Doc. no. N3018=10-0008
Date: 2010-02-14
Project: Programming Language C++
Reply to: Howard Hinnant <howard.hinnant@gmail.com>

C++ Standard Library Active Issues List (Revision R69)

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, i.e., issues which have a status of New, Open, Ready, or Review. See Library Defect Reports List for issues considered defects and Library Closed Issues List for issues considered closed.

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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 Concepts - The LWG has reached consensus that the issue is NAD for now, but represents a real issue when the library is done with language-supported concepts.

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).

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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.

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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


296. Missing descriptions and requirements of pair operators

Section: 20.3.4 [pairs] Status: Ready Submitter: Martin Sebor Opened: 2001-01-14 Last modified: 2009-10-26

View other active issues in [pairs].

View all other issues in [pairs].

View all issues with Ready status.

Discussion:

The synopsis of the header <utility> in 20.3 [utility] lists the complete set of equality and relational operators for pair but the section describing the template and the operators only describes operator==() and operator<(), and it fails to mention any requirements on the template arguments. The remaining operators are not mentioned at all.

[ 2009-09-27 Alisdair reopens. ]

The issue is a lack of wording specifying the semantics of std::pair relational operators. The rationale is that this is covered by catch-all wording in the relops component, and that as relops directly precedes pair in the document this is an easy connection to make.

Reading the current working paper I make two observations:

  1. relops no longer immediately precedes pair in the order of specification. However, even if it did, there is a lot of pair specification itself between the (apparently) unrelated relops and the relational operators for pair. (The catch-all still requires operator== and operator< to be specified explicitly)
  2. No other library component relies on the catch-all clause. The following all explicitly document all six relational operators, usually in a manner that could have deferred to the relops clause.
tuple
unique_ptr
duration
time_point
basic_string
queue
stack
move_iterator
reverse_iterator 
regex submatch
thread::id

The container components provide their own (equivalent) definition in 23.2.1 [container.requirements.general] Table 90 -- Container requirements and do so do not defer to relops.

Shared_ptr explicitly documents operator!= and does not supply the other 3 missing operators (>,>=,<=) so does not meet the reqirements of the relops clause.

Weak_ptr only supports operator< so would not be covered by relops.

At the very least I would request a note pointing to the relops clause we rely on to provide this definition. If this route is taken, I would recommend reducing many of the above listed clauses to a similar note rather than providing redundant specification.

My preference would be to supply the 4 missing specifications consistent with the rest of the library.

[ 2009-10-11 Daniel opens 1233 which deals with the same issue as it pertains to unique_ptr. ]

[ 2009-10 Santa Cruz: ]

Move to Ready

Proposed resolution:

After p20 20.3.4 [pairs] add:

template <class T1, class T2>
bool operator!=(const pair<T1,T2>& x, const pair<T1,T2>& y);
Returns: !(x==y)
template <class T1, class T2>
bool operator> (const pair<T1,T2>& x, const pair<T1,T2>& y);
Returns: y < x
template <class T1, class T2>
bool operator>=(const pair<T1,T2>& x, const pair<T1,T2>& y);
Returns: !(x < y)
template <class T1, class T2>
bool operator<=(const pair<T1,T2>& x, const pair<T1,T2>& y);
Returns: !(y < x)

Rationale:

20.3.1 [operators] paragraph 10 already specifies the semantics. That paragraph says that, if declarations of operator!=, operator>, operator<=, and operator>= appear without definitions, they are defined as specified in 20.3.1 [operators]. There should be no user confusion, since that paragraph happens to immediately precede the specification of pair.


299. Incorrect return types for iterator dereference

Section: 24.2.4 [bidirectional.iterators], 24.2.5 [random.access.iterators] Status: Open Submitter: John Potter Opened: 2001-01-22 Last modified: 2009-10-26

View all other issues in [bidirectional.iterators].

View all issues with Open status.

Discussion:

In section 24.2.4 [bidirectional.iterators], Table 75 gives the return type of *r-- as convertible to T. This is not consistent with Table 74 which gives the return type of *r++ as T&. *r++ = t is valid while *r-- = t is invalid.

In section 24.2.5 [random.access.iterators], Table 76 gives the return type of a[n] as convertible to T. This is not consistent with the semantics of *(a + n) which returns T& by Table 74. *(a + n) = t is valid while a[n] = t is invalid.

Discussion from the Copenhagen meeting: the first part is uncontroversial. The second part, operator[] for Random Access Iterators, requires more thought. There are reasonable arguments on both sides. Return by value from operator[] enables some potentially useful iterators, e.g. a random access "iota iterator" (a.k.a "counting iterator" or "int iterator"). There isn't any obvious way to do this with return-by-reference, since the reference would be to a temporary. On the other hand, reverse_iterator takes an arbitrary Random Access Iterator as template argument, and its operator[] returns by reference. If we decided that the return type in Table 76 was correct, we would have to change reverse_iterator. This change would probably affect user code.

History: the contradiction between reverse_iterator and the Random Access Iterator requirements has been present from an early stage. In both the STL proposal adopted by the committee (N0527==94-0140) and the STL technical report (HPL-95-11 (R.1), by Stepanov and Lee), the Random Access Iterator requirements say that operator[]'s return value is "convertible to T". In N0527 reverse_iterator's operator[] returns by value, but in HPL-95-11 (R.1), and in the STL implementation that HP released to the public, reverse_iterator's operator[] returns by reference. In 1995, the standard was amended to reflect the contents of HPL-95-11 (R.1). The original intent for operator[] is unclear.

In the long term it may be desirable to add more fine-grained iterator requirements, so that access method and traversal strategy can be decoupled. (See "Improved Iterator Categories and Requirements", N1297 = 01-0011, by Jeremy Siek.) Any decisions about issue 299 should keep this possibility in mind.

Further discussion: I propose a compromise between John Potter's resolution, which requires T& as the return type of a[n], and the current wording, which requires convertible to T. The compromise is to keep the convertible to T for the return type of the expression a[n], but to also add a[n] = t as a valid expression. This compromise "saves" the common case uses of random access iterators, while at the same time allowing iterators such as counting iterator and caching file iterators to remain random access iterators (iterators where the lifetime of the object returned by operator*() is tied to the lifetime of the iterator).

Note that the compromise resolution necessitates a change to reverse_iterator. It would need to use a proxy to support a[n] = t.

Note also there is one kind of mutable random access iterator that will no longer meet the new requirements. Currently, iterators that return an r-value from operator[] meet the requirements for a mutable random access iterartor, even though the expression a[n] = t will only modify a temporary that goes away. With this proposed resolution, a[n] = t will be required to have the same operational semantics as *(a + n) = t.

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-09-18 Alisdair adds: ]

Why can't we write through the reference returned from operator[] on a random access iterator?

Recommended solution:

In table Table 104 -- Random access iterator requirements, replace

a[n] : convertible to const T & T& if X is mutable, otherwise convertible to const T&

[ 2009-10 Santa Cruz: ]

Leave Open. Alisdair to spearhead a paper on revivification.

Proposed resolution:

In section 24.1.4 [lib.bidirectdional.iterators], change the return type in table 75 from "convertible to T" to T&.

In section 24.1.5 [lib.random.access.iterators], change the operational semantics for a[n] to " the r-value of a[n] is equivalent to the r-value of *(a + n)". Add a new row in the table for the expression a[n] = t with a return type of convertible to T and operational semantics of *(a + n) = t.

[Lillehammer: Real problem, but should be addressed as part of iterator redesign]

Rationale:

[ San Francisco: ]

Solved by N2758.

397. ostream::sentry dtor throws exceptions

Section: 27.7.2.4 [ostream::sentry] Status: Open Submitter: Martin Sebor Opened: 2003-01-05 Last modified: 2009-10-20

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

View all issues with Open status.

Discussion:

17.4.4.8, p3 prohibits library dtors from throwing exceptions.

27.6.2.3, p4 says this about the ostream::sentry dtor:

    -4- If ((os.flags() & ios_base::unitbuf) && !uncaught_exception())
        is true, calls os.flush().
    

27.6.2.6, p7 that describes ostream::flush() says:

    -7- If rdbuf() is not a null pointer, calls rdbuf()->pubsync().
        If that function returns ?-1 calls setstate(badbit) (which
        may throw ios_base::failure (27.4.4.3)).
    

That seems like a defect, since both pubsync() and setstate() can throw an exception.

[ The contradiction is real. Clause 17 says destructors may never throw exceptions, and clause 27 specifies a destructor that does throw. In principle we might change either one. We're leaning toward changing clause 17: putting in an "unless otherwise specified" clause, and then putting in a footnote saying the sentry destructor is the only one that can throw. PJP suggests specifying that sentry::~sentry() should internally catch any exceptions it might cause. ]

[ See 418 and 622 for related issues. ]

[ 2009-07 Frankfurt ]

Move to Review. Add "Throws: nothing" to the specification of ostream::sentry::~sentry().

[ 2009-10-13 Daniel adds: ]

The proposed resolution of 835 is written to match the outcome of this issue.

[ 2009 Santa Cruz: ]

Move to Open. Our intent is to solve this issue with 835.

Proposed resolution:

Add after 27.7.2.4 [ostream::sentry] p17:

~sentry();

-17- If ((os.flags() & ios_base::unitbuf) && !uncaught_exception()) is true, calls os.flush().

Throws: Nothing.


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

Section: 24.2 [iterator.requirements] Status: Open Submitter: Nathan Myers Opened: 2003-06-03 Last modified: 2009-11-03

View other active issues in [iterator.requirements].

View all other issues in [iterator.requirements].

View all issues with Open status.

Discussion:

I've been discussing iterator semantics with Dave Abrahams, and a surprise has popped up. I don't think this has been discussed before.

X [iterator.concepts] says that the only operation that can be performed on "singular" iterator values is to assign a non-singular value to them. (It doesn't say they can be destroyed, and that's probably a defect.) Some implementations have taken this to imply that there is no need to initialize the data member of a reverse_iterator<> in the default constructor. As a result, code like

  std::vector<std::reverse_iterator<char*> > v(7);
  v.reserve(1000);

invokes undefined behavior, because it must default-initialize the vector elements, and then copy them to other storage. Of course many other vector operations on these adapters are also left undefined, and which those are is not reliably deducible from the standard.

I don't think that 24.1 was meant to make standard-library iterator types unsafe. Rather, it was meant to restrict what operations may be performed by functions which take general user- and standard iterators as arguments, so that raw pointers would qualify as iterators. However, this is not clear in the text, others have come to the opposite conclusion.

One question is whether the standard iterator adaptors have defined copy semantics. Another is whether they have defined destructor semantics: is

  { std::vector<std::reverse_iterator<char*> >  v(7); }

undefined too?

Note this is not a question of whether algorithms are allowed to rely on copy semantics for arbitrary iterators, just whether the types we actually supply support those operations. I believe the resolution must be expressed in terms of the semantics of the adapter's argument type. It should make clear that, e.g., the reverse_iterator<T> constructor is actually required to execute T(), and so copying is defined if the result of T() is copyable.

Issue 235, which defines reverse_iterator's default constructor more precisely, has some relevance to this issue. However, it is not the whole story.

The issue was whether

  reverse_iterator() { }

is allowed, vs.

  reverse_iterator() : current() { }

The difference is when T is char*, where the first leaves the member uninitialized, and possibly equal to an existing pointer value, or (on some targets) may result in a hardware trap when copied.

8.5 paragraph 5 seems to make clear that the second is required to satisfy DR 235, at least for non-class Iterator argument types.

But that only takes care of reverse_iterator, and doesn't establish a policy for all iterators. (The reverse iterator adapter was just an example.) In particular, does my function

  template <typename Iterator>
    void f() { std::vector<Iterator>  v(7); } 

evoke undefined behavior for some conforming iterator definitions? I think it does, now, because vector<> will destroy those singular iterator values, and that's explicitly disallowed.

24.1 shouldn't give blanket permission to copy all singular iterators, because then pointers wouldn't qualify as iterators. However, it should allow copying of that subset of singular iterator values that are default-initialized, and it should explicitly allow destroying any iterator value, singular or not, default-initialized or not.

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).

[ 2009-07 Frankfurt ]

This is related to LWG 1012.

Note that there is a bug in the proposed resolution to LWG 1012. The change to [reverse.iter.con] should be modified so that the word "default" in the second sentence of the Effects clause is replaced by "value."

We believe that the proposed fix to LWG 1012 (now corrected) is sufficient to solve the problem for reverse_iterator. However, Alisdair pointed out that LWG 1012 does not solve the general problem for authors of iterator adaptors.

There are some problems with the proposed resolution. The phrase "safely copyable" is not a term of art. Also, it mentions a DefaultConstructible? concept.

Move to Review after Alisdair updates the wording.

[ 2009-07-31 Alisdair revised wording: ]

[ 2009-08-17 Alisdair and Daniel collaborate on slightly revised wording. This issue depends upon 724 ]

[ 2009-10-14 Daniel adds: ]

There is a clear dependency on 1213, because the term "singular", which is used as part of the resolution, is not properly defined yet.

[ 2009-10 Santa Cruz: ]

Moved to Open. Alisdair will provide improved wording to make this have "value semantics" and otherwise behave like a valid iterator.

Proposed resolution:

Add a new paragrpah to Iterator concepts 24.2 [iterator.requirements] after para 5 (the one describing singular iterators)

Just as a regular pointer to an array guarantees that there is a pointer value pointing past the last element of the array, so for any iterator type there is an iterator value that points past the last element of a corresponding container. These values are called past-the-end values. Values of an iterator i for which the expression *i is defined are called dereferenceable. The library never assumes that past-the-end values are dereferenceable. Iterators can also have singular values that are not associated with any container. [Example: After the declaration of an uninitialized pointer x (as with int* x;), x must always be assumed to have a singular value of a pointer. — end example] Results of most expressions are undefined for singular values; the only exceptions are destroying an iterator that holds a singular value and the assignment of a non-singular value to an iterator that holds a singular value. In this case the singular value is overwritten the same way as any other value. Dereferenceable values are always non-singular.

After value-initialization, any iterator that satisfies the DefaultConstructible requirements ([defaultconstructible]) shall not introduce undefined behaviour when used as the source of a copy or move operation, even if it would otherwise be singular. [Note: This guarantee is not offered for default-initialization (8.5 [dcl.init]), although the distinction only matters for types with trivial default constructors such as pointers. — end note]


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: 2010-02-06

View other active issues in [facet.num.get.virtuals].

View all other issues in [facet.num.get.virtuals].

View all issues with Open status.

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>.

[ 2009-07 Frankfurt ]

There was general agreement that the standard only needs to specify the behavior when the character type is char or wchar_t.

Beman: we don't need to worry about C++1x because there is a non-zero possibility that we would have a replacement facility for iostreams that would solve these problems.

We need to change the following sentence in [locale.category], paragraph 6 to specify that C is char and wchar_t:

"A template formal parameter with name C represents the set of all possible specializations on a parameter that satisfies the requirements for a character on which any member of the iostream components can be instantiated."

We also need to specify in 27 that the basic character operations, such as eq, lt, and assign use std::char_traits.

Daniel volunteered to provide wording.

[ 2009-09-19 Daniel provided wording. ]

[ 2009-10 Santa Cruz: ]

Leave as Open. Alisdair and/or Tom will provide wording based on discussions. We want to clearly state that streams and locales work just on char and wchar_t (except where otherwise specified).

[ 2010-02-06 Tom updated the proposed wording. ]

[ The original proposed wording is preserved here: ]

  1. Change 22.3.1.1.1 [locale.category]/6:

    [..] A template formal parameter with name C represents the set of all possible specializations on a char or wchar_t parameter that satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]
  2. Add the following sentence to the end of 22.4.2 [category.numeric]/2:

    [..] These specializations refer to [..], and also for the ctype<> facet to perform character classification. Implementations are encouraged but not required to use the char_traits<charT> functions for all comparisons and assignments of characters of type charT that do not belong to the set of required specializations.
  3. Change 22.4.2.1.2 [facet.num.get.virtuals]/3:

    Stage 2: If in==end then stage 2 terminates. Otherwise a charT is taken from in and local variables are initialized as if by

    char_type ct = *in;
    using tr = char_traits<char_type>;
    const char_type* pos = tr::find(atoms, sizeof(src) - 1, ct);
    char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atoms
                 pos ? pos - atoms : sizeof(src) - 1];
    if (tr::eq(ct, ct == use_facet<numpunct<charT>(loc).decimal_point()))
        c = '.';
    bool discard =
        tr::eq(ct, ct == use_facet<numpunct<charT>(loc).thousands_sep())
        && use_facet<numpunct<charT> >(loc).grouping().length() != 0;
    

    where the values src and atoms are defined as if by: [..]

    [Remark of the author: I considered to replace the initialization "char_type ct = *in;" by the sequence "char_type ct; tr::assign(ct, *in);", but decided against it, because it is a copy-initialization context, not an assignment]

  4. Add the following sentence to the end of 22.4.5 [category.time]/1:

    [..] Their members use [..] , to determine formatting details. Implementations are encouraged but not required to use the char_traits<charT> functions for all comparisons and assignments of characters of type charT that do not belong to the set of required specializations.
  5. Change 22.4.5.1.1 [locale.time.get.members]/8 bullet 4:

    • The next element of fmt is equal to '%' For the next element c of fmt char_traits<char_type>::eq(c, use_facet<ctype<char_type>>(f.getloc()).widen('%')) == true, [..]
  6. Add the following sentence to the end of 22.4.6 [category.monetary]/2:

    Their members use [..] to determine formatting details. Implementations are encouraged but not required to use the char_traits<charT> functions for all comparisons and assignments of characters of type charT that do not belong to the set of required specializations.
  7. Change 22.4.6.1.2 [locale.money.get.virtuals]/4:

    [..] The value units is produced as if by:

    for (int i = 0; i < n; ++i)
      buf2[i] = src[char_traits<charT>::find(atoms, atoms+sizeof(src), buf1[i]) - atoms];
    buf2[n] = 0;
    sscanf(buf2, "%Lf", &units);
    
  8. Change 22.4.6.2.2 [locale.money.put.virtuals]/1:

    [..] for character buffers buf1 and buf2. If for the first character c in digits or buf2 is equal to ct.widen('-')char_traits<charT>::eq(c, ct.widen('-')) == true, [..]
  9. Add a footnote to the first sentence of 27.7.1.2.2 [istream.formatted.arithmetic]/1:

    As in the case of the inserters, these extractors depend on the locale's num_get<> (22.4.2.1) object to perform parsing the input stream data.(footnote) [..]

    footnote) If the traits of the input stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.

  10. Add a footnote to the second sentence of 27.7.2.6.2 [ostream.inserters.arithmetic]/1:

    Effects: The classes num_get<> and num_put<> handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting.(footnote) [..]

    footnote) If the traits of the output stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.

  11. Add a footnote after the first sentence of 27.7.4 [ext.manip]/4:

    Returns: An object of unspecified type such that if in is an object of type basic_istream<charT, traits> then the expression in >> get_money(mon, intl) behaves as if it called f(in, mon, intl), where the function f is defined as:(footnote) [..]

    footnote) If the traits of the input stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.

  12. Add a footnote after the first sentence of 27.7.4 [ext.manip]/5:

    Returns: An object of unspecified type such that if out is an object of type basic_ostream<charT, traits> then the expression out << put_money(mon, intl) behaves as a formatted input function that calls f(out, mon, intl), where the function f is defined as:(footnote) [..]

    footnote) If the traits of the output stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.

  13. 13) Add a footnote after the first sentence of 27.7.4 [ext.manip]/8:

    Returns: An object of unspecified type such that if in is an object of type basic_istream<charT, traits> then the expression in >>get_time(tmb, fmt) behaves as if it called f(in, tmb, fmt), where the function f is defined as:(footnote) [..]

    footnote) If the traits of the input stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.

  14. Add a footnote after the first sentence of 27.7.4 [ext.manip]/10:

    Returns: An object of unspecified type such that if out is an object of type basic_ostream<charT, traits> then the expression out <<put_time(tmb, fmt) behaves as if it called f(out, tmb, fmt), where the function f is defined as:(footnote) [..]

    footnote) If the traits of the output stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.

Proposed resolution:

  1. Change 22.3.1.1.1 [locale.category]/6:

    [..] A template formal parameter with name C represents the set of all possible specializations on a of types containing char, wchar_t, and any other implementation-defined character type parameter that satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]
  2. Add the following sentence to the end of 22.4.2 [category.numeric]/2:

    [..] These specializations refer to [..], and also for the ctype<> facet to perform character classification. [Note: Implementations are encouraged but not required to use the char_traits<charT> functions for all comparisons and assignments of characters of type charT that do not belong to the set of required specializations - end note].
  3. Change 22.4.2.1.2 [facet.num.get.virtuals]/3:

    Stage 2: If in==end then stage 2 terminates. Otherwise a charT is taken from in and local variables are initialized as if by

    char_type ct = *in;
    using tr = char_traits<char_type>;
    const char_type* pos = tr::find(atoms, sizeof(src) - 1, ct);
    char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atoms
                 pos ? pos - atoms : sizeof(src) - 1];
    if (tr::eq(ct, ct == use_facet<numpunct<charT>(loc).decimal_point()))
        c = '.';
    bool discard =
        tr::eq(ct, ct == use_facet<numpunct<charT>(loc).thousands_sep())
        && use_facet<numpunct<charT> >(loc).grouping().length() != 0;
    

    where the values src and atoms are defined as if by: [..]

    [Remark of the author: I considered to replace the initialization "char_type ct = *in;" by the sequence "char_type ct; tr::assign(ct, *in);", but decided against it, because it is a copy-initialization context, not an assignment]

  4. Add the following sentence to the end of 22.4.5 [category.time]/1:

    [..] Their members use [..] , to determine formatting details. [Note: Implementations are encouraged but not required to use the char_traits<charT> functions for all comparisons and assignments of characters of type charT that do not belong to the set of required specializations - end note].
  5. Change 22.4.5.1.1 [locale.time.get.members]/8 bullet 4:

  6. Add the following sentence to the end of 22.4.6 [category.monetary]/2:

    Their members use [..] to determine formatting details. [Note: Implementations are encouraged but not required to use the char_traits<charT> functions for all comparisons and assignments of characters of type charT that do not belong to the set of required specializations - end note].
  7. Change 22.4.6.1.2 [locale.money.get.virtuals]/4:

    [..] The value units is produced as if by:

    for (int i = 0; i < n; ++i)
      buf2[i] = src[char_traits<charT>::find(atoms, atoms+sizeof(src), buf1[i]) - atoms];
    buf2[n] = 0;
    sscanf(buf2, "%Lf", &units);
    
  8. Change 22.4.6.2.2 [locale.money.put.virtuals]/1:

    [..] for character buffers buf1 and buf2. If for the first character c in digits or buf2 is equal to ct.widen('-')char_traits<charT>::eq(c, ct.widen('-')) == true, [..]
  9. Add a new paragraph after the first paragraph of 27.2.2 [iostreams.limits.pos]/1:

    In the classes of clause 27, a template formal parameter with name charT represents one of the set of types containing char, wchar_t, and any other implementation-defined character type that satisfies the requirements for a character on which any of the iostream components can be instantiated.
  10. Add a footnote to the first sentence of 27.7.1.2.2 [istream.formatted.arithmetic]/1:

    As in the case of the inserters, these extractors depend on the locale's num_get<> (22.4.2.1) object to perform parsing the input stream data.(footnote) [..]

    footnote) If the traits of the input stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.

  11. Add a footnote to the second sentence of 27.7.2.6.2 [ostream.inserters.arithmetic]/1:

    Effects: The classes num_get<> and num_put<> handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting.(footnote) [..]

    footnote) If the traits of the output stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.

  12. Add a footnote after the first sentence of 27.7.4 [ext.manip]/4:

    Returns: An object of unspecified type such that if in is an object of type basic_istream<charT, traits> then the expression in >> get_money(mon, intl) behaves as if it called f(in, mon, intl), where the function f is defined as:(footnote) [..]

    footnote) If the traits of the input stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.

  13. Add a footnote after the first sentence of 27.7.4 [ext.manip]/5:

    Returns: An object of unspecified type such that if out is an object of type basic_ostream<charT, traits> then the expression out << put_money(mon, intl) behaves as a formatted input function that calls f(out, mon, intl), where the function f is defined as:(footnote) [..]

    footnote) If the traits of the output stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.

  14. Add a footnote after the first sentence of 27.7.4 [ext.manip]/8:

    Returns: An object of unspecified type such that if in is an object of type basic_istream<charT, traits> then the expression in >>get_time(tmb, fmt) behaves as if it called f(in, tmb, fmt), where the function f is defined as:(footnote) [..]

    footnote) If the traits of the input stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.

  15. Add a footnote after the first sentence of 27.7.4 [ext.manip]/10:

    Returns: An object of unspecified type such that if out is an object of type basic_ostream<charT, traits> then the expression out <<put_time(tmb, fmt) behaves as if it called f(out, tmb, fmt), where the function f is defined as:(footnote) [..]

    footnote) If the traits of the output stream has different semantics for lt(), eq(), and assign() than char_traits<char_type>, this may give surprising results.


430. valarray subset operations

Section: 26.6.2.4 [valarray.sub] Status: Open Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2009-11-04

View all issues with Open status.

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. ]

[ 2009-07 Frankfurt ]

Move to Ready.

[ 2009-11-04 Pete opens: ]

The resolution to LWG issue 430 has not been applied --- there have been changes to the underlying text, and the resolution needs to be reworked.

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)

446. Iterator equality between different containers

Section: 24.2 [iterator.requirements], 23.2 [container.requirements] Status: Open Submitter: Andy Koenig Opened: 2003-12-16 Last modified: 2009-11-03

View other active issues in [iterator.requirements].

View all other issues in [iterator.requirements].

View all issues with Open status.

Discussion:

What requirements does the standard place on equality comparisons between iterators that refer to elements of different containers. For example, if v1 and v2 are empty vectors, is v1.end() == v2.end() allowed to yield true? Is it allowed to throw an exception?

The standard appears to be silent on both questions.

[Sydney: The intention is that comparing two iterators from different containers is undefined, but it's not clear if we say that, or even whether it's something we should be saying in clause 23 or in clause 24. Intuitively we might want to say that equality is defined only if one iterator is reachable from another, but figuring out how to say it in any sensible way is a bit tricky: reachability is defined in terms of equality, so we can't also define equality in terms of reachability. ]

[ 2009-07 Frankfurt ]

Daniel volunteered to work on this.

[ 2009-09-20 Daniel provided wording. ]

[ 2009-10 Santa Cruz: ]

Leave as Open. Alisdair has volunteered to refine the wording.

Proposed resolution:

Insert a new paragraph between 24.2 [iterator.requirements]/7+8:

[..] The result of the application of functions in the library to invalid ranges is undefined.

The result of directly or indirectly evaluating any comparison function or the binary - operator with two iterator values as arguments that were obtained from two different ranges r1 and r2 (including their past-the-end values) which are not subranges of one common range is undefined, unless explicitly described otherwise.


471. result of what() implementation-defined

Section: 18.8.1 [exception] Status: Ready Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2009-10-26

View all issues with Ready status.

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.

[ 2009-07-30 Niels adds: ]

Further discussion in the thread starting with c++std-lib-24512.

[ 2009-09-24 Niels provided updated wording: ]

I think the resolution should at least guarantee that the result of what() is independent of whether the compiler does copy-elision. And for any class derived from std::excepion that has a constructor that allows specifying a what_arg, it should make sure that the text of a user-provided what_arg is preserved, when the object is copied. Note that all the implementations I've tested already appear to satisfy the proposed resolution, including MSVC 2008 SP1, Apache stdcxx-4.2.1, GCC 4.1.2, GCC 4.3.2, and CodeGear C++ 6.13.

The proposed resolution was updated with help from Daniel Krügler; the update aims to clarify that the proposed postcondition only applies to homogeneous copying.

[ 2009-10 Santa Cruz: ]

Moved to Ready after inserting "publicly accessible" in two places.

Proposed resolution:

Change 18.8.1 [exception] to:

-1- The class exception defines the base class for the types of objects thrown as exceptions by C++ standard library components, and certain expressions, to report errors detected during program execution.

Each standard library class T that derives from class exception shall have a publicly accessible copy constructor and a publicly accessible copy assignment operator that do not exit with an exception. These member functions shall preserve the following postcondition: If two objects lhs and rhs both have dynamic type T, and lhs is a copy of rhs, then strcmp(lhs.what(), rhs.what()) == 0.

...

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

-4- Effects: Copies an exception object.

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

-5- Postcondition: If *this and rhs both have dynamic type exception then strcmp(what(), rhs.what()) == 0.


473. underspecified ctype calls

Section: 22.4.1.1 [locale.ctype] Status: Ready Submitter: Martin Sebor Opened: 2004-07-01 Last modified: 2009-10-21

View all issues with Ready status.

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.

[ 2009-07 Frankfurt, Howard provided wording directed by consensus. ]

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Add paragraph 3 to 22.4 [locale.categories]:

-3- Within this clause it is unspecified if one virtual function calls another virtual function.

Rationale:

We are explicitly not addressing bullet item #2, thus giving implementors more latitude. Users will have to override both virtual functions, not just one.


485. output iterator insufficiently constrained

Section: 24.2.2 [output.iterators] Status: Ready Submitter: Chris Jefferson Opened: 2004-10-13 Last modified: 2009-10-26

View all other issues in [output.iterators].

View all issues with Ready status.

Discussion:

The note on 24.1.2 Output iterators insufficiently limits what can be performed on output iterators. While it requires that each iterator is progressed through only once and that each iterator is written to only once, it does not require the following things:

Note: Here it is assumed that x is an output iterator of type X which has not yet been assigned to.

a) That each value of the output iterator is written to: The standard allows: ++x; ++x; ++x;

b) That assignments to the output iterator are made in order X a(x); ++a; *a=1; *x=2; is allowed

c) Chains of output iterators cannot be constructed: X a(x); ++a; X b(a); ++b; X c(b); ++c; is allowed, and under the current wording (I believe) x,a,b,c could be written to in any order.

I do not believe this was the intension of the standard?

[Lillehammer: Real issue. There are lots of constraints we intended but didn't specify. Should be solved as part of iterator redesign.]

[ 2009-07 Frankfurt ]

Bill provided wording according to consensus.

[ 2009-07-21 Alisdair requests change from Review to Open. See thread starting with c++std-lib-24459 for discussion. ]

[ 2009-10 Santa Cruz: ]

Modified wording. Set to Review.

[ 2009-10 Santa Cruz: ]

Move to Ready after looking at again in a larger group in Santa Cruz.

Proposed resolution:

Change Table 101 — Output iterator requirements in 24.2.2 [output.iterators]:

Table 101 — Output iterator requirements
Expression Return type Operational semantics Assertion/note pre-/post-condition
X(a)     a = t is equivalent to X(a) = t. note: a destructor is assumed.
X u(a);
X u = a;
     
*r = o result is not used   Post: r is not required to be dereferenceable. r is incrementable.
++r X&   &r == &++r Post: r is dereferenceable, unless otherwise specified. r is not required to be incrementable.
r++ convertible to const X& {X tmp = r;
++r;
return tmp;}
Post: r is dereferenceable, unless otherwise specified. r is not required to be incrementable.
*r++ = o; result is not used  

539. partial_sum and adjacent_difference should mention requirements

Section: 26.7.3 [partial.sum] Status: Ready Submitter: Marc Schoolderman Opened: 2006-02-06 Last modified: 2009-10-24

View all issues with Ready status.

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.

[ 2009-07 Frankfurt ]

The proposed resolution isn't quite right. For example, "the type of *first" should be changed to "iterator::value_type" or similar. Daniel volunteered to correct the wording.

[ 2009-07-29 Daniel corrected wording. ]

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

  1. Change 26.7.3 [partial.sum]/1 as indicated:

    Effects: Let VT be InputIterator's value type. For a nonempty range, initializes an accumulator acc of type VT with *first and performs *result = acc. For every iterator i in [first + 1, last) in order, acc is then modified by acc = acc + *i or acc = binary_op(acc, *i) and is assigned to *(result + (i - first)). 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)))
    

    or

    
    binary_op(binary_op(...,
       binary_op(*first, *(first + 1)),...), *(first + (i - result)))
    
  2. Change 26.7.3 [partial.sum]/3 as indicated:

    Complexity: Exactly max((last - first) - 1, 0) applications of binary_opthe binary operation.
  3. Change 26.7.3 [partial.sum]/4 as indicated:

    Requires: VT shall be constructible from the type of *first, the result of acc + *i or binary_op(acc, *i) shall be implicitly convertible to VT, and the result of the expression acc shall be writable to the result output iterator. In the ranges [first,last] and [result,result + (last - first)] [..]
  4. Change 26.7.4 [adjacent.difference]/1 as indicated:

    Effects: Let VT be InputIterator's value type. For a nonempty range, initializes an accumulator acc of type VT with *first and performs *result = acc. For every iterator i in [first + 1, last) in order, initializes a value val of type VT with *i, assigns the result of val - acc or binary_op(val, acc) to *(result + (i - first)) and modifies acc = std::move(val). Assigns to every element referred to by iterator i in the range [result + 1, result + (last - first)) a value correspondingly equal to

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

    or

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

    result gets the value of *first.

  5. Change 26.7.4 [adjacent.difference]/2 as indicated:

    Requires: VT shall be MoveAssignable ([moveassignable]) and shall be constructible from the type of *first. The result of the expression acc and the result of the expression val - acc or binary_op(val, acc) shall be writable to the result output iterator. In the ranges [first,last] [..]
  6. Change 26.7.4 [adjacent.difference]/5 as indicated:

    Complexity: Exactly max((last - first) - 1, 0) applications of binary_opthe binary operation.

556. is Compare a BinaryPredicate?

Section: 25.4 [alg.sorting] Status: Tentatively Ready Submitter: Martin Sebor Opened: 2006-02-05 Last modified: 2010-01-22

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Discussion:

In 25, p8 we allow BinaryPredicates to return a type that's convertible to bool but need not actually be bool. That allows predicates to return things like proxies and requires that implementations be careful about what kinds of expressions they use the result of the predicate in (e.g., the expression in if (!pred(a, b)) need not be well-formed since the negation operator may be inaccessible or return a type that's not convertible to bool).

Here's the text for reference:

...if an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments, it should work correctly in the construct if (binary_pred(*first1, first2)){...}.

In 25.3, p2 we require that the Compare function object return true of false, which would seem to preclude such proxies. The relevant text is here:

Compare is used as a function object which returns true if the first argument is less than the second, and false otherwise...

[ Portland: Jack to define "convertible to bool" such that short circuiting isn't destroyed. ]

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

Move to Review once wording received. Stefanus to send proposed wording.

[ 2009-10 Santa Cruz: ]

Move to Review once wording received. Stefanus to send proposed wording.

[ 2009-10-24 Stefanus supplied wording. ]

Move to Review once wording received. Stefanus to send proposed wording. Old proposed wording here:

I think we could fix this by rewording 25.3, p2 to read somthing like:

-2- Compare is used as a function object which returns true if the first argument a BinaryPredicate. The return value of the function call operator applied to an object of type Compare, when converted to type bool, yields true if the first argument of the call is less than the second, and false otherwise. Compare comp is used throughout for algorithms assuming an ordering relation. It is assumed that comp will not apply any non-constant function through the dereferenced iterator.

[ 2010-01-17: ]

Howard expresses concern that the current direction of the proposed wording outlaws expressions such as:

if (!comp(x, y))

Daniel provides wording which addresses that concern.

The previous wording is saved here:

Change 25.4 [alg.sorting] p2:

Compare is used as a function object. The return value of the function call operator applied to an object of type Compare, when converted to type bool, yields true if the first argument of the call which returns true if the first argument is less than the second, and false otherwise. Compare comp is used throughout for algorithms assuming an ordering relation. It is assumed that comp will not apply any non-constant function through the dereferenced iterator.

[ 2010-01-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

  1. Change 25.1 [algorithms.general]/7+8 as indicated. [This change is recommended to bring the return value requirements of BinaryPredicate and Compare in sync.]

    7 The Predicate parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing the corresponding iterator returns a value testable as true. In other words, if an algorithm takes Predicate pred as its argument and first as its iterator argument, it should work correctly in the construct if (pred(*first)){...} pred(*first) contextually converted to bool (4 [conv]). The function object pred shall not apply any nonconstant function through the dereferenced iterator. This function object may be a pointer to function, or an object of a type with an appropriate function call operator.

    8 The BinaryPredicate parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and type T when T is part of the signature returns a value testable as true. In other words, if an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments, it should work correctly in the construct if (binary_pred(*first1, *first2)){...} binary_pred(*first1, *first2) contextually converted to bool (4 [conv]). BinaryPredicate always takes the first iterator type as its first argument, that is, in those cases when T value is part of the signature, it should work correctly in the context of if (binary_pred(*first1, value)){...} construct binary_pred(*first1, value) contextually converted to bool (4 [conv]). binary_pred shall not apply any non-constant function through the dereferenced iterators.

  2. Change 25.4 [alg.sorting]/2 as indicated:

    2 Compare is used as a function object type (20.7 [function.objects]). The return value of the function call operation applied to an object of type Compare, when contextually converted to type bool (4 [conv]), yields true if the first argument of the call which returns true if the first argument is less than the second, and false otherwise. Compare comp is used throughout for algorithms assuming an ordering relation. It is assumed that comp will not apply any non-constant function through the dereferenced iterator.

579. erase(iterator) for unordered containers should not return an iterator

Section: 23.2.5 [unord.req] Status: Tentatively Ready Submitter: Joaquín M López Muñoz Opened: 2006-06-13 Last modified: 2010-02-11

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Discussion:

See N2023 for full discussion.

[ 2009-12-11 Paolo opens: ]

I'm asking for DR 579 to be re-opened, basing on recent discussions on the library reflector, see Message c++std-lib-26040 and replies.

[ 2010-02-07 Paolo updates wording. ]

As pointed out by Chris in c++std-lib-26040, that an erase(unordered_container, iterator) returning an iterator can easily implemented in user code, if needed; that actually returning an iterator costs nothing for the overload taking two iterators, thus that proposed change is only for consistency; that forward_list::erase_after also returns void (for different reasons, granted, but isn't that any "erase" function in the containers uniformly returns an iterator); that, also in thread started by Chris' message, Alberto pointed out that the proxy idea isn't a good one; that users both of the GNU and Boost implementations are reporting serious performance problems with the current version returning an iterator.

[ 2010-02-07 Original wording saved here: ]

Option 1:

The problem can be eliminated by omitting the requirement that a.erase(q) return an iterator. This is, however, in contrast with the equivalent requirements for other standard containers.

Option 2:

a.erase(q) can be made to compute the next iterator only when explicitly requested: the technique consists in returning a proxy object implicitly convertible to iterator, so that

iterator q1=a.erase(q);

works as expected, while

a.erase(q);

does not ever invoke the conversion-to-iterator operator, thus avoiding the associated computation. To allow this technique, some sections of TR1 along the line "return value is an iterator..." should be changed to "return value is an unspecified object implicitly convertible to an iterator..." Although this trick is expected to work transparently, it can have some collateral effects when the expression a.erase(q) is used inside generic code.

[ 2010-02-09 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Rationale:

N2023 was discussed in Portland and the consensus was that there appears to be no need for either change proposed in the paper. The consensus opinion was that since the iterator could serve as its own proxy, there appears to be no need for the change. In general, "converts to" is undesirable because it interferes with template matching.

Post Toronto: There does not at this time appear to be consensus with the Portland consensus.

[ Bellevue: ]

The Bellevue review of this issue reached consensus with the Portland consensus, in contravention of the Toronto non-consensus. Common implementations have the iterator readily available, and most common uses depend on the iterator being returned.

****

The rationale for the change in direction here is best summarized by Paolo's 2010-02-07 comment.

Proposed resolution:

In 23.2.5 [unord.req], Table 98, change return type from iterator to void for the following rows:

Table 98 — Unordered associative container requirements (in addition to container)
Expression Return type Assertion/note pre-/post-condition Complexity
a.erase(q) iterator void Erases the element pointed to by q. Return value is the iterator immediately following q prior to the erasure. Average case O(1), worst case O(a.size()).
a.erase(q1, q2) iterator void Erases all elements in the range [q1, q2). Return value is the iterator immediately following the erased elements prior to the erasure. Average case linear in distance(q1, q2), worst case O(a.size()).

Adjust the declarations accordingly in 23.5.1 [unord.map], 23.5.2 [unord.multimap], 23.5.3 [unord.set], and 23.5.4 [unord.multiset].

iterator void erase(const_iterator position);
...
iterator void erase(const_iterator first, const_iterator last);

594. Disadvantages of defining Swappable in terms of CopyConstructible and Assignable

Section: 20.2.1 [utility.arg.requirements] Status: Open Submitter: Niels Dekker Opened: 2006-11-02 Last modified: 2010-02-03

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Discussion:

It seems undesirable to define the Swappable requirement in terms of CopyConstructible and Assignable requirements. And likewise, once the MoveConstructible and MoveAssignable requirements (N1860) have made it into the Working Draft, it seems undesirable to define the Swappable requirement in terms of those requirements. Instead, it appears preferable to have the Swappable requirement defined exclusively in terms of the existence of an appropriate swap function.

Section 20.1.4 [lib.swappable] of the current Working Draft (N2009) says:

The Swappable requirement is met by satisfying one or more of the following conditions:

I can think of three disadvantages of this definition:

  1. If a client's type T satisfies the first condition (T is both CopyConstructible and Assignable), the client cannot stop T from satisfying the Swappable requirement without stopping T from satisfying the first condition.

    A client might want to stop T from satisfying the Swappable requirement, because swapping by means of copy construction and assignment might throw an exception, and she might find a throwing swap unacceptable for her type. On the other hand, she might not feel the need to fully implement her own swap function for this type. In this case she would want to be able to simply prevent algorithms that would swap objects of type T from being used, e.g., by declaring a swap function for T, and leaving this function purposely undefined. This would trigger a link error, if an attempt would be made to use such an algorithm for this type. For most standard library implementations, this practice would indeed have the effect of stopping T from satisfying the Swappable requirement.

  2. A client's type T that does not satisfy the first condition can not be made Swappable by providing a specialization of std::swap for T.

    While I'm aware about the fact that people have mixed feelings about providing a specialization of std::swap, it is well-defined to do so. It sounds rather counter-intuitive to say that T is not Swappable, if it has a valid and semantically correct specialization of std::swap. Also in practice, providing such a specialization will have the same effect as satisfying the Swappable requirement.

  3. For a client's type T that satisfies both conditions of the Swappable requirement, it is not specified which of the two conditions prevails. After reading section 20.1.4 [lib.swappable], one might wonder whether objects of T will be swapped by doing copy construction and assignments, or by calling the swap function of T.

    I'm aware that the intention of the Draft is to prefer calling the swap function of T over doing copy construction and assignments. Still in my opinion, it would be better to make this clear in the wording of the definition of Swappable.

I would like to have the Swappable requirement defined in such a way that the following code fragment will correctly swap two objects of a type T, if and only if T is Swappable:

   using std::swap;
   swap(t, u);  // t and u are of type T.

This is also the way Scott Meyers recommends calling a swap function, in Effective C++, Third Edition, item 25.

Most aspects of this issue have been dealt with in a discussion on comp.std.c++ about the Swappable requirement, from 13 September to 4 October 2006, including valuable input by David Abrahams, Pete Becker, Greg Herlihy, Howard Hinnant and others.

[ San Francisco: ]

Recommend NAD. Solved by N2774.

[ 2009-07 Frankfurt ]

Moved to Open. Waiting for non-concepts draft.

[ 2009-11-08 Howard adds: ]

This issue is very closely related to 742.

[ 2010-02-03 Sean Hunt adds: ]

While reading N3000, I independently came across Issue 594. Having seen that it's an issue under discussion, I think the proposed wording needs fixing to something more like "...function call swap(t,u) that includes std::swap in its overload set is valid...", because "...is valid within the namespace std..." does not allow other libraries to simply use the Swappable requirement by referring to the standard's definition, since they cannot actually perform any calls within std.

This wording I suggested would also make overloads visible in the same scope as the `using std::swap` valid for Swappable requirements; a more complex wording limiting the non-ADL overload set to std::swap might be required.

Proposed resolution:

Change section 20.1.4 [lib.swappable] as follows:

The Swappable requirement is met by satisfying one or more of the following conditions: the following condition:


625. mixed up Effects and Returns clauses

Section: 17 [library] Status: Open Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2009-10-20

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Duplicate of: 895

Discussion:

Many member functions of basic_string are overloaded, with some of the overloads taking a string argument, others value_type*, others size_type, and others still iterators. Often, the requirements on one of the overloads are expressed in the form of Effects, Throws, and in the Working Paper (N2134) also Remark clauses, while those on the rest of the overloads via a reference to this overload and using a Returns clause.

The difference between the two forms of specification is that per 17.5.1.4 [structure.specifications], p3, an Effects clause specifies "actions performed by the functions," i.e., its observable effects, while a Returns clause is "a description of the return value(s) of a function" that does not impose any requirements on the function's observable effects.

Since only Notes are explicitly defined to be informative and all other paragraphs are explicitly defined to be normative, like Effects and Returns, the new Remark clauses also impose normative requirements.

So by this strict reading of the standard there are some member functions of basic_string that are required to throw an exception under some conditions or use specific traits members while many other otherwise equivalent overloads, while obliged to return the same values, aren't required to follow the exact same requirements with regards to the observable effects.

Here's an example of this problem that was precipitated by the change from informative Notes to normative Remarks (presumably made to address 424):

In the Working Paper, find(string, size_type) contains a Remark clause (which is just a Note in the current standard) requiring it to use traits::eq().

find(const charT *s, size_type pos) is specified to return find(string(s), pos) by a Returns clause and so it is not required to use traits::eq(). However, the Working Paper has replaced the original informative Note about the function using traits::length() with a normative requirement in the form of a Remark. Calling traits::length() may be suboptimal, for example when the argument is a very long array whose initial substring doesn't appear anywhere in *this.

Here's another similar example, one that existed even prior to the introduction of Remarks:

insert(size_type pos, string, size_type, size_type) is required to throw out_of_range if pos > size().

insert(size_type pos, string str) is specified to return insert(pos, str, 0, npos) by a Returns clause and so its effects when pos > size() are strictly speaking unspecified.

I believe a careful review of the current Effects and Returns clauses is needed in order to identify all such problematic cases. In addition, a review of the Working Paper should be done to make sure that the newly introduced normative Remark clauses do not impose any undesirable normative requirements in place of the original informative Notes.

[ Batavia: Alan and Pete to work. ]

[ Bellevue: Marked as NAD Editorial. ]

[ Post-Sophia Antipolis: Martin indicates there is still work to be done on this issue. Reopened. ]

[ Batavia (2009-05): ]

Tom proposes we say that, unless specified otherwise, it is always the caller's responsibility to verify that supplied arguments meet the called function's requirements. If further semantics are specified (e.g., that the function throws under certain conditions), then it is up to the implementer to check those conditions. Alan feels strongly that our current use of Requires in this context is confusing, especially now that requires is a new keyword.

[ 2009-07 Frankfurt ]

Move to Tentatively NAD.

[ 2009 Santa Cruz: ]

Move to Open. Martin will work on proposed wording.

Proposed resolution:


631. conflicting requirements for BinaryPredicate

Section: 25 [algorithms] Status: Tentatively NAD Submitter: James Kanze Opened: 2007-01-31 Last modified: 2010-02-03

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Discussion:

The general requirements for BinaryPredicate (in 25 [algorithms]/8) contradict the implied specific requirements for some functions. In particular, it says that:

[...] if an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments, it should work correctly in the construct if (binary_pred (*first1 , *first2 )){...}. BinaryPredicate always takes the first iterator type as its first argument, that is, in those cases when T value is part of the signature, it should work correctly in the context of if (binary_pred (*first1 , value)){...}.

In the description of upper_bound (25.4.3.2 [upper.bound]/2), however, the use is described as "!comp(value, e)", where e is an element of the sequence (a result of dereferencing *first).

In the description of lexicographical_compare, we have both "*first1 < *first2" and "*first2 < *first1" (which presumably implies "comp( *first1, *first2 )" and "comp( *first2, *first1 )".

Logically, the BinaryPredicate is used as an ordering relationship, with the semantics of "less than". Depending on the function, it may be used to determine equality, or any of the inequality relationships; doing this requires being able to use it with either parameter first. I would thus suggest that the requirement be:

Alternatively, one could specify an order for each function. IMHO, this would be more work for the committee, more work for the implementors, and of no real advantage for the user: some functions, such as lexicographical_compare or equal_range, will still require both functions, and it seems like a much easier rule to teach that both functions are always required, rather than to have a complicated list of when you only need one, and which one.

[ Toronto: Moved to Open. ConceptGCC seems to get lower_bound and upper_bound to work withoutt these changes. ]

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

Move to Review. The small problem with the "iterator type" will be fixed. The cited functions (lower_bound, uppwer_bound, equal_range) don't actually use BinaryPredicate , and where it is used, it is consistent with [algorithm]/8, so the main complaint of the issue is moot.

[ 2010-01-16 Beman clarified wording. ]

[ 2010-01-31: Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]

Rationale:

[ post San Francisco: ]

Solved by N2759.

2010-01-31: The draft standard is well specified as is, and this specification is desired. Issues 556 and 870 solve the remaining unclearness regarding the meaning of BinaryPredicate.

