-02-26 Last modified: 2008-03-12
View all other issues in [re.results].
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Discussion:
According to the description given in 28.10 [re.results]/2 the class template match_results "shall satisfy the requirements of a Sequence, [..], except that only operations defined for const-qualified Sequences are supported". Comparing the provided operations from 28.10 [re.results]/3 with the sequence/container tables 80 and 81 one recognizes the following missing operations:
1) The members
const_iterator rbegin() const; const_iterator rend() const;
should exists because 23.1/10 demands these for containers (all sequences are containers) which support bidirectional iterators. Aren't these supported by match_result? This is not explicitely expressed, but it's somewhat implied by two arguments:
(a) Several typedefs delegate to iterator_traits<BidirectionalIterator>.
(b) The existence of const_reference operator[](size_type n) const implies even random-access iteration. I also suggest, that match_result should explicitly mention, which minimum iterator category is supported and if this does not include random-access the existence of operator[] is somewhat questionable.
2) The new "convenience" members
const_iterator cbegin() const; const_iterator cend() const; const_iterator crbegin() const; const_iterator crend() const;
should be added according to tables 80/81.
Proposed resolution:
Add the following members to the match_results synopsis after end() in 28.10 [re.results] para 3:
const_iterator cbegin() const; const_iterator cend() const;
In section 28.10.3 [re.results.acc] change:
const_iterator begin() const; const_iterator cbegin() const;-7- Returns: A starting iterator that enumerates over all the sub-expressions stored in *this.
const_iterator end() const; const_iterator cend() const;-8- Returns: A terminating iterator that enumerates over all the sub-expressions stored in *this.
[ Kona (2007): Voted to adopt proposed wording in N2409 except removing the entry in the table container requirements. Moved to Review. ]
[ Bellevue: Proposed wording now in the WP. ]
Section: 28.11.3 [re.alg.search] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-02-26 Last modified: 2007-07-26
View all issues with NAD Editorial status.
Discussion:
28.11.3 [re.alg.search]/5 declares
template <class iterator, class charT, class traits>
bool regex_search(iterator first, iterator last,
const basic_regex<charT, traits>& e,
regex_constants::match_flag_type flags =
regex_constants::match_default);
where it's not explained, which iterator category the parameter iterator belongs to. This is inconsistent to the preceding declaration in the synopsis section 28.4 [re.syn], which says:
template <class BidirectionalIterator, class charT, class traits>
bool regex_search(BidirectionalIterator first, BidirectionalIterator last,
const basic_regex<charT, traits>& e,
regex_constants::match_flag_type flags =
regex_constants::match_default);
Proposed resolution:
In 28.11.3 [re.alg.search]/5 replace all three occurences of param "iterator" with "BidirectionalIterator"
template <classiteratorBidirectionalIterator, class charT, class traits> bool regex_search(iteratorBidirectionalIterator first,iteratorBidirectionalIterator last, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default);-6- Effects: Behaves "as if" by constructing an object what of type match_results<
iteratorBidirectionalIterator> and then returning the result of regex_search(first, last, what, e, flags).
Rationale:
Applied to working paper while issue was still in New status.Section: 28.12.1.1 [re.regiter.cnstr] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-03-03 Last modified: 2007-07-02
View all issues with NAD Editorial status.
Discussion:
In 28.12.1.1 [re.regiter.cnstr]/2 the effects paragraph starts with:
Effects: Initializes begin and end to point to the beginning and the end of the target sequence, sets pregex to &re, sets flags to f,[..]
There are two issues with this description:
Proposed resolution:
In 28.12.1.1 [re.regiter.cnstr]/2 change the above quoted part by
Effects: Initializes begin and end to point to the beginning and the end of the target sequence designated by the iterator range [a, b), sets pregex to &re, sets flags to
fm, then calls regex_search(begin, end, match, *pregex, flags). If this call returns false the constructor sets *this to the end-of-sequence iterator.
Section: 28.12.2.1 [re.tokiter.cnstr] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-03-03 Last modified: 2007-07-02
View all other issues in [re.tokiter.cnstr].
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Discussion:
In 28.12.2.1 [re.tokiter.cnstr]/1+2 both the constructor declaration and the following text shows some obvious typos:
1) The third constructor form is written as
template <std::size_t N>
regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
const regex_type& re,
const int (&submatches)[R],
regex_constants::match_flag_type m =
regex_constants::match_default);
where the dimensions of submatches are specified by an unknown value R, which should be N.
2) Paragraph 2 of the same section says in its last sentence:
The third constructor initializes the member subs to hold a copy of the sequence of integer values pointed to by the iterator range [&submatches, &submatches + R).
where again R must be replaced by N.
3) Paragraph 3 of the same section says in its first sentence:
Each constructor then sets N to 0, and position to position_iterator(a, b, re, f).
where a non-existing parameter "f" is mentioned, which must be replaced by the parameter "m".
Proposed resolution:
Change 28.12.2.1 [re.tokiter.cnstr]/1:
template <std::size_t N>
regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
const regex_type& re,
const int (&submatches)[R N],
regex_constants::match_flag_type m =
regex_constants::match_default);
Change 28.12.2.1 [re.tokiter.cnstr]/2:
Effects: The first constructor initializes the member subs to hold the single value submatch. The second constructor initializes the member subs to hold a copy of the argument submatches. The third constructor initializes the member subs to hold a copy of the sequence of integer values pointed to by the iterator range [&submatches, &submatches +
RN).
Change 28.12.2.1 [re.tokiter.cnstr]/3:
Each constructor then sets N to 0, and position to position_iterator(a, b, re,
fm). If position is not an end-of-sequence iterator the constructor sets result to the address of the current match. Otherwise if any of the values stored in subs is equal to -1 the constructor sets *this to a suffix iterator that points to the range [a, b), otherwise the constructor sets *this to an end-of-sequence iterator.
Section: 1.2 [intro.refs] Status: NAD Submitter: Alisdair Meredith Opened: 2007-03-08 Last modified: 2008-02-25
View all other issues in [intro.refs].
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Discussion:
1.2 [intro.refs] Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this Interna- tional Standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.
- Ecma International, ECMAScript Language Specification, Standard Ecma-262, third edition, 1999.
- ISO/IEC 2382 (all parts), Information technology - Vocabulary
- ISO/IEC 9899:1990, Programming languages - C
- ISO/IEC 9899/Amd.1:1995, Programming languages - C, AMENDMENT 1: C Integrity
- ISO/IEC 9899:1999, Programming languages - C
- ISO/IEC 9899:1999/Cor.1:2001 Programming languages - C
- ISO/IEC 9899:1999/Cor.2:2004 Programming languages - C
- ISO/IEC 9945:2003, Information Technology-Portable Operating System Interface (POSIX)
- ISO/IEC 10646-1:1993 Information technology - Universal Multiple-Octet Coded Character Set (UCS) - Part 1: Architecture and Basic Multilingual Plane
I'm not sure how many of those reserve naming patterns that might affect us, but I am equally sure I don't own a copy of any of these to check!
The point is to list the reserved naming patterns, rather than the individual names themselves - although we may want to list C keywords that are valid identifiers in C++ but likely to cause trouble in shared headers (e.g. restrict)
[ Kona (2007): Recommend NAD. No one has identified a specific defect, just the possibility of one. ]
[
Post-Kona: Alisdair request Open. A good example of the problem was a
discussion of the system error proposal, where it was pointed out an all-caps
identifier starting with a capital E conflicted with reserved macro names for
both Posix and C. I had absolutely no idea of this rule, and suspect I was
not the only one in the room.
Resolution will require someone with access to all the listed documents to
research their respective name reservation rules, or people with access to
specific documents add their rules to this issue until the list is complete.
]
[ Bellevue: Wording is aleady present in various standards, and no-one has come forward with wording. Suggest a formal paper rather than a defect report is the correct way to proceed. ]
Proposed resolution:
Section: 26.5.2 [rand.synopsis] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-03-08 Last modified: 2007-07-02
View all other issues in [rand.synopsis].
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Discussion:
26.5.2 [rand.synopsis] the header <random> synopsis contains an unreasonable closing curly brace inside the subtract_with_carry_engine declaration.
Proposed resolution:
Change the current declaration in 26.5.2 [rand.synopsis]
template <class UIntType, size_t w}, size_t s, size_t r> class subtract_with_carry_engine;
[ Pete: Recommends editorial. ]
Section: 17.6.2.2 [using.headers] Status: NAD Submitter: Gennaro Prota Opened: 2007-03-14 Last modified: 2007-10-10
View all issues with NAD status.
Discussion:
17.6.2.2 [using.headers] states:
A translation unit shall include a header only outside of any external declaration or definition, [...]
I see three problems with this requirement:
The C++ standard doesn't define what an "external declaration" or an "external definition" are (incidentally the C99 standard does, and has a sentence very similar to the above regarding header inclusion).
I think the intent is that the #include directive shall lexically appear outside *any* declaration; instead, when the issue was pointed out on comp.std.c++ at least one poster interpreted "external declaration" as "declaration of an identifier with external linkage". If this were the correct interpretation, then the two inclusions below would be legal:
// at global scope
static void f()
{
# include <cstddef>
}
static void g()
{
# include <stddef.h>
}
(note that while the first example is unlikely to compile correctly, the second one may well do)
as the sentence stands, violations will require a diagnostic; is this the intent? It was pointed out on comp.std.c++ (by several posters) that at least one way to ensure a diagnostic exists:
[If there is an actual file for each header,] one simple way to implement this would be to insert a reserved identifier such as __begin_header at the start of each standard header. This reserved identifier would be ignored for all other purposes, except that, at the appropriate point in phase 7, if it is found inside an external definition, a diagnostic is generated. There's many other similar ways to achieve the same effect.
--James Kuyper, on comp.std.c++
is the term "header" meant to be limited to standard headers? Clause 17 is all about the library, but still the general question is interesting and affects one of the points in the explicit namespaces proposal (n1691):
Those seeking to conveniently enable argument-dependent lookups for all operators within an explicit namespace could easily create a header file that does so:
namespace mymath:: { #include "using_ops.hpp" }
Proposed resolution:
Rationale:
We believe that the existing language does not cause any real confusion and any new formulation of the rules that we could come up with are unlikely to be better than what's already in the standard.Section: 20.8 [function.objects] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-03-19 Last modified: 2007-08-05
View all other issues in [function.objects].
View all issues with NAD Editorial status.
Discussion:
The header <functional> synopsis in 20.8 [function.objects] contains the following two free comparison operator templates for the function class template
template<class Function1, class Function2> void operator==(const function<Function1>&, const function<Function2>&); template<class Function1, class Function2> void operator!=(const function<Function1>&, const function<Function2>&);
which are nowhere described. I assume that they are relicts before the corresponding two private and undefined member templates in the function template (see 20.8.14.2 [func.wrap.func] and [func.wrap.func.undef]) have been introduced. The original free function templates should be removed, because using an undefined entity would lead to an ODR violation of the user.
Proposed resolution:
Remove the above mentioned two function templates from the header <functional> synopsis (20.8 [function.objects])
template<class Function1, class Function2> void operator==(const function<Function1>&, const function<Function2>&); template<class Function1, class Function2> void operator!=(const function<Function1>&, const function<Function2>&);
Rationale:
Fixed by N2292 Standard Library Applications for Deleted Functions.Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: NAD Submitter: Cosmin Truta Opened: 2007-04-05 Last modified: 2007-07-25
View all other issues in [facet.num.get.virtuals].
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Discussion:
From Section 22.4.2.1.2 [facet.num.get.virtuals], paragraphs 11 and 12, it is implied
that the value read from a stream must be stored
even if the placement of thousands separators does not conform to the
grouping() specification from the numpunct facet.
Since incorrectly-placed thousands separators are flagged as an extraction
failure (by the means of failbit), we believe it is better not
to store the value. A consistent strategy, in which any kind of extraction
failure leaves the input item intact, is conceptually cleaner, is able to avoid
corner-case traps, and is also more understandable from the programmer's point
of view.
Here is a quote from "The C++ Programming Language (Special Edition)" by B. Stroustrup (Section D.4.2.3, pg. 897):
"If a value of the desired type could not be read, failbit is set in r. [...] An input operator will use r to determine how to set the state of its stream. If no error was encountered, the value read is assigned through v; otherwise, v is left unchanged."
This statement implies that rdstate() alone is sufficient to
determine whether an extracted value is to be assigned to the input item
val passed to do_get. However, this is in disagreement
with the current C++ Standard. The above-mentioned assumption is true in all
cases, except when there are mismatches in digit grouping. In the latter case,
the parsed value is assigned to val, and, at the same time, err
is assigned to ios_base::failbit (essentially "lying" about the
success of the operation). Is this intentional? The current behavior raises
both consistency and usability concerns.
Although digit grouping is outside the scope of scanf (on which
the virtual methods of num_get are based), handling of grouping
should be consistent with the overall behavior of scanf. The specification of
scanf makes a distinction between input failures and matching
failures, and yet both kinds of failures have no effect on the input items
passed to scanf. A mismatch in digit grouping logically falls in
the category of matching failures, and it would be more consistent, and less
surprising to the user, to leave the input item intact whenever a failure is
being signaled.
The extraction of bool is another example outside the scope of
scanf, and yet consistent, even in the event of a successful
extraction of a long but a failed conversion from
long to bool.
Inconsistency is further aggravated by the fact that, when failbit is set,
subsequent extraction operations are no-ops until failbit is
explicitly cleared. Assuming that there is no explicit handling of
rdstate() (as in cin>>i>>j) it is
counter-intuitive to be able to extract an integer with mismatched digit
grouping, but to be unable to extract another, properly-formatted integer
that immediately follows.
Moreover, setting failbit, and selectively assigning a value to
the input item, raises usability problems. Either the strategy of
scanf (when there is no extracted value in case of failure), or
the strategy of the strtol family (when there is always an
extracted value, and there are well-defined defaults in case of a failure) are
easy to understand and easy to use. On the other hand, if failbit
alone cannot consistently make a difference between a failed extraction, and a
successful but not-quite-correct extraction whose output happens to be the same
as the previous value, the programmer must resort to implementation tricks.
Consider the following example:
int i = old_i;
cin >> i;
if (cin.fail())
// can the value of i be trusted?
// what does it mean if i == old_i?
// ...
Last but not least, the current behvaior is not only confusing to the casual
reader, but it has also been confusing to some book authors. Besides
Stroustrup's book, other books (e.g. "Standard C++ IOStreams and Locales" by
Langer and Kreft) are describing the same mistaken assumption. Although books
are not to be used instead of the standard reference, the readers of these
books, as well as the people who are generally familiar to scanf,
are even more likely to misinterpret the standard, and expect the input items
to remain intact when a failure occurs.
Proposed resolution:
Change 22.4.2.1.2 [facet.num.get.virtuals]:
Stage 3: The result of stage 2 processing can be one of
- A sequence of
charshas been accumulated in stage 2 that is converted (according to the rules ofscanf) to a value of the type ofval.This value is stored invalandios_base::goodbitis stored inerr.- The sequence of
charsaccumulated in stage 2 would have causedscanfto report an input failure.ios_base::failbitis assigned toerr.In the first case,
Ddigit grouping is checked. That is, the positions of discarded separators is examined for consistency withuse_facet<numpunct<charT> >(loc).grouping(). If they are not consistent thenios_base::failbitis assigned toerr. Otherwise, the value that was converted in stage 2 is stored invalandios_base::goodbitis stored inerr.
Rationale:
post-Toronto: Changed from New to NAD at the request of the author. The preferred solution of N2327 makes this resolution obsolete.Section: 17.5.1.4 [structure.specifications] Status: NAD Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2009-05-01
View all other issues in [structure.specifications].
View all issues with NAD status.
Discussion:
17.5.1.4 [structure.specifications] para 5 says
-5- Complexity requirements specified in the library clauses are upper bounds, and implementations that provide better complexity guarantees satisfy the requirements.
The following objection has been raised:
The library clauses suggest general guidelines regarding complexity, but we have been unable to discover any absolute hard-and-fast formulae for these requirements. Unless or until the Library group standardizes specific hard-and-fast formulae, we regard all the complexity requirements as subject to a "fudge factor" without any intrinsic upper bound.
[Plum ref _23213Y31 etc]
Proposed resolution:
Rationale:
Kona (2007): No specific instances of underspecification have been identified, and big-O notation always involves constant factors.Section: 22.4.6.1.2 [locale.money.get.virtuals] Status: NAD Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2009-07-13
View all other issues in [locale.money.get.virtuals].
View all issues with NAD status.
Discussion:
22.4.6.1.2 [locale.money.get.virtuals], para 1 says:
The result is returned as an integral value stored in units or as a sequence of digits possibly preceded by a minus sign (as produced by ct.widen(c) where c is '-' or in the range from '0' through '9', inclusive) stored in digits.
The following objection has been raised:
Some implementations interpret this to mean that a facet derived from ctype<wchar_t> can provide its own member do_widen(char) which produces e.g. L'@' for the "widened" minus sign, and that the '@' symbol will appear in the resulting sequence of digits. Other implementations have assumed that one or more places in the standard permit the implementation to "hard-wire" L'-' as the "widened" minus sign. Are both interpretations permissible, or only one?
[Plum ref _222612Y14]
Furthermore: if ct.widen('9') produces L'X' (a non-digit), does a parse fail if a '9' appears in the subject string? [Plum ref _22263Y33]
[ Kona (2007): Bill and Dietmar to provide proposed wording. ]
[ post Bellevue: Bill adds: ]
The Standard is clear that the minus sign stored in digits is ct.widen('-'). The subject string must contain characters c in the set [-0123456789] which are translated by ct.widen(c) calls before being stored in digits; the widened characters are not relevant to the parsing of the subject string.
[ Batavia (2009-05): ]
We agree with Bill's comment above, in line with the first of the interpretations offered in the issue. Move to NAD.
Proposed resolution:
Section: 22.4.6.1.2 [locale.money.get.virtuals] Status: NAD Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2009-10-21
View all other issues in [locale.money.get.virtuals].
View all issues with NAD status.
Discussion:
22.4.6.1.2 [locale.money.get.virtuals], para 3 says:
If pos or neg is empty, the sign component is optional, and if no sign is detected, the result is given the sign that corresponds to the source of the empty string.
The following objection has been raised:
A negative_sign of "" means "there is no way to write a negative sign" not "any null sequence is a negative sign, so it's always there when you look for it".
[Plum ref _222612Y32]
[ Kona (2007): Bill to provide proposed wording and interpretation of existing wording. ]
Related to 669.
[ 2009-05-17 Howard adds: ]
I disagree that a negative_sign of "" means "there is no way to write a negative sign". The meaning requires the sentences of 22.4.6.1.2 [locale.money.get.virtuals] p3 following that quoted above to be taken into account:
-3- ... If pos or neg is empty, the sign component is optional, and if no sign is detected, the result is given the sign that corresponds to the source of the empty string. Otherwise, the character in the indicated position must match the first character of pos or neg, and the result is given the corresponding sign. If the first character of pos is equal to the first character of neg, or if both strings are empty, the result is given a positive sign.So a negative_sign of "" means "there is no way to write a negative sign" only when positive_sign is also "". However when negative_sign is "" and postive_sign.size() > 0, then one writes a negative value by not writing the postive_sign in the position indicated by money_base::sign. For example:
pattern = {symbol, sign, value, none} positive_sign = "+" negative_sign = "" $123 // a negative value, using optional sign $+123 // a positive value $-123 // a parse errorAnd:
pattern = {symbol, sign, value, none} positive_sign = "" negative_sign = "" $123 // a positive value, no sign possible $+123 // a parse error $-123 // a parse errorAnd (regarding 669):
pattern = {symbol, sign, value, none} positive_sign = "-" negative_sign = "-" $123 // a parse error, sign is mandatory $+123 // a parse error $-123 // a positive valueThe text seems both unambiguous and clear to me. I recommend NAD for both this issue and 669. However I would have no objection to adding examples such as those above.
[ Batavia (2009-05): ]
This discussion applies equally to issue 669 (q.v.). Howard has added examples above, and recommends either NAD or a resolution that adds his (or similar) examples to the Working Paper.
Alan would like to rewrite paragraph 3.
We recommend moving to NAD. Anyone who feels strongly about adding the examples is invited to submit corresponding wording. We further recommend issue 669 be handled identically.
[ 2009-07-14 Alan reopens with improved wording. ]
[ 2009-07 Frankfurt ]
No consensus for closing as NAD. Leave in Review.
[ 2009-10 Santa Cruz: ]
NAD. Agreed that the original assessment as NAD was correct.
Proposed resolution:
Change 22.4.6.1.2 [locale.money.get.virtuals] p3:
-3-If the first character (if any) in the string pos returned by mp.positive_sign() or the string neg returned by mp.negative_sign() is recognized in the position indicated by sign in the format pattern, it is consumed and any remaining characters in the string are required after all the other format components. [Example: If showbase is off, then for a neg value of "()" and a currency symbol of "L", in "(100 L)" the "L" is consumed; but if neg is "-", the "L" in "-100 L" is not consumed. -- end example] If pos or neg is empty, the sign component is optional, and if no sign is detected, the result is given the sign that corresponds to the source of the empty string. Otherwise, the character in the indicated position must match the first character of pos or neg, and the result is given the corresponding sign. If the first character of pos is equal to the first character of neg, or if both strings are empty, the result is given a positive sign.The sign pattern strings pos and neg are returned by mp.positive_sign() and mp.negative_sign() respectively. A sign pattern is matched if its first character is recognized in s in the position indicated by sign in the format pattern, or if the pattern is empty and there is no sign recognized in s. A match is required to occur. If both patterns are matched, the result is given a positive sign, otherwise the result is given the sign corresponding to the matched pattern. If the pattern contains more than one character, the characters after the first must be matched in s after all other format components. If any sign characters are matched, s is consumed up to and including those characters. [Example: If showbase is off, then for a neg value of "()" and a currency symbol of "L", in "(100 L)" the entire string is consumed; but for a neg value of "-", in "-100 L", the string is consumed through the second "0" (the space and "L" are not consumed). — end example]
Section: 22.4.6.1.2 [locale.money.get.virtuals] Status: NAD Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2009-07-13
View all other issues in [locale.money.get.virtuals].
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Discussion:
22.4.6.1.2 [locale.money.get.virtuals], para 3 sentence 4 says:
If the first character of pos is equal to the first character of neg, or if both strings are empty, the result is given a positive sign.
One interpretation is that an input sequence must match either the positive pattern or the negative pattern, and then in either event it is interpreted as positive. The following objections has been raised:
The input can successfully match only a positive sign, so the negative pattern is an unsuccessful match.
[Plum ref _222612Y34, 222612Y51b]
[ Bill to provide proposed wording and interpretation of existing wording. ]
[ 2009-05-17 See Howard's comments in related issue 668. ]
[ Batavia (2009-05): ]
This discussion applies equally to issue 668 (q.v.). Howard has added examples there, and recommends either NAD or a resolution that adds his (or similar) examples to the Working Paper.
We recommend moving to NAD. Anyone who feels strongly about adding the examples is invited to submit corresponding wording. We further recommend issue 668 be handled identically.
Proposed resolution:
Section: 22.4.6.3 [locale.moneypunct] Status: Dup Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2008-09-22
View all issues with Dup status.
Duplicate of: 836
Discussion:
22.4.6.3 [locale.moneypunct], para 2 says:
The value space indicates that at least one space is required at that position.
The following objection has been raised:
Whitespace is optional when matching space. (See 22.4.6.1.2 [locale.money.get.virtuals], para 2.)
[Plum ref _22263Y22]
[ Kona (2007): Bill to provide proposed wording. We agree that C++03 is ambiguous, and that we want C++0X to say "space" means 0 or more whitespace characters on input. ]
Proposed resolution:
Section: 28.12.2 [re.tokiter] Status: NAD Editorial Submitter: Eric Niebler Opened: 2007-06-02 Last modified: 2009-03-09
View all other issues in [re.tokiter].
View all issues with NAD Editorial status.
Discussion:
28.12.2 [re.tokiter], p3 says:
After it is constructed, the iterator finds and stores a value match_results<BidirectionalIterator> position and sets the internal count N to zero.
Should read:
After it is constructed, the iterator finds and stores a value
match_resultsregex_iterator<BidirectionalIterator, charT, traits> position and sets the internal count N to zero.
[ John adds: ]
Yep, looks like a typo/administrative fix to me.
Proposed resolution:
Section: 28.10 [re.results] Status: NAD Editorial Submitter: Nozomu Katoo Opened: 2007-05-27 Last modified: 2008-03-12
View all other issues in [re.results].
View all issues with NAD Editorial status.
Discussion:
In 28.4 [re.syn] of N2284, two template functions are declared here:
// 28.10, class template match_results:
<snip>
// match_results comparisons
template <class BidirectionalIterator, class Allocator>
bool operator== (const match_results<BidirectionalIterator, Allocator>& m1,
const match_results<BidirectionalIterator, Allocator>& m2);
template <class BidirectionalIterator, class Allocator>
bool operator!= (const match_results<BidirectionalIterator, Allocator>& m1,
const match_results<BidirectionalIterator, Allocator>& m2);
// 28.10.6, match_results swap:
But the details of these two bool operator functions (i.e., which members of match_results should be used in comparison) are not described in any following sections.
[ John adds: ]
That looks like a bug: operator== should return true only if the two objects refer to the same match - ie if one object was constructed as a copy of the other.
[ Kona (2007): Bill and Pete to add minor wording to that proposed in N2409. ]
Proposed resolution:
Add a new section after 28.10.6 [re.results.swap], which reads:
28.10.7 match_results non-member functions.
template<class BidirectionalIterator, class Allocator> bool operator==(const match_results<BidirectionalIterator, Allocator>& m1, const match_results<BidirectionalIterator, Allocator>& m2);Returns: true only if the two objects refer to the same match.
template<class BidirectionalIterator, class Allocator> bool operator!=(const match_results<BidirectionalIterator, Allocator>& m1, const match_results<BidirectionalIterator, Allocator>& m2);Returns: !(m1 == m2).
template<class BidirectionalIterator, class Allocator> void swap(match_results<BidirectionalIterator, Allocator>& m1, match_results<BidirectionalIterator, Allocator>& m2);Returns: m1.swap(m2).
[ Bellevue: Proposed wording now in WP. ]
Section: 20.9.10.2.4 [unique.ptr.single.observers], 20.9.11.2.5 [util.smartptr.shared.obs] Status: NAD Submitter: Beman Dawes Opened: 2007-06-14 Last modified: 2008-02-27
View all issues with NAD status.
Discussion:
The standard library uses the operator unspecified-bool-type() const idiom in five places. In three of those places (20.8.14.2.3 [func.wrap.func.cap], function capacity for example) the returned value is constrained to disallow unintended conversions to int. The standardese is
The return type shall not be convertible to int.
This constraint is omitted for unique_ptr and shared_ptr. It should be added for those.
[ Bellevue: ]
Close as NAD. Accepting paper N2435 makes it irrelevant.
Proposed resolution:
To the Returns paragraph for operator unspecified-bool-type() const of 20.9.10.2.4 [unique.ptr.single.observers] paragraph 11 and 20.9.11.2.5 [util.smartptr.shared.obs] paragraph 16, add the sentence:
The return type shall not be convertible to int.
[ Kona (2007): Uncertain if nullptr will address this issue. ]
Section: 26.8 [c.math] Status: NAD Editorial Submitter: Niels Dekker Opened: 2007-06-10 Last modified: 2007-07-25
View all other issues in [c.math].
View all issues with NAD Editorial status.
Discussion:
Quoting the latest draft (n2135), 26.8 [c.math]:
The added signatures are:
long abs(long); // labs() long abs(long long); // llabs()
Shouldn't abs(long long) have long long as return type?
Proposed resolution:
Change 26.8 [c.math]:
long long abs(long long); // llabs()
Rationale:
Had already been fixed in the WP by the time the LWG reviewed this.Section: 19.5 [syserr] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-06-24 Last modified: 2008-01-06
View all other issues in [syserr].
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Discussion:
The most recent state of N2241 as well as the current draft N2284 (section 19.5 [syserr], p.2) proposes a new enumeration type posix_errno immediatly in the namespace std. One of the enumerators has the name invalid_argument, or fully qualified: std::invalid_argument. This name clashes with the exception type std::invalid_argument, see 19.2 [std.exceptions]/p.3. This clash makes e.g. the following snippet invalid:
#include <system_error>
#include <stdexcept>
void foo() { throw std::invalid_argument("Don't call us - we call you!"); }
I propose that this enumeration type (and probably the remaining parts of <system_error> as well) should be moved into one additional inner namespace, e.g. sys or system to reduce foreseeable future clashes due to the great number of members that std::posix_errno already contains (Btw.: Why has the already proposed std::sys sub-namespace from N2066 been rejected?). A further clash candidate seems to be std::protocol_error (a reasonable name for an exception related to a std network library, I guess).
Another possible resolution would rely on the proposed strongly typed enums, as described in N2213. But maybe the forbidden implicit conversion to integral types would make these enumerators less attractive in this special case?
Proposed resolution:
Fixed by issue 7 of N2422.
Section: TR1 5.2.1.1 [tr.num.sf.Lnm] Status: NAD Submitter: Christopher Crawford Opened: 2007-06-30 Last modified: 2009-07-13
View all issues with NAD status.
Discussion:
I see that the definition the associated Laguerre polynomials TR1 5.2.1.1 [tr.num.sf.Lnm] has been corrected since N1687. However, the draft standard only specifies ranks of integer value m, while the associated Laguerre polynomials are actually valid for real values of m > -1. In the case of non-integer values of m, the definition Ln(m) = (1/n!)exx-m (d/dx)n (e-xxm+n) must be used, which also holds for integer values of m. See Abramowitz & Stegun, 22.11.6 for the general case, and 22.5.16-17 for the integer case. In fact fractional values are most commonly used in physics, for example to m = +/- 1/2 to describe the harmonic oscillator in 1 dimension, and 1/2, 3/2, 5/2, ... in 3 dimensions.
If I am correct, the calculation of the more general case is no more difficult, and is in fact the function implemented in the GNU Scientific Library. I would urge you to consider upgrading the standard, either adding extra functions for real m or switching the current ones to double.
[ Batavia (2009-05): ]
We understand the issue, and have opted not to extend as recommended.
Move to NAD.
Proposed resolution:
Section: TR1 5.2.1.2 [tr.num.sf.Plm] Status: NAD Submitter: Christopher Crawford Opened: 2007-06-30 Last modified: 2009-07-13
View all issues with NAD status.
Discussion:
One other small thing, in TR1 5.2.1.2 [tr.num.sf.Plm], the restriction should be |x| <= 1, not x >= 0.
[ Batavia (2009-05): ]
The error has been corrected in the pending IS.
Move to NAD.
Proposed resolution:
Section: 18.8.5 [propagation] Status: NAD Submitter: Jens Maurer Opened: 2007-07-20 Last modified: 2008-02-25
View all other issues in [propagation].
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Discussion:
From the Toronto Core wiki:
What do you mean by "null pointer constant"? How do you guarantee that exception_ptr() == 1 doesn't work? Do you even want to prevent that? What's the semantics? What about void *p = 0; exception_ptr() == p? Maybe disallow those in the interface, but how do you do that with portable C++? Could specify just "make it work".
Peter's response:
null pointer constant as defined in 4.10 [conv.ptr]. Intent is "just make it work", can be implemented as assignment operator taking a unique pointer to member, as in the unspecified bool type idiom.
[ Bellevue: ]
Original implementation was possible using the "unspecified-null-pointer" idiom, similar to unspecified-bool.
Even simpler now with nullptr_t.
NAD Rationale : null pointer constant is a perfectly defined term, and while API is clearly implementable there is no need to spell out implementation details.
Proposed resolution:
Section: 22 [localization] Status: NAD Future Submitter: Peter Dimov Opened: 2007-07-28 Last modified: 2009-07-16
View all other issues in [localization].
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Discussion:
The POSIX "Extended API Set Part 4,"
introduces extensions to the C locale mechanism that allow multiple concurrent locales to be used in the same application by introducing a type locale_t that is very similar to std::locale, and a number of _l functions that make use of it.
The global locale (set by setlocale) is now specified to be per- process. If a thread does not call uselocale, the global locale is in effect for that thread. It can install a per-thread locale by using uselocale.
There is also a nice querylocale mechanism by which one can obtain the name (such as "de_DE") for a specific facet, even for combined locales, with no std::locale equivalent.
std::locale should be harmonized with the new POSIX locale_t mechanism and provide equivalents for uselocale and querylocale.
[ Kona (2007): Bill and Nick to provide wording. ]
[ San Francisco: Bill and Nick still intend to provide wording, but this is a part of the task to be addressed by the group that will look into issue 860. ]
[ 2009-07 Frankfurt: ]
It's our intention to stay in sync with WG14. If WG14 makes a decision that requires a change in WG21 the issue will be reopened.
Move to NAD Future.
Proposed resolution:
Section: 26.6.2.3 [valarray.access] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-08-27 Last modified: 2008-09-22
View all other issues in [valarray.access].
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Discussion:
Since the return type of valarray's operator[] const overload has been changed to const T& as described in 389 several paragraphs of the section 26.6.2.3 [valarray.access] are now incompletely specified, because many requirements and guarantees should now also apply to the const overload. Most notably, the address and reference guarantees should be extended to the const overload case.
Proposed resolution:
Change 26.6.2.3 [valarray.access]:
-1-
When applied to a constant array, the subscript operator returns a reference to the corresponding element of the array. When applied to a non-constant array, tThe subscript operator returns a reference to the corresponding element of the array.-3- The expression &a[i+j] == &a[i] + j evaluates as true for all size_t i and size_t j such that i+j is less than the length of the
non-constantarray a.-4- Likewise, the expression &a[i] != &b[j] evaluates as true for any two
non-constantarrays a and b and for any size_t i and size_t j such that i is less than the length of a and j is less than the length of b. This property indicates an absence of aliasing and may be used to advantage by optimizing compilers.281)-5- The reference returned by the subscript operator for an
non-constantarray is guaranteed to be valid until the member function resize(size_t, T) (26.5.2.7) is called for that array or until the lifetime of that array ends, whichever happens first.
Section: 21.4 [basic.string] Status: NAD Editorial Submitter: Bo Persson Opened: 2007-08-18 Last modified: 2009-07-16
View all other issues in [basic.string].
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Discussion:
Paragraph 21.4 [basic.string]/3 states:
The class template basic_string conforms to the requirements for a Sequence (23.1.1) and for a Reversible Container (23.1).
First of all, 23.2.3 [sequence.reqmts] is no longer "Sequence" but "Sequence container". Secondly, after the resent changes to containers (emplace, push_back, const_iterator parameters to insert and erase), basic_string is not even close to conform to the current requirements.
[ Bellevue: ]
- emplace, for example, may not make sense for strings. Is also likely suboptimal
- with concepts do we need to maintain string as sequence container?
- One approach might be to say something like: string is a sequence except it doesn't have these functions
General consensus is to suggest option 2.
- basic_string already has push_back
- const_iterator parameters to insert and erase should be added to basic_string
- this leaves emplace to handle -- we have the following options:
- option 1: add it to string even though it's optional
- option 2: make emplace optional to sequences (move from table 89 to 90)
- option 3: say string not sequence (the proposal),
- option 4: add an exception to basic string wording.
[ 2009-07 Frankfurt: ]
Move to NAD Editorial
Proposed resolution:
Remove this sentence, in recognition of the fact that basic_string is not just a vector-light for literal types, but something quite different, a string abstraction in its own right.
Section: 20.7 [meta] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-08-25 Last modified: 2010-06-21
View all other issues in [meta].
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Duplicate of: 750
Discussion:
Since the inclusion of constexpr in the standard draft N2369 we have a new type category "literal", which is defined in 3.9 [basic.types]/p.11:
-11- A type is a literal type if it is:
- a scalar type; or
a class type (clause 9) with
- a trivial copy constructor,
- a trivial destructor,
- at least one constexpr constructor other than the copy constructor,
- no virtual base classes, and
- all non-static data members and base classes of literal types; or
- an array of literal type.
I strongly suggest that the standard provides a type traits for literal types in 20.7.4.3 [meta.unary.prop] for several reasons:
The special problem of reason (c) is that I don't see currently a way to portably test the condition for literal class types:
- at least one constexpr constructor other than the copy constructor,
[ Alisdair is considering preparing a paper listing a number of missing type traits, and feels that it might be useful to handle them all together rather than piecemeal. This would affect issue 719 and 750. These two issues should move to OPEN pending AM paper on type traits. ]
[ 2009-07 Frankfurt: ]
Beman, Daniel, and Alisdair will work on a paper proposing new type traits.
[ Addressed in N2947. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2984.
Proposed resolution:
In 20.7.2 [meta.type.synop] in the group "type properties", just below the line
template <class T> struct is_pod;
add a new one:
template <class T> struct is_literal;
In 20.7.4.3 [meta.unary.prop], table Type Property Predicates, just below the line for the is_pod property add a new line:
| Template | Condition | Preconditions |
|---|---|---|
| template <class T> struct is_literal; | T is a literal type (3.9) | T shall be a complete type, an array of unknown bound, or (possibly cv-qualified) void. |
Section: 22.3.3.2.2 [conversions.string] Status: NAD Submitter: Bo Persson Opened: 2007-08-27 Last modified: 2009-07-16
View all other issues in [conversions.string].
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Discussion:
Paragraph 3 says that the Codecvt template parameter shall meet the requirements of std::codecvt, even though std::codecvt itself cannot be used (because of a protected destructor).
How are we going to explain this code to beginning programmers?
template<class I, class E, class S>
struct codecvt : std::codecvt<I, E, S>
{
~codecvt()
{ }
};
void main()
{
std::wstring_convert<codecvt<wchar_t, char, std::mbstate_t> > compiles_ok;
std::wstring_convert<std::codecvt<wchar_t, char, std::mbstate_t> > not_ok;
}
[ San Francisco: ]
Bill will propose a resolution.
[ 2009-07 Frankfurt: ]
codecvt isn't intended for beginning programmers. This is a regrettable consequence of the original design of the facet.
Move to NAD.
Proposed resolution:
Section: 23.2.3 [sequence.reqmts] Status: NAD Editorial Submitter: David Abrahams Opened: 2007-09-16 Last modified: 2008-09-22
View all other issues in [sequence.reqmts].
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Discussion:
Table 90: (Optional sequence container operations) states the "assertion note pre/post-condition" of operator[] to be
*(a.begin() + n)
Surely that's meant to be "operational semantics?"
Proposed resolution:
Table 90: Optional sequence container operations expression return type assertion/note
pre/post-condition
operational semanticscontainer
Section: 28.11.4 [re.alg.replace] Status: NAD Submitter: Stephan T. Lavavej Opened: 2007-09-22 Last modified: 2010-03-08
View all other issues in [re.alg.replace].
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Discussion:
Two overloads of regex_replace() are currently provided:
template <class OutputIterator, class BidirectionalIterator,
class traits, class charT>
OutputIterator
regex_replace(OutputIterator out,
BidirectionalIterator first, BidirectionalIterator last,
const basic_regex<charT, traits>& e,
const basic_string<charT>& fmt,
regex_constants::match_flag_type flags =
regex_constants::match_default);
template <class traits, class charT>
basic_string<charT>
regex_replace(const basic_string<charT>& s,
const basic_regex<charT, traits>& e,
const basic_string<charT>& fmt,
regex_constants::match_flag_type flags =
regex_constants::match_default);
The absence of const charT * overloads prevents ordinary-looking code from compiling, such as:
const string s("kitten");
const regex r("en");
cout << regex_replace(s, r, "y") << endl;
The compiler error message will be something like "could not deduce template argument for 'const std::basic_string<_Elem> &' from 'const char[1]'".
Users expect that anything taking a basic_string<charT> can also take a const charT *. In their own code, when they write a function taking std::string (or std::wstring), they can pass a const char * (or const wchar_t *), thanks to basic_string's implicit constructor. Because the regex algorithms are templated on charT, they can't rely on basic_string's implicit constructor (as the compiler error message indicates, template argument deduction fails first).
If a user figures out what the compiler error message means, workarounds are available - but they are all verbose. Explicit template arguments could be given to regex_replace(), allowing basic_string's implicit constructor to be invoked - but charT is the last template argument, not the first, so this would be extremely verbose. Therefore, constructing a basic_string from each C string is the simplest workaround.
[ Sophia Antipolis: ]
We note that Boost already has these overloads. However, the proposed wording is provided only for 28.11.4 [re.alg.replace]; wording is needed for the synopsis as well. We also note that this has impact on match_results::format, which may require further overloads.
[ 2009-07 Frankfurt: ]
Daniel to tweak for us.
[ 2009-07-25 Daniel tweaks both this issue and 727. ]
This is solved by the proposed resolution of 727.
[ 2009-10 Santa Cruz: ]
Leave Open. Though we believe this is solved by the proposed resolution to 727.
[ 2010-01-27 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
Solved by 727.
Proposed resolution:
Provide additional overloads for regex_replace(): one additional overload of the iterator-based form (taking const charT* fmt), and three additional overloads of the convenience form (one taking const charT* str, another taking const charT* fmt, and the third taking both const charT* str and const charT* fmt). 28.11.4 [re.alg.replace]:
template <class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const basic_string<charT>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);...
template <class traits, class charT> basic_string<charT> regex_replace(const basic_string<charT>& s, const basic_regex<charT, traits>& e, const basic_string<charT>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const basic_string<charT>& s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const basic_string<charT>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);
Section: 26.5.1.4 [rand.req.eng] Status: NAD Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2008-02-27
View all other issues in [rand.req.eng].
View all issues with NAD status.
Discussion:
The 3rd table row in 26.5.1.4 [rand.req.eng]/3 requires random number engines to accept any arithmetic type as a seed, which is then casted to the engine's result_type and subsequently used for seeding the state of the engine. The requirement stated as "Creates an engine with initial state determined by static_cast<X::result_type>(s)" forces random number engines to either use a seeding method that completely depends on the result_type (see the discussion of seeding for the mersenne_twister_engine in point T2 above) or at least to throw away "bits of randomness" in the seed value if the result_type is smaller than the seed type. This seems to be inappropriate for many modern random number generators, in particular F2-linear or cryptographic ones, which operate on an internal bit array that in principle is independent of the type of numbers returned.
Posible resolution: I propose to change the wording to a version similar to "Creates an engine with initial state determined by static_cast<UintType>(s), where UintType is an implementation specific unsigned integer type."
Additionally, the definition of s in 26.5.1.4 [rand.req.eng]/1 c) could be restricted to unsigned integer types.
Similarly, the type of the seed in 26.5.1.5 [rand.req.adapt]/3 e) could be left unspecified.
See N2424 for further discussion.
[ Stephan Tolksdorf adds pre-Bellevue: ]
In reply to the discussion in N2424 regarding this issue:
The descriptions of all engines and engine adaptors given in sections 26.5.3 [rand.eng] and 26.5.4 [rand.adapt] already specify the concrete types of the integer arguments for seeding. Hence, relaxing the general requirement in 26.5.1.4 [rand.req.eng] would not affect portability and reproducibility of the standard library. Furthermore, it is not clear to me what exactly the guarantee "with initial state determined by static_cast<X::result_type>(s)" is useful for. On the other hand, relaxing the requirement would allow developers to implement other random number engines that do not have to cast all arithmetic seed arguments to their result_types.
[ Bellevue: ]
Propose close NAD for the reasons given in N2424.
Proposed resolution:
See N2424 for further discussion.
[ Stephan Tolksdorf adds pre-Bellevue: ]
Change row 3 of table 105 "Random number engine requirements" in 26.5.1.4 [rand.req.eng]/3
Creates an engine with initial state determined bystatic_cast<X::result_type>(s)Similarly, change 26.5.1.5 [rand.req.adapt]/3 e)
When X::X is invoked withan X::result_typevalue s of arithmetic type (3.9.1), ...
Section: 26.5.1.5 [rand.req.adapt] Status: NAD Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2008-02-27
View all issues with NAD status.
Discussion:
If an engine adaptor is invoked with an argument of type seed_seq, then all base engines are specified to be seeded with this seed_seq. As seed_seq's randomization method is qualified as constant, this procedure will ef fectively initialize all base engines with the same seed (though the resulting state might still dif fer to a certain degree if the engines are of different types). It is not clear whether this mode of operation is in general appropriate, hence -- as far as the stated requirements are of general nature and not just specific to the engine adaptors provided by the library -- it might be better to leave the behaviour unspecified, since the current definition of seed_seq does not allow for a generally satisfying specification.
Posssible resolution: [As above]
See N2424 for further discussion.
[ Bellevue: ]
Close NAD for the reasons given in N2424.
Proposed resolution:
See N2424 for the proposed resolution.
Section: 26.5.7.1 [rand.util.seedseq] Status: NAD Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2008-02-27
View all other issues in [rand.util.seedseq].
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Discussion:
The proper way to seed random number engines seems to be the most frequently discussed issue of the 26.5 [rand] proposal. While the new seed_seq approach is already rather general and probably sufficient for most situations, it is unlikely to be optimal in every case (one problem was pointed out in point T5 above). In some situations it might, for instance, be better to seed the state with a cryptographic generator.
In my opinion this is a pretty strong argument for extending the standard with a simple facility to customize the seeding procedure. This could, for example, be done with the following minimal changes:
Possible resolution:
Supplement the seed_seq with a traits class
template <typename T>
struct is_seed_seq { static const bool value = false; }
and the specialization
template <>
struct is_seed_seq<seed_seq> { static const bool value = true; }
which users can supplement with further specializations.
[ Bellevue: ]
See N2424. Close NAD but note that "conceptizing" the library may cause this problem to be solved by that route.
Proposed resolution:
See N2424 for the proposed resolution.
Section: X [rand.dist.samp.genpdf] Status: NAD Editorial Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2009-03-09
View all other issues in [rand.dist.samp.genpdf].
View all issues with NAD Editorial status.
Duplicate of: 795
Discussion:
X [rand.dist.samp.genpdf] describes the interface for a distribution template that is meant to simulate random numbers from any general distribution given only the density and the support of the distribution. I'm not aware of any general purpose algorithm that would be capable of correctly and efficiently implementing the described functionality. From what I know, this is essentially an unsolved research problem. Existing algorithms either require more knowledge about the distribution and the problem domain or work only under very limited circumstances. Even the state of the art special purpose library UNU.RAN does not solve the problem in full generality, and in any case, testing and customer support for such a library feature would be a nightmare.
Possible resolution: For these reasons, I propose to delete section X [rand.dist.samp.genpdf].
[ Bellevue: ]
Disagreement persists.
Objection to this issue is that this function takes a general functor. The general approach would be to normalize this function, integrate it, and take the inverse of the integral, which is not possible in general. An example function is sin(1+n*x) -- for any spatial frequency that the implementor chooses, there is a value of n that renders that choice arbitrarily erroneous.
Correction: The formula above should instead read 1+sin(n*x).
Objector proposes the following possible compromise positions:
- rand.dist.samp.genpdf takes an number of points so that implementor need not guess.
- replace rand.disk.samp.genpdf with an extension to either or both of the discrete functions to take arguments that take a functor and number of points in place of the list of probabilities. Reference issues 793 and 794.
Proposed resolution:
See N2813 for the proposed resolution.
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".Section: 26.5.1.6 [rand.req.dist] Status: NAD Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2008-02-27
View all issues with NAD status.
Discussion:
The requirement "P shall have a declaration of the form typedef X distribution_- type" effectively makes the use of inheritance for implementing distributions very inconvenient, because the child of a distribution class in general will not satisfy this requirement. In my opinion the benefits of having a typedef in the parameter class pointing back to the distribution class are not worth the hassle this requirement causes. [In my code base I never made use of the nested typedef but on several occasions could have profited from being able to use simple inheritance for the implementation of a distribution class.]
Proposed resolution: I propose to drop this requirement.
[ Bellevue: ]
Close NAD for the reasons given in N2424. In practice it is not inconvenient to meet these requirements.
Proposed resolution:
See N2424 for the proposed resolution.
Section: 26.5.8.2.2 [rand.dist.bern.bin], 26.5.8.2.4 [rand.dist.bern.negbin] Status: NAD Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2008-02-27
View all issues with NAD status.
Discussion:
In my opinion the choice of name for the t parameter of the binomial_distribution is very unfortunate. In virtually every internet reference, book and software implementation this parameter is called n instead, see for example Wikipedia, Mathworld, Evans et al. (1993) Statistical Distributions, 2nd E., Wiley, p. 38, the R statistical computing language, p. 926, Mathematica and Matlab.
Similarly, the choice of k for the parameter of the negative binomial distributions is rather unusual. The most common choice for the negative binomial distribution seems to be r instead.
Choosing unusual names for the parameters causes confusion among users and makes the interface unnecessarily inconvenient to use.
Possible resolution: For these reasons, I propose to change the name of the respective parameters to n and r.
[ Bellevue: ]
In N2424. NAD It has been around for a while. It is hardly universal, there is prior art, and this would confuse people.
Proposed resolution:
See N2424 for the proposed resolution.
Section: 26.5.8.5.1 [rand.dist.samp.discrete] Status: NAD Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2008-02-27
View all other issues in [rand.dist.samp.discrete].
View all issues with NAD status.
Discussion:
Possible resolution: I propose to change the specification such that the non-standardized probabilities need to be returned and that an additional requirement is included for the number of probabilities to be smaller than the maximum of IntType.
[ Stephan Tolksdorf adds pre-Bellevue: ]
In reply to the discussion in N2424 of this issue:
Rescaled floating-point parameter vectors can not be expected to compare equal because of the limited precision of floating-point numbers. My proposal would at least guarantee that a parameter vector (of type double) passed into the distribution would compare equal with the one returned by the probabilities() method. Furthermore, I do not understand why "the changed requirement would lead to a significant increase in the amount of state in the distribution object". A typical implementation's state would increase by exactly one number: the sum of all probabilities. The textual representation for serialization would not need to grow at all. Finally, the proposed replacement "0 < n <= numeric_limits<IntType>::max() + 1" makes the implementation unnecessarily complicated, "0 < n <= numeric_limits<IntType>::max()" would be better.
[ Bellevue: ]
In N2424. We agree with the observation and the proposed resolution to part b). We recommend the wording n > 0 be replaced with 0 < n numeric_limits::max() + 1. However, we disagree with part a), as it would interfere with the definition of parameters' equality. Further, the changed requirement would lead to a significant increase in the amount of state of the distribution object.
As it stands now, it is convenient, and the changes proposed make it much less so.
NAD. Part a the current behavior is desirable. Part b, any constructor can fail, but the rules under which it can fail do not need to be listed here.
Proposed resolution:
See N2424 for the proposed resolution.
[ Stephan Tolksdorf adds pre-Bellevue: ]
In 26.5.8.5.1 [rand.dist.samp.discrete]:
Proposed wording a):
Changae in para. 2
Constructs a discrete_distribution object with n=1 and p0 = w0 = 1and change in para. 5
Returns: A vector<double> whose size member returns n and whose operator[] member returnspkthe weight wk as a double value when invoked with argument k for k = 0, ..., n-1Proposed wording b):
Change in para. 3:
If firstW == lastW, let the sequence w have length n = 1 and consist of the single value w0 = 1. Otherwise, [firstW,lastW) shall form a sequence w of length n> 0such that 0 < n <= numeric_limits<IntType>::max(), and *firstW shall yield a value w0 convertible to double. [Note: The values wk are commonly known as the weights . -- end note]
Section: 26.5.8.5.2 [rand.dist.samp.pconst] Status: NAD Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2008-02-27
View all other issues in [rand.dist.samp.pconst].
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Discussion:
The design of the constructor
template <class InputIteratorB, class InputIteratorW>
piecewise_constant_distribution( InputIteratorB firstB, InputIteratorB lastB,
InputIteratorW firstW);
is unnecessarily unsafe, as there is no separate end-iterator given for the weights. I can't see any performance or convenience reasons that would justify the risks inherent in such a function interface, in particular the risk that input error might go unnoticed.
Possible resolution: I propose to add an InputIteratorW lastW argument to the interface.
[ Stephan Tolksdorf adds pre-Bellevue: ]
In reply to the discussion in N2424 I'd like to make the same comments as for 736.
[ Bellevue: ]
In N2424. There is already precedent elsewhere in the library. Follows existing convention. NAD.
Proposed resolution:
See N2424 for the proposed resolution.
[ Stephan Tolksdorf adds pre-Bellevue: ]
In 26.5.8.5.2 [rand.dist.samp.pconst]:
Proposed wording a)
Change in para. 2
Constructs a piecewise_constant_distribution object with n = 1, p0 = w0 = 1, b0 = 0, and b1 = 1and change in para. 5
A vector<result_type> whose size member returns n and whose operator[] member returnspkthe weight wk as a double value when invoked with argument k for k = 0, ..., n-1Proposed wording b)
Change both occurrences of
"piecewise_constant_distribution(InputIteratorB firstB, InputIteratorB lastB, InputIteratorW firstW, InputIteratorW lastW)and change in para. 3
the length of the sequence w starting from firstW shall be at least n, *firstW shall return a value w0 that is convertible to double, and any wk for k >= n shall be ignored by the distribution[firstW, lastW) shall form a sequence w of length n whose leading element w0 shall be convertible to double
Section: 26.5.4.1 [rand.adapt.disc] Status: NAD Editorial Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2008-09-22
View all issues with NAD Editorial status.
Discussion:
Since the template parameter p and r are of type size_t, the member n in the class exposition should have type size_t, too.
Proposed resolution:
See N2424 for the proposed resolution.
Section: 26.5.7.2 [rand.util.canonical] Status: NAD Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2008-02-27
View all other issues in [rand.util.canonical].
View all issues with NAD status.
Discussion:
The complexity of generate_canonical is specified to be "exactly k=max(1, ceil(b/log2 R)) invocations of g". This terms involves a logarithm that is not rounded and hence can not (in general) be computed at compile time. As this function template is performance critical, I propose to replace ceil(b/log2 R) with ceil(b/floor(log2 R)).
See N2424 for further discussion.
[ Bellevue: ]
In N2424. Close NAD as described there.
Proposed resolution:
See N2424 for the proposed resolution.
Section: 20.9.11.2.11 [util.smartptr.getdeleter] Status: NAD Submitter: Daniel Krügler Opened: 2007-09-27 Last modified: 2008-02-27
View all other issues in [util.smartptr.getdeleter].
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Discussion:
The following issue was raised by Alf P. Steinbach in c.l.c++.mod:
According to the recent draft N2369, both the header memory synopsis of 20.9 [memory] and 20.9.11.2.11 [util.smartptr.getdeleter] declare:
template<class D, class T> D* get_deleter(shared_ptr<T> const& p);
This allows to retrieve the pointer to a mutable deleter of a const shared_ptr (if that owns one) and therefore contradicts the usual philosophy that associated functors are either read-only (e.g. key_comp or value_comp of std::map) or do at least reflect the mutability of the owner (as seen for the both overloads of unique_ptr::get_deleter). Even the next similar counter-part of get_deleter - the two overloads of function::target in the class template function synopsis 20.8.14.2 [func.wrap.func] or in 20.8.14.2.5 [func.wrap.func.targ] - do properly mirror the const-state of the owner.
Possible proposed resolutions:Replace the declarations of get_deleter in the header <memory> synopsis of 20.9 [memory] and in 20.9.11.2.11 [util.smartptr.getdeleter] by one of the following alternatives (A) or (B):
template<class D, class T> const D* get_deleter(shared_ptr<T> const& p);
Alberto Ganesh Barbati adds:
Replace it with two functions:
template <class D, class T> D get_deleter(shared_ptr<T> const&); template <class D, class T> bool has_deleter(shared_ptr<T> const&);
The first one would throw if D is the wrong type, while the latter would never throw. This approach would reflect the current praxis of use_facet/has_facet, with the twist of returning the deleter by value as container::get_allocator() do.
Peter Dimov adds:
My favorite option is "not a defect". A, B and C break useful code.
[ Bellevue: ]
Concern this is similar to confusing "pointer to const" with "a constant pointer".
Proposed resolution:
Section: 20.2.1 [utility.arg.requirements] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2007-10-10 Last modified: 2010-03-11
View all other issues in [utility.arg.requirements].
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Discussion:
This issue was split from 672. 672 now just deals with changing the requirements of T in the Swappable requirement from CopyConstructible and CopyAssignable to MoveConstructible and MoveAssignable.
This issue seeks to widen the Swappable requirement to support proxy iterators. Here is example code:
namespace Mine {
template <class T>
struct proxy {...};
template <class T>
struct proxied_iterator
{
typedef T value_type;
typedef proxy<T> reference;
reference operator*() const;
...
};
struct A
{
// heavy type, has an optimized swap, maybe isn't even copyable or movable, just swappable
void swap(A&);
...
};
void swap(A&, A&);
void swap(proxy<A>, A&);
void swap(A&, proxy<A>);
void swap(proxy<A>, proxy<A>);
} // Mine
...
Mine::proxied_iterator<Mine::A> i(...)
Mine::A a;
swap(*i1, a);
The key point to note in the above code is that in the call to swap, *i1 and a are different types (currently types can only be Swappable with the same type). A secondary point is that to support proxies, one must be able to pass rvalues to swap. But note that I am not stating that the general purpose std::swap should accept rvalues! Only that overloaded swaps, as in the example above, be allowed to take rvalues.
That is, no standard library code needs to change. We simply need to have a more flexible definition of Swappable.
[ Bellevue: ]
While we believe Concepts work will define a swappable concept, we should still resolve this issue if possible to give guidance to the Concepts work.
Would an ambiguous swap function in two namespaces found by ADL break this wording? Suggest that the phrase "valid expression" means such a pair of types would still not be swappable.
Motivation is proxy-iterators, but facility is considerably more general. Are we happy going so far?
We think this wording is probably correct and probably an improvement on what's there in the WP. On the other hand, what's already there in the WP is awfully complicated. Why do we need the two bullet points? They're too implementation-centric. They don't add anything to the semantics of what swap() means, which is there in the post-condition. What's wrong with saying that types are swappable if you can call swap() and it satisfies the semantics of swapping?
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Leave as Open. Dave to provide wording.
[ 2009-11-08 Howard adds: ]
Updated wording to sync with N3000. Also this issue is very closely related to 594.
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added.
Rationale:
Solved by N3048.
Proposed resolution:
Change 20.2.1 [utility.arg.requirements]:
-1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables, T and V are
is atypes to be supplied by a C++ program instantiating a template; a, b, and c are values of type const T; s and t are modifiable lvalues of type T; u is a value of type (possibly const) T;andrv is a non-const rvalue of type T; w is a value of type T; and v is a value of type V.
Table 37: Swappable requirements [swappable] expression Return type Post-condition swap( sw,tv)void tw has the value originally held byuv, anduv has the value originally held bytwThe Swappable requirement is met by satisfying one or more of the following conditions:
- T is Swappable if T and V are the same type and T satisfies the MoveConstructible requirements (Table 33) and the MoveAssignable requirements (Table 35);
- T is Swappable with V if a namespace scope function named swap exists in the same namespace as the definition of T or V, such that the expression swap(
sw,tv) is valid and has the semantics described in this table.- T is Swappable if T is an array type whose element type is Swappable.
Rationale:
[ post San Francisco: ]
Solved by N2758.
Section: 18.8.5 [propagation] Status: NAD Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2008-02-25
View all other issues in [propagation].
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Discussion:
It could be I did not understand the design rationale, but I thought copy_exception would produce an exception_ptr to the most-derived (dynamic) type of the passed exception. Instead it slices, which appears to be less useful, and a likely source of FAQ questions in the future.
(Peter Dimov suggests NAD)
[ Bellevue: ]
How could this be implemented in a way that the dynamic type is cloned?
The feature is designed to create an exception_ptr from an object whose static type is identical to the dynamic type and thus there is no slicing involved.
Proposed resolution:
Section: 20.7.4.3 [meta.unary.prop] Status: NAD Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2009-07-16
View all other issues in [meta.unary.prop].
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Discussion:
We have 3 separate type traits to identify classes supporting no-throw operations, which are very useful when trying to provide exception safety guarantees. However, I'm not entirely clear on what the current wording requires of a conforming implementation. To quote from has_nothrow_default_constructor:
or T is a class type with a default constructor that is known not to throw any exceptions
What level of magic do we expect to deduce if this is known?
E.g.
struct test{
int x;
test() : x() {}
};
Should I expect a conforming compiler to assert( has_nothrow_constructor<test>::value )
Is this a QoI issue?
Should I expect to 'know' only if-and-only-if there is an inline definition available?
Should I never expect that to be true, and insist that the user supplies an empty throw spec if they want to assert the no-throw guarantee?
It would be helpful to maybe have a footnote explaining what is required, but right now I don't know what to suggest putting in the footnote.
(agreement since is that trivial ops and explicit no-throws are required. Open if QoI should be allowed to detect further)
[ Bellevue: ]
This looks like a QoI issue. In the case of trivial and nothrow it is known. Static analysis of the program is definitely into QoI. Move to OPEN. Need to talk to Core about this.
[ 2009-07 Frankfurt: ]
This is QoI.
Move to NAD.
Proposed resolution:
Section: 20.7.4.3 [meta.unary.prop] Status: NAD Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2009-05-01
View all other issues in [meta.unary.prop].
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Discussion:
I am trying to decide is a pure virtual function is a necessary as well as sufficient requirement to be classified as abstract?
For instance, is the following (non-polymorphic) type considered abstract?
struct abstract {
protected:
abstract(){}
abstract( abstract const & ) {}
~abstract() {}
};
(Suggested that this may be NAD, with an editorial fix-up from Pete on the core wording to make clear that abstract requires a pure virtual function)
Proposed resolution:
Core has clarified that the definition abstract is adequate. Issue withdrawn by submitter. NAD.
Section: 20.7.5 [meta.rel] Status: Dup Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2009-09-13
View all other issues in [meta.rel].
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Duplicate of: 719
Discussion:
With the pending arrival of explicit conversion functions though, I'm wondering if we want an additional trait, is_explictly_convertible?
[ Bellevue: ]
Alisdair is considering preparing a paper listing a number of missing type traits, and feels that it might be useful to handle them all together rather than piecemeal. This would affect issue 719 and 750. These two issues should move to OPEN pending AM paper on type traits.
[ 2009-07 Frankfurt: ]
Duplicate of 719 (for our purposes).
[ Addressed in N2947. ]
Proposed resolution:
Section: 23.3.7 [vector.bool] Status: NAD Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2009-07-16
View all other issues in [vector.bool].
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Discussion:
A number of vector<bool> members take const bool& as arguments. Is there any chance we could change them to pass-by-value or would I be wasting everyone's time if wrote up an issue?
[ post Bellevue: ]
As we understand it, the original requester (Martin Sebor) would like for implementations to be permitted to pass-by-value. Alisdair suggests that if this is to be resolved, it should be resolved more generally, e.g. in other containers as well.
We note that this would break ABI. However, we also suspect that this might be covered under the "as-if" rule in section 1.9.
Many in the group feel that for vector<bool>, this is a "don't care", and that at this point in the process it's not worth the bandwidth.
Issue 679 -- which was in ready status pre-Bellevue and is now in the working paper -- is related to this, though not a duplicate.
Moving to Open with a task for Alisdair to craft a informative note to be put whereever appropriate in the WP. This note would clarify places where pass-by-const-ref can be transformed to pass-by-value under the as-if rule.
[ San Francisco: ]
This is really a clause 17 issue, rather than something specific to vector<bool>.
Move to Open. Alisdair to provide a resolution. Alternately, Howard can close this as NAD and then open a new issue to handle the general issue (rather than the vector<bool> one).
Howard: Haven't yet opened new issue. Lacking wording for it.
[ 2009-07 Frankfurt: ]
NAD. Insufficient motivation to make any changes.
Proposed resolution:
Section: 20.9.9.2 [uninitialized.copy] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-10-15 Last modified: 2008-07-02
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Discussion:
14882-2003, [lib.uninitialized.copy] is currently written as follows:
template <class InputIterator, class ForwardIterator> ForwardIterator uninitialized_copy(InputIterator first, InputIterator last, ForwardIterator result);-1- Effects:
for (; first != last; ++result, ++first) new (static_cast<void*>(&*result)) typename iterator_traits<ForwardIterator>::value_type(*first);-2- Returns: result
similarily for N2369, and its corresponding section 20.9.9.2 [uninitialized.copy].
It's not clear to me what the return clause is supposed to mean, I see two possible interpretations:
The problem is: I see nothing in the standard which grants that this interpretation is correct, specifically [lib.structure.specifications] or 17.5.1.4 [structure.specifications] resp. do not clarify which "look-up" rules apply for names found in the elements of the detailed specifications - Do they relate to the corresponding synopsis or to the effects clause (or possibly other elements)? Fortunately most detailed descriptions are unambigious in this regard, e.g. this problem does not apply for std::copy.
Proposed resolution:
Change the wording of the return clause to say (20.9.9.2 [uninitialized.copy]):
-2- Returns: The value of result after effects have taken place.
[ Bellevue: ]
Resolution: NAD editorial -- project editor to decide if change is worthwhile. Concern is that there are many other places this might occur.
Section: 23.3.5 [container.adaptors] Status: NAD Editorial Submitter: Paolo Carlini Opened: 2007-10-31 Last modified: 2008-06-18
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Discussion:
After n2369 we have a single push_back overload in the sequence containers, of the "emplace" type. At variance with that, still in n2461, we have two separate overloads, the C++03 one + one taking an rvalue reference in the container adaptors. Therefore, simply from a consistency point of view, I was wondering whether the container adaptors should be aligned with the specifications of the sequence container themselves: thus have a single push along the lines:
template<typename... _Args>
void
push(_Args&&... __args)
{ c.push_back(std::forward<_Args>(__args)...); }
[ Related to 767 ]
Proposed resolution:
Change 23.3.5.1.1 [queue.defn]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Change 23.3.5.2 [priority.queue]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Change 23.3.5.2.3 [priqueue.members]:
void push(const value_type& x);
Effects:c.push_back(x);push_heap(c.begin(), c.end(), comp);template<class... Args> void push(value_typeArgs&&...xargs);Effects:
c.push_back(std::moveforward<Args>(xargs)...); push_heap(c.begin(), c.end(), comp);
Change 23.3.5.3.1 [stack.defn]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Rationale:
Addressed by N2680 Proposed Wording for Placement Insert (Revision 1).
Section: 23.3.6 [vector] Status: NAD Editorial Submitter: Paolo Carlini Opened: 2007-11-04 Last modified: 2008-07-02
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Discussion:
In the synopsis 23.3.6 [vector], there is the signature:
void insert(const_iterator position, size_type n, T&& x);
instead of:
iterator insert(const_iterator position, T&& x);
23.3.6.4 [vector.modifiers] is fine.
Proposed resolution:
Change the synopsis in 23.3.6 [vector]:
iterator insert(const_iterator position, const T& x); iterator insert(const_iterator position, T&& x); void insert(const_iterator position, size_type n, const T& x);void insert(const_iterator position, size_type n, T&& x);
Section: 23.2 [container.requirements] Status: NAD Future Submitter: Paolo Carlini Opened: 2007-11-11 Last modified: 2009-07-17
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Discussion:
In an emplace member function the function parameter pack may be bound to a priori unlimited number of objects: some or all of them can be elements of the container itself. Apparently, in order to conform to the blanket statement 23.2 [container.requirements]/11, the implementation must check all of them for that possibility. A possible solution can involve extending the exception in 23.2 [container.requirements]/12 also to the emplace member. As a side note, the push_back and push_front member functions are luckily not affected by this problem, can be efficiently implemented anyway
[ Related to 767 ]
[ Bellevue: ]
The proposed addition (13) is partially redundant with the existing paragraph 12. Why was the qualifier "rvalues" added to paragraph 12? Why does it not cover subelements and pointers?
Resolution: Alan Talbot to rework language, then set state to Review.
[ 2009-07 Frankfurt ]
The problem is broader than emplace. The LWG doesn't feel that it knows how to write wording that prohibits all of the problematic use cases at this time.
NAD Future.
Proposed resolution:
Add after 23.2 [container.requirements]/12:
-12- Objects passed to member functions of a container as rvalue references shall not be elements of that container. No diagnostic required.
-13- Objects bound to the function parameter pack of the emplace member function shall not be elements or sub-objects of elements of the container. No diagnostic required.
Section: 23.2.4 [associative.reqmts] Status: NAD Submitter: Sylvain Pion Opened: 2007-12-04 Last modified: 2008-03-12
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Discussion:
The associative containers provide 2 overloads of emplace():
template <class... Args> pair<iterator, bool> emplace(Args&&... args); template <class... Args> iterator emplace(const_iterator position, Args&&... args);
This is a problem if you mean the first overload while passing a const_iterator as first argument.
[ Related to 767 ]
[ Bellevue: ]
This can be disambiguated by passing "begin" as the first argument in the case when the non-default choice is desired. We believe that desire will be rare.
Resolution: Change state to NAD.
Proposed resolution:
Rename one of the two overloads. For example to emplace_here, hint_emplace...
Section: 23.2.5 [unord.req] Status: NAD Submitter: Joe Gottman Opened: 2007-11-29 Last modified: 2008-03-12
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Discussion:
A major attribute of the unordered containers is that iterating though them inside a bucket is very fast while iterating between buckets can be much slower. If an unordered container has a low load factor, iterating between the last iterator in one bucket and the next iterator, which is in another bucket, is O(bucket_count()) which may be much larger than O(size()).
If b is an non-const unordered container of type B and k is an object of it's key_type, then b.equal_range(k) currently returns pair<B::iterator, B::iterator>. Consider the following code:
B::iterator lb, ub;
tie(lb, ub) = b.equal_range(k);
for (B::iterator it = lb; it != ub; ++it) {
// Do something with *it
}
If b.equal_range(k) returns a non-empty range (i.e. b contains at least on element whose key is equivalent to k), then every iterator in the half-open range [lb, ub) will be in the same bucket, but ub will likely either be in a different bucket or be equal to b.end(). In either case, iterating between ub - 1 and ub could take a much longer time than iterating through the rest of the range.
If instead of returning pair<iterator, iterator>, equal_range were to return pair<local_iterator, local_iterator>, then ub (which, like lb, would now be a local_iterator) could be guaranteed to always be in the same bucket as lb. In the cases where currently ub is equal to b.end() or is in a different bucket, ub would be equal to b.end(b.bucket(key)). This would make iterating between lb and ub much faster, as every iteration would be constant time.
[ Bellevue: ]
The proposed resolution breaks consistency with other container types for dubious benefit, and iterators are already constant time.
Proposed resolution:
Change the entry for equal_range in Table 93 (23.2.5 [unord.req]) as follows:
| expression | return type | assertion/note pre/post-condition | complexity |
|---|---|---|---|
| b.equal_range(k) | pair<local_iterator,local_iterator>; pair<const_local_iterator,const_local_iterator> for const b. | Returns a range containing all elements with keys equivalent to k. Returns make_pair(b.end(b.bucket(key)),b.end(b.bucket(key))) if no such elements exist. | Average case Θ(b.count(k)). Worst case Θ(b.size()). |
Section: 23 [containers] Status: NAD Editorial Submitter: Sylvain Pion Opened: 2007-12-28 Last modified: 2008-06-18
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Discussion:
Playing with g++'s C++0X mode, I noticed that the following code, which used to compile:
#include <vector>
int main()
{
std::vector<char *> v;
v.push_back(0);
}
now fails with the following error message:
.../include/c++/4.3.0/ext/new_allocator.h: In member function 'void __gnu_cxx::new_allocator<_Tp>::construct(_Tp*, _Args&& ...) [with _Args = int, _Tp = char*]': .../include/c++/4.3.0/bits/stl_vector.h:707: instantiated from 'void std::vector<_Tp, _Alloc>::push_back(_Args&& ...) [with _Args = int, _Tp = char*, _Alloc = std::allocator<char*>]' test.cpp:6: instantiated from here .../include/c++/4.3.0/ext/new_allocator.h:114: error: invalid conversion from 'int' to 'char*'
As far as I know, g++ follows the current draft here.
Does the committee really intend to break compatibility for such cases?
[ Sylvain adds: ]
I just noticed that std::pair has the same issue. The following now fails with GCC's -std=c++0x mode:
#include <utility> int main() { std::pair<char *, char *> p (0,0); }I have not made any general audit for such problems elsewhere.
[ Related to 756 ]
[ Bellevue: ]
Motivation is to handle the old-style int-zero-valued NULL pointers. Problem: this solution requires concepts in some cases, which some users will be slow to adopt. Some discussion of alternatives involving prohibiting variadic forms and additional library-implementation complexity.
Discussion of "perfect world" solutions, the only such solution put forward being to retroactively prohibit use of the integer zero for a NULL pointer. This approach was deemed unacceptable given the large bodies of pre-existing code that do use integer zero for a NULL pointer.
Another approach is to change the member names. Yet another approach is to forbid the extension in absence of concepts.
Resolution: These issues (756, 767, 760, 763) will be subsumed into a paper to be produced by Alan Talbot in time for review at the 2008 meeting in France. Once this paper is produced, these issues will be moved to NAD.
Proposed resolution:
Add the following rows to Table 90 "Optional sequence container operations", 23.2.3 [sequence.reqmts]:
expression return type assertion/note
pre-/post-conditioncontainer a.push_front(t) void a.insert(a.begin(), t)
Requires: T shall be CopyConstructible.list, deque a.push_front(rv) void a.insert(a.begin(), rv)
Requires: T shall be MoveConstructible.list, deque a.push_back(t) void a.insert(a.end(), t)
Requires: T shall be CopyConstructible.list, deque, vector, basic_string a.push_back(rv) void a.insert(a.end(), rv)
Requires: T shall be MoveConstructible.list, deque, vector, basic_string
Change the synopsis in 23.3.2 [deque]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.3.2.3 [deque.modifiers]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change the synopsis in 23.3.4 [list]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.3.4.3 [list.modifiers]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change the synopsis in 23.3.6 [vector]:
void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.3.6.4 [vector.modifiers]:
void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Rationale:
Addressed by N2680 Proposed Wording for Placement Insert (Revision 1).
