| Doc. no. | N2577=08-0087 |
| Date: | 2008-03-16 |
| Project: | Programming Language C++ |
| Reply to: | Howard Hinnant <howard.hinnant@gmail.com> |
Reference ISO/IEC IS 14882:1998(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 14882:1998(E) document. Issues are not to be used to request new features.
This document contains only library issues which are actively being considered by the Library Working Group. That is, issues which have a status of New, Open, Ready, and Review. See Library Defect Reports List for issues considered defects and Library Closed Issues List for issues considered closed.
The issues in these lists are not necessarily formal ISO Defect Reports (DR's). While some issues will eventually be elevated to official Defect Report status, other issues will be disposed of in other ways. See Issue Status.
This document is in an experimental format designed for both viewing via a world-wide web browser and hard-copy printing. It is available as an HTML file for browsing or PDF file for printing.
Prior to Revision 14, library issues lists existed in two slightly different versions; a Committee Version and a Public Version. Beginning with Revision 14 the two versions were combined into a single version.
This document includes [bracketed italicized notes] as a reminder to the LWG of current progress on issues. Such notes are strictly unofficial and should be read with caution as they may be incomplete or incorrect. Be aware that LWG support for a particular resolution can quickly change if new viewpoints or killer examples are presented in subsequent discussions.
For the most current official version of this document see http://www.open-std.org/jtc1/sc22/wg21/. Requests for further information about this document should include the document number above, reference ISO/IEC 14882:1998(E), and be submitted to Information Technology Industry Council (ITI), 1250 Eye Street NW, Washington, DC 20005.
Public information as to how to obtain a copy of the C++ Standard, join the standards committee, submit an issue, or comment on an issue can be found in the comp.std.c++ FAQ. Public discussion of C++ Standard related issues occurs on news:comp.std.c++.
For committee members, files available on the committee's private web site include the HTML version of the Standard itself. HTML hyperlinks from this issues list to those files will only work for committee members who have downloaded them into the same disk directory as the issues list files.
New - The issue has not yet been reviewed by the LWG. Any Proposed Resolution is purely a suggestion from the issue submitter, and should not be construed as the view of LWG.
Open - The LWG has discussed the issue but is not yet ready to move the issue forward. There are several possible reasons for open status:
A Proposed Resolution for an open issue is still not be construed as the view of LWG. Comments on the current state of discussions are often given at the end of open issues in an italic font. Such comments are for information only and should not be given undue importance.
Dup - The LWG has reached consensus that the issue is a duplicate of another issue, and will not be further dealt with. A Rationale identifies the duplicated issue's issue number.
NAD - The LWG has reached consensus that the issue is not a defect in the Standard.
NAD Editorial - The LWG has reached consensus that the issue can either be handled editorially, or is handled by a paper (usually linked to in the rationale).
NAD Future - In addition to the regular status, the LWG believes that this issue should be revisited at the next revision of the standard.
Review - Exact wording of a Proposed Resolution is now available for review on an issue for which the LWG previously reached informal consensus.
Tentatively Ready - The issue has been reviewed online, but not in a meeting, and some support has been formed for the proposed resolution. Tentatively Ready issues may be moved to Ready and forwarded to full committee within the same meeting. Unlike Ready issues they will be reviewed in subcommittee prior to forwarding to full committee.
Ready - The LWG has reached consensus that the issue is a defect in the Standard, the Proposed Resolution is correct, and the issue is ready to forward to the full committee for further action as a Defect Report (DR).
DR - (Defect Report) - The full J16 committee has voted to forward the issue to the Project Editor to be processed as a Potential Defect Report. The Project Editor reviews the issue, and then forwards it to the WG21 Convenor, who returns it to the full committee for final disposition. This issues list accords the status of DR to all these Defect Reports regardless of where they are in that process.
TC - (Technical Corrigenda) - 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.
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.
Pending - This is a status qualifier. When prepended to a status this indicates the issue has been processed by the committee, and a decision has been made to move the issue to the associated unqualified status. However for logistical reasons the indicated outcome of the issue has not yet appeared in the latest working paper.
Issues are always given the status of New when they first appear on the issues list. They may progress to Open or Review while the LWG is actively working on them. When the LWG has reached consensus on the disposition of an issue, the status will then change to Dup, NAD, or Ready as appropriate. Once the full J16 committee votes to forward Ready issues to the Project Editor, they are given the status of Defect Report ( DR). These in turn may become the basis for Technical Corrigenda (TC), or are closed without action other than a Record of Response (RR ). The intent of this LWG process is that only issues which are truly defects in the Standard move to the formal ISO DR status.
Section: 22.2.2.1.2 [facet.num.get.virtuals] Status: Open Submitter: Nathan Myers Date: 1998-08-06
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 current description of numeric input does not account for the possibility of overflow. This is an implicit result of changing the description to rely on the definition of scanf() (which fails to report overflow), and conflicts with the documented behavior of traditional and current implementations.
Users expect, when reading a character sequence that results in a value unrepresentable in the specified type, to have an error reported. The standard as written does not permit this.
Further comments from Dietmar:
I don't feel comfortable with the proposed resolution to issue 23: It kind of simplifies the issue to much. Here is what is going on:
Currently, the behavior of numeric overflow is rather counter intuitive and hard to trace, so I will describe it briefly:
Further discussion from Redmond:
The basic problem is that we've defined our behavior, including our error-reporting behavior, in terms of C90. However, C90's method of reporting overflow in scanf is not technically an "input error". The strto_* functions are more precise.
There was general consensus that failbit should be set upon overflow. We considered three options based on this:
Straw poll: (1) 5; (2) 0; (3) 8.
Discussed at Lillehammer. General outline of what we want the solution to look like: we want to say that overflow is an error, and provide a way to distinguish overflow from other kinds of errors. Choose candidate field the same way scanf does, but don't describe the rest of the process in terms of format. If a finite input field is too large (positive or negative) to be represented as a finite value, then set failbit and assign the nearest representable value. Bill will provide wording.
Discussed at Toronto: N2327 is in alignment with the direction we wanted to go with in Lillehammer. Bill to work on.
Proposed resolution:
Change 22.2.2.1.2 [facet.num.get.virtuals], end of p3:
Stage 3:
The result of stage 2 processing can be one ofThe 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>:
A sequence of chars has been accumulated in stage 2 that is converted (according to the rules of scanf) to a value of the type of val. This value is stored in val and ios_base::goodbit is stored in err.For a signed integer value, the function strtoll.The sequence of chars accumulated in stage 2 would have caused scanf to report an input failure. ios_base::failbit is assigned to err.For an unsigned integer value, the function strtoull.- For a floating-point value, the function strtold.
The numeric value to be stored can be one of:
- zero, if the conversion function fails to convert the entire field. ios_base::failbit is assigned to err.
- the most positive representable value, if the field represents a value too large positive to be represented in val. ios_base::failbit is assigned to err.
- the most negative representable value (zero for unsigned integer), if the field represents a value too large negative to be represented in val. ios_base::failbit is assigned to err.
- the converted value, otherwise.
The resultant numeric value is stored in val.
Change 22.2.2.1.2 [facet.num.get.virtuals], p6-p7:
iter_type do_get(iter_type in, iter_type end, ios_base& str, ios_base::iostate& err, bool& val) const;-6- Effects: If (str.flags()&ios_base::boolalpha)==0 then input proceeds as it would for a long except that if a value is being stored into val, the value is determined according to the following: If the value to be stored is 0 then false is stored. If the value is 1 then true is stored. Otherwise
err|=ios_base::failbit is performed and no valuetrue is stored.and ios_base::failbit is assigned to err.-7- Otherwise target sequences are determined "as if" by calling the members falsename() and truename() of the facet obtained by use_facet<numpunct<charT> >(str.getloc()). Successive characters in the range [in,end) (see 23.1.1) are obtained and matched against corresponding positions in the target sequences only as necessary to identify a unique match. The input iterator in is compared to end only when necessary to obtain a character. If
and only ifa target sequence is uniquely matched, val is set to the corresponding value. Otherwise false is stored and ios_base::failbit is assigned to err.
Section: 23.2.5 [vector] Status: Open Submitter: AFNOR Date: 1998-10-07
View all other issues in [vector].
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Discussion:
vector<bool> is not a container as its reference and pointer types are not references and pointers.
Also it forces everyone to have a space optimization instead of a speed one.
See also: 99-0008 == N1185 Vector<bool> is Nonconforming, Forces Optimization Choice.
[In Santa Cruz the LWG felt that this was Not A Defect.]
[In Dublin many present felt that failure to meet Container requirements was a defect. There was disagreement as to whether or not the optimization requirements constituted a defect.]
[The LWG looked at the following resolutions in some detail:
* Not A Defect.
* Add a note explaining that vector<bool> does not meet
Container requirements.
* Remove vector<bool>.
* Add a new category of container requirements which
vector<bool> would meet.
* Rename vector<bool>.
No alternative had strong, wide-spread, support and every alternative
had at least one "over my dead body" response.
There was also mention of a transition scheme something like (1) add
vector_bool and deprecate vector<bool> in the next standard. (2)
Remove vector<bool> in the following standard.]
[Modifying container requirements to permit returning proxies (thus allowing container requirements conforming vector<bool>) was also discussed.]
[It was also noted that there is a partial but ugly workaround in that vector<bool> may be further specialized with a customer allocator.]
[Kona: Herb Sutter presented his paper J16/99-0035==WG21/N1211, vector<bool>: More Problems, Better Solutions. Much discussion of a two step approach: a) deprecate, b) provide replacement under a new name. LWG straw vote on that: 1-favor, 11-could live with, 2-over my dead body. This resolution was mentioned in the LWG report to the full committee, where several additional committee members indicated over-my-dead-body positions.]
Discussed at Lillehammer. General agreement that we should deprecate vector<bool> and introduce this functionality under a different name, e.g. bit_vector. This might make it possible to remove the vector<bool> specialization in the standard that comes after C++0x. There was also a suggestion that in C++0x we could additional say that it's implementation defined whether vector<bool> refers to the specialization or to the primary template, but there wasn't general agreement that this was a good idea.
We need a paper for the new bit_vector class.
Proposed resolution:
[ Batavia: The LWG feels we need something closer to SGI's bitvector to ease migration from vector<bool>. Although some of the funcitonality from N2050 could well be used in such a template. The concern is easing the API migration for those users who want to continue using a bit-packed container. Alan and Beman to work. ]
Section: 27.7 [string.streams], 27.8 [file.streams] Status: Open Submitter: Angelika Langer Date: 1999-02-22
View all other issues in [string.streams].
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Discussion:
The following question came from Thorsten Herlemann:
You can set a mode when constructing or opening a file-stream or filebuf, e.g. ios::in, ios::out, ios::binary, ... But how can I get that mode later on, e.g. in my own operator << or operator >> or when I want to check whether a file-stream or file-buffer object passed as parameter is opened for input or output or binary? Is there no possibility? Is this a design-error in the standard C++ library?
It is indeed impossible to find out what a stream's or stream buffer's open mode is, and without that knowledge you don't know how certain operations behave. Just think of the append mode.
Both streams and stream buffers should have a mode() function that returns the current open mode setting.
[ post Bellevue: Alisdair requested to re-Open. ]
Proposed resolution:
For stream buffers, add a function to the base class as a non-virtual function qualified as const to 27.5.2 [streambuf]:
openmode mode() const;
Returns the current open mode.
With streams, I'm not sure what to suggest. In principle, the mode could already be returned by ios_base, but the mode is only initialized for file and string stream objects, unless I'm overlooking anything. For this reason it should be added to the most derived stream classes. Alternatively, it could be added to basic_ios and would be default initialized in basic_ios<>::init().
Rationale:
This might be an interesting extension for some future, but it is not a defect in the current standard. The Proposed Resolution is retained for future reference.
Section: 23 [containers] Status: Open Submitter: Dave Abrahams Date: 1999-07-01
View other active issues in [containers].
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Discussion:
It is the constness of the container which should control whether it can be modified through a member function such as erase(), not the constness of the iterators. The iterators only serve to give positioning information.
Here's a simple and typical example problem which is currently very difficult or impossible to solve without the change proposed below.
Wrap a standard container C in a class W which allows clients to find and read (but not modify) a subrange of (C.begin(), C.end()]. The only modification clients are allowed to make to elements in this subrange is to erase them from C through the use of a member function of W.
[ post Bellevue, Alisdair adds: ]
This issue was implemented by N2350 for everything but basic_string.
Note that the specific example in this issue (basic_string) is the one place we forgot to amend in N2350, so we might open this issue for that single container?
Proposed resolution:
Change all non-const iterator parameters of standard library container member functions to accept const_iterator parameters. Note that this change applies to all library clauses, including strings.
For example, in 21.3.5.5 change:
iterator erase(iterator p);
to:
iterator erase(const_iterator p);
Rationale:
The issue was discussed at length. It was generally agreed that 1) There is no major technical argument against the change (although there is a minor argument that some obscure programs may break), and 2) Such a change would not break const correctness. The concerns about making the change were 1) it is user detectable (although only in boundary cases), 2) it changes a large number of signatures, and 3) it seems more of a design issue that an out-and-out defect.
The LWG believes that this issue should be considered as part of a general review of const issues for the next revision of the standard. Also see issue 200.
Section: 25.3.7 [alg.min.max] Status: Open Submitter: Mark Rintoul Date: 1999-08-26
View other active issues in [alg.min.max].
View all other issues in [alg.min.max].
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Discussion:
Both std::min and std::max are defined as template functions. This
is very different than the definition of std::plus (and similar
structs) which are defined as function objects which inherit
std::binary_function.
This lack of inheritance leaves std::min and std::max somewhat useless in standard library algorithms which require
a function object that inherits std::binary_function.
[ post Bellevue: Alisdair requested to re-Open. ]
Rationale:
Although perhaps an unfortunate design decision, the omission is not a defect in the current standard. A future standard may wish to consider additional function objects.
Section: 27.5.2 [streambuf] Status: Open Submitter: Martin Sebor Date: 2000-08-12
View all other issues in [streambuf].
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Discussion:
The basic_streambuf members gbump() and pbump() are specified to take an int argument. This requirement prevents the functions from effectively manipulating buffers larger than std::numeric_limits<int>::max() characters. It also makes the common use case for these functions somewhat difficult as many compilers will issue a warning when an argument of type larger than int (such as ptrdiff_t on LLP64 architectures) is passed to either of the function. Since it's often the result of the subtraction of two pointers that is passed to the functions, a cast is necessary to silence such warnings. Finally, the usage of a native type in the functions signatures is inconsistent with other member functions (such as sgetn() and sputn()) that manipulate the underlying character buffer. Those functions take a streamsize argument.
Proposed resolution:
Change the signatures of these functions in the synopsis of template class basic_streambuf (27.5.2) and in their descriptions (27.5.2.3.1, p4 and 27.5.2.3.2, p4) to take a streamsize argument.
Although this change has the potential of changing the ABI of the library, the change will affect only platforms where int is different than the definition of streamsize. However, since both functions are typically inline (they are on all known implementations), even on such platforms the change will not affect any user code unless it explicitly relies on the existing type of the functions (e.g., by taking their address). Such a possibility is IMO quite remote.
Alternate Suggestion from Howard Hinnant, c++std-lib-7780:
This is something of a nit, but I'm wondering if streamoff wouldn't be a better choice than streamsize. The argument to pbump and gbump MUST be signed. But the standard has this to say about streamsize (27.4.1/2/Footnote):
[Footnote: streamsize is used in most places where ISO C would use size_t. Most of the uses of streamsize could use size_t, except for the strstreambuf constructors, which require negative values. It should probably be the signed type corresponding to size_t (which is what Posix.2 calls ssize_t). --- end footnote]
This seems a little weak for the argument to pbump and gbump. Should we ever really get rid of strstream, this footnote might go with it, along with the reason to make streamsize signed.
Rationale:
The LWG believes this change is too big for now. We may wish to reconsider this for a future revision of the standard. One possibility is overloading pbump, rather than changing the signature.
[ [2006-05-04: Reopened at the request of Chris (Krzysztof ?elechowski)] ]
Section: 25.1.1 [alg.foreach] Status: Open Submitter: Angelika Langer Date: 2001-01-03
View all other issues in [alg.foreach].
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Discussion:
The specification of the for_each algorithm does not have a "Requires" section, which means that there are no restrictions imposed on the function object whatsoever. In essence it means that I can provide any function object with arbitrary side effects and I can still expect a predictable result. In particular I can expect that the function object is applied exactly last - first times, which is promised in the "Complexity" section.
I don't see how any implementation can give such a guarantee without imposing requirements on the function object.
Just as an example: consider a function object that removes elements from the input sequence. In that case, what does the complexity guarantee (applies f exactly last - first times) mean?
One can argue that this is obviously a nonsensical application and a theoretical case, which unfortunately it isn't. I have seen programmers shooting themselves in the foot this way, and they did not understand that there are restrictions even if the description of the algorithm does not say so.
[Lillehammer: This is more general than for_each. We don't want the function object in transform invalidiating iterators either. There should be a note somewhere in clause 17 (17, not 25) saying that user code operating on a range may not invalidate iterators unless otherwise specified. Bill will provide wording.]
Proposed resolution:
Section: 24.1.4 [bidirectional.iterators], 24.1.5 [random.access.iterators] Status: Open Submitter: John Potter Date: 2001-01-22
View all other issues in [bidirectional.iterators].
View all issues with Open status.
Discussion:
In section 24.1.4 [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 24.1.5 [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.
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]
Section: 27.6 [iostream.format] Status: Open Submitter: Martin Sebor Date: 2001-03-19
View all other issues in [iostream.format].
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Discussion:
The descriptions of the constructors of basic_istream<>::sentry (27.6.1.1.3 [istream::sentry]) and basic_ostream<>::sentry (27.6.2.4 [ostream::sentry]) do not explain what the functions do in case an exception is thrown while they execute. Some current implementations allow all exceptions to propagate, others catch them and set ios_base::badbit instead, still others catch some but let others propagate.
The text also mentions that the functions may call setstate(failbit) (without actually saying on what object, but presumably the stream argument is meant). That may have been fine for basic_istream<>::sentry prior to issue 195, since the function performs an input operation which may fail. However, issue 195 amends 27.6.1.1.3 [istream::sentry], p2 to clarify that the function should actually call setstate(failbit | eofbit), so the sentence in p3 is redundant or even somewhat contradictory.
The same sentence that appears in 27.6.2.4 [ostream::sentry], p3 doesn't seem to be very meaningful for basic_istream<>::sentry which performs no input. It is actually rather misleading since it would appear to guide library implementers to calling setstate(failbit) when os.tie()->flush(), the only called function, throws an exception (typically, it's badbit that's set in response to such an event).
Additional comments from Martin, who isn't comfortable with the current proposed resolution (see c++std-lib-11530)
The istream::sentry ctor says nothing about how the function deals with exemptions (27.6.1.1.2, p1 says that the class is responsible for doing "exception safe"(*) prefix and suffix operations but it doesn't explain what level of exception safety the class promises to provide). The mockup example of a "typical implementation of the sentry ctor" given in 27.6.1.1.2, p6, removed in ISO/IEC 14882:2003, doesn't show exception handling, either. Since the ctor is not classified as a formatted or unformatted input function, the text in 27.6.1.1, p1 through p4 does not apply. All this would seem to suggest that the sentry ctor should not catch or in any way handle exceptions thrown from any functions it may call. Thus, the typical implementation of an istream extractor may look something like [1].
The problem with [1] is that while it correctly sets ios::badbit if an exception is thrown from one of the functions called from the sentry ctor, if the sentry ctor reaches EOF while extracting whitespace from a stream that has eofbit or failbit set in exceptions(), it will cause an ios::failure to be thrown, which will in turn cause the extractor to set ios::badbit.
The only straightforward way to prevent this behavior is to move the definition of the sentry object in the extractor above the try block (as suggested by the example in 22.2.8, p9 and also indirectly supported by 27.6.1.3, p1). See [2]. But such an implementation will allow exceptions thrown from functions called from the ctor to freely propagate to the caller regardless of the setting of ios::badbit in the stream object's exceptions().
So since neither [1] nor [2] behaves as expected, the only possible solution is to have the sentry ctor catch exceptions thrown from called functions, set badbit, and propagate those exceptions if badbit is also set in exceptions(). (Another solution exists that deals with both kinds of sentries, but the code is non-obvious and cumbersome -- see [3].)
Please note that, as the issue points out, current libraries do not behave consistently, suggesting that implementors are not quite clear on the exception handling in istream::sentry, despite the fact that some LWG members might feel otherwise. (As documented by the parenthetical comment here: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1480.html#309)
Also please note that those LWG members who in Copenhagen felt that "a sentry's constructor should not catch exceptions, because sentries should only be used within (un)formatted input functions and that exception handling is the responsibility of those functions, not of the sentries," as noted here http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2001/n1310.html#309 would in effect be either arguing for the behavior described in [1] or for extractors implemented along the lines of [3].
The original proposed resolution (Revision 25 of the issues list) clarifies the role of the sentry ctor WRT exception handling by making it clear that extractors (both library or user-defined) should be implemented along the lines of [2] (as opposed to [1]) and that no exception thrown from the callees should propagate out of either function unless badbit is also set in exceptions().
[1] Extractor that catches exceptions thrown from sentry:
struct S { long i; };
istream& operator>> (istream &strm, S &s)
{
ios::iostate err = ios::goodbit;
try {
const istream::sentry guard (strm, false);
if (guard) {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
}
catch (...) {
bool rethrow;
try {
strm.setstate (ios::badbit);
rethrow = false;
}
catch (...) {
rethrow = true;
}
if (rethrow)
throw;
}
if (err)
strm.setstate (err);
return strm;
}
[2] Extractor that propagates exceptions thrown from sentry:
istream& operator>> (istream &strm, S &s)
{
istream::sentry guard (strm, false);
if (guard) {
ios::iostate err = ios::goodbit;
try {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
catch (...) {
bool rethrow;
try {
strm.setstate (ios::badbit);
rethrow = false;
}
catch (...) {
rethrow = true;
}
if (rethrow)
throw;
}
if (err)
strm.setstate (err);
}
return strm;
}
[3] Extractor that catches exceptions thrown from sentry but doesn't set badbit if the exception was thrown as a result of a call to strm.clear().
istream& operator>> (istream &strm, S &s)
{
const ios::iostate state = strm.rdstate ();
const ios::iostate except = strm.exceptions ();
ios::iostate err = std::ios::goodbit;
bool thrown = true;
try {
const istream::sentry guard (strm, false);
thrown = false;
if (guard) {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
}
catch (...) {
if (thrown && state & except)
throw;
try {
strm.setstate (ios::badbit);
thrown = false;
}
catch (...) {
thrown = true;
}
if (thrown)
throw;
}
if (err)
strm.setstate (err);
return strm;
}
[Pre-Berlin] Reopened at the request of Paolo Carlini and Steve Clamage.
[Pre-Portland] A relevant newsgroup post:
The current proposed resolution of issue #309 (http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#309) is unacceptable. I write commerical software and coding around this makes my code ugly, non-intuitive, and requires comments referring people to this very issue. Following is the full explanation of my experience.
In the course of writing software for commercial use, I constructed std::ifstream's based on user-supplied pathnames on typical POSIX systems.
It was expected that some files that opened successfully might not read successfully -- such as a pathname which actually refered to a directory. Intuitively, I expected the streambuffer underflow() code to throw an exception in this situation, and recent implementations of libstdc++'s basic_filebuf do just that (as well as many of my own custom streambufs).
I also intuitively expected that the istream code would convert these exceptions to the "badbit' set on the stream object, because I had not requested exceptions. I refer to 27.6.1.1. P4.
However, this was not the case on at least two implementations -- if the first thing I did with an istream was call operator>>( T& ) for T among the basic arithmetic types and std::string. Looking further I found that the sentry's constructor was invoking the exception when it pre-scanned for whitespace, and the extractor function (operator>>()) was not catching exceptions in this situation.
So, I was in a situation where setting 'noskipws' would change the istream's behavior even though no characters (whitespace or not) could ever be successfully read.
Also, calling .peek() on the istream before calling the extractor() changed the behavior (.peek() had the effect of setting the badbit ahead of time).
I found this all to be so inconsistent and inconvenient for me and my code design, that I filed a bugzilla entry for libstdc++. I was then told that the bug cannot be fixed until issue #309 is resolved by the committee.
Proposed resolution:
Rationale:
The LWG agrees there is minor variation between implementations, but believes that it doesn't matter. This is a rarely used corner case. There is no evidence that this has any commercial importance or that it causes actual portability problems for customers trying to write code that runs on multiple implementations.
Section: 27.6.1.3 [istream.unformatted] Status: Open Submitter: Howard Hinnant Date: 2001-10-09
View all other issues in [istream.unformatted].
View all issues with Open status.
Discussion:
I think we have a defect.
According to lwg issue 60 which is now a dr, the description of seekg in 27.6.1.3 [istream.unformatted] paragraph 38 now looks like:
Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail() != true, executes rdbuf()->pubseekpos( pos).
And according to lwg issue 243 which is also now a dr, 27.6.1.3, paragraph 1 looks like:
Each unformatted input function begins execution by constructing an object of class sentry with the default argument noskipws (second) argument true. If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input. Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of type bool, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to 0; unformatted input functions taking a character array of non-zero size as an argument shall also store a null character (using charT()) in the first location of the array. If an exception is thrown during input then ios::badbit is turned on in *this'ss error state. If (exception()&badbit)!= 0 then the exception is rethrown. It also counts the number of characters extracted. If no exception has been thrown it ends by storing the count in a member object and returning the value specified. In any event the sentry object is destroyed before leaving the unformatted input function.
And finally 27.6.1.1.2/5 says this about sentry:
If, after any preparation is completed, is.good() is true, ok_ != false otherwise, ok_ == false.
So although the seekg paragraph says that the operation proceeds if !fail(), the behavior of unformatted functions says the operation proceeds only if good(). The two statements are contradictory when only eofbit is set. I don't think the current text is clear which condition should be respected.
Further discussion from Redmond:
PJP: It doesn't seem quite right to say that seekg is "unformatted". That makes specific claims about sentry that aren't quite appropriate for seeking, which has less fragile failure modes than actual input. If we do really mean that it's unformatted input, it should behave the same way as other unformatted input. On the other hand, "principle of least surprise" is that seeking from EOF ought to be OK.
Pre-Berlin: Paolo points out several problems with the proposed resolution in Ready state:
Proposed resolution:
Change 27.6.1.3 [istream.unformatted] to:
Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted, does not affect the value returned by subsequent calls to gcount(), and does not examine the value returned by the sentry object. After constructing a sentry object, if fail() != true, executes rdbuf()->pubseekpos(pos). In case of success, the function calls clear(). In case of failure, the function calls setstate(failbit) (which may throw ios_base::failure).
[Lillehammer: Matt provided wording.]
Rationale:
In C, fseek does clear EOF. This is probably what most users would expect. We agree that having eofbit set should not deter a seek, and that a successful seek should clear eofbit. Note that fail() is true only if failbit or badbit is set, so using !fail(), rather than good(), satisfies this goal.
Section: 21 [strings], 23 [containers], 27 [input.output] Status: Open Submitter: Martin Sebor Date: 2001-10-09
View all other issues in [strings].
View all issues with Open status.
Discussion:
The synopses of the C++ library headers clearly show which names are required to be defined in each header. Since in order to implement the classes and templates defined in these headers declarations of other templates (but not necessarily their definitions) are typically necessary the standard in 17.4.4, p1 permits library implementers to include any headers needed to implement the definitions in each header.
For instance, although it is not explicitly specified in the synopsis of <string>, at the point of definition of the std::basic_string template the declaration of the std::allocator template must be in scope. All current implementations simply include <memory> from within <string>, either directly or indirectly, to bring the declaration of std::allocator into scope.
Additionally, however, some implementation also include <istream> and <ostream> at the top of <string> to bring the declarations of std::basic_istream and std::basic_ostream into scope (which are needed in order to implement the string inserter and extractor operators (21.3.7.9 [lib.string.io])). Other implementations only include <iosfwd>, since strictly speaking, only the declarations and not the full definitions are necessary.
Obviously, it is possible to implement <string> without actually providing the full definitions of all the templates std::basic_string uses (std::allocator, std::basic_istream, and std::basic_ostream). Furthermore, not only is it possible, doing so is likely to have a positive effect on compile-time efficiency.
But while it may seem perfectly reasonable to expect a program that uses the std::basic_string insertion and extraction operators to also explicitly include <istream> or <ostream>, respectively, it doesn't seem reasonable to also expect it to explicitly include <memory>. Since what's reasonable and what isn't is highly subjective one would expect the standard to specify what can and what cannot be assumed. Unfortunately, that isn't the case.
The examples below demonstrate the issue.
Example 1:
It is not clear whether the following program is complete:
#include <string>
extern std::basic_ostream<char> &strm;
int main () {
strm << std::string ("Hello, World!\n");
}
or whether one must explicitly include <memory> or <ostream> (or both) in addition to <string> in order for the program to compile.
Example 2:
Similarly, it is unclear whether the following program is complete:
#include <istream>
extern std::basic_iostream<char> &strm;
int main () {
strm << "Hello, World!\n";
}
or whether one needs to explicitly include <ostream>, and perhaps even other headers containing the definitions of other required templates:
#include <ios>
#include <istream>
#include <ostream>
#include <streambuf>
extern std::basic_iostream<char> &strm;
int main () {
strm << "Hello, World!\n";
}
Example 3:
Likewise, it seems unclear whether the program below is complete:
#include <iterator>
bool foo (std::istream_iterator<int> a, std::istream_iterator<int> b)
{
return a == b;
}
int main () { }
or whether one should be required to include <istream>.
There are many more examples that demonstrate this lack of a requirement. I believe that in a good number of cases it would be unreasonable to require that a program explicitly include all the headers necessary for a particular template to be specialized, but I think that there are cases such as some of those above where it would be desirable to allow implementations to include only as much as necessary and not more.
[ post Bellevue: ]
Position taken in prior reviews is that the idea of a table of header dependencies is a good one. Our view is that a full paper is needed to do justice to this, and we've made that recommendation to the issue author.
Proposed resolution:
For every C++ library header, supply a minimum set of other C++ library headers that are required to be included by that header. The proposed list is below (C++ headers for C Library Facilities, table 12 in 17.4.1.2, p3, are omitted):
+------------+--------------------+ | C++ header |required to include | +============+====================+ |<algorithm> | | +------------+--------------------+ |<bitset> | | +------------+--------------------+ |<complex> | | +------------+--------------------+ |<deque> |<memory> | +------------+--------------------+ |<exception> | | +------------+--------------------+ |<fstream> |<ios> | +------------+--------------------+ |<functional>| | +------------+--------------------+ |<iomanip> |<ios> | +------------+--------------------+ |<ios> |<streambuf> | +------------+--------------------+ |<iosfwd> | | +------------+--------------------+ |<iostream> |<istream>, <ostream>| +------------+--------------------+ |<istream> |<ios> | +------------+--------------------+ |<iterator> | | +------------+--------------------+ |<limits> | | +------------+--------------------+ |<list> |<memory> | +------------+--------------------+ |<locale> | | +------------+--------------------+ |<map> |<memory> | +------------+--------------------+ |<memory> | | +------------+--------------------+ |<new> |<exception> | +------------+--------------------+ |<numeric> | | +------------+--------------------+ |<ostream> |<ios> | +------------+--------------------+ |<queue> |<deque> | +------------+--------------------+ |<set> |<memory> | +------------+--------------------+ |<sstream> |<ios>, <string> | +------------+--------------------+ |<stack> |<deque> | +------------+--------------------+ |<stdexcept> | | +------------+--------------------+ |<streambuf> |<ios> | +------------+--------------------+ |<string> |<memory> | +------------+--------------------+ |<strstream> | | +------------+--------------------+ |<typeinfo> |<exception> | +------------+--------------------+ |<utility> | | +------------+--------------------+ |<valarray> | | +------------+--------------------+ |<vector> |<memory> | +------------+--------------------+
Rationale:
The portability problem is real. A program that works correctly on one implementation might fail on another, because of different header dependencies. This problem was understood before the standard was completed, and it was a conscious design choice.
One possible way to deal with this, as a library extension, would be an <all> header.
Hinnant: It's time we dealt with this issue for C++0X. Reopened.
Section: 22.2.1.4 [locale.codecvt] Status: Open Submitter: Martin Sebor Date: 2002-08-30
View all other issues in [locale.codecvt].
View all issues with Open status.
Discussion:
It seems that the descriptions of codecvt do_in() and do_out() leave sufficient room for interpretation so that two implementations of codecvt may not work correctly with the same filebuf. Specifically, the following seems less than adequately specified:
Finally, the conditions described at the end of 22.2.1.4.2 [locale.codecvt.virtuals], p4 don't seem to be possible:
"A return value of partial, if (from_next == from_end), indicates that either the destination sequence has not absorbed all the available destination elements, or that additional source elements are needed before another destination element can be produced."
If the value is partial, it's not clear to me that (from_next ==from_end) could ever hold if there isn't enough room in the destination buffer. In order for (from_next==from_end) to hold, all characters in that range must have been successfully converted (according to 22.2.1.4.2 [locale.codecvt.virtuals], p2) and since there are no further source characters to convert, no more room in the destination buffer can be needed.
It's also not clear to me that (from_next==from_end) could ever hold if additional source elements are needed to produce another destination character (not element as incorrectly stated in the text). partial is returned if "not all source characters have been converted" according to Table 53, which also implies that (from_next==from) does NOT hold.
Could it be that the intended qualifying condition was actually (from_next != from_end), i.e., that the sentence was supposed to read
"A return value of partial, if (from_next != from_end),..."
which would make perfect sense, since, as far as I understand it, partial can only occur if (from_next != from_end)?
[Lillehammer: Defer for the moment, but this really needs to be fixed. Right now, the description of codecvt is too vague for it to be a useful contract between providers and clients of codecvt facets. (Note that both vendors and users can be both providers and clients of codecvt facets.) The major philosophical issue is whether the standard should only describe mappings that take a single wide character to multiple narrow characters (and vice versa), or whether it should describe fully general N-to-M conversions. When the original standard was written only the former was contemplated, but today, in light of the popularity of utf8 and utf16, that doesn't seem sufficient for C++0x. Bill supports general N-to-M conversions; we need to make sure Martin and Howard agree.]
Proposed resolution:
Section: 26.3 [complex.numbers] Status: Ready Submitter: Gabriel Dos Reis Date: 2002-11-08
View all other issues in [complex.numbers].
View all issues with Ready status.
Discussion:
The absence of explicit description of std::complex<T> layout makes it imposible to reuse existing software developed in traditional languages like Fortran or C with unambigous and commonly accepted layout assumptions. There ought to be a way for practitioners to predict with confidence the layout of std::complex<T> whenever T is a numerical datatype. The absence of ways to access individual parts of a std::complex<T> object as lvalues unduly promotes severe pessimizations. For example, the only way to change, independently, the real and imaginary parts is to write something like
complex<T> z; // ... // set the real part to r z = complex<T>(r, z.imag()); // ... // set the imaginary part to i z = complex<T>(z.real(), i);
At this point, it seems appropriate to recall that a complex number is, in effect, just a pair of numbers with no particular invariant to maintain. Existing practice in numerical computations has it that a complex number datatype is usually represented by Cartesian coordinates. Therefore the over-encapsulation put in the specification of std::complex<> is not justified.
Proposed resolution:
Add the following requirements to 26.3 [complex.numbers] as 26.3/4:
If z is an lvalue expression of type cv std::complex<T> then
- the expression reinterpret_cast<cv T(&)[2]>(z) is well-formed; and
- reinterpret_cast<cv T(&)[2]>(z)[0]designates the real part of z; and
- reinterpret_cast<cv T(&)[2]>(z)[1]designates the imaginary part of z.
Moreover, if a is an expression of pointer type cv complex<T>* and the expression a[i] is well-defined for an integer expression i then:
- reinterpret_cast<cv T*>(a)[2*i] designates the real part of a[i]; and
- reinterpret_cast<cv T*>(a)[2*i+1] designates the imaginary part of a[i].
In 26.3.2 [complex] Add the member functions
void real(T); void imag(T);
Add to 26.3.4 [complex.members]
T real() const;Returns: the value of the real componentvoid real(T val);Assigns val to the real component.T imag() const;Returns: the value of the imaginary componentvoid imag(T val);Assigns val to the imaginary component.
[Kona: The layout guarantee is absolutely necessary for C compatibility. However, there was disagreement about the other part of this proposal: retrieving elements of the complex number as lvalues. An alternative: continue to have real() and imag() return rvalues, but add set_real() and set_imag(). Straw poll: return lvalues - 2, add setter functions - 5. Related issue: do we want reinterpret_cast as the interface for converting a complex to an array of two reals, or do we want to provide a more explicit way of doing it? Howard will try to resolve this issue for the next meeting.]
