Doc. no. N2894=09-0084
Date: 2009-06-21
Project: Programming Language C++
Reply to: Howard Hinnant <howard.hinnant@gmail.com>

C++ Standard Library Active Issues List (Revision R65)

Reference ISO/IEC IS 14882:2003(E)

Also see:

The purpose of this document is to record the status of issues which have come before the Library Working Group (LWG) of the ANSI (J16) and ISO (WG21) C++ Standards Committee. Issues represent potential defects in the ISO/IEC IS 14882:2003(E) document.

This document contains only library issues which are actively being considered by the Library Working Group. That is, issues which have a status of New, Open, Ready, and Review. See Library Defect Reports List for issues considered defects and Library Closed Issues List for issues considered closed.

The issues in these lists are not necessarily formal ISO Defect Reports (DR's). While some issues will eventually be elevated to official Defect Report status, other issues will be disposed of in other ways. See Issue Status.

Prior to Revision 14, library issues lists existed in two slightly different versions; a Committee Version and a Public Version. Beginning with Revision 14 the two versions were combined into a single version.

This document includes [bracketed italicized notes] as a reminder to the LWG of current progress on issues. Such notes are strictly unofficial and should be read with caution as they may be incomplete or incorrect. Be aware that LWG support for a particular resolution can quickly change if new viewpoints or killer examples are presented in subsequent discussions.

For the most current official version of this document see http://www.open-std.org/jtc1/sc22/wg21/. Requests for further information about this document should include the document number above, reference ISO/IEC 14882:2003(E), and be submitted to Information Technology Industry Council (ITI), 1250 Eye Street NW, Washington, DC 20005.

Public information as to how to obtain a copy of the C++ Standard, join the standards committee, submit an issue, or comment on an issue can be found in the comp.std.c++ FAQ.

Revision History

Issue Status

New - The issue has not yet been reviewed by the LWG. Any Proposed Resolution is purely a suggestion from the issue submitter, and should not be construed as the view of LWG.

Open - The LWG has discussed the issue but is not yet ready to move the issue forward. There are several possible reasons for open status:

A Proposed Resolution for an open issue is still not be construed as the view of LWG. Comments on the current state of discussions are often given at the end of open issues in an italic font. Such comments are for information only and should not be given undue importance.

Dup - The LWG has reached consensus that the issue is a duplicate of another issue, and will not be further dealt with. A Rationale identifies the duplicated issue's issue number.

NAD - The LWG has reached consensus that the issue is not a defect in the Standard.

NAD Editorial - The LWG has reached consensus that the issue can either be handled editorially, or is handled by a paper (usually linked to in the rationale).

NAD Future - In addition to the regular status, the LWG believes that this issue should be revisited at the next revision of the standard.

Review - Exact wording of a Proposed Resolution is now available for review on an issue for which the LWG previously reached informal consensus.

Ready - The LWG has reached consensus that the issue is a defect in the Standard, the Proposed Resolution is correct, and the issue is ready to forward to the full committee for further action as a Defect Report (DR).

DR - (Defect Report) - The full J16 committee has voted to forward the issue to the Project Editor to be processed as a Potential Defect Report. The Project Editor reviews the issue, and then forwards it to the WG21 Convenor, who returns it to the full committee for final disposition. This issues list accords the status of DR to all these Defect Reports regardless of where they are in that process.

TC1 - (Technical Corrigenda 1) - The full WG21 committee has voted to accept the Defect Report's Proposed Resolution as a Technical Corrigenda. Action on this issue is thus complete and no further action is possible under ISO rules.

CD1 - (Committee Draft 2008) - The full WG21 committee has voted to accept the Defect Report's Proposed Resolution into the Fall 2008 Committee Draft.

TRDec - (Decimal TR defect) - The LWG has voted to accept the Defect Report's Proposed Resolution into the Decimal TR. Action on this issue is thus complete and no further action is expected.

WP - (Working Paper) - The proposed resolution has not been accepted as a Technical Corrigendum, but the full WG21 committee has voted to apply the Defect Report's Proposed Resolution to the working paper.

Tentatively - This is a status qualifier. The issue has been reviewed online, or at an unofficial meeting, but not in an official meeting, and some support has been formed for the qualified status. Tentatively qualified issues may be moved to the unqualified status and forwarded to full committee (if Ready) within the same meeting. Unlike Ready issues, Tentatively Ready issues will be reviewed in subcommittee prior to forwarding to full committee. When a status is qualified with Tentatively, the issue is still considered active.

Pending - This is a status qualifier. When prepended to a status this indicates the issue has been processed by the committee, and a decision has been made to move the issue to the associated unqualified status. However for logistical reasons the indicated outcome of the issue has not yet appeared in the latest working paper.

Issues are always given the status of New when they first appear on the issues list. They may progress to Open or Review while the LWG is actively working on them. When the LWG has reached consensus on the disposition of an issue, the status will then change to Dup, NAD, or Ready as appropriate. Once the full J16 committee votes to forward Ready issues to the Project Editor, they are given the status of Defect Report ( DR). These in turn may become the basis for Technical Corrigenda (TC), or are closed without action other than a Record of Response (RR ). The intent of this LWG process is that only issues which are truly defects in the Standard move to the formal ISO DR status.

Active Issues


96. Vector<bool> is not a container

Section: 23.3.6 [vector] Status: Tentatively NAD Future Submitter: AFNOR Opened: 1998-10-07 Last modified: 2009-05-25

View all other issues in [vector].

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).

Proposed resolution:

We now have: N2050 and N2160.


111. istreambuf_iterator::equal overspecified, inefficient

Section: 24.6.3.5 [istreambuf.iterator::equal] Status: Open Submitter: Nathan Myers Opened: 1998-10-15 Last modified: 2009-03-13

View other active issues in [istreambuf.iterator::equal].

View all other issues in [istreambuf.iterator::equal].

View all issues with Open status.

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 24.2.2 [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.

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.


128. Need open_mode() function for file stream, string streams, file buffers, and string  buffers

Section: 27.8 [string.streams], 27.9 [file.streams] Status: Open Submitter: Angelika Langer Opened: 1999-02-22 Last modified: 2008-03-14

View all other issues in [string.streams].

View all issues with Open status.

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. ]

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.


138. Class ctype_byname<char> redundant and misleading

Section: 22.4.1.4 [locale.codecvt] Status: Open Submitter: Angelika Langer Opened: 1999-03-18 Last modified: 2009-03-09

View other active issues in [locale.codecvt].

View all other issues in [locale.codecvt].

View all issues with Open status.

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.

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.


149. Insert should return iterator to first element inserted

Section: 23.2.3 [sequence.reqmts] Status: Open Submitter: Andrew Koenig Opened: 1999-06-28 Last modified: 2009-05-01

View other active issues in [sequence.reqmts].

View all other issues in [sequence.reqmts].

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Discussion:

Suppose that c and c1 are sequential containers and i is an iterator that refers to an element of c. Then I can insert a copy of c1's elements into c ahead of element i by executing

c.insert(i, c1.begin(), c1.end());

If c is a vector, it is fairly easy for me to find out where the newly inserted elements are, even though i is now invalid:

size_t i_loc = i - c.begin();
c.insert(i, c1.begin(), c1.end());

and now the first inserted element is at c.begin()+i_loc and one past the last is at c.begin()+i_loc+c1.size().

But what if c is a list? I can still find the location of one past the last inserted element, because i is still valid. To find the location of the first inserted element, though, I must execute something like

for (size_t n = c1.size(); n; --n)
   --i;

because i is now no longer a random-access iterator.

Alternatively, I might write something like

bool first = i == c.begin();
list<T>::iterator j = i;
if (!first) --j;
c.insert(i, c1.begin(), c1.end());
if (first)
   j = c.begin();
else
   ++j;

which, although wretched, requires less overhead.

But I think the right solution is to change the definition of insert so that instead of returning void, it returns an iterator that refers to the first element inserted, if any, and otherwise is a copy of its first argument. 

[ Summit: ]

Reopened by Alisdair.

[ Post Summit Alisdair adds: ]

In addition to the original rationale for C++03, this change also gives a consistent interface for all container insert operations i.e. they all return an iterator to the (first) inserted item.

Proposed wording provided.

Proposed resolution:

Table 83 change return type from void to iterator for the following rows:

Table 83 -- Sequence container requirements (in addition to container)
Expression Return type Assertion/note pre-/post-condition
a.insert(p,n,t) void iterator Inserts n copies of t before p.
a.insert(p,i,j) void iterator Each iterator in the range [i,j) shall be dereferenced exactly once. pre: i and j are not iterators into a. Inserts copies of elements in [i, j) before p
a.insert(p,il) void iterator a.insert(p, il.begin(), il.end()).

Add after p6 23.2.3 [sequence.reqmts]:

-6- ...

The iterator returned from a.insert(p,n,t) points to the copy of the first element inserted into a, or p if n == 0.

The iterator returned from a.insert(p,i,j) points to the copy of the first element inserted into a, or p if i == j.

The iterator returned from a.insert(p,il) points to the copy of the first element inserted into a, or p if il is empty.

p9 23.2.6.1 [container.concepts.free] change return type from void to iterator:

concept RangeInsertionContainer<typename C, typename Iter> : InsertionContainer<C> {
  requires InputIterator<Iter>;
  void iterator insert(C&, const_iterator position, Iter first, Iter last);
}

p9 23.2.6.2 [container.concepts.member] change return type from void to iterator:

auto concept MemberRangeInsertionContainer<typename C, typename Iter> : MemberInsertionContainer<C> {
  requires InputIterator<Iter>;
  void iterator C::insert(const_iterator position, Iter first, Iter last);
}

p8 23.2.6.3 [container.concepts.maps] change return type from void to iterator, add return statement:

template <MemberRangeInsertionContainer C, InputIterator Iter>
concept_map RangeInsertionContainer<C, Iter> {
  void iterator insert(C& c, Container<C>::const_iterator i, Iter first, Iter last)
  { return c.insert(i, first, last); }
}

p2 23.3.2 [deque] Update class definition, change return type from void to iterator:

requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  void iterator insert(const_iterator position, size_type n, const T& x);
template <InputIterator Iter>
  requires AllocatableElement<Alloc, T, Iter::reference> && MoveAssignable<T>
  void iterator insert(const_iterator position, Iter first, Iter last);
requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  void iterator insert(const_iterator position, initializer_list<T>);

23.3.2.3 [deque.modifiers] change return type from void to iterator on following declarations:

requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  void iterator insert(const_iterator position, size_type n, const T& x);
template <InputIterator Iter>
  requires AllocatableElement<Alloc, T, Iter::reference> && MoveAssignable<T>
  void iterator insert(const_iterator position, Iter first, Iter last);

Add the following (missing) declaration

requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  iterator insert(const_iterator position, initializer_list<T>);

23.3.3 [forwardlist] Update class definition, change return type from void to iterator:

requires AllocatableElement<Alloc, T, const T&>
  void iterator insert_after(const_iterator position, initializer_list<T> il);
requires AllocatableElement<Alloc, T, const T&>
  void iterator insert_after(const_iterator position, size_type n, const T& x);
template <InputIterator Iter>
  requires AllocatableElement<Alloc, T, Iter::reference>
  void iterator insert_after(const_iterator position, Iter first, Iter last);

p8 23.3.3.4 [forwardlist.modifiers] change return type from void to iterator:

requires AllocatableElement<Alloc, T, const T&>
  void iterator insert_after(const_iterator position, size_type n, const T& x);

Add paragraph:

Returns: position.

p10 23.3.3.4 [forwardlist.modifiers] change return type from void to iterator:

template <InputIterator Iter>
  requires AllocatableElement<Alloc, T, Iter::reference>
  void iterator insert_after(const_iterator position, Iter first, Iter last);

Add paragraph:

Returns: position.

p12 23.3.3.4 [forwardlist.modifiers] change return type from void to iterator on following declarations:

requires AllocatableElement<Alloc, T, const T&>
  void iterator insert_after(const_iterator position, initializer_list<T> il);

change return type from void to iterator on following declarations:

p2 23.3.4 [list] Update class definition, change return type from void to iterator:

requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  void iterator insert(const_iterator position, size_type n, const T& x);

template <InputIterator Iter>
  requires AllocatableElement<Alloc, T, Iter::reference> && MoveAssignable<T>
  void iterator insert(const_iterator position, Iter first, Iter last);

requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  void iterator insert(const_iterator position, initializer_list<T>);

23.3.4.3 [list.modifiers] change return type from void to iterator on following declarations:

requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  void iterator insert(const_iterator position, size_type n, const T& x);

template <InputIterator Iter>
  requires AllocatableElement<Alloc, T, Iter::reference> && MoveAssignable<T>
  void iterator insert(const_iterator position, Iter first, Iter last);

Add the following (missing) declaration

requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  iterator insert(const_iterator position, initializer_list<T>);

p2 23.3.6 [vector]

Update class definition, change return type from void to iterator:

requires AllocatableElement<Alloc, T, T&&> && MoveAssignable<T>
  void iterator insert(const_iterator position, T&& x);

requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  void iterator insert(const_iterator position, size_type n, const T& x);

template <InputIterator Iter>
  requires AllocatableElement<Alloc, T, Iter::reference> && MoveAssignable<T>
  void iterator insert(const_iterator position, Iter first, Iter last);

requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  void iterator insert(const_iterator position, initializer_list<T>);

23.3.6.4 [vector.modifiers] change return type from void to iterator on following declarations:

requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  void iterator insert(const_iterator position, size_type n, const T& x);

template <InputIterator Iter>
  requires AllocatableElement<Alloc, T, Iter::reference> && MoveAssignable<T>
  void iterator insert(const_iterator position, Iter first, Iter last);

Add the following (missing) declaration

requires AllocatableElement<Alloc, T, const T&> && MoveAssignable<T>
  iterator insert(const_iterator position, initializer_list<T>);

p1 23.3.7 [vector.bool] Update class definition, change return type from void to iterator:

void iterator insert (const_iterator position, size_type n, const bool& x);

template <InputIterator Iter>
  requires Convertible<Iter::reference, bool>
  void iterator insert(const_iterator position, Iter first, Iter last);

  void iterator insert(const_iterator position, initializer_list<bool> il);

p5 21.4 [basic.string] Update class definition, change return type from void to iterator:

void iterator insert(const_iterator p, size_type n, charT c);

template<class InputIterator>
  void iterator insert(const_iterator p, InputIterator first, InputIterator last);

void iterator insert(const_iterator p, initializer_list<charT>);

p13 21.4.6.4 [string::insert] change return type from void to iterator:

void iterator insert(const_iterator p, size_type n, charT c);

Add paragraph:

Returns: an iterator which refers to the copy of the first inserted character, or p if n == 0.

p15 21.4.6.4 [string::insert] change return type from void to iterator:

template<class InputIterator>
  void iterator insert(const_iterator p, InputIterator first, InputIterator last);

Add paragraph:

Returns: an iterator which refers to the copy of the first inserted character, or p if first == last.

p17 21.4.6.4 [string::insert] change return type from void to iterator:

void iterator insert(const_iterator p, initializer_list<charT> il);

Add paragraph:

Returns: an iterator which refers to the copy of the first inserted character, or p if il is empty.

Rationale:

[ The following was the C++98/03 rationale and does not necessarily apply to the proposed resolution in the C++0X time frame: ]

The LWG believes this was an intentional design decision and so is not a defect. It may be worth revisiting for the next standard.


190. min() and max() functions should be std::binary_functions

Section: 25.5.7 [alg.min.max] Status: Open Submitter: Mark Rintoul Opened: 1999-08-26 Last modified: 2008-03-14

View other active issues in [alg.min.max].

View all other issues in [alg.min.max].

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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. ]

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.


219. find algorithm missing version that takes a binary predicate argument

Section: 25.3.5 [alg.find] Status: Open Submitter: Pablo Halpern Opened: 2000-03-06 Last modified: 2009-03-09

View all other issues in [alg.find].

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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.

Proposed resolution:

In section 25.3.5 [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".


255. Why do basic_streambuf<>::pbump() and gbump() take an int?

Section: 27.6.2 [streambuf] Status: Open Submitter: Martin Sebor Opened: 2000-08-12 Last modified: 2007-01-15

View all other issues in [streambuf].

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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.

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)] ]


290. Requirements to for_each and its function object

Section: 25.3.4 [alg.foreach] Status: Open Submitter: Angelika Langer Opened: 2001-01-03 Last modified: 2006-12-27

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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.]

Proposed resolution:


309. Does sentry catch exceptions?

Section: 27.7 [iostream.format] Status: Open Submitter: Martin Sebor Opened: 2001-03-19 Last modified: 2006-12-27

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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.

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.


342. seek and eofbit

Section: 27.7.1.3 [istream.unformatted] Status: Open Submitter: Howard Hinnant Opened: 2001-10-09 Last modified: 2007-01-15

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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:

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.


343. Unspecified library header dependencies

Section: 17 [library] Status: Open Submitter: Martin Sebor Opened: 2001-10-09 Last modified: 2009-03-11

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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.

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.


382. codecvt do_in/out result

Section: 22.4.1.4 [locale.codecvt] Status: Open Submitter: Martin Sebor Opened: 2002-08-30 Last modified: 2007-01-15

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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:

  1. the conditions under which the functions terminate
  2. precisely when the functions return ok
  3. precisely when the functions return partial
  4. the full set of conditions when the functions return error
  1. 22.4.1.4.2 [locale.codecvt.virtuals], p2 says this about the effects of the function: ...Stops if it encounters a character it cannot convert... This assumes that there *is* a character to convert. What happens when there is a sequence that doesn't form a valid source character, such as an unassigned or invalid UNICODE character, or a sequence that cannot possibly form a character (e.g., the sequence "\xc0\xff" in UTF-8)?
  2. Table 53 says that the function returns codecvt_base::ok to indicate that the function(s) "completed the conversion." Suppose that the source sequence is "\xc0\x80" in UTF-8, with from pointing to '\xc0' and (from_end==from + 1). It is not clear whether the return value should be ok or partial (see below).
  3. Table 53 says that the function returns codecvt_base::partial if "not all source characters converted." With the from pointers set up the same way as above, it is not clear whether the return value should be partial or ok (see above).
  4. Table 53, in the row describing the meaning of error mistakenly refers to a "from_type" character, without the symbol from_type having been defined. Most likely, the word "source" character is intended, although that is not sufficient. The functions may also fail when they encounter an invalid source sequence that cannot possibly form a valid source character (e.g., as explained in bullet 1 above).

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.]

Proposed resolution:


394. behavior of formatted output on failure

Section: 27.7.2.6.1 [ostream.formatted.reqmts] Status: Open Submitter: Martin Sebor Opened: 2002-12-27 Last modified: 2007-01-15

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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.]

Proposed resolution:

Rationale:


397. ostream::sentry dtor throws exceptions

Section: 27.7.2.4 [ostream::sentry] Status: Open Submitter: Martin Sebor Opened: 2003-01-05 Last modified: 2007-07-25

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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. ]

Proposed resolution:


398. effects of end-of-file on unformatted input functions

Section: 27.7.2.4 [ostream::sentry] Status: Open Submitter: Martin Sebor Opened: 2003-01-05 Last modified: 2007-01-15

View other active issues in [ostream::sentry].

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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.

Proposed resolution:


408. Is vector<reverse_iterator<char*> > forbidden?

Section: 24.2 [iterator.concepts] Status: Open Submitter: Nathan Myers Opened: 2003-06-03 Last modified: 2009-05-10

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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.

24.2 [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.

Related issues: 407, 1012

[ 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).

Proposed resolution:

Add to the end of Iterator concepts 24.2 [iterator.concepts] para 6 (the one describing singular iterators)

Any Iterator that satisfies the DefaultConstructible concept shall be safely copyable after value-initialization, 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]

417. what does ctype::do_widen() return on failure

Section: 22.4.1.1.2 [locale.ctype.virtuals] Status: Open Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2006-12-27

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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.]

Proposed resolution:


418. exceptions thrown during iostream cleanup

Section: 27.5.2.1.6 [ios::Init] Status: Open Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2007-07-25

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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. ]

Proposed resolution:


419. istream extractors not setting failbit if eofbit is already set

Section: 27.7.1.1.3 [istream::sentry] Status: Open Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2007-01-30

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Discussion:

27.7.1.1.3 [istream::sentry], p2 says that istream::sentry ctor prepares for input if is.good() is true. p4 then goes on to say that the ctor sets the sentry::ok_ member to true if the stream state is good after any preparation. 27.7.1.2.1 [istream.formatted.reqmts], p1 then says that a formatted input function endeavors to obtain the requested input if the sentry's operator bool() returns true. Given these requirements, no formatted extractor should ever set failbit if the initial stream rdstate() == eofbit. That is contrary to the behavior of all implementations I tested. The program below prints out eof = 1, fail = 0 eof = 1, fail = 1 on all of them.


#include <sstream>
#include <cstdio>

int main()
{
    std::istringstream strm ("1");

    int i = 0;

    strm >> i;

    std::printf ("eof = %d, fail = %d\n",
                 !!strm.eof (), !!strm.fail ());

    strm >> i;

    std::printf ("eof = %d, fail = %d\n",
                 !!strm.eof (), !!strm.fail ());
}


Comments from Jerry Schwarz (c++std-lib-11373):
Jerry Schwarz wrote:
I don't know where (if anywhere) it says it in the standard, but the formatted extractors are supposed to set failbit if they don't extract any characters. If they didn't then simple loops like
while (cin >> x);
would loop forever.
Further comments from Martin Sebor:
The question is which part of the extraction should prevent this from happening by setting failbit when eofbit is already set. It could either be the sentry object or the extractor. It seems that most implementations have chosen to set failbit in the sentry [...] so that's the text that will need to be corrected.

Pre Berlin: This issue is related to 342. If the sentry sets failbit when it finds eofbit already set, then you can never seek away from the end of stream.

Kona: Possibly NAD. If eofbit is set then good() will return false. We then set ok to false. We believe that the sentry's constructor should always set failbit when ok is false, and we also think the standard already says that. Possibly it could be clearer.

Proposed resolution:

Change 27.7.1.1.3 [istream::sentry], p2 to:

explicit sentry(basic_istream<charT,traits>& is , bool noskipws = false);

-2- Effects: If is.good() is true false, calls is.setstate(failbit). Otherwise prepares for formatted or unformatted input. ...


421. is basic_streambuf copy-constructible?

Section: 27.6.2.1 [streambuf.cons] Status: Open Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2007-01-15

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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.

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.


423. effects of negative streamsize in iostreams

Section: 27 [input.output] Status: Open Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2006-12-27

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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).

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.]


424. normative notes

Section: 17.5.1.2 [structure.summary] Status: Tentatively NAD Editorial Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2009-05-23

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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:
20.8.6.1 [allocator.members], p3,
20.8.6.1 [allocator.members], p10,
21.4.2 [string.cons], p11,
22.3.1.2 [locale.cons], p11,
23.3.2.3 [deque.modifiers], p2,
25.5.7 [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,
25.3.4 [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:


427. stage 2 and rationale of DR 221

Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: Open Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2007-01-15

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Discussion:

The requirements specified in Stage 2 and reiterated in the rationale of DR 221 (and echoed again in DR 303) specify that num_get<charT>:: do_get() compares characters on the stream against the widened elements of "012...abc...ABCX+-"

An implementation is required to allow programs to instantiate the num_get template on any charT that satisfies the requirements on a user-defined character type. These requirements do not include the ability of the character type to be equality comparable (the char_traits template must be used to perform tests for equality). Hence, the num_get template cannot be implemented to support any arbitrary character type. The num_get template must either make the assumption that the character type is equality-comparable (as some popular implementations do), or it may use char_traits<charT> to do the comparisons (some other popular implementations do that). This diversity of approaches makes it difficult to write portable programs that attempt to instantiate the num_get template on user-defined types.

[Kona: the heart of the problem is that we're theoretically supposed to use traits classes for all fundamental character operations like assignment and comparison, but facets don't have traits parameters. This is a fundamental design flaw and it appears all over the place, not just in this one place. It's not clear what the correct solution is, but a thorough review of facets and traits is in order. The LWG considered and rejected the possibility of changing numeric facets to use narrowing instead of widening. This may be a good idea for other reasons (see issue 459), but it doesn't solve the problem raised by this issue. Whether we use widen or narrow the num_get facet still has no idea which traits class the user wants to use for the comparison, because only streams, not facets, are passed traits classes. The standard does not require that two different traits classes with the same char_type must necessarily have the same behavior.]

Informally, one possibility: require that some of the basic character operations, such as eq, lt, and assign, must behave the same way for all traits classes with the same char_type. If we accept that limitation on traits classes, then the facet could reasonably be required to use char_traits<charT>.

Proposed resolution:


430. valarray subset operations

Section: 26.6.2.4 [valarray.sub] Status: Open Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2009-05-01

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Discussion:

The standard fails to specify the behavior of valarray::operator[](slice) and other valarray subset operations when they are passed an "invalid" slice object, i.e., either a slice that doesn't make sense at all (e.g., slice (0, 1, 0) or one that doesn't specify a valid subset of the valarray object (e.g., slice (2, 1, 1) for a valarray of size 1).

[Kona: the LWG believes that invalid slices should invoke undefined behavior. Valarrays are supposed to be designed for high performance, so we don't want to require specific checking. We need wording to express this decision.]

[ Bellevue: ]

Please note that the standard also fails to specify the behavior of slice_array and gslice_array in the valid case. Bill Plauger will endeavor to provide revised wording for slice_array and gslice_array.

[ post-Bellevue: Bill provided wording. ]

Proposed resolution:

Insert after 26.6.2.4 [valarray.sub], paragraph 1:

The member operator is overloaded to provide several ways to select sequences of elements from among those controlled by *this. The first group of five member operators work in conjunction with various overloads of operator= (and other assigning operators) to allow selective replacement (slicing) of the controlled sequence. The selected elements must exist.

The first member operator selects element off. For example:

valarray<char> v0("abcdefghijklmnop", 16);
v0[3] = 'A';
// v0 == valarray<char>("abcAefghijklmnop", 16)

The second member operator selects those elements of the controlled sequence designated by slicearr. For example:

valarray<char> v0("abcdefghijklmnop", 16);
valarray<char> v1("ABCDE", 5);
v0[slice(2, 5, 3)] = v1;
// v0 == valarray<char>("abAdeBghCjkDmnEp", 16)

The third member operator selects those elements of the controlled sequence designated by gslicearr. For example:

valarray<char> v0("abcdefghijklmnop", 16);
valarray<char> v1("ABCDEF", 6);
const size_t lv[] = {2, 3};
const size_t dv[] = {7, 2};
const valarray<size_t> len(lv, 2), str(dv, 2);
v0[gslice(3, len, str)] = v1;
// v0 == valarray<char>("abcAeBgCijDlEnFp", 16)

The fourth member operator selects those elements of the controlled sequence designated by boolarr. For example:

valarray<char> v0("abcdefghijklmnop", 16);
valarray<char> v1("ABC", 3);
const bool vb[] = {false, false, true, true, false, true};
v0[valarray<bool>(vb, 6)] = v1;
// v0 == valarray<char>("abABeCghijklmnop", 16)

The fifth member operator selects those elements of the controlled sequence designated by indarr. For example:

valarray<char> v0("abcdefghijklmnop", 16);
valarray<char> v1("ABCDE", 5);
const size_t vi[] = {7, 5, 2, 3, 8};
v0[valarray<size_t>(vi, 5)] = v1;
// v0 == valarray<char>("abCDeBgAEjklmnop", 16)

The second group of five member operators each construct an object that represents the value(s) selected. The selected elements must exist.

The sixth member operator returns the value of element off. For example:

valarray<char> v0("abcdefghijklmnop", 16);
// v0[3] returns 'd'

The seventh member operator returns an object of class valarray<Ty> containing those elements of the controlled sequence designated by slicearr. For example:

valarray<char> v0("abcdefghijklmnop", 16);
// v0[slice(2, 5, 3)] returns valarray<char>("cfilo", 5)

The eighth member operator selects those elements of the controlled sequence designated by gslicearr. For example:

valarray<char> v0("abcdefghijklmnop", 16);
const size_t lv[] = {2, 3};
const size_t dv[] = {7, 2};
const valarray<size_t> len(lv, 2), str(dv, 2);
// v0[gslice(3, len, str)] returns
//    valarray<char>("dfhkmo", 6)

The ninth member operator selects those elements of the controlled sequence designated by boolarr. For example:

valarray<char> v0("abcdefghijklmnop", 16);
const bool vb[] = {false, false, true, true, false, true};
// v0[valarray<bool>(vb, 6)] returns
//    valarray<char>("cdf", 3)

The last member operator selects those elements of the controlled sequence designated by indarr. For example:

valarray<char> v0("abcdefghijklmnop", 16);
const size_t vi[] = {7, 5, 2, 3, 8};
// v0[valarray<size_t>(vi, 5)] returns
//    valarray<char>("hfcdi", 5)

431. Swapping containers with unequal allocators

Section: X [allocator.requirements], 25 [algorithms] Status: Open Submitter: Matt Austern Opened: 2003-09-20 Last modified: 2009-05-01

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Discussion:

Clause X [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) my 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 xend note]

Proposed resolution:


446. Iterator equality between different containers

Section: 24.2 [iterator.concepts], 23.2 [container.requirements] Status: Open Submitter: Andy Koenig Opened: 2003-12-16 Last modified: 2008-09-30

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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. ]

Proposed resolution:


458. 24.1.5 contains unintented limitation for operator-

Section: 24.2.6 [random.access.iterators] Status: Tentatively NAD Submitter: Daniel Frey Opened: 2004-02-27 Last modified: 2009-05-23

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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 24.2.6 [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.

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; }

459. Requirement for widening in stage 2 is overspecification

Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: Open Submitter: Martin Sebor Opened: 2004-03-16 Last modified: 2006-12-27

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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.]

Proposed resolution:

Change stage 2 so that implementations are permitted to use either technique to perform the comparison:

  1. call widen on the atoms and compare (either by using operator== or char_traits<charT>::eq) the input with the widened atoms, or
  2. call narrow on the input and compare the narrow input with the atoms
  3. do (1) or (2) only if charT is not char or wchar_t, respectively; i.e., avoid calling widen or narrow if it the source and destination types are the same

463. auto_ptr usability issues

Section: D.9.1 [auto.ptr] Status: Open Submitter: Rani Sharoni Opened: 2003-12-07 Last modified: 2007-11-15

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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:

  1. call auto_ptr<B>::operator auto_ptr_ref<B>()
  2. call string::string(char const*) and throw

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. ]

Proposed resolution:

Change the synopsis in D.9.1 [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.9.1.3 [auto.ptr.conv].

Change D.9.1 [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.9.1.1 [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.9.1.1 [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.


466. basic_string ctor should prevent null pointer error

Section: 21.4.1 [string.require] Status: Open Submitter: Daniel Frey Opened: 2004-06-10 Last modified: 2009-05-23

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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.

Proposed resolution:

Add to the synopsis in 21.4 [basic.string]

basic_string( nullptr_t ) = delete;

471. result of what() implementation-defined

Section: 18.7.1 [type.info] Status: Open Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2009-05-23

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Discussion:

[lib.exception] specifies the following:

    exception (const exception&) throw();
    exception& operator= (const exception&) throw();

    -4- Effects: Copies an exception object.
    -5- Notes: The effects of calling what() after assignment
        are implementation-defined.

First, does the Note only apply to the assignment operator? If so, what are the effects of calling what() on a copy of an object? Is the returned pointer supposed to point to an identical copy of the NTBS returned by what() called on the original object or not?

Second, is this Note intended to extend to all the derived classes in section 19? I.e., does the standard provide any guarantee for the effects of what() called on a copy of any of the derived class described in section 19?

Finally, if the answer to the first question is no, I believe it constitutes a defect since throwing an exception object typically implies invoking the copy ctor on the object. If the answer is yes, then I believe the standard ought to be clarified to spell out exactly what the effects are on the copy (i.e., after the copy ctor was called).

[Redmond: Yes, this is fuzzy. The issue of derived classes is fuzzy too.]

[ Batavia: Howard provided wording. ]

[ Bellevue: ]

Eric concerned this is unimplementable, due to nothrow guarantees. Suggested implementation would involve reference counting.

Is the implied reference counting subtle enough to call out a note on implementation? Probably not.

If reference counting required, could we tighten specification further to require same pointer value? Probably an overspecification, especially if exception classes defer evalutation of final string to calls to what().

Remember issue moved open and not resolved at Batavia, but cannot remember who objected to canvas a disenting opinion - please speak up if you disagree while reading these minutes!

Move to Ready as we are accepting words unmodified.

[ Sophia Antipolis: ]

The issue was pulled from Ready. It needs to make clear that only homogenous copying is intended to be supported, not coping from a derived to a base.

[ Batavia (2009-05): ]

Howard supplied the following replacement wording for paragraph 7 of the proposed resolution:

-7- Postcondition: what() shall return the same NTBS as would be obtained by using static_cast to cast the rhs to the same types as the lhs and then calling what() on that possibly sliced object.

Pete asks what "the same NTBS" means.

Proposed resolution:

Change 18.8.1 [exception] to:

exception(const exception& e) throw();
exception& operator=(const exception& e) throw();

-4- Effects: Copies an exception object.

-5- Remarks: The effects of calling what() after assignment are implementation-defined.

-5- Throws: Nothing. This also applies to all standard library-defined classes that derive from exception.

-7- Postcondition: strcmp(what(), e.what()) == 0. This also applies to all standard library-defined classes that derive from exception.


473. underspecified ctype calls

Section: 22.4.1.1 [locale.ctype] Status: Open Submitter: Martin Sebor Opened: 2004-07-01 Last modified: 2006-12-27

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Discussion:

Most ctype member functions come in two forms: one that operates on a single character at a time and another form that operates on a range of characters. Both forms are typically described by a single Effects and/or Returns clause.

The Returns clause of each of the single-character non-virtual forms suggests that the function calls the corresponding single character virtual function, and that the array form calls the corresponding virtual array form. Neither of the two forms of each virtual member function is required to be implemented in terms of the other.

There are three problems:

1. One is that while the standard does suggest that each non-virtual member function calls the corresponding form of the virtual function, it doesn't actually explicitly require it.

Implementations that cache results from some of the virtual member functions for some or all values of their arguments might want to call the array form from the non-array form the first time to fill the cache and avoid any or most subsequent virtual calls. Programs that rely on each form of the virtual function being called from the corresponding non-virtual function will see unexpected behavior when using such implementations.

2. The second problem is that either form of each of the virtual functions can be overridden by a user-defined function in a derived class to return a value that is different from the one produced by the virtual function of the alternate form that has not been overriden.

Thus, it might be possible for, say, ctype::widen(c) to return one value, while for ctype::widen(&c, &c + 1, &wc) to set wc to another value. This is almost certainly not intended. Both forms of every function should be required to return the same result for the same character, otherwise the same program using an implementation that calls one form of the functions will behave differently than when using another implementation that calls the other form of the function "under the hood."

3. The last problem is that the standard text fails to specify whether one form of any of the virtual functions is permitted to be implemented in terms of the other form or not, and if so, whether it is required or permitted to call the overridden virtual function or not.

Thus, a program that overrides one of the virtual functions so that it calls the other form which then calls the base member might end up in an infinite loop if the called form of the base implementation of the function in turn calls the other form.