If there is still an issue with pair, Howard should submit another issue.
Section: 26.5.8.1 [rand.dist.uni] Status: NAD Submitter: P.J. Plauger Opened: 2008-01-14 Last modified: 2008-02-27
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Discussion:
[ Bellevue: ]
NAD. Withdrawn.
Proposed resolution:
Section: 30.4.3.2.3 [thread.lock.unique.mod] Status: NAD Submitter: Constantine Sapuntzakis Opened: 2008-02-02 Last modified: 2008-02-27
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Discussion:
unique_lock::release will probably lead to many mistakes where people call release instead of unlock. I just coded such a mistake using the boost pre-1.35 threads library last week.
In many threading libraries, a call with release in it unlocks the lock (e.g. ReleaseMutex in Win32, java.util.concurrent.Semaphore).
I don't call unique_lock::lock much at all, so I don't get to see the symmetry between ::lock and ::unlock. I usually use the constructor to lock the mutex. So I'm left to remember whether to call release or unlock during the few times I need to release the mutex before the scope ends. If I get it wrong, the compiler doesn't warn me.
An alternative name for release may be disown.
This might be a rare case where usability is hurt by consistency with the rest of the C++ standard (e.g. std::auto_ptr::release).
[ Bellevue: ]
Change a name from release to disown. However prior art uses the release name. Compatibility with prior art is more important that any possible benefit such a change might make. We do not see the benefit for changing. NAD
Proposed resolution:
Change the synopsis in 30.4.3.2 [thread.lock.unique]:
template <class Mutex>
class unique_lock
{
public:
...
mutex_type* release disown();
...
};
Change 30.4.3.2.3 [thread.lock.unique.mod]:
mutex_type *releasedisown();
Section: TR1 5.1.4.5 [tr.rand.eng.disc], TR1 5.1.4.6 [tr.rand.eng.xor] Status: NAD Submitter: John Maddock Opened: 2008-01-15 Last modified: 2009-07-13
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Discussion:
Table 16 of TR1 requires that all Pseudo Random Number generators have a
seed(integer-type s)
member function that is equivalent to:
mygen = Generator(s)
But the generators xor_combine and discard_block have no such seed member, only the
template <class Gen> seed(Gen&);
member, which will not accept an integer literal as an argument: something that appears to violate the intent of Table 16.
So... is this a bug in TR1?
This is a real issue BTW, since the Boost implementation does adhere to the requirements of Table 16, while at least one commercial implementation does not and follows a strict adherence to sections 5.1.4.5 and 5.1.4.6 instead.
[ Jens adds: ]
Both engines do have the necessary constructor, therefore the omission of the seed() member functions appears to be an oversight.
[ Post Summit Daniel adds: ]
Recommend NAD: xor_combine does no longer exist and discard_block[_engine] has now the required seed overload accepting a result_type, which shall be an unsigned integral type.
[ Batavia (2009-05): ]
Move to NAD as recommended.
Proposed resolution:
NAD Recommended.
Section: 20.10 [time] Status: NAD Editorial Submitter: Christopher Kohlhoff, Jeff Garland Opened: 2008-02-03 Last modified: 2008-09-30
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Discussion:
The draft C++0x thread library requires that the time points of type system_time and returned by get_system_time() represent Coordinated Universal Time (UTC) (section [datetime.system]). This can lead to surprising behavior when a library user performs a duration-based wait, such as condition_variable::timed_wait(). A complete explanation of the problem may be found in the Rationale for the Monotonic Clock section in POSIX, but in summary:
POSIX solves the problem by introducing a new monotonic clock, which is unaffected by changes to the system time. When a condition variable is initialized, the user may specify whether the monotonic clock is to be used. (It is worth noting that on POSIX systems it is not possible to use condition_variable::native_handle() to access this facility, since the desired clock type must be specified during construction of the condition variable object.)
In the context of the C++0x thread library, there are added dimensions to the problem due to the need to support platforms other than POSIX:
One possible minimal solution:
Proposed resolution:
Rationale:
Addressed by N2661: A Foundation to Sleep On.Section: X [rand.adapt.xor] Status: NAD Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2008-02-27
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Discussion:
xor_combine::seed(result_type) and seed(seed_seq&) don't say what happens to each of the sub-engine seeds. (Should probably do the same to both, unlike TR1.)
[ Bellevue: ]
Overcome by the previous proposal. NAD mooted by resolution of 789.
Proposed resolution:
Section: 26.5.8.5.2 [rand.dist.samp.pconst] Status: NAD Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2008-03-11
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Discussion:
piecewise_constant_distribution::densities() should be probabilities(), just like discrete_distribution. (There's no real use for weights divided by areas.)
[ Bellevue: ]
Fermilab does not agree with this summary. As defined in the equation in 26.4.8.5.2/4, the quantities are indeed probability densities not probabilities. Because we view this distribution as a parameterization of a *probability density function*, we prefer to work in terms of probability densities.
We don't think this should be changed.
If there is a technical argument about why the implementation dealing with these values can't be as efficient as one dealing with probabilities, we might reconsider. We don't care about this one member function being somewhat more or less efficient; we care about the size of the distribution object and the speed of the calls to generate variates.
Proposed resolution:
Change synopsis in 26.5.8.5.2 [rand.dist.samp.pconst]:
template <class RealType = double>
class piecewise_constant_distribution
{
public:
...
vector<double> densities probabilities() const;
...
};
Change 26.5.8.5.2 [rand.dist.samp.pconst]/6:
vector<double>densitiesprobabilities() const;
Section: 26.5.8.5.1 [rand.dist.samp.discrete] Status: NAD Editorial Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2009-03-09
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Discussion:
discrete_distribution should have a constructor like:
template<class _Fn>
discrete_distribution(result_type _Count, double _Low, double _High,
_Fn& _Func);
(Makes it easier to fill a histogram with function values over a range.)
[ Bellevue: ]
How do you specify the function so that it does not return negative values? If you do it is a bad construction. This requirement is already there. Where in each bin does one evaluate the function? In the middle. Need to revisit tomorrow.
[ Sophia Antipolis: ]
Bill is not requesting this.
Marc Paterno: _Fn cannot return negative values at the points where the function is sampled. It is sampled in the middle of each bin. _Fn cannot return 0 everywhere it is sampled.
Jens: lambda expressions are rvalues
Add a library issue to provide an initializer_list<double> constructor for discrete_distribution.
Marc Paterno: dislikes reference for _Fn parameter. Make it pass-by-value (to use lambda), use std::ref to wrap giant-state function objects.
Daniel: See random_shuffle, pass-by-rvalue-reference.
Daniel to draft wording.
[ Pre San Francisco, Daniel provided wording: ]
The here proposed changes of the WP refer to the current state of N2691. During the Sophia Antipolis meeting two different proposals came up regarding the functor argument type, either by value or by rvalue-reference. For consistence with existing conventions (state-free algorithms and the general_pdf_distribution c'tor signature) the author decided to propose a function argument that is provided by value. If severe concerns exists that stateful functions would be of dominant relevance, it should be possible to replace the two occurrences of Func by Func&& in this proposal as part of an editorial process.
Proposed resolution:
Non-concept version of the proposed resolution
In 26.5.8.5.1 [rand.dist.samp.discrete]/1, class discrete_distribution, just before the member declaration
explicit discrete_distribution(const param_type& parm);
insert:
template<typename Func> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);
Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::
template<typename Func> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);Complexity: Exactly nf invocations of fw.
Requires:
- fw shall be callable with one argument of type double, and shall return values of a type convertible to double;
- If nf > 0, the relation xmin < xmax shall hold, and for all sample values xk, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- The following relations shall hold: nf ≥ 0, and 0 < S = w0+. . .+wn-1.
Effects:
- If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value w0 = 1.
Otherwise, sets n = nf, deltax = (xmax - xmin)/n and xcent = xmin + 0.5 * deltax.
For each k = 0, . . . ,n-1, calculates: xk = xcent + k * deltax wk = fw(xk)Constructs a discrete_distribution object with probabilities:
pk = wk/S for k = 0, . . . , n-1.
Concept version of the proposed resolution
In 26.5.8.5.1 [rand.dist.samp.discrete]/1, class discrete_distribution, just before the member declaration
explicit discrete_distribution(const param_type& parm);
insert:
template<Callable<auto, double> Func> requires Convertible<Func::result_type, double> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);
Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::
template<Callable<auto, double> Func> requires Convertible<Func::result_type, double> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);Complexity: Exactly nf invocations of fw.
Requires:
- If nf > 0, the relation xmin < xmax shall hold, and for all sample values xk, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- The following relations shall hold: nf ≥ 0, and 0 < S = w0+. . .+wn-1.
Effects:
- If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value w0 = 1.
Otherwise, sets n = nf, deltax = (xmax - xmin)/n and xcent = xmin + 0.5 * deltax.
For each k = 0, . . . ,n-1, calculates: xk = xcent + k * deltax wk = fw(xk)Constructs a discrete_distribution object with probabilities:
pk = wk/S for k = 0, . . . , n-1.
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".Section: 26.5.8.5.2 [rand.dist.samp.pconst] Status: NAD Editorial Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2009-03-09
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Discussion:
piecewise_constant_distribution should have a constructor like:
template<class _Fn>
piecewise_constant_distribution(size_t _Count,
_Ty _Low, _Ty _High, _Fn& _Func);
(Makes it easier to fill a histogram with function values over a range. The two (reference 793) make a sensible replacement for general_pdf_distribution.)
[ Sophia Antipolis: ]
Marc: uses variable width of bins and weight for each bin. This is not giving enough flexibility to control both variables.
Add a library issue to provide an constructor taking an initializer_list<double> and _Fn for piecewise_constant_distribution.
Daniel to draft wording.
[ Pre San Francisco, Daniel provided wording. ]
The here proposed changes of the WP refer to the current state of N2691. For reasons explained in 793, the author decided to propose a function argument that is provided by value. The issue proposes a c'tor signature, that does not take advantage of the full flexibility of piecewise_constant_distribution, because it restricts on a constant bin width, but the use-case seems to be popular enough to justify it's introduction.
Proposed resolution:
Non-concept version of the proposed resolution
In 26.5.8.5.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution, just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert:
template<typename Func> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);
Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:
template<typename Func> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);[p5_1] Complexity: Exactly nf invocations of fw.
[p5_2] Requires:
- fw shall be callable with one argument of type RealType, and shall return values of a type convertible to double;
- For all sample values xk defined below, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- The following relations shall hold: xmin < xmax, and 0 < S = w0+. . .+wn-1.
[p5_3] Effects:
If nf == 0,
- sets deltax = xmax - xmin, and
- lets the sequence w have length n = 1 and consist of the single value w0 = 1, and
- lets the sequence b have length n+1 with b0 = xmin and b1 = xmax
Otherwise,
- sets n = nf, deltax = (xmax - xmin)/n, xcent = xmin + 0.5 * deltax, and
lets the sequences w and b have length n and n+1, resp. and
for each k = 0, . . . ,n-1, calculates: dxk = k * deltax bk = xmin + dxk xk = xcent + dxk wk = fw(xk),and
- sets bn = xmax
Constructs a piecewise_constant_distribution object with the above computed sequence b as the interval boundaries and with the probability densities:
ρk = wk/(S * deltax) for k = 0, . . . , n-1.[p5_4] [Note: In this context, the subintervals [bk, bk+1) are commonly known as the bins of a histogram. -- end note]
Concept version of the proposed resolution
In 26.5.8.5.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution, just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);
Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);[p5_1] Complexity: Exactly nf invocations of fw.
[p5_2] Requires:
- For all sample values xk defined below, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- The following relations shall hold: xmin < xmax, and 0 < S = w0+. . .+wn-1.
[p5_3] Effects:
If nf == 0,
- sets deltax = xmax - xmin, and
- lets the sequence w have length n = 1 and consist of the single value w0 = 1, and
- lets the sequence b have length n+1 with b0 = xmin and b1 = xmax
Otherwise,
- sets n = nf, deltax = (xmax - xmin)/n, xcent = xmin + 0.5 * deltax, and
lets the sequences w and b have length n and n+1, resp. and
for each k = 0, . . . ,n-1, calculates: dxk = k * deltax bk = xmin + dxk xk = xcent + dxk wk = fw(xk),and
- sets bn = xmax
Constructs a piecewise_constant_distribution object with the above computed sequence b as the interval boundaries and with the probability densities:
ρk = wk/(S * deltax) for k = 0, . . . , n-1.[p5_4] [Note: In this context, the subintervals [bk, bk+1) are commonly known as the bins of a histogram. -- end note]
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".Section: X [rand.dist.samp.genpdf] Status: Dup Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2008-03-11
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Duplicate of: 732
Discussion:
general_pdf_distribution should be dropped. (It's a research topic in adaptive numerical integration.)
[ Stephan Tolksdorf notes: ]
This appears to be a duplicate of 732.
Proposed resolution:
Section: 26.5.5 [rand.predef] Status: NAD Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2008-02-27
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Discussion:
The 10,000th value returned by ranlux48_base is supposed to be 61839128582725. We get 192113843633948. (Note that the underlying generator was changed in Kona.)
[ Bellevue: ]
Submitter withdraws defect.
Proposed resolution:
Change 26.5.5 [rand.predef]/p5:
typedef subtract_with_carry_engine<uint_fast64_t, 48, 5, 12> ranlux48_base;Required behavior: The 10000th consecutive invocation of a default-constructed object of type ranlux48_base shall produce the value61839128582725192113843633948.
Section: 26.5.5 [rand.predef] Status: NAD Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2008-02-27
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Discussion:
The 10,000th value returned by ranlux48 is supposed to be 249142670248501. We get 88229545517833. (Note that this depends on ranlux48_base.)
[ Bellevue: ]
Submitter withdraws defect.
Proposed resolution:
Change 26.5.5 [rand.predef]/p6:
typedef discard_block_engine<ranlux48_base, 389, 11> ranlux48Required behavior: The 10000th consecutive invocation of a default-constructed object of type ranlux48 shall produce the value24914267024850188229545517833.
Section: 26.5.3.2 [rand.eng.mers], TR1 5.1.4.2 [tr.rand.eng.mers] Status: NAD Submitter: Stephan Tolksdorf Opened: 2008-02-18 Last modified: 2008-03-11
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Discussion:
TR1 5.1.4.2 [tr.rand.eng.mers](10) requires that operator== for the mersenne_twister returns true if and only if the states of two mersenne_twisters, consisting each of n integers between 0 and 2w - 1, are completely equal. This is a contradiction with TR1 5.1.1 [tr.rand.req](3) because the given definition of the state also includes the lower r bits of x(i-n), which will never be used to generate a random number. If two mersenne_twisters only differ in the lower bits of x(i-n) they will not compare equal, although they will produce an identical sequence of random numbers.
26.5.3.2 [rand.eng.mers] in the latest C++ draft does not specify the behaviour of operator== but uses a similar definition of the state and, just like TR1 5.1.4.2 [tr.rand.eng.mers], requires the textual representation of a mersenne_twister_engine to consist of Xi-n to Xi-1, including the lower bits of Xi-n. This leads to two problems: First, the unsuspecting implementer is likely to erroneously compare the lower r bits of Xi-n in operator==. Second, if only the lower r bits differ, two mersenne_twister_engines will compare equal (if correctly implemented) but have different textual representations, which conceptually is a bit ugly.
I propose that a paragraph or footnote is added to 26.5.3.2 [rand.eng.mers] which clarifies that the lower r bits of Xi-n are not to be compared in operator== and operator!=. It would only be consequent if furthermore the specification for the textual respresentation was changed to Xi-n bitand ((2w - 1) - (2r - 1)), Xi-(n-1), ..., Xi-1 or something similar.
These changes would likely have no practical effect, but would allow an implementation that does the right thing to be standard-conformant.
[ Bellevue: ]
Fermi Lab has no objection to the proposed change. However it feels that more time is needed to check the details, which would suggest a change to REVIEW.
Bill feels that this is NAD, not enough practical importance to abandon the simple definition of equality, and someone would have to do a lot more study to ensure that all cases are covered for a very small payback. The submitter admits that "These changes would likely have no practical effect,", and according to Plum's razor this means that it is not worth the effort!
Revisted: Agree that the fact that there is no practical difference means that no change can be justified.
Proposed resolution:
In 26.5.3.2 [rand.eng.mers]:
Insert at the end of para 2.:
[Note: The lower r bits of Xi-n do not influence the state transition and hence should not be compared when comparing two mersenne_twister_engine objects. -- end note]In para 5. change:
The textual representation of xi consists of the values of Xi-n bitand ((2w - 1) - (2r - 1)), Xi-(n-1), ..., Xi-1, in that order.
Section: 26.5.7.1 [rand.util.seedseq] Status: NAD Editorial Submitter: Stephan Tolksdorf Opened: 2008-02-18 Last modified: 2009-03-09
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Discussion:
The for-loop in the algorithm specification has n iterations, where n is defined to be end - begin, i.e. the number of supplied w-bit quantities. Previous versions of this algorithm and the general logic behind it suggest that this is an oversight and that in the context of the for-loop n should be the number of full 32-bit quantities in b (rounded upwards). If w is 64, the current algorithm throws away half of all bits in b. If w is 16, the current algorithm sets half of all elements in v to 0.
There are two more minor issues:
[ Bellevue: ]
Move to OPEN Bill will try to propose a resolution by the next meeting.
[ post Bellevue: Bill provided wording. ]
This issue is made moot if 803 is accepted.
Proposed resolution:
Replace 26.5.7.1 [rand.util.seedseq] paragraph 6 with:
Effects: Constructs a seed_seq object by effectively concatenating the low-order u bits of each of the elements of the supplied sequence [begin, end) in ascending order of significance to make a (possibly very large) unsigned binary number b having a total of n bits, and then carrying out the following algorithm:
for( v.clear(); n > 0; n -= 32 ) v.push_back(b mod 232), b /= 232;
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".Section: 26.5.5 [rand.predef] Status: NAD Submitter: P.J. Plauger Opened: 2008-02-20 Last modified: 2008-03-17
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Discussion:
The 10,000th value returned by knuth_b is supposed to be 1112339016. We get 2126698284.
Proposed resolution:
Change 26.5.5 [rand.predef]/p8:
typedef shuffle_order_engine<minstd_rand0, 256> knuth_b;Required behavior: The 10000th consecutive invocation of a default-constructed object of type knuth_b shall produce the value11123390162126698284.
[ Bellevue: Submitter withdraws defect. "We got the wrong value for entirely the right reasons". NAD. ]
Section: 26.5.7.1 [rand.util.seedseq] Status: NAD Editorial Submitter: Charles Karney Opened: 2008-02-22 Last modified: 2009-03-09
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Discussion:
seed_seq(InputIterator begin, InputIterator end); constructs a seed_seq object repacking the bits of supplied sequence [begin, end) into a 32-bit vector.
This repacking triggers several problems:
I propose simplifying this seed_seq constructor to be "32-bit only". Despite it's being simpler, there is NO loss of functionality (see below).
Here's how the description would read
26.5.7.1 [rand.util.seedseq] Class seed_seq
template<class InputIterator> seed_seq(InputIterator begin, InputIterator end);5 Requires: NO CHANGE
6 Effects: Constructs a seed_seq object by
for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 2^32);
Discussion:
The chief virtues here are simplicity, portability, and generality.
Arguments (and counter-arguments) against making this change (and retaining the n2461 behavior) are:
The user can pass an array of unsigned char and seed_seq will nicely repack it.
Response: So what? Consider the seed string "ABC". The n2461 proposal results in
v = { 0x3, 0x434241 };
while the simplified proposal yields
v = { 0x41, 0x42, 0x43 };
The results produced by seed_seq::generate with the two inputs are different but nevertheless equivalently "mixed up" and this remains true even if the seed string is long.
With long strings (e.g., with bit-length comparable to the number of bits in the state), v is longer (by a factor of 4) with the simplified proposal and seed_seq::generate will be slower.
Response: It's unlikely that the efficiency of seed_seq::generate will be a big issue. If it is, the user is free to repack the seed vector before constructing seed_seq.
A user can pass an array of 64-bit integers and all the bits will be used.
Response: Indeed. However, there are many instances in the n2461 where integers are silently coerced to a narrower width and this should just be a case of the user needing to read the documentation. The user can of course get equivalent behavior by repacking his seed into 32-bit pieces. Furthermore, the unportability of the n2461 proposal with
unsigned long s[] = {1, 2, 3, 4};
seed_seq q(s, s+4);
which typically results in v = {1, 2, 3, 4} on 32-bit machines and in v = {1, 0, 2, 0, 3, 0, 4, 0} on 64-bit machines is a major pitfall for unsuspecting users.
Note: this proposal renders moot issues 782 and 800.
[ Bellevue: ]
Walter needs to ask Fermilab for guidance. Defer till tomorrow. Bill likes the proposed resolution.
[ Sophia Antipolis: ]
Marc Paterno wants portable behavior between 32bit and 64bit machines; we've gone to significant trouble to support portability of engines and their values.
Jens: the new algorithm looks perfectly portable
Marc Paterno to review off-line.
Modify the proposed resolution to read "Constructs a seed_seq object by the following algorithm ..."
Disposition: move to review; unanimous consent.
Proposed resolution:
Change 26.5.7.1 [rand.util.seedseq]:
template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end);-5- Requires: InputIterator shall satisfy the requirements of an input iterator (24.1.1) such that iterator_traits<InputIterator>::value_type shall denote an integral type.
-6- Constructs a seed_seq object by the following algorithm
rearranging some or all of the bits of the supplied sequence [begin,end) of w-bit quantities into 32-bit units, as if by the following:
First extract the rightmost u bits from each of the n = end - begin elements of the supplied sequence and concatenate all the extracted bits to initialize a single (possibly very large) unsigned binary number, b = ∑n-1i=0 (begin[i] mod 2u) · 2w·i (in which the bits of each begin[i] are treated as denoting an unsigned quantity). Then carry out the following algorithm:v.clear(); if ($w$ < 32) v.push_back($n$); for( ; $n$ > 0; --$n$) v.push_back(b mod 232), b /= 232;for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 232);
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".Section: 25.4.1 [alg.sort] Status: NAD Editorial Submitter: Paul McKenney Opened: 2008-02-27 Last modified: 2008-09-17
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Discussion:
Multi-threading is a good thing, but unsolicited multi-threading can potentially be harmful. For example, sort() performance might be greatly increased via a multithreaded implementation. However, such a multithreaded implementation could result in concurrent invocations of the user-supplied comparator. This would in turn result in problems given a caching comparator that might be written for complex sort keys. Please note that this is not a theoretical issue, as multithreaded implementations of sort() already exist.
Having a multithreaded sort() available is good, but it should not be the default for programs that are not explicitly multithreaded. Users should not be forced to deal with concurrency unless they have asked for it.
[ This may be covered by N2410 Thread-Safety in the Standard Library (Rev 1). ]
Proposed resolution:
Rationale:
This is already covered by 17.6.5.6/20 in N2723.Section: 20.8.14.2.4 [func.wrap.func.inv] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2008-03-16 Last modified: 2010-03-08
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Discussion:
std::function and reference_closure should use "perfect forwarding" as described in the rvalue core proposal.
[ Sophia Antipolis: ]
According to Doug Gregor, as far as std::function is concerned, perfect forwarding can not be obtained because of type erasure. Not everyone agreed with this diagnosis of forwarding.
[ 2009-05-01 Howard adds: ]
Sebastian Gesemann brought to my attention that the CopyConstructible requirement on function's ArgTypes... is an unnecessary restriction.
template<Returnable R, CopyConstructible... ArgTypes> class function<R(ArgTypes...)> ...On further investigation, this complaint seemed to be the same issue as this one. I believe the reason CopyConstructible was put on ArgTypes in the first place was because of the nature of the invoke member:
template<class R, class ...ArgTypes> R function<R(ArgTypes...)>::operator()(ArgTypes... arg) const { if (f_ == 0) throw bad_function_call(); return (*f_)(arg...); }However now with rvalue-refs, "by value" no longer implies CopyConstructible (as Sebastian correctly points out). If rvalue arguments are supplied, MoveConstructible is sufficient. Furthermore, the constraint need not be applied in function if I understand correctly. Rather the client must apply the proper constraints at the call site. Therefore, at the very least, I recommend that CopyConstructible be removed from the template class function.
Furthermore we need to mandate that the invoker is coded as:
template<class R, class ...ArgTypes> R function<R(ArgTypes...)>::operator()(ArgTypes... arg) const { if (f_ == 0) throw bad_function_call(); return (*f_)(std::forward<ArgTypes>(arg)...); }Note that ArgTypes&& (the "perfect forwarding signature") is not appropriate here as this is not a deduced context for ArgTypes. Instead the client's arguments must implicitly convert to the non-deduced ArgType type. Catching these arguments by value makes sense to enable decay.
Next forward is used to move the ArgTypes as efficiently as possible, and also with minimum requirements (not CopyConstructible) to the type-erased functor. For object types, this will be a move. For reference type ArgTypes, this will be a copy. The end result must be that the following is a valid program:
#include <functional> #include <memory> #include <cassert> std::unique_ptr<int> f(std::unique_ptr<int> p, int& i) { ++i; return std::move(p); } int main() { int i = 2; std::function<std::unique_ptr<int>(std::unique_ptr<int>, int&> g(f); std::unique_ptr<int> p = g(std::unique_ptr<int>(new int(1)), i); assert(*p == 1); assert(i == 3); }[ Tested in pre-concepts rvalue-ref-enabled compiler. ]
In the example above, the first ArgType is unique_ptr<int> and the second ArgType is int&. Both must work!
[ 2009-05-27 Daniel adds: ]
in the 2009-05-01 comment of above mentioned issue Howard
- Recommends to replace the CopyConstructible requirement by a MoveConstructible requirement
- Says: "Furthermore, the constraint need not be applied in function if I understand correctly. Rather the client must apply the proper constraints at the call site"
I'm fine with (a), but I think comment (b) is incorrect, at least in the sense I read these sentences. Let's look at Howard's example code:
function<R(ArgTypes...)>::operator()(ArgTypes... arg) const { if (f_ == 0) throw bad_function_call(); return (*f_)(std::forward<ArgTypes>(arg)...); }In the constrained scope of this operator() overload the expression "(*f_)(std::forward<ArgTypes>(arg)...)" must be valid. How can it do so, if ArgTypes aren't at least MoveConstructible?
[ 2009-07 Frankfurt: ]
Leave this open and wait until concepts are removed from the Working Draft so that we know how to write the proposed resolution in terms of diffs to otherwise stable text.
[ 2009-10 Santa Cruz: ]
Leave as open. Howard to provide wording. Howard welcomes any help.
[ 2009-12-12 Jonathan Wakely adds: ]
20.8.14.2 [func.wrap.func] says
2 A function object f of type F is Callable for argument types T1, T2, ..., TN in ArgTypes and a return type R, if, given lvalues t1, t2, ..., tN of types T1, T2, ..., TN, respectively, INVOKE (f, t1, t2, ..., tN) is well formed (20.7.2) and, if R is not void, convertible to R.N.B. lvalues, which means you can't use function<R(T&&)> or function<R(unique_ptr<T>)>
I recently implemented rvalue arguments in GCC's std::function, all that was needed was to use std::forward<ArgTypes> in a few places. The example in issue 815 works.
I think 815 could be resolved by removing the requirement that the target function be callable with lvalues. Saying ArgTypes need to be CopyConstructible is wrong, and IMHO saying MoveConstructible is unnecessary, since the by-value signature implies that already, but if it is needed it should only be on operator(), not the whole class (you could in theory instantiate std::function<R(noncopyable)> as long as you don't invoke the call operator.)
I think defining invocation in terms of INVOKE already implies perfect forwarding, so we don't need to say explicitly that std::forward should be used (N.B. the types that are forwarded are those in ArgTypes, which can differ from the actual parameter types of the target function. The actual parameter types have gone via type erasure, but that's not a problem - IMHO forwarding the arguments as ArgTypes is the right thing to do anyway.)
Is it sufficient to simply replace "lvalues" with "values"? or do we need to say something like "lvalues when Ti is an lvalue-reference and rvalues otherwise"? I prefer the former, so I propose the following resolution for 815:
Edit 20.8.14.2 [func.wrap.func] paragraph 2:
2 A function object f of type F is Callable for argument types T1, T2, ..., TN in ArgTypes and a return type R, if, givenlvalues t1, t2, ..., tN of types T1, T2, ..., TN, respectively, INVOKE (f, t1, t2, ..., tN) is well formed (20.7.2) and, if R is not void, convertible to R.
[ 2009-12-12 Daniel adds: ]
I don't like the reduction to "values" and prefer the alternative solution suggested using "lvalues when Ti is an lvalue-reference and rvalues otherwise". The reason why I dislike the shorter version is based on different usages of "values" as part of defining the semantics of requirement tables via expressions. E.g. 20.2.1 [utility.arg.requirements]/1 says "a, b, and c are values of type const T;" or similar in 23.2.1 [container.requirements.general]/4 or /14 etc. My current reading of all these parts is that both rvalues and lvalues are required to be supported, but this interpretation would violate the intention of the suggested fix of #815, if I correctly understand Jonathan's rationale.
[ 2009-12-12 Howard adds: ]
"lvalues when Ti is an lvalue-reference and rvalues otherwise"doesn't quite work here because the Ti aren't deduced. They are specified by the function type. Ti might be const int& (an lvalue reference) and a valid ti might be 2 (a non-const rvalue). I've taken another stab at the wording using "expressions" and "bindable to".
[ 2010-02-09 Wording updated by Jonathan, Ganesh and Daniel. ]
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Daniel opens to improve wording. ]
[ 2010-02-11 This issue is now addressed by 870. ]
[ 2010-02-12 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
Addressed by 870.
Proposed resolution:
Edit 20.8.14.2 [func.wrap.func] paragraph 2:
2 A function object f of type F is Callable for argument types
T1, T2, ..., TN inArgTypes andareturn type R,if, given lvalues t1, t2, ..., tN of types T1, T2, ..., TN, respectively,the expression INVOKE(f, declval<ArgTypes>()..., Rt1, t2, ..., tN), considered as an unevaluated operand (5 [expr]), is well formed (20.7.2)and, if R is not void, convertible to R.
Section: 20.8.10.1.2 [func.bind.bind] Status: NAD Editorial Submitter: Stephan T. Lavavej Opened: 2008-02-08 Last modified: 2010-03-08
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Discussion:
Library Issue 527 notes that bind(f, t1, ..., tN) should be nofail when f, t1, ..., tN have nofail copy ctors.
However, no guarantees are provided for the copy ctor of the functor returned by bind(). (It's guaranteed to have a copy ctor, which can throw implementation-defined exceptions: bind() returns a forwarding call wrapper, TR1 3.6.3/2. A forwarding call wrapper is a call wrapper, TR1 3.3/4. Every call wrapper shall be CopyConstructible, TR1 3.3/4. Everything without an exception-specification may throw implementation-defined exceptions unless otherwise specified, C++03 17.4.4.8/3.)
Should the nofail guarantee requested by Library Issue 527 be extended to cover both calling bind() and copying the returned functor?
[ Howard adds: ]
tuple construction should probably have a similar guarantee.
[ San Francisco: ]
Howard to provide wording.
[ Post Summit, Anthony provided wording. ]
[ Batavia (2009-05): ]
Part of all of this issue appears to be rendered moot by the proposed resolution to issue 817 (q.v.). We recommend the issues be considered simultaneously (or possibly even merged) to ensure there is no overlap. Move to Open, and likewise for issue 817.
[ 2009-07 Frankfurt: ]
Related to 817 (see below). Leave Open.
[ 2009-10 Santa Cruz: ]
Move to Ready. Decoupling from issue 817.
[ 2010-02-11 Moved from Ready to Tentatively NAD Editorial, rationale added below. ]
Rationale:
This issue is solved as proposed by 817.
Proposed resolution:
Add a new sentence to the end of paragraphs 2 and 4 of 20.8.10.1.2 [func.bind.bind]:
-2- Returns: A forwarding call wrapper g with a weak result type (20.6.2). The effect of g(u1, u2, ..., uM) shall be INVOKE(f, v1, v2, ..., vN, Callable<F cv,V1, V2, ..., VN>::result_type), where cv represents the cv-qualifiers of g and the values and types of the bound arguments v1, v2, ..., vN are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of F or any of the types in BoundArgs... throw an exception.
...
-5- Returns: A forwarding call wrapper g with a nested type result_type defined as a synonym for R. The effect of g(u1, u2, ..., uM) shall be INVOKE(f, v1, v2, ..., vN, R), where the values and types of the bound arguments v1, v2, ..., vN are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of F or any of the types in BoundArgs... throw an exception.
Section: 20.3.3 [forward] Status: NAD Editorial Submitter: Walter Brown Opened: 2008-04-09 Last modified: 2009-10-23
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Discussion:
N2588 seems to have added an operator() member function to the identity<> helper in 20.3.3 [forward]. I believe this change makes it no longer possible to instantiate identity<void>, as it would require forming a reference-to-void type as this operator()'s parameter type.
Suggested resolution: Specialize identity<void> so as not to require the member function's presence.
[ Sophia Antipolis: ]
Jens: suggests to add a requires clause to avoid specializing on void.
Alisdair: also consider cv-qualified void.
Alberto provided proposed wording.
[ 2009-07-30 Daniel reopens: ]
This issue became closed, because the ReferentType requirement fixed the problem - this is no longer the case. In retrospective it seems to be that the root of current issues around std::identity (823, 700, 939) is that it was standardized as something very different (an unconditional type mapper) than traditional usage indicated (a function object that should derive from std::unary_function), as the SGI definition does. This issue could be solved, if std::identity is removed (one proposal of 939), but until this has been decided, this issue should remain open. An alternative for removing it, would be, to do the following:
Let identity stay as a real function object, which would now properly derive from unary_function:
template <class T> struct identity : unary_function<T, T> { const T& operator()(const T&) const; };Invent (if needed) a generic type wrapper (corresponding to concept IdentityOf), e.g. identity_of, and move it's prototype description back to 20.3.3 [forward]:
template <class T> struct identity_of { typedef T type; };and adapt the std::forward signature to use identity_of instead of identity.
[ 2009-10 Santa Cruz: ]
Mark as NAD Editorial, fixed by 939.
Proposed resolution:
Change definition of identity in 20.3.3 [forward], paragraph 2, to:
template <class T> struct identity {
typedef T type;
requires ReferentType<T>
const T& operator()(const T& x) const;
};
...
requires ReferentType<T>
const T& operator()(const T& x) const;
Rationale:
The point here is to able to write T& given T and ReferentType is precisely the concept that guarantees so, according to N2677 (Foundational concepts). Because of this, it seems preferable than an explicit check for cv void using SameType/remove_cv as it was suggested in Sophia. In particular, Daniel remarked that there may be types other than cv void which aren't referent types (int[], perhaps?).
Section: 19.5.2.1 [syserr.errcode.overview], 20.9.11.2.8 [util.smartptr.shared.io], 22.4.8 [facets.examples], 20.5.4 [bitset.operators], 26.4.6 [complex.ops], 27.6 [stream.buffers], 28.9 [re.submatch] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2008-04-10 Last modified: 2009-10-26
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Discussion:
Addresses UK 220
Should the following use rvalues references to stream in insert/extract operators?
[ Sophia Antipolis ]
Agree with the idea in the issue, Alisdair to provide wording.
[ Daniel adds 2009-02-14: ]
The proposal given in the paper N2831 apparently resolves this issue.
[ Batavia (2009-05): ]
The cited paper is an earlier version of N2844, which changed the rvalue reference binding rules. That paper includes generic templates operator<< and operator>> that adapt rvalue streams.
We therefore agree with Daniel's observation. Move to NAD Editorial.
Proposed resolution:
Section: 22.4.2.2 [locale.nm.put] Status: NAD Submitter: Peter Dimov Opened: 2008-04-07 Last modified: 2008-06-18
View all issues with NAD status.
Discussion:
In the spirit of printf vs iostream...
POSIX printf says that %'d should insert grouping characters (and the implication is that in the absence of ' no grouping characters are inserted). The num_put facet, on the other hand, seems to always insert grouping characters. Can this be considered a defect worth fixing for C++0x? Maybe ios_base needs an additional flag?
[ Pablo Halpern: ]
I'm not sure it constitutes a defect, but I would be in favor of adding another flag (and corresponding manipulator).
[ Martin Sebor: ]
I don't know if it qualifies as a defect but I agree that there should be an easy way to control whether the thousands separator should or shouldn't be inserted. A new flag would be in line with the current design of iostreams (like boolalpha, showpos, or showbase).
[ Sophia Antipolis: ]
This is not a part of C99. LWG suggests submitting a paper may be appropriate.
Proposed resolution:
Section: 20.9.11.2.1 [util.smartptr.shared.const] Status: NAD Editorial Submitter: Peter Dimov Opened: 2008-04-11 Last modified: 2009-10-26
View all other issues in [util.smartptr.shared.const].
View all issues with NAD Editorial status.
Discussion:
Would anyone object to making the default constructor of shared_ptr (and weak_ptr and enable_shared_from_this) constexpr? This would enable static initialization for shared_ptr variables, eliminating another unfair advantage of raw pointers.
[ San Francisco: ]
It's not clear to us that you can initialize a pointer with the literal 0 in a constant expression. We need to ask CWG to make sure this works. Bjarne has been appointed to do this.
Core got back to us and assured as that nullptr would do the job nicely here.
[ 2009-05-01 Alisdair adds: ]
I don't believe that constexpr will buy anything in this case. shared_ptr/weak_ptr/enable_shared_from_this cannot be literal types as they have a non-trivial copy constructor. As they do not produce literal types, then the constexpr default constructor will not guarantee constant initialization, and so not buy the hoped for optimization.
I recommend referring this back to Core to see if we can get static initialization for types with constexpr constructors, even if they are not literal types. Otherwise this should be closed as NAD.
[ 2009-05-26 Daniel adds: ]
If Alisdair's 2009-05-01 comment is correct, wouldn't that also make constexpr mutex() useless, because this class has a non-trivial destructor? (828)
[ 2009-07-21 Alisdair adds: ]
The feedback from core is that this and similar uses of constexpr constructors to force static initialization should be supported. If there are any problems with this in the working draught, we should file core issues.
Recommend we declare the default constructor constexpr as the issue suggests (proposed wording added).
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2994.
Proposed resolution:
Change 20.9.11.2 [util.smartptr.shared] and 20.9.11.2.1 [util.smartptr.shared.const]:
consexpr shared_ptr();
Change 20.9.11.3 [util.smartptr.weak] and 20.9.11.3.1 [util.smartptr.weak.const]:
consexpr weak_ptr();
Change 20.9.11.4 [util.smartptr.enab] (2 places):
consexpr enable_shared_from_this();
Section: 30.4.1.1 [thread.mutex.class] Status: NAD Editorial Submitter: Peter Dimov Opened: 2008-04-18 Last modified: 2009-10-26
View all other issues in [thread.mutex.class].
View all issues with NAD Editorial status.
Discussion:
[Note: I'm assuming here that 3.6.2 [basic.start.init]/1 will be fixed.]
Currently std::mutex doesn't support static initialization. This is a regression with respect to pthread_mutex_t, which does. I believe that we should strive to eliminate such regressions in expressive power where possible, both to ease migration and to not provide incentives to (or force) people to forego the C++ primitives in favor of pthreads.
[ Sophia Antipolis: ]
We believe this is implementable on POSIX, because the initializer-list feature and the constexpr feature make this work. Double-check core language about static initialization for this case. Ask core for a core issue about order of destruction of statically-initialized objects wrt. dynamically-initialized objects (should come afterwards). Check non-POSIX systems for implementability.
If ubiquitous implementability cannot be assured, plan B is to introduce another constructor, make this constexpr, which is conditionally-supported. To avoid ambiguities, this new constructor needs to have an additional parameter.
[ Post Summit: ]
Jens: constant initialization seems to be ok core-language wise
Consensus: Defer to threading experts, in particular a Microsoft platform expert.
Lawrence to send e-mail to Herb Sutter, Jonathan Caves, Anthony Wiliams, Paul McKenney, Martin Tasker, Hans Boehm, Bill Plauger, Pete Becker, Peter Dimov to alert them of this issue.
Lawrence: What about header file shared with C? The initialization syntax is different in C and C++.
Recommend Keep in Review
[ Batavia (2009-05): ]
Keep in Review status pending feedback from members of the Concurrency subgroup.
[ See related comments from Alisdiar and Daniel in 827. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2994.
Proposed resolution:
Change 30.4.1.1 [thread.mutex.class]:
class mutex {
public:
constexpr mutex();
...
Section: 21.2 [char.traits] Status: NAD Editorial Submitter: Dietmar Kühl Opened: 2008-04-23 Last modified: 2009-07-13
View all other issues in [char.traits].
View all issues with NAD Editorial status.
Discussion:
Paragraph 4 of 21.2 [char.traits] mentions that this
section specifies two specializations (char_traits<char>
and (char_traits<wchar_t>). However, there are actually
four specializations provided, i.e. in addition to the two above also
char_traits<char16_t> and char_traits<char32_t>).
I guess this was just an oversight and there is nothing wrong with just
fixing this.
[ Alisdair adds: ]
char_traits< char16/32_t > should also be added to <ios_fwd> in 27.3 [iostream.forward], and all the specializations taking a char_traits parameter in that header.
[ Sophia Antipolis: ]
Idea of the issue is ok.
Alisdair to provide wording, once that wording arrives, move to review.
[ 2009-05-04 Alisdair adds: ]
The main point of the issue was resolved editorially in N2723, so we are close to NAD Editorial. However, exploring the issue we found a second tweak was necessary for <iosfwd> and that is still outstanding, so here are the words I am long overdue delivering:
[ Howard: I've put Alisdair's words into the proposed wording section and moved the issue to Review. ]
[ Original proposed wording. ]
Replace paragraph 4 of 21.2 [char.traits] by:
This subclause specifies a struct template,
char_traits<charT>, and four explicit specializations of it,char_traits<char>,char_traits<char16_t>,char_traits<char32_t>, andchar_traits<wchar_t>, all of which appear in the header <string> and satisfy the requirements below.
[ Batavia (2009-05): ]
We agree. Move to NAD Editorial.
Proposed resolution:
Change Forward declarations 27.3 [iostream.forward]:
Header <iosfwd> synopsis
namespace std { template<class charT> class char_traits; template<> class char_traits<char>; template<> class char_traits<char16_t>; template<> class char_traits<char32_t>; template<> class char_traits<wchar_t>; ... }
Section: 21.2.3 [char.traits.specializations] Status: NAD Editorial Submitter: Dietmar Kühl Opened: 2008-04-23 Last modified: 2008-06-19
View all other issues in [char.traits.specializations].
View all issues with NAD Editorial status.
Discussion:
In Table 56 (Traits requirements) the not_eof() member function
is using an argument of type e which denotes an object of
type X::int_type. However, the specializations in
21.2.3 [char.traits.specializations] all use char_type.
This would effectively mean that the argument type actually can't
represent EOF in the first place. I'm pretty sure that the type used
to be int_type which is quite obviously the only sensible
argument.
This issue is close to being editorial. I suspect that the proposal
changing this section to include the specializations for char16_t
and char32_t accidentally used the wrong type.
Proposed resolution:
In 21.2.3.1 [char.traits.specializations.char],
21.2.3.2 [char.traits.specializations.char16_t],
21.2.3.3 [char.traits.specializations.char32_t], and
[char.traits.specializations.wchar_t] correct the
argument type from char_type to int_type.
Rationale:
Already fixed in WP.Section: 19.5 [syserr] Status: NAD Submitter: Beman Dawes Opened: 2008-05-14 Last modified: 2008-09-17
View all other issues in [syserr].
View all issues with NAD status.
Discussion:
Initialization of objects of class error_code (19.5.2 [syserr.errcode]) and class error_condition (19.5.3 [syserr.errcondition]) can be made simpler and more reliable by use of the new constexpr feature [N2349] of C++0x. Less code will need to be generated for both library implementations and user programs when manipulating constant objects of these types.
This was not proposed originally because the constant expressions proposal was moving into the standard at about the same time as the Diagnostics Enhancements proposal [N2241], and it wasn't desirable to make the later depend on the former. There were also technical concerns as to how constexpr would apply to references. Those concerns are now resolved; constexpr can't be used for references, and that fact is reflected in the proposed resolution.
Thanks to Jens Maurer, Gabriel Dos Reis, and Bjarne Stroustrup for clarification of constexpr requirements.
LWG issue 804 is related in that it raises the question of whether the exposition only member cat_ of class error_code (19.5.2 [syserr.errcode]) and class error_condition (19.5.3 [syserr.errcondition]) should be presented as a reference or pointer. While in the context of 804 that is arguably an editorial question, presenting it as a pointer becomes more or less required with this proposal, given constexpr does not play well with references. The proposed resolution thus changes the private member to a pointer, which also brings it in sync with real implementations.
[ Sophia Antipolis: ]
On going question of extern pointer vs. inline functions for interface.
[ Pre-San Francisco: ]
Beman Dawes reports that this proposal is unimplementable, and thus NAD.
Implementation would require constexpr objects of classes derived from class error_category, which has virtual functions, and that is not allowed by the core language. This was determined when trying to implement the proposal using a constexpr enabled compiler provided by Gabriel Dos Reis, and subsequently verified in discussions with Gabriel and Jens Maurer.
Proposed resolution:
The proposed wording assumes the LWG 805 proposed wording has been applied to the WP, resulting in the former posix_category being renamed generic_category. If 805 has not been applied, the names in this proposal must be adjusted accordingly.
Change 19.5.1.1 [syserr.errcat.overview] Class error_category overview error_category synopsis as indicated:
const error_category& get_generic_category();const error_category& get_system_category();staticextern const error_category&* const generic_category= get_generic_category();staticextern const error_category&* constnative_categorysystem_category= get_system_category();
Change 19.5.1.5 [syserr.errcat.objects] Error category objects as indicated:
extern const error_category&* constget_generic_category();
Returns: A referencegeneric_category shall point toana statically initialized object of a type derived from class error_category.
Remarks:The object's default_error_condition and equivalent virtual functions shall behave as specified for the class error_category. The object's name virtual function shall return a pointer to the string "GENERIC".extern const error_category&* constget_system_category();
Returns: A referencesystem_category shall point toana statically initialized object of a type derived from class error_category.
Remarks:The object's equivalent virtual functions shall behave as specified for class error_category. The object's name virtual function shall return a pointer to the string "system". The object's default_error_condition virtual function shall behave as follows:If the argument ev corresponds to a POSIX errno value posv, the function shall return error_condition(posv, generic_category). Otherwise, the function shall return error_condition(ev, system_category). What constitutes correspondence for any given operating system is unspecified. [Note: The number of potential system error codes is large and unbounded, and some may not correspond to any POSIX errno value. Thus implementations are given latitude in determining correspondence. -- end note]
Change 19.5.2.1 [syserr.errcode.overview] Class error_code overview as indicated:
class error_code {
public:
...;
constexpr error_code(int val, const error_category&* cat);
...
void assign(int val, const error_category&* cat);
...
const error_category&* category() const;
...
private:
int val_; // exposition only
const error_category&* cat_; // exposition only
Change 19.5.2.2 [syserr.errcode.constructors] Class error_code constructors as indicated:
constexpr error_code(int val, const error_category&* cat);Effects: Constructs an object of type error_code.
Postconditions: val_ == val and cat_ == cat.
Throws: Nothing.
Change 19.5.2.3 [syserr.errcode.modifiers] Class error_code modifiers as indicated:
void assign(int val, const error_category&* cat);Postconditions: val_ == val and cat_ == cat.
Throws: Nothing.
Change 19.5.2.4 [syserr.errcode.observers] Class error_code observers as indicated:
const error_category&* category() const;Returns: cat_.
Throws: Nothing.
Change 19.5.3.1 [syserr.errcondition.overview] Class error_condition overview as indicated:
class error_condition {
public:
...;
constexpr error_condition(int val, const error_category&* cat);
...
void assign(int val, const error_category&* cat);
...
const error_category&* category() const;
...
private:
int val_; // exposition only
const error_category&* cat_; // exposition only
Change 19.5.3.2 [syserr.errcondition.constructors] Class error_condition constructors as indicated:
constexpr error_condition(int val, const error_category&* cat);Effects: Constructs an object of type error_condition.
Postconditions: val_ == val and cat_ == cat.
Throws: Nothing.
Change 19.5.3.3 [syserr.errcondition.modifiers] Class error_condition modifiers as indicated:
void assign(int val, const error_category&* cat);Postconditions: val_ == val and cat_ == cat.
Throws: Nothing.
Change 19.5.3.4 [syserr.errcondition.observers] Class error_condition observers as indicated:
const error_category&* category() const;Returns: cat_.
Throws: Nothing.
Throughout 19.5 [syserr] System error support, change "category()." to "category()->". Appears approximately six times.
[Partially Editorial] In 19.5.4 [syserr.compare] Comparison operators, paragraphs 2 and 4, change "category.equivalent(" to "category()->equivalent(".
Change 19.5.6.1 [syserr.syserr.overview] Class system_error overview as indicated:
public: system_error(error_code ec, const string& what_arg); system_error(error_code ec); system_error(int ev, const error_category&* ecat, const string& what_arg); system_error(int ev, const error_category&* ecat);
Change 19.5.6.2 [syserr.syserr.members] Class system_error members as indicated:
system_error(int ev, const error_category&* ecat, const string& what_arg);Effects: Constructs an object of class system_error.
Postconditions: code() == error_code(ev, ecat) and strcmp(runtime_error::what(), what_arg.c_str()) == 0.
system_error(int ev, const error_category&* ecat);Effects: Constructs an object of class system_error.
Postconditions: code() == error_code(ev, ecat) and strcmp(runtime_error::what(), "") == 0.
Rationale:
[ San Francisco: ]
NAD because Beman said so.
Section: 17.6.1.3 [compliance] Status: NAD Submitter: Beman Dawes Opened: 2008-05-14 Last modified: 2009-07-16
View all other issues in [compliance].
View all issues with NAD status.
Discussion:
Once the C++0x standard library is feature complete, the LWG needs to review 17.6.1.3 [compliance] Freestanding implementations header list to ensure it reflects LWG consensus.
[ San Francisco: ]
This is a placeholder defect to remind us to review the table once we've stopped adding headers to the library.
Three new headers that need to be added to the list:
<initializer_list> <concept> <iterator_concepts><iterator_concepts>, in particular, has lots of stuff that isn't needed, so maybe the stuff that is needed should be broken out into a separate header.
Robert: What about reference_closure? It's currently in <functional>.
[ Post Summit Daniel adds: ]
- The comment regarding reference_closure seems moot since it was just recently decided to remove that.
- A reference to proposal N2814 ("Fixing freestanding") should be added. This paper e.g. proposes to add only <initializer_list> to the include list of freestanding.
[ 2009-07 Frankfurt: ]
Addressed by paper N2814.
Move to NAD.
Proposed resolution:
Section: 20.9.10.2 [unique.ptr.single] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2008-05-14 Last modified: 2010-03-14
View all other issues in [unique.ptr.single].
View all issues with NAD Editorial status.
Discussion:
Issue 673 (including recent updates by 821) proposes a useful extension point for unique_ptr by granting support for an optional deleter_type::pointer to act as pointer-like replacement for element_type* (In the following: pointer).
Unfortunately no requirements are specified for the type pointer which has impact on at least two key features of unique_ptr:
Unique_ptr specification makes great efforts to require that essentially *all* operations cannot throw and therefore adds proper wording to the affected operations of the deleter as well. If user-provided pointer-emulating types ("smart pointers") will be allowed, either *all* throw-nothing clauses have to be replaced by weaker "An exception is thrown only if pointer's {op} throws an exception"-clauses or it has to be said explicitly that all used operations of pointer are required *not* to throw. I understand the main focus of unique_ptr to be as near as possible to the advantages of native pointers which cannot fail and thus strongly favor the second choice. Also, the alternative position would make it much harder to write safe and simple template code for unique_ptr. Additionally, I assume that a general statement need to be given that all of the expressions of pointer used to define semantics are required to be well-formed and well-defined (also as back-end for 762).
[ Sophia Antipolis: ]
Howard: We maybe need a core concept PointerLike, but we don't need the arithmetic (see shared_ptr vs. vector<T>::iterator.
Howard will go through and enumerate the individual requirements wrt. pointer for each member function.
[ 2009-07 Frankfurt: ]
Move to Ready.
[ 2009-10-15 Alisdair pulls from Ready: ]
I hate to pull an issue out of Ready status, but I don't think 834 is fully baked yet.
For reference the proposed resolution is to add the following words:
unique_ptr<T, D>::pointer's operations shall be well-formed, shall have well defined behavior, and shall not throw exceptions.This leaves me with a big question : which operations?
Are all pointer operations required to be nothrow, including operations that have nothing to do with interactions with unique_ptr? This was much simpler with concepts where we could point to operations within a certain concept, and so nail down the interactions.
[ 2009-10-15 Daniel adds: ]
I volunteer to prepare a more fine-grained solution, but I would like to ask for feedback that helps me doing so. If this question is asked early in the meeting I might be able to fix it within the week, but I cannot promise that now.
[ 2009-10 Santa Cruz: ]
Leave in open. Daniel to provide wording as already suggested.
[ 2009-12-22 Daniel provided wording and rationale. ]
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
The here proposed resolution has considerable overlap with the requirements that are used in the allocator requirements.
This might be a convincing argument to isolate the common subset into one requirement. The reason I did not do that is basically because we might find out that they are either over-constraining or under-constraining at this late point of specification. Note also that as a result of the idea of a general requirement set I added the requirement
A default-initialized object may have a singular value
even though this does not play a relevant role for unique_ptr.
One further characteristics of the resolution is that availability of relational operators of unique_ptr<T, D>::pointer is not part of the basic requirements, which is in sync with the allocator requirements on pointer-like (this means that unique_ptr can hold a void_pointer or const_void_pointer).
Solved by N3073.
Proposed resolution:
Change 20.9.10.2 [unique.ptr.single]/1 as indicated: [The intent is to replace the coupling between T* and the deleter's operator() by a coupling between unique_ptr<T, D>::pointer and this operator()]
1 - The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function pointer or functor for which, given a value d of type D and apointervalue ptr of typeT*unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of deallocating the pointer as appropriate for that deleter. D may also be an lvalue-reference to a deleter.
Change 20.9.10.2 [unique.ptr.single]/3 as indicated:
3 - If the type remove_reference<D>::type::pointer exists, then unique_ptr<T, D>::pointer shall be a synonym for remove_reference<D>::type::pointer. Otherwise unique_ptr<T, D>::pointer shall be a synonym for T*. The type unique_ptr<T, D>::pointer shall
besatisfy the requirements of EqualityComparable, DefaultConstructible, CopyConstructible(Table 34) and, CopyAssignable(Table 36), Swappable, and Destructible (20.2.1 [utility.arg.requirements]). A default-initialized object may have a singular value. A value-initialized object produces the null value of the type. The null value shall be equivalent only to itself. An object of this type can be copy-initialized with a value of type nullptr_t, compared for equality with a value of type nullptr_t, and assigned a value of type nullptr_t. The effect shall be as if a value-initialized object had been used in place of the null pointer constant. An object p of this type can be contextually converted to bool. The effect shall be as if p != nullptr had been evaluated in place of p. No operation on this type which is part of the above mentioned requirements shall exit via an exception.[Note: Given an allocator type X (20.2.5 [allocator.requirements]), the types X::pointer, X::const_pointer, X::void_pointer, and X::const_void_pointer may be used as unique_ptr<T, D>::pointer — end note]
In addition to being available via inclusion of the <utility> header, the swap function template in 20.3.2 [utility.swap] is also available within the definition of unique_ptr's swap function.
Change 20.9.10.2.1 [unique.ptr.single.ctor]/2+3 as indicated: [The first change ensures that we explicitly say, how the stored pointer is initialized. This is important for a constexpr function, because this may make a difference for user-defined pointer-like types]
constexpr unique_ptr();...
2 - Effects: Constructs a unique_ptr which owns nothing, value-initializing the stored pointer.
3 - Postconditions: get() ==
0nullptr.
Change 20.9.10.2.1 [unique.ptr.single.ctor]/6+7 as indicated: [This is a step-by-fix to ensure consistency to the changes of N2976]
unique_ptr(pointer p);...
6 - Effects: Constructs a unique_ptr which owns p, initializing the stored pointer with p.
7 - Postconditions: get() == p. get_deleter() returns a reference to a
default constructedvalue-initialized deleter D.
Insert a new effects clause in 20.9.10.2.1 [unique.ptr.single.ctor] just before p. 14: [The intent is to fix the current lack of specification in which way the stored pointer is initialized]
unique_ptr(pointer p,implementation-definedsee below d1); unique_ptr(pointer p,implementation-definedsee below d2);...
Effects: Constructs a unique_ptr which owns p, initializing the stored pointer with p and the initializing the deleter as described above.
14 - Postconditions: get() == p. get_deleter() returns a reference to the internally stored deleter. If D is a reference type then get_deleter() returns a reference to the lvalue d.
Change 20.9.10.2.1 [unique.ptr.single.ctor]/18+22 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]
unique_ptr(unique_ptr&& u);[..]
18 - Postconditions: get() == value u.get() had before the construction and u.get() == nullptr. get_deleter() returns a reference to the internally stored deleter which was constructed from u.get_deleter(). If D is a reference type then get_deleter() and u.get_deleter() both reference the same lvalue deleter.
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);[..]
22 - Postconditions: get() == value u.get() had before the construction, modulo any required offset adjustments resulting from the cast from unique_ptr<U, E>::pointer to pointer and u.get() == nullptr. get_deleter() returns a reference to the internally stored deleter which was constructed from u.get_deleter().
Change 20.9.10.2.1 [unique.ptr.single.ctor]/20 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950]
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);20 - Requires: If D is not a reference type, construction of the deleter D from an rvalue of type E shall be well formed and shall not throw an exception. If D is a reference type, then E shall be the same type as D (diagnostic required). unique_ptr<U, E>::pointer shall be implicitly convertible to pointer.[Note: These requirements imply that T and U are complete types. — end note]
Change 20.9.10.2.2 [unique.ptr.single.dtor]/2 as indicated:
~unique_ptr();...
2 - Effects: If get() ==
0nullptr there are no effects. Otherwise get_deleter()(get()).
Change 20.9.10.2.3 [unique.ptr.single.asgn]/3+8 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]
unique_ptr& operator=(unique_ptr&& u);[..]
3 - Postconditions: This unique_ptr now owns the pointer which u owned, and u no longer owns it, u.get() == nullptr. [Note: If D is a reference type, then the referenced lvalue deleters are move assigned. — end note]
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);[..]
8 - Postconditions: This unique_ptr now owns the pointer which u owned, and u no longer owns it, u.get() == nullptr.
Change 20.9.10.2.3 [unique.ptr.single.asgn]/6 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950]
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);[..]
6 - Requires: Assignment of the deleter D from an rvalue D shall not throw an exception. unique_ptr<U, E>::pointer shall be implicitly convertible to pointer.
[Note: These requirements imply that T and U are complete types. — end note]
Change 20.9.10.2.3 [unique.ptr.single.asgn] before p. 11 and p. 12 as indicated: [The first change is a simple typo fix]
unique_ptr& operator=(nullptr_t});11 - Effects: reset().
12 - Postcondition: get() ==
0nullptr
Change 20.9.10.2.4 [unique.ptr.single.observers]/1+4+12 as indicated:
typename add_lvalue_reference<T>::type operator*() const;1 - Requires: get() !=
0nullptr. The variable definition add_lvalue_reference<T>::type t = *get() shall be well formed, shall have well-defined behavior, and shall not exit via an exception.[..]
pointer operator->() const;4 - Requires: get() !=
0nullptr.[..]
explicit operator bool() const;12 - Returns: get() !=0nullptr.
Change 20.9.10.2.5 [unique.ptr.single.modifiers]/1 as indicated:
pointer release();1 - Postcondition: get() ==0nullptr.
Change 20.9.10.2.5 [unique.ptr.single.modifiers]/9 as indicated: [The intent is to ensure that potentially user-defined swaps are used. A side-step fix and harmonization with the specification of the the deleter is realized. Please note the additional requirement in bullet 2 of this proposed resolution regarding the availability of the generic swap templates within the member swap function.]
void swap(unique_ptr& u);8 - Requires: The deleter D shall be Swappable and shall not throw an exception under swap.
9 - Effects: The stored pointers of *this and u are exchanged by an unqualified call to non-member swap. The stored deleters are
swap'd (unqualified)exchanged by an unqualified call to non-member swap.
Change 20.9.10.3.2 [unique.ptr.runtime.observers]/1 as indicated:
T& operator[](size_t i) const;Requires: i < the size of the array to which the stored pointer points. The variable definition T& t = get()[i] shall be well formed, shall have well-defined behavior, and shall not exit via an exception.
Change 20.9.10.3.3 [unique.ptr.runtime.modifiers]/1 as indicated:
void reset(pointer p = pointer()); void reset(nullptr_t p);1 - Effects: If get() ==0nullptr there are no effects. Otherwise get_deleter()(get()).
Change 20.9.10.4 [unique.ptr.special] as indicated: [We don't add the relational operators to the basic requirement set, therefore we need special handling here]
template <class T1, class D1, class T2, class D2> bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition bool b = x.get() == y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.
2 - Returns: x.get() == y.get().
Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition bool b = x.get() != y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.
3 - Returns: x.get() != y.get().
Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition bool b = x.get() < y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.
4 - Returns: x.get() < y.get().
Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition bool b = x.get() <= y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.
5 - Returns: x.get() <= y.get().
Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition bool b = x.get() > y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.
6 - Returns: x.get() > y.get().
Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition bool b = x.get() >= y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.
7 - Returns: x.get() >= y.get().
Throws: nothing.
basic_ios::copyfmt() overly loosely specified
Section: 27.5.4.2 [basic.ios.members] Status: NAD Editorial Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2009-07-13
View all other issues in [basic.ios.members].
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Discussion:
The basic_ios::copyfmt() member function is specified in 27.5.4.2 [basic.ios.members] to have the following effects:
Effects: If(this == &rhs)does nothing. Otherwise assigns to the member objects of*thisthe corresponding member objects ofrhs, except that
rdstate()andrdbuf()are left unchanged;exceptions()is altered last by callingexceptions(rhs.except)- the contents of arrays pointed at by
pwordandiwordare copied not the pointers themselves
Since the rest of the text doesn't specify what the member objects
of basic_ios are this seems a little too loose.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to NAD Editorial.
Proposed resolution:
I propose to tighten things up by adding a Postcondition clause to the function like so:
Postconditions:
copyfmt()postconditionsElement Value rdbuf()unchanged tie()rhs.tie()rdstate()unchanged exceptions()rhs.exceptions()flags()rhs.flags()width()rhs.width()precision()rhs.precision()fill()rhs.fill()getloc()rhs.getloc()
The format of the table follows Table 117 (as
of N2588): basic_ios::init()
effects.
The intent of the new table is not to impose any new requirements or change existing ones, just to be more explicit about what I believe is already there.
Section: 23.4 [associative], 23.5 [unord] Status: NAD Future Submitter: Alan Talbot Opened: 2008-05-18 Last modified: 2009-09-20
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Discussion:
Splice is a very useful feature of list. This functionality is also very useful for any other node based container, and I frequently wish it were available for maps and sets. It seems like an omission that these containers lack this capability. Although the complexity for a splice is the same as for an insert, the actual time can be much less since the objects need not be reallocated and copied. When the element objects are heavy and the compare operations are fast (say a map<int, huge_thingy>) this can be a big win.
Suggested resolution:
Add the following signatures to map, set, multimap, multiset, and the unordered associative containers:
void splice(list<T,Allocator>&& x); void splice(list<T,Allocator>&& x, const_iterator i); void splice(list<T,Allocator>&& x, const_iterator first, const_iterator last);
Hint versions of these are also useful to the extent hint is useful. (I'm looking for guidance about whether hints are in fact useful.)
void splice(const_iterator position, list<T,Allocator>&& x); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator first, const_iterator last);
[ Sophia Antipolis: ]
Don't try to splice "list" into the other containers, it should be container-type.
forward_list already has splice_after.
Would "splice" make sense for an unordered_map?
Jens, Robert: "splice" is not the right term, it implies maintaining ordering in lists.
Howard: adopt?
Jens: absorb?
Alan: subsume?
Robert: recycle?
Howard: transfer? (but no direction)
Jens: transfer_from. No.
Alisdair: Can we give a nothrow guarantee? If your compare() and hash() doesn't throw, yes.
Daniel: For unordered_map, we can't guarantee nothrow.
[ San Francisco: ]
Martin: this would possibly outlaw an implementation technique that is currently in use; caching nodes in containers.
Alan: if you cache in the allocator, rather than the individual container, this proposal doesn't interfere with that.
Martin: I'm not opposed to this, but I'd like to see an implementation that demonstrates that it works.
[ 2009-07 Frankfurt: ]
NAD Future.
[ 2009-09-19 Howard adds: ]
I'm not disagreeing with the NAD Future resolution. But when the future gets here, here is a possibility worth exploring:
Add to the "unique" associative containers:
typedef details node_ptr; node_ptr remove(const_iterator p); pair<iterator, bool> insert(node_ptr&& nd); iterator insert(const_iterator p, node_ptr&& nd);And add to the "multi" associative containers:
typedef details node_ptr; node_ptr remove(const_iterator p); iterator insert(node_ptr&& nd); iterator insert(const_iterator p, node_ptr&& nd);Container::node_ptr is a smart pointer much like unique_ptr. It owns a node obtained from the container it was removed from. It maintains a reference to the allocator in the container so that it can properly deallocate the node if asked to, even if the allocator is stateful. This being said, the node_ptr can not outlive the container for this reason.
The node_ptr offers "const-free" access to the node's value_type.
With this interface, clients have a great deal of flexibility:
- A client can remove a node from one container, and insert it into another (without any heap allocation). This is the splice functionality this issue asks for.
- A client can remove a node from a container, change its key or value, and insert it back into the same container, or another container, all without the cost of allocating a node.
- If the Compare function is nothrow (which is very common), then this functionality is nothrow unless modifying the value throws. And if this does throw, it does so outside of the containers involved.
- If the Compare function does throw, the insert function will have the argument nd retain ownership of the node.
- The node_ptr should be independent of the Compare parameter so that a node can be transferred from set<T, C1, A> to set<T, C2, A> (for example).
Here is how the customer might use this functionality:
Splice a node from one container to another:
m2.insert(m1.remove(i));Change the "key" in a std::map without the cost of node reallocation:
auto p = m.remove(i); p->first = new_key; m.insert(std::move(p));Change the "value" in a std::set without the cost of node reallocation:
auto p = s.remove(i); *p = new_value; s.insert(std::move(p));Move a move-only or heavy object out of an associative container (as opposed to the proposal in 1041):
MoveOnly x = std::move(*s.remove(i));
- remove(i) transfers ownership of the node from the set to a temporary node_ptr.
- The node_ptr is dereferenced, and that non-const reference is sent to move to cast it to an rvalue.
- The rvalue MoveOnly is move constructed into x from the node_ptr.
- ~node_ptr() destructs the moved-from MoveOnly and deallocates the node.
Contrast this with the 1041 solution:
MoveOnly x = std::move(s.extract(i).first);The former requires one move construction for x while the latter requires two (one into the pair and then one into x). Either of these constructions can throw (say if there is only a copy constructor for x). With the former, the point of throw is outside of the container s, after the element has been removed from the container. With the latter, one throwing construction takes place prior to the removal of the element, and the second takes place after the element is removed.
The "node insertion" API maintains the API associated with inserting value_types so the customer can use familiar techniques for getting an iterator to the inserted node, or finding out whether it was inserted or not for the "unique" containers.
Lightly prototyped. No implementation problems. Appears to work great for the client.
Proposed resolution:
Section: 20.3.5 [pairs] Status: NAD Submitter: Thorsten Ottosen Opened: 2008-05-23 Last modified: 2008-06-18
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Discussion:
I have one issue with std::pair. Well, it might just be a very annoying historical accident, but why is there no default template argument for the second template argument? This is so annoying when the type in question is looong and hard to write (type deduction with auto won't help those cases where we use it as a return or argument type).
Proposed resolution:
Change the synopsis in 20.3 [utility] to read:
template <class T1, class T2 = T1> struct pair;
Change 20.3.5 [pairs] to read:
namespace std {
template <class T1, class T2 = T1>
struct pair {
typedef T1 first_type;
typedef T2 second_type;
...
Rationale:
std::pair is a heterogeneous container.Section: 18.4.1 [cstdint.syn] Status: NAD Editorial Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2008-09-17
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Discussion:
In specifying the names of macros and types defined in
header <stdint.h>, C99 makes use of the
symbol N to accommodate unusual platforms with
word sizes that aren't powers of two. C99
permits N to take on any positive integer value
(including, for example, 24).
In cstdint.syn Header <cstdint>
synopsis, C++ on the other hand, fixes the value
of N to 8, 16, 32, and 64, and specifies only
types with these exact widths.
In addition, paragraph 1 of the same section makes use of a rather informal shorthand notation to specify sets of macros. When interpreted strictly, the notation specifies macros such as
INT_8_MIN that are not intended to be specified.
Finally, the section is missing the usual table of symbols defined in that header, making it inconsistent with the rest of the specification.
Proposed resolution:
I propose to use the same approach in the C++ spec as C99 uses, that
is, to specify the header synopsis in terms of "exposition only" types
that make use of the symbol N to denote one or
more of a theoretically unbounded set of widths.
Further, I propose to add a new table to section listing the symbols defined in the header using a more formal notation that avoids introducing inconsistencies.
To this effect, in cstdint.syn
Header <cstdint> synopsis, replace both the
synopsis and paragraph 1 with the following text:
- In the names defined in the
<cstdint>header, the symbolNrepresents a positive decimal integer with no leading zeros (e.g., 8 or 24, but not 0, 04, or 048). With the exception of exact-width types, macros and types for values ofNin the set of 8, 16, 32, and 64 are required. Exact-width types, and any macros and types for values ofNother than 8, 16, 32, and 64 are optional. However, if an implementation provides integer types with widths of 8, 16, 32, or 64 bits, the corresponding exact-width types and macros are required.namespace std { // required types // Fastest minimum-width integer types typedef signed integer type int_fast8_t; typedef signed integer type int_fast16_t; typedef signed integer type int_fast32_t; typedef signed integer type int_fast64_t; typedef unsigned integer type uint_fast8_t; typedef unsigned integer type uint_fast16_t; typedef unsigned integer type uint_fast32_t; typedef unsigned integer type uint_fast64_t; // Minimum-width integer types typedef signed integer type int_least8_t; typedef signed integer type int_least16_t; typedef signed integer type int_least32_t; typedef signed integer type int_least64_t; typedef unsigned integer type uint_least8_t; typedef unsigned integer type uint_least16_t; typedef unsigned integer type uint_least32_t; typedef unsigned integer type uint_least64_t; // Greatest-width integer types typedef signed integer type intmax_t; typedef unsigned integer type uintmax_t; // optionally defined types // Exact-width integer types typedef signed integer type intN_t; typedef unsigned integer type uintN_t; // Fastest minimum-width integer types for values // of N other than 8, 16, 32, and 64 typedef signed integer type uint_fastN_t; typedef unsigned integer type uint_fastN_t; // Minimum-width integer types for values // of N other than 8, 16, 32, and 64 typedef signed integer type uint_leastN_t; typedef unsigned integer type uint_leastN_t; // Integer types capable of holding object pointers typedef signed integer type intptr_t; typedef signed integer type intptr_t; }
[Note to editor: Remove all of the existing paragraph 1 from cstdint.syn.]
Table ??: Header<cstdint>synopsis
Type Name(s) Macros: INTN_MIN INTN_MAX UINTN_MAX INT_FASTN_MIN INT_FASTN_MAX UINT_FASTN_MAX INT_LEASTN_MIN INT_LEASTN_MAX UINT_LEASTN_MAX INTPTR_MIN INTPTR_MAX UINTPTR_MAX INTMAX_MIN INTMAX_MAX UINTMAX_MAX PTRDIFF_MIN PTRDIFF_MAX PTRDIFF_MAX SIG_ATOMIC_MIN SIG_ATOMIC_MAX SIZE_MAX WCHAR_MIN WCHAR_MAX WINT_MIN WINT_MAX INTN_C() UINTN_C() INTMAX_C() UINTMAX_C() Types: intN_t uintN_t int_fastN_t uint_fastN_t int_leastN_t uint_leastN_t intptr_t uintptr_t intmax_t uintmax_t
Section: 20.7.6.6 [meta.trans.other] Status: NAD Submitter: Thorsten Ottosen Opened: 2008-06-05 Last modified: 2008-09-16
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Discussion:
The type traits library contains various traits to dealt with polymorphic types, e.g. std::has_virtual_destructor, std::is_polymorphic and std::is_base_of. However, there is no way to compute the unique public base class of a type if such one exists. Such a trait could be very useful if one needs to instantiate a specialization made for the root class whenever a derived class is passed as parameter. For example, imagine that you wanted to specialize std::hash for a class hierarchy---instead of specializing each class, you could specialize the std::hash<root_class> and provide a partial specialization that worked for all derived classes.