[pre-Sydney: Howard summarized the options in n1589.]
[ Bellevue: ]
Second half of proposed wording replaced and moved to Ready.
Rationale:
The LWG believes that C99 compatibility would be enough justification for this change even without other considerations. All existing implementations already have the layout proposed here.
Section: 27.6.2.6.1 [ostream.formatted.reqmts] Status: Open Submitter: Martin Sebor Date: 2002-12-27
View all issues with Open status.
Discussion:
There is a contradiction in Formatted output about what bit is supposed to be set if the formatting fails. On sentence says it's badbit and another that it's failbit.
27.6.2.5.1, p1 says in the Common Requirements on Formatted output functions:
... If the generation fails, then the formatted output function
does setstate(ios::failbit), which might throw an exception.
27.6.2.5.2, p1 goes on to say this about Arithmetic Inserters:
... The formatting conversion occurs as if it performed the following code fragment:
bool failed =
use_facet<num_put<charT,ostreambuf_iterator<charT,traits>
> >
(getloc()).put(*this, *this, fill(), val). failed();
... If failed is true then does setstate(badbit) ...
The original intent of the text, according to Jerry Schwarz (see c++std-lib-10500), is captured in the following paragraph:
In general "badbit" should mean that the stream is unusable because of some underlying failure, such as disk full or socket closure; "failbit" should mean that the requested formatting wasn't possible because of some inconsistency such as negative widths. So typically if you clear badbit and try to output something else you'll fail again, but if you clear failbit and try to output something else you'll succeed.
In the case of the arithmetic inserters, since num_put cannot report failure by any means other than exceptions (in response to which the stream must set badbit, which prevents the kind of recoverable error reporting mentioned above), the only other detectable failure is if the iterator returned from num_put returns true from failed().
Since that can only happen (at least with the required iostream specializations) under such conditions as the underlying failure referred to above (e.g., disk full), setting badbit would seem to be the appropriate response (indeed, it is required in 27.6.2.5.2, p1). It follows that failbit can never be directly set by the arithmetic (it can only be set by the sentry object under some unspecified conditions).
The situation is different for other formatted output functions which can fail as a result of the streambuf functions failing (they may do so by means other than exceptions), and which are then required to set failbit.
The contradiction, then, is that ostream::operator<<(int) will set badbit if the disk is full, while operator<<(ostream&, char) will set failbit under the same conditions. To make the behavior consistent, the Common requirements sections for the Formatted output functions should be changed as proposed below.
[Kona: There's agreement that this is a real issue. What we decided at Kona: 1. An error from the buffer (which can be detected either directly from streambuf's member functions or by examining a streambuf_iterator) should always result in badbit getting set. 2. There should never be a circumstance where failbit gets set. That represents a formatting error, and there are no circumstances under which the output facets are specified as signaling a formatting error. (Even more so for string output that for numeric because there's nothing to format.) If we ever decide to make it possible for formatting errors to exist then the facets can signal the error directly, and that should go in clause 22, not clause 27. 3. The phrase "if generation fails" is unclear and should be eliminated. It's not clear whether it's intended to mean a buffer error (e.g. a full disk), a formatting error, or something else. Most people thought it was supposed to refer to buffer errors; if so, we should say so. Martin will provide wording.]
Proposed resolution:
Rationale:
Section: 23.3.5.1 [bitset.cons] Status: Open Submitter: Martin Sebor Date: 2003-01-05
View other active issues in [bitset.cons].
View all other issues in [bitset.cons].
View all issues with Open status.
Discussion:
23.3.5.1, p6 [lib.bitset.cons] talks about a generic character having the value of 0 or 1 but there is no definition of what that means for charT other than char and wchar_t. And even for those two types, the values 0 and 1 are not actually what is intended -- the values '0' and '1' are. This, along with the converse problem in the description of to_string() in 23.3.5.2, p33, looks like a defect remotely related to DR 303.
http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html#303
23.3.5.1:
-6- An element of the constructed string has value zero if the
corresponding character in str, beginning at position pos,
is 0. Otherwise, the element has the value one.
23.3.5.2:
-33- Effects: Constructs a string object of the appropriate
type and initializes it to a string of length N characters.
Each character is determined by the value of its
corresponding bit position in *this. Character position N
?- 1 corresponds to bit position zero. Subsequent decreasing
character positions correspond to increasing bit positions.
Bit value zero becomes the character 0, bit value one becomes
the character 1.
Also note the typo in 23.3.5.1, p6: the object under construction is a bitset, not a string.
Proposed resolution:
Change the constructor's function declaration immediately before 23.3.5.1 [bitset.cons] p3 to:
template <class charT, class traits, class Allocator>
explicit
bitset(const basic_string<charT, traits, Allocator>& str,
typename basic_string<charT, traits, Allocator>::size_type pos = 0,
typename basic_string<charT, traits, Allocator>::size_type n =
basic_string<charT, traits, Allocator>::npos,
charT zero = charT('0'), charT one = charT('1'))
Change the first two sentences of 23.3.5.1 [bitset.cons] p6 to: "An element of the constructed string has value 0 if the corresponding character in str, beginning at position pos, is zero. Otherwise, the element has the value 1.
Change the text of the second sentence in 23.3.5.1, p5 to read: "The function then throws invalid_argument if any of the rlen characters in str beginning at position pos is other than zero or one. The function uses traits::eq() to compare the character values."
Change the declaration of the to_string member function immediately before 23.3.5.2 [bitset.members] p33 to:
template <class charT, class traits, class Allocator>
basic_string<charT, traits, Allocator>
to_string(charT zero = charT('0'), charT one = charT('1')) const;
Change the last sentence of 23.3.5.2 [bitset.members] p33 to: "Bit value 0 becomes the character zero, bit value 1 becomes the character one.
Change 23.3.5.3 [bitset.operators] p8 to:
Returns:
os << x.template to_string<charT,traits,allocator<charT> >(
use_facet<ctype<charT> >(os.getloc()).widen('0'),
use_facet<ctype<charT> >(os.getloc()).widen('1'));
Rationale:
There is a real problem here: we need the character values of '0' and '1', and we have no way to get them since strings don't have imbued locales. In principle the "right" solution would be to provide an extra object, either a ctype facet or a full locale, which would be used to widen '0' and '1'. However, there was some discomfort about using such a heavyweight mechanism. The proposed resolution allows those users who care about this issue to get it right.
We fix the inserter to use the new arguments. Note that we already fixed the analogous problem with the extractor in issue 303.
[ post Bellevue: ]
We are happy with the resolution as proposed, and we move this to Ready.
Section: 27.6.2.4 [ostream::sentry] Status: Open Submitter: Martin Sebor Date: 2003-01-05
View other active issues in [ostream::sentry].
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. ]
Proposed resolution:
Section: 27.6.2.4 [ostream::sentry] Status: Open Submitter: Martin Sebor Date: 2003-01-05
View other active issues in [ostream::sentry].
View all other issues in [ostream::sentry].
View all issues with Open status.
Discussion:
While reviewing unformatted input member functions of istream for their behavior when they encounter end-of-file during input I found that the requirements vary, sometimes unexpectedly, and in more than one case even contradict established practice (GNU libstdc++ 3.2, IBM VAC++ 6.0, STLPort 4.5, SunPro 5.3, HP aCC 5.38, Rogue Wave libstd 3.1, and Classic Iostreams).
The following unformatted input member functions set eofbit if they encounter an end-of-file (this is the expected behavior, and also the behavior of all major implementations):
basic_istream<charT, traits>&
get (char_type*, streamsize, char_type);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
get (char_type*, streamsize);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
getline (char_type*, streamsize, char_type);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
getline (char_type*, streamsize);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
ignore (int, int_type);
basic_istream<charT, traits>&
read (char_type*, streamsize);
Also sets failbit if it encounters end-of-file.
streamsize readsome (char_type*, streamsize);
The following unformated input member functions set failbit but not eofbit if they encounter an end-of-file (I find this odd since the functions make it impossible to distinguish a general failure from a failure due to end-of-file; the requirement is also in conflict with all major implementation which set both eofbit and failbit):
int_type get();
basic_istream<charT, traits>&
get (char_type&);
These functions only set failbit of they extract no characters, otherwise they don't set any bits, even on failure (I find this inconsistency quite unexpected; the requirement is also in conflict with all major implementations which set eofbit whenever they encounter end-of-file):
basic_istream<charT, traits>&
get (basic_streambuf<charT, traits>&, char_type);
basic_istream<charT, traits>&
get (basic_streambuf<charT, traits>&);
This function sets no bits (all implementations except for STLport and Classic Iostreams set eofbit when they encounter end-of-file):
int_type peek ();
Informally, what we want is a global statement of intent saying that eofbit gets set if we trip across EOF, and then we can take away the specific wording for individual functions. A full review is necessary. The wording currently in the standard is a mishmash, and changing it on an individual basis wouldn't make things better. Dietmar will do this work.
Proposed resolution:
Section: 24.1 [iterator.requirements] Status: Open Submitter: Nathan Myers Date: 2003-06-03
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.
24.1 [iterator.requirements] 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 issue: 407
[ 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. ]
Proposed resolution:
Section: 22.2.1.1.2 [locale.ctype.virtuals] Status: Open Submitter: Martin Sebor Date: 2003-09-18
View all other issues in [locale.ctype.virtuals].
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Discussion:
The Effects and Returns clauses of the do_widen() member function of the ctype facet fail to specify the behavior of the function on failure. That the function may not be able to simply cast the narrow character argument to the type of the result since doing so may yield the wrong value for some wchar_t encodings. Popular implementations of ctype<wchar_t> that use mbtowc() and UTF-8 as the native encoding (e.g., GNU glibc) will fail when the argument's MSB is set. There is no way for the the rest of locale and iostream to reliably detect this failure.
[Kona: This is a real problem. Widening can fail. It's unclear what the solution should be. Returning WEOF works for the wchar_t specialization, but not in general. One option might be to add a default, like narrow. But that's an incompatible change. Using traits::eof might seem like a good idea, but facets don't have access to traits (a recurring problem). We could have widen throw an exception, but that's a scary option; existing library components aren't written with the assumption that widen can throw.]
Proposed resolution:
Section: 27.4.2.1.6 [ios::Init] Status: Open Submitter: Martin Sebor Date: 2003-09-18
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Discussion:
The dtor of the ios_base::Init object is supposed to call flush() on the 6 standard iostream objects cout, cerr, clog, wcout, wcerr, and wclog. This call may cause an exception to be thrown.
17.4.4.8, p3 prohibits all library destructors from throwing exceptions.
The question is: What should this dtor do if one or more of these calls to flush() ends up throwing an exception? This can happen quite easily if one of the facets installed in the locale imbued in the iostream object throws.
[Kona: We probably can't do much better than what we've got, so the LWG is leaning toward NAD. At the point where the standard stream objects are being cleaned up, the usual error reporting mechanism are all unavailable. And exception from flush at this point will definitely cause problems. A quality implementation might reasonably swallow the exception, or call abort, or do something even more drastic.]
[ See 397 and 622 for related issues. ]
Proposed resolution:
Section: 27.6.1.1.3 [istream::sentry] Status: Open Submitter: Martin Sebor Date: 2003-09-18
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Discussion:
27.6.1.1.3 [istream::sentry], p2 says that istream::sentry ctor prepares for input if is.good() is true. p4 then goes on to say that the ctor sets the sentry::ok_ member to true if the stream state is good after any preparation. 27.6.1.2.1 [istream.formatted.reqmts], p1 then says that a formatted input function endeavors to obtain the requested input if the sentry's operator bool() returns true. Given these requirements, no formatted extractor should ever set failbit if the initial stream rdstate() == eofbit. That is contrary to the behavior of all implementations I tested. The program below prints out eof = 1, fail = 0 eof = 1, fail = 1 on all of them.
#include <sstream>
#include <cstdio>
int main()
{
std::istringstream strm ("1");
int i = 0;
strm >> i;
std::printf ("eof = %d, fail = %d\n",
!!strm.eof (), !!strm.fail ());
strm >> i;
std::printf ("eof = %d, fail = %d\n",
!!strm.eof (), !!strm.fail ());
}
Comments from Jerry Schwarz (c++std-lib-11373):
Jerry Schwarz wrote:
I don't know where (if anywhere) it says it in the standard, but the
formatted extractors are supposed to set failbit if they don't extract
any characters. If they didn't then simple loops like
while (cin >> x);
would loop forever.
Further comments from Martin Sebor:
The question is which part of the extraction should prevent this from happening
by setting failbit when eofbit is already set. It could either be the sentry
object or the extractor. It seems that most implementations have chosen to
set failbit in the sentry [...] so that's the text that will need to be
corrected.
Pre Berlin: This issue is related to 342. If the sentry sets failbit when it finds eofbit already set, then you can never seek away from the end of stream.
Kona: Possibly NAD. If eofbit is set then good() will return false. We then set ok to false. We believe that the sentry's constructor should always set failbit when ok is false, and we also think the standard already says that. Possibly it could be clearer.
Proposed resolution:
Change 27.6.1.1.3 [istream::sentry], p2 to:
explicit sentry(basic_istream<charT,traits>& is , bool noskipws = false);-2- Effects: If is.good() is
truefalse, calls is.setstate(failbit). Otherwise prepares for formatted or unformatted input. ...
Section: 27.5.2.1 [streambuf.cons] Status: Open Submitter: Martin Sebor Date: 2003-09-18
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Discussion:
The reflector thread starting with c++std-lib-11346 notes that the class template basic_streambuf, along with basic_stringbuf and basic_filebuf, is copy-constructible but that the semantics of the copy constructors are not defined anywhere. Further, different implementations behave differently in this respect: some prevent copy construction of objects of these types by declaring their copy ctors and assignment operators private, others exhibit undefined behavior, while others still give these operations well-defined semantics.
Note that this problem doesn't seem to be isolated to just the three types mentioned above. A number of other types in the library section of the standard provide a compiler-generated copy ctor and assignment operator yet fail to specify their semantics. It's believed that the only types for which this is actually a problem (i.e. types where the compiler-generated default may be inappropriate and may not have been intended) are locale facets. See issue 439.
Proposed resolution:
27.5.2 [lib.streambuf]: Add into the synopsis, public section, just above the destructor declaration:
basic_streambuf(const basic_streambuf& sb); basic_streambuf& operator=(const basic_streambuf& sb);
Insert after 27.5.2.1, paragraph 2:
basic_streambuf(const basic_streambuf& sb);Constructs a copy of sb.
Postcondtions:
eback() == sb.eback() gptr() == sb.gptr() egptr() == sb.egptr() pbase() == sb.pbase() pptr() == sb.pptr() epptr() == sb.epptr() getloc() == sb.getloc()basic_streambuf& operator=(const basic_streambuf& sb);Assigns the data members of sb to this.
Postcondtions:
eback() == sb.eback() gptr() == sb.gptr() egptr() == sb.egptr() pbase() == sb.pbase() pptr() == sb.pptr() epptr() == sb.epptr() getloc() == sb.getloc()Returns: *this.
27.7.1 [lib.stringbuf]:
Option A:
Insert into the basic_stringbuf synopsis in the private section:
basic_stringbuf(const basic_stringbuf&); // not defined basic_stringbuf& operator=(const basic_stringbuf&); // not defined
Option B:
Insert into the basic_stringbuf synopsis in the public section:
basic_stringbuf(const basic_stringbuf& sb); basic_stringbuf& operator=(const basic_stringbuf& sb);27.7.1.1, insert after paragraph 4:
basic_stringbuf(const basic_stringbuf& sb);Constructs an independent copy of sb as if with sb.str(), and with the openmode that sb was constructed with.
Postcondtions:
str() == sb.str() gptr() - eback() == sb.gptr() - sb.eback() egptr() - eback() == sb.egptr() - sb.eback() pptr() - pbase() == sb.pptr() - sb.pbase() getloc() == sb.getloc()Note: The only requirement on epptr() is that it point beyond the initialized range if an output sequence exists. There is no requirement that epptr() - pbase() == sb.epptr() - sb.pbase().
basic_stringbuf& operator=(const basic_stringbuf& sb);After assignment the basic_stringbuf has the same state as if it were initially copy constructed from sb, except that the basic_stringbuf is allowed to retain any excess capacity it might have, which may in turn effect the value of epptr().
27.8.1.1 [lib.filebuf]
Insert at the bottom of the basic_filebuf synopsis:
private: basic_filebuf(const basic_filebuf&); // not defined basic_filebuf& operator=(const basic_filebuf&); // not defined
[Kona: this is an issue for basic_streambuf itself and for its derived classes. We are leaning toward allowing basic_streambuf to be copyable, and specifying its precise semantics. (Probably the obvious: copying the buffer pointers.) We are less sure whether the streambuf derived classes should be copyable. Howard will write up a proposal.]
[Sydney: Dietmar presented a new argument against basic_streambuf being copyable: it can lead to an encapsulation violation. Filebuf inherits from streambuf. Now suppose you inhert a my_hijacking_buf from streambuf. You can copy the streambuf portion of a filebuf to a my_hijacking_buf, giving you access to the pointers into the filebuf's internal buffer. Perhaps not a very strong argument, but it was strong enough to make people nervous. There was weak preference for having streambuf not be copyable. There was weak preference for having stringbuf not be copyable even if streambuf is. Move this issue to open for now. ]
[ 2007-01-12, Howard: Rvalue Reference Recommendations for Chapter 27 recommends protected copy constructor and assignment for basic_streambuf with the same semantics as would be generated by the compiler. These members aid in derived classes implementing move semantics. A protected copy constructor and copy assignment operator do not expose encapsulation more so than it is today as each data member of a basic_streambuf is already both readable and writable by derived classes via various get/set protected member functions (eback(), setp(), etc.). Rather a protected copy constructor and copy assignment operator simply make the job of derived classes implementing move semantics less tedious and error prone. ]
Rationale:
27.5.2 [lib.streambuf]: The proposed basic_streambuf copy constructor and assignment operator are the same as currently implied by the lack of declarations: public and simply copies the data members. This resolution is not a change but a clarification of the current standard.
27.7.1 [lib.stringbuf]: There are two reasonable options: A) Make basic_stringbuf not copyable. This is likely the status-quo of current implementations. B) Reasonable copy semantics of basic_stringbuf can be defined and implemented. A copyable basic_streambuf is arguably more useful than a non-copyable one. This should be considered as new functionality and not the fixing of a defect. If option B is chosen, ramifications from issue 432 are taken into account.
27.8.1.1 [lib.filebuf]: There are no reasonable copy semantics for basic_filebuf.
Section: 27 [input.output] Status: Open Submitter: Martin Sebor Date: 2003-09-18
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Discussion:
A third party test suite tries to exercise istream::ignore(N) with a negative value of N and expects that the implementation will treat N as if it were 0. Our implementation asserts that (N >= 0) holds and aborts the test.
I can't find anything in section 27 that prohibits such values but I don't see what the effects of such calls should be, either (this applies to a number of unformatted input functions as well as some member functions of the basic_streambuf template).
Proposed resolution:
I propose that we add to each function in clause 27 that takes an argument, say N, of type streamsize a Requires clause saying that "N >= 0." The intent is to allow negative streamsize values in calls to precision() and width() but disallow it in calls to streambuf::sgetn(), istream::ignore(), or ostream::write().
[Kona: The LWG agreed that this is probably what we want. However, we need a review to find all places where functions in clause 27 take arguments of type streamsize that shouldn't be allowed to go negative. Martin will do that review.]
Section: 22.2.2.1.2 [facet.num.get.virtuals] Status: Open Submitter: Martin Sebor Date: 2003-09-18
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Discussion:
The requirements specified in Stage 2 and reiterated in the rationale of DR 221 (and echoed again in DR 303) specify that num_get<charT>:: do_get() compares characters on the stream against the widened elements of "012...abc...ABCX+-"
An implementation is required to allow programs to instantiate the num_get template on any charT that satisfies the requirements on a user-defined character type. These requirements do not include the ability of the character type to be equality comparable (the char_traits template must be used to perform tests for equality). Hence, the num_get template cannot be implemented to support any arbitrary character type. The num_get template must either make the assumption that the character type is equality-comparable (as some popular implementations do), or it may use char_traits<charT> to do the comparisons (some other popular implementations do that). This diversity of approaches makes it difficult to write portable programs that attempt to instantiate the num_get template on user-defined types.
[Kona: the heart of the problem is that we're theoretically supposed to use traits classes for all fundamental character operations like assignment and comparison, but facets don't have traits parameters. This is a fundamental design flaw and it appears all over the place, not just in this one place. It's not clear what the correct solution is, but a thorough review of facets and traits is in order. The LWG considered and rejected the possibility of changing numeric facets to use narrowing instead of widening. This may be a good idea for other reasons (see issue 459), but it doesn't solve the problem raised by this issue. Whether we use widen or narrow the num_get facet still has no idea which traits class the user wants to use for the comparison, because only streams, not facets, are passed traits classes. The standard does not require that two different traits classes with the same char_type must necessarily have the same behavior.]
Informally, one possibility: require that some of the basic character operations, such as eq, lt, and assign, must behave the same way for all traits classes with the same char_type. If we accept that limitation on traits classes, then the facet could reasonably be required to use char_traits<charT>.
Proposed resolution:
Section: 26.5.2.4 [valarray.sub] Status: Open Submitter: Martin Sebor Date: 2003-09-18
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Discussion:
The standard fails to specify the behavior of valarray::operator[](slice) and other valarray subset operations when they are passed an "invalid" slice object, i.e., either a slice that doesn't make sense at all (e.g., slice (0, 1, 0) or one that doesn't specify a valid subset of the valarray object (e.g., slice (2, 1, 1) for a valarray of size 1).
[Kona: the LWG believes that invalid slices should invoke undefined behavior. Valarrays are supposed to be designed for high performance, so we don't want to require specific checking. We need wording to express this decision.]
[ Bellevue: ]
Please note that the standard also fails to specify the behavior of slice_array and gslice_array in the valid case. Bill Plauger will endeavor to provide revised wording for slice_array and gslice_array.
[ post-Bellevue: Bill provided wording. ]
Proposed resolution:
Insert after 26.5.2.4 [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)
Section: 20.1.2 [allocator.requirements], 25 [algorithms] Status: Open Submitter: Matt Austern Date: 2003-09-20
View other active issues in [allocator.requirements].
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Discussion:
Clause 20.1.2 [allocator.requirements] paragraph 4 says that implementations are permitted to supply containers that are unable to cope with allocator instances and that container implementations may assume that all instances of an allocator type compare equal. We gave implementers this latitude as a temporary hack, and eventually we want to get rid of it. What happens when we're dealing with allocators that don't compare equal?
In particular: suppose that v1 and v2 are both objects of type vector<int, my_alloc> and that v1.get_allocator() != v2.get_allocator(). What happens if we write v1.swap(v2)? Informally, three possibilities:
1. This operation is illegal. Perhaps we could say that an implementation is required to check and to throw an exception, or perhaps we could say it's undefined behavior.
2. The operation performs a slow swap (i.e. using three invocations of operator=, leaving each allocator with its original container. This would be an O(N) operation.
3. The operation swaps both the vectors' contents and their allocators. This would be an O(1) operation. That is:
my_alloc a1(...);
my_alloc a2(...);
assert(a1 != a2);
vector<int, my_alloc> v1(a1);
vector<int, my_alloc> v2(a2);
assert(a1 == v1.get_allocator());
assert(a2 == v2.get_allocator());
v1.swap(v2);
assert(a1 == v2.get_allocator());
assert(a2 == v1.get_allocator());
[Kona: This is part of a general problem. We need a paper saying how to deal with unequal allocators in general.]
[pre-Sydney: Howard argues for option 3 in N1599. ]
[ 2007-01-12, Howard: This issue will now tend to come up more often with move constructors and move assignment operators. For containers, these members transfer resources (i.e. the allocated memory) just like swap. ]
[ Batavia: There is agreement to overload the container swap on the allocator's Swappable requirement using concepts. If the allocator supports Swappable, then container's swap will swap allocators, else it will perform a "slow swap" using copy construction and copy assignment. ]
Proposed resolution:
Section: 24.1 [iterator.requirements], 23.1 [container.requirements] Status: Open Submitter: Andy Koenig Date: 2003-12-16
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Discussion:
What requirements does the standard place on equality comparisons between iterators that refer to elements of different containers. For example, if v1 and v2 are empty vectors, is v1.end() == v2.end() allowed to yield true? Is it allowed to throw an exception?
The standard appears to be silent on both questions.
[Sydney: The intention is that comparing two iterators from different containers is undefined, but it's not clear if we say that, or even whether it's something we should be saying in clause 23 or in clause 24. Intuitively we might want to say that equality is defined only if one iterator is reachable from another, but figuring out how to say it in any sensible way is a bit tricky: reachability is defined in terms of equality, so we can't also define equality in terms of reachability. ]
Proposed resolution:
Section: 27.8.1.4 [filebuf.members] Status: Open Submitter: Bill Plauger Date: 2004-01-30
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Duplicate of: 105
Discussion:
basic_filebuf *basic_filebuf::open(const char *, ios_base::open_mode);
should be supplemented with the overload:
basic_filebuf *basic_filebuf::open(const wchar_t *, ios_base::open_mode);
Depending on the operating system, one of these forms is fundamental and the other requires an implementation-defined mapping to determine the actual filename.
[Sydney: Yes, we want to allow wchar_t filenames. Bill will provide wording.]
[ In Toronto we noted that this is issue 5 from N1569. ]
How does this interact with the newly-defined character types, and how do we avoid interface explosion considering std::string overloads that were added? Propose another solution that is different than the suggestion proposed by PJP.
Suggestion is to make a member template function for basic_string (for char, wchar_t, u16char, u32char instantiations), and then just keep a const char* member.
Goal is to do implicit conversion between character string literals to appropriate basic_string type. Not quite sure if this is possible.
Implementors are free to add specific overloads for non-char character types.
Proposed resolution:
Change from:
basic_filebuf<charT,traits>* open( const char* s, ios_base::openmode mode );Effects: If is_open() != false, returns a null pointer. Otherwise, initializes the filebuf as required. It then opens a file, if possible, whose name is the NTBS s ("as if" by calling std::fopen(s,modstr)).
to:
basic_filebuf<charT,traits>* open( const char* s, ios_base::openmode mode ); basic_filebuf<charT,traits>* open( const wchar_t* ws, ios_base::openmode mode );Effects: If is_open() != false, returns a null pointer. Otherwise, initializes the filebuf as required. It then opens a file, if possible, whose name is the NTBS s ("as if" by calling std::fopen(s,modstr)). For the second signature, the NTBS s is determined from the WCBS ws in an implementation-defined manner.
(NOTE: For a system that "naturally" represents a filename as a WCBS, the NTBS s in the first signature may instead be mapped to a WCBS; if so, it follows the same mapping rules as the first argument to open.)
Rationale:
Slightly controversial, but by a 7-1 straw poll the LWG agreed to move this to Ready. The controversy was because the mapping between wide names and files in a filesystem is implementation defined. The counterargument, which most but not all LWG members accepted, is that the mapping between narrow files names and files is also implemenation defined.
[Lillehammer: Moved back to "open" status, at Beman's urging. (1) Why just basic_filebuf, instead of also basic_fstream (and possibly other things too). (2) Why not also constructors that take std::basic_string? (3) We might want to wait until we see Beman's filesystem library; we might decide that it obviates this.]
[ post Bellevue: ]
Move again to Ready.
There is a timing issue here. Since the filesystem library will not be in C++0x, this should be brought forward. This solution would remain valid in the context of the proposed filesystem.
This issue has been kicking around for a while, and the wchar_t addition alone would help many users. Thus, we suggest putting this on the reflector list with an invitation for someone to produce proposed wording that covers basic_fstream. In the meantime, we suggest that the proposed wording be adopted as-is.
If more of the Lillehammer questions come back, they should be introduced as separate issues.
Section: 24.1.5 [random.access.iterators] Status: Open Submitter: Daniel Frey Date: 2004-02-27
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Discussion:
In 24.1.5 [lib.random.access.iterators], table 76 the operational semantics for the expression "r -= n" are defined as "return r += -n". This means, that the expression -n must be valid, which is not the case for unsigned types.
[ Sydney: Possibly not a real problem, since difference type is required to be a signed integer type. However, the wording in the standard may be less clear than we would like. ]
Proposed resolution:
To remove this limitation, I suggest to change the operational semantics for this column to:
{ Distance m = n;
if (m >= 0)
while (m--) --r;
else
while (m++) ++r;
return r; }
Section: 22.2.2.1.2 [facet.num.get.virtuals] Status: Open Submitter: Martin Sebor Date: 2004-03-16
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Discussion:
When parsing strings of wide-character digits, the standard requires the library to widen narrow-character "atoms" and compare the widened atoms against the characters that are being parsed. Simply narrowing the wide characters would be far simpler, and probably more efficient. The two choices are equivalent except in convoluted test cases, and many implementations already ignore the standard and use narrow instead of widen.
First, I disagree that using narrow() instead of widen() would necessarily have unfortunate performance implications. A possible implementation of narrow() that allows num_get to be implemented in a much simpler and arguably comparably efficient way as calling widen() allows, i.e. without making a virtual call to do_narrow every time, is as follows:
inline char ctype<wchar_t>::narrow (wchar_t wc, char dflt) const
{
const unsigned wi = unsigned (wc);
if (wi > UCHAR_MAX)
return typeid (*this) == typeid (ctype<wchar_t>) ?
dflt : do_narrow (wc, dflt);
if (narrow_ [wi] < 0) {
const char nc = do_narrow (wc, dflt);
if (nc == dflt)
return dflt;
narrow_ [wi] = nc;
}
return char (narrow_ [wi]);
}
Second, I don't think the change proposed in the issue (i.e., to use narrow() instead of widen() during Stage 2) would be at all drastic. Existing implementations with the exception of libstdc++ currently already use narrow() so the impact of the change on programs would presumably be isolated to just a single implementation. Further, since narrow() is not required to translate alternate wide digit representations such as those mentioned in issue 303 to their narrow equivalents (i.e., the portable source characters '0' through '9'), the change does not necessarily imply that these alternate digits would be treated as ordinary digits and accepted as part of numbers during parsing. In fact, the requirement in 22.2.1.1.2 [locale.ctype.virtuals], p13 forbids narrow() to translate an alternate digit character, wc, to an ordinary digit in the basic source character set unless the expression (ctype<charT>::is(ctype_base::digit, wc) == true) holds. This in turn is prohibited by the C standard (7.25.2.1.5, 7.25.2.1.5, and 5.2.1, respectively) for charT of either char or wchar_t.
[Sydney: To a large extent this is a nonproblem. As long as you're only trafficking in char and wchar_t we're only dealing with a stable character set, so you don't really need either 'widen' or 'narrow': can just use literals. Finally, it's not even clear whether widen-vs-narrow is the right question; arguably we should be using codecvt instead.]
Proposed resolution:
Change stage 2 so that implementations are permitted to use either technique to perform the comparison:
Section: D.9.1 [auto.ptr] Status: Open Submitter: Rani Sharoni Date: 2003-12-07
View all other issues in [auto.ptr].
View all issues with Open status.
Discussion:
TC1 CWG DR #84 effectively made the template<class Y> operator auto_ptr<Y>() member of auto_ptr (20.4.5.3/4) obsolete.
The sole purpose of this obsolete conversion member is to enable copy initialization base from r-value derived (or any convertible types like cv-types) case:
#include <memory>
using std::auto_ptr;
struct B {};
struct D : B {};
auto_ptr<D> source();
int sink(auto_ptr<B>);
int x1 = sink( source() ); // #1 EDG - no suitable copy constructor
The excellent analysis of conversion operations that was given in the final auto_ptr proposal (http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/1997/N1128.pdf) explicitly specifies this case analysis (case 4). DR #84 makes the analysis wrong and actually comes to forbid the loophole that was exploited by the auto_ptr designers.
I didn't encounter any compliant compiler (e.g. EDG, GCC, BCC and VC) that ever allowed this case. This is probably because it requires 3 user defined conversions and in fact current compilers conform to DR #84.
I was surprised to discover that the obsolete conversion member actually has negative impact of the copy initialization base from l-value derived case:
auto_ptr<D> dp; int x2 = sink(dp); // #2 EDG - more than one user-defined conversion applies
I'm sure that the original intention was allowing this initialization using the template<class Y> auto_ptr(auto_ptr<Y>& a) constructor (20.4.5.1/4) but since in this copy initialization it's merely user defined conversion (UDC) and the obsolete conversion member is UDC with the same rank (for the early overloading stage) there is an ambiguity between them.
Removing the obsolete member will have impact on code that explicitly invokes it:
int y = sink(source().operator auto_ptr<B>());
IMHO no one ever wrote such awkward code and the reasonable workaround for #1 is:
int y = sink( auto_ptr<B>(source()) );
I was even more surprised to find out that after removing the obsolete conversion member the initialization was still ill-formed: int x3 = sink(dp); // #3 EDG - no suitable copy constructor
This copy initialization semantically requires copy constructor which means that both template conversion constructor and the auto_ptr_ref conversion member (20.4.5.3/3) are required which is what was explicitly forbidden in DR #84. This is a bit amusing case in which removing ambiguity results with no candidates.
I also found exception safety issue with auto_ptr related to auto_ptr_ref:
int f(auto_ptr<B>, std::string); auto_ptr<B> source2(); // string constructor throws while auto_ptr_ref // "holds" the pointer int x4 = f(source2(), "xyz"); // #4
The theoretic execution sequence that will cause a leak:
According to 20.4.5.3/3 and 20.4.5/2 the auto_ptr_ref conversion member returns auto_ptr_ref<Y> that holds *this and this is another defect since the type of *this is auto_ptr<X> where X might be different from Y. Several library vendors (e.g. SGI) implement auto_ptr_ref<Y> with Y* as member which is much more reasonable. Other vendor implemented auto_ptr_ref as defectively required and it results with awkward and catastrophic code: int oops = sink(auto_ptr<B>(source())); // warning recursive on all control paths
Dave Abrahams noticed that there is no specification saying that auto_ptr_ref copy constructor can't throw.
My proposal comes to solve all the above issues and significantly simplify auto_ptr implementation. One of the fundamental requirements from auto_ptr is that it can be constructed in an intuitive manner (i.e. like ordinary pointers) but with strict ownership semantics which yield that source auto_ptr in initialization must be non-const. My idea is to add additional constructor template with sole propose to generate ill-formed, diagnostic required, instance for const auto_ptr arguments during instantiation of declaration. This special constructor will not be instantiated for other types which is achievable using 14.8.2/2 (SFINAE). Having this constructor in hand makes the constructor template<class Y> auto_ptr(auto_ptr<Y> const&) legitimate since the actual argument can't be const yet non const r-value are acceptable.
This implementation technique makes the "private auxiliary class" auto_ptr_ref obsolete and I found out that modern C++ compilers (e.g. EDG, GCC and VC) consume the new implementation as expected and allow all intuitive initialization and assignment cases while rejecting illegal cases that involve const auto_ptr arguments.
The proposed auto_ptr interface:
namespace std {
template<class X> class auto_ptr {
public:
typedef X element_type;
// 20.4.5.1 construct/copy/destroy:
explicit auto_ptr(X* p=0) throw();
auto_ptr(auto_ptr&) throw();
template<class Y> auto_ptr(auto_ptr<Y> const&) throw();
auto_ptr& operator=(auto_ptr&) throw();
template<class Y> auto_ptr& operator=(auto_ptr<Y>) throw();
~auto_ptr() throw();
// 20.4.5.2 members:
X& operator*() const throw();
X* operator->() const throw();
X* get() const throw();
X* release() throw();
void reset(X* p=0) throw();
private:
template<class U>
auto_ptr(U& rhs, typename
unspecified_error_on_const_auto_ptr<U>::type = 0);
};
}
One compliant technique to implement the unspecified_error_on_const_auto_ptr helper class is using additional private auto_ptr member class template like the following:
template<typename T> struct unspecified_error_on_const_auto_ptr;
template<typename T>
struct unspecified_error_on_const_auto_ptr<auto_ptr<T> const>
{ typedef typename auto_ptr<T>::const_auto_ptr_is_not_allowed type; };
There are other techniques to implement this helper class that might work better for different compliers (i.e. better diagnostics) and therefore I suggest defining its semantic behavior without mandating any specific implementation. IMO, and I didn't found any compiler that thinks otherwise, 14.7.1/5 doesn't theoretically defeat the suggested technique but I suggest verifying this with core language experts.
Further changes in standard text:
Remove section 20.4.5.3
Change 20.4.5/2 to read something like: Initializing auto_ptr<X> from const auto_ptr<Y> will result with unspecified ill-formed declaration that will require unspecified diagnostic.