Proposed resolution:

Change 25 [algorithms] paragraph 8 as indicated:

8 The BinaryPredicate parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and type T when T is part of the signature returns a value testable as true. BinaryPredicate always takes the first iterator value_type as one of its arguments; which argument is unspecified. In other words, if If an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments, it should work correctly both in the construct if (binary_pred(*first1, *first2)){...} and if (binary_pred (*first2, *first1)){...}. BinaryPredicate always takes the first iterator type as its first argument, that is, in In those cases when T value is part of the signature, it should work correctly in the context of if (binary_pred(*first1, value)){...} and of if (binary_pred (value, *first1)){...}. binary_pred shall not apply any non-constant function through the dereferenced iterators. [Note: if the two types are not identical, and neither is convertable to the other, this may require that the BinaryPredicate be a functional object with two overloaded operator()() functions. — end note]


671. precision of hexfloat

Section: 22.4.2.2.2 [facet.num.put.virtuals] Status: Ready Submitter: John Salmon Opened: 2007-04-20 Last modified: 2009-10-21

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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. ]

[ 2009-07 Frankfurt ]

Leave this open for Robert and Daniel to work on.

Straw poll: Disposition?

Daniel and Robert have direction to write up wording for the "always %a" solution.

[ 2009-07-15 Robert provided wording. ]

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Change 22.4.2.2.2 [facet.num.put.virtuals], Stage 1, under p5 (near the end of Stage 1):

For conversion from a floating-point type, str.precision() is specified as precision in the conversion specification if floatfield != (ios_base::fixed | ios_base::scientific), else no precision is specified.

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


676. Moving the unordered containers

Section: 23.5 [unord] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2007-05-05 Last modified: 2010-02-11

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Discussion:

Move semantics are missing from the unordered containers. The proposed resolution below adds move-support consistent with N1858 and the current working draft.

The current proposed resolution simply lists the requirements for each function. These might better be hoisted into the requirements table for unordered associative containers. Futhermore a mild reorganization of the container requirements could well be in order. This defect report is purposefully ignoring these larger issues and just focusing on getting the unordered containers "moved".

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10-17 Removed rvalue-swaps from wording. ]

[ 2009-10 Santa Cruz: ]

Move to Review. Alisdair will review proposed wording.

[ 2009-10-29 Daniel updates wording. ]

[ 2010-01-26 Alisdair updates wording. ]

[ 2010-02-10 Howard updates wording to reference the unordered container requirements table (modified by 704) as much as possible. ]

[ Voted to WP in Bellevue. ]

[ post Bellevue, Pete notes: ]

Please remind people who are reviewing issues to check that the text modifications match the current draft. Issue 676, for example, adds two overloads for unordered_map::insert taking a hint. One takes a const_iterator and returns a const_iterator, and the other takes an iterator and returns an iterator. This was correct at the time the issue was written, but was changed in Toronto so there is only one hint overload, taking a const_iterator and returning an iterator.

This issue is not ready. In addition to the relatively minor signature problem I mentioned earlier, it puts requirements in the wrong places. Instead of duplicating requirements throughout the template specifications, it should put them in the front matter that talks about requirements for unordered containers in general. This presentation problem is editorial, but I'm not willing to do the extensive rewrite that it requires. Please put it back into Open status.

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Rationale:

[ San Francisco: ]

Solved by N2776.

[ Rationale is obsolete. ]

Proposed resolution:

unordered_map

Change 23.5.1 [unord.map]:

class unordered_map
{
    ...
    unordered_map(const unordered_map&);
    unordered_map(unordered_map&&);
    unordered_map(const Allocator&);
    unordered_map(const unordered_map&, const Allocator&);
    unordered_map(unordered_map&&, const Allocator&);
    ...
    unordered_map& operator=(const unordered_map&);
    unordered_map& operator=(unordered_map&&);
    ...
    // modifiers
    ...
    std::pair<iterator, bool> insert(const value_type& obj); 
    template <class P> pair<iterator, bool> insert(P&& obj);
    iterator       insert(const_iterator hint, const value_type& obj);
    template <class P> iterator       insert(const_iterator hint, P&& obj);
    ...
    mapped_type& operator[](const key_type& k);
    mapped_type& operator[](key_type&& k);
    ...
};

Add to 23.5.1.2 [unord.map.elem]:

mapped_type& operator[](const key_type& k);

...

Requires: key_type shall be CopyConstructible and mapped_type shall be DefaultConstructible.

Complexity: Average case O(1), worst case O(size()).

mapped_type& operator[](key_type&& k);

Requires: key_type shall be MoveConstructible and mapped_type shall be DefaultConstructible.

Effects: If the unordered_map does not already contain an element whose key is equivalent to k , inserts the value value_type(std::move(k), mapped_type()).

Returns: A reference to x.second, where x is the (unique) element whose key is equivalent to k.

Complexity: Average case O(1), worst case O(size()).

Add new section [unord.map.modifiers]:

template <class P>
  pair<iterator, bool> insert(P&& x);

Requires: value_type is constructible from std::forward<P>(x).

Effects: Inserts x converted to value_type if and only if there is no element in the container with key equivalent to the key of value_type(x).

Returns: The bool component of the returned pair indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key of value_type(x).

Complexity: Average case O(1), worst case O(size()).

Remarks: P shall be implicitly convertible to value_type, else this signature shall not participate in overload resolution.

template <class P>
  iterator insert(const_iterator hint, P&& x);

Requires: value_type is constructible from std::forward<P>(x).

Effects: Inserts x converted to value_type if and only if there is no element in the container with key equivalent to the key of value_type(x). The iterator hint is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.

Returns: An iterator pointing to the element with key equivalent to the key of value_type(x).

Complexity: Average case O(1), worst case O(size()).

Remarks: P shall be implicitly convertible to value_type, else this signature shall not participate in overload resolution.

unordered_multimap

Change 23.5.2 [unord.multimap]:

class unordered_multimap
{
    ...
    unordered_multimap(const unordered_multimap&);
    unordered_multimap(unordered_multimap&&);
    unordered_multimap(const Allocator&);
    unordered_multimap(const unordered_multimap&, const Allocator&);
    unordered_multimap(unordered_multimap&&, const Allocator&);
    ...
    unordered_multimap& operator=(const unordered_multimap&);
    unordered_multimap& operator=(unordered_multimap&&);
    ...
    // modifiers
    ...
    iterator insert(const value_type& obj); 
    template <class P> iterator insert(P&& obj);
    iterator       insert(const_iterator hint, const value_type& obj);
    template <class P> iterator       insert(const_iterator hint, P&& obj);
    ...
};

Add new section [unord.multimap.modifiers]:

template <class P>
  iterator insert(P&& x);

Requires: value_type is constructible from std::forward<P>(x).

Effects: Inserts x converted to value_type.

Returns: An iterator pointing to the element with key equivalent to the key of value_type(x).

Complexity: Average case O(1), worst case O(size()).

Remarks: P shall be implicitly convertible to value_type, else this signature shall not participate in overload resolution.

template <class P>
  iterator insert(const_iterator hint, P&& x);

Requires: value_type is constructible from std::forward<P>(x).

Effects: Inserts x converted to value_type if and only if there is no element in the container with key equivalent to the key of value_type(x). The iterator hint is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.

Returns: An iterator pointing to the element with key equivalent to the key of value_type(x).

Complexity: Average case O(1), worst case O(size()).

Remarks: P shall be implicitly convertible to value_type, else this signature shall not participate in overload resolution.

unordered_set

Change 23.5.3 [unord.set]:

class unordered_set
{
    ...
    unordered_set(const unordered_set&);
    unordered_set(unordered_set&&);
    unordered_set(const Allocator&);
    unordered_set(const unordered_set&, const Allocator&);
    unordered_set(unordered_set&&, const Allocator&);
    ...
    unordered_set& operator=(const unordered_set&);
    unordered_set& operator=(unordered_set&&);
    ...
    // modifiers 
    ...
    std::pair<iterator, bool> insert(const value_type& obj); 
    pair<iterator, bool> insert(value_type&& obj);
    iterator       insert(const_iterator hint, const value_type& obj);
    iterator       insert(const_iterator hint, value_type&& obj);
    ...
};

unordered_multiset

Change 23.5.4 [unord.multiset]:

class unordered_multiset
{
    ...
    unordered_multiset(const unordered_multiset&);
    unordered_multiset(unordered_multiset&&);
    unordered_multiset(const Allocator&);
    unordered_multiset(const unordered_multiset&, const Allocator&);
    unordered_multiset(unordered_multiset&&, const Allocator&);
    ...
    unordered_multiset& operator=(const unordered_multiset&);
    unordered_multiset& operator=(unordered_multiset&&);
    ...
    // modifiers
    ...
    iterator insert(const value_type& obj); 
    iterator insert(value_type&& obj);
    iterator       insert(const_iterator hint, const value_type& obj);
    iterator       insert(const_iterator hint, value_type&& obj);
    ...
};


704. MoveAssignable requirement for container value type overly strict

Section: 23.2 [container.requirements] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2007-05-20 Last modified: 2010-02-11

View other active issues in [container.requirements].

View all other issues in [container.requirements].

View all issues with Tentatively Ready status.

Discussion:

The move-related changes inadvertently overwrote the intent of 276. Issue 276 removed the requirement of CopyAssignable from most of the member functions of node-based containers. But the move-related changes unnecessarily introduced the MoveAssignable requirement for those members which used to require CopyAssignable.

We also discussed (c++std-lib-18722) the possibility of dropping MoveAssignable from some of the sequence requirements. Additionally the in-place construction work may further reduce requirements. For purposes of an easy reference, here are the minimum sequence requirements as I currently understand them. Those items in requirements table in the working draft which do not appear below have been purposefully omitted for brevity as they do not have any requirements of this nature. Some items which do not have any requirements of this nature are included below just to confirm that they were not omitted by mistake.

Container Requirements
X u(a)value_type must be CopyConstructible
X u(rv)array requires value_type to be CopyConstructible
a = uSequences require value_type to be CopyConstructible and CopyAssignable. Associative containers require value_type to be CopyConstructible.
a = rvarray requires value_type to be CopyAssignable. Sequences containers with propagate_on_container_move_assignment == false allocators require value_type to be MoveConstructible and MoveAssignable. Associative containers with propagate_on_container_move_assignment == false allocators require value_type to be MoveConstructible.
swap(a,u)array requires value_type to be Swappable.

Sequence Requirements
X(n)value_type must be DefaultConstructible
X(n, t)value_type must be CopyConstructible
X(i, j)Sequences require value_type to be constructible from *i. Additionally if input_iterators are used, vector and deque require MoveContructible and MoveAssignable.
a.insert(p, t)The value_type must be CopyConstructible. The sequences vector and deque also require the value_type to be CopyAssignable.
a.insert(p, rv)The value_type must be MoveConstructible. The sequences vector and deque also require the value_type to be MoveAssignable.
a.insert(p, n, t)The value_type must be CopyConstructible. The sequences vector and deque also require the value_type to be CopyAssignable.
a.insert(p, i, j)If the iterators return an lvalue the value_type must be CopyConstructible. The sequences vector and deque also require the value_type to be CopyAssignable when the iterators return an lvalue. If the iterators return an rvalue the value_type must be MoveConstructible. The sequences vector and deque also require the value_type to be MoveAssignable when the iterators return an rvalue.
a.erase(p)The sequences vector and deque require the value_type to be MoveAssignable.
a.erase(q1, q2)The sequences vector and deque require the value_type to be MoveAssignable.
a.clear()
a.assign(i, j)If the iterators return an lvalue the value_type must be CopyConstructible and CopyAssignable. If the iterators return an rvalue the value_type must be MoveConstructible and MoveAssignable.
a.assign(n, t)The value_type must be CopyConstructible and CopyAssignable.
a.resize(n)The value_type must be DefaultConstructible. The sequence vector also requires the value_type to be MoveConstructible.
a.resize(n, t)The value_type must be CopyConstructible.

Optional Sequence Requirements
a.front()
a.back()
a.push_front(t)The value_type must be CopyConstructible.
a.push_front(rv)The value_type must be MoveConstructible.
a.push_back(t)The value_type must be CopyConstructible.
a.push_back(rv)The value_type must be MoveConstructible.
a.pop_front()
a.pop_back()
a[n]
a.at[n]

Associative Container Requirements
X(i, j)If the iterators return an lvalue the value_type must be CopyConstructible. If the iterators return an rvalue the value_type must be MoveConstructible.
a_uniq.insert(t)The value_type must be CopyConstructible.
a_uniq.insert(rv)The key_type and the mapped_type (if it exists) must be MoveConstructible.
a_eq.insert(t)The value_type must be CopyConstructible.
a_eq.insert(rv)The key_type and the mapped_type (if it exists) must be MoveConstructible.
a.insert(p, t)The value_type must be CopyConstructible.
a.insert(p, rv)The key_type and the mapped_type (if it exists) must be MoveConstructible.
a.insert(i, j)If the iterators return an lvalue the value_type must be CopyConstructible. If the iterators return an rvalue the key_type and the mapped_type (if it exists) must be MoveConstructible..

Unordered Associative Container Requirements
X(i, j, n, hf, eq)If the iterators return an lvalue the value_type must be CopyConstructible. If the iterators return an rvalue the value_type must be MoveConstructible.
a_uniq.insert(t)The value_type must be CopyConstructible.
a_uniq.insert(rv)The key_type and the mapped_type (if it exists) must be MoveConstructible.
a_eq.insert(t)The value_type must be CopyConstructible.
a_eq.insert(rv)The key_type and the mapped_type (if it exists) must be MoveConstructible.
a.insert(p, t)The value_type must be CopyConstructible.
a.insert(p, rv)The key_type and the mapped_type (if it exists) must be MoveConstructible.
a.insert(i, j)If the iterators return an lvalue the value_type must be CopyConstructible. If the iterators return an rvalue the key_type and the mapped_type (if it exists) must be MoveConstructible..

Miscellaneous Requirements
map[lvalue-key]The key_type must be CopyConstructible. The mapped_type must be DefaultConstructible and MoveConstructible.
map[rvalue-key]The key_type must be MoveConstructible. The mapped_type must be DefaultConstructible and MoveConstructible.

[ Kona (2007): Howard and Alan to update requirements table in issue with emplace signatures. ]

[ Bellevue: This should be handled as part of the concepts work. ]

[ 2009-07-20 Reopened by Howard: ]

This is one of the issues that was "solved by concepts" and is now no longer solved.

In a nutshell, concepts adopted the "minimum requirements" philosophy outlined in the discussion of this issue, and enforced it. My strong suggestion is that we translate the concepts specification into documentation for the containers.

What this means for vendors is that they will have to implement container members being careful to only use those characteristics of a type that the concepts specification formally allowed. Note that I am not talking about enable_if'ing everything. I am simply suggesting that (for example) we tell the vendor he can't call T's copy constructor or move constructor within the emplace member function, etc.

What this means for customers is that they will be able to use types within C++03 containers which are sometimes not CopyConstructible, and sometimes not even MoveConstructible, etc.

[ 2009-10 Santa Cruz: ]

Leave open. Howard to provide wording.

[ 2010-02-06 Howard provides wording. ]

[ 2010-02-08 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Howard opened. I neglected to reduce the requirements on value_type for the insert function of the ordered and unordered associative containers when the argument is an rvalue. Fixed it. ]

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Rationale:

[ post San Francisco: ]

Solved by N2776.

This rationale is obsolete.

Proposed resolution:

Change 23.2.1 [container.requirements.general]/4:

4 In Tables 91 and 92, X denotes a container class containing objects of type T, a and b denote values of type X, u denotes an identifier, r denotes an lvalue or a const rvalue a non-const value of type X, and rv denotes a non-const rvalue of type X.

Change the following rows in Table 91 — Container requirements 23.2.1 [container.requirements.general]:

Table 91 — Container requirements
Expression Return type Assertion/note
pre-/post-condition
Complexity
X::value_type T Requires: T is Destructible. compile time

Change 23.2.1 [container.requirements.general]/10:

Unless otherwise specified (see 23.2.4.1, 23.2.5.1, 23.3.2.3, and 23.3.6.4) all container types defined in this Clause meet the following additional requirements:

Insert a new paragraph prior to 23.2.1 [container.requirements.general]/14:

The descriptions of the requirements of the type T in this section use the terms CopyConstructible, MoveConstructible, constructible from *i, and constructible from args. These terms are equivalent to the following expression using the appropriate arguments:


allocator_traits<allocator_type>::construct(x.get_allocator(), q, args...);

where x is a non-const lvalue of some container type X and q has type X::value_type*.

[Example: The container is going to move construct a T, so will call:


allocator_traits<allocator_type>::construct(get_allocator(), q, std::move(t));

The default implementation of construct will call:


::new (q) T(std::forward<T>(t)); // where forward is the same as move here, cast to rvalue

But the allocator author may override the above definition of construct and do the construction of T by some other means. — end example]

14 ...

Add to 23.2.1 [container.requirements.general]/14:

14 In Table 93, X denotes an allocator-aware container class with a value_type of T using allocator of type A, u denotes a variable, a and b denote non-const lvalues of type X, t denotes an lvalue or a const rvalue of type X, rv denotes a non-const rvalue of type X, m is a value of type A, and Q is an allocator type.

Change or add the following rows in Table 93 — Allocator-aware container requirements in 23.2.1 [container.requirements.general]:

Table 93 — Allocator-aware container requirements
Expression Return type Assertion/note
pre-/post-condition
Complexity
X(t, m)
X u(t, m);
Requires: T is CopyConstructible.
post: u == t,
get_allocator() == m
linear
X(rv, m)
X u(rv, m);
Requires: T is MoveConstructible.
post: u shall have the same elements, or copies of the elements, that rv had before this construction,
get_allocator() == m
constant if m == rv.get_allocator(), otherwise linear
a = t X& Requires: T is CopyConstructible and CopyAssignable
post: a == t.
linear
a = rv X& Requires: If allocator_traits< allocator_type > ::propagate_on_container_move_assignment ::value is false, T is MoveConstructible and MoveAssignable.
All existing elements of a are either move assigned to or destroyed.
a shall be equal to the value that rv had before this assignment
linear
a.swap(b); void exchanges the contents of a and b constant

Change the following rows in Table 94 — Sequence container requirements (in addition to container) in 23.2.3 [sequence.reqmts]:

Table 94 — Sequence container requirements (in addition to container)
Expression Return type Assertion/note
pre-/post-condition
X(i, j)
X a(i, j)
Requires: If the iterator's dereference operation returns an lvalue or a const rvalue, T shall be CopyConstructible. T shall be constructible from *i.
If the iterator does not meet the forward iterator requirements (24.2.3 [forward.iterators]), then vector also requires T to be MoveConstructible.
Each iterator in the range [i,j) shall be dereferenced exactly once.
post: size() == distance between i and j
Constructs a sequence container equal to the range [i, j)
a = il; X& Requires: T is CopyConstructible and CopyAssignable.
a = X(il);
Assigns the range [il.begin(), il.end()) into a. All existing elements of a are either assigned or destroyed.
rReturns *this;
a.emplace(p, args); iterator Requires: ConstructibleAsElement<A, T, Args>. T is constructible from args. vector and deque also require T to be MoveConstructible and MoveAssignable. Inserts an object of type T constructed with std::forward<Args>(args)... before p.
a.insert(p, t); iterator Requires: ConstructibleAsElement<A, T, Args> and T shall be CopyAssignable. T shall be CopyConstructible. vector and deque also require T to be CopyAssignable. Inserts a copy t before p.
a.insert(p, rv); iterator Requires: ConstructibleAsElement<A, T, T&&> and T shall be MoveAssignable. T shall be MoveConstructible. vector and deque also require T to be MoveAssignable. Inserts a copy rv before p.
a.insert(p, i, j) iterator Requires: If the iterator's dereference operation returns an lvalue or a const rvalue, T shall be CopyConstructible. T shall be constructible from *i.
If the iterator does not meet the forward iterator requirements (24.2.3 [forward.iterators]), then vector also requires T to be MoveConstructible and MoveAssignable.
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.erase(q); iterator Requires: T and T shall be MoveAssignable. vector and deque require T to be MoveAssignable. Erases the element pointed to by q.
a.erase(q1, q2); iterator Requires: T and T shall be MoveAssignable. vector and deque require T to be MoveAssignable. Erases the elements in the range [q1, q2).
a.clear(); void erase(begin(), end())
Destroys all elements in a. Invalidates all references, pointers, and iterators referring to the elements of a and may invalidate the past-the-end iterator.
post: size() == 0 a.empty() == true
a.assign(i, j) void Requires: If the iterator's dereference operation returns an lvalue or a const rvalue, T shall be CopyConstructible and CopyAssignable. T shall be constructible and assignable from *i. If the iterator does not meet the forward iterator requirements (24.2.3 [forward.iterators]), then vector also requires T to be MoveConstructible.
Each iterator in the range [i,j) shall be dereferenced exactly once.
pre: i, j are not iterators into a.
Replaces elements in a with a copy of [i, j).

Change the following rows in Table 95 — Optional sequence container operations in 23.2.3 [sequence.reqmts]:

Table 95 — Optional sequence container operations
Expression Return type Operational semantics Container
a.emplace_front(args) void a.emplace(a.begin(), std::forward<Args>(args)...)
Prepends an object of type T constructed with std::forward<Args>(args)....
Requires: ConstructibleAsElement<A, T, Args> T shall be constructible from args.
list, deque, forward_list
a.emplace_back(args) void a.emplace(a.end(), std::forward<Args>(args)...)
Appends an object of type T constructed with std::forward<Args>(args)....
Requires: ConstructibleAsElement<A, T, Args> T shall be constructible from args. vector also requires T to be MoveConstructible.
list, deque, vector
a.push_front(t) void a.insert(a.begin(), t)
Prepends a copy of t.
Requires: ConstructibleAsElement<A, T, T> and T shall be CopyAssignable. T shall be CopyConstructible.
list, deque, forward_list
a.push_front(rv) void a.insert(a.begin(), t)
Prepends a copy of rv.
Requires: ConstructibleAsElement<A, T, T&&> and T shall be MoveAssignable. T shall be MoveConstructible.
list, deque, forward_list
a.push_back(t) void a.insert(a.end(), t)
Appends a copy of t.
Requires: ConstructibleAsElement<A, T, T> and T shall be CopyAssignable. T shall be CopyConstructible.
vector, list, deque, basic_string
a.push_back(rv) void a.insert(a.end(), t)
Appends a copy of rv.
Requires: ConstructibleAsElement<A, T, T&&> and T shall be MoveAssignable. T shall be MoveConstructible.
vector, list, deque, basic_string
a.pop_front() void a.erase(a.begin())
Destroys the first element.
Requires: a.empty() shall be false.
list, deque, forward_list
a.pop_back() void { iterator tmp = a.end();
--tmp;
a.erase(tmp); }

Destroys the last element.
Requires: a.empty() shall be false.
vector, list, deque, basic_string

Insert a new paragraph prior to 23.2.4 [associative.reqmts]/7, and edit paragraph 7:

The associative containers meet all of the requirements of Allocator-aware containers (23.2.1 [container.requirements.general]), except for the containers map and multimap, the requirements placed on value_type in Table 93 apply instead directly to key_type and mapped_type. [Note: For example key_type and mapped_type are sometimes required to be CopyAssignable even though the value_type (pair<const key_type, mapped_type>) is not CopyAssignable. — end note]

7 In Table 96, X denotes an associative container class, a denotes a value of X, a_uniq denotes a value of X when X supports unique keys, a_eq denotes a value of X when X supports multiple keys, u denotes an identifier, r denotes an lvalue or a const rvalue of type X, rv denotes a non-const rvalue of type X, i and j satisfy input iterator requirements and refer to elements implicitly convertible to value_type, [i,j) denotes a valid range, p denotes a valid const iterator to a, q denotes a valid dereferenceable const iterator to a, [q1, q2) denotes a valid range of const iterators in a, il designates an object of type initializer_list<value_type>, t denotes a value of X::value_type, k denotes a value of X::key_type and c denotes a value of type X::key_compare. A denotes the storage allocator used by X, if any, or std::allocator<X::value_type> otherwise, and m denotes an allocator of a type convertible to A.

Change or add the following rows in Table 96 — Associative container requirements (in addition to container) in 23.2.4 [associative.reqmts]:

Table 96 — Associative container requirements (in addition to container)
Expression Return type Assertion/note
pre-/post-condition
Complexity
X::key_type Key Requires: Key is CopyConstructible and CopyAssignable Destructible compile time
X::mapped_type (map and multimap only) T Requires: T is Destructible compile time
X(c)
X a(c);
Requires: ConstructibleAsElement<A, key_compare, key_compare>.
key_compare is CopyConstructible.
Constructs an empty container.
Uses a copy of c as a comparison object.
constant
X()
X a;
Requires: ConstructibleAsElement<A, key_compare, key_compare>.
key_compare is DefaultConstructible.
Constructs an empty container.
Uses Compare() as a comparison object.
constant
X(i, j, c)
X a(i, j, c);
Requires: ConstructibleAsElement<A, key_compare, key_compare>.
key_compare is CopyConstructible. value_type shall be constructible from *i.
Constructs an empty container ans inserts elements from the range [i, j) into it; uses c as a comparison object.
N log N in general (N is the distance from i to j); linear if [i, j) is sorted with value_comp()
X(i, j)
X a(i, j);
Requires: ConstructibleAsElement<A, key_compare, key_compare>.
value_type shall be constructible from *i. key_compare is DefaultConstructible.
Same as above, but uses Compare() as a comparison object.
same as above
a = il X& a = X(il);
return *this;

Requires: T is CopyConstructible and CopyAssignable.
Assigns the range [il.begin(), il.end()) into a. All existing elements of a are either assigned or destroyed.
Same as a = X(il). N log N in general (N is il.size() added to the existing size of a); linear if [il.begin(), il.end()) is sorted with value_comp()
a_uniq.emplace(args) pair<iterator, bool> Requires: T shall be constructible from args
inserts a T object t constructed with std::forward<Args>(args)... if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of t.
logarithmic
a_eq.emplace(args) iterator Requires: T shall be constructible from args
inserts a T object t constructed with std::forward<Args>(args)... and returns the iterator pointing to the newly inserted element.
logarithmic
a_uniq.insert(t) pair<iterator, bool> Requires: T shall be MoveConstructible if t is a non-const rvalue expression, else T shall be CopyConstructible.
inserts t if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of t.
logarithmic
a_eq.insert(t) iterator Requires: T shall be MoveConstructible if t is a non-const rvalue expression, else T shall be CopyConstructible.
inserts t and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent to t exists in a_eq, t is inserted at the end of that range.
logarithmic
a.insert(p, t) iterator Requires: T shall be MoveConstructible if t is a non-const rvalue expression, else T shall be CopyConstructible.
inserts t if and only if there is no element with key equivalent to the key of t in containers with unique keys; always inserts t in containers with equivalent keys; always returns the iterator pointing to the element with key equivalent to the key of t. t is inserted as close as possible to the position just prior to p.
logarithmic in general, but amortized constant if t is inserted right before p.
a.insert(i, j) void Requires: T shall be constructible from *i.
pre: i, j are not iterators into a. inserts each element from the range [i,j) if and only if there is no element with key equivalent to the key of that element in containers with unique keys; always inserts that element in containers with equivalent keys.
N log(size() + N ) (N is the distance from i to j)

Insert a new paragraph prior to 23.2.5 [unord.req]/9:

The unordered associative containers meet all of the requirements of Allocator-aware containers (23.2.1 [container.requirements.general]), except for the containers unordered_map and unordered_multimap, the requirements placed on value_type in Table 93 apply instead directly to key_type and mapped_type. [Note: For example key_type and mapped_type are sometimes required to be CopyAssignable even though the value_type (pair<const key_type, mapped_type>) is not CopyAssignable. — end note]

9 ...

Change or add the following rows in Table 98 — Unordered associative container requirements (in addition to container) in 23.2.5 [unord.req]:

Table 98 — Unordered associative container requirements (in addition to container)
Expression Return type Assertion/note
pre-/post-condition
Complexity
X::key_type Key Requires: Key shall be CopyAssignable and CopyConstructible Destructible compile time
X::mapped_type (unordered_map and unordered_multimap only) T Requires:T is Destructible compile time
X(n, hf, eq)
X a(n, hf, eq)
X Requires: hasher and key_equal are CopyConstructible. Constructs an empty container with at least n buckets, using hf as the hash function and eq as the key equality predicate. O(N)
X(n, hf)
X a(n, hf)
X Requires: hasher is CopyConstructible and key_equal is DefaultConstructible. Constructs an empty container with at least n buckets, using hf as the hash function and key_equal() as the key equality predicate. O(N)
X(n)
X a(n)
X Requires: hasher and key_equal are DefaultConstructible. Constructs an empty container with at least n buckets, using hasher() as the hash function and key_equal() as the key equality predicate. O(N)
X()
X a
X Requires: hasher and key_equal are DefaultConstructible. Constructs an empty container an unspecified number of buckets, using hasher() as the hash function and key_equal() as the key equality predicate. constant
X(i, j, n, hf, eq)
X a(i, j, n, hf, eq)
X Requires: value_type is constructible from *i. hasher and key_equal are CopyConstructible.
Constructs an empty container with at least n buckets, using hf as the hash function and eq as the key equality predicate, and inserts elements from [i, j) into it.
Average case O(N) (N is distance(i, j)), worst case O(N2)
X(i, j, n, hf)
X a(i, j, n, hf)
X Requires: value_type is constructible from *i. hasher is CopyConstructible and key_equal is DefaultConstructible.
Constructs an empty container with at least n buckets, using hf as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it.
Average case O(N) (N is distance(i, j)), worst case O(N2)
X(i, j, n)
X a(i, j, n)
X Requires: value_type is constructible from *i. hasher and key_equal are DefaultConstructible.
Constructs an empty container with at least n buckets, using hasher() as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it.
Average case O(N) (N is distance(i, j)), worst case O(N2)
X(i, j)
X a(i, j)
X Requires: value_type is constructible from *i. hasher and key_equal are DefaultConstructible.
Constructs an empty container with an unspecified number of buckets, using hasher() as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it.
Average case O(N) (N is distance(i, j)), worst case O(N2)
X(b)
X a(b)
X Copy constructor. In addition to the contained elements requirements of Table 93 (23.2.1 [container.requirements.general]), copies the hash function, predicate, and maximum load factor. Average case linear in b.size(), worst case quadratic.
a = b X& Copy assignment operator. In addition to the contained elements requirements of Table 93 (23.2.1 [container.requirements.general]), copies the hash function, predicate, and maximum load factor. Average case linear in b.size(), worst case quadratic.
a = il X& a = X(il); return *this;
Requires: T is CopyConstructible and CopyAssignable.
Assigns the range [il.begin(), il.end()) into a. All existing elements of a are either assigned or destroyed.
Average case linear in il.size(), worst case quadratic.
a_uniq.emplace(args) pair<iterator, bool> Requires: T shall be constructible from args
inserts a T object t constructed with std::forward<Args>(args)... if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of t.
Average case O(1), worst case O(a_uniq.size()).
a_eq.emplace(args) iterator Requires: T shall be constructible from args
inserts a T object t constructed with std::forward<Args>(args)... and returns the iterator pointing to the newly inserted element.
Average case O(1), worst case O(a_eq.size()).
a.emplace_hint(p, args) iterator Requires: T shall be constructible from args
equivalent to a.emplace( std::forward<Args>(args)...). Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. The const_iterator p is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.
Average case O(1), worst case O(a.size()).
a_uniq.insert(t) pair<iterator, bool> Requires: T shall be MoveConstructible if t is a non-const rvalue expression, else T shall be CopyConstructible.
Inserts t if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key of t.
Average case O(1), worst case O(a_uniq.size()).
a_eq.insert(t) iterator Requires: T shall be MoveConstructible if t is a non-const rvalue expression, else T shall be CopyConstructible.
Inserts t, and returns an iterator pointing to the newly inserted element.
Average case O(1), worst case O(a_uniq.size()).
a.insert(q, t) iterator Requires: T shall be MoveConstructible if t is a non-const rvalue expression, else T shall be CopyConstructible.
Equivalent to a.insert(t). Return value is an iterator pointing to the element with the key equivalent to that of t. The iterator q is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.
Average case O(1), worst case O(a_uniq.size()).
a.insert(i, j) void Requires: T shall be constructible from *i.
Pre: i and j are not iterators in a. Equivalent to a.insert(t) for each element in [i,j).
Average case O(N), where N is distance(i, j). Worst case O(N * a.size()).

Remove the entire section 23.3.2.1 [deque.cons].

[ This section is already specified by the requirements tables. ]

Change 23.3.3 [forwardlist]/2:

2 A forward_list satisfies all of the requirements of a container (table 91), except that the size() member function is not provided. A forward_list also satisfies all of the requirements of an allocator-aware container (table 93). And forward_list provides the assign member functions as specified in Table 94, Sequence container requirements, and several of the optional sequence container requirements (Table 95). Descriptions are provided here only for operations on forward_list that are not described in that table or for operations where there is additional semantic information.

Remove the entire section 23.3.3.1 [forwardlist.cons].

[ This section is already specified by the requirements tables, and is actually incorrect for the requirements on value_type under assign (lacks CopyAssignable). ]

Remove from 23.3.3.4 [forwardlist.modifiers] the redundant definitions for:

template <class... Args> void emplace_front(Args&&... args);
void push_front(const T& x);
void push_front(T&& x);
void pop_front();

Add a new paragraph after 23.3.3.4 [forwardlist.modifiers]/23:

void clear();

23 Effects: Erases all elements in the range [begin(),end()).

Remarks: Does not invalidate past-the-end iterators.

Remove the entire section 23.3.4.1 [list.cons].

[ This section is already specified by the requirements tables, and is actually incorrect for the requirements on value_type under assign (lacks CopyAssignable). ]

Remove the redundant section 23.3.6.1 [vector.cons].

Change 23.3.6.2 [vector.capacity]/13:

void resize(size_type sz, const T& c);
13 Requires: T shall be CopyConstructible. If value_type has a move constructor, that constructor shall not throw any exceptions.

In 23.5.3 [unord.set] and 23.5.4 [unord.multiset] substitute "Key" for "Value".

[ The above substitution is normative as it ties into the requirements table. ]


724. DefaultConstructible is not defined

Section: 20.2.1 [utility.arg.requirements] Status: Tentatively Ready Submitter: Pablo Halpern Opened: 2007-09-12 Last modified: 2010-02-04

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Discussion:

The DefaultConstructible requirement is referenced in several places in the August 2007 working draft N2369, but is not defined anywhere.

[ Bellevue: ]

Walking into the default/value-initialization mess...

Why two lines? Because we need both expressions to be valid.

AJM not sure what the phrase "default constructed" means. This is unfortunate, as the phrase is already used 24 times in the library!

Example: const int would not accept first line, but will accept the second.

This is an issue that must be solved by concepts, but we might need to solve it independantly first.

It seems that the requirements are the syntax in the proposed first column is valid, but not clear what semantics we need.

A table where there is no post-condition seems odd, but appears to sum up our position best.

At a minimum an object is declared and is destuctible.

Move to open, as no-one happy to produce wording on the fly.

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-08-17 Daniel adds "[defaultconstructible]" to table title. 408 depends upon this issue. ]

[ 2009-08-18 Alisdair adds: ]

Looking at the proposed table in this issue, it really needs two rows:

Table 33: DefaultConstructible requirements [defaultconstructible]
expressionpost-condition
T t;t is default-initialized.
T{}Object of type T is value-initialized.

Note I am using the new brace-initialization syntax that is unambiguous in all use cases (no most vexing parse.)

[ 2009-10-03 Daniel adds: ]

The suggested definition T{} describing it as value-initialization is wrong, because it belongs to list-initialization which would - as the current rules are - always prefer a initializer-list constructor over a default-constructor. I don't consider this as an appropriate definition of DefaultConstructible. My primary suggestion is to ask core, whether the special case T{} (which also easily leads to ambiguity situations for more than one initializer-list in a class) would always prefer a default-constructor - if any - before considering an initializer-list constructor or to provide another syntax form to prefer value-initialization over list-initialization. If that fails I would fall back to suggest to use the expression T() instead of T{} with all it's disadvantages for the meaning of the expression

T t();

[ 2009-10 Santa Cruz: ]

Leave Open. Core is looking to make Alisdair's proposed resolution correct.

[ 2010-01-24 At Alisdiar's request, moved his proposal into the proposed wording seciton. The old wording is preserved here: ]

In section 20.2.1 [utility.arg.requirements], before table 33, add the following table:

Table 33: DefaultConstructible requirements [defaultconstructible]

expression

post-condition

T t;
T()

T is default constructed.

[ 2010-02-04: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Rationale:

[ San Francisco: ]

We believe concepts will solve this problem (N2774).

[ Rationale is obsolete. ]

Proposed resolution:

In section 20.2.1 [utility.arg.requirements], before table 33, add the following table:

Table 33: DefaultConstructible requirements [defaultconstructible]
expressionpost-condition
T t;Object t is default-initialized.
T u{};Object u is value-initialized.
T()
T{}
A temporary object of type T is value-initialized.

726. Missing regex_replace() overloads

Section: 28.11.4 [re.alg.replace] Status: Tentatively NAD Submitter: Stephan T. Lavavej Opened: 2007-09-22 Last modified: 2010-01-27

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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 28.11.4 [re.alg.replace]; wording is needed for the synopsis as well. We also note that this has impact on match_results::format, which may require further overloads.

[ 2009-07 Frankfurt: ]

Daniel to tweak for us.

[ 2009-07-25 Daniel tweaks both this issue and 727. ]

This is solved by the proposed resolution of 727.

[ 2009-10 Santa Cruz: ]

Leave Open. Though we believe this is solved by the proposed resolution to 727.

[ 2010-01-27 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]

Rationale:

Solved by 727.

Proposed resolution:

Provide additional overloads for regex_replace(): one additional overload of the iterator-based form (taking const charT* fmt), and three additional overloads of the convenience form (one taking const charT* str, another taking const charT* fmt, and the third taking both const charT* str and const charT* fmt). 28.11.4 [re.alg.replace]:

template <class OutputIterator, class BidirectionalIterator, 
    class traits, class charT> 
  OutputIterator 
  regex_replace(OutputIterator out, 
                BidirectionalIterator first, BidirectionalIterator last, 
                const basic_regex<charT, traits>& e, 
                const basic_string<charT>& fmt, 
                regex_constants::match_flag_type flags = 
                  regex_constants::match_default);

template <class OutputIterator, class BidirectionalIterator, 
    class traits, class charT> 
  OutputIterator 
  regex_replace(OutputIterator out, 
                BidirectionalIterator first, BidirectionalIterator last, 
                const basic_regex<charT, traits>& e, 
                const charT* fmt, 
                regex_constants::match_flag_type flags = 
                  regex_constants::match_default);

...

template <class traits, class charT> 
  basic_string<charT> 
  regex_replace(const basic_string<charT>& s, 
                const basic_regex<charT, traits>& e, 
                const basic_string<charT>& fmt, 
                regex_constants::match_flag_type flags = 
                  regex_constants::match_default);

template <class traits, class charT> 
  basic_string<charT> 
  regex_replace(const basic_string<charT>& s, 
                const basic_regex<charT, traits>& e, 
                const charT* fmt, 
                regex_constants::match_flag_type flags = 
                  regex_constants::match_default);

template <class traits, class charT> 
  basic_string<charT> 
  regex_replace(const charT* s, 
                const basic_regex<charT, traits>& e, 
                const basic_string<charT>& fmt, 
                regex_constants::match_flag_type flags = 
                  regex_constants::match_default);

template <class traits, class charT> 
  basic_string<charT> 
  regex_replace(const charT* s, 
                const basic_regex<charT, traits>& e, 
                const charT* fmt, 
                regex_constants::match_flag_type flags = 
                  regex_constants::match_default);

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

Section: 28.11.4 [re.alg.replace] Status: Tentatively Ready Submitter: Stephan T. Lavavej Opened: 2007-09-22 Last modified: 2010-01-27

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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.

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.

[ 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.

[ 2009-07 Frankfurt: ]

Howard to ask Stephan Lavavej to provide wording.

[ 2009-07-17 Stephan provided wording. ]

[ 2009-07-25 Daniel tweaks both this issue and 726. ]

One relevant part of the proposed resolution below suggests to add a new overload of the format member function in the match_results class template that accepts two character pointers defining the begin and end of a format range. A more general approach could have proposed a pair of iterators instead, but the used pair of char pointers reflects existing practice. If the committee strongly favors an iterator-based signature, this could be simply changed. I think that the minimum requirement should be a BidirectionalIterator, but current implementations take advantage (at least partially) of the RandomAccessIterator sub interface of the char pointers.

Suggested Resolution:

[Moved into the proposed resloution]

[ 2009-07-30 Stephan agrees with Daniel's wording. Howard places Daniel's wording in the Proposed Resolution. ]

[ 2009-10 Santa Cruz: ]

Move to Review. Chair is anxious to move this to Ready in Pittsburgh.

[ 2010-01-27 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

  1. Change 28.4 [re.syn] as indicated:

    // 28.11.4, function template regex_replace:
    template <class OutputIterator, class BidirectionalIterator,
              class traits, class charT, class ST, class SA>
      OutputIterator
      regex_replace(OutputIterator out,
                    BidirectionalIterator first, BidirectionalIterator last,
                    const basic_regex<charT, traits>& e,
                    const basic_string<charT, ST, SA>& 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, class ST, class SA,
              class FST, class FSA>
      basic_string<charT, ST, SA>
      regex_replace(const basic_string<charT, ST, SA>& s,
                    const basic_regex<charT, traits>& e,
                    const basic_string<charT, FST, FSA>& fmt,
                    regex_constants::match_flag_type flags =
                      regex_constants::match_default);
    
    
    template <class traits, class charT, class ST, class SA>
      basic_string<charT, ST, SA>
      regex_replace(const basic_string<charT, ST, SA>& s,
                    const basic_regex<charT, traits>& e,
                    const charT* fmt,
                    regex_constants::match_flag_type flags =
                      regex_constants::match_default);
    
    
    
    template <class traits, class charT, class ST, class SA>
      basic_string<charT>
      regex_replace(const charT* s,
                    const basic_regex<charT, traits>& e,
                    const basic_string<charT, ST, SA>& 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);
    
    
  2. Change 28.10 [re.results]/3, class template match_results as indicated:

    
    template <class OutputIter>
      OutputIter
      format(OutputIter out,
             const char_type* fmt_first, const char_type* fmt_last,
             regex_constants::match_flag_type flags =
               regex_constants::format_default) const;
    
    
    template <class OutputIter, class ST, class SA>
      OutputIter
      format(OutputIter out,
             const string_typebasic_string<char_type, ST, SA>& fmt,
             regex_constants::match_flag_type flags =
               regex_constants::format_default) const;
    
    template <class ST, class SA>
      string_typebasic_string<char_type, ST, SA>
      format(const string_typebasic_string<char_type, ST, SA>& fmt,
             regex_constants::match_flag_type flags =
               regex_constants::format_default) const;
    
    
    string_type
    format(const char_type* fmt,
           regex_constants::match_flag_type flags =
             regex_constants::format_default) const;
    
    
  3. Insert at the very beginning of 28.10.4 [re.results.form] the following:

    
    template <class OutputIter>
      OutputIter
      format(OutputIter out,
             const char_type* fmt_first, const char_type* fmt_last,
             regex_constants::match_flag_type flags =
               regex_constants::format_default) const;
    
    

    1 Requires: The type OutputIter shall satisfy the requirements for an Output Iterator (24.2.2 [output.iterators]).

    2 Effects: Copies the character sequence [fmt_first,fmt_last) to OutputIter out. Replaces each format specifier or escape sequence in the copied range with either the character(s) it represents or the sequence of characters within *this to which it refers. The bitmasks specified in flags determine which format specifiers and escape sequences are recognized.

    3 Returns: out.

  4. Change 28.10.4 [re.results.form], before p. 1 until p. 3 as indicated:

    template <class OutputIter, class ST, class SA>
      OutputIter
      format(OutputIter out,
             const string_typebasic_string<char_type, ST, SA>& fmt,
             regex_constants::match_flag_type flags =
               regex_constants::format_default) const;
    

    1 Requires: The type OutputIter shall satisfy the requirements for an Output Iterator (24.2.3).

    2 Effects: Copies the character sequence [fmt.begin(),fmt.end()) to OutputIter out. Replaces each format specifier or escape sequence in fmt with either the character(s) it represents or the sequence of characters within *this to which it refers. The bitmasks specified in flags determines what format specifiers and escape sequences are recognized Equivalent to return format(out, fmt.data(), fmt.data() + fmt.size(), flags).

    3 Returns: out.

  5. Change 28.10.4 [re.results.form], before p. 4 until p. 4 as indicated:

    template <class ST, class SA>
      string_typebasic_string<char_type, ST, SA>
      format(const string_typebasic_string<char_type, ST, SA>& fmt,
             regex_constants::match_flag_type flags =
               regex_constants::format_default) const;
    

    Effects: Returns a copy of the string fmt. Replaces each format specifier or escape sequence in fmt with either the character(s) it represents or the sequence of characters within *this to which it refers. The bitmasks specified in flags determines what format specifiers and escape sequences are recognized. Constructs an empty string result of type basic_string<char_type, ST, SA>, and calls format(back_inserter(result), fmt, flags).

    Returns: result

  6. At the end of 28.10.4 [re.results.form] insert as indicated:

    
    string_type
      format(const char_type* fmt,
             regex_constants::match_flag_type flags =
               regex_constants::format_default) const;
    

    Effects: Constructs an empty string result of type string_type, and calls format(back_inserter(result), fmt, fmt + char_traits<char_type>::length(fmt), flags).

    Returns: result

  7. Change 28.11.4 [re.alg.replace] before p. 1 as indicated:

    template <class OutputIterator, class BidirectionalIterator,
              class traits, class charT, class ST, class SA>
      OutputIterator
      regex_replace(OutputIterator out,
                    BidirectionalIterator first, BidirectionalIterator last,
                    const basic_regex<charT, traits>& e,
                    const basic_string<charT, ST, SA>& 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);
    
    Effects: [..]. If any matches are found then, for each such match, if !(flags & regex_constants::format_no_copy) calls std::copy(m.prefix().first, m.prefix().second, out), and then calls m.format(out, fmt, flags) for the first form of the function and m.format(out, fmt, fmt + char_traits<charT>::length(fmt), flags) for the second form. [..].
  8. Change 28.11.4 [re.alg.replace] before p. 3 as indicated:

    template <class traits, class charT, class ST, class SA,
              class FST, class FSA>
      basic_string<charT, ST, SA>
      regex_replace(const basic_string<charT, ST, SA>& s,
                    const basic_regex<charT, traits>& e,
                    const basic_string<charT, FST, FSA>& fmt,
                    regex_constants::match_flag_type flags =
                      regex_constants::match_default);
    
    
    template <class traits, class charT, class ST, class SA>
      basic_string<charT, ST, SA>
      regex_replace(const basic_string<charT, ST, SA>& s,
                    const basic_regex<charT, traits>& e,
                    const charT* fmt,
                    regex_constants::match_flag_type flags =
                      regex_constants::match_default);
    
    Effects: Constructs an empty string result of type basic_string<charT, ST, SA>, calls regex_replace(back_inserter(result), s.begin(), s.end(), e, fmt, flags), and then returns result.
  9. At the end of 28.11.4 [re.alg.replace] add the following new prototype description:

    
    template <class traits, class charT, class ST, class SA>
      basic_string<charT>
      regex_replace(const charT* s,
                    const basic_regex<charT, traits>& e,
                    const basic_string<charT, ST, SA>& 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);
    
    Effects: Constructs an empty string result of type basic_string<charT>, calls regex_replace(back_inserter(result), s, s + char_traits<charT>::length(s), e, fmt, flags), and then returns result.

742. Enabling swap for proxy iterators

Section: 20.2.1 [utility.arg.requirements] Status: Open Submitter: Howard Hinnant Opened: 2007-10-10 Last modified: 2009-11-08

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Discussion:

This issue was split from 672. 672 now just deals with changing the requirements of T in the Swappable requirement from CopyConstructible and CopyAssignable to MoveConstructible and MoveAssignable.

This issue seeks to widen the Swappable requirement to support proxy iterators. Here is example code:

namespace Mine {

template <class T>
struct proxy {...};

template <class T>
struct proxied_iterator
{
   typedef T value_type;
   typedef proxy<T> reference;
   reference operator*() const;
   ...
};

struct A
{
   // heavy type, has an optimized swap, maybe isn't even copyable or movable, just swappable
   void swap(A&);
   ...
};

void swap(A&, A&);
void swap(proxy<A>, A&);
void swap(A&, proxy<A>);
void swap(proxy<A>, proxy<A>);

}  // Mine

...

Mine::proxied_iterator<Mine::A> i(...)
Mine::A a;
swap(*i1, a);

The key point to note in the above code is that in the call to swap, *i1 and a are different types (currently types can only be Swappable with the same type). A secondary point is that to support proxies, one must be able to pass rvalues to swap. But note that I am not stating that the general purpose std::swap should accept rvalues! Only that overloaded swaps, as in the example above, be allowed to take rvalues.

That is, no standard library code needs to change. We simply need to have a more flexible definition of Swappable.

[ Bellevue: ]

While we believe Concepts work will define a swappable concept, we should still resolve this issue if possible to give guidance to the Concepts work.

Would an ambiguous swap function in two namespaces found by ADL break this wording? Suggest that the phrase "valid expression" means such a pair of types would still not be swappable.

Motivation is proxy-iterators, but facility is considerably more general. Are we happy going so far?

We think this wording is probably correct and probably an improvement on what's there in the WP. On the other hand, what's already there in the WP is awfully complicated. Why do we need the two bullet points? They're too implementation-centric. They don't add anything to the semantics of what swap() means, which is there in the post-condition. What's wrong with saying that types are swappable if you can call swap() and it satisfies the semantics of swapping?

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

Leave as Open. Dave to provide wording.

[ 2009-11-08 Howard adds: ]

Updated wording to sync with N3000. Also this issue is very closely related to 594.