Lillehammer: Part of this isn't a real problem. We already talk about caching. 22.1.1/6 But part is a real problem. ctype virtuals may call each other, so users don't know which ones to override to avoid avoid infinite loops.

This is a problem for all facet virtuals, not just ctype virtuals, so we probably want a blanket statement in clause 22 for all facets. The LWG is leaning toward a blanket prohibition, that a facet's virtuals may never call each other. We might want to do that in clause 27 too, for that matter. A review is necessary. Bill will provide wording.

Proposed resolution:


485. output iterator insufficently constrained

Section: 24.2.3 [output.iterators] Status: Open Submitter: Chris Jefferson Opened: 2004-10-13 Last modified: 2006-12-27

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Discussion:

The note on 24.1.2 Output iterators insufficently 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.]

Proposed resolution:


492. Invalid iterator arithmetic expressions

Section: 17.5.1.4 [structure.specifications] Status: Open Submitter: Thomas Mang Opened: 2004-12-12 Last modified: 2009-05-10

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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.

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.]


498. Requirements for partition() and stable_partition() too strong

Section: 25.4.13 [alg.partitions] Status: Open Submitter: Sean Parent, Joe Gottman Opened: 2005-05-04 Last modified: 2009-05-01

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Discussion:

Problem: The iterator requirements for partition() and stable_partition() [25.2.12] are listed as BidirectionalIterator, however, there are efficient algorithms for these functions that only require ForwardIterator that have been known since before the standard existed. The SGI implementation includes these (see http://www.sgi.com/tech/stl/partition.html and http://www.sgi.com/tech/stl/stable_partition.html).

[ 2009-04-30 Alisdair adds: ]

Now we have concepts this is easier to express!

Proposed resolution:

Add the following signature to:

Header <algorithm> synopsis 25.2 [algorithms.syn]
p3 Partitions 25.4.13 [alg.partitions]

 template<ForwardIterator Iter, Predicate<auto, Iter::value_type> Pred>
   requires ShuffleIterator<Iter>
         && CopyConstructible<Pred>
   Iter partition(Iter first, Iter last, Pred pred);

Update p3 Partitions 25.4.13 [alg.partitions]:

Complexity: At most (last - first)/2 swaps. Exactly last - first applications of the predicate are done. If Iter satisfies BidirectionalIterator, at most (last - first)/2 swaps. Exactly last - first applications of the predicate are done.

If Iter merely satisfied ForwardIterator at most (last - first) swaps are done. Exactly (last - first) applications of the predicate are done.

[Editorial note: I looked for existing precedent in how we might call out distinct overloads overloads from a set of constrained templates, but there is not much existing practice to lean on. advance/distance were the only algorithms I could find, and that wording is no clearer.]

Proposed resolution:

Change 25.2.12 from

template<class BidirectionalIterator, class Predicate> 
BidirectionalIterator partition(BidirectionalIterato r first, 
                                BidirectionalIterator last, 
                                Predicate pred); 

to

template<class ForwardIterator, class Predicate> 
ForwardIterator partition(ForwardIterator first, 
                          ForwardIterator last, 
                          Predicate pred); 

Change the complexity from

At most (last - first)/2 swaps are done. Exactly (last - first) applications of the predicate are done.

to

If ForwardIterator is a bidirectional_iterator, at most (last - first)/2 swaps are done; otherwise at most (last - first) swaps are done. Exactly (last - first) applications of the predicate are done.

Rationale:

Partition is a "foundation" algorithm useful in many contexts (like sorting as just one example) - my motivation for extending it to include forward iterators is foward_list - without this extension you can't partition an foward_list (without writing your own partition). Holes like this in the standard library weaken the argument for generic programming (ideally I'd be able to provide a library that would refine std::partition() to other concepts without fear of conflicting with other libraries doing the same - but that is a digression). I consider the fact that partition isn't defined to work for ForwardIterator a minor embarrassment.

[Mont Tremblant: Moved to Open, request motivation and use cases by next meeting. Sean provided further rationale by post-meeting mailing.]


502. Proposition: Clarification of the interaction between a facet and an iterator

Section: 22.3.1.1.1 [locale.category] Status: Open Submitter: Christopher Conrade Zseleghovski Opened: 2005-06-07 Last modified: 2008-03-13

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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.

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. ]


503. more on locales

Section: 22.4 [locale.categories] Status: Open Submitter: P.J. Plauger Opened: 2005-06-20 Last modified: 2008-03-13

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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. ]

Proposed resolution:


523. regex case-insensitive character ranges are unimplementable as specified

Section: 28 [re] Status: Open Submitter: Eric Niebler Opened: 2005-07-01 Last modified: 2008-03-13

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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. ]

Proposed resolution:


539. partial_sum and adjacent_difference should mention requirements

Section: 26.7.3 [partial.sum] Status: Open Submitter: Marc Schoolderman Opened: 2006-02-06 Last modified: 2009-05-10

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Discussion:

There are some problems in the definition of partial_sum and adjacent_difference in 26.4 [lib.numeric.ops]

Unlike accumulate and inner_product, these functions are not parametrized on a "type T", instead, 26.4.3 [lib.partial.sum] simply specifies the effects clause as;

Assigns to every element referred to by iterator i in the range [result,result + (last - first)) a value correspondingly equal to

((...(* first + *( first + 1)) + ...) + *( first + ( i - result )))

And similarly for BinaryOperation. Using just this definition, it seems logical to expect that:

char i_array[4] = { 100, 100, 100, 100 };
int  o_array[4];

std::partial_sum(i_array, i_array+4, o_array);

Is equivalent to

int o_array[4] = { 100, 100+100, 100+100+100, 100+100+100+100 };

i.e. 100, 200, 300, 400, with addition happening in the result type, int.

Yet all implementations I have tested produce 100, -56, 44, -112, because they are using an accumulator of the InputIterator's value_type, which in this case is char, not int.

The issue becomes more noticeable when the result of the expression *i + *(i+1) or binary_op(*i, *i-1) can't be converted to the value_type. In a contrived example:

enum not_int { x = 1, y = 2 };
...
not_int e_array[4] = { x, x, y, y };
std::partial_sum(e_array, e_array+4, o_array);

Is it the intent that the operations happen in the input type, or in the result type?

If the intent is that operations happen in the result type, something like this should be added to the "Requires" clause of 26.4.3/4 [lib.partial.sum]:

The type of *i + *(i+1) or binary_op(*i, *(i+1)) shall meet the requirements of CopyConstructible (20.1.3) and Assignable (23.1) types.

(As also required for T in 26.4.1 [lib.accumulate] and 26.4.2 [lib.inner.product].)

The "auto initializer" feature proposed in N1894 is not required to implement partial_sum this way. The 'narrowing' behaviour can still be obtained by using the std::plus<> function object.

If the intent is that operations happen in the input type, then something like this should be added instead;

The type of *first shall meet the requirements of CopyConstructible (20.1.3) and Assignable (23.1) types. The result of *i + *(i+1) or binary_op(*i, *(i+1)) shall be convertible to this type.

The 'widening' behaviour can then be obtained by writing a custom proxy iterator, which is somewhat involved.

In both cases, the semantics should probably be clarified.

26.4.4 [lib.adjacent.difference] is similarly underspecified, although all implementations seem to perform operations in the 'result' type:

unsigned char i_array[4] = { 4, 3, 2, 1 };
int o_array[4];

std::adjacent_difference(i_array, i_array+4, o_array);

o_array is 4, -1, -1, -1 as expected, not 4, 255, 255, 255.

In any case, adjacent_difference doesn't mention the requirements on the value_type; it can be brought in line with the rest of 26.4 [lib.numeric.ops] by adding the following to 26.4.4/2 [lib.adjacent.difference]:

The type of *first shall meet the requirements of CopyConstructible (20.1.3) and Assignable (23.1) types."

[ Berlin: Giving output iterator's value_types very controversial. Suggestion of adding signatures to allow user to specify "accumulator". ]

[ Bellevue: ]

The intent of the algorithms is to perform their calculations using the type of the input iterator. Proposed wording provided.

[ Sophia Antipolis: ]

We did not agree that the proposed resolution was correct. For example, when the arguments are types (float*, float*, double*), the highest-quality solution would use double as the type of the accumulator. If the intent of the wording is to require that the type of the accumulator must be the input_iterator's value_type, the wording should specify it.

[ 2009-05-09 Alisdair adds: ]

Now that we have the facility, the 'best' accumulator type could probably be deduced as:

std::common_type<InIter::value_type, OutIter::reference>::type

This type would then have additional requirements of constructability and incrementability/assignability.

If this extracting an accumulator type from a pair/set of iterators (with additional requirements on that type) is a problem for multiple functions, it might be worth extracting into a SharedAccumulator concept or similar.

I'll go no further in writing up wording now, until the group gives a clearer indication of preferred direction.

Proposed resolution:

Add to section 26.7.3 [partial.sum] paragraph 4 the following two sentences:

The type of *first shall meet the requirements of CopyConstructible? (20.1.3?) and Assignable (23.1?) types. The result of *i + *(i+1) or binary_op(*i, *(i+1)) shall be convertible to this type.

Add to section 26.7.4 [adjacent.difference] paragraph 2 the following sentence:

The type of *first shall meet the requirements of CopyConstructible? (20.1.3) and Assignable (23.1) types.

546. _Longlong and _ULonglong are integer types

Section: TR1 5.1.1 [tr.rand.req] Status: Open Submitter: Matt Austern Opened: 2006-01-10 Last modified: 2007-10-09

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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.

Proposed resolution:


564. stringbuf seekpos underspecified

Section: 27.8.1.4 [stringbuf.virtuals] Status: Open Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2007-10-10

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Discussion:

The effects of the seekpos() member function of basic_stringbuf simply say that the function positions the input and/or output sequences but fail to spell out exactly how. This is in contrast to the detail in which seekoff() is described.

Proposed resolution:

Change 27.7.1.3, p13 to read:

-13- Effects: Same as seekoff(off_type(sp), ios_base::beg, which). Alters the stream position within the controlled sequences, if possible, to correspond to the stream position stored in sp (as described below).

[ Kona (2007): A pos_type is a position in a stream by definition, so there is no ambiguity as to what it means. Proposed Disposition: NAD ]

[ Post-Kona Martin adds: I'm afraid I disagree with the Kona '07 rationale for marking it NAD. The only text that describes precisely what it means to position the input or output sequence is in seekoff(). The seekpos() Effects clause is inadequate in comparison and the proposed resolution plugs the hole by specifying seekpos() in terms of seekoff(). ]


565. xsputn inefficient

Section: 27.6.2.4.5 [streambuf.virt.put] Status: Open Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2007-10-09

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Discussion:

streambuf::xsputn() is specified to have the effect of "writing up to n characters to the output sequence as if by repeated calls to sputc(c)."

Since sputc() is required to call overflow() when (pptr() == epptr()) is true, strictly speaking xsputn() should do the same. However, doing so would be suboptimal in some interesting cases, such as in unbuffered mode or when the buffer is basic_stringbuf.

Assuming calling overflow() is not really intended to be required and the wording is simply meant to describe the general effect of appending to the end of the sequence it would be worthwhile to mention in xsputn() that the function is not actually required to cause a call to overflow().

Proposed resolution:

Add the following sentence to the xsputn() Effects clause in 27.5.2.4.5, p1 (N1804):

-1- Effects: Writes up to n characters to the output sequence as if by repeated calls to sputc(c). The characters written are obtained from successive elements of the array whose first element is designated by s. Writing stops when either n characters have been written or a call to sputc(c) would return traits::eof(). It is uspecified whether the function calls overflow() when (pptr() == epptr()) becomes true or whether it achieves the same effects by other means.

In addition, I suggest to add a footnote to this function with the same text as Footnote 292 to make it extra clear that derived classes are permitted to override xsputn() for efficiency.

[ Kona (2007): We want to permit a streambuf that streams output directly to a device without making calls to sputc or overflow. We believe that has always been the intention of the committee. We believe that the proposed wording doesn't accomplish that. Proposed Disposition: Open ]


568. log2 overloads missing

Section: TR1 8.16.4 [tr.c99.cmath.over] Status: Tentatively NAD Submitter: Paolo Carlini Opened: 2006-03-07 Last modified: 2009-05-23

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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.


573. C++0x file positioning should handle modern file sizes

Section: 27.5.3 [fpos] Status: Open Submitter: Beman Dawes Opened: 2006-04-12 Last modified: 2007-10-09

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Discussion:

There are two deficiencies related to file sizes:

  1. It doesn't appear that the Standard Library is specified in a way that handles modern file sizes, which are often too large to be represented by an unsigned long.
  2. The std::fpos class does not currently have the ability to set/get file positions.

The Dinkumware implementation of the Standard Library as shipped with the Microsoft compiler copes with these issues by:

  1. Defining fpos_t be long long, which is large enough to represent any file position likely in the foreseeable future.
  2. Adding member functions to class fpos. For example,
    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 ]

Proposed resolution:


580. unused allocator members

Section: 23.2.1 [container.requirements.general] Status: Open Submitter: Martin Sebor Opened: 2006-06-14 Last modified: 2009-05-01

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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].

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 Allocator copy anthe allocator argument from their respective first parameters. All other constructors for these container types take an const Allocator& argument (20.1.6), an allocator whose value_type is the same as the container's value_type. A copy of this argument isshall 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 the allocate() member function on a copy of the allocator object of the appropriate type New Footnote), and deallocation "as if" by calling deallocate() 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's value_type, and to obtain their address. All objects residing in storage allocated by a container's allocator shall be constructed "as if" by calling the construct() member function on a copy of the allocator object of the appropriate type. The same objects shall be destroyed "as if" by calling destroy() on a copy of the same allocator object of the same type. The address of such objects shall be obtained "as if" by calling the address() 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's value_type the container may store at the same time. The container member function max_size() obtains this number from the value returned by a call to get_allocator().max_size(). In all container types defined in this clause that are parametrized on Allocator, the member get_allocator() returns a copy of the Allocator object used to construct the container.258)

New Footnote: This type may be different from Allocator: it may be derived from Allocator via Allocator::rebind<U>::other for the appropriate type U.

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. ]


582. specialized algorithms and volatile storage

Section: 20.8.11.2 [uninitialized.copy] Status: Open Submitter: Martin Sebor Opened: 2006-06-14 Last modified: 2009-03-14

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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. ]

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.


585. facet error reporting

Section: 22.4 [locale.categories] Status: Open Submitter: Martin Sebor, Paolo Carlini Opened: 2006-06-22 Last modified: 2007-10-09

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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.

Proposed resolution:

We believe the intent is for all facet member functions that take an ios_base::iostate& argument to:

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 ]


588. requirements on zero sized tr1::arrays and other details

Section: 23.3.1 [array] Status: Open Submitter: Gennaro Prota Opened: 2006-07-18 Last modified: 2009-05-30

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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: ]

  1. 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.
  2. 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 23.3.1.7 [array.tuple] does provide this functionality now.

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 ]


597. Decimal: The notion of 'promotion' cannot be emulated by user-defined types.

Section: TRDecimal 3.2 [trdec.types.types] Status: Open Submitter: Daveed Vandevoorde Opened: 2006-04-05 Last modified: 2009-05-01

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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:

  1. require that the decimal types be class types
  2. require that the decimal types be builtin types, like float and double
  3. specify a library of class types, but allow enough implementor latitude that a conforming implementation could instead provide builtin types

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.

Proposed resolution:


606. Decimal: allow narrowing conversions

Section: TRDecimal 3.2 [trdec.types.types] Status: Open Submitter: Martin Sebor Opened: 2006-06-15 Last modified: 2007-01-15

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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.

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. ]


614. std::string allocator requirements still inconsistent

Section: 21.4 [basic.string] Status: Open Submitter: Bo Persson Opened: 2006-12-05 Last modified: 2008-03-12

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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: ]

  1. If an allocator is passed in, use it, or,
  2. If a string is passed in, use its allocator.

[ 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. ]

Proposed resolution:


617. std::array is a sequence that doesn't satisfy the sequence requirements?

Section: 23.3.1 [array] Status: Open Submitter: Bo Persson Opened: 2006-12-30 Last modified: 2008-03-14

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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.

Proposed resolution:


625. mixed up Effects and Returns clauses

Section: 17 [library] Status: Open Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2009-05-23

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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.

Proposed resolution:


630. arrays of valarray

Section: 26.6.2.1 [valarray.cons] Status: Open Submitter: Martin Sebor Opened: 2007-01-28 Last modified: 2008-06-02

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Discussion:

Section 26.2 [numeric.requirements], p1 suggests that a valarray specialization on a type T that satisfies the requirements enumerated in the paragraph is itself a valid type on which valarray may be instantiated (Footnote 269 makes this clear). I.e., valarray<valarray<T> > is valid as long as T is valid. However, since implementations of valarray are permitted to initialize storage allocated by the class by invoking the default ctor of T followed by the copy assignment operator, such implementations of valarray wouldn't work with (perhaps user-defined) specializations of valarray whose assignment operator had undefined behavior when the size of its argument didn't match the size of *this. By "wouldn't work" I mean that it would be impossible to resize such an array of arrays by calling the resize() member function on it if the function used the copy assignment operator after constructing all elements using the default ctor (e.g., by invoking new value_type[N]) to obtain default-initialized storage) as it's permitted to do.

Stated more generally, the problem is that valarray<valarray<T> >::resize(size_t) isn't required or guaranteed to have well-defined semantics for every type T that satisfies all requirements in 26.2 [numeric.requirements].

I believe this problem was introduced by the adoption of the resolution outlined in N0857, Assignment of valarrays, from 1996. The copy assignment operator of the original numerical array classes proposed in N0280, as well as the one proposed in N0308 (both from 1993), had well-defined semantics for arrays of unequal size (the latter explicitly only when *this was empty; assignment of non empty arrays of unequal size was a runtime error).

The justification for the change given in N0857 was the "loss of performance [deemed] only significant for very simple operations on small arrays or for architectures with very few registers."

Since tiny arrays on a limited subset of hardware architectures are likely to be an exceedingly rare case (despite the continued popularity of x86) I propose to revert the resolution and make the behavior of all valarray assignment operators well-defined even for non-conformal arrays (i.e., arrays of unequal size). I have implemented this change and measured no significant degradation in performance in the common case (non-empty arrays of equal size). I have measured a 50% (and in some cases even greater) speedup in the case of assignments to empty arrays versus calling resize() first followed by an invocation of the copy assignment operator.

[ Bellevue: ]

If no proposed wording by June meeting, this issue should be closed NAD.

Proposed resolution:

Change 26.6.2.2 [valarray.assign], p1 as follows:

valarray<T>& operator=(const valarray<T>& x);

-1- Each element of the *this array is assigned the value of the corresponding element of the argument array. The resulting behavior is undefined if When the length of the argument array is not equal to the length of the *this array. resizes *this to make the two arrays the same length, as if by calling resize(x.size()), before performing the assignment.

And add a new paragraph just below paragraph 1 with the following text:

-2- Postcondition: size() == x.size().

Also add the following paragraph to 26.6.2.2 [valarray.assign], immediately after p4:

-?- When the length, N of the array referred to by the argument is not equal to the length of *this, the operator resizes *this to make the two arrays the same length, as if by calling resize(N), before performing the assignment.

[ pre-Sophia Antipolis, Martin adds the following compromise wording, but prefers the original proposed resolution: ]

Change 26.6.2.2 [valarray.assign], p1 as follows:

-1- Requires: size() == 0 || size() == x.size().

-2- Effects: If size() == 0 calls x.resize(x.size()) first. Each element of the *this array is assigned the value of the corresponding element of the argument array.

-3- Postcondition: size() == x.size().

Add the following paragraph to 26.6.2.2 [valarray.assign], immediately after p4:

-?- When size() == 0 and the length, N of the array referred to by the argument is not equal to the length of *this, the operator resizes *this to make the two arrays the same length, as if by calling resize(N), before performing the assignment. Otherwise, when size() > 0 and size() != N, the behavior is undefined.

[ Kona (2007): Gaby to propose wording for an alternative resolution in which you can assign to a valarray of size 0, but not to any other valarray whose size is unequal to the right hand side of the assignment. ]


632. Time complexity of size() for std::set

Section: 23.2 [container.requirements] Status: Open Submitter: Lionel B Opened: 2007-02-01 Last modified: 2009-05-23

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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.

Proposed resolution:


635. domain of allocator::address

Section: X [allocator.requirements] Status: Open Submitter: Howard Hinnant Opened: 2007-02-08 Last modified: 2009-05-01

View other active issues in [allocator.requirements].

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Discussion:

The table of allocator requirements in X [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.

Proposed resolution:

Change X [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. ]


644. Possible typos in 'function' description

Section: 20.7.16.2 [func.wrap.func] Status: Tentatively NAD Submitter: Bo Persson Opened: 2007-02-25 Last modified: 2009-05-23

View other active issues in [func.wrap.func].

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Discussion:

20.7.16.2 [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 20.8.13.3 [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 system

it 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 20.7.16.2 [func.wrap.func]

...
private:
   // 20.7.16.2 [func.wrap.func], undefined operators:
   template<class Function2> bool void operator==(const function<Function2>&);
   template<class Function2> bool void operator!=(const function<Function2>&);
};

Change 20.7.16.2 [func.wrap.func]

template<class Function2> bool void operator==(const function<Function2>&);
template<class Function2> bool void operator!=(const function<Function2>&);

659. istreambuf_iterator should have an operator->()

Section: 24.6.3 [istreambuf.iterator] Status: Open Submitter: Niels Dekker Opened: 2007-03-25 Last modified: 2009-05-30

View all other issues in [istreambuf.iterator].

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Discussion:

Greg Herlihy has clearly demonstrated that a user defined input iterator should have an operator->(), even if its value type is a built-in type (comp.std.c++, "Re: Should any iterator have an operator->() in C++0x?", March 2007). And as Howard Hinnant remarked in the same thread that the input iterator istreambuf_iterator doesn't have one, this must be a defect!

Based on Greg's example, the following code demonstrates the issue:

 #include <iostream> 
 #include <fstream>
 #include <streambuf> 

 typedef char C;
 int main ()
 {
   std::ifstream s("filename", std::ios::in);
   std::istreambuf_iterator<char> i(s);

   (*i).~C();  // This is well-formed...
   i->~C();  // ... so this should be supported!
 }

Of course, operator-> is also needed when the value_type of istreambuf_iterator is a class.

The operator-> could be implemented in various ways. For instance, by storing the current value inside the iterator, and returning its address. Or by returning a proxy, like operator_arrow_proxy, from http://www.boost.org/boost/iterator/iterator_facade.hpp

I hope that the resolution of this issue will contribute to getting a clear and consistent definition of iterator concepts.

[ Kona (2007): The proposed resolution is inconsistent because the return type of istreambuf_iterator::operator->() is specified to be pointer, but the proposed text also states that "operator-> may return a proxy." ]

[ Niels Dekker (mailed to Howard Hinnant): ]

The proposed resolution does not seem inconsistent to me. istreambuf_iterator::operator->() should have istreambuf_iterator::pointer as return type, and this return type may in fact be a proxy.

AFAIK, the resolution of 445 ("iterator_traits::reference unspecified for some iterator categories") implies that for any iterator class Iter, the return type of operator->() is Iter::pointer, by definition. I don't think Iter::pointer needs to be a raw pointer.

Still I wouldn't mind if the text "operator-> may return a proxy" would be removed from the resolution. I think it's up to the library implementation, how to implement istreambuf_iterator::operator->(). As longs as it behaves as expected: i->m should have the same effect as (*i).m. Even for an explicit destructor call, i->~C(). The main issue is just: istreambuf_iterator should have an operator->()!

[ 2009-04-30 Alisdair adds: ]

Note that operator-> is now a requirement in the InputIterator concept, so this issue cannot be ignored or existing valid programs will break when compiled with an 0x library.

[ 2009-05-29 Alisdair adds: ]

I agree with the observation that in principle the type 'pointer' may be a proxy, and the words highlighting this are redundant.

However, in the current draught pointer is required to be exactly 'charT *' by the derivation from std::iterator. At a minimum, the 4th parameter of this base class template should become unspecified. That permits the introduction of a proxy as a nested class in some further undocumented (not even exposition-only) base.

It also permits the istream_iterator approach where the cached value is stored in the iterator itself, and the iterator serves as its own proxy for post-increment operator++ - removing the need for the existing exposition-only nested class proxy.

Note that the current proxy class also has exactly the right properties to serve as the pointer proxy too. This is likely to be a common case where an InputIterator does not hold internal state but delegates to another class.

Proposed Resolution:

In addition to the current proposal:

24.6.3 [istreambuf.iterator]

template<class charT, class traits = char_traits<charT> >
class istreambuf_iterator
  : public iterator<input_iterator_tag, charT,
                    typename traits::off_type, charT* unspecified, charT> {

Proposed resolution:

Add to the synopsis in 24.6.3 [istreambuf.iterator]:

charT operator*() const;
pointer operator->() const;
istreambuf_iterator<charT,traits>& operator++();

Change 24.6.3 [istreambuf.iterator], p1:

The class template istreambuf_iterator reads successive characters from the streambuf for which it was constructed. operator* provides access to the current input character, if any. operator-> may return a proxy. Each time operator++ is evaluated, the iterator advances to the next input character. If the end of stream is reached (streambuf_type::sgetc() returns traits::eof()), the iterator becomes equal to the end of stream iterator value. 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.


667. money_get's widened minus sign

Section: 22.4.6.1.2 [locale.money.get.virtuals] Status: Tentatively NAD Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2009-05-23

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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:


668. money_get's empty minus sign

Section: 22.4.6.1.2 [locale.money.get.virtuals] Status: Tentatively NAD Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2009-05-23

View other active issues in [locale.money.get.virtuals].

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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 error

And:

pattern = {symbol, sign, value, none}
positive_sign = ""
negative_sign = ""
$123   // a positive value, no sign possible
$+123  // a parse error
$-123  // a parse error

And (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 value

The 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.

Proposed resolution:


669. Equivalent postive and negative signs in money_get

Section: 22.4.6.1.2 [locale.money.get.virtuals] Status: Tentatively NAD Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2009-05-23

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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:


671. precision of hexfloat

Section: 22.4.2.2.2 [facet.num.put.virtuals] Status: Open Submitter: John Salmon Opened: 2007-04-20 Last modified: 2009-03-12

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Discussion:

I am trying to understand how TR1 supports hex float (%a) output.

As far as I can tell, it does so via the following:

8.15 Additions to header <locale> [tr.c99.locale]

In subclause 22.4.2.2.2 [facet.num.put.virtuals], Table 58 Floating-point conversions, after the line: floatfield == ios_base::scientific %E

add the two lines:

floatfield == ios_base::fixed | ios_base::scientific && !uppercase %a
floatfield == ios_base::fixed | ios_base::scientific %A 2

[Note: The additional requirements on print and scan functions, later in this clause, ensure that the print functions generate hexadecimal floating-point fields with a %a or %A conversion specifier, and that the scan functions match hexadecimal floating-point fields with a %g conversion specifier. end note]

Following the thread, in 22.4.2.2.2 [facet.num.put.virtuals], we find:

For conversion from a floating-point type, if (flags & fixed) != 0 or if str.precision() > 0, then str.precision() is specified in the conversion specification.

This would seem to imply that when floatfield == fixed|scientific, the precision of the conversion specifier is to be taken from str.precision(). Is this really what's intended? I sincerely hope that I'm either missing something or this is an oversight. Please tell me that the committee did not intend to mandate that hex floats (and doubles) should by default be printed as if by %.6a.

[ Howard: I think the fundamental issue we overlooked was that with %f, %e, %g, the default precision was always 6. With %a the default precision is not 6, it is infinity. So for the first time, we need to distinguish between the default value of precision, and the precision value 6. ]

Proposed resolution:

[ Kona (2007): Robert volunteers to propose wording. ]


688. reference_wrapper, cref unsafe, allow binding to rvalues

Section: 20.7.5.1 [refwrap.const] Status: Tentatively Ready Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2009-05-23

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Discussion:

A reference_wrapper can be constructed from an rvalue, either by using the constructor, or via cref (and ref in some corner cases). This leads to a dangling reference being stored into the reference_wrapper object. Now that we have a mechanism to detect an rvalue, we can fix them to disallow this source of undefined behavior.

Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.

[ 2009-05-09 Alisdair adds: ]

Now that ref/cref are constained that T must be an ObjectType, I do not believe there is any risk of binding ref to a temporary (which would rely on deducing T to be an rvalue reference type)

However, the problem for cref remains, so I recommend retaining that deleted overload.

[ 2009-05-10 Howard adds: ]

Without:

template <class T> void ref(const T&& t) = delete;

I believe this program will compile:

#include <functional>

struct A {};

const A source() {return A();}

int main()
{
   std::reference_wrapper<const A> r = std::ref(source());
}

I.e. in:

template <ObjectType T> reference_wrapper<T> ref(T& t);

this:

ref(source())

deduces T as const A, and so:

ref(const A& t)

will bind to a temporary (tested with a pre-concepts rvalue-ref enabled compiler).

Therefore I think we still need the ref-protection. I respectfully disagree with Alisdair's comment and am in favor of the proposed wording as it stands. Also, CWG 606 (noted below) has now been "favorably" resolved.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 20.7 [function.objects], add the following two signatures to the synopsis:

template <class T> void ref(const T&& t) = delete;
template <class T> void cref(const T&& t) = delete;

[ N2292 addresses the first part of the resolution but not the second. ]

[ Bellevue: Doug noticed problems with the current wording. ]

[ post Bellevue: Howard and Peter provided revised wording. ]

[ This resolution depends on a "favorable" resolution of CWG 606: that is, the "special deduction rule" is disabled with the const T&& pattern. ]


696. istream::operator>>(int&) broken

Section: 27.7.1.2.2 [istream.formatted.arithmetic] Status: Open Submitter: Martin Sebor Opened: 2007-06-23 Last modified: 2009-05-28

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Discussion:

From message c++std-lib-17897:

The code shown in 27.7.1.2.2 [istream.formatted.arithmetic] as the "as if" implementation of the two arithmetic extractors that don't have a corresponding num_get interface (i.e., the short and int overloads) is subtly buggy in how it deals with EOF, overflow, and other similar conditions (in addition to containing a few typos).

One problem is that if num_get::get() reaches the EOF after reading in an otherwise valid value that exceeds the limits of the narrower type (but not LONG_MIN or LONG_MAX), it will set err to eofbit. Because of the if condition testing for (err == 0), the extractor won't set failbit (and presumably, return a bogus value to the caller).

Another problem with the code is that it never actually sets the argument to the extracted value. It can't happen after the call to setstate() since the function may throw, so we need to show when and how it's done (we can't just punt as say: "it happens afterwards"). However, it turns out that showing how it's done isn't quite so easy since the argument is normally left unchanged by the facet on error except when the error is due to a misplaced thousands separator, which causes failbit to be set but doesn't prevent the facet from storing the value.

[ Batavia (2009-05): ]

We believe this part of the Standard has been recently adjusted and that this issue was addressed during that rewrite.

Move to NAD.

[ 2009-05-28 Howard adds: ]

I've moved this issue from Tentatively NAD to Open.

The current wording of N2857 in 22.4.2.1.2 [facet.num.get.virtuals] p3, stage 3 appears to indicate that in parsing arithmetic types, the value is always set, but sometimes in addition to setting failbit.

However there is a contradictory sentence in 22.4.2.1.2 [facet.num.get.virtuals] p1.

27.7.1.2.2 [istream.formatted.arithmetic] should mimic the behavior of 22.4.2.1.2 [facet.num.get.virtuals] (whatever we decide that behavior is) for int and short, and currently does not. I believe that the correct code fragment should look like:

typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
iostate err = ios_base::goodbit;
long lval;
use_facet<numget>(loc).get(*this, 0, *this, err, lval);
if (lval < numeric_limits<int>::min())
{
  err |= ios_base::failbit;
  val = numeric_limits<int>::min();
}
else if (lval > numeric_limits<int>::max())
{
  err |= ios_base::failbit;
  val = numeric_limits<int>::max();
}
else
  val = static_cast<int>(lval);
setstate(err);

Proposed resolution:

Change 22.4.2.1.2 [facet.num.get.virtuals], p1:

-1- Effects: Reads characters from in, interpreting them according to str.flags(), use_facet<ctype<charT> >(loc), and use_facet< numpunct<charT> >(loc), where loc is str.getloc(). If an error occurs, val is unchanged; otherwise it is set to the resulting value.

Change 27.7.1.2.2 [istream.formatted.arithmetic], p2 and p3:

operator>>(short& val);

-2- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
iostate err = iostate_base::goodbit;
long lval;
use_facet<numget>(loc).get(*this, 0, *this, err, lval);
if (err != 0)
  ;
else if (lval < numeric_limits<short>::min()
  || numeric_limits<short>::max() < lval)
     err = ios_base::failbit;
if (lval < numeric_limits<short>::min())
{
  err |= ios_base::failbit;
  val = numeric_limits<short>::min();
}
else if (lval > numeric_limits<short>::max())
{
  err |= ios_base::failbit;
  val = numeric_limits<short>::max();
}
else
  val = static_cast<short>(lval);
setstate(err);
operator>>(int& val);

-3- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
iostate err = iostate_base::goodbit;
long lval;
use_facet<numget>(loc).get(*this, 0, *this, err, lval);
if (err != 0)
  ;
else if (lval < numeric_limits<int>::min()
  || numeric_limits<int>::max() < lval)
     err = ios_base::failbit;
if (lval < numeric_limits<int>::min())
{
  err |= ios_base::failbit;
  val = numeric_limits<int>::min();
}
else if (lval > numeric_limits<int>::max())
{
  err |= ios_base::failbit;
  val = numeric_limits<int>::max();
}
else
  val = static_cast<int>(lval);
setstate(err);

701. assoc laguerre poly's

Section: TR1 5.2.1.1 [tr.num.sf.Lnm] Status: Tentatively NAD Submitter: Christopher Crawford Opened: 2007-06-30 Last modified: 2009-05-23

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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:


702. Restriction in associated Legendre functions

Section: TR1 5.2.1.2 [tr.num.sf.Plm] Status: Tentatively NAD Submitter: Christopher Crawford Opened: 2007-06-30 Last modified: 2009-05-23

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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:


708. Locales need to be per thread and updated for POSIX changes

Section: 22 [localization] Status: Open Submitter: Peter Dimov Opened: 2007-07-28 Last modified: 2008-09-17

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Discussion:

The POSIX "Extended API Set Part 4,"

http://www.opengroup.org/sib/details.tpl?id=C065

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. ]

Proposed resolution:


711. Contradiction in empty shared_ptr

Section: 20.8.13.2.5 [util.smartptr.shared.obs] Status: Open Submitter: Peter Dimov Opened: 2007-08-24 Last modified: 2008-06-18

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Discussion:

A discussion on comp.std.c++ has identified a contradiction in the shared_ptr specification. The note:

[ Note: this constructor allows creation of an empty shared_ptr instance with a non-NULL stored pointer. -end note ]

after the aliasing constructor

template<class Y> shared_ptr(shared_ptr<Y> const& r, T *p);

reflects the intent of N2351 to, well, allow the creation of an empty shared_ptr with a non-NULL stored pointer.

This is contradicted by the second sentence in the Returns clause of 20.8.13.2.5 [util.smartptr.shared.obs]:

T* get() const;

Returns: the stored pointer. Returns a null pointer if *this is empty.

[ Bellevue: ]

Adopt option 1 and move to review, not ready.

There was a lot of confusion about what an empty shared_ptr is (the term isn't defined anywhere), and whether we have a good mental model for how one behaves. We think it might be possible to deduce what the definition should be, but the words just aren't there. We need to open an issue on the use of this undefined term. (The resolution of that issue might affect the resolution of issue 711.)

The LWG is getting more uncomfortable with the aliasing proposal (N2351) now that we realize some of its implications, and we need to keep an eye on it, but there isn't support for removing this feature at this time.

[ Sophia Antipolis: ]

We heard from Peter Dimov, who explained his reason for preferring solution 1.

Because it doesn't seem to add anything. It simply makes the behavior for p = 0 undefined. For programmers who don't create empty pointers with p = 0, there is no difference. Those who do insist on creating them presumably have a good reason, and it costs nothing for us to define the behavior in this case.

The aliasing constructor is sharp enough as it is, so "protecting" users doesn't make much sense in this particular case.

> Do you have a use case for r being empty and r being non-null?

I have received a few requests for it from "performance-conscious" people (you should be familiar with this mindset) who don't like the overhead of allocating and maintaining a control block when a null deleter is used to approximate a raw pointer. It is obviously an "at your own risk", low-level feature; essentially a raw pointer behind a shared_ptr facade.

We could not agree upon a resolution to the issue; some of us thought that Peter's description above is supporting an undesirable behavior.

Proposed resolution:

In keeping the N2351 spirit and obviously my preference, change 20.8.13.2.5 [util.smartptr.shared.obs]:

T* get() const;

Returns: the stored pointer. Returns a null pointer if *this is empty.

Alternative proposed resolution: (I won't be happy if we do this, but it's possible):

Change 20.8.13.2.1 [util.smartptr.shared.const]:

template<class Y> shared_ptr(shared_ptr<Y> const& r, T *p);

Requires: If r is empty, p shall be 0.

[ Note: this constructor allows creation of an empty shared_ptr instance with a non-NULL stored pointer. -- end note ]


716. Production in [re.grammar] not actually modified

Section: 28.14 [re.grammar] Status: Open Submitter: Stephan T. Lavavej Opened: 2007-08-31 Last modified: 2009-05-23

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Discussion:

TR1 7.13 [tr.re.grammar]/3 and C++0x WP 28.14 [re.grammar]/3 say:

The following productions within the ECMAScript grammar are modified as follows:

CharacterClass ::
[ [lookahead ∉ {^}] ClassRanges ]
[ ^ ClassRanges ]

This definition for CharacterClass appears to be exactly identical to that in ECMA-262.

Was an actual modification intended here and accidentally omitted, or was this production accidentally included?

[ Batavia (2009-05): ]

We agree that what is specified is identical to what ECMA-262 specifies. Pete would like to take a bit of time to assess whether we had intended, but failed, to make a change. It would also be useful to hear from John Maddock on the issue.

Move to Open.

Proposed resolution:

Remove this mention of the CharacterClass production.

CharacterClass ::
[ [lookahead ∉ {^}] ClassRanges ]
[ ^ ClassRanges ]

718. basic_string is not a sequence

Section: 21.4 [basic.string] Status: Open Submitter: Bo Persson Opened: 2007-08-18 Last modified: 2008-03-12

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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: ]

General consensus is to suggest option 2.

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.


719. std::is_literal type traits should be provided

Section: 20.6 [meta] Status: Open Submitter: Daniel Krügler Opened: 2007-08-25 Last modified: 2009-03-14

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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:

I strongly suggest that the standard provides a type traits for literal types in 20.6.4.3 [meta.unary.prop] for several reasons:

  1. To keep the traits in sync with existing types.
  2. I see many reasons for programmers to use this trait in template code to provide optimized template definitions for these types, see below.
  3. A user-provided definition of this trait is practically impossible to write portably.

The special problem of reason (c) is that I don't see currently a way to portably test the condition for literal class types:

[ 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. ]

Proposed resolution:

In 20.6.2 [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 20.6.4.3 [meta.unary.prop], table Type Property Predicates, just below the line for the is_pod property add a new line:

TemplateConditionPreconditions
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.

721. wstring_convert inconsistensies

Section: 22.3.3.2.2 [conversions.string] Status: Open Submitter: Bo Persson Opened: 2007-08-27 Last modified: 2008-09-18

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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.

Proposed resolution:


723. basic_regex should be moveable

Section: 28.9 [re.regex] Status: Open Submitter: Daniel Krügler Opened: 2007-08-29 Last modified: 2009-03-13

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Discussion:

Addresses UK 316

According to the current state of the standard draft, the class template basic_regex, as described in 28.9 [re.regex]/3, is neither MoveConstructible nor MoveAssignable. IMO it should be, because typical regex state machines tend to have a rather large data quantum and I have seen several use cases, where a factory function returns regex values, which would take advantage of moveabilities.

[ Sophia Antipolis: ]

Needs wording for the semantics, the idea is agreed upon.

[ Post Summit Daniel updated wording to reflect new "swap rules". ]

Proposed resolution:

In the class definition of basic_regex, just below 28.9 [re.regex]/3, perform the following changes:

  1. Just after basic_regex(const basic_regex&); insert:

    basic_regex(basic_regex&&);
    
  2. Just after basic_regex& operator=(const basic_regex&); insert:

    basic_regex& operator=(basic_regex&&);
    
  3. Just after basic_regex& assign(const basic_regex& that); insert:

    basic_regex& assign(basic_regex&& that);
    
  4. In 28.9.2 [re.regex.construct], just after p.11 add the following new member definition:

    basic_regex(basic_regex&& e);
    

    Effects: Move-constructs a basic_regex instance from e.

    Postconditions: flags() and mark_count() return e.flags() and e.mark_count(), respectively, that e had before construction, leaving e in a valid state with an unspecified value.

    Throws: nothing.

  5. Also in 28.9.2 [re.regex.construct], just after p.18 add the following new member definition:

    basic_regex& operator=(basic_regex&& e);
    
    Effects: Returns the result of assign(std::move(e)).
  6. In 28.9.3 [re.regex.assign], just after p. 2 add the following new member definition:

    basic_regex& assign(basic_regex&& rhs);
    

    Effects: Move-assigns a basic_regex instance from rhs and returns *this.

    Postconditions: flags() and mark_count() return rhs.flags() and rhs.mark_count(), respectively, that rhs had before assignment, leaving rhs in a valid state with an unspecified value.

    Throws: nothing.


726. Missing regex_replace() overloads

Section: 28.12.4 [re.alg.replace] Status: Open Submitter: Stephan T. Lavavej Opened: 2007-09-22 Last modified: 2008-06-18

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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);
  1. Overloads taking const charT * are provided for regex_match() and regex_search(), but not regex_replace(). This is inconsistent.
  2. 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.

  3. There is an efficiency consideration: constructing basic_strings can impose performance costs that could be avoided by a library implementation taking C strings and dealing with them directly. (Currently, for replacement sources, C strings can be converted into iterator pairs at the cost of verbosity, but for format strings, there is no way to avoid constructing a basic_string.)

[ Sophia Antipolis: ]

We note that Boost already has these overloads. However, the proposed wording is provided only for 28.12.4 [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.

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). 28.12.4 [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);

727. regex_replace() doesn't accept basic_strings with custom traits and allocators

Section: 28.12.4 [re.alg.replace] Status: Open Submitter: Stephan T. Lavavej Opened: 2007-09-22 Last modified: 2009-05-23

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Discussion:

regex_match() and regex_search() take const basic_string<charT, ST, SA>&. regex_replace() takes const basic_string<charT>&. This prevents regex_replace() from accepting basic_strings with custom traits and allocators.

[ Batavia (2009-05): ]

Bill comments, "We need to look at the depth of this change."

Pete remarks that we are here dealing with a convenience function that saves a user from calling the iterato-based overload.

Move to Open.

Proposed resolution:

Overloads of regex_replace() taking basic_string should be additionally templated on class ST, class SA and take const basic_string<charT, ST, SA>&. Consistency with regex_match() and regex_search() would place class ST, class SA as the first template arguments; compatibility with existing code using TR1 and giving explicit template arguments to regex_replace() would place class ST, class SA as the last template arguments.


747. We have 3 separate type traits to identify classes supporting no-throw operations

Section: 20.6.4.3 [meta.unary.prop] Status: Open Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2008-03-11

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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.

Proposed resolution:


750. The current definition for is_convertible requires that the type be implicitly convertible, so explicit constructors are ignored.

Section: 20.6.5 [meta.rel] Status: Open Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2008-03-11

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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.

Proposed resolution:


751. change pass-by-reference members of vector<bool> to pass-by-value?

Section: 23.3.7 [vector.bool] Status: Open Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2008-09-22

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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.

Proposed resolution:


760. The emplace issue

Section: 23.2 [container.requirements] Status: Open Submitter: Paolo Carlini Opened: 2007-11-11 Last modified: 2008-06-02

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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.

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.


765. more on iterator validity

Section: 24.2 [iterator.concepts] Status: Tentatively Ready Submitter: Martin Sebor Opened: 2007-12-14 Last modified: 2009-03-09

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Discussion:

Issue 278 defines the meaning of the term "invalid iterator" as one that may be singular.

Consider the following code:

   std::deque<int> x, y;
   std::deque<int>::iterator i = x.end(), j = y.end();
   x.swap(y);
       

Given that swap() is required not to invalidate iterators and using the definition above, what should be the expected result of comparing i and j to x.end() and y.end(), respectively, after the swap()?

I.e., is the expression below required to evaluate to true?

   i == y.end() && j == x.end()
       

(There are at least two implementations where the expression returns false.)

More generally, is the definition introduced in issue 278 meant to make any guarantees about whether iterators actually point to the same elements or be associated with the same containers after a non-invalidating operation as they did before?

Here's a motivating example intended to demonstrate the importance of the question:

   Container x, y ({ 1, 2});   // pseudocode to initialize y with { 1, 2 }
   Container::iterator i = y.begin() + 1;
   Container::iterator j = y.end();
   std::swap(x, y);
   std::find(i, j, 3);
       

swap() guarantees that i and j continue to be valid. Unless the spec says that even though they are valid they may no longer denote a valid range the code above must be well-defined. Expert opinions on this differ as does the behavior of popular implementations for some standard Containers.

[ San Francisco: ]

Pablo: add a note to the last bullet of paragraph 11 of 23.1.1 clarifying that the end() iterator doesn't refer to an element and that it can therefore be invalidated.

Proposed wording:

[Note: The end() iterator does not refer to any element and can therefore be invalidated. -- end note]

Howard will add this proposed wording to the issue and then move it to Review.

[ Post Summit: ]

Lawrence: suggestion: "Note: The end() iterator does not refer to any element"

Walter: "Note: The end() iterator can nevertheless be invalidated, because it does not refer to any element."

Nick: "The end() iterator does not refer to any element. It is therefore subject to being invalidated."

Consensus: go with Nick

With that update, Recommend Tentatively Ready.

Proposed resolution:

Add to 23.2.1 [container.requirements.general], p11:

Unless otherwise specified (see 23.1.4.1, 23.1.5.1, 23.2.2.3, and 23.2.6.4) all container types defined in this Clause meet the following additional requirements:


774. Member swap undefined for most containers

Section: 23 [containers] Status: Open Submitter: Alisdair Meredith Opened: 2008-01-14 Last modified: 2008-05-11

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Discussion:

It appears most containers declare but do not define a member-swap function.

This is unfortunate, as all overload the swap algorithm to call the member-swap function! (required for swappable guarantees [Table 37] and Container Requirements [Table 87])

Note in particular that Table 87 gives semantics of a.swap(b) as swap(a,b), yet for all containers we define swap(a,b) to call a.swap(b) - a circular definition.

A quick survey of clause 23 shows that the following containers provide a definition for member-swap:

array
queue
stack
vector

Whereas the following declare it, but do not define the semantics:

deque
list
map
multimap
multiset
priority_queue
set
unordered_map
unordered_multi_map
unordered_multi_set
unordered_set

Suggested resolution:

Provide a definition for each of the affected containers...

[ Bellevue: ]

Move to Open and ask Alisdair to provide wording.

Proposed resolution:

Wording provided in N2590.


780. std::merge() specification incorrect/insufficient

Section: 25.5.4 [alg.merge] Status: Review Submitter: Daniel Krügler Opened: 2008-01-25 Last modified: 2009-05-23

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Discussion:

Though issue 283 has fixed many open issues, it seems that some are still open:

Both 25.3.4 [lib.alg.merge] in 14882:2003 and 25.5.4 [alg.merge] in N2461 have no Requires element and the Effects element contains some requirements, which is probably editorial. Worse is that:

[ Post Summit Alisdair adds: ]

Suggest:

(where last is equal to next(result, distance(first1, last1) + distance(first2, last2)), such that resulting range will be sorted in non-decreasing order; that is, for every iterator i in [result,last) other than result, the condition *i < *prev(i) or, respectively, comp(*i, *prev(i)) will be false.

Note that this might still not be technically accurate in the case of InputIterators, depending on other resolutions working their way through the system (1011).

[ Post Summit Daniel adds: ]

If we want to use prev and next here (Note: merge is sufficiently satisfied with InputIterator) we should instead *add* more to 25 [algorithms]/6, but I can currently not propose any good wording for this.

[ Batavia (2009-05): ]

Pete points out the existing wording in [algorithms]/4 that permits the use of + in algorithm specifications.

Alisdair points out that that wording may not apply to input iterators.

Move to Review.

Proposed resolution:

In 25.5.4 [alg.merge] replace p.1+ 2:

Effects: Merges Copies all the elements of the two sorted ranges [first1,last1) and [first2,last2) into the range [result,result + (last1 - first1) + (last2 - first2)) [result, last) (where last is equal to result + (last1 - first1) + (last2 - first2)), such that resulting range will be sorted in non-decreasing order; that is, for every iterator i in [result,last) other than result, the condition *i < *(i - 1) or, respectively, comp(*i, *(i - 1)) will be false.

Requires: The resulting range shall not overlap with either of the original ranges. The list will be sorted in non-decreasing order according to the ordering defined by comp; that is, for every iterator i in [first,last) other than first, the condition *i < *(i - 1) or comp(*i, *(i - 1)) will be false.

[N.B.: I attempted to reuse the wording style of inplace_merge, therefore proposing to insert ", respectively," between both predicate tests. This is no strictly necessary as other parts of <algorithm> show, just a matter of consistency]


785. Random Number Requirements in TR1

Section: TR1 5.1.4.5 [tr.rand.eng.disc], TR1 5.1.4.6 [tr.rand.eng.xor] Status: Tentatively NAD Submitter: John Maddock Opened: 2008-01-15 Last modified: 2009-05-23

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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.


788. ambiguity in [istream.iterator]

Section: 24.6.1 [istream.iterator] Status: Open Submitter: Martin Sebor Opened: 2008-02-06 Last modified: 2009-03-14

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Discussion:

Addresses UK 287

It is not clear what the initial state of an istream_iterator should be. Is _value_ initialized by reading the stream, or default/value initialized? If it is initialized by reading the stream, what happens if the initialization is deferred until first dereference, when ideally the iterator value should have been that of an end-of-stream iterator which is not safely dereferencable?

Recommendation: Specify _value_ is initialized by reading the stream, or the iterator takes on the end-of-stream value if the stream is empty.

The description of how an istream_iterator object becomes an end-of-stream iterator is a) ambiguous and b) out of date WRT issue 468:

istream_iterator reads (using operator>>) successive elements from the input stream for which it was constructed. After it is constructed, and every time ++ is used, the iterator reads and stores a value of T. If the end of stream is reached (operator void*() on the stream returns false), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments istream_iterator() always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of operator* on an end of stream is not defined. For any other iterator value a const T& is returned. The result of operator-> on an end of stream is not defined. For any other iterator value a const T* is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that ++ operators are not equality preserving, that is, i == j does not guarantee at all that ++i == ++j. Every time ++ is used a new value is read.

istream::operator void*() returns null if istream::fail() is true, otherwise non-null. istream::fail() returns true if failbit or badbit is set in rdstate(). Reaching the end of stream doesn't necessarily imply that failbit or badbit is set (e.g., after extracting an int from stringstream("123") the stream object will have reached the end of stream but fail() is false and operator void*() will return a non-null value).

Also I would prefer to be explicit about calling fail() here (there is no operator void*() anymore.)

[ Summit: ]

Moved from Ready to Open for the purposes of using this issue to address NB UK 287. Martin to handle.

Proposed resolution:

Change 24.6.1 [istream.iterator]/1:

istream_iterator reads (using operator>>) successive elements from the input stream for which it was constructed. After it is constructed, and every time ++ is used, the iterator reads and stores a value of T. If the end of stream is reached the iterator fails to read and store a value of T (operator void*() fail() on the stream returns false true), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments istream_iterator() always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of operator* on an end of stream is not defined. For any other iterator value a const T& is returned. The result of operator-> on an end of stream is not defined. For any other iterator value a const T* is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that ++ operators are not equality preserving, that is, i == j does not guarantee at all that ++i == ++j. Every time ++ is used a new value is read.

801. tuple and pair trivial members

Section: 20.5 [tuple] Status: Open Submitter: Lawrence Crowl Opened: 2008-02-18 Last modified: 2009-05-30

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Discussion:

Classes with trivial special member functions are inherently more efficient than classes without such functions. This efficiency is particularly pronounced on modern ABIs that can pass small classes in registers. Examples include value classes such as complex numbers and floating-point intervals. Perhaps more important, though, are classes that are simple collections, like pair and tuple. When the parameter types of these classes are trivial, the pairs and tuples themselves can be trivial, leading to substantial performance wins.

The current working draft make specification of trivial functions (where possible) much easer through defaulted and deleted functions. As long as the semantics of defaulted and deleted functions match the intended semantics, specification of defaulted and deleted functions will yield more efficient programs.

There are at least two cases where specification of an explicitly defaulted function may be desirable.

First, the std::pair template has a non-trivial default constructor, which prevents static initialization of the pair even when the types are statically initializable. Changing the definition to

pair() = default;

would enable such initialization. Unfortunately, the change is not semantically neutral in that the current definition effectively forces value initialization whereas the change would not value initialize in some contexts.

** Does the committee confirm that forced value initialization was the intent? If not, does the committee wish to change the behavior of std::pair in C++0x?

Second, the same default constructor issue applies to std::tuple. Furthermore, the tuple copy constructor is current non-trivial, which effectively prevents passing it in registers. To enable passing tuples in registers, the copy constructor should be make explicitly defaulted. The new declarations are:

tuple() = default;
tuple(const tuple&) = default;

This changes is not implementation neutral. In particular, it prevents implementations based on pointers to the parameter types. It does however, permit implementations using the parameter types as bases.

** How does the committee wish to trade implementation efficiency versus implementation flexibility?

[ Bellevue: ]

General agreement; the first half of the issue is NAD.

Before voting on the second half, it was agreed that a "Strongly Favor" vote meant support for trivial tuples (assuming usual requirements met), even at the expense of other desired qualities. A "Weakly Favor" vote meant support only if not at the expense of other desired qualities.

Concensus: Go forward, but not at expense of other desired qualities.

It was agreed to Alisdair should fold this work in with his other pair/tuple action items, above, and that issue 801 should be "open", but tabled until Alisdair's proposals are disposed of.

[ 2009-05-27 Daniel adds: ]

This is partly solved by 1117.

Proposed resolution:


810. Missing traits dependencies in operational semantics of extended manipulators

Section: 27.7.4 [ext.manip] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2008-03-01 Last modified: 2009-05-23

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Discussion:

The recent draft (as well as the original proposal n2072) uses an operational semantic for get_money ([ext.manip]/4) and put_money ([ext.manip]/6), which uses

istreambuf_iterator<charT>

and

ostreambuf_iterator<charT>

resp, instead of the iterator instances, with explicitly provided traits argument (The operational semantic defined by f is also traits dependent). This is an obvious oversight because both *stream_buf c'tors expect a basic_streambuf<charT,traits> as argument.

The same problem occurs within the get_time and put_time semantic where additional to the problem we have an editorial issue in get_time (streambuf_iterator instead of istreambuf_iterator).

[ Batavia (2009-05): ]

This appears to be an issue of presentation.

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 27.7.4 [ext.manip]/4 within function f replace the first line

template <class charT, class traits, class moneyT> 
void f(basic_ios<charT, traits>& str, moneyT& mon, bool intl) { 
   typedef istreambuf_iterator<charT, traits> Iter;
   ...

In 27.7.4 [ext.manip]/5 remove the first template charT parameter:

template <class charT, class moneyT> unspecified put_money(const moneyT& mon, bool intl = false);

In 27.7.4 [ext.manip]/6 within function f replace the first line

template <class charT, class traits, class moneyT> 
void f(basic_ios<charT, traits>& str, const moneyT& mon, bool intl) { 
  typedef ostreambuf_iterator<charT, traits> Iter;
  ...

In 27.7.4 [ext.manip]/8 within function f replace the first line

template <class charT, class traits> 
void f(basic_ios<charT, traits>& str, struct tm *tmb, const charT *fmt) { 
  typedef istreambuf_iterator<charT, traits> Iter;
  ...

In 27.7.4 [ext.manip]/10 within function f replace the first line

template <class charT, class traits> 
void f(basic_ios<charT, traits>& str, const struct tm *tmb, const charT *fmt) { 
  typedef ostreambuf_iterator<charT, traits> Iter;
  ...

In 27.7 [iostream.format], Header <iomanip> synopsis change:

template <class charT, class moneyT> T8 put_money(const moneyT& mon, bool intl = false);

814. vector<bool>::swap(reference, reference) not defined

Section: 23.3.7 [vector.bool] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2008-03-17 Last modified: 2009-05-23

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Discussion:

vector<bool>::swap(reference, reference) has no definition.

[ San Francisco: ]

Move to Open. Alisdair to provide a resolution.

[ Post Summit Daniel provided wording. ]

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Just after 23.3.7 [vector.bool]/5 add the following prototype and description:

static void swap(reference x, reference y);

-6- Effects: Exchanges the contents of x and y as-if by:


bool b = x;
x = y;
y = b;

815. std::function and reference_closure do not use perfect forwarding

Section: 20.7.16.2.4 [func.wrap.func.inv] Status: Open Submitter: Alisdair Meredith Opened: 2008-03-16 Last modified: 2009-06-01

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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

  1. Recommends to replace the CopyConstructible requirement by a MoveConstructible requirement
  2. 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?

Proposed resolution:


816. Should bind()'s returned functor have a nofail copy ctor when bind() is nofail?

Section: 20.7.12.1.3 [func.bind.bind] Status: Open Submitter: Stephan T. Lavavej Opened: 2008-02-08 Last modified: 2009-05-23

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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.

Proposed resolution:

Add a new sentence to the end of paragraphs 2 and 4 of 20.7.12.1.3 [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.

...

-4- 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.


817. bind needs to be moved

Section: 20.7.12.1.3 [func.bind.bind] Status: Open Submitter: Howard Hinnant Opened: 2008-03-17 Last modified: 2009-05-23

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Discussion:

Addresses US 72, JP 38 and DE 21

The functor returned by bind() should have a move constructor that requires only move construction of its contained functor and bound arguments. That way move-only functors can be passed to objects such as thread.

This issue is related to issue 816.

US 72:

bind should support move-only functors and bound arguments.

JP 38:

add the move requirement for bind's return type.

For example, assume following th1 and th2,

void f(vector<int> v) { }

vector<int> v{ ... };
thread th1([v]{ f(v); });
thread th2(bind(f, v));

When function object are set to thread, v is moved to th1's lambda expression in a Move Constructor of lambda expression because th1's lambda expression has a Move Constructor. But bind of th2's return type doesn't have the requirement of Move, so it may not moved but copied.

Add the requirement of move to get rid of this useless copy.

And also, add the MoveConstructible as well as CopyConstructible.

DE 21

The specification for bind claims twice that "the values and types for the bound arguments v1, v2, ..., vN are determined as specified below". No such specification appears to exist.

[ San Francisco: ]

Howard to provide wording.

[ Post Summit Alisdair and Howard provided wording. ]

Several issues are being combined in this resolution. They are all touching the same words so this is an attempt to keep one issue from stepping on another, and a place to see the complete solution in one place.

  1. bind needs to be "moved".
  2. 20.7.12.1.3 [func.bind.bind]/p3, p6 and p7 were accidently removed from N2798.
  3. Issue 929 argues for a way to pass by && for efficiency but retain the decaying behavior of pass by value for the thread constructor. That same solution is applicable here.

[ Batavia (2009-05): ]

We were going to recommend moving this issue to Tentatively Ready until we noticed potential overlap with issue 816 (q.v.).

Move to Open, and recommend both issues be considered together (and possibly merged).

Proposed resolution:

Change 20.7 [function.objects] p2:

template<CopyConstructible MoveConstructible Fn, CopyConstructible MoveConstructible... Types>
  unspecified bind(Fn&&, Types&&...);
template<Returnable R, CopyConstructible MoveConstructible Fn, CopyConstructible MoveConstructible... Types>
  unspecified bind(Fn&&, Types&&...);

Change 20.7.12.1.3 [func.bind.bind]:

template<CopyConstructible MoveConstructible F, CopyConstructible MoveConstructible... BoundArgs>
  unspecified bind(F&& f, BoundArgs&&... bound_args);

Requires: unspecified return type shall be MoveConstructible.

-1- Requires: INVOKE(f, w1, w2, ..., wN) (20.6.2) shall be a valid expression for some values w1, w2, ..., wN, where N == sizeof...(bound_args).

-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.

Throws: Nothing unless the constructor of F or of one of the types in the BoundArgs... pack expansion throws an exception.

template<Returnable R, CopyConstructible MoveConstructible F, CopyConstructible MoveConstructible... BoundArgs>
  unspecified bind(F&& f, BoundArgs&&... bound_args);

Requires: unspecified return type shall be MoveConstructible.

-3- Requires: INVOKE(f, w1, w2, ..., wN) shall be a valid expression for some values w1, w2, ..., wN, where N == sizeof...(bound_args).

-4- 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.

Throws: Nothing unless the constructor of F or of one of the types in the BoundArgs... pack expansion throws an exception.

Let the values of bound arguments v1, v2, ..., vN and their corresponding types V1, V2, ..., VN depend on the type of the corresponding argument ti in bound_args in the call to bind and the cv-qualifiers cv of the call wrapper g as follows. Let Ti be an alias for the ith element of the pack expansion decay<BoundArgs>::type..., and let ti be an alias for the ith element in the function parameter pack expansion bound_args...:


819. rethrow_if_nested

Section: 18.8.6 [except.nested] Status: Open Submitter: Alisdair Meredith Opened: 2008-03-25 Last modified: 2008-09-17

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Discussion:

Looking at the wording I submitted for rethrow_if_nested, I don't think I got it quite right.

The current wording says:

template <class E> void rethrow_if_nested(const E& e);

Effects: Calls e.rethrow_nested() only if e is publicly derived from nested_exception.

This is trying to be a bit subtle, by requiring e (not E) to be publicly derived from nested_exception the idea is that a dynamic_cast would be required to be sure. Unfortunately, if e is dynamically but not statically derived from nested_exception, e.rethrow_nested() is ill-formed.

[ San Francisco: ]

Alisdair was volunteered to provide wording.

Proposed resolution:


822. Object with explicit copy constructor no longer CopyConstructible

Section: X [utility.arg.requirements] Status: Tentatively NAD Submitter: James Kanze Opened: 2008-04-01 Last modified: 2009-05-23

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Discussion:

I just noticed that the following program is legal in C++03, but is forbidden in the current draft:

#include <vector>
#include <iostream>

class Toto
{
public:
    Toto() {}
    explicit Toto( Toto const& ) {}
} ;

int
main()
{
    std::vector< Toto > v( 10 ) ;
    return 0 ;
}

Is this change intentional? (And if so, what is the justification? I wouldn't call such code good, but I don't see any reason to break it unless we get something else in return.)

[ San Francisco: ]

The subgroup that looked at this felt this was a good change, but it may already be handled by incoming concepts (we're not sure).
Original Proposed resolution:

In X [utility.arg.requirements] change Table 33: MoveConstructible requirements [moveconstructible]:

expressionpost-condition
T t(rv) = rvt is equivalent to the value of rv before the construction
...

In X [utility.arg.requirements] change Table 34: CopyConstructible requirements [copyconstructible]:

expressionpost-condition
T t(u) = uthe value of u is unchanged and is equivalent to t
...

[ Post Summit: ]

Alisdair: Proposed resolution kinda funky as these tables no longer exist. Move from direct init to copy init. Clarify with Doug, recommends NAD.

Walter: Suggest NAD via introduction of concepts.

Recommend close as NAD.

Proposed resolution:

Recommend close as NAD.


825. Missing rvalues reference stream insert/extract operators?

Section: 19.5.2.2 [syserr.errcode.overview], 20.8.13.2.8 [util.smartptr.shared.io], 22.4.8 [facets.examples], 20.3.6.3 [bitset.operators], 26.4.6 [complex.ops], 27.6 [stream.buffers], 28.10 [re.submatch] Status: Tentatively NAD Editorial Submitter: Alisdair Meredith Opened: 2008-04-10 Last modified: 2009-05-23

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Discussion:

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:


827. constexpr shared_ptr::shared_ptr()?

Section: 20.8.13.2.1 [util.smartptr.shared.const] Status: Open Submitter: Peter Dimov Opened: 2008-04-11 Last modified: 2009-05-30

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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)

Proposed resolution:


828. Static initialization for std::mutex?

Section: 30.4.1.1 [thread.mutex.class] Status: Review Submitter: Peter Dimov Opened: 2008-04-18 Last modified: 2009-05-30

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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. ]

Proposed resolution:

Change 30.4.1.1 [thread.mutex.class]:

class mutex {
public:
  constexpr mutex();
  ...

830. Incomplete list of char_traits specializations

Section: 21.2 [char.traits] Status: Tentatively NAD Editorial Submitter: Dietmar Kühl Opened: 2008-04-23 Last modified: 2009-05-23

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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>, and char_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>;
...
}

833. Freestanding implementations header list needs review for C++0x

Section: 17.6.1.3 [compliance] Status: Open Submitter: Beman Dawes Opened: 2008-05-14 Last modified: 2009-03-11

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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: ]

  1. The comment regarding reference_closure seems moot since it was just recently decided to remove that.
  2. 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.

Proposed resolution:


834. Unique_ptr::pointer requirements underspecified

Section: 20.8.12.2 [unique.ptr.single] Status: Open Submitter: Daniel Krügler Opened: 2008-05-14 Last modified: 2008-06-19

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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:

  1. Operational fail-safety.
  2. (Well-)Definedness of expressions.

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.

Proposed resolution:

Add the following sentence just at the end of the newly proposed 20.8.12.2 [unique.ptr.single]/p. 3:

unique_ptr<T, D>::pointer's operations shall be well-formed, shall have well defined behavior, and shall not throw exceptions.

835. tying two streams together (correction to DR 581)

Section: 27.5.4.2 [basic.ios.members] Status: Review Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2009-05-30

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Discussion:

The fix for issue 581, now integrated into the working paper, overlooks a couple of minor problems.

First, being an unformatted function once again, flush() is required to create a sentry object whose constructor must, among other things, flush the tied stream. When two streams are tied together, either directly or through another intermediate stream object, flushing one will also cause a call to flush() on the other tied stream(s) and vice versa, ad infinitum. The program below demonstrates the problem.

Second, as Bo Persson notes in his comp.lang.c++.moderated post, for streams with the unitbuf flag set such as std::stderr, the destructor of the sentry object will again call flush(). This seems to create an infinite recursion for std::cerr << std::flush;

#include <iostream>

int main ()
{
   std::cout.tie (&std::cerr);
   std::cerr.tie (&std::cout);
   std::cout << "cout\n";
   std::cerr << "cerr\n";
} 

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Review.

[ 2009-05-26 Daniel adds: ]

I think that the most recently suggested change in 27.7.2.4 [ostream::sentry] need some further word-smithing. As written, it would make the behavior undefined, if under conditions when pubsync() should be called, but when in this scenario os.rdbuf() returns 0.

This case is explicitly handled in flush() and needs to be taken care of. My suggested fix is:

If ((os.flags() & ios_base::unitbuf) && !uncaught_exception() && os.rdbuf() != 0) is true, calls os.flush() os.rdbuf()->pubsync().

Two secondary questions are:

  1. Should pubsync() be invoked in any case or shouldn't a base requirement for this trial be that os.good() == true as required in the original flush() case?
  2. Since uncaught_exception() is explicitly tested, shouldn't a return value of -1 of pubsync() produce setstate(badbit) (which may throw ios_base::failure)?

Proposed resolution:

I think an easy way to plug the first hole is to add a requires clause to ostream::tie(ostream *tiestr) requiring the this pointer not be equal to any pointer on the list starting with tiestr->tie() through tiestr()->tie()->tie() and so on. I am not proposing that we require implementations to traverse this list, although I think we could since the list is unlikely to be very long.

Add a Requires clause to 27.5.4.2 [basic.ios.members] withethe following text:

Requires: If (tiestr != 0) is true, tiestr must not be reachable by traversing the linked list of tied stream objects starting from tiestr->tie().

In addition, to prevent the infinite recursion that Bo writes about in his comp.lang.c++.moderated post, I propose to change 27.7.2.4 [ostream::sentry], p2 like so:

If ((os.flags() & ios_base::unitbuf) && !uncaught_exception()) is true, calls os.flush() os.rdbuf()->pubsync().

836. effects of money_base::space and money_base::none on money_get

Section: 22.4.6.1.2 [locale.money.get.virtuals] Status: Open Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2008-09-22

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Duplicate of: 670

Discussion:

In paragraph 2, 22.4.6.1.2 [locale.money.get.virtuals] specifies the following:

Where space or none appears in the format pattern, except at the end, optional white space (as recognized by ct.is) is consumed after any required space.

This requirement can be (and has been) interpreted two mutually exclusive ways by different readers. One possible interpretation is that:

  1. where money_base::space appears in the format, at least one space is required, and
  2. where money_base::none appears in the format, space is allowed but not required.

The other is that:

where either money_base::space or money_base::none appears in the format, white space is optional.

[ San Francisco: ]

Martin will revise the proposed resolution.

Proposed resolution:

I propose to change the text to make it clear that the first interpretation is intended, that is, to make following change to 22.4.6.1.2 [locale.money.get.virtuals], p2:

When money_base::space or money_base::none appears as the last element in the format pattern, except at the end, optional white space (as recognized by ct.is) is consumed after any required space. no white space is consumed. Otherwise, where money_base::space appears in any of the initial elements of the format pattern, at least one white space character is required. Where money_base::none appears in any of the initial elements of the format pattern, white space is allowed but not required. In either case, any required followed by all optional white space (as recognized by ct.is()) is consumed. If (str.flags() & str.showbase) is false, ...

837. basic_ios::copyfmt() overly loosely specified

Section: 27.5.4.2 [basic.ios.members] Status: Tentatively NAD Editorial Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2009-05-23

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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 *this the corresponding member objects of rhs, except that

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() postconditions
Element 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.


838. can an end-of-stream iterator become a non-end-of-stream one?

Section: 24.6.1 [istream.iterator] Status: Open Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2008-10-27

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Discussion:

From message c++std-lib-20003...

The description of istream_iterator in 24.6.1 [istream.iterator], p1 specifies that objects of the class become the end-of-stream (EOS) iterators under the following condition (see also issue 788 another problem with this paragraph):

If the end of stream is reached (operator void*() on the stream returns false), the iterator becomes equal to the end-of-stream iterator value.

One possible implementation approach that has been used in practice is for the iterator to set its in_stream pointer to 0 when it reaches the end of the stream, just like the default ctor does on initialization. The problem with this approach is that the Effects clause for operator++() says the iterator unconditionally extracts the next value from the stream by evaluating *in_stream >> value, without checking for (in_stream == 0).

Conformance to the requirement outlined in the Effects clause can easily be verified in programs by setting eofbit or failbit in exceptions() of the associated stream and attempting to iterate past the end of the stream: each past-the-end access should trigger an exception. This suggests that some other, more elaborate technique might be intended.

Another approach, one that allows operator++() to attempt to extract the value even for EOS iterators (just as long as in_stream is non-0) is for the iterator to maintain a flag indicating whether it has reached the end of the stream. This technique would satisfy the presumed requirement implied by the Effects clause mentioned above, but it isn't supported by the exposition-only members of the class (no such flag is shown). This approach is also found in existing practice.

The inconsistency between existing implementations raises the question of whether the intent of the specification is that a non-EOS iterator that has reached the EOS become a non-EOS one again after the stream's eofbit flag has been cleared? That is, are the assertions in the program below expected to pass?

   sstream strm ("1 ");
   istream_iterator eos;
   istream_iterator it (strm);
   int i;
   i = *it++
   assert (it == eos);
   strm.clear ();
   strm << "2 3 ";
   assert (it != eos);
   i = *++it;
   assert (3 == i);
     

Or is it intended that once an iterator becomes EOS it stays EOS until the end of its lifetime?

[ San Francisco: ]

We like the direction of the proposed resolution. We're not sure about the wording, and we need more time to reflect on it,

Move to Open. Detlef to rewrite the proposed resolution in such a way that no reference is made to exposition only members of istream_iterator.

Proposed resolution:

The discussion of this issue on the reflector suggests that the intent of the standard is for an istreambuf_iterator that has reached the EOS to remain in the EOS state until the end of its lifetime. Implementations that permit EOS iterators to return to a non-EOS state may only do so as an extension, and only as a result of calling istream_iterator member functions on EOS iterators whose behavior is in this case undefined.

To this end we propose to change 24.6.1 [istream.iterator], p1, as follows:

The result of operator-> on an end-of-stream is not defined. For any other iterator value a const T* is returned. Invoking operator++() on an end-of-stream iterator is undefined. It is impossible to store things into istream iterators...

Add pre/postconditions to the member function descriptions of istream_iterator like so:

istream_iterator();
Effects: Constructs the end-of-stream iterator.
Postcondition: in_stream == 0.
istream_iterator(istream_type &s);
Effects: Initializes in_stream with &s. value may be initialized during construction or the first time it is referenced.
Postcondition: in_stream == &s.
istream_iterator(const istream_iterator &x);
Effects: Constructs a copy of x.
Postcondition: in_stream == x.in_stream.
istream_iterator& operator++();
Requires: in_stream != 0.
Effects: *in_stream >> value.
istream_iterator& operator++(int);
Requires: in_stream != 0.
Effects:
istream_iterator tmp (*this);
*in_stream >> value;
return tmp;
     

839. Maps and sets missing splice operation

Section: 23.4 [associative], 23.5 [unord] Status: Open Submitter: Alan Talbot Opened: 2008-05-18 Last modified: 2008-09-22

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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.

Proposed resolution:


847. string exception safety guarantees

Section: 21.4.1 [string.require] Status: Open Submitter: Hervé Brönnimann Opened: 2008-06-05 Last modified: 2009-02-14

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Discussion:

In March, on comp.lang.c++.moderated, I asked what were the string exception safety guarantees are, because I cannot see *any* in the working paper, and any implementation I know offers the strong exception safety guarantee (string unchanged if a member throws exception). The closest the current draft comes to offering any guarantees is 21.4 [basic.string], para 3:

The class template basic_string conforms to the requirements for a Sequence Container (23.1.1), for a Reversible Container (23.1), and for an Allocator-aware container (91). The iterators supported by basic_string are random access iterators (24.1.5).

However, the chapter 23 only says, on the topic of exceptions: 23.2 [container.requirements], para 10:

Unless otherwise specified (see 23.2.2.3 and 23.2.6.4) all container types defined in this clause meet the following additional requirements:

I take it as saying that this paragraph has *no* implication on std::basic_string, as basic_string isn't defined in Clause 23 and this paragraph does not define a *requirement* of Sequence nor Reversible Container, just of the models defined in Clause 23. In addition, LWG Issue 718 proposes to remove 23.2 [container.requirements], para 3.

Finally, the fact that no operation on Traits should throw exceptions has no bearing, except to suggest (since the only other throws should be allocation, out_of_range, or length_error) that the strong exception guarantee can be achieved.

The reaction in that group by Niels Dekker, Martin Sebor, and Bo Persson, was all that this would be worth an LWG issue.

A related issue is that erase() does not throw. This should be stated somewhere (and again, I don't think that the 23.2 [container.requirements], para 1 applies here).

[ San Francisco: ]

Implementors will study this to confirm that it is actually possible.

[ Daniel adds 2009-02-14: ]

The proposed resolution of paper N2815 interacts with this issue (the paper does not refer to this issue).

Proposed resolution:

Add a blanket statement in 21.4.1 [string.require]:

- if any member function or operator of basic_string<charT, traits, Allocator> throws, that function or operator has no effect.

- no erase() or pop_back() function throws.

As far as I can tell, this is achieved by any implementation. If I made a mistake and it is not possible to offer this guarantee, then either state all the functions for which this is possible (certainly at least operator+=, append, assign, and insert), or add paragraphs to Effects clauses wherever appropriate.


851. simplified array construction

Section: 23.3.1 [array] Status: Open Submitter: Benjamin Kosnik Opened: 2008-06-05 Last modified: 2009-06-10

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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:

  1. Add to the array synopsis in 23.3 [sequences]:

    
    template<ReferentType... Args>
    requires ValueType<C> && Constructible<C, Args&&>...
    array<C, sizeof...(Args)>
    make_array(Args&&... args);
    
    
  2. Append after 23.3.1.7 [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> && 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))... }.

Proposed resolution:

Add to the array synopsis in 23.3 [sequences]:

template<ValueType T, ValueType... Args>
  requires Convertible<Args, T>...
  array<T, sizeof...(Args)> 
  make_array(Args&&... args);

Append after 23.3.1.7 [array.tuple] Tuple interface to class template array the following new section.

23.2.1.7 Convenience interface to class template array [array.tuple]

template<ValueType T, ValueType... Args>
  requires Convertible<Args, T>...
  array<T, sizeof...(Args)> 
  make_array(Args&&... args);

Returns: an array<T, sizeof...(Args)> initialized with {std::forward<T>(args)...}.


853. to_string needs updating with zero and one

Section: 20.3.6 [template.bitset] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2009-05-10

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Discussion:

Issue 396 adds defaulted arguments to the to_string member, but neglects to update the three newer to_string overloads.

[ post San Francisco: ]

Daniel found problems with the wording and provided fixes. Moved from Ready to Review.

[ Post Summit: ]

Alisdair: suggest to not repeat the default arguments in B, C, D (definition of to_string members)

Walter: This is not really a definition.

Consensus: Add note to the editor: Please apply editor's judgement whether default arguments should be repeated for B, C, D changes.

Recommend Tentatively Ready.

[ 2009-05-09: See alternative solution in issue 1113. ]

Proposed resolution:

  1. replace in 20.3.6 [template.bitset]/1 (class bitset)

    template <class charT, class traits>
      basic_string<charT, traits, allocator<charT> >
      to_string(charT zero = charT('0'), charT one = charT('1')) const;
    template <class charT>
      basic_string<charT, char_traits<charT>, allocator<charT> >
      to_string(charT zero = charT('0'), charT one = charT('1')) const;
    basic_string<char, char_traits<char>, allocator<char> >
      to_string(char zero = '0', char one = '1') const;
    
  2. replace in 20.3.6.2 [bitset.members]/37

    template <class charT, class traits>
      basic_string<charT, traits, allocator<charT> >
      to_string(charT zero = charT('0'), charT one = charT('1')) const;
    
    37 Returns: to_string<charT, traits, allocator<charT> >(zero, one).
  3. replace in 20.3.6.2 [bitset.members]/38

    template <class charT>
      basic_string<charT, char_traits<charT>, allocator<charT> >
      to_string(charT zero = charT('0'), charT one = charT('1')) const;
    
    38 Returns: to_string<charT, char_traits<charT>, allocator<charT> >(zero, one).
  4. replace in 20.3.6.2 [bitset.members]/39

    basic_string<char, char_traits<char>, allocator<char> >
      to_string(char zero = '0', char one = '1') const;
    
    39 Returns: to_string<char, char_traits<char>, allocator<char> >(zero, one).

854. default_delete converting constructor underspecified

Section: 20.8.12.1.1 [unique.ptr.dltr.dflt] Status: Review Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2009-05-23

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Discussion:

No relationship between U and T in the converting constructor for default_delete template.

Requirements: U* is convertible to T* and has_virtual_destructor<T>; the latter should also become a concept.

Rules out cross-casting.

The requirements for unique_ptr conversions should be the same as those on the deleter.

[ Howard adds 2008-11-26: ]

I believe we need to be careful to not outlaw the following use case, and I believe the current proposed wording (requires Convertible<U*, T*> && HasVirtualDestructor<T>) does so:

#include <memory>

int main()
{
    std::unique_ptr<int> p1(new int(1));
    std::unique_ptr<const int> p2(move(p1));
    int i = *p2;
//    *p2 = i;  // should not compile
}

I've removed "&& HasVirtualDestructor<T>" from the requires clause in the proposed wording.

[ Post Summit: ]

Alisdair: This issue has to stay in review pending a paper constraining unique_ptr.

Consensus: We agree with the resolution, but unique_ptr needs to be constrained, too.

Recommend Keep in Review.

[ Batavia (2009-05): ]

Keep in Review status for the reasons cited.

Proposed resolution:

Change 20.8.12.1.1 [unique.ptr.dltr.dflt]:

namespace std {
  template <class T> struct default_delete {
    default_delete();
    template <class U>
      requires Convertible<U*, T*>
      default_delete(const default_delete<U>&);
    void operator()(T*) const;
  };
}

...

template <class U>
  requires Convertible<U*, T*>
  default_delete(const default_delete<U>& other);

857. condition_variable::time_wait return bool error prone

Section: 30.5.1 [thread.condition.condvar] Status: Open Submitter: Beman Dawes Opened: 2008-06-13 Last modified: 2009-06-14

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Discussion:

The meaning of the bool returned by condition_variable::timed_wait is so obscure that even the class' designer can't deduce it correctly. Several people have independently stumbled on this issue.

It might be simpler to change the return type to a scoped enum:

enum class timeout { not_reached, reached };

That's the same cost as returning a bool, but not subject to mistakes. Your example below would be:

if (cv.wait_until(lk, time_limit) == timeout::reached )
  throw time_out();

[ Beman to supply exact wording. ]

[ San Francisco: ]

There is concern that the enumeration names are just as confusing, if not more so, as the bool. You might have awoken because of a signal or a spurious wakeup, for example.

Group feels that this is a defect that needs fixing.

Group prefers returning an enum over a void return.

Howard to provide wording.

[ 2009-06-14 Beman provided wording. ]

Proposed resolution:

Change Condition variables 30.5 [thread.condition], Header condition_variable synopsis, as indicated:

namespace std {
  class condition_variable;
  class condition_variable_any;

  enum class cv_status { no_timeout, timeout };
}

Change Class condition_variable 30.5.1 [thread.condition.condvar] as indicated:

class condition_variable { 
public:
  ...
  template <class Clock, class Duration>
    bool cv_status wait_until(unique_lock<mutex>& lock,
                    const chrono::time_point<Clock, Duration>& abs_time);
  template <class Clock, class Duration, class Predicate>
    bool wait_until(unique_lock<mutex>& lock,
                    const chrono::time_point<Clock, Duration>& abs_time,
                    Predicate pred);

  template <class Rep, class Period>
    bool cv_status wait_for(unique_lock<mutex>& lock,
                  const chrono::duration<Rep, Period>& rel_time);
  template <class Rep, class Period, class Predicate>
    bool wait_for(unique_lock<mutex>& lock,
                  const chrono::duration<Rep, Period>& rel_time,
                  Predicate pred);
  ...
};

...

template <class Clock, class Duration>
  bool cv_status wait_until(unique_lock<mutex>& lock,
                  const chrono::time_point<Clock, Duration>& abs_time);

-15- Precondition: lock is locked by the calling thread, and either

-16- Effects:

-17- Postcondition: lock is locked by the calling thread.

-18- Returns: Clock::now() < abs_time cv_status::timeout if the function unblocked because abs_time was reached, otherwise cv_status::no_timeout.

-19- Throws: std::system_error when the effects or postcondition cannot be achieved.

-20- Error conditions:

template <class Rep, class Period>
  bool cv_status wait_for(unique_lock<mutex>& lock,
                const chrono::duration<Rep, Period>& rel_time);

-21- Effects Returns:

wait_until(lock, chrono::monotonic_clock::now() + rel_time)

-22- Returns: false if the call is returning because the time duration specified by rel_time has elapsed, otherwise true.

[ This part of the wording may conflict with 859 in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]

template <class Clock, class Duration, class Predicate> 
  bool wait_until(unique_lock<mutex>& lock, 
                  const chrono::time_point<Clock, Duration>& abs_time, 
                  Predicate pred);

-23- Effects:

while (!pred()) 
  if (!wait_until(lock, abs_time) == cv_status::timeout) 
    return pred(); 
return true;

-24- Returns: pred().

-25- [Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. — end note].

Change Class condition_variable_any 30.5.2 [thread.condition.condvarany] as indicated:

class condition_variable_any {
public:
  ...
  template <class Lock, class Clock, class Duration>
    bool cv_status wait_until(Lock& lock,
                    const chrono::time_point<Clock, Duration>& abs_time);
  template <class Lock, class Clock, class Duration, class Predicate>
    bool wait_until(Lock& lock,
                    const chrono::time_point<Clock, Duration>& abs_time,
                    Predicate pred);

  template <class Lock, class Rep, class Period>
    bool cv_status wait_for(Lock& lock,
                  const chrono::duration<Rep, Period>& rel_time);
  template <class Lock, class Rep, class Period, class Predicate>
    bool wait_for(Lock& lock,
                  const chrono::duration<Rep, Period>& rel_time,
                  Predicate pred);
  ...
};

...

template <class Lock, class Clock, class Duration>
  bool cv_status wait_until(Lock& lock,
                  const chrono::time_point<Clock, Duration>& abs_time);

-13- Effects:

-14- Postcondition: lock is locked by the calling thread.

-15- Returns: Clock::now() < abs_time cv_status::timeout if the function unblocked because abs_time was reached, otherwise cv_status::no_timeout.

-16- Throws: std::system_error when the effects or postcondition cannot be achieved.

-17- Error conditions:

template <class Lock, class Rep, class Period>
  bool cv_status wait_for(Lock& lock,
                const chrono::duration<Rep, Period>& rel_time);

-18- Effects Returns:

wait_until(lock, chrono::monotonic_clock::now() + rel_time)

-19- Returns: false if the call is returning because the time duration specified by rel_time has elapsed, otherwise true.

[ This part of the wording may conflict with 859 in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]

template <class Lock, class Clock, class Duration, class Predicate> 
  bool wait_until(Lock& lock, 
                  const chrono::time_point<Clock, Duration>& rel_time abs_time, 
                  Predicate pred);

-20- Effects:

while (!pred()) 
  if (!wait_until(lock, abs_time) == cv_status::timeout) 
    return pred(); 
return true;

-21- Returns: pred().

-22- [Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. — end note].


859. Monotonic Clock is Conditionally Supported?

Section: 30.5 [thread.condition] Status: Open Submitter: Pete Becker Opened: 2008-06-23 Last modified: 2009-06-14

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Discussion:

Related to 958, 959.

N2661 says that there is a class named monotonic_clock. It also says that this name may be a synonym for system_clock, and that it's conditionally supported. So the actual requirement is that it can be monotonic or not, and you can tell by looking at is_monotonic, or it might not exist at all (since it's conditionally supported). Okay, maybe too much flexibility, but so be it.

A problem comes up in the threading specification, where several variants of wait_for explicitly use monotonic_clock::now(). What is the meaning of an effects clause that says

wait_until(lock, chrono::monotonic_clock::now() + rel_time)

when monotonic_clock is not required to exist?

[ San Francisco: ]

Nick: maybe instead of saying that chrono::monotonic_clock is conditionally supported, we could say that it's always there, but not necessarily supported..

Beman: I'd prefer a typedef that identifies the best clock to use for wait_for locks.

Tom: combine the two concepts; create a duration clock type, but keep the is_monotonic test.

Howard: if we create a duration_clock type, is it a typedef or an entirely true type?

There was broad preference for a typedef.

Move to Open. Howard to provide wording to add a typedef for duration_clock and to replace all uses of monotonic_clock in function calls and signatures with duration_clock.

[ Howard notes post-San Francisco: ]

After further thought I do not believe that creating a duration_clock typedef is the best way to proceed. An implementation may not need to use a time_point to implement the wait_for functions.

For example, on POSIX systems sleep_for can be implemented in terms of nanosleep which takes only a duration in terms of nanoseconds. The current working paper does not describe sleep_for in terms of sleep_until. And paragraph 2 of 30.2.4 [thread.req.timing] has the words strongly encouraging implementations to use monotonic clocks for sleep_for:

2 The member functions whose names end in _for take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions.

I believe the approach taken in describing the effects of sleep_for and try_lock_for is also appropriate for wait_for. I.e. these are not described in terms of their _until variants.

Proposed resolution:

Change 30.5.1 [thread.condition.condvar], p21-22:

template <class Rep, class Period> 
  bool wait_for(unique_lock<mutex>& lock, 
                const chrono::duration<Rep, Period>& rel_time);

Precondition: lock is locked by the calling thread, and either

21 Effects:

wait_until(lock, chrono::monotonic_clock::now() + rel_time)

Postcondition: lock is locked by the calling thread.

22 Returns: false if the call is returning because the time duration specified by rel_time has elapsed, otherwise true.

[ This part of the wording may conflict with 857 in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]

Throws: std::system_error when the effects or postcondition cannot be achieved.

Error conditions:

Change 30.5.1 [thread.condition.condvar], p26-p29:

template <class Rep, class Period, class Predicate> 
  bool wait_for(unique_lock<mutex>& lock, 
                const chrono::duration<Rep, Period>& rel_time, 
                Predicate pred);

Precondition: lock is locked by the calling thread, and either

26 Effects:

wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))
  • Executes a loop: Within the loop the function first evaluates pred() and exits the loop if the result of pred() is true.
  • Atomically calls lock.unlock() and blocks on *this.
  • When unblocked, calls lock.lock() (possibly blocking on the lock).
  • The function will unblock when signaled by a call to notify_one(), a call to notify_all(), by the elapsed time rel_time passing (30.1.4 [thread.req.timing]), or spuriously.
  • If the function exits via an exception, lock.unlock() shall be called prior to exiting the function scope.
  • The loop terminates when pred() returns true or when the time duration specified by rel_time has elapsed.

27 [Note: There is no blocking if pred() is initially true, even if the timeout has already expired. -- end note]

Postcondition: lock is locked by the calling thread.

28 Returns: pred()

29 [Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. -- end note]

Throws: std::system_error when the effects or postcondition cannot be achieved.

Error conditions:

Change 30.5.2 [thread.condition.condvarany], p18-19:

template <class Lock, class Rep, class Period> 
  bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

18 Effects:

wait_until(lock, chrono::monotonic_clock::now() + rel_time)

Postcondition: lock is locked by the calling thread.

19 Returns: false if the call is returning because the time duration specified by rel_time has elapsed, otherwise true.

Throws: std::system_error when the returned value, effects, or postcondition cannot be achieved.

Error conditions:

Change 30.5.2 [thread.condition.condvarany], p23-p26:

template <class Lock, class Rep, class Period, class Predicate> 
  bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

Precondition: lock is locked by the calling thread, and either

23 Effects:

wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))
  • Executes a loop: Within the loop the function first evaluates pred() and exits the loop if the result of pred() is true.
  • Atomically calls lock.unlock() and blocks on *this.
  • When unblocked, calls lock.lock() (possibly blocking on the lock).
  • The function will unblock when signaled by a call to notify_one(), a call to notify_all(), by the elapsed time rel_time passing (30.1.4 [thread.req.timing]), or spuriously.
  • If the function exits via an exception, lock.unlock() shall be called prior to exiting the function scope.
  • The loop terminates when pred() returns true or when the time duration specified by rel_time has elapsed.

24 [Note: There is no blocking if pred() is initially true, even if the timeout has already expired. -- end note]

Postcondition: lock is locked by the calling thread.

25 Returns: pred()

26 [Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. -- end note]

Throws: std::system_error when the effects or postcondition cannot be achieved.

Error conditions:


860. Floating-Point State

Section: 26 [numerics] Status: Open Submitter: Lawrence Crowl Opened: 2008-06-23 Last modified: 2009-03-09

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Discussion:

There are a number of functions that affect the floating point state. These function need to be thread-safe, but I'm unsure of the right approach in the standard, as we inherit them from C.

[ San Francisco: ]

Nick: I think we already say that these functions do not introduce data races; see 17.6.5.6/20

Pete: there's more to it than not introducing data races; are these states maintained per thread?

Howard: 21.5/14 says that strtok and strerror are not required to avoid data races, and 20.9/2 says the same about asctime, gmtime, ctime, and gmtime.

Nick: POSIX has a list of not-safe functions. All other functions are implicitly thread safe.

Lawrence is to form a group between meetings to attack this issue. Nick and Tom volunteered to work with Lawrence.

Move to Open.

[ Post Summit: ]

Hans: Sane oses seem ok. Sensible thing is implementable and makes sense.

Nick: Default wording seems to cover this? Hole in POSIX, these functions need to be added to list of thread-unsafe functions.

Lawrence: Not sufficient, not "thread-safe" per our definition, but think of state as a thread-local variable. Need something like "these functions only affect state in the current thread."

Hans: Suggest the following wording: "The floating point environment is maintained per-thread."

Walter: Any other examples of state being thread safe that are not already covered elsewhere?

Have thread unsafe functions paper which needs to be updated. Should just fold in 26.3 [cfenv] functions.

Recommend Open. Lawrence instead suggests leaving it open until we have suitable wording that may or may not include the thread local commentary.

Proposed resolution:


861. Incomplete specification of EqualityComparable for std::forward_list

Section: 23.2 [container.requirements] Status: Open Submitter: Daniel Krügler Opened: 2008-06-24 Last modified: 2008-11-11

View other active issues in [container.requirements].

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Discussion:

Table 89, Container requirements, defines operator== in terms of the container member function size() and the algorithm std::equal:

== is an equivalence relation. a.size() == b.size() && equal(a.begin(), a.end(), b.begin()

The new container forward_list does not provide a size member function by design but does provide operator== and operator!= without specifying it's semantic.

Other parts of the (sequence) container requirements do also depend on size(), e.g. empty() or clear(), but this issue explicitly attempts to solve the missing EqualityComparable specification, because of the special design choices of forward_list.

I propose to apply one of the following resolutions, which are described as:

  1. Provide a definition, which is optimal for this special container without previous size test. This choice prevents two O(N) calls of std::distance() with the corresponding container ranges and instead uses a special equals implementation which takes two container ranges instead of 1 1/2.
  2. The simple fix where the usual test is adapted such that size() is replaced by distance with corresponding performance disadvantages.

Both proposal choices are discussed, the preferred choice of the author is to apply (A).

[ San Francisco: ]

There's an Option C: change the requirements table to use distance().

LWG found Option C acceptable.

Martin will draft the wording for Option C.

[ post San Francisco: ]

Martin provided wording for Option C.

Proposed resolution:

Common part:

Option (A):

Option (B):

Option (C):


862. Impossible complexity for 'includes'

Section: 25.5.5.1 [includes] Status: Tentatively NAD Editorial Submitter: Alisdair Meredith Opened: 2008-07-02 Last modified: 2009-05-23

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Discussion:

In 25.5.5.1 [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 25.5.4 [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.


863. What is the state of a stream after close() succeeds

Section: 27.9.1 [fstreams] Status: Tentatively NAD Submitter: Steve Clamage Opened: 2008-07-08 Last modified: 2009-05-23

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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:


865. More algorithms that throw away information

Section: 25.4.6 [alg.fill], 25.4.7 [alg.generate] Status: Open Submitter: Daniel Krügler Opened: 2008-07-13 Last modified: 2009-05-23

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Discussion:

In regard to library defect 488 I found some more algorithms which unnecessarily throw away information. These are typically algorithms, which sequentially write into an OutputIterator, but do not return the final value of this output iterator. These cases are:

  1. template<class OutputIterator, class Size, class T>
    void fill_n(OutputIterator first, Size n, const T& value);
  2. template<class OutputIterator, class Size, class Generator>
    void generate_n(OutputIterator first, Size n, Generator gen);

In both cases the minimum requirements on the iterator are OutputIterator, which means according to the requirements of 24.2.3 [output.iterators]/2 that only single-pass iterations are guaranteed. So, if users of fill_n and generate_n have *only* an OutputIterator available, they have no chance to continue pushing further values into it, which seems to be a severe limitation to me.

[ Post Summit Daniel "conceptualized" the wording. ]

[ Batavia (2009-05): ]

Alisdair likes the idea, but has concerns about the specific wording about the returns clauses.

Alan notes this is a feature request.

Bill notes we have made similar changes to other algorithms.

Move to Open.

Proposed resolution:

  1. Replace the current declaration of fill_n in 25 [algorithms]/2, header <algorithm> synopsis and in 25.4.6 [alg.fill] by

    template<class Iter, IntegralLike Size, class T>
      requires OutputIterator<Iter, const T&>
      voidIter fill_n(Iter first, Size n, const T& value);
    

    Just after the effects clause p.1 add a new returns clause saying:

    Returns: For fill_n and n > Size(0), returns first + n. Otherwise returns first for fill_n.
  2. Replace the current declaration of generate_n in 25 [algorithms]/2, header <algorithm> synopsis and in 25.4.7 [alg.generate] by

    template<class Iter, IntegralLike Size, Callable Generator>
      requires OutputIterator<Iter, Generator::result_type>
            && CopyConstructible<Generator>
      voidIter generate_n(Iter first, Size n, Generator gen);
    

    Just after the effects clause p.1 add a new returns clause saying:

    Returns: For generate_n and n > Size(0), returns first + n. Otherwise returns first for generate_n.

867. Valarray and value-initialization

Section: 26.6.2.1 [valarray.cons] Status: Tentatively NAD Editorial Submitter: Alberto Ganesh Barbati Opened: 2008-07-20 Last modified: 2009-05-23

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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 are constructed using the default constructor for the instantiating type T value-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 be constructed using the default constructor value-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]

868. default construction and value-initialization

Section: 23 [containers] Status: Open Submitter: Alberto Ganesh Barbati Opened: 2008-07-22 Last modified: 2008-09-22

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Discussion:

The term "default constructed" is often used in wording that predates the introduction of the concept of value-initialization. In a few such places the concept of value-initialization is more correct than the current wording (for example when the type involved can be a built-in) so a replacement is in order. Two of such places are already covered by issue 867. This issue deliberately addresses the hopefully non-controversial changes in the attempt of being approved more quickly. A few other occurrences (for example in std::tuple, std::reverse_iterator and std::move_iterator) are left to separate issues. For std::reverse_iterator, see also issue 408. This issue is related with issue 724.

[ San Francisco: ]

The list provided in the proposed resolution is not complete. James Dennett will review the library and provide a complete list and will double-check the vocabulary.

This issue relates to Issue 886 tuple construction

Proposed resolution:

Change X [utility.arg.requirements], paragraph 2:

In general, a default constructor is not required. Certain container class member function signatures specify the default constructor T() as a default argument. T() shall be a well-defined expression (8.5 [dcl.init]) if one of those signatures is called using the default argument (8.3.6 [dcl.fct.default]).

In all the following paragraphs in clause 23 [containers], replace "default constructed" with "value-initialized (8.5 [dcl.init])":


869. Bucket (local) iterators and iterating past end

Section: 23.2.5 [unord.req] Status: Tentatively Ready Submitter: Sohail Somani Opened: 2008-07-22 Last modified: 2009-03-09

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Discussion:

Is there any language in the current draft specifying the behaviour of the following snippet?

unordered_set<int> s;
unordered_set<int>::local_iterator it = s.end(0);

// Iterate past end - the unspecified part
it++;

I don't think there is anything about s.end(n) being considered an iterator for the past-the-end value though (I think) it should be.

[ San Francisco: ]

We believe that this is not a substantive change, but the proposed change to the wording is clearer than what we have now.

[ Post Summit: ]

Recommend Tentatively Ready.

Proposed resolution:

Change Table 97 "Unordered associative container requirements" in 23.2.5 [unord.req]:

Table 97: Unordered associative container requirements
expressionreturn typeassertion/note pre/post-conditioncomplexity
b.begin(n) local_iterator
const_local_iterator for const b.
Pre: n shall be in the range [0,b.bucket_count()). Note: [b.begin(n), b.end(n)) is a valid range containing all of the elements in the nth bucket. b.begin(n) returns an iterator referring to the first element in the bucket. If the bucket is empty, then b.begin(n) == b.end(n). Constant
b.end(n) local_iterator
const_local_iterator for const b.
Pre: n shall be in the range [0, b.bucket_count()). b.end(n) returns an iterator which is the past-the-end value for the bucket. Constant

873. signed integral type and unsigned integral type are not clearly defined

Section: 3.9.1 [basic.fundamental] Status: Open Submitter: Travis Vitek Opened: 2008-06-30 Last modified: 2009-03-21

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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 14.10.4 [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...

  1. 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 as std::ptrdiff_t in 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. X [allocator.requirements] table 40...

Table 40: Allocator requirements
expression return type assertion/note/pre/post-condition
X::size_type unsigned integral type unsigned integer type a type that can represent the size of the largest object in the allocation model.
X::difference_type signed integral type signed integer type a 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. 20.6.6.3 [meta.trans.sign] table 48...

Table 48: Sign modifications
Template Comments
template <class T> struct make_signed; If T names a (possibly cv-qualified) signed integral typesigned integer type (3.9.1) then the member typedef type shall name the type T; otherwise, if T names a (possibly cv-qualified) unsigned integral typeunsigned integer type then type shall name the corresponding signed integral typesigned integer type, with the same cv-qualifiers as T; otherwise, type shall name the signed integral typesigned integer type with the smallest rank (4.13) for which sizeof(T) == sizeof(type), with the same cv-qualifiers as T. Requires: T shall be a (possibly cv-qualified) integral type or enumeration but not a bool type.
template <class T> struct make_unsigned; If T names a (possibly cv-qualified) unsigned integral typeunsigned integer type (3.9.1) then the member typedef type shall name the type T; otherwise, if T names a (possibly cv-qualified) signed integral typesigned integer type then type shall name the corresponding unsigned integral typeunsigned integer type, with the same cv-qualifiers as T; otherwise, type shall name the unsigned integral typeunsigned integer type with the smallest rank (4.13) for which sizeof(T) == sizeof(type), with the same cv-qualifiers as T. Requires: T shall be a (possibly cv-qualified) integral type or enumeration but not a bool type.

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 type   is identical to the difference type of X::iterator and X::const_iterator compile time
X::size_type unsigned integral typeunsigned integer type   size_type can represent any non-negative value of difference_type compile time

24.2 [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 iterators i support the expression *i, resulting in a value of some class, enumeration, or built-in type T, called the value type of the iterator. All output iterators support the expression *i = o where o is a value of some type that is in the set of types that are writable to the particular iterator type of i. All iterators i for which the expression (*i).m is well-defined, support the expression i->m with the same semantics as (*i).m. For every iterator type X for which equality is defined, there is a corresponding signed integral type signed 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 parameter UIntType shall denote an unsigned integral typeunsigned integer type large enough to store values as large as m - 1. If the template parameter m is 0, the modulus m used throughout this section 26.4.3.1 is numeric_limits<result_type>::max() plus 1. [Note: The result need not be representable as a value of type result_type. --end note] Otherwise, the following relations shall hold: a < m and c < m.

Same rationale as the previous change. X [rand.adapt.xor] paragraph 6...

Both Engine1::result_type and Engine2::result_type shall 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:RandomAccessIterator shall meet the requirements of a random access iterator (24.1.5) such that iterator_traits<RandomAccessIterator>::value_type shall denote an unsigned 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: For signed 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.

876. basic_string access operations should give stronger guarantees

Section: 21.4 [basic.string] Status: Open Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2008-09-18

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Discussion:

During the Sophia Antipolis meeting it was decided to split-off some parts of the n2647 ("Concurrency modifications for basic_string") proposal into a separate issue, because these weren't actually concurrency-related. The here proposed changes refer to the recent update document n2668 and attempt to take advantage of the stricter structural requirements.

Indeed there exists some leeway for more guarantees that would be very useful for programmers, especially if interaction with transactionary or exception-unaware C API code is important. This would also allow compilers to take advantage of more performance optimizations, because more functions can have throw() specifications. This proposal uses the form of "Throws: Nothing" clauses to reach the same effect, because there already exists a different issue in progress to clean-up the current existing "schizophrenia" of the standard in this regard.

Due to earlier support for copy-on-write, we find the following unnecessary limitations for C++0x:

  1. Missing no-throw guarantees: data() and c_str() simply return a pointer to their guts, which is a non-failure operation. This should be spelled out. It is also noteworthy to mention that the same guarantees should also be given by the size query functions, because the combination of pointer to content and the length is typically needed during interaction with low-level API.
  2. Missing complexity guarantees: data() and c_str() simply return a pointer to their guts, which is guaranteed O(1). This should be spelled out.
  3. Missing reading access to the terminating character: Only the const overload of operator[] allows reading access to the terminator char. For more intuitive usage of strings, reading access to this position should be extended to the non-const case. In contrast to C++03 this reading access should now be homogeneously an lvalue access.

The proposed resolution is split into a main part (A) and a secondary part (B) (earlier called "Adjunct Adjunct Proposal"). (B) extends (A) by also making access to index position size() of the at() overloads a no-throw operation. This was separated, because this part is theoretically observable in specifically designed test programs.

[ San Francisco: ]

We oppose part 1 of the issue but hope to address size() in issue 877.

We do not support part B. 4 of the issue because of the breaking API change.

We support part A. 2 of the issue.

On support part A. 3 of the issue:

Pete's broader comment: now that we know that basic_string will be a block of contiguous memory, we should just rewrite its specification with that in mind. The expression of the specification will be simpler and probably more correct as a result.

Proposed resolution:

    1. In 21.4.4 [string.capacity], just after p. 1 add a new paragraph:

      Throws: Nothing.
    2. In 21.4.5 [string.access] replace p. 1 by the following 4 paragraghs:

      Requires: pos ≤ size().

      Returns: If pos < size(), returns *(begin() + pos). Otherwise, returns a reference to a charT() that shall not be modified.

      Throws: Nothing.

      Complexity: Constant time.

    3. In 21.4.7.1 [string.accessors] replace the now common returns clause of c_str() and data() by the following three paragraphs:

      Returns: A pointer p such that p+i == &operator[](i) for each i in [0, size()].

      Throws: Nothing.

      Complexity: Constant time.

    1. In 21.4.5 [string.access] replace p.2 and p.3 by:

      Requires: pos ≤ size()

      Throws: out_of_range if pos > size().


877. to throw() or to Throw: Nothing.

Section: 17 [library] Status: Open Submitter: Martin Sebor Opened: 2008-08-23 Last modified: 2008-09-18

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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.

  1. Decorating functions that cannot throw with the empty exception specification can cause the compiler to generate suboptimal code for the implementation of the function when it calls other functions that aren't known to the compiler not to throw (i.e., that aren't decorated with throw() even if they don't actually throw). This is a common situation when the called function is a C or POSIX function.
  2. Alternate, proprietary mechanisms exist (such as GCC __attribute__((nothrow)) or Visual C++ __declspec(nothrow)) that let implementers mark up non-throwing functions, often without the penalty mentioned in (1) above. The C++ standard shouldn't preclude the use of these potentially more efficient mechanisms.
  3. There are functions, especially function templates, that invoke user-defined functions that may or may not be declared throw(). Declaring such functions with the empty exception specification will cause compilers to generate suboptimal code when the user-defined function isn't also declared not to throw.

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.

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.


878. forward_list preconditions

Section: 23.3.3 [forwardlist] Status: Tentatively Ready Submitter: Martin Sebor Opened: 2008-08-23 Last modified: 2009-05-10

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Discussion:

forward_list member functions that take a forward_list::iterator (denoted position in the function signatures) argument have the following precondition:

Requires: position is dereferenceable or equal to before_begin().

I believe what's actually intended is this:

Requires: position is in the range [before_begin(), end()).

That is, when it's dereferenceable, position must point into *this, not just any forward_list object.

[ San Francisco: ]

Robert suggested alternate proposed wording which had large support.

[ Post Summit: ]

Walter: "position is before_begin() or a dereferenceable": add "is" after the "or"

With that minor update, Recommend Tentatively Ready.

Proposed resolution:

Change the Requires clauses [forwardlist] , p21, p24, p26, p29, and, 23.3.3.5 [forwardlist.ops], p39, p43, p47 as follows:

Requires: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()) or equal to before_begin(). ...

879. Atomic load const qualification

Section: 29 [atomics] Status: Review Submitter: Alexander Chemeris Opened: 2008-08-24 Last modified: 2009-03-22

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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.

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);

880. Missing atomic exchange parameter

Section: 29 [atomics] Status: Open Submitter: Lawrence Crowl Opened: 2008-08-24 Last modified: 2009-03-22

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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.

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);

881. shared_ptr conversion issue

Section: 20.8.13.2.1 [util.smartptr.shared.const] Status: Open Submitter: Peter Dimov Opened: 2008-08-30 Last modified: 2008-09-18

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Discussion:

We've changed shared_ptr<Y> to not convert to shared_ptr<T> when Y* doesn't convert to T* by resolving issue 687. This only fixed the converting copy constructor though. N2351 later added move support, and the converting move constructor is not constrained.

[ San Francisco: ]

We might be able to move this to NAD, Editorial once shared_ptr is conceptualized, but we want to revisit this issue to make sure.

Proposed resolution:

We need to change the Requires clause of the move constructor:

shared_ptr(shared_ptr&& r); 
template<class Y> shared_ptr(shared_ptr<Y>&& r); 
Requires: For the second constructor Y* shall be convertible to T*. The second constructor shall not participate in overload resolution unless Y* is convertible to T*.

in order to actually make the example in 687 compile (it now resolves to the move constructor).


883. swap circular definition

Section: 23 [containers] Status: Open Submitter: Alisdair Meredith Opened: 2008-09-10 Last modified: 2009-03-11

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Discussion:

Note in particular that Table 90 "Container Requirements" gives semantics of a.swap(b) as swap(a,b), yet for all containers we define swap(a,b) to call a.swap(b) - a circular definition.

[ San Francisco: ]

Robert to propose a resolution along the lines of "Postcondition: "a = b, b = a" This will be a little tricky for the hash containers, since they don't have operator==.

[ Post Summit Anthony Williams provided proposed wording. ]

Proposed resolution:

In table 80 in section 23.2.1 [container.requirements.general], replace the postcondition of a.swap(b) with the following:

Table 80 -- Container requirements
Expression Return type Operational semantics Assertion/note pre-/post-conidtion Complexity
... ... ... ... ...
a.swap(b); void   swap(a,b) Exchange the contents of a and b as-if
X u=std::move(a);
a=std::move(b);
b=std::move(u);
(Note A)

Remove the reference to swap from the paragraph following the table.

Notes: the algorithms swap(), equal() and lexicographical_compare() are defined in Clause 25. ...

884. shared_ptr swap

Section: 20.8.13.2.4 [util.smartptr.shared.mod] Status: Tentatively NAD Editorial Submitter: Jonathan Wakely Opened: 2008-09-15 Last modified: 2009-05-23

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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:

vector<A> v = ...;
...
swap(v, vector<A>(v));
N1690.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to NAD Editorial.

Proposed resolution:

Recommend NAD Editorial, fixed by N2844.


885. pair assignment

Section: 20.3.3 [pairs] Status: Open Submitter: Alisdair Meredith Opened: 2008-09-15 Last modified: 2009-05-25

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Discussion:

20.2.3 pairs
Missing assignemnt operator:
template<class U , class V>
  requires CopyAssignable<T1, U> && CopyAssignable<T2, V>
    pair& operator=(pair<U , V> const & p );

Well, that's interesting. This assignment operator isn't in the current working paper, either. Perhaps we deemed it acceptable to build a temporary of type pair from pair<U, V>, then move-assign from that temporary?

It sounds more like an issue waiting to be opened, unless you want to plug it now. As written we risk moving from lvalues.

[ San Francisco: ]

Would be NAD if better ctors fixed it.

Related to 811.

[ post San Francisco: ]

Possibly NAD Editorial, solved by N2770.

[ 2009-05-25 Alisdair adds: ]

Issue 885 was something I reported while reviewing the library concepts documents ahead of San Francisco. The missing operator was added as part of the paper adopted at that meeting (N2770) and I can confirm this operator is present in the current working paper. I recommend NAD.

Proposed resolution:


886. tuple construction

Section: 20.5.2.1 [tuple.cnstr] Status: Open Submitter: Alisdair Meredith Opened: 2008-09-15 Last modified: 2009-05-05

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Discussion:

20.5.2.1 [tuple.cnstr]:

Effects: Default initializes each element.

Could be clarified to state each "non-trivial" element. Otherwise we have a conflict with Core deinfition of default initialization - trivial types do not get initialized (rather than initialization having no effect)

I'm going to punt on this one, because it's not an issue that's related to concepts. I suggest bringing it to Howard's attention on the reflector.

[ San Francisco: ]

Text in draft doesn't mean anything, changing to "non-trivial" makes it meaningful.

We prefer "value initializes". Present implementations use value-initialization. Users who don't want value initialization have alternatives.

Request resolution text from Alisdair.

This issue relates to Issue 868 default construction and value-initialization.

[ 2009-05-04 Alisdair provided wording and adds: ]

Note: This IS a change of semantic from TR1, although one the room agreed with during the discussion. To preserve TR1 semantics, this would have been worded:

requires DefaultConstructible<Types>... tuple();
-2- Effects: Default-initializes each non-trivial element.

Proposed resolution:

Change p2 in Construction 20.5.2.1 [tuple.cnstr]:

requires DefaultConstructible<Types>... tuple();

-2- Effects: Default Value-initializes each element.


887. issue with condition::wait_...

Section: 30.5.1 [thread.condition.condvar] Status: Open Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2009-03-09

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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;
    typename Clock::time_point  c_entry = Clock::now();
    system_clock::time_point    s_entry = system_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.

Proposed resolution:


888. this_thread::yield too strong

Section: 30.3.2 [thread.thread.this] Status: Tentatively Ready Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2009-03-09

View all issues with Tentatively Ready status.

Discussion:

I never thought I'd say this, but this_thread::yield seems to be too strong in specification. The issue is that some systems distinguish between yielding to another thread in the same process and yielding to another process. Given that the C++ standard only talks about a single program, one can infer that the specification allows yielding only to another thread within the same program. Posix has no facility for that behavior. Can you please file an issue to weaken the wording. Perhaps "Offers the operating system the opportunity to reschedule."

[ Post Summit: ]

Recommend move to Tentatively Ready.

Proposed resolution:

Change 30.3.2 [thread.thread.this]/3:

void this_thread::yield();
Effects: Offers the operating system implementation the opportunity to reschedule. another thread.

889. thread::id comparisons

Section: 30.3.1.1 [thread.thread.id] Status: Open Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2009-05-24

View all other issues in [thread.thread.id].

View all issues with Open 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> (20.7 [function.objects], 20.7.17 [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 20.7 [function.objects] does not mention hash< thread::id > nor hash< error_code >, although their existence is implied by 20.7.17 [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.

Proposed resolution:

Move to NAD.


890. Improving <system_error> initialization

Section: 19.5.1 [syserr.errcat] Status: Tentatively Ready Submitter: Beman Dawes Opened: 2008-09-14 Last modified: 2009-03-09

View all issues with Tentatively Ready status.

Discussion:

The static const error_category objects generic_category and system_category in header <system_error> are currently declared:

const error_category& get_generic_category();
const error_category& get_system_category();

static const error_category& generic_category = get_generic_category();
static const error_category& system_category = get_system_category();

This formulation has several problems:

IO streams uses a somewhat different formulation for iostream_category, but still suffer much the same problems.

The original plan was to eliminate these problems by applying the C++0x constexpr feature. See LWG issue 832. However, that approach turned out to be unimplementable, since it would require a constexpr object of a class with virtual functions, and that is not allowed by the core language.

The proposed resolution was developed as an alternative. It mitigates the above problems by removing initialization from the visible interface, allowing implementations flexibility.

Implementation experience:

Prototype implementations of the current WP interface and proposed resolution interface were tested with recent Codegear, GCC, Intel, and Microsoft compilers on Windows. The code generated by the Microsoft compiler was studied at length; the WP and proposal versions generated very similar code. For both versions the compiler did make use of static initialization; apparently the compiler applied an implicit constexpr where useful, even in cases where constexpr would not be permitted by the language!

Acknowledgements:

Martin Sebor, Chris Kohlhoff, and John Lakos provided useful ideas and comments on initialization issues.

[ San Francisco: ]

Martin: prefers not to create more file-scope static objects, and would like to see get_* functions instead.

[Pre-Summit:]

Beman: The proposed resolution has been reworked to remove the file-scope static objects, per Martin's suggestions. The get_ prefix has been eliminated from the function names as no longer necessary and to conform with standard library naming practice.

[ Post Summit: ]

Agreement that this is wise and essential, text provided works and has been implemented. Seems to be widespread consensus. Move to Tentative Ready.

Proposed resolution:

Change 17.6.4.12 [value.error.codes] Value of error codes as indicated:

Certain functions in the C++ standard library report errors via a std::error_code (19.4.2.2) object. That object's category() member shall return a reference to std::system_category() for errors originating from the operating system, or a reference to an implementation-defined error_category object for errors originating elsewhere. The implementation shall define the possible values of value() for each of these error categories. [Example: For operating systems that are based on POSIX, implementations are encouraged to define the std::system_category() values as identical to the POSIX errno values, with additional values as defined by the operating system's documentation. Implementations for operating systems that are not based on POSIX are encouraged to define values identical to the operating system's values. For errors that do not originate from the operating system, the implementation may provide enums for the associated values --end example]

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();

static storage-class-specifier const error_category& generic_category = get_generic_category();
static storage-class-specifier const error_category& system_category = get_system_category();

Change 19.5.1.5 [syserr.errcat.objects] Error category objects as indicated:

const error_category& get_generic_category();

Returns: A reference to an 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".

const error_category& get_system_category();

Returns: A reference to an 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 19.5.2.3 [syserr.errcode.constructors] Class error_code constructors as indicated:

error_code();

Effects: Constructs an object of type error_code.

Postconditions: val_ == 0 and cat_ == &system_category().

Change 19.5.2.4 [syserr.errcode.modifiers] Class error_code modifiers as indicated:

void clear();

Postconditions: value() == 0 and category() == system_category().

Change 19.5.2.6 [syserr.errcode.nonmembers] Class error_code non-member functions as indicated:

error_code make_error_code(errc e);

Returns: error_code(static_cast<int>(e), generic_category()).

Change 19.5.3.3 [syserr.errcondition.constructors] Class error_condition constructors as indicated:

error_condition();

Effects: Constructs an object of type error_condition.

Postconditions: val_ == 0 and cat_ == &generic_category().

Change 19.5.3.4 [syserr.errcondition.modifiers] Class error_condition modifiers as indicated:

void clear();

Postconditions: value() == 0 and category() == generic_category().

Change 19.5.3.6 [syserr.errcondition.nonmembers] Class error_condition non-member functions as indicated:

error_condition make_error_condition(errc e);

Returns: error_condition(static_cast<int>(e), generic_category()).

Change 27.5 [iostreams.base] Iostreams base classes, Header <ios> synopsis as indicated:

concept_map ErrorCodeEnum<io_errc> { };
error_code make_error_code(io_errc e);
error_condition make_error_condition(io_errc e);
storage-class-specifier const error_category& iostream_category();

Change 27.5.2.1.1 [ios::failure] Class ios_base::failure, paragraph 2 as indicated:

When throwing ios_base::failure exceptions, implementations should provide values of ec that identify the specific reason for the failure. [ Note: Errors arising from the operating system would typically be reported as system_category() errors with an error value of the error number reported by the operating system. Errors arising from within the stream library would typically be reported as error_code(io_errc::stream, iostream_category()). --end note ]

Change 27.5.5.5 [error.reporting] Error reporting as indicated:

error_code make_error_code(io_errc e);

Returns: error_code(static_cast<int>(e), iostream_category()).

error_condition make_error_condition(io_errc e);

Returns: error_condition(static_cast<int>(e), iostream_category()).

storage-class-specifier const error_category& iostream_category();

The implementation shall initialize iostream_category. Its storage-class-specifier may be static or extern. It is unspecified whether initialization is static or dynamic (3.6.2). If initialization is dynamic, it shall occur before completion of the dynamic initialization of the first translation unit dynamically initialized that includes header <system_error>.

Returns: A reference to an 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 "iostream".


891. std::thread, std::call_once issue

Section: 30.3.1.2 [thread.thread.constr], 30.4.5.2 [thread.once.callonce] Status: Open Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2009-03-22

View other active issues in [thread.thread.constr].

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Discussion:

I notice that the vararg overloads of std::thread and std::call_once (N2723 30.3.1.2 [thread.thread.constr] and 30.4.5.2 [thread.once.callonce]) are no longer specified in terms of std::bind; instead, some of the std::bind wording has been inlined into the specification.

There are two problems with this.

First, the specification (and implementation) in terms of std::bind allows, for example:

std::thread th( f, 1, std::bind( g ) );

which executes f( 1, g() ) in a thread. This can be useful. The "inlined" formulation changes it to execute f( 1, bind(g) ) in a thread.

Second, assuming that we don't want the above, the specification has copied the wording

INVOKE(func, w1, w2, ..., wN) (20.6.2) shall be a valid expression for some values w1, w2, ..., wN

but this is not needed since we know that our argument list is args; it should simply be

INVOKE(func, args...) (20.6.2) shall be a valid expression

[ Summit: ]

Move to open.

[ Post Summit Anthony provided proposed wording. ]

Proposed resolution:

Change paragraph 4 of 30.3.1.2 [thread.thread.constr] to:

template <class F> explicit thread(F f);
template <class F, class ...Args> thread(F&& f, Args&&... args);
-4- Requires: F and each Ti in Args shall be CopyConstructible if an lvalue and otherwise MoveConstructible. INVOKE(f, w1, w2, ..., wN args...) (20.6.2) shall be a valid expression for some values w1, w2, ..., wN, where N == sizeof...(Args).

Change paragraph 1 of 30.4.5.2 [thread.once.callonce] to:

template<class Callable, class ...Args> 
  void call_once(once_flag& flag, Callable func, Args&&... args);
-1- Requires: The template parameters Callable> and each Ti in Args shall be CopyConstructible if an lvalue and otherwise MoveConstructible. INVOKE(func, w1, w2, ..., wN args...) (20.6.2) shall be a valid expression for some values w1, w2, ..., wN, where N == sizeof...(Args).

893. std::mutex issue

Section: 30.4.1.1 [thread.mutex.class] Status: Open Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2009-03-22

View other active issues in [thread.mutex.class].

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Duplicate of: 905

Discussion:

30.4.1.1 [thread.mutex.class]/27 (in N2723) says that the behavior is undefined if:

I don't believe that this is right. Calling lock() or try_lock() on a locked mutex is well defined in the general case. try_lock() is required to fail and return false. lock() is required to either throw an exception (and is allowed to do so if it detects deadlock) or to block until the mutex is free. These general requirements apply regardless of the current owner of the mutex; they should apply even if it's owned by the current thread.

Making double lock() undefined behavior probably can be justified (even though I'd still disagree with the justification), but try_lock() on a locked mutex must fail.

[ Summit: ]

Move to open. Proposed resolution:

Proposed resolution:

In 30.4.1 [thread.mutex.requirements] paragraph 12 change:

Strike 30.4.1.1 [thread.mutex.class] paragraph 3 bullet 2:

-3- The behavior of a program is undefined if:


895. "Requires:" on std::string::at et al

Section: 17.5.1.4 [structure.specifications] Status: Open Submitter: James Dennett Opened: 2008-09-16 Last modified: 2009-03-11

View other active issues in [structure.specifications].

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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.

Proposed resolution:


896. Library thread safety issue

Section: 20.8.13.2 [util.smartptr.shared] Status: Open Submitter: Hans Boehm Opened: 2008-09-16 Last modified: 2008-09-25

View other active issues in [util.smartptr.shared].

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Discussion:

It is unclear whether shared_ptr is thread-safe in the sense that multiple threads may simultaneously copy a shared_ptr. However this is a critical piece of information for the client, and it has significant impact on usability for many applications. (Detlef Vollman thinks it is currently clear that it is not thread-safe. Hans Boehm thinks it currently requires thread safety, since the use_count is not an explicit field, and constructors and assignment take a const reference to an existing shared_ptr.)

Pro thread-safety:

Many multi-threaded usages are impossible. A thread-safe version can be used to destroy an object when the last thread drops it, something that is often required, and for which we have no other easy mechanism.

Against thread-safety:

The thread-safe version is well-known to be far more expensive, even if used by a single thread. Many applications, including all single-threaded ones, do not care.

[ San Francisco: ]

Beman: this is a complicated issue, and would like to move this to Open and await comment from Peter Dimov; we need very careful and complete rationale for any decision we make; let's go slow

Detlef: I think that shared_ptr should not be thread-safe.

Hans: When you create a thread with a lambda, it in some cases makes it very difficult for the lambda to reference anything in the heap. It's currently ambiguous as to whether you can use a shared_ptr to get at an object.

Leave in Open. Detlef will submit an alternative proposed resolution that makes shared_ptr explicitly unsafe.

A third option is to support both threadsafe and non-safe share_ptrs, and to let the programmer decide which behavior they want.

Beman: Peter, do you support the PR?

Peter:

Yes, I support the proposed resolution, and I certainly oppose any attempts to make shared_ptr thread-unsafe.

I'd mildly prefer if

[Note: This is true in spite of that fact that such functions often modify use_count() --end note]

is changed to

[Note: This is true in spite of that fact that such functions often cause a change in use_count() --end note]

(or something along these lines) to emphasise that use_count() is not, conceptually, a variable, but a return value.

Proposed resolution:

Make it explicitly thread-safe, in this weak sense, as I believe was intended:

Insert in 20.8.13.2 [util.smartptr.shared], before p5:

For purposes of determining the presence of a data race, member functions do not modify const shared_ptr and const weak_ptr arguments, nor any objects they refer to. [Note: This is true in spite of that fact that such functions often cause a change in use_count() --end note]

On looking at the text, I'm not sure we need a similar disclaimer anywhere else, since nothing else has the problem with the modified use_count(). I think Howard arrived at a similar conclusion.


897. Forward_list issues... Part 2

Section: 23.3.3.4 [forwardlist.modifiers] Status: Review Submitter: Howard Hinnant Opened: 2008-09-22 Last modified: 2009-05-23

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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:

  1. 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.

  2. 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.

  3. As stated in the discussion of 892, and reienforced by point 2 above, a splice_after should work on the source range (first, last] if the operation is to be Ο(1). When splicing an entire list x the algorithm needs (x.before_begin(), x.end()-1]. Unfortunately x.end()-1 is not available in constant time unless we specify that it must be. In order to make x.end()-1 available in constant time, the implementation would have to dedicate a pointer to it. I believe the design of N2543 intended a nominal overhead of foward_list of 1 pointer. Thus splicing one entire forward_list into another can not be Ο(1).

[ Batavia (2009-05): ]

We agree with the proposed resolution.

Move to Review.

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 exists An 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).


898. Small contradiction in n2723 to forward to committee

Section: 23.3.3.5 [forwardlist.ops] Status: Tentatively Ready Submitter: Arch Robison Opened: 2008-09-08 Last modified: 2009-05-23

View other active issues in [forwardlist.ops].

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Discussion:

I ran across a small contradiction in working draft n2723.

23.3.3 [forwardlist]p2: A forward_list satisfies all of the requirements of a container (table 90), except that the size() member function is not provided.

23.3.3.5 [forwardlist.ops]p57: Complexity: At most size() + x.size() - 1 comparisons.

Presumably 23.3.3.5 [forwardlist.ops]p57 needs to be rephrased to not use size(), or note that it is used there only for sake of notational convenience.

[ 2009-03-29 Beman provided proposed wording. ]

[ Batavia (2009-05): ]

We agree with the proposed resolution.

Move to Tentatively Ready.

Proposed resolution:

Change 23.3.3.5 [forwardlist.ops], forward_list operations, paragraph 19, merge complexity as indicated:

Complexity: At most size() + x.size() distance(begin(), end()) + distance(x.begin(), x.end()) - 1 comparisons.

899. Adjusting shared_ptr for nullptr_t

Section: 20.8.13.2.2 [util.smartptr.shared.dest] Status: Tentatively Ready Submitter: Peter Dimov Opened: 2008-09-18 Last modified: 2009-05-23

View all other issues in [util.smartptr.shared.dest].

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Discussion:

James Dennett, message c++std-lib-22442:

The wording below addresses one case of this, but opening an issue to address the need to sanity check uses of the term "pointer" in 20.8.13.2 [util.smartptr.shared] would be a good thing.

There's one more reference, in ~shared_ptr; we can apply your suggested change to it, too. That is:

Change 20.8.13.2.2 [util.smartptr.shared.dest]/1 second bullet from:

Otherwise, if *this owns a pointer p and a deleter d, d(p) is called.

to:

Otherwise, if *this owns an object p and a deleter d, d(p) is called.

[ Post Summit: ]

Recommend Review.

[ Batavia (2009-05): ]

Peter Dimov notes the analogous change has already been made to "the new nullptr_t taking constructors in 20.8.13.2.1 [util.smartptr.shared.const] p9-13."

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 20.8.13.2.2 [util.smartptr.shared.dest]/1 second bullet:


900. stream move-assignment

Section: 27.9.1.8 [ifstream.assign] Status: Open Submitter: Niels Dekker Opened: 2008-09-20 Last modified: 2009-05-23

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Discussion:

It appears that we have an issue similar to issue 675 regarding the move-assignment of stream types. For example, when assigning to an std::ifstream, ifstream1, it seems preferable to close the file originally held by ifstream1:

ifstream1 = std::move(ifstream2); 

The current Draft (N2723) specifies that the move-assignment of stream types like ifstream has the same effect as a swap:

Assign and swap 27.9.1.8 [ifstream.assign]

basic_ifstream& operator=(basic_ifstream&& rhs); 
Effects: swap(rhs).

[ Batavia (2009-05): ]

Howard agrees with the analysis and the direction proposed.

Move to Open pending specific wording to be supplied by Howard.

Proposed resolution:


901. insert iterators can move from lvalues

Section: 24.7.5 [insert.iterator] Status: Tentatively NAD Submitter: Alisdair Meredith Opened: 2008-09-24 Last modified: 2009-05-23

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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:

  1. Add another overload with a negative constraint on copy-constructible and flag it "= delete".
  2. Drop the copy-constructible overload entirely and rely on perfect forwarding to catch move issues one level deeper.
  3. This is a fundamental problem in move-syntax that relies on the presence of two overloads, and we need to look more deeply into this area as a whole - do not solve this issue in isolation.

[ 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.


902. Regular is the wrong concept to constrain numeric_limits

Section: 18.3.1 [limits] Status: Open Submitter: Alisdair Meredith Opened: 2008-09-24 Last modified: 2009-03-11

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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:


903. back_insert_iterator issue

Section: 24.7.1 [back.insert.iterator] Status: Tentatively NAD Submitter: Dave Abrahams Opened: 2008-09-19 Last modified: 2009-05-30

View all issues with Tentatively 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: ]

  1. If 1009 is accepted, OutputIterator does no longer support post increment.
  2. To support backward compatibility a second overload of operator* can be added. Note that the HasDereference concept (and the HasDereference part of concept Iterator) was specifically refactored to cope with optional const qualification and to properly reflect the dual nature of built-in operator* as of 13.5.8 [over.literal]/6.

Proposed resolution:


904. result_of argument types

Section: 20.7.4 [func.ret] Status: Tentatively Ready Submitter: Jonathan Wakely Opened: 2008-09-10 Last modified: 2009-03-09

View all issues with Tentatively Ready status.

Discussion:

The WP and TR1 have the same text regarding the argument types of a result_of expression:

The values ti are lvalues when the corresponding type Ti is a reference type, and rvalues otherwise.

I read this to mean that this compiles:

typedef int (*func)(int&);
result_of<func(int&&)>::type i = 0;

even though this doesn't:

int f(int&);
f( std::move(0) );

Should the text be updated to say "when Ti is an lvalue-reference type" or am I missing something?

I later came up with this self-contained example which won't compile, but I think it should:

struct X {
  void operator()(int&);
  int operator()(int&&);
} x;

std::result_of< X(int&&) >::type i = x(std::move(0));

[ Post Summit: ]

Recommend Tentatively Ready.

Proposed resolution:

Change 20.7.4 [func.ret], p1:

... The values ti are lvalues when the corresponding type Ti is an lvalue-reference type, and rvalues otherwise.

906. ObjectType is the wrong concept to constrain initializer_list

Section: 18.9 [support.initlist] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2009-05-23

View all issues with Tentatively Ready 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.

Proposed resolution:

  1. In 18.9 [support.initlist]/p.1 replace in "header <initializer_list> synopsis" the constraint "ObjectType" in the template parameter list by the constraint "ValueType".

907. Bitset's immutable element retrieval is inconsistently defined

Section: 20.3.6.2 [bitset.members] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2009-05-23

View all other issues in [bitset.members].

View all issues with Tentatively Ready status.

Discussion:

The current standard 14882::2003(E) as well as the current draft N2723 have in common a contradiction of the operational semantics of member function test 20.3.6.2 [bitset.members]/56-58 and the immutable member operator[] overload 20.3.6.2 [bitset.members]/64-66 (all references are defined in terms of N2723):

  1. bool test(size_t pos) const;
    

    Requires: pos is valid

    Throws: out_of_range if pos does not correspond to a valid bit position.

    Returns: true if the bit at position pos in *this has the value one.

  2. constexpr bool operator[](size_t pos) const;
    

    Requires: pos shall be valid.

    Throws: nothing.

    Returns: test(pos).

Three interpretations:

  1. The operator[] overload is indeed allowed to throw an exception (via test(), if pos corresponds to an invalid bit position) which does not leave the call frame. In this case this function cannot be a constexpr function, because test() is not, due to 5.19 [expr.const]/2, last bullet.
  2. The intend was not to throw an exception in test in case of an invalid bit position. There is only little evidence for this interpretation.
  3. The intend was that operator[] should not throw any exception, but that test has the contract to do so, if the provided bit position is invalid.

The problem became worse, because issue 720 recently voted into WP argued that member test logically must be a constexpr function, because it was used to define the semantics of another constexpr function (the operator[] overload).

Three alternatives are proposed, corresponding to the three bullets (A), (B), and (C), the author suggests to follow proposal (C).

Proposed alternatives:
  1. Remove the constexpr specifier in front of operator[] overload and undo that of member test (assuming 720 is accepted) in both the class declaration 20.3.6 [template.bitset]/1 and in the member description before 20.3.6.2 [bitset.members]/56 and before /64 to read:

    constexpr bool test(size_t pos) const;
    ..
    constexpr bool operator[](size_t pos) const;
    

    Change the throws clause of p. 65 to read:

    Throws: nothing out_of_range if pos does not correspond to a valid bit position.
  2. Replace the throws clause p. 57 to read:

    Throws: out_of_range if pos does not correspond to a valid bit position nothing.
  3. Undo the addition of the constexpr specifier to the test member function in both class declaration 20.3.6 [template.bitset]/1 and in the member description before 20.3.6.2 [bitset.members]/56, assuming that 720 was applied.

    constexpr bool test(size_t pos) const;
    

    Change the returns clause p. 66 to read:

    Returns: test(pos) true if the bit at position pos in *this has the value one, otherwise false.

[ Post Summit: ]

Lawrence: proposed resolutions A, B, C are mutually exclusive.

Recommend Review with option C.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

  1. Undo the addition of the constexpr specifier to the test member function in both class declaration 20.3.6 [template.bitset]/1 and in the member description before 20.3.6.2 [bitset.members]/56, assuming that 720 was applied.

    constexpr bool test(size_t pos) const;
    

    Change the returns clause p. 66 to read:

    Returns: test(pos) true if the bit at position pos in *this has the value one, otherwise false.

908. Deleted assignment operators for atomic types must be volatile

Section: 29.5 [atomics.types] Status: Open Submitter: Anthony Williams Opened: 2008-09-26 Last modified: 2009-03-22

View all other issues in [atomics.types].

View all issues with Open 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.

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.


909. regex_token_iterator should use initializer_list

Section: 28.13.2 [re.tokiter] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2009-05-23

View all other issues in [re.tokiter].

View all issues with Tentatively Ready status.

Discussion:

Addresses UK 319

Construction of a regex_token_iterator (28.13.2 [re.tokiter]/6+) usually requires the provision of a sequence of integer values, which can currently be done via a std::vector<int> or a C array of int. Since the introduction of initializer_list in the standard it seems much more reasonable to provide a corresponding constructor that accepts an initializer_list<int> instead. This could be done as a pure addition or one could even consider replacement. The author suggests the replacement strategy (A), but provides an alternative additive proposal (B) as a fall-back, because of the handiness of this range type:

[ Batavia (2009-05): ]

We strongly recommend alternative B of the proposed resolution in order that existing code not be broken. With that understanding, move to Tentatively Ready.

Original proposed wording:


    1. In 28.13.2 [re.tokiter]/6 and the list 28.13.2.1 [re.tokiter.cnstr]/10-11 change the constructor declaration:

      template <std::size_t N>
      regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
                           const regex_type& re,
                           const int (&submatches)[N] initializer_list<int> submatches,
                           regex_constants::match_flag_type m =
                             regex_constants::match_default);
      
    2. In 28.13.2.1 [re.tokiter.cnstr]/12 change the 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.begin(), &submatches.end() + N).

    1. In 28.13.2 [re.tokiter]/6 and the list 28.13.2.1 [re.tokiter.cnstr]/10-11 insert the following constructor declaration between the already existing ones accepting a std::vector and a C array of int, resp.:

      regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
                           const regex_type& re,
                           initializer_list<int> submatches,
                           regex_constants::match_flag_type m =
                             regex_constants::match_default);
      
    2. In 28.13.2.1 [re.tokiter.cnstr]/12 change the last sentence

      The third and fourth constructor initializes the member subs to hold a copy of the sequence of integer values pointed to by the iterator range [&submatches,&submatches + N) and [submatches.begin(),submatches.end()), respectively.

Proposed resolution:


    1. In 28.13.2 [re.tokiter]/6 and the list 28.13.2.1 [re.tokiter.cnstr]/10-11 insert the following constructor declaration between the already existing ones accepting a std::vector and a C array of int, resp.:

      regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
                           const regex_type& re,
                           initializer_list<int> submatches,
                           regex_constants::match_flag_type m =
                             regex_constants::match_default);
      
    2. In 28.13.2.1 [re.tokiter.cnstr]/12 change the last sentence

      The third and fourth constructor initializes the member subs to hold a copy of the sequence of integer values pointed to by the iterator range [&submatches,&submatches + N) and [submatches.begin(),submatches.end()), respectively.

910. Effects of MoveAssignable

Section: 20.2.9 [concept.copymove] Status: Tentatively Ready Submitter: Alberto Ganesh Barbati Opened: 2008-09-29 Last modified: 2009-05-23

View all issues with Tentatively Ready status.

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.

Proposed resolution:

In 20.2.9 [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]

911. I/O streams and move/swap semantic

Section: 27.7.1 [input.streams], 27.7.2 [output.streams] Status: Open Submitter: Alberto Ganesh Barbati Opened: 2008-09-29 Last modified: 2009-05-23

View all issues with Open status.

Discussion:

Class template basic_istream, basic_ostream and basic_iostream implements public move constructors, move assignment operators and swap method and free functions. This might induce both the user and the compiler to think that those types are MoveConstructible, MoveAssignable and Swappable. However, those class templates fail to fulfill the user expectations. For example:

std::ostream os(std::ofstream("file.txt"));
assert(os.rdbuf() == 0); // buffer object is not moved to os, file.txt has been closed

std::vector<std::ostream> v;
v.push_back(std::ofstream("file.txt"));
v.reserve(100); // causes reallocation
assert(v[0].rdbuf() == 0); // file.txt has been closed!

std::ostream&& os1 = std::ofstream("file1.txt");
os1 = std::ofstream("file2.txt");
os1 << "hello, world"; // still writes to file1.txt, not to file2.txt!

std::ostream&& os1 = std::ofstream("file1.txt");
std::ostream&& os2 = std::ofstream("file2.txt");
std::swap(os1, os2);
os1 << "hello, world"; // writes to file1.txt, not to file2.txt!

This is because the move constructor, the move assignment operator and swap are all implemented through calls to std::basic_ios member functions move() and swap() that do not move nor swap the controlled stream buffers. That can't happen because the stream buffers may have different types.

Notice that for basic_streambuf, the member function swap() is protected. I believe that is correct and all of basic_istream, basic_ostream, basic_iostream should do the same as the move ctor, move assignment operator and swap member function are needed by the derived fstreams and stringstreams template. The free swap functions for basic_(i|o|io)stream templates should be removed for the same reason.

[ Batavia (2009-05): ]

We note that the rvalue swap functions have already been removed.

Bill is unsure about making the affected functions protected; he believes they may need to be public.

We are also unsure about removing the lvalue swap functions as proposed.

Move to Open.

Proposed resolution:

27.7.1.1 [istream]: make the following member functions protected:

basic_istream(basic_istream&&  rhs);
basic_istream&  operator=(basic_istream&&  rhs);
void  swap(basic_istream&&  rhs);

Ditto: remove the three swap free functions signatures

// swap: 
template <class charT, class traits> 
  void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y); 
template <class charT, class traits> 
  void swap(basic_istream<charT, traits>&& x, basic_istream<charT, traits>& y); 
template <class charT, class traits> 
  void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>&& y);

27.7.1.1.2 [istream.assign]: remove paragraph 4

template <class charT, class traits> 
  void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y); 
template <class charT, class traits> 
  void swap(basic_istream<charT, traits>&& x, basic_istream<charT, traits>& y); 
template <class charT, class traits> 
  void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>&& y);
Effects: x.swap(y).

27.7.1.5 [iostreamclass]: make the following member function protected:

basic_iostream(basic_iostream&&  rhs);
basic_iostream&  operator=(basic_iostream&&  rhs);
void  swap(basic_iostream&&  rhs);

Ditto: remove the three swap free functions signatures

template <class charT, class traits> 
  void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y); 
template <class charT, class traits> 
  void swap(basic_iostream<charT, traits>&& x, basic_iostream<charT, traits>& y); 
template <class charT, class traits> 
  void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>&& y);

27.7.1.5.3 [iostream.assign]: remove paragraph 3

template <class charT, class traits> 
  void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y); 
template <class charT, class traits> 
  void swap(basic_iostream<charT, traits>&& x, basic_iostream<charT, traits>& y); 
template <class charT, class traits> 
  void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>&& y);
Effects: x.swap(y).

27.7.2.1 [ostream]: make the following member function protected:

basic_ostream(basic_ostream&&  rhs);
basic_ostream&  operator=(basic_ostream&&  rhs);
void  swap(basic_ostream&&  rhs);

Ditto: remove the three swap free functions signatures

// swap: 
template <class charT, class traits> 
  void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y);
template <class charT, class traits> 
  void swap(basic_ostream<charT, traits>&& x, basic_ostream<charT, traits>& y); 
template <class charT, class traits> 
  void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>&& y);

27.7.2.3 [ostream.assign]: remove paragraph 13 (The paragraphs seems to be misnumbered in the whole section 27.7.2 [output.streams] in N2723. The paragraph to remove is the one that describes the three swap free functions).

template <class charT, class traits> 
  void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y); 
template <class charT, class traits> 
  void swap(basic_ostream<charT, traits>&& x, basic_ostream<charT, traits>& y); 
template <class charT, class traits> 
  void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>&& y);
Effects: x.swap(y).

912. Array swap needs to be conceptualized

Section: 25.4.3 [alg.swap] Status: Tentatively NAD Submitter: Daniel Krügler Opened: 2008-10-01 Last modified: 2009-05-23

View all other issues in [alg.swap].

View all issues with Tentatively NAD status.

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 25.4.3 [alg.swap] before p. 3 until p. 4 by

template <class ValueType 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);


913. Superfluous requirements for replace algorithms

Section: 25.4.5 [alg.replace] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2008-10-03 Last modified: 2009-05-23

View other active issues in [alg.replace].

View all other issues in [alg.replace].

View all issues with Tentatively Ready status.

Discussion:

(A) 25.4.5 [alg.replace]/1:

Requires: The expression *first = new_value shall be valid.

(B) 25.4.5 [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:

  1. Remove 25.4.5 [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.
  2. 25.4.5 [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.

914. Superfluous requirement for unique

Section: 25.4.9 [alg.unique] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2008-10-03 Last modified: 2009-05-23

View other active issues in [alg.unique].

View all other issues in [alg.unique].

View all issues with Tentatively Ready status.

Discussion:

25.4.9 [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 25.4.9 [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.


915. minmax with initializer_list should return pair of T, not pair of const T&

Section: 25.5.7 [alg.min.max] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2008-10-04 Last modified: 2009-05-23

View other active issues in [alg.min.max].

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:

25.5.7 [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.

Proposed resolution:

  1. In 25 [algorithms]/2, header <algorithm> synopsis change as indicated:

    template<classLessThanComparable T>
    requires CopyConstructible<T>
    pair<const T&, const T&>
    minmax(initializer_list<T> t);
    
    template<class T, classStrictWeakOrder<auto, T> Compare>
    requires CopyConstructible<T>
    pair<const T&, const T&>
    minmax(initializer_list<T> t, Compare comp);
    
  2. In 25.5.7 [alg.min.max] change as indicated (Begin: Just before p.20):

    template<classLessThanComparable T>
      requires CopyConstructible<T>
      pair<const T&, const T&>
      minmax(initializer_list<T> t);
    

    -20- Requires: T is LessThanComparable and CopyConstructible.

    -21- Returns: pair<const T&, const T&>(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<const T&, const T&>
      minmax(initializer_list<T> t, Compare comp);
    

    -24- Requires: type T is LessThanComparable and CopyConstructible.

    -25- Returns: pair<const T&, const T&>(x, y) where x is the smallest value and y largest value in the initializer_list.


916. Redundant move-assignment operator of pair should be removed

Section: 20.3.3 [pairs] Status: Open Submitter: Daniel Krügler Opened: 2008-10-04 Last modified: 2009-05-23

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Discussion:

see also 917.

The current WP provides the following assignment operators for pair in 20.3.3 [pairs]/1:

  1. template<class U , class V>
    requires HasAssign<T1, const U&> && HasAssign<T2, const V&>
    pair& operator=(const pair<U , V>& p);
    
  2. requires MoveAssignable<T1> && MoveAssignable<T2> pair& operator=(pair&& p );
    
  3. 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.

Proposed resolution:

  1. In 20.3.3 [pairs] p. 1, class pair and just before p. 13 remove the declaration:

    requires MoveAssignable<T1> && MoveAssignable<T2> pair& operator=(pair&& p );
    
  2. Remove p.13+p.14

917. Redundant move-assignment operator of tuple should be removed

Section: 20.5.2.1 [tuple.cnstr] Status: Open Submitter: Daniel Krügler Opened: 2008-10-04 Last modified: 2009-05-23

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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.

Proposed resolution:

  1. In 20.5.2 [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...>&&);
    
  2. In 20.5.2.1 [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.


918. Swap for tuple needs to be conceptualized

Section: 20.5.2.6 [tuple.swap] Status: Tentatively NAD Submitter: Daniel Krügler Opened: 2008-10-04 Last modified: 2009-05-23

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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:

  1. In both 20.5.1 [tuple.general]/2 and 20.5.2.7 [tuple.special] change

    template <class Swappable... Types>
    void swap(tuple<Types...>& x, tuple<Types...>& y);
    
  2. In 20.5.2 [tuple.tuple], class tuple definition and in 20.5.2.6 [tuple.swap], change

    requires Swappable<Types>...void swap(tuple&);
    
  3. In 20.5.2.6 [tuple.swap] remove the current requires-clause, which says:

    Requires: Each type in Types shall be Swappable

919. (forward_)list specialized remove algorithms are over constrained

Section: 23.3.3.5 [forwardlist.ops], 23.3.4.4 [list.ops] Status: Review Submitter: Daniel Krügler Opened: 2008-10-06 Last modified: 2009-05-23

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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.

Proposed resolution:

  1. Replace in 23.3.3.5 [forwardlist.ops] before 11 and in 23.3.4.4 [list.ops] before 15

    requires EqualityComparable<T> HasEqualTo<T, T> void remove(const T& value);
    

920. Ref-qualification support in the library

Section: 20.7.15 [func.memfn] Status: Open Submitter: Bronek Kozicki Opened: 2008-10-06 Last modified: 2009-05-23

View all issues with Open status.

Discussion:

Daniel Krügler wrote:

Shouldn't above list be completed for &- and &&-qualified member functions This would cause to add:

template<Returnable R, class T, CopyConstructible... Args>
unspecified mem_fn(R (T::* pm)(Args...) &);
template<Returnable R, class T, CopyConstructible... Args>
unspecified mem_fn(R (T::* pm)(Args...) const &);
template<Returnable R, class T, CopyConstructible... Args>
unspecified mem_fn(R (T::* pm)(Args...) volatile &);
template<Returnable R, class T, CopyConstructible... Args>
unspecified mem_fn(R (T::* pm)(Args...) const volatile &);
template<Returnable R, class T, CopyConstructible... Args>
unspecified mem_fn(R (T::* pm)(Args...) &&);
template<Returnable R, class T, CopyConstructible... Args>
unspecified mem_fn(R (T::* pm)(Args...) const &&);
template<Returnable R, class T, CopyConstructible... Args>
unspecified mem_fn(R (T::* pm)(Args...) volatile &&);
template<Returnable R, class T, CopyConstructible... Args>
unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);

yes, absolutely. Thanks for spotting this. Without this change mem_fn cannot be initialized from pointer to ref-qualified member function. I believe semantics of such function pointer is well defined.

[ Post Summit Daniel provided wording. ]

[ Batavia (2009-05): ]

We need to think about whether we really want to go down the proposed path of combinatorial explosion. Perhaps a Note would suffice.

We would really like to have an implementation before proceeding.

Move to Open, and recommend this be deferred until after the next Committee Draft has been issued.

Proposed resolution:

  1. In 20.7 [function.objects]/2, header <functional> synopsis, just after the section "// 20.6.15, member function adaptors::" add the following declarations to the existing list:

    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) &);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) const &);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) volatile &);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) const volatile &);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) &&);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) const &&);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) volatile &&);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);
    
  2. In 20.7.15 [func.memfn] add the following declarations to the existing list:

    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) &);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) const &);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) volatile &);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) const volatile &);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) &&);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) const &&);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) volatile &&);
    template<Returnable R, class T, CopyConstructible... Args>
      unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);
    

The following text, most notably p.2 and p.3 which discuss influence of the cv-qualification on the definition of the base class's first template parameter remains unchanged.


921. Rational Arithmetic should use template aliases

Section: 20.4.1 [ratio.ratio] Status: Review Submitter: Pablo Halpern Opened: 2008-10-07 Last modified: 2009-05-23

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Discussion:

The compile-time functions that operate on ratio<N,D> require the cumbersome and error-prone "evaluation" of a type member using a meta-programming style that predates the invention of template aliases. Thus, multiplying three ratios a, b, and c requires the expression:

ratio_multiply<a, ratio_multiply<b, c>::type>::type

The simpler expression:

ratio_multiply<a, ratio_multiply<b, c>>

Could be used by if template aliases were employed in the definitions.

[ Post Summit: ]

Jens: not a complete proposed resolution: "would need to make similar change"

Consensus: We agree with the direction of the issue.

Recommend Open.

[ 2009-05-11 Daniel adds: ]

Personally I'm not in favor for the addition of:

typedef ratio type;

For a reader of the standard it's usage or purpose is unclear. I haven't seen similar examples of attempts to satisfy non-feature complete compilers.

[ 2009-05-11 Pablo adds: ]

The addition of type to the ratio template allows the previous style (i.e., in the prototype implementations) to remain valid and permits the use of transitional library implementations for C++03 compilers. I do not feel strongly about its inclusion, however, and leave it up to the reviewers to decide.

[ Batavia (2009-05): ]

Bill asks for additional discussion in the issue that spells out more details of the implementation. Howard points us to issue 948 which has at least most of the requested details. Tom is strongly in favor of overflow-checking at compile time. Pete points out that there is no change of functionality implied. We agree with the proposed resolution, but recommend moving the issue to Review to allow time to improve the discussion if needed.

Proposed resolution:

  1. In 20.4 [ratio]/3 change as indicated:

    // ratio arithmetic
    template <class R1, class R2> structusing ratio_add = see below;
    template <class R1, class R2> structusing ratio_subtract = see below;
    template <class R1, class R2> structusing ratio_multiply = see below;
    template <class R1, class R2> structusing ratio_divide = see below;
    
  2. In 20.4.1 [ratio.ratio], change as indicated:

    namespace std {
      template <intmax_t N, intmax_t D = 1>
      class ratio {
      public:
        typedef ratio type;
        static const intmax_t num;
        static const intmax_t den;
      };
    }
    
  3. In 20.4.2 [ratio.arithmetic] change as indicated:

    template <class R1, class R2> structusing ratio_add = see below{
      typedef see below type;
    };
    

    1 The nested typedef type ratio_add<R1, R2> shall be a synonym for ratio<T1, T2> where T1 has the value R1::num * R2::den + R2::num * R1::den and T2 has the value R1::den * R2::den.

    template <class R1, class R2> structusing ratio_subtract = see below{
      typedef see below type;
    };
    

    2 The nested typedef type ratio_subtract<R1, R2> shall be a synonym for ratio<T1, T2> where T1 has the value R1::num * R2::den - R2::num * R1::den and T2 has the value R1::den * R2::den.

    template <class R1, class R2> structusing ratio_multiply = see below{
      typedef see below type;
    };
    

    3 The nested typedef type ratio_multiply<R1, R2> shall be a synonym for ratio<T1, T2> where T1 has the value R1::num * R2::num and T2 has the value R1::den * R2::den.

    template <class R1, class R2> structusing ratio_divide = see below{
      typedef see below type;
    };
    

    4 The nested typedef type ratio_divide<R1, R2> shall be a synonym for ratio<T1, T2> where T1 has the value R1::num * R2::den and T2 has the value R1::den * R2::num.

  4. In 20.9.3.1 [time.duration.cons]/4 change as indicated:

    Requires: treat_as_floating_point<rep>::value shall be true or ratio_divide<Period2, period>::type::den shall be 1.[..]

  5. In 20.9.3.7 [time.duration.cast]/2 change as indicated:

    Returns: Let CF be ratio_divide<Period, typename ToDuration::period>::type, and [..]


922. [func.bind.place] Number of placeholders

Section: B [implimits] Status: Tentatively Ready Submitter: Sohail Somani Opened: 2008-10-11 Last modified: 2009-03-13

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Discussion:

Addresses DE 24

With respect to the section 20.7.12.1.4 [func.bind.place]:

TR1 dropped some suggested implementation quantities for the number of placeholders. The purpose of this defect is to put these back for C++0x.

[ Post Summit: ]

see DE 24

Recommend applying the proposed resolution from DE 24, with that Tentatively Ready.

Original proposed resolution:

Add 20.7.12.1.4 [func.bind.place]/2:

While the exact number of placeholders (_M) is implementation defined, this number shall be at least 10.

Proposed resolution:

Add to B [implimits]:


923. atomics with floating-point

Section: 29 [atomics] Status: Open Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2009-05-01

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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:

  1. ill-formed, or
  2. well-defined.

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]

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]


924. structs with internal padding

Section: 29 [atomics] Status: Open Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2009-03-22

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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:

  1. ill-formed, or
  2. well-defined.

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.

Proposed resolution:


925. shared_ptr's explicit conversion from unique_ptr

Section: 20.8.13.2.1 [util.smartptr.shared.const] Status: Tentatively Ready Submitter: Rodolfo Lima Opened: 2008-10-12 Last modified: 2009-05-23

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Discussion:

The current working draft (N2798), section 20.8.13.2.1 [util.smartptr.shared.const] declares shared_ptr's constructor that takes a rvalue reference to unique_ptr and auto_ptr as being explicit, affecting several valid smart pointer use cases that would take advantage of this conversion being implicit, for example:

class A;
std::unique_ptr<A> create();
void process(std::shared_ptr<A> obj);

int main()
{
   process(create());                  // use case #1
   std::unique_ptr<A> uobj = create();
   process(std::move(uobj));           // use case #2
   return 0;
}

If unique_ptr to shared_ptr conversions are explicit, the above lines should be written:

process(std::shared_ptr<A>(create()));        // use case #1
process(std::shared_ptr<A>(std::move(uobj))); // use case #2

The extra cast required doesn't seems to give any benefits to the user, nor protects him of any unintended conversions, this being the raison d'etre of explicit constructors.

It seems that this constructor was made explicit to mimic the conversion from auto_ptr in pre-rvalue reference days, which accepts both lvalue and rvalue references. Although this decision was valid back then, C++0x allows the user to express in a clear and non verbose manner when he wants move semantics to be employed, be it implicitly (use case 1) or explicitly (use case 2).

[ Batavia (2009-05): ]

Howard and Alisdair like the motivating use cases and the proposed resolution.

Move to Tentatively Ready.

Proposed resolution:

In both 20.8.13.2 [util.smartptr.shared] paragraph 1 and 20.8.13.2.1 [util.smartptr.shared.const] change:

template <class Y> explicit shared_ptr(auto_ptr<Y> &&r);
template <class Y, class D> explicit shared_ptr(unique_ptr<Y, D> &&r);

926. Sequentially consistent fences, relaxed operations and modification order

Section: 29.3 [atomics.order] Status: Open Submitter: Anthony Williams Opened: 2008-10-19 Last modified: 2009-03-22

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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.

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.

927. Dereferenceable should be HasDereference

Section: 20.8.2.2 [allocator.concepts] Status: Tentatively NAD Editorial Submitter: Pablo Halpern Opened: 2008-10-23 Last modified: 2009-05-23

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Discussion:

20.8.2.2 [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 20.8.2.2 [allocator.concepts].


928. Wrong concepts used for tuple's comparison operators

Section: 20.5.2.5 [tuple.rel] Status: Tentatively Ready Submitter: Joe Gottman Opened: 2008-10-28 Last modified: 2009-03-09

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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 20.5.1 [tuple.general] and 20.5.2.5 [tuple.rel] change:

template<class... TTypes, class... UTypes>
  requires EqualityComparableHasEqualTo<TTypes, UTypes>...
  bool operator==(const tuple<TTypes...>&, const tuple<UTypes...>&);

template<class... TTypes, class... UTypes>
  requires LessThanComparableHasLess<TTypes, UTypes>... && HasLess<UTypes, TTypes>...
  bool operator<(const tuple<TTypes...>&, const tuple<UTypes...>&);

template<class... TTypes, class... UTypes>
  requires EqualityComparableHasEqualTo<TTypes, UTypes>...
  bool operator!=(const tuple<TTypes...>&, const tuple<UTypes...>&);

template<class... TTypes, class... UTypes>
  requires LessThanComparableHasLess<UTTypes, TUTypes>... && HasLess<UTypes, TTypes>...
  bool operator>(const tuple<TTypes...>&, const tuple<UTypes...>&);

template<class... TTypes, class... UTypes>
  requires LessThanComparableHasLess<UTTypes, TUTypes>... && HasLess<UTypes, TTypes>...
  bool operator<=(const tuple<TTypes...>&, const tuple<UTypes...>&);

template<class... TTypes, class... UTypes>
  requires LessThanComparableHasLess<TTypes, UTypes>... && HasLess<UTypes, TTypes>...
  bool operator>=(const tuple<TTypes...>&, const tuple<UTypes...>&);

929. Thread constructor

Section: 30.3.1.2 [thread.thread.constr] Status: Review Submitter: Anthony Williams Opened: 2008-10-23 Last modified: 2009-05-23

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Discussion:

Addresses UK 323

The thread constructor for starting a new thread with a function and arguments is overly constrained by the signature requiring rvalue references for func and args and the CopyConstructible requirements for the elements of args. The use of an rvalue reference for the function restricts the potential use of a plain function name, since the type of the bound parameter will be deduced to be a function reference and decay to pointer-to-function will not happen. This therefore complicates the implementation in order to handle a simple case. Furthermore, the use of rvalue references for args prevents the array to pointer decay. Since arrays are not CopyConstructible or even MoveConstructible, this essentially prevents the passing of arrays as parameters. In particular it prevents the passing of string literals. Consequently a simple case such as

void f(const char*);
std::thread t(f,"hello");

is ill-formed since the type of the string literal is const char[6].

By changing the signature to take all parameters by value we can eliminate the CopyConstructible requirement and permit the use of arrays, as the parameter passing semantics will cause the necessary array-to-pointer decay. They will also cause the function name to decay to a pointer to function and allow the implementation to handle functions and function objects identically.

The new signature of the thread constructor for a function and arguments is thus:

template<typename F,typename... Args>
thread(F,Args... args);

Since the parameter pack Args can be empty, the single-parameter constructor that takes just a function by value is now redundant.

[ Howard adds: ]

I agree with everything Anthony says in this issue. However I believe we can optimize in such a way as to get the pass-by-value behavior with the pass-by-rvalue-ref performance. The performance difference is that the latter removes a move when passing in an lvalue.

This circumstance is very analogous to make_pair (20.3.3 [pairs]) where we started with passing by const reference, changed to pass by value to get pointer decay, and then changed to pass by rvalue reference, but modified with decay<T> to retain the pass-by-value behavior. If we were to apply the same solution here it would look like:

template <class F> explicit thread(F f);
template <class F, class ...Args> thread(F&& f, Args&&... args);

-4- Requires: F and each Ti in Args shall be CopyConstructible if an lvalue and otherwise MoveConstructible. INVOKE(f, w1, w2, ..., wN) (20.7.2 [func.require]) shall be a valid expression for some values w1, w2, ... , wN, where N == sizeof...(Args).

-5- Effects: Constructs an object of type thread and executes INVOKE(f, t1, t2, ..., tN) in a new thread of execution, where t1, t2, ..., tN are the values in args.... Constructs the following objects in memory which is accessible to a new thread of execution as if:

typename decay<F>::type g(std::forward<F>(f));
tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);

The new thread of execution executes INVOKE(g, wi...) where the wi... refers to the elements stored in the tuple w. Any return value from g is ignored. If f terminates with an uncaught exception, std::terminate() shall be called. If the evaluation of INVOKE(g, wi...) terminates with an uncaught exception, std::terminate() shall be called [Note: std::terminate() could be called before entering g. -- end note]. Any exception thrown before the evaluation of INVOKE has started shall be catchable in the calling thread.

Text referring to when terminate() is called was contributed by Ganesh.

[ Batavia (2009-05): ]

We agree with the proposed resolution, but would like the final sentence to be reworded since "catchable" is not a term of art (and is used nowhere else).

Proposed resolution:

Modify the class definition of std::thread in 30.3.1 [thread.thread.class] to remove the following signature:

template<class F> explicit thread(F f);
template<class F, class ... Args> explicit thread(F&& f, Args&& ... args);

Modify 30.3.1.2 [thread.thread.constr] to replace the constructors prior to paragraph 4 with the single constructor as above. Replace paragraph 4 - 6 with the following:

-4- Requires: F and each Ti in Args shall be CopyConstructible if an lvalue and otherwise MoveConstructible. INVOKE(f, w1, w2, ..., wN) (20.7.2 [func.require]) shall be a valid expression for some values w1, w2, ... , wN, where N == sizeof...(Args).

-5- Effects: Constructs an object of type thread and executes INVOKE(f, t1, t2, ..., tN) in a new thread of execution, where t1, t2, ..., tN are the values in args.... Constructs the following objects:

typename decay<F>::type g(std::forward<F>(f));
tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);

and executes INVOKE(g, wi...) in a new thread of execution. These objects shall be destroyed when the new thread of execution completes. Any return value from g is ignored. If f terminates with an uncaught exception, std::terminate() shall be called. If the evaluation of INVOKE(g, wi...) terminates with an uncaught exception, std::terminate() shall be called [Note: std::terminate() could be called before entering g. -- end note]. Any exception thrown before the evaluation of INVOKE has started shall be catchable in the calling thread.

-6- Synchronization: The invocation of the constructor happens before the invocation of f g.


930. Access to std::array data as built-in array type

Section: 23.3.1 [array] Status: Open Submitter: Niels Dekker Opened: 2008-11-17 Last modified: 2009-06-01

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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;
  1. 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:

  1. Remove the words "exposition only" from the definition of std::array::elems, as well as the note saying that "elems is shown for exposition only."
  2. Change the signature of std::array::data(), so that it would return a reference to the built-in array, instead of a pointer to its first element.
  3. Add extra member functions, returning a reference to the built-in array.

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 conversion

Recommended 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. ]

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.

Add to Zero sized arrays 23.3.1.6 [array.zero]:

4. The presence of c_array_type and c_array() and their semantics are implementation defined, for a zero-sized array.

931. type trait extent<T, I>

Section: 20.6.4.3 [meta.unary.prop] Status: Tentatively Ready Submitter: Yechezkel Mett Opened: 2008-11-04 Last modified: 2009-03-09

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Discussion:

The draft (N2798) says in 20.6.4.3 [meta.unary.prop] Table 44:

Table 44 -- Type property queries
TemplateValue
template <class T, unsigned I = 0> struct extent; If T is not an array type (8.3.4), or if it has rank less than I, or if I is 0 and T has type "array of unknown bound of U", then 0; otherwise, the size of the I'th dimension of T

Firstly it isn't clear from the wording if I is 0-based or 1-based ("the I'th dimension" sort of implies 1-based). From the following example it is clear that the intent is 0-based, in which case it should say "or if it has rank less than or equal to I".

Sanity check:

The example says assert((extent<int[2], 1>::value) == 0);

Here the rank is 1 and I is 1, but the desired result is 0.

[ Post Summit: ]

Do not use "size" or "value", use "bound". Also, move the cross-reference to 8.3.4 to just after "bound".

Recommend Tentatively Ready.

Proposed resolution:

In Table 44 of 20.6.4.3 [meta.unary.prop], third row, column "Value", change the cell content:

Table 44 -- Type property queries
TemplateValue
template <class T, unsigned I = 0> struct extent; If T is not an array type (8.3.4), or if it has rank less than or equal to I, or if I is 0 and T has type "array of unknown bound of U", then 0; otherwise, the size bound (8.3.4) of the I'th dimension of T, where indexing of I is zero-based.

[ Wording supplied by Daniel. ]


932. unique_ptr(pointer p) for pointer deleter types

Section: 20.8.12.2.1 [unique.ptr.single.ctor] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2008-11-26 Last modified: 2009-03-12

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Discussion:

Addresses US 79

20.8.12.2.1 [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.

Proposed resolution:

Change 20.8.12.2.1 [unique.ptr.single.ctor]/5:

unique_ptr(pointer p);
Requires: D shall not be a pointer type (diagnostic required). D shall be default constructible, and that construction shall not throw an exception.

933. Unique_ptr defect

Section: 20.8.12.2.5 [unique.ptr.single.modifiers] Status: Open Submitter: Alisdair Meredith Opened: 2008-11-27 Last modified: 2009-05-23

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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.

Proposed resolution:


934. duration is missing operator%

Section: 20.9.3 [time.duration] Status: Tentatively Ready Submitter: Terry Golubiewski Opened: 2008-11-30 Last modified: 2009-05-23

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Discussion:

Addresses US 81

duration is missing operator%. This operator is convenient for computing where in a time frame a given duration lies. A motivating example is converting a duration into a "broken-down" time duration such as hours::minutes::seconds:

class ClockTime
{
    typedef std::chrono::hours hours;
    typedef std::chrono::minutes minutes;
    typedef std::chrono::seconds seconds;
public:
    hours hours_;
    minutes minutes_;
    seconds seconds_;

    template <class Rep, class Period>
      explicit ClockTime(const std::chrono::duration<Rep, Period>& d)
        : hours_  (std::chrono::duration_cast<hours>  (d)),
          minutes_(std::chrono::duration_cast<minutes>(d % hours(1))),
          seconds_(std::chrono::duration_cast<seconds>(d % minutes(1)))
          {}
};

[ Summit: ]

Agree except that there is a typo in the proposed resolution. The member operators should be operator%=.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add to the synopsis in 20.9 [time]:

template <class Rep1, class Period, class Rep2>
  duration<typename common_type<Rep1, Rep2>::type, Period>
  operator%(const duration<Rep1, Period>& d, const Rep2& s);
template <class Rep1, class Period1, class Rep2, class Period2>
  typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
  operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

Add to the synopsis of duration in 20.9.3 [time.duration]:

template <class Rep, class Period = ratio<1>>
class duration {
public:
  ...
  duration& operator%=(const rep& rhs);
  duration& operator%=(const duration& d);
  ...
};

Add to 20.9.3.3 [time.duration.arithmetic]:

duration& operator%=(const rep& rhs);

Effects: rep_ %= rhs.

Returns: *this.

duration& operator%=(const duration& d);

Effects: rep_ %= d.count().

Returns: *this.

Add to 20.9.3.5 [time.duration.nonmember]:

template <class Rep1, class Period, class Rep2>
  duration<typename common_type<Rep1, Rep2>::type, Period>
  operator%(const duration<Rep1, Period>& d, const Rep2& s);

Requires: Rep2 shall be implicitly convertible to CR(Rep1, Rep2) and Rep2 shall not be an instantiation of duration. Diagnostic required.

Returns: duration<CR, Period>(d) %= s.

template <class Rep1, class Period1, class Rep2, class Period2>
  typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
  operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

Returns: common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type(lhs) %= rhs.


935. clock error handling needs to be specified

Section: 20.9.5 [time.clock] Status: Open Submitter: Beman Dawes Opened: 2008-11-24 Last modified: 2009-05-23

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Discussion:

Each of the three clocks specified in Clocks 20.9.5 [time.clock] provides the member function:

static time_point now();

The semantics specified by Clock requirements 20.9.1 [time.clock.req] make no mention of error handling. Thus the function may throw bad_alloc or an implementation-defined exception (17.6.4.10 [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.

Proposed resolution:

Accept the proposed wording of N2828, Library Support for hybrid error handling (Rev 1).

Change Clock requirements 20.9.1 [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 (19.5.2.2 [syserr.errcode.overview]).

Table 55 -- Clock requirements
ExpressionReturn typeOperational 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 20.9.5.1 [time.clock.system] as indicated:

static time_point now(error_code& ec=throws());

Change Class monotonic_clock 20.9.5.2 [time.clock.monotonic] as indicated:

static time_point now(error_code& ec=throws());

Change Class high_resolution_clock 20.9.5.3 [time.clock.hires] as indicated:

static time_point now(error_code& ec=throws());

936. Mutex type overspecified

Section: 30.4.1 [thread.mutex.requirements] Status: Open Submitter: Pete Becker Opened: 2008-12-05 Last modified: 2009-03-22

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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.

[ 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:


938. default_delete<T[]>::operator() should only accept T*

Section: 20.8.12.1.2 [unique.ptr.dltr.dflt1] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2008-12-07 Last modified: 2009-05-23

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Discussion:

Consider:

derived* p = new derived[3];
std::default_delete<base[]> d;
d(p);  // should fail

Currently the marked line is a run time failure. We can make it a compile time failure by "poisoning" op(U*).

[ Post Summit: ]

Recommend Review.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add to 20.8.12.1.2 [unique.ptr.dltr.dflt1]:

namespace std {
  template <class T> struct default_delete<T[]> {
    void operator()(T*) const;
  template <class U> void operator()(U*) const = delete;
};
}

939. Problem with std::identity and reference-to-temporaries

Section: 20.7.6 [identity.operation] Status: Review Submitter: Alisdair Meredith Opened: 2008-12-11 Last modified: 2009-06-02

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Discussion:

std::identity takes an argument of type T const & and returns a result of T const &.

Unfortunately, this signature will accept a value of type other than T that is convertible-to-T, and then return a reference to the dead temporary. The constraint in the concepts version simply protects against returning reference-to-void.

Solutions:

i/ Return-by-value, potentially slicing bases and rejecting non-copyable types

ii/ Provide an additional overload:

template< typename T >
template operator( U & ) = delete;

This seems closer on intent, but moves beyond the original motivation for the operator, which is compatibility with existing (non-standard) implementations.

iii/ Remove the operator() overload. This restores the original definition of the identity, although now effectively a type_trait rather than part of the perfect forwarding protocol.

iv/ Remove std::identity completely; its original reason to exist is replaced with the IdentityOf concept.

My own preference is somewhere between (ii) and (iii) - although I stumbled over the issue with a specific application hoping for resolution (i)!

[ Batavia (2009-05): ]

We dislike options i and iii, and option ii seems like overkill. If we remove it (option iv), implementers can still provide it under a different name.

Move to Open pending wording (from Alisdair) for option iv.

[ 2009-05-23 Alisdair provided wording for option iv. ]

Proposed resolution:

Strike 20.2.1 [concept.transform] p3:

-4- Note: concept form of the identity type metafunction (20.7.6).

Strike from 20.7 [function.objects] p2:

// 20.7.6, identity operation:
template <IdentityOf T> struct identity;

Remove 20.7.6 [identity.operation] (whole subclause):

template <IdentityOf 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;
-1- Returns: x

940. std::distance

Section: 24.4 [iterator.operations] Status: Tentatively Ready Submitter: Thomas Opened: 2008-12-14 Last modified: 2009-05-23

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Discussion:

Addresses UK 270

Regarding the std::distance - function, 24.4 [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 [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 [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 24.2 [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 [iterator.operations] p4 as follows:

Effects: Returns the number of increments or decrements needed 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.

Proposed resolution:

Change 24.4 [iterator.operations]:

template <InputIterator Iter>
  Iter::difference_type
  distance(Iter first, Iter last);
template <RandomAccessIterator Iter>
  Iter::difference_type distance(Iter first, Iter last);

-4- Effects: Returns the number of increments or decrements needed to get from first to last.

-5- Requires: last shall be reachable from first.

template <RandomAccessIterator Iter>
  Iter::difference_type distance(Iter first, Iter last);

-6- Effects: Returns the number of increments or decrements needed to get from first to last.

-7- Requires: last shall be reachable from first or first shall be reachable from last.


941. Ref-qualifiers for assignment operators

Section: 17 [library] Status: Open Submitter: Niels Dekker Opened: 2008-12-18 Last modified: 2009-05-23

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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.

Proposed resolution:

A proposed resolution is provided by the paper on this subject, N2819, Ref-qualifiers for assignment operators of the Standard Library


943. ssize_t undefined

Section: 29.5.2 [atomics.types.address] Status: Tentatively Ready Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2009-05-23

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Discussion:

There is a row in "Table 122 - Atomics for standard typedef types" in 29.5.1 [atomics.types.integral] with atomic_ssize_t and ssize_t. Unless, I'm missing something ssize_t is not defined by the standard.

[ Summit: ]

Move to review. Proposed resolution: Remove the typedef. Note: ssize_t is a POSIX type.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Remove the row containing ssize_t from Table 119 "Atomics for standard typedef types" in 29.5.2 [atomics.types.address].


944. atomic<bool> derive from atomic_bool?

Section: 29.5.3 [atomics.types.generic] Status: Open Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2009-03-22

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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. ]

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.

945. system_clock::rep not specified

Section: 20.9.5.1 [time.clock.system] Status: Tentatively NAD Editorial Submitter: Pete Becker Opened: 2008-12-19 Last modified: 2009-05-23

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Discussion:

In 20.9.5.1 [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 20.9.5.1 [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].

946. duration_cast improperly specified

Section: 20.9.3.7 [time.duration.cast] Status: Tentatively NAD Submitter: Pete Becker Opened: 2008-12-20 Last modified: 2009-05-23

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Discussion:

20.9.3.7 [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>::type

The 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:


947. duration arithmetic: contradictory requirements

Section: 20.9.3.5 [time.duration.nonmember] Status: Open Submitter: Pete Becker Opened: 2008-12-20 Last modified: 2009-05-23

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Discussion:

In 20.9.3.5 [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).

Proposed resolution:


948. ratio arithmetic tweak

Section: 20.4.2 [ratio.arithmetic] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2008-12-26 Last modified: 2009-03-09

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Discussion:

N2800, 20.4.2 [ratio.arithmetic] lacks a paragraph from the proposal N2661:

ratio arithmetic [ratio.arithmetic]

... If the implementation is unable to form the indicated ratio due to overflow, a diagnostic shall be issued.

The lack of a diagnostic on compile-time overflow is a significant lack of functionality. This paragraph could be put back into the WP simply editorially. However in forming this issue I realized that we can do better than that. This paragraph should also allow alternative formulations which go to extra lengths to avoid overflow when possible. I.e. we should not mandate overflow when the implementation can avoid it.

For example:

template <class R1, class R2> struct ratio_multiply {
  typedef see below} type; 
The nested typedef type shall be a synonym for ratio<T1, T2> where T1 has the value R1::num * R2::num and T2 has the value R1::den * R2::den.

Consider the case where intmax_t is a 64 bit 2's complement signed integer, and we have:

typedef std::ratio<0x7FFFFFFFFFFFFFFF, 0x7FFFFFFFFFFFFFF0> R1;
typedef std::ratio<8, 7> R2;
typedef std::ratio_multiply<R1, R2>::type RT;

According to the present formulation the implementaiton will multiply 0x7FFFFFFFFFFFFFFF * 8 which will result in an overflow and subsequently require a diagnostic.

However if the implementation is first allowed to divde 0x7FFFFFFFFFFFFFFF by 7 obtaining 0x1249249249249249 / 1 and divide 8 by 0x7FFFFFFFFFFFFFF0 obtaining 1 / 0x0FFFFFFFFFFFFFFE, then the exact result can then be computed without overflow:

[0x7FFFFFFFFFFFFFFF/0x7FFFFFFFFFFFFFF0] * [8/7] = [0x1249249249249249/0x0FFFFFFFFFFFFFFE]

Example implmentation which accomplishes this:

template <class R1, class R2>
struct ratio_multiply
{
private:
    typedef ratio<R1::num, R2::den> _R3;
    typedef ratio<R2::num, R1::den> _R4;
public:
    typedef ratio<__ll_mul<_R3::num, _R4::num>::value,
                  __ll_mul<_R3::den, _R4::den>::value> type;
};

[ Post Summit: ]

Recommend Tentatively Ready.

Proposed resolution:

Add a paragraph prior to p1 in 20.4.2 [ratio.arithmetic]:

Implementations may use other algorithms to compute the indicated ratios to avoid overflow. If overflow occurs, a diagnostic shall be issued.

949. owner_less

Section: 20.8.13.4 [util.smartptr.ownerless] Status: Tentatively Ready Submitter: Thomas Plum Opened: 2008-12-30 Last modified: 2009-03-10

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Discussion:

20.8.13.4 [util.smartptr.ownerless] (class template owner_less) says that operator()(x,y) shall return x.before(y).

However, shared_ptr and weak_ptr have an owner_before() but not a before(), and there's no base class to provide a missing before().

Being that the class is named owner_less , I'm guessing that "before()" should be "owner_before()", right?

[ Herve adds: ]

Agreed with the typo, it should be "shall return x.owner_before(y)".

[ Post Summit: ]

Recommend Tentatively Ready.

Proposed resolution:

Change 20.8.13.4 [util.smartptr.ownerless] p2:

-2- operator()(x,y) shall return x.owner_before(y). [Note: ...

950. unique_ptr converting ctor shouldn't accept array form

Section: 20.8.12.2.1 [unique.ptr.single.ctor] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2009-01-07 Last modified: 2009-05-23

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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.

Proposed resolution:

Change 20.8.12.2.1 [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 (diagnostic required). U shall not be an array type (diagnostic required). [Note: These requirements imply that T and U are complete types. -- end note]

Change 20.8.12.2.3 [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 (diagnostic required). U shall not be an array type (diagnostic required). [Note: These requirements imply that T and U are complete types. -- end note]


951. Various threading bugs #1

Section: 20.9.2.1 [time.traits.is_fp] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-05-23

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Discussion:

Related to 953.

20.9.2.1 [time.traits.is_fp] says that the type Rep "is assumed to be ... a class emulating an integral type." What are the requirements for such a type?

[ 2009-05-10 Howard adds: ]

IntegralLike.

[ Batavia (2009-05): ]

As with issue 953, we recommend this issue be addressed in the context of providing concepts for the entire thread header.

We look forward to proposed wording.

Move to Open.

Proposed resolution:


952. Various threading bugs #2

Section: 20.9.3.7 [time.duration.cast] Status: Tentatively NAD Editorial Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-05-23

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Discussion:

20.9.3.7 [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:


953. Various threading bugs #3

Section: 20.9.1 [time.clock.req] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-05-23

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Discussion:

Related to 951.

20.9.1 [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.

Proposed resolution:


954. Various threading bugs #4

Section: 20.9.1 [time.clock.req] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-05-30

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Discussion:

Table 55 -- Clock Requirements (in 20.9.1 [time.clock.req])

  1. the requirements for C1::time_point require C1 and C2 to "refer to the same epoch", but "epoch" is not defined.
  2. "Different clocks may share a time_point definition if it is valid to compare their time_points by comparing their respective durations." What does "valid" mean here? And, since C1::rep is "**THE** representation type of the native duration and time_point" (emphasis added), there doesn't seem to be much room for some other representation.
  3. C1::is_monotonic has type "const bool". The "const" should be removed.
  4. C1::period has type ratio. ratio isn't a type, it's a template. What is the required type?

[ 2009-05-10 Howard adds: ]

  1. "epoch" is purposefully not defined beyond the common English definition. The C standard also chose not to define epoch, though POSIX did. I believe it is a strength of the C standard that epoch is not defined. When it is known that two time_points refer to the same epoch, then a definition of the epoch is not needed to compare the two time_points, or subtract them.

    A time_point and a Clock implicitly refer to an (unspecified) epoch. The time_point represents an offset (duration) from an epoch.

  2. The sentence:

    Different clocks may share a time_point definition if it is valid to compare their time_points by comparing their respective durations.

    is redundant and could be removed. I believe the sentence which follows the above:

    C1 and C2 shall refer to the same epoch.

    is sufficient. If two clocks share the same epoch, then by definition, comparing their time_points is valid.

  3. is_monotonic is meant to never change (be const). It is also desired that this value be usable in compile-time computation and branching.
  4. This should probably instead be worded:

    An instantiation of ratio.

[ Batavia (2009-05): ]

Re (a): It is not clear to us whether "epoch" is a term of art.

Re (b), (c), and (d): We agree with Howard's comments, and would consider adding to (c) a static constexpr requirement.

Move to Open pending proposed wording.

[ 2009-05-25 Daniel adds: ]

In regards to (d) I suggest to say "a specialization of ratio" instead of "An instantiation of ratio". This seems to be the better matching standard core language term for this kind of entity.

[ 2009-05-25 Ganesh adds: ]

Regarding (a), I found this paper on the ISO website using the term "epoch" consistently with the current wording:

http://standards.iso.org/ittf/PubliclyAvailableStandards/C030811e_FILES/MAIN_C030811e/text/ISOIEC_18026E_TEMPORAL_CS.HTM

which is part of ISO/IEC 18026 "Information technology -- Spatial Reference Model (SRM)".

Proposed resolution:

  1. Change 20.9.1 [time.clock.req] p1:

    -1- A clock is a bundle consisting of a native duration, a native time_point, and a function now() to get the current time_point. The origin of the clock's time_point is referred to as the clock's epoch as defined in section 6.3 of ISO/IEC 18026. A clock shall meet the requirements in Table 45.
  2. Remove the sentence from the time_point row of the table "Clock Requirements":

    Clock requirements
    C1::time_point chrono::time_point<C1> or chrono::time_point<C2, C1::duration> The native time_point type of the clock. Different clocks may share a time_point definition if it is valid to compare their time_points by comparing their respective durations. C1 and C2 shall refer to the same epoch.
  1. Change the row starting with C1::period of the table "Clock Requirements":

    Clock requirements
    C1::period a specialization of ratio The tick period of the clock in seconds.

955. Various threading bugs #5

Section: 20.9.1 [time.clock.req] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-06-07

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Discussion:

20.9.1 [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.

Proposed resolution:

Change 20.9 [time]:

...
template <class Clock, class Duration = typename Clock::duration_type> class time_point;
...

Change 20.9.1 [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 20.9.5.1 [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 20.9.5.2 [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 20.9.5.3 [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();
};

956. Various threading bugs #6

Section: 20.9.1 [time.clock.req] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-05-23

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Discussion:

20.9.1 [time.clock.req] uses the word "native" in several places, but doesn't define it. What is a "native duration"?

[ 2009-05-10 Howard adds: ]

The standard uses "native" in several places without defining it (e.g. 2.14.3 [lex.ccon]). It is meant to mean "that which is defined by the facility", or something along those lines. In this case it refers to the nested time_point and duration types of the clock. Better wording is welcome.

[ Batavia (2009-05): ]

Move to Open pending proposed wording from Pete.

Proposed resolution:


957. Various threading bugs #7

Section: 20.9.5.1 [time.clock.system] Status: Review Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-05-23

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Discussion:

20.9.5.1 [time.clock.system]: to_time_t is overspecified. It requires truncation, but should allow rounding. For example, suppose a system has a clock that gives times in milliseconds, but time() rounds those times to the nearest second. Then system_clock can't use any resolution finer than one second, because if it did, truncating times between half a second and a full second would produce the wrong time_t value.

[ Post Summit Anthony Williams provided proposed wording. ]

[ Batavia (2009-05): ]

Move to Review pending input from Howard. and other stakeholders.

[ 2009-05-23 Howard adds: ]

I am in favor of the wording provided by Anthony.

Proposed resolution:

In 20.9.5.1 [time.clock.system] replace paragraphs 3 and 4 with:

time_t to_time_t(const time_point& t);
-3- Returns: A time_t object that represents the same point in time as t when both values are truncated restricted to the coarser of the precisions of time_t and time_point. It is implementation defined whether values are rounded or truncated to the required precision.
time_point from_time_t(time_t t);
-4- Returns: A time_point object that represents the same point in time as t when both values are truncated restricted to the coarser of the precisions of time_t and time_point. It is implementation defined whether values are rounded or truncated to the required precision.

958. Various threading bugs #8

Section: 30.5.1 [thread.condition.condvar] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-03-22

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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.

Proposed resolution:


959. Various threading bugs #9

Section: 30.5.1 [thread.condition.condvar] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-03-22

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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.

Proposed resolution:


960. Various threading bugs #10

Section: 30.4.1 [thread.mutex.requirements] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-03-27

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Discussion:

30.4.1 [thread.mutex.requirements]: paragraph 4 is entitled "Error conditions", but according to 17.5.1.4 [structure.specifications], "Error conditions:" specifies "the error conditions for error codes reported by the function." It's not clear what this should mean when there is no function in sight.

[ Summit: ]

Move to open.

[ Beman provided proposed wording. ]

Proposed resolution:

Change 30.4.1 [thread.mutex.requirements] Mutex requirements, paragraph 4 as indicated:

-4- Error conditions: The error conditions for error codes, if any, reported by member functions of type Mutex shall be:


961. Various threading bugs #11

Section: 30.4.1 [thread.mutex.requirements] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-03-22

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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.

Proposed resolution:


962. Various threading bugs #12

Section: 30.4.3.2.2 [thread.lock.unique.locking] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-03-22

View all issues with Open status.

Discussion:

30.4.3.2.2 [thread.lock.unique.locking]: unique_lock::lock is required to throw an object of type std::system_error "when the postcondition cannot be achieved." The postcondition is owns == true, and this is trivial to achieve. Presumably, the requirement is intended to mean something more than that.

[ Summit: ]

Move to open.

[ Beman has volunteered to provide proposed wording. ]

Proposed resolution:


963. Various threading bugs #13

Section: 30.3.1.5 [thread.thread.member] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-03-22

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Discussion:

30.3.1.5 [thread.thread.member]: thread::detach is required to throw an exception if the thread is "not a detachable thread". "Detachable" is never defined.

[ Howard adds: ]

Due to a mistake on my part, 3 proposed resolutions appeared at approximately the same time. They are all three noted below in the discussion.

[ Summit, proposed resolution: ]

In 30.3.1.5 [thread.thread.member] change:

void detach();

...

-14- Error conditions:

  • no_such_process -- if the thread is not a valid thread.
  • invalid_argument -- if the thread is not a detachable joinable thread.

[ Post Summit, Jonathan Wakely adds: ]

A thread is detachable if it is joinable. As we've defined joinable, we can just use that.

This corresponds to the pthreads specification, where pthread_detach fails if the thread is not joinable:

EINVAL: The implementation has detected that the value specified by thread does not refer to a joinable thread.

Jonathan recommends this proposed wording:

In 30.3.1.5 [thread.thread.member] change:

void detach();

...

-14- Error conditions:

  • ...
  • invalid_argument -- not a detachable joinable thread.

[ Post Summit, Anthony Williams adds: ]

This is covered by the precondition that joinable() be true.

Anthony recommends this proposed wording:

In 30.3.1.5 [thread.thread.member] change:

void detach();

...

-14- Error conditions:

  • ...
  • invalid_argument -- not a detachable thread.

Proposed resolution:


964. Various threading bugs #14

Section: 30.5.2 [thread.condition.condvarany] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-03-22

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Discussion:

The requirements for the constructor for condition_variable has several error conditions, but the requirements for the constructor for condition_variable_any has none. Is this difference intentional?

[ Summit: ]

Move to open, pass to Howard. If this is intentional, a note may be helpful. If the error conditions are to be copied from condition_variable, this depends on LWG 965.

[ Post Summit Howard adds: ]

The original intention (N2447) was to let the OS return whatever errors it was going to return, and for those to be translated into exceptions, for both condition_variable and condition_variable_any. I have not received any complaints about specific error conditions from vendors on non-POSIX platforms, but such complaints would not surprise me if they surfaced.

Proposed resolution:


965. Various threading bugs #15

Section: 30.5.1 [thread.condition.condvar] Status: Tentatively Ready Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-05-23

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Discussion:

30.5.1 [thread.condition.condvar]: the constructor for condition_variable throws an exception with error code device_or_resource_busy "if attempting to initialize a previously-initialized but as of yet undestroyed condition_variable." How can this occur?

[ Summit: ]

Move to review. Proposed resolution: strike the device_or_resource_busy error condition from the constructor of condition_variable.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 30.5.1 [thread.condition.condvar] p3:


966. Various threading bugs #16

Section: 30.5.1 [thread.condition.condvar] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-03-22

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Discussion:

30.5.1 [thread.condition.condvar]: condition_variable::wait and condition_variable::wait_until both have a postcondition that lock is locked by the calling thread, and a throws clause that requires throwing an exception if this postcondition cannot be achieved. How can the implementation detect that this lock can never be obtained?

[ Summit: ]

Move to open. Requires wording. Agreed this is an issue, and the specification should not require detecting deadlocks.

Proposed resolution:


967. Various threading bugs #17

Section: 30.3.1.2 [thread.thread.constr] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-03-22

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Discussion:

the error handling for the constructor for condition_variable distinguishes lack of memory from lack of other resources, but the error handling for the thread constructor does not. Is this difference intentional?

[ Beman has volunteered to provide proposed wording. ]

Proposed resolution:


968. Various threading bugs #18

Section: 30.4.1 [thread.mutex.requirements] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-03-22

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Discussion:

30.4.1 [thread.mutex.requirements]: several functions are required to throw exceptions "if the thread does not have the necessary permission ...". "The necessary permission" is not defined.

[ Summit: ]

Move to open.

[ Beman has volunteered to provide proposed wording. ]

Proposed resolution:


969. What happened to Library Issue 475?

Section: 25.3.4 [alg.foreach] Status: Tentatively NAD Editorial Submitter: Stephan T. Lavavej Opened: 2009-01-12 Last modified: 2009-05-23

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Discussion:

Library Issue 475 has CD1 status, but the non-normative note in N2723 was removed in N2798 (25.3.4 [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.


970. addressof overload unneeded

Section: 20.8.11.1 [object.addressof] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2009-01-16 Last modified: 2009-05-23

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Discussion:

20.8.11.1 [object.addressof] specifies:

template <ObjectType T> T* addressof(T& r);
template <ObjectType T> T* addressof(T&& r);

The two signatures are ambiguous when the argument is an lvalue. The second signature seems not useful: what does it mean to take the address of an rvalue?

[ Post Summit: ]

Recommend Review.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 20.8.11.1 [object.addressof]:

template <ObjectType T> T* addressof(T& r);
template <ObjectType T> T* addressof(T&& r);

971. Spurious diagnostic conversion function

Section: 19.5.2.6 [syserr.errcode.nonmembers] Status: Open Submitter: Beman Dawes Opened: 2009-01-19 Last modified: 2009-05-23

View all issues with Open 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.

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 19.5.2.6 [syserr.errcode.nonmembers]:

error_code make_error_code(errc e);
Returns: error_code(static_cast<int>(e), generic_category).

972. The term "Assignable" undefined but still in use

Section: 17 [library] Status: Tentatively NAD Editorial Submitter: Niels Dekker Opened: 2009-01-07 Last modified: 2009-05-23

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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 20.7.5 [refwrap]:

reference_wrapper<T> is a CopyConstructible and CopyAssignable wrapper around a reference to an object of type T.

Change Placeholders 20.7.12.1.4 [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 20.8.13.2 [util.smartptr.shared]:

Specializations of shared_ptr shall be CopyConstructible, CopyAssignable, and LessThanComparable...

Change Class template weak_ptr 20.8.13.3 [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.9.1 [auto.ptr], as this is dealt with more specifically by LWG 973, "auto_ptr characteristics", submitted by Maarten Hilferink.


973. auto_ptr characteristics

Section: D.9.1 [auto.ptr] Status: Tentatively NAD Editorial Submitter: Maarten Hilferink Opened: 2009-01-21 Last modified: 2009-05-23

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Discussion:

I think that the Note of D.9.1 [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.9.1 [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]

974. duration<double> should not implicitly convert to duration<int>

Section: 20.9.3.1 [time.duration.cons] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2009-01-21 Last modified: 2009-05-23

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Discussion:

The following code should not compile because it involves implicit truncation errors (against the design philosophy of the duration library).

duration<double> d(3.5);
duration<int> i = d;  // implicit truncation, should not compile

This intent was codified in the example implementation which drove this proposal but I failed to accurately translate the code into the specification in this regard.

[ Batavia (2009-05): ]

We agree with the proposed resolution.

Move to Tentatively Ready.

Proposed resolution:

Change 20.9.3.1 [time.duration.cons], p4:

template <class Rep2, class Period2> 
  duration(const duration<Rep2, Period2>& d);
-4- Requires: treat_as_floating_point<rep>::value shall be true or both ratio_divide<Period2, period>::type::den shall be 1 and treat_as_floating_point<Rep2>::value shall be false. Diagnostic required. [Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. -- end note]

975. is_convertible cannot be instantiated for non-convertible types

Section: 20.6.5 [meta.rel] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2009-01-25 Last modified: 2009-05-23

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Discussion:

Addresses UK 206

Related to 1114.

The current specification of std::is_convertible (reference is draft N2798) is basically defined by 20.6.5 [meta.rel]/4:

In order to instantiate the template is_convertible<From, To>, the following code shall be well formed:

template <class T>
  typename add_rvalue_reference<T>::type create();

To test() {
  return create<From>();
}

[Note: This requirement gives well defined results for reference types, void types, array types, and function types. --end note]

The first sentence can be interpreted, that e.g. the expression

std::is_convertible<double, int*>::value

is ill-formed because std::is_convertible<double, int*> could not be instantiated, or in more general terms: The wording requires that std::is_convertible<X, Y> cannot be instantiated for otherwise valid argument types X and Y if X is not convertible to Y.

This semantic is both unpractical and in contradiction to what the last type traits paper N2255 proposed:

If the following test function is well formed code b is true, else it is false.

template <class T>
  typename add_rvalue_reference<T>::type create();

To test() {
  return create<From>();
}

[Note: This definition gives well defined results for reference types, void types, array types, and function types. --end note]

[ Post Summit: ]

Jens: Checking that code is well-formed and then returning true/false sounds like speculative compilation. John Spicer would really dislike this. Please find another wording suggesting speculative compilation.

Recommend Open.

[ Post Summit, Howard adds: ]

John finds the following wording clearer:

TemplateConditionComments
template <class From, class To>
struct is_convertible;
see below From and To shall be complete types, arrays of unknown bound, or (possibly cv-qualified) void types.

Given the following function prototype:

template <class T>
  typename add_rvalue_reference<T>::type create();

is_convertible<From, To>::value shall be true if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, else is_convertible<From, To>::value shall be false.

To test() {
  return create<From>();
}
Original proposed wording:

In 20.6.5 [meta.rel]/4 change:

In order to instantiate the template is_convertible<From, To>, the following code shall be well formed If the following code is well formed is_convertible<From, To>::value is true, otherwise false:[..]

Revision 2

In 20.6.5 [meta.rel] change:

TemplateConditionComments
.........
template <class From, class To>
struct is_convertible;
The code set out below shall be well formed. see below From and To shall be complete types, arrays of unknown bound, or (possibly cv-qualified) void types.

-4- In order to instantiate the template is_convertible<From, To>, the following code shall be well formed: Given the following function prototype:

template <class T> 
  typename add_rvalue_reference<T>::type create();

is_convertible<From, To>::value inherits either directly or indirectly from true_type if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, else is_convertible<From, To>::value inherits either directly or indirectly from false_type.

To test() { 
  return create<From>(); 
}

[Note: This requirement gives well defined results for reference types, void types, array types, and function types. -- end note]

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 20.6.5 [meta.rel] change:

TemplateConditionComments
.........
template <class From, class To>
struct is_convertible;
The code set out below shall be well formed. see below From and To shall be complete types, arrays of unknown bound, or (possibly cv-qualified) void types.

-4- In order to instantiate the template is_convertible<From, To>, the following code shall be well formed: Given the following function prototype:

template <class T> 
  typename add_rvalue_reference<T>::type create();

the predicate condition for a template specialization is_convertible<From, To> shall be satisfied, if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function.

To test() { 
  return create<From>(); 
}

[Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]


976. Class template std::stack should be movable

Section: 23.3.5.3.1 [stack.defn] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2009-02-01 Last modified: 2009-05-23

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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.

Proposed resolution:

In the class stack synopsis of 23.3.5.3.1 [stack.defn] insert:

template <ObjectType T, StackLikeContainer Cont = deque<T> > 
  requires SameType<Cont::value_type, T> 
        && NothrowDestructible<Cont> 
class stack { 
public: 
   ...
   requires CopyConstructible<Cont> explicit stack(const Cont&); 
   requires MoveConstructible<Cont> explicit stack(Cont&& = Cont()); 
   requires MoveConstructible<Cont> stack(stack&&);
   requires MoveAssignable<Cont> stack& operator=(stack&&);
   template <class Alloc> 
     requires Constructible<Cont, const Alloc&> 
     explicit stack(const Alloc&);
   ...
};

[Remark: This change should be done in sync with the resolution of paper N2819]


977. insert iterators inefficient for expensive to move types

Section: 24.7 [insert.iterators] Status: Open Submitter: Howard Hinnant Opened: 2009-02-02 Last modified: 2009-05-23

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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.

Proposed resolution:

Change 23.2.6.1 [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_type Value 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_type Value v) { 
    v == *insert(c, position, v); 
  } 
}

Change 23.2.6.2 [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_type Value 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_type Value v) { 
    v == *c.insert(position, v); 
  } 
}

Change 23.2.6.3 [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 24.7.1 [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 24.7.2.2 [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 24.7.3 [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 24.7.4.2 [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 24.7.5 [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 24.7.6.2 [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;

978. Hashing smart pointers

Section: 20.7.17 [unord.hash] Status: Open Submitter: Alisdair Meredith Opened: 2009-02-02 Last modified: 2009-05-31

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Discussion:

I don't see an open issue on supporting std::hash for smart pointers (unique_ptr and shared_ptr at least).

It seems reasonable to at least expect support for the smart pointers, especially as they support comparison for use in ordered associative containers.

[ Batavia (2009-05): ]

Howard points out that the client can always supply a custom hash function.

Alisdair replies that the smart pointer classes are highly likely to be frequently used as hash keys.

Bill would prefer to be conservative.

Alisdair mentions that this issue may also be viewed as a subissue or duplicate of issue 1025.

Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.

[ 2009-05-31 Peter adds: ]

Howard points out that the client can always supply a custom hash function.

Not entirely true. The client cannot supply the function that hashes the address of the control block (the equivalent of the old operator<, now proudly carrying the awkward name of 'owner_before'). Only the implementation can do that, not necessarily via specializing hash<>, of course.

This hash function makes sense in certain situations for shared_ptr (when one needs to switch from set/map using ownership ordering to unordered_set/map) and is the only hash function that makes sense for weak_ptr.

Proposed resolution:


979. Bad example

Section: 24.5.3 [move.iterators] Status: Tentatively NAD Editorial Submitter: Howard Hinnant Opened: 2009-02-03 Last modified: 2009-05-23

View all issues with Tentatively NAD Editorial status.

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 the setlist 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]


981. Unordered container requirements should add initializer_list support

Section: 23.2.5 [unord.req] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2009-02-08 Last modified: 2009-05-23

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Discussion:

Refering to N2800 all container requirements tables (including those for associative containers) provide useful member function overloads accepting std::initializer_list as argument, the only exception is Table 87. There seems to be no reason for not providing them, because 23.5 [unord] is already initializer_list-aware. For the sake of library interface consistency and user-expectations corresponding overloads should be added to the table requirements of unordered containers as well.

[ Batavia (2009-05): ]

We agree with the proposed resolution.

Move to Tentatively Ready.

Proposed resolution:

In 23.2.5 [unord.req]/9 insert:

... [q1, q2) is a valid range in a, il designates an object of type initializer_list<value_type>, t is a value of type X::value_type, ...

In 23.2.5 [unord.req], Table 87 insert:

Table 87 - Unordered associative container requirements (in addition to container)
Expression Return type Assertion/note
pre-/post-condition
Complexity
X(i, j)
X a(i, j)
X ... ...
X(il) X Same as X(il.begin(), il.end()). Same as X(il.begin(), il.end()).
... ... ... ...
a = b X ... ...
a = il X& a = X(il); return *this; Same as a = X(il).
... ... ... ...
a.insert(i, j) void ... ...
a.insert(il) void Same as a.insert(il.begin(), il.end()). Same as a.insert(il.begin(), il.end()).

982. Wrong complexity for initializer_list assignment in Table 85

Section: 23.2.4 [associative.reqmts] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2009-02-08 Last modified: 2009-05-23

View other active issues in [associative.reqmts].

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Discussion:

According to N2800, the associative container requirements table 85 says that assigning an initializer_list to such a container is of constant complexity, which is obviously wrong.

[ Batavia (2009-05): ]

We agree with the proposed resolution.

Move to Tentatively Ready.

Proposed resolution:

In 23.2.4 [associative.reqmts], Table 85 change:

Table 85 - Associative container requirements (in addition to container)
Expression Return type Assertion/note
pre-/post-condition
Complexity
a = il X& a = X(il);
return *this;
constantSame as a = X(il).

983. unique_ptr reference deleters should not be moved from

Section: 20.8.12.2 [unique.ptr.single] Status: Review Submitter: Howard Hinnant Opened: 2009-02-10 Last modified: 2009-05-23

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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.

Proposed resolution:

Change 20.8.12.2.1 [unique.ptr.single.ctor], p20-21

template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);

-20- Requires: If D E 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, it this deleter is move constructed from u's deleter, otherwise the reference this 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 20.8.12.2.3 [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 move assignment from u's deleter to this deleter std::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 20.8.12.2.3 [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 rvalue DE 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 move assignment from u's deleter to this deleter std::forward<E>(u.get_deleter()). If either D or E is a reference type, then the referenced lvalue deleter participates in the move assignment.


984. Does <cinttypes> have macro guards?

Section: 27.9.2 [c.files] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2009-02-12 Last modified: 2009-05-23

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Discussion:

The C standard says about <inttypes.h>:

C++ implementations should define these macros only when __STDC_FORMAT_MACROSis defined before <inttypes.h> is included.

The C standard has a similar note about <stdint.h>. For <cstdint> we adopted a "thanks but no thanks" policy and documented that fact in 18.4.1 [cstdint.syn]:

... [Note: The macros defined by <stdint> are provided unconditionally. In particular, the symbols __STDC_LIMIT_MACROS and __STDC_CONSTANT_MACROS (mentioned in C99 footnotes 219, 220, and 222) play no role in C++. -- end note]

I recommend we put a similar note in 27.9.2 [c.files] regarding <cinttypes>.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add to 27.9.2 [c.files]:

Table 112 describes header <cinttypes>. [Note: The macros defined by <cintypes> are provided unconditionally. In particular, the symbol __STDC_FORMAT_MACROS (mentioned in C99 footnote 182) plays no role in C++. -- end note]

985. Allowing throwing move

Section: 23.2.1 [container.requirements.general] Status: Open Submitter: Rani Sharoni Opened: 2009-02-12 Last modified: 2009-05-23

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Discussion:

Introduction

This proposal is meant to resolve potential regression of the N2800 draft, see next section, and to relax the requirements for containers of types with throwing move constructors.

The basic problem is that some containers operations, like push_back, have a strong exception safety guarantee (i.e. no side effects upon exception) that are not achievable when throwing move constructors are used since there is no way to guarantee revert after partial move. For such operations the implementation can at most provide the basic guarantee (i.e. valid but unpredictable) as it does with multi copying operations (e.g. range insert).

For example, vector<T>::push_back() (where T has a move constructor) might resize the vector and move the objects to the new underlying buffer. If move constructor throws it might not be possible to recover the throwing object or to move the old objects back to the original buffer.

The current draft is explicit by disallowing throwing move for some operations (e.g. vector<>::reserve) and not clear about other operations mentioned in 23.2.1 [container.requirements.general]/10 (e.g. single element insert): it guarantees strong exception safety without explicitly disallowing a throwing move constructor.

Regression

This section only refers to cases in which the contained object is by itself a standard container.

Move constructors of standard containers are allowed to throw and therefore existing operations are broken, compared with C++03, due to move optimization. (In fact existing implementations like Dinkumware are actually throwing).

For example, vector< list<int> >::reserve yields undefined behavior since list<int>'s move constructor is allowed to throw. On the other hand, the same operation has strong exception safety guarantee in C++03.

There are few options to solve this regression:

  1. Disallow throwing move and throwing default constructor
  2. Disallow throwing move but disallowing usage after move
  3. Special casing
  4. Disallow throwing move and making it optional

Option 1 is suggested by proposal N2815 but it might not be applicable for existing implementations for which containers default constructors are throwing.

Option 2 limits the usage significantly and it's error prone by allowing zombie objects that are nothing but destructible (e.g. no clear() is allowed after move). It also potentially complicates the implementation by introducing special state.

Option 3 is possible, for example, using default construction and swap instead of move for standard containers case. The implementation is also free to provide special hidden operation for non throwing move without forcing the user the cope with the limitation of option-2 when using the public move.

Option 4 impact the efficiency in all use cases due to rare throwing move.

The proposed wording will imply option 1 or 3 though option 2 is also achievable using more wording. I personally oppose to option 2 that has impact on usability.

Relaxation for user types

Disallowing throwing move constructors in general seems very restrictive since, for example, common implementation of move will be default construction + swap so move will throw if the default constructor will throw. This is currently the case with the Dinkumware implementation of node based containers (e.g. std::list) though this section doesn't refer to standard types.

For throwing move constructors it seem that the implementation should have no problems to provide the basic guarantee instead of the strong one. It's better to allow throwing move constructors with basic guarantee than to disallow it silently (compile and run), via undefined behavior.

There might still be cases in which the relaxation will break existing generic code that assumes the strong guarantee but it's broken either way given a throwing move constructor since this is not a preserving optimization.

[ Batavia (2009-05): ]

Bjarne comments (referring to his draft paper): "I believe that my suggestion simply solves that. Thus, we don't need a throwing move."

Move to Open and recommend it be deferred until after the next Committee Draft is issued.

Proposed resolution:

23.2.1 [container.requirements.general] paragraph 10 add footnote:

-10- Unless otherwise specified (see 23.1.4.1, 23.1.5.1, 23.2.2.3, and 23.2.6.4) all container types defined in this Clause meet the following additional requirements:

[Note: for compatibility with C++ 2003, when "no effect" is required, standard containers should not use the value_type's throwing move constructor when the contained object is by itself a standard container. -- end note]

23.2.5.1 [unord.req.except] change paragraph 2 to say:

-2- For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an insert() function inserting a single element, the insert() function has no effect unless the exception is thrown by the contained object move constructor.

-4- For unordered associative containers, if an exception is thrown from within a rehash() function other than by the container's hash function or comparison function, the rehash() function has no effect unless the exception is thrown by the contained object move constructor.

23.3.2.3 [deque.modifiers] change paragraph 2 to say:

-2- Remarks: If an exception is thrown other than by the copy constructor, move constructor or assignment operator of T there are no effects. If an exception is thrown by push_back() or emplace_back() function, that function has no effects unless the exception is thrown by the move constructor of T.

23.3.2.3 [deque.modifiers] change paragraph 6 to say:

-6- Throws: Nothing unless an exception is thrown by the copy constructor, move constructor or assignment operator of T.

23.3.6.2 [vector.capacity] remove paragraph 2

-2- Requires: If value_type has a move constructor, that constructor shall not throw any exceptions.

23.3.6.2 [vector.capacity] paragraph 3 change to say:

-3- Effects: A directive that informs a vector of a planned change in size, so that it can manage the storage allocation accordingly. After reserve(), capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of reserve(). If an exception is thrown, there are no effects unless the exception is thrown by the contained object move constructor.

23.3.6.2 [vector.capacity] paragraph 12 change to say:

-12- Requires: If value_type has a move constructor, that constructor shall not throw any exceptions. If an exception is thrown, there are no effects unless the exception is thrown by the contained object move constructor.

23.3.6.4 [vector.modifiers] change paragraph 1 to say:

-1- Requires: If value_type has a move constructor, that constructor shall not throw any exceptions. Remarks: If an exception is thrown by push_back() or emplace_back() function, that function has no effect unless the exception is thrown by the move constructor of T.

23.3.6.4 [vector.modifiers] change paragraph 2 to say:

-2- Remarks: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor or assignment operator of T or by any InputIterator operation there are no effects.

23.3.6.4 [vector.modifiers] change paragraph 6 to say:

-6- Throws: Nothing unless an exception is thrown by the copy constructor, move constructor or assignment operator of T.

986. Generic try_lock contradiction

Section: 30.4.4 [thread.lock.algorithm] Status: Tentatively Ready Submitter: Chris Fairles Opened: 2009-02-14 Last modified: 2009-05-23

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Discussion:

In 30.4.4 [thread.lock.algorithm], the generic try_lock effects (p2) say that a failed try_lock is when it either returns false or throws an exception. In the event a call to try_lock does fail, by either returning false or throwing an exception, it states that unlock shall be called for all prior arguments. Then the returns clause (p3) goes on to state in a note that after returning, either all locks are locked or none will be. So what happens if multiple locks fail on try_lock?

Example:

#include <mutex>

int main() {
 std::mutex m0, m1, m2;
 std::unique_lock<std::mutex> l0(m0, std::defer_lock);
 std::unique_lock<std::mutex> l1(m1); //throws on try_lock
 std::unique_lock<std::mutex> l2(m2); //throws on try_lock

 int result = std::try_lock(l0, l1, l2);

 assert( !l0.owns_lock() );
 assert( l1.owns_lock() ); //??
 assert( l2.owns_lock() ); //??
}

The first lock's try_lock succeeded but, being a prior argument to a lock whose try_lock failed, it gets unlocked as per the effects clause of 30.4.4 [thread.lock.algorithm]. However, 2 locks remain locked in this case but the return clause states that either all arguments shall be locked or none will be. This seems to be a contradiction unless the intent is for implementations to make an effort to unlock not only prior arguments, but the one that failed and those that come after as well. Shouldn't the note only apply to the arguments that were successfully locked?

Further discussion and possible resolutions in c++std-lib-23049.

[ Summit: ]

Move to review. Agree with proposed resolution.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 30.4.4 [thread.lock.algorithm], p2:

-2- Effects: Calls try_lock() for each argument in order beginning with the first until all arguments have been processed or a call to try_lock() fails, either by returning false or by throwing an exception. If a call to try_lock() fails, unlock() shall be called for all prior arguments and there shall be no further calls to try_lock().

Delete the note from 30.4.4 [thread.lock.algorithm], p3

-3- Returns: -1 if all calls to try_lock() returned true, otherwise a 0-based index value that indicates the argument for which try_lock() returned false. [Note: On return, either all arguments will be locked or none will be locked. -- end note]

987. reference_wrapper and function types

Section: 20.7.5 [refwrap] Status: Review Submitter: Howard Hinnant Opened: 2009-02-18 Last modified: 2009-05-23

View all other issues in [refwrap].

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Discussion:

The synopsis in 20.7.5 [refwrap] says:

template <ObjectType T> class reference_wrapper
...

And then paragraph 3 says:

The template instantiation reference_wrapper<T> shall be derived from std::unary_function<T1, R> only if the type T is any of the following:

But function types are not ObjectTypes.

Paragraph 4 contains the same contradiction.

[ Post Summit: ]

Jens: restricted reference to ObjectType

Recommend Review.

[ Post Summit, Peter adds: ]

In https://svn.boost.org/trac/boost/ticket/1846 however Eric Niebler makes the very reasonable point that reference_wrapper<F>, where F is a function type, represents a reference to a function, a legitimate entity. So boost::ref was changed to allow it.

https://svn.boost.org/trac/boost/browser/trunk/libs/bind/test/ref_fn_test.cpp

Therefore, I believe an alternative proposed resolution for issue 987 could simply allow reference_wrapper to be used with function types.

[ Post Summit, Howard adds: ]

I agree with Peter (and Eric). I got this one wrong on my first try. Here is code that demonstrates how easy (and useful) it is to instantiate reference_wrapper with a function type:

#include <functional>

template <class F>
void test(F f);

void f() {}

int main()
{
    test(std::ref(f));
}

Output (link time error shows type of reference_wrapper instantiated with function type):

Undefined symbols:
  "void test<std::reference_wrapper<void ()()> >(std::reference_wrapper<void ()()>)",...

I've taken the liberty of changing the proposed wording to allow function types and set to Open. I'll also freely admit that I'm not positive ReferentType is the correct concept.

[ Batavia (2009-05): ]

Howard observed that FunctionType, a concept not (yet?) in the Working Paper, is likely the correct constraint to be applied. However, the proposed resolution provides an adequate approximation.

Move to Review.

[ 2009-05-23 Alisdair adds: ]

By constraining to PointeeType we rule out the ability for T to be a reference, and call in reference-collapsing. I'm not sure if this is correct and intended, but would like to be sure the case was considered.

Is dis-allowing reference types and the implied reference collapsing the intended result?

Proposed resolution:

Change the synopsis in 20.7 [function.objects]:

// 20.6.5, reference_wrapper:
template <ObjectType ReferentType T>
  requires PointeeType<T>
  class reference_wrapper;

template <ObjectType PointeeType T>
  reference_wrapper<T> ref(T&);

template <ObjectType PointeeType T>
  reference_wrapper<const T> cref(const T&);

template <ObjectType PointeeType T>
  reference_wrapper<T> ref(reference_wrapper<T>);
template <ObjectType PointeeType T>
  reference_wrapper<const T> cref(reference_wrapper<T>);

Change the synopsis in 20.7.5 [refwrap]:

template <ObjectType ReferentType T>
  requires PointeeType<T>
  class reference_wrapper
   ...

Change the prototypes in 20.7.5.5 [refwrap.helpers]:

template <ObjectType PointeeType T>
  reference_wrapper<T> ref(T&);
...
template <ObjectType PointeeType T>
  reference_wrapper<const T> cref(const T&);
...
template <ObjectType PointeeType T>
  reference_wrapper<T> ref(reference_wrapper<T>);
...
template <ObjectType PointeeType T>
  reference_wrapper<const T> cref(reference_wrapper<T>);

Rationale:

a) The occurrence of T& in the function signature auto-implies std::ReferentType, this is due to 14.11.1.2 [temp.req.impl]/4 bullet 4

b) The occurrence of the constrained template reference_wrapper<T> in the remaining signatures lets kick in 14.11.1.2 [temp.req.impl]/4 bullet 1 and adds *all* requirements of this template. But we need to add at least *one* requirement (and it was an arbitrary, but natural decision to require std::PointeeType here) to *activate* this. If we hadn't done this, we were in unconstrained mode!


988. Reflexivity meaningless?

Section: 20.2.6 [concept.comparison] Status: Tentatively NAD Submitter: Alisdair Meredith Opened: 2009-02-24 Last modified: 2009-05-23

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Discussion:

20.2.6 [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 20.2.6 [concept.comparison]:

axiom Reflexivity(T a) { (a == a) == true; }

[ Post Summit: ]

Alisdair: I was wrong.

Recommend NAD.

Proposed resolution:

NAD.


989. late_check and library

Section: 17 [library] Status: Open Submitter: Alisdair Meredith Opened: 2009-02-24 Last modified: 2009-05-23

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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:


990. monotonic_clock::is_monotonic must be true

Section: 20.9.5.2 [time.clock.monotonic] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2009-03-09 Last modified: 2009-05-23

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Discussion:

There is some confusion over what the value of monotonic_clock::is_monotonic when monotonic_clock is a synonym for system_clock. The intent is that if monotonic_clock exists, then monotonic_clock::is_monotonic is true.

[ Batavia (2009-05): ]

We agree with the proposed resolution.

Move to Tentatively Ready.

Proposed resolution:

Change 20.9.5.2 [time.clock.monotonic], p1:

-1- Objects of class monotonic_clock represent clocks for which values of time_point never decrease as physical time advances. monotonic_clock may be a synonym for system_clock if and only if system_clock::is_monotonic is true.

991. Response to JP 50

Section: 22.3.3.2.2 [conversions.string] Status: Tentatively Ready Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2009-05-23

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Discussion:

Add custom allocator parameter to wstring_convert, since we cannot allocate memory for strings from a custom allocator.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 22.3.3.2.2 [conversions.string]:

template<class Codecvt, class Elem = wchar_t,
         class Wide_alloc = std::allocator<Elem>,
         class Byte_alloc = std::allocator<char> > class wstring_convert {
  public:
    typedef std::basic_string<char, char_traits<char>, Byte_alloc> byte_string;
    typedef std::basic_string<Elem, char_traits<Elem>, Wide_alloc> wide_string;
     ...

Change 22.3.3.2.2 [conversions.string], p3:

-3- The class template describes an ob ject that controls conversions between wide string ob jects of class std::basic_string<Elem, char_traits<Elem>, Wide_alloc> and byte string objects of class std::basic_string<char, char_traits<char>, Byte_alloc> (also known as std::string).

992. Response to UK 169

Section: 17.6.1.1 [contents] Status: Open Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2009-05-23

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Discussion:

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.

Proposed resolution:

Change 17.6.1.1 [contents], p2:

-2- All library entities except those from the Standard C library, macros, operator new and operator delete are defined within the namespace std or namespaces nested within namespace std.

993. Response to UK 188

Section: 18.5 [support.start.term] Status: Tentatively Ready Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2009-05-23

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Discussion:

The function _Exit does not appear to be defined in this standard. Should it be added to the table of functions included-by-reference to the C standard?

[ 2009-05-09 Alisdair fixed some minor issues in the wording. ]

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add to 18.5 [support.start.term] Table 20 (Header <cstdlib> synopsis) Functions:

_Exit

Add before the description of abort(void):

void _Exit [[noreturn]] (int status)

The function _Exit(int status) has additional behavior in this International Standard:


994. Response to UK 193

Section: 18.6.2.2 [new.handler] Status: Tentatively Ready Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2009-05-23

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Discussion:

quick_exit has been added as a new valid way to terminate a program in a well defined way

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 18.6.2.2 [new.handler], p2:

-2- Required behavior: ...


995. Operational Semantics Unclear

Section: 17.5.1.3 [structure.requirements] Status: Tentatively NAD Submitter: David Abrahams Opened: 2009-03-06 Last modified: 2009-05-23

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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.


996. Move operation not well specified

Section: 17 [library] Status: Open Submitter: David Abrahams Opened: 2009-03-06 Last modified: 2009-05-23

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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.

Proposed resolution:


997. Response to UK 163

Section: 17.5.1.4 [structure.specifications] Status: Tentatively Ready Submitter: Thomas Plum Opened: 2009-03-03 Last modified: 2009-05-23

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Discussion:

Many functions are defined as "Effects: Equivalent to a...", which seems to also define the preconditions, effects, etc. But this is not made clear.

After studying the occurrences of "Effects: Equivalent to", I agree with the diagnosis but disagree with the solution. In 21.4.2 [string.cons] we find

14 Effects: If InputIterator is an integral type, equivalent to basic_string(static_cast<size_type>(begin), static_cast<value_type>(end), a)

15 Otherwise constructs a string from the values in the range [begin, end), as indicated in the Sequence Requirements table (see 23.1.3).

This would be devishly difficult to re-write with an explicit "Equivalent to:" clause. Instead, I propose the following, which will result in much less editorial re-work.

[ 2009-05-09 Alisdair adds: ]

This issue is related to 492.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add a new paragraph after 17.5.1.4 [structure.specifications], p3:

-3- Descriptions of function semantics contain the following elements (as appropriate):154

Whenever the Effects element specifies that the semantics of some function F are Equivalent to some code-sequence, then the various elements are interpreted as follows. If F's semantics specifies a Requires element, then that requirement is logically imposed prior to the equivalent-to semantics. Then, the semantics of the code-sequence are determined by the Requires, Effects, Postconditions, Returns, Throws, Complexity, Remarks, Error Conditions and Notes specified for the (one or more) function invocations contained in the code-sequence. The value returned from F is specified by F's Returns element, or if F has no Returns element, a non-void return from F is specified by the Returns elements in code-sequence. If F's semantics contains a Throws (or Postconditions, or Complexity) element, then that supersedes any occurrences of that element in the code-sequence.


998. Smart pointer referencing its owner

Section: 20.8.12.2.5 [unique.ptr.single.modifiers] Status: Tentatively Ready Submitter: Pavel Minaev Opened: 2009-02-26 Last modified: 2009-05-23

View other active issues in [unique.ptr.single.modifiers].

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Discussion:

Consider the following (simplified) implementation of std::auto_ptr<T>::reset():

void reset(T* newptr = 0) { 
   if (this->ptr && this->ptr != newptr) { 
     delete this->ptr; 
   } 
   this->ptr = newptr; 
} 

Now consider the following code which uses the above implementation:

struct foo { 
   std::auto_ptr<foo> ap; 
   foo() : ap(this) {} 
   void reset() { ap.reset(); } 
}; 
int main() { 
   (new foo)->reset(); 
} 

With the above implementation of auto_ptr, this results in U.B. at the point of auto_ptr::reset(). If this isn't obvious yet, let me explain how this goes step by step:

  1. foo::reset() entered
  2. auto_ptr::reset() entered
  3. auto_ptr::reset() tries to delete foo
  4. foo::~foo() entered, tries to destruct its members
  5. auto_ptr::~auto_ptr() executed - auto_ptr is no longer a valid object!
  6. foo::~foo() left
  7. auto_ptr::reset() sets its "ptr" field to 0 <- U.B.! auto_ptr is not a valid object here already!

[ Thanks to Peter Dimov who recognized the connection to unique_ptr and brought this to the attention of the LWG, and helped with the solution. ]

[ Howard adds: ]

To fix this behavior reset must be specified such that deleting the pointer is the last action to be taken within reset.

[ Alisdair adds: ]

The example providing the rationale for LWG 998 is poor, as it relies on broken semantics of having two object believing they are unique owners of a single resource. It should not be surprising that UB results from such code, and I feel no need to go out of our way to support such behaviour.

If an example is presented that does not imply multiple ownership of a unique resource, I would be much more ready to accept the proposed resolution.

[ Batavia (2009-05): ]

Howard summarizes:

This issue has to do with circular ownership, and affects auto_ptr, too (but we don't really care about that). It is intended to spell out the order in which operations must be performed so as to avoid the possibility of undefined behavior in the self-referential case.

Howard points to message c++std-lib-23175 for another example, requested by Alisdair.

We agree with the issue and with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 20.8.12.2.5 [unique.ptr.single.modifiers], p5 (Effects clause for reset), and p6:

-5- Effects: If get() == nullptr there are no effects. Otherwise get_deleter()(get()). Assigns p to the stored pointer, and then if the old value of the pointer is not equal to nullptr, calls get_deleter()(the old value of the pointer). [Note: The order of these operations is significant because the call to get_deleter() may destroy *this. -- end note]

-6- Postconditions: get() == p. [Note: The postcondition does not hold if the call to get_deleter() destroys *this since this->get() is no longer a valid expression. -- end note]


999. Taking the address of a function

Section: 20.8.11.1 [object.addressof] Status: Tentatively Ready Submitter: Peter Dimov Opened: 2009-03-09 Last modified: 2009-05-23

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Discussion:

The same fix (reference 987) may be applied to addressof, which is also constrained to ObjectType. (That was why boost::ref didn't work with functions - it tried to apply boost::addressof and the reinterpret_cast<char&> implementation of addressof failed.)

[ Batavia (2009-05): ]

We agree.

Move to Tentatively Ready.

Proposed resolution:

Change the synopsis in 20.8 [memory]:

template <ObjectType PointeeType T>
  T* addressof(T& r);

Change 20.8.11.1 [object.addressof]:

template <ObjectType PointeeType T>
  T* addressof(T& r);

Rationale:

a) The occurrence of T& in the function signature auto-implies std::ReferentType, this is due to 14.11.1.2 [temp.req.impl]/4 bullet 4


1000. adjacent_find is over-constrained

Section: 25.3.8 [alg.adjacent.find] Status: Open Submitter: Chris Jefferson Opened: 2009-03-09 Last modified: 2009-03-13

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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 25.3.8 [alg.adjacent.find] to:

template<ForwardIterator Iter> 
  requires EqualityComparableHasEqualTo<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);

1001. Pointers, concepts and headers

Section: 17 [library] Status: Review Submitter: Alisdair Meredith Opened: 2009-03-10 Last modified: 2009-06-16

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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: ]

Proposed resolution:

Change 17.6.4.2 [res.on.headers], Headers, paragraph 1, as indicated:

A C++ header shall provide definitions for any names that appear in its synopsis (3.2 [basic.def.odr]). A C++ header may include other C++ headers.[footnote] A C++ header shown in its synopsis as including other C++ headers shall provide definitions for the same names as if those other headers were included. A C++ header that uses a concept (14.10 [concept]) shall provide the definition for that concept as if it included the C++ header that defines that concept in its synopsis. The mechanism and ordering of such definitions is unspecified.

[Example: If C++ header <a> contains a concept defined in C++ header <b>, and header <b> contains a concept defined in C++ header <c>, then inclusion of <a> is equivalent to inclusion of <a>, <b>, and <c>. — end example]

[footnote] C++ headers must include a C++ header that contains any needed definition (3.2).


1002. Response to UK 170

Section: 17.6.1.2 [headers] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

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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.

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]

1003. Response to JP 23

Section: 17.6.1.3 [compliance] Status: Review Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

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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.

Proposed resolution:

Add <type_traits> to Table 15.


1004. Response to UK 179

Section: 17.6.3.8 [res.on.functions] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

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Discussion:

Addresses UK 179

According to the 4th bullet there is a problem if "if any replacement function or handler function or destructor operation throws an exception". There should be no problem throwing exceptions so long as they are caught within the function.

[ Batavia (2009-05): ]

The phrasing "throws an exception" is commonly used elsewhere to mean "throws or propagates an exception." Move to Open pending a possible more general resolution.

Proposed resolution:

Change the 4th bullet of 17.6.3.8 [res.on.functions], p2:


1005. Response to JP 26

Section: 18.3.1.1 [numeric.limits] Status: Review Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-25

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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<class Regular T> class numeric_limits<const T>;
template<class Regular T> class numeric_limits<volatile T>;
template<class Regular T> class numeric_limits<const volatile T>;

1006. Response to UK 190

Section: 18.6.1 [new.delete] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

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Discussion:

Addresses UK 190

It is not entirely clear how the current specification acts in the presence of a garbage collected implementation.

[ Summit: ]

Agreed.

[ 2009-05-09 Alisdair adds: ]

Proposed wording is too strict for implementations that do not support garbage collection. Updated wording supplied.

[ Batavia (2009-05): ]

We recommend advancing this to Tentatively Ready with the understanding that it will not be moved for adoption unless and until the proposed resolution to Core issue #853 is adopted.

Proposed resolution:

(Editorial note: This wording ties into the proposed resolution for Core #853)

Add paragraphs to 18.6.1.1 [new.delete.single]:

void operator delete(void* ptr) throw();
void operator delete(void* ptr, const std::nothrow_t&) throw();

[ The second signature deletion above is editorial. ]

Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then ptr shall be a safely-derived pointer.

-10- ...

void operator delete(void* ptr, const std::nothrow_t&) throw();

Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then ptr shall be a safely-derived pointer.

-15- ...

Add paragraphs to 18.6.1.2 [new.delete.array]:

void operator delete[](void* ptr) throw();
void operator delete[](void* ptr, const std::nothrow_t&) throw();

[ The second signature deletion above is editorial. ]

Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then ptr shall be a safely-derived pointer.

-9- ...

void operator delete[](void* ptr, const std::nothrow_t&) throw();

Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then ptr shall be a safely-derived pointer.

-13- ...

Add paragraphs to 18.6.1.3 [new.delete.placement]:

void operator delete(void* ptr, void*) throw();

Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then ptr shall be a safely-derived pointer.

-7- ...

void operator delete[](void* ptr, void*) throw();

Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then ptr shall be a safely-derived pointer.

-9- ...


1007. Response to JP 29

Section: 18.8.6 [except.nested] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-13

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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.


1008. Response to JP 31

Section: 18.8.6 [except.nested] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-13

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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.

Proposed resolution:


1009. Response to UK 251

Section: 24.2.1 [iterator.iterators] Status: Review Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-22

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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.

Proposed resolution:

Change 24.2.1 [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 24.2.2 [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 24.2.3 [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 24.2.4 [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; }.


1010. Response to UK 263

Section: 24.2.6 [random.access.iterators] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

View other active issues in [random.access.iterators].

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View all issues with Open status.

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 24.2.6 [random.access.iterators]:

concept RandomAccessIterator<typename X> : BidirectionalIterator<X>, LessThanComparable<X> {
  ...
  X& operator-=(X& x, difference_type n) { return x += -n; }
  ...
}

1011. Response to UK 271

Section: 24.4 [iterator.operations] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

View other active issues in [iterator.operations].

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Discussion:

Addresses UK 271

next/prev return an incremented iterator without changing the value of the original iterator. However, even this may invalidate an InputIterator. A ForwardIterator is required to guarantee the 'multipass' property.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change [iterator.synopsis]:

template <InputIterator ForwardIterator Iter> 
  Iter next(Iter x, 
    Iter::difference_type n = 1);

Change 24.4 [iterator.operations], p6:

template <InputIterator ForwardIterator Iter> 
  Iter next(Iter x, 
    Iter::difference_type n = 1);

1012. Response to UK 277

Section: 24.5.1.2.1 [reverse.iter.cons] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

View all issues with Open status.

Discussion:

Addresses UK 277

The default constructor default-initializes current, rather than value-initializes. This means that when Iterator corresponds to a trivial type, the current member is left un-initialized, even when the user explictly requests value intialization! At this point, it is not safe to perform any operations on the reverse_iterator other than assign it a new value or destroy it. Note that this does correspond to the basic definition of a singular iterator.

[ Summit: ]

Agree with option i.

Related issue: 408

[ Batavia (2009-05): ]

We believe this should be revisited in conjunction with issue 408, which nearly duplicates this issue. Move to Open.

Proposed resolution:

Change [reverse.iter.con]:

reverse_iterator();
-1- Effects: Default Value initializes current. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a default constructed iterator of type Iterator.

Change 24.5.3.2.1 [move.iter.op.const]:

move_iterator();
-1- Effects: Constructs a move_iterator, default value initializing current.

1013. Response to UK 305

Section: 25.5.7 [alg.min.max] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

View other active issues in [alg.min.max].

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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.

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 25.5.7 [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);

1014. Response to UK 317 and JP 74

Section: 28.9.2 [re.regex.construct] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

View all other issues in [re.regex.construct].

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Discussion:

Addresses UK 317 and JP 74

UK 317:

basic_string has both a constructor and an assignment operator that accepts an initializer list, basic_regex should have the same.

JP 74:

basic_regx & operator= (initializer_list<T>); is not defined.

[ Batavia (2009-05): ]

UK 317 asks for both assignment and constructor, but the requested constructor is already present in the current Working Paper. We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 28.9 [re.regex]:

template <class charT,
          class traits = regex_traits<charT> >
class basic_regex {
  ...
  basic_regex& operator=(const charT* ptr);
  basic_regex& operator=(initializer_list<charT> il);
  template <class ST, class SA>
    basic_regex& operator=(const basic_string<charT, ST, SA>& p);
  ...
};

Add in 28.9.2 [re.regex.construct]:

-20- ...
basic_regex& operator=(initializer_list<charT> il);
-21- Effects: returns assign(il.begin(), il.end());

1015. Response to UK 199

Section: 20.2.1 [concept.transform] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

View other active issues in [concept.transform].

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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 14.10.4 [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 20.2.1 [concept.transform] p2:

-2- A program user shall not provide concept maps for any concept in 20.1.1.

Change 20.2.2 [concept.true] p2:

-2- Requires: a program user shall not provide a concept map for the True concept.

Change 20.2.3 [concept.classify] p2:

-2- Requires: a programuser shall not provide concept maps for any concept in this section.

1016. Response to JP 33

Section: 20.2.6 [concept.comparison] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-12

View other active issues in [concept.comparison].

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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.


1017. Response to US 66

Section: 20.2.11 [concept.regular] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-12

View all issues with Open status.

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:


1018. Response to US 70

Section: 20.6 [meta] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-12

View other active issues in [meta].

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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:


1019. Response to UK 205

Section: 20.6.3 [meta.help] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

View other active issues in [meta.help].

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View all issues with Open status.

Discussion:

Addresses UK 205

integral_constant objects should be usable in integral-constant-expressions. The addition to the language of literal types and the enhanced rules for constant expressions make this possible.

[ Batavia (2009-05): ]

We agree that the static data member ought be declared constexpr, but do not see a need for the proposed operator value_type(). (A use case would be helpful.) Move to Open.

[ 2009-05-23 Alisdair adds: ]

The motivating case in my mind is that we can then use true_type and false_type as integral Boolean expressions, for example inside a static_assert declaration. In that sense it is purely a matter of style.

Note that Boost has applied the non-explicit conversion operator for many years as it has valuable properties for extension into other metaprogramming libraries, such as MPL. If additional rationale is desired I will poll the Boost lists for why this extension was originally applied. I would argue that explicit conversion is more appropriate for 0x though.

Proposed resolution:

Add to the integral_constant struct definition in 20.6.3 [meta.help]:

template <class 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; }
};

1020. Response to UK 204

Section: 20.6.7 [meta.trans.other] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-12

View other active issues in [meta.trans.other].

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View all issues with Open status.

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.

Proposed resolution:


1021. Response to UK 211

Section: 20.8.12.2.3 [unique.ptr.single.asgn] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

View all issues with Tentatively Ready status.

Discussion:

Addresses UK 211

The nullptr_t type was introduced to resolve the null pointer literal problem. It should be used for the assignemnt operator, as with the constructor and elsewhere through the library.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change the synopsis in 20.8.12.2 [unique.ptr.single]:

unique_ptr& operator=(unspecified-pointer-type nullptr_t);

Change 20.8.12.2.3 [unique.ptr.single.asgn]:

unique_ptr& operator=(unspecified-pointer-type nullptr_t);
Assigns from the literal 0 or NULL. [Note: The unspecified-pointer-type is often implemented as a pointer to a private data member, avoiding many of the implicit conversion pitfalls. -- end note]

1023. Response to DE 22

Section: 20.7.16.2 [func.wrap.func] Status: Tentatively NAD Editorial Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

View other active issues in [func.wrap.func].

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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 20.7.16.2 [func.wrap.func]:

template<Returnable R, CopyConstructible... ArgTypes> 
class function<R(ArgTypes...)> 
  : public unary_function<T1, R>      // iff sizeof...(ArgTypes) == 1 and see below
                                      // ArgTypes contains T1
  : public binary_function<T1, T2, R> // iff sizeof...(ArgTypes) == 2 and see below
                                      // ArgTypes contains T1 and T2
{
   ...

Add new p1/p2 before 20.7.16.2.1 [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();

1024. Response to JP 39

Section: 20.7.16.2 [func.wrap.func] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-05-23

View other active issues in [func.wrap.func].

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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.

Proposed resolution:

Correct as follows in 20.7.16.2 [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&&);

1026. Response to UK 209

Section: 20.8 [memory] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-12

View other active issues in [memory].

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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:


1027. Response to UK 213

Section: 20.8.6 [default.allocator] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-13

View all issues with Open status.

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:


1028. Response to UK 214

Section: 20.8.8 [storage.iterator] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-15

View all issues with Open status.

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:

20.8 [memory] p2

Update the synopsis for <memory>

// 20.7.8, raw storage iterator:
template <class ForwardIterator OutputIterator, class ObjectType 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 > > { }

20.8.8 [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:

  1. The initial iterator passed by value is expected to remain valid, pointing to the initialized region of memory.
  2. to avoid breaking the declaration of post-increment operator which would require some kind of proxy formulation to support generalised InputIterators.

1029. Response to UK 210

Section: 20.8.11 [specialized.algorithms] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-15

View all other issues in [specialized.algorithms].

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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:

  1. 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.
  2. 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:

    1. for uninitialized_copy_n: for ( ; n > Size(0); ++result, ++first, --n) {
    2. for uninitialized_fill_n: for (; n > Size(0); ++first, --n) {

Alisdair adds:

For the record I agree with Daniel's suggestion.

Proposed resolution:

20.8 [memory] p2

Update the synopsis for <memory>

template <class InputIterator InIter,
         class ForwardIterator OutputIterator<auto, InIter::reference> OutIter> 
   requires ForwardIterator<OutIter>
   ForwardIterator OutIter
   uninitialized_copy(InputIterator InIter first, InputIterator InIter last, 
                      ForwardIterator OutIter result);

template <class InputIterator InIter,
          class IntegralLike Size,
          class ForwardIterator OutputIterator<auto, InIter::reference> OutIter> 
  requires ForwardIterator<OutIter>
  ForwardIterator OutIter
  uninitialized_copy_n(InputIterator InIter first, Size n, 
                       ForwardIterator OutIter result);

template <class ForwardIterator Iter, class ObjectType T>
  requires Constructible< Iter::value_type, const T& >
  void uninitialized_fill(ForwardIterator Iter first, ForwardIterator Iter last, 
                          const T& x);

template <class ForwardIterator Iter, class IntegralLike Size, class ObjectType T> 
  requires Constructible< Iter::value_type, const T& >
  void
  uninitialized_fill_n(ForwardIterator Iter first, Size n, const T& x);

Update as follows:

uninitialized_copy 20.8.11.2 [uninitialized.copy]

template <class InputIterator InIter,
         class ForwardIterator OutputIterator<auto, InIter::reference> OutIter> 
   requires ForwardIterator<OutIter>
   ForwardIterator OutIter
   uninitialized_copy(InputIterator InIter first, InputIterator InIter last, 
                      ForwardIterator OutIter 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 <class InputIterator InIter,
          class IntegralLike Size,
          class ForwardIterator OutputIterator<auto, InIter::reference> OutIter> 
  requires ForwardIterator<OutIter>
  ForwardIterator OutIter
  uninitialized_copy_n(InputIterator InIter first, Size n, 
                       ForwardIterator OutIter 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 20.8.11.3 [uninitialized.fill]

template <class ForwardIterator Iter, class ObjectType T>
  requires Constructible< Iter::value_type, const T& >
  void uninitialized_fill(ForwardIterator Iter first, ForwardIterator Iter 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 20.8.11.4 [uninitialized.fill.n]

template <class ForwardIterator Iter, class IntegralLike Size, class ObjectType T> 
  requires Constructible< Iter::value_type, const T& >
  void
  uninitialized_fill_n(ForwardIterator Iter 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);
}

1030. Response to JP 44

Section: 20.8.13.6 [util.smartptr.shared.atomic] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-12

View all issues with Open status.

Discussion:

Addresses JP 44

The 1st parameter p and 2nd parameter v is now shared_ptr<T>*.

It should be shared_ptr<T>&, or if these are shared_ptr<T>* then add the "p shall not be a null pointer" at the requires.

[ Summit: ]

Agree. All of the functions need a requirement that p (or v) is a pointer to a valid object.

Proposed resolution:


1031. Response to US 78

Section: 20.8.13.2 [util.smartptr.shared] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-12

View other active issues in [util.smartptr.shared].

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View all issues with Open status.

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.

Proposed resolution:


1032. Response to JP 45

Section: 20.9 [time] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-03-12

View all other issues in [time].

View all issues with Open status.

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 20.9 [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:


1033. thread::join() effects?

Section: 30.3.1.5 [thread.thread.member] Status: Open Submitter: Alberto Ganesh Barbati Opened: 2009-03-12 Last modified: 2009-05-23

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Discussion:

While looking at thread::join() I think I spotted a couple of possible defects in the specifications. I could not find a previous issue or NB comment about that, but I might have missed it.

The postconditions clause for thread::join() is:

Postconditions: If join() throws an exception, the value returned by get_id() is unchanged. Otherwise, get_id() == id().

and the throws clause is:

Throws: std::system_error when the postconditions cannot be achieved.

Now... how could the postconditions not be achieved? It's just a matter of resetting the value of get_id() or leave it unchanged! I bet we can always do that. Moreover, it's a chicken-and-egg problem: in order to decide whether to throw or not I depend on the postconditions, but the postconditions are different in the two cases.

I believe the throws clause should be:

Throws: std::system_error when the effects or postconditions cannot be achieved.

as it is in detach(), or, even better, as the postcondition is trivially satisfiable and to remove the circular dependency:

Throws: std::system_error if the effects cannot be achieved.

Problem is that... ehm... join() has no "Effects" clause. Is that intentional?

[ See the thread starting at c++std-lib-23204 for more discussion. ]

[ Batavia (2009-05): ]

Pete believes there may be some more general language (in frontmatter) that can address this and related issues such as 962.

Move to Open.

Proposed resolution:


1034. Response to UK 222

Section: 23.2.1 [container.requirements.general] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-03-12

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Discussion:

Addresses UK 222

It is not clear what purpose the Requirement tables serve in the Containers clause. Are they the definition of a library Container? Or simply a conventient shorthand to factor common semantics into a single place, simplifying the description of each subsequent container? This becomes an issue for 'containers' like array, which does not meet the default-construct-to-empty requirement, or forward_list which does not support the size operation. Are these components no longer containers? Does that mean the remaining requirements don't apply? Or are these contradictions that need fixing, despite being a clear design decision?

Recommend:

Clarify all the tables in 23.2 [container.requirements] are there as a convenience for documentation, rather than a strict set of requirements. Containers should be allowed to relax specific requirements if they call attention to them in their documentation. The introductory text for array should be expanded to mention a default constructed array is not empty, and forward_list introduction should mention it does not provide the required size operation as it cannot be implemented efficiently.

[ Summit: ]

Agree in principle.

Proposed resolution:


1035. Response to UK 226

Section: 23.2.1 [container.requirements.general] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-05-02

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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.

Proposed resolution:


1036. Response to UK 231

Section: 23.2.3 [sequence.reqmts] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-03-12

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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.


1037. Response to UK 232

Section: 23.2.3 [sequence.reqmts] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-05-23

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Discussion:

Addresses UK 232

match_results may follow the requirements but is not listed a general purpose library container.

Remove reference to match_results against a[n] operation.

[ Summit: ]

Agree. operator[] is defined elsewhere.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 23.2.3 [sequence.reqmts] Table 84, remove reference to match_results in the row describing the a[n] operation.


1038. Response to UK 233

Section: 23.2.3 [sequence.reqmts] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-05-23

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Discussion:

Addresses UK 233

Table 84 is missing references to several new container types.

[ Summit: ]

Agree.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 23.2.3 [sequence.reqmts] Table 84, Add reference to listed containers to the following rows:

Table 84 -- Optional sequence container operations
Expression Return type Operational semantics Container
a.front() ... ... vector, list, deque, basic_string, array, forward_list
a.back() ... ... vector, list, deque, basic_string, array
a.emplace_front(args) ... ... list, deque, forward_list
a.push_front(t) ... ... list, deque, forward_list
a.push_front(rv) ... ... list, deque, forward_list
a.pop_front() ... ... list, deque, forward_list
a[n] ... ... vector, deque, basic_string, array
a.at(n) ... ... vector, deque, basic_string, array

1039. Response to UK 234

Section: 23.2.3 [sequence.reqmts] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-05-23

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Discussion:

Addresses UK 234

The reference to iterator in semantics for back should also allow for const_iterator when called on a const-qualified container. This would be ugly to specify in the 03 standard, but is quite easy with the addition of auto in this new standard.

[ Summit: ]

Agree.

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 23.2.3 [sequence.reqmts] Table 84, replace iterator with auto in semantics for back:

Table 84 -- Optional sequence container operations
Expression Return type Operational semantics Container
a.back() reference; const_reference for constant a { iterator auto tmp = a.end();
--tmp;
return *tmp; }
vector, list, deque, basic_string

1040. Response to UK 238

Section: 23.2.4 [associative.reqmts] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-05-23

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Discussion:

Addresses UK 238

Leaving it unspecified whether or not iterator<