Change 20.4.5.1/4,5,6 to read:
template<class Y> auto_ptr(auto_ptr<Y> const& a) throw();
4 Requires: Y* can be implicitly converted to X*.
5 Effects: Calls const_cast<auto_ptr<Y>&>(a).release().
6 Postconditions: *this holds the pointer returned from a.release().
Change 20.4.5.1/10
template<class Y> auto_ptr& operator=(auto_ptr<Y> a) throw();
10 Requires: Y* can be implicitly converted to X*. The expression delete get() is well formed.
LWG TC DR #127 is obsolete.
Notice that the copy constructor and copy assignment operator should remain as before and accept non-const auto_ptr& since they have effect on the form of the implicitly declared copy constructor and copy assignment operator of class that contains auto_ptr as member per 12.8/5,10:
struct X {
// implicit X(X&)
// implicit X& operator=(X&)
auto_ptr<D> aptr_;
};
In most cases this indicates about sloppy programming but preserves the current auto_ptr behavior.
Dave Abrahams encouraged me to suggest fallback implementation in case that my suggestion that involves removing of auto_ptr_ref will not be accepted. In this case removing the obsolete conversion member to auto_ptr<Y> and 20.4.5.3/4,5 is still required in order to eliminate ambiguity in legal cases. The two constructors that I suggested will co exist with the current members but will make auto_ptr_ref obsolete in initialization contexts. auto_ptr_ref will be effective in assignment contexts as suggested in DR #127 and I can't see any serious exception safety issues in those cases (although it's possible to synthesize such). auto_ptr_ref<X> semantics will have to be revised to say that it strictly holds pointer of type X and not reference to an auto_ptr for the favor of cases in which auto_ptr_ref<Y> is constructed from auto_ptr<X> in which X is different from Y (i.e. assignment from r-value derived to base).
[Redmond: punt for the moment. We haven't decided yet whether we want to fix auto_ptr for C++-0x, or remove it and replace it with move_ptr and unique_ptr.]
[ Oxford 2007: Recommend NAD. We're just going to deprecate it. It still works for simple use cases and people know how to deal with it. Going forward unique_ptr is the recommended tool. ]
[ 2007-11-09: Reopened at the request of David Abrahams, Alisdair Meredith and Gabriel Dos Reis. ]
Proposed resolution:
Change the synopsis in D.9.1 [auto.ptr]:
namespace std {
template <class Y> struct auto_ptr_ref {};
// exposition only
template <class T> struct constant_object;
// exposition only
template <class T>
struct cannot_transfer_ownership_from
: constant_object<T> {};
template <class X> class auto_ptr {
public:
typedef X element_type;
// D.9.1.1 construct/copy/destroy:
explicit auto_ptr(X* p =0) throw();
auto_ptr(auto_ptr&) throw();
template<class Y> auto_ptr(auto_ptr<Y> const&) throw();
auto_ptr& operator=(auto_ptr&) throw();
template<class Y> auto_ptr& operator=(auto_ptr<Y>&) throw();
auto_ptr& operator=(auto_ptr_ref<X> r) throw();
~auto_ptr() throw();
// D.9.1.2 members:
X& operator*() const throw();
X* operator->() const throw();
X* get() const throw();
X* release() throw();
void reset(X* p =0) throw();
// D.9.1.3 conversions:
auto_ptr(auto_ptr_ref<X>) throw();
template<class Y> operator auto_ptr_ref<Y>() throw();
template<class Y> operator auto_ptr<Y>() throw();
// exposition only
template<class U>
auto_ptr(U& rhs, typename cannot_transfer_ownership_from<U>::error = 0);
};
template <> class auto_ptr<void>
{
public:
typedef void element_type;
};
}
Remove D.9.1.3 [auto.ptr.conv].
Change D.9.1 [auto.ptr], p3:
The auto_ptr provides a semantics of strict ownership. An auto_ptr owns the object it holds a pointer to. Copying an auto_ptr copies the pointer and transfers ownership to the destination. If more than one auto_ptr owns the same object at the same time the behavior of the program is undefined. Templates constant_object and cannot_transfer_ownership_from, and the final constructor of auto_ptr are for exposition only. For any types X and Y, initializing auto_ptr<X> from const auto_ptr<Y> is ill-formed, diagnostic required. [Note: The uses of auto_ptr include providing temporary exception-safety for dynamically allocated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function. auto_ptr does not meet the CopyConstructible and Assignable requirements for Standard Library container elements and thus instantiating a Standard Library container with an auto_ptr results in undefined behavior. -- end note]
Change D.9.1.1 [auto.ptr.cons], p5:
template<class Y> auto_ptr(auto_ptr<Y> const& a) throw();Requires: Y* can be implicitly converted to X*.
Effects: Calls const_cast<auto_ptr<Y>&>(a).release().
Postconditions: *this holds the pointer returned from a.release().
Change D.9.1.1 [auto.ptr.cons], p10:
template<class Y> auto_ptr& operator=(auto_ptr<Y>&a) throw();Requires: Y* can be implicitly converted to X*. The expression delete get() is well formed.
Effects: Calls reset(a.release()).
Returns: *this.
Section: 18.6.1 [type.info] Status: Ready Submitter: Martin Sebor Date: 2004-06-28
View all other issues in [type.info].
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Discussion:
[lib.exception] specifies the following:
exception (const exception&) throw();
exception& operator= (const exception&) throw();
-4- Effects: Copies an exception object.
-5- Notes: The effects of calling what() after assignment
are implementation-defined.
First, does the Note only apply to the assignment operator? If so, what are the effects of calling what() on a copy of an object? Is the returned pointer supposed to point to an identical copy of the NTBS returned by what() called on the original object or not?
Second, is this Note intended to extend to all the derived classes in section 19? I.e., does the standard provide any guarantee for the effects of what() called on a copy of any of the derived class described in section 19?
Finally, if the answer to the first question is no, I believe it constitutes a defect since throwing an exception object typically implies invoking the copy ctor on the object. If the answer is yes, then I believe the standard ought to be clarified to spell out exactly what the effects are on the copy (i.e., after the copy ctor was called).
[Redmond: Yes, this is fuzzy. The issue of derived classes is fuzzy too.]
[ Batavia: Howard provided wording. ]
[ Bellevue: ]
Eric concerned this is unimplementable, due to nothrow guarantees. Suggested implementation would involve reference counting.
Is the implied reference counting subtle enough to call out a note on implementation? Probably not.
If reference counting required, could we tighten specification further to require same pointer value? Probably an overspecification, especially if exception classes defer evalutation of final string to calls to what().
Remember issue moved open and not resolved at Batavia, but cannot remember who objected to canvas a disenting opinion - please speak up if you disagree while reading these minutes!
Move to Ready as we are accepting words unmodified.
Proposed resolution:
Change 18.7.1 [exception] to:
exception(const exception& e) throw(); exception& operator=(const exception& e) throw();-4- Effects: Copies an exception object.
-5- Remarks: The effects of calling what() after assignment are implementation-defined.-5- Throws: Nothing. This also applies to all standard library-defined classes that derive from exception.
-7- Postcondition: strcmp(what(), e.what()) == 0. This also applies to all standard library-defined classes that derive from exception.
Section: 22.2.1.1 [locale.ctype] Status: Open Submitter: Martin Sebor Date: 2004-07-01
View all issues with Open 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.
Proposed resolution:
Section: 24.1.1 [input.iterators] Status: Open Submitter: Chris Jefferson Date: 2004-09-16
View all other issues in [input.iterators].
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Discussion:
From comp.std.c++:
I note that given an input iterator a for type T, then *a only has to be "convertable to T", not actually of type T.
Firstly, I can't seem to find an exact definition of "convertable to T". While I assume it is the obvious definition (an implicit conversion), I can't find an exact definition. Is there one?
Slightly more worryingly, there doesn't seem to be any restriction on the this type, other than it is "convertable to T". Consider two input iterators a and b. I would personally assume that most people would expect *a==*b would perform T(*a)==T(*b), however it doesn't seem that the standard requires that, and that whatever type *a is (call it U) could have == defined on it with totally different symantics and still be a valid inputer iterator.
Is this a correct reading? When using input iterators should I write T(*a) all over the place to be sure that the object i'm using is the class I expect?
This is especially a nuisance for operations that are defined to be "convertible to bool". (This is probably allowed so that implementations could return say an int and avoid an unnessary conversion. However all implementations I have seen simply return a bool anyway. Typical implemtations of STL algorithms just write things like while(a!=b && *a!=0). But strictly speaking, there are lots of types that are convertible to T but that also overload the appropriate operators so this doesn't behave as expected.
If we want to make code like this legal (which most people seem to expect), then we'll need to tighten up what we mean by "convertible to T".
[Lillehammer: The first part is NAD, since "convertible" is well-defined in core. The second part is basically about pathological overloads. It's a minor problem but a real one. So leave open for now, hope we solve it as part of iterator redesign.]
Proposed resolution:
Section: 24.1.2 [output.iterators] Status: Open Submitter: Chris Jefferson Date: 2004-10-13
View all other issues in [output.iterators].
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Discussion:
The note on 24.1.2 Output iterators insufficently limits what can be performed on output iterators. While it requires that each iterator is progressed through only once and that each iterator is written to only once, it does not require the following things:
Note: Here it is assumed that x is an output iterator of type X which has not yet been assigned to.
a) That each value of the output iterator is written to: The standard allows: ++x; ++x; ++x;
b) That assignments to the output iterator are made in order X a(x); ++a; *a=1; *x=2; is allowed
c) Chains of output iterators cannot be constructed: X a(x); ++a; X b(a); ++b; X c(b); ++c; is allowed, and under the current wording (I believe) x,a,b,c could be written to in any order.
I do not believe this was the intension of the standard?
[Lillehammer: Real issue. There are lots of constraints we intended but didn't specify. Should be solved as part of iterator redesign.]
Proposed resolution:
Section: 23 [containers], 24 [iterators], 25 [algorithms] Status: Open Submitter: Thomas Mang Date: 2004-12-12
View other active issues in [containers].
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Discussion:
Various clauses other than clause 25 make use of iterator arithmetic not supported by the iterator category in question. Algorithms in clause 25 are exceptional because of 25 [lib.algorithms], paragraph 9, but this paragraph does not provide semantics to the expression "iterator - n", where n denotes a value of a distance type between iterators.
1) Examples of current wording:
Current wording outside clause 25:
23.2.2.4 [lib.list.ops], paragraphs 19-21: "first + 1", "(i - 1)", "(last - first)" 23.3.1.1 [lib.map.cons], paragraph 4: "last - first" 23.3.2.1 [lib.multimap.cons], paragraph 4: "last - first" 23.3.3.1 [lib.set.cons], paragraph 4: "last - first" 23.3.4.1 [lib.multiset.cons], paragraph 4: "last - first" 24.4.1 [lib.reverse.iterators], paragraph 1: "(i - 1)"
[Important note: The list is not complete, just an illustration. The same issue might well apply to other paragraphs not listed here.]
None of these expressions is valid for the corresponding iterator category.
Current wording in clause 25:
25.1.1 [lib.alg.foreach], paragraph 1: "last - 1" 25.1.3 [lib.alg.find.end], paragraph 2: "[first1, last1 - (last2-first2))" 25.2.8 [lib.alg.unique], paragraph 1: "(i - 1)" 25.2.8 [lib.alg.unique], paragraph 5: "(i - 1)"
However, current wording of 25 [lib.algorithms], paragraph 9 covers neither of these four cases:
Current wording of 25 [lib.algorithms], paragraph 9:
"In the description of the algorithms operator + and - are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics of a+n is the same as that of
{X tmp = a;
advance(tmp, n);
return tmp;
}
and that of b-a is the same as of return distance(a, b)"
This paragrpah does not take the expression "iterator - n" into account, where n denotes a value of a distance type between two iterators [Note: According to current wording, the expression "iterator - n" would be resolved as equivalent to "return distance(n, iterator)"]. Even if the expression "iterator - n" were to be reinterpreted as equivalent to "iterator + -n" [Note: This would imply that "a" and "b" were interpreted implicitly as values of iterator types, and "n" as value of a distance type], then 24.3.4/2 interfers because it says: "Requires: n may be negative only for random access and bidirectional iterators.", and none of the paragraphs quoted above requires the iterators on which the algorithms operate to be of random access or bidirectional category.
2) Description of intended behavior:
For the rest of this Defect Report, it is assumed that the expression "iterator1 + n" and "iterator1 - iterator2" has the semantics as described in current 25 [lib.algorithms], paragraph 9, but applying to all clauses. The expression "iterator1 - n" is equivalent to an result-iterator for which the expression "result-iterator + n" yields an iterator denoting the same position as iterator1 does. The terms "iterator1", "iterator2" and "result-iterator" shall denote the value of an iterator type, and the term "n" shall denote a value of a distance type between two iterators.
All implementations known to the author of this Defect Report comply with these assumptions. No impact on current code is expected.
3) Proposed fixes:
Change 25 [lib.algorithms], paragraph 9 to:
"In the description of the algorithms operator + and - are used for some of the iterator categories for which they do not have to be defined. In this paragraph, a and b denote values of an iterator type, and n denotes a value of a distance type between two iterators. In these cases the semantics of a+n is the same as that of
{X tmp = a;
advance(tmp, n);
return tmp;
}
,the semantics of a-n denotes the value of an iterator i for which the following condition holds: advance(i, n) == a, and that of b-a is the same as of return distance(a, b)".
Comments to the new wording:
a) The wording " In this paragraph, a and b denote values of an iterator type, and n denotes a value of a distance type between two iterators." was added so the expressions "b-a" and "a-n" are distinguished regarding the types of the values on which they operate. b) The wording ",the semantics of a-n denotes the value of an iterator i for which the following condition holds: advance(i, n) == a" was added to cover the expression 'iterator - n'. The wording "advance(i, n) == a" was used to avoid a dependency on the semantics of a+n, as the wording "i + n == a" would have implied. However, such a dependency might well be deserved. c) DR 225 is not considered in the new wording.
Proposed fixes regarding invalid iterator arithmetic expressions outside clause 25:
Either a) Move modified 25 [lib.algorithms], paragraph 9 (as proposed above) before any current invalid iterator arithmetic expression. In that case, the first sentence of 25 [lib.algorithms], paragraph 9, need also to be modified and could read: "For the rest of this International Standard, ...." / "In the description of the following clauses including this ...." / "In the description of the text below ..." etc. - anyways substituting the wording "algorithms", which is a straight reference to clause 25. In that case, 25 [lib.algorithms] paragraph 9 will certainly become obsolete. Alternatively, b) Add an appropiate paragraph similar to resolved 25 [lib.algorithms], paragraph 9, to the beginning of each clause containing invalid iterator arithmetic expressions. Alternatively, c) Fix each paragraph (both current wording and possible resolutions of DRs) containing invalid iterator arithmetic expressions separately.
5) References to other DRs:
See DR 225. See DR 237. The resolution could then also read "Linear in last - first".
[ Bellevue: ]
Keep open and ask Bill to provide wording.
Proposed resolution:
[Lillehammer: Minor issue, but real. We have a blanket statement about this in 25/11. But (a) it should be in 17, not 25; and (b) it's not quite broad enough, because there are some arithmetic expressions it doesn't cover. Bill will provide wording.]
Section: 25.2.13 [alg.partitions] Status: Open Submitter: Sean Parent, Joe Gottman Date: 2005-05-04
View all issues with Open status.
Discussion:
Problem: The iterator requirements for partition() and stable_partition() [25.2.12] are listed as BidirectionalIterator, however, there are efficient algorithms for these functions that only require ForwardIterator that have been known since before the standard existed. The SGI implementation includes these (see http://www.sgi.com/tech/stl/partition.html and http://www.sgi.com/tech/stl/stable_partition.html).
Proposed resolution:
Change 25.2.12 from
template<class BidirectionalIterator, class Predicate>
BidirectionalIterator partition(BidirectionalIterato r first,
BidirectionalIterator last,
Predicate pred);
to
template<class ForwardIterator, class Predicate>
ForwardIterator partition(ForwardIterator first,
ForwardIterator last,
Predicate pred);
Change the complexity from
At most (last - first)/2 swaps are done. Exactly (last - first) applications of the predicate are done.
to
If ForwardIterator is a bidirectional_iterator, at most (last - first)/2 swaps are done; otherwise at most (last - first) swaps are done. Exactly (last - first) applications of the predicate are done.
Rationale:
Partition is a "foundation" algorithm useful in many contexts (like sorting as just one example) - my motivation for extending it to include forward iterators is slist - without this extension you can't partition an slist (without writing your own partition). Holes like this in the standard library weaken the argument for generic programming (ideally I'd be able to provide a library that would refine std::partition() to other concepts without fear of conflicting with other libraries doing the same - but that is a digression). I consider the fact that partition isn't defined to work for ForwardIterator a minor embarrassment.
[Mont Tremblant: Moved to Open, request motivation and use cases by next meeting. Sean provided further rationale by post-meeting mailing.]
Section: 22.1.1.1.1 [locale.category] Status: Open Submitter: Christopher Conrade Zseleghovski Date: 2005-06-07
View all other issues in [locale.category].
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Discussion:
Motivation:
This requirement seems obvious to me, it is the essence of code modularity. I have complained to Mr. Plauger that the Dinkumware library does not observe this principle but he objected that this behaviour is not covered in the standard.
Proposed resolution:
Append the following point to 22.1.1.1.1:
6. The implementation of a facet of Table 52 parametrized with an InputIterator/OutputIterator should use that iterator only as character source/sink respectively. For a *_get facet, it means that the value received depends only on the sequence of input characters and not on how they are accessed. For a *_put facet, it means that the sequence of characters output depends only on the value to be formatted and not of how the characters are stored.
[ Berlin: Moved to Open, Need to clean up this area to make it clear locales don't have to contain open ended sets of facets. Jack, Howard, Bill. ]
Section: 22.2 [locale.categories] Status: Open Submitter: P.J. Plauger Date: 2005-06-20
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Discussion:
a) In 22.2.1.1 para. 2 we refer to "the instantiations required in Table 51" to refer to the facet *objects* associated with a locale. And we almost certainly mean just those associated with the default or "C" locale. Otherwise, you can't switch to a locale that enforces a different mapping between narrow and wide characters, or that defines additional uppercase characters.
b) 22.2.1.5 para. 3 (codecvt) has the same issues.
c) 22.2.1.5.2 (do_unshift) is even worse. It *forbids* the generation of a homing sequence for the basic character set, which might very well need one.
d) 22.2.1.5.2 (do_length) likewise dictates that the default mapping between wide and narrow characters be taken as one-for-one.
e) 22.2.2 para. 2 (num_get/put) is both muddled and vacuous, as far as I can tell. The muddle is, as before, calling Table 51 a list of instantiations. But the constraint it applies seems to me to cover *all* defined uses of num_get/put, so why bother to say so?
f) 22.2.3.1.2 para. 1(do_decimal_point) says "The required instantiations return '.' or L'.'.) Presumably this means "as appropriate for the character type. But given the vague definition of "required" earlier, this overrules *any* change of decimal point for non "C" locales. Surely we don't want to do that.
g) 22.2.3.1.2 para. 2 (do_thousands_sep) says "The required instantiations return ',' or L','.) As above, this probably means "as appropriate for the character type. But this overrules the "C" locale, which requires *no* character ('\0') for the thousands separator. Even if we agree that we don't mean to block changes in decimal point or thousands separator, we should also eliminate this clear incompatibility with C.
h) 22.2.3.1.2 para. 2 (do_grouping) says "The required instantiations return the empty string, indicating no grouping." Same considerations as for do_decimal_point.
i) 22.2.4.1 para. 1 (collate) refers to "instantiations required in Table 51". Same bad jargon.
j) 22.2.4.1.2 para. 1 (do_compare) refers to "instantiations required in Table 51". Same bad jargon.
k) 22.2.5 para. 1 (time_get/put) uses the same muddled and vacuous as num_get/put.
l) 22.2.6 para. 2 (money_get/put) uses the same muddled and vacuous as num_get/put.
m) 22.2.6.3.2 (do_pos/neg_format) says "The instantiations required in Table 51 ... return an object of type pattern initialized to {symbol, sign, none, value}." This once again *overrides* the "C" locale, as well as any other locale."
3) We constrain the use_facet calls that can be made by num_get/put, so why don't we do the same for money_get/put? Or for any of the other facets, for that matter?
4) As an almost aside, we spell out when a facet needs to use the ctype facet, but several also need to use a codecvt facet and we don't say so.
[ Berlin: Bill to provide wording. ]
Proposed resolution:
Section: 23.1.3 [unord.req], TR1 6.3.1 [tr.unord.req] Status: Ready Submitter: Matt Austern Date: 2005-07-03
View all other issues in [unord.req].
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Discussion:
Issue 371 deals with stability of multiset/multimap under insert and erase (i.e. do they preserve the relative order in ranges of equal elements). The same issue applies to unordered_multiset and unordered_multimap.
[ Moved to open (from review): There is no resolution. ]
[ Toronto: We have a resolution now. Moved to Review. Some concern was noted as to whether this conflicted with existing practice or not. An additional concern was in specifying (partial) ordering for an unordered container. ]
Proposed resolution:
Wording for the proposed resolution is taken from the equivalent text for associative containers.
Change 23.1.3 [unord.req], Unordered associative containers, paragraph 6 to:
An unordered associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys. unordered_set and unordered_map support unique keys. unordered_multiset and unordered_multimap support equivalent keys. In containers that support equivalent keys, elements with equivalent keys are adjacent to each other. For unordered_multiset and unordered_multimap, insert and erase preserve the relative ordering of equivalent elements.
Change 23.1.3 [unord.req], Unordered associative containers, paragraph 8 to:
The elements of an unordered associative container are organized into buckets. Keys with the same hash code appear in the same bucket. The number of buckets is automatically increased as elements are added to an unordered associative container, so that the average number of elements per bucket is kept below a bound. Rehashing invalidates iterators, changes ordering between elements, and changes which buckets elements appear in, but does not invalidate pointers or references to elements. For unordered_multiset and unordered_multimap, rehashing preserves the relative ordering of equivalent elements.
Section: 20.3 [tuple], TR1 6.1 [tr.tuple] Status: Open Submitter: Andy Koenig Date: 2005-07-03
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Discussion:
Tuple doesn't define swap(). It should.
[ Berlin: Doug to provide wording. ]
[ Batavia: Howard to provide wording. ]
[ Toronto: Howard to provide wording (really this time). ]
[ Bellevue: Alisdair provided wording. ]
Proposed resolution:
Add these signatures to 20.3 [tuple]
template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>&& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>&& y);
Add this signature to 20.3.1 [tuple.tuple]
void swap(tuple&&);
Add the following two sections to the end of the tuple clauses
20.3.1.7 tuple swap [tuple.swap]
void swap(tuple&& rhs);Requires: Each type in Types shall be Swappable.
Effects: Calls swap for each element in *this and its corresponding element in rhs.
Throws: Nothing, unless one of the element-wise swap calls throw an exception.
20.3.1.8 tuple specialized algorithms [tuple.special]
template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>&& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>&& y);Effects: x.swap(y)
Section: 28 [re] Status: Open Submitter: Eric Niebler Date: 2005-07-01
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Discussion:
A problem with TR1 regex is currently being discussed on the Boost developers list. It involves the handling of case-insensitive matching of character ranges such as [Z-a]. The proper behavior (according to the ECMAScript standard) is unimplementable given the current specification of the TR1 regex_traits<> class template. John Maddock, the author of the TR1 regex proposal, agrees there is a problem. The full discussion can be found at http://lists.boost.org/boost/2005/06/28850.php (first message copied below). We don't have any recommendations as yet.
-- Begin original message --
The situation of interest is described in the ECMAScript specification (ECMA-262), section 15.10.2.15:
"Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case ASCII letters as well as the symbols [, \, ], ^, _, and `."
A more interesting case is what should happen when doing a case-insentitive match on a range such as [Z-a]. It should match z, Z, a, A and the symbols [, \, ], ^, _, and `. This is not what happens with Boost.Regex (it throws an exception from the regex constructor).
The tough pill to swallow is that, given the specification in TR1, I don't think there is any effective way to handle this situation. According to the spec, case-insensitivity is handled with regex_traits<>::translate_nocase(CharT) -- two characters are equivalent if they compare equal after both are sent through the translate_nocase function. But I don't see any way of using this translation function to make character ranges case-insensitive. Consider the difficulty of detecting whether "z" is in the range [Z-a]. Applying the transformation to "z" has no effect (it is essentially std::tolower). And we're not allowed to apply the transformation to the ends of the range, because as ECMA-262 says, "the case of the two ends of a range is significant."
So AFAICT, TR1 regex is just broken, as is Boost.Regex. One possible fix is to redefine translate_nocase to return a string_type containing all the characters that should compare equal to the specified character. But this function is hard to implement for Unicode, and it doesn't play nice with the existing ctype facet. What a mess!
-- End original message --
[ John Maddock adds: ]
One small correction, I have since found that ICU's regex package does implement this correctly, using a similar mechanism to the current TR1.Regex.
Given an expression [c1-c2] that is compiled as case insensitive it:
Enumerates every character in the range c1 to c2 and converts it to it's case folded equivalent. That case folded character is then used a key to a table of equivalence classes, and each member of the class is added to the list of possible matches supported by the character-class. This second step isn't possible with our current traits class design, but isn't necessary if the input text is also converted to a case-folded equivalent on the fly.
ICU applies similar brute force mechanisms to character classes such as [[:lower:]] and [[:word:]], however these are at least cached, so the impact is less noticeable in this case.
Quick and dirty performance comparisons show that expressions such as "[X-\\x{fff0}]+" are indeed very slow to compile with ICU (about 200 times slower than a "normal" expression). For an application that uses a lot of regexes this could have a noticeable performance impact. ICU also has an advantage in that it knows the range of valid characters codes: code points outside that range are assumed not to require enumeration, as they can not be part of any equivalence class. I presume that if we want the TR1.Regex to work with arbitrarily large character sets enumeration really does become impractical.
Finally note that Unicode has:
Three cases (upper, lower and title). One to many, and many to one case transformations. Character that have context sensitive case translations - for example an uppercase sigma has two different lowercase forms - the form chosen depends on context(is it end of a word or not), a caseless match for an upper case sigma should match either of the lower case forms, which is why case folding is often approximated by tolower(toupper(c)).
Probably we need some way to enumerate character equivalence classes, including digraphs (either as a result or an input), and some way to tell whether the next character pair is a valid digraph in the current locale.
Hoping this doesn't make this even more complex that it was already,
[ Portland: Alisdair: Detect as invalid, throw an exception. Pete: Possible general problem with case insensitive ranges. ]
Proposed resolution:
Section: 26.6.3 [partial.sum] Status: Review Submitter: Marc Schoolderman Date: 2006-02-06
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Discussion:
There are some problems in the definition of partial_sum and adjacent_difference in 26.4 [lib.numeric.ops]
Unlike accumulate and inner_product, these functions are not parametrized on a "type T", instead, 26.4.3 [lib.partial.sum] simply specifies the effects clause as;
Assigns to every element referred to by iterator i in the range [result,result + (last - first)) a value correspondingly equal to
((...(* first + *( first + 1)) + ...) + *( first + ( i - result )))
And similarly for BinaryOperation. Using just this definition, it seems logical to expect that:
char i_array[4] = { 100, 100, 100, 100 };
int o_array[4];
std::partial_sum(i_array, i_array+4, o_array);
Is equivalent to
int o_array[4] = { 100, 100+100, 100+100+100, 100+100+100+100 };
i.e. 100, 200, 300, 400, with addition happening in the result type, int.
Yet all implementations I have tested produce 100, -56, 44, -112, because they are using an accumulator of the InputIterator's value_type, which in this case is char, not int.
The issue becomes more noticeable when the result of the expression *i + *(i+1) or binary_op(*i, *i-1) can't be converted to the value_type. In a contrived example:
enum not_int { x = 1, y = 2 };
...
not_int e_array[4] = { x, x, y, y };
std::partial_sum(e_array, e_array+4, o_array);
Is it the intent that the operations happen in the input type, or in the result type?
If the intent is that operations happen in the result type, something like this should be added to the "Requires" clause of 26.4.3/4 [lib.partial.sum]:
The type of *i + *(i+1) or binary_op(*i, *(i+1)) shall meet the requirements of CopyConstructible (20.1.3) and Assignable (23.1) types.
(As also required for T in 26.4.1 [lib.accumulate] and 26.4.2 [lib.inner.product].)
The "auto initializer" feature proposed in N1894 is not required to implement partial_sum this way. The 'narrowing' behaviour can still be obtained by using the std::plus<> function object.
If the intent is that operations happen in the input type, then something like this should be added instead;
The type of *first shall meet the requirements of CopyConstructible (20.1.3) and Assignable (23.1) types. The result of *i + *(i+1) or binary_op(*i, *(i+1)) shall be convertible to this type.
The 'widening' behaviour can then be obtained by writing a custom proxy iterator, which is somewhat involved.
In both cases, the semantics should probably be clarified.
26.4.4 [lib.adjacent.difference] is similarly underspecified, although all implementations seem to perform operations in the 'result' type:
unsigned char i_array[4] = { 4, 3, 2, 1 };
int o_array[4];
std::adjacent_difference(i_array, i_array+4, o_array);
o_array is 4, -1, -1, -1 as expected, not 4, 255, 255, 255.
In any case, adjacent_difference doesn't mention the requirements on the value_type; it can be brought in line with the rest of 26.4 [lib.numeric.ops] by adding the following to 26.4.4/2 [lib.adjacent.difference]:
The type of *first shall meet the requirements of CopyConstructible (20.1.3) and Assignable (23.1) types."
[ Berlin: Giving output iterator's value_types very controversial. Suggestion of adding signatures to allow user to specify "accumulator". ]
[ Bellevue: ]
The intent of the algorithms is to perform their calculations using the type of the input iterator. Proposed wording provided.
Proposed resolution:
Add to section 26.6.3 [partial.sum] paragraph 4 the following two sentences:
The type of *first shall meet the requirements of CopyConstructible? (20.1.3?) and Assignable (23.1?) types. The result of *i + *(i+1) or binary_op(*i, *(i+1)) shall be convertible to this type.
Add to section 26.6.4 [adjacent.difference] paragraph 2 the following sentence:
The type of *first shall meet the requirements of CopyConstructible? (20.1.3) and Assignable (23.1) types.
Section: TR1 5.1.1 [tr.rand.req] Status: Open Submitter: Matt Austern Date: 2006-01-10
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Discussion:
The TR sneaks in two new integer types, _Longlong and _Ulonglong, in [tr.c99]. The rest of the TR should use that type. I believe this affects two places. First, the random number requirements, 5.1.1/10-11, lists all of the types with which template parameters named IntType and UIntType may be instantiated. _Longlong (or "long long", assuming it is added to C++0x) should be added to the IntType list, and UIntType (again, or "unsigned long long") should be added to the UIntType list. Second, 6.3.2 lists the types for which hash<> is required to be instantiable. _Longlong and _Ulonglong should be added to that list, so that people may use long long as a hash key.
Proposed resolution:
Section: 26.7 [c.math] Status: Ready Submitter: Howard Hinnant Date: 2006-01-12
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Discussion:
Assuming we adopt the C compatibility package from C99 what should be the return type of the following signature be:
? pow(float, int);
C++03 says that the return type should be float. TR1 and C90/99 say the return type should be double. This can put clients into a situation where C++03 provides answers that are not as high quality as C90/C99/TR1. For example:
#include <math.h>
int main()
{
float x = 2080703.375F;
double y = pow(x, 2);
}
Assuming an IEEE 32 bit float and IEEE 64 bit double, C90/C99/TR1 all suggest:
y = 4329326534736.390625
which is exactly right. While C++98/C++03 demands:
y = 4329326510080.
which is only approximately right.
I recommend that C++0X adopt the mixed mode arithmetic already adopted by Fortran, C and TR1 and make the return type of pow(float,int) be double.
[ Kona (2007): Other functions that are affected by this issue include ldexp, scalbln, and scalbn. We also believe that there is a typo in 26.7/10: float nexttoward(float, long double); [sic] should be float nexttoward(float, float); Proposed Disposition: Review (the proposed resolution appears above, rather than below, the heading "Proposed resolution") ]
[
Howard, post Kona:
]Unfortunately I strongly disagree with a part of the resolution from Kona. I am moving from New to Open instead of to Review because I do not believe we have consensus on the intent of the resolution.
This issue does not include ldexp, scalbln, and scalbn because the second integral parameter in each of these signatures (from C99) is not a generic parameter according to C99 7.22p2. The corresponding C++ overloads are intended (as far as I know) to correspond directly to C99's definition of generic parameter.
For similar reasons, I do not believe that the second long double parameter of nexttoward, nor the return type of this function, is in error. I believe the correct signature is:
float nexttoward(float, long double);which is what both the C++0X working paper and C99 state (as far as I currently understand).
This is really only about pow(float, int). And this is because C++98 took one route (with pow only) and C99 took another (with many math functions in <tgmath.h>. The proposed resolution basically says: C++98 got it wrong and C99 got it right; let's go with C99.
[ Bellevue: ]
This signature was not picked up from C99. Instead, if one types pow(2.0f,2), the promotion rules will invoke "double pow(double, double)", which generally gives special treatment for integral exponents, preserving full accuracy of the result. New proposed wording provided.
Proposed resolution:
Change 26.7 [c.math] p10:
The added signatures are:
...float pow(float, int);...double pow(double, int);...long double pow(long double, int);
Section: 25.3 [alg.sorting] Status: Open Submitter: Martin Sebor Date: 2006-02-05
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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...
Proposed resolution:
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 argumenta 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.
[ Portland: Jack to define "convertible to bool" such that short circuiting isn't destroyed. ]
Section: 27.7.1.4 [stringbuf.virtuals] Status: Open Submitter: Martin Sebor Date: 2006-02-23
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Discussion:
The effects of the seekpos() member function of
basic_stringbuf simply say that the function positions
the input and/or output sequences but fail to spell out exactly
how. This is in contrast to the detail in which seekoff()
is described.
Proposed resolution:
Change 27.7.1.3, p13 to read:
-13- Effects: Same as seekoff(off_type(sp), ios_base::beg, which).
Alters the stream position within the controlled sequences, if possible, to correspond to the stream position stored in sp (as described below).
If (which & ios_base::in) != 0, positions the input sequence.If (which & ios_base::out) != 0, positions the output sequence.If sp is an invalid stream position, or if the function positions neither sequence, the positioning operation fails. If sp has not been obtained by a previous successful call to one of the positioning functions (seekoff, seekpos, tellg, tellp) the effect is undefined.
[ Kona (2007): A pos_type is a position in a stream by definition, so there is no ambiguity as to what it means. Proposed Disposition: NAD ]
[ Post-Kona Martin adds: I'm afraid I disagree with the Kona '07 rationale for marking it NAD. The only text that describes precisely what it means to position the input or output sequence is in seekoff(). The seekpos() Effects clause is inadequate in comparison and the proposed resolution plugs the hole by specifying seekpos() in terms of seekoff(). ]
Section: 27.5.2.4.5 [streambuf.virt.put] Status: Open Submitter: Martin Sebor Date: 2006-02-23
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Discussion:
streambuf::xsputn() is specified to have the effect of "writing up to n characters to the output sequence as if by repeated calls to sputc(c)."
Since sputc() is required to call overflow() when (pptr() == epptr()) is true, strictly speaking xsputn() should do the same. However, doing so would be suboptimal in some interesting cases, such as in unbuffered mode or when the buffer is basic_stringbuf.
Assuming calling overflow() is not really intended to be required and the wording is simply meant to describe the general effect of appending to the end of the sequence it would be worthwhile to mention in xsputn() that the function is not actually required to cause a call to overflow().
Proposed resolution:
Add the following sentence to the xsputn() Effects clause in 27.5.2.4.5, p1 (N1804):
-1- Effects: Writes up to n characters to the output sequence as if by repeated calls to sputc(c). The characters written are obtained from successive elements of the array whose first element is designated by s. Writing stops when either n characters have been written or a call to sputc(c) would return traits::eof(). It is uspecified whether the function calls overflow() when (pptr() == epptr()) becomes true or whether it achieves the same effects by other means.
In addition, I suggest to add a footnote to this function with the same text as Footnote 292 to make it extra clear that derived classes are permitted to override xsputn() for efficiency.
[ Kona (2007): We want to permit a streambuf that streams output directly to a device without making calls to sputc or overflow. We believe that has always been the intention of the committee. We believe that the proposed wording doesn't accomplish that. Proposed Disposition: Open ]
Section: TR1 8.16.4 [tr.c99.cmath.over] Status: New Submitter: Paolo Carlini Date: 2006-03-07
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Discussion:
log2 is missing from the list of "additional overloads" in TR1 8.16.4 [tr.c99.cmath.over] p1.