Proposed resolution:

Change 20.2.1 [utility.arg.requirements]:

-1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables, T and V are is a types to be supplied by a C++ program instantiating a template; a, b, and c are values of type const T; s and t are modifiable lvalues of type T; u is a value of type (possibly const) T; and rv is a non-const rvalue of type T; w is a value of type T; and v is a value of type V.

Table 37: Swappable requirements [swappable]
expressionReturn typePost-condition
swap(sw,tv)void tw has the value originally held by uv, and uv has the value originally held by tw

The Swappable requirement is met by satisfying one or more of the following conditions:

  • T is Swappable if T and V are the same type and T satisfies the MoveConstructible requirements (Table 33) and the MoveAssignable requirements (Table 35);
  • T is Swappable with V if a namespace scope function named swap exists in the same namespace as the definition of T or V, such that the expression swap(sw,t v) is valid and has the semantics described in this table.
  • T is Swappable if T is an array type whose element type is Swappable.

Rationale:

[ post San Francisco: ]

Solved by N2758.

774. Member swap undefined for most containers

Section: 23 [containers] Status: Open Submitter: Alisdair Meredith Opened: 2008-01-14 Last modified: 2009-10-31

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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.

[ 2009-07 Frankfurt: ]

Daniel to provide wording. N2590 is no longer applicable.

[ 2009-07-28 Daniel provided wording. ]

  1. It assumes that the proposed resolution for 883 is applied, which breaks the circularity of definition between member swap and free swap.
  2. It uses the notation of the pre-concept allocator trait allocator_propagation_map, which might be renamed after the next refactoring phase of generalized allocators.
  3. It requires that compare objects, key equal functions and hash functions in containers are swapped via unqualified free swap according to 594.

[ 2009-09-30 Daniel adds: ]

The outcome of this issue should be considered with the outcome of 1198 both in style and in content (e.g. bullet 9 suggests to define the semantic of void priority_queue::swap(priority_queue&) in terms of the member swap of the container).

[ 2009-10 Santa Cruz: ]

Looked at, but took no action on as it overlaps too much with N2982. Waiting for a new draft WP.

[ 2009-10 Santa Cruz: ]

Leave as open. Pablo to provide wording.

[ 2009-10-26 Pablo updated wording. Here is the wording he replaced: ]

  1. Add a new Throws clause just after X [allocator.propagation.map]/5:

    static void swap(Alloc& a, Alloc& b);
    

    Effects: [..]

    Throws: Nothing.

    [ This exception requirement is added, such that it's combination with the general container requirements of N2723 [container.requirements.general]/9 make it unambiguously clear that the following descriptions of "swaps the allocators" have the following meaning: (a) This swap is done by calling allocator_propagation_map<allocator_type>::swap and (b) This allocator swap does never propagate an exception ]

  2. Change 23.2.4.1 [associative.reqmts.except]/3 as indicated:

    For associative containers, no swap function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator swap of the container's Pred objects (if any).
  3. Change 23.2.5.1 [unord.req.except]/3 as indicated:

    For unordered associative containers, no swap function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator swap of the container's Hash or Pred objects, respectively (if any).
  4. Insert a new paragraph just after 23.3 [sequences]/1:

    In addition to being available via inclusion of the <algorithm> header, the swap function templates in 25.3.3 [alg.swap] are also available when the header <queue> is included.

    [ There is a new issue in process that will suggest a minimum header for swap and move. If this one is provided, this text can be removed and the header dependency should be added to <queue> ]

  5. Add one further clause at the end of 23.3.1.2 [array.special]:

    [This part is added, because otherwise array::swap would otherwise contradict the general contract of 23.2.1 [container.requirements.general] p. 10 b. 5]

    Throws: Nothing, unless one of the element-wise swap calls throws an exception.
    1. In 23.3.2 [deque], class template deque synopsis change as indicated:

      void swap(deque<T,Alloc>&);
      
    2. At the end of 23.3.2.3 [deque.modifiers] add as indicated:

      void swap(deque& x);
      

      Effects: Exchanges the contents and swaps the allocators of *this with that of x.

      Complexity: Constant time.

    1. In 23.3.3 [forwardlist], class template forward_list synposis change as indicated:

      void swap(forward_list<T,Allocator>&);
      
    2. At the end of 23.3.3.4 [forwardlist.modifiers] add as indicated:

      void swap(forward_list& x);
      

      Effects: Exchanges the contents and swaps the allocators of *this with that of x.

      Complexity: Constant time.

    1. In 23.3.4 [list], class template list synopsis change as indicated:

      void swap(list<T,Allocator>&);
      
    2. At the end of 23.3.4.3 [list.modifiers] add as indicated:

      void swap(list& x);
      

      Effects: Exchanges the contents and swaps the allocators of *this with that of x.

      Complexity: Constant time.

  6. At the end of 23.3.5.2.2 [priqueue.members] add a new prototype description:

    void swap(priority_queue& q);
    

    Requires: Compare shall satisfy the Swappable requirements ( [swappable]).

    [ This requirement is added to ensure that even a user defined swap which is found by ADL for Compare satisfies the Swappable requirements ]

    Effects: this->c.swap(q.c); swap(this->comp, q.comp);

    Throws: What and if c.swap(q.c) and swap(comp, q.comp) throws.

    [ This part is added, because otherwise priority_queue::swap would otherwise contradict the general contract of 23.2.1 [container.requirements.general] p. 10 b. 5 ]

    1. In 23.3.6 [vector], class template vector synopsis change as indicated:

      void swap(vector<T,Allocator>&);
      
    2. Change 23.3.6.2 [vector.capacity]/8 as indicated:

      void swap(vector<T,Allocator>& x);
      
      Effects: Exchanges the contents and capacity() and swaps the allocators of *this with that of x.
  7. Insert a new paragraph just before 23.4 [associative]/1:

    In addition to being available via inclusion of the <algorithm> header, the swap function templates in 25.3.3 [alg.swap] are also available when any of the headers <map> or <set> are included.
    1. In 23.4.1 [map], class template map synopsis change as indicated:

      void swap(map<Key,T,Compare,Allocator>&);
      
    2. At the end of 23.4.1.3 [map.modifiers] add as indicated:

      void swap(map& x);
      

      Requires: Compare shall satisfy the Swappable requirements ( [swappable]).

      [ This requirement is added to ensure that even a user defined swap which is found by ADL for Compare satisfies the Swappable requirements ]

      Effects: Exchanges the contents and swaps the allocators of *this with that of x, followed by an unqualified swap of the comparison objects of *this and x.

      Complexity: Constant time

    1. In 23.4.2 [multimap], class template multimap synopsis change as indicated:

      void swap(multimap<Key,T,Compare,Allocator>&);
      
    2. At the end of 23.4.2.2 [multimap.modifiers] add as indicated:

      void swap(multimap& x);
      

      Requires: Compare shall satisfy the Swappable requirements ( [swappable]).

      Effects: Exchanges the contents and swaps the allocators of *this with that of x, followed by an unqualified swap of the comparison objects of *this and x.

      Complexity: Constant time

    1. In 23.4.3 [set], class template set synopsis change as indicated:

      void swap(set<Key,Compare,Allocator>&);
      
    2. After section 23.4.3.1 [set.cons] add a new section set modifiers [set.modifiers] and add the following paragraphs:

      void swap(set& x);
      

      Requires: Compare shall satisfy the Swappable requirements ( [swappable]).

      Effects: Exchanges the contents and swaps the allocators of *this with that of x, followed by an unqualified swap of the comparison objects of *this and x.

      Complexity: Constant time

    1. In 23.4.4 [multiset], class template multiset synosis, change as indicated:

      void swap(multiset<Key,Compare,Allocator>&);
      
    2. After section 23.4.4.1 [multiset.cons] add a new section multiset modifiers [multiset.modifiers] and add the following paragraphs:

      void swap(multiset& x);
      

      Requires: Compare shall satisfy the Swappable requirements ( [swappable]).

      Effects: Exchanges the contents and swaps the allocators of *this with that of x, followed by an unqualified swap of the comparison objects of *this and x.

      Complexity: Constant time

  8. Insert a new paragraph just before 23.5 [unord]/1:

    In addition to being available via inclusion of the <algorithm> header, the swap function templates in 25.3.3 [alg.swap] are also available when any of the headers <unordered_map> or <unordered_set> are included.
  9. After section 23.5.1.2 [unord.map.elem] add a new section unordered_map modifiers [unord.map.modifiers] and add the following paragraphs:

    void swap(unordered_map& x);
    

    Requires: Hash and Pred shall satisfy the Swappable requirements ( [swappable]).

    [ This requirement is added to ensure that even a user defined swap which is found by ADL for Hash and Pred satisfies the Swappable requirements ]

    Effects: Exchanges the contents and hash policy and swaps the allocators of *this with that of x, followed by an unqualified swap of the Pred objects and an unqualified swap of the Hash objects of *this and x.

    Complexity: Constant time

  10. After section 23.5.2.1 [unord.multimap.cnstr] add a new section unordered_multimap modifiers [unord.multimap.modifiers] and add the following paragraphs:

    void swap(unordered_multimap& x);
    

    Requires: Hash and Pred shall satisfy the Swappable requirements ( [swappable]).

    Effects: Exchanges the contents and hash policy and swaps the allocators of *this with that of x, followed by an unqualified swap of the Pred objects and an unqualified swap of the Hash objects of *this and x

    Complexity: Constant time

  11. After section 23.5.3.1 [unord.set.cnstr] add a new section unordered_set modifiers [unord.set.modifiers] and add the following paragraphs:

    void swap(unordered_set& x);
    

    Requires: Hash and Pred shall satisfy the Swappable requirements ( [swappable]).

    Effects: Exchanges the contents and hash policy and swaps the allocators of *this with that of x, followed by an unqualified swap of the Pred objects and an unqualified swap of the Hash objects of *this and x

    Complexity: Constant time

  12. After section 23.5.4.1 [unord.multiset.cnstr] add a new section unordered_multiset modifiers [unord.multiset.modifiers] and add the following paragraphs:

    void swap(unordered_multiset& x);
    

    Requires: Hash and Pred shall satisfy the Swappable requirements ( [swappable]).

    Effects: Exchanges the contents and hash policy and swaps the allocators of *this with that of x, followed by an unqualified swap of the Pred objects and an unqualified swap of the Hash objects of *this and x

    Complexity: Constant time

[ 2009-10-30 Pablo and Daniel updated wording. ]

Proposed resolution:

[ This resolution is based on the September 2009 WP, N2960, except that it assumes that N2982 and issues 883 and 1232 have already been applied. Note in particular that Table 91 in N2960 is refered to as Table 90 because N2982 removed the old Table 90. This resolution also addresses issue 431. ]

In 23.2.1 [container.requirements.general], replace the a.swap(b) row in table 90, "container requirements" (was table 91 before the application of N2982 to the WP):

a.swap(b) void     swap(a,b)Exchange the contents of a and b. (Note A)
swap(a,b) void     a.swap(b) (Note A)

Modify the notes immediately following Table 90 in 23.2.1 [container.requirements.general] as follows (The wording below is after the application of N2982 to N2960. The editor might also want to combine Notes A and B into one.):

Notes: the algorithms swap(), equal() and lexicographical_compare() are defined in Clause 25. Those entries marked "(Note A)" or "(Note B)" should have linear complexity for array and constant complexity for all other standard containers.

In 23.2.1 [container.requirements.general], after paragraph 9, add:

The expression a.swap(b), for containers a and b of a standard container type other than array, exchanges the values of a and b without invoking any move, copy, or swap operations on the individual container elements. Any Compare, Pred, or Hash function objects belonging to a and b shall satisfy the Swappable requirements and are exchanged by unqualified calls to non-member swap. If allocator_traits<allocator_type>::propagate_on_container_swap::value == true, then the allocators of a and b are also exchanged using an unqualified call to non-member swap. Otherwise, the behavior is undefined unless a.get_allocator() == b.get_allocator(). Each iterator refering to an element in one container before the swap shall refer to the same element in the other container after the swap. It is unspecified whether an iterator with value a.end() before the swap will have value b.end() after the swap. In addition to being available via inclusion of the <utility> header, the swap function template in 25.3.3 [alg.swap] is also available within the definition of every standard container's swap function.

[ Note to the editor: Paragraph 2 starts with a sentence fragment, clearly from an editing or source-control error. ]

Modify 23.2.4.1 [associative.reqmts.except] as follows:

23.2.4.1 Exception safety guarantees 23.2.4.1 [associative.reqmts.except]

For associative containers, no clear() function throws an exception. erase(k) does not throw an exception unless that exception is thrown by the container's PredCompare object (if any).

For associative containers, if an exception is thrown by any operation from within an insert() function inserting a single element, the insert() function has no effect.

For associative containers, no swap function throws an exception unless that exception is thrown by the copy constructor or copy assignment operatorswap of the container's PredCompare object (if any).

Modify 23.2.5.1 [unord.req.except], paragraph 3 as follows:

For unordered associative containers, no swap function throws an exception unless that exception is thrown by the copy constructor or copy assignment operatorswap of the container's Hash or Pred object (if any).

Modify section 23.3.1.2 [array.special]:

array specialized algorithms 23.3.1.2 [array.special]

template <class T, size_t N> void swap(array<T,N>& x,array<T,N>& y);

Effects: swap_ranges(x.begin(), x.end(), y.begin() );x.swap(y);

Add a new section after 23.3.1.5 [array.fill] (Note to the editor: array::fill make use of a concept requirement that must be removed or changed to text.):

array::swap [array.swap]

void swap(array& y);

Effects: swap_ranges(this->begin(), this->end(), y.begin() );

Throws: Nothing unless one of the element-wise swap calls throws an exception.

[Note: Unlike other containers' swap functions, array::swap takes linear, not constant, time, may exit via an exception, and does not cause iterators to become associated with the other container. — end note]

Insert a new paragraph just after 23.3.5 [container.adaptors]/1:

For container adaptors, no swap function throws an exception unless that exception is thrown by the swap of the adaptor's Container or Compare object (if any).


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

Section: 25.4.4 [alg.merge] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2008-01-25 Last modified: 2010-02-10

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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 25.4.4 [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.

[ 2009-07 Frankfurt: ]

Move to Ready.

[ 2009-08-23 Daniel reopens: ]

The proposed wording must be rephrased, because the part

for every iterator i in [result,last) other than result, the condition *i < *(i - 1) or, respectively, comp(*i, *(i - 1)) will be false"

isn't meaningful, because the range [result,last) is that of a pure OutputIterator, which is not readable in general.

[Howard: Proposed wording updated by Daniel, status moved from Ready to Review.]

[ 2009-10 Santa Cruz: ]

Matt has some different words to propose. Those words have been moved into the proposed wording section, and the original proposed wording now appears here:

In 25.4.4 [alg.merge] replace p.1+ 2:

Effects: MergesCopies all the elements of the two sorted ranges [first1,last1) and [first2,last2) into the range [result,result + (last1 - first1) + (last2 - first2)) , such that resulting range will be sorted in non-decreasing order; that is for every pair of iterators i and j of either input ranges, where *i was copied to the output range before *j was copied to the output range, the condition *j < *i or, respectively, comp(*j, *i) 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.

[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 25.4.4 [alg.merge] 1 and 2:

1 Effects: Merges two sorted ranges [first1,last1) and [first2,last2) into the range [result, result + (last1 - first1) + (last2 - first2)).

Effects: Copies all the elements of the two ranges [first1,last1) and [first2,last2) into the range [result, result_last), where result_last is result + (last1 - first1) + (last2 - first2), such that the resulting range satisfies is_sorted(result, result_last) or is_sorted(result, result_last, comp), respectively.

2 Requires: The ranges [first1,last1) and [first2,last2) shall be sorted with respect to operator< or comp. 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.

Change 25.4.4 [alg.merge]/6+7 as indicated [This ensures harmonization between inplace_merge and merge]

6 Effects: Merges two sorted consecutive ranges [first,middle) and [middle,last), putting the result of the merge into the range [first,last). The resulting range will be in non-decreasing order; that is, for every iterator i in [first,last) other than first, the condition *i < *(i - 1) or, respectively, comp(*i, *(i - 1)) will be false.

7 Requires: The ranges [first,middle) and [middle,last) shall be sorted with respect to operator< or comp. The type of *first shall satisfy the Swappable requirements (37), the MoveConstructible requirements (Table 33), and the the MoveAssignable requirements (Table 35).


801. tuple and pair trivial members

Section: 20.5 [tuple] Status: Open Submitter: Lawrence Crowl Opened: 2008-02-18 Last modified: 2009-10-23

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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.

[ 2009-07 Frankfurt: ]

Wait for dust to settle from fixing exception safety problem with rvalue refs.

[ 2009-07-20 Alisdair adds: ]

Basically, this issue is what should we do with the default constructor for pairs and tuples of trivial types. The motivation of the issue was to force static initialization rather than dynamic initialization, and was rejected in the case of pair as it would change the meaning of existing programs. The advice was "do the best we can" for tuple without changing existing meaning.

Frankfurt seems to simply wait and see the resolution on no-throw move constructors, which (I believe) is only tangentially related to this issue, but as good as any to defer until Santa Cruz.

Looking again now, I think constant (static) initialization for pair can be salvaged by making the default construct constexpr. I have a clarification from Core that this is intended to work, even if the constructor is not trivial/constexpr, so long as no temporaries are implied in the process (even if elided).

[ 2009-10 Santa Cruz: ]

Leave as open. Alisdair to provide wording.

Proposed resolution:


811. pair of pointers no longer works with literal 0

Section: 20.3.4 [pairs] Status: Tentatively Ready Submitter: Doug Gregor Opened: 2008-03-14 Last modified: 2010-02-09

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Discussion:

#include <utility>

int main()
{
   std::pair<char *, char *> p (0,0);
}

I just got a bug report about that, because it's valid C++03, but not C++0x. The important realization, for me, is that the emplace proposal---which made push_back variadic, causing the push_back(0) issue---didn't cause this break in backward compatibility. The break actually happened when we added this pair constructor as part of adding rvalue references into the language, long before variadic templates or emplace came along:

template<class U, class V> pair(U&& x, V&& y);

Now, concepts will address this issue by constraining that pair constructor to only U's and V's that can properly construct "first" and "second", e.g. (from N2322):

template<class U , class V >
requires Constructible<T1, U&&> && Constructible<T2, V&&>
pair(U&& x , V&& y );

[ San Francisco: ]

Suggested to resolve using pass-by-value for that case.

Side question: Should pair interoperate with tuples? Can construct a tuple of a pair, but not a pair from a two-element tuple.

Related to 885.

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

Leave as open. Howard to provide wording.

[ 2010-02-06 Howard provided wording. ]

[ 2010-02-09 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Rationale:

[ San Francisco: ]

Solved by N2770.

[ The rationale is obsolete. ]

Proposed resolution:

Add a paragraph to 20.3.4 [pairs]:

template<class U, class V> pair(U&& x, V&& y);

6 Effects: The constructor initializes first with std::forward<U>(x) and second with std::forward<V>(y).

Remarks: U shall be implicitly convertible to first_type and V shall be implicitly convertible to second_type, else this constructor shall not participate in overload resolution.


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

Section: 20.7.15.2.4 [func.wrap.func.inv] Status: Tentatively NAD Editorial Submitter: Alisdair Meredith Opened: 2008-03-16 Last modified: 2010-02-12

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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?

[ 2009-07 Frankfurt: ]

Leave this open and wait until concepts are removed from the Working Draft so that we know how to write the proposed resolution in terms of diffs to otherwise stable text.

[ 2009-10 Santa Cruz: ]

Leave as open. Howard to provide wording. Howard welcomes any help.

[ 2009-12-12 Jonathan Wakely adds: ]

20.7.15.2 [func.wrap.func] says

2 A function object f of type F is Callable for argument types T1, T2, ..., TN in ArgTypes and a return type R, if, given lvalues t1, t2, ..., tN of types T1, T2, ..., TN, respectively, INVOKE (f, t1, t2, ..., tN) is well formed (20.7.2) and, if R is not void, convertible to R.

N.B. lvalues, which means you can't use function<R(T&&)> or function<R(unique_ptr<T>)>

I recently implemented rvalue arguments in GCC's std::function, all that was needed was to use std::forward<ArgTypes> in a few places. The example in issue 815 works.

I think 815 could be resolved by removing the requirement that the target function be callable with lvalues. Saying ArgTypes need to be CopyConstructible is wrong, and IMHO saying MoveConstructible is unnecessary, since the by-value signature implies that already, but if it is needed it should only be on operator(), not the whole class (you could in theory instantiate std::function<R(noncopyable)> as long as you don't invoke the call operator.)

I think defining invocation in terms of INVOKE already implies perfect forwarding, so we don't need to say explicitly that std::forward should be used (N.B. the types that are forwarded are those in ArgTypes, which can differ from the actual parameter types of the target function. The actual parameter types have gone via type erasure, but that's not a problem - IMHO forwarding the arguments as ArgTypes is the right thing to do anyway.)

Is it sufficient to simply replace "lvalues" with "values"? or do we need to say something like "lvalues when Ti is an lvalue-reference and rvalues otherwise"? I prefer the former, so I propose the following resolution for 815:

Edit 20.7.15.2 [func.wrap.func] paragraph 2:

2 A function object f of type F is Callable for argument types T1, T2, ..., TN in ArgTypes and a return type R, if, given lvalues t1, t2, ..., tN of types T1, T2, ..., TN, respectively, INVOKE (f, t1, t2, ..., tN) is well formed (20.7.2) and, if R is not void, convertible to R.

[ 2009-12-12 Daniel adds: ]

I don't like the reduction to "values" and prefer the alternative solution suggested using "lvalues when Ti is an lvalue-reference and rvalues otherwise". The reason why I dislike the shorter version is based on different usages of "values" as part of defining the semantics of requirement tables via expressions. E.g. 20.2.1 [utility.arg.requirements]/1 says "a, b, and c are values of type const T;" or similar in 23.2.1 [container.requirements.general]/4 or /14 etc. My current reading of all these parts is that both rvalues and lvalues are required to be supported, but this interpretation would violate the intention of the suggested fix of #815, if I correctly understand Jonathan's rationale.

[ 2009-12-12 Howard adds: ]

"lvalues when Ti is an lvalue-reference and rvalues otherwise"

doesn't quite work here because the Ti aren't deduced. They are specified by the function type. Ti might be const int& (an lvalue reference) and a valid ti might be 2 (a non-const rvalue). I've taken another stab at the wording using "expressions" and "bindable to".

[ 2010-02-09 Wording updated by Jonathan, Ganesh and Daniel. ]

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Daniel opens to improve wording. ]

[ 2010-02-11 This issue is now addressed by 870. ]

[ 2010-02-12 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. Rationale added below. ]

Rationale:

Addressed by 870.

Proposed resolution:

Edit 20.7.15.2 [func.wrap.func] paragraph 2:

2 A function object f of type F is Callable for argument types T1, T2, ..., TN in ArgTypes and a return type R, if, given lvalues t1, t2, ..., tN of types T1, T2, ..., TN, respectively, the expression INVOKE(f, declval<ArgTypes>()..., Rt1, t2, ..., tN), considered as an unevaluated operand (5 [expr]), is well formed (20.7.2) and, if R is not void, convertible to R.


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

Section: 20.7.11.1.3 [func.bind.bind] Status: Tentatively NAD Editorial Submitter: Stephan T. Lavavej Opened: 2008-02-08 Last modified: 2010-02-11

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Discussion:

Library Issue 527 notes that bind(f, t1, ..., tN) should be nofail when f, t1, ..., tN have nofail copy ctors.

However, no guarantees are provided for the copy ctor of the functor returned by bind(). (It's guaranteed to have a copy ctor, which can throw implementation-defined exceptions: bind() returns a forwarding call wrapper, TR1 3.6.3/2. A forwarding call wrapper is a call wrapper, TR1 3.3/4. Every call wrapper shall be CopyConstructible, TR1 3.3/4. Everything without an exception-specification may throw implementation-defined exceptions unless otherwise specified, C++03 17.4.4.8/3.)

Should the nofail guarantee requested by Library Issue 527 be extended to cover both calling bind() and copying the returned functor?

[ Howard adds: ]

tuple construction should probably have a similar guarantee.

[ San Francisco: ]

Howard to provide wording.

[ Post Summit, Anthony provided wording. ]

[ Batavia (2009-05): ]

Part of all of this issue appears to be rendered moot by the proposed resolution to issue 817 (q.v.). We recommend the issues be considered simultaneously (or possibly even merged) to ensure there is no overlap. Move to Open, and likewise for issue 817.

[ 2009-07 Frankfurt: ]

Related to 817 (see below). Leave Open.

[ 2009-10 Santa Cruz: ]

Move to Ready. Decoupling from issue 817.

[ 2010-02-11 Moved from Ready to Tentatively NAD Editorial, rationale added below. ]

Rationale:

This issue is solved as proposed by 817.

Proposed resolution:

Add a new sentence to the end of paragraphs 2 and 4 of 20.7.11.1.3 [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.

...

-5- Returns: A forwarding call wrapper g with a nested type result_type defined as a synonym for R. The effect of g(u1, u2, ..., uM) shall be INVOKE(f, v1, v2, ..., vN, R), where the values and types of the bound arguments v1, v2, ..., vN are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of F or any of the types in BoundArgs... throw an exception.


817. bind needs to be moved

Section: 20.7.11.1.3 [func.bind.bind] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2008-03-17 Last modified: 2009-11-16

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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. 20.7.11.1.3 [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).

[ 2009-07 Frankfurt: ]

The proposed resolution uses concepts. Leave Open.

[ 2009-10 Santa Cruz: ]

Leave as Open. Howard to provide deconceptified wording.

[ 2009-11-07 Howard updates wording. ]

[ 2009-11-15 Further updates by Peter, Chris and Daniel. ]

[ Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

Change 20.7 [function.objects] p2:

template<class Fn, class... Types BoundArgs>
  unspecified bind(Fn&&, Types BoundArgs&&...);
template<class R, class Fn, class... Types BoundArgs>
  unspecified bind(Fn&&, Types BoundArgs&&...);

Change 20.7.2 [func.require]:

4 Every call wrapper (20.7.1 [func.def]) shall be CopyMoveConstructible. A simple call wrapper is a call wrapper that is CopyConstructible and CopyAssignable and whose copy constructor, move constructor and assignment operator do not throw exceptions. A forwarding call wrapper is a call wrapper that can be called with an argument list. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

template<class... ArgTypesUnBoundsArgs>
R operator()(ArgTypesUnBoundsArgs&&... unbound_args) cv-qual;

end note]

Change 20.7.11.1.3 [func.bind.bind]:

Within this clause:

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

-1- Requires: is_constructible<FD, F>::value shall be true. For each Ti in BoundArgs, is_constructible<TiD, Ti>::value shall be true. F and each Ti in BoundArgs shall be CopyConstructible. INVOKE(fd, w1, w2, ..., wN) (20.7.2 [func.require]) 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 (20.7.2 [func.require]). The effect of g(u1, u2, ..., uM) shall be INVOKE(fd, v1, v2, ..., vN, result_of<FD cv (V1, V2, ..., VN)>::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 FD or of any of the types TiD throws an exception.

-3- Throws: Nothing unless the copy constructionor of Ffd or of one of the values tid types in the BoundArgs... pack expansion throws an exception.

Remarks: The unspecified return type shall satisfy the requirements of MoveConstructible. If all of FD and TiD satisfy the requirements of CopyConstructible then the unspecified return type shall satisfy the requirements of CopyConstructible. [Note: This implies that all of FD and TiD shall be MoveConstructibleend note]

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

-4- Requires: is_constructible<FD, F>::value shall be true. For each Ti in BoundArgs, is_constructible<TiD, Ti>::value shall be true. F and each Ti in BoundArgs shall be CopyConstructible. INVOKE(fd, w1, w2, ..., wN) shall be a valid expression for some values w1, w2, ..., wN, where N == sizeof...(bound_args).

-5- Returns: A forwarding call wrapper g with a nested type result_type defined as a synonym for R. The effect of g(u1, u2, ..., uM) shall be INVOKE(fd, 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 FD or of any of the types TiD throws an exception.

-6- Throws: Nothing unless the copy constructionor of Ffd or of one of the values tid types in the BoundArgs... pack expansion throws an exception.

Remarks: The unspecified return type shall satisfy the requirements of MoveConstructible. If all of FD and TiD satisfy the requirements of CopyConstructible then the unspecified return type shall satisfy the requirements of CopyConstructible. [Note: This implies that all of FD and TiD shall be MoveConstructibleend note]

-7- The values of the bound arguments v1, v2, ..., vN and their corresponding types V1, V2, ..., VN depend on the types TiD derived from of the corresponding argument ti in bound_args of type Ti in BoundArgs in the call to bind and the cv-qualifiers cv of the call wrapper g as follows:


819. rethrow_if_nested

Section: 18.8.6 [except.nested] Status: Open Submitter: Alisdair Meredith Opened: 2008-03-25 Last modified: 2009-11-09

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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.

[ 2009-10 Santa Cruz: ]

Leave as Open. Alisdair to provide wording.

[ 2009-11-09 Alisdair provided wording. ]

Proposed resolution:

Change 18.8.6 [except.nested], p8:

template <class E> void rethrow_if_nested(const E& e);
-8- Effects: Calls e.rethrow_nested() only if the dynamic type of e is publicly and unambiguously derived from nested_exception.

834. Unique_ptr::pointer requirements underspecified

Section: 20.8.14.2 [unique.ptr.single] Status: Open Submitter: Daniel Krügler Opened: 2008-05-14 Last modified: 2010-01-16

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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.

[ 2009-07 Frankfurt: ]

Move to Ready.

[ 2009-10-15 Alisdair pulls from Ready: ]

I hate to pull an issue out of Ready status, but I don't think 834 is fully baked yet.

For reference the proposed resolution is to add the following words:

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

This leaves me with a big question : which operations?

Are all pointer operations required to be nothrow, including operations that have nothing to do with interactions with unique_ptr? This was much simpler with concepts where we could point to operations within a certain concept, and so nail down the interactions.

[ 2009-10-15 Daniel adds: ]

I volunteer to prepare a more fine-grained solution, but I would like to ask for feedback that helps me doing so. If this question is asked early in the meeting I might be able to fix it within the week, but I cannot promise that now.

[ 2009-10 Santa Cruz: ]

Leave in open. Daniel to provide wording as already suggested.

[ 2009-12-22 Daniel provided wording and rationale. ]

Rationale:

The here proposed resolution has considerable overlap with the requirements that are used in the allocator requirements.

This might be a convincing argument to isolate the common subset into one requirement. The reason I did not do that is basically because we might find out that they are either over-constraining or under-constraining at this late point of specification. Note also that as a result of the idea of a general requirement set I added the requirement

A default-initialized object may have a singular value

even though this does not play a relevant role for unique_ptr.

One further characteristics of the resolution is that availability of relational operators of unique_ptr<T, D>::pointer is not part of the basic requirements, which is in sync with the allocator requirements on pointer-like (this means that unique_ptr can hold a void_pointer or const_void_pointer).

Proposed resolution:

  1. Change 20.8.14.2 [unique.ptr.single]/1 as indicated: [The intent is to replace the coupling between T* and the deleter's operator() by a coupling between unique_ptr<T, D>::pointer and this operator()]

    1 - The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function pointer or functor for which, given a value d of type D and a pointer value ptr of type T* unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of deallocating the pointer as appropriate for that deleter. D may also be an lvalue-reference to a deleter.
  2. Change 20.8.14.2 [unique.ptr.single]/3 as indicated:

    3 - If the type remove_reference<D>::type::pointer exists, then unique_ptr<T, D>::pointer shall be a synonym for remove_reference<D>::type::pointer. Otherwise unique_ptr<T, D>::pointer shall be a synonym for T*. The type unique_ptr<T, D>::pointer shall be satisfy the requirements of EqualityComparable, DefaultConstructible, CopyConstructible (Table 34) and, CopyAssignable (Table 36), Swappable, and Destructible (20.2.1 [utility.arg.requirements]). A default-initialized object may have a singular value. A value-initialized object produces the null value of the type. The null value shall be equivalent only to itself. An object of this type can be copy-initialized with a value of type nullptr_t, compared for equality with a value of type nullptr_t, and assigned a value of type nullptr_t. The effect shall be as if a value-initialized object had been used in place of the null pointer constant. An object p of this type can be contextually converted to bool. The effect shall be as if p != nullptr had been evaluated in place of p. No operation on this type which is part of the above mentioned requirements shall exit via an exception.

    [Note: Given an allocator type X (20.2.2 [allocator.requirements]), the types X::pointer, X::const_pointer, X::void_pointer, and X::const_void_pointer may be used as unique_ptr<T, D>::pointerend note]

    In addition to being available via inclusion of the <utility> header, the swap function template in 20.3.2 [utility.swap] is also available within the definition of unique_ptr's swap function.

  3. Change 20.8.14.2.1 [unique.ptr.single.ctor]/2+3 as indicated: [The first change ensures that we explicitly say, how the stored pointer is initialized. This is important for a constexpr function, because this may make a difference for user-defined pointer-like types]

    constexpr unique_ptr();
    

    ...

    2 - Effects: Constructs a unique_ptr which owns nothing, value-initializing the stored pointer.

    3 - Postconditions: get() == 0 nullptr.

  4. Change 20.8.14.2.1 [unique.ptr.single.ctor]/6+7 as indicated: [This is a step-by-fix to ensure consistency to the changes of N2976]

    unique_ptr(pointer p);
    

    ...

    6 - Effects: Constructs a unique_ptr which owns p, initializing the stored pointer with p.

    7 - Postconditions: get() == p. get_deleter() returns a reference to a default constructed value-initialized deleter D.

  5. Insert a new effects clause in 20.8.14.2.1 [unique.ptr.single.ctor] just before p. 14: [The intent is to fix the current lack of specification in which way the stored pointer is initialized]

    unique_ptr(pointer p, implementation-defined see below d1);
    unique_ptr(pointer p, implementation-defined see below d2);
    

    ...

    Effects: Constructs a unique_ptr which owns p, initializing the stored pointer with p and the initializing the deleter as described above.

    14 - Postconditions: get() == p. get_deleter() returns a reference to the internally stored deleter. If D is a reference type then get_deleter() returns a reference to the lvalue d.

  6. Change 20.8.14.2.1 [unique.ptr.single.ctor]/18+22 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]

    unique_ptr(unique_ptr&& u);
    

    [..]

    18 - Postconditions: get() == value u.get() had before the construction and u.get() == nullptr. get_deleter() returns a reference to the internally stored deleter which was constructed from u.get_deleter(). If D is a reference type then get_deleter() and u.get_deleter() both reference the same lvalue deleter.

    template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);
    

    [..]

    22 - Postconditions: get() == value u.get() had before the construction, modulo any required offset adjustments resulting from the cast from unique_ptr<U, E>::pointer to pointer and u.get() == nullptr. get_deleter() returns a reference to the internally stored deleter which was constructed from u.get_deleter().

  7. Change 20.8.14.2.1 [unique.ptr.single.ctor]/20 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950]

    template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);
    
    20 - Requires: If D is not a reference type, construction of the deleter D from an rvalue of type E shall be well formed and shall not throw an exception. If D is a reference type, then E shall be the same type as D (diagnostic required). unique_ptr<U, E>::pointer shall be implicitly convertible to pointer. [Note: These requirements imply that T and U are complete types. — end note]
  8. Change 20.8.14.2.2 [unique.ptr.single.dtor]/2 as indicated:

    ~unique_ptr();
    

    ...

    2 - Effects: If get() == 0 nullptr there are no effects. Otherwise get_deleter()(get()).

  9. Change 20.8.14.2.3 [unique.ptr.single.asgn]/3+8 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]

    unique_ptr& operator=(unique_ptr&& u);
    

    [..]

    3 - Postconditions: This unique_ptr now owns the pointer which u owned, and u no longer owns it, u.get() == nullptr. [Note: If D is a reference type, then the referenced lvalue deleters are move assigned. — end note]

    template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);
    

    [..]

    8 - Postconditions: This unique_ptr now owns the pointer which u owned, and u no longer owns it, u.get() == nullptr.

  10. Change 20.8.14.2.3 [unique.ptr.single.asgn]/6 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950]

    template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);
    

    [..]

    6 - Requires: Assignment of the deleter D from an rvalue D shall not throw an exception. unique_ptr<U, E>::pointer shall be implicitly convertible to pointer. [Note: These requirements imply that T and U are complete types. — end note]

  11. Change 20.8.14.2.3 [unique.ptr.single.asgn] before p. 11 and p. 12 as indicated: [The first change is a simple typo fix]

    unique_ptr& operator=(nullptr_t});
    

    11 - Effects: reset().

    12 - Postcondition: get() == 0 nullptr

  12. Change 20.8.14.2.4 [unique.ptr.single.observers]/1+4+12 as indicated:

    typename add_lvalue_reference<T>::type operator*() const;
    

    1 - Requires: get() != 0 nullptr. The variable definition add_lvalue_reference<T>::type t = *get() shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    [..]

    pointer operator->() const;
    

    4 - Requires: get() != 0 nullptr.

    [..]

    explicit operator bool() const;
    
    12 - Returns: get() != 0nullptr.
  13. Change 20.8.14.2.5 [unique.ptr.single.modifiers]/1 as indicated:

    pointer release();
    
    1 - Postcondition: get() == 0 nullptr.
  14. Change 20.8.14.2.5 [unique.ptr.single.modifiers]/9 as indicated: [The intent is to ensure that potentially user-defined swaps are used. A side-step fix and harmonization with the specification of the the deleter is realized. Please note the additional requirement in bullet 2 of this proposed resolution regarding the availability of the generic swap templates within the member swap function.]

    void swap(unique_ptr& u);
    

    8 - Requires: The deleter D shall be Swappable and shall not throw an exception under swap.

    9 - Effects: The stored pointers of *this and u are exchanged by an unqualified call to non-member swap. The stored deleters are swap'd (unqualified) exchanged by an unqualified call to non-member swap.

  15. Change 20.8.14.3.2 [unique.ptr.runtime.observers]/1 as indicated:

    T& operator[](size_t i) const;
    
    Requires: i < the size of the array to which the stored pointer points. The variable definition T& t = get()[i] shall be well formed, shall have well-defined behavior, and shall not exit via an exception.
  16. Change 20.8.14.3.3 [unique.ptr.runtime.modifiers]/1 as indicated:

    void reset(pointer p = pointer());
    void reset(nullptr_t p);
    
    1 - Effects: If get() == 0 nullptr there are no effects. Otherwise get_deleter()(get()).
  17. Change 20.8.14.4 [unique.ptr.special] as indicated: [We don't add the relational operators to the basic requirement set, therefore we need special handling here]

    template <class T1, class D1, class T2, class D2>
      bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
    

    Requires: The variable definition bool b = x.get() == y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    2 - Returns: x.get() == y.get().

    Throws: nothing.

    template <class T1, class D1, class T2, class D2>
      bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
    

    Requires: The variable definition bool b = x.get() != y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    3 - Returns: x.get() != y.get().

    Throws: nothing.

    template <class T1, class D1, class T2, class D2>
      bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
    

    Requires: The variable definition bool b = x.get() < y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    4 - Returns: x.get() < y.get().

    Throws: nothing.

    template <class T1, class D1, class T2, class D2>
      bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
    

    Requires: The variable definition bool b = x.get() <= y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    5 - Returns: x.get() <= y.get().

    Throws: nothing.

    template <class T1, class D1, class T2, class D2>
      bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
    

    Requires: The variable definition bool b = x.get() > y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    6 - Returns: x.get() > y.get().

    Throws: nothing.

    template <class T1, class D1, class T2, class D2>
      bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
    

    Requires: The variable definition bool b = x.get() >= y.get(); shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    7 - Returns: x.get() >= y.get().

    Throws: nothing.


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

Section: 27.5.4.2 [basic.ios.members] Status: Open Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2009-10-20

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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)?

[ 2009-07 Frankfurt: ]

Daniel volunteered to modify the proposed resolution to address his two questions.

Move back to Open.

[ 2009-07-26 Daniel provided wording. Moved to Review. ]

[ 2009-10-13 Daniel adds: ]

This proposed wording is written to match the outcome of 397.

[ 2009 Santa Cruz: ]

Move to Open. Martin to propose updated wording that will also resolve issue 397 consistently.

Proposed resolution:

[ based on N2960 numbering ]

  1. Just before 27.5.4.2 [basic.ios.members]/2 insert a new paragraph:

    Requires: If (tiestr != 0) is true, tiestr must not be reachable by traversing the linked list of tied stream objects starting from tiestr->tie().
  2. Change 27.7.2.4 [ostream::sentry]/4 as indicated:

    If ((os.flags() & ios_base::unitbuf) && !uncaught_exception()&& os.good()) is true, calls os.flush() os.rdbuf()->pubsync(). If that function returns -1 sets badbit in os.rdstate() without propagating an exception.

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

Section: 22.4.6.1.2 [locale.money.get.virtuals] Status: Ready Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2009-10-21

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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.

[ 2009-07 Frankfurt: ]

There is a noun missing from the proposed resolution. It's not clear that the last sentence would be helpful, even if the word were not missing:

In either case, any required MISSINGWORD followed by all optional whitespace (as recognized by ct.is()) is consumed.

Strike this sentence and move to Review.

[ Howard: done. ]

[ 2009-10 Santa Cruz: ]

Move to Ready.

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. If (str.flags() & str.showbase) is false, ...

854. default_delete converting constructor underspecified

Section: 20.8.14.1.1 [unique.ptr.dltr.dflt] Status: Ready Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2009-10-21

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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.

[ 2009-07 Frankfurt: ]

The proposed resolution uses concepts. Howard needs to rewrite the proposed resolution.

Move back to Open.

[ 2009-07-26 Howard provided rewritten proposed wording and moved to Review. ]

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Add after 20.8.14.1.1 [unique.ptr.dltr.dflt], p1:

template <class U> default_delete(const default_delete<U>& other);

-1- Effects: ...

Remarks: This constructor shall participate in overload resolution if and only if U* is implicitly convertible to T*.


860. Floating-Point State

Section: 26 [numerics] Status: Ready Submitter: Lawrence Crowl Opened: 2008-06-23 Last modified: 2009-10-23

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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.

[ 2009-09-23 Hans provided wording. ]

If I understand the history correctly, Nick, as the Posix liaison, should probably get a veto on this, since I think it came from Posix (?) via WG14 and should probably really be addressed there (?). But I think we are basically in agreement that there is no other sane way to do this, and hence we don't have to worry too much about stepping on toes. As far as I can tell, this same issue also exists in the latest Posix standard (?).

[ 2009-10 Santa Cruz: ]

Moved to Ready.

Proposed resolution:

Add at the end of 26.3.1 [cfenv.syn]:

2 The header defines all functions, types, and macros the same as C99 7.6.

A separate floating point environment shall be maintained for each thread. Each function accesses the environment corresponding to its calling thread.


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: 2010-01-30

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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.

[ 2009-07 Frankfurt ]

Other operational semantics (see, for example, Tables 82 and 83) are written in terms of a container's size() member. Daniel to update proposed resolution C.

[ Howard: Commented out options A and B. ]

[ 2009-07-26 Daniel updated proposed resolution C. ]

[ 2009-10 Santa Cruz: ]

Mark NAD Editorial. Addressed by N2986.

[ 2009-10 Santa Cruz: ]

Reopened. N2986 was rejected in full committee on procedural grounds.

[ 2010-01-30 Howard updated Table numbers. ]

Proposed resolution:

Option (C):

  1. In 23.2.1 [container.requirements.general] change Table 90 -- Container requirements as indicated:

    1. Change the text in the Assertion/note column in the row for "X u;" as follows:

      post: u.size() == 0empty() == true
    2. Change the text in the Assertion/note column in the row for "X();" as follows:

      X().size() == 0empty() == true
    3. Change the text in the Operational Semantics column in the row for "a == b" as follows:

      == is an equivalence relation. a.size()distance(a.begin(), a.end()) == b.size()distance(b.begin(), b.end()) && equal(a.begin(), a.end(), b.begin())
    4. Add text in the Ass./Note/pre-/post-condition column in the row for "a == b" as follows:

      Requires: T is EqualityComparable
    5. Change the text in the Operational Semantics column in the row for "a.size()" as follows:

      a.end() - a.begin()distance(a.begin(), a.end())
    6. Change the text in the Operational Semantics column in the row for "a.max_size()" as follows:

      size()distance(begin(), end()) of the largest possible container
    7. Change the text in the Operational Semantics column in the row for "a.empty()" as follows:

      a.size() == 0a.begin() == a.end()
  2. In 23.2.1 [container.requirements.general] change Table 93 -- Allocator-aware container requirements as indicated:

    1. Change the text in the Assertion/note column in the row for "X() / X u;" as follows:

      Requires: A is DefaultConstructible post: u.size() == 0u.empty() == true, get_allocator() == A()
    2. Change the text in the Assertion/note column in the row for "X(m) / X u(m);" as follows:

      post: u.size() == 0u.empty() == true, get_allocator() == m
  3. In 23.2.3 [sequence.reqmts] change Table 94 -- Sequence container requirements as indicated:

    1. Change the text in the Assertion/note column in the row for "X(n, t) / X a(n, t)" as follows:

      post: size()distance(begin(), end()) == n [..]
    2. Change the text in the Assertion/note column in the row for "X(i, j) / X a(i, j)" as follows:

      [..] post: size() == distance between i and jdistance(begin(), end()) == distance(i, j) [..]
    3. Change the text in the Assertion/note column in the row for "a.clear()" as follows:

      a.erase(a.begin(), a.end()) post: size() == 0a.empty() == true
  4. In 23.2.4 [associative.reqmts] change Table 96 -- Associative container requirements as indicated:

    [ Not every occurrence of size() was replaced, because all current associative containers have a size. The following changes ensure consistency regarding the semantics of "erase" for all tables and adds some missing objects ]

    1. Change the text in the Complexity column in the row for X(i,j,c)/X a(i,j,c); as follows:

      N log N in general (N == distance(i, j)is the distance from i to j); ...
    2. Change the text in the Complexity column in the row for "a.insert(i, j)" as follows:

      N log(a.size() + N) (N is the distance from i to j) where N == distance(i, j)
    3. Change the text in the Complexity column in the row for "a.erase(k)" as follows:

      log(a.size()) + a.count(k)
    4. Change the text in the Complexity column in the row for "a.erase(q1, q2)" as follows:

      log(a.size()) + N where N is the distance from q1 to q2 == distance(q1, q2).
    5. Change the text in the Assertion/note column in the row for "a.clear()" as follows:

      a.erase(a.begin(),a.end()) post: size() == 0a.empty() == true
    6. Change the text in the Complexity column in the row for "a.clear()" as follows:

      linear in a.size()
    7. Change the text in the Complexity column in the row for "a.count(k)" as follows:

      log(a.size()) + a.count(k)
  5. In 23.2.5 [unord.req] change Table 98 -- Unordered associative container requirements as indicated:

    [ The same rational as for Table 96 applies here ]

    1. Change the text in the Assertion/note column in the row for "a.clear()" as follows:

      [..] Post: a.size() == 0empty() == true

865. More algorithms that throw away information

Section: 25.3.6 [alg.fill], 25.3.7 [alg.generate] Status: Ready Submitter: Daniel Krügler Opened: 2008-07-13 Last modified: 2009-10-23

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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 24.2.2 [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.

[ 2009-07 Frankfurt ]

We have a consensus for moving forward on this issue, but Daniel needs to deconceptify it.

[ 2009-07-25 Daniel provided non-concepts wording. ]

[ 2009-10 Santa Cruz: ]

Moved to Ready.

Proposed resolution:

  1. Replace the current declaration of fill_n in 25 [algorithms]/2, header <algorithm> synopsis and in 25.3.6 [alg.fill] by

    template<class OutputIterator, class Size, class T>
      voidOutputIterator fill_n(OutputIterator first, Size n, const T& value);
    

    Just after the effects clause add a new returns clause saying:

    Returns: For fill_n and positive n, 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 25.3.7 [alg.generate] by

    template<class OutputIterator, class Size, class Generator>
      voidOutputIterator generate_n(OutputIterator first, Size n, Generator gen);
    

    Just after the effects clause add a new returns clause saying:

    For generate_n and positive n, returns first + n. Otherwise returns first for generate_n.

868. default construction and value-initialization

Section: 23 [containers] Status: Ready Submitter: Alberto Ganesh Barbati Opened: 2008-07-22 Last modified: 2009-10-20

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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

[ 2009-07 Frankfurt ]

The proposed resolution is incomplete.

Move to Tentatively NAD Future. Howard will contact Ganesh for wording. If wording is forthcoming, Howard will move it back to Review.

[ 2009-07-18 Ganesh updated the proposed wording. ]

Howard: Moved back to Review. Note that 20.2.1 [utility.arg.requirements] refers to a section that is not in the current working paper, but does refer to a section that we expect to reappear after the de-concepts merge. This was a point of confusion we did not recognize when we reviewed this issue in Frankfurt.

Howard: Ganesh also includes a survey of places in the WP surveyed for changes of this nature and purposefully not treated:

Places where changes are not being proposed

In the following paragraphs, we are not proposing changes because it's not clear whether we actually prefer value-initialization over default-initialization (now partially covered by 1012):

In the following paragraphs, the expression "default constructed" need not be changed, because the relevant type does not depend on a template parameter and has a user-provided constructor:

[ 2009-08-18 Daniel adds: ]

I have no objections against the currently suggested changes, but I also cross-checked with the list regarding intentionally excluded changes, and from this I miss the discussion of

  1. 21.4.1 [string.require]/2:

    "[..] 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."
  2. N2723, X [rand.req.eng], Table 109, expression "T()":

    Pre-/post-condition: "Creates an engine with the same initial state as all other default-constructed engines of type X."

    as well as in 26.5.5 [rand.predef]/1-9 (N2914), 26.5.7.1 [rand.util.seedseq]/3, 27.7.1.1.1 [istream.cons]/3, 27.7.2.2 [ostream.cons]/9 (N2914), 28.13 [re.grammar]/2, 30.3.1.4 [thread.thread.assign]/1 (N2914),

    [ Candidates for the "the expression "default constructed" need not be changed" list ]

    I'm fine, if these would be added to the intentionally exclusion list, but mentioning them makes it easier for other potential reviewers to decide on the relevance or not-relevance of them for this issue.