This ability---to specify operations in terms of their equivalent in the root class---can be done with e.g. normal functions, but there is, AFAIK, no way to do it for class templates. Being able to access compile-time information about the type-hierachy can be very powerful, and I therefore also suggest traits that computes the directly derived class whenever that is possible.
If the computation can not be done, the traits should fall back on an identity transformation. I expect this gives the best overall usability.
Proposed resolution:
Add the following to the synopsis in 20.7.2 [meta.type.synop] under "other transformations":
template< class T > struct direct_base_class; template< class T > struct direct_derived_class; template< class T > struct root_base_class;
Add three new entries to table 51 (20.7.6.6 [meta.trans.other]) with the following content
Template Condition Comments template< class T > struct direct_base_class; T shall be a complete type. The member typedef type shall equal the accessible unambiguous direct base class of T. If no such type exists, the member typedef type shall equal T. template< class T > struct direct_derived_class; T shall be a complete type. The member typedef type shall equal the unambiguous type which has T as an accessible unambiguous direct base class. If no such type exists, the member typedef type shall equal T. template< class T > struct root_base_class; T shall be a complete type. The member typedef type shall equal the accessible unambiguous most indirect base class of T. If no such type exists, the member typedef type shall equal T.
Rationale:
2008-9-16 San Francisco: Issue pulled by author prior to being reviewed by the LWG.Section: 23.3.1 [array] Status: NAD Future Submitter: Benjamin Kosnik Opened: 2008-06-05 Last modified: 2009-10-23
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Discussion:
This is an issue that came up on the libstdc++ list, where a discrepancy between "C" arrays and C++0x's std::array was pointed out.
In "C," this array usage is possible:
int ar[] = {1, 4, 6};
But for C++,
std::array<int> a = { 1, 4, 6 }; // error
Instead, the second parameter of the array template must be explicit, like so:
std::array<int, 3> a = { 1, 4, 6 };
Doug Gregor proposes the following solution, that assumes generalized initializer lists.
template<typename T, typename... Args>
inline array<T, sizeof...(Args)>
make_array(Args&&... args)
{ return { std::forward<Args>(args)... }; }
Then, the way to build an array from a list of unknown size is:
auto a = make_array<T>(1, 4, 6);
[ San Francisco: ]
Benjamin: Move to Ready?
Bjarne: I'm not convinced this is useful enough to add, so I'd like us to have time to reflect on it.
Alisdair: the constraints are wrong, they should be
template<ValueType T, ValueType... Args> requires Convertible<Args, T>... array<T, sizeof...(Args)> make_array(Args&&... args);Alidair: this would be useful if we had a constexpr version.
Bjarne: this is probably useful for arrays with a small number of elements, but it's not clearly useful otherwise.
Consensus is to move to Open.
[ 2009-06-07 Daniel adds: ]
I suggest a fix and a simplification of the current proposal: Recent prototyping by Howard showed, that a fix is required because narrowing conversion 8.5.4 [dcl.init.list]/6 b.3 would severely limit the possible distribution of argument types, e.g. the expression make_array<double>(1, 2.0) is ill-formed, because the narrowing happens inside the function body where no constant expressions exist anymore. Furthermore given e.g.
int f(); double g();we probably want to support
make_array<double>(f(), g());as well. To make this feasible, the currently suggested expansion
{ std::forward<Args>(args)... }needs to be replaced by
{ static_cast<T>(std::forward<Args>(args))... }which is safe, because we already ensure convertibility via the element-wise Convertible<Args, T> requirement. Some other fixes are necessary: The ValueType requirement for the function parameters is invalid, because all lvalue arguments will deduce to an lvalue-reference, thereby no longer satisfying this requirement.
The suggested simplification is to provide a default-computed effective type for the result array based on common_type and decay, in unconstrained form:
template<typename... Args> array<typename decay<typename common_type<Args...>::type>::type, sizeof...(Args)> make_array(Args&&... args);The approach used below is similar to that of make_pair and make_tuple using a symbol C to represent the decayed common type [Note: Special handling of reference_wrapper types is intentionally not provided, because our target has so satisfy ValueType, thus under the revised proposal only an all-reference_wrapper-arguments would be well-formed and an array of reference_wrapper will be constructed]. I do currently not suggest to add new concepts reflecting decay and common_type, but an implementor will need something like this to succeed. Note that we use a similar fuzziness for make_pair and make_tuple currently. This fuzziness is not related to the currently missing Constructible<Vi, Ti&&> requirement for those functions. The following proposal fixes that miss for make_array. If the corresponding C type deduction is explicitly wanted for standardization, here the implementation
auto concept DC<typename... T> { typename type = typename decay<typename common_type<T...>::type>::type; }where C is identical to DC<Args...>::type in the proposed resolution below.
I intentionally added no further type relation between type and the concept template parameters, but instead added this requirement below to make the specification as transparent as possible. As written this concept is satisfied, if the corresponding associated type exists.
Suggested Resolution:
Add to the array synopsis in 23.3 [sequences]:
template<ReferentType... Args> requires ValueType<C> && IdentityOf<Args> && Constructible<C, Args&&>... array<C, sizeof...(Args)> make_array(Args&&... args);Append after 23.3.1.8 [array.tuple] Tuple interface to class template array the following new section:
23.4.1.7 Array creation functions [array.creation]
template<ReferentType... Args> requires ValueType<C> && IdentityOf<Args> && Constructible<C, Args&&>... array<C, sizeof...(Args)> make_array(Args&&... args);Let C be decay<common_type<Args...>::type>::type.
Returns: an array<C, sizeof...(Args)> initialized with { static_cast<C>(std::forward<Args>(args))... }.
[ 2009-07 Frankfurt: ]
The proposed resolution uses concepts.
Daniel to rewrite the proposed resolution.
Leave Open.
[ 2009-07-25 Daniel provides rewritten proposed resolution. ]
[ 2009-10 Santa Cruz: ]
Argument for NAD future: everything about this could be added on. This does not require changes to the existing text.
Proposed resolution:
Add to the array synopsis in 23.3 [sequences]:
template<class... Args> array<CT, sizeof...(Args)> make_array(Args&&... args);
Append after 23.3.1.8 [array.tuple] "Tuple interface to class template array" the following new section:
XX.X.X.X Array creation functions [array.creation]
template<class... Args> array<CT, sizeof...(Args)> make_array(Args&&... args)Let CT be decay<common_type<Args...>::type>::type.
Returns: An array<CT, sizeof...(Args)> initialized with { static_cast<CT>(std::forward<Args>(args))... }.
[Example:
int i = 0; int& ri = i; make_array(42u, i, 2.78, ri);returns an array of type
array<double, 4>—end example]
Section: 23.3.2.2 [deque.capacity] Status: NAD Submitter: Hervé Brönnimann Opened: 2008-06-11 Last modified: 2008-09-22
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Discussion:
The main point is that capacity can be viewed as a mechanism to guarantee the validity of iterators when only push_back/pop_back operations are used. For vector, this goes with reallocation. For deque, this is a bit more subtle: capacity() of a deque may shrink, whereas that of vector doesn't. In a circular buffer impl. of the map, as Howard did, there is very similar notion of capacity: as long as size() is less than B * (total size of the map - 2), it is guaranteed that no iterator is invalidated after any number of push_front/back and pop_front/back operations. But this does not hold for other implementations.
Still, I believe, capacity() can be defined by size() + how many push_front/back minus pop_front/back that can be performed before terators are invalidated. In a classical impl., capacity() = size() + the min distance to either "physical" end of the deque (i.e., counting the empty space in the last block plus all the blocks until the end of the map of block pointers). In Howard's circular buffer impl., capacity() = B * (total size of the map - 2) still works with this definition, even though the guarantee could be made stronger.
A simple picture of a deque:
A-----|----|-----|---F+|++++|++B--|-----|-----Z
(A,Z mark the beginning/end, | the block boundaries, F=front, B=back, and - are uninitialized, + are initialized) In that picture: capacity = size() + min(dist(A,F),dist(B,Z)) = min (dist(A,B),dist(F,Z)).
Reserve(n) can grow the map of pointers and add possibly a number of empty blocks to it, in order to guarantee that the next n-size() push_back/push_front operations will not invalidate iterators, and also will not allocate (i.e. cannot throw). The second guarantee is not essential and can be left as a QoI. I know well enough existing implementations of deque (sgi/stl, roguewave, stlport, and dinkumware) to know that either can be implemented with no change to the existing class layout and code, and only a few modifications if blocks are pre-allocated (instead of always allocating a new block, check if the next entry in the map of block pointers is not zero).
Due to the difference with vector, wording is crucial. Here's a proposed wording to make things concrete; I tried to be reasonably careful but please double-check me:
[ San Francisco: ]
Hans: should the Returns clause for capacity read "1 Returns: A lower bound..." rather than "1 Returns: An upper bound..."
Howard: maybe what's needed is capacity_front and capacity_back. In fact, I think I implemented a deque that had these members as implementation details.
Proposed resolution:
Add new signatures to synopsis in 23.3.2 [deque]:
size_type capacity() const; bool reserve(size_type n);
Add new signatures to 23.3.2.2 [deque.capacity]:
size_type capacity() const;1 Returns: An upper bound on n + max(n_f - m_f, n_b - m_b) such that, for any sequence of n_f push_front, m_f pop_front, n_b push_back, and m_b pop_back operations, interleaved in any order, starting with the current deque of size n, the deque does not invalidate any of its iterators except to the erased elements.
2 Remarks: Unlike a vector's capacity, the capacity of a deque can decrease after a sequence of insertions at both ends, even if none of the operations caused the deque to invalidate any of its iterators except to the erased elements.
bool reserve(size_type n);2 Effects: A directive that informs a deque of a planned sequence of push_front, pop_front, push_back, and pop_back operations, so that it can manage iterator invalidation accordingly. After reserve(), capacity() is greater or equal to the argument of reserve if this operation returns true; and equal to the previous value of capacity() otherwise. If an exception is thrown, there are no effects.
3 Returns: true if iterators are invalidated as a result of this operation, and false otherwise.
4 Complexity: It does not change the size of the sequence and takes at most linear time in n.
5 Throws: length_error if n > max_size().
6 Remarks: It is guaranteed that no invalidation takes place during a sequence of insert or erase operations at either end that happens after a call to reserve() except to the erased elements, until the time when an insertion would make max(n_f-m_f, n_b-m_b) larger than capacity(), where n_f is the number of push_front, m_f of pop_front, n_b of push_back, and m_b of pop_back operations since the call to reserve().
7 An implementation is free to pre-allocate buffers so as to offer the additional guarantee that no exception will be thrown during such a sequence other than by the element constructors.
And 23.3.2.3 [deque.modifiers] para 1, can be enhanced:
1 Effects: An insertion in the middle of the deque invalidates all the iterators and references to elements of the deque. An insertion at either end of the deque invalidates all the iterators to the deque, unless provisions have been made with reserve, but has no effect on the validity of references to elements of the deque.
Rationale:
Complication outweighs the benefit.Section: 25.4.5.1 [includes] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2008-07-02 Last modified: 2009-07-13
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Discussion:
In 25.4.5.1 [includes] the complexity is "at most -1 comparisons" if passed two empty ranges. I don't know how to perform a negative number of comparisions!
This same issue also applies to:
[ 2009-03-30 Beman adds: ]
Suggest NAD. The complexity of empty ranges is -1 in other places in the standard. See 25.4.4 [alg.merge] merge and inplace_merge, and forward_list merge, for example. The time and effort to find and fix all places in the standard where empty range[s] result in negative complexity isn't worth the very limited benefit.
[ 2009-05-09 Alisdair adds: ]
I'm not happy with NAD if we can find a simple solution.
How about adding a rider somewhere in clause 17 suggesting that complexities that specify a negative number of operations are treated as specifying zero operations? That should generically solve the issue without looking for further cases.
[ Batavia (2009-05): ]
Pete to provide "straightforward" wording. Move to NAD Editorial.
Proposed resolution:
Recommend NAD.
Section: 27.9.1 [fstreams] Status: NAD Submitter: Steve Clamage Opened: 2008-07-08 Last modified: 2009-07-13
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Discussion:
Suppose writing to an [o]fstream fails and you later close the stream. The overflow() function is called to flush the buffer (if it exists). Then the file is unconditionally closed, as if by calling flcose.
If either overflow or fclose fails, close() reports failure, and clearly the stream should be in a failed or bad state.
Suppose the buffer is empty or non-existent (so that overflow() does not fail), and fclose succeeds. The close() function reports success, but what is the state of the stream?
[ Batavia (2009-05): ]
Tom's impression is that the issue is about the failbit, etc.
Bill responds that the stream is now closed, and any status bits remain unchanged.
See the description of close() in 27.9.1.17 [fstream.members].
We prefer not to add wording to say that nothing changes. Move to NAD.
Proposed resolution:
Section: 29.6 [atomics.types.operations] Status: NAD Editorial Submitter: Anthony Williams Opened: 2008-07-10 Last modified: 2008-09-17
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Discussion:
There's an error in 29.6 [atomics.types.operations]/p9:
C atomic_load(const volatile A * object); C atomic_load_explicit(const volatile A * object, memory_order); C A ::load(memory_order order = memory_order_seq_cst) const volatile;Requires: The order argument shall not be memory_order_acquire nor memory_order_acq_rel.
I believe that this should state
shall not be memory_order_release.
There's also an error in 29.6 [atomics.types.operations]/p17:
... When only one memory_order argument is supplied, the value of success is order, and the value of failure is order except that a value of memory_order_acq_rel shall be replaced by the value memory_order_require ...
I believe this should state
shall be replaced by the value memory_order_acquire ...
Proposed resolution:
Change 29.6 [atomics.types.operations]/p9:
C atomic_load(const volatile A * object); C atomic_load_explicit(const volatile A * object, memory_order); C A ::load(memory_order order = memory_order_seq_cst) const volatile;Requires: The order argument shall not be
memory_order_acquirememory_order_release nor memory_order_acq_rel.
Change 29.6 [atomics.types.operations]/p17:
... When only one memory_order argument is supplied, the value of success is order, and the value of failure is order except that a value of memory_order_acq_rel shall be replaced by the valuememory_order_requirememory_order_acquire ...
Rationale:
Already fixed by the time the LWG processed it.Section: 26.6.2.1 [valarray.cons] Status: NAD Editorial Submitter: Alberto Ganesh Barbati Opened: 2008-07-20 Last modified: 2009-07-13
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Discussion:
From 26.6.2.1 [valarray.cons], paragraph 2:
explicit valarray(size_t);The array created by this constructor has a length equal to the value of the argument. The elements of the array are constructed using the default constructor for the instantiating type T.
The problem is that the most obvious Ts for valarray are float and double, they don't have a default constructor. I guess the intent is to value-initialize the elements, so I suggest replacing:
The elements of the array are constructed using the default constructor for the instantiating type T.
with
The elements of the array are value-initialized.
There is another reference to the default constructor of T in the non-normative note in paragraph 9. That reference should also be replaced. (The normative wording in paragraph 8 refers to T() and so it doesn't need changes).
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to NAD Editorial.
Proposed resolution:
Change 26.6.2.1 [valarray.cons], paragraph 2:
explicit valarray(size_t);The array created by this constructor has a length equal to the value of the argument. The elements of the array areconstructed using the default constructor for the instantiating type Tvalue-initialized (8.5 [dcl.init]).
Change 26.6.2.7 [valarray.members], paragraph 9:
[Example: If the argument has the value -2, the first two elements of the result will beconstructed using the default constructorvalue-initialized (8.5 [dcl.init]); the third element of the result will be assigned the value of the first element of the argument; etc. -- end example]
Section: 3.9.1 [basic.fundamental] Status: NAD Editorial Submitter: Travis Vitek Opened: 2008-06-30 Last modified: 2009-07-17
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Discussion:
Neither the term "signed integral type" nor the term "unsigned integral type" is defined in the core language section of the standard, therefore the library section should avoid its use. The terms signed integer type and unsigned integer type are indeed defined (in 3.9.1 [basic.fundamental]), thus the usages should be replaced accordingly.
Note that the key issue here is that "signed" + "integral type" !=
"signed integral type".
The types bool, char, char16_t,
char32_t and wchar_t are all listed as
integral types, but are neither of signed integer type or
unsigned integer type. According to 3.9 [basic.types] p7, a synonym for
integral type is integer type.
Given this, one may choose to assume that an integral type that
can represent values less than zero is a signed integral type.
Unfortunately this can cause ambiguities.
As an example, if T is unsigned char, the
expression make_signed<T>::type, is supposed to
name a signed integral type. There are potentially two types that
satisfy this requirement, namely signed char and
char (assuming CHAR_MIN < 0).
[ San Francisco: ]
Plum, Sebor to review.
[ Post Summit Daniel adds: ]
The proposed resolution needs to be "conceptualized". Currently we have in [concept.support] only concept IntegralType for all "integral types", thus indeed the current Container concept and Iterator concepts are sufficiently satisfied with "integral types". If the changes are applied, we might ask core for concept BilateralIntegerType and add proper restrictions to the library concepts.
Proposed resolution:
I propose to use the terms "signed integer type" and "unsigned integer type" in place of "signed integral type" and "unsigned integral type" to eliminate such ambiguities.
The proposed change makes it absolutely clear that the difference between two pointers cannot be char or wchar_t, but could be any of the signed integer types. 5.7 [expr.add] paragraph 6...
- When two pointers to elements of the same array object are subtracted, the result is the difference of the subscripts of the two array elements. The type of the result is an implementation-defined
signed integral typesigned integer type; this type shall be the same type that is defined asstd::ptrdiff_tin the<cstdint>header (18.1)...
The proposed change makes it clear that X::size_type and X::difference_type cannot be char or wchar_t, but could be one of the signed or unsigned integer types as appropriate. 20.2.5 [allocator.requirements] table 40...
Table 40: Allocator requirements
expression return type assertion/note/pre/post-condition X::size_type unsigned integral typeunsigned integer typea type that can represent the size of the largest object in the allocation model. X::difference_type signed integral typesigned integer typea type that can represent the difference between any two pointers in the allocation model.
The proposed change makes it clear that make_signed<T>::type must be one of the signed integer types as defined in 3.9.1. Ditto for make_unsigned<T>type and unsigned integer types. 20.7.6.3 [meta.trans.sign] table 48...
Table 48: Sign modifications
Template Comments template <class T> struct make_signed; If Tnames a (possibly cv-qualified)signed integral typesigned integer type (3.9.1) then the member typedeftypeshall name the typeT; otherwise, ifTnames a (possibly cv-qualified)unsigned integral typeunsigned integer type thentypeshall name the correspondingsigned integral typesigned integer type, with the same cv-qualifiers asT; otherwise,typeshall name thesigned integral typesigned integer type with the smallest rank (4.13) for whichsizeof(T) == sizeof(type), with the same cv-qualifiers asT. Requires:Tshall be a (possibly cv-qualified) integral type or enumeration but not abooltype.template <class T> struct make_unsigned; If Tnames a (possibly cv-qualified)unsigned integral typeunsigned integer type (3.9.1) then the member typedeftypeshall name the typeT; otherwise, ifTnames a (possibly cv-qualified)signed integral typesigned integer type thentypeshall name the correspondingunsigned integral typeunsigned integer type, with the same cv-qualifiers asT; otherwise,typeshall name theunsigned integral typeunsigned integer type with the smallest rank (4.13) for whichsizeof(T) == sizeof(type), with the same cv-qualifiers asT. Requires:Tshall be a (possibly cv-qualified) integral type or enumeration but not abooltype.
Note: I believe that the basefield values should probably be prefixed with ios_base:: as they are in 22.4.2.2.2 [facet.num.put.virtuals] The listed virtuals are all overloaded on signed and unsigned integer types, the new wording just maintains consistency. 22.4.2.1.2 [facet.num.get.virtuals] table 78...
Table 78: Integer Conversions
State stdio equivalent basefield == oct %o basefield == hex %X basefield == 0 %i signed integral typesigned integer type%d unsigned integral typeunsigned integer type%u
Rationale is same as above. 22.4.2.2.2 [facet.num.put.virtuals] table 80...
Table 80: Integer Conversions
State stdio equivalent basefield == ios_base::oct %o (basefield == ios_base::hex) && !uppercase %x (basefield == ios_base::hex) %X basefield == 0 %i for a signed integral typesigned integer type%d for a unsigned integral typeunsigned integer type%u
23.2 [container.requirements] table 80...
Table 89: Container requirements
expression return type operational semantics assertion/note/pre/post-condition complexity X::difference_type signed integral typesigned integer typeis identical to the difference type of X::iterator and X::const_iterator compile time X::size_type unsigned integral typeunsigned integer typesize_type can represent any non-negative value of difference_type compile time
X [iterator.concepts] paragraph 1...
Iterators are a generalization of pointers that allow a C++ program to work with different data structures (containers) in a uniform manner. To be able to construct template algorithms that work correctly and efficiently on different types of data structures, the library formalizes not just the interfaces but also the semantics and complexity assumptions of iterators. All input iteratorsisupport the expression*i, resulting in a value of some class, enumeration, or built-in typeT, called the value type of the iterator. All output iterators support the expression*i = owhereois a value of some type that is in the set of types that are writable to the particular iterator type ofi. All iteratorsifor which the expression(*i).mis well-defined, support the expressioni->mwith the same semantics as(*i).m. For every iterator typeXfor which equality is defined, there is a correspondingsigned integral typesigned integer type called the difference type of the iterator.
I'm a little unsure of this change. Previously this paragraph would allow instantiations of linear_congruential_engine on char, wchar_t, bool, and other types. The new wording prohibits this. 26.5.3.1 [rand.eng.lcong] paragraph 2...
The template parameterUIntTypeshall denote anunsigned integral typeunsigned integer type large enough to store values as large asm - 1. If the template parametermis 0, the modulusmused throughout this section 26.4.3.1 isnumeric_limits<result_type>::max()plus 1. [Note: The result need not be representable as a value of typeresult_type. --end note] Otherwise, the following relations shall hold:a < mandc < m.
Same rationale as the previous change. X [rand.adapt.xor] paragraph 6...
BothEngine1::result_typeandEngine2::result_typeshall denote (possibly different)unsigned integral typesunsigned integer types. The member result_type shall denote either the type Engine1::result_type or the type Engine2::result_type, whichever provides the most storage according to clause 3.9.1.
26.5.7.1 [rand.util.seedseq] paragraph 7...
Requires:RandomAccessIteratorshall meet the requirements of a random access iterator (24.1.5) such thatiterator_traits<RandomAccessIterator>::value_typeshall denote anunsigned integral typeunsigned integer type capable of accomodating 32-bit quantities.
By making this change, integral types that happen to have a signed representation, but are not signed integer types, would no longer be required to use a two's complement representation. This may go against the original intent, and should be reviewed. 29.6 [atomics.types.operations] paragraph 24...
Remark: Forsigned integral typessigned integer types, arithmetic is defined using two's complement representation. There are no undefined results. For address types, the result may be an undefined address, but the operations otherwise have no undefined behavior.
Section: 26.5.8.5.1 [rand.dist.samp.discrete] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2009-03-09
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Discussion:
During the Sophia Antipolis meeting it was decided to separate from 793 a subrequest that adds initializer list support to discrete_distribution, specifically, the issue proposed to add a c'tor taking a initializer_list<double>.
Proposed resolution:
In 26.5.8.5.1 [rand.dist.samp.discrete]/1, class discrete_distribution, just before the member declaration
explicit discrete_distribution(const param_type& parm);
insert
discrete_distribution(initializer_list<double> wl);
Between p.4 and p.5 of the same section insert a new paragraph as part of the new member description:
discrete_distribution(initializer_list<double> wl);Effects: Same as discrete_distribution(wl.begin(), wl.end()).
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".Section: 26.5.8.5.2 [rand.dist.samp.pconst] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2009-03-09
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Discussion:
During the Sophia Antipolis meeting it was decided to separate from 794 a subrequest that adds initializer list support to piecewise_constant_distribution, specifically, the issue proposed to add a c'tor taking a initializer_list<double> and a Callable to evaluate weight values. For consistency with the remainder of this class and the remainder of the initializer_list-aware library the author decided to change the list argument type to the template parameter RealType instead. For the reasoning to use Func instead of Func&& as c'tor function argument see issue 793.
Proposed resolution:
Non-concept version of the proposed resolution
In 26.5.8.5.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution, just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert
template<typename Func> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);
Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:
template<typename Func> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);[p5_1] Complexity: Exactly nf = max(bl.size(), 1) - 1 invocations of fw.
[p5_2] Requires:
- fw shall be callable with one argument of type RealType, and shall return values of a type convertible to double;
- The relation 0 < S = w0+. . .+wn-1 shall hold. For all sampled values xk defined below, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- If nf > 0 let bk = *(bl.begin() + k), k = 0, . . . , bl.size()-1 and the following relations shall hold for k = 0, . . . , nf-1: bk < bk+1.
[p5_3] Effects:
If nf == 0,
- lets the sequence w have length n = 1 and consist of the single value w0 = 1, and
- lets the sequence b have length n+1 with b0 = 0 and b1 = 1.
Otherwise,
- sets n = nf, and [bl.begin(), bl.end()) shall form the sequence b of length n+1, and
lets the sequences w have length n and for each k = 0, . . . ,n-1, calculates:
xk = 0.5*(bk+1 + bk) wk = fw(xk)Constructs a piecewise_constant_distribution object with the above computed sequence b as the interval boundaries and with the probability densities:
ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.
Concept version of the proposed resolution
In 26.5.8.5.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution, just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);
Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);[p5_1] Complexity: Exactly nf = max(bl.size(), 1) - 1 invocations of fw.
[p5_2] Requires:
- The relation 0 < S = w0+. . .+wn-1 shall hold. For all sampled values xk defined below, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- If nf > 0 let bk = *(bl.begin() + k), k = 0, . . . , bl.size()-1 and the following relations shall hold for k = 0, . . . , nf-1: bk < bk+1.
[p5_3] Effects:
If nf == 0,
- lets the sequence w have length n = 1 and consist of the single value w0 = 1, and
- lets the sequence b have length n+1 with b0 = 0 and b1 = 1.
Otherwise,
- sets n = nf, and [bl.begin(), bl.end()) shall form the sequence b of length n+1, and
lets the sequences w have length n and for each k = 0, . . . ,n-1, calculates:
xk = 0.5*(bk+1 + bk) wk = fw(xk)Constructs a piecewise_constant_distribution object with the above computed sequence b as the interval boundaries and with the probability densities:
ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".Section: 17 [library] Status: NAD Future Submitter: Martin Sebor Opened: 2008-08-23 Last modified: 2009-07-17
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Discussion:
Recent changes to the working draft have introduced a gratuitous inconsistency with the C++ 2003 version of the specification with respect to exception guarantees provided by standard functions. While the C++ 2003 standard consistenly uses the empty exception specification, throw(), to declare functions that are guaranteed not to throw exceptions, the current working draft contains a number of "Throws: Nothing." clause to specify essentially the same requirement. The difference between the two approaches is that the former specifies the behavior of programs that violate the requirement (std::unexpected() is called) while the latter leaves the behavior undefined.
A survey of the working draft reveals that there are a total of 209 occurrences of throw() in the library portion of the spec, the majority in clause 18, a couple (literally) in 19, a handful in 20, a bunch in 22, four in 24, one in 27, and about a dozen in D.9.
There are also 203 occurrences of "Throws: Nothing." scattered throughout the spec.
While sometimes there are good reasons to use the "Throws: Nothing." approach rather than making use of throw(), these reasons do not apply in most of the cases where this new clause has been introduced and the empty exception specification would be a better approach.
First, functions declared with the empty exception specification permit compilers to generate better code for calls to such functions. In some cases, the compiler might even be able to eliminate whole chunks of user-written code when instantiating a generic template on a type whose operations invoked from the template specialization are known not to throw. The prototypical example are the std::uninitialized_copy() and std::uninitialized_fill() algorithms where the entire catch(...) block can be optimized away.
For example, given the following definition of the std::uninitialized_copy function template and a user-defined type SomeType:
template <class InputIterator, class ForwardIterator>
ForwardIterator
uninitialized_copy (InputIterator first, InputIterator last, ForwardIterator res)
{
typedef iterator_traits<ForwardIterator>::value_type ValueType;
ForwardIterator start = res;
try {
for (; first != last; ++first, ++res)
::new (&*res) ValueType (*first);
}
catch (...) {
for (; start != res; --start)
(&*start)->~ValueType ();
throw;
}
return res;
}
struct SomeType {
SomeType (const SomeType&) throw ();
}
compilers are able to emit the following efficient specialization of std::uninitialized_copy<const SomeType*, SomeType*> (note that the catch block has been optimized away):
template <> SomeType*
uninitialized_copy (const SomeType *first, const SomeType *last, SomeType *res)
{
for (; first != last; ++first, ++res)
::new (res) SomeType (*first);
return res;
}
Another general example is default constructors which, when decorated with throw(), allow the compiler to eliminate the implicit try and catch blocks that it otherwise must emit around each the invocation of the constructor in new-expressions.
For example, given the following definitions of class MayThrow and WontThrow and the two statements below:
struct MayThrow {
MayThrow ();
};
struct WontThrow {
WontThrow () throw ();
};
MayThrow *a = new MayThrow [N];
WontThrow *b = new WontThrow [N];
the compiler generates the following code for the first statement:
MayThrow *a;
{
MayThrow *first = operator new[] (N * sizeof (*a));
MayThrow *last = first + N;
MayThrow *next = first;
try {
for ( ; next != last; ++next)
new (next) MayThrow;
}
catch (...) {
for ( ; first != first; --next)
next->~MayThrow ();
operator delete[] (first);
throw;
}
a = first;
}
but it is can generate much more compact code for the second statement:
WontThrow *b = operator new[] (N * sizeof (*b)); WontThrow *last = b + N; for (WontThrow *next = b; next != last; ++next) new (next) WontThrow;
Second, in order for users to get the maximum benefit out of the new std::has_nothrow_xxx traits when using standard library types it will be important for implementations to decorate all non throwing copy constructors and assignment operators with throw(). Note that while an optimizer may be able to tell whether a function without an explicit exception specification can throw or not based on its definition, it can only do so when it can see the source code of the definition. When it can't it must assume that the function may throw. To prevent violating the One Definition Rule, the std::has_nothrow_xxx trait must return the most pessimistic guess across all translation units in the program, meaning that std::has_nothrow_xxx<T>::value must evaluate to false for any T whose xxx (where xxx is default or copy ctor, or assignment operator) is defined out-of-line.
Counterarguments:
During the discussion of this issue on c++std-lib@accu.org (starting with post c++std-lib-21950) the following arguments in favor of the "Throws: Nothing." style have been made.
The answer to point (1) above is that implementers can (and some have) declare functions with throw() to indicate to the compiler that calls to the function can safely be assumed not to throw in order to allow it to generate efficient code at the call site without also having to define the functions the same way and causing the compiler to generate suboptimal code for the function definition. That is, the function is declared with throw() in a header but it's defined without it in the source file. The throw() declaration is suppressed when compiling the definition to avoid compiler errors. This technique, while strictly speaking no permitted by the language, is safe and has been employed in practice. For example, the GNU C library takes this approach. Microsoft Visual C++ takes a similar approach by simply assuming that no function with C language linkage can throw an exception unless it's explicitly declared to do so using the language extension throw(...).
Our answer to point (2) above is that there is no existing practice where C++ Standard Library implementers have opted to make use of the proprietary mechanisms to declare functions that don't throw. The language provides a mechanism specifically designed for this purpose. Avoiding its use in the specification itself in favor of proprietary mechanisms defeats the purpose of the feature. In addition, making use of the empty exception specification inconsistently, in some areas of the standard, while conspicuously avoiding it and making use of the "Throws: Nothing." form in others is confusing to users.
The answer to point (3) is simply to exercise caution when declaring functions and especially function templates with the empty exception specification. Functions that required not to throw but that may call back into user code are poor candidates for the empty exception specification and should instead be specified using "Throws: Nothing." clause.
[ 2009-07 Frankfurt ]
We need someone to do an extensive review.
NAD Future.
Proposed resolution:
We propose two possible solutions. Our recommendation is to adopt Option 1 below.
Option 1:
Except for functions or function templates that make calls back to user-defined functions that may not be declared throw() replace all occurrences of the "Throws: Nothing." clause with the empty exception specification. Functions that are required not to throw but that make calls back to user code should be specified to "Throw: Nothing."
Option 2:
For consistency, replace all occurrences of the empty exception specification with a "Throws: Nothing." clause.
Section: 29 [atomics] Status: NAD Editorial Submitter: Alexander Chemeris Opened: 2008-08-24 Last modified: 2009-10-26
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Discussion:
The atomic_address type and atomic<T*> specialization provide atomic updates to pointers. However, the current specification requires that the types pointer be to non-const objects. This restriction is unnecessary and unintended.
[ Summit: ]
Move to review. Lawrence will first check with Peter whether the current examples are sufficient, or whether they need to be expanded to include all cases.
[ 2009-07 Frankfurt ]
Lawrence will handle all issues relating to atomics in a single paper.
LWG will defer discussion on atomics until that paper appears.
Move to Open.
[ 2009-08-17 Handled by N2925. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Add const qualification to the pointer values of the atomic_address and atomic<T*> specializations. E.g.
typedef struct atomic_address {
void store(const void*, memory_order = memory_order_seq_cst) volatile;
void* exchange( const void*, memory_order = memory_order_seq_cst) volatile;
bool compare_exchange( const void*&, const void*,
memory_order, memory_order) volatile;
bool compare_exchange( const void*&, const void*,
memory_order = memory_order_seq_cst ) volatile;
void* operator=(const void*) volatile;
} atomic_address;
void atomic_store(volatile atomic_address*, const void*);
void atomic_store_explicit(volatile atomic_address*, const void*,
memory_order);
void* atomic_exchange(volatile atomic_address*, const void*);
void* atomic_exchange_explicit(volatile atomic_address*, const void*,
memory_order);
bool atomic_compare_exchange(volatile atomic_address*,
const void**, const void*);
bool atomic_compare_exchange_explicit(volatile atomic_address*,
const void**, const void*,
memory_order, memory_order);
Section: 29 [atomics] Status: NAD Editorial Submitter: Lawrence Crowl Opened: 2008-08-24 Last modified: 2009-10-26
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Duplicate of: 942
Discussion:
The atomic_exchange and atomic_exchange_explicit functions seem to be inconsistently missing parameters.
[ Post Summit: ]
Lawrence: Need to write up a list for Pete with details.
Detlef: Should not be New, we already talked about in Concurrency group.
Recommend Open.
[ 2009-07 Frankfurt ]
Lawrence will handle all issues relating to atomics in a single paper.
LWG will defer discussion on atomics until that paper appears.
Move to Open.
[ 2009-08-17 Handled by N2925. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Add the appropriate parameters. For example,
bool atomic_exchange(volatile atomic_bool*, bool); bool atomic_exchange_explicit(volatile atomic_bool*, bool, memory_order);
Section: 20.9.11.2.4 [util.smartptr.shared.mod] Status: NAD Editorial Submitter: Jonathan Wakely Opened: 2008-09-15 Last modified: 2009-07-13
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Discussion:
#include <memory>
#include <cassert>
struct A { };
struct B : A { };
int main()
{
std::shared_ptr<A> pa(new A);
std::shared_ptr<B> pb(new B);
std::swap<A>(pa, pb); // N.B. no argument deduction
assert( pa.get() == pb.get() );
return 0;
}
Is this behaviour correct (I believe it is) and if so, is it unavoidable, or not worth worrying about?
This calls the lvalue/rvalue swap overload for shared_ptr:
template<class T> void swap( shared_ptr<T> & a, shared_ptr<T> && b );
silently converting the second argument from shared_ptr<B> to shared_ptr<A> and binding the rvalue ref to the produced temporary.
This is not, in my opinion, a shared_ptr problem; it is a general issue with the rvalue swap overloads. Do we want to prevent this code from compiling? If so, how?
Perhaps we should limit rvalue args to swap to those types that would benefit from the "swap trick". Or, since we now have shrink_to_fit(), just eliminate the rvalue swap overloads altogether. The original motivation was:
N1690.vector<A> v = ...; ... swap(v, vector<A>(v));
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to NAD Editorial.
Proposed resolution:
Recommend NAD Editorial, fixed by N2844.
Section: 30.5.1 [thread.condition.condvar] Status: NAD Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2010-03-08
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Discussion:
The Posix/C++ working group has identified an inconsistency between Posix and the C++ working draft in that Posix requires the clock to be identified at creation, whereas C++ permits identifying the clock at the call to wait. The latter cannot be implemented with the former.
[ San Francisco: ]
Howard recommends NAD with the following explanation:
The intent of the current wording is for the condtion_variable::wait_until be able to handle user-defined clocks as well as clocks the system knows about. This can be done by providing overloads for the known clocks, and another overload for unknown clocks which synchs to a known clock before waiting. For example:
template <class Duration> bool condition_variable::wait_until(unique_lock<mutex>& lock, const chrono::time_point<chrono::system_clock, Duration>& abs_time) { using namespace chrono; nanoseconds d = __round_up<nanoseconds>(abs_time.time_since_epoch()); __do_timed_wait(lock.mutex()->native_handle(), time_point<system_clock, nanoseconds>(d)); return system_clock::now() < abs_time; } template <class Clock, class Duration> bool condition_variable::wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time) { using namespace chrono; system_clock::time_point s_entry = system_clock::now(); typename Clock::time_point c_entry = Clock::now(); nanoseconds dn = __round_up<nanoseconds>(abs_time.time_since_epoch() - c_entry.time_since_epoch()); __do_timed_wait(lock.mutex()->native_handle(), s_entry + dn); return Clock::now() < abs_time; }In the above example, system_clock is the only clock which the underlying condition variable knows how to deal with. One overload just passes that clock through. The second overload (approximately) converts the unknown clock into a system_clock time_point prior to passing it down to the native condition variable.
On Posix systems vendors are free to add implementation defined constructors which take a clock. That clock can be stored in the condition_variable, and converted to (or not as necessary) as shown above.
If an implementation defined constructor takes a clock (for example), then part of the semantics for that implementation defined ctor might include that a wait_until using a clock other than the one constructed with results in an error (exceptional condition) instead of a conversion to the stored clock. Such a design is up to the vendor as once an implementation defined ctor is used, the vendor is free to specifiy the behavior of waits and/or notifies however he pleases (when the cv is constructed in an implementation defined manner).
[ Post Summit: ]
"POSIX people will review the proposed NAD resolution at their upcoming NY meeting.
See the minutes at: http://wiki.dinkumware.com/twiki/bin/view/Posix/POSIX-CppBindingWorkingGroupNewYork2009.
[ 2009-07 Frankfurt ]
Move to NAD.
[ 2009-07-18 Detlef reopens the issue: ]
On Friday afternoon in Frankfurt is was decided that 887 is NAD. This decision was mainly based on a sample implementation presented by Howard that implemented one clock on top of another. Unfortunately this implementation doesn't work for the probably most important case where a system has a monotonic clock and a real-time clock (or "wall time" clock):
If the underlying "system_clock" is a monotonic clock, and the program waits on the real-time clock, and the real-time clock is set forward, the wait will unblock too late.
If the underlying "system_clock" is a real-time clock, and the program waits on the monotonic clock, and the real-time clock is set back, the wait again will unblock too late.
Sorry that I didn't remember this on Friday, but it was Friday afternoon after a busy week...
So as the decision was made on a wrong asumption, I propose to re-open the issue.
[ 2009-07-26 Howard adds: ]
Detlef correctly argues that condition_variable::wait_until could return "too late" in the context of clocks being adjusted during the wait. I agree with his logic. But I disagree that this makes this interface unimplementable on POSIX.
The POSIX spec also does not guarantee that pthread_cond_timedwait does not return "too late" when clocks are readjusted during the wait. Indeed, the POSIX specification lacks any requirements at all concerning how soon pthread_cond_timedwait returns after a time out. This is evidently a QOI issue by the POSIX standard. Here is a quote of the most relevant normative text concerning pthread_cond_timedwait found here.
The pthread_cond_timedwait() function shall be equivalent to pthread_cond_wait(), except that an error is returned if the absolute time specified by abstime passes (that is, system time equals or exceeds abstime) before the condition cond is signaled or broadcasted, or if the absolute time specified by abstime has already been passed at the time of the call.I.e. the POSIX specification speaks of the error code returned in case of a time out, but not on the timeliness of that return.
Might this simply be an oversight, or minor defect in the POSIX specification?
I do not believe so. This same section goes on to say in non-normative text:
For cases when the system clock is advanced discontinuously by an operator, it is expected that implementations process any timed wait expiring at an intervening time as if that time had actually occurred.Here is non-normative wording encouraging the implementation to ignore an advancing underlying clock and subsequently causing an early (spurious) return. There is no wording at all which addresses Detlef's example of a "late return". With pthread_cond_timedwait this would be caused by setting the system clock backwards. It seems reasonable to assume, based on the wording that is already in the POSIX spec, that again, the discontinuously changed clock would be ignored by pthread_cond_timedwait.
A noteworthy difference between pthread_cond_timedwait and condition_variable::wait_until is that the POSIX spec appears to say that ETIMEDOUT should be returned if pthread_cond_timedwait returns because of timeout signal, whether or not the system clock was discontinuously advanced during the wait. In contrast condition_variable::wait_until always returns:
Clock::now() < abs_timeThat is, the C++ spec requires that the clock be rechecked (detecting discontinuous adjustments during the wait) at the time of return. condition_variable::wait_until may indeed return early or late. But regardless it will return a value reflecting timeout status at the time of return (even if clocks have been adjusted). Of course the clock may be adjusted after the return value is computed but before the client has a chance to read the result of the return. Thus there are no iron-clad guarantees here.
condition_variable::wait_until (and pthread_cond_timedwait) is little more than a convenience function for making sure condition_variable::wait doesn't hang for an unreasonable amount of time (where the client gets to define "unreasonable"). I do not think it is in anyone's interest to try to make it into anything more than that.
I maintain that this is a useful and flexible specification in the spirit of C++, and is implementable on POSIX. The implementation technique described above is a reasonable approach. There may also be higher quality approaches. This specification, like the POSIX specification, gives a wide latitude for QOI.
I continue to recommend NAD, but would not object to a clarifying note regarding the behavior of condition_variable::wait_until. At the moment, I do not have good wording for such a note, but welcome suggestions.
[ 2009-09-30: See N2969. ]
[ 2009-10 Santa Cruz: ]
The LWG is in favor of Detlef to supply revision which adopts Option 2 from N2969 but is modified by saying that system_clock must be available for wait_until.
[ 2010-02-11 Anthony provided wording. ]
[ 2010-02-22 Anthony adds: ]
I am strongly against N2999.
Firstly, I think that the most appropriate use of a timed wait on a condition variable is with a monotonic clock, so it ought to be guaranteed to be available on systems that support such a clock. Also, making the set of supported clocks implementation defined essentially kills portability around the use of user-defined clocks.
I also think that wait_for is potentially useful, and trivially implementable given a working templated wait_until and a monotonic clock.
I also disagree with many of Detlef's points in the rationale. In a system with hard latency limits there is likely to be a monotonic clock, otherwise you have no way of measuring against these latency limits since the system_clock may change arbitrarily. In such systems, you want to be able to use wait_for, or wait_until with a monotonic clock.
I disagree that the wait_* functions cannot be implemented correctly on top of POSIX: I have done so. The only guarantee in the working draft is that when the function returns certain properties are true; there is no guarantee that the function will return immediately that the properties are true. My resolution to issue 887 makes this clear. How small the latency is is QoI.
On systems without a monotonic clock, you cannot measure the problem since the system clock can change arbitrarily so any timing calculations you make may be wrong due to clock changes.
On systems with a monotonic clock, you can choose to use it for your condition variables. If you are waiting against a system_clock::time_point then you can check the clock when waking, and either return as a timeout or spurious wake depending on whether system_clock::now() is before or after the specified time_point.
Windows does provide condition variables from Vista onwards. I choose not to use them, but they are there. If people are concerned about implementation difficulty, the Boost implementation can be used for most purposes; the Boost license is pretty liberal in that regard.
My preferred resolution to issue 887 is currently the PR in the issues list.
[ 2010 Pittsburgh: ]
There is no consensus for moving the related paper N2999 into the WP.
There was support for moving this issue as proposed to Ready, but the support was insufficient to call a consensus.
There was consensus for moving this issue to NAD as opposed to leaving it open. Rationale added.
Rationale:
The standard as written is sufficiently implementable and self consistent.
Proposed resolution:
Add a new paragraph after 30.2.4 [thread.req.timing]p3:
3 The resolution of timing provided by an implementation depends on both operating system and hardware. The finest resolution provided by an implementation is called the native resolution.
If a function in this clause takes a timeout argument, and the time point or elapsed time specified passes before the function returns, the latency between the timeout occurring and the function returning is unspecified [Note: Implementations should strive to keep such latency as small as possible, but portable code should not rely on any specific upper limits — end note]
Section: 30.3.1.1 [thread.thread.id] Status: NAD Editorial Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2010-03-08
View all other issues in [thread.thread.id].
View all issues with NAD Editorial status.
Discussion:
Addresses UK 324
The thread::id type supports the full set of comparison operators. This is substantially more than is required for the associative containers that justified them. Please place an issue against the threads library.
[ San Francisco: ]
Would depend on proposed extension to POSIX, or non-standard extension. What about hash? POSIX discussing op. POSIX not known to be considering support needed for hash, op.
Group expresses support for putting ids in both unordered and ordered containers.
[ post San Francisco: ]
Howard: It turns out the current working paper N2723 already has hash<thread::id> (20.8 [function.objects], 20.8.15 [unord.hash]). We simply overlooked it in the meeting. It is a good thing we voted in favor of it (again). :-)
Recommend NAD.
[ Post Summit: ]
Recommend to close as NAD. For POSIX, see if we need to add a function to convert pthread_t to integer.
[ Post Summit, Alisdair adds: ]
The recommendation for LWG-889/UK-324 is NAD, already specified.
It is not clear to me that the specification is complete.
In particular, the synopsis of <functional> in 20.8 [function.objects] does not mention hash< thread::id > nor hash< error_code >, although their existence is implied by 20.8.15 [unord.hash], p1.
I am fairly uncomfortable putting the declaration for the thread_id specialization into <functional> as id is a nested class inside std::thread, so it implies that <functional> would require the definition of the thread class template in order to forward declared thread::id and form this specialization.
It seems better to me that the dependency goes the other way around (<thread> will more typically make use of <functional> than vice-versa) and the hash<thread::id> specialization be declared in the <thread> header.
I think hash<error_code> could go into either <system_error> or <functional> and have no immediate preference either way. However, it should clearly appear in the synopsis of one of these two.
Recommend moving 889 back to open, and tying in a reference to UK-324.
[ Batavia (2009-05): ]
Howard observes that thread::id need not be a nested class; it could be a typedef for a more visible type.
[ 2009-05-24 Alisdair adds: ]
I do not believe this is correct. thread::id is explicitly documents as a nested class, rather than as an unspecified typedef analogous to an iterator. If the intent is that this is not implemented as a nested class (under the as-if freedoms) then this is a novel form of standardese.
[ 2009-07 Frankfurt ]
Decided we want to move hash specialization for thread_id to the thread header. Alisdair to provide wording.
[ 2009-07-28 Alisdair provided wording, moved to Review. ]
[ 2009-10 Santa Cruz: ]
Add a strike for hash<thread::id>. Move to Ready
[ 2009-11-13 The proposed wording of 1182 is a superset of the wording in this issue. ]
[ 2010-02-09 Moved from Ready to Open: ]
Issue 1182 is not quite a superset of this issue and it is controversial whether or not the note:
hash template specialization allows thread::id objects to be used as keys in unordered containers.should be added to the WP.
[ 2010-02-09 Objections to moving this to NAD Editorial, solved by 1182 have been removed. Set to Tentatively NAD Editorial. ]
Rationale:
Solved by 1182.
Proposed resolution:
Remove the following prototype from the synopsis in 20.8 [function.objects]:
template <> struct hash<std::thread::id>;
Add to 30.3 [thread.threads], p1 Header <thread> synopsis:
template <class T> struct hash; template <> struct hash<thread::id>;
Add template specialization below class definition in 30.3.1.1 [thread.thread.id]
template <>
struct hash<thread::id> : public unary_function<thread::id, size_t> {
size_t operator()(thread::id val) const;
};
Extend note in p2 30.3.1.1 [thread.thread.id] with second sentence:
[Note: Relational operators allow thread::id objects to be used as keys in associative containers. hash template specialization allows thread::id objects to be used as keys in unordered containers. — end note]
Add new paragraph to end of 30.3.1.1 [thread.thread.id]
template <> struct hash<thread::id>;An explicit specialization of the class template hash (20.8.15 [unord.hash]) shall be provided for the type thread::id.
Section: 23.3.3.5 [forwardlist.ops] Status: NAD Editorial Submitter: Ed Smith-Rowland Opened: 2008-09-15 Last modified: 2009-03-09
View all other issues in [forwardlist.ops].
View all issues with NAD Editorial status.
Discussion:
I was looking at the latest draft on forward_list. Especially the splice methods.
The first one splices a whole list after a given iterator in this. The name is splice_after. I think in 23.3.3.5 [forwardlist.ops] paragraph 40 change:
Effect: Insert the contents of xbeforeafter position, ...
A deeper issue involves the complexity. forward_list has no size and we don't know when we've reached the end except to walk up to it. To splice we would need to hook the end of the source list to the item after position in this list. This would involve walking length of the source list until we got to the last dereference-able element in source. There's no way we could do this in O(1) unless we stored a bogus end in forward_list.
OTOH, the last version of splice_after with iterator ranges we could do in O(1) because we know how to hook the end of the source range to ...
Unless I'm misconceiving the whole thing. Which is possible. I'll look at it again.
I'm pretty sure about the first part though.
[ San Francisco: ]
This issue is more complicated than it looks.
paragraph 47: replace each (first, last) with (first, last]
add a statement after paragraph 48 that complexity is O(1)
remove the complexity statement from the first overload of splice_after
We may have the same problems with other modifiers, like erase_after. Should it require that all iterators in the range (position, last] be dereferenceable?
We do, however, like the proposed changes and consider them Editorial. Move to NAD Editorial, Pending. Howard to open a new issue to handle the problems with the complexity requirements.
Opened 897.
Proposed resolution:
In 23.3.3.5 [forwardlist.ops] paragraph 40 change:
Effect: Insert the contents of xbeforeafter position, ...
Section: 17.5.1.4 [structure.specifications] Status: Dup Submitter: James Dennett Opened: 2008-09-16 Last modified: 2009-07-17
View all other issues in [structure.specifications].
View all issues with Dup status.
Duplicate of: 625
Discussion:
Per discussion, we need an issue open to cover looking at "Requires" clauses which are not constraints on user code, such as that on std::basic_string::at.
[ 2009-07 Frankfurt ]
Alan to address in paper.
Proposed resolution:
Section: 23.3.3.4 [forwardlist.modifiers] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2008-09-22 Last modified: 2009-10-20
View all other issues in [forwardlist.modifiers].
View all issues with NAD Editorial status.
Discussion:
This issue was split off from 892 at the request of the LWG.
[ San Francisco: ]
This issue is more complicated than it looks.
paragraph 47: replace each (first, last) with (first, last]
add a statement after paragraph 48 that complexity is O(1)
remove the complexity statement from the first overload of splice_after
We may have the same problems with other modifiers, like erase_after. Should it require that all iterators in the range (position, last] be dereferenceable?
There are actually 3 issues here:
What value should erase_after return? With list, code often looks like:
for (auto i = l.begin(); i != l.end();)
{
// inspect *i and decide if you want to erase it
// ...
if (I want to erase *i)
i = l.erase(i);
else
++i;
}
I.e. the iterator returned from erase is useful for setting up the logic for operating on the next element. For forward_list this might look something like:
auto i = fl.before_begin();
auto ip1 = i;
for (++ip1; ip1 != fl.end(); ++ip1)
{
// inspect *(i+1) and decide if you want to erase it
// ...
if (I want to erase *(i+1))
i = fl.erase_after(i);
else
++i;
ip1 = i;
}
In the above example code, it is convenient if erase_after returns the element prior to the erased element (range) instead of the element after the erase element (range).
Existing practice:
There is not a strong technical argument for either solution over the other.
With all other containers, operations always work on the range [first, last) and/or prior to the given position.
With forward_list, operations sometimes work on the range (first, last] and/or after the given position.
This is simply due to the fact that in order to operate on *first (with forward_list) one needs access to *(first-1). And that's not practical with forward_list. So the operating range needs to start with (first, not [first (as the current working paper says).
Additionally, if one is interested in splicing the range (first, last), then (with forward_list), one needs practical (constant time) access to *(last-1) so that one can set the next field in this node to the proper value. As this is not possible with forward_list, one must specify the last element of interest instead of one past the last element of interest. The syntax for doing this is to pass (first, last] instead of (first, last).
With erase_after we have a choice of either erasing the range (first, last] or (first, last). Choosing the latter enables:
x.erase_after(pos, x.end());
With the former, the above statement is inconvenient or expensive due to the lack of constant time access to x.end()-1. However we could introduce:
iterator erase_to_end(const_iterator position);
to compensate.
The advantage of the former ((first, last]) for erase_after is a consistency with splice_after which uses (first, last] as the specified range. But this either requires the addition of erase_to_end or giving up such functionality.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Review.
[ 2009-07 Frankfurt ]
We may need a new issue to correct splice_after, because it may no longer be correct to accept an rvalues as an argument. Merge may be affected, too. This might be issue 1133. (Howard: confirmed)
Move this to Ready, but the Requires clause of the second form of splice_after should say "(first, last)," not "(first, last]" (there are three occurrences). There was considerable discussion on this. (Howard: fixed)
Alan suggested removing the "foward_last<T. Alloc>&& x" parameter from the second form of splice_after, because it is redundant. PJP wanted to keep it, because it allows him to check for bad ranges (i.e. "Granny knots").
We prefer to keep x.
Beman. Whenever we deviate from the customary half-open range in the specification, we should add a non-normative comment to the standard explaining the deviation. This clarifies the intention and spares the committee much confusion in the future.
Alan to write a non-normative comment to explain the use of fully-closed ranges.
Move to Ready, with the changes described above. (Howard: awaiting note from Alan)
[ 2009-10 Santa Cruz: ]
NAD Editorial, addressed by N2988.
Proposed resolution:
Wording below assumes issue 878 is accepted, but this issue is independent of that issue.
Change 23.3.3.4 [forwardlist.modifiers]:
iterator erase_after(const_iterator position);Requires: The iterator following position is dereferenceable.
Effects: Erases the element pointed to by the iterator following position.
Returns:
An iterator pointing to the element following the one that was erased, or end() if no such element existsAn iterator equal to position.iterator erase_after(const_iterator position, const_iterator last);Requires: All iterators in the range
[(position,last) are dereferenceable.Effects: Erases the elements in the range
[(position,last).Returns: An iterator equal to position
last
Change 23.3.3.5 [forwardlist.ops]:
void splice_after(const_iterator position, forward_list<T,Allocator>&& x);Requires: position is before_begin() or a dereferenceable iterator in the range [begin(), end)). &x != this.
Effects: Inserts the contents of x after position, and x becomes empty. Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.
Throws: Nothing.
Complexity:
Ο(1)Ο(distance(x.begin(), x.end()))...
void splice_after(const_iterator position, forward_list<T,Allocator>&& x, const_iterator first, const_iterator last);Requires: position is before_begin() or a dereferenceable iterator in the range [begin(), end)). (first,last) is a valid range in x, and all iterators in the range (first,last) are dereferenceable. position is not an iterator in the range (first,last).
Effects: Inserts elements in the range (first,last) after position and removes the elements from x. Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.
Complexity: Ο(1).
Section: 24.5.2.5 [insert.iterator] Status: NAD Submitter: Alisdair Meredith Opened: 2008-09-24 Last modified: 2009-07-13
View all issues with NAD status.
Discussion:
Addresses UK 282
The requires clause on the const T & overloads in back_insert_iterator/front_insert_iterator/insert_iterator mean that the assignment operator will implicitly move from lvalues of a move-only type.
Suggested resolutions are:
[ Post Summit, Alisdair adds: ]
Both comment and issue have been resolved by the adoption of N2844 (rvalue references safety fix) at the last meeting.
Suggest resolve as NAD Editorial with a reference to the paper.
[ Batavia (2009-05): ]
We agree that this has been resolved in the latest Working Draft. Move to NAD.
Proposed resolution:
Recommend NAD, addressed by N2844.
Section: 18.3.1 [limits] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2008-09-24 Last modified: 2009-07-16
View all other issues in [limits].
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Discussion:
Addresses FR 32 and DE 16
numeric_limits has functions specifically designed to return NaNs, which break the model of Regular (via its axioms.) While floating point types will be acceptible in many algorithms taking Regular values, it is not appopriate for this specific API and we need a less refined constraint.
FR 32:
The definition of numeric_limits<> as requiring a regular type is both conceptually wrong and operationally illogical. As we pointed before, this mistake needs to be corrected. For example, the template can be left unconstrained. In fact this reflects a much more general problem with concept_maps/axioms and their interpretations. It appears that the current text heavily leans toward experimental academic type theory.
DE 16:
The class template numeric_limits should not specify the Regular concept requirement for its template parameter, because it contains functions returning NaN values for floating-point types; these values violate the semantics of EqualityComparable.
[ Summit: ]
Move to Open. Alisdair and Gaby will work on a solution, along with the new treatment of axioms in clause 14.
Proposed resolution:
Section: 24.5.2.1 [back.insert.iterator] Status: NAD Submitter: Dave Abrahams Opened: 2008-09-19 Last modified: 2009-07-16
View all issues with NAD status.
Discussion:
I just noticed this; don't know how far the problem(?) extends or whether it's new or existing: back_insert_iterator's operator* is not const, so you can't dereference a const one.
[ Post Summit Daniel adds: ]
If done, this change should be applied for front_insert_iterator, insert_iterator, ostream_iterator, and ostreambuf_iterator as well.
[ Batavia (2009-05): ]
Alisdair notes that these all are output iterators. Howard points out that ++*i would no longer work if we made this change.
Move to NAD.
[ 2009-05-25 Daniel adds: ]
Proposed resolution:
Section: 30.4.1.1 [thread.mutex.class] Status: Dup Submitter: Herb Sutter Opened: 2008-09-18 Last modified: 2009-03-22
View all other issues in [thread.mutex.class].
View all issues with Dup status.
Duplicate of: 893
Discussion:
A few questions on the current WP, N2723:
30.4.1 [thread.mutex.requirements]/24 says an expression mut.unlock() "Throws: Nothing." I'm assuming that, per 17.6.3.11 [res.on.required], errors that violate the precondition "The calling thread shall own the mutex" opens the door for throwing an exception anyway, such as to report unbalanced unlock operations and unlocking from a thread that does not have ownership. Right?
30.4.1.1 [thread.mutex.class]/3 (actually numbered paragraph "27" in the WP; this is just a typo I think) says
The behavior of a program is undefined if:
- it destroys a mutex object owned by any thread,
- a thread that owns a mutex object calls lock() or try_lock() on that object, or
- a thread terminates while owning a mutex object.
As already discussed, I think the second bullet should be removed, and such a lock() or try_lock() should fail with an exception or returning false, respectively.
A potential addition to the list would be
but without that the status quo text endorses the technique of the program logically transferring ownership of a mutex to another thread with correctness enforced by programming discipline. Was that intended?
[ Summit: ]
Two resolutions: "not a defect" and "duplicate", as follows:
- 30.4.1 [thread.mutex.requirements]/24: NAD. If the precondition fails the program has undefined behaviour and therefore an implementation may throw an exception already.
- 30.4.1.1 [thread.mutex.class]/3 bullet 2: Already addressed by issue 893.
- 30.4.1.1 [thread.mutex.class]/3 proposed addition: NAD. This is already covered by the mutex requirements, which have ownership as a Precondition.
Proposed resolution:
Section: 18.9 [support.initlist] Status: NAD Concepts Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2009-07-16
View all issues with NAD Concepts status.
Discussion:
The currently proposed constraint on initializer_list's element type E is that is has to meet ObjectType. This is an underspecification, because both core language and library part of initializer_list make clear, that it references an implicitly allocated array:
8.5.4 [dcl.init.list]/4:
When an initializer list is implicitly converted to a std::initializer_list<E>, the object passed is constructed as if the implementation allocated an array of N elements of type E, where N is the number of elements in the initializer list.[..]
18.9 [support.initlist]/2.
An object of type initializer_list<E> provides access to an array of objects of type const E.[..]
Therefore, E needs to fulfill concept ValueType (thus excluding abstract class types). This stricter requirement should be added to prevent deep instantiation errors known from the bad old times, as shown in the following example:
// Header A: (Should concept-check even in stand-alone modus)
template <DefaultConstructible T>
requires MoveConstructible<T>
void generate_and_do_3(T a) {
std::initializer_list<T> list{T(), std::move(a), T()};
...
}
void do_more();
void do_more_or_less();
template <DefaultConstructible T>
requires MoveConstructible<T>
void more_generate_3() {
do_more();
generate_and_do_3(T());
}
template <DefaultConstructible T>
requires MoveConstructible<T>
void something_and_generate_3() {
do_more_or_less();
more_generate_3();
}
// Test.cpp
#include "A.h"
class Abstract {
public:
virtual ~Abstract();
virtual void foo() = 0; // abstract type
Abstract(Abstract&&){} // MoveConstructible
Abstract(){} // DefaultConstructible
};
int main() {
// The restricted template *accepts* the argument, but
// causes a deep instantiation error in the internal function
// generate_and_do_3:
something_and_generate_3<Abstract>();
}
The proposed stricter constraint does not minimize the aim to support more general containers for which ObjectType would be sufficient. If such an extended container (lets assume it's still a class template) provides a constructor that accepts an initializer_list only this constructor would need to be restricted on ValueType:
template<ObjectType T>
class ExtContainer {
public:
requires ValueType<T>
ExtContainer(std::initializer_list<T>);
...
};
[ Batavia (2009-05): ]
Move to Tentatively Ready.
[ 2009-07 Frankfurt: ]
Need to look at again without concepts.
Proposed resolution:
Section: 29.5 [atomics.types] Status: NAD Editorial Submitter: Anthony Williams Opened: 2008-09-26 Last modified: 2009-10-26
View all other issues in [atomics.types].
View all issues with NAD Editorial status.
Discussion:
Addresses US 90
The deleted copy-assignment operators for the atomic types are not marked as volatile in N2723, whereas the assignment operators from the associated non-atomic types are. e.g.
atomic_bool& operator=(atomic_bool const&) = delete; atomic_bool& operator=(bool) volatile;
This leads to ambiguity when assigning a non-atomic value to a non-volatile instance of an atomic type:
atomic_bool b; b=false;
Both assignment operators require a standard conversions: the copy-assignment operator can use the implicit atomic_bool(bool) conversion constructor to convert false to an instance of atomic_bool, or b can undergo a qualification conversion in order to use the assignment from a plain bool.
This is only a problem once issue 845 is applied.
[ Summit: ]
Move to open. Assign to Lawrence. Related to US 90 comment.
[ 2009-08-17 Handled by N2925. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Add volatile qualification to the deleted copy-assignment operator of all the atomic types:
atomic_bool& operator=(atomic_bool const&) volatile = delete; atomic_itype& operator=(atomic_itype const&) volatile = delete;
etc.
This will mean that the deleted copy-assignment operator will require two conversions in the above example, and thus be a worse match than the assignment from plain bool.
Section: 20.2.1 [utility.arg.requirements] Status: NAD Concepts Submitter: Alberto Ganesh Barbati Opened: 2008-09-29 Last modified: 2010-03-08
View all other issues in [utility.arg.requirements].
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Discussion:
Addresses UK 150
The description of the effect of operator= in the MoveAssignable concept, given in paragraph 7 is:
result_type T::operator=(T&& rv); // inherited from HasAssign<T, T&&>Postconditions: the constructed T object is equivalent to the value of rv before the assignment. [Note: there is no requirement on the value of rv after the assignment. --end note]
The sentence contains a typo (what is the "constructed T object"?) probably due to a cut&paste from MoveConstructible. Moreover, the discussion of LWG issue 675 shows that the postcondition is too generic and might not reflect the user expectations. An implementation of the move assignment that just calls swap() would always fulfill the postcondition as stated, but might have surprising side-effects in case the source rvalue refers to an object that is not going to be immediately destroyed. See LWG issue 900 for another example. Due to the sometimes intangible nature of the "user expectation", it seems difficult to have precise normative wording that could cover all cases without introducing unnecessary restrictions. However a non-normative clarification could be a very helpful warning sign that swapping is not always the correct thing to do.
[ 2009-05-09 Alisdair adds: ]
Issue 910 is exactly the reason BSI advanced the Editorial comment UK-150.
The post-conditions after assignment are at a minimum that the object referenced by rv must be safely destructible, and the transaction should not leak resources. Ideally it should be possible to simply assign rv a new valid state after the call without invoking undefined behaviour, but any other use of the referenced object would depend upon additional guarantees made by that type.
[ 2009-05-09 Howard adds: ]
The intent of the rvalue reference work is that the moved from rv is a valid object. Not one in a singular state. If, for example, the moved from object is a vector, one should be able to do anything on that moved-from vector that you can do with any other vector. However you would first have to query it to find out what its current state is. E.g. it might have capacity, it might not. It might have a non-zero size, it might not. But regardless, you can push_back on to it if you want.
That being said, most standard code is now conceptized. That is, the concepts list the only operations that can be done with templated types - whether or not the values have been moved from.
Here is user-written code which must be allowed to be legal:
#include <vector> #include <cstdio> template <class Allocator> void inspect(std::vector<double, Allocator>&& v) { std::vector<double, Allocator> result(move(v)); std::printf("moved from vector has %u size and %u capacity\n", v.size(), v.capacity()); std::printf("The contents of the vector are:\n"); typedef typename std::vector<double, Allocator>::iterator I; for (I i = v.begin(), e = v.end(); i != e; ++i) printf("%f\n", *i); } int main() { std::vector<double> v1(100, 5.5); inspect(move(v1)); }The above program does not treat the moved-from vector as singular. It only treats it as a vector with an unknown value.
I believe the current proposed wording is consistent with my view on this.
[ Batavia (2009-05): ]
We agree that the proposed resolution is an improvement over the current wording.
[ 2009-07 Frankfurt: ]
Need to look at again without concepts.
[ 2009-07 Frankfurt: ]
Walter will consult with Dave and Doug.
[ 2009-10 Santa Cruz: ]
We believe this is handled by the resolution to issue 1204, but there is to much going on in this area to be sure. Defer for now.
[ 2010-01-23 Moved to Tentatively NAD Concepts after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
The current MoveAssignable requirements say everything that can be said in general. Each std-defined type has a more detailed specification of move assignment.
Proposed resolution:
In [concept.copymove], replace the postcondition in paragraph 7 with:
Postconditions: *this is equivalent to the value of rv before the assignment. [Note: there is no requirement on the value of rv after the assignment, but the effect should be unsurprising to the user even in case rv is not immediately destroyed. This may require that resources previously owned by *this are released instead of transferred to rv. -- end note]
Section: 25.3.3 [alg.swap] Status: NAD Concepts Submitter: Daniel Krügler Opened: 2008-10-01 Last modified: 2009-07-13
View all other issues in [alg.swap].
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Discussion:
With the adaption of 809 we have a new algorithm swap for C-arrays, which needs to be conceptualized.
[ Post Summit Daniel adds: ]
Recommend as NAD Editorial: The changes have already been applied to the WP N2800.
[ Batavia (2009-05): ]
Move to NAD; the changes have already been made.
Proposed resolution:
Replace in 25.3.3 [alg.swap] before p. 3 until p. 4 by
template <classValueType T, size_t N> requires Swappable<T> void swap(T (&a)[N], T (&b)[N]);
Requires: T shall be Swappable.Effects: swap_ranges(a, a + N, b);
Section: 25.3.5 [alg.replace] Status: NAD Concepts Submitter: Daniel Krügler Opened: 2008-10-03 Last modified: 2009-07-14
View all other issues in [alg.replace].
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Discussion:
(A) 25.3.5 [alg.replace]/1:
Requires: The expression *first = new_value shall be valid.
(B) 25.3.5 [alg.replace]/4:
Requires: The results of the expressions *first and new_value shall be writable to the result output iterator.[..]
Since conceptualization, the quoted content of these clauses is covered by the existing requirements
(A) OutputIterator<Iter, const T&>
and
(B) OutputIterator<OutIter, InIter::reference> && OutputIterator<OutIter, const T&>
resp, and thus should be removed.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
Remove 25.3.5 [alg.replace]/1.
template<ForwardIterator Iter, class T> requires OutputIterator<Iter, Iter::reference> && OutputIterator<Iter, const T&> && HasEqualTo<Iter::value_type, T> void replace(Iter first, Iter last, const T& old_value, const T& new_value); template<ForwardIterator Iter, Predicate<auto, Iter::value_type> Pred, class T> requires OutputIterator<Iter, Iter::reference> && OutputIterator<Iter, const T&> && CopyConstructible<Pred> void replace_if(Iter first, Iter last, Pred pred, const T& new_value);1 Requires: The expression *first = new_value shall be valid.
25.3.5 [alg.replace]/4: Remove the sentence "The results of the expressions *first and new_value shall be writable to the result output iterator.".
template<InputIterator InIter, typename OutIter, class T> requires OutputIterator<OutIter, InIter::reference> && OutputIterator<OutIter, const T&> && HasEqualTo<InIter::value_type, T> OutIter replace_copy(InIter first, InIter last, OutIter result, const T& old_value, const T& new_value); template<InputIterator InIter, typename OutIter, Predicate<auto, InIter::value_type> Pred, class T> requires OutputIterator<OutIter, InIter::reference> && OutputIterator<OutIter, const T&> && CopyConstructible<Pred> OutIter replace_copy_if(InIter first, InIter last, OutIter result, Pred pred, const T& new_value);4 Requires:The results of the expressions *first and new_value shall be writable to the result output iterator.The ranges [first,last) and [result,result + (last - first)) shall not overlap.
Section: 25.3.9 [alg.unique] Status: NAD Concepts Submitter: Daniel Krügler Opened: 2008-10-03 Last modified: 2009-07-14
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Discussion:
25.3.9 [alg.unique]/2: "Requires: The comparison function shall be an equivalence relation."
The essence of this is already covered by the given requirement
EquivalenceRelation<auto, Iter::value_type> Pred
and should thus be removed.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Remove 25.3.9 [alg.unique]/2
template<ForwardIterator Iter> requires OutputIterator<Iter, Iter::reference> && EqualityComparable<Iter::value_type> Iter unique(Iter first, Iter last); template<ForwardIterator Iter, EquivalenceRelation<auto, Iter::value_type> Pred> requires OutputIterator<Iter, RvalueOf<Iter::reference>::type> && CopyConstructible<Pred> Iter unique(Iter first, Iter last, Pred pred);1 Effects: ...
2 Requires: The comparison function shall be an equivalence relation.
Section: 25.4.7 [alg.min.max] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2008-10-04 Last modified: 2010-03-15
View all other issues in [alg.min.max].
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Discussion:
It seems that the proposed changes for N2772 were not clear enough in this point:
25.4.7 [alg.min.max], before p.23 + p.24 + before p. 27 + p. 28 say that the return type of the minmax overloads with an initializer_list is pair<const T&, const T&>, which is inconsistent with the decision for the other min/max overloads which take a initializer_list as argument and return a T, not a const T&. Doing otherwise for minmax would easily lead to unexpected life-time problems by using minmax instead of min and max separately.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-08-18 Daniel adds: ]
Recommend NAD since the proposed changes have already been performed as part of editorial work of N2914.
[ 2009-10 Santa Cruz: ]
Can't find initializer_list form of minmax anymore, only variadic version. Seems like we had an editing clash with concepts. Leave Open, at least until editorial issues resolved. Bring this to Editor's attention.
[ 2010 Pittsburgh: Pete to reapply N2772. ]
Rationale:
Solved by reapplying N2772.Proposed resolution:
In 25 [algorithms]/2, header <algorithm> synopsis change as indicated:
template<classLessThanComparable T> requires CopyConstructible<T> pair<constT&,constT&> minmax(initializer_list<T> t); template<class T,classStrictWeakOrder<auto, T> Compare> requires CopyConstructible<T> pair<constT&,constT&> minmax(initializer_list<T> t, Compare comp);
In 25.4.7 [alg.min.max] change as indicated (Begin: Just before p.20):
template<classLessThanComparable T> requires CopyConstructible<T> pair<constT&,constT&> minmax(initializer_list<T> t);
-20- Requires: T is LessThanComparable and CopyConstructible.-21- Returns: pair<
constT&,constT&>(x, y) where x is the smallest value and y the largest value in the initializer_list.[..]
template<class T,classStrictWeakOrder<auto, T> Compare> requires CopyConstructible<T> pair<constT&,constT&> minmax(initializer_list<T> t, Compare comp);
-24- Requires: type T is LessThanComparable and CopyConstructible.-25- Returns: pair<
constT&,constT&>(x, y) where x is the smallest value and y largest value in the initializer_list.
Section: 20.3.5 [pairs] Status: NAD Submitter: Daniel Krügler Opened: 2008-10-04 Last modified: 2009-10-23
View all other issues in [pairs].
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Discussion:
see also 917.
The current WP provides the following assignment operators for pair in 20.3.5 [pairs]/1:
template<class U , class V> requires HasAssign<T1, const U&> && HasAssign<T2, const V&> pair& operator=(const pair<U , V>& p);
requires MoveAssignable<T1> && MoveAssignable<T2> pair& operator=(pair&& p );
template<class U , class V> requires HasAssign<T1, RvalueOf<U>::type> && HasAssign<T2, RvalueOf<V>::type> pair& operator=(pair<U , V>&& p);
It seems that the functionality of (2) is completely covered by (3), therefore (2) should be removed.