Hinnant: This is a TR1 issue only. It is fixed in the current (N2135) WD.
Proposed resolution:
Add log2 to the list of functions in TR1 8.16.4 [tr.c99.cmath.over] p1.
Section: 21.1 [char.traits] Status: Open Submitter: Jack Reeves Date: 2006-04-06
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Discussion:
Currently, the Standard Library specifies only a declaration for template class char_traits<> and requires the implementation provide two explicit specializations: char_traits<char> and char_traits<wchar_t>. I feel the Standard should require explicit specializations for all built-in character types, i.e. char, wchar_t, unsigned char, and signed char.
I have put together a paper (N1985) that describes this in more detail and includes all the necessary wording.
[ Portland: Jack will rewrite N1985 to propose a primary template that will work with other integral types. ]
[ Toronto: issue has grown with addition of char16_t and char32_t. ]
[ post Bellevue: ]
We suggest that Jack be asked about the status of his paper, and if it is not forthcoming, the work-item be assigned to someone else. If no one steps forward to do the paper before the next meeting, we propose to make this NAD without further discussion. We leave this Open for now, but our recommendation is NAD.
Note: the issue statement should be updated, as the Toronto comment has already been resolved. E.g., char_traits specializations for char16_t and char32_t are now in the working paper.
Proposed resolution:
Section: 27.4.3 [fpos] Status: Open Submitter: Beman Dawes Date: 2006-04-12
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Discussion:
There are two deficiencies related to file sizes:
The Dinkumware implementation of the Standard Library as shipped with the Microsoft compiler copes with these issues by:
fpos_t seekpos() const;
Because there are so many types relating to file positions and offsets (fpos_t, fpos, pos_type, off_type, streamoff, streamsize, streampos, wstreampos, and perhaps more), it is difficult to know if the Dinkumware extensions are sufficient. But they seem a useful starting place for discussions, and they do represent existing practice.
[ Kona (2007): We need a paper. It would be nice if someone proposed clarifications to the definitions of pos_type and off_type. Currently these definitions are horrible. Proposed Disposition: Open ]
Proposed resolution:
Section: 27.3 [iostream.objects] Status: Ready Submitter: Pete Becker Date: 2006-04-18
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Discussion:
lib.iostream.objects requires that the standard stream objects are never destroyed, and it requires that they be destroyed.
DR 369 adds words to say that we really mean for ios_base::Init objects to force construction of standard stream objects. It ends, though, with the phrase "these stream objects shall be destroyed after the destruction of dynamically ...". However, the rule for destruction is stated in the standard: "The objects are not destroyed during program execution."
Proposed resolution:
Change 27.3 [iostream.objects]/1:
-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.290) The objects are not destroyed during program execution.291) If a translation unit includes <iostream&t; 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
, and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit.
[ Kona (2007): From 27.3 [iostream.objects]/2, strike the words "...and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit." Proposed Disposition: Review ]
Section: 20.1.2 [allocator.requirements] Status: Open Submitter: Martin Sebor Date: 2006-06-14
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Duplicate of: 479
Discussion:
C++ Standard Library templates that take an allocator as an argument
are required to call the allocate() and
deallocate() members of the allocator object to obtain
storage. However, they do not appear to be required to call any other
allocator members such as construct(),
destroy(), address(), and
max_size(). This makes these allocator members less than
useful in portable programs.
It's unclear to me whether the absence of the requirement to use these allocator members is an unintentional omission or a deliberate choice. However, since the functions exist in the standard allocator and since they are required to be provided by any user-defined allocator I believe the standard ought to be clarified to explictly specify whether programs should or should not be able to rely on standard containers calling the functions.
I propose that all containers be required to make use of these functions.
[ Batavia: We support this resolution. Martin to provide wording. ]
[ pre-Oxford: Martin provided wording. ]
Proposed resolution:
Specifically, I propose to change 23.1 [container.requirements], p9 as follows:
-9- Copy constructors for all container types defined in this clause that are parametrized on
Allocatorcopyanthe allocator argument from their respective first parameters. All other constructors for these container types take anconstAllocator&argument (20.1.6), an allocator whosevalue_typeis the same as the container'svalue_type. A copy of this argumentisshall be used for any memory allocation and deallocation performed,by these constructors and by all member functions,during the lifetime of each container object. Allocation shall be performed "as if" by calling theallocate()member function on a copy of the allocator object of the appropriate type New Footnote), and deallocation "as if" by callingdeallocate()on a copy of the same allocator object of the corresponding type. A copy of this argument shall also be used to construct and destroy objects whose lifetime is managed by the container, including but not limited to those of the container'svalue_type, and to obtain their address. All objects residing in storage allocated by a container's allocator shall be constructed "as if" by calling theconstruct()member function on a copy of the allocator object of the appropriate type. The same objects shall be destroyed "as if" by callingdestroy()on a copy of the same allocator object of the same type. The address of such objects shall be obtained "as if" by calling theaddress()member function on a copy of the allocator object of the appropriate type. Finally, a copy of this argument shall be used by its container object to determine the maximum number of objects of the container'svalue_typethe container may store at the same time. The container member functionmax_size()obtains this number from the value returned by a call toget_allocator().max_size(). In all container types defined in this clause that are parametrized onAllocator, the memberget_allocator()returns a copy of theAllocatorobject used to construct the container.258)New Footnote: This type may be different from
Allocator: it may be derived fromAllocatorviaAllocator::rebind<U>::otherfor the appropriate typeU.
The proposed wording seems cumbersome but I couldn't think of a better
way to describe the requirement that containers use their
Allocator to manage only objects (regardless of their
type) that persist over their lifetimes and not, for example,
temporaries created on the stack. That is, containers shouldn't be
required to call Allocator::construct(Allocator::allocate(1),
elem) just to construct a temporary copy of an element, or
Allocator::destroy(Allocator::address(temp), 1) to
destroy temporaries.
[ Howard: This same paragraph will need some work to accommodate 431. ]
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
Section: 20.6.4.1 [uninitialized.copy] Status: Open Submitter: Martin Sebor Date: 2006-06-14
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Discussion:
The specialized algorithms [lib.specialized.algorithms] are specified as having the general effect of invoking the following expression:
new (static_cast<void*>(&*i))
typename iterator_traits<ForwardIterator>::value_type (x)
This expression is ill-formed when the type of the subexpression
&*i is some volatile-qualified T.
[ Batavia: Lack of support for proposed resolution but agree there is a defect. Howard to look at wording. Concern that move semantics properly expressed if iterator returns rvalue. ]
Proposed resolution:
In order to allow these algorithms to operate on volatile storage I propose to change the expression so as to make it well-formed even for pointers to volatile types. Specifically, I propose the following changes to clauses 20 and 24. Change 20.6.4.1, p1 to read:
Effects:
typedef typename iterator_traits<ForwardIterator>::pointer pointer;
typedef typename iterator_traits<ForwardIterator>::value_type value_type;
for (; first != last; ++result, ++first)
new (static_cast<void*>(const_cast<pointer>(&*result))
value_type (*first);
change 20.6.4.2, p1 to read
Effects:
typedef typename iterator_traits<ForwardIterator>::pointer pointer;
typedef typename iterator_traits<ForwardIterator>::value_type value_type;
for (; first != last; ++result, ++first)
new (static_cast<void*>(const_cast<pointer>(&*first))
value_type (*x);
and change 20.6.4.3, p1 to read
Effects:
typedef typename iterator_traits<ForwardIterator>::pointer pointer;
typedef typename iterator_traits<ForwardIterator>::value_type value_type;
for (; n--; ++first)
new (static_cast<void*>(const_cast<pointer>(&*first))
value_type (*x);
In addition, since there is no partial specialization for
iterator_traits<volatile T*> I propose to add one
to parallel such specialization for <const T*>. Specifically, I
propose to add the following text to the end of 24.3.1, p3:
and for pointers to volatile as
namespace std {
template<class T> struct iterator_traits<volatile T*> {
typedef ptrdiff_t difference_type;
typedef T value_type;
typedef volatile T* pointer;
typedef volatile T& reference;
typedef random_access_iterator_tag iterator_category;
};
}
Note that the change to iterator_traits isn't necessary
in order to implement the specialized algorithms in a way that allows
them to operate on volatile strorage. It is only necesassary in order
to specify their effects in terms of iterator_traits as
is done here. Implementations can (and some do) achieve the same
effect by means of function template overloading.
Section: 22.2 [locale.categories] Status: Open Submitter: Martin Sebor, Paolo Carlini Date: 2006-06-22
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Discussion:
Section 22.2, paragraph 2 requires facet get() members
that take an ios_base::iostate& argument,
err, to ignore the (initial) value of the
argument, but to set it to ios_base::failbit in case of a
parse error.
We believe there are a few minor problems with this blanket
requirement in conjunction with the wording specific to each
get() member function.
First, besides get() there are other member functions
with a slightly different name (for example,
get_date()). It's not completely clear that the intent of
the paragraph is to include those as well, and at least one
implementation has interpreted the requirement literally.
Second, the requirement to "set the argument to
ios_base::failbit suggests that the functions are not
permitted to set it to any other value (such as
ios_base::eofbit, or even ios_base::eofbit |
ios_base::failbit).
However, 22.2.2.1.2, p5 (Stage 3 of num_get parsing) and
p6 (bool parsing) specifies that the do_get
functions perform err |= ios_base::eofbit, which
contradicts the earlier requirement to ignore err's initial
value.
22.2.6.1.2, p1 (the Effects clause of the money_get
facet's do_get member functions) also specifies that
err's initial value be used to compute the final
value by ORing it with either ios_base::failbit or
withios_base::eofbit | ios_base::failbit.
Proposed resolution:
We believe the intent is for all facet member functions that take an
ios_base::iostate& argument to:
err argument,
err to ios_base::goodbit prior
to any further processing,
ios_base::eofbit, or
ios_base::failbit, or both in err, as
appropriate, in response to reaching the end-of-file or on parse
error, or both.
To that effect we propose to change 22.2, p2 as follows:
The put() members make no provision for error
reporting. (Any failures of the OutputIterator argument must be
extracted from the returned iterator.) Unless otherwise
specified, the get() members that
take an ios_base::iostate& argument whose value
they ignore, but set to ios_base::failbit in case of a parse
error., err, start by evaluating
err = ios_base::goodbit, and may subsequently set
err to either ios_base::eofbit, or
ios_base::failbit, or ios_base::eofbit |
ios_base::failbit in response to reaching the end-of-file or in
case of a parse error, or both, respectively.
[ Kona (2007): We need to change the proposed wording to clarify that the phrase "the get members" actually denotes get(), get_date(), etc. Proposed Disposition: Open ]
Section: 23.2.1 [array] Status: Open Submitter: Gennaro Prota Date: 2006-07-18
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Discussion:
The wording used for section 23.2.1 [lib.array] seems to be subtly ambiguous about zero sized arrays (N==0). Specifically:
* "An instance of array<T, N> stores N elements of type T, so that [...]"
Does this imply that a zero sized array object stores 0 elements, i.e. that it cannot store any element of type T? The next point clarifies the rationale behind this question, basically how to implement begin() and end():
* 23.2.1.5 [lib.array.zero], p2: "In the case that N == 0, begin() == end() == unique value."
What does "unique" mean in this context? Let's consider the following possible implementations, all relying on a partial specialization:
a)
template< typename T >
class array< T, 0 > {
....
iterator begin()
{ return iterator( reinterpret_cast< T * >( this ) ); }
....
};
This has been used in boost, probably intending that the return value had to be unique to the specific array object and that array couldn't store any T. Note that, besides relying on a reinterpret_cast, has (more than potential) alignment problems.
b)
template< typename T >
class array< T, 0 > {
T t;
iterator begin()
{ return iterator( &t ); }
....
};
This provides a value which is unique to the object and to the type of the array, but requires storing a T. Also, it would allow the user to mistakenly provide an initializer list with one element.
A slight variant could be returning *the* null pointer of type T
return static_cast<T*>(0);
In this case the value would be unique to the type array<T, 0> but not to the objects (all objects of type array<T, 0> with the same value for T would yield the same pointer value).
Furthermore this is inconsistent with what the standard requires from allocation functions (see library issue 9).
c) same as above but with t being a static data member; again, the value would be unique to the type, not to the object.
d) to avoid storing a T *directly* while disallowing the possibility to use a one-element initializer list a non-aggregate nested class could be defined
struct holder { holder() {} T t; } h;
and then begin be defined as
iterator begin() { return &h.t; }
But then, it's arguable whether the array stores a T or not. Indirectly it does.
-----------------------------------------------------
Now, on different issues:
* what's the effect of calling assign(T&) on a zero-sized array? There seems to be only mention of front() and back(), in 23.2.1 [lib.array] p4 (I would also suggest to move that bullet to section 23.2.1.5 [lib.array.zero], for locality of reference)
* (minor) the opening paragraph of 23.2.1 [lib.array] wording is a bit inconsistent with that of other sequences: that's not a problem in itself, but compare it for instance with "A vector is a kind of sequence that supports random access iterators"; though the intent is obvious one might argue that the wording used for arrays doesn't tell what an array is, and relies on the reader to infer that it is what the <array> header defines.
* it would be desiderable to have a static const data member of type std::size_t, with value N, for usage as integral constant expression
* section 23.1 [lib.container.requirements] seem not to consider fixed-size containers at all, as it says: "[containers] control allocation and deallocation of these objects [the contained objects] through constructors, destructors, *insert and erase* operations"
* max_size() isn't specified: the result is obvious but, technically, it relies on table 80: "size() of the largest possible container" which, again, doesn't seem to consider fixed size containers
Proposed resolution:
[ Kona (2007): requirements on zero sized tr1::arrays and other details Issue 617: std::array is a sequence that doesn't satisfy the sequence requirements? Alisdair will prepare a paper. Proposed Disposition: Open ]
Section: 26.3.7 [complex.value.ops] Status: Ready Submitter: Stefan Große Pawig Date: 2006-09-24
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Discussion:
TR1 introduced, in the C compatibility chapter, the function fabs(complex<T>):
----- SNIP -----
8.1.1 Synopsis [tr.c99.cmplx.syn]
namespace std {
namespace tr1 {
[...]
template<class T> complex<T> fabs(const complex<T>& x);
} // namespace tr1
} // namespace std
[...]
8.1.8 Function fabs [tr.c99.cmplx.fabs]
1 Effects: Behaves the same as C99 function cabs, defined in
subclause 7.3.8.1.
----- SNIP -----
The current C++0X draft document (n2009.pdf) adopted this definition in chapter 26.3.1 (under the comment // 26.3.7 values) and 26.3.7/7.
But in C99 (ISO/IEC 9899:1999 as well as the 9899:TC2 draft document n1124), the referenced subclause reads
----- SNIP ----- 7.3.8.1 The cabs functions Synopsis 1 #include <complex.h> double cabs(double complex z); float cabsf(float complex z); long double cabsl(long double z); Description 2 The cabs functions compute the complex absolute value (also called norm, modulus, or magnitude) of z. Returns 3 The cabs functions return the complex absolute value. ----- SNIP -----
Note that the return type of the cabs*() functions is not a complex type. Thus, they are equivalent to the already well established template<class T> T abs(const complex<T>& x); (26.2.7/2 in ISO/IEC 14882:1998, 26.3.7/2 in the current draft document n2009.pdf).
So either the return value of fabs() is specified wrongly, or fabs() does not behave the same as C99's cabs*().
Possible ResolutionsThis depends on the intention behind the introduction of fabs().
If the intention was to provide a /complex/ valued function that calculates the magnitude of its argument, this should be explicitly specified. In TR1, the categorization under "C compatibility" is definitely wrong, since C99 does not provide such a complex valued function.
Also, it remains questionable if such a complex valued function is really needed, since complex<T> supports construction and assignment from real valued arguments. There is no difference in observable behaviour between
complex<double> x, y; y = fabs(x); complex<double> z(fabs(x));
and
complex<double> x, y; y = abs(x); complex<double> z(abs(x));
If on the other hand the intention was to provide the intended functionality of C99, fabs() should be either declared deprecated or (for C++0X) removed from the standard, since the functionality is already provided by the corresponding overloads of abs().
[ Bellevue: ]
Bill believes that abs() is a suitable overload. We should remove fabs().
Proposed resolution:
Change the synopsis in 26.3.1 [complex.synopsis]:
template<class T> complex<T> fabs(const complex<T>&);
Remove 26.3.7 [complex.value.ops], p7:
template<class T> complex<T> fabs(const complex<T>& x);
-7- Effects: Behaves the same as C99 function cabs, defined in subclause 7.3.8.1.
[ Kona (2007): Change the return type of fabs(complex) to T. Proposed Disposition: Ready ]
Section: 27.8.1.4 [filebuf.members] Status: Ready Submitter: Thomas Plum Date: 2006-09-26
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Discussion:
In testing 27.8.1.4 [filebuf.members], Table 112 (in the latest N2009 draft), we invoke
ostr.open("somename", ios_base::out | ios_base::in | ios_base::app)
and we expect the open to fail, because out|in|app is not listed in Table 92, and just before the table we see very specific words:
If mode is not some combination of flags shown in the table then the open fails.
But the corresponding table in the C standard, 7.19.5.3, provides two modes "a+" and "a+b", to which the C++ modes out|in|app and out|in|app|binary would presumably apply.
We would like to argue that the intent of Table 112 was to match the semantics of 7.19.5.3 and that the omission of "a+" and "a+b" was unintentional. (Otherwise there would be valid and useful behaviors available in C file I/O which are unavailable using C++, for no valid functional reason.)
We further request that the missing modes be explicitly restored to the WP, for inclusion in C++0x.
[ Martin adds: ]
...besides "a+" and "a+b" the C++ table is also missing a row for a lone app bit which in at least two current implementation as well as in Classic Iostreams corresponds to the C stdio "a" mode and has been traditionally documented as implying ios::out. Which means the table should also have a row for in|app meaning the same thing as "a+" already proposed in the issue.
Proposed resolution:
Add to the table "File open modes" in 27.8.1.4 [filebuf.members]:
File open modes ios_base Flag combination stdio equivalent binary in out trunc app + "w" + + "a" + "a" + + "w" + "r" + + "r+" + + + "w+" + + + "a+" + + "a+" + + "wb" + + + "ab" + + "ab" + + + "wb" + + "rb" + + + "r+b" + + + + "w+b" + + + + "a+b" + + + "a+b"
[ Kona (2007) Added proposed wording and moved to Review. ]
Section: TRDecimal 3.2 [trdec.types.types] Status: Open Submitter: Daveed Vandevoorde Date: 2006-04-05
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Discussion:
In a private email, Daveed writes:
I am not familiar with the C TR, but my guess is that the class type approach still won't match a built-in type approach because the notion of "promotion" cannot be emulated by user-defined types.
Here is an example:
struct S {
S(_Decimal32 const&); // Converting constructor
};
void f(S);
void f(_Decimal64);
void g(_Decimal32 d) {
f(d);
}
If _Decimal32 is a built-in type, the call f(d) will likely resolve to f(_Decimal64) because that requires only a promotion, whereas f(S) requires a user-defined conversion.
If _Decimal32 is a class type, I think the call f(d) will be ambiguous because both the conversion to _Decimal64 and the conversion to S will be user-defined conversions with neither better than the other.
Robert comments:
In general, a library of arithmetic types cannot exactly emulate the behavior of the intrinsic numeric types. There are several ways to tell whether an implementation of the decimal types uses compiler intrinisics or a library. For example:
_Decimal32 d1;
d1.operator+=(5); // If d1 is a builtin type, this won't compile.
In preparing the decimal TR, we have three options:
We decided as a group to pursue option #3, but that approach implies that implementations may not agree on the semantics of certain use cases (first example, above), or on whether certain other cases are well-formed (second example). Another potentially important problem is that, under the present definition of POD, the decimal classes are not POD types, but builtins will be.
Note that neither example above implies any problems with respect to C-to-C++ compatibility, since neither example can be expressed in C.
Proposed resolution:
Section: TRDecimal 3.2 [trdec.types.types] Status: Open Submitter: Martin Sebor Date: 2006-06-15
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Discussion:
In c++std-lib-17205, Martin writes:
...was it a deliberate design choice to make narrowing assignments ill-formed while permitting narrowing compound assignments? For instance:
decimal32 d32;
decimal64 d64;
d32 = 64; // error
d32 += 64; // okay
In c++std-lib-17229, Robert responds:
It is a vestige of an old idea that I forgot to remove from the paper. Narrowing assignments should be permitted. The bug is that the converting constructors that cause narrowing should not be explicit. Thanks for pointing this out.
Proposed resolution:
1. In "3.2.2 Class decimal32" synopsis, remove the explicit specifier from the narrowing conversions:
// 3.2.2.2 conversion from floating-point type:
explicit decimal32(decimal64 d64);
explicit decimal32(decimal128 d128);
2. Do the same thing in "3.2.2.2. Conversion from floating-point type."
3. In "3.2.3 Class decimal64" synopsis, remove the explicit specifier from the narrowing conversion:
// 3.2.3.2 conversion from floating-point type:
explicit decimal64(decimal128 d128);
4. Do the same thing in "3.2.3.2. Conversion from floating-point type."
[ Redmond: We prefer explicit conversions for narrowing and implicit for widening. ]
Section: 18.2.1.2 [numeric.limits.members] Status: Ready Submitter: Chris Jefferson Date: 2006-11-10
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Discussion:
18.2.1.2 55 states that "A type is modulo if it is possible to add two positive numbers together and have a result that wraps around to a third number that is less". This seems insufficient for the following reasons:
[ Batavia: Related to N2144. Pete: is there an ISO definition of modulo? Underflow on signed behavior is undefined. ]
[ Bellevue: accept resolution, move to ready status. Does this mandate that is_modulo be true on platforms for which int happens to b modulo? A: the standard already seems to require that. ]
Proposed resolution:
Suggest 18.2.1.2 [numeric.limits.members[numeric.limits.members], paragraph 57 is amended to:
A type is modulo if,
it is possible to add two positive numbers and have a result that wraps around to a third number that is less.given any operation involving +,- or * on values of that type whose value would fall outside the range [min(), max()], then the value returned differs from the true value by an integer multiple of (max() - min() + 1).
Section: 21.3 [basic.string] Status: Open Submitter: Bo Persson Date: 2006-12-05
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Discussion:
This is based on N2134, where 21.3.1/2 states: "... The Allocator object used shall be a copy of the Allocator object passed to the basic_string object's constructor or, if the constructor does not take an Allocator argument, a copy of a default-constructed Allocator object."
Section 21.3.2/1 lists two constructors:
basic_string(const basic_string<charT,traits,Allocator>& str );
basic_string(const basic_string<charT,traits,Allocator>& str ,
size_type pos , size_type n = npos,
const Allocator& a = Allocator());
and then says "In the first form, the Allocator value used is copied from str.get_allocator().", which isn't an option according to 21.3.1.
[ Batavia: We need blanket statement to the effect of: ]
[ Review constructors and functions that return a string; make sure we follow these rules (substr, operator+, etc.). Howard to supply wording. ]
[
Bo adds: The new container constructor which takes only a size_type is not
consistent with 23.1 [container.requirements], p9 which says in part:
All other constructors for these container types take an
Allocator& argument (20.1.2), an allocator whose value type
is the same as the container's value type. A copy of this argument is
used for any memory allocation performed, by these constructors and by
all member functions, during the lifetime of each container object.
]
[ post Bellevue: We re-confirm that the issue is real. Pablo will provide wording. ]
Proposed resolution:
Section: 23.2.1 [array] Status: Open Submitter: Bo Persson Date: 2006-12-30
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Discussion:
The <array> header is given under 23.2 [sequences]. 23.2.1 [array]/paragraph 3 says:
"Unless otherwise specified, all array operations are as described in 23.1 [container.requirements]".
However, array isn't mentioned at all in section 23.1 [container.requirements]. In particular, Table 82 "Sequence requirements" lists several operations (insert, erase, clear) that std::array does not have in 23.2.1 [array].
Also, Table 83 "Optional sequence operations" lists several operations that std::array does have, but array isn't mentioned.
Proposed resolution:
Section: 26.5.2.7 [valarray.members] Status: Ready Submitter: Gabriel Dos Reis Date: 2007-01-10
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Discussion:
I would respectfully request an issue be opened with the intention to clarify the wording for size() == 0 for cshift.
Proposed resolution:
Change 26.5.2.7 [valarray.members], paragraph 10:
valarray<T> cshift(int n) const;This function returns an object of class valarray<T>, of length size(),
each of whose elements I is (*this)[(I + n ) % size()]. Thus, if element zero is taken as the leftmost element, a positive value of n shifts the elements circularly left n places.that is a circular shift of *this. If element zero is taken as the leftmost element, a non-negative value of n shifts the elements circularly left n places and a negative value of n shifts the elements circularly right -n places.
Rationale:
We do not believe that there is any real ambiguity about what happens when size() == 0, but we do believe that spelling this out as a C++ expression causes more trouble that it solves. The expression is certainly wrong when n < 0, since the sign of % with negative arguments is implementation defined.
[ Kona (2007) Changed proposed wording, added rationale and set to Review. ]
Section: 26.3.6 [complex.ops] Status: Ready Submitter: Gabriel Dos Reis Date: 2007-01-28
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Discussion:
is there an issue opened for (0,3) as complex number with the French local? With the English local, the above parses as an imaginery complex number. With the French locale it parses as a real complex number.
Further notes/ideas from the lib-reflector, messages 17982-17984:
Add additional entries in num_punct to cover the complex separator (French would be ';').
Insert a space before the comma, which should eliminate the ambiguity.
Solve the problem for ordered sequences in general, perhaps with a dedicated facet. Then complex should use that solution.
[ Bellevue: ]
After much discussion, we agreed on the following: Add a footnote:
[In a locale in which comma is being used as a decimal point character, inserting "showbase" into the output stream forces all outputs to show an explicit decimal point character; then all inserted complex sequences will extract unambiguously.]
And move this to READY status.
Proposed resolution:
Add a footnote to 26.3.6 [complex.ops] p16:
[In a locale in which comma is being used as a decimal point character, inserting "showbase" into the output stream forces all outputs to show an explicit decimal point character; then all inserted complex sequences will extract unambiguously.]
Section: 26.5.2.1 [valarray.cons] Status: Open Submitter: Martin Sebor Date: 2007-01-28
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Discussion:
Section 26.1 [numeric.requirements], p1 suggests that a
valarray specialization on a type T that
satisfies the requirements enumerated in the paragraph is itself a
valid type on which valarray may be instantiated
(Footnote 269 makes this clear). I.e.,
valarray<valarray<T> > is valid as long as
T is valid. However, since implementations of
valarray are permitted to initialize storage allocated by
the class by invoking the default ctor of T followed by
the copy assignment operator, such implementations of
valarray wouldn't work with (perhaps user-defined)
specializations of valarray whose assignment operator had
undefined behavior when the size of its argument didn't match the size
of *this. By "wouldn't work" I mean that it would
be impossible to resize such an array of arrays by calling the
resize() member function on it if the function used the
copy assignment operator after constructing all elements using the
default ctor (e.g., by invoking new value_type[N]) to
obtain default-initialized storage) as it's permitted to do.
Stated more generally, the problem is that
valarray<valarray<T> >::resize(size_t) isn't
required or guaranteed to have well-defined semantics for every type
T that satisfies all requirements in
26.1 [numeric.requirements].
I believe this problem was introduced by the adoption of the
resolution outlined in N0857,
Assignment of valarrays, from 1996. The copy assignment
operator of the original numerical array classes proposed in N0280,
as well as the one proposed in N0308
(both from 1993), had well-defined semantics for arrays of unequal
size (the latter explicitly only when *this was empty;
assignment of non empty arrays of unequal size was a runtime error).
The justification for the change given in N0857 was the "loss of performance [deemed] only significant for very simple operations on small arrays or for architectures with very few registers."
Since tiny arrays on a limited subset of hardware architectures are
likely to be an exceedingly rare case (despite the continued
popularity of x86) I propose to revert the resolution and make the
behavior of all valarray assignment operators
well-defined even for non-conformal arrays (i.e., arrays of unequal
size). I have implemented this change and measured no significant
degradation in performance in the common case (non-empty arrays of
equal size). I have measured a 50% (and in some cases even greater)
speedup in the case of assignments to empty arrays versus calling
resize() first followed by an invocation of the copy
assignment operator.
[ Bellevue: ]
If no proposed wording by June meeting, this issue should be closed NAD.
Proposed resolution:
Change 26.5.2.2 [valarray.assign], p1 as follows:
valarray<T>& operator=(const valarray<T>& x);-1- Each element of the
*thisarray is assigned the value of the corresponding element of the argument array.The resulting behavior is undefined ifWhen the length of the argument array is not equal to the length of the *this array.resizes*thisto make the two arrays the same length, as if by callingresize(x.size()), before performing the assignment.
And add a new paragraph just below paragraph 1 with the following text:
-2- Postcondition:
size() == x.size().
Also add the following paragraph to 26.5.2.2 [valarray.assign], immediately after p4:
-?- When the length,
Nof the array referred to by the argument is not equal to the length of*this, the operator resizes*thisto make the two arrays the same length, as if by callingresize(N), before performing the assignment.
[ Kona (2007): Gaby to propose wording for an alternative resolution in which you can assign to a valarray of size 0, but not to any other valarray whose size is unequal to the right hand side of the assignment. ]
Section: 25 [algorithms] Status: Open Submitter: James Kanze Date: 2007-01-31
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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 (25.3.3.2 [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 )".
[ Toronto: Moved to Open. ConceptGCC seems to get lower_bound and upper_bound to work withoutt these changes. ]
Proposed resolution:
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:
[...] BinaryPredicate always takes the first iterator value_type as one of its arguments, it is unspecified which. 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)){...}. 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)){...}. [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]
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.
Section: 23.1 [container.requirements] Status: Open Submitter: Lionel B Date: 2007-02-01
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Discussion:
A recent news group discussion:
Anyone know if the Standard has anything to say about the time complexity of size() for std::set? I need to access a set's size (/not/ to know if it is empty!) heavily during an algorithm and was thus wondering whether I'd be better off tracking the size "manually" or whether that'd be pointless.
That would be pointless. size() is O(1).
Nit: the standard says "should" have constant time. Implementations may take license to do worse. I know that some do this for std::list<> as a part of some trade-off with other operation.
I was aware of that, hence my reluctance to use size() for std::set.
However, this reason would not apply to std::set<> as far as I can see.
Ok, I guess the only option is to try it and see...
If I have any recommendation to the C++ Standards Committee it is that implementations must (not "should"!) document clearly[1], where known, the time complexity of *all* container access operations.
[1] In my case (gcc 4.1.1) I can't swear that the time complexity of size() for std::set is not documented... but if it is it's certainly well hidden away.
Proposed resolution:
[ Kona (2007): This issue affects all the containers. We'd love to see a paper dealing with the broad issue. We think that the complexity of the size() member of every container -- except possibly list -- should be O(1). Alan has volunteered to provide wording. ]
[ Bellevue: ]
Mandating O(1) size will not fly, too many implementations would be invalidated. Alan to provide wording that toughens wording, but that does not absolutely mandate O(1).
Section: 20.1.2 [allocator.requirements] Status: Open Submitter: Howard Hinnant Date: 2007-02-08
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Discussion:
The table of allocator requirements in 20.1.2 [allocator.requirements] describes allocator::address as:
a.address(r) a.address(s)
where r and s are described as:
a value of type X::reference obtained by the expression *p.
and p is
a value of type X::pointer, obtained by calling a1.allocate, where a1 == a
This all implies that to get the address of some value of type T that value must have been allocated by this allocator or a copy of it.
However sometimes container code needs to compare the address of an external value of type T with an internal value. For example list::remove(const T& t) may want to compare the address of the external value t with that of a value stored within the list. Similarly vector or deque insert may want to make similar comparisons (to check for self-referencing calls).
Mandating that allocator::address can only be called for values which the allocator allocated seems overly restrictive.
Proposed resolution:
Change 20.1.2 [allocator.requirements]:
r : a value of type X::reference
obtained by the expression *p.s : a value of type X::const_reference
obtained by the expression *q or by conversion from a value r.
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
[ Kona (2007): This issue is section 8 of N2387. There was some discussion of it but no resolution to this issue was recorded. Moved to Open. ]
Section: 23.2.2.3 [deque.modifiers] Status: Ready Submitter: Steve LoBasso Date: 2007-02-17
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Discussion:
The standard states at 23.2.2.3 [deque.modifiers]/4:
deque erase(...)Effects: ... An erase at either end of the deque invalidates only the iterators and the references to the erased elements.
This does not state that iterators to end will be invalidated. It needs to be amended in such a way as to account for end invalidation.
Something like:
Any time the last element is erased, iterators to end are invalidated.
This would handle situations like:
erase(begin(), end()) erase(end() - 1) pop_back() resize(n, ...) where n < size() pop_front() with size() == 1
[ Post Kona, Steve LoBasso notes: ]
My only issue with the proposed resolution is that it might not be clear that pop_front() [where size() == 1] can invalidate past-the-end iterators.
Proposed resolution:
Change 23.2.2.3 [deque.modifiers], p4:
iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last);-4- Effects: An erase in the middle of the deque invalidates all the iterators and references to elements of the deque and the past-the-end iterator. An erase at either end of the deque invalidates only the iterators and the references to the erased elements, except that erasing at the end also invalidates the past-the-end iterator.
[ Kona (2007): Proposed wording added and moved to Review. ]
[ Bellevue: ]
Note that there is existing code that relies on iterators not being invalidated, but there are also existing implementations that do invalidate iterators. Thus, such code is not portable in any case. There is a pop_front() note, which should possibly be a separate issue. Mike Spertus to evaluate and, if need be, file an issue.
Section: 24.5.3 [istreambuf.iterator] Status: Open Submitter: Niels Dekker Date: 2007-03-25
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Discussion:
Greg Herlihy has clearly demonstrated that a user defined input iterator should have an operator->(), even if its value type is a built-in type (comp.std.c++, "Re: Should any iterator have an operator->() in C++0x?", March 2007). And as Howard Hinnant remarked in the same thread that the input iterator istreambuf_iterator doesn't have one, this must be a defect!
Based on Greg's example, the following code demonstrates the issue:
#include <iostream>
#include <fstream>
#include <streambuf>
typedef char C;
int main ()
{
std::ifstream s("filename", std::ios::in);
std::istreambuf_iterator<char> i(s);
(*i).~C(); // This is well-formed...
i->~C(); // ... so this should be supported!
}
Of course, operator-> is also needed when the value_type of istreambuf_iterator is a class.
The operator-> could be implemented in various ways. For instance, by storing the current value inside the iterator, and returning its address. Or by returning a proxy, like operator_arrow_proxy, from http://www.boost.org/boost/iterator/iterator_facade.hpp
I hope that the resolution of this issue will contribute to getting a clear and consistent definition of iterator concepts.
Proposed resolution:
Add to the synopsis in 24.5.3 [istreambuf.iterator]:
charT operator*() const; pointer operator->() const; istreambuf_iterator<charT,traits>& operator++();
Change 24.5.3 [istreambuf.iterator], p1:
The class template istreambuf_iterator reads successive characters from the streambuf for which it was constructed. operator* provides access to the current input character, if any. operator-> may return a proxy. Each time operator++ is evaluated, the iterator advances to the next input character. If the end of stream is reached (streambuf_type::sgetc() returns traits::eof()), the iterator becomes equal to the end of stream iterator value. The default constructor istreambuf_iterator() and the constructor istreambuf_iterator(0) both construct an end of stream iterator object suitable for use as an end-of-range.
[ Kona (2007): The proposed resolution is inconsistent because the return type of istreambuf_iterator::operator->() is specified to be pointer, but the proposed text also states that "operator-> may return a proxy." ]
[ Niels Dekker (mailed to Howard Hinnant): ]
The proposed resolution does not seem inconsistent to me. istreambuf_iterator::operator->() should have istreambuf_iterator::pointer as return type, and this return type may in fact be a proxy.
AFAIK, the resolution of 445 ("iterator_traits::reference unspecified for some iterator categories") implies that for any iterator class Iter, the return type of operator->() is Iter::pointer, by definition. I don't think Iter::pointer needs to be a raw pointer.
Still I wouldn't mind if the text "operator-> may return a proxy" would be removed from the resolution. I think it's up to the library implementation, how to implement istreambuf_iterator::operator->(). As longs as it behaves as expected: i->m should have the same effect as (*i).m. Even for an explicit destructor call, i->~C(). The main issue is just: istreambuf_iterator should have an operator->()!
Section: 22.2.6.1.2 [locale.money.get.virtuals] Status: Open Submitter: Thomas Plum Date: 2007-04-16
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Discussion:
22.2.6.1.2 [locale.money.get.virtuals], para 1 says:
The result is returned as an integral value stored in units or as a sequence of digits possibly preceded by a minus sign (as produced by ct.widen(c) where c is '-' or in the range from '0' through '9', inclusive) stored in digits.