  3. I suggest to remove the reference of [func.referenceclosure.invoke] in the "it's not clear" list, because this component does no longer exist.

  4. I also suggest to add a short comment that all paragraphs in the resolution whether they refer to N2723 or to N2914 numbering, because e.g. "Change 23.3.2.1 [deque.cons] para 5" is an N2723 coordinate, while "Change 23.3.2.2 [deque.capacity] para 1" is an N2914 coordinate. Even better would be to use one default document for the numbering (probably N2914) and mention special cases (e.g. "Change 20.2.1 [utility.arg.requirements] para 2" as referring to N2723 numbering).

[ 2009-08-18 Alisdair adds: ]

I strongly believe the term "default constructed" should not appear in the library clauses unless we very clearly define a meaning for it, and I am not sure what that would be.

In those cases where we do not want to replace "default constructed" with "vale initialized" we should be using "default initialized". If we have a term that could mean either, we reduce portability of programs.

I have not done an exhaustive review to clarify if that is a vendor freedom we have reason to support (e.g. value-init in debug, default-init in release) so I may yet be convinced that LWG has reason to define this new term of art, but generally C++ initialization is confusing enough without supporting further ill-defined terms.

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Change 20.2.1 [utility.arg.requirements] para 2:

In general, a default constructor is not required. Certain container class member function signatures specify the default constructorT() as a default argument. T() shall be a well-defined expression (8.5) if one of those signatures is called using the default argument (8.3.6).

Change 23.3.2.1 [deque.cons] para 5:

Effects: Constructs a deque with n default constructedvalue-initialized elements.

Change 23.3.2.2 [deque.capacity] para 1:

Effects: If sz < size(), equivalent to erase(begin() + sz, end());. If size() < sz, appends sz - size() default constructedvalue-initialized elements to the sequence.

Change 23.3.3.1 [forwardlist.cons] para 5:

Effects: Constructs a forward_list object with n default constructedvalue-initialized elements.

Change 23.3.3.4 [forwardlist.modifiers] para 21:

Effects: [...] For the first signature the inserted elements are default constructedvalue-initialized, and for the second signature they are copies of c.

Change 23.3.4.1 [list.cons] para 5:

Effects: Constructs a list with n default constructedvalue-initialized elements.

Change 23.3.4.2 [list.capacity] para 15:

Effects: If sz < size(), equivalent to list<T>::iterator it = begin(); advance(it, sz); erase(it, end());. If size() < sz, appends sz - size() default constructedvalue-initialized elements to the sequence.

Change 23.3.6.1 [vector.cons] para 3:

Effects: Constructs a vector with n default constructedvalue-initialized elements.

Change 23.3.6.2 [vector.capacity] para 24:

Effects: If sz < size(), equivalent to erase(begin() + sz, end());. If size() < sz, appends sz - size() default constructedvalue-initialized elements to the sequence.

870. Do unordered containers not support function pointers for predicate/hasher?

Section: 23.2.5 [unord.req] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2008-08-17 Last modified: 2010-02-11

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Discussion:

Good ol' associative containers allow both function pointers and function objects as feasible comparators, as described in 23.2.4 [associative.reqmts]/2:

Each associative container is parameterized on Key and an ordering relation Compare that induces a strict weak ordering (25.3) on elements of Key. [..]. The object of type Compare is called the comparison object of a container. This comparison object may be a pointer to function or an object of a type with an appropriate function call operator.[..]

The corresponding wording for unordered containers is not so clear, but I read it to disallow function pointers for the hasher and I miss a clear statement for the equality predicate, see 23.2.5 [unord.req]/3+4+5:

Each unordered associative container is parameterized by Key, by a function object Hash that acts as a hash function for values of type Key, and by a binary predicate Pred that induces an equivalence relation on values of type Key.[..]

A hash function is a function object that takes a single argument of type Key and returns a value of type std::size_t.

Two values k1 and k2 of type Key are considered equal if the container's equality function object returns true when passed those values.[..]

and table 97 says in the column "assertion...post-condition" for the expression X::hasher:

Hash shall be a unary function object type such that the expression hf(k) has type std::size_t.

Note that 20.7 [function.objects]/1 defines as "Function objects are objects with an operator() defined.[..]"

Does this restriction exist by design or is it an oversight? If an oversight, I suggest that to apply the following

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

Ask Daniel to provide proposed wording that: makes it explicit that function pointers are function objects at the beginning of 20.7 [function.objects]; fixes the "requirements" for typedefs in 20.7.5 [refwrap] to instead state that the function objects defined in that clause have these typedefs, but not that these typedefs are requirements on function objects; remove the wording that explicitly calls out that associative container comparators may be function pointers.

[ 2009-12-19 Daniel updates wording and rationale. ]

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Rationale:

The below provided wording also affects some part of the library which is involved with callable types (20.7.1 [func.def]/3). Reason for this is that callable objects do have a lot in common with function objects. A simple formula seems to be:

callable objects = function objects + pointers to member

The latter group is excluded from function objects because of the expression-based usage of function objects in the algorithm clause, which is incompatible with the notation to dereference pointers to member without a concept map available in the language.

This analysis showed some currently existing normative definition differences between the above subset of callable objects and function objects which seem to be unintended: Backed by the Santa Cruz outcome function objects should include both function pointers and "object[s] with an operator() defined". This clearly excludes class types with a conversion function to a function pointer or all similar conversion function situations described in 13.3 [over.match]/2 b. 2. In contrast to this, the wording for callable types seems to be less constrained (20.7.1 [func.def]/3):

A callable type is a [..] class type whose objects can appear immediately to the left of a function call operator.

The rationale given in N1673 and a recent private communication with Peter Dimov revealed that the intention of this wording was to cover the above mentioned class types with conversion functions as well. To me the current wording of callable types can be read either way and I suggest to make the intention more explicit by replacing

[..] class type whose objects can appear immediately to the left of a function call operator
by
[..] class type whose objects can appear as the leftmost subexpression of a function call expression 5.2.2 [expr.call].

and to use the same definition for the class type part of function objects, because there is no reason to exclude class types with a conversion function to e.g. pointer to function from being used in algorithms.

Now this last term "function objects" itself brings us to a third unsatisfactory state: The term is used both for objects (e.g. "Function objects are objects[..]" in 20.7 [function.objects]/1) and for types (e.g. "Each unordered associative container is parameterized [..] by a function object Hash that acts as a hash function [..]" in 23.2.5 [unord.req]/3). This impreciseness should be fixed and I suggest to introduce the term function object type as the counter part to callable type. This word seems to be a quite natural choice, because the library already uses it here and there (e.g. "Hash shall be a unary function object type such that the expression hf(k) has type std::size_t." in Table 98, "X::hasher" or "Requires: T shall be a function object type [..]" in 20.7.15.2.5 [func.wrap.func.targ]/3).

Finally I would like to add that part of the issue 870 discussion related to the requirements for typedefs in 20.7.5 [refwrap] during the Santa Cruz meeting is now handled by the new issue 1290.

Obsolete rationale:

[ San Francisco: ]

This is fixed by N2776.

Proposed resolution:

  1. Change 20.7 [function.objects]/1 as indicated:

    1 Function objects are objects with an operator() defined. An object type (3.9 [basic.types]) that can be the type of the postfix-expression in a function call (5.2.2 [expr.call], 13.3.1.1 [over.match.call]) is called a function object type*. A function object is an object of a function object type. In the places where one would expect to pass a pointer to a function to an algorithmic template (Clause 25 [algorithms]), the interface is specified to accept an object with an operator() defineda function object. This not only makes algorithmic templates work with pointers to functions, but also enables them to work with arbitrary function objects.

    * Such a type is either a function pointer or a class type which often has a member operator(), but in some cases it can omit that member and provide a conversion to a pointer to function.
  2. Change 20.7.1 [func.def]/3 as indicated: [The intent is to make the commonality of callable types and function object types more explicit and to get rid of wording redundancies]

    3 A callable type is a pointer to function, a pointer to member function, a pointer to member data, or a class type whose objects can appear immediately to the left of a function call operator function object type (20.7 [function.objects]).
  3. Change 20.7.11 [bind]/1 as indicated:

    1 The function template bind returns an object that binds a function callable object passed as an argument to additional arguments.
  4. Change 20.7.11.1 [func.bind]/1 as indicated:

    1 This subclause describes a uniform mechanism for binding arguments of function callable objects.
  5. Change 20.7.15 [func.wrap]/1 as indicated:

    1 This subclause describes a polymorphic wrapper class that encapsulates arbitrary function callable objects.
  6. Change 20.7.15.2 [func.wrap.func]/2 as indicated [The reason for this change is that 20.7.15.2 [func.wrap.func]/1 clearly says that all callable types may be wrapped by std::function and current implementations indeed do provide support for pointer to members as well. One further suggested improvement is to set the below definition of Callable in italics]:

    2 A functioncallable object f of type F is Callable Callable for argument types T1, T2, ..., TN in ArgTypes and a return type R, if, given lvalues t1, t2, ..., tN of types T1, T2, ..., TN, respectively, the expression INVOKE(f, declval<ArgTypes>()..., Rt1, t2, ..., tN), considered as an unevaluated operand (5 [expr]), is well formed (20.7.2) and, if R is not void, convertible to R.
  7. Change 20.7.15.2.1 [func.wrap.func.con]/7 as indicated:

    function(const function& f);
    template <class A> function(allocator_arg_t, const A& a, const function& f);
    

    ...

    7 Throws: shall not throw exceptions if f's target is a function pointer or a function callable object passed via reference_wrapper. Otherwise, may throw bad_alloc or any exception thrown by the copy constructor of the stored function callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small function callable objects, e.g., where f's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

  8. Change 20.7.15.2.1 [func.wrap.func.con]/11 as indicated:

    template<class F> function(F f);
    template <class F, class A> function(allocator_arg_t, const A& a, F f);
    

    ...

    11 [..] [Note: implementations are encouraged to avoid the use of dynamically allocated memory for small function callable objects, for example, where f's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

  9. Change 20.7.15.2.4 [func.wrap.func.inv]/3 as indicated:

    R operator()(ArgTypes... args) const
    

    ...

    3 Throws: bad_function_call if !*this; otherwise, any exception thrown by the wrapped function callable object.

  10. Change 20.7.15.2.5 [func.wrap.func.targ]/3 as indicated:

    template<typename T>       T* target();
    template<typename T> const T* target() const;
    

    ...

    3 Requires: T shall be a function object type that is Callable (20.7.15.2 [func.wrap.func]) for parameter types ArgTypes and return type R.

  11. Change 23.2.4 [associative.reqmts]/2 as indicated: [The suggested removal seems harmless, because 25.4 [alg.sorting]1 already clarifies that Compare is a function object type. Nevertheless it is recommended, because the explicit naming of "pointer to function" is misleading]

    2 Each associative container is parameterized on Key and an ordering relation Compare that induces a strict weak ordering (25.4 [alg.sorting]) on elements of Key. In addition, map and multimap associate an arbitrary type T with the Key. The object of type Compare is called the comparison object of a container. This comparison object may be a pointer to function or an object of a type with an appropriate function call operator.
  12. Change 23.2.5 [unord.req]/3 as indicated:

    3 Each unordered associative container is parameterized by Key, by a function object type Hash that acts as a hash function for values of type Key, and by a binary predicate Pred that induces an equivalence relation on values of type Key. [..]
  13. Change 25.1 [algorithms.general]/7 as indicated: [The intent is to bring this part in sync with 20.7 [function.objects]]

    7 The Predicate parameter is used whenever an algorithm expects a function object (20.7 [function.objects]) that when applied to the result of dereferencing the corresponding iterator returns a value testable as true. In other words, if an algorithm takes Predicate pred as its argument and first as its iterator argument, it should work correctly in the construct if (pred(*first)){...}. The function object pred shall not apply any nonconstant function through the dereferenced iterator. This function object may be a pointer to function, or an object of a type with an appropriate function call operator.
  14. Change 20.8.14.2 [unique.ptr.single]/1 as indicated:

    1 The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function pointer or functor object type for which, given a value d of type D and a pointer ptr of type T*, the expression d(ptr) is valid and has the effect of deallocating the pointer as appropriate for that deleter. D may also be an lvalue-reference to a deleter.

871. Iota's requirements on T are too strong

Section: 26.7.5 [numeric.iota] Status: Ready Submitter: Daniel Krügler Opened: 2008-08-20 Last modified: 2009-10-22

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Discussion:

According to the recent WP N2691, 26.7.5 [numeric.iota]/1, the requires clause of std::iota says:

T shall meet the requirements of CopyConstructible and Assignable types, and shall be convertible to ForwardIterator's value type.[..]

Neither CopyConstructible nor Assignable is needed, instead MoveConstructible seems to be the correct choice. I guess the current wording resulted as an artifact from comparing it with similar numerical algorithms like accumulate.

Note: If this function will be conceptualized, the here proposed MoveConstructible requirement can be removed, because this is an implied requirement of function arguments, see N2710/[temp.req.impl]/3, last bullet.

[ post San Francisco: ]

Issue pulled by author prior to review.

[ 2009-07-30 Daniel reopened: ]

with the absence of concepts, this issue (closed) is valid again and I suggest to reopen it. I also revised by proposed resolution based on N2723 wording:

[ 2009-10 Santa Cruz: ]

Change 'convertible' to 'assignable', Move To Ready.

Proposed resolution:

Change the first sentence of 26.7.5 [numeric.iota]/1:

Requires: T shall meet the requirements of CopyConstructible and Assignable types, and shall be assignable to ForwardIterator's value type. [..]

872. move_iterator::operator[] has wrong return type

Section: 24.5.3.3.12 [move.iter.op.index] Status: Ready Submitter: Doug Gregor Opened: 2008-08-21 Last modified: 2009-10-23

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Discussion:

move_iterator's operator[] is declared as:

reference operator[](difference_type n) const;

This has the same problem that reverse_iterator's operator[] used to have: if the underlying iterator's operator[] returns a proxy, the implicit conversion to value_type&& could end up referencing a temporary that has already been destroyed. This is essentially the same issue that we dealt with for reverse_iterator in DR 386.

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-08-15 Howard adds: ]

I recommend closing this as a duplicate of 1051 which addresses this issue for both move_iterator and reverse_iterator.

[ 2009-10 Santa Cruz: ]

Move to Ready. Note that if 1051 is reopened, it may yield a better resolution, but 1051 is currently marked NAD.

Proposed resolution:

In 24.5.3.1 [move.iterator] and 24.5.3.3.12 [move.iter.op.index], change the declaration of move_iterator's operator[] to:

reference unspecified operator[](difference_type n) const;

Rationale:

[ San Francisco: ]

NAD Editorial, see N2777.

885. pair assignment

Section: 20.3.4 [pairs] Status: Open Submitter: Alisdair Meredith Opened: 2008-09-15 Last modified: 2009-07-17

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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.

[ 2009-07 Frankfurt ]

We agree with the intent, but we need to wait for the dust to settle on concepts.

Proposed resolution:


887. issue with condition::wait_...

Section: 30.5.1 [thread.condition.condvar] Status: Open Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2010-02-11

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Discussion:

The Posix/C++ working group has identified an inconsistency between Posix and the C++ working draft in that Posix requires the clock to be identified at creation, whereas C++ permits identifying the clock at the call to wait. The latter cannot be implemented with the former.

[ San Francisco: ]

Howard recommends NAD with the following explanation:

The intent of the current wording is for the condtion_variable::wait_until be able to handle user-defined clocks as well as clocks the system knows about. This can be done by providing overloads for the known clocks, and another overload for unknown clocks which synchs to a known clock before waiting. For example:

template <class Duration>
bool
condition_variable::wait_until(unique_lock<mutex>& lock,
                               const chrono::time_point<chrono::system_clock, Duration>& abs_time)
{
    using namespace chrono;
    nanoseconds d = __round_up<nanoseconds>(abs_time.time_since_epoch());
    __do_timed_wait(lock.mutex()->native_handle(), time_point<system_clock, nanoseconds>(d));
    return system_clock::now() < abs_time;
}

template <class Clock, class Duration>
bool
condition_variable::wait_until(unique_lock<mutex>& lock,
                               const chrono::time_point<Clock, Duration>& abs_time)
{
    using namespace chrono;
    system_clock::time_point    s_entry = system_clock::now();
    typename Clock::time_point  c_entry = Clock::now();
    nanoseconds dn = __round_up<nanoseconds>(abs_time.time_since_epoch() -
                                              c_entry.time_since_epoch());
    __do_timed_wait(lock.mutex()->native_handle(), s_entry + dn);
    return Clock::now() < abs_time;
}

In the above example, system_clock is the only clock which the underlying condition variable knows how to deal with. One overload just passes that clock through. The second overload (approximately) converts the unknown clock into a system_clock time_point prior to passing it down to the native condition variable.

On Posix systems vendors are free to add implementation defined constructors which take a clock. That clock can be stored in the condition_variable, and converted to (or not as necessary) as shown above.

If an implementation defined constructor takes a clock (for example), then part of the semantics for that implementation defined ctor might include that a wait_until using a clock other than the one constructed with results in an error (exceptional condition) instead of a conversion to the stored clock. Such a design is up to the vendor as once an implementation defined ctor is used, the vendor is free to specifiy the behavior of waits and/or notifies however he pleases (when the cv is constructed in an implementation defined manner).

[ Post Summit: ]

"POSIX people will review the proposed NAD resolution at their upcoming NY meeting.

See the minutes at: http://wiki.dinkumware.com/twiki/bin/view/Posix/POSIX-CppBindingWorkingGroupNewYork2009.

[ 2009-07 Frankfurt ]

Move to NAD.

[ 2009-07-18 Detlef reopens the issue: ]

On Friday afternoon in Frankfurt is was decided that 887 is NAD. This decision was mainly based on a sample implementation presented by Howard that implemented one clock on top of another. Unfortunately this implementation doesn't work for the probably most important case where a system has a monotonic clock and a real-time clock (or "wall time" clock):

If the underlying "system_clock" is a monotonic clock, and the program waits on the real-time clock, and the real-time clock is set forward, the wait will unblock too late.

If the underlying "system_clock" is a real-time clock, and the program waits on the monotonic clock, and the real-time clock is set back, the wait again will unblock too late.

Sorry that I didn't remember this on Friday, but it was Friday afternoon after a busy week...

So as the decision was made on a wrong asumption, I propose to re-open the issue.

[ 2009-07-26 Howard adds: ]

Detlef correctly argues that condition_variable::wait_until could return "too late" in the context of clocks being adjusted during the wait. I agree with his logic. But I disagree that this makes this interface unimplementable on POSIX.

The POSIX spec also does not guarantee that pthread_cond_timedwait does not return "too late" when clocks are readjusted during the wait. Indeed, the POSIX specification lacks any requirements at all concerning how soon pthread_cond_timedwait returns after a time out. This is evidently a QOI issue by the POSIX standard. Here is a quote of the most relevant normative text concerning pthread_cond_timedwait found here.

The pthread_cond_timedwait() function shall be equivalent to pthread_cond_wait(), except that an error is returned if the absolute time specified by abstime passes (that is, system time equals or exceeds abstime) before the condition cond is signaled or broadcasted, or if the absolute time specified by abstime has already been passed at the time of the call.

I.e. the POSIX specification speaks of the error code returned in case of a time out, but not on the timeliness of that return.

Might this simply be an oversight, or minor defect in the POSIX specification?

I do not believe so. This same section goes on to say in non-normative text:

For cases when the system clock is advanced discontinuously by an operator, it is expected that implementations process any timed wait expiring at an intervening time as if that time had actually occurred.

Here is non-normative wording encouraging the implementation to ignore an advancing underlying clock and subsequently causing an early (spurious) return. There is no wording at all which addresses Detlef's example of a "late return". With pthread_cond_timedwait this would be caused by setting the system clock backwards. It seems reasonable to assume, based on the wording that is already in the POSIX spec, that again, the discontinuously changed clock would be ignored by pthread_cond_timedwait.

A noteworthy difference between pthread_cond_timedwait and condition_variable::wait_until is that the POSIX spec appears to say that ETIMEDOUT should be returned if pthread_cond_timedwait returns because of timeout signal, whether or not the system clock was discontinuously advanced during the wait. In contrast condition_variable::wait_until always returns:

Clock::now() < abs_time

That is, the C++ spec requires that the clock be rechecked (detecting discontinuous adjustments during the wait) at the time of return. condition_variable::wait_until may indeed return early or late. But regardless it will return a value reflecting timeout status at the time of return (even if clocks have been adjusted). Of course the clock may be adjusted after the return value is computed but before the client has a chance to read the result of the return. Thus there are no iron-clad guarantees here.

condition_variable::wait_until (and pthread_cond_timedwait) is little more than a convenience function for making sure condition_variable::wait doesn't hang for an unreasonable amount of time (where the client gets to define "unreasonable"). I do not think it is in anyone's interest to try to make it into anything more than that.

I maintain that this is a useful and flexible specification in the spirit of C++, and is implementable on POSIX. The implementation technique described above is a reasonable approach. There may also be higher quality approaches. This specification, like the POSIX specification, gives a wide latitude for QOI.

I continue to recommend NAD, but would not object to a clarifying note regarding the behavior of condition_variable::wait_until. At the moment, I do not have good wording for such a note, but welcome suggestions.

[ 2009-09-30: See N2969. ]

[ 2009-10 Santa Cruz: ]

The LWG is in favor of Detlef to supply revision which adopts Option 2 from N2969 but is modified by saying that system_clock must be available for wait_until.

[ 2010-02-11 Anthony provided wording. ]

Proposed resolution:

Add a new paragraph after 30.2.4 [thread.req.timing]p3:

3 The resolution of timing provided by an implementation depends on both operating system and hardware. The finest resolution provided by an implementation is called the native resolution.

If a function in this clause takes a timeout argument, and the time point or elapsed time specified passes before the function returns, the latency between the timeout occurring and the function returning is unspecified [Note: Implementations should strive to keep such latency as small as possible, but portable code should not rely on any specific upper limits — end note]


889. thread::id comparisons

Section: 30.3.1.1 [thread.thread.id] Status: Tentatively NAD Editorial Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2010-02-09

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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], 20.7.16 [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 20.7.16 [unord.hash], p1.

I am fairly uncomfortable putting the declaration for the thread_id specialization into <functional> as id is a nested class inside std::thread, so it implies that <functional> would require the definition of the thread class template in order to forward declared thread::id and form this specialization.

It seems better to me that the dependency goes the other way around (<thread> will more typically make use of <functional> than vice-versa) and the hash<thread::id> specialization be declared in the <thread> header.

I think hash<error_code> could go into either <system_error> or <functional> and have no immediate preference either way. However, it should clearly appear in the synopsis of one of these two.

Recommend moving 889 back to open, and tying in a reference to UK-324.

[ Batavia (2009-05): ]

Howard observes that thread::id need not be a nested class; it could be a typedef for a more visible type.

[ 2009-05-24 Alisdair adds: ]

I do not believe this is correct. thread::id is explicitly documents as a nested class, rather than as an unspecified typedef analogous to an iterator. If the intent is that this is not implemented as a nested class (under the as-if freedoms) then this is a novel form of standardese.

[ 2009-07 Frankfurt ]

Decided we want to move hash specialization for thread_id to the thread header. Alisdair to provide wording.

[ 2009-07-28 Alisdair provided wording, moved to Review. ]

[ 2009-10 Santa Cruz: ]

Add a strike for hash<thread::id>. Move to Ready

[ 2009-11-13 The proposed wording of 1182 is a superset of the wording in this issue. ]

[ 2010-02-09 Moved from Ready to Open: ]

Issue 1182 is not quite a superset of this issue and it is controversial whether or not the note:

hash template specialization allows thread::id objects to be used as keys in unordered containers.

should be added to the WP.

[ 2010-02-09 Objections to moving this to NAD Editorial, solved by 1182 have been removed. Set to Tentatively NAD Editorial. ]

Proposed resolution:

Remove the following prototype from the synopsis in 20.7 [function.objects]:


template <> struct hash<std::thread::id>;

Add to 30.3 [thread.threads], p1 Header <thread> synopsis:

template <class T> struct hash;
template <> struct hash<thread::id>;

Add template specialization below class definition in 30.3.1.1 [thread.thread.id]

template <>
struct hash<thread::id> : public unary_function<thread::id, size_t> {
   size_t operator()(thread::id val) const;
};

Extend note in p2 30.3.1.1 [thread.thread.id] with second sentence:

[Note: Relational operators allow thread::id objects to be used as keys in associative containers. hash template specialization allows thread::id objects to be used as keys in unordered containers.end note]

Add new paragraph to end of 30.3.1.1 [thread.thread.id]

template <> struct hash<thread::id>;
An explicit specialization of the class template hash (20.7.16 [unord.hash]) shall be provided for the type thread::id.

891. std::thread, std::call_once issue

Section: 30.3.1.2 [thread.thread.constr], 30.4.5.2 [thread.once.callonce] Status: Tentatively Ready Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2010-02-12

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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. ]

[ 2009-07 Frankfurt ]

Leave Open. Await decision for thread variadic constructor.

[ 2009-10 Santa Cruz: ]

See proposed wording for 929 for this, for the formulation on how to solve this. 929 modifies the thread constructor to have "pass by value" behavior with pass by reference efficiency through the use of the decay trait. This same formula would be useful for call_once.

[ 2010-02-11 Anthony updates wording. ]

[ 2010-02-12 Moved to Tentatively Ready after 5 postive votes on c++std-lib. ]

Proposed resolution:

Modify 30.4.5.2 [thread.once.callonce] p1-p2 with the following:

template<class Callable, class ...Args>
  void call_once(once_flag& flag, Callable&& func, Args&&... args);

Given a function as follows:


template<typename T> typename decay<T>::type decay_copy(T&& v)
   { return std::forward<T>(v); }

1 Requires: The template parameters Callable and each Ti in Args shall be CopyConstructible if an lvalue and otherwise satisfy the MoveConstructible requirements. INVOKE(decay_copy(std::forward<Callable>(func), w1, w2, ..., wN decay_copy(std::forward<Args>(args))...) (20.7.2 [func.require]) shall be a valid expression for some values w1, w2, ..., wN, where N == sizeof...(Args).

2 Effects: Calls to call_once on the same once_flag object are serialized. If there has been a prior effective call to call_once on the same once_flag object, the call to call_once returns without invoking func. If there has been no prior effective call to call_once on the same once_flag object, the argument func (or a copy thereof) is called as if by invoking func(args) INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...) is executed. The call to call_once is effective if and only if func(args) INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...) returns without throwing an exception. If an exception is thrown it is propagated to the caller.


893. std::mutex issue

Section: 30.4.1.1 [thread.mutex.class] Status: Tentatively Ready Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2009-11-18

View all other 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:

[ 2009-07 Frankfurt ]

Move to Review. Alisdair to provide note.

[ 2009-07-31 Alisdair provided note. ]

[ 2009-10 Santa Cruz: ]

Moved to Ready.

[ 2009-11-18 Peter Opens: ]

I don't believe that the proposed note:

[Note: a program may deadlock if the thread that owns a mutex object calls lock() or try_lock() on that object. If the program can detect the deadlock, a resource_deadlock_would_occur error condition may be observed. — end note]

is entirely correct. "or try_lock()" should be removed, because try_lock is non-blocking and doesn't deadlock; it just returns false when it fails to lock the mutex.

[ Howard: I've set to Open and updated the wording per Peter's suggestion. ]

[ 2009-11-18 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

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:

Add the following note after p3 30.4.1.1 [thread.mutex.class]

[Note: a program may deadlock if the thread that owns a mutex object calls lock() on that object. If the implementation can detect the deadlock, a resource_deadlock_would_occur error condition may be observed. — end note]

896. Library thread safety issue

Section: 20.8.15.2 [util.smartptr.shared] Status: Open Submitter: Hans Boehm Opened: 2008-09-16 Last modified: 2010-01-20

View all other 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.

[ 2009-07 Frankfurt ]

Vote: Do we want one thread-safe shared pointer or two? If two, one would allow concurrent construction and destruction of shared pointers, and one would not be thread-safe. If one, then it would be thread-safe.

No concensus on that vote.

Hans to improve wording in consultation with Pete. Leave Open.

[ 2009-10 Santa Cruz: ]

Move to Ready. Ask Editor to clear up wording a little when integrating to make it clear that the portion after the first comma only applies for the presence of data races.

[ 2009-10-24 Hans adds: ]

I think we need to pull 896 back from ready, unfortunately. My wording doesn't say the right thing.

I suspect we really want to say something along the lines of:

For purposes of determining the presence of a data race, member functions access and modify only the shared_ptr and weak_ptr objects themselves and not objects they refer to. Changes in use_count() do not reflect modifications that can introduce data races.

But I think this needs further discussion by experts to make sure this is right.

Detlef and I agree continue to disagree on the resolution, but I think we agree that it would be good to try to expedite this so that it can be in CD2, since it's likely to generate NB comments no matter what we do. And lack of clarity of intent is probably the worst option. I think it would be good to look at this between meetings.

[ 2010-01-20 Howard: ]

I've moved Hans' suggested wording above into the proposed resolution section and preserved the previous wording here:

Make it explicitly thread-safe, in this weak sense, as I believe was intended:

Insert in 20.8.15.2 [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.

Proposed resolution:

Insert a new paragraph at the end of 20.8.15.2 [util.smartptr.shared]:

For purposes of determining the presence of a data race, member functions access and modify only the shared_ptr and weak_ptr objects themselves and not objects they refer to. Changes in use_count() do not reflect modifications that can introduce data races.

900. stream move-assignment

Section: 27.9.1.8 [ifstream.assign] Status: Open Submitter: Niels Dekker Opened: 2008-09-20 Last modified: 2009-10-20

View all issues with Open status.

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.

[ 2009-07 Frankfurt: ]

Howard is going to write wording.

[ 2009-07-26 Howard provided wording. ]

[ 2009-09-13 Niels adds: ]

Note: The proposed change of 27.9.1.3 [filebuf.assign]/1 depends on the resolution of LWG 1204, which allows implementations to assume that *this and rhs refer to different objects.

[ 2009 Santa Cruz: ]

Leave as Open. Too closely related to 911 to move on at this time.

Proposed resolution:

Change 27.8.1.2 [stringbuf.assign]/1:

basic_stringbuf& operator=(basic_stringbuf&& rhs);
-1- Effects: swap(rhs). After the move assignment *this reflects the same observable state it would have if it had been move constructed from rhs (27.8.1.1 [stringbuf.cons]).

Change 27.8.2.2 [istringstream.assign]/1:

basic_istringstream& operator=(basic_istringstream&& rhs);
-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.

Change 27.8.3.2 [ostringstream.assign]/1:

basic_ostringstream& operator=(basic_ostringstream&& rhs);
-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.

Change 27.8.5.1 [stringstream.assign]/1:

basic_stringstream& operator=(basic_stringstream&& rhs);
-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.

Change 27.9.1.3 [filebuf.assign]/1:

basic_filebuf& operator=(basic_filebuf&& rhs);
-1- Effects: swap(rhs). Begins by calling this->close(). After the move assignment *this reflects the same observable state it would have if it had been move constructed from rhs (27.9.1.2 [filebuf.cons]).

Change 27.9.1.8 [ifstream.assign]/1:

basic_ifstream& operator=(basic_ifstream&& rhs);
-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.

Change 27.9.1.12 [ofstream.assign]/1:

basic_ofstream& operator=(basic_ofstream&& rhs);
-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.

Change 27.9.1.16 [fstream.assign]/1:

basic_fstream& operator=(basic_fstream&& rhs);
-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.

910. Effects of MoveAssignable

Section: 20.2.1 [utility.arg.requirements] Status: Tentatively NAD Concepts Submitter: Alberto Ganesh Barbati Opened: 2008-09-29 Last modified: 2010-01-23

View other active issues in [utility.arg.requirements].

View all other issues in [utility.arg.requirements].

View all issues with Tentatively NAD Concepts 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.

[ 2009-07 Frankfurt: ]

Need to look at again without concepts.

[ 2009-07 Frankfurt: ]

Walter will consult with Dave and Doug.

[ 2009-10 Santa Cruz: ]

We believe this is handled by the resolution to issue 1204, but there is to much going on in this area to be sure. Defer for now.

[ 2010-01-23 Moved to Tentatively NAD Concepts after 5 positive votes on c++std-lib. Rationale added below. ]

Rationale:

The current MoveAssignable requirements say everything that can be said in general. Each std-defined type has a more detailed specification of move assignment.

Proposed resolution:

In [concept.copymove], replace the postcondition in paragraph 7 with:

Postconditions: *this is equivalent to the value of rv before the assignment. [Note: there is no requirement on the value of rv after the assignment, but the effect should be unsurprising to the user even in case rv is not immediately destroyed. This may require that resources previously owned by *this are released instead of transferred to rv. -- end note]

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: 2009-10-20

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.

[ 2009-07 Frankfurt: ]

It's not clear that the use case is compelling.

Howard: This needs to be implemented and tested.

[ 2009-07-26 Howard adds: ]

I started out thinking I would recommend NAD for this one. I've turned around to agree with the proposed resolution (which I've updated to the current draft). I did not fully understand Ganesh's rationale, and attempt to describe my improved understanding below.

The move constructor, move assignment operator, and swap function are different for basic_istream, basic_ostream and basic_iostream than other classes. A timely conversation with Daniel reminded me of this long forgotten fact. These members are sufficiently different that they would be extremely confusing to use in general, but they are very much needed for derived clients.

The reason for this behavior is that for the std-derived classes (stringstreams, filestreams), the rdbuf pointer points back into the class itself (self referencing). It can't be swapped or moved. But this fact isn't born out at the stream level. Rather it is born out at the fstream/sstream level. And the lower levels just need to deal with that fact by not messing around with the rdbuf pointer which is stored down at the lower levels.

In a nutshell, it is very confusing for all of those who are not so intimately related with streams that they've implemented them. And it is even fairly confusing for some of those who have (including myself). I do not think it is safe to swap or move istreams or ostreams because this will (by necessary design) separate stream state from streambuffer state. Derived classes (such as fstream and stringstream must be used to keep the stream state and stream buffer consistently packaged as one unit during a move or swap.

I've implemented this proposal and am living with it day to day.

[ 2009 Santa Cruz: ]

Leave Open. Pablo expected to propose alternative wording which would rename move construction, move assignment and swap, and may or may not make them protected. This will impact issue 900.

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 swap free function signature

// swap: 
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);
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 swap free function signature

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);
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 swap free function signature

// swap: 
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 4

template <class charT, class traits> 
  void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y);
Effects: x.swap(y).

915. minmax with initializer_list should return pair of T, not pair of const T&

Section: 25.4.7 [alg.min.max] Status: Open Submitter: Daniel Krügler Opened: 2008-10-04 Last modified: 2009-10-23

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

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

View all issues with Open status.

Discussion:

It seems that the proposed changes for N2772 were not clear enough in this point:

25.4.7 [alg.min.max], before p.23 + p.24 + before p. 27 + p. 28 say that the return type of the minmax overloads with an initializer_list is pair<const T&, const T&>, which is inconsistent with the decision for the other min/max overloads which take a initializer_list as argument and return a T, not a const T&. Doing otherwise for minmax would easily lead to unexpected life-time problems by using minmax instead of min and max separately.

[ Batavia (2009-05): ]

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

[ 2009-07 Frankfurt ]

Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-08-18 Daniel adds: ]

Recommend NAD since the proposed changes have already been performed as part of editorial work of N2914.

[ 2009-10 Santa Cruz: ]

Can't find initializer_list form of minmax anymore, only variadic version. Seems like we had an editing clash with concepts. Leave Open, at least until editorial issues resolved. Bring this to Editor's attention.

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 25.4.7 [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.


920. Ref-qualification support in the library

Section: 20.7.14 [func.memfn] Status: Ready Submitter: Bronek Kozicki Opened: 2008-10-06 Last modified: 2009-10-23

View all other issues in [func.memfn].

View all issues with Ready status.

Duplicate of: 1230

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.

[ 2009-10-10 Daniel updated wording to post-concepts. ]

1230 has a similar proposed resolution

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

  1. Change 20.7 [function.objects]/2, header <functional> synopsis as follows:

    // 20.7.14, member function adaptors:
    template<class R, class T> unspecified mem_fn(R T::*);
    
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...));
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile);
    
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) &);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const &);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile &);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile &);
    
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) &&);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const &&);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile &&);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile &&);
    
  2. Change the prototype list of 20.7.14 [func.memfn] as follows [NB: 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. ]:

    template<class R, class T> unspecified mem_fn(R T::* pm);
    
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...));
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile);
    
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) &);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const &);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &);
    
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) &&);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const &&);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &&);
    template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);
    
  3. Remove 20.7.14 [func.memfn]/5:

    Remarks: Implementations may implement mem_fn as a set of overloaded function templates.

921. Rational Arithmetic should use template aliases

Section: 20.4.1 [ratio.ratio] Status: Ready Submitter: Pablo Halpern Opened: 2008-10-07 Last modified: 2009-10-21

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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.

[ 2009-07-21 Alisdair adds: ]

See 1121 for a potentially incompatible proposal.

[ 2009-10 Santa Cruz: ]

Move to Ready.

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 [..]


929. Thread constructor

Section: 30.3.1.2 [thread.thread.constr] Status: Tentatively Ready Submitter: Anthony Williams Opened: 2008-10-23 Last modified: 2010-02-04

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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 (20.3.4 [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).

[ 2009-07 Frankfurt: ]

This is linked to N2901.

Howard to open a separate issue to remove (1176).

In Frankfurt there is no consensus for removing the variadic constructor.

[ 2009-10 Santa Cruz: ]

We want to move forward with this issue. If we later take it out via 1176 then that's ok too. Needs small group to improve wording.

[ 2009-10 Santa Cruz: ]

Stefanus provided revised wording. Moved to Review Here is the original wording:

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.

[ 2010-01-19 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

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:

Given a function as follows:


template<typename T> typename decay<T>::type decay_copy(T&& v)
    { return std::forward<T>(v); }

-4- Requires: F and each Ti in Args shall be CopyConstructible if an lvalue and otherwise satisfy the MoveConstructible requirements. 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). INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...) (20.7.2 [func.require]) shall be a valid expression.

-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.... Any return value from f is ignored. If f terminates with an uncaught exception, std::terminate() shall be called. The new thread of execution executes INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...) with the calls to decay_copy() being evaluated in the constructing thread. Any return value from this invocation is ignored. [Note: this implies any exceptions not thrown from the invocation of the copy of f will be thrown in the constructing thread, not the new thread. — end note]. If the invocation of INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...) terminates with an uncaught exception, std::terminate shall be called.

-6- Synchronization: The invocation of the constructor happens before the invocation of the copy of f.


932. unique_ptr(pointer p) for pointer deleter types

Section: 20.8.14.2.1 [unique.ptr.single.ctor] Status: Ready Submitter: Howard Hinnant Opened: 2008-11-26 Last modified: 2009-10-22

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Discussion:

Addresses US 79

20.8.14.2.1 [unique.ptr.single.ctor]/5 no longer requires for D not to be a pointer type. I believe this restriction was accidently removed when we relaxed the completeness reuqirements on T. The restriction needs to be put back in. Otherwise we have a run time failure that could have been caught at compile time:

{
unique_ptr<int, void(*)(void*)> p1(malloc(sizeof(int)));  // should not compile
}  // p1.~unique_ptr() dereferences a null function pointer
unique_ptr<int, void(*)(void*)> p2(malloc(sizeof(int)), free);  // ok

[ Post Summit: ]

Recommend Tentatively Ready.

[ 2009-07 Frankfurt ]

Moved from Tentatively Ready to Open only because the wording needs to be improved for enable_if type constraining, possibly following Robert's formula.

[ 2009-07 Frankfurt: ]

We need to consider whether some requirements in the Requires paragraphs of [unique.ptr] should instead be Remarks.

Leave Open. Howard to provide wording, and possibly demonstrate how this can be implemented using enable_if.

[ 2009-07-27 Howard adds: ]

The two constructors to which this issue applies are not easily constrained with enable_if as they are not templated:

unique_ptr();
explicit unique_ptr(pointer p);

To "SFINAE" these constructors away would take heroic effort such as specializing the entire unique_ptr class template on pointer deleter types. There is insufficient motivation for such heroics. Here is the expected and reasonable implementation for these constructors:

unique_ptr()
    : ptr_(pointer())
    {
        static_assert(!is_pointer<deleter_type>::value,
            "unique_ptr constructed with null function pointer deleter");
    }
explicit unique_ptr(pointer p)
    : ptr_(p)
    {
        static_assert(!is_pointer<deleter_type>::value,
            "unique_ptr constructed with null function pointer deleter");
    }

I.e. just use static_assert to verify that the constructor is not instantiated with a function pointer for a deleter. The compiler will automatically take care of issuing a diagnostic if the deleter is a reference type (uninitialized reference error).

In keeping with our discussions in Frankfurt, I'm moving this requirement on the implementation from the Requires paragraph to a Remarks paragraph.

[ 2009-08-17 Daniel adds: ]

It is insufficient to require a diagnostic. This doesn't imply an ill-formed program as of 1.3.3 [defns.diagnostic] (a typical alternative would be a compiler warning), but exactly that seems to be the intend. I suggest to use the following remark instead:

Remarks: The program shall be ill-formed if this constructor is instantiated when D is a pointer type or reference type.

Via the general standard rules of 1.4 [intro.compliance] the "diagnostic required" is implied.

[ 2009-10 Santa Cruz: ]

Moved to Ready.

Proposed resolution:

Change the description of the default constructor in 20.8.14.2.1 [unique.ptr.single.ctor]:

unique_ptr();

-1- Requires: D shall be default constructible, and that construction shall not throw an exception. D shall not be a reference type or pointer type (diagnostic required).

...

Remarks: The program shall be ill-formed if this constructor is instantiated when D is a pointer type or reference type.

Add after 20.8.14.2.1 [unique.ptr.single.ctor]/8:

unique_ptr(pointer p);

...

Remarks: The program shall be ill-formed if this constructor is instantiated when D is a pointer type or reference type.


939. Problem with std::identity and reference-to-temporaries

Section: 20.3.3 [forward] Status: Ready Submitter: Alisdair Meredith Opened: 2008-12-11 Last modified: 2009-10-29

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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. ]

[ 2009-07-20 Alisdair adds: ]

I'm not sure why this issue was not discussed at Frankfurt (or I missed the discussion) but the rationale is now fundamentally flawed. With the removal of concepts, std::identity again becomes an important library type so we cannot simply remove it.

At that point, we need to pick one of the other suggested resolutions, but have no guidance at the moment.

[ 2009-07-20 Howard adds: ]

I believe the rationale for not addressing this issue in Frankfurt was that it did not address a national body comment.

I also believe that removal of identity is still a practical option as my latest reformulation of forward, which is due to comments suggested at Summit, no longer uses identity. :-)

template <class T, class U,
    class = typename enable_if
            <
                !is_lvalue_reference<T>::value || 
                 is_lvalue_reference<T>::value &&
                 is_lvalue_reference<U>::value
            >::type,
    class = typename enable_if
            <
                is_same<typename remove_all<T>::type,
                        typename remove_all<U>::type>::value
            >::type>
inline
T&&
forward(U&& t)
{
    return static_cast<T&&>(t);

}

[ The above code assumes acceptance of 1120 for the definition of remove_all. This is just to make the syntax a little more palatable. Without this trait the above is still very implementable. ]

Paper with rationale is on the way ... really, I promise this time! ;-)

[ 2009-07-30 Daniel adds: See 823 for an alternative resolution. ]

[ 2009-10 Santa Cruz: ]

Move to Ready. Howard will update proposed wording to reflect current draft.

Proposed resolution:

Strike from 20.3 [utility]:

template <class T> struct identity;

Remove from 20.3.3 [forward]:

template <class T> struct identity {
  typedef T type;

  const T& operator()(const T& x) const;
};

const T& operator()(const T& x) const;
-2- Returns: x

940. std::distance

Section: 24.4.4 [iterator.operations] Status: Ready Submitter: Thomas Opened: 2008-12-14 Last modified: 2009-10-22

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Discussion:

Addresses UK 270

Regarding the std::distance - function, 24.4.4 [iterator.operations] / 4 says:

Returns the number of increments or decrements needed to get from first to last.

This sentence is completely silent about the sign of the return value. 24.4.4 [iterator.operations] / 1 gives more information about the underlying operations, but again no inferences about the sign can be made. Strictly speaking, that is taking that sentence literally, I think this sentence even implies a positive return value in all cases, as the number of increments or decrements is clearly a ratio scale variable, with a natural zero bound.

Practically speaking, my implementations did what common sense and knowledge based on pointer arithmetic forecasts, namely a positive sign for increments (that is, going from first to last by operator++), and a negative sign for decrements (going from first to last by operator--).

Here are my two questions:

First, is that paragraph supposed to be interpreted in the way what I called 'common sense', that is negative sign for decrements ? I am fairly sure that's the supposed behavior, but a double-check here in this group can't hurt.

Second, is the present wording (2003 standard version - no idea about the draft for the upcoming standard) worth an edit to make it a bit more sensible, to mention the sign of the return value explicitly ?

[ Daniel adds: ]

My first thought was that resolution 204 would already cover the issue report, but it seems that current normative wording is in contradiction to that resolution:

Referring to N2798, 24.4.4 [iterator.operations]/ p.4 says:

Effects: Returns the number of increments or decrements needed to get from first to last.

IMO the part " or decrements" is in contradiction to p. 5 which says

Requires: last shall be reachable from first.

because "reachable" is defined in X [iterator.concepts]/7 as

An iterator j is called reachable from an iterator i if and only if there is a finite sequence of applications of the expression ++i that makes i == j.[..]

Here is wording that would be consistent with this definition of "reachable":

Change 24.4.4 [iterator.operations] p4 as follows:

Effects: Returns the number of 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.

[ 2009-07 Frankfurt ]

Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-07 Frankfurt: ]

Leave Open pending arrival of a post-Concepts WD.

[ 2009-10-14 Daniel provided de-conceptified wording. ]

[ 2009-10 Santa Cruz: ]

Move to Ready, replacing the Effects clause in the proposed wording with "If InputIterator meets the requirements of random access iterator then returns (last - first), otherwise returns the number of increments needed to get from first to list.".

Proposed resolution:

  1. Change 24.2.5 [random.access.iterators], Table 105 as indicated [This change is not essential but it simplifies the specification] for the row with expression "b - a" and the column Operational semantics:

    (a < b) ? distance(a,b)
    : -distance(b,a)
    
  2. Change 24.4.4 [iterator.operations]/4+5 as indicated:

    template<class InputIterator>
      typename iterator_traits<InputIterator>::difference_type
        distance(InputIterator first, InputIterator last);
    

    4 Effects: If InputIterator meets the requirements of random access iterator then returns (last - first), otherwise Rreturns the number of increments or decrements needed to get from first to last.

    5 Requires: If InputIterator meets the requirements of random access iterator then last shall be reachable from first or first shall be reachable from last, otherwise last shall be reachable from first.


950. unique_ptr converting ctor shouldn't accept array form

Section: 20.8.14.2.1 [unique.ptr.single.ctor] Status: Ready Submitter: Howard Hinnant Opened: 2009-01-07 Last modified: 2009-10-21

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Discussion:

unique_ptr's of array type should not convert to unique_ptr's which do not have an array type.

struct Deleter
{
   void operator()(void*) {}
};

int main()
{
   unique_ptr<int[], Deleter> s;
   unique_ptr<int, Deleter> s2(std::move(s));  // should not compile
}

[ Post Summit: ]

Walter: Does the "diagnostic required" apply to both arms of the "and"?

Tom Plum: suggest to break into several sentences

Walter: suggest "comma" before the "and" in both places

Recommend Review.

[ Batavia (2009-05): ]

The post-Summit comments have been applied to the proposed resolution. We now agree with the proposed resolution. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

Moved from Tentatively Ready to Open only because the wording needs to be improved for enable_if type constraining, possibly following Robert's formula.

[ 2009-08-01 Howard updates wording and sets to Review. ]

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Change 20.8.14.2.1 [unique.ptr.single.ctor]:

template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);

-20- Requires: If D is not a reference type, construction of the deleter D from an rvalue of type E shall be well formed and shall not throw an exception. If D is a reference type, then E shall be the same type as D (diagnostic required). unique_ptr<U, E>::pointer shall be implicitly convertible to pointer. [Note: These requirements imply that T and U are complete types. — end note]

Remarks: If D is a reference type, then E shall be the same type as D, else this constructor shall not participate in overload resolution. unique_ptr<U, E>::pointer shall be implicitly convertible to pointer, else this constructor shall not participate in overload resolution. U shall not be an array type, else this constructor shall not participate in overload resolution. [Note: These requirements imply that T and U are complete types. — end note]

Change 20.8.14.2.3 [unique.ptr.single.asgn]:

template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);

-6- Requires: Assignment of the deleter D from an rvalue D shall not throw an exception. unique_ptr<U, E>::pointer shall be implicitly convertible to pointer. [Note: These requirements imply that T and U are complete types. — end note]

Remarks: unique_ptr<U, E>::pointer shall be implicitly convertible to pointer, else this operator shall not participate in overload resolution. U shall not be an array type, else this operator shall not participate in overload resolution. [Note: These requirements imply that T and U are complete types. — end note]


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: 2010-02-11

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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.

[ 2009-08-01 Howard adds: ]

I have surveyed all clauses of 20.9.2.2 [time.traits.duration_values], 20.9.2.3 [time.traits.specializations] and 20.9.3 [time.duration]. I can not find any clause which involves the use of a duration::rep type where the requirements on the rep type are not clearly spelled out. These requirements were carefully crafted to allow any arithmetic type, or any user-defined type emulating an arithmetic type.

Indeed, treat_as_floating_point becomes completely superfluous if duration::rep can never be a class type.

There will be some Rep types which will not meet the requirements of every duration operation. This is no different than the fact that vector<T> can easily be used for types T which are not DefaultConstructible, even though some members of vector<T> require T to be DefaultConstructible. This is why the requirements on Rep are specified for each operation individually.

In 20.9.2.1 [time.traits.is_fp] p1:

template <class Rep> struct treat_as_floating_point 
  : is_floating_point<Rep> { };
The duration template uses the treat_as_floating_point trait to help determine if a duration object can be converted to another duration with a different tick period. If treat_as_floating_point<Rep>::value is true, then Rep is a floating-point type and implicit conversions are allowed among durations. Otherwise, the implicit convertibility depends on the tick periods of the durations. If Rep is a class type which emulates a floating-point type, the author of Rep can specialize treat_as_floating_point so that duration will treat this Rep as if it were a floating-point type. Otherwise Rep is assumed to be an integral type or a class emulating an integral type.

The phrases "a class type which emulates a floating-point type" and "a class emulating an integral type" are clarifying phrases which refer to the summation of all the requirements on the Rep type specified in detail elsewhere (and should not be repeated here).

This specification has been implemented, now multiple times, and the experience has been favorable. The current specification clearly specifies the requirements at each point of use (though I'd be happy to fix any place I may have missed, but none has been pointed out).

I am amenable to improved wording of this paragraph (and any others), but do not have any suggestions for improved wording at this time. I am strongly opposed to changes which would significantly alter the semantics of the specification under 20.9 [time] without firmly grounded and documented rationale, example implementation, testing, and user experience which relates a positive experience.

I recommend NAD unless someone wants to produce some clarifying wording.

[ 2009-10 Santa Cruz: ]

Stefanus to provide wording to turn this into a note.

[ 2010-02-11 Stefanus provided wording. ]

Proposed resolution:

Change 20.9.2.1 [time.traits.is_fp]/1:

1 The duration template uses the treat_as_floating_point trait to help determine if a duration object can be converted to another duration with a different tick period. If treat_as_floating_point<Rep>::value is true, then Rep is a floating-point type and implicit conversions are allowed among durations. Otherwise, the implicit convertibility depends on the tick periods of the durations. If Rep is a class type which emulates a floating-point type, the author of Rep can specialize treat_as_floating_point so that duration will treat this Rep as if it were a floating-point type. Otherwise Rep is assumed to be an integral type or a class emulating an integral type. [Note: The intention of this trait is to indicate whether a given class behaves like a floating point type, and thus allows division of one value by another with acceptable loss of precision. If treat_as_floating_point<Rep>::value is false, Rep will be treated as if it behaved like an integral type for the purpose of these conversions. — end note]

953. Various threading bugs #3

Section: 20.9.1 [time.clock.req] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2010-02-11

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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.

[ 2009-08-01 Howard adds: ]

See commented dated 2009-08-01 in 951.

[ 2009-10 Santa Cruz: ]

Stefanus to provide wording to turn this into a note.

[ 2010-02-11 Stephanus provided wording for 951 which addresses this issue as well. ]

Proposed resolution:


954. Various threading bugs #4

Section: 20.9.1 [time.clock.req] Status: Ready Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-10-21

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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)".

[ 2009-08-01 Howard: Moved to Reivew as the wording requested in Batavia has been provided. ]

[ 2009-10 Santa Cruz: ]

Move to Ready.

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.

956. Various threading bugs #6

Section: 20.9.1 [time.clock.req] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-11-18

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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.

[ 2009-10-23 Pete provides wording: ]

[ 2009-11-18 Daniel adds: ]

I see that 30.4.2 [thread.timedmutex.requirements]/3 says:

Precondition: If the tick period of rel_time is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period.

I would prefer to see that adapted as well. Following the same style as the proposed resolution I come up with

Precondition: If the tick period of rel_time is not exactly convertible to the native tick period of the execution environment, the duration shall be rounded up to the nearest native tick period of the execution environment.

Proposed resolution:

Remove every occurrence of "native" in 20.9.1 [time.clock.req].

Add the following sentence at the end of 20.9.1 [time.clock.req]/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. A clock shall meet the requirements in Table 55. The duration and time_point types have the natural size and resolution suggested by the architecture of the execution environment.

957. Various threading bugs #7

Section: 20.9.5.1 [time.clock.system] Status: Ready Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-10-21

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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.

[ 2009-10 Santa Cruz: ]

Move to Ready.

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.

959. Various threading bugs #9

Section: 30.5.1 [thread.condition.condvar] Status: Tentatively NAD Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-11-18

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Discussion:

30.5.1 [thread.condition.condvar]: condition_variable::wait_for is required to compute the absolute time by adding the duration value to chrono::monotonic_clock::now(), but monotonic_clock is not required to exist.

[ Summit: ]

Move to open. Associate with LWG 859 and any other monotonic-clock related issues.

[ 2009-08-01 Howard adds: ]

I believe that 859 (currently Ready) addresses this issue, and that this issue should be marked NAD, solved by 859 (assuming it moves to WP).

[ 2009-10 Santa Cruz: ]

Leave open, but expect to be fixed by N2969 revision that Detlef is writing.

[ 2009-11-18 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]

Proposed resolution:

Rationale:

condition_variable::wait_for no longer refers to monotonic_clock, so this issue is moot.


960. Various threading bugs #10

Section: 30.4.1 [thread.mutex.requirements] Status: Ready Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-10-26

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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. ]

[ 2009-10 Santa Cruz: ]

Move to Ready. Fix the proposed wording with "functions of type Mutex" -> "functions of Mutex type"

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 Mutex type shall be:


962. Various threading bugs #12

Section: 30.4.3.2.2 [thread.lock.unique.locking] Status: Ready Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-10-22

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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. ]

[ 2009-07-21 Beman added wording to address 30.2.2 [thread.req.exception] in response to the Frankfurt notes in 859. ]

[ 2009-09-25 Beman: minor update to wording. ]

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Change Exceptions 30.2.2 [thread.req.exception] as indicated:

Some functions described in this Clause are specified to throw exceptions of type system_error (19.5.5). Such exceptions shall be thrown if any of the Error conditions are detected or a call to an operating system or other underlying API results in an error that prevents the library function from satisfying its postconditions or from returning a meaningful value meeting its specifications. Failure to allocate storage shall be reported as described in 17.6.4.11 [res.on.exception.handling].

Change thread assignment 30.3.1.5 [thread.thread.member], join(), paragraph 8 as indicated:

Throws: std::system_error when the postconditions cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change thread assignment 30.3.1.5 [thread.thread.member], detach(), paragraph 13 as indicated:

Throws: std::system_error when the effects or postconditions cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change Mutex requirements 30.4.1 [thread.mutex.requirements], paragraph 11, as indicated:

Throws: std::system_error when the effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change unique_lock locking 30.4.3.2.2 [thread.lock.unique.locking], paragraph 3, as indicated:

Throws: std::system_error when the postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change unique_lock locking 30.4.3.2.2 [thread.lock.unique.locking], paragraph 8, as indicated:

Throws: std::system_error when the postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change unique_lock locking 30.4.3.2.2 [thread.lock.unique.locking], paragraph 13, as indicated:

Throws: std::system_error when the postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change unique_lock locking 30.4.3.2.2 [thread.lock.unique.locking], paragraph 18, as indicated:

Throws: std::system_error when the postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change unique_lock locking 30.4.3.2.2 [thread.lock.unique.locking], paragraph 22, as indicated:

Throws: std::system_error when the postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change Function call_once 30.4.5.2 [thread.once.callonce], paragraph 4, as indicated

Throws: std::system_error when the effects cannot be achieved an exception is required (30.2.2 [thread.req.exception]), or any exception thrown by func.

Change Class condition_variable 30.5.1 [thread.condition.condvar], paragraph 12, as indicated:

Throws: std::system_error when the effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change Class condition_variable 30.5.1 [thread.condition.condvar], paragraph 19, as indicated:

Throws: std::system_error when the effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change Class condition_variable_any 30.5.2 [thread.condition.condvarany], paragraph 10, as indicated:

Throws: std::system_error when the effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change Class condition_variable_any 30.5.2 [thread.condition.condvarany], paragraph 16, as indicated:

Throws: std::system_error when the returned value, effects, or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Assuming issue 859, Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change Change 30.5.1 [thread.condition.condvar] as indicated:

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

Throws: std::system_error when the effects or postcondition cannot be achieved an exception is required ([thread.req.exception]).

Assuming issue 859, Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change Change 30.5.1 [thread.condition.condvar] as indicated:

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

Throws: std::system_error when the effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Assuming issue 859, Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change 30.5.2 [thread.condition.condvarany] as indicated:

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

Throws: std::system_error when the returned value, effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Assuming issue 859, Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change 30.5.2 [thread.condition.condvarany] as indicated:

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

Throws: std::system_error when the returned value, effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).


963. Various threading bugs #13

Section: 30.3.1.5 [thread.thread.member] Status: Ready Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-10-26

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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.

[ 2009-10 Santa Cruz: ]

Mark as Ready with proposed resolution from Summit.

Proposed resolution:

In 30.3.1.5 [thread.thread.member] change:

void detach();

...

-14- Error conditions:


964. Various threading bugs #14

Section: 30.5.2 [thread.condition.condvarany] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-10-26

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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.

[ 2009-10 Santa Cruz: ]

Leave open. Benjamin to provide wording.

Proposed resolution:


966. Various threading bugs #16

Section: 30.5.1 [thread.condition.condvar] Status: Open Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2010-02-12

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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.

[ 2009-08-01 Howard provides wording. ]

The proposed wording is inspired by the POSIX spec which says:

[EINVAL]
The value specified by cond or mutex is invalid.
[EPERM]
The mutex was not owned by the current thread at the time of the call.

I do not believe [EINVAL] is possible without memory corruption (which we don't specify). [EPERM] is possible if this thread doesn't own the mutex, which is listed as a precondition. "May" is used instead of "Shall" because not all OS's are POSIX.

[ 2009-10 Santa Cruz: ]

Leave open, Detlef to provide improved wording.

[ 2009-10-23 Detlef Provided wording. ]

Detlef's wording put in Proposed resolution. Original wording here:

Change 30.5.1 [thread.condition.condvar] p12, p19 and 30.5.2 [thread.condition.condvarany] p10, p16:

Throws: May throw std::system_error if a precondition is not met. when the effects or postcondition cannot be achieved.

[ 2009-10 Santa Cruz: ]

Leave open, Detlef to provide improved wording.

[ 2009-11-18 Anthony adds: ]

condition_variable::wait takes a unique_lock<mutex>. We know whether or not a unique_lock owns a lock, through use of its owns_lock() member.

I would like to propose the following resolution:

Modify the first sentence of 30.5.1 [thread.condition.condvar] p9:

void wait(unique_lock<mutex>& lock);
9 Precondition: lock is locked by the calling thread lock.owns_lock() is true, and either

...

Replace 30.5.1 [thread.condition.condvar] p11-13 with:

void wait(unique_lock<mutex>& lock);

...

11 Postcondition: lock is locked by the calling thread lock.owns_lock() is true.

12 Throws: std::system_error when the effects or postcondition cannot be achieved if the implementation detects that the preconditions are not met or the effects cannot be achieved. Any exception thrown by lock.lock() or lock.unlock().

13 Error Conditions: The error conditions are implementation defined.

  • equivalent error condition from lock.lock() or lock.unlock().

Proposed resolution:

Replace 30.5.1 [thread.condition.condvar] p12, p19 and 30.5.2 [thread.condition.condvarany] p10, p16:

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

Error conditions:

Throws: It is implementation-defined whether a std::system_error with implementation-defined error condition is thrown if the precondition is not met.


967. Various threading bugs #17

Section: 30.3.1.2 [thread.thread.constr] Status: Ready Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-10-22

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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. ]

[ 2009-09-25 Beman provided proposed wording. ]

The proposed resolution assumes 962 has been accepted and its proposed resolution applied to the working paper.

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Change Mutex requirements 30.4.1 [thread.mutex.requirements], paragraph 4, as indicated:

Error conditions:

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

condition_variable();

Effects: Constructs an object of type condition_variable.

Throws: std::system_error when an exception is required (30.2.2 [thread.req.exception]).

Error conditions:

  • not_enough_memory — if a memory limitation prevents initialization.
  • resource_unavailable_try_again — if some non-memory resource limitation prevents initialization.
  • device_or_resource_busy — if attempting to initialize a previously-initialized but as of yet undestroyed condition_variable.

968. Various threading bugs #18

Section: 30.4.1 [thread.mutex.requirements] Status: Ready Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2009-10-22

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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. ]

[ 2009-10 Santa Cruz: ]

Moved to Ready with minor word-smithing in the example.

Proposed resolution:

Change Exceptions 30.2.2 [thread.req.exception] as indicated:

Some functions described in this Clause are specified to throw exceptions of type system_error (19.5.5). Such exceptions shall be thrown if any of the Error conditions are detected or a call to an operating system or other underlying API results in an error that prevents the library function from meeting its specifications. [Note: See 17.6.4.11 [res.on.exception.handling] for exceptions thrown to report storage allocation failures. —end note]

[Example:

Consider a function in this clause that is specified to throw exceptions of type system_error and specifies Error conditions that include operation_not_permitted for a thread that does not have the privilege to perform the operation. Assume that, during the execution of this function, an errno of EPERM is reported by a POSIX API call used by the implementation. Since POSIX specifies an errno of EPERM when "the caller does not have the privilege to perform the operation", the implementation maps EPERM  to an error_condition of operation_not_permitted (19.5 [syserr]) and an exception of type system_error is thrown.

—end example]

Editorial note: For the sake of exposition, the existing text above is shown with the changes proposed in issues 962 and 967. The proposed additional example is independent of whether or not the 962 and 967 proposed resolutions are accepted.

Change Mutex requirements 30.4.1 [thread.mutex.requirements], paragraph 4, as indicated:

operation_not_permitted — if the thread does not have the necessary permission to change the state of the mutex object privilege to perform the operation.

Change Mutex requirements 30.4.1 [thread.mutex.requirements], paragraph 12, as indicated:

operation_not_permitted — if the thread does not have the necessary permission to change the state of the mutex privilege to perform the operation.


974. duration<double> should not implicitly convert to duration<int>

Section: 20.9.3.1 [time.duration.cons] Status: Ready Submitter: Howard Hinnant Opened: 2009-01-21 Last modified: 2009-10-26

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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.

[ 2009-07 Frankfurt ]

Moved from Tentatively Ready to Open only because the wording needs to be improved for enable_if type constraining, possibly following Robert's formula.

[ 2009-08-01 Howard adds: ]

Addressed by 1177.

[ 2009-10 Santa Cruz: ]

Not completely addressed by 1177. Move to 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]

978. Hashing smart pointers

Section: 20.7.16 [unord.hash] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-02-02 Last modified: 2010-02-09

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Discussion:

Addresses UK 208

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.

[ 2009-07-28 Alisdair provides wording. ]

[ 2009-10 Santa Cruz: ]

Move to Ready.

[ 2009-11-16 Moved from Ready to Open: ]

Pete writes:

As far as I can see, "...suitable for using this type as key in unordered associative containers..." doesn't define any semantics. It's advice to the reader, and if it's present at all it should be in a note. But we have far too much of this sort of editorial commentary as it is.

And in the resolution of 978 it's clearly wrong: it says that if there is no hash specialization available for D::pointer, the implementation may provide hash<unique_ptr<T,D>> if the result is not suitable for use in unordered containers.

Howard writes:

Is this a request to pull 978 from Ready?

Barry writes:

I read this as more than a request. The PE says it's wrong, so it can't be Ready.

[ 2010-01-31 Alisdair: related to 1245 and 1182. ]

[ 2010-02-08 Beman updates wording. ]

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add the following declarations to the synopsis of <memory> in 20.8 [memory]

// [util.smartptr.hash] hash support
template <class T> struct hash;
template <class T, class D> struct hash<unique_ptr<T,D>>;
template <class T> struct hash<shared_ptr<T>>;

Add a new subclause under 20.8.15 [util.smartptr] called hash support

hash support [util.smartptr.hash]

template <class T, class D> struct hash<unique_ptr<T,D>>;

Specialization meeting the requirements of class template hash (20.7.16 [unord.hash]). For an object p of type UP, where UP is a type unique_ptr<T,D>, hash<UP>()(p) shall evaluate to the same value as hash<typename UP::pointer>()(p.get()). The specialization hash<typename UP::pointer> is required to be well-formed.

template <class T> struct hash<shared_ptr<T>>;

Specialization meeting the requirements of class template hash (20.7.16 [unord.hash]). For an object p of type shared_ptr<T>, hash<shared_ptr<T>>()(p) shall evaluate to the same value as hash<T*>()(p.get()).


983. unique_ptr reference deleters should not be moved from

Section: 20.8.14.2 [unique.ptr.single] Status: Ready Submitter: Howard Hinnant Opened: 2009-02-10 Last modified: 2009-10-21

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Discussion:

Dave brought to my attention that when a unique_ptr has a non-const reference type deleter, move constructing from it, even when the unique_ptr containing the reference is an rvalue, could have surprising results:

D d(some-state);
unique_ptr<A, D&> p(new A, d);
unique_ptr<A, D> p2 = std::move(p);
// has d's state changed here?

I agree with him. It is the unique_ptr that is the rvalue, not the deleter. When the deleter is a reference type, the unique_ptr should respect the "lvalueness" of the deleter.

Thanks Dave.

[ Batavia (2009-05): ]

Seems correct, but complicated enough that we recommend moving to Review.

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Change 20.8.14.2.1 [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 20.8.14.2.3 [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 20.8.14.2.3 [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.


985. Allowing throwing move

Section: 23.2.1 [container.requirements.general] Status: Open Submitter: Rani Sharoni Opened: 2009-02-12 Last modified: 2009-10-20

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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.

[ 2009-10 Santa Cruz: ]

Should wait to get direction from Dave/Rani (N2983).

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.

987. reference_wrapper and function types

Section: 20.7.5 [refwrap] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2009-02-18 Last modified: 2009-10-26

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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?

[ 2009-07 Frankfurt ]

Moved from Review to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-10-14 Daniel provided de-conceptified wording. ]

[ 2009-10 post-Santa Cruz: ]

Move to Tentatively Ready.

Proposed resolution:

Change 20.7.5 [refwrap]/1 as indicated:

reference_wrapper<T> is a CopyConstructible and CopyAssignable wrapper around a reference to an object or function of type T.

Rationale:

a) The occurrence of T& in the function signature auto-implies std::ReferentType, this is due to [temp.req.impl]/4 bullet 4

b) The occurrence of the constrained template reference_wrapper<T> in the remaining signatures lets kick in [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!


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:


999. Taking the address of a function

Section: 20.8.13 [specialized.algorithms] Status: Tentatively Ready Submitter: Peter Dimov Opened: 2009-03-09 Last modified: 2009-10-26

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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.

[ 2009-07 Frankfurt ]

Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-10-10 Daniel updates wording to concept-free. ]

[ 2009-10 post-Santa Cruz: ]

Move to Tentatively Ready.

Proposed resolution:

[ The resolution assumes that addressof is reintroduced as described in n2946 ]

In 20.8.13 [specialized.algorithms] change as described:

template <class T> T* addressof(T& r);
Returns: The actual address of the object or function referenced by r, even in the presence of an overloaded operator&.

Rationale:

a) The occurrence of T& in the function signature auto-implies std::ReferentType, this is due to [temp.req.impl]/4 bullet 4


1008. nested_exception wording unclear

Section: 18.8.6 [except.nested] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-10-22

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Discussion:

Addresses JP 31

It is difficult to understand in which case nested_exception is applied.

[ Summit: ]

Alisdair will add an example in an update to N2619.

[ 2009-10 Santa Cruz: ]

It doesn't appear that N2619 really addresses this. Alisdair to propose wording.

Proposed resolution:


1011. next/prev wrong iterator type

Section: 24.4.4 [iterator.operations] Status: Ready Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-10-22

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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.

[ 2009-07 Frankfurt ]

Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-10-14 Daniel provided de-conceptified wording. ]

[ 2009-10 Santa Cruz: ]

Moved to Ready.

Proposed resolution:

  1. Change header <iterator> synopsis 24.3 [iterator.synopsis] as indicated:

    // 24.4.4, iterator operations:
    ...
    template <class InputForwardIterator>
      InputForwardIterator
      next(InputForwardIterator x, typename std::iterator_traits<InputForwardIterator>::difference_type n = 1);
    
  2. Change 24.4.4 [iterator.operations] before p.6 as indicated:

    template <class InputForwardIterator>
      InputForwardIterator
      next(InputForwardIterator x, typename std::iterator_traits<InputForwardIterator>::difference_type n = 1);
    

1030. Response to JP 44

Section: 20.8.15.5 [util.smartptr.shared.atomic] Status: Ready Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2009-10-23

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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.

[ 2009-07 post-Frankfurt: ]

Lawrence explained that these signatures match the regular atomics. The regular atomics must not use references because these signatures are shared with C. The decision to pass shared_ptrs by pointer rather than by reference was deliberate and was motivated by the principle of least surprise.

Lawrence to write wording that requires that the pointers not be null.

[ 2009-09-20 Lawrence provided wording: ]

The parameter types for atomic shared pointer access were deliberately chosen to be pointers to match the corresponding parameters of the atomics chapter. Those in turn were deliberately chosen to match C functions, which do not have reference parameters.

We adopt the second suggestion, to require that such pointers not be null.

[ 2009-10 Santa Cruz: ]

Moved to Ready.

Proposed resolution:

In section "shared_ptr atomic access" 20.8.15.5 [util.smartptr.shared.atomic], add to each function the following clause.

Requires: p shall not be null.


1033. thread::join() effects?

Section: 30.3.1.5 [thread.thread.member] Status: Tentatively Ready Submitter: Alberto Ganesh Barbati Opened: 2009-03-12 Last modified: 2010-02-12

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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.

[ 2009-11-18 Anthony provides wording. ]

[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Edit 30.3.1.5 [thread.thread.member] as indicated:

void join();

5 Precondition: joinable() is true.

Effects: Blocks until the thread represented by *this has completed.

6 Synchronization: The completion of the thread represented by *this happens before (1.10 [intro.multithread]) join() returns. [Note: Operations on *this are not synchronized. — end note]

7 Postconditions: If join() throws an exception, the value returned by get_id() is unchanged. Otherwise, The thread represented by *this has completed. get_id() == id().

8 ...


1034. Response to UK 222

Section: 23.2.1 [container.requirements.general] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2010-02-12

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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.

[ 2009-07 post-Frankfurt: ]

We agree in principle, but we have a timetable. This group feels that the issue should be closed as NAD unless a proposed resolution is submitted prior to the March 2010 meeting.

[ 2009-10 Santa Cruz: ]

Looked at this and still intend to close as NAD in March 2010 unless there is proposed wording that we like.

[ 2010-02-02 Nicolai M. Josuttis updates proposed wording and adds: ]

I just came across issue #1034 (response to UK 222), which covers the role of container requirements. The reason I found this issue was that I am wondering why array<> is specified to be a sequence container. For me, currently, this follows from Sequence containers 23.2.3 [sequence.reqmts] saying:

The library provides five basic kinds of sequence containers: array, vector, forward_list, list, and deque. while later on in Table 94 "Sequence container requirements" are defined.

IMO, you can hardly argue that this is NAD. We MUST say somewhere that either array is not a sequence container or does not provide all operations of a sequence container (even not all requirements of a container in general).

Here is the number of requirements array<> does not meet (AFAIK):

general container requirements:

Note also that swap not only has linear complexity it also invalidates iterators (or to be more precise, assigns other values to the elements), which is different from the effect swap has for other containers. For this reason, I must say that i tend to propose to remove swap() for arrays.

sequence container requirements:

In fact, out of all sequence container requirements array<> only provides the following operations: from sequence requirements (Table 94):

X(il);
a = il;

and from optional requirements (Table 95):

[], at(), front(), back()

This is almost nothing!

Note in addition, that due to the fact that array is an aggregate and not a container with initializer_lists a construction or assignment with an initializer list is valid for all sequence containers but not valid for array:

vector<int>  v({1,2,3});   // OK
v = {4,5,6};               // OK

array<int,3> a({1,2,3});   // Error
array<int,3> a = {1,2,3};  // OK
a = {4,5,6};               // Error

BTW, for this reason, I am wondering, why <array> includes <initializer_list>.

IMO, we can't really say that array is a sequence container. array is special. As the solution to this issue seemed to miss some proposed wording where all could live with, let me try to suggest some.

[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

In Sequence containers 23.2.3 [sequence.reqmts] modify paragraph 1 as follows:

1 A sequence container organizes a finite set of objects, all of the same type, into a strictly linear arrangement. The library provides five four basic kinds of sequence containers: array, vector, forward_list, list, and deque. In addition, array is provided as a sequence container, which only provides limited sequence operations because it has a fixed number of elements. It also provides container adaptors that make it easy to construct abstract data types, such as stacks or queues, out of the basic sequence container kinds (or out of other kinds of sequence containers that the user might define).

BTW, modify paragraph 2 as follows (just editorial):

2 list, and deque The five basic sequence containers offer the programmer different complexity trade-offs and should be used accordingly. vector or array is the type of sequence container that should be used by default. list or forward_list should be used when there are frequent insertions and deletions from the middle of the sequence. deque is the data structure of choice when most insertions and deletions take place at the beginning or at the end of the sequence.

In Class template array 23.3.1 [array] replace paragraph 3 by:

3 Unless otherwise specified, all array operations are as described in 23.2. An array satisfies all of the requirements of a container and of a reversible container (given in two tables in 23.2 [container.requirements]) except that a default constructed array is not empty and that swap has no constant complexity. An array satisfies some of the requirements of a sequence container (given in 23.2.3 [sequence.reqmts]). Descriptions are provided here only for operations on array that are not described in that Clause in one of these tables or for operations where there is additional semantic information.

In array specialized algorithms 23.3.1.2 [array.special] add to the specification of swap():

template <class T, size_t N> void swap(array<T,N>& x, array<T,N>& y);

1 Effects: ...

Complexity: Linear in N.

OPEN: remove from 23.3 [sequences]:

Header <array> synopsis

namespace std {
  #include <initializer_list>
  ...

1052. Response to UK 281

Section: 24.5.1.3.5 [reverse.iter.opref] Status: Ready Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-10-22

View all issues with Ready status.

Discussion:

Addresses UK 281

The current specification for return value for reverse_iterator::operator-> will always be a true pointer type, but reverse_iterator supports proxy iterators where the pointer type may be some kind of 'smart pointer'.

[ Summit: ]

move_iterator avoids this problem by returning a value of the wrapped Iterator type. study group formed to come up with a suggested resolution.

move_iterator solution shown in proposed wording.

[ 2009-07 post-Frankfurt: ]

Howard to deconceptize. Move to Review after that happens.

[ 2009-08-01 Howard deconceptized: ]

[ 2009-10 Santa Cruz: ]

We can't think of any reason we can't just define reverse iterator's pointer types to be the same as the underlying iterator's pointer type, and get it by calling the right arrow directly.

Here is the proposed wording that was replaced:

template <class Iterator> 
class reverse_iterator { 
  ...
  typedef typename iterator_traits<Iterator>::pointer pointer;

Change 24.5.1.3.5 [reverse.iter.opref]:

pointer operator->() const;
Returns:
&(operator*());
this->tmp = current;
--this->tmp;
return this->tmp;

Proposed resolution:

Change 24.5.1.3.5 [reverse.iter.opref]:

pointer operator->() const;
Returns:
&(operator*());
deref_tmp = current;
--deref_tmp;
return deref_tmp::operator->();

1056. Must all Engines and Distributions be Streamable?

Section: 26.5 [rand] Status: Tentatively NAD Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2009-11-03

View all other issues in [rand].

View all issues with Tentatively NAD status.

Discussion:

Both the concepts RandomNumberEngine and RandomNumberDistribution have requirements to be InputStreamable and OutputStreamable.

I have no problems leaving the WP in an inconsistent state on the best-faith assumption these concepts will be provided later, however disagree with the proposers that these constraints are not separable, orthogonal to the basic concepts of generating random number distributions.

These constraints should be dropped, and applied to specific algorithms as needed.

If a more refined concept (certainly deemed useful by the proposers) is proposed there is no objection, but the basic concept should not require persistence via streaming.

[ Batavia (2009-05): ]

Move to Open.

[ 2009-05-31 Alisdair adds: ]

Working on constraining the stream iterators, I have a few more observations to make on the concepts proposed while constraining the random number facility.

While I still believe the concerns are orthogonal, I don't believe the existing constraints go far enough either! The goal we want to achieve is not that a RandomNumberEngine / RandomNumberDistribution supports the stream operators, but that it is Serializable. I.e. there is a relationship between the insert and extract operations that guarantees to restore the state of the original object. This implies a coupling of the concepts together in a broader concept (Serializable) with at least one axiom to assert the semantics.

One problem is that istream and ostream may be fundamentally different types, although we can hook a relation if we are prepared to drop down to the char type and char_traits template parameters. Doing so ties us to a form of serialization that demands implementation via the std iostreams framework, which seems overly prescriptive. I believe the goal is generally to support serialization without regard to how it is expressed - although this is getting even more inventive in terms of concepts we do not have today.

[ 2009-11-03 Alisdair adds: ]

I can't find the record in the wiki minutes, but it was agreed at both Frankfurt and Santa Cruz that this issue is NAD.

The agreement in SC was that I would provide you with the rationale (see below) to include when moving to NAD.

[ 2009-11-03 Howard adds: ]

Moved to Tentatively NAD after 5 positive votes on c++std-lib.

Proposed resolution:

Rationale:

The issue suggests a more refined concept should be used if we want to require streaming, to separate concerns from the basic RandomNumberEngine behaviour. In Frankfurt it was observed that RandomNumberEngine is that more refined concept, and the basic concept used in the framework is UniformRandomNumberGenerator, which it refines.

We concur, and expect this to have no repurcussions re-writing this clause now concepts are removed.


1068. class random_device should be movable

Section: 26.5.6 [rand.device] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-18 Last modified: 2009-10-26

View all other issues in [rand.device].

View all issues with Open status.

Discussion:

class random_device should be movable.

[ Batavia (2009-05): ]

Move to Open, and recommend this issue be deferred until after the next Committee Draft is issued.

[ 2009-10 post-Santa Cruz: ]

Leave open. Walter to provide drafting as part of his planned paper.

Proposed resolution:


1069. class seed_seq should support efficient move operations

Section: 26.5.7.1 [rand.util.seedseq] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-18 Last modified: 2009-10-26

View other active issues in [rand.util.seedseq].

View all other issues in [rand.util.seedseq].

View all issues with Open status.

Discussion:

class seed_seq should support efficient move operations.

[ Batavia (2009-05): ]

Move to Open, and recommend this issue be deferred until after the next Committee Draft is issued.

[ 2009-10 post-Santa Cruz: ]

Leave open. Walter to provide drafting as part of his planned paper.

Proposed resolution:


1071. is_bind_expression should derive from integral_constant<bool>

Section: 20.7.11.1.1 [func.bind.isbind] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2009-10-26

View all issues with Tentatively Ready status.

Discussion:

Class template is_bind_expression 20.7.11.1.1 [func.bind.isbind]:

namespace std {
  template<class T> struct is_bind_expression {
    static const bool value = see below;
  };
}

is_bind_expression should derive from std::integral_constant<bool> like other similar trait types.

[ Daniel adds: ]

We need the same thing for the trait is_placeholder as well.

[ 2009-03-22 Daniel provided wording. ]

[ Batavia (2009-05): ]

We recommend this be deferred until after the next Committee Draft is issued.

Move to Open.

[ 2009-05-31 Peter adds: ]

I am opposed to the proposed resolution and to the premise of the issue in general. The traits's default definitions should NOT derive from integral_constant, because this is harmful, as it misleads people into thinking that is_bind_expression<E> always derives from integral_constant, whereas it may not.

is_bind_expression and is_placeholder allow user specializations, and in fact, this is their primary purpose. Such user specializations may not derive from integral_constant, and the places where is_bind_expression and is_placeholder are used intentionally do not require such derivation.

The long-term approach here is to switch to BindExpression<E> and Placeholder<P> explicit concepts, of course, but until that happens, I say leave them alone.

[ 2009-10 post-Santa Cruz: ]

Move to Tentatively Ready. We are comfortable with requiring user specializations to derive from integral_constant.

Proposed resolution:

  1. In 20.7.11.1.1 [func.bind.isbind] change as indicated:

    namespace std {
     template<class T> struct is_bind_expression : integral_constant<bool, see below> { };{
       static const bool value = see below;
     };
    }
    
  2. In 20.7.11.1.1 [func.bind.isbind]/2 change as indicated:

    static const bool value;
    
    -2- true if T is a type returned from bind, false otherwise. If T is a type returned from bind, is_bind_expression<T> shall be publicly derived from integral_constant<bool, true>, otherwise it shall be publicly derived from integral_constant<bool, false>.
  3. In 20.7.11.1.2 [func.bind.isplace] change as indicated:

    namespace std {
     template<class T> struct is_placeholder : integral_constant<int, see below> { };{
       static const int value = see below;
     };
    }
    
  4. In 20.7.11.1.2 [func.bind.isplace]/2 change as indicated:

    static const int value;
    
    -2- value is J if T is the type of std::placeholders::_J, 0 otherwise. If T is the type of std::placeholders::_J, is_placeholder<T> shall be publicly derived from integral_constant<int, J> otherwise it shall be publicly derived from integral_constant<int, 0>.

1076. unary/binary_negate need constraining and move support

Section: 20.7.10 [negators] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-20 Last modified: 2010-01-31

View all issues with Open status.

Discussion:

The class templates unary/binary_negate need constraining and move support.

Ideally these classes would be deprecated, allowing unary/binary_function to also be deprecated. However, until a generic negate adaptor is introduced that can negate any Callable type, they must be supported so should be constrained. Likewise, they should be movable, and support adopting a move-only predicate type.

In order to preserve ABI compatibility, new rvalue overloads are supplied in preference to changing the existing pass-by-const-ref to pass-by-value.

Do not consider the issue of forwarding mutable lvalues at this point, although remain open to another issue on the topic.

[ 2009-05-01 Daniel adds: ]

IMO the currently proposed resolution needs some updates because it is ill-formed at several places:

  1. In concept AdaptableUnaryFunction change

    typename X::result_type;
    typename X::argument_type;
    

    to

    Returnable result_type = typename X::result_type;
    typename argument_type = typename X::argument_type;
    

    [The replacement "Returnable result_type" instead of "typename result_type" is non-editorial, but maybe you prefer that as well]

  2. In concept AdaptableBinaryFunction change

    typename X::result_type;
    typename X::first_argument_type;
    typename X::second_argument_type;
    

    to

    Returnable result_type = typename X::result_type;
    typename first_argument_type = typename X::first_argument_type;
    typename second_argument_type = typename X::second_argument_type;
    

    [The replacement "Returnable result_type" instead of "typename result_type" is non-editorial, but maybe you prefer that as well.]

  3. In class unary/binary_function

    1. I suggest to change "ReturnType" to "Returnable" in both cases.
    2. I think you want to replace the remaining occurrences of "Predicate" by "P" (in both classes in copy/move from a predicate)
  4. I think you need to change the proposed signatures of not1 and not2, because they would still remain unconstrained: To make them constrained at least a single requirement needs to be added to enable requirement implication. This could be done via a dummy ("requires True<true>") or just explicit as follows:

    1. template <AdaptableUnaryFunction P>
      requires Predicate< P, P::argument_type>
      unary_negate<P> not1(const P&& pred);
      template <AdaptableUnaryFunction P>
      requires Predicate< P, P::argument_type >
      unary_negate<P> not1(P&& pred);
      
      -3- Returns: unary_negate<P>(pred).

      [Don't we want a move call for the second overload as in

      unary_negate<P>(std::move(pred))
      

      in the Returns clause ?]

    2. template <AdaptableBinaryFunction P>
      requires Predicate< P, P::first_argument_type, P::second_argument_type >
      binary_negate<P> not2(const P& pred);
      template <AdaptableBinaryFunction P>
      requires Predicate< P, P::first_argument_type, P::second_argument_type >
      binary_negate<P> not2(P&& pred);
      

      -5- Returns: binary_negate<P>(pred).

      [Don't we want a move call for the second overload as in

      binary_negate<P>(std::move(pred))
      

      in the Returns clause ?]

[ Batavia (2009-05): ]

There is concern that complicating the solution to preserve the ABI seems unnecessary, since we're not in general preserving the ABI.

We would prefer a separate paper consolidating all Clause 20 issues that are for the purpose of providing constrained versions of the existing facilities.

Move to Open.

[ 2009-10 post-Santa Cruz: ]

Leave open pending the potential move constructor paper. Note that we consider the "constraining" part NAD Concepts.

[ 2010-01-31 Alisdair removes the current proposed wording from the proposed wording section because it is based on concepts. That wording is proposed here: ]

Add new concepts where appropriate::

auto concept AdaptableUnaryFunction< typename X > {
  typename X::result_type;
  typename X::argument_type;
}

auto concept AdaptableBinaryFunction< typename X > {
  typename X::result_type;
  typename X::first_argument_type;
  typename X::second_argument_type;
}

Revise as follows:

Base 20.7.3 [base] (Only change is constrained Result)

-1- The following classes are provided to simplify the typedefs of the argument and result types:

namespace std {
  template <class Arg, class ReturnType Result>
  struct unary_function {
     typedef Arg    argument_type;
     typedef Result result_type;
  };

  template <class Arg1, class Arg2, class ReturnType Result>
  struct binary_function {
     typedef Arg1   first_argument_type;
     typedef Arg2   second_argument_type;
     typedef Result result_type;
  };
}

Negators 20.7.10 [negators]:

-1- Negators not1 and not2 take a unary and a binary predicate, respectively, and return their complements (5.3.1).

template <class AdaptableUnaryFunction Predicate>
  requires Predicate< P, P::argument_type >
  class unary_negate
    : public unary_function<typename Predicate::argument_type,bool> {
  public:
    unary_negate(const unary_negate & ) = default;
    unary_negate(unary_negate && );

    requires CopyConstructible< P >
       explicit unary_negate(const Predicate& pred); 
    requires MoveConstructible< P >
       explicit unary_negate(Predicate && pred);

    bool operator()(const typename Predicate::argument_type& x) const;
  };
-2 operator() returns !pred(x).
template <class Predicate>
  unary_negate<Predicate> not1(const Predicate&amp; pred);
template <class Predicate>
  unary_negate<Predicate> not1(Predicate&& pred);
-3- Returns: unary_negate<Predicate>(pred).
template <class AdaptableBinaryFunction Predicate >
  requires Predicate< P, P::first_argument_type, P::second_argument_type >
  class binary_negate
    : public binary_function<typename Predicate::first_argument_type,
                              typename Predicate::second_argument_type, bool> {
  public:
    biary_negate(const binary_negate & ) = default;
    binary_negate(binary_negate && );

    requires CopyConstructible< P >
       explicit binary_negate(const Predicate& pred);
    requires MoveConstructible< P >
       explicit binary_negate(const Predicate& pred);

    bool operator()(const typename Predicate::first_argument_type& x,
                    const typename Predicate::second_argument_type& y) const;
  };
-4- operator() returns !pred(x,y).
template <class Predicate>
  binary_negate<Predicate> not2(const Predicate& pred);
template <class Predicate>
  binary_negate<Predicate> not2(Predicate&& pred);
-5- Returns: binary_negate<Predicate>(pred).

Proposed resolution:


1079. UK-265: RandomAccessIterator's operator- has nonsensical effects clause

Section: 24.2.5 [random.access.iterators] Status: Open Submitter: Doug Gregor Opened: 2009-03-20 Last modified: 2010-02-13

View all other issues in [random.access.iterators].

View all issues with Open status.

Discussion:

Addresses UK 265

UK-265:

This effects clause is nonesense. It looks more like an axiom stating equivalence, and certainly an effects clause cannot change the state of two arguments passed by const reference

[ 2009-09-18 Alisdair adds: ]

For random access iterators, the definitions of (b-a) and (a<b) are circular:

From table Table 104 -- Random access iterator requirements:

b - a :==>  (a < b) ? distance(a,b) : -distance(b,a)

a < b :==>  b - a > 0

[ 2009-10 Santa Cruz: ]

Moved to Ready.

[ 2010-02-13 Alisdair opens. ]

Looking again at LWG #1079, the wording in the issue no longer exists, and appears to be entirely an artefact of the concepts wording.

This issue is currently on our Ready list (not even Tentative!) but I think it has to be pulled as there is no way to apply the resolution.

Looking at the current paper, I think this issue is now "NAD, solved by the removal of concepts". Unfortunately it is too late to poll again, so we will have to perform that review in Pittsburgh.

[ 2010-02-13 Daniel updates the wording to address the circularity problem. ]

[ The previous wording is preserved here: ]

Modify 24.2.5 [random.access.iterators]p7-9 as follows:

difference_type operator-(const X& a, const X& b);
  1. Precondition: there exists a value n of difference_type such that a == b + n.
  2. Effects: b == a + (b - a)
  3. Returns: (a < b) ? distance(a,b) : -distance(b,a)n

Proposed resolution:

Modify Table 105 in 24.2.5 [random.access.iterators]:

Table 105 — Random access iterator requirements (in addition to bidirectional iterator)
Expression Return type Operational semantics Assertion/note
pre-/post-condition
b - a Distance distance(a,b) Returns: n pre: there exists a value n of Distance such that a + n == b. b == a + (b - a).

1089. Response to JP 76

Section: 30 [thread] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2010-02-11

View all other issues in [thread].

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Discussion:

Addresses JP 76

A description for "Throws: Nothing." are not unified.

At the part without throw, "Throws: Nothing." should be described.

Add "Throws: Nothing." to the following.

[ Summit: ]

Pass on to editor.

[ Post Summit: Editor declares this non-editorial. ]

[ 2009-08-01 Howard provided wording: ]

The definition of "Throws: Nothing." that I added is probably going to be controversial, but I beg you to consider it seriously.

In C++ there are three "flow control" options for a function:

  1. It can return, either with a value, or with void.
  2. It can call a function which never returns, such as std::exit or std::terminate.
  3. It can throw an exception.
The above list can be abbreviated with:
  1. Returns.
  2. Ends program.
  3. Throws exception.

In general a function can have the behavior of any of these 3, or any combination of any of these three, depending upon run time data.

  1. R
  2. E
  3. T
  4. RE
  5. RT
  6. ET
  7. RET

A function with no throw spec, and no documentation, is in general a RET function. It may return, it may end the program, or it may throw. When we specify a function with an empty throw spec:

void f() throw();

We are saying that f() is an RE function: It may return or end the program, but it will not throw.

I posit that there are very few places in the library half of the standard where we intend for functions to be able to end the program (call terminate). And none of those places where we do say terminate could be called, do we currently say "Throws: Nothing.".

I believe that if we define "Throws: Nothing." to mean R, we will both clarify many, many places in the standard, and give us a good rationale for choosing between "Throws: Nothing." (R) and throw() (RE) in the future. Indeed, this may give us motivation to change several throw()s to "Throws: Nothing.".

I did not add the following changes as JP 76 requested as I believe we want to allow these functions to throw:

Add a paragraph under 30.4.3.1 [thread.lock.guard] p4:

explicit lock_guard(mutex_type& m);

Throws: Nothing.

Add a paragraph under 30.4.3.2.1 [thread.lock.unique.cons] p6:

explicit unique_lock(mutex_type& m);

Throws: Nothing.

Add a paragraph under 30.5.2 [thread.condition.condvarany] p19, p21 and p25:

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

Throws: Nothing.

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

Throws: Nothing.

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

Throws: Nothing.

[ 2009-10 Santa Cruz: ]

Defer pending further developments with exception restriction annotations.

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add a paragraph after 17.5.1.4 [structure.specifications] p4:

-3- Descriptions of function semantics contain the following elements (as appropriate):158

-4- For non-reserved replacement and handler functions, ...

A "Throws: Nothing." element indicates that the function shall return ordinarily, and not exit via an exception. This element also indicates that the function shall return. [Note: This differs from an empty throw specification which may cause a function to call unexpected and subsequently terminate. — end note]

Add a paragraph under 30.3.1.6 [thread.thread.static] p1:

unsigned hardware_concurrency();

-1- Returns: ...

Throws: Nothing.

Add a paragraph under 30.5.1 [thread.condition.condvar] p7 and p8:

[Informational, not to be incluced in the WP: The POSIX spec allows only:

[EINVAL]
The value cond does not refer to an initialized condition variable. — end informational]
void notify_one();

-7- Effects: ...

Throws: Nothing.

void notify_all();

-8- Effects: ...

Throws: Nothing.

Add a paragraph under 30.5.2 [thread.condition.condvarany] p6 and p7:

void notify_one();

-6- Effects: ...

Throws: Nothing.

void notify_all();

-7- Effects: ...

Throws: Nothing.


1090. Missing description of packaged_task member swap, missing non-member swap

Section: 30.6.10 [futures.task] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2009-03-22 Last modified: 2009-10-26

View all issues with Tentatively Ready status.

Discussion:

Class template packaged_task in 30.6.10 [futures.task] shows a member swap declaration, but misses to document it's effects (No prototype provided). Further on this class misses to provide a non-member swap.

[ Batavia (2009-05): ]

Alisdair notes that paragraph 2 of the proposed resolution has already been applied in the current Working Draft.

We note a pending future-related paper by Detlef; we would like to wait for this paper before proceeding.

Move to Open.

[ 2009-05-24 Daniel removed part 2 of the proposed resolution. ]

[ 2009-10 post-Santa Cruz: ]

Move to Tentatively Ready, removing bullet 3 from the proposed resolution but keeping the other two bullets.

Proposed resolution:

  1. In 30.6.10 [futures.task], immediately after the definition of class template packaged_task add:

    
    template<class R, class... Argtypes>
    void swap(packaged_task<R(ArgTypes...)>&, packaged_task<R(ArgTypes...)>&);
    
    
  1. At the end of 30.6.10 [futures.task] (after p. 20), add add the following prototype description:

    
    template<class R, class... Argtypes>
    void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y);
    

    Effects: x.swap(y)

    Throws: Nothing.


1093. Multiple definitions for random_shuffle algorithm

Section: 25.3.12 [alg.random.shuffle] Status: Open Submitter: Alisdair Meredith Opened: 2009-03-22 Last modified: 2009-10-26

View all other issues in [alg.random.shuffle].

View all issues with Open status.

Discussion:

There are a couple of issues with the declaration of the random_shuffle algorithm accepting a random number engine.

  1. The Iterators must be shuffle iterators, yet this requirement is missing.
  2. The RandomNumberEngine concept is now provided by the random number library (n2836) and the placeholder should be removed.

[ 2009-05-02 Daniel adds: ]

this issue completes adding necessary requirement to the third new random_shuffle overload. The current suggestion is:

template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand>
requires ShuffleIterator<Iter>
void random_shuffle(Iter first, Iter last, Rand&& g);

IMO this is still insufficient and I suggest to add the requirement

Convertible<Rand::result_type, Iter::difference_type>

to the list (as the two other overloads already have).

Rationale:

Its true that this third overload is somewhat different from the remaining two. Nevertheless we know from UniformRandomNumberGenerator, that it's result_type is an integral type and that it satisfies UnsignedIntegralLike<result_type>.

To realize it's designated task, the algorithm has to invoke the Callable aspect of g and needs to perform some algebra involving it's min()/max() limits to compute another index value that at this point is converted into Iter::difference_type. This is so, because 24.2.5 [random.access.iterators] uses this type as argument of it's algebraic operators. Alternatively consider the equivalent iterator algorithms in 24.4.4 [iterator.operations] with the same result.

This argument leads us to the conclusion that we also need Convertible<Rand::result_type, Iter::difference_type> here.

[ Batavia (2009-05): ]

Alisdair notes that point (ii) has already been addressed.

We agree with the proposed resolution to point (i) with Daniel's added requirement.

Move to Review.

[ 2009-06-05 Daniel updated proposed wording as recommended in Batavia. ]

[ 2009-07-28 Alisdair adds: ]

Revert to Open, with a note there is consensus on direction but the wording needs updating to reflect removal of concepts.

[ 2009-10 post-Santa Cruz: ]

Leave Open, Walter to work on it.

Proposed resolution:

Change in [algorithms.syn] and 25.3.12 [alg.random.shuffle]:

concept UniformRandomNumberGenerator<typename Rand> { }
template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand>
  requires ShuffleIterator<Iter> &&
  Convertible<Rand::result_type, Iter::difference_type>
  void random_shuffle(Iter first, Iter last, Rand&& g);

1094. Response to JP 65 and JP 66

Section: 27.5.4.3 [iostate.flags] Status: Ready Submitter: P.J. Plauger Opened: 2009-03-24 Last modified: 2009-10-20

View all other issues in [iostate.flags].

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Discussion:

Addresses JP 65 and JP 66

Switch from "unspecified-bool-type" to "explicit operator bool() const".

Replace operator unspecified-bool-type() const;" with explicit operator bool() const;

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Review.

[ 2009 Santa Cruz: ]

Moved to Ready.

Proposed resolution:

Change the synopis in 27.5.4 [ios]:

explicit operator unspecified-bool-type bool() const;

Change 27.5.4.3 [iostate.flags]:

explicit operator unspecified-bool-type bool() const;

-1- Returns: !fail() If fail() then a value that will evaluate false in a boolean context; otherwise a value that will evaluate true in a boolean context. The value type returned shall not be convertible to int.

[Note: This conversion can be used in contexts where a bool is expected (e.g., an if condition); however, implicit conversions (e.g., to int) that can occur with bool are not allowed, eliminating some sources of user error. One possible implementation choice for this type is pointer-to-member. -- end note]


1095. Shared objects and the library wording unclear

Section: 17.6.3.10 [res.on.objects] Status: Ready Submitter: Beman Dawes Opened: 2009-03-27 Last modified: 2009-10-21

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Discussion:

N2775, Small library thread-safety revisions, among other changes, removed a note from 17.6.3.10 [res.on.objects] that read:

[Note: This prohibition against concurrent non-const access means that modifying an object of a standard library type shared between threads without using a locking mechanism may result in a data race. --end note.]

That resulted in wording which is technically correct but can only be understood by reading the lengthy and complex 17.6.4.8 [res.on.data.races] Data race avoidance. This has the effect of making 17.6.3.10 [res.on.objects] unclear, and has already resulted in a query to the LWG reflector. See c++std-lib-23194.

[ Batavia (2009-05): ]

The proposed wording seems to need a bit of tweaking ("really bad idea" isn't quite up to standardese). We would like feedback as to whether the original Note's removal was intentional.

Change the phrase "is a really bad idea" to "risks undefined behavior" and move to Review status.

[ 2009-10 Santa Cruz: ]

Note: Change to read: "Modifying...", Delete 'thus', move to Ready

Proposed resolution:

Change 17.6.3.10 [res.on.objects] as indicated:

The behavior of a program is undefined if calls to standard library functions from different threads may introduce a data race. The conditions under which this may occur are specified in 17.6.4.7.

[Note: Modifying an object of a standard library type shared between threads risks undefined behavior unless objects of the type are explicitly specified as being sharable without data races or the user supplies a locking mechanism. --end note]


1097. #define __STDCPP_THREADS

Section: 18.2 [support.types] Status: Ready Submitter: Jens Maurer Opened: 2009-04-03 Last modified: 2009-10-21

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Discussion:

Addresses DE 18

Freestanding implementations do not (necessarily) have support for multiple threads (see 1.10 [intro.multithread]). Applications and libraries may want to optimize for the absence of threads. I therefore propose a preprocessor macro to indicate whether multiple threads can occur.

There is ample prior implementation experience for this feature with various spellings of the macro name. For example, gcc implicitly defines _REENTRANT if multi-threading support is selected on the compiler command-line.

While this is submitted as a library issue, it may be more appropriate to add the macro in 16.8 cpp.predefined in the core language.

See also N2693.

[ Batavia (2009-05): ]

We agree with the issue, and believe it is properly a library issue.

We prefer that the macro be conditionally defined as part of the <thread> header.

Move to Review.

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Insert a new subsection before 18.2 [support.types], entitled "Feature Macros" (support.macros):

The standard library defines the following macros; no explicit prior inclusion of any header file is necessary.

__STDCPP_THREADS
The macro __STDCPP_THREADS shall be defined if and only if a program can have more than one thread of execution (1.10 [intro.multithread]). If the macro is defined, it shall have the same value as the predefined macro __cplusplus (16.8 [cpp.predefined]).

1098. definition of get_pointer_safety()

Section: 20.8.15.6 [util.dynamic.safety] Status: Ready Submitter: Jens Maurer Opened: 2009-04-03 Last modified: 2009-10-23

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Discussion:

Addresses DE 18

In 20.8.15.6 [util.dynamic.safety], get_pointer_safety() purports to define behavior for non-safely derived pointers (3.7.4.3 [basic.stc.dynamic.safety]). However, the cited core-language section in paragraph 4 specifies undefined behavior for the use of such pointer values. This seems an unfortunate near-contradiction. I suggest to specify the term relaxed pointer safety in the core language section and refer to it from the library description. This issue deals with the library part, the corresponding core issue (c++std-core-13940) deals with the core modifications.

See also N2693.

[ Batavia (2009-05): ]

We recommend if this issue is to be moved, the issue be moved concurrently with the cited Core issue.

We agree with the intent of the proposed resolution. We would like input from garbage collection specialists.

Move to Open.

[ 2009-10 Santa Cruz: ]

The core issue is 853 and is in Ready status.

Proposed resolution:

In 20.8.15.6 [util.dynamic.safety] p16, replace the description of get_pointer_safety() with:

pointer_safety get_pointer_safety();

Returns: an enumeration value indicating the implementation's treatment of pointers that are not safely derived (3.7.4.3). Returns pointer_safety::relaxed if pointers that are not safely derived will be treated the same as pointers that are safely derived for the duration of the program. Returns pointer_safety::preferred if pointers that are not safely derived will be treated the same as pointers that are safely derived for the duration of the program but allows the implementation to hint that it could be desirable to avoid dereferencing pointers that are not safely derived as described. [Example: pointer_safety::preferred might be returned to detect if a leak detector is running to avoid spurious leak reports. -- end note] Returns pointer_safety::strict if pointers that are not safely derived might be treated differently than pointers that are safely derived.

Returns: Returns pointer_safety::strict if the implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]). It is implementation-defined whether get_pointer_safety returns pointer_safety::relaxed or pointer_safety::preferred if the implementation has relaxed pointer safety (3.7.4.3 [basic.stc.dynamic.safety]).Footnote

Throws: nothing

Footnote) pointer_safety::preferred might be returned to indicate to the program that a leak detector is running so that the program can avoid spurious leak reports.


1099. Various issues

Section: 17 [library] Status: Tentatively NAD Submitter: David Abrahams Opened: 2009-03-21 Last modified: 2009-10-26

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Discussion:

Notes

[2009-03-21 Sat] p. 535 at the top we need MoveConstructible V1, MoveConstructible V2 (where V1,V2 are defined on 539). Also make_tuple on 550

CD-1 reads:

template <MoveConstructible T1, MoveConstructible T2> 
pair<V1, V2> make_pair(T1&&, T2&&); 

Actually I'm guessing we need something like MoveConstructible<V1,T1>, i.e. "V1 can be constructed from an rvalue of type T1."

Ditto for make_tuple

[2009-03-21 Sat] p1183 thread ctor, and in general, we need a way to talk about "copiable from generalized rvalue ref argument" for cases where we're going to forward and copy.

This issue may well be quite large. Language in para 4 about "if an lvalue" is wrong because types aren't expressions.

Maybe we should define the term "move" so we can just say in the effects, "f is moved into the newly-created thread" or something, and agree (and ideally document) that saying "f is moved" implies

F x(move(f))

is required to work. That would cover both ctors at once.

p1199, call_once has all the same issues.

[2009-03-21 Sat] p869 InputIterator pointer type should not be required to be convertible to const value_type*, rather it needs to have a operator-> of its own that can be used for the value type.

This one is serious and unrelated to the move issue.

[2009-03-21 Sat] p818 stack has the same problem with default ctor.

[2009-03-21 Sat] p816 priority_queue has the same sorts of problems as queue, only more so

   requires MoveConstructible<Cont> 
     explicit priority_queue(const Compare& x = Compare(), Cont&& = Cont()); 

Don't require MoveConstructible when default constructing Cont. Also missing semantics for move ctor.

[2009-03-21 Sat] Why are Allocators required to be CopyConstructible as opposed to MoveConstructible?

[2009-03-21 Sat] p813 queue needs a separate default ctor (Cont needn't be MoveConstructible). No documented semantics for move c'tor. Or *any* of its 7 ctors!

[2009-03-21 Sat] std::array should have constructors for C++0x, consequently must consider move construction.

[ 2009-05-01 Daniel adds: ]

This could be done as part of 1035, which already handles deviation of std::array from container tables.

[2009-03-21 Sat] p622 all messed up.

para 8 "implementation-defined" is the wrong term; should be "see below" or something.

para 12 "will be selected" doesn't make any sense because we're not talking about actual arg types.

paras 9-13 need to be totally rewritten for concepts.

[2009-03-21 Sat] Null pointer comparisons (p587) have all become unconstrained. Need to fix that

[2009-03-21 Sat] mem_fun_t etc. definition doesn't match declaration. We think CopyConstructible is the right reqt.

make_pair needs Constructible<V1, T1&&> requirements!

make_tuple needs something similar

tuple bug in synopsis:

   template <class... UTypes>
   requires Constructible<Types, const UTypes&>...
   template <class... UTypes>
   requires Constructible<Types, RvalueOf<UTypes>::type>...

Note: removal of MoveConstructible requirements in std::function makes these routines unconstrained!

[ 2009-05-02 Daniel adds: ]

This part of the issue is already covered by 1077.

these unique_ptr constructors are broken [ I think this is covered in "p622 all messed up" ]

 unique_ptr(pointer p, implementation-defined d);
 unique_ptr(pointer p, implementation-defined d);

multimap range constructor should not have MoveConstructible<value_type> requirement.

same with insert(..., P&&); multiset has the same issue, as do unordered_multiset and unordered_multimap. Review these!

[ Batavia (2009-05): ]

Move to Open, pending proposed wording from Dave for further review.

[ 2009-10 post-Santa Cruz: ]

Tentatively NAD. We are not sure what has been addressed and what hasn't. Recommend closing unless someone sorts this out into something more readable.

Proposed resolution:


1100. auto_ptr to unique_ptr conversion

Section: 20.8.14.2.1 [unique.ptr.single.ctor] Status: Ready Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2009-10-21

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Discussion:

Message c++std-lib-23182 led to a discussion in which several people expressed interest in being able to convert an auto_ptr to a unique_ptr without the need to call release. Below is wording to accomplish this.

[ Batavia (2009-05): ]

Pete believes it not a good idea to separate parts of a class's definition. Therefore, if we do this, it should be part of unique-ptr's specification.

Alisdair believes the lvalue overload may be not necessary.

Marc believes it is more than just sugar, as it does ease the transition to unique-ptr.

We agree with the resolution as presented. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-08-01 Howard deconceptifies wording: ]

I also moved the change from D.10 [depr.auto.ptr] to 20.8.14.2 [unique.ptr.single] per the Editor's request in Batavia (as long as I was making changes anyway). Set back to Review.

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Add to 20.8.14.2 [unique.ptr.single]:

template <class T, class D>
class unique_ptr
{
public:
    template <class U>
      unique_ptr(auto_ptr<U>& u);
    template <class U>
      unique_ptr(auto_ptr<U>&& u);
};

Add to 20.8.14.2.1 [unique.ptr.single.ctor]:

template <class U>
  unique_ptr(auto_ptr<U>& u);
template <class U>
  unique_ptr(auto_ptr<U>&& u);

Effects: Constructs a unique_ptr with u.release().

Postconditions: get() == the value u.get() had before the construciton, modulo any required offset adjustments resulting from the cast from U* to T*. u.get() == nullptr.

Throws: nothing.

Remarks: U* shall be implicitly convertible to T* and D shall be the same type as default_delete<T>, else these constructors shall not participate in overload resolution.


1104. basic_ios::move should accept lvalues

Section: 27.5.4.2 [basic.ios.members] Status: Ready Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2009-10-20

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Discussion:

With the rvalue reference changes in N2844 basic_ios::move no longer has the most convenient signature:

void move(basic_ios&& rhs);

This signature should be changed to accept lvalues. It does not need to be overloaded to accept rvalues. This is a special case that only derived clients will see. The generic move still needs to accept rvalues.

[ Batavia (2009-05): ]

Tom prefers, on general principles, to provide both overloads. Alisdair agrees.

Howard points out that there is no backward compatibility issue as this is new to C++0X.

We agree that both overloads should be provided, and Howard will provide the additional wording. Move to Open.

[ 2009-05-23 Howard adds: ]

Added overload, moved to Review.

[ 2009 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Add a signature to the existing prototype in the synopsis of 27.5.4 [ios] and in 27.5.4.2 [basic.ios.members]:

void move(basic_ios& rhs);
void move(basic_ios&& rhs);

1106. Multiple exceptions from connected shared_future::get()?

Section: 30.6.7 [future.shared_future] Status: Tentatively NAD Editorial Submitter: Thomas J. Gritzan Opened: 2009-04-03 Last modified: 2010-01-23

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Discussion:

It is not clear, if multiple threads are waiting in a shared_future::get() call, if each will rethrow the stored exception.

Paragraph 9 reads:

Throws: the stored exception, if an exception was stored and not retrieved before.

The "not retrieved before" suggests that only one exception is thrown, but one exception for each call to get() is needed, and multiple calls to get() even on the same shared_future object seem to be allowed.

I suggest removing "and not retrieved before" from the Throws paragraph. I recommend adding a note that explains that multiple calls on get() are allowed, and each call would result in an exception if an exception was stored.

[ Batavia (2009-05): ]

We note there is a pending paper by Detlef on such future-related issues; we would like to wait for his paper before proceeding.

Alisdair suggests we may want language to clarify that this get() function can be called from several threads with no need for explicit locking.

Move to Open.

[ 2010-01-23 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 30.6.7 [future.shared_future]:

const R& shared_future::get() const; 
R& shared_future<R&>::get() const; 
void shared_future<void>::get() const;

...

-9- Throws: the stored exception, if an exception was stored and not retrieved before. [Note: Multiple calls on get() are allowed, and each call would result in an exception if an exception was stored. — end note]


1108. thread.req.exception overly constrains implementations

Section: 30.2.2 [thread.req.exception] Status: Tentatively Ready Submitter: Christopher Kohlhoff Opened: 2009-04-25 Last modified: 2009-10-26

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Discussion:

The current formulation of 30.2.2 [thread.req.exception]/2 reads:

The error_category of the error_code reported by such an exception's code() member function is as specified in the error condition Clause.

This constraint on the code's associated error_categor means an implementation must perform a mapping from the system-generated error to a generic_category() error code. The problems with this include:

The latter was one of Peter Dimov's main objections (in a private email discussion) to the original error_code-only design, and led to the creation of error_condition in the first place. Specifically, error_code and error_condition are intended to perform the following roles:

Any mapping determining correspondence of the returned error code to the conditions listed in the error condition clause falls under the "latitude" granted to implementors in 19.5.1.5 [syserr.errcat.objects]. (Although obviously their latitude is restricted a little by the need to match the right error condition when returning an error code from a library function.)

It is important that this error_code/error_condition usage is done correctly for the thread library since it is likely to set the pattern for future TR libraries that interact with the operating system.

[ Batavia (2009-05): ]

Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.

[ 2009-10 post-Santa Cruz: ]

Move to Tentatively Ready.

Proposed resolution:

Change 30.2.2 [thread.req.exception]/2:

-2- The error_category (19.5.1.1) of the error_code reported by such an exception's code() member function is as specified in the error condition Clause. The error_code reported by such an exception's code() member function shall compare equal to one of the conditions specified in the function's error condition Clause. [Example: When the thread constructor fails:


ec.category() == implementation-defined // probably system_category
ec == errc::resource_unavailable_try_again // holds true

end example]


1110. Is for_each overconstrained?

Section: 25.2.4 [alg.foreach] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-04-29 Last modified: 2009-11-22

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Discussion:

Quoting working paper for reference (25.2.4 [alg.foreach]):

template<InputIterator Iter, Callable<auto, Iter::reference> Function>
  requires CopyConstructible<Function>
  Function for_each(Iter first, Iter last, Function f);

1 Effects: Applies f to the result of dereferencing every iterator in the range [first,last), starting from first and proceeding to last - 1.

2 Returns: f.

3 Complexity: Applies f exactly last - first times.

P2 implies the passed object f should be invoked at each stage, rather than some copy of f. This is important if the return value is to usefully accumulate changes. So the requirements are an object of type Function can be passed-by-value, invoked multiple times, and then return by value. In this case, MoveConstructible is sufficient. This would open support for move-only functors, which might become important in concurrent code as you can assume there are no other references (copies) of a move-only type and so freely use them concurrently without additional locks.

[ See further discussion starting with c++std-lib-23686. ]

[ Batavia (2009-05): ]

Pete suggests we may want to look at this in a broader context involving other algorithms. We should also consider the implications of parallelism.

Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.

[ 2009-10-14 Daniel de-conceptified the proposed resolution. ]

The note in 25.1 [algorithms.general]/9 already says the right thing:

Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely.

So we only need to ensure that the wording for for_each is sufficiently clear, which is the intend of the following rewording.

[ 2009-10-15 Daniel proposes: ]

[ 2009-10 post-Santa Cruz: ]

Move to Tentatively Ready, using Daniel's wording without the portion saying "CopyConstructible is not required".

[ 2009-10-27 Daniel adds: ]

I see that during the Santa Cruz meeting the originally proposed addition

, CopyConstructible is not required.

was removed. I don't think that this removal was a good idea. The combination of 25.1 [algorithms.general]/9

[Note: Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely.[..]

with the fact that CopyConstructible is a refinement MoveConstructible makes it necessary that such an explicit statement is given. Even the existence of the usage of std::move in the Returns clause doesn't help much, because this would still be well-formed for a CopyConstructible without move constructor. Let me add that the originally proposed addition reflects current practice in the standard, e.g. 25.3.9 [alg.unique]/5 usages a similar terminology.

For similar wording need in case for auto_ptr see 973.

[ Howard: Moved from Tentatively Ready to Open. ]

[ 2009-11-20 Howard restores "not CopyConstructible" to the spec. ]

[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:


1112. bitsets and new style for loop

Section: 20.3.7 [template.bitset] Status: Tentatively NAD Future Submitter: Alisdair Meredith Opened: 2009-05-06 Last modified: 2009-10-26

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Discussion:

Std::bitset is a homogeneous container-like sequence of bits, yet it does not model the Range concept so cannot be used with the new for-loop syntax. It is the only such type in the library that does NOT support the new for loop.

The obvious reason is that bitset does not support iterators.

At least two reasonable solutions are available:

  1. Add an iterator interface to bitset, bringing its interface close to that of std::array
  2. Provide an unspecified concept_map for Range<bitset>.

The latter will still need some kind of iterator-like adapter for bitset, but gives implementers greater freedom on the details. E.g. begin/end return some type that simply invokes operator[] on the object it wraps, and increments its index on operator++. A vendor can settle for InputIterator support, rather than wrapping up a full RandomAccessIterator.

I have a mild preference for option (ii) as I think it is less work to specify at this stage of the process, although (i) is probably more useful in the long run.

Hmm, my wording looks a little woolly, as it does not say what the element type of the range is. Do I get a range of bool, bitset<N>::reference, or something else entirely?

I guess most users will assume the behaviour of reference, but expect to work with bool. Bool is OK for read-only traversal, but you really need to take a reference to a bitset::reference if you want to write back.

[ Batavia (2009-05): ]

Move to Open. We further recommend this be deferred until after the next Committee Draft.

[ 2009-05-25 Alisdair adds: ]

I just stumbled over the Range concept_map for valarray and this should probably set the precedent on how to write the wording.

[ Howard: I've replaced the proposed wording with Alisdair's suggestion. ]

[ 2009-07-24 Daniel modifies the proposed wording for non-concepts. ]

[ 2009-10 post-Santa Cruz: ]

Mark as Tentatively NAD Future due to the loss of concepts.

Proposed resolution:

  1. Modify the section 20.3.7 [template.bitset] <bitset> synopsis by adding the following at the end of the synopsis:

    
    // XX.X.X bitset range access [bitset.range]
    template<size_t N> unspecified-1 begin(bitset<N>&);
    template<size_t N> unspecified-2 begin(const bitset<N>&);
    template<size_t N> unspecified-1 end(bitset<N>&);
    template<size_t N> unspecified-2 end(const bitset<N>&);
    
    
  2. Add a new section "bitset range access" [bitset.range] after the current section 20.3.7.3 [bitset.operators] with the following series of paragraphs:

    1. In the begin and end function templates that follow, unspecified-1 is a type that meets the requirements of a mutable random access iterator (24.2.5 [random.access.iterators]) whose value_type is bool and whose reference type is bitset<N>::reference. unspecified-2 is a type that meets the requirements of a constant random access iterator (24.2.5 [random.access.iterators]) whose value_type is bool and whose reference type is bool.

    
    template<size_t N> unspecified-1 begin(bitset<N>&);
    template<size_t N> unspecified-2 begin(const bitset<N>&);
    
    
    2. Returns: an iterator referencing the first bit in the bitset.
    
    template<size_t N> unspecified-1 end(bitset<N>&);
    template<size_t N> unspecified-2 end(const bitset<N>&);
    
    3. Returns: an iterator referencing one past the last bit in the bitset.

1113. bitset::to_string could be simplified

Section: 20.3.7 [template.bitset] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-05-09 Last modified: 2009-10-26

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Discussion:

In 853 our resolution is changing the signature by adding two defaulting arguments to 3 calls. In principle, this means that ABI breakage is not an issue, while API is preserved.

With that observation, it would be very nice to use the new ability to supply default template parameters to function templates to collapse all 3 signatures into 1. In that spirit, this issue offers an alternative resolution than that of 853.

[ Batavia (2009-05): ]

Move to Open, and look at the issue again after 853 has been accepted. We further recommend this be deferred until after the next Committee Draft.

[ 2009-10 post-Santa Cruz: ]

Move to Tentatively Ready.

Proposed resolution:

  1. In 20.3.7 [template.bitset]/1 (class bitset) ammend:

    template <class charT = char,
                class traits = char_traits<charT>,
                class Allocator = allocator<charT>> 
      basic_string<charT, traits, Allocator>
      to_string(charT zero = charT('0'), charT one = charT('1')) const;
    template <class charT, class traits> 
      basic_string<charT, traits, allocator<charT> > to_string() const; 
    template <class charT> 
      basic_string<charT, char_traits<charT>, allocator<charT> > to_string() const; 
    basic_string<char, char_traits<char>, allocator<char> > to_string() const;
    
  2. In 20.3.7.2 [bitset.members] prior to p35 ammend:

    template <class charT = char,
                class traits = char_traits<charT>,
                class Allocator = allocator<charT>> 
      basic_string<charT, traits, Allocator>
      to_string(charT zero = charT('0'), charT one = charT('1')) const;
    
  3. Strike 20.3.7.2 [bitset.members] paragraphs 37 -> 39 (including signature above 37)

1114. Type traits underspecified

Section: 20.6 [meta] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2009-05-12 Last modified: 2009-10-26

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Discussion:

Related to 975 and 1023.

The current wording in 20.6.1 [meta.rqmts] is still unclear concerning it's requirements on the type traits classes regarding ambiguities. Specifically it's unclear

[ Batavia (2009-05): ]

Alisdair would prefer to factor some of the repeated text, but modulo a corner case or two, he believes the proposed wording is otherwise substantially correct.

Move to Open.

[ 2009-10 post-Santa Cruz: ]

Move to Tentatively Ready.

Proposed resolution:

[ The usage of the notion of a BaseCharacteristic below might be useful in other places - e.g. to define the base class relation in 20.7.5 [refwrap], 20.7.14 [func.memfn], or 20.7.15.2 [func.wrap.func]. In this case it's definition should probably be moved to Clause 17 ]

  1. Change 20.6.1 [meta.rqmts]/1 as indicated:

    [..] It shall be DefaultConstructible, CopyConstructible, and publicly and unambiguously derived, directly or indirectly, from its BaseCharacteristic, which is a specialization of the template integral_constant (20.6.3), with the arguments to the template integral_constant determined by the requirements for the particular property being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the UnaryTypeTrait.
  2. Change 20.6.1 [meta.rqmts]/2 as indicated:

    [..] It shall be DefaultConstructible, CopyConstructible, and publicly and unambiguously derived, directly or indirectly, from an instance its BaseCharacteristic, which is a specialization of the template integral_constant (20.6.3), with the arguments to the template integral_constant determined by the requirements for the particular relationship being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the BinaryTypeTrait.
  3. Change 20.6.4 [meta.unary]/2 as indicated:

    Each of these templates shall be a UnaryTypeTrait (20.6.1), publicly derived directly or indirectly from true_type if the corresponding condition is true, otherwise from false_type where its BaseCharacteristic shall be true_type if the corresponding condition is true, otherwise false_type.
  4. Change 20.6.5 [meta.rel]/2 as indicated:

    Each of these templates shall be a BinaryTypeTrait (20.6.1), publicly derived directly or indirectly from true_type if the corresponding condition is true, otherwise from false_type where its BaseCharacteristic shall be true_type if the corresponding condition is true, otherwise false_type.

1115. va_copy missing from Standard macros table

Section: C.2 [diff.library] Status: Tentatively NAD Editorial Submitter: Miles Zhao Opened: 2009-05-23 Last modified: 2009-10-26

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Discussion:

In "Table 122 -- Standard macros" of C.2 [diff.library], which lists the 56 macros inherited from C library, va_copy seems to be missing. But in "Table 21 -- Header <cstdarg> synopsis" (18.10 [support.runtime]), there is.

[ 2009-10 post-Santa Cruz: ]

Mark as Tentatively NAD Editorial, if Pete disagrees, Howard will move to Tentatively Ready

Proposed resolution:

Add va_copy to Table 122 -- Standard macros in C.2 [diff.library].


1118. tuple query APIs do not support cv-qualification

Section: 20.5.2.5 [tuple.helper] Status: Open Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2009-10-26

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Discussion:

The APIs tuple_size and tuple_element do not support cv-qualified tuples, pairs or arrays.

The most generic solution would be to supply partial specializations once for each cv-type in the tuple header. However, requiring this header for cv-qualified pairs/arrays seems unhelpful. The BSI editorial suggestion (UK-198/US-69, N2533) to merge tuple into <utility> would help with pair, but not array. That might be resolved by making a dependency between the <array> header and <utility>, or simply recognising the dependency be fulfilled in a Remark.

[ 2009-05-24 Daniel adds: ]

All tuple_size templates with a base class need to derive publicly, e.g.

template <IdentityOf T> class tuple_size< const T > :
   public tuple_size<T> {};

The same applies to the tuple_element class hierarchies.

What is actually meant with the comment

this solution relies on 'metafunction forwarding' to inherit the nested typename type

?

I ask, because all base classes are currently unconstrained and their instantiation is invalid in the constrained context of the tuple_element partial template specializations.

[ 2009-05-24 Alisdair adds: ]

I think a better solution might be to ask Pete editorially to change all declarations of tupling APIs to use the struct specifier instead of class.

"metafunction forwarding" refers to the MPL metafunction protocol, where a metafunction result is declared as a nested typedef with the name "type", allowing metafunctions to be chained by means of inheritance. It is a neater syntax than repeatedly declaring a typedef, and inheritance syntax is slightly nicer when it comes to additional typename keywords.

The constrained template with an unconstrained base is a good observation though.

[ 2009-10 post-Santa Cruz: ]

Move to Open, Alisdair to provide wording. Once wording is provided, Howard will move to Review.

Proposed resolution:

Add to 20.5.1 [tuple.general] p2 (Header <tuple> synopsis)

// 20.5.2.3, tuple helper classes:
template <IdentityOf T> class tuple_size; // undefined
template <IdentityOf T> class tuple_size< const T > : tuple_size<T> {};
template <IdentityOf T> class tuple_size< volatile T > : tuple_size<T> {};
template <IdentityOf T> class tuple_size< const volatile T > : tuple_size<T> {};

template <VariableType... Types> class tuple_size<tuple<Types...> >;

template <size_t I, IdentityOf T> class tuple_element; // undefined
template <size_t I, IdentityOf T> class tuple_element<I, const T>;
template <size_t I, IdentityOf T> class tuple_element<I, volatile T>;
template <size_t I, IdentityOf T> class tuple_element<I, const volatile T>;

template <size_t I, VariableType... Types>
  requires True<(I < sizeof...(Types))> class tuple_element<I, tuple<Types...> >;

Add to 20.5.2.5 [tuple.helper]

[ (note that this solution relies on 'metafunction forwarding' to inherit the nested typename type) ]

template <class... Types>
class tuple_size<tuple<Types...> >
  : public integral_constant<size_t, sizeof...(Types)> { };

template <size_t I, class... Types>
requires True<(I < sizeof...(Types))>
class tuple_element<I, tuple<Types...> > {
public:
  typedef TI type;
};

template <size_t I, IdentityOf T>
  class tuple_element<I, const T> : add_const<tuple_element<I,T>> {};

template <size_t I, IdentityOf T>
  class tuple_element<I, volatile T> : add_volatile<tuple_element<I,T>> {};

template <size_t I, IdentityOf T>
  class tuple_element<I, const volatile T> : add_cv<tuple_element<I,T>> {};

1119. tuple query APIs do not support references

Section: 20.5.2.5 [tuple.helper] Status: Open Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2009-10-26

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Discussion:

The tuple query APIs tuple_size and tuple_element do not support references-to-tuples. This can be annoying when a template deduced a parameter type to be a reference, which must be explicitly stripped with remove_reference before calling these APIs.

I am not proposing a resolution at this point, as there is a combinatorial explosion with lvalue/rvalue references and cv-qualification (see previous issue) that suggests some higher refactoring is in order. This might be something to kick back over to Core/Evolution.

Note that we have the same problem in numeric_limits.

[ 2009-10 post-Santa Cruz: ]

Move to Open. Alisdair to provide wording.

Proposed resolution:


1121. Support for multiple arguments

Section: 20.4.2 [ratio.arithmetic] Status: Tentatively NAD Future Submitter: Alisdair Meredith Opened: 2009-05-25 Last modified: 2009-11-02

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Discussion:

Both add and multiply could sensibly be called with more than two arguments. The variadic template facility makes such declarations simple, and is likely to be frequently wrapped by end users if we do not supply the variant ourselves.

We deliberately ignore divide at this point as it is not transitive. Likewise, subtract places special meaning on the first argument so I do not suggest extending that immediately. Both could be supported with analogous wording to that for add/multiply below.

Note that the proposed resolution is potentially incompatible with that proposed for 921, although the addition of the typedef to ratio would be equally useful.

[ 2009-10-30 Alisdair adds: ]

The consensus of the group when we reviewed this in Santa Cruz was that 921 would proceed to Ready as planned, and the multi-paramater add/multiply templates should be renamed as ratio_sum and ratio_product to avoid the problem mixing template aliases with partial specializations.

It was also suggested to close this issue as NAD Future as it does not correspond directly to any NB comment. NBs are free to submit a specific comment (and re-open) in CD2 though.

Walter Brown also had concerns on better directing the order of evaluation to avoid overflows if we do proceed for 0x rather than TR1, so wording may not be complete yet.

[ Alisdair updates wording. ]

[ 2009-10-30 Howard: ]

Moved to Tentatively NAD Future after 5 positive votes on c++std-lib.

Proposed resolution:

Add the following type traits to p3 20.4 [ratio]

// ratio arithmetic
template <class R1, class R2> struct ratio_add;
template <class R1, class R2> struct ratio_subtract;
template <class R1, class R2> struct ratio_multiply;
template <class R1, class R2> struct ratio_divide;
template <class R1, class ... RList> struct ratio_sum;
template <class R1, class ... RList> struct ratio_product;

after 20.4.2 [ratio.arithmetic] p1: add

template <class R1, class ... RList> struct ratio_sum; // declared, never defined

template <class R1> struct ratio_sum<R1> : R1 {};
Requires: R1 is a specialization of class template ratio
template <class R1, class R2, class ... RList> 
 struct ratio_sum<R1, R2, RList...>
   : ratio_add< R1, ratio_sum<R2, RList...>> {
};
Requires: R1 and each element in parmater pack RList is a specialization of class template ratio

after 20.4.2 [ratio.arithmetic] p3: add

template <class R1, class ... RList> struct ratio_product; // declared, never defined

template <class R1> struct ratio_product<R1> : R1 {};
Requires: R1 is a specialization of class template ratio
template <class R1, class R2, class ... RList> 
 struct ratio_sum<R1, R2, RList...>
   : ratio_add< R1, ratio_product<R2, RList...>> {
};
Requires: R1 and each element in parmater pack RList is a specialization of class template ratio

1123. no requirement that standard streams be flushed

Section: 27.5.2.1.6 [ios::Init] Status: Ready Submitter: James Kanze Opened: 2009-05-14 Last modified: 2009-10-20

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Discussion:

As currently formulated, the standard doesn't require that there is ever a flush of cout, etc. (This implies, for example, that the classical hello, world program may have no output.) In the current draft (N2798), there is a requirement that the objects be constructed before main, and before the dynamic initialization of any non-local objects defined after the inclusion of <iostream> in the same translation unit. The only requirement that I can find concerning flushing, however, is in 27.5.2.1.6 [ios::Init], where the destructor of the last std::ios_base::Init object flushes. But there is, as far as I can see, no guarantee that such an object ever exists.

Also, the wording in [iostreams.objects] says that:

The objects are constructed and the associations are established at some time prior to or during the first time an object of class ios_base::Init is constructed, and in any case before the body of main begins execution.

In 27.5.2.1.6 [ios::Init], however, as an effect of the constructor, it says that

If init_cnt is zero, the function stores the value one in init_cnt, then constructs and initializes the objects cin, cout, cerr, clog wcin, wcout, wcerr, and wclog"

which seems to forbid earlier construction.

(Note that with these changes, the exposition only "static int init_cnt" in ios_base::Init can be dropped.)

Of course, a determined programmer can still inhibit the flush with things like:

new std::ios_base::Init ;       //  never deleted 

or (in a function):

std::ios_base::Init ensureConstruction ; 
//  ... 
exit( EXIT_SUCCESS ) ; 

Perhaps some words somewhere to the effect that all std::ios_base::Init objects should have static lifetime would be in order.

[ 2009 Santa Cruz: ]

Moved to Ready. Some editorial changes are expected (in addition to the proposed wording) to remove init_cnt from Init.

Proposed resolution:

Change 27.4 [iostream.objects]/2:

-2- The objects are constructed and the associations are established at some time prior to or during the first time an object of class ios_base::Init is constructed, and in any case before the body of main begins execution.292 The objects are not destroyed during program execution.293 If a translation unit includes <iostream> or explicitly constructs an ios_base::Init object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit. The results of including <iostream> in a translation unit shall be as if <iostream> defined an instance of ios_base::Init with static lifetime. Similarly, the entire program shall behave as if there were at least one instance of ios_base::Init with static lifetime.

Change 27.5.2.1.6 [ios::Init]/3:

Init();
-3- Effects: Constructs an object of class Init. If init_cnt is zero, the function stores the value one in init_cnt, then constructs and initializes the objects cin, cout, cerr, clog (27.4.1), wcin, wcout, wcerr, and wclog (27.4.2). In any case, the function then adds one to the value stored in init_cnt. Constructs and initializes the objects cin, cout, cerr, clog, wcin, wcout, wcerr and wclog if they have not already been constructed and initialized.

Change 27.5.2.1.6 [ios::Init]/4:

~Init();
-4- Effects: Destroys an object of class Init. The function subtracts one from the value stored in init_cnt and, if the resulting stored value is one, If there are no other instances of the class still in existance, calls cout.flush(), cerr.flush(), clog.flush(), wcout.flush(), wcerr.flush(), wclog.flush().

1125. ostream_iterator does not work with movable types

Section: 24.6.2.2 [ostream.iterator.ops] Status: Tentatively NAD Submitter: Alisdair Meredith Opened: 2009-05-28 Last modified: 2009-11-10

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Discussion:

ostream_iterator has not been updated to support moveable types, in a similar manner to the insert iterators. Note that this is not a problem for ostreambuf_iterator, as the types it is restricted to dealing with do not support extra-efficient moving.

[ 2009-11-10 Howard adds: ]

Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below.

Proposed resolution:

Add second operator= overload to class template ostream_iterator in 24.6.2 [ostream.iterator], para 2:

ostream_iterator<T,charT,traits>& operator=(const T& value);
ostream_iterator<T,charT,traits>& operator=(T&& value);

Add a new paragraph: in 24.6.2.2 [ostream.iterator.ops]:

ostream_iterator& operator=(T&& value);

-2- Effects:

*out_stream << std::move(value);
if(delim != 0)
  *out_stream << delim;
return (*this);

Rationale:

Several objections to move forward with this issue were voiced in the thread starting with c++std-lib-25438. Among them is that we know of no motivating use case to make streaming rvalues behave differently than streaming const lvalues.


1126. istreambuff_iterator::equal needs a const & parameter

Section: 24.6.3.5 [istreambuf.iterator::equal] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-05-28 Last modified: 2009-11-02

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Discussion:

The equal member function of istreambuf_iterator is declared const, but takes its argument by non-const reference.

This is not compatible with the operator== free function overload, which is defined in terms of calling equal yet takes both arguments by reference to const.

[ The proposed wording is consistent with 110 with status TC1. ]

[ 2009-11-02 Howard adds: ]

Set to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Ammend in both:
24.6.3 [istreambuf.iterator]
24.6.3.5 [istreambuf.iterator::equal]

bool equal(const istreambuf_iterator& b) const;

1130. copy_exception name misleading

Section: 18.8.5 [propagation] Status: Tentatively Ready Submitter: Peter Dimov Opened: 2009-05-13 Last modified: 2009-11-16

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Discussion:

The naming of std::copy_exception misleads almost everyone (experts included!) to think that it is the function that copies an exception_ptr:

exception_ptr p1 = current_exception();
exception_ptr p2 = copy_exception( p1 );

But this is not at all what it does. The above actually creates an exception_ptr p2 that contains a copy of p1, not of the exception to which p1 refers!

This is, of course, all my fault; in my defence, I used copy_exception because I was unable to think of a better name.

But I believe that, based on what we've seen so far, any other name would be better.

Therefore, I propose copy_exception to be renamed to create_exception:

template<class E> exception_ptr create_exception(E e);

with the following explanatory paragraph after it:

Creates an exception_ptr that refers to a copy of e.

[ 2009-05-13 Daniel adds: ]

What about

make_exception_ptr

in similarity to make_pair and make_tuple, make_error_code and make_error_condition, or make_shared? Or, if a stronger symmetry to current_exception is preferred:

make_exception

We have not a single create_* function in the library, it was always make_* used.

[ 2009-05-13 Peter adds: ]

make_exception_ptr works for me.

[ 2009-06-02 Thomas J. Gritzan adds: ]

To avoid surprises and unwanted recursion, how about making a call to std::make_exception_ptr with an exception_ptr illegal?

It might work like this:

template<class E>
exception_ptr make_exception_ptr(E e);
template<>
exception_ptr make_exception_ptr<exception_ptr>(exception_ptr e) = delete;

[ 2009 Santa Cruz: ]

Move to Review for the time being. The subgroup thinks this is a good idea, but doesn't want to break compatibility unnecessarily if someone is already shipping this. Let's talk to Benjamin and PJP tomorrow to make sure neither objects.

[ 2009-11-16 Jonathan Wakely adds: ]

GCC 4.4 shipped with copy_exception but we could certainly keep that symbol in the library (but not the headers) so it can still be found by any apps foolishly relying on the experimental C++0x mode being ABI stable.

[ 2009-11-16 Peter adopts wording supplied by Daniel. ]

[ 2009-11-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

  1. Change 18.8 [support.exception]/1, header <exception> synopsis as indicated:

    exception_ptr current_exception();
    void rethrow_exception [[noreturn]] (exception_ptr p);
    template<class E> exception_ptr copy_exceptionmake_exception_ptr(E e);
    
  2. Change 18.8.5 [propagation]:

    template<class E> exception_ptr copy_exceptionmake_exception_ptr(E e);
    

    -11- Effects: Creates an exception_ptr that refers to a copy of e, as if

    try {
      throw e;
    } catch(...) {
      return current_exception();
    }
    

    ...

  3. Change 30.6.5 [futures.promise]/7 as indicated:

    Effects: if the associated state of *this is not ready, stores an exception object of type future_error with an error code of broken_promise as if by this->set_exception(copy_exceptionmake_exception_ptr( future_error(future_errc::broken_promise)). Destroys ...

1131. C++0x does not need alignment_of

Section: 20.6.4.3 [meta.unary.prop] Status: Tentatively Ready Submitter: Niels Dekker Opened: 2009-06-01 Last modified: 2010-01-30

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Discussion:

The alignment_of template is no longer necessary, now that the core language will provide alignof. Scott Meyers raised this issue at comp.std.c++, C++0x: alignof vs. alignment_of, May 21, 2009. In a reply, Daniel Krügler pointed out that alignof was added to the working paper after alignment_of. So it appears that alignment_of is only part of the current Working Draft (N2857) because it is in TR1.

Having both alignof and alignment_of would cause unwanted confusion. In general, I think TR1 functionality should not be brought into C++0x if it is entirely redundant with other C++0x language or library features.

[ 2009-11-16 Chris adds: ]

I would like to suggest the following new wording for this issue, based on recent discussions. Basically this doesn't delete alignment_of, it just makes it clear that it is just a wrapper for alignof. This deletes the first part of the proposed resolution, changes the second part by leaving in alignof(T) but changing the precondition and leaves the 3rd part unchanged.

Suggested Resolution:

Change the first row of Table 44 ("Type property queries"), from Type properties 20.6.4.3 [meta.unary.prop]:

Table 44 — Type property queries
template <class T> struct alignment_of; alignof(T).
Precondition: T shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified) void. alignof(T) shall be defined

Change text in Table 51 ("Other transformations"), from Other transformations 20.6.7 [meta.trans.other], as follows:

Table 51 — Other transformations
...aligned_storage; Len shall not be zero. Align shall be equal to alignment_of<T>::value alignof(T) for some type T or to default-alignment. ...

[ 2010-01-30 Alisdair proposes that Chris' wording be moved into the proposed wording section and tweaks it on the way. ]

Original proposed wording saved here:

Remove from Header <type_traits> synopsis 20.6.2 [meta.type.synop]:

template <class T> struct alignment_of;

Remove the first row of Table 44 ("Type property queries"), from Type properties 20.6.4.3 [meta.unary.prop]:

Table 44 — Type property queries
template <class T> struct alignment_of; alignof(T).
Precondition: T shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified) void.

Change text in Table 51 ("Other transformations"), from Other transformations 20.6.7 [meta.trans.other], as follows:

Table 51 — Other transformations
...aligned_storage; Len shall not be zero. Align shall be equal to alignment_of<T>::value alignof(T) for some type T or to default-alignment. ...

[ 2010-01-30 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change the first row of Table 43 ("Type property queries"), from Type properties 20.6.4.3 [meta.unary.prop]:

Table 43 — Type property queries
template <class T> struct alignment_of; alignof(T).
Precondition: T shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified) void. alignof(T) is a valid expression (5.3.6 [expr.alignof])

Change text in Table 51 ("Other transformations"), from Other transformations 20.6.7 [meta.trans.other], as follows:

Table 51 — Other transformations
...aligned_storage; Len shall not be zero. Align shall be equal to alignment_of<T>::value alignof(T) for some type T or to default-alignment. ...

1133. Does N2844 break current specification of list::splice?

Section: 23.3.3.5 [forwardlist.ops], 23.3.4.4 [list.ops] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-05-09 Last modified: 2009-12-13

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Discussion:

IIUC, N2844 means that lvalues will no longer bind to rvalue references. Therefore, the current specification of list::splice (list operations 23.3.4.4 [list.ops]) will be a breaking change of behaviour for existing programs. That is because we changed the signature to swallow via an rvalue reference rather than the lvalue reference used in 03.

Retaining this form would be safer, requiring an explicit move when splicing from lvalues. However, this will break existing programs. We have the same problem with forward_list, although without the risk of breaking programs so here it might be viewed as a positive feature.

The problem signatures:

void splice_after(const_iterator position, forward_list<T,Alloc>&& x);
void splice_after(const_iterator position, forward_list<T,Alloc>&& x,
                  const_iterator i);
void splice_after(const_iterator position, forward_list<T,Alloc>&& x,
                  const_iterator first, const_iterator last);

void splice(const_iterator position, list<T,Alloc>&& x);
void splice(const_iterator position, list<T,Alloc>&& x,
            const_iterator i);
void splice(const_iterator position, list<T,Alloc>&& x,
            const_iterator first, const_iterator last);
Possible resolutions:

Option A. Add an additional (non-const) lvalue-reference overload in each case

Option B. Change rvalue reference back to (non-const) lvalue-reference overload in each case

Option C. Add an additional (non-const) lvalue-reference overload in just the std::list cases

I think (B) would be very unfortunate, I really like the forward_list behaviour in (C) but feel (A) is needed for consistency.

My actual preference would be NAD, ship with this as a breaking change as it is a more explicit interface. I don't think that will fly though!

See the thread starting with c++std-lib-23725 for more discussion.

[ 2009-10-27 Christopher Jefferson provides proposed wording for Option C. ]

[ 2009-12-08 Jonathan Wakely adds: ]

As Bill Plauger pointed out, list::merge needs similar treatment.

[ Wording updated. ]

[ 2009-12-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

In 23.3.4 [list]

Add lvalue overloads before rvalue ones:

void splice(const_iterator position, list<T,Allocator>& x);
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 i);
void splice(const_iterator position, list<T,Allocator>& x,
            const_iterator first, const_iterator last);
void splice(const_iterator position, list<T,Allocator>&& x,
            const_iterator first, const_iterator last);
void merge(list<T,Allocator>& x);
template <class Compare> void merge(list<T,Allocator>& x, Compare comp);
void merge(list<T,Allocator>&& x);
template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

In 23.3.4.4 [list.ops], similarly add lvalue overload before each rvalue one:

(After paragraph 2)

void splice(const_iterator position, list<T,Allocator>& x);
void splice(const_iterator position, list<T,Allocator>&& x);

(After paragraph 6)

void splice(const_iterator position, list<T,Allocator>& x, const_iterator i);
void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i);

(After paragraph 10)

void splice(const_iterator position, list<T,Allocator>& x,
            const_iterator first, const_iterator last);
void splice(const_iterator position, list<T,Allocator>&& x,
            const_iterator first, const_iterator last);

In 23.3.4.4 [list.ops], after paragraph 21

void merge(list<T,Allocator>& x);
template <class Compare> void merge(list<T,Allocator>& x, Compare comp);
void merge(list<T,Allocator>&& x);
template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

1134. Redundant specification of stdint.h, fenv.h, tgmath.h, and maybe complex.h

Section: 18.4.2 [stdinth], 26.3.2 [fenv], 26.8 [c.math], 26.4.11 [cmplxh] Status: Ready Submitter: Robert Klarer Opened: 2009-05-26 Last modified: 2009-10-20

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Discussion:

This is probably editorial.

The following items should be removed from the draft, because they're redundant with Annex D, and they arguably make some *.h headers non-deprecated:

18.4.2 [stdinth] (regarding <stdint.h>)

26.3.2 [fenv] (regarding <fenv.h>

Line 3 of 26.8 [c.math] (regarding <tgmath.h>)

26.4.11 [cmplxh] (regarding <complex.h>, though the note in this subclause is not redundant)

[ 2009-06-10 Ganesh adds: ]

While searching for stdint in the CD, I found that <stdint.h> is also mentioned in 3.9.1 [basic.fundamental] /5. It guess it should refer to <cstdint> instead.

[ 2009 Santa Cruz: ]

Real issue. Maybe just editorial, maybe not. Move to Ready.

Proposed resolution:

Remove the section 18.4.2 [stdinth].

Remove the section 26.3.2 [fenv].

Remove 26.8 [c.math], p3:

-3- The header <tgmath.h> effectively includes the headers <complex.h> and <math.h>.

Remove the section 26.4.11 [cmplxh].


1135. exception_ptr should support contextual conversion to bool

Section: 18.8.5 [propagation] Status: Ready Submitter: Daniel Krügler Opened: 2007-06-06 Last modified: 2009-10-20

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Discussion:

As of N2857 18.8.5 [propagation]/5, the implementation-defined type exception_ptr does provide the following ways to check whether it is a null value:

void f(std::exception_ptr p) {
  p == nullptr;
  p == 0;
  p == exception_ptr();
}

This is rather cumbersome way of checking for the null value and I suggest to require support for evaluation in a boolean context like so:

void g(std::exception_ptr p) {
  if (p) {}
  !p;
}

[ 2009 Santa Cruz: ]

Move to Ready. Note to editor: considering putting in a cross-reference to 4 [conv], paragraph 3, which defines the phrase "contextually converted to bool".

Proposed resolution:

In section 18.8.5 [propagation] insert a new paragraph between p.5 and p.6:

An object e of type exception_ptr can be contextually converted to bool. The effect shall be as if e != exception_ptr() had been evaluated in place of e. There shall be no implicit conversion to arithmetic type, to enumeration type or to pointer type.

1136. Incomplete specification of nested_exception::rethrow_nested()

Section: 18.8.6 [except.nested] Status: Ready Submitter: Daniel Krügler Opened: 2007-06-06 Last modified: 2009-10-20

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Discussion:

It was recently mentioned in a newsgroup article http://groups.google.de/group/comp.std.c++/msg/f82022aff68edf3d that the specification of the member function rethrow_nested() of the class nested_exception is incomplete, specifically it remains unclear what happens, if member nested_ptr() returns a null value. In 18.8.6 [except.nested] we find only the following paragraph related to that:

void rethrow_nested() const; // [[noreturn]]
-4- Throws: the stored exception captured by this nested_exception object.

This is a problem, because it is possible to create an object of nested_exception with exactly such a state, e.g.

#include <exception>
#include <iostream>

int main() try {
  std::nested_exception e; // OK, calls current_exception() and stores it's null value
  e.rethrow_nested(); // ?
  std::cout << "A" << std::endl;
}
catch(...) {
  std::cout << "B" << std::endl;
}

I suggest to follow the proposal of the reporter, namely to invoke terminate() if nested_ptr() return a null value of exception_ptr instead of relying on the fallback position of undefined behavior. This would be consistent to the behavior of a throw; statement when no exception is being handled.

[ 2009 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Change around 18.8.6 [except.nested]/4 as indicated:

-4- Throws: the stored exception captured by this nested_exception object, if nested_ptr() != nullptr

- Remarks: If nested_ptr() == nullptr, terminate() shall be called.


1137. Return type of conj and proj

Section: 26.4.9 [cmplx.over] Status: Tentatively Ready Submitter: Marc Steinbach Opened: 2009-06-11 Last modified: 2009-11-11

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Discussion:

In clause 1, the Working Draft (N2857) specifies overloads of the functions

arg, conj, imag, norm, proj, real

for non-complex arithmetic types (float, double, long double, and integers). The only requirement (clause 2) specifies effective type promotion of arguments.

I strongly suggest to add the following requirement on the return types:

All the specified overloads must return real (i.e., non-complex) values, specifically, the nested value_type of effectively promoted arguments.

(This has no effect on arg, imag, norm, real: they are real-valued anyway.)

Rationale:

Mathematically, conj() and proj(), like the transcendental functions, are complex-valued in general but map the (extended) real line to itself. In fact, both functions act as identity on the reals. A typical user will expect conj() and proj() to preserve this essential mathematical property in the same way as exp(), sin(), etc. A typical use of conj(), e.g., is the generic scalar product of n-vectors:

template<typename T>
inline T
scalar_product(size_t n, T const* x, T const* y) {
  T result = 0;
  for (size_t i = 0; i < n; ++i)
    result += x[i] * std::conj(y[i]);
  return result;
}

This will work equally well for real and complex floating-point types T if conj() returns T. It will not work with real types if conj() returns complex values.

Instead, the implementation of scalar_product becomes either less efficient and less useful (if a complex result is always returned), or unnecessarily complicated (if overloaded versions with proper return types are defined). In the second case, the real-argument overload of conj() cannot be used. In fact, it must be avoided.

Overloaded conj() and proj() are principally needed in generic programming. All such use cases will benefit from the proposed return type requirement, in a similar way as the scalar_product example. The requirement will not harm use cases where a complex return value is expected, because of implicit conversion to complex. Without the proposed return type guarantee, I find overloaded versions of conj() and proj() not only useless but actually troublesome.

[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Insert a new paragraph after 26.4.9 [cmplx.over]/2:

All of the specified overloads shall have a return type which is the nested value_type of the effectively cast arguments.

1138. unusual return value for operator+

Section: 21.4.8.1 [string::op+] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-06-12 Last modified: 2009-11-05

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Discussion:

Many of the basic_string operator+ overloads return an rvalue-reference. Is that really intended?

I'm considering it might be a mild performance tweak to avoid making un-necessary copies of a cheaply movable type, but it opens risk to dangling references in code like:

auto && s = string{"x"} + string{y};

and I'm not sure about:

auto s = string{"x"} + string{y};

[ 2009-10-11 Howard updated Returns: clause for each of these. ]

[ 2009-11-05 Howard adds: ]

Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Strike the && from the return type in the following function signatures:

21.3 [string.classes] p2 Header Synopsis

template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(basic_string<charT,traits,Allocator>&& lhs,
              const basic_string<charT,traits,Allocator>& rhs);

template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(const basic_string<charT,traits,Allocator>& lhs,
              basic_string<charT,traits,Allocator>&& rhs);

template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(basic_string<charT,traits,Allocator>&& lhs,
              basic_string<charT,traits,Allocator>&& rhs);


template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(const charT* lhs,
              basic_string<charT,traits,Allocator>&& rhs);

template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(charT lhs, basic_string<charT,traits,Allocator>&& rhs);

template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(basic_string<charT,traits,Allocator>&& lhs,
              const charT* rhs);

template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(basic_string<charT,traits,Allocator>&& lhs, charT rhs);

21.4.8.1 [string::op+]

template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(basic_string<charT,traits,Allocator>&& lhs,
              const basic_string<charT,traits,Allocator>& rhs);
Returns: std::move(lhs.append(rhs))
template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(const basic_string<charT,traits,Allocator>& lhs,
              basic_string<charT,traits,Allocator>&& rhs);
Returns: std::move(rhs.insert(0, lhs))
template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(basic_string<charT,traits,Allocator>&& lhs,
              basic_string<charT,traits,Allocator>&& rhs);
Returns: std::move(lhs.append(rhs)) [Note: Or equivalently std::move(rhs.insert(0, lhs))end note]
template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(const charT* lhs,
              basic_string<charT,traits,Allocator>&& rhs);
Returns: std::move(rhs.insert(0, lhs)).
template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(charT lhs, basic_string<charT,traits,Allocator>&& rhs);
Returns: std::move(rhs.insert(0, 1, lhs)).
template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(basic_string<charT,traits,Allocator>&& lhs,
              const charT* rhs);
Returns: std::move(lhs.append(rhs)).
template<class charT, class traits, class Allocator>
  basic_string<charT,traits,Allocator>&&
    operator+(basic_string<charT,traits,Allocator>&& lhs, charT rhs);
Returns: std::move(lhs.append(1, rhs)).

1144. "thread safe" is undefined

Section: 18.5 [support.start.term] Status: Ready Submitter: LWG Opened: 2009-06-16 Last modified: 2009-10-20

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Discussion:

Addresses UK 187

The term "thread safe" is not defined nor used in this context anywhere else in the standard.

Suggested action:

Clarify the meaning of "thread safe".

[ 2009 Santa Cruz: ]

The "thread safe" language has already been change in the WP. It was changed to "happen before", but the current WP text is still a little incomplete: "happen before" is binary, but the current WP text only mentions one thing.

Move to Ready.

Proposed resolution:

For the following functions in 18.5 [support.start.term].


extern "C" int at_quick_exit(void (*f)(void));
extern "C++" int at_quick_exit(void (*f)(void));

Edit paragraph 10 as follows. The intent is to provide the other half of the happens before relation; to note indeterminate ordering; and to clean up some formatting.

Effects: The at_quick_exit() functions register the function pointed to by f to be called without arguments when quick_exit is called. It is unspecified whether a call to at_quick_exit() that does not happen-before happen before (1.10) all calls to quick_exit will succeed. [Note: the at_quick_exit() functions shall not introduce a data race (17.6.4.7). exitnote end note] [Note: The order of registration may be indeterminate if at_quick_exit was called from more than one thread. —end note] [Note: The at_quick_exit registrations are distinct from the atexit registrations, and applications may need to call both registration functions with the same argument. —end note]

For the following function.


void quick_exit [[noreturn]] (int status)

Edit paragraph 13 as follows. The intent is to note that thread-local variables may be different.

Effects: Functions registered by calls to at_quick_exit are called in the reverse order of their registration, except that a function shall be called after any previously registered functions that had already been called at the time it was registered. Objects shall not be destroyed as a result of calling quick_exit. If control leaves a registered function called by quick_exit because the function does not provide a handler for a thrown exception, terminate() shall be called. [Note: Functions registered by one thread may be called by any thread, and hence should not rely on the identity of thread-storage-duration objects. —end note] After calling registered functions, quick_exit shall call _Exit(status). [Note: The standard file buffers are not flushed. See: ISO C 7.20.4.4. —end note]


1151. Behavior of the library in the presence of threads is incompletely specified

Section: 17 [library] Status: Open Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-20

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Discussion:

Addresses US 63

Description

The behavior of the library in the presence of threads is incompletely specified.

For example, if thread 1 assigns to X, then writes data to file f, which is read by thread 2, and then accesses variable X, is thread 2 guaranteed to be able to see the value assigned to X by thread 1? In other words, does the write of the data "happen before" the read?

Another example: does simultaneous access using operator at() to different characters in the same non-const string really introduce a data race?

Suggestion

Notes

17 SG: should go to threads group; misclassified in document

Concurrency SG: Create an issue. Hans will look into it.

[ 2009 Santa Cruz: ]

Move to "Open". Hans and the rest of the concurrency working group will study this. We can't make progress without a thorough review and a paper.

Proposed resolution:


1152. expressions parsed differently than intended

Section: 22.4.2.2.2 [facet.num.put.virtuals] Status: Tentatively Ready Submitter: Seungbeom Kim Opened: 2009-06-27 Last modified: 2009-10-28

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Discussion:

In Table 73 -- Floating-point conversions, 22.4.2.2.2 [facet.num.put.virtuals], in N2914, we have the following entries:

Table 73 — Floating-point conversions
State stdio equivalent
floatfield == ios_base::fixed | ios_base::scientific && !uppercase %a
floatfield == ios_base::fixed | ios_base::scientific %A

These expressions are supposed to mean:

floatfield == (ios_base::fixed | ios_base::scientific) && !uppercase 
floatfield == (ios_base::fixed | ios_base::scientific) 

but technically parsed as:

((floatfield == ios_base::fixed) | ios_base::scientific) && (!uppercase) 
((floatfield == ios_base::fixed) | ios_base::scientific) 

and should be corrected with additional parentheses, as shown above.

[ 2009-10-28 Howard: ]

Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Change Table 83 — Floating-point conversions in 22.4.2.2.2 [facet.num.put.virtuals]:

Table 83 — Floating-point conversions
State stdio equivalent
floatfield == (ios_base::fixed | ios_base::scientific) && !uppercase %a
floatfield == (ios_base::fixed | ios_base::scientific) %A

1153. Standard library needs review for constructors to be explicit to avoid treatment as initializer-list constructor

Section: 17 [library], 30 [thread], D [depr] Status: Open Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-20

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Discussion:

Addresses DE 2

Description

Marking a constructor with explicit has semantics even for a constructor with zero or several parameters: Such a constructor cannot be used with list-initialization in a copy-initialization context, see 13.3.1.7 [over.match.list]. The standard library apparently has not been reviewed for marking non-single-parameter constructors as explicit.

Suggestion

Consider marking zero-parameter and multi-parameter constructors explicit in classes that have at least one constructor marked explicit and that do not have an initializer-list constructor.

Notes

Robert Klarer to address this one.

[ 2009 Santa Cruz: ]

Move to "Open". Robert Klarer has promised to provide wording.

Proposed resolution:


1154. complex should accept integral types

Section: 26.4 [complex.numbers] Status: New Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-26

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Discussion:

Addresses FR 35

Description

Instantiations of the class template complex<> have to be allowed for integral types, to reflect existing practice and ISO standards (LIA-III).

Suggestion

[ 2009-10-26 Proposed wording in N3002. ]

Proposed resolution:

Adopt N3002.

1156. Constraints on bitmask and enumeration types to be tightened

Section: 17.5.2.1.2 [enumerated.types], 17.5.2.1.3 [bitmask.types] Status: Open Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-27

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Discussion:

Addresses UK 165

Description

Constraints on bitmask and enumeration types were supposed to be tightened up as part of the motivation for the constexpr feature - see paper N2235 for details

Suggestion

Adopt wording in line with the motivation described in N2235

Notes

Robert Klarer to review

[ 2009 Santa Cruz: ]

Move to Open. Ping Robert Klarer to provide wording, using N2235 as guidance.

Proposed resolution:


1157. Local types can now instantiate templates

Section: 17.6.3.2.1 [namespace.std] Status: Ready Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-21

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Discussion:

Addresses UK 175

Description

Local types can now be used to instantiate templates, but don't have external linkage.

Suggestion

Remove the reference to external linkage.

Notes

We accept the proposed solution. Martin will draft an issue.

[ 2009-07-28 Alisdair provided wording. ]

[ 2009-10 Santa Cruz: ]

Moved to Ready.

Proposed resolution:

17.6.3.2.1 [namespace.std]

Strike "of external linkage" in p1 and p2:

-1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std unless otherwise specified. A program may add a concept map for any standard library concept or a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type of external linkage and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.179

-2- The behavior of a C++ program is undefined if it declares

A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.


1158. Encouragement to use monotonic clock

Section: 30.2.4 [thread.req.timing] Status: Tentatively Ready Submitter: LWG Opened: 2009-06-28 Last modified: 2009-10-31

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Discussion:

Addresses UK 322, US 96

Description

Not all systems can provide a monotonic clock. How are they expected to treat a _for function?

Suggestion

Add at least a note explaining the intent for systems that do not support a monotonic clock.

Notes

Create an issue, together with UK 96. Note that the specification as is already allows a non-monotonic clock due to the word “should” rather than “shall”. If this wording is kept, a footnote should be added to make the meaning clear.

[ 2009-06-29 Beman provided a proposed resolution. ]

[ 2009-10-31 Howard adds: ]

Set to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Change Timing specifications 30.2.4 [thread.req.timing] as indicated:

The member functions whose names end in _for take an argument that specifies a relative time. Implementations should are encouraged but not required to use a monotonic clock to measure time for these functions.


1159. Unclear spec for resource_deadlock_would_occur

Section: 30.4.3.2.2 [thread.lock.unique.locking] Status: New Submitter: LWG Opened: 2009-06-28 Last modified: 2010-02-12

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Duplicate of: 1219

Discussion:

Addresses UK 327, UK 328

UK 327 Description

Not clear what the specification for error condition resource_deadlock_would_occur means. It is perfectly possible for this thread to own the mutex without setting owns to true on this specific lock object. It is also possible for lock operations to succeed even if the thread does own the mutex, if the mutex is recursive. Likewise, if the mutex is not recursive and the mutex has been locked externally, it is not always possible to know that this error condition should be raised, depending on the host operating system facilities. It is possible that 'i.e.' was supposed to be 'e.g.' and that suggests that recursive locks are not allowed. That makes sense, as the exposition-only member owns is boolean and not a integer to count recursive locks.

UK 327 Suggestion

Add a precondition !owns. Change the 'i.e.' in the error condition to be 'e.g.' to allow for this condition to propogate deadlock detection by the host OS.

UK 327 Notes

Create an issue. Assigned to Lawrence Crowl. Note: not sure what try_lock means for recursive locks when you are the owner. POSIX has language on this, which should ideally be followed. Proposed fix is not quite right, for example, try_lock should have different wording from lock.

UK 328 Description

There is a missing precondition that owns is true, or an if(owns) test is missing from the effect clause

UK 328 Suggestion

Add a precondition that owns == true. Add an error condition to detect a violation, rather than yield undefined behaviour.

UK 328 Notes

Handle in same issue as UK 327. Also uncertain that the proposed resolution is the correct one.

[ 2009-11-11 Alisdair notes that this issue is very closely related to 1219, if not a dup. ]

[ 2010-02-12 Anthony provided wording. ]

Proposed resolution:

Modify 30.4.3.2.2 [thread.lock.unique.locking] p3 to say:

void lock();

...

3 Throws: Any exception thrown by pm->lock(). std::system_error if an exception is required (30.2.2 [thread.req.exception]). std::system_error when the postcondition cannot be achieved.

Modify 30.4.3.2.2 [thread.lock.unique.locking] p8 to say:

bool try_lock();

...

8 Throws: Any exception thrown by pm->try_lock(). std::system_error if an exception is required (30.2.2 [thread.req.exception]). std::system_error when the postcondition cannot be achieved.

Modify 30.4.3.2.2 [thread.lock.unique.locking] p13 to say:

template <class Clock, class Duration>
  bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);

...

13 Throws: Any exception thrown by pm->try_lock_until(). std::system_error if an exception is required (30.2.2 [thread.req.exception]). std::system_error when the postcondition cannot be achieved.

Modify 30.4.3.2.2 [thread.lock.unique.locking] p18 to say:

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

...

18 Throws: Any exception thrown by pm->try_lock_for(). std::system_error if an exception is required (30.2.2 [thread.req.exception]). std::system_error when the postcondition cannot be achieved.


1169. num_get not fully compatible with strto*

Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: New Submitter: Cosmin Truta Opened: 2009-07-04 Last modified: 2009-07-07

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Discussion:

As specified in the latest draft, N2914, num_get is still not fully compatible with the following C functions: strtoul, strtoull, strtof and strtod.

In C, when conversion of a string to an unsigned integer type falls outside the representable range, strtoul and strtoull return ULONG_MAX and ULLONG_MAX, respectively, regardless whether the input field represents a positive or a negative value. On the other hand, the result of num_get conversion of negative values to unsigned integer types is zero. This raises a compatibility issue.

Moreover, in C, when conversion of a string to a floating-point type falls outside the representable range, strtof, strtod and strtold return ±HUGE_VALF, ±HUGE_VAL and ±HUGE_VALL, respectively. On the other hand, the result of num_get conversion of such out-of-range floating-point values results in the most positive/negative representable value. Although many C library implementations do implement HUGE_VAL (etc.) as the highest representable (which is, usually, the infinity), this isn't required by the C standard. The C library specification makes no statement regarding the value of HUGE_VAL and friends, which potentially raises the same compatibility issue as in the above case of unsigned integers. In addition, neither C nor C++ define symbolic constants for the maximum representable floating-point values (they only do so only for the maximum representable finite floating-point values), which raises a usability issue (it would be hard for the programmer to check the result of num_get against overflow).

As such, we propose to adjust the specification of num_get to closely follow the behavior of all of its underlying C functions.

Proposed resolution:

Change 22.4.2.1.2 [facet.num.get.virtuals] as follows:

Stage 3: The sequence of chars accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header <cstdlib>:

The numeric value to be stored can be one of:

The resultant numeric value is stored in val. If the conversion function fails to convert the entire field, or if the field represents a value outside the range of representable values, ios_base::failbit is assigned to err.


1170. String char-like types no longer PODs

Section: 21.1 [strings.general] Status: Tentatively Ready Submitter: Beman Dawes Opened: 2009-06-22 Last modified: 2010-01-23

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Discussion:

Addresses UK 218

Prior to the introduction of constant expressions into the library, basic_string elements had to be POD types, and thus had to be both trivially copyable and standard-layout. This ensured that they could be memcpy'ed and would be compatible with other libraries and languages, particularly the C language and its library.

N2349, Constant Expressions in the Standard Library Revision 2, changed the requirement in 21/1 from "POD type" to "literal type". That change had the effect of removing the trivially copyable and standard-layout requirements from basic_string elements.

This means that basic_string elements no longer are guaranteed to be memcpy'able, and are no longer guaranteed to be standard-layout types:

3.9/p2 and 3.9/p3 both make it clear that a "trivially copyable type" is required for memcpy to be guaranteed to work.

Literal types (3.9p12) may have a non-trivial copy assignment operator, and that violates the trivially copyable requirements given in 9/p 6, bullet item 2.

Literal types (3.9p12) have no standard-layout requirement, either.

This situation probably arose because the wording for "Constant Expressions in the Standard Library" was in process at the same time the C++ POD deconstruction wording was in process.

Since trivially copyable types meet the C++0x requirements for literal types, and thus work with constant expressions, it seems an easy fix to revert the basic_string element wording to its original state.

[ 2009-07-28 Alisdair adds: ]

When looking for any resolution for this issue, consider the definition of "character container type" in 17.3.4 [defns.character.container]. This does require the character type to be a POD, and this term is used in a number of places through clause 21 and 28. This suggests the PODness constraint remains, but is much more subtle than before. Meanwhile, I suspect the change from POD type to literal type was intentional with the assumption that trivially copyable types with non-trivial-but-constexpr constructors should serve as well. I don't believe the current wording offers the right guarantees for either of the above designs.

[ 2009-11-04 Howard modifies proposed wording to disallow array types as char-like types. ]

[ 2010-01-23 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change General 21.1 [strings.general] as indicated:

This Clause describes components for manipulating sequences of any literal non-array POD (3.9) type. In this Clause such types are called char-like types, and objects of char-like types are called char-like objects or simply characters.


1171. duration types should be literal

Section: 20.9.3 [time.duration] Status: Open Submitter: Alisdair Meredith Opened: 2009-07-06 Last modified: 2009-10-31

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Discussion:

The duration types in 20.9.3 [time.duration] are exactly the sort of type that should be "literal types" in the new standard. Likewise, arithmetic operations on durations should be declared constexpr.

[ 2009-09-21 Daniel adds: ]

An alternative (and possibly preferable solution for potentially heap-allocating big_int representation types) would be to ask the core language to allow references to const literal types as feasible arguments for constexpr functions.

[ 2009-10-30 Alisdair adds: ]

I suggest this issue moves from New to Open.

Half of this issue was dealt with in paper n2994 on constexpr constructors.

The other half (duration arithmetic) is on hold pending Core support for const & in constexpr functions.

Proposed resolution:

Add constexpr to declaration of following functions and constructors:

p1 20.9 [time]

Header <chrono> synopsis

[Draughting note - observe switch to pass-by-value to support constexpr]

// duration arithmetic
template <class Rep1, class Period1, class Rep2, class Period2>
   typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
   constexpr operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
template <class Rep1, class Period1, class Rep2, class Period2>
   typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
   constexpr operator-(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
template <class Rep1, class Period, class Rep2>
   duration<typename common_type<Rep1, Rep2>::type, Period>
   constexpr operator*(const duration<Rep1, Period>& d, const Rep2& s);
template <class Rep1, class Period, class Rep2>
   duration<typename common_type<Rep1, Rep2>::type, Period>
   constexpr operator*(const Rep1& s, const duration<Rep2, Period>& d);
template <class Rep1, class Period, class Rep2>
   duration<typename common_type<Rep1, Rep2>::type, Period>
   constexpr operator/(const duration<Rep1, Period>& d, const Rep2& s);
template <class Rep1, class Period1, class Rep2, class Period2>
   typename common_type<Rep1, Rep2>::type
   constexpr operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

// duration comparisons
template <class Rep1, class Period1, class Rep2, class Period2>
   constexpr bool operator==(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
template <class Rep1, class Period1, class Rep2, class Period2>
   constexpr bool operator!=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
template <class Rep1, class Period1, class Rep2, class Period2>
   constexpr bool operator< (const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
template <class Rep1, class Period1, class Rep2, class Period2>
   constexpr bool operator<=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
template <class Rep1, class Period1, class Rep2, class Period2>
   constexpr bool operator> (const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
template <class Rep1, class Period1, class Rep2, class Period2>
   constexpr bool operator>=(const  duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

// duration_cast
template <class ToDuration, class Rep, class Period>
   constexpr ToDuration duration_cast(const duration<Rep, Period>& d);

20.9.3 [time.duration]

template <class Rep, class Period = ratio<1>>
class duration {
  ....
public:
  // 20.9.3.1, construct/copy/destroy:
 constexpr duration() = default;

 template <class Rep2>
   constexpr explicit duration(const Rep2& r);
 template <class Rep2, class Period2>
   constexpr duration(const duration<Rep2, Period2>& d);

  constexpr duration(const duration&) = default;

  // 20.9.3.2, observer:
  constexpr rep count() const;

  // 20.9.3.3, arithmetic:
  constexpr duration operator+() const;
  constexpr duration operator-() const;
  ...

};

[ Note - this edit already seems assumed by definition of the duration static members zero/min/max. They cannot meaningfully be constexpr without this change. ]


1173. "Equivalence" wishy-washiness

Section: 17 [library] Status: Open Submitter: David Abrahams Opened: 2009-07-14 Last modified: 2009-10-20

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Discussion:

Issue: The CopyConstructible requirements are wishy-washy. It requires that the copy is "equivalent" to the original, but "equivalent" is never defined.

I believe this to be an example of a more general lack of rigor around copy and assignment, although I haven't done the research to dig up all the instances.

It's a problem because if you don't know what CopyConstructible means, you also don't know what it means to copy a pair of CopyConstructible types. It doesn't prevent us from writing code, but it is a hole in our ability to understand the meaning of copy.

Furthermore, I'm pretty sure that vector's copy constructor doesn't require the elements to be EqualityComparable, so that table is actually referring to some ill-defined notion of equivalence when it uses ==.

[ 2009 Santa Cruz: ]

Move to "Open". Dave is right that this is a big issue. Paper D2987 ("Defining Move Special Member Functions", Bjarne Stroustrup and Lawrence Crowl) touches on this but does not solve it. This issue is discussed in Elements of Programming.

Proposed resolution:


1175. unordered complexity

Section: 23.2.5 [unord.req] Status: New Submitter: Pablo Halpern Opened: 2009-07-17 Last modified: 2009-07-19

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Discussion:

When I look at the unordered_* constructors, I think the complexity is poorly described and does not follow the style of the rest of the standard.

The complexity for the default constructor is specified as constant. Actually, it is proportional to n, but there are no invocations of value_type constructors or other value_type operations.

For the iterator-based constructor the complexity should be:

Complexity: exactly n calls to construct value_type from InputIterator::value_type (where n = distance(f,l)). The number of calls to key_equal::operator() is proportional to n in the average case and n*n in the worst case.

Proposed resolution:


1176. Make thread constructor non-variadic

Section: 30.3.1.2 [thread.thread.constr] Status: Tentatively NAD Submitter: Howard Hinnant Opened: 2009-07-18 Last modified: 2009-11-17

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Discussion:

The variadic thread constructor is causing controversy, e.g. N2901. This issue has been created as a placeholder for this course of action.

template <class F, class ...Args> thread(F&& f, Args&&... args);

See 929 for wording which specifies an rvalue-ref signature but with "decay behavior", but using variadics.

[ 2009-11-17 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]

Proposed resolution:

Rationale:

The (tentative) concensus of the LWG is to keep the variadic thread constructor.


1177. Improve "diagnostic required" wording

Section: 20.9.3 [time.duration] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2009-07-18 Last modified: 2009-11-22

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Discussion:

"diagnostic required" has been used (by me) for code words meaning "use enable_if to constrain templated functions. This needs to be improved by referring to the function signature as not participating in the overload set, and moving this wording to a Remarks paragraph.

[ 2009-10 Santa Cruz: ]

Moved to Ready.

[ 2009-11-19 Pete opens: ]

Oh, and speaking of 1177, most of the changes result in rather convoluted prose. Instead of saying

A shall be B, else C

it should be

C if A is not B

That is:

Rep2 shall be implicitly convertible to CR(Rep1, Rep2), else this signature shall not participate in overload resolution.

should be

This signature shall not participate in overload resolution if Rep2 is not implicitly convertible to CR(Rep1, Rep2).

That is clearer, and eliminates the false requirement that Rep2 "shall be" convertible.

[ 2009-11-19 Howard adds: ]

I've updated the wording to match Pete's suggestion and included bullet 16 from 1195.

[ 2009-11-19 Jens adds: ]

Further wording suggestion using "unless":

This signature shall not participate in overload resolution unless Rep2 is implicitly convertible to CR(Rep1, Rep2).

[ 2009-11-20 Howard adds: ]

I've updated the wording to match Jens' suggestion.

[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

[ This proposed resolution addresses 947 and 974. ]

  1. Change 20.9.3.1 [time.duration.cons] (and reorder the Remarks paragraphs per 17.5.1.4 [structure.specifications]):

    template <class Rep2> 
      explicit duration(const Rep2& r);
    

    Requires: Remarks: This constructor shall not participate in overload resolution unless Rep2 shall be is implicitly convertible to rep and

    • treat_as_floating_point<rep>::value shall be is true, or
    • treat_as_floating_point<Rep2>::value shall be is false.

    Diagnostic required [Example:

    duration<int, milli> d(3); // OK 
    duration<int, milli> d(3.5); // error 
    

    end example]

    Effects: Constructs an object of type duration.

    Postcondition: count() == static_cast<rep>(r).

    template <class Rep2, class Period2>
      duration(const duration<Rep2, Period2>& d);
    

    Requires: Remarks: This constructor shall not participate in overload resolution unless treat_as_floating_point<rep>::value shall be is true or ratio_divide<Period2, period>::type::den shall be is 1. 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] [Example:

    duration<int, milli> ms(3); 
    duration<int, micro> us = ms; // OK 
    duration<int, milli> ms2 = us; // error 
    

    end example]

    Effects: Constructs an object of type duration, constructing rep_ from duration_cast<duration>(d).count().

  2. Change the following paragraphs in 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 Remarks: This operator shall not participate in overload resolution unless Rep2 shall be is implicitly convertible to CR(Rep1, Rep2). Diagnostic required.
    template <class Rep1, class Period, class Rep2> 
      duration<typename common_type<Rep1, Rep2>::type, Period> 
      operator*(const Rep1& s, const duration<Rep2, Period>& d);
    
    Requires Remarks: This operator shall not participate in overload resolution unless Rep1 shall be is implicitly convertible to CR(Rep1, Rep2). Diagnostic required.
    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 Remarks: This operator shall not participate in overload resolution unless Rep2 shall be is implicitly convertible to CR(Rep1, Rep2) and Rep2 shall not be is not an instantiation of duration. Diagnostic required.
    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 Remarks: This operator shall not participate in overload resolution unless Rep2 shall be is implicitly convertible to CR(Rep1, Rep2) and Rep2 shall not be is not an instantiation of duration. Diagnostic required.
  3. Change the following paragraphs in 20.9.3.7 [time.duration.cast]:

    template <class ToDuration, class Rep, class Period> 
      ToDuration duration_cast(const duration<Rep, Period>& d);
    
    Requires Remarks: This function shall not participate in overload resolution unless ToDuration shall be is an instantiation of duration. Diagnostic required.
  4. Change 20.9.4.1 [time.point.cons]/3 as indicated:

    Requires: Duration2 shall be implicitly convertible to duration. Diagnostic required.

    Remarks: This constructor shall not participate in overload resolution unless Duration2 is implicitly convertible to duration.

  5. Change the following paragraphs in 20.9.4.7 [time.point.cast]:

    template <class ToDuration, class Clock, class Duration> 
      time_point<Clock, ToDuration> time_point_cast(const time_point<Clock, Duration>& t);
    
    Requires Remarks: This function shall not participate in overload resolution unless ToDuration shall be is an instantiation of duration. Diagnostic required.

1180. Missing string_type member typedef in class sub_match

Section: 28.9.1 [re.submatch.members] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2009-07-25 Last modified: 2009-11-15

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Discussion:

The definition of class template sub_match is strongly dependent on the type basic_string<value_type>, both in interface and effects, but does not provide a corresponding typedef string_type, as e.g. class match_results does, which looks like an oversight to me that should be fixed.

[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

  1. In the class template sub_match synopsis 28.9 [re.submatch]/1 change as indicated:

    template <class BidirectionalIterator>
    class sub_match : public std::pair<BidirectionalIterator, BidirectionalIterator> {
    public:
      typedef typename iterator_traits<BidirectionalIterator>::value_type value_type;
      typedef typename iterator_traits<BidirectionalIterator>::difference_type difference_type;
      typedef BidirectionalIterator iterator;
      typedef basic_string<value_type> string_type;
    
      bool matched;
    
      difference_type length() const;
      operator basic_string<value_type>string_type() const;
      basic_string<value_type>string_type str() const;
      int compare(const sub_match& s) const;
      int compare(const basic_string<value_type>string_type& s) const;
      int compare(const value_type* s) const;
    };
    
  2. In 28.9.1 [re.submatch.members]/2 change as indicated:

    operator basic_string<value_type>string_type() const;
    
    Returns: matched ? basic_string<value_type> string_type(first, second) : basic_string<value_type> string_type().
  3. In 28.9.1 [re.submatch.members]/3 change as indicated:

    basic_string<value_type>string_type str() const;
    
    Returns: matched ? basic_string<value_type> string_type(first, second) : basic_string<value_type> string_type().
  4. In 28.9.1 [re.submatch.members]/5 change as indicated:

    int compare(const basic_string<value_type>string_type& s) const;
    

1181. Invalid sub_match comparison operators

Section: 28.9.2 [re.submatch.op] Status: New Submitter: Daniel Krügler Opened: 2009-07-25 Last modified: 2009-07-28

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Discussion:

Several heterogeneous comparison operators of class template sub_match are specified by return clauses that are not valid in general. E.g. 28.9.2 [re.submatch.op]/7:

template <class BiIter, class ST, class SA>
bool operator==(
  const basic_string<
    typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
  const sub_match<BiIter>& rhs);
Returns: lhs == rhs.str().

The returns clause would be ill-formed for all cases where ST != std::char_traits<iterator_traits<BiIter>::value_type> or SA != std::allocator<iterator_traits<BiIter>::value_type>.

The generic character of the comparison was intended, so there are basically two approaches to fix the problem: The first one would define the semantics of the comparison using the traits class ST (The semantic of basic_string::compare is defined in terms of the compare function of the corresponding traits class), the second one would define the semantics of the comparison using the traits class

std::char_traits<iterator_traits<BiIter>::value_type>

which is essentially identical to

std::char_traits<sub_match<BiIter>::value_type>

I suggest to follow the second approach, because this emphasizes the central role of the sub_match object as part of the comparison and would also make sure that a sub_match comparison using some basic_string<char_t, ..> always is equivalent to a corresponding comparison with a string literal because of the existence of further overloads (beginning from 28.9.2 [re.submatch.op]/19). If users really want to take advantage of their own traits::compare, they can simply write a corresponding compare function that does so.

Proposed resolution:

  1. In 28.9.2 [re.submatch.op] change as indicated:

    1. If 1180 is accepted:

      template <class BiIter, class ST, class SA>
        bool operator==(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      7 Returns: lhstypename sub_match<BiIter>::string_type(lhs.begin(), lhs.end()) == rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator!=(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      8 Returns: lhstypename sub_match<BiIter>::string_type(lhs.begin(), lhs.end()) != rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator<(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      9 Returns: lhstypename sub_match<BiIter>::string_type(lhs.begin(), lhs.end()) < rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator>(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      10 Returns: lhstypename sub_match<BiIter>::string_type(lhs.begin(), lhs.end()) > rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator>=(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      11 Returns: lhstypename sub_match<BiIter>::string_type(lhs.begin(), lhs.end()) >= rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator<=(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      12 Returns: lhstypename sub_match<BiIter>::string_type(lhs.begin(), lhs.end()) <= rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator==(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      13 Returns: lhs.str() == rhstypename sub_match<BiIter>::string_type(rhs.begin(), rhs.end()).
      template <class BiIter, class ST, class SA>
        bool operator!=(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      14 Returns: lhs.str() != rhstypename sub_match<BiIter>::string_type(rhs.begin(), rhs.end()).
      template <class BiIter, class ST, class SA>
        bool operator<(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      15 Returns: lhs.str() < rhstypename sub_match<BiIter>::string_type(rhs.begin(), rhs.end()).
      template <class BiIter, class ST, class SA>
        bool operator>(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      16 Returns: lhs.str() > rhstypename sub_match<BiIter>::string_type(rhs.begin(), rhs.end()).
      template <class BiIter, class ST, class SA>
        bool operator>=(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      17 Returns: lhs.str() >= rhstypename sub_match<BiIter>::string_type(rhs.begin(), rhs.end()).
      template <class BiIter, class ST, class SA>
        bool operator<=(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      18 Returns: lhs.str() <= rhstypename sub_match<BiIter>::string_type(rhs.begin(), rhs.end()).
    2. If 1180 is not accepted:

      template <class BiIter, class ST, class SA>
        bool operator==(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      7 Returns: lhsbasic_string<typename sub_match<BiIter>::value_type>(lhs.begin(), lhs.end()) == rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator!=(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      8 Returns: lhsbasic_string<typename sub_match<BiIter>::value_type>(lhs.begin(), lhs.end()) != rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator<(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      9 Returns: lhsbasic_string<typename sub_match<BiIter>::value_type>(lhs.begin(), lhs.end()) < rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator>(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      10 Returns: lhsbasic_string<typename sub_match<BiIter>::value_type>(lhs.begin(), lhs.end()) > rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator>=(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      11 Returns: lhsbasic_string<typename sub_match<BiIter>::value_type>(lhs.begin(), lhs.end()) >= rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator<=(
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
          const sub_match<BiIter>& rhs);
      
      12 Returns: lhsbasic_string<typename sub_match<BiIter>::value_type>(lhs.begin(), lhs.end()) <= rhs.str().
      template <class BiIter, class ST, class SA>
        bool operator==(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      13 Returns: lhs.str() == rhsbasic_string<typename sub_match<BiIter>::value_type>(rhs.begin(), rhs.end()).
      template <class BiIter, class ST, class SA>
        bool operator!=(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      14 Returns: lhs.str() != rhsbasic_string<typename sub_match<BiIter>::value_type>(rhs.begin(), rhs.end()).
      template <class BiIter, class ST, class SA>
        bool operator<(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      15 Returns: lhs.str() < rhsbasic_string<typename sub_match<BiIter>::value_type>(rhs.begin(), rhs.end()).
      template <class BiIter, class ST, class SA>
        bool operator>(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      16 Returns: lhs.str() > rhsbasic_string<typename sub_match<BiIter>::value_type>(rhs.begin(), rhs.end()).
      template <class BiIter, class ST, class SA>
        bool operator>=(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      17 Returns: lhs.str() >= rhsbasic_string<typename sub_match<BiIter>::value_type>(rhs.begin(), rhs.end()).
      template <class BiIter, class ST, class SA>
        bool operator<=(const sub_match<BiIter>& lhs,
          const basic_string<
            typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);
      
      18 Returns: lhs.str() <= rhsbasic_string<typename sub_match<BiIter>::value_type>(rhs.begin(), rhs.end()).

1182. Unfortunate hash dependencies

Section: 20.7.16 [unord.hash] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-07-28 Last modified: 2010-02-09

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Discussion:

Addresses UK 324

The implied library dependencies created by spelling out all the hash template specializations in the <functional> synopsis are unfortunate. The potential coupling is greatly reduced if the hash specialization is declared in the appropriate header for each library type, as it is much simpler to forward declare the primary template and provide a single specialization than it is to implement a hash function for a string or vector without providing a definition for the whole string/vector template in order to access the necessary bits.

Note that the proposed resolution purely involves moving the declarations of a few specializations, it specifically does not make any changes to 20.7.16 [unord.hash].

[ 2009-09-15 Daniel adds: ]

I suggest to add to the current existing proposed resolution the following items.

[ 2009-11-13 Alisdair adopts Daniel's suggestion and the extended note from 889. ]

[ 2010-01-31 Alisdair: related to 1245 and 978. ]

[ 2010-02-07 Proposed wording updated by Beman, Daniel, Alisdair and Ganesh. ]

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Strike the following specializations declared in the <functional> synopsis p2 20.7 [function.objects]

template <> struct hash<std::string>;
template <> struct hash<std::u16string>;
template <> struct hash<std::u32string>;
template <> struct hash<std::wstring>;

template <> struct hash<std::error_code>;
template <> struct hash<std::thread::id>;
template <class Allocator> struct hash<std::vector<bool, Allocator> >;
template <std::size_t N> struct hash<std::bitset<N> >;

Add the following at the end of 20.7.16 [unord.hash]:

template <> struct hash<bool>;
template <> struct hash<char>;
template <> struct hash<signed char>;
template <> struct hash<unsigned char>;
template <> struct hash<char16_t>;
template <> struct hash<char32_t>;
template <> struct hash<wchar_t>;
template <> struct hash<short>;
template <> struct hash<unsigned short>;
template <> struct hash<int>;
template <> struct hash<unsigned int>;
template <> struct hash<long>;
template <> struct hash<long long>;
template <> struct hash<unsigned long>;
template <> struct hash<unsigned long long>;
template <> struct hash<float>;
template <> struct hash<double>;
template <> struct hash<long double>;
template<class T> struct hash<T*>;

Specializations meeting the requirements of class template hash 20.7.16 [unord.hash].

Add the following declarations to 19.5 [syserr], header <system_error> synopsis after // 19.5.4:

// [syserr.hash] hash support
template <class T> struct hash;
template <> struct hash<error_code>;

Add a new clause 19.5.X (probably after 19.5.4):

19.5.X Hash support [syserr.hash]

template <> struct hash<error_code>;

Specialization meeting the requirements of class template hash 20.7.16 [unord.hash].

Add the following declarations to the synopsis of <string> in 21.3 [string.classes]

// [basic.string.hash] hash support
template <class T> struct hash;
template <> struct hash<string>;
template <> struct hash<u16string>;
template <> struct hash<u32string>;
template <> struct hash<wstring>;

Add a new clause 21.4.X

21.4.X Hash support [basic.string.hash]>

template <> struct hash<string>;
template <> struct hash<u16string>;
template <> struct hash<u32string>;
template <> struct hash<wstring>;

Specializations meeting the requirements of class template hash 20.7.16 [unord.hash].

Add the following declarations to the synopsis of <vector> in 23.3 [sequences]

// 21.4.x hash support
template <class T> struct hash;
template <class Allocator> struct hash<vector<bool, Allocator>>;

Add a new paragraph to the end of 23.3.7 [vector.bool]

template <class Allocator> struct hash<vector<bool, Allocator>>;

Specialization meeting the requirements of class template hash 20.7.16 [unord.hash].

Add the following declarations to the synopsis of <bitset> in 20.3.7 [template.bitset]

// [bitset.hash] hash support
template <class T> struct hash;
template <size_t N> struct hash<bitset<N> >;

Add a new subclause 20.3.7.X [bitset.hash]

20.3.7.X bitset hash support [bitset.hash]

template <size_t N> struct hash<bitset<N> >;

Specialization meeting the requirements of class template hash 20.7.16 [unord.hash].

Add the following declarations to 30.3.1.1 [thread.thread.id] synopsis just after the declaration of the comparison operators:

template <class T> struct hash;
template <> struct hash<thread::id>;

Add a new paragraph at the end of 30.3.1.1 [thread.thread.id]:

template <> struct hash<thread::id>;

Specialization meeting the requirements of class template hash 20.7.16 [unord.hash].

Change Header <typeindex> synopsis 20.11.1 [type.index.synopsis] as indicated:

namespace std {
class type_index;
  // [type.index.hash] hash support
  template <class T> struct hash;
  template<> struct hash<type_index>;  : public unary_function<type_index, size_t> {
    size_t operator()(type_index index) const;
  }
}

Change Template specialization hash<type_index> 20.11.4 [type.index.templ] as indicated:

20.11.4 Template specialization hash<type_index> [type.index.templ] Hash support [type.index.hash]

size_t operator()(type_index index) const;

Returns: index.hash_code()

template<> struct hash<type_index>;

Specialization meeting the requirements of class template hash [unord.hash]. For an object index of type type_index, hash<type_index>()(index) shall evaluate to the same value as index.hash_code().


1183. basic_ios::set_rdbuf may break class invariants

Section: 27.5.4.2 [basic.ios.members] Status: Open Submitter: Daniel Krügler Opened: 2009-07-28 Last modified: 2009-10-22

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Discussion:

The protected member function set_rdbuf had been added during the process of adding move and swap semantics to IO classes. A relevant property of this function is described by it's effects in 27.5.4.2 [basic.ios.members]/19:

Effects: Associates the basic_streambuf object pointed to by sb with this stream without calling clear().

This means that implementors of or those who derive from existing IO classes could cause an internal state where the stream buffer could be 0, but the IO class has the state good(). This would break several currently existing implementations which rely on the fact that setting a stream buffer via the currently only ways, i.e. either by calling

void init(basic_streambuf<charT,traits>* sb);

or by calling

basic_streambuf<charT,traits>* rdbuf(basic_streambuf<charT,traits>* sb);

to set rdstate() to badbit, if the buffer is 0. This has the effect that many internal functions can simply check rdstate() instead of rdbuf() for being 0.

I therefore suggest that a requirement is added for callers of set_rdbuf to set a non-0 value.

[ 2009-10 Santa Cruz: ]

Moved to Open. Martin volunteers to provide new wording, where set_rdbuf() sets the badbit but does not cause an exception to be thrown like a call to clear() would.

[ 2009-10-20 Martin provides wording: ]

Proposed resolution:

Change 27.5.4.2 [basic.ios.members] around p. 19 as indicated:

void set_rdbuf(basic_streambuf<charT, traits>* sb);

Effects: Associates the basic_streambuf object pointed to by sb with this stream without calling clear(). Postconditions: rdbuf() == sb.

Effects: As if:


iostate state = rdstate();
try { rdbuf(sb); }
catch(ios_base::failure) {
   if (0 == (state & ios_base::badbit))
       unsetf(badbit);
}

Throws: Nothing.

Rationale:

We need to be able to call set_rdbuf() on stream objects for which (rdbuf() == 0) holds without causing ios_base::failure to be thrown. We also don't want badbit to be set as a result of setting rdbuf() to 0 if it wasn't set before the call. This changed Effects clause maintains the current behavior (as of N2914) without requiring that sb be non-null.

1185. iterator categories and output iterators

Section: 24.2 [iterator.requirements] Status: New Submitter: Alisdair Meredith Opened: 2009-07-31 Last modified: 2009-07-31

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Discussion:

(wording relative to N2723 pending new working paper)

According to p3 24.2 [iterator.requirements], Forward iterators, Bidirectional iterators and Random Access iterators all satisfy the requirements for an Output iterator:

XXX iterators satisfy all the requirements of the input and output iterators and can be used whenever either kind is specified ...

Meanwhile, p4 goes on to contradict this:

Besides its category, a forward, bidirectional, or random access iterator can also be mutable or constant...
... Constant iterators do not satisfy the requirements for output iterators

The latter seems to be the overriding concern, as the iterator tag hierarchy does not define forward_iterator_tag as multiply derived from both input_iterator_tag and output_iterator_tag.

The work on concepts for iterators showed us that output iterator really is fundamentally a second dimension to the iterator categories, rather than part of the linear input -> forward -> bidirectional -> random-access sequence. It would be good to clear up these words to reflect that, and separately list output iterator requirements in the requires clauses for the appropriate algorithms and operations.

Proposed resolution:


1186. Forward list could model a stack

Section: 23.3.5.3 [stack] Status: Tentatively NAD Concepts Submitter: Alisdair Meredith Opened: 2009-07-31 Last modified: 2009-11-02

View all issues with Tentatively NAD Concepts status.

Discussion:

The library template forward_list could easily model the idea of a stack, where the operations work on the front of the list rather than the back. However, the standard library stack adaptor cannot support this.

It would be relatively easy to write a partial specialization for stack to support forward_list, but that opens the question of which header to place it in. A much better solution would be to add a concept_map for the StackLikeContainer concept to the <forward_list> header and then everything just works, including a user's own further uses in a stack-like context.

Therefore while I am submitting the issue now so that it is on record, I strongly recommend we resolve as "NAD Concepts" as any non-concepts based solution will be inferior to the final goal, and the feature is not so compelling it must be supported ahead of the concepts-based library.

[ 2009-11-02 Howard adds: ]

Moved to Tentatively NAD Concepts after 5 positive votes on c++std-lib.

Proposed resolution:


1187. std::decay

Section: 20.6.7 [meta.trans.other] Status: New Submitter: Jason Merrill Opened: 2009-08-07 Last modified: 2009-08-22

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Discussion:

I notice that std::decay is specified to strip the cv-quals from anything but an array or pointer. This seems incorrect for values of class type, since class rvalues can have cv-qualified type (3.10 [basic.lval]/9).

[ 2009-08-09 Howard adds: ]

See the thread starting with c++std-lib-24568 for further discussion. And here is a convenience link to the original proposal. Also see the closely related issue 705.

Proposed resolution:

Add a note to decay in 20.6.7 [meta.trans.other]:

[Note: This behavior is similar to the lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) conversions applied when an lvalue expression is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing. — end note]

1188. Unordered containers should have a minimum load factor as well as a maximum

Section: 23.2.5 [unord.req], 23.5 [unord] Status: New Submitter: Matt Austern Opened: 2009-08-10 Last modified: 2009-08-11

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Discussion:

Unordered associative containers have a notion of a maximum load factor: when the number of elements grows large enough, the containers automatically perform a rehash so that the number of elements per bucket stays below a user-specified bound. This ensures that the hash table's performance characteristics don't change dramatically as the size increases.

For similar reasons, Google has found it useful to specify a minimum load factor: when the number of elements shrinks by a large enough, the containers automatically perform a rehash so that the number of elements per bucket stays above a user-specified bound. This is useful for two reasons. First, it prevents wasting a lot of memory when an unordered associative container grows temporarily. Second, it prevents amortized iteration time from being arbitrarily large; consider the case of a hash table with a billion buckets and only one element. (This was discussed even before TR1 was published; it was TR issue 6.13, which the LWG closed as NAD on the grounds that it was a known design feature. However, the LWG did not consider the approach of a minimum load factor.)

The only interesting question is when shrinking is allowed. In principle the cleanest solution would be shrinking on erase, just as we grow on insert. However, that would be a usability problem; it would break a number of common idioms involving erase. Instead, Google's hash tables only shrink on insert and rehash.

The proposed resolution allows, but does not require, shrinking in rehash, mostly because a postcondition for rehash that involves the minimum load factor would be fairly complicated. (It would probably have to involve a number of special cases and it would probably have to mention yet another parameter, a minimum bucket count.)

The current behavior is equivalent to a minimum load factor of 0. If we specify that 0 is the default, this change will have no impact on backward compatibility.

Proposed resolution:

Add two new rows, and change rehash's postcondition in the unordered associative container requirements table in 23.2.5 [unord.req]:

Table 87 — Unordered associative container requirements (in addition to container)
ExpressionReturn typeAssertion/note pre-/post-condition Complexity
a.min_load_factor() float Returns a non-negative number that the container attempts to keep the load factor greater than or equal to. The container automatically decreases the number of buckets as necessary to keep the load factor above this number. constant
a.min_load_factor(z) void Pre: z shall be non-negative. Changes the container's minimum load factor, using z as a hint. [Footnote: the minimum load factor should be significantly smaller than the maximum. If z is too large, the implementation may reduce it to a more sensible value.] constant
a.rehash(n) void Post: a.bucket_count() >= n, and a.size() <= a.bucket_count() * a.max_load_factor(). [Footnote: It is intentional that the postcondition does not mention the minimum load factor. This member function is primarily intended for cases where the user knows that the container's size will increase soon, in which case the container's load factor will temporarily fall below a.min_load_factor().] a.bucket_cout > a.size() / a.max_load_factor() and a.bucket_count() >= n. Average case linear in a.size(), worst case quadratic.

Add a footnote to 23.2.5 [unord.req] p12:

The insert members shall not affect the validity of references to container elements, but may invalidate all iterators to the container. The erase members shall invalidate only iterators and references to the erased elements.

[A consequence of these requirements is that while insert may change the number of buckets, erase may not. The number of buckets may be reduced on calls to insert or rehash.]

Change paragraph 13:

The insert members shall not affect the validity of iterators if (N+n) < z * B zmin * B <= (N+n) <= zmax * B, where N is the number of elements in the container prior to the insert operation, n is the number of elements inserted, B is the container's bucket count, zmin is the container's minimum load factor, and zmax is the container's maximum load factor.

Add to the unordered_map class synopsis in section 23.5.1 [unord.map], the unordered_multimap class synopsis in 23.5.2 [unord.multimap], the unordered_set class synopsis in 23.5.3 [unord.set], and the unordered_multiset class synopsis in 23.5.4 [unord.multiset]:


float min_load_factor() const;
void min_load_factor(float z);

In 23.5.1.1 [unord.map.cnstr], 23.5.2.1 [unord.multimap.cnstr], 23.5.3.1 [unord.set.cnstr], and 23.5.4.1 [unord.multiset.cnstr], change:

... max_load_factor() returns 1.0 and min_load_factor() returns 0.

1189. Awkward interface for changing the number of buckets in an unordered associative container

Section: 23.2.5 [unord.req], 23.5 [unord] Status: Tentatively Ready Submitter: Matt Austern Opened: 2009-08-10 Last modified: 2009-10-28

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Discussion:

Consider a typical use case: I create an unordered_map and then start adding elements to it one at a time. I know that it will eventually need to store a few million elements, so, for performance reasons, I would like to reserve enough capacity that none of the calls to insert will trigger a rehash.

Unfortunately, the existing interface makes this awkward. The user naturally sees the problem in terms of the number of elements, but the interface presents it as buckets. If m is the map and n is the expected number of elements, this operation is written m.rehash(n / m.max_load_factor()) — not very novice friendly.

[ 2009-09-30 Daniel adds: ]

I recommend to replace "resize" by a different name like "reserve", because that would better match the intended use-case. Rational: Any existing resize function has the on-success post-condition that the provided size is equal to size(), which is not satisfied for the proposal. Reserve seems to fit the purpose of the actual renaming suggestion.

[ 2009-10-28 Ganesh summarizes alternative resolutions and expresses a strong preference for the second (and opposition to the first): ]

  1. In the unordered associative container requirements (23.2.5 [unord.req]), remove the row for rehash and replace it with:

    Table 87 — Unordered associative container requirements (in addition to container)
    ExpressionReturn typeAssertion/note pre-/post-condition Complexity
    a.rehashreserve(n) void Post: a.bucket_count > max(a.size(), n) / a.max_load_factor() and a.bucket_count() >= n. Average case linear in a.size(), worst case quadratic.

    Make the corresponding change in the class synopses in 23.5.1 [unord.map], 23.5.2 [unord.multimap], 23.5.3 [unord.set], and 23.5.4 [unord.multiset].

  2. In 23.2.5 [unord.req]/9, table 98, append a new row after the last one:

    Table 87 — Unordered associative container requirements (in addition to container)
    ExpressionReturn typeAssertion/note pre-/post-condition Complexity
    a.rehash(n) void Post: a.bucket_count > a.size() / a.max_load_factor() and a.bucket_count() >= n. Average case linear in a.size(), worst case quadratic.
    a.reserve(n) void Same as a.rehash(ceil(n / a.max_load_factor())) Average case linear in a.size(), worst case quadratic.

    In 23.5.1 [unord.map]/3 in the definition of class template unordered_map, in 23.5.2 [unord.multimap]/3 in the definition of class template unordered_multimap, in 23.5.3 [unord.set]/3 in the definition of class template unordered_set and in 23.5.4 [unord.multiset]/3 in the definition of class template unordered_multiset, add the following line after member function rehash():

    void reserve(size_type n);
    

[ 2009-10-28 Howard: ]

Moved to Tentatively Ready after 5 votes in favor of Ganesh's option 2 above. The original proposed wording now appears here:

Informally: instead of providing rehash(n) provide resize(n), with the semantics "make the container a good size for n elements".

In the unordered associative container requirements (23.2.5 [unord.req]), remove the row for rehash and replace it with:

Table 87 — Unordered associative container requirements (in addition to container)
ExpressionReturn typeAssertion/note pre-/post-condition Complexity
a.rehashresize(n) void Post: a.bucket_count > max(a.size(), n) / a.max_load_factor() and a.bucket_count() >= n. Average case linear in a.size(), worst case quadratic.

Make the corresponding change in the class synopses in 23.5.1 [unord.map], 23.5.2 [unord.multimap], 23.5.3 [unord.set], and 23.5.4 [unord.multiset].

Proposed resolution:

In 23.2.5 [unord.req]/9, table 98, append a new row after the last one:

Table 87 — Unordered associative container requirements (in addition to container)
ExpressionReturn typeAssertion/note pre-/post-condition Complexity
a.rehash(n) void Post: a.bucket_count > a.size() / a.max_load_factor() and a.bucket_count() >= n. Average case linear in a.size(), worst case quadratic.
a.reserve(n) void Same as a.rehash(ceil(n / a.max_load_factor())) Average case linear in a.size(), worst case quadratic.

In 23.5.1 [unord.map]/3 in the definition of class template unordered_map, in 23.5.2 [unord.multimap]/3 in the definition of class template unordered_multimap, in 23.5.3 [unord.set]/3 in the definition of class template unordered_set and in 23.5.4 [unord.multiset]/3 in the definition of class template unordered_multiset, add the following line after member function rehash():

void reserve(size_type n);

1190. Setting the maximum load factor should return the previous value

Section: 23.2.5 [unord.req], 23.5 [unord] Status: New Submitter: Matt Austern Opened: 2009-08-10 Last modified: 2009-08-11

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Discussion:

The unordered associative container requirements table specifies that a.set_max_load_factor(z) has return type void. However, there is a useful piece of information to return: the previous value. Users who don't need it can always ignore it.

Proposed resolution:

In the unordered associative container requirements table, change:

Table 87 — Unordered associative container requirements (in addition to container)
ExpressionReturn typeAssertion/note pre-/post-condition Complexity
a.max_load_factor(z) void float Pre: z shall be positive. Changes the container's maximum load load factor, using z as a hint. Returns: the previous value of a.max_load_factor(). constant

Change the return type of set_max_load_factor in the class synopses in 23.5.1 [unord.map], 23.5.2 [unord.multimap], 23.5.3 [unord.set], and 23.5.4 [unord.multiset].

If issue 1188 is also accepted, make the same changes for min_load_factor.


1191. tuple get API should respect rvalues

Section: 20.5.2.6 [tuple.elem] Status: Open Submitter: Alisdair Meredith Opened: 2009-08-18 Last modified: 2009-10-31

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Discussion:

The tuple get API should respect rvalues. This would allow for moving a single element out of a tuple-like type.

[ 2009-10-30 Alisdair adds: ]

The issue of rvalue overloads of get for tuple-like types was briefly discussed in Santa Cruz.

The feedback was this would be welcome, but we need full wording for the other types (pair and array) before advancing.

I suggest the issue moves to Open from New as it has been considered, feedback given, and it has not (yet) been rejected as NAD.

Proposed resolution:

Add the following signature to p2 20.5.1 [tuple.general]


template <size_t I, class ... Types>
typename tuple_element<I, tuple<Types...> >::type&& get(tuple<Types...> &&);

And again to 20.5.2.6 [tuple.elem].


template <size_t I, class ... Types>
typename tuple_element<I, tuple<Types...> >::type&& get(tuple<Types...>&& t);

Effects: Equivalent to return std::forward<typename tuple_element<I, tuple<Types...> >::type&&>(get<I>(t));

[Note: If a T in Types is some reference type X&, the return type is X&, not X&&. However, if the element type is non-reference type T, the return type is T&&. — end note]

Add the following signature to p1 20.3 [utility]


template <size_t I, class T1, class T2>
typename tuple_element<I, pair<T1,T2> >::type&& get(pair<T1, T2>&&);

And to p5 20.3.5 [pair.astuple]


template <size_t I, class T1, class T2>
typename tuple_element<I, pair<T1,T2> >::type&& get(pair<T1, T2>&& p);

Returns: If I == 0 returns std::forward<T1&&>(p.first); if I == 1 returns std::forward<T2&&>(p.second); otherwise the program is ill-formed.

Throws: Nothing.

Add the following signature to 23.3 [sequences] <array> synopsis

template <size_t I, class T, size_t N>
T&& get(array<T,N> &&);

And after p8 23.3.1.7 [array.tuple]

template <size_t I, class T, size_t N>
T&& get(array<T,N> && a);
Effects: Equivalent to return std::move(get<I>(a));

1192. basic_string missing definitions for cbegin / cend / crbegin / crend

Section: 21.4.3 [string.iterators] Status: Tentatively Ready Submitter: Jonathan Wakely Opened: 2009-08-14 Last modified: 2009-10-29

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Discussion:

Unlike the containers in clause 23, basic_string has definitions for begin() and end(), but these have not been updated to include cbegin, cend, crbegin and crend.

[ 2009-10-28 Howard: ]

Moved to Tentatively NAD after 5 positive votes on c++std-lib. Added rationale.

[ 2009-10-28 Alisdair disagrees: ]

I'm going to have to speak up as the dissenting voice.

I agree the issue could be handled editorially, and that would be my preference if Pete feels this is appropriate. Failing that, I really think this issue should be accepted and moved to ready. The other begin/end functions all have a semantic definition for this template, and it is confusing if a small few are missing.

I agree that an alternative would be to strike all the definitions for begin/end/rbegin/rend and defer completely to the requirements tables in clause 23. I think that might be confusing without a forward reference though, as those tables are defined in a *later* clause than the basic_string template itself. If someone wants to pursue this I would support it, but recommend it as a separate issue.

So my preference is strongly to move Ready over NAD, and a stronger preference for NAD Editorial if Pete is happy to make these changes.

[ 2009-10-29 Howard: ]

Moved to Tentatively Ready after 5 positive votes on c++std-lib. Removed rationale to mark it NAD. :-)

Proposed resolution:

Add to 21.4.3 [string.iterators]

iterator       begin();
const_iterator begin() const;
const_iterator cbegin() const;

...

iterator       end();
const_iterator end() const;
const_iterator cend() const;

...

reverse_iterator       rbegin();
const_reverse_iterator rbegin() const;
const_reverse_iterator crbegin() const;

...

reverse_iterator       rend();
const_reverse_iterator rend() const;
const_reverse_iterator crend() const;

1193. default_delete cannot be instantiated with incomplete types

Section: 20.8.14.1 [unique.ptr.dltr] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2009-08-18 Last modified: 2010-01-24

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Discussion:

According to the general rules of 17.6.3.8 [res.on.functions]/2 b 5 the effects are undefined, if an incomplete type is used to instantiate a library template. But neither in 20.8.14.1 [unique.ptr.dltr] nor in any other place of the standard such explicit allowance is given. Since this template is intended to be instantiated with incomplete types, this must be fixed.

[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2009-11-17 Alisdair Opens: ]

LWG 1193 tries to support unique_ptr for incomplete types. I believe the proposed wording goes too far:

The template parameter T of default_delete may be an incomplete type.

Do we really want to support cv-void? Suggested ammendment:

The template parameter T of default_delete may be an incomplete type other than cv-void.

We might also consider saying something about arrays of incomplete types.

Did we lose support for unique_ptr<function-type> when the concept-enabled work was shelved? If so, we might want a default_delete partial specialization for function types that does nothing. Alternatively, function types should not be supported by default, but there is no reason a user cannot support them via their own deletion policy.

Function-type support might also lead to conditionally supporting a function-call operator in the general case, and that seems way too inventive at this stage to me, even if we could largely steal wording directly from reference_wrapper. shared_ptr would have similar problems too.

[ 2010-01-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add two new paragraphs directly to 20.8.14.1 [unique.ptr.dltr] (before 20.8.14.1.1 [unique.ptr.dltr.dflt]) with the following content:

The class template default_delete serves as the default deleter (destruction policy) for the class template unique_ptr.

The template parameter T of default_delete may be an incomplete type.


1194. Unintended queue constructor

Section: 23.3.5 [container.adaptors] Status: Ready Submitter: Howard Hinnant Opened: 2009-08-20 Last modified: 2009-10-20

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Discussion:

23.3.5.1.1 [queue.defn] has the following queue constructor:

template <class Alloc> explicit queue(const Alloc&);

This will be implemented like so:

template <class Alloc> explicit queue(const Alloc& a) : c(a) {}

The issue is that Alloc can be anything that a container will construct from, for example an int. Is this intended to compile?

queue<int> q(5);

Before the addition of this constructor, queue<int>(5) would not compile. I ask, not because this crashes, but because it is new and appears to be unintended. We do not want to be in a position of accidently introducing this "feature" in C++0X and later attempting to remove it.

I've picked on queue. priority_queue and stack have the same issue. Is it useful to create a priority_queue of 5 identical elements?

[ Daniel, Howard and Pablo collaborated on the proposed wording. ]

[ 2009-10 Santa Cruz: ]

Move to Ready.

Proposed resolution:

[ This resolution includes a semi-editorial clean up, giving definitions to members which in some cases weren't defined since C++98. This resolution also offers editorially different wording for 976, and it also provides wording for 1196. ]

Change container.adaptors, p1:

The container adaptors each take a Container template parameter, and each constructor takes a Container reference argument. This container is copied into the Container member of each adaptor. If the container takes an allocator, then a compatible allocator may be passed in to the adaptor's constructor. Otherwise, normal copy or move construction is used for the container argument. [Note: it is not necessary for an implementation to distinguish between the one-argument constructor that takes a Container and the one- argument constructor that takes an allocator_type. Both forms use their argument to construct an instance of the container. — end note]

Change queue.defn, p1:

template <class T, class Container = deque<T> >
class queue {
public:
  typedef typename Container::value_type      value_type;
  typedef typename Container::reference       reference;
  typedef typename Container::const_reference const_reference;
  typedef typename Container::size_type       size_type;
  typedef Container                           container_type;
protected:
  Container c;

public:
  explicit queue(const Container&);
  explicit queue(Container&& = Container());
  queue(queue&& q); : c(std::move(q.c)) {}
  template <class Alloc> explicit queue(const Alloc&);
  template <class Alloc> queue(const Container&, const Alloc&);
  template <class Alloc> queue(Container&&, const Alloc&);
  template <class Alloc> queue(queue&&, const Alloc&);
  queue& operator=(queue&& q); { c = std::move(q.c); return *this; }

  bool empty() const          { return c.empty(); }
  ...
};

Add a new section after 23.3.5.1.1 [queue.defn], [queue.cons]:

queue constructors [queue.cons]

explicit queue(const Container& cont);

Effects: Initializes c with cont.

explicit queue(Container&& cont = Container());

Effects: Initializes c with std::move(cont).

queue(queue&& q)

Effects: Initializes c with std::move(q.c).

For each of the following constructors, if uses_allocator<container_type, Alloc>::value is false, then the constructor shall not participate in overload resolution.

template <class Alloc> 
  explicit queue(const Alloc& a);

Effects: Initializes c with a.

template <class Alloc> 
  queue(const container_type& cont, const Alloc& a);

Effects: Initializes c with cont as the first argument and a as the second argument.

template <class Alloc> 
  queue(container_type&& cont, const Alloc& a);

Effects: Initializes c with std::move(cont) as the first argument and a as the second argument.

template <class Alloc> 
  queue(queue&& q, const Alloc& a);

Effects: Initializes c with std::move(q.c) as the first argument and a as the second argument.

queue& operator=(queue&& q);

Effects: Assigns c with std::move(q.c).

Returns: *this.

Add to 23.3.5.2.1 [priqueue.cons]:

priority_queue(priority_queue&& q);

Effects: Initializes c with std::move(q.c) and initializes comp with std::move(q.comp).

For each of the following constructors, if uses_allocator<container_type, Alloc>::value is false, then the constructor shall not participate in overload resolution.

template <class Alloc>
  explicit priority_queue(const Alloc& a);

Effects: Initializes c with a and value-initializes comp.

template <class Alloc>
  priority_queue(const Compare& compare, const Alloc& a);

Effects: Initializes c with a and initializes comp with compare.

template <class Alloc>
  priority_queue(const Compare& compare, const Container& cont, const Alloc& a);

Effects: Initializes c with cont as the first argument and a as the second argument, and initializes comp with compare.

template <class Alloc>
  priority_queue(const Compare& compare, Container&& cont, const Alloc& a);

Effects: Initializes c with std::move(cont) as the first argument and a as the second argument, and initializes comp with compare.

template <class Alloc>
  priority_queue(priority_queue&& q, const Alloc& a);

Effects: Initializes c with std::move(q.c) as the first argument and a as the second argument, and initializes comp with std::move(q.comp).

priority_queue& operator=(priority_queue&& q);

Effects: Assigns c with std::move(q.c) and assigns comp with std::move(q.comp).

Returns: *this.

Change 23.3.5.3.1 [stack.defn]:

template <class T, class Container = deque<T> >
class stack {
public:
  typedef typename Container::value_type      value_type;
  typedef typename Container::reference       reference;
  typedef typename Container::const_reference const_reference;
  typedef typename Container::size_type       size_type;
  typedef Container                           container_type;
protected:
  Container c;

public:
  explicit stack(const Container&);
  explicit stack(Container&& = Container());
  stack(stack&& s);
  template <class Alloc> explicit stack(const Alloc&);
  template <class Alloc> stack(const Container&, const Alloc&);
  template <class Alloc> stack(Container&&, const Alloc&);
  template <class Alloc> stack(stack&&, const Alloc&);
  stack& operator=(stack&& s);

  bool empty() const          { return c.empty(); }
  ...
};

Add a new section after 23.3.5.3.1 [stack.defn], [stack.cons]:

stack constructors [stack.cons]

stack(stack&& s);

Effects: Initializes c with std::move(s.c).

For each of the following constructors, if uses_allocator<container_type, Alloc>::value is false, then the constructor shall not participate in overload resolution.

template <class Alloc> 
  explicit stack(const Alloc& a);

Effects: Initializes c with a.

template <class Alloc> 
  stack(const container_type& cont, const Alloc& a);

Effects: Initializes c with cont as the first argument and a as the second argument.

template <class Alloc> 
  stack(container_type&& cont, const Alloc& a);

Effects: Initializes c with std::move(cont) as the first argument and a as the second argument.

template <class Alloc> 
  stack(stack&& s, const Alloc& a);

Effects: Initializes c with std::move(s.c) as the first argument and a as the second argument.

stack& operator=(stack&& s);

Effects: Assigns c with std::move(s.c).

Returns: *this.


1195. "Diagnostic required" wording is insufficient to prevent UB

Section: 17 [library] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2009-08-18 Last modified: 2010-02-14

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Discussion:

Several parts of the library use the notion of "Diagnostic required" to indicate that in the corresponding situation an error diagnostic should occur, e.g. 20.8.14.1.1 [unique.ptr.dltr.dflt]/2

void operator()(T *ptr) const;
Effects: calls delete on ptr. A diagnostic is required if T is an incomplete type.

The problem with this approach is that such a requirement is insufficient to prevent undefined behavior, if this situation occurs. According to 1.3.3 [defns.diagnostic] a diagnostic message is defined as

a message belonging to an implementation-defined subset of the implementation's output messages.

which doesn't indicate any relation to an ill-formed program. In fact, "compiler warnings" are a typical expression of such diagnostics. This means that above wording can be interpreted by compiler writers that they satisfy the requirements of the standard if they just produce such a "warning", if the compiler happens to compile code like this:

#include <memory>

struct Ukn; // defined somewhere else
Ukn* create_ukn(); // defined somewhere else

int main() {
 std::default_delete<Ukn>()(create_ukn());
}

In this and other examples discussed here it was the authors intent to guarantee that the program is ill-formed with a required diagnostic, therefore such wording should be used instead. According to the general rules outlined in 1.4 [intro.compliance] it should be sufficient to require that these situations produce an ill-formed program and the "diagnostic required" part should be implied. The proposed resolution also suggests to remove several redundant wording of "Diagnostics required" to ensure that the absence of such saying does not cause a misleading interpretation.

[ 2009 Santa Cruz: ]

Move to NAD.

It's not clear that there's any important difference between "ill-formed" and "diagnostic required". From 1.4 [intro.compliance], 1.3.5 [defns.ill.formed], and 1.3.15 [defns.well.formed] it appears that an ill-formed program is one that is not correctly constructed according to the syntax rules and diagnosable semantic rules, which means that... "a conforming implementation shall issue at least one diagnostic message." The author's intent seems to be that we should be requiring a fatal error instead of a mere warning, but the standard just doesn't have language to express that distinction. The strongest thing we can ever require is a "diagnostic".

The proposed rewording may be a clearer way of expressing the same thing that the WP already says, but such a rewording is editorial.

[ 2009 Santa Cruz: ]

Considered again. Group disagrees that the change is technical, but likes it editorially. Moved to NAD Editorial.

[ 2009-11-19: Moved from NAD Editorial to Open. Please see the thread starting with Message c++std-lib-25916. ]

[ 2009-11-20 Daniel updated wording. ]

The following resolution differs from the previous one by avoiding the unusual and misleading term "shall be ill-formed", which does also not follow the core language style. This resolution has the advantage of a minimum impact on the current wording, but I would like to mention that a more intrusive solution might be preferrable - at least as a long-term solution: Jens Maurer suggested the following approach to get rid of the usage of the term "ill-formed" from the library by introducing a new category to existing elements to the list of 17.5.1.4 [structure.specifications]/3, e.g. "type requirements" or "static constraints" that define conditions that can be checked during compile-time and any violation would make the program ill-formed. As an example, the currently existing phrase 20.5.2.5 [tuple.helper]/1

Requires: I < sizeof...(Types). The program is ill-formed if I is out of bounds.

could then be written as

Static constraints: I < sizeof...(Types).

[ 2009-11-21 Daniel updated wording. ]

[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

  1. Change 20.4 [ratio]/2 as indicated:

    Throughout this subclause, if the template argument types R1 and R2 shall be are not specializations of the ratio template, the program is ill-formed. Diagnostic required.
  2. Change 20.4.1 [ratio.ratio]/1 as indicated:

    If tThe template argument D shall not be is zero, and or the absolute values of the template arguments N and D shall be are not representable by type intmax_t, the program is ill-formed. Diagnostic required. [..]

  3. Change 20.4.2 [ratio.arithmetic]/1 as indicated:

    Implementations may use other algorithms to compute these values. If overflow occurs, the program is ill-formed a diagnostic shall be issued.
  4. Change 20.4.3 [ratio.comparison]/2 as indicated:

    [...] Implementations may use other algorithms to compute this relationship to avoid overflow. If overflow occurs, the program is ill-formed a diagnostic is required.
  5. Change 20.8.14.1.1 [unique.ptr.dltr.dflt]/2 as indicated:

    Effects: calls delete on ptr. A diagnostic is required if T is an incomplete type.

    Remarks: If T is an incomplete type, the program is ill-formed.

  6. Change 20.8.14.1.2 [unique.ptr.dltr.dflt1]/1 as indicated:

    void operator()(T* ptr) const;
    

    Effects: operator() calls delete[] on ptr. A diagnostic is required if T is an incomplete type.

    Remarks: If T is an incomplete type, the program is ill-formed.

  7. Change 20.8.14.2.1 [unique.ptr.single.ctor] as indicated: [Note: This editorially improves the currently suggested wording of 932 by replacing

    "shall be ill-formed" by "is ill-formed"]

    [If N3025 is accepted this bullet is applied identically in that paper as well.]

    -1- Requires: D shall be default constructible, and that construction shall not throw an exception. D shall not be a reference type or pointer type (diagnostic required).

    ...

    Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument D, the program is ill-formed.

  8. Change 20.8.14.2.1 [unique.ptr.single.ctor]/8 as indicated: [Note: This editorially improves the currently suggested wording of 932 by replacing

    "shall be ill-formed" by "is ill-formed"]

    [If N3025 is accepted this bullet is applied identically in that paper as well.]

    unique_ptr(pointer p);
    

    ...

    Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument D, the program is ill-formed.

  9. Change 20.8.14.2.1 [unique.ptr.single.ctor]/13 as indicated:

    [..] If d is an rvalue, it will bind to the second constructor of this pair and the program is ill-formed. That constructor shall emit a diagnostic. [Note: The diagnostic could be implemented using a static_assert which assures that D is not a reference type. — end note] Else d is an lvalue and will bind to the first constructor of this pair. [..]
  10. 20.8.14.2.1 [unique.ptr.single.ctor]/20: Solved by 950.
  11. Change 20.8.14.3 [unique.ptr.runtime]/1 as indicated:

    A specialization for array types is provided with a slightly altered interface.

    • Conversions among different types of unique_ptr<T[], D> or to or from the non-array forms of unique_ptr are disallowed (diagnostic required) produce an ill-formed program.
    • ...
  12. Change 20.9.3 [time.duration]/2-4 as indicated:

    2 Requires: Rep shall be an arithmetic type or a class emulating an arithmetic type. If a program instantiates duration with a duration type for the template argument Rep a diagnostic is required.

    3 Remarks: If duration is instantiated with a duration type for the template argument Rep, the program is ill-formed.

    3 4 Requires Remarks: If Period shall be is not a specialization of ratio, diagnostic required the program is ill-formed.

    4 5 Requires Remarks: If Period::num shall be is not positive, diagnostic required the program is ill-formed.

  13. 20.9.3.1 [time.duration.cons]/1+4: Apply 1177
  14. 20.9.3.5 [time.duration.nonmember]/4+6+8+11: Apply 1177
  15. 20.9.3.7 [time.duration.cast]/1: Apply 1177
  16. Change 20.9.4 [time.point]/2 as indicated:

    If Duration shall be is not an instance of duration, the program is ill-formed. Diagnostic required.
  17. 20.9.4.1 [time.point.cons]/3: Apply 1177
  18. 20.9.4.7 [time.point.cast]/1: Apply 1177

1197. Can unordered containers have bucket_count() == 0?

Section: 23.2.5 [unord.req] Status: Tentatively Ready Submitter: Howard Hinnant Opened: 2009-08-24 Last modified: 2010-01-31

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Discussion:

Table 97 "Unordered associative container requirements" in 23.2.5 [unord.req] says:

Table 97 — Unordered associative container requirements (in addition to container)
Expression Return type Assertion/note pre-/post-condition Complexity
b.bucket(k) size_type Returns the index of the bucket in which elements with keys equivalent to k would be found, if any such element existed. Post: the return value shall be in the range [0, b.bucket_count()). Constant

What should b.bucket(k) return if b.bucket_count() == 0?

I believe allowing b.bucket_count() == 0 is important. It is a very reasonable post-condition of the default constructor, or of a moved-from container.

I can think of several reasonable results from b.bucket(k) when b.bucket_count() == 0:

  1. Return 0.
  2. Return numeric_limits<size_type>::max().
  3. Throw a domain_error.
  4. Requires: b.bucket_count() != 0.

[ 2009-08-26 Daniel adds: ]

A forth choice would be to add the pre-condition "b.bucket_count() != 0" and thus imply undefined behavior if this is violated.

[ Howard: I like this option too, added to the list. ]

Further on here my own favorite solution (rationale see below):

Suggested resolution:

[Rationale: I suggest to follow choice (1). The main reason is that all associative container functions which take a key argument, are basically free of pre-conditions and non-disrupting, therefore excluding choices (3) and (4). Option (2) seems a bit unexpected to me. It would be more natural, if several similar functions would exist which would also justify the existence of a symbolic constant like npos for this situation. The value 0 is both simple and consistent, it has exactly the same role as a past-the-end iterator value. A typical use-case is:

size_type pos = m.bucket(key);
if (pos != m.bucket_count()) {
 ...
} else {
 ...
}

— end Rationale]

- Change Table 97 in 23.2.5 [unord.req] as follows (Row b.bucket(k), Column "Assertion/..."):

Table 97 — Unordered associative container requirements (in addition to container)
Expression Return type Assertion/note pre-/post-condition Complexity
b.bucket(k) size_type Returns the index of the bucket in which elements with keys equivalent to k would be found, if any such element existed. Post: if b.bucket_count() != 0, the return value shall be in the range [0, b.bucket_count()), otherwise 0. Constant

[ 2010-01-25 Choice 4 put into proposed resolution section. ]

[ 2010-01-31 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change Table 97 in 23.2.5 [unord.req] as follows (Row b.bucket(k), Column "Assertion/..."):

Table 97 — Unordered associative container requirements (in addition to container)
Expression Return type Assertion/note pre-/post-condition Complexity
b.bucket(k) size_type Pre: b.bucket_count() > 0 Returns the index of the bucket in which elements with keys equivalent to k would be found, if any such element existed. Post: the return value shall be in the range [0, b.bucket_count()). Constant

1198. Container adaptor swap: member or non-member?

Section: 23.3.5 [container.adaptors] Status: New Submitter: Pablo Halpern Opened: 2009-08-26 Last modified: 2009-09-30

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Discussion:

Under 23.3.5 [container.adaptors] of N2914 the member function of swap of queue and stack call:

swap(c, q.c);

But under 23.3.5 [container.adaptors] of N2723 these members are specified to call:

c.swap(q.c);

Neither draft specifies the semantics of member swap for priority_queue though it is declared.

Although the distinction between member swap and non-member swap is not important when these adaptors are adapting standard containers, it may be important for user-defined containers.

We (Pablo and Howard) feel that it is more likely for a user-defined container to support a namespace scope swap than a member swap, and therefore these adaptors should use the container's namespace scope swap.

[ 2009-09-30 Daniel adds: ]

The outcome of this issue should be considered with the outcome of 774 both in style and in content (e.g. 774 bullet 9 suggests to define the semantic of void priority_queue::swap(priority_queue&) in terms of the member swap of the container).

Proposed resolution:

[ Changes written with respect to N2723. ]

Change 23.3.5.1.1 [queue.defn]:

template <class T, class Container = deque<T> > 
class queue {
   ...
   void swap(queue&& q) { using std::swap;
                          c.swap(c, q.c); }
   ...
};

Change 23.3.5.2 [priority.queue]:

template <class T, class Container = vector<T>, 
          class Compare = less<typename Container::value_type> > 
class priority_queue { 
    ...
    void swap(priority_queue&& q); { using std::swap;
                                     swap(c, q.c);
                                     swap(comp, q.comp); }
    ...
};

Change 23.3.5.3.1 [stack.defn]:

template <class T, class Container = deque<T> > 
class stack {
   ...
   void swap(stack&& s) { using std::swap;
                          c.swap(c, s.c); }
   ...
};

1199. Missing extended copy constructor in container adaptors

Section: 23.3.5 [container.adaptors] Status: Tentatively Ready Submitter: Pablo Halpern Opened: 2009-08-26 Last modified: 2010-02-01

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Discussion:

queue has a constructor:

template <class Alloc>
  queue(queue&&, const Alloc&);

but it is missing a corresponding constructor:

template <class Alloc>
  queue(const queue&, const Alloc&);

The same is true of priority_queue, and stack. This "extended copy constructor" is needed for consistency and to ensure that the user of a container adaptor can always specify the allocator for his adaptor.

[ 2010-02-01 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

[ This resolution has been harmonized with the proposed resolution to issue 1194 ]

Change 23.3.5.1.1 [queue.defn], p1:

template <class T, class Container = deque<T> >
class queue {
public:
  typedef typename Container::value_type      value_type;
  typedef typename Container::reference       reference;
  typedef typename Container::const_reference const_reference;
  typedef typename Container::size_type       size_type;
  typedef Container                           container_type;
protected:
  Container c;

public:
  explicit queue(const Container&);
  explicit queue(Container&& = Container());
  queue(queue&& q);

  template <class Alloc> explicit queue(const Alloc&);
  template <class Alloc> queue(const Container&, const Alloc&);
  template <class Alloc> queue(Container&&, const Alloc&);
  template <class Alloc> queue(const queue&, const Alloc&);
  template <class Alloc> queue(queue&&, const Alloc&);
  queue& operator=(queue&& q);

  bool empty() const          { return c.empty(); }
  ...
};

To the new section [queue.cons], introduced in 1194, add:

template <class Alloc> 
  queue(const queue& q, const Alloc& a);

Effects: Initializes c with q.c as the first argument and a as the second argument.

Change 23.3.5.2 [priority.queue] as follows (I've an included an editorial change to move the poorly-placed move-assignment operator):

template <class T, class Container = vector<T>,
          class Compare = less<typename Container::value_type> >
class priority_queue {
public:
  typedef typename Container::value_type      value_type;
  typedef typename Container::reference       reference;
  typedef typename Container::const_reference const_reference;
  typedef typename Container::size_type       size_type;
  typedef          Container                  container_type;
protected:
  Container c;
  Compare comp;

public:
  priority_queue(const Compare& x, const Container&);
  explicit priority_queue(const Compare& x = Compare(), Container&& = Container());
  template <class InputIterator>
    priority_queue(InputIterator first, InputIterator last,
                   const Compare& x, const Container&);
  template <class InputIterator>
    priority_queue(InputIterator first, InputIterator last,
                   const Compare& x = Compare(), Container&& = Container());
  priority_queue(priority_queue&&);
  priority_queue& operator=(priority_queue&&);
  template <class Alloc> explicit priority_queue(const Alloc&);
  template <class Alloc> priority_queue(const Compare&, const Alloc&);
  template <class Alloc> priority_queue(const Compare&,
                                        const Container&, const Alloc&);
  template <class Alloc> priority_queue(const Compare&,
                                        Container&&, const Alloc&);
  template <class Alloc> priority_queue(const priority_queue&, const Alloc&);
  template <class Alloc> priority_queue(priority_queue&&, const Alloc&);

  priority_queue& operator=(priority_queue&&);
  ...
};

Add to 23.3.5.2.1 [priqueue.cons]:

template <class Alloc>
  priority_queue(const priority_queue& q, const Alloc& a);

Effects: Initializes c with q.c as the first argument and a as the second argument, and initializes comp with q.comp.

Change 23.3.5.3.1 [stack.defn]:

template <class T, class Container = deque<T> >
class stack {
public:
  typedef typename Container::value_type      value_type;
  typedef typename Container::reference       reference;
  typedef typename Container::const_reference const_reference;
  typedef typename Container::size_type       size_type;
  typedef Container                           container_type;
protected:
  Container c;

public:
  explicit stack(const Container&);
  explicit stack(Container&& = Container());
  stack(stack&& s);

  template <class Alloc> explicit stack(const Alloc&);
  template <class Alloc> stack(const Container&, const Alloc&);
  template <class Alloc> stack(Container&&, const Alloc&);
  template <class Alloc> stack(const stack&, const Alloc&);
  template <class Alloc> stack(stack&&, const Alloc&);
  stack& operator=(stack&& s);

  bool empty() const          { return c.empty(); }
  ...
};

To the new section [stack.cons], introduced in 1194, add:

template <class Alloc> 
  stack(const stack& s, const Alloc& a);

Effects: Initializes c with s.c as the first argument and a as the second argument.


1200. "surprising" char_traits<T>::int_type requirements

Section: 21.2.2 [char.traits.typedefs] Status: New Submitter: Sean Hunt Opened: 2009-09-03 Last modified: 2009-10-28

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Discussion:

The footnote for int_type in 21.2.2 [char.traits.typedefs] says that

If eof() can be held in char_type then some iostreams implementations may give surprising results.

This implies that int_type should be a superset of char_type. However, the requirements for char16_t and char32_t define int_type to be equal to int_least16_t and int_least32_t respectively. int_least16_t is likely to be the same size as char_16_t, which may lead to surprising behavior, even if eof() is not a valid UTF-16 code unit. The standard should not prescribe surprising behavior, especially without saying what it is (it's apparently not undefined, just surprising). The same applies for 32-bit types.

I personally recommend that behavior be undefined if eof() is a member of char_type, and another type be chosen for int_type (my personal favorite has always been a struct {bool eof; char_type c;}). Alternatively, the exact results of such a situation should be defined, at least so far that I/O could be conducted on these types as long as the code units remain valid. Note that the argument that no one streams char16_t or char32_t is not really valid as it would be perfectly reasonable to use a basic_stringstream in conjunction with UTF character types.

[ 2009-10-28 Ganesh provides two possible resolutions and expresses a preference for the second: ]

  1. Replace 21.2.3.2 [char.traits.specializations.char16_t] para 3 with:

    The member eof() shall return an implementation-defined constant that cannot appear as a valid UTF-16 code unit UINT_LEAST16_MAX [Note: this value is guaranteed to be a permanently reserved UCS-2 code position if UINT_LEAST16_MAX == 0xFFFF and it's not a UCS-2 code position otherwise — end note].

    Replace 21.2.3.3 [char.traits.specializations.char32_t] para 3 with:

    The member eof() shall return an implementation-defined constant that cannot appear as a Unicode code point UINT_LEAST32_MAX [Note: this value is guaranteed to be a permanently reserved UCS-4 code position if UINT_LEAST32_MAX == 0xFFFFFFFF and it's not a UCS-4 code position otherwise — end note].
  2. In 21.2.3.2 [char.traits.specializations.char16_t], in the definition of char_traits<char16_t> replace the definition of nested typedef int_type with:

    namespace std {
      template<> struct char_traits<char16_t> {
        typedef char16_t         char_type;
        typedef uint_least16_t uint_fast16_t int_type;
         ...
    

    Replace 21.2.3.2 [char.traits.specializations.char16_t] para 3 with:

    The member eof() shall return an implementation-defined constant that cannot appear as a valid UTF-16 code unit UINT_FAST16_MAX [Note: this value is guaranteed to be a permanently reserved UCS-2 code position if UINT_FAST16_MAX == 0xFFFF and it's not a UCS-2 code position otherwise — end note].

    In 21.2.3.3 [char.traits.specializations.char32_t], in the definition of char_traits<char32_t> replace the definition of nested typedef int_type with:

    namespace std {
      template<> struct char_traits<char32_t> {
        typedef char32_t         char_type;
        typedef uint_least32_t uint_fast32_t int_type;
         ...
    

    Replace 21.2.3.3 [char.traits.specializations.char32_t] para 3 with:

    The member eof() shall return an implementation-defined constant that cannot appear as a Unicode code point UINT_FAST32_MAX [Note: this value is guaranteed to be a permanently reserved UCS-4 code position if UINT_FAST32_MAX == 0xFFFFFFFF and it's not a UCS-4 code position otherwise — end note].

Proposed resolution:


1201. Do we always want to unwrap ref-wrappers in make_tuple

Section: 20.5.2.4 [tuple.creation], 20.3.4 [pairs] Status: Tentatively NAD Future Submitter: Alisdair Meredith Opened: 2009-09-05 Last modified: 2009-10-26

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Discussion:

Spotting a recent thread on the boost lists regarding collapsing optional representations in optional<optional<T>> instances, I wonder if we have some of the same issues with make_tuple, and now make_pair?

Essentially, if my generic code in my own library is handed a reference_wrapper by a user, and my library in turn delegates some logic to make_pair or make_tuple, then I am going to end up with a pair/tuple holding a real reference rather than the intended reference wrapper.

There are two things as a library author I can do at this point:

  1. document my library also has the same reference-wrapper behaviour as std::make_tuple
  2. roll my own make_tuple that does not unwrap rereferences, a lost opportunity to re-use the standard library.

(There may be some metaprogramming approaches my library can use to wrap the make_tuple call, but all will be significantly more complex than simply implementing a simplified make_tuple.)

Now I don't propose we lose this library facility, I think unwrapping references will be the common behaviour. However, we might want to consider adding another overload that does nothing special with ref-wrappers. Note that we already have a second overload of make_tuple in the library, called tie.

[ 2009-09-30 Daniel adds: ]

I suggest to change the currently proposed paragraph for make_simple_pair

template<typename... Types>
  pair<typename decay<Types>::type...> make_simple_pair(Types&&... t);

Type requirements: sizeof...(Types) == 2. Remarks: The program shall be ill-formed, if sizeof...(Types) != 2.

...

or alternatively (but with a slightly different semantic):

Remarks: If sizeof...(Types) != 2, this function shall not participate in overload resolution.

to follow a currently introduced style and because the library does not have yet a specific "Type requirements" element. If such thing would be considered as useful this should be done as a separate issue. Given the increasing complexity of either of these wordings it might be preferable to use the normal two-argument-declaration style again in either of the following ways:

  1. template<class T1, class T2>
    pair<typename decay<T1>::type, typename decay<T2>::type>
    make_simple_pair(T1&& t1, T2&& t2);
    
  2. template<class T1, class T2>
    pair<V1, V2> make_simple_pair(T1&& t1, T2&& t2);
    
    Let V1 be typename decay<T1>::type and V2 be typename decay<T2>::type.

[ 2009-10 post-Santa Cruz: ]

Mark as Tentatively NAD Future.

Proposed resolution:

Add the following function to 20.3.4 [pairs] and signature in appropriate synopses:

template<typename... Types>
  pair<typename decay<Types>::type...> make_simple_pair(Types&&... t);

Type requirements: sizeof...(Types) == 2.

Returns: pair<typename decay<Types>::type...>(std::forward<Types>(t)...).

[ Draughting note: I chose a variadic representation similar to make_tuple rather than naming both types as it is easier to read through the clutter of metaprogramming this way. Given there are exactly two elements, the committee may prefer to draught with two explicit template type parameters instead ]

Add the following function to 20.5.2.4 [tuple.creation] and signature in appropriate synopses:

template<typename... Types>
  tuple<typename decay<Types>::type...> make_simple_tuple(Types&&... t);

Returns: tuple<typename decay<Types>::type...>(std::forward<Types>(t)...).


1202. integral_constant needs a spring clean

Section: 20.6.3 [meta.help] Status: Tentatively NAD Submitter: Alisdair Meredith Opened: 2009-09-05 Last modified: 2010-01-14

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Discussion:

The specification of integral_constant has been inherited essentially unchanged from TR1:

template <class T, T v>
struct integral_constant {
  static const T value = v;
  typedef T value_type;
  typedef integral_constant<T,v> type;
};

In light of 0x language changes there are several things we might consider changing, notably the form of specification for value.

The current form requires a static data member have storage allocated for it, where we could now implement without this using the new enum syntax:

template <class T, T v>
struct integral_constant {
  enum : T { value = v };
  typedef T value_type;
  typedef integral_constant type;
};

The effective difference between these two implementation is:

  1. No requirement to allocate storage for data member (which we hope but do not guarantee compilers strip today)
  2. You can no longer take the address of the constant as &integral_constant<T,v>::value;

Also note the editorial change to drop the explicit qualification of integral_constant in the typedef type. This makes it quite clear we mean the current instantiation, and cannot be mistaken for a recursive metaprogram.

Even if we don't mandate this implementation, it would be nice to give vendors freedom under QoI to choose their preferred representation.

The other side of this issue is if we choose to retain the static constant form. In that case we should go further and insist on constexpr, much like we did throughout numeric_limits:

template <class T, T v>
struct integral_constant {
  static constexpr T value = v;
  typedef T value_type;
  typedef integral_constant type;
};

[Footnote] It turns out constexpr is part of the Tentatively Ready resolution for 1019. I don't want to interfere with that issue, but would like a new issue to consider if the fixed-base enum implementation should be allowed.

[ 2009-09-05 Daniel adds: ]

I think that the suggested resolution is incomplete and may have some possible unwanted side-effects. To understand why, note that integral_constant is completely specified by code in 20.6.3 [meta.help]. While this is usually considered as a good thing, let me give a possible user-defined specialization that would break given the suggested changes:

enum NodeColor { Red, Black };

std::integral_constant<NodeColor, Red> red;

The reason why that breaks is due to the fact that current core language rules does only allow integral types as enum-bases, see 7.2 [dcl.enum]/2.

So, I think that we cannot leave the implementation the freedom to decide which way they would like to provide the implementation, because that is easily user-visible (I don't speak of addresses, but of instantiation errors), therefore if applied, this should be either specified or wording must be added that gives a note about this freedom of implementation.

Another possible disadvantage seems to me that user-expectations are easy to disappoint if they see a failure of the test

assert(typeid(std::integral_constant<int, 0>::value) == typeid(int));

or of

static_assert(std::is_same<decltype(std::integral_constant<int, 0>::value), const int>::value, "Bad library");

[ 2010-01-14 Moved to Tentatively NAD after 5 positive votes on c++std-lib. ]

Proposed resolution:


1204. Global permission to move

Section: 17.6.3.9 [res.on.arguments] Status: Ready Submitter: Howard Hinnant Opened: 2009-09-12 Last modified: 2009-10-20

View all issues with Ready status.

Discussion:

When a library function binds an rvalue reference parameter to an argument, the library must be able to assume that the bound argument is a temporary, and not a moved-from lvalue. The reason for this is that the library function must be able to modify that argument without concern that such modifications will corrupt the logic of the calling code. For example:

template <class T, class A>
void
vector<T, A>::push_back(value_type&& v)
{
    // This function should move from v, potentially modifying
    //   the object v is bound to.
}

If v is truly bound to a temporary, then push_back has the only reference to this temporary in the entire program. Thus any modifications will be invisible to the rest of the program.

If the client supplies std::move(x) to push_back, the onus is on the client to ensure that the value of x is no longer important to the logic of his program after this statement. I.e. the client is making a statement that push_back may treat x as a temporary.

The above statement is the very foundation upon which move semantics is based.

The standard is currently lacking a global statement to this effect. I propose the following addition to 17.6.3.9 [res.on.arguments]:

Each of the following statements applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.

Such a global statement will eliminate the need for piecemeal statements such as 23.2.1 [container.requirements.general]/13:

An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.

Additionally this clarifies that move assignment operators need not perform the traditional if (this != &rhs) test commonly found (and needed) in copy assignment operators.

[ 2009-09-13 Niels adds: ]

Note: This resolution supports the change of 27.9.1.3 [filebuf.assign]/1, proposed by LWG 900.

[ 2009 Santa Cruz: ]

Move to Ready.

Proposed resolution:

Add a bullet to 17.6.3.9 [res.on.arguments]:

Each of the following statements applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.

Delete 23.2.1 [container.requirements.general]/13:

An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.

1205. Some algorithms could more clearly document their handling of empty ranges

Section: 25 [algorithms] Status: Tentatively Ready Submitter: Alisdair Meredith Opened: 2009-09-13 Last modified: 2009-11-12

View other active issues in [algorithms].

View all other issues in [algorithms].

View all issues with Tentatively Ready status.

Discussion:

There are a number of algorithms whose result might depend on the handling of an empty range. In some cases the result is not clear, while in others it would help readers to clearly mention the result rather than require some subtle intuition of the supplied wording.

25.2.1 [alg.all_of]

Returns: true if pred(*i) is true for every iterator i in the range [first,last), ...

What does this mean if the range is empty?

I believe that we intend this to be true and suggest a non-normative note to clarify:

Add to p1 25.2.1 [alg.all_of]:

[Note: Returns true if [first,last) is empty. — end note]

25.2.3 [alg.none_of]

Returns: true if pred(*i) is false for every iterator i in the range [first,last), ...

What does this mean if the range empty?

I believe that we intend this to be true and suggest a non-normative note to clarify:

Add to p1 25.2.3 [alg.none_of]:

[Note: Returns true if [first,last) is empty. — end note]

25.2.2 [alg.any_of]

The specification for an empty range is actually fairly clear in this case, but a note wouldn't hurt and would be consistent with proposals for all_of/none_of algorithms.

Add to p1 25.2.2 [alg.any_of]:

[Note: Returns false if [first,last) is empty. — end note]

25.2.6 [alg.find.end]

what does this mean if [first2,last2) is empty?

I believe the wording suggests the algorithm should return last1 in this case, but am not 100% sure. Is this in fact the correct result anyway? Surely an empty range should always match and the naive expected result would be first1?

My proposed wording is a note to clarify the current semantic:

Add to p2 25.2.6 [alg.find.end]:

[Note: Returns last1 if [first2,last2) is empty. — end note]

I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:

Alternative wording: (NOT a note)

Add to p2 25.2.6 [alg.find.end]:

Returns first1 if [first2,last2) is empty.

25.2.7 [alg.find.first.of]

The phrasing seems precise when [first2, last2) is empty, but a small note to confirm the reader's understanding might still help.

Add to p2 25.2.7 [alg.find.first.of]

[Note: Returns last1 if [first2,last2) is empty. — end note]

25.2.12 [alg.search]

What is the expected result if [first2, last2) is empty?

I believe the wording suggests the algorithm should return last1 in this case, but am not 100% sure. Is this in fact the correct result anyway? Surely an empty range should always match and the naive expected result would be first1?

My proposed wording is a note to clarify the current semantic:

Add to p2 25.2.12 [alg.search]:

[Note: Returns last1 if [first2,last2) is empty. — end note]

Again, I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:

Alternative wording: (NOT a note)

Add to p2 25.2.12 [alg.search]:

Returns first1 if [first2,last2) is empty.

25.3.13 [alg.partitions]

Is an empty range partitioned or not?

Proposed wording:

Add to p1 25.3.13 [alg.partitions]:

[Note: Returns true if [first,last) is empty. — end note]

25.4.5.1 [includes]

Returns: true if every element in the range [first2,last2) is contained in the range [first1,last1). ...

I really don't know what this means if [first2,last2) is empty. I could loosely guess that this implies empty ranges always match, and my proposed wording is to clarify exactly that:

Add to p1 25.4.5.1 [includes]:

[Note: Returns true if [first2,last2) is empty. — end note]

25.4.6.2 [pop.heap]

The effects clause is invalid if the range [first,last) is empty, unlike all the other heap alogorithms. The should be called out in the requirements.

Proposed wording:

Revise p2 25.4.6.2 [pop.heap]

Requires: The range [first,last) shall be a valid non-empty heap.

[Editorial] Reverse order of 25.4.6.2 [pop.heap] p1 and p2.

25.4.7 [alg.min.max]

minmax_element does not clearly specify behaviour for an empty range in the same way that min_element and max_element do.

Add to p31 25.4.7 [alg.min.max]:

Returns make_pair(first, first) if first == last.

25.4.8 [alg.lex.comparison]

The wording here seems quite clear, especially wit