[ Batavia (2009-05): ]
Bill believes the extra assignment operators are necessary for resolving ambiguities, but that does not mean it needs to be part of the specification.
Move to Open. We recommend this be looked at in the context of the ongoing work related to the pair templates.
[ 2009-07 Frankfurt: ]
Leave this open pending the removal of concepts from the WD.
[ 2009-10 Santa Cruz: ]
Mark as NAD, see issue 801.
Proposed resolution:
In 20.3.5 [pairs] p. 1, class pair and just before p. 13 remove the declaration:
requires MoveAssignable<T1> && MoveAssignable<T2> pair& operator=(pair&& p );
Section: 20.4.2.1 [tuple.cnstr] Status: NAD Submitter: Daniel Krügler Opened: 2008-10-04 Last modified: 2009-10-23
View all other issues in [tuple.cnstr].
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Discussion:
see also 916.
N2770 (and thus now the WP) removed the non-template move-assignment operator from tuple's class definition, but the latter individual member description does still provide this operator. Is this (a) an oversight and can it (b) be solved as part of an editorial process?
[ Post Summit Daniel provided wording. ]
[ Batavia (2009-05): ]
We believe that the proposed resolution's part 1 is editorial.
Regarding part 2, we either remove the specification as proposed, or else add back the declaration to which the specification refers. Alisdair and Bill prefer the latter. It is not immediately obvious whether the function is intended to be present.
We recommend that the Project Editor restore the missing declaration and that we keep part 2 of the issue alive.
Move to Open.
[ 2009-07 Frankfurt: ]
Leave this open pending the removal of concepts from the WD.
[ 2009-10 Santa Cruz: ]
Mark as NAD, see issue 801.
Proposed resolution:
In 20.4.2 [tuple.tuple], class tuple just before member swap please change as indicated:
[ This fixes an editorial loss between N2798 to N2800 ]
template <class... UTypes> requires HasAssign<Types, const UTypes&>... tuple& operator=(const pair<UTypes...>&); template <class... UTypes> requires HasAssign<Types, RvalueOf<UTypes>::type>... tuple& operator=(pair<UTypes...>&&);
In 20.4.2.1 [tuple.cnstr], starting just before p. 11 please remove as indicated:
requires MoveAssignable<Types>... tuple& operator=(tuple&& u);
-11- Effects: Move-assigns each element of u to the corresponding element of *this.
-12- Returns: *this.
Section: 20.4.2.3 [tuple.swap] Status: NAD Concepts Submitter: Daniel Krügler Opened: 2008-10-04 Last modified: 2009-07-13
View all issues with NAD Concepts status.
Discussion:
Issue 522 was accepted after tuple had been conceptualized, therefore this step needs to be completed.
[ Post Summit Daniel adds ]
This is now NAD Editorial (addressed by N2844) except for item 3 in the proposed wording.
[ 2009-05-01 Daniel adds: ]
As of the recent WP (N2857), this issue is now completely covered by editorial changes (including the third bullet), therefore I unconditionally recommend NAD.
[ Batavia (2009-05): ]
We observed that all the proposed changes have already been applied to the Working Draft, rendering this issue moot.
Move to NAD.
Proposed resolution:
In both 20.4.1 [tuple.general]/2 and 20.4.2.9 [tuple.special] change
template <classSwappable... Types> void swap(tuple<Types...>& x, tuple<Types...>& y);
In 20.4.2 [tuple.tuple], class tuple definition and in 20.4.2.3 [tuple.swap], change
requires Swappable<Types>...void swap(tuple&);
In 20.4.2.3 [tuple.swap] remove the current requires-clause, which says:
Requires: Each type in Types shall be Swappable
Section: 23.3.3.5 [forwardlist.ops], 23.3.4.4 [list.ops] Status: NAD Submitter: Daniel Krügler Opened: 2008-10-06 Last modified: 2009-10-23
View all other issues in [forwardlist.ops].
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Discussion:
The signatures of forwardlist::remove and list::remove defined in 23.3.3.5 [forwardlist.ops] before 11 + 23.3.4.4 [list.ops] before 15:
requires EqualityComparable<T> void remove(const T& value);
are asymmetric to their predicate variants (which only require Predicate, not EquivalenceRelation) and with the free algorithm remove (which only require HasEqualTo). Also, nothing in the pre-concept WP N2723 implies that EqualityComparable should be the intended requirement.
[ Batavia (2009-05): ]
We agree with the proposed resolution, but would like additional input from concepts experts.
Move to Review.
[ 2009-07-21 Alisdair adds: ]
Current rationale and wording for this issue is built around concepts. I suggest the issue reverts to Open status. I believe there is enough of an issue to review after concepts are removed from the WP to re-examine the issue in Santa Cruz, rather than resolve as NAD Concepts.
[ 2009-10-10 Daniel adds: ]
Recommend NAD: The concept-free wording as of N2960 has no longer the over-specified requirement EqualityComparable for the remove function that uses ==. In fact, now the same test conditions exists as for the free algorithm remove (25.3.8 [alg.remove]). The error was introduced in the process of conceptifying.
[ 2009-10 Santa Cruz: ]
NAD, solved by the removal of concepts.
Proposed resolution:
Replace in 23.3.3.5 [forwardlist.ops] before 11 and in 23.3.4.4 [list.ops] before 15
requiresEqualityComparable<T>HasEqualTo<T, T> void remove(const T& value);
Section: 29 [atomics] Status: NAD Editorial Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2009-10-26
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Discussion:
Right now, C++0x doesn't have atomic<float>. We're thinking of adding the words to support it for TR2 (note: that would be slightly post-C++0x). If we need it, we could probably add the words.
Proposed resolutions: Using atomic<FP>::compare_exchange (weak or strong) should be either:
I propose Option 1 for C++0x for expediency. If someone wants to argue for Option 2, they need to say what exactly they want compare_exchange to mean in this case (IIRC, C++0x doesn't even assume IEEE 754).
[ Summit: ]
Move to open. Blocked until concepts for atomics are addressed.
[ Post Summit Anthony adds: ]
Recommend NAD. C++0x does have std::atomic<float>, and both compare_exchange_weak and compare_exchange_strong are well-defined in this case. Maybe change the note in 29.6 [atomics.types.operations] paragraph 20 to:
[Note: The effect of the compare-and-exchange operations is
if (!memcmp(object,expected,sizeof(*object))) *object = desired; else *expected = *object;This may result in failed comparisons for values that compare equal if the underlying type has padding bits or alternate representations of the same value. -- end note]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Change the note in 29.6 [atomics.types.operations] paragraph 20 to:
[Note: The effect of the compare-and-exchange operations is
if (*object == *expected!memcmp(object,expected,sizeof(*object))) *object = desired; else *expected = *object;This may result in failed comparisons for values that compare equal if the underlying type has padding bits or alternate representations of the same value. -- end note]
Section: 29 [atomics] Status: NAD Editorial Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2009-10-26
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Discussion:
Right now, the compare_exchange_weak loop should rapidly converge on the padding contents. But compare_exchange_strong will require a bit more compiler work to ignore padding for comparison purposes.
Note that this isn't a problem for structs with no padding, and we do already have one portable way to ensure that there is no padding that covers the key use cases: Have elements be the same type. I suspect that the greatest need is for a structure of two pointers, which has no padding problem. I suspect the second need is a structure of a pointer and some form of an integer. If that integer is intptr_t, there will be no padding.
Related but separable issue: For unused bitfields, or other unused fields for that matter, we should probably say it's the programmer's responsibility to set them to zero or otherwise ensure they'll be ignored by memcmp.
Proposed resolutions: Using atomic<struct-with-padding>::compare_exchange_strong should be either:
I propose Option 1 for C++0x for expediency, though I'm not sure how to say it. I would be happy with Option 2, which I believe would mean that compare_exchange_strong would be implemented to avoid comparing padding bytes, or something equivalent such as always zeroing out padding when loading/storing/comparing. (Either implementation might require compiler support.)
[ Summit: ]
Move to open. Blocked until concepts for atomics are addressed.
[ Post Summit Anthony adds: ]
The resoultion of LWG 923 should resolve this issue as well.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Section: 29.3 [atomics.order] Status: NAD Editorial Submitter: Anthony Williams Opened: 2008-10-19 Last modified: 2009-10-26
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Discussion:
Addresses UK 313
There was an interesting issue raised over on comp.programming.threads today regarding the following example
// Thread 1: x.store(1, memory_order_relaxed); // SX atomic_thread_fence(memory_order_seq_cst); // F1 y.store(1, memory_order_relaxed); // SY1 atomic_thread_fence(memory_order_seq_cst); // F2 r1 = y.load(memory_order_relaxed); // RY // Thread 2: y.store(0, memory_order_relaxed); // SY2 atomic_thread_fence(memory_order_seq_cst); // F3 r2 = x.load(memory_order_relaxed); // RX
is the outcome r1 == 0 and r2 == 0 possible?
I think the intent is that this is not possible, but I am not sure the wording guarantees that. Here is my analysis:
Since all the fences are SC, there must be a total order between them. F1 must be before F2 in that order since they are in the same thread. Therefore F3 is either before F1, between F1 and F2 or after F2.
If F3 is after F2, then we can apply 29.3 [atomics.order]p5 from N2798:
For atomic operations A and B on an atomic object M, where A modifies M and B takes its value, if there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B observes either the effects of A or a later modification of M in its modification order.
In this case, A is SX, B is RX, the fence X is F2 and the fence Y is F3, so RX must see 1.
If F3 is before F2, this doesn't apply, but F3 can therefore be before or after F1.
If F3 is after F1, the same logic applies, but this time the fence X is F1. Therefore again, RX must see 1.
Finally we have the case that F3 is before F1 in the SC ordering. There are now no guarantees about RX, and RX can see r2==0.
We can apply 29.3 [atomics.order]p5 again. This time, A is SY2, B is RY, X is F3 and Y is F1. Thus RY must observe the effects of SY2 or a later modification of y in its modification order.
Since SY1 is sequenced before RY, RY must observe the effects of SY1 or a later modification of y in its modification order.
In order to ensure that RY sees (r1==1), we must see that SY1 is later in the modification order of y than SY2.
We're now skating on thin ice. Conceptually, SY2 happens-before F3, F3 is SC-ordered before F1, F1 happens-before SY1, so SY1 is later in the modification order M of y, and RY must see the result of SY1 (r1==1). However, I don't think the words are clear on that.
[ Post Summit Hans adds: ]
In my (Hans') view, our definition of fences will always be weaker than what particular hardware will guarantee. Memory_order_seq_cst fences inherently don't guarantee sequential consistency anyway, for good reasons (e.g. because they can't enforce a total order on stores). Hence I don't think the issue demonstrates a gross failure to achieve what we intended to achieve. The example in question is a bit esoteric. Hence, in my view, living with the status quo certainly wouldn't be a disaster either.
In any case, we should probably add text along the lines of the following between p5 and p6 in 29.3 [atomics.order]:
[Note: Memory_order_seq_cst only ensures sequential consistency for a data-race-free program that uses exclusively memory_order_seq_cst operations. Any use of weaker ordering will invalidate this guarantee unless extreme care is used. In particular, memory_order_seq_cst fences only ensure a total order for the fences themselves. They cannot, in general, be used to restore sequential consistency for atomic operations with weaker ordering specifications.]Also see thread beginning at c++std-lib-23271.
[ Herve's correction: ]
Minor point, and sorry for the knee jerk reaction: I admit to having no knowledge of Memory_order_seq_cst, but my former boss (John Lakos) has ingrained an automatic introspection on the use of "only". I think you meant:
[Note: Memory_order_seq_cst ensures sequential consistency only for . . . . In particular, memory_order_seq_cst fences ensure a total order only for . . .Unless, of course, Memory_order_seq_cst really do nothing but ensure sequential consistency for a data-race-free program that uses exclusively memory_order_seq_cst operations.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Add a new paragraph after 29.3 [atomics.order]p5 that says
For atomic operations A and B on an atomic object M, where A and B modify M, if there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modifiction order of M.
Section: X [allocator.concepts] Status: NAD Concepts Submitter: Pablo Halpern Opened: 2008-10-23 Last modified: 2009-07-13
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Discussion:
X [allocator.concepts] contains a reference to a concept named Dereferenceable. No such concept exists.
[ Daniel adds 2009-02-14: ]
The proposal given in the paper N2829 would automatically resolve this issue.
[ Batavia (2009-05): ]
This particular set of changes has already been made. There are two related changes later on (and possibly also an earlier Example); these can be handled editorially.
Move to NAD Editorial.
Proposed resolution:
Change all uses of the concept Dereferenceable to HasDereference in X [allocator.concepts].
Section: 20.4.2.7 [tuple.rel] Status: NAD Concepts Submitter: Joe Gottman Opened: 2008-10-28 Last modified: 2009-07-13
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Discussion:
In the latest working draft for C++0x, tuple's operator== and operator< are declared as
template<class... TTypes, class... UTypes> requires EqualityComparable<TTypes, UTypes>... bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
and
template<class... TTypes, class... UTypes> requires LessThanComparable<TTypes, UTypes>... bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
But the concepts EqualityComparable and LessThanComparable only take one parameter, not two. Also, even if LessThanComparable could take two parameters, the definition of tuple::operator<() should also require
LessThanComparable<UTypes, TTypes>... // (note the order)
since the algorithm for tuple::operator< is the following (pseudo-code)
for (size_t N = 0; N < sizeof...(TTypes); ++N) {
if (get<N>(t) < get<N>(u) return true;
else if ((get<N>(u) < get<N>(t)) return false;
}
return false;
Similar problems hold for tuples's other comparison operators.
[ Post Summit: ]
Recommend Tentatively Ready.
Proposed resolution:
In 20.4.1 [tuple.general] and 20.4.2.7 [tuple.rel] change:
template<class... TTypes, class... UTypes> requiresEqualityComparableHasEqualTo<TTypes, UTypes>... bool operator==(const tuple<TTypes...>&, const tuple<UTypes...>&); template<class... TTypes, class... UTypes> requiresLessThanComparableHasLess<TTypes, UTypes>... && HasLess<UTypes, TTypes>... bool operator<(const tuple<TTypes...>&, const tuple<UTypes...>&); template<class... TTypes, class... UTypes> requiresEqualityComparableHasEqualTo<TTypes, UTypes>... bool operator!=(const tuple<TTypes...>&, const tuple<UTypes...>&); template<class... TTypes, class... UTypes> requiresLessThanComparableHasLess<UTTypes,TUTypes>... && HasLess<UTypes, TTypes>... bool operator>(const tuple<TTypes...>&, const tuple<UTypes...>&); template<class... TTypes, class... UTypes> requiresLessThanComparableHasLess<UTTypes,TUTypes>... && HasLess<UTypes, TTypes>... bool operator<=(const tuple<TTypes...>&, const tuple<UTypes...>&); template<class... TTypes, class... UTypes> requiresLessThanComparableHasLess<TTypes, UTypes>... && HasLess<UTypes, TTypes>... bool operator>=(const tuple<TTypes...>&, const tuple<UTypes...>&);
Section: 23.3.1 [array] Status: NAD Submitter: Niels Dekker Opened: 2008-11-17 Last modified: 2009-10-20
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Discussion:
The Working Draft (N2798) allows access to the elements of std::array by its data() member function:
23.2.1.4 array::data [array.data]
T *data(); const T *data() const;
- Returns: elems.
Unfortunately, the result of std::array::data() cannot be bound to a reference to a built-in array of the type of array::elems. And std::array provides no other way to get a reference to array::elems. This hampers the use of std::array, for example when trying to pass its data to a C style API function:
// Some C style API function. void set_path( char (*)[MAX_PATH] ); std::array<char,MAX_PATH> path; set_path( path.data() ); // error set_path( &(path.data()) ); // error
Another example, trying to pass the array data to an instance of another C++ class:
// Represents a 3-D point in space.
class three_d_point {
public:
explicit three_d_point(const double (&)[3]);
};
const std::array<double,3> coordinates = { 0, 1, 2 };
three_d_point point1( coordinates.data() ); // error.
three_d_point point2( *(coordinates.data()) ); // error.
A user might be tempted to use std::array::elems instead, but doing so isn't recommended, because std::array::elems is "for exposition only". Note that Boost.Array users might already use boost::array::elems, as its documentation doesn't explicitly state that boost::array::elems is for exposition only: http://www.boost.org/doc/libs/1_36_0/doc/html/boost/array.html
I can think of three options to solve this issue:
Lawrence Crowl wrote me that it might be better to leave std::array::elems "for exposition only", to allow alternate representations to allocate the array data dynamically. This might be of interest to the embedded community, having to deal with very limited stack sizes.
The second option, changing the return type of std::array::data(), would break backward compatible to current Boost and TR1 implementations, as well as to the other contiguous container (vector and string) in a very subtle way. For example, the following call to std::swap currently swap two locally declared pointers (data1, data2), for any container type T that has a data() member function. When std::array::data() is changed to return a reference, the std::swap call may swap the container elements instead.
template <typename T>
void func(T& container1, T& container2)
{
// Are data1 and data2 pointers or references?
auto data1 = container1.data();
auto data2 = container2.data();
// Will this swap two local pointers, or all container elements?
std::swap(data1, data2);
}
The following concept is currently satisfied by all contiguous containers, but it no longer is for std::array, when array::data() is changed to return a reference (tested on ConceptGCC Alpha 7):
auto concept ContiguousContainerConcept<typename T>
{
typename value_type = typename T::value_type;
const value_type * T::data() const;
}
Still it's worth considering having std::array::data() return a reference, because it might be the most intuitive option, from a user's point of view. Nicolai Josuttis (who wrote boost::array) mailed me that he very much prefers this option.
Note that for this option, the definition of data() would also need to be revised for zero-sized arrays, as its return type cannot be a reference to a zero-sized built-in array. Regarding zero-sized array, data() could throw an exception. Or there could be a partial specialization of std::array where data() returns T* or gets removed.
Personally I prefer the third option, adding a new member function to std::array, overloaded for const and non-const access, returning a reference to the built-in array, to avoid those compatible issues. I'd propose naming the function std::array::c_array(), which sounds intuitive to me. Note that boost::array already has a c_array() member, returning a pointer, but Nicolai told me that this one is only there for historical reasons. (Otherwise a name like std::array::native_array() or std::array::builtin_array() would also be fine with me.) According to my proposed resolution, a zero-sized std::array does not need to have c_array(), while it is still required to have data() functions.
[ Post Summit: ]
Alisdair: Don't like p4 suggesting implementation-defined behaviour.
Walter: What about an explicit conversion operator, instead of adding the new member function?
Alisdair: Noodling about:
template<size_t N, ValueType T> struct array { T elems[N]; // fantasy code starts here // crazy decltype version for grins only //requires True<(N>0)> //explict operator decltype(elems) & () { return elems; } // conversion to lvalue ref requires True<(N>0)> explict operator T(&)[N] () & { return elems; } // conversion to const lvalue ref requires True<(N>0)> explict operator const T(&)[N] () const & { return elems; } // conversion to rvalue ref using ref qualifiers requires True<(N>0)> explict operator T(&&)[N] () && { return elems; } // fantasy code ends here explicit operator bool() { return true; } };This seems legal but odd. Jason Merrill says currently a CWG issue 613 on the non-static data member that fixes the error that current G++ gives for the non-explicit, non-conceptualized version of this. Verdict from human compiler: seems legal.
Some grumbling about zero-sized arrays being allowed and supported.
Walter: Would this address the issue? Are we inclined to go this route?
Alan: What would usage look like?
// 3-d point in space struct three_d_point { explicit three_d_point(const double (&)[3]); }; void sink(double*); const std::array<double, 3> coordinates = { 0, 1, 2 }; three_d_point point1( coordinates.data() ); //error three_d_point point2( *(coordinates.data()) ); // error three_d_point point3( coordinates ); // yay! sink(cooridinates); // error, no conversionRecommended Open with new wording. Take the required clause and add the explicit conversion operators, not have a typedef. At issue still is use decltype or use T[N]. In favour of using T[N], even though use of decltype is specially clever.
[ Post Summit, further discussion in the thread starting with c++std-lib-23215. ]
[ 2009-07 post-Frankfurt (Saturday afternoon group): ]
The idea to resolve the issue by adding explicit conversion operators was abandoned, because it would be inconvenient to use, especially when passing the array to a template function, as mentioned by Daniel. So we reconsidered the original proposed resolution, which appeared acceptable, except for its proposed changes to 23.3.1.7 [array.zero], which allowed c_array_type and c_array() to be absent for a zero-sized array. Alisdair argued that such wording would disallow certain generic use cases. New wording for 23.3.1.7 [array.zero] was agreed upon (Howard: and is reflected in the proposed resolution).
Move to Review
[ 2009-07-31 Alisdair adds: ]
I will be unhappy voting the proposed resolution for 930 past review until we have implementation experience with reference qualifiers. Specifically, I want to understand the impact of the missing overload for const && (if any.)
If we think the issue is important enough it might be worthwhile stripping the ref qualifiers for easy progress next meeting, and opening yet another issue to put them back with experience.
Recommend deferring any decision on splitting the issue until we get LWG feedback next meeting - I may be the lone dissenting voice if others are prepared to proceed without it.
[ 2009-10 Santa Cruz: ]
Mark as NAD. There was not enough consensus that this was sufficiently useful. There are known other ways to do this, such as small inline conversion functions.
Proposed resolution:
Add to the template definition of array, 23.3.1 [array]/3:
typedef T c_array_type[N]; c_array_type & c_array() &; c_array_type && c_array() &&; const c_array_type & c_array() const &;
Add the following subsection to 23.3.1 [array], after 23.3.1.4 [array.data]:
23.2.1.5 array::c_array [array.c_array]
c_array_type & c_array() &; c_array_type && c_array() &&; const c_array_type & c_array() const &;Returns: elems.
Change Zero sized arrays 23.3.1.7 [array.zero]:
-2- ...
The type c_array_type is unspecified for a zero-sized array.
-3- The effect of calling c_array(), front(), or back() for a zero-sized array is implementation defined.
Section: 20.9.10.2.1 [unique.ptr.single.ctor] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2008-11-26 Last modified: 2010-03-14
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Discussion:
Addresses US 79
20.9.10.2.1 [unique.ptr.single.ctor]/5 no longer requires for D not to be a pointer type. I believe this restriction was accidently removed when we relaxed the completeness reuqirements on T. The restriction needs to be put back in. Otherwise we have a run time failure that could have been caught at compile time:
{
unique_ptr<int, void(*)(void*)> p1(malloc(sizeof(int))); // should not compile
} // p1.~unique_ptr() dereferences a null function pointer
unique_ptr<int, void(*)(void*)> p2(malloc(sizeof(int)), free); // ok
[ Post Summit: ]
Recommend Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be improved for enable_if type constraining, possibly following Robert's formula.
[ 2009-07 Frankfurt: ]
We need to consider whether some requirements in the Requires paragraphs of [unique.ptr] should instead be Remarks.
Leave Open. Howard to provide wording, and possibly demonstrate how this can be implemented using enable_if.
[ 2009-07-27 Howard adds: ]
The two constructors to which this issue applies are not easily constrained with enable_if as they are not templated:
unique_ptr(); explicit unique_ptr(pointer p);To "SFINAE" these constructors away would take heroic effort such as specializing the entire unique_ptr class template on pointer deleter types. There is insufficient motivation for such heroics. Here is the expected and reasonable implementation for these constructors:
unique_ptr() : ptr_(pointer()) { static_assert(!is_pointer<deleter_type>::value, "unique_ptr constructed with null function pointer deleter"); } explicit unique_ptr(pointer p) : ptr_(p) { static_assert(!is_pointer<deleter_type>::value, "unique_ptr constructed with null function pointer deleter"); }I.e. just use static_assert to verify that the constructor is not instantiated with a function pointer for a deleter. The compiler will automatically take care of issuing a diagnostic if the deleter is a reference type (uninitialized reference error).
In keeping with our discussions in Frankfurt, I'm moving this requirement on the implementation from the Requires paragraph to a Remarks paragraph.
[ 2009-08-17 Daniel adds: ]
It is insufficient to require a diagnostic. This doesn't imply an ill-formed program as of 1.3.3 [defns.diagnostic] (a typical alternative would be a compiler warning), but exactly that seems to be the intend. I suggest to use the following remark instead:
Remarks: The program shall be ill-formed if this constructor is instantiated when D is a pointer type or reference type.Via the general standard rules of 1.4 [intro.compliance] the "diagnostic required" is implied.
[ 2009-10 Santa Cruz: ]
Moved to Ready.
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
Change the description of the default constructor in 20.9.10.2.1 [unique.ptr.single.ctor]:
unique_ptr();-1- Requires: D shall be default constructible, and that construction shall not throw an exception.
D shall not be a reference type or pointer type (diagnostic required)....
Remarks: The program shall be ill-formed if this constructor is instantiated when D is a pointer type or reference type.
Add after 20.9.10.2.1 [unique.ptr.single.ctor]/8:
unique_ptr(pointer p);...
Remarks: The program shall be ill-formed if this constructor is instantiated when D is a pointer type or reference type.
Section: 20.9.10.2.5 [unique.ptr.single.modifiers] Status: NAD Future Submitter: Alisdair Meredith Opened: 2008-11-27 Last modified: 2009-10-23
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Discussion:
If we are supporting stateful deleters, we need an overload for reset that takes a deleter as well.
void reset( pointer p, deleter_type d);
We probably need two overloads to support move-only deleters, and this sounds uncomfortably like the two constructors I have been ignoring for now...
[ Batavia (2009-05): ]
Howard comments that we have the functionality via move-assigment.
Move to Open.
[ 2009-10 Santa Cruz: ]
Mark as NAD Future.
Proposed resolution:
Section: 20.10.5 [time.clock] Status: NAD Future Submitter: Beman Dawes Opened: 2008-11-24 Last modified: 2009-10-23
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Discussion:
Each of the three clocks specified in Clocks 20.10.5 [time.clock] provides the member function:
static time_point now();
The semantics specified by Clock requirements 20.10.1 [time.clock.req] make no mention of error handling. Thus the function may throw bad_alloc or an implementation-defined exception (17.6.4.11 [res.on.exception.handling] paragraph 4).
Some implementations of these functions on POSIX, Windows, and presumably on other operating systems, may fail in ways only detectable at runtime. Some failures on Windows are due to supporting chipset errata and can even occur after successful calls to a clock's now() function.
These functions are used in cases where exceptions are not appropriate or where the specifics of the exception or cause of error need to be available to the user. See N2828, Library Support for hybrid error handling (Rev 1), for more specific discussion of use cases. Thus some change in the interface of now is required.
The proposed resolution has been implemented in the Boost version of the chrono library. No problems were encountered.
[ Batavia (2009-05): ]
We recommend this issue be deferred until the next Committee Draft has been issued and the prerequisite paper has been accepted.
Move to Open.
[ 2009-10 Santa Cruz: ]
Mark as NAD future. Too late to make this change without having already accepted the hybrid error handling proposal.
Proposed resolution:
Accept the proposed wording of N2828, Library Support for hybrid error handling (Rev 1).
Change Clock requirements 20.10.1 [time.clock.req] as indicated:
-2- In Table 55 C1 and C2 denote clock types. t1 and t2 are values returned by C1::now() where the call returning t1 happens before (1.10) the call returning t2 and both of these calls happen before C1::time_point::max(). ec denotes an object of type error_code (19.5.2.1 [syserr.errcode.overview]).
Table 55 -- Clock requirements Expression Return type Operational semantics ... ... ... C1::now() C1::time_point Returns a time_point object representing the current point in time. C1::now(ec) C1::time_point Returns a time_point object representing the current point in time.
Change Class system_clock 20.10.5.1 [time.clock.system] as indicated:
static time_point now(error_code& ec=throws());
Change Class monotonic_clock 20.10.5.2 [time.clock.monotonic] as indicated:
static time_point now(error_code& ec=throws());
Change Class high_resolution_clock 20.10.5.3 [time.clock.hires] as indicated:
static time_point now(error_code& ec=throws());
Section: 30.4.1 [thread.mutex.requirements] Status: NAD Future Submitter: Pete Becker Opened: 2008-12-05 Last modified: 2009-10-23
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Duplicate of: 961
Discussion:
30.4.1 [thread.mutex.requirements] describes the requirements for a type to be a "Mutex type". A Mutex type can be used as the template argument for the Lock type that's passed to condition_variable_any::wait (although Lock seems like the wrong name here, since Lock is given a different formal meaning in 30.4.3 [thread.lock]) and, although the WD doesn't quite say so, as the template argument for lock_guard and unique_lock.
The requirements for a Mutex type include:
Also, a Mutex type "shall not be copyable nor movable".
The latter requirement seems completely irrelevant, and the three requirements on return types are tighter than they need to be. For example, there's no reason that lock_guard can't be instantiated with a type that's copyable. The rule is, in fact, that lock_guard, etc. won't try to copy objects of that type. That's a constraint on locks, not on mutexes. Similarly, the requirements for void return types are unnecessary; the rule is, in fact, that lock_guard, etc. won't use any returned value. And with the return type of bool, the requirement should be that the return type is convertible to bool.
[ Summit: ]
Move to open. Related to conceptualization and should probably be tackled as part of that.
- The intention is not only to place a constraint on what types such as lock_guard may do with mutex types, but on what any code, including user code, may do with mutex types. Thus the constraints as they are apply to the mutex types themselves, not the current users of mutex types in the standard.
- This is a low priority issue; the wording as it is may be overly restrictive but this may not be a real issue.
[ Post Summit Anthony adds: ]
Section 30.4.1 [thread.mutex.requirements] conflates the requirements on a generic Mutex type (including user-supplied mutexes) with the requirements placed on the standard-supplied mutex types in an attempt to group everything together and save space.
When applying concepts to chapter 30, I suggest that the concepts Lockable and TimedLockable embody the requirements for *use* of a mutex type as required by unique_lock/lock_guard/condition_variable_any. These should be relaxed as Pete describes in the issue. The existing words in 30.4.1 [thread.mutex.requirements] are requirements on all of std::mutex, std::timed_mutex, std::recursive_mutex and std::recursive_timed_mutex, and should be rephrased as such.
Proposed resolution:
Section: 29 [atomics] Status: NAD Editorial Submitter: Clark Nelson Opened: 2008-12-05 Last modified: 2009-05-23
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Discussion:
Addresses US 89
The types in the table "Atomics for standard typedef types" should be typedefs, not classes. These semantics are necessary for compatibility with C.
Change the classes to typedefs.
N2427 specified different requirements for atomic analogs of fundamental integer types (such as atomic_int) and for atomic analogs of <cstdint> typedefs (such as atomic_size_t). Specifically, atomic_int et al. were specified to be distinct classes, whereas atomic_size_t et al. were specified to be typedefs. Unfortunately, in applying N2427 to the WD, that distinction was erased, and the atomic analog of every <cstdint> typedef is required to be a distinct class.
It shouldn't be required that the atomic analog of every <cstdint> typedef be a typedef for some fundamental integer type. After all, <cstdint> is supposed to provide standard names for extended integer types. So there was a problem in N2427, which certainly could have been interpreted to require that. But the status quo in the WD is even worse, because it's unambiguously wrong.
What is needed are words to require the existence of a bunch of type names, without specifying whether they are class names or typedef names.
[ Summit: ]
Change status to NAD, editorial. See US 89 comment notes above.
Direct the editor to turn the types into typedefs as proposed in the comment. Paper approved by committee used typedefs, this appears to have been introduced as an editorial change. Rationale: for compatibility with C.
Proposed resolution:
Section: 24.4.4 [iterator.operations] Status: NAD Editorial Submitter: Thomas Opened: 2008-12-14 Last modified: 2010-03-11
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Discussion:
Addresses UK 270
Regarding the std::distance - function, 24.4.4 [iterator.operations] / 4 says:
Returns the number of increments or decrements needed to get from first to last.
This sentence is completely silent about the sign of the return value. 24.4.4 [iterator.operations] / 1 gives more information about the underlying operations, but again no inferences about the sign can be made. Strictly speaking, that is taking that sentence literally, I think this sentence even implies a positive return value in all cases, as the number of increments or decrements is clearly a ratio scale variable, with a natural zero bound.
Practically speaking, my implementations did what common sense and knowledge based on pointer arithmetic forecasts, namely a positive sign for increments (that is, going from first to last by operator++), and a negative sign for decrements (going from first to last by operator--).
Here are my two questions:
First, is that paragraph supposed to be interpreted in the way what I called 'common sense', that is negative sign for decrements ? I am fairly sure that's the supposed behavior, but a double-check here in this group can't hurt.
Second, is the present wording (2003 standard version - no idea about the draft for the upcoming standard) worth an edit to make it a bit more sensible, to mention the sign of the return value explicitly ?
[ Daniel adds: ]
My first thought was that resolution 204 would already cover the issue report, but it seems that current normative wording is in contradiction to that resolution:
Referring to N2798, 24.4.4 [iterator.operations]/ p.4 says:
Effects: Returns the number of increments or decrements needed to get from first to last.IMO the part " or decrements" is in contradiction to p. 5 which says
Requires: last shall be reachable from first.because "reachable" is defined in X [iterator.concepts]/7 as
An iterator j is called reachable from an iterator i if and only if there is a finite sequence of applications of the expression ++i that makes i == j.[..]Here is wording that would be consistent with this definition of "reachable":
Change 24.4.4 [iterator.operations] p4 as follows:
Effects: Returns the number of incrementsor decrementsneeded to get from first to last.
Thomas adds more discussion and an alternative view point here.
[ Summit: ]
The proposed wording below was verbally agreed to. Howard provided.
[ Batavia (2009-05): ]
Pete reports that a recent similar change has been made for the advance() function.
We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-07 Frankfurt: ]
Leave Open pending arrival of a post-Concepts WD.
[ 2009-10-14 Daniel provided de-conceptified wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready, replacing the Effects clause in the proposed wording with "If InputIterator meets the requirements of random access iterator then returns (last - first), otherwise returns the number of increments needed to get from first to list.".
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added below.
Rationale:
Solved by N3066.
Proposed resolution:
Change 24.2.7 [random.access.iterators], Table 105 as indicated [This change is not essential but it simplifies the specification] for the row with expression "b - a" and the column Operational semantics:
(a < b) ?distance(a,b): -distance(b,a)
Change 24.4.4 [iterator.operations]/4+5 as indicated:
template<class InputIterator> typename iterator_traits<InputIterator>::difference_type distance(InputIterator first, InputIterator last);4 Effects: If InputIterator meets the requirements of random access iterator then returns (last - first), otherwise
Rreturns the number of incrementsor decrementsneeded to get from first to last.5 Requires: If InputIterator meets the requirements of random access iterator then last shall be reachable from first or first shall be reachable from last, otherwise last shall be reachable from first.
Section: 17 [library] Status: NAD Submitter: Niels Dekker Opened: 2008-12-18 Last modified: 2009-07-17
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Discussion:
The assignment and equality operators = and == are easily confused, just because of their visual similarity, and in this case a simple typo can cause a serious bug. When the left side of an operator= is an rvalue, it's highly unlikely that the assignment was intended by the programmer:
if ( func() = value ) // Typical typo: == intended!
Built-in types don't support assignment to an rvalue, but unfortunately, a lot of types provided by the Standard Library do.
Fortunately the language now offers a syntax to prevent a certain member function from having an rvalue as *this: by adding a ref-qualifier (&) to the member function declaration. Assignment operators are explicitly mentioned as a use case of ref-qualifiers, in "Extending Move Semantics To *this (Revision 1)", N1821 by Daveed Vandevoorde and Bronek Kozicki
Hereby I would like to propose adding ref-qualifiers to all appropriate assignment operators in the library.
[ Batavia (2009-05): ]
Move to Open. We recommend this be deferred until after the next Committee Draft.
[ Frankfurt 2009-07: ]
The LWG declined to move forward with N2819.
Moved to NAD.
Proposed resolution:
A proposed resolution is provided by the paper on this subject, N2819, Ref-qualifiers for assignment operators of the Standard Library
Section: 29 [atomics] Status: Dup Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2009-03-22
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Duplicate of: 880
Discussion:
I'm looking at 29 [atomics] and can't really make sense of a couple of things.
Firstly, there appears to be a typo in the <cstdatomic> synopsis:
The atomic_exchange overload taking an atomic_address is missing the second parameter:
void* atomic_exchange(volatile atomic_address*);should be
void* atomic_exchange(volatile atomic_address*, void*);Note, that this is not covered by 880 "Missing atomic exchange parameter", which only talks about the atomic_bool.
Proposed resolution:
Change the synopsis in 29 [atomics]/2:
void* atomic_exchange(volatile atomic_address*, void*);
Section: 29.5.3 [atomics.types.generic] Status: NAD Editorial Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2009-10-26
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Discussion:
I think it's fairly obvious that atomic<bool> is supposed to be derived from atomic_bool (and otherwise follow the atomic<integral> interface), though I think the current wording doesn't support this. I raised this point along with atomic<floating-point> privately with Herb and I seem to recall it came up in the resulting discussion on this list. However, I don't see anything on the current libs issue list mentioning this problem.
29.5.3 [atomics.types.generic]/3 reads
There are full specializations over the integral types on the atomic class template. For each integral type integral in the second column of table 121 or table 122, the specialization atomic<integral> shall be publicly derived from the corresponding atomic integral type in the first column of the table. These specializations shall have trivial default constructors and trivial destructors.
Table 121 does not include (atomic_bool, bool), so that this should probably be mentioned explicitly in the quoted paragraph.
[ Summit: ]
Move to open. Lawrence will draft a proposed resolution. Also, ask Howard to fix the title.
[ Post Summit Anthony provided proposed wording. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Replace paragraph 3 in 29.5.3 [atomics.types.generic] with
-3- There are full specializations over the integral types on the atomic class template. For each integral type integral in the second column of table 121 or table 122, the specialization atomic<integral> shall be publicly derived from the corresponding atomic integral type in the first column of the table. In addition, the specialization atomic<bool> shall be publicly derived from atomic_bool. These specializations shall have trivial default constructors and trivial destructors.
Section: 20.10.5.1 [time.clock.system] Status: NAD Editorial Submitter: Pete Becker Opened: 2008-12-19 Last modified: 2009-07-13
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Discussion:
In 20.10.5.1 [time.clock.system], the declaration of system_clock::rep says "see below", but there is nothing below that describes it.
[ Howard adds: ]
This note refers to:
-2- system_clock::duration::min() < system_clock::duration::zero() shall be true.I.e. this is standardeze for "system_clock::rep is signed". Perhaps an editorial note along the lines of:
-2- system_clock::duration::min() < system_clock::duration::zero() shall be true. [Note: system_clock::rep shall be signed. -- end note].?
[ Batavia (2009-05): ]
We agree with the direction of the proposed resolution. Move to NAD Editorial.
Proposed resolution:
Add a note to 20.10.5.1 [time.clock.system], p2:
-2- system_clock::duration::min() < system_clock::duration::zero() shall be true. [Note: system_clock::rep shall be signed. -- end note].
Section: 20.10.3.7 [time.duration.cast] Status: NAD Submitter: Pete Becker Opened: 2008-12-20 Last modified: 2009-07-13
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Discussion:
20.10.3.7 [time.duration.cast]/3:.... All intermediate computations shall be carried out in the widest possible representation... .
So ignoring floating-point types for the moment, all this arithmetic has to be done using the implementation's largest integral type, even if both arguments use int for their representation. This seems excessive. And it's not at all clear what this means if we don't ignore floating-point types.
This issue is related to 952.
[ Howard adds: ]
The intent of this remark is that intermediate computations are carried out using:
common_type<typename ToDuration::rep, Rep, intmax_t>::typeThe Remark was intended to be clarifying prose supporting the rather algorithmic description of the previous paragraph. I'm open to suggestions. Perhaps the entire paragraph 3 (Remarks) would be better dropped?
[ Batavia (2009-05): ]
We view this as a specific case of issue 952, and should be resolved when that issue is resolved.
Move to NAD.
Proposed resolution:
Section: 20.10.3.5 [time.duration.nonmember] Status: NAD Editorial Submitter: Pete Becker Opened: 2008-12-20 Last modified: 2009-10-26
View all other issues in [time.duration.nonmember].
View all issues with NAD Editorial status.
Discussion:
In 20.10.3.5 [time.duration.nonmember], paragraph 8 says that calling dur / rep when rep is an instantiation of duration requires a diagnostic. That's followed by an operator/ that takes two durations. So dur1 / dur2 is legal under the second version, but requires a diagnostic under the first.
[ Howard adds: ]
Please see the thread starting with c++std-lib-22980 for more information.
[ Batavia (2009-05): ]
Move to Open, pending proposed wording (and preferably an implementation).
[ 2009-07-27 Howard adds: ]
I've addressed this issue under the proposed wording for 1177 which cleans up several places under 20.10.3 [time.duration] which used the phrase "diagnostic required".
For clarity's sake, here is an example implementation of the constrained operator/:
template <class _Duration, class _Rep, bool = __is_duration<_Rep>::value> struct __duration_divide_result { }; template <class _Duration, class _Rep2, bool = is_convertible<_Rep2, typename common_type<typename _Duration::rep, _Rep2>::type>::value> struct __duration_divide_imp { }; template <class _Rep1, class _Period, class _Rep2> struct __duration_divide_imp<duration<_Rep1, _Period>, _Rep2, true> { typedef duration<typename common_type<_Rep1, _Rep2>::type, _Period> type; }; template <class _Rep1, class _Period, class _Rep2> struct __duration_divide_result<duration<_Rep1, _Period>, _Rep2, false> : __duration_divide_imp<duration<_Rep1, _Period>, _Rep2> { }; template <class _Rep1, class _Period, class _Rep2> inline typename __duration_divide_result<duration<_Rep1, _Period>, _Rep2>::type operator/(const duration<_Rep1, _Period>& __d, const _Rep2& __s) { typedef typename common_type<_Rep1, _Rep2>::type _Cr; duration<_Cr, _Period> __r = __d; __r /= static_cast<_Cr>(__s); return __r; }__duration_divide_result is basically a custom-built enable_if that will contain type only if Rep2 is not a duration and if Rep2 is implicitly convertible to common_type<typename Duration::rep, Rep2>::type. __is_duration is simply a private trait that answers false, but is specialized for duration to answer true.
The constrained operator% works identically.
[ 2009-10 Santa Cruz: ]
Mark NAD Editorial, fixed by 1177.
Proposed resolution:
Section: 20.9.10.2.1 [unique.ptr.single.ctor] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2009-01-07 Last modified: 2010-03-14
View all other issues in [unique.ptr.single.ctor].
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Discussion:
unique_ptr's of array type should not convert to unique_ptr's which do not have an array type.
struct Deleter
{
void operator()(void*) {}
};
int main()
{
unique_ptr<int[], Deleter> s;
unique_ptr<int, Deleter> s2(std::move(s)); // should not compile
}
[ Post Summit: ]
Walter: Does the "diagnostic required" apply to both arms of the "and"?
Tom Plum: suggest to break into several sentences
Walter: suggest "comma" before the "and" in both places
Recommend Review.
[ Batavia (2009-05): ]
The post-Summit comments have been applied to the proposed resolution. We now agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be improved for enable_if type constraining, possibly following Robert's formula.
[ 2009-08-01 Howard updates wording and sets to Review. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010-02-27 Pete Opens: ]
The proposed replacement text doesn't make sense.
If D is a reference type, then E shall be the same type as D, else this constructor shall not participate in overload resolution.This imposes two requirements. 1. If D is a reference type, E has to be D. 2. If D is not a reference type, the constructor shall not participate in overload resolution. If the latter apples, the language in the preceding paragraph that this constructor shall not throw an exception if D is not a reference type is superfluous. I suspect that's not the intention, but I can't parse this text any other way.
U shall not be an array type, else this constructor shall not participate in overload resolution.I don't know what this means.
[ 2010-02-27 Peter adds: ]
I think that the intent is (proposed text):
Remarks: this constructor shall only participate in overload resolution if:
- unique_ptr<U, E>::pointer is implicitly convertible to pointer,
- U is not an array type, and
- if D is a reference type, E is the same type as D.
[ 2010-02-28 Howard adds: ]
I like Peter's proposal. Here is a tweak of it made after looking at my implementation. I believe this fixes a further defect not addressed by the current proposed wording:
Remarks: this constructor shall only participate in overload resolution if:
- unique_ptr<U, E>::pointer is implicitly convertible to pointer, and
- U is not an array type, and
- if D is a reference type, E is the same type as D, else E shall be implicitly convertible to D.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3073.
Proposed resolution:
Change 20.9.10.2.1 [unique.ptr.single.ctor]:
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);-20- Requires: If D is not a reference type, construction of the deleter D from an rvalue of type E shall be well formed and shall not throw an exception.
If D is a reference type, then E shall be the same type as D (diagnostic required). unique_ptr<U, E>::pointer shall be implicitly convertible to pointer. [Note: These requirements imply that T and U are complete types. — end note]Remarks: If D is a reference type, then E shall be the same type as D, else this constructor shall not participate in overload resolution. unique_ptr<U, E>::pointer shall be implicitly convertible to pointer, else this constructor shall not participate in overload resolution. U shall not be an array type, else this constructor shall not participate in overload resolution. [Note: These requirements imply that T and U are complete types. — end note]
Change 20.9.10.2.3 [unique.ptr.single.asgn]:
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);-6- Requires: Assignment of the deleter D from an rvalue D shall not throw an exception.
unique_ptr<U, E>::pointer shall be implicitly convertible to pointer. [Note: These requirements imply that T and U are complete types. — end note]Remarks: unique_ptr<U, E>::pointer shall be implicitly convertible to pointer, else this operator shall not participate in overload resolution. U shall not be an array type, else this operator shall not participate in overload resolution. [Note: These requirements imply that T and U are complete types. — end note]
Section: 20.10.3.7 [time.duration.cast] Status: NAD Editorial Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-07-13
View all other issues in [time.duration.cast].
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Discussion:
20.10.3.7 [time.duration.cast] specifies an implementation and imposes requirements in text (and the implementation doesn't satisfy all of the text requirements). Pick one.
This issue is related to 946.
[ 2009-05-10 Howard adds: ]
The Remarks paragraph is an English re-statement of the preceeding Returns clause. It was meant to be clarifying and motivating, not confusing. I'm not aware with how the Remarks contradicts the Returns clause but I'm ok with simply removing the Remarks.
[ Batavia (2009-05): ]
Pete suggests that this could be resolved by rephrasing the Remarks to Notes.
Move to NAD Editorial.
Proposed resolution:
Section: 20.10.1 [time.clock.req] Status: NAD Editorial Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2010-08-25
View all other issues in [time.clock.req].
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Discussion:
Related to 951.
20.10.1 [time.clock.req] says that a clock's rep member is "an arithmetic type or a class emulating an arithmetic type." What are the requirements for such a type?
[ 2009-05-10 Howard adds: ]
This wording was aimed directly at the ArithmeticLike concept.
[ Batavia (2009-05): ]
We recommend this issue be addressed in the context of providing concepts for the entire thread header.
May resolve for now by specifying arithmetic types, and in future change to ArithmeticLike. However, Alisdair believes this is not feasible.
Bill disagrees.
We look forward to proposed wording. Move to Open.
[ 2009-08-01 Howard adds: ]
See commented dated 2009-08-01 in 951.
[ 2009-10 Santa Cruz: ]
Stefanus to provide wording to turn this into a note.
[ 2010-02-11 Stephanus provided wording for 951 which addresses this issue as well. ]
[ 2010 Rapperswil: ]
Move to NAD Editorial, resolved by 951.
Proposed resolution:
Section: 20.10.1 [time.clock.req] Status: NAD Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-10-26
View all other issues in [time.clock.req].
View all issues with NAD status.
Discussion:
20.10.1 [time.clock.req] requires that a clock type have a member typedef named time_point that names an instantiation of the template time_point, and a member named duration that names an instantiation of the template duration. This mixing of levels is confusing. The typedef names should be different from the template names.
[ Post Summit, Anthony provided proposed wording. ]
[ 2009-05-04 Howard adds: ]
The reason that the typedef names were given the same name as the class templates was so that clients would not have to stop and think about whether they were using the clock's native time_point / duration or the class template directly. In this case, one person's confusion is another person's encapsulation. The detail that sometimes one is referring to the clock's native types, and sometimes one is referring to an independent type is purposefully "hidden" because it is supposed to be an unimportant detail. It can be confusing to have to remember when to type duration and when to type duration_type, and there is no need to require the client to remember something like that.
For example, here is code that I once wrote in testing out the usability of this facility:
template <class Clock, class Duration> void do_until(const std::chrono::time_point<Clock, Duration>& t) { typename Clock::time_point now = Clock::now(); if (t > now) { typedef typename std::common_type < Duration, typename std::chrono::system_clock::duration >::type CD; typedef std::chrono::duration<double, std::nano> ID; CD d = t - now; ID us = duration_cast<ID>(d); if (us < d) ++us; ... } }I see no rationale to require the client to append _type to some of those declarations. It seems overly burdensome on the author of do_until:
template <class Clock, class Duration> void do_until(const std::chrono::time_point<Clock, Duration>& t) { typename Clock::time_point_type now = Clock::now(); if (t > now) { typedef typename std::common_type < Duration, typename std::chrono::system_clock::duration_type >::type CD; typedef std::chrono::duration<double, std::nano> ID; CD d = t - now; ID us = duration_cast<ID>(d); if (us < d) ++us; ... } }Additionally I'm fairly certain that this suggestion hasn't been implemented. If it had, it would have been discovered that it is incomplete. time_point also has a nested type (purposefully) named duration.
That is, the current proposed wording would put the WP into an inconsistent state.In contrast, the current WP has been implemented and I've received very favorable feedback from people using this interface in real-world code.
[ Batavia (2009-05): ]
Bill agrees that distinct names should be used for distinct kinds of entities.
Walter would prefer not to suffix type names, especially for such well-understood terms as "duration".
Howard reminds us that the proposed resolution is incomplete, per his comment in the issue.
Move to Open.
[ 2009-06-07 Howard adds: ]
Not meaning to be argumentative, but we have a decade of positive experience with the precedent of using the same name for the nested type as an external class representing an identical concept.
template<class Category, class T, class Distance = ptrdiff_t, class Pointer = T*, class Reference = T&> struct iterator { ... }; template <BidirectionalIterator Iter> class reverse_iterator { ... }; template <ValueType T, Allocator Alloc = allocator<T> > requires NothrowDestructible<T> class list { public: typedef implementation-defined iterator; ... typedef reverse_iterator<iterator> reverse_iterator; ... };I am aware of zero complaints regarding the use of iterator and reverse_iterator as nested types of the containers despite these names also having related meaning at namespace std scope.
Would we really be doing programmers a favor by renaming these nested types?
template <ValueType T, Allocator Alloc = allocator<T> > requires NothrowDestructible<T> class list { public: typedef implementation-defined iterator_type; ... typedef reverse_iterator<iterator> reverse_iterator_type; ... };I submit that such design contributes to needless verbosity which ends up reducing readability.
[ 2009-10 Santa Cruz: ]
Mark as NAD. No concensus for changing the WP.
Proposed resolution:
Change 20.10 [time]:
... template <class Clock, class Duration = typename Clock::duration_type> class time_point; ...
Change 20.10.1 [time.clock.req]:
Table 45 -- Clock requirements Expression Return type Operational semantics ... ... ... C1::duration_type chrono::duration<C1::rep, C1::period> The native duration type of the clock. C1::time_point_type chrono::time_point<C1> or chrono::time_point<C2, C1::duration_type< The native time_point type of the clock. Different clocks may share a time_point_type definition if it is valid to compare their time_point_types by comparing their respective duration_types. C1 and C2 shall refer to the same epoch. ... ... ... C1::now() C1::time_point_type Returns a time_point_type object representing the current point in time.
Change 20.10.5.1 [time.clock.system]:
-1- Objects of class system_clock represent wall clock time from the system-wide realtime clock.
class system_clock { public: typedef see below rep; typedef ratio<unspecified, unspecified> period; typedef chrono::duration<rep, period> duration_type; typedef chrono::time_point<system_clock> time_point_type; static const bool is_monotonic = unspecified ; static time_point_type now(); // Map to C API static time_t to_time_t (const time_point_type& t); static time_point_type from_time_t(time_t t); };-2- system_clock::duration_type::min() < system_clock::duration_type::zero() shall be true.
time_t to_time_t(const time_point_type& t);-3- Returns: A time_t object that represents the same point in time as t when both values are truncated to the coarser of the precisions of time_t and time_point_type.time_point_type from_time_t(time_t t);-4- Returns: A time_point_type object that represents the same point in time as t when both values are truncated to the coarser of the precisions of time_t and time_point_type.
Change 20.10.5.2 [time.clock.monotonic]:
class monotonic_clock {
public:
typedef unspecified rep;
typedef ratio<unspecified , unspecified> period;
typedef chrono::duration<rep, period> duration_type;
typedef chrono::time_point<unspecified , duration_type> time_point_type;
static const bool is_monotonic = true;
static time_point_type now();
};
Change 20.10.5.3 [time.clock.hires]:
class high_resolution_clock {
public:
typedef unspecified rep;
typedef ratio<unspecified , unspecified> period;
typedef chrono::duration<rep, period> duration_type;
typedef chrono::time_point<unspecified , duration_type> time_point_type;
static const bool is_monotonic = true;
static time_point_type now();
};
Section: 30.5.1 [thread.condition.condvar] Status: NAD Editorial Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-10-23
View all other issues in [thread.condition.condvar].
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Discussion:
30.5.1 [thread.condition.condvar]: the specification for wait_for with no predicate has an effects clause that says it calls wait_until, and a returns clause that sets out in words how to determine the return value. Is this description of the return value subtly different from the description of the value returned by wait_until? Or should the effects clause and the returns clause be merged?
[ Summit: ]
Move to open. Associate with LWG 859 and any other monotonic-clock related issues.
[ 2009-08-01 Howard adds: ]
I believe that 859 (currently Ready) addresses this issue, and that this issue should be marked NAD, solved by 859 (assuming it moves to WP).
[ 2009-10 Santa Cruz: ]
Mark as NAD Editorial, solved by resolution of Issue 859.
Proposed resolution:
Section: 30.5.1 [thread.condition.condvar] Status: NAD Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2010-03-08
View all other issues in [thread.condition.condvar].
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Discussion:
30.5.1 [thread.condition.condvar]: condition_variable::wait_for is required to compute the absolute time by adding the duration value to chrono::monotonic_clock::now(), but monotonic_clock is not required to exist.
[ Summit: ]
Move to open. Associate with LWG 859 and any other monotonic-clock related issues.
[ 2009-08-01 Howard adds: ]
I believe that 859 (currently Ready) addresses this issue, and that this issue should be marked NAD, solved by 859 (assuming it moves to WP).
[ 2009-10 Santa Cruz: ]
Leave open, but expect to be fixed by N2969 revision that Detlef is writing.
[ 2009-11-18 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]
Proposed resolution:
Rationale:
condition_variable::wait_for no longer refers to monotonic_clock, so this issue is moot.
Section: 30.4.1 [thread.mutex.requirements] Status: NAD Future Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-10-26
View all other issues in [thread.mutex.requirements].
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Duplicate of: 936
Discussion:
30.4.1 [thread.mutex.requirements] describes required member functions of mutex types, and requires that they throw exceptions under certain circumstances. This is overspecified. User-defined types can abort on such errors without affecting the operation of templates supplied by standard-library.
[ Summit: ]
Move to open. Related to conceptualization and should probably be tackled as part of that.
[ 2009-10 Santa Cruz: ]
Would be OK to leave it as is for time constraints, could loosen later.
Mark as NAD Future.
Proposed resolution:
Section: 25.2.4 [alg.foreach] Status: NAD Editorial Submitter: Stephan T. Lavavej Opened: 2009-01-12 Last modified: 2009-07-13
View all other issues in [alg.foreach].
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Discussion:
Library Issue 475 has CD1 status, but the non-normative note in N2723 was removed in N2798 (25.2.4 [alg.foreach] in both drafts).
[ Batavia (2009-05): ]
Move to NAD Editorial.
Proposed resolution:
Restore the non-normative note. It might need to be expressed in terms of concepts.
Section: 19.5.2.5 [syserr.errcode.nonmembers] Status: NAD Submitter: Beman Dawes Opened: 2009-01-19 Last modified: 2009-10-20
View all issues with NAD status.
Discussion:
Anthony Williams raised the question in c++std-lib-22987 "why is there std::make_error_code(std::errc)? What purpose does this serve?"
The function make_error_code(errc e) is not required, since make_error_condition(errc e) is the function that is needed for errc conversions. make_error_code(errc e) appears to be a holdover from my initial confusion over the distinction between POSIX and operating systems that conform to the POSIX spec.
[ Post Summit: ]
Recommend Review.
[ Batavia (2009-05): ]
The designer of the facility (Christopher Kohlhoff) strongly disagrees that there is an issue here, and especially disagrees with the proposed resolution. Bill would prefer to be conservative and not apply this proposed resolution. Move to Open, and recommend strong consideration for NAD status.
[ 2009-05-21 Beman adds: ]
My mistake. Christopher and Bill are correct and the issue should be NAD. The function is needed by users.
[ 2009-07-21 Christopher Kohlhoff adds rationale for make_error_code: ]
Users (and indeed library implementers) may need to use the errc codes in portable code. For example:
void do_foo(error_code& ec) { #if defined(_WIN32) // Windows implementation ... #elif defined(linux) // Linux implementation ... #else // do_foo not supported on this platform ec = make_error_code(errc::not_supported); #endif }
[ 2009 Santa Cruz: ]
Moved to NAD.
Proposed resolution:
Change System error support 19.5 [syserr], Header <system_error> synopsis, as indicated:
error_code make_error_code(errc e);error_condition make_error_condition(errc e);
Delete from Class error_code non-member functions 19.5.2.5 [syserr.errcode.nonmembers]:
error_code make_error_code(errc e);Returns: error_code(static_cast<int>(e), generic_category).
Section: 17 [library] Status: NAD Editorial Submitter: Niels Dekker Opened: 2009-01-07 Last modified: 2009-07-13
View other active issues in [library].
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Discussion:
Previous versions of the Draft had a table, defining the Assignable requirement. For example N2134 Table 79, "Assignable requirements". But I guess the term "Assignable" is outdated by now, because the current Committee Draft provides MoveAssignable, CopyAssignable, and TriviallyCopyAssignable concepts instead. And as far as I can see, it no longer has a definition of Assignable. (Please correct me if I'm wrong.) Still the word "Assignable" is used in eight places in the Draft, N2800.
Are all of those instances of "Assignable" to be replaced by "CopyAssignable"?
[ Batavia (2009-05): ]
Move to NAD Editorial.
Proposed resolution:
Change Exception Propagation 18.8.5 [propagation]:
exception_ptr shall be DefaultConstructible, CopyConstructible, CopyAssignable and EqualityComparable.
Change Class template reference_wrapper 20.8.4 [refwrap]:
reference_wrapper<T> is a CopyConstructible and CopyAssignable wrapper around a reference to an object of type T.
Change Placeholders 20.8.10.1.3 [func.bind.place]:
It is implementation defined whether placeholder types are CopyAssignable. CopyAssignable placeholders' copy assignment operators shall not throw exceptions.
Change Class template shared_ptr 20.9.11.2 [util.smartptr.shared]:
Specializations of shared_ptr shall be CopyConstructible, CopyAssignable, and LessThanComparable...
Change Class template weak_ptr 20.9.11.3 [util.smartptr.weak]:
Specializations of weak_ptr shall be CopyConstructible, CopyAssignable, and LessThanComparable...
Change traits typedefs 21.2.2 [char.traits.typedefs] (note: including deletion of reference to 23.1!):
Requires: state_type shall meet the requirements of CopyAssignable(23.1), CopyConstructible (20.1.8), and DefaultConstructible types.
Change Class seed_seq 26.5.7.1 [rand.util.seedseq] (note again: including deletion of reference to 23.1!):
In addition to the requirements set forth below, instances of seed_seq shall meet the requirements of CopyConstructible (20.1.8) and of CopyAssignable(23.1).
Note: The proposed resolution of this issue does not deal with the instance of the term "Assignable" in D.10.1 [auto.ptr], as this is dealt with more specifically by LWG 973, "auto_ptr characteristics", submitted by Maarten Hilferink.
Section: D.10.1 [auto.ptr] Status: NAD Editorial Submitter: Maarten Hilferink Opened: 2009-01-21 Last modified: 2009-07-13
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Discussion:
I think that the Note of D.10.1 [auto.ptr], paragraph 3 needs a rewrite since "Assignable" is no longer defined as a concept. The relationship of auto_ptr with the new CopyAssignable, MoveAssignable, and MoveConstructible concepts should be clarified. Furthermore, since the use of auto_ptr is depreciated anyway, we can also omit a description of its intended use.
[ Batavia (2009-05): ]
We agree with the intent of the proposed resolution. Move to NAD Editorial.
Proposed resolution:
Change D.10.1 [auto.ptr], paragraph 3:
The auto_ptr provides a semantics of strict ownership. An auto_ptr owns the ob ject it holds a pointer to. Copying an auto_ptr copies the pointer and transfers ownership to the destination. If more than one auto_ptr owns the same ob ject at the same time the behavior of the program is undefined. [Note: The uses of auto_ptr include providing temporary exception-safety for dynamically allocated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function.auto_ptr does not meet the CopyConstructible and Assignable requirements for standard library container elements and thus instantiating a standard library container with an auto_ptr results in undefined behavior.Instances of auto_ptr shall meet the MoveConstructible and MoveAssignable requirements, but do not meet the CopyConstructible and CopyAssignable requirements. -- end note]
Section: 23.3.5.3.1 [stack.defn] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2009-02-01 Last modified: 2009-10-20
View all issues with NAD Editorial status.
Discussion:
The synopsis given in 23.3.5.3.1 [stack.defn] does not show up
requires MoveConstructible<Cont> stack(stack&&); requires MoveAssignable<Cont> stack& operator=(stack&&);
although the other container adaptors do provide corresponding members.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-08-18 Daniel updates the wording and Howard sets to Review. ]
[ 2009-08-23 Howard adds: ]
1194 also adds these move members using an editorially different style.
[ 2009-10 Santa Cruz: ]
Mark NAD Editorial, solved by issue 1194.
Proposed resolution:
In the class stack synopsis of 23.3.5.3.1 [stack.defn] insert:
template <class T, class Container = deque<T> >
class stack {
[..]
explicit stack(const Container&);
explicit stack(Container&& = Container());
stack(stack&& s) : c(std::move(s.c)) {}
stack& operator=(stack&& s) { c = std::move(s.c); return *this; }
[..]
};
Section: 24.5.2 [insert.iterators] Status: NAD Submitter: Howard Hinnant Opened: 2009-02-02 Last modified: 2009-10-26
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Discussion:
The new concepts for the insert iterators mandate an extra copy when inserting an lvalue:
requires CopyConstructible<Cont::value_type> back_insert_iterator<Cont>& operator=(const Cont::value_type& value);-1- Effects: push_back(*container, Cont::value_type(value));
The reason is to convert value into an rvalue because the current BackInsertionContainer concept only handles push_back-ing rvalues:
concept BackInsertionContainer<typename C> : Container<C> {
void push_back(C&, value_type&&);
}
Without the conversion of value to an rvalue, the assignment operator fails to concept check.
A solution is to modify the BackInsertionContainer concept so that the client can pass in the parameter type for push_back similar to what is already done for the OutputIterator concept:
concept BackInsertionContainer<typename C, typename Value = C::value_type&&>
: Container<C> {
void push_back(C&, Value);
}
This allows the assignment operator to be adjusted appropriately:
requires BackInsertionContainer<Cont, Cont::value_type const&> && CopyConstructible<Cont::value_type> back_insert_iterator<Cont>& operator=(const Cont::value_type& value);-1- Effects: push_back(*container, value);
[ We may want to propagate this fix to other concepts such as StackLikeContainer. ]
[ Solution and wording collaborated on by Doug and Howard. ]
[ Batavia (2009-05): ]
Howard notes that "these operations behaved efficiently until concepts were added."
Alisdair is uncertain that the proposed resolution is syntactically correct.
Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.
[ 2009-10 Santa Cruz: ]
NAD, solved by the removal of concepts.
Proposed resolution:
Change [container.concepts.free]:
concept FrontInsertionContainer<typename C, typename Value = C::value_type&&> : Container<C> { void push_front(C&,value_type&&Value); axiom FrontInsertion(C c,value_typeValue x) { x == (push_front(c, x), front(c)); } }...
concept BackInsertionContainer<typename C, typename Value = C::value_type&&> : Container<C> { void push_back(C&,value_type&&Value); }...
concept InsertionContainer<typename C, typename Value = C::value_type&&> : Container<C> { iterator insert(C&, const_iterator,value_type&&Value); axiom Insertion(C c, const_iterator position,value_typeValue v) { v == *insert(c, position, v); } }
Change [container.concepts.member]:
auto concept MemberFrontInsertionContainer<typename C, typename Value = C::value_type&&> : MemberContainer<C> { void C::push_front(value_type&&Value); axiom MemberFrontInsertion(C c,value_typeValue x) { x == (c.push_front(x), c.front()); } }...
auto concept MemberBackInsertionContainer<typename C, typename Value = C::value_type&&> : MemberContainer<C> { void C::push_back(value_type&&Value); }...
auto concept MemberInsertionContainer<typename C, typename Value = C::value_type&&> : MemberContainer<C> { iterator C::insert(const_iterator,value_type&&Value); axiom MemberInsertion(C c, const_iterator position,value_typeValue v) { v == *c.insert(position, v); } }
Change [container.concepts.maps]:
template <MemberFrontInsertionContainer C, typename Value = C::value_type&&> concept_map FrontInsertionContainer<C, Value> { typedef Container<C>::value_type value_type; void push_front(C& c,value_type&&Value v) { c.push_front(static_cast<value_type&&Value>(v)); } }...
template <MemberBackInsertionContainer C, typename Value = C::value_type&&> concept_map BackInsertionContainer<C, Value> { typedef Container<C>::value_type value_type; void push_back(C& c,value_type&&Value v) { c.push_back(static_cast<value_type&&Value>(v)); } }...
template <MemberInsertionContainer C, typename Value = C::value_type&&> concept_map InsertionContainer<C, Value> { typedef Container<C>::value_type value_type; Container<C>::iterator insert(C& c, Container<C>::const_iterator i,value_type&&Value v) { return c.insert(i, static_cast<value_type&&Value>(v)); } }
Change 24.5.2.1 [back.insert.iterator]:
template <BackInsertionContainer Cont>
class back_insert_iterator {
...
requires BackInsertionContainer<Cont, const Cont::value_type&>
CopyConstructible<Cont::value_type>
back_insert_iterator<Cont>&
operator=(const Cont::value_type& value);
...
Change 24.5.2.2.2 [back.insert.iter.op=]:
requires BackInsertionContainer<Cont, const Cont::value_type&>CopyConstructible<Cont::value_type>back_insert_iterator<Cont>& operator=(const Cont::value_type& value);-1- Effects: push_back(*container,Cont::value_type(value));
Change 24.5.2.3 [front.insert.iterator]:
template <FrontInsertionContainer Cont>
class front_insert_iterator {
...
requires FrontInsertionContainer<Cont, const Cont::value_type&>
CopyConstructible<Cont::value_type>
front_insert_iterator<Cont>&
operator=(const Cont::value_type& value);
...
Change 24.5.2.4.2 [front.insert.iter.op=]:
requires FrontInsertionContainer<Cont, const Cont::value_type&>CopyConstructible<Cont::value_type>front_insert_iterator<Cont>& operator=(const Cont::value_type& value);-1- Effects: push_front(*container,Cont::value_type(value));
Change 24.5.2.5 [insert.iterator]:
template <InsertionContainer Cont>
class insert_iterator {
...
requires InsertionContainer<Cont, const Cont::value_type&>
CopyConstructible<Cont::value_type>
insert_iterator<Cont>&
operator=(const Cont::value_type& value);
...
Change 24.5.2.6.2 [insert.iter.op=]:
requires InsertionContainer<Cont, const Cont::value_type&>CopyConstructible<Cont::value_type>insert_iterator<Cont>& operator=(const Cont::value_type& value);-1- Effects:
iter = insert(*container, iter,Cont::value_type(value)); ++iter;
Section: 24.5.3 [move.iterators] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2009-02-03 Last modified: 2009-07-13
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Discussion:
24.5.3 [move.iterators] has an incorrect example:
-2- [Example:
set<string> s; // populate the set s vector<string> v1(s.begin(), s.end()); // copies strings into v1 vector<string> v2(make_move_iterator(s.begin()), make_move_iterator(s.end())); // moves strings into v2-- end example]
One can not move from a set because the iterators return const references.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to NAD Editorial.
Proposed resolution:
Change 24.5.3 [move.iterators]/2:
-2- [Example:
setlist<string> s; // populate thesetlist s vector<string> v1(s.begin(), s.end()); // copies strings into v1 vector<string> v2(make_move_iterator(s.begin()), make_move_iterator(s.end())); // moves strings into v2-- end example]
Section: 30.4.1 [thread.mutex.requirements] Status: NAD Submitter: Ion Gaztañaga Opened: 2009-02-07 Last modified: 2009-03-22
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Discussion:
POSIX 2008 adds two return values for pthread_mutex_xxxlock(): EOWNERDEAD (owner_dead) and ENOTRECOVERABLE (state_not_recoverable). In the first case the mutex is locked, in the second case the mutex is not locked.
Throwing an exception in the first case can be incompatible with the use of Locks, since the Lock::owns_lock() will be false when the lock is being destroyed.
Consider:
//Suppose mutex.lock() throws "owner_dead" unique_lock ul(&mutex); //mutex left locked if "owner_dead" is thrown
Throwing an exception with owner_dead might be also undesirable if robust-mutex support is added to C++ and the user has the equivalent of pthread_mutex_consistent() to notify the user has fixed the corrupted data and the mutex state should be marked consistent.
[ Summit: ]
Not a defect. Handling these error conditions is an implementation detail and must be handled below the C++ interface.
Proposed resolution:
Add to 30.4.1 [thread.mutex.requirements], p12:
-12- Error conditions:
- operation_not_permitted -- if the thread does not have the necessary permission to change the state of the mutex.
- resource_deadlock_would_occur -- if the current thread already owns the mutex and is able to detect it.
- device_or_resource_busy -- if the mutex is already locked and blocking is not possible.
- state_not_recoverable -- if the state protected by the mutex is not recoverable.
Section: 20.9.10.2 [unique.ptr.single] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2009-02-10 Last modified: 2010-03-14
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Discussion:
Dave brought to my attention that when a unique_ptr has a non-const reference type deleter, move constructing from it, even when the unique_ptr containing the reference is an rvalue, could have surprising results:
D d(some-state); unique_ptr<A, D&> p(new A, d); unique_ptr<A, D> p2 = std::move(p); // has d's state changed here?
I agree with him. It is the unique_ptr that is the rvalue, not the deleter. When the deleter is a reference type, the unique_ptr should respect the "lvalueness" of the deleter.
Thanks Dave.
[ Batavia (2009-05): ]
Seems correct, but complicated enough that we recommend moving to Review.
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
Change 20.9.10.2.1 [unique.ptr.single.ctor], p20-21
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);-20- Requires: If
DE is not a reference type, construction of the deleter D from an rvalue of type E shall be well formed and shall not throw an exception. Otherwise E is a reference type and construction of the deleter D from an lvalue of type E shall be well formed and shall not throw an exception. If D is a reference type, then E shall be the same type as D (diagnostic required). unique_ptr<U, E>::pointer shall be implicitly convertible to pointer. [Note: These requirements imply that T and U are complete types. -- end note]-21- Effects: Constructs a unique_ptr which owns the pointer which u owns (if any). If the deleter E is not a reference type,
itthis deleter is move constructed from u's deleter, otherwisethe referencethis deleter is copy constructed from u.'s deleter. After the construction, u no longer owns a pointer. [Note: The deleter constructor can be implemented with std::forward<DE>. -- end note]
Change 20.9.10.2.3 [unique.ptr.single.asgn], p1-3
unique_ptr& operator=(unique_ptr&& u);-1- Requires: If the deleter D is not a reference type,
Aassignment of the deleter D from an rvalue D shall not throw an exception. Otherwise the deleter D is a reference type, and assignment of the deleter D from an lvalue D shall not throw an exception.-2- Effects: reset(u.release()) followed by an
moveassignment fromu's deleter to this deleterstd::forward<D>(u.get_deleter()).-3- Postconditions: This unique_ptr now owns the pointer which u owned, and u no longer owns it.
[Note: If D is a reference type, then the referenced lvalue deleters are move assigned. -- end note]
Change 20.9.10.2.3 [unique.ptr.single.asgn], p6-7
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);Requires: If the deleter E is not a reference type,
Aassignment of the deleter D from an rvalueDE shall not throw an exception. Otherwise the deleter E is a reference type, and assignment of the deleter D from an lvalue E shall not throw an exception. unique_ptr<U, E>::pointer shall be implicitly convertible to pointer. [Note: These requirements imply that T and U> are complete types. -- end note]Effects: reset(u.release()) followed by an
moveassignment fromu's deleter to this deleterstd::forward<E>(u.get_deleter()).If either D or E is a reference type, then the referenced lvalue deleter participates in the move assignment.
Section: X [concept.comparison] Status: NAD Submitter: Alisdair Meredith Opened: 2009-02-24 Last modified: 2009-07-16
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Discussion:
X [concept.comparison] p2:
Due to the subtle meaning of == inside axioms, the Reflexivity axiom does not do anything as written. It merely states that a value is substitutable with itself, rather than asserting a property of the == operator.
Original proposed resolution:Change the definition of Reflexivity in X [concept.comparison]:
axiom Reflexivity(T a) { (a == a) == true; }
[ Post Summit: ]
Alisdair: I was wrong.
Recommend NAD.
Proposed resolution:
NAD.
Section: 17 [library] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-02-24 Last modified: 2009-07-16
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Discussion:
The example in 6.9p2 shows how late_check blocks inhibit concept_map lookup inside a constrained context, and so inhibit concept map adaption by users to meet template requirements.
Do we need some text in clause 17 prohibitting use of late_check in library template definitions unless otherwise documented?
[ Doug adds: ]
We need something like this, but it should be a more general statement about implementations respecting the concept maps provided by the user. Use of late_check is one way in which implementations can subvert the concept maps provided by the user, but there are other ways as well ("pattern-based" overloading, tricks with "auto" concept maps and defaulted associated type arguments).
[ Batavia (2009-05): ]
Move to Open, pending proposed wording from Alisdair and/or Doug for further review.
Proposed resolution:
Section: 17.6.1.1 [contents] Status: NAD Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2009-07-22
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Discussion:
Addresses UK 169
This phrasing contradicts later freedom to implement the C standard library portions in the global namespace as well as std. (17.6.2.3p4)
[ Batavia (2009-05): ]
The proposed wording seems to go too far. Move back to Open.
[ 2009-07 Frankfurt: ]
Howard to add NB reference to the description of this issue.
Move to NAD. This comment is informative and not normative by the use of the word "are" instead of the word "shall."
A note linking to Annex D would help clarify the intention, here.
Robert to Open a separate issue proposing that the standard C headers be undeprecated, for the purpose of clarifying the standard.
[ 2009-07-22 Bill modified the proposed wording with a clarifying footnote. ]
Proposed resolution:
Add a footnote to 17.6.1.1 [contents], p2:
-2- All library entities except macros, operator new and operator delete are defined within the namespace std or namespaces nested within namespace std*.
*The C standard library headers D.6 [depr.c.headers] also define names within the global namespace, while the C++ headers for C library facilities 17.6.1.2 [headers] may also define names within the global namespace.
Section: 17.5.1.3 [structure.requirements] Status: NAD Submitter: David Abrahams Opened: 2009-03-06 Last modified: 2009-07-13
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Discussion:
As a practical matter there's disagreement on the meaning of operational semantics. If the text in 17.5.1.3 [structure.requirements]p4 isn't clear, it should be clarified. However, it's not clear whether the disagreement is merely due to people not being aware of the text.
[ Batavia (2009-05): ]
Agree with the recommended NAD resolution.
Proposed resolution:
Recommend NAD. The text in 17.5.1.3 [structure.requirements] is perfectly clear.
Section: 17 [library] Status: NAD Submitter: David Abrahams Opened: 2009-03-06 Last modified: 2010-08-25
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Discussion:
There are lots of places in the standard where we talk about "the move constructor" but where we mean "the move operation," i.e. T( move( x ) ).
We also don't account for whether that operation modifies x or not, and we need to.
[ Batavia (2009-05): ]
Move to Open, pending proposed wording from Dave for further review.
[ 2010 Rapperswil: ]
Move to NAD. We define what we expect from a moved-from object in Table 34 [movesconstructible].
Proposed resolution:
Section: 25.2.8 [alg.adjacent.find] Status: NAD Concepts Submitter: Chris Jefferson Opened: 2009-03-09 Last modified: 2009-07-15
View all other issues in [alg.adjacent.find].
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Discussion:
Addresses UK 296
adjacent_find in C++03 allows an arbitrary predicate, but in C++0x EqualityComparable/EquivalenceRelation is required. This forbids a number of use cases, including:
adjacent_find(begin, end, less<double>) Find the first place where a range is not ordered in decreasing order - in use to check for sorted ranges. adjacent_find(begin, end, DistanceBiggerThan(6) ) ) Find the first place in a range where values differ by more than a given value - in use to check an algorithm which produces points in space does not generate points too far apart.
A number of books use predicate which are not equivalence relations in examples, including "Thinking in C++" and "C++ Primer".
Adding the requirement that the predicate is an EquivalenceRelation does not appear to open up any possibility for a more optimised algorithm.
Proposed resolution:
Change the definition of adjacent_find in the synopsis of 25 [algorithms] and 25.2.8 [alg.adjacent.find] to:
template<ForwardIterator Iter> requiresEqualityComparableHasEqualTo<Iter::value_type, Iter::value_type> Iter adjacent_find(Iter first, Iter last); template<ForwardIterator Iter,EquivalenceRelationPredicate<auto, Iter::value_type, Iter::value_type> Pred> requires CopyConstructible<Pred> Iter adjacent_find(Iter first, Iter last, Pred pred);
Section: 17 [library] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-10 Last modified: 2009-07-18
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Discussion:
Addresses UK 78
Related to 1063.
This is effectively an extension of LWG issue 343.
We know there is an increasing trend (encouraged by conformance testers and some users) that each library header should supply no more than required to satisfy the synopsis in the standard. This is typically achieved by breaking larger headers into smaller subsets, and judicious use of forward declarations.
If we apply this policy to C++0x (per N2800) it will be very surprising for people using library algorithms over ranges defined by pointers that they must #include <iterator_concepts> for their code to compile again. That is because pointers do not satisfy any of the iterator concepts without the concept_map supplied in this header.
Therefore, I suggest we should require all library headers that make use of iterator concepts are specifically required to #include <iterator_concepts>.
At a minimum, the list of headers would be: (assuming all are constrained by concepts)
algorithm array deque forward_list initializer_list iterator locale list map memory // if 1029 is adopted memory_concepts numeric random regex set string tuple unordered_map unordered_set utility vector
[ Ganesh adds: ]
The same problems exists for <memory_concepts> and <container_concepts>.
In order to compile <vector> you just need the definitions of the concepts in <memory_concepts>, the concept maps defined there are not necessary. Yet, from the user point of view, if the concept map template for AllocatableElement are not in scope, <vector> is pretty useless. Same for <tuple> and ConstructibleWithAllocator.
Similarly, <queue> is not very useful if the concept map template for QueueLikeContainer is not in scope, although the definition of concept alone is theoretically sufficient.
There's a pattern here: if a concept has concept maps "attached", they should never be separated.
[ Beman provided the proposed resolution for the May 2009 mailing. He comments: ]
Initially I tried to specify exactly what header should include what other headers. This was verbose, error prone, hard to maintain, and appeared to add little value compared to just stating the general rule.
[ Batavia (2009-05): ]
Pete believes the proposed wording overconstrains implementers. Instead of specifying the mechanism, he prefers a solution that spells out what needs to be declared, rather than how those declarations are to be provided, e.g.,
A C++ header shall provide the names that are required to be defined in that header.Bill suggests approaching the wording from a programmer's perspective. We may want to consider promising that certain widely-used headers (e.g., the concept headers) are included when needed by other headers. He feels, however, there is nothing broken now, although we may want to consider "something nicer."
Move to Open status.
[ 2009-06-16 Beman updated the proposed resolution: ]
- The mechanism is no longer specified, as requested in Batavia.
- The footnote has been removed since it specified mechanism and also did not reflect existing practice.
- A sentence was added that makes it clear that the existing practice is permitted.
[ 2009-07-15 Beman updated the proposed resolution: ]
[ 2009-07-17 Beman updated the proposed resolution based on feedback from the LWG in Frankfurt: ]
- Strike two pieces of text considered unnecessary.
- Change "definitions" to "declarations and definitions" in two places.
- Wording tightened slightly.
[ 2009-07 Frankfurt: ]
Revised Proposed Resolution:
A C++ header may include other C++ headers. A C++ header shall provide the declarations and definitions that appear in its synopsis (3.2 [basic.def.odr]). A C++ header shown in its synopsis as including other C++ headers shall provide the declarations and definitions that appear in the synopses of those other headers.
Alisdair: Does this address the BSI comment?
Beman: There were several overlapping comments. I tried to handle them all with one resolution.
Alisdair: I'd prefer to see this closed as NAD and have this resolution be the subject of some other, new issue.
Move to NAD Concepts. Howard to open a new issue (1178) in Ready state with the Proposed Resolution above. Beman will write up a discussion for the new issue.
Proposed resolution:
Change 17.6.4.2 [res.on.headers], Headers, paragraph 1, as indicated:
A C++ header may include other C++ headers.
[footnote]A C++ header shall provide the declarations and definitions that appear in its synopsis (3.2 [basic.def.odr]). A C++ header shown in its synopsis as including other C++ headers shall provide the same declarations and definitions as if those other headers were included.
[footnote] C++ headers must include a C++ header that contains any needed definition (3.2).
Section: 17.6.1.2 [headers] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-15
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Discussion:
Addresses UK 170
One of goals of C++0x is to make language easier to teach and for 'incidental' programmers. The fine-grained headers of the C++ library are valuable in large scale systems for managing dependencies and optimising build times, but overcomplicated for simple development and tutorials. Add additional headers to support the whole library through a single include statement.
[ Batavia (2009-05): ]
We do not all agree that this is an issue, but we agree that if it needs solving this is the right way to do it. Move to Tentatively Ready.
[ 2009-07-06 Beman notes: ]
This issue adds a header <std>.
There is a paper to be looked at, N2905 Aggregation headers, that adds a header <std-all> that is the same thing except it excludes deprecated headers. N2905 also proposes a second aggregation header.
Seems like this issue should be held in abeyance until the LWG has had a chance to look at N2905.
[ 2009-07-06 Howard: I've pulled this issue back to Review. ]
[ 2009-07 Frankfurt ]
No consensus for change.
Proposed resolution:
Insert a new paragraph in 17.6.1.2 [headers] between p4 and p5
An additional header <std> shall have the effect of supplying the entire standard library. [Note: for example, it might be implemented as a file with an #include statement for each of the headers listed in tables 13 and 14. -- end note]
Section: 17.6.1.3 [compliance] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-18
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Discussion:
Addresses JP 23
There is a freestanding implementation including <type_traits>, <array>, <ratio>, lately added to Table 13, C++ library headers. Programmers think them useful and hope that these headers are also added to Table 15, C++ headers for freestanding implementations, that shows the set of headers which a freestanding implementation shall include at least.
Original proposed resolution
Add <type_traits>, <array>, <ratio> to Table 15.
[ Summit: ]
The <array> header has far too many dependencies to require for a free-standing implementation.
The <ratio> header would be useful, has no dependencies, but is not strictly necessary.
The <type_traits> header is fundamentally a core language facility with a library interface, so should be supported.
(it is anticipated the resolution will come via an update to paper N2814) (see also LWG 833)
[ Batavia (2009-05): ]
Leave in Review status pending a paper on freestanding implementations by Martin Tasker.
[ 2009-07 Frankfurt: ]
Move this to NAD.
We considered all of the listed headers, and found a compelling case only for the inclusion of <type_traits> in the list of headers required of a freestanding implementation.
See Martin Tasker's paper Fixing Freestanding which provides the wording to include <type_traits> into freestanding implementations.
Proposed resolution:
Add <type_traits> to Table 15.
Section: 18.3.1.1 [numeric.limits] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-15
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Discussion:
Addresses JP 26
numeric_limits [partial specializations] does not use concept.
[ Summit: ]
Alisdair will provide a soltion as part of treatment of axioms and LWG 902.
[ Post Summit: ]
Alisdair recommends NAD as the partial specializations are already constrained by requirements on the primary template.
[ Batavia (2009-05): ]
The Working Draft does not in general repeat a primary template's constraints in any specializations. Move to NAD.
[ 2009-05-25 Howard adds: ]
A c++std-lib thread starting at c++std-lib-23880 has cast doubt that NAD is the correct resolution of this issue. Indeed the discussion also casts doubt that the current proposed wording is the correct resolution as well. Personally I'm inclined to reset the status to Open. However I'm reverting the status to that which it had prior to the Batavia recommendation. I'm setting back to Review.
Proposed resolution:
Change 18.3.1.1 [numeric.limits]:
template<classRegular T> class numeric_limits<const T>; template<classRegular T> class numeric_limits<volatile T>; template<classRegular T> class numeric_limits<const volatile T>;
Section: 18.8.6 [except.nested] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-15
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Discussion:
Addresses JP 29
throw_with_nested does not use concept.
[ Summit: ]
Agreed.
Proposed resolution:
Alisdair initially proposed wording in N2619.
We are awaiting an updated paper based on feedback from the San Francisco review.
Section: 18.8.6 [except.nested] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2010-03-09
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Discussion:
Addresses JP 31
It is difficult to understand in which case nested_exception is applied.
[ Summit: ]
Alisdair will add an example in an update to N2619.
[ 2009-10 Santa Cruz: ]
It doesn't appear that N2619 really addresses this. Alisdair to propose wording.
[ 2010 Pittsburgh: ]
Mark issue 1008 as NAD, the type is adequately described.
Rationale:
nested_exception is intended to be inherited from by exception classes that are to be thrown during the handling of another exception, i.e. when translating from one exception type to another. nested_exception allows the originally thrown exception to be easily retained in that scenario.
Proposed resolution:
Section: 24.2.2 [iterator.iterators] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-10-22
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Discussion:
Addresses UK 251
The post-increment operator is dangerous for a general InputIterator. The multi-pass guarantees that make it meaningful are defined as part of the ForwardIterator refinement. Any change will affect only constrained templates that have not yet been written, so should not break existing user iterators which remain free to add these operations. This change will also affect the generalised OutputIterator, although there is no percieved need for the post-increment operator in this case either.
[ 2009-07-28 Alisdair adds: ]
We still think the issue is relevant, but needs totally rewording in non-concept language. We would like to see the issue retained as Open, rather than deferred as NAD Concepts. Review status is no longer appropriate.
[ 2009-10 Santa Cruz: ]
NAD. Without concepts we do not feel that input iterator post increment is broken.
Proposed resolution:
Change 24.2.2 [iterator.iterators]:
concept Iterator<typename X> : Semiregular<X> { MoveConstructible reference = typename X::reference;MoveConstructible postincrement_result;requires HasDereference<postincrement_result>;reference operator*(X&&); X& operator++(X&);postincrement_result operator++(X&, int);}...
postincrement_result operator++(X& r, int);-3- Effects: equivalent to { X tmp = r; ++r; return tmp; }.
Change 24.2.3 [input.iterators]:
concept InputIterator<typename X> : Iterator<X>, EqualityComparable<X> {
ObjectType value_type = typename X::value_type;
MoveConstructible pointer = typename X::pointer;
SignedIntegralLike difference_type = typename X::difference_type;
requires IntegralType<difference_type>
&& Convertible<reference, const value_type &>;
&& Convertible<pointer, const value_type*>;
requires Convertible<HasDereference<postincrement_result>::result_type, const value_type&>;
pointer operator->(const X&);
}
Change 24.2.4 [output.iterators]:
auto concept OutputIterator<typename X, typename Value> {
requires Iterator<X>;
typename reference = Iterator<X>::reference;
typename postincrement_result = Iterator<X>::postincrement_result;
requires SameType<reference, Iterator<X>::reference>
&& SameType<postincrement_result, Iterator<X>::postincrement_result>
&& Convertible<postincrement_result, const X&>
&& HasAssign<reference, Value>
&& HasAssign<HasDereference<postincrement_result>::result_type, Value>;
}
Change 24.2.5 [forward.iterators]:
[ See 1084 which is attempting to change this same area in a compatible way. ]
concept ForwardIterator<typename X> : InputIterator<X>, Regular<X> {requires Convertible<postincrement_result, const X&>;MoveConstructible postincrement_result; requires HasDereference<postincrement_result> && Convertible<HasDereference<postincrement_result>::result_type, const value_type&>; postincrement_result operator++(X&, int); axiom MultiPass(X a, X b) { if (a == b) *a == *b; if (a == b) ++a == ++b; } }-4- ...
postincrement_result operator++(X& r, int);-5- Effects: equivalent to { X tmp = r; ++r; return tmp; }.
Section: 24.2.7 [random.access.iterators] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-16
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Discussion:
Addresses UK 263
This requirement on operator-= would be better expressed as a default implementation in the concept, with a matching axiom.
[ Batavia (2009-05): ]
The proposed resolution should also remove paragraph 5 and the declaration that precedes it. Further, we should provide an axiom that captures the desired semantics. This may be a broader policy to be applied. Move to Open.
Proposed resolution:
Change 24.2.7 [random.access.iterators]:
concept RandomAccessIterator<typename X> : BidirectionalIterator<X>, LessThanComparable<X> {
...
X& operator-=(X& x, difference_type n) { return x += -n; }
...
}
Section: 25.4.7 [alg.min.max] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-16
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Discussion:
Addresses UK 305
The negative requirement on IsSameType is a hold-over from an earlier draught with a variadic template form of min/max algorith. It is no longer necessary.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
We believe this is NAD, but this needs to be reviewed against the post-remove-concepts draft.
Proposed resolution:
Change 25 [algorithms]:
template<class T, StrictWeakOrder<auto, T> Compare>requires !SameType<T, Compare> && CopyConstructible<Compare>const T& min(const T& a, const T& b, Compare comp); ... template<class T, StrictWeakOrder<auto, T> Compare>requires !SameType<T, Compare> && CopyConstructible<Compare>const T& max(const T& a, const T& b, Compare comp); ... template<class T, StrictWeakOrder<auto, T> Compare>requires !SameType<T, Compare> && CopyConstructible<Compare>pair<const T&, const T&> minmax(const T& a, const T& b, Compare comp);
Change 25.4.7 [alg.min.max], p1, p9 and p17:
template<class T, StrictWeakOrder<auto, T> Compare>requires !SameType<T, Compare> && CopyConstructible<Compare>const T& min(const T& a, const T& b, Compare comp); ... template<class T, StrictWeakOrder<auto, T> Compare>requires !SameType<T, Compare> && CopyConstructible<Compare>const T& max(const T& a, const T& b, Compare comp); ... template<class T, StrictWeakOrder<auto, T> Compare>requires !SameType<T, Compare> && CopyConstructible<Compare>pair<const T&, const T&> minmax(const T& a, const T& b, Compare comp);
Section: X [concept.transform] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-15
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Discussion:
Addresses UK 199
The requirement that programs do not supply concept_maps should probably be users do not supply their own concept_map specializations. The program will almost certainly supply concept_maps - the standard itself supplies a specialization for RvalueOf references. Note that the term program is defined in 3.5 [basic.link]p1 and makes no account of the standard library being treated differently to user written code.
[ 2009-05-09 Alisdair adds: ]
The same problem is present in the words added for the LvalueReference/RvalueReference concepts last meeting.
With three subsections requiring the same constraint, I'm wondering if there is a better way to organise this section. Possible 20.2.1 -> 20.2.3 belong in the fundamental concepts clause in [concept.support]? While they can be implemented purely as a library feature without additional compiler support, they are pretty fundamental and we want the same restriction on user-concept maps as is mandated there.
[ Batavia (2009-05): ]
We agree with the issue, but believe the wording needs further improvement. We want to investigate current definitions for nomenclature such as "user" and "program." Move to Open pending the recommended investigation.
Proposed resolution:
Change X [concept.transform] p2:
-2- Aprogramuser shall not provide concept maps for any concept in 20.1.1.
Change [concept.true] p2:
-2- Requires: aprogramuser shall not provide a concept map for the True concept.
Change [concept.classify] p2:
-2- Requires: aprogramuser shall not provide concept maps for any concept in this section.
Section: X [concept.comparison] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-15
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Discussion:
Addresses JP 33
LessThanComparable and EqualityComparable don't correspond to NaN.
Original proposed resolution:
Apply concept_map to these concepts at FloatingPointType.
[ Post Summit, Alisdair adds: ]
I don't understand the proposed resolution - there is no such thing as a 'negative' concept_map, and these concepts are auto concepts that match float/double etc. Also not clear how we are supposed to match values to concepts.
Recommend NAD and treat as a subset of issue 902.
Proposed resolution:
Recommend NAD.
Section: X [concept.regular] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-15
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Discussion:
Addresses US 66
Application of the Regular concept to floating-point types appears to be controversial (see long discussion on std-lib reflector).
Original proposed resolution:
State that the Regular concept does not apply to floating-point types.
[ Summit: ]
Recommend that we handle the same as JP 33 / 1016.
[ Post Summit, Alisdair adds: ]
Recommend Open, and review after resolution of 902 and revised axiom feature.
Proposed resolution:
Section: 20.7 [meta] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-15
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Discussion:
Addresses US 70
Specifications now expressed via narrative text are more accurately and clearly expressed via executable code.
Wherever concepts are available that directly match this section's type traits, express the traits in terms of the concepts instead of via narrative text. Where the type traits do not quite match the corresponding concepts, bring the two into alignment so as to avoid two nearly-identical notions.
[ Summit: ]
We think that this is a good idea, but it requires a lot of work. If someone submits a paper proposing specific changes, we would be happy to review it at the next meeting.
Proposed resolution:
Section: 20.7.6.6 [meta.trans.other] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-10-23
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Discussion:
Addresses UK 204
It is not possible to create a variant union based on a parameter pack expansion, e.g. to implement a classic discriminated union template.
Original proposed resolutuion:
Restore aligned_union template that was removed by LWG issue 856.
[ Summit: ]
Agree. The need for aligned_union is compelling enough to reinstate.
[ Post Summit, Alisdair adds: ]
paper N2843 proposes an extension to the [[align]] attribute that further diminishes the need for this template. Recommend NAD.
[ 2009-10 Santa Cruz: ]
Mark NAD as suggested.
Proposed resolution:
Section: 20.9.12 [util.dynamic.safety] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-12
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Discussion:
Addresses UK 212
The pointer-safety API is nothing to do with smart pointers, so does not belong in 20.9.11 [util.smartptr]. In fact it is a set of language support features are really belongs in clause 18 [language.support], with the contents declared in a header that deals with language-support of memory management.
[ Summit: ]
Agree in principle, but not with the proposed resolution. We believe it belongs either a subsection of either 20 [utilities] or 20.9 [memory] as part of the general reorganization of 20 [utilities]. The declaration should stay in <memory>.
Proposed resolution:
Section: 20.8.14.2 [func.wrap.func] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-13
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Discussion:
Addresses DE 22
Related to issue 1114.
The conditions for deriving from std::unary_function and std::binary_function are unclear: The condition would also be satisfied if ArgTypes were std::vector<T1>, because it (arguably) "contains" T1.
[ Summit: ]
Agree. std::reference_wrapper has the same structure, and we suggest that std::function be presented in the same way as std::reference_wrapper.
[ 2009-05-09 Alisdair adds: ]
Phrasing should be "publicly and unambiguously derived from" and probably back in reference_wrapper too. Updated wording supplied.
[ Batavia (2009-05): ]
We agree with the proposed wording. Move to NAD Editorial.
Proposed resolution:
(no changes to <functional> synopsis required)
Change synopsis in Class template function 20.8.14.2 [func.wrap.func]:
template<Returnable R, CopyConstructible... ArgTypes> class function<R(ArgTypes...)> : public unary_function<T1, R> //iff sizeof...(ArgTypes) == 1 andsee below// ArgTypes contains T1: public binary_function<T1, T2, R> //iff sizeof...(ArgTypes) == 2 andsee below// ArgTypes contains T1 and T2{ ...
Add new p1/p2 before 20.8.14.2.1 [func.wrap.func.con]:
The template instantiation function<R(T1)> shall be publicly and unambiguously derived from std::unary_function<T1,R> if and only if the template type parameter is a function type taking one argument of type T1 and returning R.
The template instantiation function<R(T1,T2)> shall be publicly and unambiguously derived from std::binary_function<T1,T2,R> if and only if the template type parameter is a function type taking two arguments of type T1 and T2 and returning R.
explicit function();
Section: 20.8.14.2 [func.wrap.func] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-16
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Discussion:
Addresses JP 39
There are no requires corresponding to F of std::function.
[ 2009-05-01 Daniel adds: ]
1070 removes the second constructor.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready. If issue 1070 is accepted, the changes to the second constructor in this issue are moot.
[ 2009-07 Frankfurt: ]
Constructors have no definition.
Proposed resolution:
Correct as follows in 20.8.14.2 [func.wrap.func] (class definition)
template<class F, Allocator Alloc>
requires ConstructibleWithAllocator<F, Alloc>
&& call=Callable<F, ArgTypes...>
&& Convertible<call::result_type, R>
function(allocator_arg_t, const Alloc&, F);
template<class F, Allocator Alloc>
requires ConstructibleWithAllocator<F,Alloc>
&& call=Callable<F, ArgTypes...>
&& Convertible<call::result_type, R>
function(allocator_arg_t, const Alloc&, F&&);
Section: 20.8.15 [unord.hash] Status: NAD Future Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-12
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Discussion:
Addresses UK 208
std::hash should be implemented for much more of the standard library. In particular for pair, tuple and all the standard containers.
Proposed resolution:
Section: 20.9 [memory] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-15
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Discussion:
Addresses UK 209
Smart pointers cannot be used in constrained templates.
[ Summit: ]
We look forward to a paper on this topic. We recommend no action until a paper is available. We understand that a paper is forthcoming.
[ Peter Dimov adds: ]
shared_ptr<T> and weak_ptr<T> support all types T for which T* is valid. In other words, a possible (partial) resolution is to change class T to PointeeType T for shared_ptr, weak_ptr and possibly enable_shared_from_this.
Proposed resolution:
Section: 20.9.5 [default.allocator] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-15
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Discussion:
Addresses UK 213
std::allocator should be constrained to simplify its use on constrained contexts. This library component models allocation from free store via the new operator so choose constraints to match. The Allocator concept allows for a wider variety of allocators that users may choose to supply if their allocation model does not require operator new, without impacting the requirements of this template.
Suggested direction:
The primary allocator template should be constrained to require ObjectType<T> and FreeStoreAllocatable<T>. Further operations to be constrained as required.
[ Summit: ]
Agree as stated. A future paper will address additional related issues.
Proposed resolution:
Section: 20.9.7 [storage.iterator] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-16
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Discussion:
Addresses UK 214
raw_storage_iterator needs constraining as an iterator adaptor to be safely used in constrained templates
[ Summit: ]
We look forward to a paper on this topic. We recommend no action until a paper is available.
[ Post Summit Alisdair provided wording and rationale. ]
Proposed resolution:
20.9 [memory] p2
Update the synopsis for <memory>
// 20.7.8, raw storage iterator: template <classForwardIterator OutputIterator,classObjectType T> requires OutputIterator< OutIter, T > class raw_storage_iterator; template <ForwardIterator OutIter, ObjectType T> requires OutputIterator< OutIter, T > concept_map Iterator<raw_storage_iterator< OutIter, T > > { }
20.9.7 [storage.iterator] p1
Replace class template definition with:
namespace std {
template <class ForwardIterator OutputIterator, class ObjectType T>
requires OutputIterator< OutIter, T >
class raw_storage_iterator
: public iterator<output_iterator_tag,void,void,void,void> {
public:
explicit raw_storage_iterator(OutputIterator x);
raw_storage_iterator<OutputIterator,T>& operator*();
raw_storage_iterator<OutputIterator,T>& operator=(const T& element);
raw_storage_iterator<OutputIterator,T>& operator++();
raw_storage_iterator<OutputIterator,T> operator++(int);
};
template <ForwardIterator OutIter, ObjectType T>
requires OutputIterator< OutIter, T >
concept_map Iterator<raw_storage_iterator< OutIter, T > > { }
}
Rationale:
raw_storage_iterator has to adapt a ForwardIterator, rather than just an InputIterator for two reasons:
Section: 20.9.9 [specialized.algorithms] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-16
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Discussion:
Addresses UK 210
Related to 582
Specialized algorithms for memory managenment need requirements to be easily usable in constrained templates.
[ Summit: ]
We look forward to a paper on this topic. We recommend no action until a paper is available.
[ Post Summit Alisdair provided wording. ]
[ Post Summit: ]
Daniel adds:
- I suggest Size should require IntegralLike and not UnsignedIntegralLike, because otherwise simple int-literals could not be provided as arguments and it would conflict with other algorithms that only require IntegralLike.
The current for-loop-test relies on evaluation in boolean context which is not provided by ArithmeticLike and it's refinements. I propose to change the corresponding for-loop-headers to:
- for uninitialized_copy_n: for ( ; n > Size(0); ++result, ++first, --n) {
- for uninitialized_fill_n: for (; n > Size(0); ++first, --n) {
Alisdair adds:
For the record I agree with Daniel's suggestion.
Proposed resolution:
20.9 [memory] p2
Update the synopsis for <memory>
template <classInputIterator InIter,class ForwardIteratorOutputIterator<auto, InIter::reference> OutIter> requires ForwardIterator<OutIter>ForwardIteratorOutIter uninitialized_copy(InputIteratorInIter first,InputIteratorInIter last,ForwardIteratorOutIter result); template <classInputIterator InIter,classIntegralLike Size,class ForwardIteratorOutputIterator<auto, InIter::reference> OutIter> requires ForwardIterator<OutIter>ForwardIteratorOutIter uninitialized_copy_n(InputIteratorInIter first, Size n,ForwardIteratorOutIter result); template <classForwardIterator Iter,classObjectType T> requires Constructible< Iter::value_type, const T& > void uninitialized_fill(ForwardIteratorIter first,ForwardIteratorIter last, const T& x); template <classForwardIterator Iter,classIntegralLike Size,classObjectType T> requires Constructible< Iter::value_type, const T& > void uninitialized_fill_n(ForwardIteratorIter first, Size n, const T& x);
Update as follows:
uninitialized_copy 20.9.9.2 [uninitialized.copy]
template <classInputIterator InIter,class ForwardIteratorOutputIterator<auto, InIter::reference> OutIter> requires ForwardIterator<OutIter>ForwardIteratorOutIter uninitialized_copy(InputIteratorInIter first,InputIteratorInIter last,ForwardIteratorOutIter result);-1- Effects:
for (; first != last; ++result, ++first) { new (static_cast<void*>(&*result))typename iterator_traits<ForwardIterator>OutIter::value_type(*first); }-2- Returns: result
template <classInputIterator InIter,classIntegralLike Size,class ForwardIteratorOutputIterator<auto, InIter::reference> OutIter> requires ForwardIterator<OutIter>ForwardIteratorOutIter uninitialized_copy_n(InputIteratorInIter first, Size n,ForwardIteratorOutIter result);-3- Effects:
for ( ; n > Size(0); ++result, ++first, --n) { new (static_cast<void*>(&*result))typename iterator_traits<ForwardIterator>OutIter::value_type(*first); }-4- Returns: result
uninitialized_fill 20.9.9.3 [uninitialized.fill]
template <classForwardIterator Iter,classObjectType T> requires Constructible< Iter::value_type, const T& > void uninitialized_fill(ForwardIteratorIter first,ForwardIteratorIter last, const T& x);-1- Effects:
for (; first != last; ++first) { new ( static_cast<void*>( &*first) )typename iterator_traits<ForwardIterator>Iter::value_type(x); }
uninitialized_fill_n 20.9.9.4 [uninitialized.fill.n]
template <classForwardIterator Iter,classIntegralLike Size,classObjectType T> requires Constructible< Iter::value_type, const T& > void uninitialized_fill_n(ForwardIteratorIter first, Size n, const T& x);-1- Effects:
for (; n--> Size(0); ++first, --n) { new ( static_cast<void*>( &*first) )typename iterator_traits<ForwardIterator>Iter::value_type(x); }
Section: 20.9.11.2 [util.smartptr.shared] Status: NAD Future Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-10-20
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Discussion:
Addresses US 78
There is presently no way to convert directly from a shared_ptr to a unique_ptr. Add an interface that performs the conversion.
[ Summit: ]
We look forward to a paper on this topic. We recommend no action until a paper is available. We believe that the shared pointer must use the default deleter for the conversion to succeed.
[ Peter Dimov adds: ]
This is basically a request for shared_ptr<>::release in disguise, with all the associated problems. Not a good idea.
[ 2009-07 post-Frankfurt: ]
The rationale for the omission of a release() member function from shared_ptr is given in: http://www.boost.org/doc/libs/1_39_0/libs/smart_ptr/shared_ptr.htm
The implementation of such a member is non-trivial (and maybe impossible), because it would need to account for the deleter.
[ 2009-07-26 Howard sets to Tentatively NAD Future. ]
I took an online poll and got 3 votes for NAD and 3 for NAD Future. Personally I prefer NAD Future as this does refer to an extension that could conceivably be considered beyond C++0X.
However such an extension would need to solve a couple of problems:
- What is the interface for such a conversion when the shared_ptr does not have unique ownership? Throw an exception? Create a null unique_ptr? Undefined behavior?
How does one handle custom deleters given to the shared_ptr constructor?
I do not believe it is possible to implement a general answer to this question. The shared_ptr deleter is a run time (or construction time) characteristic. The unique_ptr deleter is a compile time characteristic. In general one can not know to what type of unqiue_ptr you are converting to.
One answer is for the user of the conversion to specify the deleter type and perhaps throw an exception if the specification turns out to be incorrect.
Another answer is for the conversion to only be valid when the underlying deleter is default_delete. We would probalby need to specify that this is indeed the underlying deleter of a shared_ptr when a custom deleter is not given in the constructor.
At any rate, there are non-trivial design issues which would need to be implemented and tested in the field for usability prior to standardization.
[ 2009 Santa Cruz: ]
Moved to NAD Future.
Proposed resolution:
Section: 20.10 [time] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-07-16
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Discussion:
Addresses JP 45
Rep, Period, Clock and Duration don't correspond to concept.
template <class Rep, class Period = ratio<1>> class duration; template <class Clock, class Duration = typename Clock::duration> class time_point;
Make concept for Rep, Period, Clock and Duration. Fix 20.10 [time] and wait_until and wait_for's template parameter at 30 [thread].
[ Summit: ]
We agree that this section needs concepts. We look forward to a paper on this topic. We recommend no action until a paper is available.
Proposed resolution:
Section: 23.2.1 [container.requirements.general] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-20
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Discussion:
Addresses UK 226
<array> must be added to this list. In particular it doesn't satisfy: - no swap() function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. and probably doesn't satisfy: - no swap() function throws an exception.
If <array> remains a container, this will have to also reference array, which will then have to say which of these points it satisfies.
[ Summit: ]
Agree. The proposed resolution is incomplete. Further work required.
[ 2009-05-01 Daniel adds: ]
Issue 1099 also suggests adding move constructor to this.
[ 2009-07 post-Frankfurt: ]
Howard is to draft a note that explains what happens to references.
[ 2009-10 Santa Cruz: ]
Mark as NAD. No consensus for change.
[ 2009-08-01 Howard provided wording. ]
Proposed resolution:
Add a paragraph to 23.3.1.2 [array.special]:
template <Swappable T, size_t N> void swap(array<T,N>& x, array<T,N>& y);Effects:
swap_ranges(x.begin(), x.end(), y.begin());[Note: Outstanding iterators, references and pointers may be invalidated. — end note]
Section: 23.2.3 [sequence.reqmts] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-07-15
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Discussion:
Addresses UK 231
p9-p11 are redundant now that Concepts define what it means to be an Iterator and guide overload resolution accordingly.
[ Summit: ]
Agree with issue and change to 23.2.3 [sequence.reqmts]. The changes required to 21 [strings] will be part of the general concept support for that clause.
Proposed resolution:
Strike 23.2.3 [sequence.reqmts]p9-11. Make sure std::basic_string has constraints similar to std::vector to meet this old guarantee.
Section: 23.2.4 [associative.reqmts] Status: NAD Future Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-20
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Discussion:
Addresses UK 239
It is not possible to take a move-only key out of an unordered container, such as (multi)set or (multi)map, or the new unordered containers.
Add below a.erase(q), a.extract(q), with the following notation:
a.extract(q)>, Return type pair<key, iterator> Extracts the element pointed to by q and erases it from the set. Returns a pair containing the value pointed to by q and an iterator pointing to the element immediately following q prior to the element being erased. If no such element exists,returns a.end().
[ Summit: ]
We look forward to a paper on this topic. We recommend no action until a paper is available. The paper would need to address exception safety.
[ Post Summit Alisdair adds: ]
Would value_type be a better return type than key_type?
[ 2009-07 post-Frankfurt: ]
Leave Open. Alisdair to contact Chris Jefferson about this.
[ 2009-09-20 Howard adds: ]
See the 2009-09-19 comment of 839 for an API which accomplishes this functionality and also addresses several other use cases which this proposal does not.
[ 2009-10 Santa Cruz: ]
Mark as NAD Future. No consensus to make the change at this time.
Proposed resolution:
In 23.2.4 [associative.reqmts] Table 85, add:
Table 85 -- Associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity a.erase(q) ... ... ... a.extract(q) pair<key_type, iterator> Extracts the element pointed to by q and erases it from the set. Returns a pair containing the value pointed to by q and an iterator pointing to the element immediately following q prior to the element being erased. If no such element exists, returns a.end(). amortized constant
In 23.2.5 [unord.req] Table 87, add:
Table 87 -- Unordered associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity a.erase(q) ... ... ... a.extract(q) pair<key_type, iterator> Extracts the element pointed to by q and erases it from the set. Returns a pair containing the value pointed to by q and an iterator pointing to the element immediately following q prior to the element being erased. If no such element exists, returns a.end(). amortized constant
Section: 23.3 [sequences] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-23
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Discussion:
Addresses UK 244
The validity of the expression &a[n] == &a[0] + n is contingent on operator& doing the "right thing" (as captured by the CopyConstructible requirements in table 30 in C++2003). However this constraint has been lost in the Concepts of C++0x. This applies to vector and array (it actually applies to string also, but that's a different chapter, so I'll file a separate comment there and cross-reference).
Suggested solution:
Define a ContiguousStrorage and apply it to vector, array and string.
[ Summit: ]
Agree with the issue but not the details of the proposed solution. Walter to provide wording for the new concept.
[ Post Summit Alisdair adds: ]
Another LWG subgroup wondered if this concept should extend to complex<T>, and so not be built on the container concept at all?
[ 2009-07 post-Frankfurt: ]
Leave Open, pending a post-Concepts Working Draft.
[ 2009-10 Santa Cruz: ]
Mark issue 1042 as NAD, in rationale state that this was solved by removal of concepts.
Proposed resolution:
Add to <container_concepts> synopsis in [container.concepts]
concept< typename C > ContiguousStorageContainer see below;
Add a new section to the end of [container.concepts]
23.1.6.x ContiguousStorageContainer concept [container.concepts.contiguous]
concept ContiguousStorageContainer< typename C > : Container<C> { value_type* data(C&); axiom Contiguity(C& c, size_type i) { if( i < size(c) ) { addressof( * (data(c) + i) ) == addressof( * advance(data(c), i) ); } } }The ContiguousStorageContainer concept describes a container whose elements are allocated in a single region of memory, and are stored sequentially without intervening padding other than to meet alignment requirements. For example, the elements may be stored in a single array of suitable length.
value_type * data( C& );Returns: a pointer to the first element in the region of storage. Result is unspecified for an empty container.
Change 23.3.1 [array] p1:
-1- The header <array> defines a class template for storing fixed-size sequences of objects. An array supports random access iterators. An instance of array<T, N> stores N elements of type T, so that size() == N is an invariant. The elements of an array are stored contiguously, meaning thatif a isan array<T, N>then it obeys the identity &a[n] == &a[0] + n for all 0 <= n < Nsatisfies the concept ContiguousStorageContainer< array<T, N>>.
Add to the synopsis in 23.3.1 [array]:
...
T * data();
const T * data() const;
};
template< typename T, size_t N >
concept_map ContiguousStorageContainer< array<T, N>> {};
}
Change 23.3.6 [vector] p1:
A vector is a sequence container that supports random access iterators. In addition, it supports (amortized) constant time insert and erase operations at the end; insert and erase in the middle take linear time. Storage management is handled automatically, though hints can be given to improve efficiency. The elements of a vector are stored contiguously, meaning thatif v isa vector<T, Alloc> (where T is some type other than bool), then it obeys the identity &v[n] == &v[0] + n for all 0 <= n < v.size()satisfies the concept ContiguousStorageContainer< vector< T, Alloc>>.
Add at the end of the synopsis in 23.3.6 [vector] p2:
template< typename T, typename A >
requires !SameType< T, bool >
concept_map ContiguousStorageContainer< vector<T, A>> {};
Rationale:
Solved by removal of concepts.Section: 29.6 [atomics.types.operations] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-26
View all other issues in [atomics.types.operations].
View all issues with NAD Editorial status.
Discussion:
Addresses US 91
It is unclear whether or not a failed compare_exchange is a RMW operation (as used in 1.10 [intro.multithread]).
Suggested solution:
Make failing compare_exchange operations not be RMW.
[ Anthony Williams adds: ]
In 29.6 [atomics.types.operations] p18 it says that "These operations are atomic read-modify-write operations" (final sentence). This is overly restrictive on the implementations of compare_exchange_weak and compare_exchange_strong on platforms without a native CAS instruction.
[ Summit: ]
Group agrees with the resolution as proposed by Anthony Williams in the attached note.
[ Batavia (2009-05): ]
We recommend the proposed resolution be reviewed by members of the Concurrency Subgroup.
[ 2009-07 post-Frankfurt: ]
This is likely to be addressed by Lawrence's upcoming paper. He will adopt the proposed resolution.
[ 2009-08-17 Handled by N2925. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Change 29.6 [atomics.types.operations] p18:
-18- Effects: Atomically, compares the value pointed to by object or by this for equality with that in expected, and if true, replaces the value pointed to by object or by this with desired, and if false, updates the value in expected with the value pointed to by object or by this. Further, if the comparison is true, memory is affected according to the value of success, and if the comparison is false, memory is affected according to the value of failure. When only one memory_order argument is supplied, the value of success is order, and the value of failure is order except that a value of memory_order_acq_rel shall be replaced by the value memory_order_acquire and a value of memory_order_release shall be replaced by the value memory_order_relaxed. If the comparison is true,Tthese operations are atomic read-modify-write operations (1.10). If the comparison is false, these operations are atomic load operations.
Section: 30.6 [futures] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-26
View all other issues in [futures].
View all issues with NAD Editorial status.
Discussion:
Addresses UK 329
future, promise and packaged_task provide a framework for creating future values, but a simple function to tie all three components together is missing. Note that we only need a *simple* facility for C++0x. Advanced thread pools are to be left for TR2.
Simple Proposal:
Provide a simple function along the lines of:
template< typename F, typename ... Args >
requires Callable< F, Args... >
future< Callable::result_type > async( F&& f, Args && ... );
Semantics are similar to creating a thread object with a packaged_task invoking f with forward<Args>(args...) but details are left unspecified to allow different scheduling and thread spawning implementations.
It is unspecified whether a task submitted to async is run on its own thread or a thread previously used for another async task. If a call to async succeeds, it shall be safe to wait for it from any thread.
The state of thread_local variables shall be preserved during async calls.
No two incomplete async tasks shall see the same value of this_thread::get_id().
[Note: this effectively forces new tasks to be run on a new thread, or a fixed-size pool with no queue. If the library is unable to spawn a new thread or there are no free worker threads then the async call should fail. --end note]
[ Summit: ]
The concurrency subgroup has revisited this issue and decided that it could be considered a defect according to the Kona compromise. A task group was formed lead by Lawrence Crowl and Bjarne Stroustrup to write a paper for Frankfort proposing a simple asynchronous launch facility returning a future. It was agreed that the callable must be run on a separate thread from the caller, but not necessarily a brand-new thread. The proposal might or might not allow for an implementation that uses fixed-size or unlimited thread pools.
Bjarne in c++std-lib-23121: I think that what we agreed was that to avoid deadlock async() would almost certainly be specified to launch in a different thread from the thread that executed async(), but I don't think it was a specific design constraint.
[ 2009-10 Santa Cruz: ]
Proposed resolution: see N2996 (Herb's and Lawrence's paper on Async). Move state to NAD editorial.
Proposed resolution:
Section: 30.6.6 [futures.unique_future] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-26
View all other issues in [futures.unique_future].
View all issues with NAD Editorial status.
Discussion:
Addresses UK 334
Behaviour of get() is undefined if calling get() while not is_ready(). The intent is that get() is a blocking call, and will wait for the future to become ready.
[ Summit: ]
Agree, move to Review.
[ 2009-04-03 Thomas J. Gritzan adds: ]
This issue also applies to shared_future::get().
Suggested wording:
Add a paragraph to 30.6.7 [futures.shared_future]:
void shared_future<void>::get() const;Effects: If is_ready() would return false, block on the asynchronous result associated with *this.
[ Batavia (2009-05): ]
It is not clear to us that this is an issue, because the proposed resolution's Effects clause seems to duplicate information already present in the Synchronization clause. Keep in Review status.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2997.
Proposed resolution:
Add a paragraph to 30.6.6 [futures.unique_future]:
R&& unique_future::get(); R& unique_future<R&>::get(); void unique_future<void>::get();Note:...
Effects: If is_ready() would return false, block on the asynchronous result associated with *this.
Synchronization: if *this is associated with a promise object, the completion of set_value() or set_exception() to that promise happens before (1.10) get() returns.
Section: 30.6.6 [futures.unique_future] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-26
View all other issues in [futures.unique_future].
View all issues with NAD Editorial status.
Discussion:
Addresses UK 335
std::unique_future is MoveConstructible, so you can transfer the association with an asynchronous result from one instance to another. However, there is no way to determine whether or not an instance has been moved from, and therefore whether or not it is safe to wait for it.
std::promise<int> p; std::unique_future<int> uf(p.get_future()); std::unique_future<int> uf2(std::move(uf)); uf.wait(); // oops, uf has no result to wait for.
Suggest we add a waitable() function to unique_future (and shared_future) akin to std::thread::joinable(), which returns true if there is an associated result to wait for (whether or not it is ready).
Then we can say:
if(uf.waitable()) uf.wait();
[ Summit: ]
Create an issue. Requires input from Howard. Probably NAD.
[ Post Summit, Howard thows in his two cents: ]
Here is a copy/paste of my last prototype of unique_future which was several years ago. At that time I was calling unique_future future:
template <class R> class future { public: typedef R result_type; private: future(const future&);// = delete; future& operator=(const future&);// = delete; template <class R1, class F1> friend class prommise; public: future(); ~future(); future(future&& f); future& operator=(future&& f); void swap(future&& f); bool joinable() const; bool is_normal() const; bool is_exceptional() const; bool is_ready() const; R get(); void join(); template <class ElapsedTime> bool timed_join(const ElapsedTime&); };shared_future had a similar interface. I intentionally reused the thread interface where possible to lessen the learning curve std::lib clients will be faced with.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2997.
Proposed resolution:
Section: 30.6.5 [futures.promise] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-26
View all other issues in [futures.promise].
View all issues with NAD Editorial status.
Discussion:
Addresses UK 339
Move assignment is goiing in the wrong direction, assigning from *this to the passed rvalue, and then returning a reference to an unusable *this.
[ Summit: ]
Agree, move to Review.
[ Batavia (2009-05): ]
We recommend deferring this issue until after Detlef's paper (on futures) has been issued.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2997.
Proposed resolution:
Strike 30.6.5 [futures.promise] p6 and change p7:
promise& operator=(promise&& rhs);
-6- Effects: move assigns its associated state to rhs.-7- Postcondition:
*this has no associated state.associated state of *this is the same as the associated state of rhs before the call. rhs has no associated state.
Section: 30.6.5 [futures.promise] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-26
View all other issues in [futures.promise].
View all issues with NAD Editorial status.
Discussion:
Addresses UK 340
There is an implied postcondition for get_future() that the state of the promise is transferred into the future leaving the promise with no associated state. It should be spelled out.
[ Summit: ]
Agree, move to Review.
[ 2009-04-03 Thomas J. Gritzan adds: ]
promise::get_future() must not invalidate the state of the promise object.
A promise is used like this:
promise<int> p; unique_future<int> f = p.get_future(); // post 'p' to a thread that calculates a value // use 'f' to retrieve the value.So get_future() must return an object that shares the same associated state with *this.
But still, this function should throw an future_already_retrieved error when it is called twice.
packaged_task::get_future() throws std::bad_function_call if its future was already retrieved. It should throw future_error(future_already_retrieved), too.
Suggested resolution:
Replace p12/p13 30.6.5 [futures.promise]:
-12- Throws: future_error if
*this has no associated statethe future has already been retrieved.-13- Error conditions: future_already_retrieved if
*this has no associated statethe future associated with the associated state has already been retrieved.Postcondition: The returned object and *this share the associated state.
Replace p14 30.6.10 [futures.task]:
-14- Throws:
std::bad_function_callfuture_error if the futureassociated with the taskhas already been retrieved.Error conditions: future_already_retrieved if the future associated with the task has already been retrieved.
Postcondition: The returned object and *this share the associated task.
[ Batavia (2009-05): ]
Keep in Review status pending Detlef's forthcoming paper on futures.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2997.
Proposed resolution:
Add after p13 30.6.5 [futures.promise]:
unique_future<R> get_future();-13- ...
Postcondition: *this has no associated state.
Section: 24.5.1.3.12 [reverse.iter.opindex], 24.5.3.3.12 [move.iter.op.index] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-24
View all issues with NAD status.
Discussion:
Addresses UK 279
The reason the return type became unspecified is LWG issue 386. This reasoning no longer applies as there are at least two ways to get the right return type with the new language facilities added since the previous standard.
Proposal: Specify the return type using either decltype or the Iter concept_map.
[ Summit: ]
Under discussion. This is a general question about all iterator adapters.
[ Howard adds post Summit: ]
I am requesting test cases to demonstrate a position.
[ 2009-07-24 Daniel adds: ]
I recommend NAD. Without concepts we can no longer restrict this member in a trivial way. Using decltype the declaration would be along the lines of
static const Iter& __base(); // not defined auto operator[](difference_type n) const -> decltype(__base()[-n-1]);but once reverse_iterator is instantiated for some given type Iter which cannot form a well-formed expression __base()[-n-1] this would cause an ill-formed function declaration, diagnostic required, and no silent SFINAE elimination.
[ 2009-10 Santa Cruz: ]
Moved to NAD.
[ 2009-10-22 Daniel adds: ]
IMO, my original comment regarding ill-formedness of the described construction is still correct, but I must add that I should weaken my assertion "Without concepts we can no longer restrict this member in a trivial way".
In fact with the existence of default template arguments for function templates it is not too hard to implement this like as follows, which shows that we can indeed simulate to some sense constrained member functions in C++0x.
My example does not really proof that the specification is easy, but it should be possible. I assume that the implementation would not be ABI compatible, though.
It is now your own decision how to proceed ;-)
#include <type_traits> #include <cstddef> template<class T> typename std::add_rvalue_reference<T>::type declval(); template<class It> struct reverse_iterator { It base; typedef std::ptrdiff_t difference_type; template<class U = It, class Res = decltype(declval<const U&>()[declval<difference_type>()]) > Res operator[](difference_type n) const { return base[-n-1]; } }; struct MyIter { }; int main() { reverse_iterator<int*> ri; ri[0] = 2; reverse_iterator<MyIter> ri2; }The above declaration could be simplified, but the ideal solution
template<class U = It> decltype(declval<const U&>()[declval<difference_type>()]) operator[](difference_type n) const;does not work yet on gcc 4.4.1.
Proposed resolution:
Section: 24.5.1.3.5 [reverse.iter.opref] Status: NAD Future Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2010-03-11
View all issues with NAD Future status.
Discussion:
Addresses UK 281
The current specification for return value for reverse_iterator::operator-> will always be a true pointer type, but reverse_iterator supports proxy iterators where the pointer type may be some kind of 'smart pointer'.
[ Summit: ]
move_iterator avoids this problem by returning a value of the wrapped Iterator type. study group formed to come up with a suggested resolution.
move_iterator solution shown in proposed wording.
[ 2009-07 post-Frankfurt: ]
Howard to deconceptize. Move to Review after that happens.
[ 2009-08-01 Howard deconceptized: ]
[ 2009-10 Santa Cruz: ]
We can't think of any reason we can't just define reverse iterator's pointer types to be the same as the underlying iterator's pointer type, and get it by calling the right arrow directly.
Here is the proposed wording that was replaced:
template <class Iterator> class reverse_iterator { ... typedeftypename iterator_traits<Iterator>::pointerpointer;Change 24.5.1.3.5 [reverse.iter.opref]:
pointer operator->() const;Returns:&(operator*());this->tmp = current; --this->tmp; return this->tmp;
[ 2010-03-03 Daniel opens: ]
- There is a minor problem with the exposition-only declaration of the private member deref_tmp which is modified in a const member function (and the same problem occurs in the specification of operator*). The fix is to make it a mutable member.
The more severe problem is that the resolution for some reasons does not explain in the rationale why it was decided to differ from the suggested fix (using deref_tmp instead of tmp) in the [ 2009-10 Santa Cruz] comment:
this->deref_tmp = current; --this->deref_tmp; return this->deref_tmp;combined with the change of
typedef typename iterator_traits<Iterator>::pointer pointer;to
typedef Iterator pointer;The problem of the agreed on wording is that the following rather typical example, that compiled with the wording before 1052 had been applied, won't compile anymore:
#include <iterator> #include <utility> int main() { typedef std::pair<int, double> P; P op; std::reverse_iterator<P*> ri(&op + 1); ri->first; // Error }Comeau online returns (if a correspondingly changed reverse_iterator is used):
"error: expression must have class type return deref_tmp.operator->(); ^ detected during instantiation of "Iterator reverse_iterator<Iterator>::operator->() const [with Iterator=std::pair<int, double> *]""Thus the change will break valid, existing code based on std::reverse_iterator.
IMO the suggestion proposed in the comment is a necessary fix, which harmonizes with the similar specification of std::move_iterator and properly reflects the recursive nature of the evaluation of operator-> overloads.
Suggested resolution:
In the class template reverse_iterator synopsis of 24.5.1.1 [reverse.iterator] change as indicated:
namespace std { template <class Iterator> class reverse_iterator : public iterator<typename iterator_traits<Iterator>::iterator_category, typename iterator_traits<Iterator>::value_type, typename iterator_traits<Iterator>::difference_type,typename iterator_traits<Iterator>::pointer, typename iterator_traits<Iterator>::reference> { public: [..] typedeftypename iterator_traits<Iterator>::pointerpointer; [..] protected: Iterator current; private: mutable Iterator deref_tmp; // exposition only };- Change 24.5.1.3.5 [reverse.iter.opref]/1 as indicated:
pointer operator->() const;1ReturnsEffects:&(operator*()).deref_tmp = current; --deref_tmp; return deref_tmp;
[ 2010 Pittsburgh: ]
We prefer to make to use a local variable instead of deref_tmp within operator->(). And although this means that the mutable change is no longer needed, we prefer to keep it because it is needed for operator*() anyway.
Here is the proposed wording that was replaced:
Change 24.5.1.3.5 [reverse.iter.opref]:
pointer operator->() const;Returns:&(operator*());deref_tmp = current; --deref_tmp; return deref_tmp::operator->();
[ 2010-03-10 Howard adds: ]
Here are three tests that the current proposed wording passes, and no other solution I've seen passes all three:
Proxy pointer support:
#include <iterator> #include <cassert> struct X { int m; }; X x; struct IterX { typedef std::bidirectional_iterator_tag iterator_category; typedef X& reference; struct pointer { pointer(X& v) : value(v) {} X& value; X* operator->() const {return &value;} }; typedef std::ptrdiff_t difference_type; typedef X value_type; // additional iterator requirements not important for this issue reference operator*() const { return x; } pointer operator->() const { return pointer(x); } IterX& operator--() {return *this;} }; int main() { std::reverse_iterator<IterX> ix; assert(&ix->m == &(*ix).m); }Raw pointer support:
#include <iterator> #include <utility> int main() { typedef std::pair<int, double> P; P op; std::reverse_iterator<P*> ri(&op + 1); ri->first; // Error }Caching iterator support:
#include <iterator> #include <cassert> struct X { int m; }; struct IterX { typedef std::bidirectional_iterator_tag iterator_category; typedef X& reference; typedef X* pointer; typedef std::ptrdiff_t difference_type; typedef X value_type; // additional iterator requirements not important for this issue reference operator*() const { return value; } pointer operator->() const { return &value; } IterX& operator--() {return *this;} private: mutable X value; }; int main() { std::reverse_iterator<IterX> ix; assert(&ix->m == &(*ix).m); }
[ 2010 Pittsburgh: ]
Moved to NAD Future, rationale added.
Rationale:
The LWG did not reach a consensus for a change to the WP.
Proposed resolution:
In the class template reverse_iterator synopsis of 24.5.1.1 [reverse.iterator] change as indicated:
namespace std {
template <class Iterator>
class reverse_iterator : public
iterator<typename iterator_traits<Iterator>::iterator_category,
typename iterator_traits<Iterator>::value_type,
typename iterator_traits<Iterator>::difference_type,
typename iterator_traits<Iterator&>::pointer,
typename iterator_traits<Iterator>::reference> {
public:
[..]
typedef typename iterator_traits<Iterator&>::pointer pointer;
[..]
protected:
Iterator current;
private:
mutable Iterator deref_tmp; // exposition only
};
pointer operator->() const;1ReturnsEffects:&(operator*()).deref_tmp = current; --deref_tmp; return deref_tmp;
Section: 25 [algorithms] Status: NAD Future Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-23
View all other issues in [algorithms].
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Discussion:
Addresses UK 295
There is a level of redundancy in the library specification for many algorithms that can be eliminated with the combination of concepts and default parameters for function templates. Eliminating redundancy simplified specification and reduces the risk of introducing accidental inconsistencies.
Proposed resolution: Adopt N2743.
[ Summit: ]
NAD, this change would break code that takes the address of an algorithm.
[ Post Summit Alisdair adds: ]
Request 'Open'. The issues in the paper go beyond just reducing the number of signatures, but cover unifying the idea of the ordering operation used by algorithms, containers and other library components. At least, it takes a first pass at the problem.
For me (personally) that was the more important part of the paper, and not clearly addressed by the Summit resolution.
[ 2009-10 Santa Cruz: ]
Too inventive, too late, would really need a paper. Moved to NAD Future.
Proposed resolution:
Section: 20.3.3 [forward] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2009-03-13 Last modified: 2009-10-26
View all other issues in [forward].
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Discussion:
This is a placeholder issue to track the fact that we (well I) put the standard into an inconsistent state by requesting that we accept N2844 except for the proposed changes to [forward].
There will exist in the post meeting mailing N2835 which in its current state reflects the state of affairs prior to the Summit meeting. I hope to update it in time for the post Summit mailing, but as I write this issue I have not done so yet.
[ Batavia (2009-05): ]
Move to Open, awaiting the promised paper.
[ 2009-08-02 Howard adds: ]
My current preferred solution is:
template <class T> struct __base_type { typedef typename remove_cv<typename remove_reference<T>::type>::type type; }; template <class T, class U, class = typename enable_if< !is_lvalue_reference<T>::value || is_lvalue_reference<T>::value && is_lvalue_reference<U>::value>::type, class = typename enable_if< is_same<typename __base_type<T>::type, typename __base_type<U>::type>::value>::type> inline T&& forward(U&& t) { return static_cast<T&&>(t); }This has been tested by Bill, Jason and myself.
It allows the following lvalue/rvalue casts:
- Cast an lvalue t to an lvalue T (identity).
- Cast an lvalue t to an rvalue T.
- Cast an rvalue t to an rvalue T (identity).
It disallows:
- Cast an rvalue t to an lvalue T.
- Cast one type t to another type T (such as int to double).
"a." is disallowed as it can easily lead to dangling references. "b." is disallowed as this function is meant to only change the lvalue/rvalue characteristic of an expression.
Jason has expressed concern that "b." is not dangerous and is useful in contexts where you want to "forward" a derived type as a base type. I find this use case neither dangerous, nor compelling. I.e. I could live with or without the "b." constraint. Without it, forward would look like:
template <class T, class U, class = typename enable_if< !is_lvalue_reference<T>::value || is_lvalue_reference<T>::value && is_lvalue_reference<U>::value>::type> inline T&& forward(U&& t) { return static_cast<T&&>(t); }Or possibly:
template <class T, class U, class = typename enable_if< !is_lvalue_reference<T>::value || is_lvalue_reference<T>::value && is_lvalue_reference<U>::value>::type, class = typename enable_if< is_base_of<typename __base_type<U>::type, typename __base_type<T>::type>::value>::type> inline T&& forward(U&& t) { return static_cast<T&&>(t); }The "promised paper" is not in the post-Frankfurt mailing only because I'm waiting for the non-concepts draft. But I'm hoping that by adding this information here I can keep people up to date.
[ 2009-08-02 David adds: ]
forward was originally designed to do one thing: perfect forwarding. That is, inside a function template whose actual argument can be a const or non-const lvalue or rvalue, restore the original "rvalue-ness" of the actual argument:
template <class T> void f(T&& x) { // x is an lvalue here. If the actual argument to f was an // rvalue, pass static_cast<T&&>(x) to g; otherwise, pass x. g( forward<T>(x) ); }Attempting to engineer forward to accomodate uses other than perfect forwarding dilutes its idiomatic meaning. The solution proposed here declares that forward<T>(x) means nothing more than static_cast<T&&>(x), with a patchwork of restrictions on what T and x can be that can't be expressed in simple English.
I would be happy with either of two approaches, whose code I hope (but can't guarantee) I got right.
Use a simple definition of forward that accomplishes its original purpose without complications to accomodate other uses:
template <class T, class U> T&& forward(U& x) { return static_cast<T&&>(x); }Use a definition of forward that protects the user from as many potential mistakes as possible, by actively preventing all other uses:
template <class T, class U> boost::enable_if_c< // in forward<T>(x), x is a parameter of the caller, thus an lvalue is_lvalue_reference<U>::value // in caller's deduced T&& argument, T can only be non-ref or lvalue ref && !is_rvalue_reference<T>::value // Must not cast cv-qualifications or do any type conversions && is_same<T&,U&>::value , T&&>::type forward(U&& a) { return static_cast<T&&>(a); }
[ 2009-09-27 Howard adds: ]
A paper, N2951, is available which compares several implementations (including David's) with respect to several use cases (including Jason's) and provides wording for one implementation.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2951.
Proposed resolution:
Section: 20.7.6.6 [meta.trans.other] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-26
View all other issues in [meta.trans.other].
View all issues with NAD Editorial status.
Discussion:
Addresses UK 98
It would be useful to be able to determine the underlying type of an arbitrary enumeration type. This would allow safe casting to an integral type (especially needed for scoped enums, which do not promote), and would allow use of numeric_limits. In general it makes generic programming with enumerations easier.
[ Batavia (2009-05): ]
Pete observes (and Tom concurs) that the proposed resolution seems to require compiler support for its implementation, as it seems necessary to look at the range of values of the enumerated type. To a first approximation, a library solution could give an answer based on the size of the type. If the user has specialized numeric_limits for the enumerated type, then the library might be able to do better, but there is no such requirement. Keep status as Open and solicit input from CWG.
[ 2009-05-23 Alisdair adds: ]
Just to confirm that the BSI originator of this comment assumed it did indeed imply a compiler intrinsic. Rather than request a Core extension, it seemed in keeping with that the type traits interface provides a library API to unspecified compiler features - where we require several other traits (e.g. has_trivial_*) to get the 'right' answer now, unlike in TR1.
[ Addressed in N2947. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2984.
Proposed resolution:
Add a new row to the table in 20.7.6.6 [meta.trans.other]:
Table 41 -- Other transformations Template Condition Comments template< class T > struct enum_base; T shall be an enumeration type (7.2 [dcl.enum]) The member typedef type shall name the underlying type of the enum T.
Section: 26.5 [rand] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2010-03-08
View all other issues in [rand].
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Discussion:
Both the concepts RandomNumberEngine and RandomNumberDistribution have requirements to be InputStreamable and OutputStreamable.
I have no problems leaving the WP in an inconsistent state on the best-faith assumption these concepts will be provided later, however disagree with the proposers that these constraints are not separable, orthogonal to the basic concepts of generating random number distributions.
These constraints should be dropped, and applied to specific algorithms as needed.
If a more refined concept (certainly deemed useful by the proposers) is proposed there is no objection, but the basic concept should not require persistence via streaming.
[ Batavia (2009-05): ]
Move to Open.
[ 2009-05-31 Alisdair adds: ]
Working on constraining the stream iterators, I have a few more observations to make on the concepts proposed while constraining the random number facility.
While I still believe the concerns are orthogonal, I don't believe the existing constraints go far enough either! The goal we want to achieve is not that a RandomNumberEngine / RandomNumberDistribution supports the stream operators, but that it is Serializable. I.e. there is a relationship between the insert and extract operations that guarantees to restore the state of the original object. This implies a coupling of the concepts together in a broader concept (Serializable) with at least one axiom to assert the semantics.
One problem is that istream and ostream may be fundamentally different types, although we can hook a relation if we are prepared to drop down to the char type and char_traits template parameters. Doing so ties us to a form of serialization that demands implementation via the std iostreams framework, which seems overly prescriptive. I believe the goal is generally to support serialization without regard to how it is expressed - although this is getting even more inventive in terms of concepts we do not have today.
[ 2009-11-03 Alisdair adds: ]
I can't find the record in the wiki minutes, but it was agreed at both Frankfurt and Santa Cruz that this issue is NAD.
The agreement in SC was that I would provide you with the rationale (see below) to include when moving to NAD.
[ 2009-11-03 Howard adds: ]
Moved to Tentatively NAD after 5 positive votes on c++std-lib.
Proposed resolution:
Rationale:
The issue suggests a more refined concept should be used if we want to require streaming, to separate concerns from the basic RandomNumberEngine behaviour. In Frankfurt it was observed that RandomNumberEngine is that more refined concept, and the basic concept used in the framework is UniformRandomNumberGenerator, which it refines.
We concur, and expect this to have no repurcussions re-writing this clause now concepts are removed.
Section: 26.5 [rand] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-07-15
View all other issues in [rand].
View all issues with NAD Concepts status.
Discussion:
The RandomNumberEngineAdaptor concept breaks precedent in the way the library has been specified by grouping requirements into a concept that is never actually used in the library.
This is undoubtedly a very helpful device for documentation, but we are not comfortable with the precedent - especially as we have rejected national body comments on the same grounds.
Suggest either removing the concept, or providing an algorithm/type that requires this concept in their definition (such as a factory function to create new engines).
The preference is to create a single new algorithm and retain the value of the existing documentation.
[ Batavia (2009-05): ]
Walter points out that it is unlikely that any algorithm would ever require this concept, but that the concept nonetheless is useful as documentation, and (via concept maps) as a means of checking specific adapters.
Alisdair disagrees as to the concept's value as documentation.
Marc points out that the RandomNumberDistribution is also a concept not used elsewhere in the Standard.
Pete agrees that a policy of not inventing concepts that aren't used in the Standard is a good starting point, but should not be used as a criterion for rejecting a concept.
Move to Open.
Proposed resolution:
Section: 23.2.3 [sequence.reqmts] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-07-13
View all other issues in [sequence.reqmts].
View all issues with NAD Editorial status.
Discussion:
Sequence containers 23.2.3 [sequence.reqmts]:
The return value of new calls added to table 83 are not specified.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to NAD Editorial.
Proposed resolution:
Add after p6 23.2.3 [sequence.reqmts]:
-6- ...
The iterator returned from a.insert(p,rv) points to the copy of rv inserted into a.
The iterator returned from a.emplace(p, args) points to the new element constructed from args inserted into a.
Section: 20.8.14.2 [func.wrap.func] Status: NAD Concepts Submitter: Daniel Krügler Opened: 2009-03-13 Last modified: 2009-07-16
View all other issues in [func.wrap.func].
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Discussion:
Due to a deliberate core language decision, the earlier called "foundation" concept std::FunctionType had been removed in N2773 shortly before the first "conceptualized" version of the WP (N2798) had been prepared. This caused a break of the library, which already used this concept in the adapted definition of std::function (20.8 [function.objects]/2, header <functional> synopsis and 20.8.14.2 [func.wrap.func]).
A simple fix would be to either (a) make std::function's primary template unconstrained or to (b) add constraints based on existing (support) concepts. A more advanced fix would (c) introduce a new library concept.
The big disadvantage of (a) is, that users can define templates which cause compiler errors during instantiation time because of under-constrainedness and would thus violate the basic advantage of constrained code.
For (b), the ideal constraints for std::function's template parameter would be one which excludes everything else but the single provided partial specialization that matches every "free function" type (i.e. any function type w/o cv-qualifier-seq and w/o ref-qualifier). Expressing such a type as as single requirement would be written as
template<typename T>
requires ReferentType<T> // Eliminate cv void and function types with cv-qual-seq
// or ref-qual (depending on core issue #749)
&& PointeeType<T> // Eliminate reference types
&& !ObjectType<T> // Eliminate object types
Just for completeness approach (c), which would make sense, if the library has more reasons to constrain for free function types:
auto concept FreeFunctionType<typename T>
: ReferentType<T>, PointeeType<T>, MemberPointeeType<T>
{
requires !ObjectType<T>;
}
I mention that approach because I expect that free function types belong to the most natural type categories for every days coders. Potential candidates in the library are addressof and class template packaged_task.
[ Batavia (2009-05): ]
Alisdair would prefer to have a core-supported FunctionType concept in order that any future changes be automatically correct without need for a library solution to catch up; he points to type traits as a precedent. Further, he believes that a published concept can't in the future be changed.
Bill feels this category of entity would change sufficiently slowly that he would be willing to take the risk.
Of the discussed solutions, we tend toward option (c). We like the idea of having a complete taxonomy of native types, and perhaps erred in trimming the set.
We would like to have this issue reviewed by Core and would like their feedback. Move to Open.
Proposed resolution:
Change in 20.8 [function.objects]/2, Header <functional> synopsis:
// 20.6.16 polymorphic function wrappers: class bad_function_call; template<FunctionTypeReferentType F> requires PointeeType<F> && !ObjectType<F> class function; // undefined
Change in 20.8.14.2 [func.wrap.func]:
namespace std {
template<FunctionTypeReferentType F>
requires PointeeType<F> && !ObjectType<F>
class function; // undefined
Section: 17.5.2.1.4.1 [byte.strings] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-13 Last modified: 2009-07-13
View all issues with NAD Editorial status.
Discussion:
Definition of null-terminated sequences allow for embedded nulls. This is surprising, and probably not supportable with the intended use cases.
[ Batavia (2009-05): ]
We agree with the issue, but believe this can be handled editorially. Move to NAD Editorial.
Proposed resolution:
Section: 20.3.5.3 [pair.astuple] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-13 Last modified: 2009-07-13
View all issues with NAD Editorial status.
Discussion:
The definition of get implies that get must return the second element if given a negative integer.
[ Batavia (2009-05): ]
Move to NAD Editorial.
Proposed resolution:
20.3.5.3 [pair.astuple] p5:
template<intsize_t I, class T1, class T2> requires True<(I < 2)> const P& get(const pair<T1, T2>&);
Section: 24.5.2 [insert.iterators] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-13 Last modified: 2009-10-20
View all other issues in [insert.iterators].
View all issues with NAD status.
Discussion:
It is odd that we have an iterator to insert into a vector, but not an iterator to insert into a vector that is adapted as a stack. The standard container adapters all have a common interface to push and pop so it should be simple to create an iterator adapter to complete the library support.
We should provide an AdaptedContainer concept supporting push and pop operations. Create a new insert iterator and factory function that inserts values into the container by calling push.
[ Batavia (2009-05): ]
Walter recommends NAD Future.
Move to Open, and recommend deferring the issue until after the next Committee Draft is issued.
[ 2009-07-29 Howard moves to Tentatively NAD Future. ]
A poll on the LWG reflector voted unanimously to move this issue to Tentatively NAD Future.
[ 2009 Santa Cruz: ]
Moved to NAD. The intent of these adapters are to restrict the interfaces.
Proposed resolution:
Section: X [iterator.backward] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-15 Last modified: 2009-07-16
View all issues with NAD Concepts status.
Discussion:
Which header must a user #include to obtain the library-supplied concept_maps declared in this paragraph?
This is important information, as existing user code will break if this header is not included, and we should make a point of mandating this header is #include-d by library headers likely to make use of it, notably <algorithm>. See issue 1001 for more details.
[ Batavia (2009-05): ]
We agree with the direction of the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
We believe this is NAD Concepts, but this needs to be reviewed against the post-remove-concepts draft.
Proposed resolution:
Change [depr.lib.iterator.primitives], Iterator primitives, as indicated:
To simplify the use of iterators and provide backward compatibility with previous C++ Standard Libraries, the library provides several classes and functions. Unless otherwise specified, these classes and functions shall be defined in header <iterator>.
Change X [iterator.backward], Iterator backward compatibility, as indicated:
The library provides concept maps that allow iterators specified with iterator_traits to interoperate with algorithms that require iterator concepts. These concept maps shall be defined in the same header that defines the iterator. [Example:
Section: 17.3.15 [defns.obj.state] Status: NAD Submitter: Howard Hinnant Opened: 2009-03-15 Last modified: 2009-10-23
View all issues with NAD status.
Discussion:
Addresses UK 152
Object state is using a definition of object (instance of a class) from outside the standard, rather than the 'region of storage' definiton in 1.8 [intro.object]p1
[ Summit: ]
We think we're removing this; See X [func.referenceclosure.cons].
[ 2009-10 Santa Cruz: ]
Mark as NAD. This will not affect user or implementer code
Proposed resolution:
Section: 26.7 [numeric.ops] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-17 Last modified: 2009-07-14
View all issues with NAD Concepts status.
Discussion:
One of the motivating examples for introducing requirements-aliases was to simplify the wording of the inner_product requirements. As the paper adopting the feature and constrained wording for the library went through in the same meeting, it was not possible to make the change at the time. The simpler form should be adopted now though. Similarly, most the other numerical algorithms can benefit from a minor cleanup.
Note that in each case, the second more generalised form of the algorithm does not benefit, as there are already named constraints supplied by the template type parameters.
[ 2009-05-02 Daniel adds: ]
one part of the suggested resolution suggests the removal of the MoveConstructible<T> requirement from inner_product. According to 26.7.2 [inner.product]
Computes its result by initializing the accumulator acc with the initial value initthis step requires at least MoveConstructible.
Therefore I strongly suggest to take this removal back (Note also that the corresponding overload with a functor argument still has the same MoveConstructible<T> requirement).
[ Batavia (2009-05): ]
We agree with the proposed resolution as amended by Daniel's suggestion to restore MoveConstructible, reflected in the updated proposed resolution below.
Move to Tentatively Ready.
Proposed resolution:
Change in 26.7 [numeric.ops] and 26.7.1 [accumulate]:
template <InputIterator Iter, MoveConstructible T>
requires add = HasPlus<T, Iter::reference>
&& HasAssign<T, HasPlus<T, Iter::reference> add::result_type>
T accumulate(Iter first, Iter last, T init);
Change in 26.7 [numeric.ops] and 26.7.2 [inner.product]:
template <InputIterator Iter1, InputIterator Iter2, MoveConstructible T>
requires mult = HasMultiply<Iter1::reference, Iter2::reference>
&& add = HasPlus<T, HasMultiply<Iter1::reference, Iter2::reference> mult::result_type>
&& HasAssign<
T,
HasPlus<T,
HasMultiply<Iter1::reference, Iter2::reference>::result_type> add::result_type>
T inner_product(Iter1 first1, Iter1 last1, Iter2 first2, T init);
Change in 26.7 [numeric.ops] and 26.7.3 [partial.sum]:
template <InputIterator InIter, OutputIterator<auto, const InIter::value_type&> OutIter>
requires add = HasPlus<InIter::value_type, InIter::reference>
&& HasAssign<InIter::value_type,
HasPlus<InIter::value_type, InIter::reference> add::result_type>
&& Constructible<InIter::value_type, InIter::reference>
OutIter partial_sum(InIter first, InIter last, OutIter result);
Change in 26.7 [numeric.ops] and 26.7.4 [adjacent.difference]:
template <InputIterator InIter, OutputIterator<auto, const InIter::value_type&> OutIter>
requires sub = HasMinus<InIter::value_type, InIter::value_type>
&& Constructible<InIter::value_type, InIter::reference>
&& OutputIterator<OutIter, HasMinus<InIter::value_type, InIter::value_type> sub::result_type>
&& MoveAssignable<InIter::value_type>
OutIter adjacent_difference(InIter first, InIter last, OutIter result);
Section: 26.5.6 [rand.device] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-18 Last modified: 2010-03-08
View all other issues in [rand.device].
View all issues with NAD status.
Discussion:
class random_device should be movable.
[ Batavia (2009-05): ]
Move to Open, and recommend this issue be deferred until after the next Committee Draft is issued.
[ 2009-10 post-Santa Cruz: ]
Leave open. Walter to provide drafting as part of his planned paper.
[ 2010 Pittsburgh: Moved to NAD. ]
Rationale:
WP is correct as written.
Proposed resolution:
Section: 26.5.7.1 [rand.util.seedseq] Status: NAD Submitter: Alisdair Meredith Opened: 2009-03-18 Last modified: 2010-03-09
View all other issues in [rand.util.seedseq].
View all issues with NAD status.
Discussion:
class seed_seq should support efficient move operations.
[ Batavia (2009-05): ]
Move to Open, and recommend this issue be deferred until after the next Committee Draft is issued.
[ 2009-10 post-Santa Cruz: ]
Leave open. Walter to provide drafting as part of his planned paper.
[ 2010 Pittsburgh: ]
seed_seq is explicitly not copyable, so, much like LWG issue 1068, LWG issue 1069 could be marked NAD to be consistent with this.
Proposed resolution:
Section: 20.8.15 [unord.hash] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2009-07-16
View all other issues in [unord.hash].
View all issues with NAD Concepts status.
Discussion:
Is std::hash a constrained template or not?
According to Class template hash 20.8.15 [unord.hash], the definition is:
template <class T>
struct hash : public std::unary_function<T, std::size_t> {
std::size_t operator()(T val) const;
};
And so unconstrained.
According to the <functional> synopsis in p2 Function objects 20.8 [function.objects] the template is declared as:
template <ReferentType T> struct hash;
which would make hash a constrained template.
[ 2009-03-22 Daniel provided wording. ]
[ Batavia (2009-05): ]
Alisdair is not certain that Daniel's proposed resolution is sufficient, and recommends we leave the hash template unconstrained for now.
Recommend that the Project Editor make the constrained declaration consistent with the definition in order to make the Working Paper internally consistent, and that the issue then be revisited.
Move to Open.
Proposed resolution:
[To the editor: This resolution is merge-compatible to the resolution of 1078]
In 20.8 [function.objects]/2, header <functional> synopsis, change as indicated:
// 20.6.17, hash function base template: template <ReferentType T> struct hash; // undefined
In 20.8.15 [unord.hash]/1 change as indicated:
namespace std {
template <class T>
struct hash : public std::unary_function<T, std::size_t> {
std::size_t operator()(T val) const;
};
template <ReferentType T> struct hash; // undefined
}
In 20.8.15 [unord.hash]/2 change as indicated:
-2- For all library-provided specializations, the template instantiation hash<T> shall provide a public operator() with return type std::size_t to satisfy the concept requirement Callable<const hash<T>, const T&>. If T is an object type or reference to object, hash<T> shall be publicly derived from std::unary_function<T, std::size_t>. The return value of operator() is unspecified, except that equal arguments shall yield the same result. operator() shall not throw exceptions.
In 18.7 [support.rtti]/1, header <typeinfo> synopsis change as indicated:
namespace std {
class type_info;
class type_index;
template <classReferentType T> struct hash;
Section: X [allocator.element.concepts] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2009-07-13
View all issues with NAD Concepts status.
Discussion:
p7 Allocator-related element concepts X [allocator.element.concepts]
The changes to the AllocatableElement concept mean this concept_map specialization no longer matches the original concept:
template <Allocator Alloc, class T, class ... Args>
requires HasConstructor<T, Args...>
concept_map AllocatableElement<Alloc, T, Args&&...> {
void construct_element(Alloc& a, T* t, Args&&... args) {
Alloc::rebind<T>(a).construct(t, forward(args)...);
}
}
[ 2009-03-23 Pablo adds: ]
Actually, this is incorrect, N2840 says. "In section X [allocator.element.concepts] paragraph 8, modify the definition of the AllocatableElement concept and eliminate the related concept map:" but then neglects to include the red-lined text of the concept map that was to be eliminated. Pete also missed this, but I caught it he asked me to review his edits. Pete's updated WP removes the concept map entirely, which was the original intent. The issue is, therefore, moot. Note, as per my presentation of N2840 in summit, construct() no longer has a default implementation. This regrettable fact was deemed (by David Abrahams, Doug, and myself) to be preferable to the complexity of providing a default implementation that would not under-constrain a more restrictive allocator (like the scoped allocators).
[ 2009-05-01 Daniel adds: ]
it seems to me that #1074 should be resolved as a NAD, because the current WP has already removed the previous AllocatableElement concept map. It introduced auto concept AllocatableElement instead, but as of X [allocator.element.concepts]/7 this guy contains now
requires FreeStoreAllocatable<T>; void Alloc::construct(T*, Args&&...);
[ Batavia (2009-05): ]
The affected code is no longer part of the Working Draft.
Move to NAD.
Proposed resolution:
Change X [allocator.element.concepts]:
template <Allocator Alloc, class T, class ... Args>
requires HasConstructor<T, Args...>
concept_map AllocatableElement<Alloc, T, Args&&...> {
void construct_element(Alloc& a, T* t, Args&&... args) {
Alloc::rebind<T>(a).construct(t, forward(args)...);
}
}
Section: 20 [utilities], 23 [containers] Status: NAD Editorial Submitter: Alan Talbot Opened: 2009-03-20 Last modified: 2009-10-26
View all other issues in [utilities].
View all issues with NAD Editorial status.
Discussion:
Addresses US 65 and US 74.1
US 65:
Scoped allocators and allocator propagation traits add a small amount of utility at the cost of a great deal of machinery. The machinery is user visible, and it extends to library components that don't have any obvious connection to allocators, including basic concepts and simple components like pair and tuple.Suggested resolution:
Sketch of proposed resolution: Eliminate scoped allocators, replace allocator propagation traits with a simple uniform rule (e.g. always propagate on copy and move), remove all mention of allocators from components that don't explicitly allocate memory (e.g. pair), and adjust container interfaces to reflect this simplification.
Components that I propose eliminating include HasAllocatorType, is_scoped_allocator, allocator_propagation_map, scoped_allocator_adaptor, and ConstructibleAsElement.
US 74.1:
Scoped allocators represent a poor trade-off for standardization, since (1) scoped-allocator--aware containers can be implemented outside the C++ standard library but used with its algorithms, (2) scoped allocators only benefit a tiny proportion of the C++ community (since few C++ programmers even use today's allocators), and (3) all C++ users, especially the vast majority of the C++ community that won't ever use scoped allocators are forced to cope with the interface complexity introduced by scoped allocators.
In essence, the larger community will suffer to support a very small subset of the community who can already implement their own data structures outside of the standard library. Therefore, scoped allocators should be removed from the working paper.
Some evidence of the complexity introduced by scoped allocators:
20.3.5 [pairs], 20.4 [tuple]: Large increase in the number of pair and tuple constructors.
23 [containers]: Confusing "AllocatableElement" requirements throughout.
Suggested resolution:
Remove support for scoped allocators from the working paper. This includes at least the following changes:
Remove X [allocator.element.concepts]
Remove 20.9.6 [allocator.adaptor]
Remove [construct.element]
In Clause 23 [containers]: replace requirements naming the AllocatableElement concept with requirements naming CopyConstructible, MoveConstructible, DefaultConstructible, or Constructible, as appropriate.
[ Post Summit Alan moved from NAD to Open. ]
[ 2009-05-15 Ganesh adds: ]
The requirement AllocatableElement should not be replaced with Constructible on the emplace_xxx() functions as suggested. In the one-parameter case the Constructible requirement is not satisfied when the constructor is explicit (as per [concept.map.fct], twelfth bullet) but we do want to allow explicit constructors in emplace, as the following example shows:
vector<shared_ptr<int>> v; v.emplace_back(new int); // should be allowedIf the issue is accepted and scoped allocators are removed, I suggest to add a new pair of concepts to [concept.construct], namely:
auto concept HasExplicitConstructor<typename T, typename... Args> { explicit T::T(Args...); } auto concept ExplicitConstructible<typename T, typename... Args> : HasExplicitConstructor<T, Args...>, NothrowDestructible<T> { }We should then use ExplicitConstructible as the requirement for all emplace_xxx() member functions.
For coherence and consistency with the similar concepts Convertible/ExplicitlyConvertible, we might also consider changing Constructible to:
auto concept Constructible<typename T, typename... Args> : HasConstructor<T, Args...>, ExplicitConstructible<T, Args...> { }Moreover, all emplace-related concepts in [container.concepts] should also use ExplicitConstructible instead of Constructible in the definitions of their axioms. In fact the concepts in [container.concepts] should be corrected even if the issue is not accepted.
On the other hand, if the issue is not accepted, the scoped allocator adaptors should be fixed because the following code:
template <typename T> using scoped_allocator = scoped_allocator_adaptor<allocator<T>>; vector<shared_ptr<int>, scoped_allocator<shared_ptr<int>>> v; v.emplace_back(new int); // ops! doesn't compiledoesn't compile, as the member function construct() of the scoped allocator requires non-explicit constructors through concept ConstructibleWithAllocator. Fixing that is not difficult but probably more work than it's worth and is therefore, IMHO, one more reason in support of the complete removal of scoped allocators.
[ 2009-06-09 Alan adds: ]
I reopened this issue because I did not think that these National Body comments were adequately addressed by marking them NAD. My understanding is that something can be marked NAD if it is clearly a misunderstanding or trivial, but a substantive issue that has any technical merit requires a disposition that addresses the concerns.
The notes in the NB comment list (US 65 & US 74.1) say that:
- this issue has not introduced any new arguments not previously discussed,
- the vote (4-9-3) was not a consensus for removing scoped allocators,
- the issue is resolved by N2840.
My opinion is:
- there are new arguments in both comments regarding concepts (which were not present in the library when the scoped allocator proposal was voted in),
- the vote was clearly not a consensus for removal, but just saying there was a vote does not provide a rationale,
- I do not believe that N2840 addresses these comments (although it does many other things and was voted in with strong approval).
My motivation to open the issue was to ensure that the NB comments were adequately addressed in a way that would not risk a "no" vote on our FCD. If there are responses to the technical concerns raised, then perhaps they should be recorded. If the members of the NB who authored the comments are satisfied with N2840 and the other disposition remarks in the comment list, then I am sure they will say so. In either case, this issue can be closed very quickly in Frankfurt, and hopefully will have helped make us more confident of approval with little effort. If in fact there is controversy, my thought is that it is better to know now rather than later so there is more time to deal with it.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Addressed by N2982.
Proposed resolution:
Rationale:
Scoped allocators have been revised significantly.Section: 20.8.9 [negators] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-03-20 Last modified: 2010-08-25
View all issues with NAD Concepts status.
Discussion:
The class templates unary/binary_negate need constraining and move support.
Ideally these classes would be deprecated, allowing unary/binary_function to also be deprecated. However, until a generic negate adaptor is introduced that can negate any Callable type, they must be supported so should be constrained. Likewise, they should be movable, and support adopting a move-only predicate type.
In order to preserve ABI compatibility, new rvalue overloads are supplied in preference to changing the existing pass-by-const-ref to pass-by-value.
Do not consider the issue of forwarding mutable lvalues at this point, although remain open to another issue on the topic.
[ 2009-05-01 Daniel adds: ]
IMO the currently proposed resolution needs some updates because it is ill-formed at several places:
In concept AdaptableUnaryFunction change
typename X::result_type; typename X::argument_type;to
Returnable result_type = typename X::result_type; typename argument_type = typename X::argument_type;[The replacement "Returnable result_type" instead of "typename result_type" is non-editorial, but maybe you prefer that as well]
In concept AdaptableBinaryFunction change
typename X::result_type; typename X::first_argument_type; typename X::second_argument_type;to
Returnable result_type = typename X::result_type; typename first_argument_type = typename X::first_argument_type; typename second_argument_type = typename X::second_argument_type;[The replacement "Returnable result_type" instead of "typename result_type" is non-editorial, but maybe you prefer that as well.]
In class unary/binary_function
- I suggest to change "ReturnType" to "Returnable" in both cases.
- I think you want to replace the remaining occurrences of "Predicate" by "P" (in both classes in copy/move from a predicate)
I think you need to change the proposed signatures of not1 and not2, because they would still remain unconstrained: To make them constrained at least a single requirement needs to be added to enable requirement implication. This could be done via a dummy ("requires True<true>") or just explicit as follows:
[Don't we want a move call for the second overload as in
unary_negate<P>(std::move(pred))in the Returns clause ?]
-5- Returns: binary_negate<P>(pred).
[Don't we want a move call for the second overload as in
binary_negate<P>(std::move(pred))in the Returns clause ?]
[ Batavia (2009-05): ]
There is concern that complicating the solution to preserve the ABI seems unnecessary, since we're not in general preserving the ABI.
We would prefer a separate paper consolidating all Clause 20 issues that are for the purpose of providing constrained versions of the existing facilities.
Move to Open.
[ 2009-10 post-Santa Cruz: ]
Leave open pending the potential move constructor paper. Note that we consider the "constraining" part NAD Concepts.
[ 2010-01-31 Alisdair removes the current proposed wording from the proposed wording section because it is based on concepts. That wording is proposed here: ]
Add new concepts where appropriate::
auto concept AdaptableUnaryFunction< typename X > { typename X::result_type; typename X::argument_type; } auto concept AdaptableBinaryFunction< typename X > { typename X::result_type; typename X::first_argument_type; typename X::second_argument_type; }Revise as follows:
Base 20.8.3 [base] (Only change is constrained Result)
-1- The following classes are provided to simplify the typedefs of the argument and result types:
namespace std { template <class Arg,classReturnType Result> struct unary_function { typedef Arg argument_type; typedef Result result_type; }; template <class Arg1, class Arg2,classReturnType Result> struct binary_function { typedef Arg1 first_argument_type; typedef Arg2 second_argument_type; typedef Result result_type; }; }Negators 20.8.9 [negators]:
-1- Negators not1 and not2 take a unary and a binary predicate, respectively, and return their complements (5.3.1).
template <classAdaptableUnaryFunction Predicate> requires Predicate< P, P::argument_type > class unary_negate : public unary_function<typenamePredicate::argument_type,bool> { public: unary_negate(const unary_negate & ) = default; unary_negate(unary_negate && ); requires CopyConstructible< P > explicit unary_negate(const Predicate& pred); requires MoveConstructible< P > explicit unary_negate(Predicate && pred); bool operator()(consttypenamePredicate::argument_type& x) const; };-2 operator() returns !pred(x).template <class Predicate> unary_negate<Predicate> not1(const Predicate& pred); template <class Predicate> unary_negate<Predicate> not1(Predicate&& pred);-3- Returns: unary_negate<Predicate>(pred).template <classAdaptableBinaryFunction Predicate> requires Predicate< P, P::first_argument_type, P::second_argument_type > class binary_negate : public binary_function<typenamePredicate::first_argument_type,typenamePredicate::second_argument_type, bool> { public: biary_negate(const binary_negate & ) = default; binary_negate(binary_negate && ); requires CopyConstructible< P > explicit binary_negate(const Predicate& pred); requires MoveConstructible< P > explicit binary_negate(const Predicate& pred); bool operator()(consttypenamePredicate::first_argument_type& x, consttypenamePredicate::second_argument_type& y) const; };-4- operator() returns !pred(x,y).template <class Predicate> binary_negate<Predicate> not2(const Predicate& pred); template <class Predicate> binary_negate<Predicate> not2(Predicate&& pred);-5- Returns: binary_negate<Predicate>(pred).
[ 2010 Rapperswil: ]
Move to NAD Concepts. The move-semantic part has been addressed by a core language change, which implicitly generates appropriate move constructors and move-assignment operators.
Proposed resolution:
Section: 20.4.2 [tuple.tuple] Status: NAD Editorial Submitter: Pete Becker Opened: 2009-03-20 Last modified: 2009-07-13
View all other issues in [tuple.tuple].
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Discussion:
Class template tuple 20.4.2 [tuple.tuple]:
template <class... UTypes> requires Constructible<Types, const UTypes&>... template <class... UTypes> requires Constructible<Types, RvalueOf<UTypes>::type>...
Somebody needs to look at this and say what it should be.
[ 2009-03-21 Daniel provided wording. ]
[ Batavia (2009-05): ]
The resolution looks correct; move to NAD Editorial.
Proposed resolution:
In 20.4.2 [tuple.tuple], class tuple, change as indicated:
template <class... UTypes> requires Constructible<Types, const UTypes&>... tuple(const pair<UTypes...>&); template <class... UTypes> requires Constructible<Types, RvalueOf<UTypes>::type>... tuple(pair<UTypes...>&&);
[NB.: The corresponding prototypes do already exist in 20.4.2.1 [tuple.cnstr]/7+8]
Section: 20.12 [type.index] Status: NAD Concepts Submitter: Doug Gregor Opened: 2009-03-20 Last modified: 2009-07-16
View all issues with NAD Concepts status.
Discussion:
Addresses DE 17
DE-17:
The class type_index should be removed; it provides no additional functionality beyond providing appropriate concept maps.
[ 2009-03-31 Peter adds: ]
It is not true, in principle, that std::type_index provides no utility compared to bare std::type_info*.
std::type_index can avoid the lifetime issues with type_info when the DLL that has produced the type_info object is unloaded. A raw type_info* does not, and cannot, provide any protection in this case. A type_index can (if the implementor so chooses) because it can wrap a smart (counted or even cloning) pointer to the type_info data that is needed for name() and before() to work.
Proposed resolution:
Modify the header <typeinfo> synopsis in 18.7 [support.rtti]p1 as follows:
namespace std {
class type_info;
class type_index;
template <class T> struct hash;
template<> struct hash<type_indexconst type_info *> : public std::unary_function<type_indexconst type_info *, size_t> {
size_t operator()(type_indexconst type_info * indext) const;
};
concept_map LessThanComparable<const type_info *> see below
class bad_cast;
class bad_typeid;
}
Add the following new subsection
18.7.1.1 Template specialization
hash<const type_info *>[type.info.hash]size_t operator()(const type_info *x) const;
- Returns:
x->hash_code()
Add the following new subsection
18.7.1.2
type_infoconcept map [type.info.concepts]concept_map LessThanComparable<const type_info *> { bool operator<(const type_info *x, const type_info *y) { return x->before(*y); } bool operator<=(const type_info *x, const type_info *y) { return !y->before(*x); } bool operator>(const type_info *x, const type_info *y) { return y->before(*x); } bool operator>=(const type_info *x, const type_info *y) { return !x->before(*y); } }
- Note: provides a well-defined ordering among
type_info constpointers, which makes such pointers usable in associative containers (23.4).
Remove section 20.12 [type.index]
Section: X [concept.arithmetic] Status: NAD Concepts Submitter: Daniel Krügler Opened: 2009-03-21 Last modified: 2009-07-15
View all issues with NAD Concepts status.
Discussion:
Astonishingly, the current concept ArithmeticLike as specified in X [concept.arithmetic] does not provide explicit conversion to bool although this is a common property of arithmetic types (4.12 [conv.bool]). Recent proposals that introduced such types (integers of arbitrary precision, n2143, decimals n2732 indirectly via conversion to long long) also took care of such a feature.
Adding such an explicit conversion associated function would also partly solve a currently invalid effects clause in library, which bases on this property, 24.2.7 [random.access.iterators]/2:
{ difference_type m = n;
if (m >= 0) while (m--) ++r;
else while (m++) --r;
return r; }
Both while-loops take advantage of a contextual conversion to bool (Another problem is that the >= comparison uses the no longer supported existing implicit conversion from int to IntegralLike).
Original proposed resolution:In X [concept.arithmetic], add to the list of less refined concepts one further concept:
concept ArithmeticLike<typename T>
: Regular<T>, LessThanComparable<T>, HasUnaryPlus<T>, HasNegate<T>,
HasPlus<T, T>, HasMinus<T, T>, HasMultiply<T, T>, HasDivide<T, T>,
HasPreincrement<T>, HasPostincrement<T>, HasPredecrement<T>,
HasPostdecrement<T>,
HasPlusAssign<T, const T&>, HasMinusAssign<T, const T&>,
HasMultiplyAssign<T, const T&>,
HasDivideAssign<T, const T&>, ExplicitlyConvertible<T, bool> {
In 24.2.7 [random.access.iterators]/2 change the current effects clause as indicated [The proposed insertion fixes the problem that the previous implicit construction from integrals has been changed to an explicit constructor]:
{ difference_type m = n;
if (m >= difference_type(0)) while (m--) ++r;
else while (m++) --r;
return r; }
[ Batavia (2009-05): ]
We agree that arithmetic types ought be convertible to bool, and we therefore agree with the proposed resolution's paragraph 1.
We do not agree that the cited effects clause is invalid, as it expresses intent rather than specific code.
Move to Review, pending input from concepts experts.
Proposed resolution:
In X [concept.arithmetic], add to the list of less refined concepts one further concept:
concept ArithmeticLike<typename T>
: Regular<T>, LessThanComparable<T>, HasUnaryPlus<T>, HasNegate<T>,
HasPlus<T, T>, HasMinus<T, T>, HasMultiply<T, T>, HasDivide<T, T>,
HasPreincrement<T>, HasPostincrement<T>, HasPredecrement<T>,
HasPostdecrement<T>,
HasPlusAssign<T, const T&>, HasMinusAssign<T, const T&>,
HasMultiplyAssign<T, const T&>,
HasDivideAssign<T, const T&>, ExplicitlyConvertible<T, bool> {
Section: 21 [strings] Status: NAD Concepts Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2009-07-15
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Discussion:
Addresses UK 216, JP 46, JP 48
All the containers use concepts for their iterator usage, exect for basic_string. This needs fixing.
Use concepts for iterator template parameters throughout the chapter.
[ Summit: ]
NB comments to be handled by Dave Abrahams and Howard Hinnant with advice from PJP: UK216 (which duplicates) JP46, JP48. JP46 supplies extensive proposed wording; start there.
Proposed resolution:
Section: 22 [localization] Status: NAD Concepts Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2009-07-15
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Discussion:
Addresses JP 49
codecvt does not use concept. For example, create CodeConvert concept and change as follows.
template<CodeConvert Codecvt, class Elem = wchar_t>
class wstring_convert {
[ Summit: ]
To be handled by Howard Hinnant, Dave Abrahams, Martin Sebor, PJ Plauger.
Proposed resolution:
Section: 22 [localization] Status: NAD Concepts Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2009-07-15
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Discussion:
Addresses JP 52, JP 53
InputIterator does not use concept.
OutputIterator does not use concept.
Comments include proposed wording.
[ Summit: ]
To be handled by Howard Hinnant, Dave Abrahams, Martin Sebor, PJ Plauger.
Proposed resolution:
Section: 24.2.5 [forward.iterators] Status: NAD Concepts Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2009-07-16
View all other issues in [forward.iterators].
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Discussion:
Addresses UK 250
A default implementation should be supplied for the post-increment operator to simplify implementation of iterators by users.
Copy the Effects clause into the concept description as the default implementation. Assumes a default value for postincrement_result
[ Summit: ]
Howard will open an issue.
[ 2009-06-07 Daniel adds: ]
This issue cannot currently be resolved as suggested, because that would render auto-detection of the return type postincrement_result invalid, see [concept.map.assoc]/4+5. The best fix would be to add a default type to that associated type, but unfortunately any default type will prevent auto-deduction of types of associated functions as quoted above. A corresponding core issue is in preparation.
Proposed resolution:
[ This wording assumes the acceptance of UK 251 / 1009. Both wordings change the same paragraphs. ]
Change 24.2.5 [forward.iterators]:
concept ForwardIterator<typename X> : InputIterator<X>, Regular<X> {
MoveConstructible postincrement_result;
requires HasDereference<postincrement_result>
&& Convertible<HasDereference<postincrement_result>::result_type, const value_type&>;
postincrement_result operator++(X& r, int); {
X tmp = r;
++r;
return tmp;
}
axiom MultiPass(X a, X b) {
if (a == b) *a == *b;
if (a == b) ++a == ++b;
}
}
Section: 24.2.6 [bidirectional.iterators] Status: NAD Concepts Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2009-07-16
View all other issues in [bidirectional.iterators].
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Discussion:
Addresses UK 258
A default implementation should be supplied for the post-decrement operator to simplify implementation of iterators by users.
Copy the Effects clause into the concept description as the default implementation. Assumes a default value for postincrement_result
[ Summit: ]
Howard will open an issue.
[ 2009-06-07 Daniel adds: ]
This issue cannot currently be resolved as suggested, because that would render auto-detection of the return type postdecrement_result invalid, see 1084.
Proposed resolution:
Change 24.2.6 [bidirectional.iterators]:
concept BidirectionalIterator<typename X> : ForwardIterator<X> {
MoveConstructible postdecrement_result;
requires HasDereference<postdecrement_result>
&& Convertible<HasDereference<postdecrement_result>::result_type, const value_type&>
&& Convertible<postdecrement_result, const X&>;
X& operator--(X&);
postdecrement_result operator--(X& r, int); {
X tmp = r;
--r;
return tmp;
}
}
Section: 24.6 [stream.iterators] Status: NAD Concepts Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2009-07-15
View all issues with NAD Concepts status.
Discussion:
Addresses UK 284
The stream iterators need constraining with concepts/requrires clauses.
[ Summit: ]
We agree. To be handled by Howard, Martin and PJ.
Proposed resolution:
Section: 25.3.5 [alg.replace] Status: NAD Concepts Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2009-07-15
View all other issues in [alg.replace].
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Discussion:
Addresses UK 301
replace and replace_if have the requirement: OutputIterator<Iter, Iter::reference> Which implies they need to copy some values in the range the algorithm is iterating over. This is not however the case, the only thing that happens is const T&s might be copied over existing elements (hence the OutputIterator<Iter, const T&>.
Remove OutputIterator<Iter, Iter::reference> from replace and replace_if.
[ Summit: ]
We agree. To be handled by Howard.
Proposed resolution:
Change in [algorithms.syn] and 25.3.5 [alg.replace]:
template<ForwardIterator Iter, class T> requiresOutputIterator<Iter, Iter::reference> &&OutputIterator<Iter, const T&> && HasEqualTo<Iter::value_type, T> void replace(Iter first, Iter last, const T& old_value, const T& new_value); template<ForwardIterator Iter, Predicate<auto, Iter::value_type> Pred, class T> requiresOutputIterator<Iter, Iter::reference> &&OutputIterator<Iter, const T&> && CopyConstructible<Pred> void replace_if(Iter first, Iter last, Pred pred, const T& new_value);
Section: 30.6.5 [futures.promise] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2009-10-26
View all other issues in [futures.promise].
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Discussion:
Addresses UK 342
std::promise is missing a non-member overload of swap. This is inconsistent with other types that provide a swap member function.
Add a non-member overload void swap(promise&& x,promise&& y){ x.swap(y); }
[ Summit: ]
Create an issue. Move to review, attention: Howard. Detlef will also look into it.
[ Post Summit Daniel provided wording. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2997.
Proposed resolution:
In 30.6.5 [futures.promise], before p.1, immediately after class template promise add:
template <class R> void swap(promise<R>& x, promise<R>& y);
Change 30.6.5 [futures.promise]/10 as indicated (to fix a circular definition):
-10- Effects:
swap(*this, other)Swaps the associated state of *this and otherThrows: Nothing.
After the last paragraph in 30.6.5 [futures.promise] add the following prototype description:
template <class R> void swap(promise<R>& x, promise<R>& y);Effects: x.swap(y)
Throws: Nothing.
Section: 30.6.10 [futures.task] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2009-03-22 Last modified: 2010-03-11
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Discussion:
Class template packaged_task in 30.6.10 [futures.task] shows a member swap declaration, but misses to document it's effects (No prototype provided). Further on this class misses to provide a non-member swap.
[ Batavia (2009-05): ]
Alisdair notes that paragraph 2 of the proposed resolution has already been applied in the current Working Draft.
We note a pending future-related paper by Detlef; we would like to wait for this paper before proceeding.
Move to Open.
[ 2009-05-24 Daniel removed part 2 of the proposed resolution. ]
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready, removing bullet 3 from the proposed resolution but keeping the other two bullets.
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
In 30.6.10 [futures.task], immediately after the definition of class template packaged_task add:
template<class R, class... Argtypes> void swap(packaged_task<R(ArgTypes...)>&, packaged_task<R(ArgTypes...)>&);
At the end of 30.6.10 [futures.task] (after p. 20), add add the following prototype description:
template<class R, class... Argtypes> void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y);Effects: x.swap(y)
Throws: Nothing.
Section: 23.4.2.2 [multimap.modifiers] Status: NAD Submitter: LWG Opened: 2009-03-22 Last modified: 2009-10-20
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Discussion:
Addresses UK 246
The content of this sub-clause is purely trying to describe in words the effect of the requires clauses on these operations, now that we have Concepts. As such, the description is more confusing than the signature itself. The semantic for these functions is adequately covered in the requirements tables in 23.2.4 [associative.reqmts].
[ Beman adds: ]
Pete is clearly right that this one is technical rather than editorial.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Review.
[ 2009-10 Santa Cruz: ]
Mark as NAD, solved by removing concepts.
Proposed resolution:
Strike 23.4.2.2 [multimap.modifiers] entirely (but do NOT strike these signatures from the class template definition!).
Section: 20.7.3 [meta.help] Status: NAD Concepts Submitter: Daniel Krügler Opened: 2009-03-22 Last modified: 2009-07-15
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Discussion:
A first step to change the type traits predicates to constrained templates is to constrain their common base template integral_constant. This can be done, without enforcing depending classes to be constrained as well, but not vice versa without brute force late_check usages. The following proposed resolution depends on the resolution of LWG issue 1019.
[ Batavia (2009-05): ]
Move to Open, pending a paper that looks at constraints for the entirety of the type traits and their relationship to the foundation concepts. We recommend this be deferred until after the next Committee Draft is issued.
Proposed resolution:
In 20.7.2 [meta.type.synop], Header <type_traits> synopsis change as indicated:
namespace std {
// 20.5.3, helper class:
template <classIntegralConstantExpressionType T, T v> struct integral_constant;
In 20.7.3 [meta.help] change as indicated:
template <classIntegralConstantExpressionType T, T v> struct integral_constant { static constexpr T value = v; typedef T value_type; typedef integral_constant<T,v> type; constexpr operator value_type() { return value; } };
Section: 25.3.12 [alg.random.shuffle] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-22 Last modified: 2010-03-15
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Discussion:
There are a couple of issues with the declaration of the random_shuffle algorithm accepting a random number engine.
[ 2009-05-02 Daniel adds: ]
this issue completes adding necessary requirement to the third new random_shuffle overload. The current suggestion is:
template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand> requires ShuffleIterator<Iter> void random_shuffle(Iter first, Iter last, Rand&& g);IMO this is still insufficient and I suggest to add the requirement
Convertible<Rand::result_type, Iter::difference_type>to the list (as the two other overloads already have).
Rationale:
Its true that this third overload is somewhat different from the remaining two. Nevertheless we know from UniformRandomNumberGenerator, that it's result_type is an integral type and that it satisfies UnsignedIntegralLike<result_type>.
To realize it's designated task, the algorithm has to invoke the Callable aspect of g and needs to perform some algebra involving it's min()/max() limits to compute another index value that at this point is converted into Iter::difference_type. This is so, because 24.2.7 [random.access.iterators] uses this type as argument of it's algebraic operators. Alternatively consider the equivalent iterator algorithms in 24.4.4 [iterator.operations] with the same result.
This argument leads us to the conclusion that we also need Convertible<Rand::result_type, Iter::difference_type> here.
[ Batavia (2009-05): ]
Alisdair notes that point (ii) has already been addressed.
We agree with the proposed resolution to point (i) with Daniel's added requirement.
Move to Review.
[ 2009-06-05 Daniel updated proposed wording as recommended in Batavia. ]
[ 2009-07-28 Alisdair adds: ]
Revert to Open, with a note there is consensus on direction but the wording needs updating to reflect removal of concepts.
[ 2009-10 post-Santa Cruz: ]
Leave Open, Walter to work on it.
[ 2010 Pittsburgh: Moved to NAD Editorial, solved by N3056. ]
Rationale:
Solved by N3056.
Proposed resolution:
Change in [algorithms.syn] and 25.3.12 [alg.random.shuffle]:
concept UniformRandomNumberGenerator<typename Rand> { }template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand> requires ShuffleIterator<Iter> && Convertible<Rand::result_type, Iter::difference_type> void random_shuffle(Iter first, Iter last, Rand&& g);
Section: 17 [library] Status: NAD Concepts Submitter: David Abrahams Opened: 2009-03-21 Last modified: 2009-07-16
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Discussion:
TODO: Look at all cases of unconstrained rvalue ref parameters and check that concept req'ts work when T deduced as reference.
We found some instances where that was not done correctly and we figure the possibility of deducing T to be an lvalue reference was probably overlooked elsewhere.
[ Batavia (2009-05): ]
Move to Open, pending proposed wording from Dave for further review.
Proposed resolution:
Section: 17 [library] Status: NAD Submitter: David Abrahams Opened: 2009-03-21 Last modified: 2010-03-08
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Discussion:
Notes
[2009-03-21 Sat] p. 535 at the top we need MoveConstructible V1, MoveConstructible V2 (where V1,V2 are defined on 539). Also make_tuple on 550
CD-1 reads:
template <MoveConstructible T1, MoveConstructible T2> pair<V1, V2> make_pair(T1&&, T2&&);Actually I'm guessing we need something like MoveConstructible<V1,T1>, i.e. "V1 can be constructed from an rvalue of type T1."
Ditto for make_tuple
[2009-03-21 Sat] p1183 thread ctor, and in general, we need a way to talk about "copiable from generalized rvalue ref argument" for cases where we're going to forward and copy.
This issue may well be quite large. Language in para 4 about "if an lvalue" is wrong because types aren't expressions.
Maybe we should define the term "move" so we can just say in the effects, "f is moved into the newly-created thread" or something, and agree (and ideally document) that saying "f is moved" implies
F x(move(f))is required to work. That would cover both ctors at once.
p1199, call_once has all the same issues.
[2009-03-21 Sat] p869 InputIterator pointer type should not be required to be convertible to const value_type*, rather it needs to have a operator-> of its own that can be used for the value type.
This one is serious and unrelated to the move issue.[2009-03-21 Sat] p818 stack has the same problem with default ctor.
[2009-03-21 Sat] p816 priority_queue has the same sorts of problems as queue, only more so
requires MoveConstructible<Cont> explicit priority_queue(const Compare& x = Compare(), Cont&& = Cont());Don't require MoveConstructible when default constructing Cont. Also missing semantics for move ctor.
[2009-03-21 Sat] Why are Allocators required to be CopyConstructible as opposed to MoveConstructible?
[2009-03-21 Sat] p813 queue needs a separate default ctor (Cont needn't be MoveConstructible). No documented semantics for move c'tor. Or *any* of its 7 ctors!
[2009-03-21 Sat] std::array should have constructors for C++0x, consequently must consider move construction.
[ 2009-05-01 Daniel adds: ]
This could be done as part of 1035, which already handles deviation of std::array from container tables.[2009-03-21 Sat] p622 all messed up.
para 8 "implementation-defined" is the wrong term; should be "see below" or something.
para 12 "will be selected" doesn't make any sense because we're not talking about actual arg types.
paras 9-13 need to be totally rewritten for concepts.
[2009-03-21 Sat] Null pointer comparisons (p587) have all become unconstrained. Need to fix that
[2009-03-21 Sat] mem_fun_t etc. definition doesn't match declaration. We think CopyConstructible is the right reqt.
make_pair needs Constructible<V1, T1&&> requirements!
make_tuple needs something similar
tuple bug in synopsis:
template <class... UTypes> requires Constructible<Types, const UTypes&>... template <class... UTypes> requires Constructible<Types, RvalueOf<UTypes>::type>...Note: removal of MoveConstructible requirements in std::function makes these routines unconstrained!
[ 2009-05-02 Daniel adds: ]
This part of the issue is already covered by 1077.these unique_ptr constructors are broken [ I think this is covered in "p622 all messed up" ]
unique_ptr(pointer p, implementation-defined d); unique_ptr(pointer p, implementation-defined d);multimap range constructor should not have MoveConstructible<value_type> requirement.
same with insert(..., P&&); multiset has the same issue, as do unordered_multiset and unordered_multimap. Review these!
[ Batavia (2009-05): ]
Move to Open, pending proposed wording from Dave for further review.
[ 2009-10 post-Santa Cruz: ]
Tentatively NAD. We are not sure what has been addressed and what hasn't. Recommend closing unless someone sorts this out into something more readable.
Rationale:
The issue(s) at hand not adequately communicated.
Proposed resolution:
Section: 20.9.10.2.1 [unique.ptr.single.ctor] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2010-03-14
View all other issues in [unique.ptr.single.ctor].
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Discussion:
Message c++std-lib-23182 led to a discussion in which several people expressed interest in being able to convert an auto_ptr to a unique_ptr without the need to call release. Below is wording to accomplish this.
[ Batavia (2009-05): ]
Pete believes it not a good idea to separate parts of a class's definition. Therefore, if we do this, it should be part of unique-ptr's specification.
Alisdair believes the lvalue overload may be not necessary.
Marc believes it is more than just sugar, as it does ease the transition to unique-ptr.
We agree with the resolution as presented. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-08-01 Howard deconceptifies wording: ]
I also moved the change from D.10 [depr.auto.ptr] to 20.9.10.2 [unique.ptr.single] per the Editor's request in Batavia (as long as I was making changes anyway). Set back to Review.
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
Add to 20.9.10.2 [unique.ptr.single]:
template <class T, class D>
class unique_ptr
{
public:
template <class U>
unique_ptr(auto_ptr<U>& u);
template <class U>
unique_ptr(auto_ptr<U>&& u);
};
Add to 20.9.10.2.1 [unique.ptr.single.ctor]:
template <class U> unique_ptr(auto_ptr<U>& u); template <class U> unique_ptr(auto_ptr<U>&& u);Effects: Constructs a unique_ptr with u.release().
Postconditions: get() == the value u.get() had before the construciton, modulo any required offset adjustments resulting from the cast from U* to T*. u.get() == nullptr.
Throws: nothing.
Remarks: U* shall be implicitly convertible to T* and D shall be the same type as default_delete<T>, else these constructors shall not participate in overload resolution.
Section: 25.3.9 [alg.unique] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2009-07-13
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Discussion:
From Message c++std-core-14160 Howard wrote:
It was the intent of the rvalue reference proposal for unique to only require MoveAssignable: N1860.
And Pete replied:
That was overridden by the subsequent changes made for concepts in N2573, which reimposed the C++03 requirements.
My impression is that this overwrite was a simple (unintentional) mistake. Wording below to correct it.
[ Batavia (2009-05): ]
Howard notes this issue resolves a discrepancy between the synopsis and the description.
Move to NAD Editorial.
Proposed resolution:
Change 25.3.9 [alg.unique]:
template<ForwardIterator Iter>
requires OutputIterator<Iter, RvalueOf<Iter::reference>::type>
&& EqualityComparable<Iter::value_type>
Iter unique(Iter first, Iter last);
template<ForwardIterator Iter, EquivalenceRelation<auto, Iter::value_type> Pred>
requires OutputIterator<Iter, RvalueOf<Iter::reference>::type>
&& CopyConstructible<Pred>
Iter unique(Iter first, Iter last, Pred pred);
Note that the synopsis in [algorithms.syn] is already correct.
Section: 23.3.6.2 [vector.capacity] Status: NAD Submitter: Daniel Krügler Opened: 2009-04-20 Last modified: 2009-10-20
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Discussion:
I have the impression that even the wording of current draft N2857 does insufficiently express the intent of vector's reallocation strategy. This has produced not too old library implementations which release memory in the clear() function and even modern articles about C++ programming cultivate the belief that clear is allowed to do exactly this. A typical example is something like this:
const int buf_size = ...;
std::vector<T> buf(buf_size);
for (int i = 0; i < some_condition; ++i) {
buf.resize(buf_size);
write_or_read_data(buf.data());
buf.clear(); // Ensure that the next round get's 'zeroed' elements
}
where still the myth is ubiquitous that buf might be allowed to reallocate it's memory *inside* the for loop.
IMO the problem is due to the fact, that
the effects clause of std::vector's erase overloads in 23.3.6.4 [vector.modifiers]/4 is silent about capacity changes. This easily causes a misunderstanding, because the counter parting insert functions described in 23.3.6.4 [vector.modifiers]/2 explicitly say, that
Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid.
It requires a complex argumentation chain about four different places in the standard to provide the - possibly weak - proof that calling clear() also does never change the capacity of the std::vector container. Since std::vector is the de-facto replacement of C99's dynamic arrays this type is near to a built-in type and it's specification should be clear enough that usual programmers can trust their own reading.
[ Batavia (2009-05): ]
Bill believes paragraph 1 of the proposed resolution is unnecessary because it is already implied (even if tortuously) by the current wording.
Move to Review.
[ 2009-10 Santa Cruz: ]
Mark as NAD. Rationale: there is no consensus to clarify the standard, general consensus that the standard is correct as written.
Proposed resolution:
[ This is a minimum version. I also suggest that the wording explaining the allocation strategy of std::vector in 23.3.6.2 [vector.capacity]/3 and /6 is moved into a separate sub paragraph of 23.3.6.2 [vector.capacity] before any of the prototype's are discussed, but I cannot provide reasonable wording changes now ]
Change 23.3.6.2 [vector.capacity]/6 as follows:
It is guaranteed that no reallocation takes place during insertions or erasures that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the value of capacity().
Change 23.3.6.4 [vector.modifiers]/4 as follows:
Effects: The capacity shall remain unchanged and no reallocation shall happen. Invalidates iterators and references at or after the point of the erase.
Section: X [iterator.concepts.range] Status: NAD Concepts Submitter: David Abrahams Opened: 2009-04-23 Last modified: 2009-07-15
View all issues with NAD Concepts status.
Discussion:
[ 2009-04-26 Herb adds: ]
Here's a common example: We have many ISV customers who have built lots of in-house STL-like containers. Imagine that, for the past ten years, the user has been happily using his XYZCorpContainer<T> that has begin() and end() and an iterator typedef, and indeed satisfies nearly all of Container, though maybe not quite all just like valarray. The user upgrades to a range-enabled version of a library, and now lib_algo( xyz.begin(), xyz.end()); no longer works -- compiler error.
Even though XYZCorpContainer matches the pre-conceptized version of the algorithm, and has been working for years, it appears the user has to write at least this:
template<class T> concept_map Range<XYZCorpContainer<T>> {}; template<class T> concept_map Range<const XYZCorpContainer<T>> {};Is that correct?
But he may actually have to write this as we do for initializer list:
template<class T> concept_map Range<XYZCorpContainer<T>> { typedef T* iterator; iterator begin(XYZCorpContainer<T> c) { return c.begin(); } iterator end(XYZCorpContainer<T> c) { return c.end(); } }; template<class T> concept_map Range<const XYZCorpContainer<T>> { typedef T* iterator; iterator begin(XYZCorpContainer<T> c) { return c.begin(); } iterator end(XYZCorpContainer<T> c) { return c.end(); } };
[ 2009-04-28 Alisdair adds: ]
I recommend NAD, although remain concerned about header organisation.
A user container will satisfy the MemberContainer concept, which IS auto. There is a concept_map for all MemberContainers to Container, and then a further concept_map for all Container to Range, so the stated problem is not actually true. User defined containers will automatically match the Range concept without explicitly declaring a concept_map.
The problem is that they should now provide an additional two headers, <iterator_concepts> and <container_concepts>. The only difference from making Range an auto concept would be this reduces to a single header, <iterator_concepts>.
I am strongly in favour of any resolution that tackles the issue of explicitly requiring concept headers to make these concept maps available.
[ Batavia (2009-05): ]
We observe there is a recent paper by Bjarne that overlaps this issue.
Alisdair continues to recommend NAD.
Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.
Proposed resolution:
Section: 30.6.7 [futures.shared_future] Status: NAD Editorial Submitter: Thomas J. Gritzan Opened: 2009-04-03 Last modified: 2010-03-08
View all other issues in [futures.shared_future].
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Discussion:
It is not clear, if multiple threads are waiting in a shared_future::get() call, if each will rethrow the stored exception.
Paragraph 9 reads:
Throws: the stored exception, if an exception was stored and not retrieved before.
The "not retrieved before" suggests that only one exception is thrown, but one exception for each call to get() is needed, and multiple calls to get() even on the same shared_future object seem to be allowed.
I suggest removing "and not retrieved before" from the Throws paragraph. I recommend adding a note that explains that multiple calls on get() are allowed, and each call would result in an exception if an exception was stored.
[ Batavia (2009-05): ]
We note there is a pending paper by Detlef on such future-related issues; we would like to wait for his paper before proceeding.
Alisdair suggests we may want language to clarify that this get() function can be called from several threads with no need for explicit locking.
Move to Open.
[ 2010-01-23 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. ]
Rationale:
Resolved by paper N2997.
Proposed resolution:
Change 30.6.7 [futures.shared_future]:
const R& shared_future::get() const; R& shared_future<R&>::get() const; void shared_future<void>::get() const;...
-9- Throws: the stored exception, if an exception was stored
and not retrieved before. [Note: Multiple calls on get() are allowed, and each call would result in an exception if an exception was stored. — end note]
Section: 30.6.7 [futures.shared_future] Status: NAD Editorial Submitter: Thomas J. Gritzan Opened: 2009-04-03 Last modified: 2010-02-19
View all other issues in [futures.shared_future].
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Discussion:
In the shared_future class definition in 30.6.7 [futures.shared_future] the move constructor that constructs a shared_future from an unique_future receives the parameter by value. In paragraph 3, the same constructor receives it as const value.
I think that is a mistake and the constructor should take a r-value reference:
shared_future(unique_future<R>&& rhs);
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
[ 2009-07-05 Daniel notes: ]
The proposed change has already been incorported into the current working draft N2914.
Proposed resolution:
Change the synopsis in 30.6.7 [futures.shared_future]:
shared_future(unique_future<R>&& rhs);
Change the definition of the constructor in 30.6.7 [futures.shared_future]:
shared_future(constunique_future<R>&& rhs);
Section: 25.4.5.1 [includes] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-04-28 Last modified: 2009-07-13
View all other issues in [includes].
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Discussion:
All the set operation algorithms require a CopyConstructible predicate, with the exception of std::includes. This looks like a typo as much as anything, given the general library requirement that predicates are copy constructible, and wording style of other set-like operations.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to NAD Editorial.
Proposed resolution:
Change [algorithms.syn] and 25.4.5.1 [includes]:
template<InputIterator Iter1, InputIterator Iter2,
typename CopyConstructible Compare>
requires Predicate<Compare, Iter1::value_type, Iter2::value_type>
&& Predicate<Compare, Iter2::value_type, Iter1::value_type>
bool includes(Iter1 first1, Iter1 last1,
Iter2 first2, Iter2 last2,
Compare comp);
Section: 23.4 [associative] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-04-29 Last modified: 2009-07-16
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Discussion:
According to table 87 (n2857) the expression X::key_equal for an unordered container shall return a value of type Pred, where Pred is an equivalence relation.
However, all 4 containers constrain Pred to be merely a Predicate, and not EquivalenceRelation.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Review.
Proposed resolution:
For ordered containers, replace
Predicate<auto, Key, Key> Compare = less<Key>
with
StrictWeakOrder<auto, Key, Key> Compare = less<Key>
For unordered containers, replace
Predicate<auto, Key, Key> Compare = less<Key>
with
EquivalenceRelation<auto, Key, Key> Compare = less<Key>
As in the following declarations:
Associative containers 23.4 [associative]
1 Headers <map> and <set>:
Header <map> synopsis
namespace std { template <ValueType Key, ValueType T,PredicateStrictWeakOrder<auto, Key, Key> Compare = less<Key>, Allocator Alloc = allocator<pair<<b>const Key, T> > > requires NothrowDestructible<Key> && NothrowDestructible<T> && CopyConstructible<Compare> && AllocatableElement<Alloc, Compare, const Compare&> && AllocatableElement<Alloc, Compare, Compare&&> class map; ... template <ValueType Key, ValueType T,PredicateStrictWeakOrder<auto, Key, Key> Compare = less<Key>, Allocator Alloc = allocator<pair<<b>const Key, T> > > requires NothrowDestructible<Key> && NothrowDestructible<T> && CopyConstructible<Compare> && AllocatableElement<Alloc, Compare, const Compare&> && AllocatableElement<Alloc, Compare, Compare&&> class multimap; ... }Header <set> synopsis
namespace std { template <ValueType Key,PredicateStrictWeakOrder<auto, Key, Key> Compare = less<Key>, Allocator Alloc = allocator<Key> > requires NothrowDestructible<Key> && CopyConstructible<Compare> && AllocatableElement<Alloc, Compare, const Compare&> && AllocatableElement<Alloc, Compare, Compare&&> class set; ... template <ValueType Key,PredicateStrictWeakOrder<auto, Key, Key> Compare = less<Key>, Allocator Alloc = allocator<Key> > requires NothrowDestructible<Key> && CopyConstructible<Compare> && AllocatableElement<Alloc, Compare, const Compare&> && AllocatableElement<Alloc, Compare, Compare&&> class multiset; ... }23.4.1p2 Class template map [map]
namespace std { template <ValueType Key, ValueType T,PredicateStrictWeakOrder<auto, Key, Key> Compare = less<Key>, Allocator Alloc = allocator<pair<<b>const Key, T> > > requires NothrowDestructible<Key> && NothrowDestructible<T> && CopyConstructible<Compare> && AllocatableElement<Alloc, Compare, const Compare&> && AllocatableElement<Alloc, Compare, Compare&&> class map { ... }; }23.4.2p2 Class template multimap [multimap]
namespace std { template <ValueType Key, ValueType T,PredicateStrictWeakOrder<auto, Key, Key> Compare = less<Key>, Allocator Alloc = allocator<pair<<b>const Key, T> > > requires NothrowDestructible<Key> && NothrowDestructible<T> && CopyConstructible<Compare> && AllocatableElement<Alloc, Compare, const Compare&> && AllocatableElement<Alloc, Compare, Compare&&> class multimap { ... }; }23.4.3p2 Class template set [set]
namespace std { template <ValueType Key,PredicateStrictWeakOrder<auto, Key, Key> Compare = less<Key>, Allocator Alloc = allocator<Key> > requires NothrowDestructible<Key> && CopyConstructible<Compare> && AllocatableElement<Alloc, Compare, const Compare&> && AllocatableElement<Alloc, Compare, Compare&&> class set { ... }; }23.4.4p2 Class template multiset [multiset]
namespace std { template <ValueType Key,PredicateStrictWeakOrder<auto, Key, Key> Compare = less<Key>, Allocator Alloc = allocator<Key> > requires NothrowDestructible<Key> && CopyConstructible<Compare> && AllocatableElement<Alloc, Compare, const Compare&> && AllocatableElement<Alloc, Compare, Compare&&> class multiset { ... }; }23.5 Unordered associative containers [unord]
1 Headers <unordered_map> and <unordered_set>:
Header <unordered_map> synopsis
namespace std { // 23.5.1, class template unordered_map: template <ValueType Key, ValueType T, Callable<auto, const Key&> Hash = hash<Key>,PredicateEquivalenceRelation<auto, Key, Key> Pred = equal_to<Key>, Allocator Alloc = allocator<pair<<b>const Key, T> > > requires NothrowDestructible<Key> && NothrowDestructible<T> && SameType<Hash::result_type, size_t> && CopyConstructible<Hash> && CopyConstructible<Pred> && AllocatableElement<Alloc, Pred, const Pred&> && AllocatableElement<Alloc, Pred, Pred&&> && AllocatableElement<Alloc, Hash, const Hash&> && AllocatableElement<Alloc, Hash, Hash&&> class unordered_map; // 23.5.2, class template unordered_multimap: template <ValueType Key, ValueType T, Callable<auto, const Key&> Hash = hash<Key>,PredicateEquivalenceRelation<auto, Key, Key> Pred = equal_to<Key>, Allocator Alloc = allocator<pair<<b>const Key, T> > > requires NothrowDestructible<Key> && NothrowDestructible<T> && SameType<Hash::result_type, size_t> && CopyConstructible<Hash> && CopyConstructible<Pred> && AllocatableElement<Alloc, Pred, const Pred&> && AllocatableElement<Alloc, Pred, Pred&&> && AllocatableElement<Alloc, Hash, const Hash&> && AllocatableElement<Alloc, Hash, Hash&&> class unordered_multimap; ... }Header <unordered_set> synopsis
namespace std { // 23.5.3, class template unordered_set: template <ValueType Value, Callable<auto, const Value&> Hash = hash<Value>,PredicateEquivalenceRelation<auto, Value, Value> class Pred = equal_to<Value>, Allocator Alloc = allocator<Value> > requires NothrowDestructible<Value> && SameType<Hash::result_type, size_t> && CopyConstructible<Hash> && CopyConstructible<Pred> && AllocatableElement<Alloc, Pred, const Pred&> && AllocatableElement<Alloc, Pred, Pred&&> && AllocatableElement<Alloc, Hash, const Hash&> && AllocatableElement<Alloc, Hash, Hash&&> class unordered_set; // 23.5.4, class template unordered_multiset: template <ValueType Value, Callable<auto, const Value&> Hash = hash<Value>,PredicateEquivalenceRelation<auto, Value, Value> class Pred = equal_to<Value>, Allocator Alloc = allocator<Value> > requires NothrowDestructible<Value> && SameType<Hash::result_type, size_t> && CopyConstructible<Hash> && CopyConstructible<Pred> && AllocatableElement<Alloc, Pred, const Pred&> && AllocatableElement<Alloc, Pred, Pred&&> && AllocatableElement<Alloc, Hash, const Hash&> && AllocatableElement<Alloc, Hash, Hash&&> class unordered_multiset; ... }23.5.1p3 Class template unordered_map [unord.map]
namespace std { template <ValueType Key, ValueType T, Callable<auto, const Key&> Hash = hash<Key>,PredicateEquivalenceRelation<auto, Key, Key> Pred = equal_to<Key>, Allocator Alloc = allocator<pair<<b>const Key, T> > > requires NothrowDestructible<Key> && NothrowDestructible<T> && SameType<Hash::result_type, size_t> && CopyConstructible<Hash> && CopyConstructible<Pred> && AllocatableElement<Alloc, Pred, const Pred&> && AllocatableElement<Alloc, Pred, Pred&&> && AllocatableElement<Alloc, Hash, const Hash&> && AllocatableElement<Alloc, Hash, Hash&&> class unordered_map { ... }; }23.5.2p3 Class template unordered_multimap [unord.multimap]
namespace std { template <ValueType Key, ValueType T, Callable<auto, const Key&> Hash = hash<Key>,PredicateEquivalenceRelation<auto, Key, Key> Pred = equal_to<Key>, Allocator Alloc = allocator<pair<<b>const Key, T> > > requires NothrowDestructible<Key> && NothrowDestructible<T> && SameType<Hash::result_type, size_t> && CopyConstructible<Hash> && CopyConstructible<Pred> && AllocatableElement<Alloc, Pred, const Pred&> && AllocatableElement<Alloc, Pred, Pred&&> && AllocatableElement<Alloc, Hash, const Hash&> && AllocatableElement<Alloc, Hash, Hash&&> class unordered_multimap { ... }; }23.5.3p3 Class template unordered_set [unord.set]
namespace std { template <ValueType Value, Callable<auto, const Value&> Hash = hash<Value>,PredicateEquivalenceRelation<auto, Value, Value> class Pred = equal_to<Value>, Allocator Alloc = allocator<Value> > requires NothrowDestructible<Value> && SameType<Hash::result_type, size_t> && CopyConstructible<Hash> && CopyConstructible<Pred> && AllocatableElement<Alloc, Pred, const Pred&> && AllocatableElement<Alloc, Pred, Pred&&> && AllocatableElement<Alloc, Hash, const Hash&> && AllocatableElement<Alloc, Hash, Hash&&> class unordered_set { ... }; }23.5.4p3 Class template unordered_multiset [unord.multiset]
namespace std { template <ValueType Value, Callable<auto, const Value&> Hash = hash<Value>,PredicateEquivalenceRelation<auto, Value, Value> class Pred = equal_to<Value>, Allocator Alloc = allocator<Value> > requires NothrowDestructible<Value> && SameType<Hash::result_type, size_t> && CopyConstructible<Hash> && CopyConstructible<Pred> && AllocatableElement<Alloc, Pred, const Pred&> && AllocatableElement<Alloc, Pred, Pred&&> && AllocatableElement<Alloc, Hash, const Hash&> && AllocatableElement<Alloc, Hash, Hash&&> class unordered_multiset { ... }; }
Section: 20.5 [template.bitset] Status: NAD Future Submitter: Alisdair Meredith Opened: 2009-05-06 Last modified: 2010-03-08
View all other issues in [template.bitset].
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Discussion:
Std::bitset is a homogeneous container-like sequence of bits, yet it does not model the Range concept so cannot be used with the new for-loop syntax. It is the only such type in the library that does NOT support the new for loop.
The obvious reason is that bitset does not support iterators.
At least two reasonable solutions are available:
The latter will still need some kind of iterator-like adapter for bitset, but gives implementers greater freedom on the details. E.g. begin/end return some type that simply invokes operator[] on the object it wraps, and increments its index on operator++. A vendor can settle for InputIterator support, rather than wrapping up a full RandomAccessIterator.
I have a mild preference for option (ii) as I think it is less work to specify at this stage of the process, although (i) is probably more useful in the long run.
Hmm, my wording looks a little woolly, as it does not say what the element type of the range is. Do I get a range of bool, bitset<N>::reference, or something else entirely?
I guess most users will assume the behaviour of reference, but expect to work with bool. Bool is OK for read-only traversal, but you really need to take a reference to a bitset::reference if you want to write back.
[ Batavia (2009-05): ]
Move to Open. We further recommend this be deferred until after the next Committee Draft.
[ 2009-05-25 Alisdair adds: ]
I just stumbled over the Range concept_map for valarray and this should probably set the precedent on how to write the wording.
[ Howard: I've replaced the proposed wording with Alisdair's suggestion. ]
[ 2009-07-24 Daniel modifies the proposed wording for non-concepts. ]
[ 2009-10 post-Santa Cruz: ]
Mark as Tentatively NAD Future due to the loss of concepts.
Rationale:
All concepts-related text has been removed from the draft.
Proposed resolution:
Modify the section 20.5 [template.bitset] <bitset> synopsis by adding the following at the end of the synopsis:
// XX.X.X bitset range access [bitset.range] template<size_t N> unspecified-1 begin(bitset<N>&); template<size_t N> unspecified-2 begin(const bitset<N>&); template<size_t N> unspecified-1 end(bitset<N>&); template<size_t N> unspecified-2 end(const bitset<N>&);
Add a new section "bitset range access" [bitset.range] after the current section 20.5.4 [bitset.operators] with the following series of paragraphs:
1. In the begin and end function templates that follow, unspecified-1 is a type that meets the requirements of a mutable random access iterator (24.2.7 [random.access.iterators]) whose value_type is bool and whose reference type is bitset<N>::reference. unspecified-2 is a type that meets the requirements of a constant random access iterator (24.2.7 [random.access.iterators]) whose value_type is bool and whose reference type is bool.
template<size_t N> unspecified-1 begin(bitset<N>&); template<size_t N> unspecified-2 begin(const bitset<N>&);2. Returns: an iterator referencing the first bit in the bitset.template<size_t N> unspecified-1 end(bitset<N>&); template<size_t N> unspecified-2 end(const bitset<N>&);3. Returns: an iterator referencing one past the last bit in the bitset.
Section: C.2 [diff.library] Status: NAD Editorial Submitter: Miles Zhao Opened: 2009-05-23 Last modified: 2010-03-08
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Discussion:
In "Table 122 -- Standard macros" of C.2 [diff.library], which lists the 56 macros inherited from C library, va_copy seems to be missing. But in "Table 21 -- Header <cstdarg> synopsis" (18.10 [support.runtime]), there is.
[ 2009-10 post-Santa Cruz: ]
Mark as Tentatively NAD Editorial, if Pete disagrees, Howard will move to Tentatively Ready
Proposed resolution:
Add va_copy to Table 122 -- Standard macros in C.2 [diff.library].
Section: 20.4.2 [tuple.tuple] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2009-10-26
View all other issues in [tuple.tuple].
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Discussion:
It is not currently possible to construct tuple literal values, even if the elements are all literal types. This is because parameters are passed to constructor by reference.
An alternative would be to pass all constructor arguments by value, where it is known that *all* elements are literal types. This can be determined with concepts, although note that the negative constraint really requires factoring out a separate concept, as there is no way to provide an 'any of these fails' constraint inline.
Note that we will have similar issues with pair (and tuple constructors from pair) although I am steering clear of that class while other constructor-related issues settle.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2994.
Proposed resolution:
Ammend the tuple class template declaration in 20.4.2 [tuple.tuple] as follows
Add the following concept:
auto concept AllLiteral< typename ... Types > { requires LiteralType<Types>...; }ammend the constructor
template <class... UTypes> requires AllLiteral<Types...> && Constructible<Types, UTypes>... explicit tuple(UTypes...); template <class... UTypes> requires !AllLiteral<Types...> && Constructible<Types, UTypes&&>... explicit tuple(UTypes&&...);ammend the constructor
template <class... UTypes> requires AllLiteral<Types...> && Constructible<Types, UTypes>... tuple(tuple<UTypes...>); template <class... UTypes> requires !AllLiteral<Types...> && Constructible<Types, const UTypes&>... tuple(const tuple<UTypes...>&);
Update the same signatures in 20.4.2.1 [tuple.cnstr], paras 3 and 5.
Section: 20.4.2.1 [tuple.cnstr] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2009-10-26
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Discussion:
The copy constructor for the tuple template is constrained. This seems an unusual strategy, as the copy constructor will be implicitly deleted if the constraints are not met. This is exactly the same effect as requesting an =default; constructor. The advantage of the latter is that it retains triviality, and provides support for tuples as literal types if issue 1116 is also accepted.
Actually, it might be worth checking with core if a constrained copy constructor is treated as a constructor template, and as such does not suppress the implicit generation of the copy constructor which would hide the template in this case.
[ 2009-05-27 Daniel adds: ]
This would solve one half of the suggested changes in 801.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2994.
Proposed resolution:
Change 20.4.2 [tuple.tuple] and 20.4.2.1 [tuple.cnstr] p4:
requires CopyConstructible<Types>...tuple(const tuple&) = default;
Section: 20.4.2.5 [tuple.helper] Status: NAD Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2010-08-25
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Discussion:
The tuple query APIs tuple_size and tuple_element do not support references-to-tuples. This can be annoying when a template deduced a parameter type to be a reference, which must be explicitly stripped with remove_reference before calling these APIs.
I am not proposing a resolution at this point, as there is a combinatorial explosion with lvalue/rvalue references and cv-qualification (see previous issue) that suggests some higher refactoring is in order. This might be something to kick back over to Core/Evolution.
Note that we have the same problem in numeric_limits.
[ 2009-10 post-Santa Cruz: ]
Move to Open. Alisdair to provide wording.
[ 2010 Rapperswil: ]
Move to NAD. This is an extension after the FCD, without a clear motivation. May consider as NAD Future if motivating examples come forward.
Proposed resolution:
Section: 20.7 [meta] Status: NAD Future Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2009-10-26
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Discussion:
Sometimes it is necessary to remove all qualifiers from a type before passing on to a further API. A good example would be calling the tuple query APIs tuple_size or tuple_element with a deduced type inside a function template. If the deduced type is cv-qualified or a reference then the call will fail. The solution is to chain calls to remove_cv<remove_reference<T>::type>::type, and note that the order matters.
Suggest it would be helpful to add a new type trait, remove_all, that removes all top-level qualifiers from a type i.e. cv-qualification and any references. Define the term in such a way that if additional qualifiers are added to the language, then remove_all is defined as stripping those as well.
[ 2009-10-14 Daniel adds: ]
remove_all seems too generic, a possible alternative matching the current naming style could be remove_cv_reference or remove_reference_cv. It should also be considered whether this trait should also remove 'extents', or pointer 'decorations'. Especially if the latter situations are considered as well, it might be easier to chose the name not in terms of what it removes (which might be a lot), but in terms of it creates. In this case I could think of e.g. extract_value_type.
[ 2009-10 Santa Cruz: ]
NAD Future.
Proposed resolution:
Section: 20.6.2 [ratio.arithmetic] Status: NAD Future Submitter: Alisdair Meredith Opened: 2009-05-25 Last modified: 2010-03-08
View all other issues in [ratio.arithmetic].
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Discussion:
Both add and multiply could sensibly be called with more than two arguments. The variadic template facility makes such declarations simple, and is likely to be frequently wrapped by end users if we do not supply the variant ourselves.
We deliberately ignore divide at this point as it is not transitive. Likewise, subtract places special meaning on the first argument so I do not suggest extending that immediately. Both could be supported with analogous wording to that for add/multiply below.
Note that the proposed resolution is potentially incompatible with that proposed for 921, although the addition of the typedef to ratio would be equally useful.
[ 2009-10-30 Alisdair adds: ]
The consensus of the group when we reviewed this in Santa Cruz was that 921 would proceed to Ready as planned, and the multi-paramater add/multiply templates should be renamed as ratio_sum and ratio_product to avoid the problem mixing template aliases with partial specializations.
It was also suggested to close this issue as NAD Future as it does not correspond directly to any NB comment. NBs are free to submit a specific comment (and re-open) in CD2 though.
Walter Brown also had concerns on better directing the order of evaluation to avoid overflows if we do proceed for 0x rather than TR1, so wording may not be complete yet.
[ Alisdair updates wording. ]
[ 2009-10-30 Howard: ]
Moved to Tentatively NAD Future after 5 positive votes on c++std-lib.
Rationale:
Does not have sufficient support at this time. May wish to reconsider for a future standard.
Proposed resolution:
Add the following type traits to p3 20.6 [ratio]
// ratio arithmetic template <class R1, class R2> struct ratio_add; template <class R1, class R2> struct ratio_subtract; template <class R1, class R2> struct ratio_multiply; template <class R1, class R2> struct ratio_divide; template <class R1, class ... RList> struct ratio_sum; template <class R1, class ... RList> struct ratio_product;
after 20.6.2 [ratio.arithmetic] p1: add
template <class R1, class ... RList> struct ratio_sum; // declared, never defined template <class R1> struct ratio_sum<R1> : R1 {};Requires: R1 is a specialization of class template ratiotemplate <class R1, class R2, class ... RList> struct ratio_sum<R1, R2, RList...> : ratio_add< R1, ratio_sum<R2, RList...>> { };Requires: R1 and each element in parmater pack RList is a specialization of class template ratio
after 20.6.2 [ratio.arithmetic] p3: add
template <class R1, class ... RList> struct ratio_product; // declared, never defined template <class R1> struct ratio_product<R1> : R1 {};Requires: R1 is a specialization of class template ratiotemplate <class R1, class R2, class ... RList> struct ratio_sum<R1, R2, RList...> : ratio_add< R1, ratio_product<R2, RList...>> { };Requires: R1 and each element in parmater pack RList is a specialization of class template ratio
Section: 20.6.1 [ratio.ratio] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-05-25 Last modified: 2009-10-26
View all other issues in [ratio.ratio].
View all issues with NAD Editorial status.
Discussion:
The values num and den in the ratio template should be declared constexpr.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2994.
Proposed resolution:
20.6.1 [ratio.ratio]
namespace std {
template <intmax_t N, intmax_t D = 1>
class ratio {
public:
static constexpr intmax_t num;
static constexpr intmax_t den;
};
}
Section: X [concept.transform] Status: NAD Concepts Submitter: Daniel Krügler Opened: 2009-05-28 Last modified: 2009-07-15
View all other issues in [concept.transform].
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Discussion:
A recent news group article points to several defects in the specification of reference-related concepts.
One problem of the concept RvalueOf as currently defined in X [concept.transform]:
concept RvalueOf<typename T> {
typename type = T&&;
requires ExplicitlyConvertible<T&,type> && Convertible<T&&,type>;
}
template<typename T> concept_map RvalueOf<T&> {
typedef T&& type;
}
is that if T is an lvalue-reference, the requirement Convertible<T&&,type> isn't satisfied for lvalue-references, because after reference-collapsing in the concept definition we have Convertible<T&,type> in this case, which isn't satisfied in the concept map template and also is not the right constraint either. I think that the reporter is right that SameType requirements should do the job and that we also should use the new RvalueReference concept to specify a best matching type requirement.
Proposed resolution:
In X [concept.transform] before p. 4 change as indicated:
auto concept RvalueOf<typename T> {
typenameRvalueReference type = T&&;
requires ExplicitlyConvertible<T&, type> && Convertible<T&&, type>SameType<T&, type&>;
}
Section: 24.6.2.2 [ostream.iterator.ops] Status: NAD Submitter: Alisdair Meredith Opened: 2009-05-28 Last modified: 2010-03-08
View all issues with NAD status.
Discussion:
ostream_iterator has not been updated to support moveable types, in a similar manner to the insert iterators. Note that this is not a problem for ostreambuf_iterator, as the types it is restricted to dealing with do not support extra-efficient moving.
[ 2009-11-10 Howard adds: ]
Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below.
Proposed resolution:
Add second operator= overload to class template ostream_iterator in 24.6.2 [ostream.iterator], para 2:
ostream_iterator<T,charT,traits>& operator=(const T& value); ostream_iterator<T,charT,traits>& operator=(T&& value);
Add a new paragraph: in 24.6.2.2 [ostream.iterator.ops]:
ostream_iterator& operator=(T&& value);-2- Effects:
*out_stream << std::move(value); if(delim != 0) *out_stream << delim; return (*this);
Rationale:
Several objections to move forward with this issue were voiced in the thread starting with c++std-lib-25438. Among them is that we know of no motivating use case to make streaming rvalues behave differently than streaming const lvalues.
Section: 24.4.1 [iterator.traits] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-05-28 Last modified: 2009-07-15
View all other issues in [iterator.traits].
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Discussion:
The deprecated support for iterator_traits and legacy (unconstrained) iterators features the (exposition only) concept:
concept IsReference<typename T> { } // exposition only
template<typename T> concept_map IsReference<T&> { }
Now this looks exactly like the LvalueReference concept recently added to clause 20, so I wonder if we should use that instead? Then I consider the lack of rvalue-reference support, which means that move_iterator would always flag as merely supporting the input_iterator_tag category. This suggests we retain the exposition concept, but add a second concept_map to support rvalue references.
I would suggest adding the extra concept_map is the right way forward, but still wonder if the two exposition-only concepts in this clause might be worth promoting to clause 20. That question might better be answered with a fuller investigation of type_trait/concept unification though.
Proposed resolution:
In Iterator traits 24.4.1 [iterator.traits] para 4 add:
concept IsReference<typename T> { } // exposition only
template<typename T> concept_map IsReference<T&> { }
template<typename T> concept_map IsReference<T&&> { }
Section: X [iterator.syn] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-05-28 Last modified: 2009-07-16
View all issues with NAD Concepts status.
Discussion:
The <iterator> header synopsis declares a partial specialization of iterator_traits to support pointers, X [iterator.syn]. The implication is that specialization will be described in D10, yet it did not follow the rest of the deprecated material into this clause.
However, this is not as bad as it first seems! There are partial specializations of iterator_traits for types that satisfy the various Iterator concepts, and there are concept_maps for pointers to explicitly support the RandomAccessIterator concept, so the required template will be present - just not in the manner advertised.
I can see two obvious solutions:
I recommend option (ii) in the wording below
Option (ii) could be extended to strike all the declarations of deprecated material from the synopsis, as it is effectively duplicating D.10 anyway. This is the approach taken for deprecated library components in the 98/03 standards. This is probably a matter best left to the Editor though.
Proposed resolution:
In X [iterator.syn] strike:
template<class T> struct iterator_traits<T*>;
Section: 24.6.1.1 [istream.iterator.cons], 24.6.3 [istreambuf.iterator] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-05-30 Last modified: 2009-10-26
View all other issues in [istream.iterator.cons].
View all issues with NAD Editorial status.
Discussion:
istream_iterator and istreambuf_iterator should support literal sentinel values. The default constructor is frequently used to terminate ranges, and could easily be a literal value for istreambuf_iterator, and istream_iterator when iterating value types. A little more work using a suitably sized/aligned char-array for storage (or an updated component like boost::optional proposed for TR2) would allow istream_iterator to support constexpr default constructor in all cases, although we might leave this tweak as a QoI issue. Note that requiring constexpr be supported also allows us to place no-throw guarantees on this constructor too.
[ 2009-06-02 Daniel adds: ]
I agree with the usefulness of the issue suggestion, but we need to ensure that istream_iterator can satisfy be literal if needed. Currently this is not clear, because 24.6.1 [istream.iterator]/3 declares a copy constructor and a destructor and explains their semantic in 24.6.1.1 [istream.iterator.cons]/3+4.
The prototype semantic specification is ok (although it seems somewhat redundant to me, because the semantic doesn't say anything interesting in both cases), but for support of trivial class types we also need a trivial copy constructor and destructor as of 9 [class]/6. The current non-informative specification of these two special members suggests to remove their explicit declaration in the class and add explicit wording that says that if T is trivial a default constructed iterator is also literal, alternatively it would be possible to mark both as defaulted and add explicit (memberwise) wording that guarantees that they are trivial.
Btw.: I'm quite sure that the istreambuf_iterator additions to ensure triviality are not sufficient as suggested, because the library does not yet give general guarantees that a defaulted special member declaration makes this member also trivial. Note that e.g. the atomic types do give a general statement!
Finally there is a wording issue: There does not exist something like a "literal constructor". The core language uses the term "constexpr constructor" for this.
Suggestion:
Change 24.6.1 [istream.iterator]/3 as indicated:
constexpr istream_iterator(); istream_iterator(istream_type& s); istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default; ~istream_iterator() = default;Change 24.6.1.1 [istream.iterator.cons]/1 as indicated:
constexpr istream_iterator();-1- Effects: Constructs the end-of-stream iterator. If T is a literal type, then this constructor shall be a constexpr constructor.Change 24.6.1.1 [istream.iterator.cons]/3 as indicated:
istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default;-3- Effects: Constructs a copy of x. If T is a literal type, then this constructor shall be a trivial copy constructor.Change 24.6.1.1 [istream.iterator.cons]/4 as indicated:
~istream_iterator() = default;-4- Effects: The iterator is destroyed. If T is a literal type, then this destructor shall be a trivial destructor.Change 24.6.3 [istreambuf.iterator] before p. 1 as indicated:
constexpr istreambuf_iterator() throw(); istreambuf_iterator(const istreambuf_iterator&) throw() = default; ~istreambuf_iterator() throw() = default;Change 24.6.3 [istreambuf.iterator]/1 as indicated:
[..] The default constructor istreambuf_iterator() and the constructor istreambuf_iterator(0) both construct an end of stream iterator object suitable for use as an end-of-range. All specializations of istreambuf_iterator shall have a trivial copy constructor, a constexpr default constructor and a trivial destructor.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2994.
Proposed resolution:
24.6.1 [istream.iterator] para 3
constexpr istream_iterator();
24.6.1.1 [istream.iterator.cons]
constexpr istream_iterator();-1- Effects: Constructs the end-of-stream iterator. If T is a literal type, then this constructor shall be a literal constructor.
24.6.3 [istreambuf.iterator]
constexpr istreambuf_iterator() throw();
Section: 18.8.6 [except.nested] Status: NAD Submitter: Seiji Hayashida Opened: 2009-06-01 Last modified: 2009-10-23
View all other issues in [except.nested].
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Discussion:
Addresses JP 30
C++0x nested_exception cannot handle a structured exception well. The following codes show two types of tree structured exception handling.
The first one is based on nested_exception in C++0x, while the second one is based on my library trickerr.h (in Japanese). http://tricklib.com/cxx/dagger/trickerr.h
Assume that Function A() calls two sub functions A_a() and A_b(), both might throw tree structured exceptions, and A_b() must be called even if A_a() throws an exception.
List A (code of tree structured exception handling based on nested_exception in C++0x)
void A()
{
try
{
std::vector<exception_ptr> exception_list;
try
{
// A_a() does a similar processing as A().
A_a();
}
catch(...)
{
exception_list.push_back(current_exception());
}
// ***The processing A() has to do even when A_a() fails. ***
try
{
// A_b() does a similar processing as A().
A_b();
}
catch(...)
{
exception_list.push_back(current_exception());
}
if (!exception_list.empty())
{
throw exception_list;
}
}
catch(...)
{
throw_with_nested(A_exception("someone error"));
}
}
void print_tree_exception(exception_ptr e, const std::string & indent ="")
{
const char * indent_unit = " ";
const char * mark = "- ";
try
{
rethow_exception(e);
}
catch(const std::vector<exception_ptr> e)
{
for(std::vector<exception_ptr>::const_iterator i = e.begin(); i!=e.end(); ++i)
{
print_tree_exception(i, indent);
}
}
catch(const std::nested_exception e)
{
print_tree_exception(evil_i(e), indent +indent_unit);
}
catch(const std::exception e)
{
std::cout << indent << mark << e.what() << std::endl;
}
catch(...)
{
std::cout << indent << mark << "unknown exception" << std::endl;
}
}
int main(int, char * [])
{
try
{
A();
}
catch()
{
print_tree_exception(current_exception());
}
return EXIT_SUCCESS;
}
List B ( code of tree structured exception handling based on trickerr.h. ) "trickerr.h" (in Japanese), refer to: http://tricklib.com/cxx/dagger/trickerr.h.
void A()
{
tricklib::error_listener_type error_listener;
// A_a() is like A(). A_a() can throw tree structured exception.
A_a();
// *** It must do process so that A_a() throws exception in A(). ***
// A_b() is like A(). A_b() can throw tree structured exception.
A_b();
if (error_listener.has_error()) // You can write this "if block" in destructor
// of class derived from error_listener_type.
{
throw_error(new A_error("someone error",error_listener.listener_off().extract_pending_error()));
}
}
void print_tree_error(const tricklib::error_type &a_error, const std::string & indent = "")
{
const char * indent_unit = " ";
const char * mark = "- ";
tricklib::error_type error = a_error;
while(error)
{
std::cout << indent << mark << error->message << std::endl;
if (error->children)
{
print_tree_error(error->children, indent +indent_unit);
}
error = error->next;
}
}
int main(int, char * [])
{
tricklib::error_thread_power error_thread_power_on; // This object is necessary per thread.
try
{
A();
}
catch(error_type error)
{
print_tree_error(error);
}
catch(...)
{
std::cout << "- unknown exception" << std::endl;
}
return EXIT_SUCCESS;
}
Prospect
We will focus on the method A() since the other methods, also main(), occur only once respectively.
According to the above observation, we cannot help concluding that it is not so easy to use the nested_exception handling as a tree structured exception handling mechanism in a practical sense.
This text is based on the web page below (in Japanese). http://d.hatena.ne.jp/wraith13/20081231/1230715424
[ 2009-10 Santa Cruz: ]
Mark as NAD. The committee agrees that nested_exception is not a good match for this usage model. The committee did not see a way of improving this within the timeframe allowed.
Proposed resolution:
Section: 18.8.5 [propagation] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2007-06-06 Last modified: 2010-03-14
View all other issues in [propagation].
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Discussion:
As of N2857 18.8.5 [propagation]/5, the implementation-defined type exception_ptr does provide the following ways to check whether it is a null value:
void f(std::exception_ptr p) {
p == nullptr;
p == 0;
p == exception_ptr();
}
This is rather cumbersome way of checking for the null value and I suggest to require support for evaluation in a boolean context like so:
void g(std::exception_ptr p) {
if (p) {}
!p;
}
[ 2009 Santa Cruz: ]
Move to Ready. Note to editor: considering putting in a cross-reference to 4 [conv], paragraph 3, which defines the phrase "contextually converted to bool".
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
In section 18.8.5 [propagation] insert a new paragraph between p.5 and p.6:
An object e of type exception_ptr can be contextually converted to bool. The effect shall be as if e != exception_ptr() had been evaluated in place of e. There shall be no implicit conversion to arithmetic type, to enumeration type or to pointer type.
Section: 30 [thread] Status: NAD Concepts Submitter: LWG Opened: 2009-06-15 Last modified: 2009-07-15
View all other issues in [thread].
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Discussion:
Addresses US 93, JP 79, UK 333, JP 81
The thread chapter is not concept enabled.
Proposed resolution:
Section: 26 [numerics] Status: NAD Concepts Submitter: LWG Opened: 2009-06-15 Last modified: 2009-07-15
View all other issues in [numerics].
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Discussion:
Addresses US 84
The numerics chapter is not concept enabled.
The portion of this comment dealing with random numbers was resolved by N2836, which was accepted in Summit.
Proposed resolution:
Section: 27 [input.output] Status: NAD Concepts Submitter: LWG Opened: 2009-06-15 Last modified: 2009-07-15
View all other issues in [input.output].
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Discussion:
Addresses US 85, JP 67, JP 68, JP 69, JP 72, UK 308
The input/output chapter is not concept enabled.
Proposed resolution:
Section: 28 [re] Status: NAD Concepts Submitter: LWG Opened: 2009-06-15 Last modified: 2009-07-15
View all other issues in [re].
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Discussion:
Addresses US 86, UK 309, UK 310
The regular expressions chapter is not concept enabled.
Proposed resolution:
Section: 29 [atomics] Status: NAD Editorial Submitter: LWG Opened: 2009-06-15 Last modified: 2009-10-26
View all other issues in [atomics].
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Discussion:
Addresses US 87, UK 311
The atomics chapter is not concept enabled.
Needs to also consider issues 923 and 924.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Section: 29 [atomics] Status: NAD Editorial Submitter: LWG Opened: 2009-06-16 Last modified: 2009-10-26
View all other issues in [atomics].
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Discussion:
Addresses UK 312
The contents of the <stdatomic.h> header are not listed anywhere, and <cstdatomic> is listed as a C99 header in chapter 17. If we intend to use these for compatibility with a future C standard, we should not use them now.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Remove <cstdatomic> from the C99 headers in table 14. Add a new header <atomic> to the headers in table 13. Update chapter 29 to remove reference to <stdatomic.h> and replace the use of <cstdatomic> with <atomic>.
[ If and when WG14 adds atomic operations to C we can add corresponding headers to table 14 with a TR. ]
Section: 29.4 [atomics.lockfree] Status: NAD Editorial Submitter: Jeffrey Yasskin Opened: 2009-06-16 Last modified: 2009-10-26
View all issues with NAD Editorial status.
Discussion:
Addresses US 88
The "lockfree" facilities do not tell the programmer enough.
There are 2 problems here.
First, at least on x86,
it's less important to me whether some integral types are lock free
than what is the largest type I can pass to atomic and have it be lock-free.
For example, if long longs are not lock-free,
ATOMIC_INTEGRAL_LOCK_FREE is probably 1,
but I'd still be interested in knowing whether longs are always lock-free.
Or if long longs at any address are lock-free,
I'd expect ATOMIC_INTEGRAL_LOCK_FREE to be 2,
but I may actually care whether I have access to
the cmpxchg16b instruction.
None of the support here helps with that question.
(There are really 2 related questions here:
what alignment requirements are there for lock-free access;
and what processor is the program actually running on,
as opposed to what it was compiled for?)
Second, having atomic_is_lock_free only apply to individual objects is pretty useless (except, as Lawrence Crowl points out, for throwing an exception when an object is unexpectedly not lock-free). I'm likely to want to use its result to decide what algorithm to use, and that algorithm is probably going to allocate new memory containing atomic objects and then try to act on them. If I can't predict the lock-freedom of the new object by checking the lock-freedom of an existing object, I may discover after starting the algorithm that I can't continue.
[ 2009-06-16 Jeffrey Yasskin adds: ]
To solve the first problem, I think 2 macros would help: MAX_POSSIBLE_LOCK_FREE_SIZE and MAX_GUARANTEED_LOCK_FREE_SIZE, which expand to the maximum value of sizeof(T) for which atomic may (or will, respectively) use lock-free operations. Lawrence points out that this "relies heavily on implementations using word-size compare-swap on sub-word-size types, which in turn requires address modulation." He expects that to be the end state anyway, so it doesn't bother him much.
To solve the second, I think one could specify that equally aligned objects of the same type will return the same value from atomic_is_lock_free(). I don't know how to specify "equal alignment". Lawrence suggests an additional function, atomic_is_always_lock_free().
[ 2009-10-22 Benjamin Kosnik: ]
In the evolution discussion of N2925, "More Collected Issues with Atomics," there is an action item with respect to LWG 1146, US 88
This is stated in the paper as:
Relatedly, Mike Sperts will create an issue to propose adding a traits mechanism to check the compile-time properties through a template mechanism rather than macros
Here is my attempt to do this. I don't believe that a separate trait is necessary for this, and that instead atomic_integral::is_lock_free can be re-purposed with minimal work as follows.
[ Howard: Put Benjamin's wording in the proposed wording section. ]
[ 2009-10-22 Alberto Ganesh Barbati: ]
Just a thought... wouldn't it be better to use a scoped enum instead of plain integers? For example:
enum class is_lock_free { never = 0, sometimes = 1, always = 2; };if compatibility with C is deemed important, we could use an unscoped enum with suitably chosen names. It would still be more descriptive than 0, 1 and 2.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Header <cstdatomic> synopsis [atomics.synopsis]
Edit as follows:
namespace std {
...
// 29.4, lock-free property
#define ATOMIC_INTEGRAL_LOCK_FREE unspecified
#define ATOMIC_CHAR_LOCK_FREE unspecified
#define ATOMIC_CHAR16_T_LOCK_FREE unspecified
#define ATOMIC_CHAR32_T_LOCK_FREE unspecified
#define ATOMIC_WCHAR_T_LOCK_FREE unspecified
#define ATOMIC_SHORT_LOCK_FREE unspecified
#define ATOMIC_INT_LOCK_FREE unspecified
#define ATOMIC_LONG_LOCK_FREE unspecified
#define ATOMIC_LLONG_LOCK_FREE unspecified
#define ATOMIC_ADDRESS_LOCK_FREE unspecified
Lock-free Property 29.4 [atomics.lockfree]
Edit the synopsis as follows.
namespace std {
#define ATOMIC_INTEGRAL_LOCK_FREE unspecified
#define ATOMIC_CHAR_LOCK_FREE unspecified
#define ATOMIC_CHAR16_T_LOCK_FREE unspecified
#define ATOMIC_CHAR32_T_LOCK_FREE unspecified
#define ATOMIC_WCHAR_T_LOCK_FREE unspecified
#define ATOMIC_SHORT_LOCK_FREE unspecified
#define ATOMIC_INT_LOCK_FREE unspecified
#define ATOMIC_LONG_LOCK_FREE unspecified
#define ATOMIC_LLONG_LOCK_FREE unspecified
#define ATOMIC_ADDRESS_LOCK_FREE unspecified
}
Edit paragraph 1 as follows.
The ATOMIC_...._LOCK_FREE macrosATOMIC_INTEGRAL_LOCK_FREE and ATOMIC_ADDRESS_LOCK_FREEindicate the general lock-free property ofintegral and address atomicthe corresponding atomic integral types, with the signed and unsigned variants grouped together.The properties also apply to the corresponding specializations of the atomic template.A value of 0 indicates that the types are never lock-free. A value of 1 indicates that the types are sometimes lock-free. A value of 2 indicates that the types are always lock-free.
Operations on Atomic Types 29.6 [atomics.types.operations]
Edit as follows.
voidstatic constexpr bool A::is_lock_free() const volatile;Returns: True if theobject'stypes's operations are lock-free, false otherwise. [Note: In the same way that <limits> std::numeric_limits<short>::max() is related to <limits.h> __LONG_LONG_MAX__, <atomic> std::atomic_short::is_lock_free is related to <stdatomic.h> and ATOMIC_SHORT_LOCK_FREE — end note]
Section: 29.6 [atomics.types.operations] Status: NAD Editorial Submitter: Jeffrey Yasskin Opened: 2009-06-16 Last modified: 2009-10-26
View all other issues in [atomics.types.operations].
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Discussion:
Addresses US 90
The C++0X draft declares all of the functions dealing with atomics (section 29.6 [atomics.types.operations]) to take volatile arguments. Yet it also says (29.4-3),
[ Note: Many operations are volatile-qualified. The "volatile as device register" semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects. It does not mean that operations on non-volatile objects become volatile. Thus, volatile qualified operations on non-volatile objects may be merged under some conditions. —end note ]
I was thinking about how to implement this in gcc, and I believe that we'll want to overload most of the functions on volatile and non-volatile. Here's why:
To let the compiler take advantage of the permission to merge non-volatile atomic operations and reorder atomics in certain, we'll need to tell the compiler backend about exactly which atomic operation was used. So I expect most of the functions of the form atomic_<op>_explicit() (e.g. atomic_load_explicit, atomic_exchange_explicit, atomic_fetch_add_explicit, etc.) to become compiler builtins. A builtin can tell whether its argument was volatile or not, so those functions don't really need extra explicit overloads. However, I don't expect that we'll want to add builtins for every function in chapter 29, since most can be implemented in terms of the _explicit free functions:
class atomic_int {
__atomic_int_storage value;
public:
int fetch_add(int increment, memory_order order = memory_order_seq_cst) volatile {
// &value has type "volatile __atomic_int_storage*".
atomic_fetch_add_explicit(&value, increment, order);
}
...
};
But now this always calls the volatile builtin version of atomic_fetch_add_explicit(), even if the atomic_int wasn't declared volatile. To preserve volatility and the compiler's permission to optimize, I'd need to write:
class atomic_int {
__atomic_int_storage value;
public:
int fetch_add(int increment, memory_order order = memory_order_seq_cst) volatile {
atomic_fetch_add_explicit(&value, increment, order);
}
int fetch_add(int increment, memory_order order = memory_order_seq_cst) {
atomic_fetch_add_explicit(&value, increment, order);
}
...
};
But this is visibly different from the declarations in the standard because it's now overloaded. (Consider passing &atomic_int::fetch_add as a template parameter.)
The implementation may already have permission to add overloads to the member functions:
17.6.4.5 [member.functions] An implementation may declare additional non-virtual member function signatures within a class:
...
- by adding a member function signature for a member function name.
but I don't see an equivalent permission to add overloads to the free functions.
[ 2009-06-16 Lawrence adds: ]
I recommend allowing non-volatile overloads.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2992.
Proposed resolution:
Section: 27.7 [iostream.format] Status: NAD Submitter: Marc Steinbach Opened: 2009-06-20 Last modified: 2009-10-20
View all other issues in [iostream.format].
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Discussion:
The header <iomanip> synopsis in 27.7 [iostream.format] specifies
T5 setprecision(int n); T6 setw(int n);
The argument types should be streamsize, as in class ios_base (see 27.5.2 [ios.base]):
streamsize precision() const; streamsize precision(streamsize prec); streamsize width() const; streamsize width(streamsize wide);
(Editorial: 'wide' should probably be renamed as 'width', or maybe just 'w'.)
[ 2009-07-29 Daniel clarified wording. ]
[ 2009 Santa Cruz: ]
No concensus for this change. There was some interest in doing the opposite fix: Change the streamsize in <ios> to int. But ultimately there was no concensus for that change either.
Proposed resolution:
In 27.7 [iostream.format], header <iomanip> synopsis change as indicated:
T5 setprecision(intstreamsize n); T6 setw(intstreamsize n);
In 27.7.3 [std.manip], just before p. 6 change as indicated:
unspecified setprecision(intstreamsize n);
In 27.7.3 [std.manip], just before p. 7 change as indicated:
unspecified setw(intstreamsize n);
Section: X [rand.concept.urng] Status: NAD Concepts Submitter: Walter Brown Opened: 2009-06-25 Last modified: 2009-07-15
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Discussion:
In X [rand.concept.urng], we have the following:
concept UniformRandomNumberGenerator<typename G> : Callable<G> {
...
axiom NonemptyRange(G& g) {
G::min() < G::max();
}
...
}
Since the parameter G is in scope throughout the concept, there is no need for the axiom to be further parameterized, and so the axiom can be slightly simplified as:
axiom NonemptyRange() {
G::min() < G::max();
}
We can further reformulate so as to avoid any axiom machinery as:
requires True< G::min() < G::max() >;
This is not only a simpler statement of the same requirement, but also forces the requirement to be checked.
Proposed resolution:
In X [rand.concept.urng], replace the NonemptyRange axiom by:
axiom NonemptyRange(G& g) { G::min() < G::max(); }requires True< G::min() < G::max() >;
Section: 27.9.1.14 [fstream] Status: NAD Future Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-20
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Discussion:
Addresses JP 73
Description
It is a problem from C++98, fstream cannot appoint a filename of wide character string(const wchar_t and const wstring&).
Suggestion
Add interface corresponding to wchar_t, char16_t and char32_t.
[ 2009-07-01 Alisdair notes that this is a duplicate of 454 which has more in-depth rationale. ]
[ 2009-09-21 Daniel adds: ]
I suggest to mark this issue as NAD Future with the intend to solve the issue with a single file path c'tor template assuming a provision of a TR2 filesystem library.
[ 2009 Santa Cruz: ]
NAD Future. This is a duplicate of 454.
Proposed resolution:
Section: 17 [library] Status: NAD Editorial Submitter: LWG Opened: 2009-06-28 Last modified: 2010-03-14
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Discussion:
Addresses US 63
Description
The behavior of the library in the presence of threads is incompletely specified.
For example, if thread 1 assigns to X, then writes data to file f, which is read by thread 2, and then accesses variable X, is thread 2 guaranteed to be able to see the value assigned to X by thread 1? In other words, does the write of the data "happen before" the read?
Another example: does simultaneous access using operator at() to different characters in the same non-const string really introduce a data race?
Suggestion
Notes
17 SG: should go to threads group; misclassified in document
Concurrency SG: Create an issue. Hans will look into it.
[ 2009 Santa Cruz: ]
Move to "Open". Hans and the rest of the concurrency working group will study this. We can't make progress without a thorough review and a paper.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3069.
Proposed resolution:
Section: 17 [library], 30 [thread], D [depr] Status: NAD Submitter: LWG Opened: 2009-06-28 Last modified: 2010-03-15
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Discussion:
Addresses DE 2
Description
Marking a constructor with explicit has semantics even for a constructor with zero or several parameters: Such a constructor cannot be used with list-initialization in a copy-initialization context, see 13.3.1.7 [over.match.list]. The standard library apparently has not been reviewed for marking non-single-parameter constructors as explicit.
Suggestion
Consider marking zero-parameter and multi-parameter constructors explicit in classes that have at least one constructor marked explicit and that do not have an initializer-list constructor.
Notes
Robert Klarer to address this one.
[ 2009 Santa Cruz: ]
Move to "Open". Robert Klarer has promised to provide wording.
[ 2010 Pittsburgh: Moved to NAD, rationale added below. ]
Rationale:
We are unaware of any cases where initializer lists cause problem in this context, but if problems arise in the future the issue can be reopened.
Proposed resolution:
Section: 26.4 [complex.numbers] Status: NAD Future Submitter: LWG Opened: 2009-06-28 Last modified: 2010-03-15
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Discussion:
Addresses FR 35
Description
Instantiations of the class template complex<> have to be allowed for integral types, to reflect existing practice and ISO standards (LIA-III).
Suggestion
[ 2009-10-26 Proposed wording in N3002. ]
[ 2010 Pittsburgh: ]
Moved to NAD Future. Rationale added.
Rationale:
There is no consensus for making this change at this time.
Proposed resolution:
Adopt N3002.Section: C.2 [diff.library] Status: NAD Editorial Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-23
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Discussion:
Addresses FR 38
Description
What is ISO/IEC 1990:9899/DAM 1? My guess is that's a typo for ISO/IEC 9899/Amd.1:1995 which I'd have expected to be referenced here (the tables make reference to things which were introduced by Amd.1).
Suggestion
One need probably a reference to the document which introduce char16_t and char32_t in C (ISO/IEC TR 19769:2004?).
Notes
Create issue. Document in question should be C99, not C90+amendment1. The rest of the section requires careful review for completeness. Example <cstdint> 18.4.1 [cstdint.syn]. Assign to C liasons.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Already fixed.
Proposed resolution:
Section: 17.5.2.1.2 [enumerated.types], 17.5.2.1.3 [bitmask.types] Status: NAD Submitter: LWG Opened: 2009-06-28 Last modified: 2010-03-09
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Discussion:
Addresses UK 165
Description
Constraints on bitmask and enumeration types were supposed to be tightened up as part of the motivation for the constexpr feature - see paper N2235 for details
Suggestion
Adopt wording in line with the motivation described in N2235
Notes
Robert Klarer to review
[ 2009 Santa Cruz: ]
Move to Open. Ping Robert Klarer to provide wording, using N2235 as guidance.
[ 2010 Pittsburgh: ]
Moved to NAD. Rationale added.
Rationale:
UK NB did not sufficiently describe how to resolve their comment, and therefore we cannot make a change for the FCD. If a resolution were provided in the future, we would be happy to apply it.
Proposed resolution:
Section: 30.6.3 [futures.future_error] Status: NAD Editorial Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-26
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Discussion:
Addresses UK 331
Description
Not clear what it means for a public constructor to be 'exposition only'. If the intent is purely to support the library calling this constructor then it can be made private and accessed through friendship. Otherwise it should be documented for public consumption.
Suggestion
Declare the constructor as private with a note about intended friendship, or remove the exposition-only comment and document the semantics.
Notes
Create an issue. Assigned to Detlef. Suggested resolution probably makes sense.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detleff Volleman.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2997.
Proposed resolution:
Section: 30.6.6 [futures.unique_future] Status: NAD Editorial Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-26
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Discussion:
Addresses UK 336
Description
It is possible to transfer ownership of the asynchronous result from one unique_future instance to another via the move-constructor. However, it is not possible to transfer it back, and nor is it possible to create a default-constructed unique_future instance to use as a later move target. This unduly limits the use of unique_future in code. Also, the lack of a move-assignment operator restricts the use of unique_future in containers such as std::vector - vector::insert requires move-assignable for example.
Suggestion
Add a default constructor with the semantics that it creates a unique_future with no associated asynchronous result. Add a move-assignment operator which transfers ownership.
Notes
Create an issue. Detlef will look into it.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detleff Volleman.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2997.
Proposed resolution:
Section: 30.6.7 [futures.shared_future] Status: NAD Editorial Submitter: LWG Opened: 2009-06-28 Last modified: 2010-02-19
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Discussion:
Addresses UK 337
Description
shared_future should support an efficient move constructor that can avoid unnecessary manipulation of a reference count, much like shared_ptr
Suggestion
Add a move constructor
Notes
Create an issue. Detlef will look into it.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detleff Volleman.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2997.
Proposed resolution:
Section: 30.6.7 [futures.shared_future] Status: NAD Editorial Submitter: LWG Opened: 2009-06-28 Last modified: 2010-02-19
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Discussion:
Addresses UK 338
Description
shared_future is currently CopyConstructible, but not CopyAssignable. This is inconsistent with shared_ptr, and will surprise users. Users will then write work-arounds to provide this behaviour. We should provide it simply and efficiently as part of shared_future. Note that since the shared_future member functions for accessing the state are all declared const, the original usage of an immutable shared_future value that can be freely copied by multiple threads can be retained by declaring such an instance as "const shared_future".
Suggestion
Remove "=delete" from the copy-assignment operator of shared_future. Add a move-constructor shared_future(shared_future&& rhs), and a move-assignment operator shared_future& operator=(shared_future&& rhs). The postcondition for the copy-assignment operator is that *this has the same associated state as rhs. The postcondition for the move-constructor and move assignment is that *this has the same associated as rhs had before the constructor/assignment call and that rhs has no associated state.
Notes
Create an issue. Detlef will look into it.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detleff Volleman.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2997.
Proposed resolution:
Section: 30.6.5 [futures.promise] Status: NAD Submitter: LWG Opened: 2009-06-28 Last modified: 2009-07-17
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Discussion:
Addresses UK 341
Description
promise::swap accepts its parameter by lvalue reference. This is inconsistent with other types that provide a swap member function, where those swap functions accept an rvalue reference
Suggestion
Change promise::swap to take an rvalue reference.
Notes
Create an issue. Detlef will look into it. Probably ready as it.
[ 2009-07 Frankfurt ]
NAD, by virtue of the changed rvalue rules and swap signatures from Summit.
Proposed resolution:
Section: 30.6.5 [futures.promise] Status: NAD Editorial Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-26
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Discussion:
Addresses UK 343
Description
The move constructor of a std::promise object does not need to allocate any memory, so the move-construct-with-allocator overload of the constructor is superfluous.
Suggestion
Remove the constructor with the signature template <class Allocator> promise(allocator_arg_t, const Allocator& a, promise& rhs);
Notes
Create an issue. Detlef will look into it. Will solicit feedback from Pablo. Note that “rhs” argument should also be an rvalue reference in any case.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detleff Volleman.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2997.
Proposed resolution:
Section: X [allocator.propagation], X [allocator.propagation.map], 23 [containers] Status: NAD Editorial Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-26
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Discussion:
Addresses US 77
Description
Allocator-specific move and copy behavior for containers (N2525) complicates a little-used and already-complicated portion of the standard library (allocators), and breaks the conceptual model of move-constructor and move-assignment operations on standard containers being efficient operations. The extensions for allocator-specific move and copy behavior should be removed from the working paper.
With the introduction of rvalue references, we are teaching programmers that moving from a standard container (e.g., a vector<string>) is an efficient, constant-time operation. The introduction of N2525 removed that guarantee; depending on the behavior of four different traits (20.8.4), the complexity of copy and move operations can be constant or linear time. This level of customization greatly increases the complexity of standard containers, and benefits only a tiny fraction of the C++ community.
Suggestion
Remove 20.8.4.
Remove 20.8.5.
Remove all references to the facilities in 20.8.4 and 20.8.5 from clause 23.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Addressed by N2982.
Proposed resolution:
Section: 20.3.5 [pairs] Status: NAD Concepts Submitter: Dave Abrahams Opened: 2009-07-01 Last modified: 2009-07-16
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Discussion:
LessThanComparable requires (and provides default implementations for) <=,>, and >=. However, the defaults don't take effect in unconstrained code.
Still, it's a problem to have types acting one way in constrained code and another in unconstrained code, except in cases of syntax adaptation. It's also inconsistent with the containers, which supply all those operators.
Totally Unbiased Suggested Resolution:
accept the exported concept maps proposal and change the way this stuff is handled to use an explicit exported concept map rather than nested function templates
e.g., remove from the body of std::list
template <LessThanComparable T, class Allocator> bool operator< (const list<T,Allocator>& x, const list<T,Allocator>& y); template <LessThanComparable T, class Allocator> bool operator> (const list<T,Allocator>& x, const list<T,Allocator>& y); template <LessThanComparable T, class Allocator> bool operator>=(const list<T,Allocator>& x, const list<T,Allocator>& y); template <LessThanComparable T, class Allocator> bool operator<=(const list<T,Allocator>& x, const list<T,Allocator>& y);
and add this concept_map afterwards:
template <LessThanComparable T, class Allocator>
export concept_map LessThanComparable<list<T,Allocator> >
{
bool operator<(const list<T,Allocator>& x, const list<T,Allocator>& y);
}
do similarly for std::pair. While you're at it, do the same for operator== and != everywhere, and seek out other such opportunities.
Alternative Resolution: keep the ugly, complex specification and add the missing operators to std::pair.
Proposed resolution:
Section: 20.5.2 [bitset.members] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-07-02 Last modified: 2009-07-27
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Discussion:
The following wording seems a little unusual to me:
p42/43 20.5.2 [bitset.members]
bool operator==(const bitset<N>& rhs) const;-42- Returns: A nonzero value if the value of each bit in *this equals the value of the corresponding bit in rhs.bool operator!=(const bitset<N>& rhs) const;-43- Returns: A nonzero value if !(*this == rhs).
"A nonzero value" may be well defined as equivalent to the literal 'true' for Booleans, but the wording is clumsy. I suggest replacing "A nonzero value" with the literal 'true' (in appropriate font) in each case.
[ 2009-07-24 Alisdair recommends NAD Editorial. ]
[ 2009-07-27 Pete adds: ]
It's obviously editorial. There's no need for further discussion.
[ 2009-07-27 Howard sets to NAD Editorial. ]
Proposed resolution:
Change 20.5.2 [bitset.members] p42-43:
bool operator==(const bitset<N>& rhs) const;-42- Returns:A nonzero valuetrue if the value of each bit in *this equals the value of the corresponding bit in rhs.bool operator!=(const bitset<N>& rhs) const;-43- Returns:A nonzero valuetrue if !(*this == rhs).
Section: X [allocator.concepts.members] Status: NAD Editorial Submitter: Alberto Ganesh Barbati Opened: 2009-07-08 Last modified: 2009-10-26
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Discussion:
I believe the two functions select_on_container_(copy|move)_construction() are over-constrained. For example, the return value of the "copy" version is (see X [allocator.concepts.members]/21):
Returns: x if the allocator should propagate from the existing container to the new container on copy construction, otherwise X().
Consider the case where a user decides to provide an explicit concept map for Allocator to adapt some legacy allocator class, as he wishes to provide customizations that the LegacyAllocator concept map template does not provide. Now, although it's true that the legacy class is required to have a default constructor, the user might have reasons to prefer a different constructor to implement select_on_container_copy_construction(). However, the current wording requires the use of the default constructor.
Moreover, it's not said explicitly that x is supposed to be the allocator of the existing container. A clarification would do no harm.
[ 2009-10 Santa Cruz: ]
NAD Editorial. Addressed by N2982.
Proposed resolution:
Replace X [allocator.concepts.members]/21 with:
X select_on_container_copy_construction(const X& x);-21- Returns:
x if the allocator should propagate from the existing container to the new container on copy construction, otherwise X().an allocator object to be used by the new container on copy construction. [Note: x is the allocator of the existing container that is being copied. The most obvious choices for the return value are x, if the allocator should propagate from the existing container, and X(). — end note]
Replace X [allocator.concepts.members]/25 with:
X select_on_container_move_construction(X&& x);-25- Returns:
move(x) if the allocator should propagate from the existing container to the new container on move construction, otherwise X().an allocator object to be used by the new container on move construction. [Note: x is the allocator of the existing container that is being moved. The most obvious choices for the return value are move(x), if the allocator should propagate from the existing container, and X(). — end note]
Section: 20.7.4.3 [meta.unary.prop] Status: NAD Editorial Submitter: Jason Merrill Opened: 2009-07-16 Last modified: 2009-10-26
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Discussion:
I've been implementing compiler support for is_standard_layout, and noticed a few nits about 20.7.4.3 [meta.unary.prop]:
[ See the thread starting with c++std-lib-24420 for further discussion. ]
[ Addressed in N2947. ]
[ 2009-10 Santa Cruz: ]
NAD Editorial. Solved by N2984.
Proposed resolution:
Section: 30.3.1.2 [thread.thread.constr] Status: NAD Submitter: Howard Hinnant Opened: 2009-07-18 Last modified: 2010-03-08
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Discussion:
The variadic thread constructor is causing controversy, e.g. N2901. This issue has been created as a placeholder for this course of action.
template <class F, class ...Args> thread(F&& f, Args&&... args);
See 929 for wording which specifies an rvalue-ref signature but with "decay behavior", but using variadics.
[ 2009-11-17 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]
Proposed resolution:
Rationale:
The (tentative) concensus of the LWG is to keep the variadic thread constructor.
Section: 17.5.1.4 [structure.specifications] Status: NAD Editorial Submitter: Robert Klarer Opened: 2009-07-21 Last modified: 2009-10-20
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Discussion:
While reviewing 971 I noted that 17.5.1.4 [structure.specifications]/7 says:
-7- Error conditions specify conditions where a function may fail. The conditions are listed, together with a suitable explanation, as the enum class errc constants (19.5) that could be used as an argument to function make_error_condition (19.5.3.6).
This paragraph should mention make_error_code or the text "that could be used as an argument to function make_error_condition (19.5.3.6)" should be deleted. I believe this is editorial.
[ 2009-07-21 Chris adds: ]
I'm not convinced there's a problem there, because as far as the "Error conditions" clauses are concerned, make_error_condition() is used by a user to test for the condition, whereas make_error_code is not. For example:
void foobar(error_code& ec = throws());Error conditions:
permission_denied - Insufficient privilege to perform operation.When a user writes:
error_code ec; foobar(ec); if (ec == errc::permission_denied) ...the implicit conversion errc->error_condition makes the if-test equivalent to:
if (ec == make_error_condition(errc::permission_denied))On the other hand, if the user had written:
if (ec == make_error_code(errc::permission_denied))the test is now checking for a specific error code. The test may evaluate to false even though foobar() failed due to the documented error condition "Insufficient privilege".
[ 2009 Santa Cruz: ]
NAD Editorial.
What the WP says right now is literally true: these codes can be used as an argument to make_error_condition. (It is also true that they can be used as an argument to make_error_code, which the WP doesn't say.) Maybe it would be clearer to just delete "that could be used as an argument to function make_error_condition", since that fact is already implied by other things that we say. We believe that this is editorial.
Proposed resolution:
Section: 23.3.6 [vector] Status: NAD Future Submitter: Alisdair Meredith Opened: 2009-07-29 Last modified: 2009-10-26
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Discussion:
Opened at Alisdair's request, steming from 96. Alisdair recommends NAD Future.
[ 2009-10 Santa Cruz: ]
NAD Future. We want a heap allocated bitset, but we don't have one today and don't have time to add one.
Proposed resolution:
Section: 24.2 [iterator.requirements] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-07-31 Last modified: 2010-03-14
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Discussion:
(wording relative to N2723 pending new working paper)
According to p3 24.2 [iterator.requirements], Forward iterators, Bidirectional iterators and Random Access iterators all satisfy the requirements for an Output iterator:
XXX iterators satisfy all the requirements of the input and output iterators and can be used whenever either kind is specified ...
Meanwhile, p4 goes on to contradict this:
Besides its category, a forward, bidirectional, or random access iterator can also be mutable or constant...
... Constant iterators do not satisfy the requirements for output iterators
The latter seems to be the overriding concern, as the iterator tag hierarchy does not define forward_iterator_tag as multiply derived from both input_iterator_tag and output_iterator_tag.
The work on concepts for iterators showed us that output iterator really is fundamentally a second dimension to the iterator categories, rather than part of the linear input -> forward -> bidirectional -> random-access sequence. It would be good to clear up these words to reflect that, and separately list output iterator requirements in the requires clauses for the appropriate algorithms and operations.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3066.
Proposed resolution:
Section: 23.3.5.3 [stack] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2009-07-31 Last modified: 2010-03-08
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Discussion:
The library template forward_list could easily model the idea of a stack, where the operations work on the front of the list rather than the back. However, the standard library stack adaptor cannot support this.
It would be relatively easy to write a partial specialization for stack to support forward_list, but that opens the question of which header to place it in. A much better solution would be to add a concept_map for the StackLikeContainer concept to the <forward_list> header and then everything just works, including a user's own further uses in a stack-like context.
Therefore while I am submitting the issue now so that it is on record, I strongly recommend we resolve as "NAD Concepts" as any non-concepts based solution will be inferior to the final goal, and the feature is not so compelling it must be supported ahead of the concepts-based library.
[ 2009-11-02 Howard adds: ]
Moved to Tentatively NAD Concepts after 5 positive votes on c++std-lib.
Rationale:
Any non-concepts based solution will be inferior to the final goal, and the feature is not so compelling it must be supported ahead of the concepts-based library.
Proposed resolution:
Section: 23.3.5.2.1 [priqueue.cons] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-08-19 Last modified: 2009-10-20
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Discussion:
The class template priority_queue declares signatures for a move constructor and move assignment operator in its class definition. However, it does not provide a definition (unlike std::queue, and proposed resolution for std::stack.) Nor does it provide a text clause specifying their behaviour.
[ 2009-08-23 Daniel adds: ]
1194 provides wording that solves this issue.
[ 2009-10 Santa Cruz: ]
Mark NAD Editorial, solved by issue 1194.
Proposed resolution:
Section: 20.4.2.4 [tuple.creation], 20.3.5 [pairs] Status: NAD Future Submitter: Alisdair Meredith Opened: 2009-09-05 Last modified: 2010-03-08
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Discussion:
Spotting a recent thread on the boost lists regarding collapsing optional representations in optional<optional<T>> instances, I wonder if we have some of the same issues with make_tuple, and now make_pair?
Essentially, if my generic code in my own library is handed a reference_wrapper by a user, and my library in turn delegates some logic to make_pair or make_tuple, then I am going to end up with a pair/tuple holding a real reference rather than the intended reference wrapper.
There are two things as a library author I can do at this point:
(There may be some metaprogramming approaches my library can use to wrap the make_tuple call, but all will be significantly more complex than simply implementing a simplified make_tuple.)
Now I don't propose we lose this library facility, I think unwrapping references will be the common behaviour. However, we might want to consider adding another overload that does nothing special with ref-wrappers. Note that we already have a second overload of make_tuple in the library, called tie.
[ 2009-09-30 Daniel adds: ]
I suggest to change the currently proposed paragraph for make_simple_pair
template<typename... Types> pair<typename decay<Types>::type...> make_simple_pair(Types&&... t);
Type requirements: sizeof...(Types) == 2.Remarks: The program shall be ill-formed, if sizeof...(Types) != 2....
or alternatively (but with a slightly different semantic):
Remarks: If sizeof...(Types) != 2, this function shall not participate in overload resolution.to follow a currently introduced style and because the library does not have yet a specific "Type requirements" element. If such thing would be considered as useful this should be done as a separate issue. Given the increasing complexity of either of these wordings it might be preferable to use the normal two-argument-declaration style again in either of the following ways:
[ 2009-10 post-Santa Cruz: ]
Mark as Tentatively NAD Future.
Rationale:
Does not have sufficient support at this time. May wish to reconsider for a future standard.
Proposed resolution:
Add the following function to 20.3.5 [pairs] and signature in appropriate synopses:
template<typename... Types> pair<typename decay<Types>::type...> make_simple_pair(Types&&... t);Type requirements: sizeof...(Types) == 2.
Returns: pair<typename decay<Types>::type...>(std::forward<Types>(t)...).
[ Draughting note: I chose a variadic representation similar to make_tuple rather than naming both types as it is easier to read through the clutter of metaprogramming this way. Given there are exactly two elements, the committee may prefer to draught with two explicit template type parameters instead ]
Add the following function to 20.4.2.4 [tuple.creation] and signature in appropriate synopses:
template<typename... Types> tuple<typename decay<Types>::type...> make_simple_tuple(Types&&... t);Returns: tuple<typename decay<Types>::type...>(std::forward<Types>(t)...).
Section: 20.7.3 [meta.help] Status: NAD Submitter: Alisdair Meredith Opened: 2009-09-05 Last modified: 2010-03-08
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Discussion:
The specification of integral_constant has been inherited essentially unchanged from TR1:
template <class T, T v>
struct integral_constant {
static const T value = v;
typedef T value_type;
typedef integral_constant<T,v> type;
};
In light of 0x language changes there are several things we might consider changing, notably the form of specification for value.
The current form requires a static data member have storage allocated for it, where we could now implement without this using the new enum syntax:
template <class T, T v>
struct integral_constant {
enum : T { value = v };
typedef T value_type;
typedef integral_constant type;
};
The effective difference between these two implementation is:
Also note the editorial change to drop the explicit qualification of integral_constant in the typedef type. This makes it quite clear we mean the current instantiation, and cannot be mistaken for a recursive metaprogram.
Even if we don't mandate this implementation, it would be nice to give vendors freedom under QoI to choose their preferred representation.
The other side of this issue is if we choose to retain the static constant form. In that case we should go further and insist on constexpr, much like we did throughout numeric_limits:
template <class T, T v>
struct integral_constant {
static constexpr T value = v;
typedef T value_type;
typedef integral_constant type;
};
[Footnote] It turns out constexpr is part of the Tentatively Ready resolution for 1019. I don't want to interfere with that issue, but would like a new issue to consider if the fixed-base enum implementation should be allowed.
[ 2009-09-05 Daniel adds: ]
I think that the suggested resolution is incomplete and may have some possible unwanted side-effects. To understand why, note that integral_constant is completely specified by code in 20.7.3 [meta.help]. While this is usually considered as a good thing, let me give a possible user-defined specialization that would break given the suggested changes:
enum NodeColor { Red, Black }; std::integral_constant<NodeColor, Red> red;The reason why that breaks is due to the fact that current core language rules does only allow integral types as enum-bases, see 7.2 [dcl.enum]/2.
So, I think that we cannot leave the implementation the freedom to decide which way they would like to provide the implementation, because that is easily user-visible (I don't speak of addresses, but of instantiation errors), therefore if applied, this should be either specified or wording must be added that gives a note about this freedom of implementation.
Another possible disadvantage seems to me that user-expectations are easy to disappoint if they see a failure of the test
assert(typeid(std::integral_constant<int, 0>::value) == typeid(int));or of
static_assert(std::is_same<decltype(std::integral_constant<int, 0>::value), const int>::value, "Bad library");
[ 2010-01-14 Moved to Tentatively NAD after 5 positive votes on c++std-lib. ]
Rationale:
We think that the suggested resolution is incomplete and may have some possible unwanted side-effects. (see Daniel's 2009-09-05 comment for details).
Proposed resolution:
Section: 27.7.2.9 [ostream.rvalue], 27.7.1.6 [istream.rvalue] Status: NAD Future Submitter: Howard Hinnant Opened: 2009-09-06 Last modified: 2009-10-20
View all issues with NAD Future status.
Discussion:
27.7.2.9 [ostream.rvalue] was created to preserve the ability to insert into (and extract from 27.7.1.6 [istream.rvalue]) rvalue streams:
template <class charT, class traits, class T> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const T& x);1 Effects: os << x
2 Returns: os
This is good as it allows code that wants to (for example) open, write to, and close an ofstream all in one statement:
std::ofstream("log file") << "Some message\n";
However, I think we can easily make this "rvalue stream helper" even easier to use. Consider trying to quickly create a formatted string. With the current spec you have to write:
std::string s = static_cast<std::ostringstream&>(std::ostringstream() << "i = " << i).str();
This will store "i = 10" (for example) in the string s. Note the need to cast the stream back to ostringstream& prior to using the member .str(). This is necessary because the inserter has cast the ostringstream down to a more generic ostream during the insertion process.
I believe we can re-specify the rvalue-inserter so that this cast is unnecessary. Thus our customer now has to only type:
std::string s = (std::ostringstream() << "i = " << i).str();
This is accomplished by having the rvalue stream inserter return an rvalue of the same type, instead of casting it down to the base class. This is done by making the stream generic, and constraining it to be an rvalue of a type derived from ios_base.
The same argument and solution also applies to the inserter. This code has been implemented and tested.
[ 2009 Santa Cruz: ]
NAD Future. No concensus for change.
Proposed resolution:
Change 27.7.1.6 [istream.rvalue]:
template <classcharT, class traitsIstream, class T>basic_istream<charT, traits>&Istream&& operator>>(basic_istream<charT, traits>Istream&& is, T& x);1 Effects: is >> x
2 Returns: std::move(is)
3 Remarks: This signature shall participate in overload resolution if and only if Istream is not an lvalue reference type and is derived from ios_base.
Change 27.7.2.9 [ostream.rvalue]:
template <classcharT, class traitsOstream, class T>basic_ostream<charT, traits>&Ostream&& operator<<(basic_ostream<charT, traits>Ostream&& os, const T& x);1 Effects: os << x
2 Returns: std::move(os)
3 Remarks: This signature shall participate in overload resolution if and only if Ostream is not an lvalue reference type and is derived from ios_base.
Section: 24.2 [iterator.requirements] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2010-03-14
View all other issues in [iterator.requirements].
View all issues with NAD Editorial status.
Discussion:
p6 Iterator requirements 24.2 [iterator.requirements]
An iterator j is called reachable from an iterator i if and only if there is a finite sequence of applications of the expression ++i that makes i == j. If j is reachable from i, they refer to the same container.
A good example would be stream iterators, which do not refer to a container. Typically, the end iterator from a range of stream iterators will compare equal for many such ranges. I suggest striking the second sentence.
An alternative wording might be:
If j is reachable from i, and both i and j are dereferencable iterators, then they refer to the same range.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3066.
Proposed resolution:
Change 24.2 [iterator.requirements], p6:
An iterator j is called reachable from an iterator i if and only if there is a finite sequence of applications of the expression ++i that makes i == j.If j is reachable from i, they refer to the same container.
Section: 24.5.3.1 [move.iterator] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2010-03-14
View all other issues in [move.iterator].
View all issues with NAD Editorial status.
Discussion:
I contend that while we can support both bidirectional and random access traversal, the category of a move iterator should never be better than input_iterator_tag.
The contentious point is that you cannot truly have a multipass property when values are moved from a range. This is contentious if you view a moved-from object as still holding a valid value within the range.
The second reason comes from the Forward Iterator requirements table:
Forward iterators 24.2.5 [forward.iterators]
Table 102 -- Forward iterator requirements
For expression *a the return type is: "T& if X is mutable, otherwise const T&"
There is a similar constraint on a->m.
There is no support for rvalue references, nor do I believe their should be. Again, opinions may vary but either this table or the definition of move_iterator need updating.
Note: this requirement probably need updating anyway if we wish to support proxy iterators but I am waiting to see a new working paper before filing that issue.
[ 2009-10 post-Santa Cruz: ]
Move to Open. Howard to put his rationale mentioned above into the issue as a note.
[ 2009-10-26 Howard adds: ]
vector::insert(pos, iter, iter) is significantly more effcient when iter is a random access iterator, as compared to when it is an input iterator.
When iter is an input iterator, the best algorithm is to append the inserted range to the end of the vector using push_back. This may involve several reallocations before the input range is exhausted. After the append, then one can use std::rotate to place the inserted range into the correct position in the vector.
But when iter is a random access iterator, the best algorithm is to first compute the size of the range to be inserted (last - first), do a buffer reallocation if necessary, scoot existing elements in the vector down to make the "hole", and then insert the new elements directly to their correct place.
The insert-with-random-access-iterators algorithm is considerably more efficient than the insert-with-input-iterators algorithmNow consider:
vector<A> v; // ... build up a large vector of A ... vector<A> temp; // ... build up a large temporary vector of A to later be inserted ... typedef move_iterator<vector<A>::iterator> MI; // Now insert the temporary elements: v.insert(v.begin() + N, MI(temp.begin()), MI(temp.end()));A major motivation for using move_iterator in the above example is the expectation that A is cheap to move but expensive to copy. I.e. the customer is looking for high performance. If we allow vector::insert to subtract two MI's to get the distance between them, the customer enjoys substantially better performance, compared to if we say that vector::insert can not subtract two MI's.
I can find no rationale for not giving this performance boost to our customers. Therefore I am strongly against restricting move_iterator to the input_iterator_tag category.
I believe that the requirement that forward iterators have a dereference that returns an lvalue reference to cause unacceptable pessimization. For example vector<bool>::iterator also does not return a bool& on dereference. Yet I am not aware of a single vendor that is willing to ship vector<bool>::iterator as an input iterator. Everyone classifies it as a random access iterator. Not only does this not cause any problems, it prevents significant performance problems.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3066.
Proposed resolution:
Class template move_iterator 24.5.3.1 [move.iterator]
namespace std {
template <class Iterator>
class move_iterator {
public:
...
typedef typename iterator_traits<Iterator>::iterator_category input_iterator_tag iterator_category;
Section: 24.2 [iterator.requirements] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2010-03-14
View all other issues in [iterator.requirements].
View all issues with NAD Editorial status.
Discussion:
Forward iterator and bidirectional iterator place different requirements on the result of post-increment/decrement operator. The same form should be used in each case.
Merging row from:
Table 102 -- Forward iterator requirements
Table 103 -- Bidirectional iterator requirements
r++ : convertible to const X&
r-- : convertible to const X&
*r++ : T& if X is mutable, otherwise const T&
*r-- : convertible to T
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3066.
Proposed resolution:
Section: 26.4 [complex.numbers] Status: NAD Future Submitter: Ted Shaneyfelt Opened: 2009-09-26 Last modified: 2009-10-26
View all other issues in [complex.numbers].
View all issues with NAD Future status.
Discussion:
Concerning mathematically proper operation of the type:
complex<complex<T> >
Generally accepted mathematical semantics of such a construct correspond to quaternions through Cayly-Dickson construct
(w+xi) + (y+zi) j
The proper implementation seems straightforward by adding a few declarations like those below. I have included operator definition for combining real scalars and complex types, as well, which seems appropriate, as algebra of complex numbers allows mixing complex and real numbers with operators. It also allows for constructs such as complex<double> i=(0,1), x = 12.34 + 5*i;
Quaternions are often used in areas such as computer graphics, where, for example, they avoid the problem of Gimbal lock when rotating objects in 3D space, and can be more efficient than matrix multiplications, although I am applying them to a different field.
/////////////////////////ALLOW OPERATORS TO COMBINE REAL SCALARS AND COMPLEX VALUES /////////////////////////
template<typename T,typename S> complex<T> operator+(const complex<T> x,const S a) {
complex<T> result(x.real()+a, x.imag());
return result;
}
template<typename T,typename S> complex<T> operator+(const S a,const complex<T> x) {
complex<T> result(a+x.real(), x.imag());
return result;
}
template<typename T,typename S> complex<T> operator-(const complex<T> x,const S a) {
complex<T> result(x.real()-a, x.imag());
return result;
}
template<typename T,typename S> complex<T> operator-(const S a,const complex<T> x) {
complex<T> result(a-x.real(), x.imag());
return result;
}
template<typename T,typename S> complex<T> operator*(const complex<T> x,const S a) {
complex<T> result(x.real()*a, x.imag()*a);
return result;
}
template<typename T,typename S> complex<T> operator*(const S a,const complex<T> x) {
complex<T> result(a*x.real(), a*x.imag());
return result;
}
/////////////////////////PROPERLY IMPLEMENT QUATERNION SEMANTICS/////////////////////////
template<typename T> double normSq(const complex<complex<T> >q) {
return q.real().real()*q.real().real()
+ q.real().imag()*q.real().imag()
+ q.imag().real()*q.imag().real()
+ q.imag().imag()*q.imag().imag();
}
template<typename T> double norm(const complex<complex<T> >q) {
return sqrt(normSq(q));
}
/////// Cayley-Dickson Construction
template<typename T> complex<complex<T> > conj(const complex<complex<T> > x) {
complex<complex<T> > result(conj(x.real()),-x.imag());
return result;
}
template<typename T> complex<complex<T> > operator*(const complex<complex<T> > ab,const complex<complex<T> > cd) {
complex<T> re(ab.real()*cd.real()-conj(cd.imag())*ab.imag());
complex<T> im(cd.imag()*ab.real()+ab.imag()*conj(cd.real()));
complex<complex<double> > q(re,im);
return q;
}
//// Quaternion division
template<typename S,typename T> complex<complex<T> > operator/(const complex<complex<T> > q,const S a) {
return q * (1/a);
}
template<typename S,typename T> complex<complex<T> > operator/(const S a,const complex<complex<T> > q) {
return a*conj(q)/normSq(q);
}
template<typename T> complex<complex<T> > operator/(const complex<complex<T> > n, const complex<complex<T> > d) {
return n * (conj(d)/normSq(d));
}
[ 2009-10 Santa Cruz: ]
NAD Future. There is no consensus or time to move this into C++0X.
Proposed resolution:
Section: 30.4.3.2.2 [thread.lock.unique.locking] Status: Dup Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2010-03-08
View all other issues in [thread.lock.unique.locking].
View all issues with Dup status.
Duplicate of: 1159
Discussion:
unique_lock::lock and friends raise "resource_deadlock_would_occur -- if the current thread already owns the mutex (i.e., on entry, owns is true)." 1) The current thread owning a mutex is not the same as any particular unique_lock::owns being true. 2) There's no need to raise this exception for a recursive_mutex if owns is false. 3) If owns is true, we need to raise some exception or the unique_lock will lose track of whether to unlock itself on destruction, but "deadlock" isn't it. For (3), s/bool owns/int ownership_level/ would fix it.
[ 2009-11-11 Alisdair notes that this issue is very closely related to 1159, if not a dup. ]
[ 2009-11-14 Moved to Tentatively Dup after 5 positive votes on c++std-lib. ]
Proposed resolution:
Section: 30.5.2 [thread.condition.condvarany] Status: NAD Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2010-03-08
View all other issues in [thread.condition.condvarany].
View all issues with NAD status.
Discussion:
For condition_variable_any, must all lock arguments to concurrent wait calls "match" in some way, similar to the requirement in 30.5.1 [thread.condition.condvar] that lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting threads (via wait or timed_wait)?
[ 2010-02-12 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
The rationale is that it doesn't matter, and you can't check: the lock types may be different, or the same and user-defined, so the implementation must provide internal synchronization anyway.
Proposed resolution:
Section: 30.5.2 [thread.condition.condvarany] Status: NAD Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2010-03-08
View all other issues in [thread.condition.condvarany].
View all issues with NAD status.
Discussion:
For condition_variable_any, are recursive mutexes allowed? (I think "no")
[ 2009-11-17 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]
Proposed resolution:
Rationale:
condition_variable_any::wait accepts any type of mutex. It calls unlock precisely once on entry and lock precisely once on exit. It is up to the user to ensure that this provides the required synchronization. Use of a recursive mutex is safe if either its lock count is 1, so after the single unlock it can be acquired by another thread, or another mechanism is used to synchronize the data.
Section: X [func.ret] Status: NAD Editorial Submitter: Sebastian Gesemann Opened: 2009-10-05 Last modified: 2010-03-15
View all other issues in [func.ret].
View all issues with NAD Editorial status.
Discussion:
I think the text about std::result_of could be a little more precise. Quoting from N2960...
X [func.ret] Function object return types
template<class> class result_of; template<class Fn, class... ArgTypes> class result_of<Fn(ArgTypes...)> { public: typedef see below type; };Given an rvalue fn of type Fn and values t1, t2, ..., tN of types T1, T2, ... TN in ArgTypes respectivly, the type member is the result type of the expression fn(t1,t2,...,tN). the values ti are lvalues when the corresponding type Ti is an lvalue-reference type, and rvalues otherwise.
This text doesn't seem to consider lvalue reference types for Fn. Also, it's not clear whether this class template can be used for "SFINAE" like std::enable_if. Example:
template<typename Fn, typename... Args>
typename std::result_of<Fn(Args...)>::type
apply(Fn && fn, Args && ...args)
{
// Fn may be an lvalue reference, too
return std::forward<Fn>(fn)(std::forward<Args>(args)...);
}
Either std::result_of<...> can be instantiated and simply may not have a typedef "type" (-->SFINAE) or instantiating the class template for some type combinations will be a "hard" compile-time error.
[ 2010-02-14 Daniel adds: ]
This issue should be considered resolved by 1255 and 1270. The wish to change result_of into a compiler-support trait was beyond the actual intention of the submitter Sebastian.
[ 2010 Pittsburgh: Moved to NAD Editorial, rationale added below. ]
Rationale:
Solved by 1270.
Proposed resolution:
[ These changes will require compiler support ]
Change X [func.ret]:
template<class> class result_of; // undefined template<class Fn, class... ArgTypes> class result_of<Fn(ArgTypes...)> { public:typedefsee belowtype;};
Given an rvalue fn of type Fn and values t1, t2, ..., tN of types T1, T2, ... TN in ArgTypes respectivly, the type member is the result type of the expression fn(t1,t2,...,tN). the values ti are lvalues when the corresponding type Ti is an lvalue-reference type, and rvalues otherwise.The class template result_of shall meet the requirements of a TransformationTrait: Given the types Fn, T1, T2, ..., TN every template specialization result_of<Fn(T1,T2,...,TN)> shall define the member typedef type equivalent to decltype(RE) if and only if the expression RE
value<Fn>() ( value<T1>(), value<T2>(), ... value<TN>() )would be well-formed. Otherwise, there shall be no member typedef type defined.
[ The value<> helper function is a utility Daniel Krügler proposed in N2958. ]
Section: 30.6.2 [futures.errors] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2009-10-05 Last modified: 2010-03-11
View all issues with NAD Editorial status.
Discussion:
Defect issue 890 overlooked to adapt the future_category from 30.6.1 [futures.overview] and 30.6.2 [futures.errors]:
extern const error_category* const future_category;
which should be similarly transformed into function form.
[ 2009-10-27 Howard: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ 2009-11-11 Daniel adds: ]
I just observe that the proposed resolution of this issue is incomplete and needs to reworded. The problem is that the corresponding declarations
constexpr error_code make_error_code(future_errc e); constexpr error_condition make_error_condition(future_errc e);as constexpr functions are incompatible to the requirements of constexpr functions given their specified implementation. Note that the incompatibility is not a result of the modifications proposed by the issue resolution, but already existed within the N2960 state where we have
extern const error_category* const future_category;combined with
constexpr error_code make_error_code(future_errc e);3 Returns: error_code(static_cast<int>(e), *future_category).constexpr error_code make_error_condition(future_errc e);4 Returns: error_condition(static_cast<int>(e), *future_category).Neither is any of the constructors of error_code and error_condition constexpr, nor does the expression *future_category satisfy the requirements for a constant expression (5.19 [expr.const]/2 bullet 6 in N3000).
The simple solution is just to remove the constexpr qualifiers for both functions, which makes sense, because none of the remaining make_error_* overloads in the library is constexpr. One might consider to realize that those make_* functions could satisfy the constexpr requirements, but this looks not like an easy task to me, because it would need to rely on a not yet existing language feature. If such a change is wanted, a new issue should be opened after the language extension approval (if at all) [1].
If no-one complaints I would like to ask Howard to add the following modifications to this issue, alternatively a new issue could be opened but I don't know what the best solution is that would cause as little overhead as possible.
What-ever the route is, the following is my proposed resolution for this issue interaction part of the story:
In 30.6.1 [futures.overview]/1, Header <future> synopsis and in 30.6.2 [futures.errors]/3+4 change as indicated:
constexprerror_code make_error_code(future_errc e);constexprerror_condition make_error_condition(future_errc e);[1] Let me add that we have a related NAD issue here: 832 so the chances for realization are little IMO.
[ Howard: I've updated the proposed wording as Daniel suggests and set to Review. ]
[ 2009-11-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Change in 30.6.1 [futures.overview], header <future> synopsis:
externconst error_category&* constfuture_category();
In 30.6.1 [futures.overview]/1, Header <future> synopsis change as indicated:
constexprerror_code make_error_code(future_errc e);constexprerror_condition make_error_condition(future_errc e);
Change in 30.6.2 [futures.errors]:
externconst error_category&* constfuture_category();
1- future_category shall point to a statically initialized object of a type derived from class error_category.1- Returns: A reference to an object of a type derived from class error_category.
constexprerror_code make_error_code(future_errc e);3 Returns: error_code(static_cast<int>(e),*future_category()).constexprerror_codecondition make_error_condition(future_errc e);4 Returns: error_condition(static_cast<int>(e),*future_category()).
Section: 20.7.4.3 [meta.unary.prop] Status: NAD Submitter: Alisdair Meredith Opened: 2009-10-07 Last modified: 2010-03-15
View all other issues in [meta.unary.prop].
View all issues with NAD status.
Discussion:
According to p1 20.7.2 [meta.type.synop]:
The behavior of a program that adds specializations for any of the class templates defined in this subclause is undefined unless otherwise specified.
I believe we should 'otherwise specify' for the nothrow traits, are these are exactly the use cases where the end user actually has more information than the compiler.
[ 2009-10 Santa Cruz: ]
Moved to Open. Definitely need to give the users the ability to ensure that the traits give the right answers. Unsure we want to give them the ability to say this in more than one way. Believes the noexcept proposal already gives this.
[ 2010 Pittsburgh: Moved to NAD, rationale added below. ]
Rationale:
We believe the solution offered by N3050 is superior.
Proposed resolution:
Add the following comment:
user specialization permitted to derive from std::true_type when the operation is known not to throw.
to the following traits in 20.7.4.3 [meta.unary.prop] Table 43 Type property predicates.
[ This may require a new Comments column ]
has_nothrow_default_constructor has_nothrow_copy_constructor has_nothrow_assign
Section: 19.5.2.3 [syserr.errcode.modifiers] Status: NAD Submitter: Stephan T. Lavavej Opened: 2009-10-08 Last modified: 2009-10-26
View all issues with NAD status.
Discussion:
N2960 19.5.2.1 [syserr.errcode.overview] and 19.5.2.3 [syserr.errcode.modifiers] say:
template <class ErrorCodeEnum>
typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type&
operator=(ErrorCodeEnum e);
They should say:
template <class ErrorCodeEnum>
typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code>::type&
operator=(ErrorCodeEnum e);
Or (I prefer this form):
template <class ErrorCodeEnum>
typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code&>::type
operator=(ErrorCodeEnum e);
This is because enable_if is declared as (20.7.6.6 [meta.trans.other]):
template <bool B, class T = void> struct enable_if;
So, the current wording makes operator= return void&, which is not good.
19.5.2.3 [syserr.errcode.modifiers]/4 says
Returns: *this.
which is correct.
Additionally,
19.5.3.1 [syserr.errcondition.overview]/1 says:
template<typename ErrorConditionEnum>
typename enable_if<is_error_condition_enum<ErrorConditionEnum>, error_code>::type &
operator=( ErrorConditionEnum e );
Which contains several problems (typename versus class inconsistency, lack of ::value, error_code instead of error_condition), while 19.5.3.3 [syserr.errcondition.modifiers] says:
template <class ErrorConditionEnum>
typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::type&
operator=(ErrorConditionEnum e);
Which returns void&. They should both say:
template <class ErrorConditionEnum>
typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_condition>::type&
operator=(ErrorConditionEnum e);
Or (again, I prefer this form):
template <class ErrorConditionEnum>
typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_condition&>::type
operator=(ErrorConditionEnum e);
Additionally, 19.5.3.3 [syserr.errcondition.modifiers] lacks a "Returns: *this." paragraph, which is presumably necessary.
[ 2009-10-18 Beman adds: ]
The proposed resolution for issue 1237 makes this issue moot, so it should become NAD.
[ 2009-10 Santa Cruz: ]
NAD, solved by 1237.
Proposed resolution:
Change 19.5.2.1 [syserr.errcode.overview] and 19.5.2.3 [syserr.errcode.modifiers]:
template <class ErrorCodeEnum> typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code&>::type&operator=(ErrorCodeEnum e);
Change 19.5.3.1 [syserr.errcondition.overview]:
template<typenameclass ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_conditionde&>::type&operator=( ErrorConditionEnum e );
Change 19.5.3.3 [syserr.errcondition.modifiers]:
template <class ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_condition&>::type&operator=(ErrorConditionEnum e);Postcondition: *this == make_error_condition(e).
Returns: *this.
Throws: Nothing.
Section: 20.8.13 [func.memfn] Status: Dup Submitter: Alisdair Meredith Opened: 2009-10-09 Last modified: 2009-10-23
View all other issues in [func.memfn].
View all issues with Dup status.
Duplicate of: 920
Discussion:
Since we have removed the entry in B [implimits] for the library-specific limit for number of arguments passed to function/tuple/etc. I believe we need to update the spec for mem_fn to reflect this.
The "Remarks: Implementations may implement mem_fn as a set of overloaded function templates." no longer holds, as we cannot create an arbitrary number of such overloads. I believe we should strike the remark and add a second signature:
template<class R, class T, typename ... ArgTypes> unspecified mem_fn(R (T::*pm)(ArgTypes...));
I believe we need two signatures as pointer-to-data-member and pointer-to-member-function-taking-no-args appear to use subtly different syntax.
[ 920 as a similar proposed resolution. ]
Proposed resolution:
Add to 20.8 [function.objects] and 20.8.13 [func.memfn]:template<class R, class T> unspecified mem_fn(R T::* pm) template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...)); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...)&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...)&&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const&&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile&&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile&&);
Strike 20.8.13 [func.memfn], p5:
Remarks: Implementations may implement mem_fn as a set of overloaded function templates.
Section: 17 [library] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2009-10-11 Last modified: 2009-10-29
View other active issues in [library].
View all other issues in [library].
View all issues with NAD Editorial status.
Discussion:
The current library contains still rvalue reference-swaps that seem to be overlooked in the process of switching back to lvalue-ref swaps.
[ 2009-10 Santa Cruz: ]
Editor accepts as NAD Editorial.
Proposed resolution:
Change 20.3.5 [pairs]/1 as indicated:
template <class T1, class T2>
struct pair {
...
void swap(pair&& p);
};
Change 20.3.5 [pairs] before p. 17 as indicated:
void swap(pair&&p);
Change 20.3.5 [pairs] before p. 21 as indicated:
template<class T1, class T2> void swap(pair<T1, T2>& x, pair<T1, T2>& y);template<class T1, class T2> void swap(pair<T1, T2>&& x, pair<T1, T2>& y);template<class T1, class T2> void swap(pair<T1, T2>& x, pair<T1, T2>&& y);
Change 20.4.1 [tuple.general]/2, header <tuple> synopsis, as indicated:
// 20.5.2.9, specialized algorithms: template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y);template <class... Types> void swap(tuple<Types...>&& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>&& y);
Change 20.4.2 [tuple.tuple] as indicated:
// 20.5.2.3, tuple swap void swap(tuple&&)
Change 20.4.2.3 [tuple.swap] before 1 as indicated:
void swap(tuple&&rhs);
Change 20.8 [function.objects]/2, header <functional> synopsis, as indicated:
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&);template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&); template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)&&);
Change 20.8.14.2 [func.wrap.func], as indicated:
// 20.7.15.2.2, function modifiers: void swap(function&&); template<class F, class A> void assign(F, const A&); [..] // 20.7.15.2.7, specialized algorithms: template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&);template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&); template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&&);
Change 20.8.14.2.7 [func.wrap.func.alg] before 1 as indicated:
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>& f2);template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&& f1, function<R(ArgTypes...)>& f2); template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>&& f2);
Change 20.9.11.2 [util.smartptr.shared]/1 as indicated:
// 20.8.12.2.4, modifiers: void swap(shared_ptr&&r); [..] // 20.8.12.2.9, shared_ptr specialized algorithms: template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b);template<class T> void swap(shared_ptr<T>&& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>&& b);
Change 21.3 [string.classes]/1, header <string> synopsis, as indicated:
// 21.4.8.8: swap template<class charT, class traits, class Allocator> void swap(basic_string<charT,traits,Allocator>& lhs, basic_string<charT,traits,Allocator>& rhs);template<class charT, class traits, class Allocator> void swap(basic_string<charT,traits,Allocator>&& lhs, basic_string<charT,traits,Allocator>& rhs); template<class charT, class traits, class Allocator> void swap(basic_string<charT,traits,Allocator>& lhs, basic_string<charT,traits,Allocator>&& rhs);
Change 23.3 [sequences]/1, header <deque> synopsis, as indicated:
template <class T, class Allocator> void swap(deque<T,Allocator>& x, deque<T,Allocator>& y);template <class T, class Allocator> void swap(deque<T,Allocator>&& x, deque<T,Allocator>& y); template <class T, class Allocator> void swap(deque<T,Allocator>& x, deque<T,Allocator>&& y);
Change 23.3 [sequences]/1, header <list> synopsis, as indicated:
template <class T, class Allocator> void swap(list<T,Allocator>& x, list<T,Allocator>& y);template <class T, class Allocator> void swap(list<T,Allocator>&& x, list<T,Allocator>& y); template <class T, class Allocator> void swap(list<T,Allocator>& x, list<T,Allocator>&& y);
Change 23.3 [sequences]/1, header <queue> synopsis, as indicated:
template <class T, class Allocator> void swap(queue<T, Container>& x, queue<T, Container>& y);template <class T, class Container> void swap(queue<T, Container>&& x, queue<T, Container>& y); template <class T, class Container> void swap(queue<T, Container>& x, queue<T, Container>&& y);template <class T, class Container = vector<T>, class Compare = less<typename Container::value_type> > class priority_queue; template <class T, class Container, class Compare> void swap(priority_queue<T, Container, Compare>& x, priority_queue<T, Container, Compare>& y);template <class T, class Container, class Compare> void swap(priority_queue<T, Container, Compare>&& x, priority_queue<T, Container, Compare>& y); template <class T, class Container, class Compare> void swap(priority_queue<T, Container, Compare>& x, priority_queue<T, Container, Compare>&& y);
Change 23.3 [sequences]/1, header <stack> synopsis, as indicated:
template <class T, class Container> void swap(stack<T, Container>& x, stack<T, Container>& y);template <class T, class Container> void swap(stack<T, Container>&& x, stack<T, Container>& y); template <class T, class Container> void swap(stack<T, Container>& x, stack<T, Container>&& y);
Change 23.3 [sequences]/1, header <vector> synopsis, as indicated:
template <class T, class Allocator> void swap(vector<T,Allocator>& x, vector<T,Allocator>& y);template <class T, class Allocator> void swap(vector<T,Allocator>&& x, vector<T,Allocator>& y); template <class T, class Allocator> void swap(vector<T,Allocator>& x, vector<T,Allocator>&& y);
Change 23.3.2 [deque]/2 as indicated:
iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last); void swap(deque<T,Allocator>&&); void clear(); [..] // specialized algorithms: template <class T, class Allocator> void swap(deque<T,Allocator>& x, deque<T,Allocator>& y);template <class T, class Allocator> void swap(deque<T,Allocator>&& x, deque<T,Allocator>& y); template <class T, class Allocator> void swap(deque<T,Allocator>& x, deque<T,Allocator>&& y);
Change 23.3.2.4 [deque.special] as indicated:
template <class T, class Allocator> void swap(deque<T,Allocator>& x, deque<T,Allocator>& y);template <class T, class Allocator> void swap(deque<T,Allocator>&& x, deque<T,Allocator>& y); template <class T, class Allocator> void swap(deque<T,Allocator>& x, deque<T,Allocator>&& y);
Change 23.3.3 [forwardlist]/2 as indicated:
iterator erase_after(const_iterator position); iterator erase_after(const_iterator position, iterator last); void swap(forward_list<T,Allocator>&&); [..] // 23.3.3.6 specialized algorithms: template <class T, class Allocator> void swap(forward_list<T,Allocator>& x, forward_list<T,Allocator>& y);template <class T, class Allocator> void swap(forward_list<T,Allocator>&& x, forward_list<T,Allocator>& y); template <class T, class Allocator> void swap(forward_list<T,Allocator>& x, forward_list<T,Allocator>&& y);
Change 23.3.3.6 [forwardlist.spec] as indicated:
template <class T, class Allocator> void swap(forward_list<T,Allocator>& x, forward_list<T,Allocator>& y);template <class T, class Allocator> void swap(forward_list<T,Allocator>&& x, forward_list<T,Allocator>& y); template <class T, class Allocator> void swap(forward_list<T,Allocator>& x, forward_list<T,Allocator>&& y);
Change 23.3.4 [list]/2 as indicated:
iterator erase(const_iterator position); iterator erase(const_iterator position, const_iterator last); void swap(list<T,Allocator>&&); void clear(); [..] // specialized algorithms: template <class T, class Allocator> void swap(list<T,Allocator>& x, list<T,Allocator>& y);template <class T, class Allocator> void swap(list<T,Allocator>&& x, list<T,Allocator>& y); template <class T, class Allocator> void swap(list<T,Allocator>& x, list<T,Allocator>&& y);
Change 23.3.4.5 [list.special] as indicated:
template <class T, class Allocator> void swap(list<T,Allocator>& x, list<T,Allocator>& y);template <class T, class Allocator> void swap(list<T,Allocator>&& x, list<T,Allocator>& y); template <class T, class Allocator> void swap(list<T,Allocator>& x, list<T,Allocator>&& y);
Change 23.3.5.1.1 [queue.defn] as indicated:
void swap(queue&&q) { c.swap(q.c); } [..] template <class T, class Container> void swap(queue<T, Container>& x, queue<T, Container>& y);template <class T, class Container> void swap(queue<T, Container>&& x, queue<T, Container>& y); template <class T, class Container> void swap(queue<T, Container>& x, queue<T, Container>&& y);
Change 23.3.5.1.5 [queue.special] as indicated:
template <class T, class Container> void swap(queue<T, Container>& x, queue<T, Container>& y);template <class T, class Container> void swap(queue<T, Container>&& x, queue<T, Container>& y); template <class T, class Container> void swap(queue<T, Container>& x, queue<T, Container>&& y);
Change 23.3.5.2 [priority.queue]/1 as indicated:
void swap(priority_queue&&); // no equality is provided template <class T, class Container, class Compare> void swap(priority_queue<T, Container, Compare>& x, priority_queue<T, Container, Compare>& y);template <class T, class Container, class Compare> void swap(priority_queue<T, Container, Compare>&& x, priority_queue<T, Container, Compare>& y); template <class T, class Container, class Compare> void swap(priority_queue<T, Container, Compare>& x, priority_queue<T, Container, Compare>&& y);
Change 23.3.5.2.4 [priqueue.special] as indicated:
template <class T, class Container, Compare> void swap(priority_queue<T, Container, Compare>& x, priority_queue<T, Container, Compare>& y);template <class T, class Container, Compare> void swap(priority_queue<T, Container, Compare>&& x, priority_queue<T, Container, Compare>& y); template <class T, class Container, Compare> void swap(priority_queue<T, Container, Compare>& x, priority_queue<T, Container, Compare>&& y);
Change 23.3.5.3.1 [stack.defn] as indicated:
void swap(stack&&s) { c.swap(s.c); } [..] template <class T, class Allocator> void swap(stack<T,Allocator>& x, stack<T,Allocator>& y);template <class T, class Allocator> void swap(stack<T,Allocator>&& x, stack<T,Allocator>& y); template <class T, class Allocator> void swap(stack<T,Allocator>& x, stack<T,Allocator>&& y);
Change 23.3.5.3.5 [stack.special] as indicated:
template <class T, class Container> void swap(stack<T, Container>& x, stack<T, Container>& y);template <class T, class Container> void swap(stack<T, Container>&& x, stack<T, Container>& y); template <class T, class Container> void swap(stack<T, Container>& x, stack<T, Container>&& y);
Change 23.3.6 [vector]/2 as indicated:
void swap(vector<T,Allocator>&&); void clear(); [..] // specialized algorithms: template <class T, class Allocator> void swap(vector<T,Allocator>& x, vector<T,Allocator>& y);template <class T, class Allocator> void swap(vector<T,Allocator>&& x, vector<T,Allocator>& y); template <class T, class Allocator> void swap(vector<T,Allocator>& x, vector<T,Allocator>&& y);
Change 23.3.6.2 [vector.capacity] before p. 8 as indicated:
void swap(vector<T,Allocator>&&x);
Change 23.3.6.5 [vector.special] as indicated:
template <class T, class Allocator> void swap(vector<T,Allocator>& x, vector<T,Allocator>& y);template <class T, class Allocator> void swap(vector<T,Allocator>&& x, vector<T,Allocator>& y); template <class T, class Allocator> void swap(vector<T,Allocator>& x, vector<T,Allocator>&& y);
Change 23.3.7 [vector.bool]/1 as indicated:
iterator erase(const_iterator first, const_iterator last); void swap(vector<bool,Allocator>&&); static void swap(reference x, reference y);
Change 23.4 [associative]/1, header <map> synopsis as indicated:
template <class Key, class T, class Compare, class Allocator> void swap(map<Key,T,Compare,Allocator>& x, map<Key,T,Compare,Allocator>& y);template <class Key, class T, class Compare, class Allocator> void swap(map<Key,T,Compare,Allocator&& x, map<Key,T,Compare,Allocator>& y); template <class Key, class T, class Compare, class Allocator> void swap(map<Key,T,Compare,Allocator& x, map<Key,T,Compare,Allocator>&& y);[..] template <class Key, class T, class Compare, class Allocator> void swap(multimap<Key,T,Compare,Allocator>& x, multimap<Key,T,Compare,Allocator>& y);template <class Key, class T, class Compare, class Allocator> void swap(multimap<Key,T,Compare,Allocator&& x, multimap<Key,T,Compare,Allocator>& y); template <class Key, class T, class Compare, class Allocator> void swap(multimap<Key,T,Compare,Allocator& x, multimap<Key,T,Compare,Allocator>&& y);
Change 23.4 [associative]/1, header <set> synopsis as indicated:
template <class Key, class Compare, class Allocator> void swap(set<Key,Compare,Allocator>& x, set<Key,Compare,Allocator>& y);template <class Key, class T, class Compare, class Allocator> void swap(set<Key,T,Compare,Allocator&& x, set<Key,T,Compare,Allocator>& y); template <class Key, class T, class Compare, class Allocator> void swap(set<Key,T,Compare,Allocator& x, set<Key,T,Compare,Allocator>&& y);[..] template <class Key, class Compare, class Allocator> void swap(multiset<Key,Compare,Allocator>& x, multiset<Key,Compare,Allocator>& y);template <class Key, class T, class Compare, class Allocator> void swap(multiset<Key,T,Compare,Allocator&& x, multiset<Key,T,Compare,Allocator>& y); template <class Key, class T, class Compare, class Allocator> void swap(multiset<Key,T,Compare,Allocator& x, multiset<Key,T,Compare,Allocator>&& y);
Change 23.4.1 [map]/2 as indicated:
iterator erase(const_iterator first, const_iterator last); void swap(map<Key,T,Compare,Allocator>&&); void clear(); [..] // specialized algorithms: template <class Key, class T, class Compare, class Allocator> void swap(map<Key,T,Compare,Allocator>& x, map<Key,T,Compare,Allocator>& y);template <class Key, class T, class Compare, class Allocator> void swap(map<Key,T,Compare,Allocator&& x, map<Key,T,Compare,Allocator>& y); template <class Key, class T, class Compare, class Allocator> void swap(map<Key,T,Compare,Allocator& x, map<Key,T,Compare,Allocator>&& y);
Change 23.4.1.5 [map.special] as indicated:
template <class Key, class T, class Compare, class Allocator> void swap(map<Key,T,Compare,Allocator>& x, map<Key,T,Compare,Allocator>& y);template <class Key, class T, class Compare, class Allocator> void swap(map<Key,T,Compare,Allocator>&& x, map<Key,T,Compare,Allocator>& y); template <class Key, class T, class Compare, class Allocator> void swap(map<Key,T,Compare,Allocator>& x, map<Key,T,Compare,Allocator>&& y);
Change 23.4.2 [multimap]/2 as indicated:
iterator erase(const_iterator first, const_iterator last); void swap(multimap<Key,T,Compare,Allocator>&&); void clear(); [..] // specialized algorithms: template <class Key, class T, class Compare, class Allocator> void swap(multimap<Key,T,Compare,Allocator>& x, multimap<Key,T,Compare,Allocator>& y);template <class Key, class T, class Compare, class Allocator> void swap(multimap<Key,T,Compare,Allocator&& x, multimap<Key,T,Compare,Allocator>& y); template <class Key, class T, class Compare, class Allocator> void swap(multimap<Key,T,Compare,Allocator& x, multimap<Key,T,Compare,Allocator>&& y);
Change 23.4.2.4 [multimap.special] as indicated:
template <class Key, class T, class Compare, class Allocator> void swap(multimap<Key,T,Compare,Allocator>& x, multimap<Key,T,Compare,Allocator>& y);template <class Key, class T, class Compare, class Allocator> void swap(multimap<Key,T,Compare,Allocator>&& x, multimap<Key,T,Compare,Allocator>& y); template <class Key, class T, class Compare, class Allocator> void swap(multimap<Key,T,Compare,Allocator>& x, multimap<Key,T,Compare,Allocator>&& y);
Change 23.4.3 [set]/2 and 23.4.3.2 [set.special] as indicated: (twice!)
// specialized algorithms: template <class Key, class Compare, class Allocator> void swap(set<Key,Compare,Allocator>& x, set<Key,Compare,Allocator>& y);template <class Key, class Compare, class Allocator> void swap(set<Key,Compare,Allocator&& x, set<Key,Compare,Allocator>& y); template <class Key, class Compare, class Allocator> void swap(set<Key,Compare,Allocator& x, set<Key,Compare,Allocator>&& y);
Change 23.4.4 [multiset]/2 as indicated:
iterator erase(const_iterator first, const_iterator last); void swap(multiset<Key,Compare,Allocator>&&); void clear(); [..] // specialized algorithms: template <class Key, class Compare, class Allocator> void swap(multiset<Key,Compare,Allocator>& x, multiset<Key,Compare,Allocator>& y);template <class Key, class Compare, class Allocator> void swap(multiset<Key,Compare,Allocator&& x, multiset<Key,Compare,Allocator>& y); template <class Key, class Compare, class Allocator> void swap(multiset<Key,Compare,Allocator& x, multiset<Key,Compare,Allocator>&& y);
Change 23.4.4.2 [multiset.special] as indicated:
template <class Key, class Compare, class Allocator> void swap(multiset<Key,Compare,Allocator>& x, multiset<Key,Compare,Allocator>& y);template <class Key, class Compare, class Allocator> void swap(multiset<Key,Compare,Allocator>&& x, multiset<Key,Compare,Allocator>& y); template <class Key, class Compare, class Allocator> void swap(multiset<Key,Compare,Allocator>& x, multiset<Key,Compare,Allocator>&& y);
Section: 20.9 [memory] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2009-10-11 Last modified: 2010-03-08
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Discussion:
Related to 296. Some unique_ptr signatures are missing from the synopsis in 20.9 [memory].
[ 2009-11-04 Howard adds: ]
Moved to Tentatively NAD Editorial. The editor has adopted the fix.
Proposed resolution:
Add in 20.9 [memory], Header <memory> synopsis missing declarations as shown below:
// 20.8.11 Class unique_ptr: template <class X> class default_delete; template<class T> struct default_delete<T[]>; template <class X, class D = default_delete<T>> class unique_ptr; template<class T, class D> class unique_ptr<T[], D>; template<class T, class D> void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y); template<class T1, class D1, class T2, class D2> bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template<class T1, class D1, class T2, class D2> bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template<class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template<class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template<class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template<class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Section: X [rand.concept.dist] Status: NAD Future Submitter: Matthias Troyer Opened: 2009-10-12 Last modified: 2009-10-26
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Discussion:
There exist optimized, vectorized vendor libraries for the creation of random number generators, such as Intel's MKL [1] and AMD's ACML [2]. In timing tests we have seen a performance gain of a factor of up to 80 (eighty) compared to a pure C++ implementation (in Boost.Random) when using these generator to generate a sequence of normally distributed random numbers. In codes dominated by the generation of random numbers (we have application codes where random number generation is more than 50% of the CPU time) this factor 80 is very significant.
To make use of these vectorized generators, we use a C++ class modeling the RandomNumberEngine concept and forwarding the generation of random numbers to those optimized generators. For example:
namespace mkl {
class mt19937 {.... };
}
For the generation of random variates we also want to dispatch to optimized vectorized functions in the MKL or ACML libraries. See this example:
mkl::mt19937 eng; std::normal_distribution<double> dist; double n = dist(eng);
Since the variate generation is done through the operator() of the distribution there is no customization point to dispatch to Intel's or AMD's optimized functions to generate normally distributed numbers based on the mt19937 generator. Hence, the performance gain of 80 cannot be achieved.
Contrast this with TR1:
mkl::mt19937 eng; std::tr1::normal_distribution<double> dist; std::tr1::variate_generator<mkl::mt19937,std::tr1::normal_distribution<double> > rng(eng,dist); double n = rng();
This - admittedly much uglier from an aestethic point of view - design allowed optimization by specializing the variate_generator template for mkl::mt19937:
namespace std { namespace tr1 {
template<>
class variate_generator<mkl::mt19937,std::tr1::normal_distribution<double> > { .... };
} }
A similar customization point is missing in the C++0x design and prevents the optimized vectorized version to be used.
Suggested resolution:
Add a customization point to the distribution concept. Instead of the variate_generator template this can be done through a call to a free function generate_variate found by ADL instead of operator() of the distribution:
template <RandomNumberDistribution, class RandomNumberEngine> typename RandomNumberDistribution ::result_type generate_variate(RandomNumberDistribution const& dist, RandomNumberEngine& eng);
This function can be overloaded for optimized enginges like mkl::mt19937.
[ 2009-10 Santa Cruz: ]
NAD Future. No time to add this feature for C++0X.
Proposed resolution:
Section: 17 [library] Status: NAD Submitter: Sean Hunt Opened: 2009-10-13 Last modified: 2009-10-20
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Discussion:
I wasn't sure whether to consider this a library or a language issue, because the issue is I think it's incorrectly categorized as being part of the library, so I thought I'd send a message to both of you and let you sort it out.
Most reserved identifiers are treated as unilaterally available to the implementation, such as to implement language extensions, or provide macros documenting its functionality. However, the requirements for reserved identifers are in 17.6.3.3 [reserved.names], which are a subsection of 17.6.3 [constraints]. 17.6.3.1 [constraints.overview] appears only to apply to "C++ programs that use the facilities of the C++ standard library", meaning that, in theory, all implementations are erroneous in having any non-standard identifiers predefined for programs that do not, at some point, include a standard library header.
Furthermore, it's unclear whether the use of certain identifiers is UB or results in an ill-formed program. In particular, 17.6.3.3.1 [macro.names] uses a "shall not", where 17.6.3.3.2 [global.names] says that names are "reserved to the implementation". 17.6.3.3 [reserved.names] seems only to cover the instance of a name being described as "reserved", so are implementations required to diagnose a program that performs, as an example, "#undef get"?
[ 2009 Santa Cruz: ]
Move to NAD. There may in theory be multiple interpretations possible, but there's no evidence that this causes any genuine problems or uncertainty about what implementations are allowed to do. We do not believe this rises to the level of a defect.
Proposed resolution:
Section: 25 [algorithms] Status: NAD Future Submitter: Alisdair Meredith Opened: 2009-10-15 Last modified: 2010-03-08
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Discussion:
The library has many algorithms that take a source range represented by a pair of iterators, and the start of some second sequence given by a single iterator. Internally, these algorithms will produce undefined behaviour if the second 'range' is not as large as the input range, but none of the algorithms spell this out in Requires clauses, and there is no catch-all wording to cover this in clause 17 or the front matter of 25.
There was an attempt to provide such wording in paper n2944 but this seems incidental to the focus of the paper, and getting the wording of this issue right seems substantially more difficult than the simple approach taken in that paper. Such wording will be removed from an updated paper, and hopefully tracked via the LWG issues list instead.
It seems there are several classes of problems here and finding wording to solve all in one paragraph could be too much. I suspect we need several overlapping requirements that should cover the desired range of behaviours.
Motivating examples:
A good initial example is the swap_ranges algorithm. Here there is a clear requirement that first2 refers to the start of a valid range at least as long as the range [first1, last1). n2944 tries to solve this by positing a hypothetical last2 iterator that is implied by the signature, and requires distance(first2,last2) < distance(first1,last1). This mostly works, although I am uncomfortable assuming that last2 is clearly defined and well known without any description of how to obtain it (and I have no idea how to write that).
A second motivating example might be the copy algorithm. Specifically, let us image a call like:
copy(istream_iterator<int>(is),istream_iterator(),ostream_iterator<int>(os));
In this case, our input iterators are literally simple InputIterators, and the destination is a simple OutputIterator. In neither case am I happy referring to std::distance, in fact it is not possible for the ostream_iterator at all as it does not meet the requirements. However, any wording we provide must cover both cases. Perhaps we might deduce last2 == ostream_iterator<int>{}, but that might not always be valid for user-defined iterator types. I can well imagine an 'infinite range' that writes to /dev/null and has no meaningful last2.
The motivating example in n2944 is std::equal, and that seems to fall somewhere between the two.
Outlying examples might be partition_copy that takes two output iterators, and the _n algorithms where a range is specified by a specific number of iterations, rather than traditional iterator pair. We should also not accidentally apply inappropriate constraints to std::rotate which takes a third iterator that is not intended to be a separate range at all.
I suspect we want some wording similar to:
For algorithms that operate on ranges where the end iterator of the second range is not specified, the second range shall contain at least as many elements as the first.
I don't think this quite captures the intent yet though. I am not sure if 'range' is the right term here rather than sequence. More awkwardly, I am not convinced we can describe an Output sequence such as produce by an ostream_iterator as "containing elements", at least not as a precondition to the call before they have been written.
Another idea was to describe require that the trailing iterator support at least distance(input range) applications of operator++ and may be written through the same number of times if a mutable/output iterator.
We might also consider handling the case of an output range vs. an input range in separate paragraphs, if that simplifies how we describe some of these constraints.
[ 2009-11-03 Howard adds: ]
Moved to Tentatively NAD Future after 5 positive votes on c++std-lib.
Rationale:
Does not have sufficient support at this time. May wish to reconsider for a future standard.
Proposed resolution:
Section: 20.7.4.3 [meta.unary.prop] Status: NAD Editorial Submitter: David Abrahams Opened: 2009-10-16 Last modified: 2010-03-08
View all other issues in [meta.unary.prop].
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Discussion:
Table 43 defines a number of traits that yield true for arrays of class types with the trait's property, but not arrays of other types with that property. For example, has_trivial_default_constructor:
T is a trivial type (3.9) or a class type with a trivial default constructor (12.1) or an array of such a class type.
[ 2009-10 post-Santa Cruz: ]
An array of a trivial type is a trivial type.
Mark as Tentatively NAD Editorial. The wording is OK as is, since an array of a trivial type is a trivial type, but the wording as proposed might be clearer.
Rationale:
The wording is OK as is, since an array of a trivial type is a trivial type. Project editor may wish to accept the suggested wording as editorial.
Proposed resolution:
Change all the traits in question following this pattern:
T is a trivial type (3.9) or a class type with a trivial default constructor (12.1), or an array of such aclasstype.
i.e., add a comma and delete a "class."
Section: 23 [containers] Status: NAD Future Submitter: Herb Sutter Opened: 2009-10-21 Last modified: 2009-10-21
View all other issues in [containers].
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Discussion:
See N2980.
Proposed resolution:
Section: 26.6.2.6 [valarray.cassign] Status: NAD Submitter: Daniel Krügler Opened: 2009-10-22 Last modified: 2009-10-26
View all other issues in [valarray.cassign].
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Discussion:
Addresses JP 64
During the additions of initializer_list overloads basic_string added
basic_string& operator+=(initializer_list<charT>);
but
valarray<T>& operator+= (initializer_list<T>);
was not defined.
[ Daniel adds on opening: ]
Recommend NAD. The operator+= overload of basic_string behaves as-if calling append, which is completely different in meaning as the existing operator+= overloads in valarray which just sum the value or values to the existing elements. The suggestion to add a corresponding append function to valarray was not considered as appropriate and the request was withdrawn (c++std-lib-24968).
[ 2009-10 Santa Cruz: ]
Mark as NAD. Request has been withdrawn by NB.
Proposed resolution:
Add to 26.6.2.6 [valarray.cassign]:
valarray<T>& operator+= (initializer_list<T>);
Section: 30.6 [futures] Status: NAD Editorial Submitter: Detlef Vollmann Opened: 2009-10-22 Last modified: 2010-03-14
View all other issues in [futures].
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Discussion:
With the addition of async(), a future might be associated with a function that is not running in a different thread but is stored to by run synchronously on the get() call. It's not clear what the wait() functions should do in this case.
Suggested resolution:
Throw an exception.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3058.
Proposed resolution:
Section: 23.3.6.2 [vector.capacity] Status: NAD Submitter: David Abrahams Opened: 2009-10-24 Last modified: 2010-03-08
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Discussion:
If v is a vector, I think repeated calls to v.resize( v.size() + 1 ) should be amortized O(1), but it's not clear that's true from the text of the standard:
void resize(size_type sz);Effects: If sz < size(), equivalent to erase(begin() + sz, end());. If size() < sz, appends sz - size() default constructed elements to the sequence.
Seems to me if we used push_back instead of appends, we might be giving the guarantee I'd like. Thoughts?
[ 2009-11-10 Howard adds: ]
Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below.
Proposed resolution:
In 23.3.6.2 [vector.capacity]/10, change
void resize(size_type sz);Effects: If sz < size(), equivalent to erase(begin() + sz, end());. If size() < sz,appends sz - size() default constructed elements to the sequenceequivalent to sz - size() consecutive evaluations of push_back(T()).
Rationale:
The description in terms of push_back led some to believe that one could expect the exact same growth pattern from both resize and push_back (e.g.) which could lead to sub-optimal implementations. Additionally, 23.3.6 [vector], p1 includes a statement that this container "supports (amortized) constant time insert and erase operations at the end;", therefore addressing the concern of this issue.
Section: 23.5 [unord] Status: NAD Editorial Submitter: Herb Sutter Opened: 2009-10-25 Last modified: 2010-03-14
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Discussion:
See N2986.
[ 2010-01-22 Alisdair Opens. ]
[ 2010-01-24 Alisdair provides wording. ]
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3068.
Proposed resolution:
Apply paper N2986.
Section: 27.8.1.1 [stringbuf.cons] Status: NAD Submitter: Martin Sebor Opened: 2009-10-29 Last modified: 2010-03-08
View all other issues in [stringbuf.cons].
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Discussion:
I just came across issue 1204 -- Global permission to move, which seems to address the concern raised by the example in c++std-lib-25030.
IIUC, the example violates the permission to assume that arguments bound to rvalue references are unnamed temporaries granted to implementations by the resolution of issue 1204 - Global permission to move.
I.e., the ostringstream(ostringstream &&rhs) ctor can leave the rhs pointers pointing to the newly constructed object's buffer just as long as the dtor doesn't change or invalidate the buffer. The caller may not make any assumptions about rhs after the move beyond it being safe to destroy or reassign.
So unless I misunderstood something, I still think the basic_stringbuf move ctor is overspecified. Specifically, I think the third sentence in the Effects clause and the last 6 bullets in the Postconditions clause can, and IMO should, be stricken.
[ 2010-01-31 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
The sense of 1251 appears to be that the basic_stringbuf move constructor offers more guarantees than the minimum. This is true, and quite correct. The additional words guarantee that the internal buffer has genuinely transferred from one object to another, and further operations on the original will not affect the buffer of the newly created object. This is a very important guarantee, much as we see that a moved-from unique_ptr is guaranteed to be empty.
Proposed resolution:
Strike from 27.8.1.1 [stringbuf.cons]:
basic_stringbuf(basic_stringbuf&& rhs);Effects: Move constructs from the rvalue rhs. It is implementation-defined whether the sequence pointers in *this (eback(), gptr(), egptr(), pbase(), pptr(), epptr()) obtain the values which rhs had.
Whether they do or not, *this and rhs reference separate buffers (if any at all) after the construction.The openmode, locale and any other state of rhs is also copied.Postconditions: Let rhs_p refer to the state of rhs just prior to this construction and let rhs_a referto the state of rhs just after this construction.
- str() == rhs_p.str()
- gptr() - eback() == rhs_p.gptr() - rhs_p.eback()
- egptr() - eback() == rhs_p.egptr() - rhs_p.eback()
- pptr() - pbase() == rhs_p.pptr() - rhs_p.pbase()
- epptr() - pbase() == rhs_p.epptr() - rhs_p.pbase()
if (eback()) eback() != rhs_a.eback()if (gptr()) gptr() != rhs_a.gptr()if (egptr()) egptr() != rhs_a.egptr()if (pbase()) pbase() != rhs_a.pbase()if (pptr()) pptr() != rhs_a.pptr()if (epptr()) epptr() != rhs_a.epptr()
Section: 20.8.14.2.2 [func.wrap.func.mod] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2009-11-05 Last modified: 2010-03-08
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Discussion:
As of 20.8.14.2.2 [func.wrap.func.mod]/2+ we have the following prototype description:
template<class F, Allocator Alloc> void assign(F, const Alloc&);Effects: function(f, a).swap(*this)
Two things: First the concept debris needs to be removed, second and much more importantly, the effects clause is now impossible to satisfy, because there is no constructor that would match the parameter sequence (FunctionObject, Allocator) [plus the fact that no f and no a is part of the signature]. The most probable candidate is
template<class F, class A> function(allocator_arg_t, const A&, F);
and the effects clause needs to be adapted to use this signature.
[ 2009-11-13 Daniel brought wording up to date. ]
[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-11 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. It was noted that this issue was in partial conflict with 1288, and the two issues were merged in 1288. ]
Rationale:
Addressed by 1288.
Proposed resolution:
Change in 20.8.14.2.2 [func.wrap.func.mod] the complete prototype description as indicated
[ Question to the editor: Shouldn't there a paragraph number in front of the Effects clause? ]
template<class F,Allocator Allocclass A> void assign(F f, const Alloc& a);3 Effects: function(f, aallocator_arg, a, f).swap(*this)
Section: 23.2.3 [sequence.reqmts] Status: NAD Submitter: Sean Hunt Opened: 2009-11-05 Last modified: 2010-03-08
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Discussion:
According to 23.2.3 [sequence.reqmts], X(il) is equivalent to X(il.begin(), il.end()). Should it instead be equivalent to X(move_iterator(il.begin()), move_iterator(il.end())) so that needless copies are not made? This doesn't seem ideal either - it may make more sense to provide two overloads for the constructor, one for move and one for copy.
[ 2009-11-10 Howard adds: ]
I've moved this issue to Tentatively NAD after 5 positive votes on c++std-lib, and added a rationale below.
Proposed resolution:
Rationale:
There is no consensus at this time within EWG or CWG to make the required language changes. Therefore this is not something that the LWG can even consider. Should such language changes be made for a future standard, no doubt there would need to be an accompanying library impact survey.
Section: 28.4 [re.syn] Status: NAD Submitter: Howard Hinnant Opened: 2009-11-12 Last modified: 2010-03-08
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Discussion:
Addresses: UK 314
In Message c++std-lib-25529, Alisdair writes:
UK comment 314 requests rvalue swap overloads in a couple of places they were missed.
We have in general reverted to the single swap signature taking lvalue references, which could be seen as the alternative solution to UK 314, bringing consistency to the standard <g>
Either way, I no longer expect to see any work to resolve this comment - the work is complete and it should be either marked Rejected, or Accepted with Modifications (namely, removing all other rvalue swaps!)
[ Moved to Tentatively NAD after 5 positive votes on c++std-lib. ]
Proposed resolution:
Rationale:
We have in general reverted to the single swap signature taking lvalue references, which could be seen as the alternative solution to UK 314, bringing consistency to the standard.
Section: 18.10 [support.runtime] Status: NAD Submitter: Sean Hunt Opened: 2009-11-16 Last modified: 2010-03-08
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Discussion:
18.10 [support.runtime]/4 says that longjmp is undefined if unwinding by the mechanism used by catch and throw would invoke any nontrivial destructors. However, the text as written is rather vague, in particular when dealing with catch(...):
void foo() {
jump_buf buf;
non_trivial_dtor n1; // 1
if (!setjmp(buf)) {
non_trivial_dtor n2; // 2
try {
longjmp(buf, 1);
} catch (...) {
}
}
}
My interpretation of the meaning of 18.10 [support.runtime]/4 is that declaration 2, but not 1, would cause the longjmp to be undefined behavior. However, it's not entirely clear from the text. Arguably, replacing the setjmp and longjmp with catch would still cause the destructor for n1 to be called after the unwinding, which would lead to undefined behavior. This is clearly not an intended consequence of the wording. However, it is probably still UB, as n1 now has "indeterminate" value, and running its destructor on foo's exit will cause Bad Things.
Declarations 2 has a more interesting issue. The catch(...) muddles up the definition that uses throw and catch - if longjmp() were indeed a throw, control would never return to the setjmp. As such, n2's destructor wouldn't be called (except by the argument for n1, which is that the destructor would be called later as the frame was left in the normal control flow).
I suggest that paragraph 4 of 18.10 [support.runtime] should be replaced with the following, or something that reads better but has the same effect:
The function signature longjmp(jmp_buf jbuf, int val) has more restricted behavior in this International Standard. A call to longjmp has undefined behavior if any non-trivial destructors would be called were the longjmp call replaced with a throw-expression whose nearest matching handler were a (possibly imaginary) function-try-block on the function containing the corresponding setjmp call.
[ 2009-11-17 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]
Proposed resolution:
Change 18.10 [support.runtime]/4:
The function signature longjmp(jmp_buf jbuf, int val) has more restricted behavior in this International Standard.A setjmp/longjmp call pair has undefined behavior if replacing the setjmp and longjmp by catch and throw would invoke any non-trivial destructors for any automatic objects.A call to longjmp has undefined behavior if any non-trivial destructors would be called were the longjmp call replaced with a throw-expression whose nearest matching handler were a (possibly imaginary) function-try-block on the function containing the corresponding setjmp call.
Rationale:
In the given example, it is clear that it is only n2 and not n1 that is destroyed by the longjmp.
At this late stage in the standards process, we are focusing on issues that impact users or implementers. Trying to rewrite complex wording just for the sake of improved clarity is likely to do more harm than good.
Section: 30.6.7 [futures.shared_future] Status: NAD Editorial Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2010-03-14
View all other issues in [futures.shared_future].
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Discussion:
If a shared_future is constructed with the result of an async call with a deferred function, and two or more copies of that shared_future are created, with multiple threads calling get(), it is not clear which thread runs the deferred function. 30.6.7 [futures.shared_future]p22 from N3000 says (minus editor's note):
E ffects: if the associated state contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready.
In the absence of wording to the contrary, this implies that every thread that calls wait() will execute the deferred function.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3058.
Proposed resolution:
Replace 30.6.7 [futures.shared_future]p22 with the following:
Effects: If the associated state
contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready.was created by a promise or packaged_task object, block until the associated state is ready. If the associated state is associated with a thread created for an async call (30.6.9 [futures.async]), as if associated-thread.join().If the associated state contains a deferred function, calls to wait() on all shared_future objects that share the same associated state are serialized. The first call to wait() that shares a given associated state executes the deferred function and stores the return value or exception in the associated state.
Synchronization: if the associated state was created by a promise object, the completion of set_value() or set_exception() to that promise happens before (1.10 [intro.multithread]) wait() returns. If the associated state was created by a packaged_task object, the completion of the associated task happens before wait() returns. If the associated state is associated with a thread created for an async call (30.6.9 [futures.async]), the completion of the associated thread happens-before wait() returns.
If the associated state contained a deferred function, the invocation of the deferred function happens-before any call to wait() on a future that shares that state returns.
Section: 30.6.4 [futures.state] Status: NAD Editorial Submitter: Anthony Williams Opened: 2009-11-18 Last modified: 2010-03-14
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Discussion:
The current description of the associated state in 30.6.4 [futures.state] does not allow for futures created by an async call. The description therefore needs to be extended to cover that.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3058.
Proposed resolution:
Add a new sentence to 30.6.4 [futures.state] p2:
2 This associated state consists of some state information and some (possibly not yet evaluated) result, which can be a (possibly void) value or an exception. If the associated state was created by a call to async (30.6.9 [futures.async]) then it may also contain a deferred function or an associated thread.
Add an extra bullet to 30.6.4 [futures.state] p3:
The result of an associated state can be set by calling:
- promise::set_value,
- promise::set_exception,
or- packaged_task::operator()
., or- a call to async (30.6.9 [futures.async]).
Section: 30.6.5 [futures.promise] Status: NAD Editorial Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2010-03-14
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Discussion:
The definitions of promise::set_value need tidying up, the synopsis says:
// setting the result void set_value(const R& r); void set_value(see below);
Why is the first one there? It implies it is always present for all specialisations of promise, which is not true.
The definition says:
void set_value(const R& r); void promise::set_value(R&& r); void promise<R&>::set_value(R& r); void promise<void>::set_value();
The lack of qualification on the first one again implies it's present for all specialisations, again not true.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3058.
Proposed resolution:
Change the synopsis in 30.6.5 [futures.promise]:
// setting the resultvoid set_value(const R& r);void set_value(see below);
And the definition be changed by qualifying the first signature:
void promise::set_value(const R& r); void promise::set_value(R&& r); void promise<R&>::set_value(R& r); void promise<void>::set_value();
Section: 30.6.6 [futures.unique_future] Status: NAD Editorial Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2010-03-11
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Discussion:
30.6.6 [futures.unique_future]/3 says:
The effect of calling any member function other than the destructor or the move-assignment operator on a future object for which valid() == false is undefined.
This means calling future::valid() is undefined unless it will return true, so you can only use it if you know the answer!
[ 2009-12-08 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Change 30.6.6 [futures.unique_future]/3:
The effect of calling any member function other than the destructor, or the move-assignment operator, or valid, on a future object for which valid() == false is undefined.
Section: 30.6.8 [futures.atomic_future] Status: NAD Editorial Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2010-03-11
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Discussion:
In 30.6.8 [futures.atomic_future] this constructor:
atomic_future(future<R>&&);
is declared in the synopsis, but not defined. Instead n2997 defines:
atomic_future(const future<R>&& rhs);
and n3000 defines
atomic_future(atomic_future<R>&& rhs);
both of which are wrong. The constructor definition should be changed to match the synopsis.
[ 2009-12-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Adjust the signature above 30.6.8 [futures.atomic_future]/6 like so:
atomic_future(atomic_future<R>&& rhs);
Section: 30.6 [futures] Status: NAD Editorial Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2010-03-14
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Discussion:
30.6.6 [futures.unique_future]/1 should be updated to mention async.
30.6.7 [futures.shared_future]/1 should also be updated for async. That paragraph also says
... Its value or exception can be set by use of a shared_future, promise (30.6.5 [futures.promise]), or packaged_task (30.6.10 [futures.task]) object that shares the same associated state.
How can the value be set by a shared_future?
30.6.8 [futures.atomic_future]/1 says
An atomic_future object can only be created by use of a promise (30.6.5 [futures.promise]) or packaged_task (30.6.10 [futures.task]) object.
which is wrong, it's created from a std::future, which could have been default-cosntructed. That paragraph should be closer to the text of 30.6.7 [futures.shared_future]/1, and should also mention async.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3058.
Proposed resolution:
Section: 25 [algorithms] Status: NAD Future Submitter: Igor Semenov Opened: 2009-12-07 Last modified: 2010-03-08
View all other issues in [algorithms].
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Discussion:
Motivation and Scope
Splitting strings into parts by some set of delimiters is an often task, but there is no simple and generalized solution in C++ Standard. Usually C++ developers use std::basic_stringstream<> to split string into parts, but there are several inconvenient restrictions:
Impact on the Standard
This algorithm doesn't interfere with any of current standard algorithms.
Design Decisions
This algorithm is implemented in terms of input/output iterators. Also, there is one additional wrapper for const CharType * specified delimiters.
Example implementation
template< class It, class DelimIt, class OutIt >
void split( It begin, It end, DelimIt d_begin, DelimIt d_end, OutIt out )
{
while ( begin != end )
{
It it = std::find_first_of( begin, end, d_begin, d_end );
*out++ = std::make_pair( begin, it );
begin = std::find_first_of( it, end, d_begin, d_end,
std::not2( std::equal_to< typename It::value_type >() ) );
}
}
template< class It, class CharType, class OutIt >
void split( It begin, It end, const CharType * delim, OutIt out )
{
split( begin, end, delim, delim + std::strlen( delim ), out );
}
Usage
std::string ss( "word1 word2 word3" );
std::vector< std::pair< std::string::const_iterator, std::string::const_iterator > > v;
split( ss.begin(), ss.end(), " ", std::back_inserter( v ) );
for ( int i = 0; i < v.size(); ++i )
{
std::cout << std::string( v[ i ].first, v[ i ].second ) << std::endl;
}
// word1
// word2
// word3
[ 2010-01-22 Moved to Tentatively NAD Future after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
The LWG is not considering completely new features for standardization at this time. We would like to revisit this good suggestion for a future TR and/or standard.
Proposed resolution:
Add to the synopsis in 25.1 [algorithms.general]:
template< class ForwardIterator1, class ForwardIterator2, class OutputIterator >
void split( ForwardIterator1 first, ForwardIterator1 last,
ForwardIterator2 delimiter_first, ForwardIterator2 delimiter_last,
OutputIterator result );
template< class ForwardIterator1, class CharType, class OutputIterator >
void split( ForwardIterator1 first, ForwardIterator1 last,
const CharType * delimiters, OutputIterator result );
Add a new section [alg.split]:
template< class ForwardIterator1, class ForwardIterator2, class OutputIterator > void split( ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 delimiter_first, ForwardIterator2 delimiter_last, OutputIterator result );1. Effects: splits the range [first, last) into parts, using any element of [delimiter_first, delimiter_last) as a delimiter. Results are pushed to output iterator in the form of std::pair<ForwardIterator1, ForwardIterator1>. Each of these pairs specifies a maximal subrange of [first, last) which does not contain a delimiter.
2. Returns: nothing.
3. Complexity: Exactly last - first assignments.
template< class ForwardIterator1, class CharType, class OutputIterator > void split( ForwardIterator1 first, ForwardIterator1 last, const CharType * delimiters, OutputIterator result );1. Effects: split the range [first, last) into parts, using any element of delimiters (interpreted as zero-terminated string) as a delimiter. Results are pushed to output iterator in the form of std::pair<ForwardIterator1, ForwardIterator1>. Each of these pairs specifies a maximal subrange of [first, last) which does not contain a delimiter.
2. Returns: nothing.
3. Complexity: Exactly last - first assignments.
Section: 20.2.1 [utility.arg.requirements] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2009-12-12 Last modified: 2010-03-08
View all other issues in [utility.arg.requirements].
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Discussion:
Addresses UK 150
There is on going confusion over what one can and can not do with a moved-from object (e.g. UK 150, 910). This issue attempts to clarify that moved-from objects are valid objects with an unknown state.
[ 2010-01-22 Wording tweaked by Beman. ]
[ 2010-01-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-01-23 Alisdair opens: ]
I'm afraid I must register an objection.
My primary objection is that I have always been opposed to this kind of a resolution as over-constraining. My preferred example is a call implementing the pImpl idiom via unique_ptr. Once the pImpl has been moved from, it is no longer safe to call the vast majority of the object's methods, yet I see no reason to make such a type unusable in the standard library. I would prefer a resolution along the lines suggested in the UK comment, which only requires that the object can be safely destroyed, and serve as the target of an assignment operator (if supported.)
However, I will not hold the issue up if I am a lone dissenting voice on this (yes, that is a call to hear more support, or I will drop that objection in Pittsburgh)
With the proposed wording, I'm not clear what the term 'valid object' means. In my example above, is a pImpl holding a null pointer 'valid'? What about a float holding a signalling NaN? What determines if an object is valid? Without a definition of a valid/invalid object, I don't think this wording adds anything, and this is an objection that I do want resolved.
[ 2010-01-24 Alisdair removes his objection. ]
[ 2010-01-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Reopened. The wording here has been merged into 1309. ]
[ 2010-02-10 Moved to Tentatively NAD Editorial after 5 postive votes on c++std-lib. Rationale added below. ]
Rationale:
This issue is now addressed by 1309.
Proposed resolution:
Change the follwing tables in 20.2.1 [utility.arg.requirements] as shown:
Table 33 — MoveConstructible requirements [moveconstructible] Expression Post-condition T t(rv) t is equivalent to the value of rv before the construction. [Note: There is no requirement on the value of rv after the construction.rv remains a valid object. Its state is unspecified. — end note]
Table 35 — MoveAssignable requirements [moveassignable] Expression Return type Return value Post-condition t = rv T& t t is equivalent to the value of rv before the assigment. [Note: There is no requirement on the value of rv after the assignment.rv remains a valid object. Its state is unspecified. — end note]
Section: 30.6.5 [futures.promise] Status: NAD Editorial Submitter: Jonathan Wakely Opened: 2009-12-18 Last modified: 2010-03-14
View all other issues in [futures.promise].
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Discussion:
In 30.6.5 [futures.promise]
Does promise<R>::set_value return normally if the copy/move constructor of R throws?
The exception could be caught and set using promise<R>::set_exception, or it could be allowed to leave the set_value call, but it's not clear which is intended. I suggest the exception should not be caught.
N.B. This doesn't apply to promise<R&>::set_value or promise<void>::set_value because they don't construct a new object.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3058.
Proposed resolution:
Change 30.6.5 [futures.promise]/18:
18 Throws: future_error if its associated state is already ready or, for the first version an exception thrown by the copy constructor of R, or for the second version an exception thrown by the move constructor of R.
Section: 20.9.10.3 [unique.ptr.runtime] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2009-12-20 Last modified: 2010-03-14
View all issues with NAD Editorial status.
Discussion:
Addresses UK 211
As a response to UK 211 LWG issue 1021 has replaced the unspecified-pointer-type by nullptr_t to allow assignment of type-safe null-pointer literals in the non-array form of unique_ptr::operator=, but did not the same for the specialization for arrays of runtime length. But without this parallel change of the signature we have a status quo, where unique_ptr<T[], D> declares a member function which is completely unspecified.
[ 2009-12-21 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
In 20.9.10.3 [unique.ptr.runtime], class template unique_ptr<T[], D> synopsis, change as indicated:
// assignment unique_ptr& operator=(unique_ptr&& u); unique_ptr& operator=(unspecified-pointer-typenullptr_t);
Section: 23.4.1 [map] Status: NAD Submitter: Alisdair Meredith Opened: 2009-12-22 Last modified: 2010-03-08
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Discussion:
The container class templates map and multimap both contain a nested type called value_compare, that is used to compare the value_type pair elements, an adaptor of the user-supplied comparison function-like object.
I believe these types are over-specified, as we require a distinct type for each template, even though the allocator plays no part in the comparator, and map and multimap value_compare classes could easily be shared. The benefits are similar to the SCARY iterator proposal (although on a much smaller scale!) but unlike SCARY, this is not a QoI issue today but actively prohibited.
If the value_compare classes were marked 'exposition only', a vendor would be free to experiment with implementations that do not produce so many template instantiations with negligible impact on conforming programs. (There is a minor risk that programs could no longer portably overload functions taking value_compare types. This scenario is extremely unlikely outside conformance suites.)
(Note that there are no similar problems for unordered maps, nor any of the set variants)
[ 2010-01-31 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
The value_compare specification is an unfortunate bit from the past that we have to live with. Fortunately vendors can work around the problems mentioned in this issue.
Proposed resolution:
p2 23.4.1 [map]: Above the declaration of class value_compare in the map synopsis, add:
template <class Key, class T, class Compare = less<Key>,
class Allocator = allocator<pair<const Key, T> > >
class map {
public:
// types:
...
// exposition only.
class value_compare
: public binary_function<value_type,value_type,bool> {
...
p2 23.4.2 [multimap]: Above the declaration of class value_compare in the map synopsis, add:
template <class Key, class T, class Compare = less<Key>,
class Allocator = allocator<pair<const Key, T> > >
class multimap {
public:
// types:
...
// exposition only.
class value_compare
: public binary_function<value_type,value_type,bool> {
...
Section: 30.6.5 [futures.promise] Status: NAD Editorial Submitter: Jonathan Wakely Opened: 2009-12-26 Last modified: 2010-03-11
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Discussion:
30.6.5 [futures.promise]/12 defines the effects of promise::swap(promise&) as
void swap(promise& other);12 Effects: swap(*this, other)
and 30.6.5 [futures.promise]/25 defines swap(promise<R&>, promise<R>&) as
template <class R> void swap(promise<R>& x, promise<R>& y);25 Effects: x.swap(y).
[ 2010-01-13 Daniel added "Throws: Nothing." ]
[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Change 30.6.5 [futures.promise] paragraph 12
void swap(promise& other);12 Effects:
swap(*this, other)Exchanges the associated states of *this and other.13 ...
Throws: Nothing.
Section: 23.2.3 [sequence.reqmts] Status: NAD Editorial Submitter: Nicolai Josuttis Opened: 2010-01-01 Last modified: 2010-03-08
View all other issues in [sequence.reqmts].
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Discussion:
I propose that clear() be defined to be equivalent to erase(begin(),end()) except not using copy or move of elements.
To: C++ libraries mailing list
Message c++std-lib-26465and specifiying as post: size()==0 might also not be appropriate because forward-Lists provide no size(), this it should be: post: empty()==true
Bjarne Stroustrup schrieb/wrote:
To: C++ libraries mailing list
Message c++std-lib-26458in table 94 we define clear() as:
a.clear() void erase(begin(), end()) post: size() == 0Now erase requires assignment (MoveAssignable) which makes sense if we have to move an element, but why should that be required from clear() where all elements are destroyed?
[ 2010-01-23 Alisdiar provides wording. ]
[ 2010-01-30 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-01-30 Daniel opens: ]
First, I read the newly proposed spec for clear() that it does in general not invalidate a previous past-the-end iterator value, but deque says in 23.3.2.3 [deque.modifiers] for the semantics of erase that erasures at the end will invalidate the past-the-end iterator. With removal of a direct binding between clear() and erase() there seem to be some fixes necessary. One way to fix that would be to mention in Table 94 that this "may also invalidate the past-the-end iterator" and then to mention for all specific containers where this does not happen, the exception, [1] e.g. in std::vector. std::vector has no own specification of clear() and one aspect of the closed issue 1102 was to realize just that (indirectly via erase). IMO we should now add an extra specification for clear(). Btw.: std::vector::erase reads to me that it would invalidate previous past-the-end values (and that seems correct in general).
Before I will provide explicit wording, I would like to discuss these points.
[1] std::list does fortunately specify that clear does not invalidate the past-the-end iterator.
[ 2010-02-08 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
Solved as proposed by LWG 704.
Proposed resolution:
Change 23.2.1 [container.requirements.general]/10:
Unless otherwise specified (see 23.2.4.1, 23.2.5.1, 23.3.2.3, and 23.3.6.4) all container types defined in this Clause meet the following additional requirements:
- ..
- no erase(), clear(), pop_back() or pop_front() function throws an exception.
- ...
Replace the following words from Table 94 — Sequence container requirements (in addition to container) in 23.2.3 [sequence.reqmts]:
Table 94 — Sequence container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditiona.clear() void erase(begin(), end())
Destroys all elements in the container a. Invalidates all references, pointers, and iterators referring to the elements of a and may invalidate the past-the-end iterator.
post:size() == 0a.empty() == true.
Add a new paragraph after 23.3.3.4 [forwardlist.modifiers]/23:
void clear();23 Effects: Erases all elements in the range [begin(),end()).
Remarks: Does not invalidate past-the-end iterators.
Section: 23.2.4 [associative.reqmts], 23.2.5 [unord.req] Status: NAD Submitter: Nicolai Josuttis Opened: 2010-01-03 Last modified: 2010-03-15
View other active issues in [associative.reqmts].
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Discussion:
According to the new naming scheme introduced with N2680
vector<T> v; v.emplace(v.begin(),x,y,z)
now has a different semantics than
set<T> s; s.emplace(s.begin(),x,y,z);
While the version for vectors takes the first argument as position and the remaining for construction, the version for sets takes all arguments for construction.
IMO, this is a serious design mistake for a couple of reasons:
First, in principle, all STL member functions should have the same behavior with the same member function to avoid confusion and allow to write proper generic code.
In fact, when I write the following simple function template:
template <typename T>
void doEmplace (T& cont)
{
cont.emplace(cont.begin(),"nico","josuttis",42);
}
the semantics depends on the type of the container.
In addition, I also guess using the name emplace_hint() instead of emplace() for associative containers is a design mistake. According to my knowledge, it was a design goal of the original STL to provide ONE insert function, which works for ALL containers. This was insert(pos,val).
The trick to declare pos as a hint, allowed that we could implement a generic insert for all containers. Now, with the new emplace naming scheme, this trick is gone for the new kind of insertion.
I consider this to be a serious design penalty because once this is specified we can't fix that without breaking backward compatibility.
However, we have two choices for a fix:
[ 2010 Pittsburgh: Moved to NAD, rationale added below. ]
Rationale:
There was no consensus to make this change.
Proposed resolution:
In 23.2.5 [unord.req], change:
Table 96 — Associative container requirements (in addition to container) expression Return type Assertion/note pre-/post-condition Post-condition ... a_uniq.emplace_value(args) pair<iterator, bool> inserts a T object t constructed with std::forward<Args>(args)...
if and only if there is no element in the container with key equivalent to the key of t.
The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of t.logarithmic a_eq.emplace_value(args) iterator inserts a T object t constructed with std::forward<Args>(args)... and returns the iterator pointing to the newly inserted element. logarithmic a.emplace _hint(p,args)iterator equivalent to a.emplace_value(std::forward<Args>(args)...). Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. The const_iterator p is a hint pointing to where the search should start. Implementations are permitted to ignore the hint. logarithmic in general, but amortized constant if the element is inserted right after p ...
In 23.2.5 [unord.req], change:
Table 98 — Unordered associative container requirements (in addition to container) expression Return type Assertion/note pre-/post-condition Post-condition ... a_uniq.emplace_value(args) pair<iterator, bool> inserts a T object t constructed with std::forward<Args>(args)... if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of t. Average case O(1), worst case O(a_uniq.size()). a_eq.emplace_value(args) iterator inserts a T object t constructed with std::forward<Args>(args)... and returns the iterator pointing to the newly inserted element. Average case O(1), worst case O(a_eq.size()). a.emplace _hint(p,args)iterator equivalent to a.emplace_value(std::forward<Args>(args)...). Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. The const_iterator p is a hint pointing to where the search should start. Implementations are permitted to ignore the hint. Average case O(1), worst case O(a.size()). ...
In 23.4.1 [map], 23.4.3 [set], 23.5.1 [unord.map], 23.5.3 [unord.set], change:
// modifiers:
template <class... Args> pair<iterator, bool> emplace_value(Args&&... args);
template <class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
In 23.4.2 [multimap], 23.4.4 [multiset], 23.5.2 [unord.multimap], 23.5.4 [unord.multiset], change:
// modifiers:
template <class... Args> iterator emplace_value(Args&&... args);
template <class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
Section: 30.6.7 [futures.shared_future] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2010-01-23 Last modified: 2010-03-11
View all other issues in [futures.shared_future].
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Discussion:
The revised futures package in the current working paper simplified the is_ready/has_exception/has_value set of APIs, replacing them with a single 'valid' method. This method is used in many places to signal pre- and post- conditions, but that edit is not complete. Each method on a shared_future that requires an associated state should have a pre-condition that valid() == true.
[ 2010-01-28 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Insert the following extra paragraphs:
In 30.6.7 [futures.shared_future]
shared_future();4 Effects: constructs ...
Postcondition: valid() == false.
Throws: nothing.
void wait() const;Requires: valid() == true.
22 Effects: if the associated ...
template <class Rep, class Period> bool wait_for(const chrono::duration<Rep, Period>& rel_time) const;Requires: valid() == true.
23 Effects: if the associated ...
template <class Clock, class Duration> bool wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;Requires: valid() == true.
25 Effects: blocks until ...
Section: 30.6.8 [futures.atomic_future] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2010-01-23 Last modified: 2010-03-14
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Discussion:
The revised futures package in the current working paper simplified the is_ready/has_exception/has_value set of APIs, replacing them with a single 'valid' method. This method is used in many places to signal pre- and post- conditions, but that edit is not complete.
Atomic future retains the extended earlier API, and provides defined, synchronized behaviour for all calls. However, some preconditions and throws clauses are missing, which can easily be built around the new valid() api. Note that for consistency, I suggest is_ready/has_exception/has_value throw an exception if valid() is not true, rather than return false. I think this is implied by the existing pre-condition on is_ready.
[ 2010-01-23 See discussion starting with Message c++std-lib-26666. ]
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3058.
Proposed resolution:
Insert the following extra paragraphs:
In 30.6.8 [futures.atomic_future]
bool is_ready() const;17
PreconditionRequires: valid() == true.18 Returns: true only if the associated state is ready.
Throws: future_error with an error condition of no_state if the precondition is not met.
bool has_exception() const;Requires: valid() == true.
19 Returns: true only if the associated state is ready and contains an exception.
Throws: future_error with an error condition of no_state if the precondition is not met.
bool has_value() const;Requires: valid() == true.
20 Returns: true only if the associated state is ready and contains a value.
Throws: future_error with an error condition of no_state if the precondition is not met.
void wait() const;Requires: valid() == true.
22 Effects: blocks until ...
Throws: future_error with an error condition of no_state if the precondition is not met.
template <class Rep, class Period> bool wait_for(const chrono::duration<Rep, Period>& rel_time) const;Requires: valid() == true.
23 Effects: blocks until ...
24 Returns: true only if ...
Throws: future_error with an error condition of no_state if the precondition is not met.
template <class Clock, class Duration> bool wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;Requires: valid() == true.
25 Effects: blocks until ...
26 Returns: true only if ...
Throws: future_error with an error condition of no_state if the precondition is not met.
Section: 18.8.5 [propagation] Status: NAD Editorial Submitter: Daniel Krügler Opened: 2010-01-26 Last modified: 2010-03-14
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Discussion:
The current requirements for a conforming implementation of std::exception_ptr (18.8.5 [propagation]/1-6) does not clarify whether the expression
e1 != e2 e1 != nullptr
with e1 and e2 being two values of type std::exception_ptr are supported or not. Reason for this oddity is that the concept EqualityComparable does not provide operator !=.
For the same reason programmers working against the types X::pointer, X::const_pointer, X::void_pointer, and X::const_void_pointer of any allocator concept X (20.2.5 [allocator.requirements]/4 + Table 40) in a generic context can not rely on the availability of the != operation, which is rather unnatural and error-prone.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3073.
Proposed resolution:
Section: 25.4.7 [alg.min.max] Status: NAD Submitter: Niels Dekker Opened: 2010-02-02 Last modified: 2010-03-15
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Discussion:
In San Francisco, June 2008, N2722 was adopted, replacing the variadic templates min, max, and minmax by overloads that have an initializer_list<T> parameter. The paper showed benchmark results wherein initializer_list versions of min appeared to outperform the corresponding variadic template. Unfortunately, in October 2009 a very serious error was detected in the benchmark. (c++std-lib-25210). In fact, an initializer_list<T> version of min often appears to perform worse than the corresponding variadic template, especially when T has an expensive copy constructor (c++std-lib-25253, http://www.xs4all.nl/~nd/dekkerware/issues/n2772_fix).
IMO, the biggest problem of the initializer_list overloads is that they pass and return T objects by value. Which has the following consequences:
In the future, this problem might be solvable by using an initializer_list of const references, instead:
const T& min(initializer_list<const T&>); const T& max(initializer_list<const T&>); pair<const T&, const T&> minmax(initializer_list<const T&>);
It is unlikely that C++0x will support initializer_list<const T&>, but technically it seems possible to add such a language feature after C++0x (c++std-core-15428).
Variadic templates of min, max, and minmax, as proposed by N2551 (Sylvain Pion), do have some other advantages over initializer_list overloads:
Unfortunately, the variadic templates of min, max, and minmax may still need further improvement, before having them in the Standard Library. Especially the optional Compare parameter appears to be a concern. So for this moment I recommend to keep both versions out of C++0x, and postpone further discussion until after C++0x.
[ 2010 Pittsburgh: Discussed and the LWG still prefers the initializer list solution of N2772. ]
Rationale:
We prefer the solution of N2772 which will be reapplied.Proposed resolution:
Remove both variadic templates and initializer_list overloads of min, max, and minmax from the synopsis in 25.1 [algorithms.general] and from 25.4.7 [alg.min.max].
[ Note: This proposed resolution will resolve LWG 915 as NAD. ]
Section: 24.2.5 [forward.iterators] Status: NAD Editorial Submitter: Alisdair Meredith Opened: 2010-02-07 Last modified: 2010-03-14
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Discussion:
The following example demonstrates code that would meet the guarantees of a Forward Iterator, but only permits a single traversal of the underlying sequence:
template< typename ForwardIterator>
struct bad_iterator {
shared_ptr<ForwardIterator> impl;
bad_iterator( ForwardIterator iter ) {
: impl{new ForwardIterator{iter} }
{
}
auto operator*() const -> decltype(*ForwardIterator{}) {
return **impl;
}
auto operator->() const -> ForwardIterator {
return *impl;
}
auto operator==(bad_iterator const & rhs) {
return impl == rhs.impl;
}
auto operator++() {
++(*imp);
}
// other operations as necessary...
};
Here, we use shared_ptr to wrap a forward iterator, so all iterators constructed from the same original iterator share the same 'value', and incrementing any one copy increments all others.
There is a missing guarantee, expressed by the following code sequence
FwdIter x = seq.begin(); // obtain forward iterator from a sequence FwdIter y = x; // copy the iterator assert(x == y); // iterators must be the same ++x; // increment *just one* iterator assert(x != y); // iterators *must now be different* ++y; // increment the other iterator assert(x == y); // now the iterators must be the same again
That inequality in the middle is an essential guarantee. Note that this list is simplified, as each assertion should also note that they refer to exactly the same element (&*x == &*y) but I am not complicating the issue with tests to support proxy iterators, or value types overloading unary operator+.
I have not yet found a perverse example that can meet this additional constraint, and not meet the multi-pass expectations of a Forward Iterator without also violating other Forward Iterator requirements.
Note that I do not yet have standard-ready wording to resolve the problem, as saying this neatly and succinctly in 'standardese' is more difficult.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3066.
Proposed resolution:
Section: 26.5.7.1 [rand.util.seedseq] Status: NAD Submitter: Daniel Krügler Opened: 2010-02-07 Last modified: 2010-03-09
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Discussion:
The Seed sequence requirements (26.5.1.2 [rand.req.seedseq]) require the existence of a member function
template<typename OutputIterator> void param(OutputIterator ob);
The fact that this function returns void instead of the value of ob after accepting the sequence data leads to the same problem as in issue 865 - In case of pure output iterators there is no way to serialize further data into that data sink.
[ 2010-02-07 Howard adds: ]
At the time this issue was opened, the suggested changes are with respect to an anticipated draft which does not yet exist.
[ 2010 Pittsburgh: ]
No technical counterarguments, but it is simply too late in the process to make this change at this point.
Proposed resolution:
In Table 109 — Seed sequence requirements, expression "r.param(ob)"
change the
Return type entry:
voidOutputIterator
In 26.5.7.1 [rand.util.seedseq], class seed_seq synopsis change
template<class OutputIterator>voidOutputIterator param(OutputIterator dest) const;
Section: 18.2 [support.types] Status: NAD Submitter: Sean Hunt Opened: 2010-02-07 Last modified: 2010-03-15
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Discussion:
Currently, the 18.2 [support.types]/3 allows NULL to be any null pointer constant. The footnote marks that 0 or 0L might be appropriate. However, this definition also allows the implementation to define NULL to be nullptr. This may lead to overload and conversion issues more serious than with the C++98 version:
void f(void*);
void f(int);
void g()
{
// calls f(int) if NULL is integral
// calls f(void*) if NULL is nullptr
f(NULL);
}
Possible resolutions:
Making NULL nullptr would improve code correctness, and breaking backwards compatibility shouldn't be a huge concern as NULL shouldn't be used except as a null pointer constant anyways.
[ 2010-02-10 Chris provided wording. ]
[ 2010 Pittsburgh: Moved to NAD, rationale added below. ]
Rationale:
The LWG discussed the proposed resolution and several other options. There was no concensus to make this or any other changes.
Proposed resolution:
18.2 [support.types]
3 The macro NULL is defined to be nullptr.
is an implementation-defined C++ null pointer constant in this International Standard (4.10).196
196) Possible definitions include 0 and 0L, but not (void*)0.
20.9.14 [c.malloc]
7 The contents are the same as the Standard C library header <string.h>, with the change to memchr() specified in 21.6 and the macro NULL defined to be nullptr.
20.11 [date.time]
2 The contents are the same as the Standard C library header <time.h>.232 except the macro NULL, which is defined to be nullptr. The functions asctime, ctime, gmtime, and localtime are not required to avoid data races (17.6.4.8).
22.6 [c.locales]
2 The contents are the same as the Standard C library header <locale.h> except the macro NULL, which is defined to be nullptr.
C.2.2.4 [diff.null]
1 The macro NULL, defined in any of <clocale>, <cstddef>, <cstdio>, <cstdlib>, <cstring>, <ctime>, or <cwchar>, is nullptran implementation-defined C++ null pointer constant in this International Standard (18.2).
Section: 30.6.9 [futures.async] Status: NAD Editorial Submitter: Jonathan Wakely Opened: 2009-02-09 Last modified: 2010-03-11
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Discussion:
Both overloads of async return future<typename F::result_type> which requires that F has a nested type. This prevents async being used with function pointers and makes the example in 30.6.9 [futures.async] invalid. I believe this is unintentional.
The proposed resolution also addresses editorial issues with the launch_policy function parameter.
For the first overload it is not sufficient to return future<typename result_of<F(ArgTypes...)>::type>. Calling async(launch::xxx, foo, bar) performs argument deduction on both async overloads, which for the first overload attempts to instantiate result_of<launch(F, ArgTypes...)>, which is invalid. SFINAE must be used to prevent that.
[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-12 Daniel opens: ]
[..] if decay<F>::type is of type std::launch.
or
[..] if remove_cv<remove_reference<F>::type>::type is of type std::launch.
The latter is the more specific form, but the former is equivalent to the latter for all cases that can occur here. I suggest to use the former for simplicity, but expect that implementations can effectively use the latter.
[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
In 30.6.1 [futures.overview] paragraph 1:
template <class F, class... Args>future<typename F::result_type>future<typename result_of<F(Args...)>::type> async(F&& f, Args&&... args); template <class F, class... Args>future<typename F::result_type>future<typename result_of<F(Args...)>::type> async(launch policy, F&& f, Args&&... args);
In 30.6.9 [futures.async] before paragraph 1
template <class F, class... Args>future<typename F::result_type>future<typename result_of<F(Args...)>::type> async(F&& f, Args&&... args); template <class F, class... Args>future<typename F::result_type>future<typename result_of<F(Args...)>::type> async(launch policy, F&& f, Args&&... args);...
Remarks: The first signature shall not participate in overload resolution if decay<F>::type is std::launch.
Section: 20.8.15 [unord.hash] Status: NAD Future Submitter: Nicolai M. Josuttis Opened: 2010-02-10 Last modified: 2010-03-15
View all other issues in [unord.hash].
View all issues with NAD Future status.
Discussion:
Currently, the library lacks a convenient way to provide a hash function that can be used with the provided unordered containers to allow the usage of non trivial element types.
While we can easily declare an
std::unordered_set<int>
or
std::unordered_set<std::string>
we have no easy way to declare an unordered_set for a user defined type. IMO, this is a big obstacle to use unordered containers in practice. Note that in Java, the wide usage of HashMap is based on the fact that there is always a default hash function provided.
Of course, a default hash function implies the risk to provide poor hash functions. But often even poor hash functions are good enough.
While I really would like to see a default hash function, I don't propose it here because this would probably introduce a discussion that's too big for this state of C++0x.
However, I strongly suggest at least to provide a convenience variadic template function make_hash<>() to allow an easy definition of a (possibly poor) hash function.
As a consequence for a user-defined type such as
class Customer {
friend class CustomerHash;
private:
string firstname;
string lastname;
long no;
...
};
would allow to specify:
class CustomerHash : public std::unary_function<Customer, std::size_t>
{
public:
std::size_t operator() (const Customer& c) const {
return make_hash(c.firstname,c.lastname,c.no);
}
};
instead of:
class CustomerHash : public std::unary_function<Customer, std::size_t>
{
public:
std::size_t operator() (const Customer& c) const {
return std::hash<std::string>()(c.firstname) +
std::hash<std::string>()(c.lastname) +
std::hash<long>()(c.no);
}
};
Note that, in principle, we can either specify that
make_hash returns the sum of a call of std::hash<T>()(x) for each argument x of type T
or we can specify that
make_hash provides a hash value for each argument, for which a std::hash() function is provided
with the possible note that the hash value may be poor or only a good hash value if the ranges of all passed arguments is equally distributed.
For my convenience, I propose wording that describes the concrete implementation.
[ 2010 Pittsburgh: Moved to NAD Editorial, rationale added below. ]
Rationale:
There is no consensus to make this change at this time.
Proposed resolution:
In Function objects 20.8 [function.objects] in paragraph 2 at the end of the Header <functional> synopsis insert:
// convenience functions template <class T> size_t make_hash (const T&); template <class T, class... Types> size_t make_hash (const T&, const Types&...);
In Class template hash 20.8.15 [unord.hash] add:
20.7.16.1 Hash creation functions [hash.creation]
template <class T> size_t make_hash (const T& val);Returns: hash<T>()(val);template <class T, class... Types> size_t make_hash (const T& val, const Types&... args);Returns: hash<T>()(val) + std::make_hash(args...)
Section: 20.9.6.3 [allocator.adaptor.members] Status: NAD Editorial Submitter: Howard Hinnant Opened: 2010-02-16 Last modified: 2010-03-14
View all issues with NAD Editorial status.
Discussion:
20.9.6.3 [allocator.adaptor.members] p8-9 says:
template <class T, class... Args> void construct(T* p, Args&&... args);8 Effects: let OUTERMOST(x) be x if x does not have an outer_allocator() function and OUTERMOST(x.outer_allocator()) otherwise.
- If uses_allocator<T, inner_allocator_type>::value is false and is_constructible<T, Args...>::value is true, calls OUTERMOST(*this).construct(p, std::forward<Args>(args)...).
- Otherwise, if uses_allocator<T, inner_allocator_type>::value is true and is_constructible<T, allocator_arg_t, inner_allocator_type, Args...>::value is true, calls OUTERMOST(*this).construct(p, allocator_arg, inner_allocator(),std::forward<Args>(args)...).
- Otherwise, if uses_allocator<T, inner_allocator_type>::value is true and is_constructible<T, Args..., inner_allocator_type>::value is true, calls OUTERMOST(*this).construct(p, std::forward<Args>(args)..., inner_allocator()).
- Otherwise, the program is ill-formed. [Note: an error will result if uses_allocator evaluates to true but the specific constructor does not take an allocator. This definition prevents a silent failure to pass an inner allocator to a contained element. — end note]
template <class T> void destroy(T* p);9 Effects: calls outer_allocator().destroy(p).
In all other calls where applicable scoped_allocator_adaptor does not call members of an allocator directly, but rather does so indirectly via allocator_traits. For example:
size_type max_size() const;7 Returns: allocator_traits<OuterAlloc>::max_size(outer_allocator()).
Indeed, without the indirection through allocator_traits the definitions for construct and destroy are likely to fail at compile time since the outer_allocator() may not have the members construct and destroy.
[ The proposed wording is a product of Pablo, Daniel and Howard. ]
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3059.
Proposed resolution:
In 20.9.6.3 [allocator.adaptor.members] move and change p8 as indicated, and change p9 as indicated:
Let OUTERMOST(x) be x if x does not have an outer_allocator() member function and OUTERMOST(x.outer_allocator()) otherwise. Let OUTERMOST_ALLOC_TRAITS(x) be allocator_traits<decltype(OUTERMOST(x))>. [Note: OUTERMOST(x) and OUTERMOST_ALLOC_TRAITS(x) are recursive operations. It is incumbent upon the definition of outer_allocator() to ensure that the recursion terminates. It will terminate for all instantiations of scoped_allocator_adaptor. — end note]
template <class T, class... Args> void construct(T* p, Args&&... args);8 Effects:
let OUTERMOST(x) be x if x does not have an outer_allocator() function and OUTERMOST(x.outer_allocator()) otherwise.
- If uses_allocator<T, inner_allocator_type>::value is false and is_constructible<T, Args...>::value is true, calls
OUTERMOST(*this).OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, std::forward<Args>(args)... ).- Otherwise, if uses_allocator<T, inner_allocator_type>::value is true and is_constructible<T, allocator_arg_t, inner_allocator_type, Args...>::value is true, calls
OUTERMOST(*this).OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, allocator_arg, inner_allocator(), std::forward<Args>(args)... ).- Otherwise, if uses_allocator<T, inner_allocator_type>::value is true and is_constructible<T, Args..., inner_allocator_type>::value is true, calls
OUTERMOST(*this).OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, std::forward<Args>(args)..., inner_allocator() ).- Otherwise, the program is ill-formed. [Note: an error will result if uses_allocator evaluates to true but the specific constructor does not take an allocator. This definition prevents a silent failure to pass an inner allocator to a contained element. — end note]
template <class T> void destroy(T* p);9 Effects: callsouter_allocator().OUTERMOST_ALLOC_TRAITS(outer_allocator())::destroy( OUTERMOST(outer_allocator()), p).
vector<bool>Section: 23.2.2 [container.requirements.dataraces] Status: NAD Editorial Submitter: Jeffrey Yaskin Opened: 2010-03-09 Last modified: 2010-03-14
View all issues with NAD Editorial status.
Discussion:
The common implementation of vector<bool> is as an unsynchronized bitfield. The addition of 23.2.2 [container.requirements.dataraces]/2 would require either a change in representation or a change in access synchronization, both of which are undesireable with respect to compatibility and performance.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3069.
Proposed resolution:
Container data races 23.2.2 [container.requirements.dataraces]
Paragraph 1 is unchanged as follows:
1 For purposes of avoiding data races (17.6.4.8), implementations shall consider the following functions to beconst:begin,end,rbegin,rend,front,back,data,find,lower_bound,upper_bound,equal_range, and, except in associative containers,operator[].
Edit paragraph 2 as follows:
2 Notwithstanding (17.6.4.8), implementations are required to avoid data
races when the contents of the contained object in different elements in
the same sequence, excepting vector<bool>,
are modified concurrently.
Edit paragraph 3 as follows:
3 [Note: For avector<int> xwith a size greater than one,x[1] = 5and*x.begin() = 10can be executed concurrently without a data race, butx[0] = 5and*x.begin() = 10executed concurrently may result in a data race. As an exception to the general rule, for avector<bool> y,y[i] = truemay race withy[j] = true. —end note]