The following objection has been raised:
Some implementations interpret this to mean that a facet derived from ctype<wchar_t> can provide its own member do_widen(char) which produces e.g. L'@' for the "widened" minus sign, and that the '@' symbol will appear in the resulting sequence of digits. Other implementations have assumed that one or more places in the standard permit the implementation to "hard-wire" L'-' as the "widened" minus sign. Are both interpretations permissible, or only one?
[Plum ref _222612Y14]
Furthermore: if ct.widen('9') produces L'X' (a non-digit), does a parse fail if a '9' appears in the subject string? [Plum ref _22263Y33]
[ Kona (2007): Bill and Dietmar to provide proposed wording. ]
[ post Bellevue: Bill adds: ]
The Standard is clear that the minus sign stored in digits is ct.widen('-'). The subject string must contain characters c in the set [-0123456789] which are translated by ct.widen(c) calls before being stored in digits; the widened characters are not relevant to the parsing of the subject string.
Proposed resolution:
Section: 22.2.6.1.2 [locale.money.get.virtuals] Status: Open Submitter: Thomas Plum Date: 2007-04-16
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Discussion:
22.2.6.1.2 [locale.money.get.virtuals], para 3 says:
If pos or neg is empty, the sign component is optional, and if no sign is detected, the result is given the sign that corresponds to the source of the empty string.
The following objection has been raised:
A negative_sign of "" means "there is no way to write a negative sign" not "any null sequence is a negative sign, so it's always there when you look for it".
[Plum ref _222612Y32]
[ Kona (2007): Bill to provide proposed wording and interpretation of existing wording. ]
Proposed resolution:
Section: 22.2.6.1.2 [locale.money.get.virtuals] Status: Open Submitter: Thomas Plum Date: 2007-04-16
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Discussion:
22.2.6.1.2 [locale.money.get.virtuals], para 3 sentence 4 says:
If the first character of pos is equal to the first character of neg, or if both strings are empty, the result is given a positive sign.
One interpretation is that an input sequence must match either the positive pattern or the negative pattern, and then in either event it is interpreted as positive. The following objections has been raised:
The input can successfully match only a positive sign, so the negative pattern is an unsuccessful match.
[Plum ref _222612Y34, 222612Y51b]
[ Bill to provide proposed wording and interpretation of existing wording. ]
Proposed resolution:
Section: 22.2.6.3 [locale.moneypunct] Status: Open Submitter: Thomas Plum Date: 2007-04-16
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Discussion:
22.2.6.3 [locale.moneypunct], para 2 says:
The value space indicates that at least one space is required at that position.
The following objection has been raised:
Whitespace is optional when matching space. (See 22.2.6.1.2 [locale.money.get.virtuals], para 2.)
[Plum ref _22263Y22]
[ Kona (2007): Bill to provide proposed wording. We agree that C++03 is ambiguous, and that we want C++0X to say "space" means 0 or more whitespace characters on input. ]
Proposed resolution:
Section: 22.2.2.2.2 [facet.num.put.virtuals] Status: Open Submitter: John Salmon Date: 2007-04-20
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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 22.2.2.2.2 [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 22.2.2.2.2 [facet.num.put.virtuals], we find:
For conversion from a floating-point type, if (flags & fixed) != 0 or if str.precision() > 0, then str.precision() is specified in the conversion specification.
This would seem to imply that when floatfield == fixed|scientific, the precision of the conversion specifier is to be taken from str.precision(). Is this really what's intended? I sincerely hope that I'm either missing something or this is an oversight. Please tell me that the committee did not intend to mandate that hex floats (and doubles) should by default be printed as if by %.6a.
[ Howard: I think the fundamental issue we overlooked was that with %f, %e, %g, the default precision was always 6. With %a the default precision is not 6, it is infinity. So for the first time, we need to distinguish between the default value of precision, and the precision value 6. ]
Proposed resolution:
[ Kona (2007): Robert volunteers to propose wording. ]
Section: 20.1.1 [utility.arg.requirements] Status: Ready Submitter: Howard Hinnant Date: 2007-05-04
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Discussion:
The current Swappable is:
Table 37: Swappable requirements [swappable] expression return type post-condition swap(s,t) void t has the value originally held by u, and u has the value originally held by t The Swappable requirement is met by satisfying one or more of the following conditions:
- T is Swappable if T satisfies the CopyConstructible requirements (Table 34) and the CopyAssignable requirements (Table 36);
- T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in this table.
With the passage of rvalue reference into the language, Swappable needs to be updated to require only MoveConstructible and MoveAssignable. This is a minimum.
Additionally we may want to support proxy references such that the following code is acceptable:
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);
I.e. here is a call to swap which the user enables swapping between a proxy to a class and the class itself. We do not need to anything in terms of implementation except not block their way with overly constrained concepts. That is, the Swappable concept should be expanded to allow swapping between two different types for the case that one is binding to a user-defined swap.
Proposed resolution:
Change 20.1.1 [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 is a type 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.
Table 37: Swappable requirements [swappable] expression return type post-condition swap(s,t) void t has the value originally held by u, and u has the value originally held by t The Swappable requirement is met by satisfying one or more of the following conditions:
- T is Swappable if T satisfies the
CopyConstructibleMoveConstructible requirements (Table3433) and theCopyAssignableMoveAssignable requirements (Table3635);- T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in this table.
[ Kona (2007): We like the change to the Swappable requirements to use move semantics. The issue relating to the support of proxies is separable from the one relating to move semantics, and it's bigger than just swap. We'd like to address only the move semantics changes under this issue, and open a separated issue (742) to handle proxies. Also, there may be a third issue, in that the current definition of Swappable does not permit rvalues to be operands to a swap operation, and Howard's proposed resolution would allow the right-most operand to be an rvalue, but it would not allow the left-most operand to be an rvalue (some swap functions in the library have been overloaded to permit left operands to swap to be rvalues). ]
Section: 20.6.5 [unique.ptr] Status: Ready Submitter: Howard Hinnant Date: 2007-05-04
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Discussion:
Since the publication of N1856 there have been a few small but significant advances which should be included into unique_ptr. There exists a example implmenation for all of these changes.
Even though unique_ptr<void> is not a valid use case (unlike for shared_ptr<void>), unexpected cases to crop up which require the instantiation of the interface of unique_ptr<void> even if it is never used. For example see LWG 541 for how this accidently happened to auto_ptr. I believe the most robust way to protect unique_ptr against this type of failure is to augment the return type of unique_ptr<T>:operator*() with add_lvalue_reference<T>::type. This means that given an instantiated unique_ptr<void> the act of dereferencing it will simply return void instead of causing a compile time failure. This is simpler than creating a unique_ptr<void> specialization which isn't robust in the face of cv-qualified void types.
This resolution also supports instantiations such as unique_ptr<void, free_deleter> which could be very useful to the client.
Efforts have been made to better support containers and smart pointers in shared memory contexts. One of the key hurdles in such support is not assuming that a pointer type is actually a T*. This can easily be accomplished for unique_ptr by having the deleter define the pointer type: D::pointer. Furthermore this type can easily be defaulted to T* should the deleter D choose not to define a pointer type (example implementation here). This change has no run time overhead. It has no interface overhead on authors of custom delter types. It simply allows (but not requires) authors of custom deleter types to define a smart pointer for the storage type of unique_ptr if they find such functionality useful. std::default_delete is an example of a deleter which defaults pointer to T* by simply ignoring this issue and not including a pointer typedef.
When the deleter type is a function pointer then it is unsafe to construct a unique_ptr without specifying the function pointer in the constructor. This case is easy to check for with a static_assert assuring that the deleter is not a pointer type in those constructors which do not accept deleters.
unique_ptr<A, void(*)(void*)> p(new A); // error, no function given to delete the pointer!
[ Kona (2007): We don't like the solution given to the first bullet in light of concepts. The second bullet solves the problem of supporting fancy pointers for one library component only. The full LWG needs to decide whether to solve the problem of supporting fancy pointers piecemeal, or whether a paper addressing the whole library is needed. We think that the third bullet is correct. ]
[ Post Kona: Howard adds example user code related to the first bullet: ]
void legacy_code(void*, std::size_t);
void foo(std::size_t N)
{
std::unique_ptr<void, void(*)(void*)> ptr(std::malloc(N), std::free);
legacy_code(ptr.get(), N);
} // unique_ptr used for exception safety purposes
I.e. unique_ptr<void> is a useful tool that we don't want to disable with concepts. The only part of unique_ptr<void> we want to disable (with concepts or by other means) are the two member functions:
T& operator*() const; T* operator->() const;
Proposed resolution:
[ I am grateful for the generous aid of Peter Dimov and Ion Gaztañaga in helping formulate and review the proposed resolutions below. ]
Change 20.6.5.2 [unique.ptr.single]:
template <class T, class D = default_delete<T>> class unique_ptr {
...
T& typename add_lvalue_reference<T>::type operator*() const;
...
};
Change 20.6.5.2.4 [unique.ptr.single.observers]:
T&typename add_lvalue_reference<T>::type operator*() const;
Change 20.6.5.2 [unique.ptr.single]:
template <class T, class D = default_delete<T>> class unique_ptr {
public:
typedef implementation (see description below) pointer;
...
explicit unique_ptr(T* pointer p);
...
unique_ptr(T* pointer p, implementation defined (see description below) d);
unique_ptr(T* pointer p, implementation defined (see description below) d);
...
T* pointer operator->() const;
T* pointer get() const;
...
T* pointer release();
void reset(T* pointer p = 0 pointer());
};
-3- If the type remove_reference<D>::type::pointer exists, then unique_ptr<T, D>::pointer is a typedef to remove_reference<D>::type::pointer. Otherwise unique_ptr<T, D>::pointer is a typedef to T*. The type unique_ptr<T, D>::pointer shall be CopyConstructible and CopyAssignable.
Change 20.6.5.2.1 [unique.ptr.single.ctor]:
unique_ptr(T*pointer p); ... unique_ptr(T*pointer p, implementation defined d); unique_ptr(T*pointer p, implementation defined d); ... unique_ptr(T*pointer p, const A& d); unique_ptr(T*pointer p, A&& d); ... unique_ptr(T*pointer p, A& d); unique_ptr(T*pointer p, A&& d); ... unique_ptr(T*pointer p, const A& d); unique_ptr(T*pointer p, const A&& d); ...
-23- Requires: If D is not a reference type,
construction of the deleter D from an rvalue of type E
must shall be well formed and not throw an exception. If D is a
reference type, then E must shall be the same type as D
(diagnostic required). U* unique_ptr<U,E>::pointer
must shall be implicitly convertible to T*
pointer.
-25- Postconditions: get() == value u.get() had before
the construction, modulo any required offset adjustments resulting from
the cast from U*
unique_ptr<U,E>::pointer to T*
pointer. get_deleter() returns a reference to the
internally stored deleter which was constructed from
u.get_deleter().
Change 20.6.5.2.3 [unique.ptr.single.asgn]:
-8- Requires: Assignment of the deleter D from an rvalue D
mustshall not throw an exception.U*unique_ptr<U,E>::pointermustshall be implicitly convertible toT*pointer.
Change 20.6.5.2.4 [unique.ptr.single.observers]:
...T*pointer operator->() const;T*pointer get() const;
Change 20.6.5.2.5 [unique.ptr.single.modifiers]:
...T*pointer release();void reset(T*pointer p =0pointer());
Change 20.6.5.3 [unique.ptr.runtime]:
template <class T, class D> class unique_ptr<T[], D> {
public:
typedef implementation pointer;
...
explicit unique_ptr(T* pointer p);
...
unique_ptr(T* pointer p, implementation defined d);
unique_ptr(T* pointer p, implementation defined d);
...
T* pointer get() const;
...
T* pointer release();
void reset(T* pointer p = 0 pointer());
};
Change 20.6.5.3.1 [unique.ptr.runtime.ctor]:
unique_ptr(T*pointer p); unique_ptr(T*pointer p, implementation defined d); unique_ptr(T*pointer p, implementation defined d);These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to
T*pointer. [Note: One implementation technique is to create private templated overloads of these members. -- end note]
Change 20.6.5.3.3 [unique.ptr.runtime.modifiers]:
void reset(T*pointer p =0pointer());-1- Requires: Does not accept pointer types which are convertible to
T*pointer (diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]
Change 20.6.5.2.1 [unique.ptr.single.ctor]:
unique_ptr();Requires: D
mustshall be default constructible, and that constructionmustshall not throw an exception. Dmustshall not be a reference type or pointer type (diagnostic required).unique_ptr(T*pointer p);Requires: The expression D()(p)
mustshall be well formed. The default constructor of Dmustshall not throw an exception. Dmustshall not be a reference type or pointer type (diagnostic required).
Change 20.6.5.2.1 [unique.ptr.single.ctor]:
unique_ptr();Requires: D
mustshall be default constructible, and that constructionmustshall not throw an exception. Dmustshall not be a reference type or pointer type (diagnostic required).unique_ptr(T*pointer p);Requires: The expression D()(p)
mustshall be well formed. The default constructor of Dmustshall not throw an exception. Dmustshall not be a reference type or pointer type (diagnostic required).
Section: 23.1 [container.requirements] Status: Open Submitter: Howard Hinnant Date: 2007-05-05
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Discussion:
James Hopkin pointed out to me that if vector<T> move assignment is O(1) (just a swap) then containers such as vector<shared_ptr<ostream>> might have the wrong semantics under move assignment when the source is not truly an rvalue, but a moved-from lvalue (destructors could run late).
vector<shared_ptr<ostream>> v1; vector<shared_ptr<ostream>> v2; ... v1 = v2; // #1 v1 = std::move(v2); // #2
Move semantics means not caring what happens to the source (v2 in this example). It doesn't mean not caring what happens to the target (v1). In the above example both assignments should have the same effect on v1. Any non-shared ostream's v1 owns before the assignment should be closed, whether v1 is undergoing copy assignment or move assignment.
This implies that the semantics of move assignment of a generic container should be clear, swap instead of just swap. An alternative which could achieve the same effect would be to move assign each element. In either case, the complexity of move assignment needs to be relaxed to O(v1.size()).
The performance hit of this change is not nearly as drastic as it sounds. In practice, the target of a move assignment has always just been move constructed or move assigned from. Therefore under clear, swap semantics (in this common use case) we are still achieving O(1) complexity.
Proposed resolution:
Change 23.1 [container.requirements]:
Table 86: Container requirements expression return type operational semantics assertion/note pre/post-condition complexity a = rv; X& All existing elements of a are either move assigned or destructed a shall be equal to the value that rv had before this construction constantlinear in a.size()
[ post Bellevute Howard adds: ]
This issue was voted to WP in Bellevue, but accidently got stepped on by N2525 which was voted to WP simulataneously. Moving back to Open for the purpose of getting the wording right. The intent of this issue and N2525 are not in conflict.
Section: 23.4 [unord] Status: Open Submitter: Howard Hinnant Date: 2007-05-05
View other active issues in [unord].
View all other issues in [unord].
View all issues with Open status.
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".
Proposed resolution:
Add to 23.4 [unord]:
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
unordered_map<Key, T, Hash, Pred, Alloc>& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
unordered_map<Key, T, Hash, Pred, Alloc>&& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_map<Key, T, Hash, Pred, Alloc>&& x,
unordered_map<Key, T, Hash, Pred, Alloc>& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
unordered_multimap<Key, T, Hash, Pred, Alloc>& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
unordered_multimap<Key, T, Hash, Pred, Alloc>&& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>&& x,
unordered_multimap<Key, T, Hash, Pred, Alloc>& y);
...
template <class Value, class Hash, class Pred, class Alloc>
void swap(unordered_set<Value, Hash, Pred, Alloc>& x,
unordered_set<Value, Hash, Pred, Alloc>& y);
template <class Value, class Hash, class Pred, class Alloc>
void swap(unordered_set<Value, Hash, Pred, Alloc>& x,
unordered_set<Value, Hash, Pred, Alloc>&& y);
template <class Value, class Hash, class Pred, class Alloc>
void swap(unordered_set<Value, Hash, Pred, Alloc>&& x,
unordered_set<Value, Hash, Pred, Alloc>& y);
template <class Value, class Hash, class Pred, class Alloc>
void swap(unordered_multiset<Value, Hash, Pred, Alloc>& x,
unordered_multiset<Value, Hash, Pred, Alloc>& y);
template <class Value, class Hash, class Pred, class Alloc>
void swap(unordered_multiset<Value, Hash, Pred, Alloc>& x,
unordered_multiset<Value, Hash, Pred, Alloc>&& y);
template <class Value, class Hash, class Pred, class Alloc>
void swap(unordered_multiset<Value, Hash, Pred, Alloc>&& x,
unordered_multiset<Value, Hash, Pred, Alloc>& y);
unordered_map
Change 23.4.1 [unord.map]:
class unordered_map
{
...
unordered_map(const unordered_map&);
unordered_map(unordered_map&&);
~unordered_map();
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(iterator hint, const value_type& obj);
template <class P> iterator insert(iterator hint, P&& obj);
const_iterator insert(const_iterator hint, const value_type& obj);
template <class P> const_iterator insert(const_iterator hint, P&& obj);
...
void swap(unordered_map&&);
...
mapped_type& operator[](const key_type& k);
mapped_type& operator[](key_type&& k);
...
};
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
unordered_map<Key, T, Hash, Pred, Alloc>& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
unordered_map<Key, T, Hash, Pred, Alloc>&& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_map<Key, T, Hash, Pred, Alloc>&& x,
unordered_map<Key, T, Hash, Pred, Alloc>& y);
Add to 23.4.1.1 [unord.map.cnstr]:
template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n = implementation-defined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());Requires: If the iterator's dereference operator returns an lvalue or a const rvalue pair<key_type, mapped_type>, then both key_type and mapped_type shall be CopyConstructible.
Add to 23.4.1.2 [unord.map.elem]:
mapped_type& operator[](const key_type& k);...
Requires: key_type shall be CopyConstructible and mapped_type shall be DefaultConstructible.
mapped_type& operator[](key_type&& k);Effects: If the unordered_map does not already contain an element whose key is equivalent to k , inserts the value std::pair<const key_type, mapped_type>(std::move(k), mapped_type()).
Requires: mapped_type shall be DefaultConstructible.
Returns: A reference to x.second, where x is the (unique) element whose key is equivalent to k.
Add new section [unord.map.modifiers]:
pair<iterator, bool> insert(const value_type& x); template <class P> pair<iterator, bool> insert(P&& x); iterator insert(iterator hint, const value_type& x); template <class P> iterator insert(iterator hint, P&& x); const_iterator insert(const_iterator hint, const value_type& x); template <class P> const_iterator insert(const_iterator hint, P&& x); template <class InputIterator> void insert(InputIterator first, InputIterator last);Requires: Those signatures taking a const value_type& parameter requires both the key_type and the mapped_type to be CopyConstructible.
P shall be convertible to value_type. If P is instantiated as a reference type, then the argument x is copied from. Otherwise x is considered to be an rvalue as it is converted to value_type and inserted into the unordered_map. Specifically, in such cases CopyConstructible is not required of key_type or mapped_type unless the conversion from P specifically requires it (e.g. if P is a tuple<const key_type, mapped_type>, then key_type must be CopyConstructible).
The signature taking InputIterator parameters requires CopyConstructible of both key_type and mapped_type if the dereferenced InputIterator returns an lvalue or const rvalue value_type.
Add to 23.4.1.3 [unord.map.swap]:
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
unordered_map<Key, T, Hash, Pred, Alloc>& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
unordered_map<Key, T, Hash, Pred, Alloc>&& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_map<Key, T, Hash, Pred, Alloc>&& x,
unordered_map<Key, T, Hash, Pred, Alloc>& y);
unordered_multimap
Change 23.4.2 [unord.multimap]:
class unordered_multimap
{
...
unordered_multimap(const unordered_multimap&);
unordered_multimap(unordered_multimap&&);
~unordered_multimap();
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(iterator hint, const value_type& obj);
template <class P> iterator insert(iterator hint, P&& obj);
const_iterator insert(const_iterator hint, const value_type& obj);
template <class P> const_iterator insert(const_iterator hint, P&& obj);
...
void swap(unordered_multimap&&);
...
};
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
unordered_multimap<Key, T, Hash, Pred, Alloc>& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
unordered_multimap<Key, T, Hash, Pred, Alloc>&& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>&& x,
unordered_multimap<Key, T, Hash, Pred, Alloc>& y);
Add to 23.4.2.1 [unord.multimap.cnstr]:
template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n = implementation-defined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());Requires: If the iterator's dereference operator returns an lvalue or a const rvalue pair<key_type, mapped_type>, then both key_type and mapped_type shall be CopyConstructible.
Add new section [unord.multimap.modifiers]:
iterator insert(const value_type& x); template <class P> iterator insert(P&& x); iterator insert(iterator hint, const value_type& x); template <class P> iterator insert(iterator hint, P&& x); const_iterator insert(const_iterator hint, const value_type& x); template <class P> const_iterator insert(const_iterator hint, P&& x); template <class InputIterator> void insert(InputIterator first, InputIterator last);Requires: Those signatures taking a const value_type& parameter requires both the key_type and the mapped_type to be CopyConstructible.
P shall be convertible to value_type. If P is instantiated as a reference type, then the argument x is copied from. Otherwise x is considered to be an rvalue as it is converted to value_type and inserted into the unordered_multimap. Specifically, in such cases CopyConstructible is not required of key_type or mapped_type unless the conversion from P specifically requires it (e.g. if P is a tuple<const key_type, mapped_type>, then key_type must be CopyConstructible).
The signature taking InputIterator parameters requires CopyConstructible of both key_type and mapped_type if the dereferenced InputIterator returns an lvalue or const rvalue value_type.
Add to 23.4.2.2 [unord.multimap.swap]:
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
unordered_multimap<Key, T, Hash, Pred, Alloc>& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
unordered_multimap<Key, T, Hash, Pred, Alloc>&& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>&& x,
unordered_multimap<Key, T, Hash, Pred, Alloc>& y);
unordered_set
Change 23.4.3 [unord.set]:
class unordered_set
{
...
unordered_set(const unordered_set&);
unordered_set(unordered_set&&);
~unordered_set();
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(iterator hint, const value_type& obj);
iterator insert(iterator hint, value_type&& obj);
const_iterator insert(const_iterator hint, const value_type& obj);
const_iterator insert(const_iterator hint, value_type&& obj);
...
void swap(unordered_set&&);
...
};
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_set<Key, T, Hash, Pred, Alloc>& x,
unordered_set<Key, T, Hash, Pred, Alloc>& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_set<Key, T, Hash, Pred, Alloc>& x,
unordered_set<Key, T, Hash, Pred, Alloc>&& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_set<Key, T, Hash, Pred, Alloc>&& x,
unordered_set<Key, T, Hash, Pred, Alloc>& y);
Add to 23.4.3.1 [unord.set.cnstr]:
template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n = implementation-defined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());Requires: If the iterator's dereference operator returns an lvalue or a const rvalue value_type, then the value_type shall be CopyConstructible.
Add new section [unord.set.modifiers]:
pair<iterator, bool> insert(const value_type& x); pair<iterator, bool> insert(value_type&& x); iterator insert(iterator hint, const value_type& x); iterator insert(iterator hint, value_type&& x); const_iterator insert(const_iterator hint, const value_type& x); const_iterator insert(const_iterator hint, value_type&& x); template <class InputIterator> void insert(InputIterator first, InputIterator last);Requires: Those signatures taking a const value_type& parameter requires the value_type to be CopyConstructible.
The signature taking InputIterator parameters requires CopyConstructible of value_type if the dereferenced InputIterator returns an lvalue or const rvalue value_type.
Add to 23.4.3.2 [unord.set.swap]:
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_set<Key, T, Hash, Pred, Alloc>& x,
unordered_set<Key, T, Hash, Pred, Alloc>& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_set<Key, T, Hash, Pred, Alloc>& x,
unordered_set<Key, T, Hash, Pred, Alloc>&& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_set<Key, T, Hash, Pred, Alloc>&& x,
unordered_set<Key, T, Hash, Pred, Alloc>& y);
unordered_multiset
Change 23.4.4 [unord.multiset]:
class unordered_multiset
{
...
unordered_multiset(const unordered_multiset&);
unordered_multiset(unordered_multiset&&);
~unordered_multiset();
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(iterator hint, const value_type& obj);
iterator insert(iterator hint, value_type&& obj);
const_iterator insert(const_iterator hint, const value_type& obj);
const_iterator insert(const_iterator hint, value_type&& obj);
...
void swap(unordered_multiset&&);
...
};
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multiset<Key, T, Hash, Pred, Alloc>& x,
unordered_multiset<Key, T, Hash, Pred, Alloc>& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multiset<Key, T, Hash, Pred, Alloc>& x,
unordered_multiset<Key, T, Hash, Pred, Alloc>&& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multiset<Key, T, Hash, Pred, Alloc>&& x,
unordered_multiset<Key, T, Hash, Pred, Alloc>& y);
Add to 23.4.4.1 [unord.multiset.cnstr]:
template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n = implementation-defined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());Requires: If the iterator's dereference operator returns an lvalue or a const rvalue value_type, then the value_type shall be CopyConstructible.
Add new section [unord.multiset.modifiers]:
iterator insert(const value_type& x); iterator insert(value_type&& x); iterator insert(iterator hint, const value_type& x); iterator insert(iterator hint, value_type&& x); const_iterator insert(const_iterator hint, const value_type& x); const_iterator insert(const_iterator hint, value_type&& x); template <class InputIterator> void insert(InputIterator first, InputIterator last);Requires: Those signatures taking a const value_type& parameter requires the value_type to be CopyConstructible.
The signature taking InputIterator parameters requires CopyConstructible of value_type if the dereferenced InputIterator returns an lvalue or const rvalue value_type.
Add to 23.4.4.2 [unord.multiset.swap]:
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multiset<Key, T, Hash, Pred, Alloc>& x,
unordered_multiset<Key, T, Hash, Pred, Alloc>& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multiset<Key, T, Hash, Pred, Alloc>& x,
unordered_multiset<Key, T, Hash, Pred, Alloc>&& y);
template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multiset<Key, T, Hash, Pred, Alloc>&& x,
unordered_multiset<Key, T, Hash, Pred, Alloc>& y);
[ 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.
Section: 24.4.1.3.19 [reverse.iter.opdiff], 24.4.3.3.14 [move.iter.nonmember] Status: Ready Submitter: Bo Persson Date: 2007-06-10
View all issues with Ready status.
Discussion:
In C++03 the difference between two reverse_iterators
ri1 - ri2
is possible to compute only if both iterators have the same base iterator. The result type is the difference_type of the base iterator.
In the current draft, the operator is defined as 24.4.1.3.19 [reverse.iter.opdiff]
template<class Iterator1, class Iterator2>
typename reverse_iterator<Iterator>::difference_type
operator-(const reverse_iterator<Iterator1>& x,
const reverse_iterator<Iterator2>& y);
The return type is the same as the C++03 one, based on the no longer present Iterator template parameter.
Besides being slightly invalid, should this operator work only when Iterator1 and Iterator2 has the same difference_type? Or should the implementation choose one of them? Which one?
The same problem now also appears in operator-() for move_iterator 24.4.3.3.14 [move.iter.nonmember].
Proposed resolution:
Change the synopsis in 24.4.1.1 [reverse.iterator]:
template <class Iterator1, class Iterator2>typename reverse_iterator<Iterator>::difference_typeauto operator-( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y) -> decltype(y.current - x.current);
Change 24.4.1.3.19 [reverse.iter.opdiff]:
template <class Iterator1, class Iterator2>typename reverse_iterator<Iterator>::difference_typeauto operator-( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y) -> decltype(y.current - x.current);Returns: y.current - x.current.
Change the synopsis in 24.4.3.1 [move.iterator]:
template <class Iterator1, class Iterator2>typename move_iterator<Iterator>::difference_typeauto operator-( const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y) -> decltype(x.base() - y.base());
Change 24.4.3.3.14 [move.iter.nonmember]:
template <class Iterator1, class Iterator2>typename move_iterator<Iterator>::difference_typeauto operator-( const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y) -> decltype(x.base() - y.base());Returns: x.base() - y.base().
[ Pre Bellevue: This issue needs to wait until the auto -> return language feature goes in. ]
Section: 20.5.5.1 [refwrap.const] Status: Open Submitter: Peter Dimov Date: 2007-05-10
View all other issues in [refwrap.const].
View all issues with Open status.
Discussion:
A reference_wrapper can be constructed from an rvalue, either by using the constructor, or via cref (and ref in some corner cases). This leads to a dangling reference being stored into the reference_wrapper object. Now that we have a mechanism to detect an rvalue, we can fix them to disallow this source of undefined behavior.
Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.
Proposed resolution:
In 20.5 [function.objects], add the following two signatures to the synopsis:
void ref(const T&& t) = delete; void cref(const T&& t) = delete;
[ N2292 addresses the first part of the resolution but not the second. ]
[ Bellevue: Doug noticed problems with the current wording. ]
[ post Bellevue: Howard and Peter provided revised wording. ]
Section: 23.4 [unord], TR1 6.3 [tr.hash] Status: Review Submitter: Joaquín M López Muñoz Date: 2007-06-14
View other active issues in [unord].
View all other issues in [unord].
View all issues with Review status.
Discussion:
The last version of TR1 does not include the following member functions for unordered containers:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
which looks like an oversight to me. I've checked th TR1 issues lists and the latest working draft of the C++0x std (N2284) and haven't found any mention to these menfuns or to their absence.
Is this really an oversight, or am I missing something?
Proposed resolution:
Add the following two rows to table 93 (unordered associative container requirements) in section 23.1.3 [unord.req]:
Unordered associative container requirements (in addition to container) expression return type assertion/note pre/post-condition complexity b.cbegin(n) const_local_iterator n shall be in the range [0, bucket_count()). Note: [b.cbegin(n), b.cend(n)) is a valid range containing all of the elements in the nth bucket. Constant b.cend(n) const_local_iterator n shall be in the range [0, bucket_count()). Constant
Add to the synopsis in 23.4.1 [unord.map]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
Add to the synopsis in 23.4.2 [unord.multimap]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
Add to the synopsis in 23.4.3 [unord.set]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
Add to the synopsis in 23.4.4 [unord.multiset]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
get_money and put_money should be formatted I/O functionsSection: 27.6.4 [ext.manip] Status: New Submitter: Martin Sebor Date: 2007-06-22
View other active issues in [ext.manip].
View all other issues in [ext.manip].
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Discussion:
In a private email Bill Plauger notes:
I believe that the function that implements
get_money[from N2072] should behave as a formatted input function, and the function that implementsput_moneyshould behave as a formatted output function. This has implications regarding the skipping of whitespace and the handling of errors, among other things.The words don't say that right now and I'm far from convinced that such a change is editorial.
Martin's response:
I agree that the manipulators should handle exceptions the same way as formatted I/O functions do. The text in N2072 assumes so but the Returns clause explicitly omits exception handling for the sake of brevity. The spec should be clarified to that effect.
As for dealing with whitespace, I also agree it would make sense for the extractors and inserters involving the new manipulators to treat it the same way as formatted I/O.
Proposed resolution:
Add a new paragraph immediately above p4 of 27.6.4 [ext.manip] with the following text:
Effects: The expression
in >> get_money(mon, intl)described below behaves as a formatted input function (as described in 27.6.1.2.1 [istream.formatted.reqmts]).
Also change p4 of 27.6.4 [ext.manip] as follows:
Returns: An object
sof unspecified type such that ifinis an object of typebasic_istream<charT, traits>then the expressionin >> get_money(mon, intl)behaves as a formatted input function that callsf(in, mon, intl)were called. The functionfcan be defined as...
[ post Bellevue: ]
We recommend moving immediately to Review. We've looked at the issue and have a consensus that the proposed resolution is correct, but want an iostream expert to sign off. Alisdair has taken the action item to putt this up on the reflector for possible movement by Howard to Tenatively Ready.
istream::operator>>(int&) brokenSection: 27.6.1.2.2 [istream.formatted.arithmetic] Status: New Submitter: Martin Sebor Date: 2007-06-23
View all other issues in [istream.formatted.arithmetic].
View all issues with New status.
Discussion:
From message c++std-lib-17897:
The code shown in 27.6.1.2.2 [istream.formatted.arithmetic] as the "as if"
implementation of the two arithmetic extractors that don't have a
corresponding num_get interface (i.e., the
short and int overloads) is subtly buggy in
how it deals with EOF, overflow, and other similar
conditions (in addition to containing a few typos).
One problem is that if num_get::get() reaches the EOF
after reading in an otherwise valid value that exceeds the limits of
the narrower type (but not LONG_MIN or
LONG_MAX), it will set err to
eofbit. Because of the if condition testing for
(err == 0), the extractor won't set
failbit (and presumably, return a bogus value to the
caller).
Another problem with the code is that it never actually sets the
argument to the extracted value. It can't happen after the call to
setstate() since the function may throw, so we need to
show when and how it's done (we can't just punt as say: "it happens
afterwards"). However, it turns out that showing how it's done isn't
quite so easy since the argument is normally left unchanged by the
facet on error except when the error is due to a misplaced thousands
separator, which causes failbit to be set but doesn't
prevent the facet from storing the value.
Proposed resolution:
Section: 19.4.5.1 [syserr.syserr.overview] Status: New Submitter: Daniel Krügler Date: 2007-06-24
View all issues with New status.
Discussion:
In 19.4.5.1 [syserr.syserr.overview] we have the class definition of std::system_error. In contrast to all exception classes, which are constructible with a what_arg string (see 19.1 [std.exceptions], or ios_base::failure in 27.4.2.1.1 [ios::failure]), only overloads with with const string& are possible. For consistency with the re-designed remaining exception classes this class should also provide c'tors which accept a const char* what_arg string.
Please note that this proposed addition makes sense even considering the given implementation hint for what(), because what_arg is required to be set as what_arg of the base class runtime_error, which now has the additional c'tor overload accepting a const char*.
Proposed resolution:
Section: TR1 5.2.1.1 [tr.num.sf.Lnm] Status: New Submitter: Christopher Crawford Date: 2007-06-30
View all issues with New status.
Discussion:
I see that the definition the associated Laguerre polynomials TR1 5.2.1.1 [tr.num.sf.Lnm] has been corrected since N1687. However, the draft standard only specifies ranks of integer value m, while the associated Laguerre polynomials are actually valid for real values of m > -1. In the case of non-integer values of m, the definition Ln(m) = (1/n!)exx-m (d/dx)n (e-xxm+n) must be used, which also holds for integer values of m. See Abramowitz & Stegun, 22.11.6 for the general case, and 22.5.16-17 for the integer case. In fact fractional values are most commonly used in physics, for example to m = +/- 1/2 to describe the harmonic oscillator in 1 dimension, and 1/2, 3/2, 5/2, ... in 3 dimensions.
If I am correct, the calculation of the more general case is no more difficult, and is in fact the function implemented in the GNU Scientific Library. I would urge you to consider upgrading the standard, either adding extra functions for real m or switching the current ones to double.
Proposed resolution:
Section: TR1 5.2.1.2 [tr.num.sf.Plm] Status: New Submitter: Christopher Crawford Date: 2007-06-30
View all issues with New status.
Discussion:
One other small thing, in TR1 5.2.1.2 [tr.num.sf.Plm], the restriction should be |x| <= 1, not x >= 0.
Proposed resolution:
Section: 23.1 [container.requirements] Status: Open Submitter: Howard Hinnant Date: 2007-05-20
View other active issues in [container.requirements].
View all other issues in [container.requirements].
View all issues with Open 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.
| X u(a) | value_type must be CopyConstructible |
| X u(rv) | array requires value_type to be MoveConstructible |
| a = u | Sequences require value_type to be CopyConstructible and CopyAssignable. Associative containers require value_type to be CopyConstructible. |
| a = rv | array requires value_type to be MoveAssignable. Sequences with non-Swappable allocators require value_type to be MoveConstructible and MoveAssignable. Associative containers with non-Swappable allocators require value_type to be MoveConstructible. |
| swap(a,u) | array requires value_type to be Swappable. Sequences with non-Swappable allocators require value_type to be Swappable, MoveConstructible and MoveAssignable. Associative containers with non-Swappable allocators require value_type to be MoveConstructible. |
| X(n) | value_type must be DefaultConstructible |
| X(n, t) | value_type must be CopyConstructible |
| 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.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 sequences vector and deque also require the value_type to be MoveConstructible. |
| a.resize(n, t) | The value_type must be CopyConstructible. |
| 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] |
| 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.. |
| 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.. |
| 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. ]
Proposed resolution:
Section: 22 [localization] Status: Open Submitter: Peter Dimov Date: 2007-07-28
View all other issues in [localization].
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Discussion:
The POSIX "Extended API Set Part 4,"
introduces extensions to the C locale mechanism that allow multiple concurrent locales to be used in the same application by introducing a type locale_t that is very similar to std::locale, and a number of _l functions that make use of it.
The global locale (set by setlocale) is now specified to be per- process. If a thread does not call uselocale, the global locale is in effect for that thread. It can install a per-thread locale by using uselocale.
There is also a nice querylocale mechanism by which one can obtain the name (such as "de_DE") for a specific facet, even for combined locales, with no std::locale equivalent.
std::locale should be harmonized with the new POSIX locale_t mechanism and provide equivalents for uselocale and querylocale.
[ Kona (2007): Bill and Nick to provide wording. ]
Proposed resolution:
Section: 20.6.6.2 [util.smartptr.shared] Status: Ready Submitter: Peter Dimov Date: 2007-08-24
View other active issues in [util.smartptr.shared].
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Discussion:
A discussion on comp.std.c++ has identified a contradiction in the shared_ptr specification. The shared_ptr move constructor and the cast functions are missing postconditions for the get() accessor.
[ Bellevue: ]
Move to "ready", adopting the first (Peter's) proposed resolution.
Note to the project editor: there is an editorial issue here. The wording for the postconditions of the casts is slightly awkward, and the editor should consider rewording "If w is the return value...", e. g. as "For a return value w...".
Proposed resolution:
Add to 20.6.6.2.1 [util.smartptr.shared.const]:
shared_ptr(shared_ptr&& r); template<class Y> shared_ptr(shared_ptr<Y>&& r);Postconditions: *this shall contain the old value of r. r shall be empty. r.get() == 0.
Add to 20.6.6.2.10 [util.smartptr.shared.cast]:
template<class T, class U> shared_ptr<T> static_pointer_cast(shared_ptr<U> const& r);Postconditions: If w is the return value, w.get() == static_cast<T*>(r.get()) && w.use_count() == r.use_count().
template<class T, class U> shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> const& r);Postconditions: If w is the return value, w.get() == dynamic_cast<T*>(r.get()).
template<class T, class U> shared_ptr<T> const_pointer_cast(shared_ptr<U> const& r);Postconditions: If w is the return value, w.get() == const_cast<T*>(r.get()) && w.use_count() == r.use_count().
Alberto Ganesh Barbati has written an alternative proposal where he suggests (among other things) that the casts be respecified in terms of the aliasing constructor as follows:
Change 20.6.6.2.10 [util.smartptr.shared.cast]:
-2- Returns:
If r is empty, an empty shared_ptr<T>; otherwise, a shared_ptr<T> object that stores static_cast<T*>(r.get()) and shares ownership with r.shared_ptr<T>(r, static_cast<T*>(r.get()).
-6- Returns:
When dynamic_cast<T*>(r.get()) returns a nonzero value, a shared_ptr<T> object that stores a copy of it and shares ownership with r;Otherwise, an empty shared_ptr<T> object.- If p = dynamic_cast<T*>(r.get()) is a non-null pointer, shared_ptr<T>(r, p);
- Otherwise, shared_ptr<T>().
-10- Returns:
If r is empty, an empty shared_ptr<T>; otherwise, a shared_ptr<T> object that stores const_cast<T*>(r.get()) and shares ownership with r.shared_ptr<T>(r, const_cast<T*>(r.get()).
This takes care of the missing postconditions for the casts by bringing in the aliasing constructor postcondition "by reference".
Section: 20.6.6.2.5 [util.smartptr.shared.obs] Status: Review Submitter: Peter Dimov Date: 2007-08-24
View all other issues in [util.smartptr.shared.obs].
View all issues with Review status.
Discussion:
A discussion on comp.std.c++ has identified a contradiction in the shared_ptr specification. The note:
[ Note: this constructor allows creation of an empty shared_ptr instance with a non-NULL stored pointer. -end note ]
after the aliasing constructor
template<class Y> shared_ptr(shared_ptr<Y> const& r, T *p);
reflects the intent of N2351 to, well, allow the creation of an empty shared_ptr with a non-NULL stored pointer.
This is contradicted by the second sentence in the Returns clause of 20.6.6.2.5 [util.smartptr.shared.obs]:
T* get() const;Returns: the stored pointer. Returns a null pointer if *this is empty.
[ Bellevue: ]
Adopt option 1 and move to review, not ready.
There was a lot of confusion about what an empty shared_ptr is (the term isn't defined anywhere), and whether we have a good mental model for how one behaves. We think it might be possible to deduce what the definition should be, but the words just aren't there. We need to open an issue on the use of this undefined term. (The resolution of that issue might affect the resolution of issue 711.)
The LWG is getting more uncomfortable with the aliasing proposal (N2351) now that we realize some of its implications, and we need to keep an eye on it, but there isn't support for removing this feature at this time.
Proposed resolution:
In keeping the N2351 spirit and obviously my preference, change 20.6.6.2.5 [util.smartptr.shared.obs]:
T* get() const;Returns: the stored pointer.
Returns a null pointer if *this is empty.
Alternative proposed resolution: (I won't be happy if we do this, but it's possible):
Change 20.6.6.2.1 [util.smartptr.shared.const]:
template<class Y> shared_ptr(shared_ptr<Y> const& r, T *p);Requires: If r is empty, p shall be 0.
[ Note: this constructor allows creation of an empty shared_ptr instance with a non-NULL stored pointer. -- end note ]
Section: 25.3.1.1 [sort] Status: New Submitter: Matt Austern Date: 2007-08-30
View all issues with New status.
Discussion:
The complexity of sort() is specified as "Approximately N log(N) (where N == last - first ) comparisons on the average", with no worst case complicity specified. The intention was to allow a median-of-three quicksort implementation, which is usually O(N log N) but can be quadratic for pathological inputs. However, there is no longer any reason to allow implementers the freedom to have a worst-cast-quadratic sort algorithm. Implementers who want to use quicksort can use a variant like David Musser's "Introsort" (Software Practice and Experience 27:983-993, 1997), which is guaranteed to be O(N log N) in the worst case without incurring additional overhead in the average case. Most C++ library implementers already do this, and there is no reason not to guarantee it in the standard.
Proposed resolution:
In 25.3.1.1 [sort], change the complexity to "O(N log N)", and remove footnote 266:
Complexity:
ApproximatelyO(N log(N))(where N == last - first )comparisonson the average.266)
266) If the worst case behavior is important stable_sort() (25.3.1.2) or partial_sort() (25.3.1.3) should be used.
Section: 25.1.9 [alg.search] Status: New Submitter: Matt Austern Date: 2007-08-30
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Discussion:
The complexity for search_n (25.1.9 [alg.search] par 7) is specified as "At most (last - first ) * count applications of the corresponding predicate if count is positive, or 0 otherwise." This is unnecessarily pessimistic. Regardless of the value of count, there is no reason to examine any element in the range more than once.
Proposed resolution:
Change the complexity to "At most (last - first) applications of the corresponding predicate".
template<class ForwardIterator, class Size, class T> ForwardIterator search_n(ForwardIterator first , ForwardIterator last , Size count , const T& value ); template<class ForwardIterator, class Size, class T, class BinaryPredicate> ForwardIterator search_n(ForwardIterator first , ForwardIterator last , Size count , const T& value , BinaryPredicate pred );Complexity: At most (last - first )
* countapplications of the corresponding predicateif count is positive, or 0 otherwise.
Section: 25.3.7 [alg.min.max] Status: Ready Submitter: Matt Austern Date: 2007-08-30
View other active issues in [alg.min.max].
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Discussion:
The complexity for minmax_element (25.3.7 [alg.min.max] par 16) says "At most max(2 * (last - first ) - 2, 0) applications of the corresponding comparisons", i.e. the worst case complexity is no better than calling min_element and max_element separately. This is gratuitously inefficient. There is a well known technique that does better: see section 9.1 of CLRS (Introduction to Algorithms, by Cormen, Leiserson, Rivest, and Stein).
Proposed resolution:
Change 25.3.7 [alg.min.max] to:
template<class ForwardIterator> pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first , ForwardIterator last); template<class ForwardIterator, class Compare> pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first , ForwardIterator last , Compare comp);Returns: make_pair(m, M), where m is
min_element(first, last) or min_element(first, last, comp)the first iterator in [first, last) such that no iterator in the range refers to a smaller element, and where M ismax_element(first, last) or max_element(first, last, comp)the last iterator in [first, last) such that no iterator in the range refers to a larger element.Complexity: At most
max(2 * (last - first ) - 2, 0)max(⌊(3/2) (N-1)⌋, 0) applications of the correspondingcomparisonspredicate, where N is distance(first, last).
Section: 28.13 [re.grammar] Status: New Submitter: Stephan T. Lavavej Date: 2007-08-31
View all issues with New status.
Discussion:
TR1 7.13 [tr.re.grammar]/3 and C++0x WP 28.13 [re.grammar]/3 say:
The following productions within the ECMAScript grammar are modified as follows:
CharacterClass :: [ [lookahead ∉ {^}] ClassRanges ] [ ^ ClassRanges ]
This definition for CharacterClass appears to be exactly identical to that in ECMA-262.
Was an actual modification intended here and accidentally omitted, or was this production accidentally included?
Proposed resolution:
Remove this mention of the CharacterClass production.
CharacterClass :: [ [lookahead ∉ {^}] ClassRanges ] [ ^ ClassRanges ]
Section: 21.3 [basic.string] Status: Open Submitter: Bo Persson Date: 2007-08-18
View other active issues in [basic.string].
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Discussion:
Paragraph 21.3 [basic.string]/3 states:
The class template basic_string conforms to the requirements for a Sequence (23.1.1) and for a Reversible Container (23.1).
First of all, 23.1.1 [sequence.reqmts] is no longer "Sequence" but "Sequence container". Secondly, after the resent changes to containers (emplace, push_back, const_iterator parameters to insert and erase), basic_string is not even close to conform to the current requirements.
[ Bellevue: ]
- emplace, for example, may not make sense for strings. Is also likely suboptimal
- with concepts do we need to maintain string as sequence container?
- One approach might be to say something like: string is a sequence except it doesn't have these functions
General consensus is to suggest option 2.
- basic_string already has push_back
- const_iterator parameters to insert and erase should be added to basic_string
- this leaves emplace to handle -- we have the following options:
- option 1: add it to string even though it's optional
- option 2: make emplace optional to sequences (move from table 89 to 90)
- option 3: say string not sequence (the proposal),
- option 4: add an exception to basic string wording.
Proposed resolution:
Remove this sentence, in recognition of the fact that basic_string is not just a vector-light for literal types, but something quite different, a string abstraction in its own right.
Section: 20.4 [meta] Status: Open Submitter: Daniel Krügler Date: 2007-08-25
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Discussion:
Since the inclusion of constexpr in the standard draft N2369 we have a new type category "literal", which is defined in 3.9 [basic.types]/p.11:
-11- A type is a literal type if it is:
- a scalar type; or
a class type (clause 9) with
- a trivial copy constructor,
- a trivial destructor,
- at least one constexpr constructor other than the copy constructor,
- no virtual base classes, and
- all non-static data members and base classes of literal types; or
- an array of literal type.
I strongly suggest that the standard provides a type traits for literal types in 20.4.4.3 [meta.unary.prop] for several reasons:
The special problem of reason (c) is that I don't see currently a way to portably test the condition for literal class types:
- at least one constexpr constructor other than the copy constructor,
Here follows a simply example to demonstrate it's usefulness:
template <typename T>
constexpr typename std::enable_if<std::is_literal<T>::value, T>::type
abs(T x) {
return x < T() ? -x : x;
}
template <typename T>
typename std::enable_if<!std::is_literal<T>::value, T>::type
abs(const T& x) {
return x < T() ? -x : x;
}
Here we have the possibility to provide a general abs function template that can be used in ICE's if it's argument is a literal type which's value is a constant expression, otherwise we have an optimized version for arguments which are expensive to copy and therefore need the usage of arguments of reference type (instead of const T& we could decide to use T&&, but that is another issue).
[ Alisdair is considering preparing a paper listing a number of missing type traits, and feels that it might be useful to handle them all together rather than piecemeal. This would affect issue 719 and 750. These two issues should move to OPEN pending AM paper on type traits. ]
Proposed resolution:
In 20.4.2 [meta.type.synop] in the group "type properties", just below the line
template <class T> struct is_pod;
add a new one:
template <class T> struct is_literal;
In 20.4.4.3 [meta.unary.prop], table Type Property Predicates, just below the line for the is_pod property add a new line:
| Template | Condition | Preconditions |
|---|---|---|
| template <class T> struct is_literal; | T is a literal type (3.9) | T shall be a complete type, an array of unknown bound, or (possibly cv-qualified) void. |
Section: 23.2.1 [array], 23.3.5 [template.bitset] Status: Open Submitter: Daniel Krügler Date: 2007-08-25
View other active issues in [array].
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Discussion:
Proposed resolution:
In the class template definition of 23.2.1 [array]/p. 3 change
constexpr bool empty() const;
In the class template definition of 23.3.5 [template.bitset]/p. 1 change
constexpr bool test(size_t pos ) const;
and in 23.3.5.2 [bitset.members] change
constexpr bool test(size_t pos ) const;
Section: 22.1.3.2.2 [conversions.string] Status: New Submitter: Bo Persson Date: 2007-08-27
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Discussion:
Paragraph 3 says that the Codecvt template parameter shall meet the requirements of std::codecvt, even though std::codecvt itself cannot be used (because of a protected destructor).
How are we going to explain this code to beginning programmers?
template<class I, class E, class S>
struct codecvt : std::codecvt<I, E, S>
{
~codecvt()
{ }
};
void main()
{
std::wstring_convert<codecvt<wchar_t, char, std::mbstate_t> > compiles_ok;
std::wstring_convert<std::codecvt<wchar_t, char, std::mbstate_t> > not_ok;
}
Proposed resolution:
Section: 26.7 [c.math] Status: Ready Submitter: Daniel Krügler Date: 2007-08-27
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Discussion:
In the listing of 26.7 [c.math], table 108: Header <cmath> synopsis I miss the following C99 functions (from 7.12.11.2):
float nanf(const char *tagp); long double nanl(const char *tagp);
(Note: These functions cannot be overloaded and they are also not listed anywhere else)
Proposed resolution:
In 26.7 [c.math], table 108, section "Functions", add nanf and nanl just after the existing entry nan.
Section: 28.8 [re.regex] Status: New Submitter: Daniel Krügler Date: 2007-08-29
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Discussion:
According to the current state of the standard draft, the class template basic_regex, as described in 28.8 [re.regex]/3, is neither MoveConstructible nor MoveAssignable. IMO it should be, because typical regex state machines tend to have a rather large data quantum and I have seen several use cases, where a factory function returns regex values, which would take advantage of moveabilities.
Proposed resolution:
In the header <regex> synopsis 28.4 [re.syn], just below the function template swap add two further overloads:
template <class charT, class traits> void swap(basic_regex<charT, traits>& e1, basic_regex<charT, traits>& e2); template <class charT, class traits> void swap(basic_regex<charT, traits>&& e1, basic_regex<charT, traits>& e2); template <class charT, class traits> void swap(basic_regex<charT, traits>& e1, basic_regex<charT, traits>&& e2);
In the class definition of basic_regex, just below 28.8 [re.regex]/3, perform the following changes:
Just after the copy c'tor:
basic_regex(basic_regex&&);
Just after the copy-assignment op.:
basic_regex& operator=(basic_regex&&);
Just after the first assign overload insert:
basic_regex& assign(basic_regex&& that);
Change the current swap function to read:
void swap(basic_regex&&);
In 28.8.2 [re.regex.construct], just below the copy c'tor add a corresponding member definition of:
basic_regex(basic_regex&&);
Also in 28.8.2 [re.regex.construct], just below the copy assignment c'tor add a corresponding member definition of:
basic_regex& operator=(basic_regex&&);
In 28.8.3 [re.regex.assign], just below the first assign overload add a corresponding member definition of:
basic_regex& assign(basic_regex&& that);
In 28.8.6 [re.regex.swap], change the signature of swap to say:
void swap(basic_regex&& e);
In 28.8.7.1 [re.regex.nmswap], just below the single binary swap function, add the two missing overloads:
template <class charT, class traits> void swap(basic_regex<charT, traits>&& e1, basic_regex<charT, traits>& e2); template <class charT, class traits> void swap(basic_regex<charT, traits>& e1, basic_regex<charT, traits>&& e2);
Of course there would be need of corresponding proper standardese to describe these additions.
Section: 20.1.1 [utility.arg.requirements] Status: Open Submitter: Pablo Halpern Date: 2007-09-12
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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.
Proposed resolution:
In section 20.1.1 [utility.arg.requirements], before table 33, add the following table:
Table 33: DefaultConstructible requirements
|
expression |
post-condition |
|
T
t; |
T is default constructed. |
Section: 28.11.4 [re.alg.replace] Status: New Submitter: Stephan T. Lavavej Date: 2007-09-22
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Discussion:
Two overloads of regex_replace() are currently provided:
template <class OutputIterator, class BidirectionalIterator,
class traits, class charT>
OutputIterator
regex_replace(OutputIterator out,
BidirectionalIterator first, BidirectionalIterator last,
const basic_regex<charT, traits>& e,
const basic_string<charT>& fmt,
regex_constants::match_flag_type flags =
regex_constants::match_default);
template <class traits, class charT>
basic_string<charT>
regex_replace(const basic_string<charT>& s,
const basic_regex<charT, traits>& e,
const basic_string<charT>& fmt,
regex_constants::match_flag_type flags =
regex_constants::match_default);
The absence of const charT * overloads prevents ordinary-looking code from compiling, such as:
const string s("kitten");
const regex r("en");
cout << regex_replace(s, r, "y") << endl;
The compiler error message will be something like "could not deduce template argument for 'const std::basic_string<_Elem> &' from 'const char[1]'".
Users expect that anything taking a basic_string<charT> can also take a const charT *. In their own code, when they write a function taking std::string (or std::wstring), they can pass a const char * (or const wchar_t *), thanks to basic_string's implicit constructor. Because the regex algorithms are templated on charT, they can't rely on basic_string's implicit constructor (as the compiler error message indicates, template argument deduction fails first).
If a user figures out what the compiler error message means, workarounds are available - but they are all verbose. Explicit template arguments could be given to regex_replace(), allowing basic_string's implicit constructor to be invoked - but charT is the last template argument, not the first, so this would be extremely verbose. Therefore, constructing a basic_string from each C string is the simplest workaround.
Proposed resolution:
Provide additional overloads for regex_replace(): one additional overload of the iterator-based form (taking const charT* fmt), and three additional overloads of the convenience form (one taking const charT* str, another taking const charT* fmt, and the third taking both const charT* str and const charT* fmt). 28.11.4 [re.alg.replace]:
template <class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const basic_string<charT>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);...
template <class traits, class charT> basic_string<charT> regex_replace(const basic_string<charT>& s, const basic_regex<charT, traits>& e, const basic_string<charT>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const basic_string<charT>& s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const basic_string<charT>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);
Section: 28.11.4 [re.alg.replace] Status: New Submitter: Stephan T. Lavavej Date: 2007-09-22
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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.
Proposed resolution:
Overloads of regex_replace() taking basic_string should be additionally templated on class ST, class SA and take const basic_string<charT, ST, SA>&. Consistency with regex_match() and regex_search() would place class ST, class SA as the first template arguments; compatibility with existing code using TR1 and giving explicit template arguments to regex_replace() would place class ST, class SA as the last template arguments.
Section: 26.4.3.2 [rand.eng.mers] Status: Review Submitter: Stephan Tolksdorf Date: 2007-09-21
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Discussion:
The mersenne_twister_engine is required to use a seeding method that is given as an algorithm parameterized over the number of bits W. I doubt whether the given generalization of an algorithm that was originally developed only for unsigned 32-bit integers is appropriate for other bit widths. For instance, W could be theoretically 16 and UIntType a 16-bit integer, in which case the given multiplier would not fit into the UIntType. Moreover, T. Nishimura and M. Matsumoto have chosen a dif ferent multiplier for their 64 bit Mersenne Twister [reference].
I see two possible resolutions:
See N2424 for further discussion.
[ Bellevue: ]
Stephan Tolksdorf has additional comments on N2424. He comments: "there is a typo in the required behaviour for mt19937_64: It should be the 10000th (not 100000th) invocation whose value is given, and the value should be 9981545732273789042 (not 14002232017267485025)." These values need checking.
Take the proposed recommendation in N2424 and move to REVIEW.
Proposed resolution:
See N2424 for the proposed resolution.
[ Stephan Tolksdorf adds pre-Bellevue: ]
I support the proposed resolution in N2424, but there is a typo in the required behaviour for mt19937_64: It should be the 10000th (not 100000th) invocation whose value is given, and the value should be 9981545732273789042 (not 14002232017267485025). The change to para. 8 proposed by Charles Karney should also be included in the proposed wording.
Section: 26.4.8.5.3 [rand.dist.samp.genpdf] Status: Open Submitter: Stephan Tolksdorf Date: 2007-09-21
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Duplicate of: 795
Discussion:
26.4.8.5.3 [rand.dist.samp.genpdf] describes the interface for a distribution template that is meant to simulate random numbers from any general distribution given only the density and the support of the distribution. I'm not aware of any general purpose algorithm that would be capable of correctly and efficiently implementing the described functionality. From what I know, this is essentially an unsolved research problem. Existing algorithms either require more knowledge about the distribution and the problem domain or work only under very limited circumstances. Even the state of the art special purpose library UNU.RAN does not solve the problem in full generality, and in any case, testing and customer support for such a library feature would be a nightmare.
Possible resolution: For these reasons, I propose to delete section 26.4.8.5.3 [rand.dist.samp.genpdf].
[ Bellevue: ]
Disagreement persists.
Objection to this issue is that this function takes a general functor. The general approach would be to normalize this function, integrate it, and take the inverse of the integral, which is not possible in general. An example function is sin(1+n*x) -- for any spatial frequency that the implementor chooses, there is a value of n that renders that choice arbitrarily erroneous.
Correction: The formula above should instead read 1+sin(n*x).
Objector proposes the following possible compromise positions:
- rand.dist.samp.genpdf takes an number of points so that implementor need not guess.
- replace rand.disk.samp.genpdf with an extension to either or both of the discrete functions to take arguments that take a functor and number of points in place of the list of probabilities. Reference issues 793 and 794.
Proposed resolution:
See N2424 for the proposed resolution.
Section: 26.4.8.4.3 [rand.dist.norm.chisq] Status: Open Submitter: Stephan Tolksdorf Date: 2007-09-21
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Discussion:
chi_squared_distribution, fisher_f_distribution and student_t_distribution have parameters for the "degrees of freedom" n and m that are specified as integers. For the following two reasons this is an unnecessary restriction: First, in many applications such as Bayesian inference or Monte Carlo simulations it is more convenient to treat the respective param- eters as continuous variables. Second, the standard non-naive algorithms (i.e. O(1) algorithms) for simulating from these distributions work with floating-point parameters anyway (all three distributions could be easily implemented using the Gamma distribution, for instance).
Similar arguments could in principle be made for the parameters t and k of the discrete binomial_distribution and negative_binomial_distribution, though in both cases continuous parameters are less frequently used in practice and in case of the binomial_distribution the implementation would be significantly complicated by a non-discrete parameter (in most implementations one would need an approximation of the log-gamma function instead of just the log-factorial function).
Possible resolution: For these reasons, I propose to change the type of the respective parameters to double.
[ Bellevue: ]
In N2424. Not wildly enthusiastic, not really felt necessary. Less frequently used in practice. Not terribly bad either. Move to OPEN.
Proposed resolution:
See N2424 for the proposed resolution.
[ Stephan Tolksdorf adds pre-Bellevue: ]
In 26.4.8.4.3 [rand.dist.norm.chisq]:
Delete ", where n is a positive integer" in the first paragraph.
Replace both occurrences of "explicit chi_squared_distribution(int n = 1);" with "explicit chi_squared_distribution(RealType n = 1);".
Replace both occurrences of "int n() const;" with "RealType n() const;".
In 26.4.8.4.5 [rand.dist.norm.f]:
Delete ", where m and n are positive integers" in the first paragraph.
Replace both occurrences of
explicit fisher_f_distribution(int m = 1, int n = 1);with
explicit fisher_f_distribution(RealType m = 1, RealType n = 1);Replace both occurrences of "int m() const;" with "RealType m() const;".
Replace both occurrences of "int n() const;" with "RealType n() const;".
In 26.4.8.4.6 [rand.dist.norm.t]:
Delete ", where n is a positive integer" in the first paragraph.
Replace both occurrences of "explicit student_t_distribution(int n = 1);" with "explicit student_t_distribution(RealType n = 1);".
Replace both occurrences of "int n() const;" with "RealType n() const;".
Section: 20.6.5.4 [unique.ptr.compiletime] Status: Ready Submitter: Herb Sutter Date: 2007-10-04
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Discussion:
Please don't provide *_ptr<T[N]>. It doesn't enable any useful bounds-checking (e.g., you could imagine that doing op++ on a shared_ptr<T[N]> yields a shared_ptr<T[N-1]>, but that promising path immediately falters on op-- which can't reliably dereference because we don't know the lower bound). Also, most buffers you'd want to point to don't have a compile-time known size.
To enable any bounds-checking would require run-time information, with the usual triplet: base (lower bound), current offset, and max offset (upper bound). And I can sympathize with the point of view that you wouldn't want to require this on *_ptr itself. But please let's not follow the <T[N]> path, especially not with additional functions to query the bounds etc., because this sets wrong user expectations by embarking on a path that doesn't go all the way to bounds checking as it seems to imply.
If bounds checking is desired, consider a checked_*_ptr instead (e.g., checked_shared_ptr). And make the interfaces otherwise identical so that user code could easily #define/typedef between prepending checked_ on debug builds and not doing so on release builds (for example).
Note that some may object that checked_*_ptr may seem to make the smart pointer more like vector, and we don't want two ways to spell vector. I don't agree, but if that were true that would be another reason to remove *_ptr<T[N]> which equally makes the smart pointer more like std::array. :-)
[ Bellevue: ]
Suggestion that fixed-size array instantiations are going to fail at compile time anyway (if we remove specialization) due to pointer decay, at least that appears to be result from available compilers.
So concerns about about requiring static_assert seem unfounded.
After a little more experimentation with compiler, it appears that fixed size arrays would only work at all if we supply these explicit specialization. So removing them appears less breaking than originally thought.
straw poll unanimous move to Ready.
Proposed resolution:
Change the synopsis under 20.6.5 [unique.ptr] p2:
... template<class T> struct default_delete; template<class T> struct default_delete<T[]>;template<class T, size_t N> struct default_delete<T[N]>;template<class T, class D = default_delete<T>> class unique_ptr; template<class T, class D> class unique_ptr<T[], D>;template<class T, class D, size_t N> class unique_ptr<T[N], D>;...
Remove the entire section 20.6.5.1.3 [unique.ptr.dltr.dflt2] default_delete<T[N]>.
Remove the entire section 20.6.5.4 [unique.ptr.compiletime] unique_ptr for array objects with a compile time length and its subsections: 20.6.5.4.1 [unique.ptr.compiletime.dtor], 20.6.5.4.2 [unique.ptr.compiletime.observers], 20.6.5.4.3 [unique.ptr.compiletime.modifiers].
Section: 20.1.1 [utility.arg.requirements] Status: Open Submitter: Howard Hinnant Date: 2007-10-10
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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?
Proposed resolution:
Change 20.1.1 [utility.arg.requirements]:
-1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables, T and V are
is atypes to be supplied by a C++ program instantiating a template; a, b, and c are values of type const T; s and t are modifiable lvalues of type T; u is a value of type (possibly const) T;andrv is a non-const rvalue of type T; w is a value of type T; and v is a value of type V.
Table 37: Swappable requirements [swappable] expression return type post-condition swap( sw,tv)void tw has the value originally held byuv, anduv has the value originally held bytwThe Swappable requirement is met by satisfying one or more of the following conditions:
- T is Swappable if T and V are the same type and T satisfies the
CopyConstructibleMoveConstructible requirements (Table3433) and theCopyAssignableMoveAssignable requirements (Table3635);- 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(
tw,uv) is valid and has the semantics described in this table.
Section: 20.6.6.2.9 [util.smartptr.shared.spec] Status: Ready Submitter: Howard Hinnant Date: 2007-10-10
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Discussion:
When the LWG looked at 674 in Kona the following note was made:
We may need to open an issue to deal with the question of whether shared_ptr needs an rvalue swap.
This issue was opened in response to that note.
I believe allowing rvalue shared_ptrs to swap is both appropriate, and consistent with how other library components are currently specified.
[ Bellevue: ]
Concern that the three signatures for swap is needlessly complicated, but this issue merely brings shared_ptr into equal complexity with the rest of the library. Will open a new issue for concern about triplicate signatures.
Adopt issue as written.
Proposed resolution:
Change the synopsis in 20.6.6.2 [util.smartptr.shared]:
void swap(shared_ptr&& r); ... template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>&& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>&& b);
Change 20.6.6.2.4 [util.smartptr.shared.mod]:
void swap(shared_ptr&& r);
Change 20.6.6.2.9 [util.smartptr.shared.spec]:
template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>&& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>&& b);
Section: 18.7.5 [propagation] Status: Ready Submitter: Alisdair Meredith Date: 2007-10-10
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Discussion:
Without some lifetime guarantee, it is hard to know how this type can be used. Very specifically, I don't see how the current wording would guarantee and exception_ptr caught at the end of one thread could be safely stored and rethrown in another thread - the original motivation for this API.
(Peter Dimov agreed it should be clearer, maybe a non-normative note to explain?)
[ Bellevue: ]
Agree the issue is real.
Intent is lifetime is similar to a shared_ptr (and we might even want to consider explicitly saying that it is a shared_ptr< unspecified type >).
We expect that most implementations will use shared_ptr, and the standard should be clear that the exception_ptr type is intended to be something whose semantics are smart-pointer-like so that the user does not need to worry about lifetime management. We still need someone to draught those words - suggest emailing Peter Dimov.
Move to Open.
Proposed resolution:
Change 18.7.5 [propagation]/7:
-7- Returns: An exception_ptr object that refers to the currently handled exception or a copy of the currently handled exception, or a null exception_ptr object if no exception is being handled. The referenced object remains valid at least as long as there is an exception_ptr that refers to it. If the function needs to allocate memory and the attempt fails, it returns an exception_ptr object that refers to an instance of bad_alloc. It is unspecified whether the return values of two successive calls to current_exception refer to the same exception object. [Note: that is, it is unspecified whether current_exception creates a new copy each time it is called. --end note]
Section: 18.7.5 [propagation] Status: Ready Submitter: Alisdair Meredith Date: 2007-10-10
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Discussion:
I understand that the attempt to copy an exception may run out of memory, but I believe this is the only part of the standard that mandates failure with specifically bad_alloc, as opposed to allowing an implementation-defined type derived from bad_alloc. For instance, the Core language for a failed new expression is:
Any other allocation function that fails to allocate storage shall indicate failure only by throwing an exception of a type that would match a handler (15.3) of type std::bad_alloc (18.5.2.1).
I think we should allow similar freedom here (or add a blanket compatible-exception freedom paragraph in 17)
I prefer the clause 17 approach myself, and maybe clean up any outstanding wording that could also rely on it.
Although filed against a specific case, this issue is a problem throughout the library.
[ Bellevue: ]
Is issue bigger than library?
No - Core are already very clear about their wording, which is inspiration for the issue.
While not sold on the original 18.7.5 use case, the generalised 17.4.4.8 wording is the real issue.
Accept the broad view and move to ready
Proposed resolution:
Add the following exemption clause to 17.4.4.8 [res.on.exception.handling]:
A function may throw a type not listed in its Throws clause so long as it is derived from a class named in the Throws clause, and would be caught by an exception handler for the base type.
Section: 20.4.4.3 [meta.unary.prop] Status: Open Submitter: Alisdair Meredith Date: 2007-10-10
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Discussion:
We have 3 separate type traits to identify classes supporting no-throw operations, which are very useful when trying to provide exception safety guarantees. However, I'm not entirely clear on what the current wording requires of a conforming implementation. To quote from has_nothrow_default_constructor:
or T is a class type with a default constructor that is known not to throw any exceptions
What level of magic do we expect to deduce if this is known?
E.g.
struct test{
int x;
test() : x() {}
};
Should I expect a conforming compiler to assert( has_nothrow_constructor<test>::value )
Is this a QoI issue?
Should I expect to 'know' only if-and-only-if there is an inline definition available?
Should I never expect that to be true, and insist that the user supplies an empty throw spec if they want to assert the no-throw guarantee?
It would be helpful to maybe have a footnote explaining what is required, but right now I don't know what to suggest putting in the footnote.
(agreement since is that trivial ops and explicit no-throws are required. Open if QoI should be allowed to detect further)
[ Bellevue: ]
This looks like a QoI issue. In the case of trivial and nothrow it is known. Static analysis of the program is definitely into QoI. Move to OPEN. Need to talk to Core about this.
Proposed resolution:
Section: 20.4.4.3 [meta.unary.prop] Status: Ready Submitter: Alisdair Meredith Date: 2007-10-10
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Discussion:
Unfortunately a class can have multiple copy constructors, and I believe to be useful this trait should only return true is ALL copy constructors are no-throw.
For instance:
struct awkward {
awkward( const awkward & ) throw() {}
awkward( awkward & ) { throw "oops"; } };
Proposed resolution:
Change 20.4.4.3 [meta.unary.prop]:
has_trivial_copy_constructorT is a trivial type (3.9) or a reference type or a class typewith a trivial copy constructorwhere all copy constructors are trivial (12.8).
has_trivial_assignT is neither const nor a reference type, and T is a trivial type (3.9) or a class typewith a trivial copy assignment operatorwhere all copy assignment operators are trivial (12.8).
has_nothrow_copy_constructorhas_trivial_copy_constructor<T>::value is true or T is a class typewith awhere all copy constructorsthat isare known not to throw any exceptions or T is an array of such a class type
has_nothrow_assignT is neither const nor a reference type, and has_trivial_assign<T>::value is true or T is a class typewith awhere all copy assignment operators takeingan lvalue of type T that is known not to throw any exceptions or T is an array of such a class type.
Section: 20.4.5 [meta.rel] Status: Open Submitter: Alisdair Meredith Date: 2007-10-10
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Discussion:
With the pending arrival of explicit conversion functions though, I'm wondering if we want an additional trait, is_explictly_convertible?
[ Bellevue: ]
Alisdair is considering preparing a paper listing a number of missing type traits, and feels that it might be useful to handle them all together rather than piecemeal. This would affect issue 719 and 750. These two issues should move to OPEN pending AM paper on type traits.
Proposed resolution:
Section: 23.2.6 [vector.bool] Status: New Submitter: Alisdair Meredith Date: 2007-10-10
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Discussion:
A number of vector<bool> members take const bool& as arguments. Is there any chance we could change them to pass-by-value or would I be wasting everyone's time if wrote up an issue?
[ post Bellevue: ]
As we understand it, the original requester (Martin Sebor) would like for implementations to be permitted to pass-by-value. Alisdair suggests that if this is to be resolved, it should be resolved more generally, e.g. in other containers as well.
We note that this would break ABI. However, we also suspect that this might be covered under the "as-if" rule in section 1.9.
Many in the group feel that for vector<bool>, this is a "don't care", and that at this point in the process it's not worth the bandwidth.
Issue 679 -- which was in ready status pre-Bellevue and is now in the working paper -- is related to this, though not a duplicate.
Moving to Open with a task for Alisdair to craft a informative note to be put whereever appropriate in the WP. This note would clarify places where pass-by-const-ref can be transformed to pass-by-value under the as-if rule.
Proposed resolution:
Section: 20.1.2 [allocator.requirements] Status: New Submitter: Hans Boehm Date: 2007-10-11
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Discussion:
Did LWG recently discuss 20.1.2 [allocator.requirements]-2, which states that "All the operations on the allocators are expected to be amortized constant time."?
As I think I pointed out earlier, this is currently fiction for allocate() if it has to obtain memory from the OS, and it's unclear to me how to interpret this for construct() and destroy() if they deal with large objects. Would it be controversial to officially let these take time linear in the size of the object, as they already do in real life?
Allocate() more blatantly takes time proportional to the size of the object if you mix in GC. But it's not really a new problem, and I think we'd be confusing things by leaving the bogus requirements there. The current requirement on allocate() is generally not important anyway, since it takes O(size) to construct objects in the resulting space. There are real performance issues here, but they're all concerned with the constants, not the asymptotic complexity.
Proposed resolution:
Change 20.1.2 [allocator.requirements]/2:
-2- Table 39 describes the requirements on types manipulated through allocators. All the operations on the allocators are expected to be amortized constant time, except that allocate and construct may require time proportional to the size of the object allocated or constructed. Table 40 describes the requirements on allocator types.
Section: 20.1.1 [utility.arg.requirements] Status: Open Submitter: Yechezkel Mett Date: 2007-10-14
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Discussion:
The draft standard n2369 uses the term move constructor in a few places, but doesn't seem to define it.
MoveConstructible requirements are defined in Table 33 in 20.1.1 [utility.arg.requirements] as follows:
MoveConstructible requirements expression post-condition T t = rv t is equivalent to the value of rv before the construction [Note: There is no requirement on the value of rv after the construction. -- end note]
(where rv is a non-const rvalue of type T).
So I assume the move constructor is the constructor that would be used in filling the above requirement.
For vector::reserve, vector::resize and the vector modifiers given in 23.2.5.4 [vector.modifiers] we have
Requires: If value_type has a move constructor, that constructor shall not throw any exceptions.
Firstly "If value_type has a move constructor" is superfluous; every type which can be put into a vector has a move constructor (a copy constructor is also a move constructor). Secondly it means that for any value_type which has a throwing copy constructor and no other move constructor these functions cannot be used -- which I think will come as a shock to people who have been using such types in vector until now!
I can see two ways to correct this. The simpler, which is presumably what was intended, is to say "If value_type has a move constructor and no copy constructor, the move constructor shall not throw any exceptions" or "If value_type has a move constructor which changes the value of its parameter,".
The other alternative is add to MoveConstructible the requirement that the expression does not throw. This would mean that not every type that satisfies the CopyConstructible requirements also satisfies the MoveConstructible requirements. It would mean changing requirements in various places in the draft to allow either MoveConstructible or CopyConstructible, but I think the result would be clearer and possibly more concise too.
Proposed resolution:
Add new defintions to 17.1 [definitions]:
move constructor
a constructor which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the construction.
move assignment operator
an assignment operator which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the assignment.
move assignment
use of the move assignment operator.
[ Howard adds post-Bellevue: ]
Unfortunately I believe the wording recommended by the LWG in Bellevue is incorrect. reserve et. al. will use a move constructor if one is available, else it will use a copy constructor. A type may have both. If the move constructor is used, it must not throw. If the copy constructor is used, it can throw. The sentence in the proposed wording is correct without the recommended insertion. The Bellevue LWG recommended moving this issue to Ready. I am unfortunately pulling it back to Open. But I'm drafting wording to atone for this egregious action. :-)
Section: 23.2.5.2 [vector.capacity], 21.3.4 [string.capacity] Status: Ready Submitter: Beman Dawes Date: 2007-10-31
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Discussion:
A std::vector can be shrunk-to-fit via the swap idiom:
vector<int> v; ... v.swap(vector<int>(v)); // shrink to fitor:
vector<int>(v).swap(v); // shrink to fitor:
swap(v, vector<int>(v)); // shrink to fit
A non-binding request for shrink-to-fit can be made to a std::string via:
string s; ... s.reserve(0);
Neither of these is at all obvious to beginners, and even some experienced C++ programmers are not aware that shrink-to-fit is trivially available.
Lack of explicit functions to perform these commonly requested operations makes vector and string less usable for non-experts. Because the idioms are somewhat obscure, code readability is impaired. It is also unfortunate that two similar vector-like containers use different syntax for the same operation.
The proposed resolution addresses these concerns. The proposed function takes no arguments to keep the solution simple and focused.
Proposed resolution:
To Class template basic_string 21.3 [basic.string] synopsis, Class template vector 23.2.5 [vector] synopsis, and Class vector<bool> 23.2.6 [vector.bool] synopsis, add:
void shrink_to_fit();
To basic_string capacity 21.3.4 [string.capacity] and vector capacity 23.2.5.2 [vector.capacity], add:
void shrink_to_fit();Remarks: shrink_to_fit is a non-binding request to reduce capacity() to size(). [Note: The request is non-binding to allow latitude for implementation-specific optimizations. -- end note]
Section: 23.2.4 [container.adaptors] Status: Open Submitter: Paolo Carlini Date: 2007-10-31
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Discussion:
After n2369 we have a single push_back overload in the sequence containers, of the "emplace" type. At variance with that, still in n2461, we have two separate overloads, the C++03 one + one taking an rvalue reference in the container adaptors. Therefore, simply from a consistency point of view, I was wondering whether the container adaptors should be aligned with the specifications of the sequence container themselves: thus have a single push along the lines:
template<typename... _Args>
void
push(_Args&&... __args)
{ c.push_back(std::forward<_Args>(__args)...); }
[ Related to 767 ]
Proposed resolution:
Change 23.2.4.1.1 [queue.defn]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Change 23.2.4.2 [priority.queue]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Change 23.2.4.2.2 [priqueue.members]:
void push(const value_type& x);
Effects:c.push_back(x);push_heap(c.begin(), c.end(), comp);template<class... Args> void push(value_typeArgs&&...xargs);Effects:
c.push_back(std::moveforward<Args>(xargs)...); push_heap(c.begin(), c.end(), comp);
Change 23.2.4.3.1 [stack.defn]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Section: 20.6.6.2 [util.smartptr.shared] Status: Ready Submitter: Joe Gottman Date: 2007-10-31
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Discussion:
Consider the following program:
int main() {
shared_ptr<int> p(nullptr);
return 0;
}
This program will fail to compile because shared_ptr uses the following template constructor to construct itself from pointers:
template <class Y> shared_ptr(Y *);
According to N2431, the conversion from nullptr_t to Y * is not deducible, so the above constructor will not be found. There are similar problems with the constructors that take a pointer and a deleter or a pointer, a deleter and an allocator, as well as the corresponding forms of reset(). Note that N2435 will solve this problem for constructing from just nullptr, but not for constructors that use deleters or allocators or for reset().
In the case of the functions that take deleters, there is the additional question of what argument should be passed to the deleter when it is eventually called. There are two reasonable possibilities: nullptr or static_cast<T *>(0), where T is the template argument of the shared_ptr. It is not immediately clear which of these is better. If D::operator() is a template function similar to shared_ptr's constructor, then d(static_cast<T*>(0)) will compile and d(nullptr) will not. On the other hand, if D::operator()() takes a parameter that is a pointer to some type other that T (for instance U* where U derives from T) then d(nullptr) will compile and d(static_cast<T *>(0)) may not.
[ Bellevue: ]
The general idea is right, we need to be able to pass a nullptr to a shared_ptr, but there are a few borderline editorial issues here. (For example, the single-argument nullptr_t constructor in the class synopsis isn't marked explicit, but it is marked explicit in the proposed wording for 20.6.6.2.1. There is a missing empty parenthesis in the form that takes a nullptr_t, a deleter, and an allocator.)
More seriously: this issue says that a shared_ptr constructed from a nullptr is empty. Since "empty" is undefined, it's hard to know whether that's right. This issue is pending on handling that term better.
Peter suggests definition of empty should be "does not own anything"
Is there an editorial issue that post-conditions should refer to get() = nullptr, rather than get() = 0?
No strong feeling towards accept or NAD, but prefer to make a decision than leave it open.
Seems there are no technical merits between NAD and Ready, comes down to "Do we intentially want to allow/disallow null pointers with these functions". Staw Poll - support null pointers 5 - No null pointers 0
Move to Ready, modulo editorial comments
[ post Bellevue Peter adds: ]
The following wording changes are less intrusive:
In 20.6.6.2.1 [util.smartptr.shared.const], add:
shared_ptr(nullptr_t);after:
shared_ptr();(Absence of explicit intentional.)
px.reset( nullptr ) seems a somewhat contrived way to write px.reset(), so I'm not convinced of its utility.
It's similarly not clear to me whether the deleter constructors need to be extended to take nullptr, but if they need to:
Add
template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);after
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);Note that this changes the semantics of the new constructors such that they consistently call d(p) instead of d((T*)0) when p is nullptr.
The ability to be able to pass 0/NULL to a function that takes a shared_ptr has repeatedly been requested by users, but the other additions that the proposed resolution makes are not supported by real world demand or motivating examples.
It might be useful to split the obvious and non-controversial nullptr_t constructor into a separate issue. Waiting for "empty" to be clarified is unnecessary; this is effectively an alias for the default constructor.
Proposed resolution:
Add the following constructors to 20.6.6.2 [util.smartptr.shared]:
shared_ptr(nullptr_t); template <class D> shared_ptr(nullptr_t, D d); template <class D, class A> shared_ptr(nullptr_t, D d, A a);
Add the following methods to 20.6.6.2 [util.smartptr.shared]:
void reset(nullptr_t); template <class D> void reset(nullptr_t, D d); template <class D, class A> void reset(nullptr_t, D d, A a);
Add the following constructor definitions to 20.6.6.2.1 [util.smartptr.shared.const]:
explicit shared_ptr(nullptr_t);Effects: Constructs an empty shared_ptr object.
Postconditions: use_count() == 0 && get() == 0.
Throws: nothing.
template <class D> shared_ptr(nullptr_t, D d); template <class D, class A> shared_ptr<nullptr_t, D d, A a);Requires: D shall be CopyConstructible. The copy constructor and destructor of D shall not throw exceptions. The expression d(static_cast<T *>(0)) shall be well-formed, shall have well defined behavior, and shall not throw exceptions. A shall be an allocator (20.1.2 [allocator.requirements]). The copy constructor and destructor of A shall not throw exceptions.
Effects: Constructs a shared_ptr object that owns a null pointer of type T * and deleter d. The second constructor shall use a copy of a to allocate memory for internal use.
Postconditions: use_count() == 1 and get() == 0.
Throws: bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.
Exception safety: If an exception is thrown, d(static_cast<Y *>(nullptr)) is called.
Add the following method definitions to 20.6.6.2.4 [util.smartptr.shared.mod]:
void reset(nullptr_t);Effects: Equivalent to shared_ptr(nullptr).swap(*this).
template <class D> void reset(nullptr_t, const D d)Effects: Equivalent to shared_ptr(nullptr, d).swap(*this).
template <class D, class A> void reset(nullptr_t, D d, A a);Effects: Equivalent to shared_ptr(nullptr, d, a).swap(*this).
Section: 23.1 [container.requirements] Status: Ready Submitter: Jens Maurer Date: 2007-11-06
View other active issues in [container.requirements].
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Discussion:
23.1 [container.requirements] says:
-12- Objects passed to member functions of a container as rvalue references shall not be elements of that container. No diagnostic required.
A reference is not an object, but this sentence appears to claim so.
What is probably meant here:
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.
Proposed resolution:
Change 23.1 [container.requirements]:
-12-Objects passed to member functions of a container as rvalue references shall not be elementsAn object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container.;Nno diagnostic required.
Section: 23.1 [container.requirements] Status: Open Submitter: Paolo Carlini Date: 2007-11-11
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Discussion:
In an emplace member function the function parameter pack may be bound to a priori unlimited number of objects: some or all of them can be elements of the container itself. Apparently, in order to conform to the blanket statement 23.1 [container.requirements]/11, the implementation must check all of them for that possibility. A possible solution can involve extending the exception in 23.1 [container.requirements]/12 also to the emplace member. As a side note, the push_back and push_front member functions are luckily not affected by this problem, can be efficiently implemented anyway
[ Related to 767 ]
[ Bellevue: ]
The proposed addition (13) is partially redundant with the existing paragraph 12. Why was the qualifier "rvalues" added to paragraph 12? Why does it not cover subelements and pointers?
Resolution: Alan Talbot to rework language, then set state to Review.
Proposed resolution:
Add after 23.1 [container.requirements]/12:
-12- Objects passed to member functions of a container as rvalue references shall not be elements of that container. No diagnostic required.
-13- Objects bound to the function parameter pack of the emplace member function shall not be elements or sub-objects of elements of the container. No diagnostic required.
Section: 23.4.1.2 [unord.map.elem] Status: Ready Submitter: Joe Gottman Date: 2007-11-15
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Discussion:
The new member function at() was recently added to std::map(). It acts like operator[](), except it throws an exception when the input key is not found. It is useful when the map is const, the value_type of the key doesn't have a default constructor, it is an error if the key is not found, or the user wants to avoid accidentally adding an element to the map. For exactly these same reasons, at() would be equally useful in std::unordered_map.
Proposed resolution:
Add the following functions to the definition of unordered_map under "lookup" (23.4.1 [unord.map]):
mapped_type& at(const key_type& k); const mapped_type &at(const key_type &k) const;
Add the following definitions to 23.4.1.2 [unord.map.elem]:
mapped_type& at(const key_type& k); const mapped_type &at(const key_type &k) const;Returns: A reference to x.second, where x is the (unique) element whose key is equivalent to k.
Throws: An exception object of type out_of_range if no such element is present.
[ Bellevue: Editorial note: the "(unique)" differs from map. ]
Section: 20.6.5 [unique.ptr] Status: Open Submitter: Daniel Krügler Date: 2007-11-30
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Discussion:
In contrast to the proposed std::shared_ptr, std::unique_ptr does currently not support incomplete types, because it gives no explicit grant - thus instantiating unique_ptr with an incomplete pointee type T automatically belongs to undefined behaviour according to 17.4.3.6 [res.on.functions]/2, last bullet. This is an unnecessary restriction and prevents many well-established patterns - like the bridge pattern - for std::unique_ptr.
[ Bellevue: ]
Move to open. The LWG is comfortable with the intent of allowing incomplete types and making unique_ptr more like shared_ptr, but we are not comfortable with the wording. The specification for unique_ptr should be more like that of shared_ptr. We need to know, for individual member functions, which ones require their types to be complete. The shared_ptr specification is careful to say that for each function, and we need the same level of care here. We also aren't comfortable with the "part of the operational semantic" language; it's not used elsewhere in the standard, and it's not clear what it means. We need a volunteer to produce new wording.
Proposed resolution:
In 20.6.5 [unique.ptr]/2 add as the last sentence to the existing para:
The unique_ptr provides a semantics of strict ownership. A unique_ptr owns the object it holds a pointer to. A unique_ptr is not CopyConstructible, nor CopyAssignable, however it is MoveConstructible and MoveAssignable. The template parameter T of unique_ptr may be an incomplete type. [ Note: The uses of unique_ptr include providing exception safety for dynamically allcoated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function. -- end note ]
Change the 2nd sentence of 20.6.5.2 [unique.ptr.single]/1
The default type for the template parameter D is default_delete. A client-supplied template argument Dmustshall be a function pointer or functor for which, given a value d of type D and a pointer ptr of type T*, the expression d(ptr) isvalidwell-formed and has well-defined behavior when its evaluation is part of the operational semantic, and has the effect of deallocating the pointer as appropriate for that deleter. D may also be an lvalue-reference to a deleter.
Add the following sentence at the end of 20.6.5.2 [unique.ptr.single]/2:
If the deleter D maintains state, it is intended that this state stay with the associated pointer as ownership is transferred from unique_ptr to unique_ptr. The deleter state need never be copied, only moved or swapped as pointer ownership is moved around. That is, the deleter need only be MoveConstructible, MoveAssignable, and Swappable, and need not be CopyConstructible (unless copied into the unique_ptr) nor CopyAssignable. If any of these operations is part of the operational semantic, the corresponding expression shall be well-formed and shall have well-defined behavior.
Section: 24.1 [iterator.requirements] Status: New Submitter: Martin Sebor Date: 2007-12-14
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Discussion:
Issue 278 defines the meaning of the term "invalid iterator" as one that may be singular.
Consider the following code:
std::deque<int> x, y;
std::deque<int>::iterator i = x.end(), j = y.end();
x.swap(y);
Given that swap() is required not to invalidate iterators
and using the definition above, what should be the expected result of
comparing i and j to x.end()
and y.end(), respectively, after the swap()?
I.e., is the expression below required to evaluate
to true?
i == y.end() && j == x.end()
(There are at least two implementations where the expression
returns false.)
More generally, is the definition introduced in issue 278 meant to make any guarantees about whether iterators actually point to the same elements or be associated with the same containers after a non-invalidating operation as they did before?
Here's a motivating example intended to demonstrate the importance of the question:
Container x, y ({ 1, 2}); // pseudocode to initialize y with { 1, 2 }
Container::iterator i = y.begin() + 1;
Container::iterator j = y.end();
std::swap(x, y);
std::find(i, j, 3);
swap() guarantees that i and j
continue to be valid. Unless the spec says that even though they are
valid they may no longer denote a valid range the code above must be
well-defined. Expert opinions on this differ as does the behavior of
popular implementations for some standard Containers.
Proposed resolution:
Section: 23.1 [container.requirements], 23.1.3.1 [unord.req.except] Status: Ready Submitter: Ion Gaztañaga Date: 2007-12-22
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Discussion:
23.1 [container.requirements]p10 states:
Unless otherwise specified (see 23.2.2.3 and 23.2.5.4) all container types defined in this clause meet the following additional requirements:
- [...]
- no erase(), pop_back() or pop_front() function throws an exception.
23.2.2.3 [deque.modifiers] and 23.2.5.4 [vector.modifiers] offer additional guarantees for deque/vector insert() and erase() members. However, 23.1 [container.requirements]p10 does not mention 23.1.3.1 [unord.req.except] that specifies exception safety guarantees for unordered containers. In addition, 23.1.3.1 [unord.req.except]p1 offers the following guaratee for erase():
No erase() function throws an exception unless that exception is thrown by the container's Hash or Pred object (if any).
Summary:
According to 23.1 [container.requirements]p10 no erase() function should throw an exception unless otherwise specified. Although does not explicitly mention 23.1.3.1 [unord.req.except], this section offers additional guarantees for unordered containers, allowing erase() to throw if predicate or hash function throws.
In contrast, associative containers have no exception safety guarantees section so no erase() function should throw, including erase(k) that needs to use the predicate function to perform its work. This means that the predicate of an associative container is not allowed to throw.
So:
Proposed resolution:
Create a new sub-section of 23.1.2 [associative.reqmts] (perhaps [associative.req.except]) titled "Exception safety guarantees".
1 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 Pred object (if any).
2 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.
3 For associative containers, no swap function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator of the container's Pred object (if any).
Change 23.1.3.1 [unord.req.except]p1:
For unordered associative containers, no clear() function throws an exception.Noerase(k)functiondoes not throwsan exception unless that exception is thrown by the container's Hash or Pred object (if any).
Change 23.1 [container.requirements]p10 to add references to new sections:
Unless otherwise specified (see [deque.modifiers],and[vector.modifiers], [associative.req.except], [unord.req.except]) all container types defined in this clause meet the following additional requirements:
Change 23.1 [container.requirements]p10 referring to swap:
- no swap() function throws an exception
unless that exception is thrown by the copy constructor or assignment operator of the container's Compare object (if any; see [associative.reqmts]).
Section: 23 [containers] Status: Open Submitter: Sylvain Pion Date: 2007-12-28
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Discussion:
Playing with g++'s C++0X mode, I noticed that the following code, which used to compile:
#include <vector>
int main()
{
std::vector<char *> v;
v.push_back(0);
}
now fails with the following error message:
.../include/c++/4.3.0/ext/new_allocator.h: In member function 'void __gnu_cxx::new_allocator<_Tp>::construct(_Tp*, _Args&& ...) [with _Args = int, _Tp = char*]': .../include/c++/4.3.0/bits/stl_vector.h:707: instantiated from 'void std::vector<_Tp, _Alloc>::push_back(_Args&& ...) [with _Args = int, _Tp = char*, _Alloc = std::allocator<char*>]' test.cpp:6: instantiated from here .../include/c++/4.3.0/ext/new_allocator.h:114: error: invalid conversion from 'int' to 'char*'
As far as I know, g++ follows the current draft here.
Does the committee really intend to break compatibility for such cases?
[ Sylvain adds: ]
I just noticed that std::pair has the same issue. The following now fails with GCC's -std=c++0x mode:
#include <utility> int main() { std::pair<char *, char *> p (0,0); }I have not made any general audit for such problems elsewhere.
[ Related to 756 ]
[ Bellevue: ]
Motivation is to handle the old-style int-zero-valued NULL pointers. Problem: this solution requires concepts in some cases, which some users will be slow to adopt. Some discussion of alternatives involving prohibiting variadic forms and additional library-implementation complexity.
Discussion of "perfect world" solutions, the only such solution put forward being to retroactively prohibit use of the integer zero for a NULL pointer. This approach was deemed unacceptable given the large bodies of pre-existing code that do use integer zero for a NULL pointer.
Another approach is to change the member names. Yet another approach is to forbid the extension in absence of concepts.
Resolution: These issues (756, 767, 760, 763) will be subsumed into a paper to be produced by Alan Talbot in time for review at the 2008 meeting in France. Once this paper is produced, these issues will be moved to NAD.
Proposed resolution:
Add the following rows to Table 90 "Optional sequence container operations", 23.1.1 [sequence.reqmts]:
expression return type assertion/note
pre-/post-conditioncontainer a.push_front(t) void a.insert(a.begin(), t)
Requires: T shall be CopyConstructible.list, deque a.push_front(rv) void a.insert(a.begin(), rv)
Requires: T shall be MoveConstructible.list, deque a.push_back(t) void a.insert(a.end(), t)
Requires: T shall be CopyConstructible.list, deque, vector, basic_string a.push_back(rv) void a.insert(a.end(), rv)
Requires: T shall be MoveConstructible.list, deque, vector, basic_string
Change the synopsis in 23.2.2 [deque]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.2.2.3 [deque.modifiers]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change the synopsis in 23.2.3 [list]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.2.3.3 [list.modifiers]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change the synopsis in 23.2.5 [vector]:
void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.2.5.4 [vector.modifiers]:
void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Section: 29.4.3 [atomics.types.generic] Status: Ready Submitter: Alberto Ganesh Barbati Date: 2007-12-28
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Discussion:
in the latest publicly available draft, paper N2641, in section 29.4.3 [atomics.types.generic], the following specialization of the template atomic<> is provided for pointers:
template <class T> struct atomic<T*> : atomic_address {
T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;
T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;
atomic() = default;
constexpr explicit atomic(T);
atomic(const atomic&) = delete;
atomic& operator=(const atomic&) = delete;
T* operator=(T*) volatile;
T* operator++(int) volatile;
T* operator--(int) volatile;
T* operator++() volatile;
T* operator--() volatile;
T* operator+=(ptrdiff_t) volatile;
T* operator-=(ptrdiff_t) volatile;
};
First of all, there is a typo in the non-default constructor which should take a T* rather than a T.
As you can see, the specialization redefine and therefore hide a few methods from the base class atomic_address, namely fetch_add, fetch_sub, operator=, operator+= and operator-=. That's good, but... what happened to the other methods, in particular these ones:
void store(T*, memory_order = memory_order_seq_cst) volatile; T* load( memory_order = memory_order_seq_cst ) volatile; T* swap( T*, memory_order = memory_order_seq_cst ) volatile; bool compare_swap( T*&, T*, memory_order, memory_order ) volatile; bool compare_swap( T*&, T*, memory_order = memory_order_seq_cst ) volatile;
By reading paper N2427 "C++ Atomic Types and Operations", I see that the definition of the specialization atomic<T*> matches the one in the draft, but in the example implementation the methods load(), swap() and compare_swap() are indeed present.
Strangely, the example implementation does not redefine the method store(). It's true that a T* is always convertible to void*, but not hiding the void* signature from the base class makes the class error-prone to say the least: it lets you assign pointers of any type to a T*, without any hint from the compiler.
Is there a true intent to remove them from the specialization or are they just missing from the definition because of a mistake?
[ Bellevue: ]
The proposed revisions are accepted.
Further discussion: why is the ctor labeled "constexpr"? Lawrence said this permits the object to be statically initialized, and that's important because otherwise there would be a race condition on initialization.
Proposed resolution:
Change the synopsis in 29.4.3 [atomics.types.generic]:
template <class T> struct atomic<T*> : atomic_address {
void store(T*, memory_order = memory_order_seq_cst) volatile;
T* load( memory_order = memory_order_seq_cst ) volatile;
T* swap( T*, memory_order = memory_order_seq_cst ) volatile;
bool compare_swap( T*&, T*, memory_order, memory_order ) volatile;
bool compare_swap( T*&, T*, memory_order = memory_order_seq_cst ) volatile;
T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;
T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;
atomic() = default;
constexpr explicit atomic(T*);
atomic(const atomic&) = delete;
atomic& operator=(const atomic&) = delete;
T* operator=(T*) volatile;
T* operator++(int) volatile;
T* operator--(int) volatile;
T* operator++() volatile;
T* operator--() volatile;
T* operator+=(ptrdiff_t) volatile;
T* operator-=(ptrdiff_t) volatile;
};
Section: 20.5.15.2 [func.wrap.func] Status: New Submitter: Daniel Krügler Date: 2008-01-10
View all issues with New status.
Discussion:
N2461 already replaced in 20.5.15.2 [func.wrap.func] it's originally proposed (implicit) conversion operator to "unspecified-bool-type" by the new explicit bool conversion, but the inverse conversion should also use the new std::nullptr_t type instead of "unspecified-null-pointer- type".
Proposed resolution:
In 20.5 [function.objects], header <functional> synopsis replace:
template<class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template<class R, class... ArgTypes> bool operator==(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&); template<class R, class... ArgTypes> bool operator!=(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template<class R, class... ArgTypes> bool operator!=(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&);
In the class function synopsis of 20.5.15.2 [func.wrap.func] replace
function(unspecified-null-pointer-typenullptr_t); ... function& operator=(unspecified-null-pointer-typenullptr_t);
In 20.5.15.2 [func.wrap.func], "Null pointer comparisons" replace:
template <class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator==(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&); template <class R, class... ArgTypes> bool operator!=(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator!=(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&);
In 20.5.15.2.1 [func.wrap.func.con], replace
function(unspecified-null-pointer-typenullptr_t); ... function& operator=(unspecified-null-pointer-typenullptr_t);
In 20.5.15.2.6 [func.wrap.func.nullptr], replace
template <class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>& f,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator==(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>& f);
and replace
template <class R, class... ArgTypes> bool operator!=(const function<R(ArgTypes...)>& f,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator!=(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>& f);
Section: 20.5.15 [func.wrap] Status: Ready Submitter: Daniel Krügler Date: 2008-01-10
View all issues with Ready status.
Discussion:
It is expected that typical implementations of std::function will use dynamic memory allocations at least under given conditions, so it seems appropriate to change the current lvalue swappabilty of this class to rvalue swappability.
Proposed resolution:
In 20.5 [function.objects], header <functional> synopsis, just below of
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&); template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&); template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&&);
In 20.5.15.2 [func.wrap.func] class function definition, change
void swap(function&&);
In 20.5.15.2 [func.wrap.func], just below of
template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&); template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&); template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&&);
In 20.5.15.2.2 [func.wrap.func.mod] change
void swap(function&& other);
In 20.5.15.2.7 [func.wrap.func.alg] add the two overloads
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&& f1, function<R(ArgTypes...)>& f2); template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>&& f2);
Section: 21.4 [string.conversions] Status: Review Submitter: Daniel Krügler Date: 2008-01-13
View other active issues in [string.conversions].
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Discussion:
The new to_string and to_wstring functions described in 21.4 [string.conversions] have throws clauses (paragraphs 8 and 16) which say:
Throws: nothing
Since all overloads return either a std::string or a std::wstring by value this throws clause is impossible to realize in general, since the basic_string constructors can fail due to out-of-memory conditions. Either these throws clauses should be removed or should be more detailled like:
Throws: Nothing if the string construction throws nothing
Further there is an editorial issue in p. 14: All three to_wstring overloads return a string, which should be wstring instead (The header <string> synopsis of 21.2 [string.classes] is correct in this regard).
Proposed resolution:
In 21.4 [string.conversions], remove the paragraphs 8 and 16.
string to_string(long long val); string to_string(unsigned long long val); string to_string(long double val);Throws: nothing
wstring to_wstring(long long val); wstring to_wstring(unsigned long long val); wstring to_wstring(long double val);Throws: nothing
Section: 21.4 [string.conversions] Status: New Submitter: Daniel Krügler Date: 2008-01-13
View other active issues in [string.conversions].
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Discussion:
The return clause 21.4 [string.conversions]/paragraph 15 of the new to_wstring overloads says:
Returns: each function returns a wstring object holding the character representation of the value of its argument that would be generated by calling wsprintf(buf, fmt, val) with a format specifier of L"%lld", L"%ulld", or L"%f", respectively.
Problem is: There does not exist any wsprintf function in C99 (I checked the 2nd edition of ISO 9899, and the first and the second corrigenda from 2001-09-01 and 2004-11-15). What probably meant here is the function swprintf from <wchar.h>/<cwchar>, but this has the non-equivalent declaration:
int swprintf(wchar_t * restrict s, size_t n, const wchar_t * restrict format, ...);
therefore the paragraph needs to mention the size_t parameter n.
Proposed resolution:
Change the current wording of 21.4 [string.conversions]/p. 15 to:
Returns:eEach function returns a wstring object holding the character representation of the value of its argument that would be generated by callingwsswprintf(buf, bufsz, fmt, val) with a format specifier fmt of L"%lld", L"%ulld", or L"%f", respectively, where buf designates an internal character buffer of sufficient size bufsz.
[Hint to the editor: The resolution also adds to mention the name of the format specifier "fmt"]
I also would like to remark that the current wording of it's equivalent paragraph 7 should also mention the meaning of buf and fmt.
Change the current wording of 21.4 [string.conversions]/p. 7 to:
Returns:eEach function returns a string object holding the character representation of the value of its argument that would be generated by calling sprintf(buf, fmt, val) with a format specifier fmt of "%lld", "%ulld", or "%f", respectively, where buf designates an internal character buffer of sufficient size.
Section: 23 [containers] Status: Open Submitter: Alisdair Meredith Date: 2008-01-14
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Discussion:
It appears most containers declare but do not define a member-swap function.
This is unfortunate, as all overload the swap algorithm to call the member-swap function! (required for swappable guarantees [Table 37] and Container Requirements [Table 87])
Note in particular that Table 87 gives semantics of a.swap(b) as swap(a,b), yet for all containers we define swap(a,b) to call a.swap(b) - a circular definition.
A quick survey of clause 23 shows that the following containers provide a definition for member-swap:
array queue stack vector
Whereas the following declare it, but do not define the semantics:
deque list map multimap multiset priority_queue set unordered_map unordered_multi_map unordered_multi_set unordered_set
Suggested resolution:
Provide a definition for each of the affected containers...
[ Bellevue: ]
Move to Open and ask Alisdair to provide wording.
Proposed resolution:
Section: 20.3.1.3 [tuple.helper] Status: Ready Submitter: Alisdair Meredith Date: 2008-01-16
View all issues with Ready status.
Discussion:
The tuple element access API identifies the element in the sequence using signed integers, and then goes on to enforce the requirement that I be >= 0. There is a much easier way to do this - declare I as unsigned.
In fact the proposal is to use std::size_t, matching the type used in the tuple_size API.
A second suggestion is that it is hard to imagine an API that deduces and index at compile time and returns a reference throwing an exception. Add a specific Throws: Nothing paragraph to each element access API.
In addition to tuple, update the API applies to
pair and array, and should be updated
accordingly.
A third observation is that the return type of the get
functions for std::pair is pseudo-code, but it is not
clearly marked as such. There is actually no need for pseudo-code as
the return type can be specified precisely with a call to
tuple_element. This is already done for
std::tuple, and std::array does not have a
problem as all elements are of type T.
Proposed resolution:
Update header <utility> synopsis in 20.2 [utility]
// 20.2.3, tuple-like access to pair: template <class T> class tuple_size; template <intsize_t I, class T> class tuple_element; template <class T1, class T2> struct tuple_size<std::pair<T1, T2> >; template <class T1, class T2> struct tuple_element<0, std::pair<T1, T2> >; template <class T1, class T2> struct tuple_element<1, std::pair<T1, T2> >; template<intsize_t I, class T1, class T2>Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(std::pair<T1, T2>&); template<intsize_t I, class T1, class T2> constPtypename tuple_element<I, std::pair<T1, T2> >::type & get(const std::pair<T1, T2>&);
Update 20.2.3 [pairs] Pairs
template<intsize_t I, class T1, class T2>Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(pair<T1, T2>&); template<intsize_t I, class T1, class T2> constPtypename tuple_element<I, std::pair<T1, T2> >::type & get(const pair<T1, T2>&);
24 Return type: If
I == 0 then P is T1, if I == 1 then P is T2, and otherwise the program is ill-formed.
25 Returns: If I == 0 returns p.first, otherwise if I == 1 returns p.second, and otherwise the program is ill-formed.
Throws: Nothing.
Update header <tuple> synopsis in 20.3 [tuple] with a APIs as below:
template <intsize_t I, class T> class tuple_element; // undefined template <intsize_t I, class... Types> class tuple_element<I, tuple<Types...> >; // 20.3.1.4, element access: template <intsize_t I, class... Types> typename tuple_element<I, tuple<Types...> >::type& get(tuple<Types...>&); template <intsize_t I, class ... types> typename tuple_element<I, tuple<Types...> >::type const& get(const tuple<Types...>&);
Update 20.3.1.3 [tuple.helper] Tuple helper classes
template <intsize_t I, class... Types> class tuple_element<I, tuple<Types...> > { public: typedef TI type; };
1 Requires: 0 <= I and I < sizeof...(Types)I is out of bounds.
2 Type: TI is the type of the Ith element of Types, where indexing is zero-based.
Update 20.3.1.4 [tuple.elem] Element access
template <1 Requires:intsize_t I, class... types > typename tuple_element<I, tuple<Types...> >::type& get(tuple<Types...>& t);
0 <= I and I < sizeof...(Types)I is out of bounds.
2 Returns: A reference to the Ith element of t, where indexing is zero-based.
template <intsize_t I, class... types> typename tuple_element<I, tuple<Types...> >::type const& get(const tuple<Types...>& t);
3 Requires: 0 <= I and I < sizeof...(Types)I is out of bounds.
4 Returns: A const reference to the Ith element of t, where indexing is zero-based.
Throws: Nothing.
Update header <array> synopsis in 20.2 [utility]
template <class T> class tuple_size; // forward declaration template <intsize_t I, class T> class tuple_element; // forward declaration template <class T, size_t N> struct tuple_size<array<T, N> >; template <intsize_t I, class T, size_t N> struct tuple_element<I, array<T, N> >; template <intsize_t I, class T, size_t N> T& get(array<T, N>&); template <intsize_t I, class T, size_t N> const T& get(const array<T, N>&);
Update 23.2.1.6 [array.tuple] Tuple interface to class template array
tuple_element<size_t I, array<T, N> >::type
3 Requires: 0 <= I < N.I is out of bounds.
4 Value: The type T.
template <intsize_t I, class T, size_t N> T& get(array<T, N>& a);
5 Requires: 0 <= I < NI is out of bounds.
Returns: A reference to the Ith element of a, where indexing is zero-based.
Throws: Nothing.
template <intsize_t I, class T, size_t N> const T& get(const array<T, N>& a);
6 Requires: 0 <= I < NI is out of bounds.
7 Returns: A const reference to the Ith element of a, where indexing is zero-based.
Throws: Nothing.
[ Bellevue: Note also that the phrase "The program is ill-formed if I is out of bounds" in the requires clauses are probably unnecessary, and could be removed at the editor's discretion. Also std:: qualification for pair is also unnecessary. ]
Section: 23.2.1 [array] Status: Review Submitter: Daniel Krügler Date: 2008-01-20
View other active issues in [array].
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Discussion:
The class template array synopsis in 23.2.1 [array]/3 declares a member function
void assign(const T& u);
which's semantic is no-where described. Since this signature is not part of the container requirements, such a semantic cannot be derived by those.
I found only one reference to this function in the issue list, 588 where the question is raised:
what's the effect of calling assign(T&) on a zero-sized array?
which does not answer the basic question of this issue.
If this function shall be part of the std::array, it's probable semantic should correspond to that of boost::array, but of course such wording must be added.
Proposed resolution:
Just after the section 23.2.1.4 [array.data] add the following new section:
23.2.1.5 array::fill [array.fill]
void fill(const T& u);1: Effects: fill_n(begin(), N, u)
[N.B: I wonder, why class array does not have a "modifiers" section. If it had, then assign would naturally belong to it]
Change the synopsis in 23.2.1 [array]/3:
template <class T, size_t N>
struct array {
...
void assign fill(const T& u);
...
[ Bellevue: ]
Suggest substituting "fill" instead of "assign".
Set state to Review given substitution of "fill" for "assign".
Section: 29.4.4 [atomics.types.operations] Status: Ready Submitter: Lawrence Crowl Date: 2008-01-21
View all issues with Ready status.
Discussion:
The load functions are defined as
C atomic_load(volatile A* object); C atomic_load_explicit(volatile A* object, memory_order); C A::load(memory_order order = memory_order_seq_cst) volatile;
which prevents their use in const contexts.
[ post Bellevue Peter adds: ]
Issue 777 suggests making atomic_load operate on const objects. There is a subtle point here. Atomic loads do not generally write to the object, except potentially for the memory_order_seq_cst constraint. Depending on the architecture, a dummy write with the same value may be required to be issued by the atomic load to maintain sequential consistency. This, in turn, may make the following code:
const atomic_int x{}; int main() { x.load(); }dump core under a straightforward implementation that puts const objects in a read-only section.
There are ways to sidestep the problem, but it needs to be considered.
The tradeoff is between making the data member of the atomic types mutable and requiring the user to explicitly mark atomic members as mutable, as is already the case with mutexes.
Proposed resolution:
Add the const qualifier to *object and *this.
C atomic_load(const volatile A* object); C atomic_load_explicit(const volatile A* object, memory_order); C A::load(memory_order order = memory_order_seq_cst) const volatile;
Section: 23.3.5.1 [bitset.cons] Status: Ready Submitter: Thorsten Ottosen Date: 2008-01-24
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Duplicate of: 116
Discussion:
A small issue with std::bitset: it does not have any constructor taking a string literal, which is clumsy and looks like an oversigt when we tried to enable uniform use of string and const char* in the library.
Suggestion: Add
explicit bitset( const char* str );
to std::bitset.
Proposed resolution:
Add to synopsis in 23.3.5 [template.bitset]
explicit bitset( const char* str );
Add to synopsis in 23.3.5.1 [bitset.cons]
explicit bitset( const char* str );Effects: Constructs a bitset as if bitset(string(str)).
Section: 25.2.8 [alg.remove] Status: New Submitter: Daniel Krügler Date: 2008-01-25
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Discussion:
The resolution of 283 did not resolve similar necessary changes for algorithm remove_copy[_if], which seems to be an oversight.
Proposed resolution:
In 25.2.8 [alg.remove]/p.6, replace the N2461 requires clause with one of:
Requires:Type T is EqualityComparable (31).The ranges [first,last) and [result,result + (last - first)) shall not overlap. The expression *result = *first shall be valid.
or
Requires:Type T is EqualityComparable (31).The ranges [first,last) and [result,result + (last - first)) shall not overlap. The result of the expression *first shall be writable to the output iterator.
Section: 25.3.4 [alg.merge] Status: New Submitter: Daniel Krügler Date: 2008-01-25
View all issues with New status.
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 25.3.4 [alg.merge] in N2461 have no Requires element and the Effects element contains some requirements, which is probably editorial. Worse is that:
Proposed resolution:
In 25.3.4 [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))[result, last) (where last is equal to result + (last1 - first1) + (last2 - first2)), such that resulting range will be sorted in non-decreasing order; that is, for every iterator i in [result,last) other than result, the condition *i < *(i - 1) or, respectively, comp(*i, *(i - 1)) will be false.Requires: The resulting range shall not overlap with either of the original ranges.
The list will be sorted in non-decreasing order according to the ordering defined by comp; that is, for every iterator i in [first,last) other than first, the condition *i < *(i - 1) or comp(*i, *(i - 1)) will be false.The results of the expressions *first1 and *first2 shall be writable to the output iterator.
[N.B.: I attempted to reuse the wording style of inplace_merge, therefore proposing to insert ", respectively," between both predicate tests. This is no strictly necessary as other parts of <algorithm> show, just a matter of consistency]
Section: 26.3.7 [complex.value.ops] Status: Ready Submitter: Daniel Krügler Date: 2008-01-26
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Discussion:
A comparision of the N2461 header <complex> synopsis ([complex.synopsis]) with the C99 standard (ISO 9899, 2nd edition and the two corrigenda) show some complex functions that are missing in C++. These are:
cerf cerfc cexp2 cexpm1 clog10 clog1p clog2 clgamma ctgamma
I propose that at least the required cproj overloads are provided as equivalent C++ functions. This addition is easy to do in one sentence (delegation to C99 function).
Please note also that the current entry polar in 26.3.9 [cmplx.over]/1 should be removed from the mentioned overload list. It does not make sense to require that a function already expecting scalar arguments should cast these arguments into corresponding complex<T> arguments, which are not accepted by this function.
Proposed resolution:
In 26.3.1 [complex.synopsis] add just between the declaration of conj and fabs:
template<class T> complex<T> conj(const complex<T>&); template<class T> complex<T> proj(const complex<T>&); template<class T> complex<T> fabs(const complex<T>&);
In 26.3.7 [complex.value.ops] just after p.6 (return clause of conj) add:
template<class T> complex<T> proj(const complex<T>& x);Effects: Behaves the same as C99 function cproj, defined in subclause 7.3.9.4."
In 26.3.9 [cmplx.over]/1, add one further entry proj to the overload list.
The following function templates shall have additional overloads:
arg norm conjpolarproj imag real
Section: 26.4.7.1 [rand.util.seedseq] Status: Ready Submitter: Daniel Krügler Date: 2008-01-27
View other active issues in [rand.util.seedseq].
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Discussion:
Part of the resolution of n2423, issue 8 was the proposal to extend the seed_seq constructor accepting an input range as follows (which is now part of N2461):
template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end);
First, the expression iterator_traits<InputIterator>::value_type is invalid due to missing typename keyword, which is easy to fix.
Second (and worse), while the language now supports default template arguments of function templates, this customization point via the second size_t template parameter is of no advantage, because u can never be deduced, and worse - because it is a constructor function template - it can also never be explicitly provided (14.8.1 [temp.arg.explicit]/7).
The question arises, which advantages result from a compile-time knowledge of u versus a run time knowledge? If run time knowledge suffices, this parameter should be provided as normal function default argument [Resolution marked (A)], if compile-time knowledge is important, this could be done via a tagging template or more user-friendly via a standardized helper generator function (make_seed_seq), which allows this [Resolution marked (B)].
[ Bellevue: ]
Fermilab does not have a strong opinion. Would prefer to go with solution A. Bill agrees that solution A is a lot simpler and does the job.
Proposed Resolution: Accept Solution A.
Issue 803 claims to make this issue moot.
Proposed resolution:
In 26.4.7.1 [rand.util.seedseq]/2, class seed_seq synopsis replace:
class seed_seq
{
public:
...
template<class InputIterator,
size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits>
seed_seq(InputIterator begin, InputIterator end,
size_t u = numeric_limits<typename iterator_traits<InputIterator>::value_type>::digits);
...
};
and do a similar replacement in the member description between p.3 and p.4.
In 26.4.7.1 [rand.util.seedseq]/2, class seed_seq synopsis and in the member description between p.3 and p.4 replace:
template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end); template<class InputIterator, size_t u> seed_seq(InputIterator begin, InputIterator end, implementation-defined s);
In 26.4.2 [rand.synopsis], header <random> synopsis, immediately after the class seed_seq declaration and in 26.4.7.1 [rand.util.seedseq]/2, immediately after the class seed_seq definition add:
template<size_t u, class InputIterator> seed_seq make_seed_seq(InputIterator begin, InputIterator end);
In 26.4.7.1 [rand.util.seedseq], just before p.5 insert two paragraphs:
The first constructor behaves as if it would provide an integral constant expression u of type size_t of value numeric_limits<typename iterator_traits<InputIterator>::value_type>::digits.
The second constructor uses an implementation-defined mechanism to provide an integral constant expression u of type size_t and is called by the function make_seed_seq.
In 26.4.7.1 [rand.util.seedseq], just after the last paragraph add:
template<size_t u, class InputIterator> seed_seq make_seed_seq(InputIterator begin, InputIterator end);where u is used to construct an object s of implementation-defined type.
Returns: seed_seq(begin, end, s);
Section: 30.2.1.1 [thread.thread.id] Status: Ready Submitter: Hans Boehm Date: 2008-02-01
View all issues with Ready status.
Discussion:
The current working paper (N2497, integrated just before Bellevue) is not completely clear whether a given thread::id value may be reused once a thread has exited and has been joined or detached. Posix allows thread ids (pthread_t values) to be reused in this case. Although it is not completely clear whether this originally was the right decision, it is clearly the established practice, and we believe it was always the intent of the C++ threads API to follow Posix and allow this. Howard Hinnant's example implementation implicitly relies on allowing reuse of ids, since it uses Posix thread ids directly.
It is important to be clear on this point, since it the reuse of thread ids often requires extra care in client code, which would not be necessary if thread ids were unique across all time. For example, a hash table indexed by thread id may have to be careful not to associate data values from an old thread with a new one that happens to reuse the id. Simply removing the old entry after joining a thread may not be sufficient, if it creates a visible window between the join and removal during which a new thread with the same id could have been created and added to the table.
[ post Bellevue Peter adds: ]
There is a real issue with thread::id reuse, but I urge the LWG to reconsider fixing this by disallowing reuse, rather than explicitly allowing it. Dealing with thread id reuse is an incredibly painful exercise that would just force the world to reimplement a non-conflicting thread::id over and over.
In addition, it would be nice if a thread::id could be manipulated atomically in a lock-free manner, as motivated by the recursive lock example:
http://www.decadentplace.org.uk/pipermail/cpp-threads/2006-August/001091.html
Proposed resolution:
Add a sentence to 30.2.1.1 [thread.thread.id]/p1:
An object of type
thread::idprovides a unique identifier for each thread of execution and a single distinct value for all thread objects that do not represent a thread of execution ([thread.threads.class]). Each thread of execution has athread::idthat is not equal to thethread::idof other threads of execution and that is not equal to thethread::idofstd::threadobjects that do not represent threads of execution. The library may reuse the value of athread::idof a terminated thread that can no longer be joined.
Section: TR1 5.1.4.5 [tr.rand.eng.disc], TR1 5.1.4.6 [tr.rand.eng.xor] Status: New Submitter: John Maddock Date: 2008-01-15
View all issues with New status.
Discussion:
Table 16 of TR1 requires that all Pseudo Random Number generators have a
seed(integer-type s)
member function that is equivalent to:
mygen = Generator(s)
But the generators xor_combine and discard_block have no such seed member, only the
template <class Gen> seed(Gen&);
member, which will not accept an integer literal as an argument: something that appears to violate the intent of Table 16.
So... is this a bug in TR1?
This is a real issue BTW, since the Boost implementation does adhere to the requirements of Table 16, while at least one commercial implementation does not and follows a strict adherence to sections 5.1.4.5 and 5.1.4.6 instead.
[ Jens adds: ]
Both engines do have the necessary constructor, therefore the omission of the seed() member functions appears to be an oversight.
Proposed resolution:
Section: 31.10 [datetime.system] Status: New Submitter: Christopher Kohlhoff, Jeff Garland Date: 2008-02-03
View all issues with New status.
Discussion:
The draft C++0x thread library requires that the time points of type system_time and returned by get_system_time() represent Coordinated Universal Time (UTC) (section 31.10 [datetime.system]). This can lead to surprising behavior when a library user performs a duration-based wait, such as condition_variable::timed_wait(). A complete explanation of the problem may be found in the Rationale for the Monotonic Clock section in POSIX, but in summary:
POSIX solves the problem by introducing a new monotonic clock, which is unaffected by changes to the system time. When a condition variable is initialized, the user may specify whether the monotonic clock is to be used. (It is worth noting that on POSIX systems it is not possible to use condition_variable::native_handle() to access this facility, since the desired clock type must be specified during construction of the condition variable object.)
In the context of the C++0x thread library, there are added dimensions to the problem due to the need to support platforms other than POSIX:
One possible minimal solution:
Proposed resolution:
Section: 25.3.3.4 [binary.search] Status: New Submitter: Daniel Krügler Date: 2007-09-08
View all issues with New status.
Discussion:
In 25.3.3.4 [binary.search]/3 the complexity of binary_search is described as
At most log(last - first) + 2 comparisons.
This should be precised and brought in line with the nomenclature used for lower_bound, upper_bound, and equal_range.
All existing libraries I'm aware of, delegate to lower_bound (+ one further comparison). Since issue 384 has now WP status, the resolution of #787 should be brought in-line with 384 by changing the + 2 to + O(1).
Proposed resolution:
Change 25.3.3.4 [binary.search]/3
Complexity: At most log2(last - first) +2O(1) comparisons.
Section: 24.5.1 [istream.iterator] Status: New Submitter: Martin Sebor Date: 2008-02-06
View all other issues in [istream.iterator].
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Discussion:
The description of how an istream_iterator object becomes an end-of-stream iterator is a) ambiguous and b) out of date WRT issue 468:
istream_iterator reads (using operator>>) successive elements from the input stream for which it was constructed. After it is constructed, and every time ++ is used, the iterator reads and stores a value of T. If the end of stream is reached (operator void*() on the stream returns false), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments istream_iterator() always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of operator* on an end of stream is not defined. For any other iterator value a const T& is returned. The result of operator-> on an end of stream is not defined. For any other iterator value a const T* is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that ++ operators are not equality preserving, that is, i == j does not guarantee at all that ++i == ++j. Every time ++ is used a new value is read.
istream::operator void*() returns null if istream::fail() is true, otherwise non-null. istream::fail() returns true if failbit or badbit is set in rdstate(). Reaching the end of stream doesn't necessarily imply that failbit or badbit is set (e.g., after extracting an int from stringstream("123") the stream object will have reached the end of stream but fail() is false and operator void*() will return a non-null value).
Also I would prefer to be explicit about calling fail() here (there is no operator void*() anymore.)
Proposed resolution:
Change 24.5.1 [istream.iterator]/1:
istream_iterator reads (using operator>>) successive elements from the input stream for which it was constructed. After it is constructed, and every time ++ is used, the iterator reads and stores a value of T. Ifthe end of stream is reachedthe iterator fails to read and store a value of T (operator void*()fail() on the stream returnsfalsetrue), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments istream_iterator() always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of operator* on an end of stream is not defined. For any other iterator value a const T& is returned. The result of operator-> on an end of stream is not defined. For any other iterator value a const T* is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that ++ operators are not equality preserving, that is, i == j does not guarantee at all that ++i == ++j. Every time ++ is used a new value is read.
Section: 26.4.4.4 [rand.adapt.xor] Status: Ready Submitter: P.J. Plauger Date: 2008-02-09
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Discussion:
xor_combine_engine(result_type) should be explicit. (Obvious oversight.)
[ Bellevue: ]
Non-controversial. Bill is right, but Fermilab believes that this is easy to use badly and hard to use right, and so it should be removed entirely. Got into TR1 by well defined route, do we have permission to remove stuff? Should probably check with Jens, as it is believed he is the originator. Broad consensus that this is not a robust engine adapter.
Proposed resolution:
Remove xor_combine_engine from synopsis of 26.4.2 [rand.synopsis].
Remove 26.4.4.4 [rand.adapt.xor] xor_combine_engine.
Section: 26.4.8.5.2 [rand.dist.samp.pconst] Status: Ready Submitter: P.J. Plauger Date: 2008-02-09
View other active issues in [rand.dist.samp.pconst].
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Discussion:
piecewise_constant_distribution is undefined for a range with just one endpoint. (Probably should be the same as an empty range.)
Proposed resolution:
Change 26.4.8.5.2 [rand.dist.samp.pconst] paragraph 3b:
b) If firstB == lastB or the sequence w has the length zero,
Section: 26.4.8.5.1 [rand.dist.samp.discrete] Status: Open Submitter: P.J. Plauger Date: 2008-02-09
View all other issues in [rand.dist.samp.discrete].
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Discussion:
discrete_distribution should have a constructor like:
template<class _Fn>
discrete_distribution(result_type _Count, double _Low, double _High,
_Fn& _Func);
(Makes it easier to fill a histogram with function vaues over a range.)
[ Bellevue: ]
How do you specify the function so that it does not return negative values? If you do it is a bad construction. This requirement is already there. Where in each bin does one evaluate the function? In the middle. Need to revisit tomorrow.
Proposed resolution:
Section: 26.4.8.5.2 [rand.dist.samp.pconst] Status: New Submitter: P.J. Plauger Date: 2008-02-09
View other active issues in [rand.dist.samp.pconst].
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Discussion:
piecewise_constant_distribution should have a constructor like:
template<class _Fn>
piecewise_constant_distribution(size_t _Count,
_Ty _Low, _Ty _High, _Fn& _Func);
(Makes it easier to fill a histogram with function vaues over a range. The two (reference 793) make a sensible replacement for general_pdf_distribution.)
Proposed resolution:
Section: D.8 [depr.lib.binders] Status: Ready Submitter: Daniel Krügler Date: 2008-02-14
View all other issues in [depr.lib.binders].
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Discussion:
N2521 and its earlier predecessors have moved the old binders from [lib.binders] to D.8 [depr.lib.binders] thereby introducing some renaming of the template parameter names (Operation -> Fn). During this renaming process the protected data member op was also renamed to fn, which seems as an unnecessary interface breakage to me - even if this user access point is probably rarely used.
Proposed resolution:
Change D.8.1 [depr.lib.binder.1st]:
template <class Fn> class binder1st : public unary_function<typename Fn::second_argument_type, typename Fn::result_type> { protected: Fnfnop; typename Fn::first_argument_type value; public: binder1st(const Fn& x, const typename Fn::first_argument_type& y); typename Fn::result_type operator()(const typename Fn::second_argument_type& x) const; typename Fn::result_type operator()(typename Fn::second_argument_type& x) const; };-1- The constructor initializes
fnop with x and value with y.-2- operator() returns
fnop(value,x).
Change D.8.3 [depr.lib.binder.2nd]:
template <class Fn> class binder2nd : public unary_function<typename Fn::first_argument_type, typename Fn::result_type> { protected: Fnfnop; typename Fn::second_argument_type value; public: binder2nd(const Fn& x, const typename Fn::second_argument_type& y); typename Fn::result_type operator()(const typename Fn::first_argument_type& x) const; typename Fn::result_type operator()(typename Fn::first_argument_type& x) const; };-1- The constructor initializes
fnop with x and value with y.-2- operator() returns
fnop(value,x).
Section: 26.4.7.1 [rand.util.seedseq] Status: Open Submitter: Stephan Tolksdorf Date: 2008-02-18
View other active issues in [rand.util.seedseq].
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Discussion:
The for-loop in the algorithm specification has n iterations, where n is defined to be end - begin, i.e. the number of supplied w-bit quantities. Previous versions of this algorithm and the general logic behind it suggest that this is an oversight and that in the context of the for-loop n should be the number of full 32-bit quantities in b (rounded upwards). If w is 64, the current algorithm throws away half of all bits in b. If w is 16, the current algorithm sets half of all elements in v to 0.
There are two more minor issues:
[ Bellevue: ]
Move to OPEN Bill will try to propose a resolution by the next meeting.
[ post Bellevue: Bill provided wording. ]
This issue is made moot if 803 is accepted.
Proposed resolution:
Replace 26.4.7.1 [rand.util.seedseq] paragraph 6 with:
Effects: Constructs a seed_seq object by effectively concatenating the low-order u bits of each of the elements of the supplied sequence [begin, end) in ascending order of significance to make a (possibly very large) unsigned binary number b having a total of n bits, and then carrying out the following algorithm:
for( v.clear(); n > 0; n -= 32 ) v.push_back(b mod 232), b /= 232;
Section: 20.3 [tuple] Status: Open Submitter: Lawrence Crowl Date: 2008-02-18
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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.
Proposed resolution:
Section: 26.4.5 [rand.predef] Status: New Submitter: P.J. Plauger Date: 2008-02-20
View all other issues in [rand.predef].
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Discussion:
The 10,000th value returned by knuth_b is supposed to be 1112339016. We get 2126698284.
Proposed resolution:
Change 26.4.5 [rand.predef]/p8:
typedef shuffle_order_engine<minstd_rand0, 256> knuth_b;Required behavior: The 10000th consecutive invocation of a default-constructed object of type knuth_b shall produce the value11123390162126698284.
Section: 26.4.7.1 [rand.util.seedseq] Status: Open Submitter: Charles Karney Date: 2008-02-22
View other active issues in [rand.util.seedseq].
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Discussion:
seed_seq(InputIterator begin, InputIterator end); constructs a seed_seq object repacking the bits of supplied sequence [begin, end) into a 32-bit vector.
This repacking triggers several problems:
I propose simplifying this seed_seq constructor to be "32-bit only". Despite it's being simpler, there is NO loss of functionality (see below).
Here's how the description would read
26.4.7.1 [rand.util.seedseq] Class seed_seq
template<class InputIterator> seed_seq(InputIterator begin, InputIterator end);5 Requires: NO CHANGE
6 Effects: Constructs a seed_seq object by
for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 2^32);
Discussion:
The chief virtues here are simplicity, portability, and generality.
Arguments (and counter-arguments) against making this change (and retaining the n2461 behavior) are:
The user can pass an array of unsigned char and seed_seq will nicely repack it.
Response: So what? Consider the seed string "ABC". The n2461 proposal results in
v = { 0x3, 0x434241 };
while the simplified proposal yields
v = { 0x41, 0x42, 0x43 };
The results produced by seed_seq::generate with the two inputs are different but nevertheless equivalently "mixed up" and this remains true even if the seed string is long.
With long strings (e.g., with bit-length comparable to the number of bits in the state), v is longer (by a factor of 4) with the simplified proposal and seed_seq::generate will be slower.
Response: It's unlikely that the efficiency of seed_seq::generate will be a big issue. If it is, the user is free to repack the seed vector before constructing seed_seq.
A user can pass an array of 64-bit integers and all the bits will be used.
Response: Indeed. However, there are many instances in the n2461 where integers are silently coerced to a narrower width and this should just be a case of the user needing to read the documentation. The user can of course get equivalent behavior by repacking his seed into 32-bit pieces. Furthermore, the unportability of the n2461 proposal with
unsigned long s[] = {1, 2, 3, 4};
seed_seq q(s, s+4);
which typically results in v = {1, 2, 3, 4} on 32-bit machines and in v = {1, 0, 2, 0, 3, 0, 4, 0} on 64-bit machines is a major pitfall for unsuspecting users.
Note: this proposal renders moot issues 782 and 800.
[ Bellevue: ]
Walter needs to ask Fermilab for guidance. Defer till tomorrow. Bill likes the proposed resolution.
Proposed resolution:
Change 26.4.7.1 [rand.util.seedseq]:
template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end);-5- Requires: InputIterator shall satisfy the requirements of an input iterator (24.1.1) such that iterator_traits<InputIterator>::value_type shall denote an integral type.
-6- Constructs a seed_seq object by
rearranging some or all of the bits of the supplied sequence [begin,end) of w-bit quantities into 32-bit units, as if by the following:
First extract the rightmost u bits from each of the n = end - begin elements of the supplied sequence and concatenate all the extracted bits to initialize a single (possibly very large) unsigned binary number, b = ∑n-1i=0 (begin[i] mod 2u) · 2w·i (in which the bits of each begin[i] are treated as denoting an unsigned quantity). Then carry out the following algorithm:v.clear(); if ($w$ < 32) v.push_back($n$); for( ; $n$ > 0; --$n$) v.push_back(b mod 232), b /= 232;for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 232);
Section: 19.4 [syserr] Status: New Submitter: Daniel Krügler Date: 2008-02-24
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Discussion:
19.4.2.1 [syserr.errcode.overview]/1, class error_code and 19.4.3.1 [syserr.errcondition.overview]/, class error_condition synopses declare an expository data member cat_:
const error_category& cat_; // exposition only
which is used to define the semantics of several members. The decision to use a member of reference type lead to several problems:
The simple fix would be to replace the reference by a pointer member.
Proposed resolution:
In 19.4.1.2 [syserr.errcat.virtuals], remove the throws clause p. 10.
virtual string message(int ev) const = 0;Returns: A string that describes the error condition denoted by ev.
Throws: Nothing.
In 19.4.2.1 [syserr.errcode.overview]/1, class error_code synopsis, modifiers section, replace the current operator= overload by the following:
template <class ErrorCodeEnum>
typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code>::type&
operator=(ErrorCodeEnum e);
In the private section of the same class replace the current data member cat_ definition by:
const error_category&* cat_; // exposition only
In 19.4.2.2 [syserr.errcode.constructors], change p. 2 to read:
error_code();
...
Postconditions: val_ == 0 and cat_ == &system_category.
Change 19.4.2.2 [syserr.errcode.constructors] p. 5 to read:
error_code(int val, const error_category& cat);...
Postconditions: val_ == val and cat_ == &cat.
In 19.4.2.3 [syserr.errcode.modifiers], change p. 1 to read:
void assign(int val, const error_category& cat);
...
Postconditions: val_ == val and cat_ == &cat.
In 19.4.2.3 [syserr.errcode.modifiers], change the operator= signature to read:
template <class ErrorCodeEnum>
typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code>::type&
operator=(ErrorCodeEnum e);
In 19.4.2.4 [syserr.errcode.observers], change p. 3 to read:
const error_category& category() const;
...
Returns: *cat_.
In 19.4.2.4 [syserr.errcode.observers], remove the throws clause p. 8.
string message() const;
...
Throws: Nothing.
In 19.4.3.1 [syserr.errcondition.overview]/1, class error_condition synopsis, constructors section, replace the template constructor overload declaration by one with an added "::value"
template <class ErrorConditionEnum>
error_condition(ErrorConditionEnum e,
typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::type* = 0);
In 19.4.3.1 [syserr.errcondition.overview]/1, class error_condition synopsis, modifiers section, replace the operator= overload declaration by:
template<typename ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value,error_codeerror_condition>::type & operator=( ErrorConditionEnum e );
In the private section of the same class replace the current data member cat_ definition by:
const error_category&* cat_; // exposition only
In 19.4.3.2 [syserr.errcondition.constructors], change p. 2 to read:
error_condition();
...
Postconditions: val_ == 0 and cat_ == &posix_category.
In the same section, change p. 5 to read:
error_condition(int val, const error_category& cat);
...
Postconditions: val_ == val and cat_ == &cat.
In 19.4.3.3 [syserr.errcondition.modifiers], change p. 1 to read:
void assign(int val, const error_category& cat);Postconditions: val_ == val and cat_ == &cat.
In the same section replace the current operator= overload declaration by:
template <class ErrorConditionEnum>
typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_condition>::type&
operator=(ErrorConditionEnum e);
In 19.4.3.4 [syserr.errcondition.observers], change p. 3 to read:
const error_category& category() const;Returns: *cat_.
In 19.4.3.4 [syserr.errcondition.observers], remove the throws clause p. 6.
string message() const;
...
Throws: Nothing.
Section: 19.4 [syserr] Status: New Submitter: Jens Maurer Date: 2008-02-24
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Discussion:
19.4 [syserr]
namespace posix_error {
enum posix_errno {
address_family_not_supported, // EAFNOSUPPORT
...
should rather use the new strong-using facility, which would avoid the necessity for a new posix_error namespace, if I understand correctly.
Proposed resolution:
Section: 20.6.5.2.5 [unique.ptr.single.modifiers] Status: New Submitter: Peter Dimov Date: 2008-03-13
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Discussion:
void unique_ptr::reset(T* p = 0) is currently specified as:
Effects: If p == get() there are no effects. Otherwise get_deleter()(get()).
There are two problems with this. One, if get() == 0 and p != 0, the deleter is called with a NULL pointer, and this is probably not what's intended (the destructor avoids calling the deleter with 0.)
Two, the special check for get() == p is generally not needed and such a situation usually indicates an error in the client code, which is being masked. As a data point, boost::shared_ptr was changed to assert on such self-resets in 2001 and there were no complaints.
One might think that self-resets are necessary for operator= to work; it's specified to perform
reset( u.release() );
and the self-assignment
p = move(p);
might appear to result in a self-reset. But it doesn't; the release() is performed first, zeroing the stored pointer. In other words, p.reset( q.release() ) works even when p and q are the same unique_ptr, and there is no need to special-case p.reset( q.get() ) to work in a similar scenario, as it definitely doesn't when p and q are separate.
Proposed resolution:
Change 20.6.5.2.5 [unique.ptr.single.modifiers]:
void reset(T* p = 0);-4- Effects: Ifp ==get() == 0 there are no effects. Otherwise get_deleter()(get()).
Change 20.6.5.3.3 [unique.ptr.runtime.modifiers]:
void reset(T* p = 0);...
-2- Effects: If
p ==get() == 0 there are no effects. Otherwise get_deleter()(get()).
Section: 20.3.1.1 [tuple.cnstr] Status: New Submitter: Howard Hinnant Date: 2008-03-13
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Discussion:
527 Added a throws clause to bind constructors. I believe the same throws clause should be added to tuple except it ought to take into account move constructors as well.
Proposed resolution:
Add to 20.3.1.1 [tuple.cnstr]:
For each tuple constructor and assignment operator, an exception is thrown only if the construction or assignment of one of the types in Types throws an exception.
Section: 20.2.2 [forward] Status: New Submitter: Jens Maurer Date: 2008-03-13
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Discussion:
p4 (forward) says:
Return type: If T is an lvalue-reference type, an lvalue; otherwise, an rvalue.
First of all, lvalue-ness and rvalue-ness are properties of an expression, not of a type (see 3.10 [basic.lval]). Thus, the phrasing "Return type" is wrong. Second, the phrase says exactly what the core language wording says for folding references in 14.3.1 [temp.arg.type]/p4 and for function return values in 5.2.2 [expr.call]/p10. (If we feel the wording should be retained, it should at most be a note with cross-references to those sections.)
The prose after the example talks about "forwarding as an int& (an lvalue)" etc. In my opinion, this is a category error: "int&" is a type, "lvalue" is a property of an expression, orthogonal to its type. (Btw, expressions cannot have reference type, ever.)
Similar with move:
Return type: an rvalue.
is just wrong and also redundant.
Proposed resolution:
Change 20.2.2 [forward] as indicated:
template <class T> T&& forward(typename identity<T>::type&& t);...
Return type: If T is an lvalue-reference type, an lvalue; otherwise, an rvalue....
-7- In the first call to factory, A1 is deduced as int, so 2 is forwarded to A's constructor as
an int&& (an rvalue). In the second call to factory, A1 is deduced as int&, so i is forwarded to A's constructor asan int& (an lvalue). In both cases, A2 is deduced as double, so 1.414 is forwarded to A's constructor asdouble&& (an rvalue).template <class T> typename remove_reference<T>::type&& move(T&& t);...
Return type: an rvalue.
Section: 25.2.3 [alg.swap] Status: New Submitter: Niels Dekker Date: 2008-02-28
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Discussion:
For the sake of generic programming, the header <algorithm> should provide an
overload of std::swap for array types:
template<class T, size_t N> void swap(T (&a)[N], T (&b)[N]);
It became apparent to me that this overload is missing, when I considered how to write a swap
function for a generic wrapper class template.
(Actually I was thinking of Boost's value_initialized.)
Please look at the following template, W, and suppose that is intended to be a very
generic wrapper:
template<class T> class W {
public:
T data;
};
Clearly W<T> is CopyConstructible and CopyAssignable, and therefore
Swappable, whenever T is CopyConstructible and CopyAssignable.
Moreover, W<T> is also Swappable when T is an array type
whose element type is CopyConstructible and CopyAssignable.
Still it is recommended to add a custom swap function template to such a class template,
for the sake of efficiency and exception safety.
(E.g., Scott Meyers, Effective C++, Third Edition, item 25: Consider support for a non-throwing
swap.)
This function template is typically written as follows:
template<class T> void swap(W<T>& x, W<T>& y) {
using std::swap;
swap(x.data, y.data);
}
Unfortunately, this will introduce an undesirable inconsistency, when T is an array.
For instance, W<std::string[8]> is Swappable, but the current Standard does not
allow calling the custom swap function that was especially written for W!
W<std::string[8]> w1, w2; // Two objects of a Swappable type. std::swap(w1, w2); // Well-defined, but inefficient. using std::swap; swap(w1, w2); // Ill-formed, just because ADL finds W's swap function!!!
W's swap function would try to call std::swap for an array,
std::string[8], which is not supported by the Standard Library.
This issue is easily solved by providing an overload of std::swap for array types.
This swap function should be implemented in terms of swapping the elements of the arrays, so that
it would be non-throwing for arrays whose element types have a non-throwing swap.
Note that such an overload of std::swap should also support multi-dimensional
arrays. Fortunately that isn't really an issue, because it would do so automatically, by
means of recursion.
For your information, there was a discussion on this issue at comp.lang.c++.moderated: [Standard Library] Shouldn't std::swap be overloaded for C-style arrays?
Proposed resolution:
Add an extra condition to the definition of Swappable requirements [swappable] in 20.1.1 [utility.arg.requirements]:
- T is Swappable if T is an array type whose element type is Swappable.
Add the following to 25.2.3 [alg.swap]:
template<class T, size_t N> void swap(T (&a)[N], T (&b)[N]);Requires: TypeTshall be Swappable.Effects:swap_ranges(a, a + N, b);
Section: 27.6.4 [ext.manip] Status: New Submitter: Daniel Krügler Date: 2008-03-01
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Discussion:
The recent draft (as well as the original proposal n2072) uses an operational semantic for get_money ([ext.manip]/3) and put_money ([ext.manip]/5), which uses
istreambuf_iterator<charT>
and
ostreambuf_iterator<charT>
resp, instead of the iterator instances, with explicitly provided traits argument (The operational semantic defined by f is also traits dependent). This is an obvious oversight because both *stream_buf c'tors expect a basic_streambuf<charT,traits> as argument.
The same problem occurs within the get_time and put_time semantic (p. 7 and p. 9) of n2071 incorporated in N2521, where additional to the problem we have an editorial issue in get_time (streambuf_iterator instead of istreambuf_iterator).
Proposed resolution:
In 27.6.4 [ext.manip]/3 within function f replace the first line
template <class charT, class traits, class moneyT>
void f(basic_ios<charT, traits>& str, moneyT& mon, bool intl) {
typedef istreambuf_iterator<charT, traits> Iter;
...
In 27.6.4 [ext.manip]/4 remove the first template charT parameter:
template <class charT,class moneyT> unspecified put_money(const moneyT& mon, bool intl = false);
In 27.6.4 [ext.manip]/5 within function f replace the first line
template <class charT, class traits, class moneyT>
void f(basic_ios<charT, traits>& str, const moneyT& mon, bool intl) {
typedef ostreambuf_iterator<charT, traits> Iter;
...
In 27.6.4 [ext.manip]/7 within function f replace the first line
template <class charT, class traits>
void f(basic_ios<charT, traits>& str, struct tm *tmb, const charT *fmt) {
typedef istreambuf_iterator<charT, traits> Iter;
...
In 27.6.4 [ext.manip]/8 add const:
template <class charT> unspecified put_time(const struct tm *tmb, const charT *fmt);
In 27.6.4 [ext.manip]/9 within function f replace the first line
template <class charT, class traits>
void f(basic_ios<charT, traits>& str, const struct tm *tmb, const charT *fmt) {
typedef ostreambuf_iterator<charT, traits> Iter;
...
Add to the <iomanip> synopsis in 27.6 [iostream.format]
template <class moneyT> unspecified get_money(moneyT& mon, bool intl = false); template <class moneyT> unspecified put_money(const moneyT& mon, bool intl = false); template <class charT> unspecified get_time(struct tm *tmb, const charT *fmt); template <class charT> unspecified put_time(const struct tm *tmb, const charT *fmt);
Section: 20.2.3 [pairs] Status: New Submitter: Doug Gregor Date: 2008-03-14
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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 );
Proposed resolution:
Section: 25.3.1 [alg.sort] Status: New Submitter: Paul McKenney Date: 2008-02-27
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Discussion:
Multi-threading is a good thing, but unsolicited multi-threading can potentially be harmful. For example, sort() performance might be greatly increased via a multithreaded implementation. However, such a multithreaded implementation could result in concurrent invocations of the user-supplied comparator. This would in turn result in problems given a caching comparator that might be written for complex sort keys. Please note that this is not a theoretical issue, as multithreaded implementations of sort() already exist.
Having a multithreaded sort() available is good, but it should not be the default for programs that are not explicitly multithreaded. Users should not be forced to deal with concurrency unless they have asked for it.
[ This may be covered by N2410 Thread-Safety in the Standard Library (Rev 1). ]
Proposed resolution:
Section: 20.6.6.2 [util.smartptr.shared] Status: New Submitter: Matt Austern Date: 2008-02-26
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Discussion:
Several places in 20.6.6.2 [util.smartptr.shared] refer to an "empty" shared_ptr. However, that term is nowhere defined. The closest thing we have to a definition is that the default constructor creates an empty shared_ptr and that a copy of a default-constructed shared_ptr is empty. Are any other shared_ptrs empty? For example, is shared_ptr((T*) 0) empty? What are the properties of an empty shared_ptr? We should either clarify this term or stop using it.
One reason it's not good enough to leave this term up to the reader's intuition is that, in light of N2351 and issue 711, most readers' intuitive understanding is likely to be wrong. Intuitively one might expect that an empty shared_ptr is one that doesn't store a pointer, but, whatever the definition is, that isn't it.
Proposed resolution: