| Doc. no. | J16/02-0048 = WG21 N1390 |
| Date: | 10 Sep 2002 |
| Project: | Programming Language C++ |
| Reply to: | Matt Austern <austern@apple.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 or other extensions.
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.dkuug.dk/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 C++ FAQ at http://www.research.att.com/~austern/csc/faq.html. 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, and the issue is ready to forward to the full committee as a proposed record of response. A Rationale discusses the LWG's reasoning.
Review - Exact wording of a Proposed Resolution is now available for review on an issue for which the LWG previously reached informal consensus.
Ready - The LWG has reached consensus that the issue is a defect in the Standard, the Proposed Resolution is correct, and the issue is ready to forward to the full committee for further action as a Defect Report (DR).
DR - (Defect Report) - The full J16 committee has voted to forward the issue to the Project Editor to be processed as a Potential Defect Report. The Project Editor reviews the issue, and then forwards it to the WG21 Convenor, who returns it to the full committee for final disposition. This issues list accords the status of DR to all these Defect Reports regardless of where they are in that process.
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.
RR - (Record of Response) - The full WG21 committee has determined that this issue is not a defect in the Standard. Action on this issue is thus complete and no further action is possible under ISO rules.
Future - In addition to the regular status, the LWG believes that this issue should be revisited at the next revision of the standard. It is usually paired with NAD.
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 [lib.facet.num.get.virtuals] Status: Open Submitter: Nathan Myers Date: 6 Aug 1998
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.
PJP will provide wording.
Proposed resolution:
Section: 27 [lib.input.output] Status: Open Submitter: Nathan Myers Date: 6 Aug 1998
Many of the specifications for iostreams specify that character values or their int_type equivalents are compared using operators == or !=, though in other places traits::eq() or traits::eq_int_type is specified to be used throughout. This is an inconsistency; we should change uses of == and != to use the traits members instead.
Proposed resolution:
[Kona: Nathan to supply proposed wording]
[ Tokyo: the LWG reaffirmed that this is a defect, and requires careful review of clause 27 as the changes are context sensitive. ]
Section: 21.3.7.9 [lib.string.io] Status: Ready Submitter: Nico Josuttis Date: 29 Sep 1998
Operator >> and getline() for strings read until eof() in the input stream is true. However, this might never happen, if the stream can't read anymore without reaching EOF. So shouldn't it be changed into that it reads until !good() ?
Proposed resolution:
In 21.3.7.9 [lib.string.io], paragraph 1, replace:
Effects: Begins by constructing a sentry object k as if k were constructed by typename basic_istream<charT,traits>::sentry k( is). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:
with:
Effects: Behaves as a formatted input function (27.6.1.2.1 [lib.istream.formatted.reqmts]). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:
In 21.3.7.9 [lib.string.io], paragraph 6, replace
Effects: Begins by constructing a sentry object k as if by typename basic_istream<charT,traits>::sentry k( is, true). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c) until any of the following occurs:
with:
Effects: Behaves as an unformatted input function (27.6.1.3 [lib.istream.unformatted]), except that it does not affect the value returned by subsequent calls to basic_istream<>::gcount(). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c) until any of the following occurs:
[Redmond: Made changes in proposed resolution. operator>> should be a formatted input function, not an unformatted input function. getline should not be required to set gcount, since there is no mechanism for gcount to be set except by one of basic_istream's member functions.]
[Curaçao: Nico agrees with proposed resolution.]
Rationale:
The real issue here is whether or not these string input functions get their characters from a streambuf, rather than by calling an istream's member functions, a streambuf signals failure either by returning eof or by throwing an exception; there are no other possibilities. The proposed resolution makes it clear that these two functions do get characters from a streambuf.
Section: 25 [lib.algorithms] Status: Open Submitter: Nico Josuttis Date: 29 Sep 1998
The standard does not state, how often a function object is copied, called, or the order of calls inside an algorithm. This may lead to surprising/buggy behavior. Consider the following example:
class Nth { // function object that returns true for the nth element
private:
int nth; // element to return true for
int count; // element counter
public:
Nth (int n) : nth(n), count(0) {
}
bool operator() (int) {
return ++count == nth;
}
};
....
// remove third element
list<int>::iterator pos;
pos = remove_if(coll.begin(),coll.end(), // range
Nth(3)), // remove criterion
coll.erase(pos,coll.end());
This call, in fact removes the 3rd AND the 6th element. This happens because the usual implementation of the algorithm copies the function object internally:
template <class ForwIter, class Predicate>
ForwIter std::remove_if(ForwIter beg, ForwIter end, Predicate op)
{
beg = find_if(beg, end, op);
if (beg == end) {
return beg;
}
else {
ForwIter next = beg;
return remove_copy_if(++next, end, beg, op);
}
}
The algorithm uses find_if() to find the first element that should be removed. However, it then uses a copy of the passed function object to process the resulting elements (if any). Here, Nth is used again and removes also the sixth element. This behavior compromises the advantage of function objects being able to have a state. Without any cost it could be avoided (just implement it directly instead of calling find_if()).
Proposed resolution:
In [lib.function.objects] 20.3 Function objects add as new paragraph 6 (or insert after paragraph 1):
Option 1:
Predicates are functions or function objects that fulfill the following requirements:
- They return a Boolean value (bool or a value convertible to bool)
- It doesn't matter for the behavior of a predicate how often it is copied or assigned and how often it is called.
Option 2:
- if it's a function:
- All calls with the same argument values yield the same result.
- if it's a function object:
- In any sequence of calls to operator () without calling any non-constant member function, all calls with the same argument values yield the same result.
- After an assignment or copy both objects return the same result for the same values.
[Santa Cruz: The LWG believes that there may be more to this than meets the eye. It applies to all function objects, particularly predicates. Two questions: (1) must a function object be copyable? (2) how many times is a function object called? These are in effect questions about state. Function objects appear to require special copy semantics to make state work, and may fail if calling alters state and calling occurs an unexpected number of times.]
[Dublin: Pete Becker felt that this may not be a defect, but rather something that programmers need to be educated about. There was discussion of adding wording to the effect that the number and order of calls to function objects, including predicates, not affect the behavior of the function object.]
[Pre-Kona: Nico comments: It seems the problem is that we don't have a clear statement of "predicate" in the standard. People including me seemed to think "a function returning a Boolean value and being able to be called by an STL algorithm or be used as sorting criterion or ... is a predicate". But a predicate has more requirements: It should never change its behavior due to a call or being copied. IMHO we have to state this in the standard. If you like, see section 8.1.4 of my library book for a detailed discussion.]
[Kona: Nico will provide wording to the effect that "unless otherwise specified, the number of copies of and calls to function objects by algorithms is unspecified". Consider placing in 25 [lib.algorithms] after paragraph 9.]
[Pre-Tokyo: Angelika Langer comments: if the resolution is that algorithms are free to copy and pass around any function objects, then it is a valid question whether they are also allowed to change the type information from reference type to value type.]
[Tokyo: Nico will discuss this further with Matt as there are multiple problems beyond the underlying problem of no definition of "Predicate".]
[Post-Tokyo: Nico provided the above proposed resolutions.]
[Curaçao: Nico will provide wording to make options clearer: are the exclusive, or is one a superset of the other?]
Section: 23.2.5 [lib.vector.bool] Status: Open Submitter: AFNOR Date: 7 Oct 1998
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.
Proposed resolution:
[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.]
[Tokyo: Not discussed by the full LWG; no one claimed new insights and so time was more productively spent on other issues. In private discussions it was asserted that requirements for any solution include 1) Increasing the full committee's understanding of the problem, and 2) providing compiler vendors, authors, teachers, and of course users with specific suggestions as to how to apply the eventual solution.]
Section: 24.1.1 [lib.input.iterators] Status: Open Submitter: AFNOR Date: 7 Oct 1998
Table 72 in 24.1.1 [lib.input.iterators] specifies semantics for *r++ of:
{ T tmp = *r; ++r; return tmp; }
There are two problems with this. First, the return type is specified to be "T", as opposed to something like "convertible to T". This is too specific: we want to allow *r++ to return an lvalue.
Second, writing the semantics in terms of code misleadingly suggests that the effects *r++ should precisely replicate the behavior of this code, including side effects. (What if it's a user-defined type whose copy constructor has observable behavior?) We should replace the code with words, or else put some blanket statement in clause 17 saying that code samples aren't intended to specify exactly how many times a copy constructor is called, even if the copy constructor has observable behavior. (See issue 334 for a similar problem.)
[Issue still isn't clear. Matt will try to explain it more clearly at the next meeting.]
Proposed resolution:
Section: 17.4.3.1 [lib.reserved.names] Status: Open Submitter: Judy Ward Date: 15 Dec 1998
The original issue asked whether a library implementor could specialize standard library templates for built-in types. (This was an issue because users are permitted to explicitly instantiate standard library templates.)
Specializations are no longer a problem, because of the resolution to core issue 259. Under the proposed resolution, it will be legal for a translation unit to contain both a specialization and an explicit instantiation of the same template, provided that the specialization comes first. In such a case, the explicit instantiation will be ignored. Further discussion of library issue 120 assumes that the core 259 resolution will be adopted.
However, as noted in lib-7047, one piece of this issue still remains: what happens if a standard library implementor explicitly instantiates a standard library templates? It's illegal for a program to contain two different explicit instantiations of the same template for the same type in two different translation units (ODR violation), and the core working group doesn't believe it is practical to relax that restriction.
The issue, then, is: are users allowed to implicitly instantiate standard library templates for non-user defined types? The status quo answer is 'yes'. Changing it to 'no' would give library implementors more freedom.
This is an issue because, for performance reasons, library implementors often need to explicitly instantiate standard library templates. (for example, std::basic_string<char>) Does giving users freedom to explicitly instantiate standard library templates for non-user defined types make it impossible or painfully difficult for library implementors to do this?
John Spicer suggests, in lib-8957, that library implementors have a mechanism they can use for explicit instantiations that doesn't prevent users from performing their own explicit instantiations: put each explicit instantiation in its own object file. (Different solutions might be necessary for Unix DSOs or MS-Windows DLLs.) On some platforms, library implementors might not need to do anything special: the "undefined behavior" that results from having two different explicit instantiations might be harmless.
Proposed resolution:
Option 1.
Append to 17.4.3.1 [lib.reserved.names] paragraph 1:
A program may explicitly instantiate any templates in the standard library only if the declaration depends on a user-defined name of external linkage and the instantiation meets the standard library requirements for the original template.
Option 2.
In light of the resolution to core issue 259, no normative changes in the library clauses are necessary. Add the following non-normative note to the end of 17.4.3.1 [lib.reserved.names] paragraph 1:
[Note: A program may explicitly instantiate standard library templates, even when an explicit instantiation does not depend on a user-defined name. --end note]
[Copenhagen: LWG discussed three options. (1) Users may not explicitly instantiate standard library templates, except on user-defined types. Consequence: library implementors may freely specialize or instantiate templates. (2) Users may explicitly instantiate any standard library template. Consequence: if implementors specialize or instantiate library templates, they may need to take special steps to make sure users can do it too. (3) It is implementation defined whether users may explicitly instantiate standard library templates on non-user-defined types. Consequence: library implementors may freely specialize or instantiate templates, but may need to document some or all templates that have been explicitly instantiated. ]
[Straw poll (first number is favor, second is strongly oppose): 1 - 4, 0; 2 - 9, 1; 3 - 0, 9. (Proposed resolution 1 was the original proposed resolution.) Because there was no support for option 3, no wording is provided.]
[Redmond: discussed again; straw poll had results similar to those of Copenhagen (1 - 1, 3; 2 - 8, 4; 3 - 6, 2). Most people said they could live with any option. The only objection to option 2 is potential implementation difficulty. Steve Clamage volunteered do a survey to see if there are any popular platforms where option 2 would present a real problem for implementors. See his reflector message, c++std-lib-9002. ]
[Steve and Pete Becker will talk to Jonathan Caves. The Microsoft linker might present a problem if there are multiple copies, some of which have static data and some of which are in DLLs. There may be similar problems with the Apple linker; Matt will clarify that.]
Section: 26.3.5.4 [lib.slice.arr.fill], 26.3.7.4 [lib.gslice.array.fill], 26.3.8.4 [lib.mask.array.fill], 26.3.9.4 [lib.indirect.array.fill] Status: Ready Submitter: Judy Ward Date: 15 Dec 1998
One of the operator= in the valarray helper arrays is const and one is not. For example, look at slice_array. This operator= in Section 26.3.5.2 [lib.slice.arr.assign] is const:
void operator=(const valarray<T>&) const;
but this one in Section 26.3.5.4 [lib.slice.arr.fill] is not:
void operator=(const T&);
The description of the semantics for these two functions is similar.
Proposed resolution:
26.3.5 [lib.template.slice.array] Template class slice_array
In the class template definition for slice_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
26.3.5.4 [lib.slice.arr.fill] slice_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
26.3.7 [lib.template.gslice.array] Template class gslice_array
In the class template definition for gslice_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
26.3.7.4 [lib.gslice.array.fill] gslice_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
26.3.8 [lib.template.mask.array] Template class mask_array
In the class template definition for mask_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
26.3.8.4 [lib.mask.array.fill] mask_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
26.3.9 [lib.template.indirect.array] Template class indirect_array
In the class template definition for indirect_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
26.3.9.4 [lib.indirect.array.fill] indirect_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
[Redmond: Robert provided wording.]
Rationale:
There's no good reason for one version of operator= being const and another one not. Because of issue 253, this now matters: these functions are now callable in more circumstances. In many existing implementations, both versions are already const.
Section: 27.6.2.5.4 [lib.ostream.inserters.character] Status: Open Submitter: Dietmar Kühl Date: 20 Jul 1999
Paragraph 4 states that the length is determined using traits::length(s). Unfortunately, this function is not defined for example if the character type is wchar_t and the type of s is char const*. Similar problems exist if the character type is char and the type of s is either signed char const* or unsigned char const*.
Proposed resolution:
Change 27.6.2.5.4 [lib.ostream.inserters.character] paragraph 4 from:
Effects: Behaves like an formatted inserter (as described in lib.ostream.formatted.reqmts) of out. After a sentry object is constructed it inserts characters. The number of characters starting at s to be inserted is traits::length(s). Padding is determined as described in lib.facet.num.put.virtuals. The traits::length(s) characters starting at s are widened using out.widen (lib.basic.ios.members). The widened characters and any required padding are inserted into out. Calls width(0).
to:
Effects: Behaves like an formatted inserter (as described in lib.ostream.formatted.reqmts) of out. After a sentry object is constructed it inserts characters. The number len of characters starting at s to be inserted is
- traits::length((const char*)s) if the second argument is of type const charT*
- char_traits<char>::length(s) if the second argument is of type const char*, const signed char*, or const unsigned char* and and charT is not char.
Padding is determined as described in lib.facet.num.put.virtuals. The len characters starting at s are widened using out.widen (lib.basic.ios.members). The widened characters and any required padding are inserted into out. Calls width(0).
[Kona: It is clear to the LWG there is a defect here. Dietmar will supply specific wording.]
[Post-Tokyo: Dietmar supplied the above wording.]
[Toronto: The original proposed resolution involved char_traits<signed char> and char_traits<unsigned char>. There was strong opposition to requiring that library implementors provide those specializations of char_traits.]
[Copenhagen: This still isn't quite right: proposed resolution text got garbled when the signed char/unsigned char specializations were removed. Dietmar will provide revised wording.]
Section: 23.1 [lib.container.requirements] Status: Ready Submitter: Judy Ward Date: 2 Jul 1998
Currently the following will not compile on two well-known standard library implementations:
#include <set>
using namespace std;
void f(const set<int> &s)
{
set<int>::iterator i;
if (i==s.end()); // s.end() returns a const_iterator
}
The reason this doesn't compile is because operator== was implemented as a member function of the nested classes set:iterator and set::const_iterator, and there is no conversion from const_iterator to iterator. Surprisingly, (s.end() == i) does work, though, because of the conversion from iterator to const_iterator.
I don't see a requirement anywhere in the standard that this must work. Should there be one? If so, I think the requirement would need to be added to the tables in section 24.1.1. I'm not sure about the wording. If this requirement existed in the standard, I would think that implementors would have to make the comparison operators non-member functions.
This issues was also raised on comp.std.c++ by Darin Adler. The example given was:
bool check_equal(std::deque<int>::iterator i,
std::deque<int>::const_iterator ci)
{
return i == ci;
}
Comment from John Potter:
In case nobody has noticed, accepting it will break reverse_iterator.
The fix is to make the comparison operators templated on two types.
template <class Iterator1, class Iterator2> bool operator== (reverse_iterator<Iterator1> const& x, reverse_iterator<Iterator2> const& y);Obviously: return x.base() == y.base();
Currently, no reverse_iterator to const_reverse_iterator compares are valid.
BTW, I think the issue is in support of bad code. Compares should be between two iterators of the same type. All std::algorithms require the begin and end iterators to be of the same type.
Proposed resolution:
Insert this paragraph after 23.1 [lib.container.requirements] paragraph 7:
In the expressions
i == j i != j i < j i <= j i >= j i > j i - jWhere i and j denote objects of a container's iterator type, either or both may be replaced by an object of the container's const_iterator type referring to the same element with no change in semantics.
[post-Toronto: Judy supplied a proposed resolution saying that iterator and const_iterator could be freely mixed in iterator comparison and difference operations.]
[Redmond: Dave and Howard supplied a new proposed resolution which explicitly listed expressions; there was concern that the previous proposed resolution was too informal.]
Rationale:
The LWG believes it is clear that the above wording applies only to the nested types X::iterator and X::const_iterator, where X is a container. There is no requirement that X::reverse_iterator and X::const_reverse_iterator can be mixed. If mixing them is considered important, that's a separate issue. (Issue 280.)
Section: 25.2.2 [lib.alg.swap] Status: Open Submitter: Andrew Koenig Date: 14 Aug 1999
The description of iter_swap in 25.2.2 paragraph 7,says that it ``exchanges the values''
of the objects to which two iterators refer.
What it doesn't say is whether it does so using swap or using the assignment operator and copy constructor.
This question is an important one to answer, because swap is specialized to work efficiently for standard containers.
For example:
vector<int> v1, v2; iter_swap(&v1, &v2);
Is this call to iter_swap equivalent to calling swap(v1, v2)? Or is it equivalent to
{
vector<int> temp = v1;
v1 = v2;
v2 = temp;
}
The first alternative is O(1); the second is O(n).
A LWG member, Dave Abrahams, comments:
Not an objection necessarily, but I want to point out the cost of that requirement:
iter_swap(list<T>::iterator, list<T>::iterator)
can currently be specialized to be more efficient than iter_swap(T*,T*) for many T (by using splicing). Your proposal would make that optimization illegal.
[Kona: The LWG notes the original need for iter_swap was proxy iterators which are no longer permitted.]
Proposed resolution:
Change the effect clause of iter_swap in 25.2.2 paragraph 7 from:
Exchanges the values pointed to by the two iterators a and b.
to
swap(*a, *b).
[post-Toronto: The LWG is concerned about possible overspecification: there may be cases, such as Dave Abrahams's example above, and such as vector<bool>'s iterators, where it makes more sense for iter_swap to do something other than swap. If performance is a concern, it may be better to have explicit complexity requirements than to say how iter_swap should be implemented.]
[Redmond: Discussed, with no consensus. There was very little support for the proposed resolution. Some people favored closing this issue as NAD. Others favored a more complicated specification of iter_swap, which might distinguish between ordinary iterators and proxies. A possible new issue: how do we know that the iterators passed to iter_swap have Assignable value types? (If this new issue is real, it extends far beyond just iter_swap.)]
Section: 20.1.5 [lib.allocator.requirements], 23.1 [lib.container.requirements] Status: Open Submitter: Andy Sawyer Date: 21 Oct 1999
Must the value returned by max_size() be unchanged from call to call?
Must the value returned from max_size() be meaningful?
Possible meanings identified in lib-6827:
1) The largest container the implementation can support given "best
case" conditions - i.e. assume the run-time platform is "configured to
the max", and no overhead from the program itself. This may possibly
be determined at the point the library is written, but certainly no
later than compile time.
2) The largest container the program could create, given "best case"
conditions - i.e. same platform assumptions as (1), but take into
account any overhead for executing the program itself. (or, roughly
"storage=storage-sizeof(program)"). This does NOT include any resource
allocated by the program. This may (or may not) be determinable at
compile time.
3) The largest container the current execution of the program could
create, given knowledge of the actual run-time platform, but again,
not taking into account any currently allocated resource. This is
probably best determined at program start-up.
4) The largest container the current execution program could create at
the point max_size() is called (or more correctly at the point
max_size() returns :-), given it's current environment (i.e. taking
into account the actual currently available resources). This,
obviously, has to be determined dynamically each time max_size() is
called.
Proposed resolution:
Change 20.1.5 [lib.allocator.requirements] table 32 max_size() wording from:
the largest value that can meaningfully be
passed to X::allocate
to:
the value of the largest constant expression
(5.19 [expr.const]) that could ever meaningfully be passed to X::allocate
Change 23.1 [lib.container.requirements] table 65 max_size() wording from:
size() of the largest possible container.
to:
the value of the largest constant expression
(5.19 [expr.const]) that could ever meaningfully be returned by X::size().
[Kona: The LWG informally discussed this and asked Andy Sawyer to submit an issue.]
[Tokyo: The LWG believes (1) above is the intended meaning.]
[Post-Tokyo: Beman Dawes supplied the above resolution at the request of the LWG. 21.3.3 [lib.string.capacity] was not changed because it references max_size() in 23.1. The term "compile-time" was avoided because it is not defined anywhere in the standard (even though it is used several places in the library clauses).]
[Copenhagen: Exactly what max_size means is still unclear. It may have a different meaning as a container member function than as an allocator member function. For the latter, it is probably best thought of as an architectural limit. Nathan will provide new wording.]
Section: 24.1.3 [lib.forward.iterators] Status: Ready Submitter: Matt Austern Date: 19 Nov 1999
In table 74, the return type of the expression *a is given as T&, where T is the iterator's value type. For constant iterators, however, this is wrong. ("Value type" is never defined very precisely, but it is clear that the value type of, say, std::list<int>::const_iterator is supposed to be int, not const int.)
Proposed resolution:
In table 74, in the *a and *r++ rows, change the return type from "T&" to "T& if X is mutable, otherwise const T&". In the a->m row, change the return type from "U&" to "U& if X is mutable, otherwise const U&".
[Tokyo: The LWG believes this is the tip of a larger iceberg; there are multiple const problems with the STL portion of the library and that these should be addressed as a single package. Note that issue 180 has already been declared NAD Future for that very reason.]
[Redmond: the LWG thinks this is separable from other constness issues. This issue is just cleanup; it clarifies language that was written before we had iterator_traits. Proposed resolution was modified: the original version only discussed *a. It was pointed out that we also need to worry about *r++ and a->m.]
Section: 18.2.1 [lib.limits] Status: Open Submitter: Stephen Cleary Date: 21 Dec 1999
In some places in this section, the terms "fundamental types" and "scalar types" are used when the term "arithmetic types" is intended. The current usage is incorrect because void is a fundamental type and pointers are scalar types, neither of which should have specializations of numeric_limits.
Proposed resolution:
Change 18.2 [lib.support.limits] para 1 from:
The headers <limits>, <climits>, and <cfloat> supply characteristics of implementation-dependent fundamental types (3.9.1).
to:
The headers <limits>, <climits>, and <cfloat> supply characteristics of implementation-dependent arithmetic types (3.9.1).
Change 18.2.1 [lib.limits] para 1 from:
The numeric_limits component provides a C++ program with information about various properties of the implementation's representation of the fundamental types.
to:
The numeric_limits component provides a C++ program with information about various properties of the implementation's representation of the arithmetic types.
Change 18.2.1 [lib.limits] para 2 from:
Specializations shall be provided for each fundamental type. . .
to:
Specializations shall be provided for each arithmetic type. . .
Change 18.2.1 [lib.limits] para 4 from:
Non-fundamental standard types. . .
to:
Non-arithmetic standard types. . .
Change 18.2.1.1 [lib.numeric.limits] para 1 from:
The member is_specialized makes it possible to distinguish between fundamental types, which have specializations, and non-scalar types, which do not.
to:
The member is_specialized makes it possible to distinguish between arithmetic types, which have specializations, and non-arithmetic types, which do not.
[post-Toronto: The opinion of the LWG is that the wording in the standard, as well as the wording of the proposed resolution, is flawed. The term "arithmetic types" is well defined in C and C++, and it is not clear that the term is being used correctly. It is also not clear that the term "implementation dependent" has any useful meaning in this context. The biggest problem is that numeric_limits seems to be intended both for built-in types and for user-defined types, and the standard doesn't make it clear how numeric_limits applies to each of those cases. A wholesale review of numeric_limits is needed. A paper would be welcome.]
Section: 25.2.8 [lib.alg.unique] Status: Ready Submitter: Andrew Koenig Date: 13 Jan 2000
What should unique() do if you give it a predicate that is not an equivalence relation? There are at least two plausible answers:
1. You can't, because 25.2.8 says that it it "eliminates all but the first element from every consecutive group of equal elements..." and it wouldn't make sense to interpret "equal" as meaning anything but an equivalence relation. [It also doesn't make sense to interpret "equal" as meaning ==, because then there would never be any sense in giving a predicate as an argument at all.]
2. The word "equal" should be interpreted to mean whatever the predicate says, even if it is not an equivalence relation (and in particular, even if it is not transitive).
The example that raised this question is from Usenet:
int f[] = { 1, 3, 7, 1, 2 };
int* z = unique(f, f+5, greater<int>());
If one blindly applies the definition using the predicate greater<int>, and ignore the word "equal", you get:
Eliminates all but the first element from every consecutive group of elements referred to by the iterator i in the range [first, last) for which *i > *(i - 1).
The first surprise is the order of the comparison. If we wanted to
allow for the predicate not being an equivalence relation, then we
should surely compare elements the other way: pred(*(i - 1), *i). If
we do that, then the description would seem to say: "Break the
sequence into subsequences whose elements are in strictly increasing
order, and keep only the first element of each subsequence". So the
result would be 1, 1, 2. If we take the description at its word, it
would seem to call for strictly DEcreasing order, in which case the
result should be 1, 3, 7, 2.
In fact, the SGI implementation of unique() does neither: It yields 1,
3, 7.
Proposed resolution:
Change 25.2.8 [lib.alg.unique] paragraph 1 to:
For a nonempty range, eliminates all but the first element from every consecutive group of equivalent elements referred to by the iterator i in the range [first+1, last) for which the following conditions hold: *(i-1) == *i or pred(*(i-1), *i) != false.
Also insert a new paragraph, paragraph 2a, that reads: "Requires: The comparison function must be an equivalence relation."
[Redmond: discussed arguments for and against requiring the comparison function to be an equivalence relation. Straw poll: 14-2-5. First number is to require that it be an equivalence relation, second number is to explicitly not require that it be an equivalence relation, third number is people who believe they need more time to consider the issue. A separate issue: Andy Sawyer pointed out that "i-1" is incorrect, since "i" can refer to the first iterator in the range. Matt provided wording to address this problem.]
[Curaçao: The LWG changed "... the range (first, last)..." to "... the range [first+1, last)..." for clarity. They considered this change close enough to editorial to not require another round of review.]
Rationale:
The LWG also considered an alternative resolution: change 25.2.8 [lib.alg.unique] paragraph 1 to:
For a nonempty range, eliminates all but the first element from every consecutive group of elements referred to by the iterator i in the range (first, last) for which the following conditions hold: *(i-1) == *i or pred(*(i-1), *i) != false.
Also insert a new paragraph, paragraph 1a, that reads: "Notes: The comparison function need not be an equivalence relation."
Informally: the proposed resolution imposes an explicit requirement that the comparison function be an equivalence relation. The alternative resolution does not, and it gives enough information so that the behavior of unique() for a non-equivalence relation is specified. Both resolutions are consistent with the behavior of existing implementations.
Section: 17.4.4.3 [lib.global.functions] Status: Review Submitter: Dave Abrahams Date: 01 Apr 2000
Are algorithms in std:: allowed to use other algorithms without qualification, so functions in user namespaces might be found through Koenig lookup?
For example, a popular standard library implementation includes this implementation of std::unique:
namespace std {
template <class _ForwardIter>
_ForwardIter unique(_ForwardIter __first, _ForwardIter __last) {
__first = adjacent_find(__first, __last);
return unique_copy(__first, __last, __first);
}
}
Imagine two users on opposite sides of town, each using unique on his own sequences bounded by my_iterators . User1 looks at his standard library implementation and says, "I know how to implement a more efficient unique_copy for my_iterators", and writes:
namespace user1 {
class my_iterator;
// faster version for my_iterator
my_iterator unique_copy(my_iterator, my_iterator, my_iterator);
}
user1::unique_copy() is selected by Koenig lookup, as he intended.
User2 has other needs, and writes:
namespace user2 {
class my_iterator;
// Returns true iff *c is a unique copy of *a and *b.
bool unique_copy(my_iterator a, my_iterator b, my_iterator c);
}
User2 is shocked to find later that his fully-qualified use of std::unique(user2::my_iterator, user2::my_iterator, user2::my_iterator) fails to compile (if he's lucky). Looking in the standard, he sees the following Effects clause for unique():
Effects: Eliminates all but the first element from every consecutive group of equal elements referred to by the iterator i in the range [first, last) for which the following corresponding conditions hold: *i == *(i - 1) or pred(*i, *(i - 1)) != false
The standard gives user2 absolutely no reason to think he can interfere with std::unique by defining names in namespace user2. His standard library has been built with the template export feature, so he is unable to inspect the implementation. User1 eventually compiles his code with another compiler, and his version of unique_copy silently stops being called. Eventually, he realizes that he was depending on an implementation detail of his library and had no right to expect his unique_copy() to be called portably.
On the face of it, and given above scenario, it may seem obvious that the implementation of unique() shown is non-conforming because it uses unique_copy() rather than ::std::unique_copy(). Most standard library implementations, however, seem to disagree with this notion.
[Tokyo: Steve Adamczyk from the core working group indicates that "std::" is sufficient; leading "::" qualification is not required because any namespace qualification is sufficient to suppress Koenig lookup.]
Proposed resolution:
Add a paragraph and a note at the end of 17.4.4.3 [lib.global.functions]:
Unless otherwise specified, no global or non-member function in the standard library shall use a function from another namespace which is found through argument-dependent name lookup (3.4.2 [basic.lookup.koenig]).
[Note: the phrase "unless otherwise specified" is intended to allow Koenig lookup in cases like that of ostream_iterators:
Effects:*out_stream << value;
if(delim != 0) *out_stream << delim;
return (*this);--end note]
[Tokyo: The LWG agrees that this is a defect in the standard, but is as yet unsure if the proposed resolution is the best solution. Furthermore, the LWG believes that the same problem of unqualified library names applies to wording in the standard itself, and has opened issue 229 accordingly. Any resolution of issue 225 should be coordinated with the resolution of issue 229.]
[Toronto: The LWG is not sure if this is a defect in the standard. Most LWG members believe that an implementation of std::unique like the one quoted in this issue is already illegal, since, under certain circumstances, its semantics are not those specified in the standard. The standard's description of unique does not say that overloading adjacent_find should have any effect.]
[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. The proposed resolution for this issue is in accordance with Howard's paper.]
Section: 17.4.3.1 [lib.reserved.names] Status: Review Submitter: Dave Abrahams Date: 01 Apr 2000
The issues are:
1. How can a 3rd party library implementor (lib1) write a version of a standard algorithm which is specialized to work with his own class template?
2. How can another library implementor (lib2) write a generic algorithm which will take advantage of the specialized algorithm in lib1?
This appears to be the only viable answer under current language rules:
namespace lib1 { // arbitrary-precision numbers using T as a basic unit template <class T> class big_num { //... };// defining this in namespace std is illegal (it would be an // overload), so we hope users will rely on Koenig lookup template <class T> void swap(big_int<T>&, big_int<T>&); }#include <algorithm> namespace lib2 { template <class T> void generic_sort(T* start, T* end) { ... // using-declaration required so we can work on built-in types using std::swap; // use Koenig lookup to find specialized algorithm if available swap(*x, *y); } }
This answer has some drawbacks. First of all, it makes writing lib2 difficult and somewhat slippery. The implementor needs to remember to write the using-declaration, or generic_sort will fail to compile when T is a built-in type. The second drawback is that the use of this style in lib2 effectively "reserves" names in any namespace which defines types which may eventually be used with lib2. This may seem innocuous at first when applied to names like swap, but consider more ambiguous names like unique_copy() instead. It is easy to imagine the user wanting to define these names differently in his own namespace. A definition with semantics incompatible with the standard library could cause serious problems (see issue 225).
Why, you may ask, can't we just partially specialize std::swap()? It's because the language doesn't allow for partial specialization of function templates. If you write:
namespace std
{
template <class T>
void swap(lib1::big_int<T>&, lib1::big_int<T>&);
}
You have just overloaded std::swap, which is illegal under the current language rules. On the other hand, the following full specialization is legal:
namespace std
{
template <>
void swap(lib1::other_type&, lib1::other_type&);
}
This issue reflects concerns raised by the "Namespace issue with specialized swap" thread on comp.lang.c++.moderated. A similar set of concerns was earlier raised on the boost.org mailing list and the ACCU-general mailing list. Also see library reflector message c++std-lib-7354.
J. C. van Winkel points out (in c++std-lib-9565) another unexpected fact: it's impossible to output a container of std::pair's using copy and an ostream_iterator, as long as both pair-members are built-in or std:: types. That's because a user-defined operator<< for (for example) std::pair<const std::string, int> will not be found: lookup for operator<< will be performed only in namespace std. Opinions differed on whether or not this was a defect, and, if so, whether the defect is that something is wrong with user-defined functionality and std, or whether it's that the standard library does not provide an operator<< for std::pair<>.
Proposed resolution:
Adopt the wording in the Customization Points section of Howard Hinnant's paper, N1387=02-0045.
[Tokyo: Summary, "There is no conforming way to extend std::swap for user defined templates." The LWG agrees that there is a problem. Would like more information before proceeding. This may be a core issue. Core issue 229 has been opened to discuss the core aspects of this problem. It was also noted that submissions regarding this issue have been received from several sources, but too late to be integrated into the issues list. ]
[Post-Tokyo: A paper with several proposed resolutions, J16/00-0029==WG21/N1252, "Shades of namespace std functions " by Alan Griffiths, is in the Post-Tokyo mailing. It should be considered a part of this issue.]
[Toronto: Dave Abrahams and Peter Dimov have proposed a resolution that involves core changes: it would add partial specialization of function template. The Core Working Group is reluctant to add partial specialization of function templates. It is viewed as a large change, CWG believes that proposal presented leaves some syntactic issues unanswered; if the CWG does add partial specialization of function templates, it wishes to develop its own proposal. The LWG continues to believe that there is a serious problem: there is no good way for users to force the library to use user specializations of generic standard library functions, and in certain cases (e.g. transcendental functions called by valarray and complex) this is important. Koenig lookup isn't adequate, since names within the library must be qualified with std (see issue 225), specialization doesn't work (we don't have partial specialization of function templates), and users aren't permitted to add overloads within namespace std. ]
[Copenhagen: Discussed at length, with no consensus. Relevant papers in the pre-Copenhagen mailing: N1289, N1295, N1296. Discussion focused on four options. (1) Relax restrictions on overloads within namespace std. (2) Mandate that the standard library use unqualified calls for swap and possibly other functions. (3) Introduce helper class templates for swap and possibly other functions. (4) Introduce partial specialization of function templates. Every option had both support and opposition. Straw poll (first number is support, second is strongly opposed): (1) 6, 4; (2) 6, 7; (3) 3, 8; (4) 4, 4.]
[Redmond: Discussed, again no consensus. Herb presented an argument that a user who is defining a type T with an associated swap should not be expected to put that swap in namespace std, either by overloading or by partial specialization. The argument is that swap is part of T's interface, and thus should to in the same namespace as T and only in that namespace. If we accept this argument, the consequence is that standard library functions should use unqualified call of swap. (And which other functions? Any?) A small group (Nathan, Howard, Jeremy, Dave, Matt, Walter, Marc) will try to put together a proposal before the next meeting.]
[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. The proposed resolution is the one proposed by Howard.]
Section: 17.4.1.1 [lib.contents] Status: Review Submitter: Steve Clamage Date: 19 Apr 2000
Throughout the library chapters, the descriptions of library entities refer to other library entities without necessarily qualifying the names.
For example, section 25.2.2 "Swap" describes the effect of swap_ranges in terms of the unqualified name "swap". This section could reasonably be interpreted to mean that the library must be implemented so as to do a lookup of the unqualified name "swap", allowing users to override any ::std::swap function when Koenig lookup applies.
Although it would have been best to use explicit qualification with "::std::" throughout, too many lines in the standard would have to be adjusted to make that change in a Technical Corrigendum.
Issue 182, which addresses qualification of size_t, is a special case of this.
Proposed resolution:
To section 17.4.1.1 "Library contents" Add the following paragraph:
Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise. For example, if the Effects section for library function F is described as calling library function G, the function ::std::G is meant.
[Post-Tokyo: Steve Clamage submitted this issue at the request of the LWG to solve a problem in the standard itself similar to the problem within implementations of library identified by issue 225. Any resolution of issue 225 should be coordinated with the resolution of this issue.]
[post-Toronto: Howard is undecided about whether it is appropriate for all standard library function names referred to in other standard library functions to be explicitly qualified by std: it is common advice that users should define global functions that operate on their class in the same namespace as the class, and this requires argument-dependent lookup if those functions are intended to be called by library code. Several LWG members are concerned that valarray appears to require argument-dependent lookup, but that the wording may not be clear enough to fall under "unless explicitly described otherwise".]
[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. This paper resolves issues 225 and 226. In light of that resolution, the proposed resolution for the current issue makes sense.]
Section: 22.2.2.2.2 [lib.facet.num.put.virtuals] Status: Review Submitter: James Kanze, Stephen Clamage Date: 25 Apr 2000
What is the following program supposed to output?
#include <iostream>
int
main()
{
std::cout.setf( std::ios::scientific , std::ios::floatfield ) ;
std::cout.precision( 0 ) ;
std::cout << 1.00 << '\n' ;
return 0 ;
}
From my C experience, I would expect "1e+00"; this is what printf("%.0e" , 1.00 ); does. G++ outputs "1.000000e+00".
The only indication I can find in the standard is 22.2.2.2.2/11, where it says "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 is an obvious error, however, fixed is not a mask for a field, but a value that a multi-bit field may take -- the results of and'ing fmtflags with ios::fixed are not defined, at least not if ios::scientific has been set. G++'s behavior corresponds to what might happen if you do use (flags & fixed) != 0 with a typical implementation (floatfield == 3 << something, fixed == 1 << something, and scientific == 2 << something).
Presumably, the intent is either (flags & floatfield) != 0, or (flags & floatfield) == fixed; the first gives something more or less like the effect of precision in a printf floating point conversion. Only more or less, of course. In order to implement printf formatting correctly, you must know whether the precision was explicitly set or not. Say by initializing it to -1, instead of 6, and stating that for floating point conversions, if precision < -1, 6 will be used, for fixed point, if precision < -1, 1 will be used, etc. Plus, of course, if precision == 0 and flags & floatfield == 0, 1 should be = used. But it probably isn't necessary to emulate all of the anomalies of printf:-).
Proposed resolution:
Replace 22.2.2.2.2 [lib.facet.num.put.virtuals], paragraph 11, with the following sentence:
For conversion from a floating-point type, str.precision() is specified in the conversion specification.
Rationale:
The floatfield determines whether numbers are formatted as if with %f, %e, or %g. If the fixed bit is set, it's %f, if scientific it's %e, and if both bits are set, or neither, it's %g.
Turning to the C standard, a precision of 0 is meaningful for %f and %e. For %g, precision 0 is taken to be the same as precision 1.
The proposed resolution has the effect that if neither fixed nor scientific is set we'll be specifying a precision of 0, which will be internally turned into 1. There's no need to call it out as a special case.
The output of the above program will be "1e+00".
[Post-Curaçao: Howard provided improved wording covering the case where precision is 0 and mode is %g.]
Section: 23.1.2 [lib.associative.reqmts] Status: Open Submitter: Andrew Koenig Date: 30 Apr 2000
If mm is a multimap and p is an iterator into the multimap, then mm.insert(p, x) inserts x into mm with p as a hint as to where it should go. Table 69 claims that the execution time is amortized constant if the insert winds up taking place adjacent to p, but does not say when, if ever, this is guaranteed to happen. All it says it that p is a hint as to where to insert.
The question is whether there is any guarantee about the relationship between p and the insertion point, and, if so, what it is.
I believe the present state is that there is no guarantee: The user can supply p, and the implementation is allowed to disregard it entirely.
Additional comments from Nathan:
The vote [in Redmond] was on whether to elaborately specify the use of
the hint, or to require behavior only if the value could be inserted
adjacent to the hint. I would like to ensure that we have a chance to
vote for a deterministic treatment: "before, if possible, otherwise
after, otherwise anywhere appropriate", as an alternative to the
proposed "before or after, if possible, otherwise [...]".
Proposed resolution:
In table 69 "Associative Container Requirements" in 23.1.2 [lib.associative.reqmts], in the row for a.insert(p, t), change
iterator p is a hint pointing to where the insert should start to search.
to
insertion adjacent to iterator p is preferred if more than one insertion point is valid.
and change
logarithmic in general, but amortized constant if t is inserted right after p.
to
logarithmic in general, but amortized constant if t is inserted adjacent to iterator p.
[Toronto: there was general agreement that this is a real defect: when inserting an element x into a multiset that already contains several copies of x, there is no way to know whether the hint will be used. The proposed resolution was that the new element should always be inserted as close to the hint as possible. So, for example, if there is a subsequence of equivalent values, then providing a.begin() as the hint means that the new element should be inserted before the subsequence even if a.begin() is far away. JC van Winkel supplied precise wording for this proposed resolution, and also for an alternative resolution in which hints are only used when they are adjacent to the insertion point.]
[Copenhagen: the LWG agreed to the original proposed resolution, in which an insertion hint would be used even when it is far from the insertion point. This was contingent on seeing a reference implementation showing that it is possible to implement this requirement without loss of efficiency. John Potter provided such a reference implementation.]
[Redmond: The LWG was reluctant to adopt the proposal that emerged from Copenhagen: it seemed excessively complicated, and went beyond fixing the defect that we identified in Toronto. PJP provided the new wording described in this issue. Nathan agrees that we shouldn't adopt the more detailed semantics, and notes: "we know that you can do it efficiently enough with a red-black tree, but there are other (perhaps better) balanced tree techniques that might differ enough to make the detailed semantics hard to satisfy."]
[Curaçao: Nathan should give us the alternative wording he suggests so the LWG can decide between the two options.]
Section: 25.2.8 [lib.alg.unique] Status: Ready Submitter: Angelika Langer Date: May 15 2000
Some popular implementations of unique_copy() create temporary copies of values in the input sequence, at least if the input iterator is a pointer. Such an implementation is built on the assumption that the value type is CopyConstructible and Assignable.
It is common practice in the standard that algorithms explicitly specify any additional requirements that they impose on any of the types used by the algorithm. An example of an algorithm that creates temporary copies and correctly specifies the additional requirements is accumulate(), 26.4.1 [lib.accumulate].
Since the specifications of unique() and unique_copy() do not require CopyConstructible and Assignable of the InputIterator's value type the above mentioned implementations are not standard-compliant. I cannot judge whether this is a defect in the standard or a defect in the implementations.
Proposed resolution:
In 25.2.8 change:
-4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap.
to:
-4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap. The expression *result = *first must be valid. If neither InputIterator nor OutputIterator meets the requirements of forward iterator then the value type of InputIterator must be copy constructible. Otherwise copy constructible is not required.
[Redmond: the original proposed resolution didn't impose an explicit requirement that the iterator's value type must be copy constructible, on the grounds that an input iterator's value type must always be copy constructible. Not everyone in the LWG thought that this requirement was clear from table 72. It has been suggested that it might be possible to implement unique_copy without requiring assignability, although current implementations do impose that requirement. Howard provided new wording.]
[ Curaçao: The LWG changed the PR editorially to specify "neither...nor...meet..." as clearer than "both...and...do not meet...". Change believed to be so minor as not to require re-review. ]
Section: 23.2.4.3 [lib.vector.modifiers] Status: Open Submitter: Lisa Lippincott Date: 06 June 2000
Paragraph 2 of 23.2.4.3 [lib.vector.modifiers] describes the complexity of vector::insert:
Complexity: If first and last are forward iterators, bidirectional iterators, or random access iterators, the complexity is linear in the number of elements in the range [first, last) plus the distance to the end of the vector. If they are input iterators, the complexity is proportional to the number of elements in the range [first, last) times the distance to the end of the vector.
First, this fails to address the non-iterator forms of insert.
Second, the complexity for input iterators misses an edge case -- it requires that an arbitrary number of elements can be added at the end of a vector in constant time.
At the risk of strengthening the requirement, I suggest simply
Complexity: The complexity is linear in the number of elements inserted plus the distance to the end of the vector.
For input iterators, one may achieve this complexity by first inserting at the end of the vector, and then using rotate.
I looked to see if deque had a similar problem, and was surprised to find that deque places no requirement on the complexity of inserting multiple elements (23.2.1.3 [lib.deque.modifiers], paragraph 3):
Complexity: In the worst case, inserting a single element into a deque takes time linear in the minimum of the distance from the insertion point to the beginning of the deque and the distance from the insertion point to the end of the deque. Inserting a single element either at the beginning or end of a deque always takes constant time and causes a single call to the copy constructor of T.
I suggest:
Complexity: The complexity is linear in the number of elements inserted plus the shorter of the distances to the beginning and end of the deque. Inserting a single element at either the beginning or the end of a deque causes a single call to the copy constructor of T.
Proposed resolution:
[Toronto: It's agreed that there is a defect in complexity of multi-element insert for vector and deque. For vector, the complexity should probably be something along the lines of c1 * N + c2 * distance(i, end()). However, there is some concern about whether it is reasonable to amortize away the copies that we get from a reallocation whenever we exceed the vector's capacity. For deque, the situation is somewhat less clear. Deque is notoriously complicated, and we may not want to impose complexity requirements that would imply any implementation technique more complicated than a while loop whose body is a single-element insert.]
Section: 26.3.2.1 [lib.valarray.cons], 26.3.2.2 [lib.valarray.assign] Status: Ready Submitter: Robert Klarer Date: 31 Jul 2000
This discussion is adapted from message c++std-lib-7056 posted November 11, 1999. I don't think that anyone can reasonably claim that the problem described below is NAD.
These valarray constructors can never be called:
template <class T>
valarray<T>::valarray(const slice_array<T> &);
template <class T>
valarray<T>::valarray(const gslice_array<T> &);
template <class T>
valarray<T>::valarray(const mask_array<T> &);
template <class T>
valarray<T>::valarray(const indirect_array<T> &);
Similarly, these valarray assignment operators cannot be called:
template <class T>
valarray<T> valarray<T>::operator=(const slice_array<T> &);
template <class T>
valarray<T> valarray<T>::operator=(const gslice_array<T> &);
template <class T>
valarray<T> valarray<T>::operator=(const mask_array<T> &);
template <class T>
valarray<T> valarray<T>::operator=(const indirect_array<T> &);
Please consider the following example:
#include <valarray>
using namespace std;
int main()
{
valarray<double> va1(12);
valarray<double> va2(va1[slice(1,4,3)]); // line 1
}
Since the valarray va1 is non-const, the result of the sub-expression va1[slice(1,4,3)] at line 1 is an rvalue of type const std::slice_array<double>. This slice_array rvalue is then used to construct va2. The constructor that is used to construct va2 is declared like this:
template <class T>
valarray<T>::valarray(const slice_array<T> &);
Notice the constructor's const reference parameter. When the constructor is called, a slice_array must be bound to this reference. The rules for binding an rvalue to a const reference are in 8.5.3, paragraph 5 (see also 13.3.3.1.4). Specifically, paragraph 5 indicates that a second slice_array rvalue is constructed (in this case copy-constructed) from the first one; it is this second rvalue that is bound to the reference parameter. Paragraph 5 also requires that the constructor that is used for this purpose be callable, regardless of whether the second rvalue is elided. The copy-constructor in this case is not callable, however, because it is private. Therefore, the compiler should report an error.
Since slice_arrays are always rvalues, the valarray constructor that has a parameter of type const slice_array<T> & can never be called. The same reasoning applies to the three other constructors and the four assignment operators that are listed at the beginning of this post. Furthermore, since these functions cannot be called, the valarray helper classes are almost entirely useless.
Proposed resolution:
slice_array:
gslice_array:
mask_array:
indirect_array:
[This wording is taken from Robert Klarer's reflector message, c++std-lib-7827. Gabriel Dos Reis agrees that this general solution is correct.]
Rationale:
Keeping the valarray constructors private is untenable. Merely making valarray a friend of the helper classes isn't good enough, because access to the copy constructor is checked in the user's environment.
Making the assignment operator public is not strictly necessary to solve this problem. A majority of the LWG (straw poll: 13-4) believed we should make the assignment operators public, in addition to the copy constructors, for reasons of symmetry and user expectation.
Section: 19.1 [lib.std.exceptions] Status: Open Submitter: Dave Abrahams Date: 01 Aug 2000
Many of the standard exception types which implementations are required to throw are constructed with a const std::string& parameter. For example:
19.1.5 Class out_of_range [lib.out.of.range]
namespace std {
class out_of_range : public logic_error {
public:
explicit out_of_range(const string& what_arg);
};
}
1 The class out_of_range defines the type of objects thrown as excep-
tions to report an argument value not in its expected range.
out_of_range(const string& what_arg);
Effects:
Constructs an object of class out_of_range.
Postcondition:
strcmp(what(), what_arg.c_str()) == 0.
There are at least two problems with this:
There may be no cure for (1) other than changing the interface to out_of_range, though one could reasonably argue that (1) is not a defect. Personally I don't care that much if out-of-memory is reported when I only have 20 bytes left, in the case when out_of_range would have been reported. People who use exception-specifications might care a lot, though.
There is a cure for (2), but it isn't completely obvious. I think a note for implementors should be made in the standard. Avoiding possible termination in this case shouldn't be left up to chance. The cure is to use a reference-counted "string" implementation in the exception object. I am not necessarily referring to a std::string here; any simple reference-counting scheme for a NTBS would do.
Further discussion, in email:
...I'm not so concerned about (1). After all, a library implementation can add const char* constructors as an extension, and users don't need to avail themselves of the standard exceptions, though this is a lame position to be forced into. FWIW, std::exception and std::bad_alloc don't require a temporary basic_string.
...I don't think the fixed-size buffer is a solution to the problem,
strictly speaking, because you can't satisfy the postcondition
strcmp(what(), what_arg.c_str()) == 0
For all values of what_arg (i.e. very long values). That means that
the only truly conforming solution requires a dynamic allocation.
Further discussion, from Redmond:
The most important progress we made at the Redmond meeting was realizing that there are two separable issues here: the const string& constructor, and the copy constructor. If a user writes something like throw std::out_of_range("foo"), the const string& constructor is invoked before anything gets thrown. The copy constructor is potentially invoked during stack unwinding.
The copy constructor is a more serious problem, becuase failure during stack unwinding invokes terminate. The copy constructor must be nothrow. Curaçao: Howard thinks this requirement is already present.
The fundamental problem is that it's difficult to get the nothrow requirement to work well with the requirement that the exception objects store a string of unbounded size, particularly if you also try to make the const string& constructor nothrow. Options discussed include:
(Not all of these options are mutually exclusive.)
Proposed resolution:
NAD/Future
Rationale:
Throwing a bad_alloc while trying to construct a message for another exception-derived class is not necessarily a bad thing. And the bad_alloc constructor already has a no throw spec on it (18.4.2.1).
The copy constructors of all exception-derived classes already have a no throw spec. Reference 18.6.1, 19.1 and 15.4/13.
Future:
All involved would like to see const char* constructors added, but this should probably be done for C++0X as opposed to a DR.
I believe the no throw specs currently decorating these functions could be improved by some kind of static no throw spec checking mechanism (in a future C++ language). As they stand, the copy constructors might fail via a call to unexpected. I think what is intended here is that the copy constructors can't fail.
[Toronto: some LWG members thought this was merely a QoI issue, but most believed that it was at least a borderline defect. There was more support for nonnormative advice to implementors than for a normative change.]
[Redmond: discussed, without definite conclusion. Most LWG members thought there was a real defect lurking here. The above proposed resolution/rationale is from Howard, Herb, Kevlin, Martin, and Dave.]
Section: 20.1.5 [lib.allocator.requirements] Status: Open Submitter: Matt Austern Date: 22 Aug 2000
From lib-7752:
I've been assuming (and probably everyone else has been assuming) that allocator instances have a particular property, and I don't think that property can be deduced from anything in Table 32.
I think we have to assume that allocator type conversion is a homomorphism. That is, if x1 and x2 are of type X, where X::value_type is T, and if type Y is X::template rebind<U>::other, then Y(x1) == Y(x2) if and only if x1 == x2.
Further discussion: Howard Hinnant writes, in lib-7757:
I think I can prove that this is not provable by Table 32. And I agree it needs to be true except for the "and only if". If x1 != x2, I see no reason why it can't be true that Y(x1) == Y(x2). Admittedly I can't think of a practical instance where this would happen, or be valuable. But I also don't see a need to add that extra restriction. I think we only need:
if (x1 == x2) then Y(x1) == Y(x2)
If we decide that == on allocators is transitive, then I think I can prove the above. But I don't think == is necessarily transitive on allocators. That is:
Given x1 == x2 and x2 == x3, this does not mean x1 == x3.
Example:
x1 can deallocate pointers from: x1, x2, x3
x2 can deallocate pointers from: x1, x2, x4
x3 can deallocate pointers from: x1, x3
x4 can deallocate pointers from: x2, x4x1 == x2, and x2 == x4, but x1 != x4
Proposed resolution:
[Toronto: LWG members offered multiple opinions. One opinion is that it should not be required that x1 == x2 implies Y(x1) == Y(x2), and that it should not even be required that X(x1) == x1. Another opinion is that the second line from the bottom in table 32 already implies the desired property. This issue should be considered in light of other issues related to allocator instances.]
Section: 23.2.2.4 [lib.list.ops] Status: Open Submitter: P.J. Plauger Date: 27 Nov 2000
Section 23.2.2.4 [lib.list.ops] states that
void splice(iterator position, list<T, Allocator>& x);
invalidates all iterators and references to list x.
But what does the C++ Standard mean by "invalidate"? You can still dereference the iterator to a spliced list element, but you'd better not use it to delimit a range within the original list. For the latter operation, it has definitely lost some of its validity.
If we accept the proposed resolution to issue 250, then we'd better clarify that a "valid" iterator need no longer designate an element within the same container as it once did. We then have to clarify what we mean by invalidating a past-the-end iterator, as when a vector or string grows by reallocation. Clearly, such an iterator has a different kind of validity. Perhaps we should introduce separate terms for the two kinds of "validity."
Proposed resolution:
Add the following text to the end of section 24.1 [lib.iterator.requirements], after paragraph 5:
An invalid iterator is an iterator that may be singular. [Footnote: This definition applies to pointers, since pointers are iterators. The effect of dereferencing an iterator that has been invalidated is undefined.]
[post-Copenhagen: Matt provided wording.]
[Redmond: General agreement with the intent, some objections to the wording. Dave provided new wording.]
[Curaçao: The definition of "singular" is contentious. The 278 resolution must be made consistent with issue 208 and 24.1/5. Furthermore, a Rationale paragraph is required.]
Section: 24.4.1 [lib.reverse.iterators] Status: Open Submitter: Steve Cleary Date: 27 Nov 2000
This came from an email from Steve Cleary to Fergus in reference to issue 179. The library working group briefly discussed this in Toronto and believed it should be a separate issue. There was also some reservations about whether this was a worthwhile problem to fix.
Steve said: "Fixing reverse_iterator. std::reverse_iterator can (and should) be changed to preserve these additional requirements." He also said in email that it can be done without breaking user's code: "If you take a look at my suggested solution, reverse_iterator doesn't have to take two parameters; there is no danger of breaking existing code, except someone taking the address of one of the reverse_iterator global operator functions, and I have to doubt if anyone has ever done that. . . But, just in case they have, you can leave the old global functions in as well -- they won't interfere with the two-template-argument functions. With that, I don't see how any user code could break."
Proposed resolution:
Section: 24.4.1.1 [lib.reverse.iterator] add/change the following declarations:
A) Add a templated assignment operator, after the same manner
as the templated copy constructor, i.e.:
template < class U >
reverse_iterator < Iterator >& operator=(const reverse_iterator< U >& u);
B) Make all global functions (except the operator+) have
two template parameters instead of one, that is, for
operator ==, !=, <, >, <=, >=, - replace:
template < class Iterator >
typename reverse_iterator< Iterator >::difference_type operator-(
const reverse_iterator< Iterator >& x,
const reverse_iterator< Iterator >& y);
with:
template < class Iterator1, class Iterator2 >
typename reverse_iterator < Iterator1 >::difference_type operator-(
const reverse_iterator < Iterator1 > & x,
const reverse_iterator < Iterator2 > & y);
Also make the addition/changes for these signatures in 24.4.1.3 [lib.reverse.iter.ops].
[ Copenhagen: The LWG is concerned that the proposed resolution introduces new overloads. Experience shows that introducing overloads is always risky, and that it would be inappropriate to make this change without implementation experience. It may be desirable to provide this feature in a different way. ]
Section: 22.2.2.2.2 [lib.facet.num.put.virtuals] Status: Review Submitter: Howard Hinnant Date: 5 Dec 2000
Paragraph 16 mistakenly singles out integral types for inserting thousands_sep() characters. This conflicts with the syntax for floating point numbers described under 22.2.3.1/2.
Proposed resolution:
Change paragraph 16 from:
For integral types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 22.2.3.1.2 [lib.facet.numpunct.virtuals].
To:
For arithmetic types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 22.2.3.1.2 [lib.facet.numpunct.virtuals].
[ Copenhagen: Opinions were divided about whether this is actually an inconsistency, but at best it seems to have been unintentional. This is only an issue for floating-point output: The standard is unambiguous that implementations must parse thousands_sep characters when performing floating-point. The standard is also unambiguous that this requirement does not apply to the "C" locale. ]
[ A survey of existing practice is needed; it is believed that some implementations do insert thousands_sep characters for floating-point output and others fail to insert thousands_sep characters for floating-point input even though this is unambiguously required by the standard. ]
[Post-Curaçao: the above proposed resolution is the consensus of Howard, Bill, Pete, Benjamin, Nathan, Dietmar, Boris, and Martin.]
Section: 25.2.4 [lib.alg.replace] Status: Open Submitter: Martin Sebor Date: 15 Dec 2000
The requirements in 25.2.4 [lib.alg.replace], p1 that T to be Assignable (23.1 [lib.container.requirements]) is not necessary or sufficient for either of the algorithms. The algorithms require that std::iterator_traits<ForwardIterator>::value_type be Assignable and that both std::iterator_traits<ForwardIterator>::value_type and be EqualityComparable (20.1.1 [lib.equalitycomparable]) with respect to one another.
Further discussion, from Jeremy:
There are a number of problems with the requires clauses for the algorithms in 25.1 [lib.alg.nonmodifying] and 25.2 [lib.alg.modifying.operations]. The requires clause of each algorithm should describe the necessary and sufficient requirements on the inputs to the algorithm such that the algorithm compiles and runs properly. Many of the requires clauses fail to do this. Here is a summary of the kinds of mistakes:
Here is the list of algorithms that contain mistakes:
Also, in the requirements for EqualityComparable, the requirement that the operator be defined for const objects is lacking.
Proposed resolution:
20.1.1 [lib.equalitycomparable] Change p1 from
In Table 28, T is a type to be supplied by a C++ program instantiating a template, a, b, and c are values of type T.
to
In Table 28, T is a type to be supplied by a C++ program instantiating a template, a, b, and c are values of type const T.
25.1.2 [lib.alg.find] Change p1 from
Requires: Type T is EqualityComparable (20.1.1).
to
Requires: There must be a equality operator defined that accepts type std::iterator_traits<InputIterator>::reference for the left operand and const T for the right operand.
25.1.3 [lib.alg.find.end] Add the following requires clause
Requires: There must be an equality operator defined that accepts type const std::iterator_traits<ForwardIterator1>::value_type for the left operand and const std::iterator_traits<ForwardIterator2>::value_type for the right operand.
25.1.4 [lib.alg.find.first.of] Add the following requires clause
Requires: There must be an equality operator defined that accepts type const std::iterator_traits<ForwardIterator1>::value_type for the left operand and const std::iterator_traits<ForwardIterator2>::value_type for the right operand.
25.1.5 [lib.alg.adjacent.find] Add the following requires clause
Requires: T must be EqualityComparable (20.1.1).
25.1.6 [lib.alg.count] Change p1 from
Requires: Type T is EqualityComparable (20.1.1).
to
Requires: There must be a equality operator defined that accepts type std::iterator_traits<InputIterator>::reference for the left operand and const T for the right operand.
25.1.7 [lib.mismatch] Add the following requires clause
Requires: There must be an equality operator defined that accepts type std::iterator_traits<InputIterator1>::reference for the left operand and std::iterator_traits<InputIterator2>::reference for the right operand.
25.1.8 [lib.alg.equal] Add the following requires clause
Requires: There must be an equality operator defined that accepts type std::iterator_traits<InputIterator1>::reference for the left operand and std::iterator_traits<InputIterator2>::reference for the right operand.
25.1.9 [lib.alg.search] Add the following requires clause
Requires: There must be an equality operator defined that accepts type const std::iterator_traits<ForwardIterator1>::value_type for the left operand and const std::iterator_traits<ForwardIterator2>::value_type for the right operand.
Change change p4 from
Requires: Type T is EqualityComparable (20.1.1), type Size is convertible to integral type (4.7.12.3).
to
Requires: There must be an equality operator defined that accepts const std::iterator_traits<ForwardIterator>::value_type for the left operand and const T for the right operand. The type Size is convertible to integral type (4.7.12.3).
25.2.4 [lib.alg.replace] Change p1 from
Requires: Type T is Assignable (23.1 [lib.container.requirements]) (and, for replace(), EqualityComparable (20.1.1 [lib.equalitycomparable])).
to
Requires: Type std::iterator_traits<ForwardIterator>::value_type is Assignable (23.1 [lib.container.requirements]) and the type const T is convertible to std::iterator_traits<ForwardIterator>::value_type. For replace(), an equality operator must be defined that accepts type std::iterator_traits<ForwardIterator>::reference for the left operand and const T for the right operand.
and change p4 from
Requires: Type T is Assignable (23.1 [lib.container.requirements]) (and, for replace_copy(), EqualityComparable (20.1.1 [lib.equalitycomparable])). The ranges [first, last) and [result, result + (last - first)) shall not overlap.
to
Requires: Both types const T and std::iterator_traits<InputIterator>::reference are writable to the OutputIterator type. For replace_copy() an equality operator must be defined that accepts type std::iterator_traits<InputIterator>::reference for the left operand and const T for the right operand. The ranges [first, last) and [result, result + (last - first)) shall not overlap.
25.2.5 [lib.alg.fill] Change p1 from
Requires: Type T is Assignable (23.1 [lib.container.requirements] ). Size is convertible to an integral type (3.9.1 [basic.fundamental] ).
to
Requires: Type const T is writable to the OutputIterator. Size is convertible to an integral type (3.9.1 [basic.fundamental] ).
25.2.7 [lib.alg.remove] Change p1 from
Requires: Type T is EqualityComparable (20.1.1 [lib.equalitycomparable]).
to
Requires: There must be an equality operator defined that accepts type const std::iterator_traits<ForwardIterator>::value_type for the left operand and const T for the right operand. The type std::iterator_traits<ForwardIterator>::value_type must be Assignable (23.1 [lib.container.requirements]).
[Curaçao: Jeremy reports he has run the changes through his automated test tools. At the request of the LWG, Jeremy will reword the PR in terms of valid expressions rather than "equality operator".]
Section: 25.1.1 [lib.alg.foreach] Status: Open Submitter: Angelika Langer Date: 03 Jan 2001
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.
Proposed resolution:
Add a "Requires" section to section 25.1.1 similar to those proposed for transform and the numeric algorithms (see issue 242):
-2- Requires: In the range [first, last], f shall not invalidate iterators or subranges.
[Copenhagen: The LWG agrees that a function object passed to an algorithm should not invalidate iterators in the range that the algorithm is operating on. The LWG believes that this should be a blanket statement in Clause 25, not just a special requirement for for_each. ]
Section: 25.3.5 [lib.alg.set.operations] Status: Review Submitter: Matt Austern Date: 03 Jan 2001
The standard library contains four algorithms that compute set operations on sorted ranges: set_union, set_intersection, set_difference, and set_symmetric_difference. Each of these algorithms takes two sorted ranges as inputs, and writes the output of the appropriate set operation to an output range. The elements in the output range are sorted.
The ordinary mathematical definitions are generalized so that they apply to ranges containing multiple copies of a given element. Two elements are considered to be "the same" if, according to an ordering relation provided by the user, neither one is less than the other. So, for example, if one input range contains five copies of an element and another contains three, the output range of set_union will contain five copies, the output range of set_intersection will contain three, the output range of set_difference will contain two, and the output range of set_symmetric_difference will contain two.
Because two elements can be "the same" for the purposes of these set algorithms, without being identical in other respects (consider, for example, strings under case-insensitive comparison), this raises a number of unanswered questions:
The standard should either answer these questions, or explicitly say that the answers are unspecified. I prefer the former option, since, as far as I know, all existing implementations behave the same way.
Proposed resolution:
Add the following to the end of 25.3.5.2 [lib.set.union] paragraph 5:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, then max(m, n) of these elements will be copied to the output range: all m of these elements from [first1, last1), and the last max(n-m, 0) of them from [first2, last2), in that order.
Add the following to the end of 25.3.5.3 [lib.set.intersection] paragraph 5:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the first min(m, n) of those elements from [first1, last1) are copied to the output range.
Add a new paragraph, Notes, after 25.3.5.4 [lib.set.difference] paragraph 4:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the last max(m-n, 0) elements from [first1, last1) are copied to the output range.
Add a new paragraph, Notes, after 25.3.5.5 [lib.set.symmetric.difference] paragraph 4:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, then |m - n| of those elements will be copied to the output range: the last m - n of these elements from [first1, last1) if m > n, and the last n - m of these elements from [first2, last2) if m < n.
[Curaçao: Missing Rationale and missing status comments from Redmond made discussion difficult. For union, doesn't the standard already say this? Howard, others think maybe so. Several thought the PR may be "too complicated".]
Section: 17.4.3.1.1 [lib.macro.names] Status: Open Submitter: James Kanze Date: 11 Jan 2001
Paragraph 2 of 17.4.3.1.1 [lib.macro.names] reads: "A translation unit that includes a header shall not contain any macros that define names declared in that header." As I read this, it would mean that the following program is legal:
#define npos 3.14 #include <sstream>
since npos is not defined in <sstream>. It is, however, defined in <string>, and it is hard to imagine an implementation in which <sstream> didn't include <string>.
I think that this phrase was probably formulated before it was decided that a standard header may freely include other standard headers. The phrase would be perfectly appropriate for C, for example. In light of 17.4.4.1 [lib.res.on.headers] paragraph 1, however, it isn't stringent enough.
Proposed resolution:
In paragraph 2 of 17.4.3.1.1 [lib.macro.names], change "A translation unit that includes a header shall not contain any macros that define names declared in that header." to "A translation unit that includes a header shall not contain any macros that define names declared in any standard header."
[Copenhagen: the general idea is clearly correct, but there is concern about making sure that the two paragraphs in 17.4.3.1.1 [lib.macro.names] remain consistent. Nathan will provide new wording.]
Section: 24.1.4 [lib.bidirectional.iterators], 24.1.5 [lib.random.access.iterators] Status: Open Submitter: John Potter Date: 22 Jan 2001
In section 24.1.4 [lib.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 [lib.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.
Proposed resolution:
In section 24.1.4 [lib.bidirectional.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 return type in table 76 from "convertible to T" to T&.
[Curaçao: Jeremy volunteered to work on this issue.]
Section: 23.2.2.4 [lib.list.ops] Status: Open Submitter: John Pedretti Date: 23 Jan 2001
The "Effects" clause for list::merge() (23.2.2.4, p23) appears to be incomplete: it doesn't cover the case where the argument list is identical to *this (i.e., this == &x). The requirement in the note in p24 (below) is that x be empty after the merge which is surely unintended in this case.
Proposed resolution:
In 23.2.2.4 [lib.list.ops], replace paragraps 23-25 with:
23 Effects: if (&x == this) does nothing; otherwise, merges the two sorted ranges [begin(), end()) and [x.begin(), x.end()). The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by comp; that is, for every iterator i in the range other than the first, the condition comp(*i, *(i - 1)) will be false.
24 Notes: Stable: if (&x != this), then for equivalent elements in the two original ranges, the elements from the original range [begin(), end()) always precede the elements from the original range [x.begin(), x.end()). If (&x != this) the range [x.begin(), x.end()) is empty after the merge.
25 Complexity: At most size() + x.size() - 1 applications of comp if (&x ! = this); otherwise, no applications of comp are performed. If an exception is thrown other than by a comparison there are no effects.
[Copenhagen: The original proposed resolution did not fix all of the problems in 23.2.2.4 [lib.list.ops], p22-25. Three different paragraphs (23, 24, 25) describe the effects of merge. Changing p23, without changing the other two, appears to introduce contradictions. Additionally, "merges the argument list into the list" is excessively vague.]
[Post-Curaçao: Robert Klarer provided new wording.]
Section: 24.1 [lib.iterator.requirements] Status: Open Submitter: Dave Abrahams Date: 5 Feb 2001
We all "know" that input iterators are allowed to produce values when dereferenced of which there is no other in-memory copy.
But: Table 72, with a careful reading, seems to imply that this can only be the case if the value_type has no members (e.g. is a built-in type).
The problem occurs in the following entry:
a->m pre: (*a).m is well-defined
Equivalent to (*a).m
*a.m can be well-defined if *a is not a reference type, but since operator->() must return a pointer for a->m to be well-formed, it needs something to return a pointer to. This seems to indicate that *a must be buffered somewhere to make a legal input iterator.
I don't think this was intentional.
Proposed resolution:
[Copenhagen: the two obvious possibilities are to keep the operator-> requirement for Input Iterators, and put in a non-normative note describing how it can be implemented with proxies, or else moving the operator-> requirement from Input Iterator to Forward Iterator. If we do the former we'll also have to change istreambuf_iterator, because it has no operator->. A straw poll showed roughly equal support for the two options.]
Section: 22.2.1.5.2 [lib.locale.codecvt.virtuals] Status: Ready Submitter: Howard Hinnant Date: 24 Jan 2001
22.2.1.5/3 introduces codecvt in part with:
codecvt<wchar_t,char,mbstate_t> converts between the native character sets for tiny and wide characters. Instantiations on mbstate_t perform conversion between encodings known to the library implementor.
But 22.2.1.5.2/10 describes do_length in part with:
... codecvt<wchar_t, char, mbstate_t> ... return(s) the lesser of max and (from_end-from).
The semantics of do_in and do_length are linked. What one does must be consistent with what the other does. 22.2.1.5/3 leads me to believe that the vendor is allowed to choose the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in performs so that it makes his customers happy on a given platform. But 22.2.1.5.2/10 explicitly says what codecvt<wchar_t,char,mbstate_t>::do_length must return. And thus indirectly specifies the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in must perform. I believe that this is not what was intended and is a defect.
Discussion from the -lib reflector:
This proposal would have the effect of making the semantics of
all of the virtual functions in codecvt<wchar_t, char,
mbstate_t> implementation specified. Is that what we want, or
do we want to mandate specific behavior for the base class virtuals
and leave the implementation specified behavior for the codecvt_byname
derived class? The tradeoff is that former allows implementors to
write a base class that actually does something useful, while the
latter gives users a way to get known and specified---albeit
useless---behavior, and is consistent with the way the standard
handles other facets. It is not clear what the original intention
was.
Nathan has suggest a compromise: a character that is a widened version of the characters in the basic execution character set must be converted to a one-byte sequence, but there is no such requirement for characters that are not part of the basic execution character set.
Proposed resolution:
Change 22.2.1.5.2/5 from:
The instantiations required in Table 51 (lib.locale.category), namely codecvt<wchar_t,char,mbstate_t> and codecvt<char,char,mbstate_t>, store no characters. Stores no more than (to_limit-to) destination elements. It always leaves the to_next pointer pointing one beyond the last element successfully stored.
to:
Stores no more than (to_limit-to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored. codecvt<char,char,mbstate_t> stores no characters.
Change 22.2.1.5.2/10 from:
-10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiations required in Table 51 (21.1.1.1.1), namely codecvt<wchar_t, char, mbstate_t> and codecvt<char, char, mbstate_t>, return the lesser of max and (from_end-from).
to:
-10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiation codecvt<char, char, mbstate_t> returns the lesser of max and (from_end-from).
[Redmond: Nathan suggested an alternative resolution: same as above, but require that, in the default encoding, a character from the basic execution character set would map to a single external character. The straw poll was 8-1 in favor of the proposed resolution.]
Rationale:
The default encoding should be whatever users of a given platform would expect to be the most natural. This varies from platform to platform. In many cases there is a preexisting C library, and users would expect the default encoding to be whatever C uses in the default "C" locale. We could impose a guarantee like the one Nathan suggested (a character from the basic execution character set must map to a single external character), but this would rule out important encodings that are in common use: it would rule out JIS, for example, and it would rule out a fixed-width encoding of UCS-4.
[Curaçao: fixed rationale typo at the request of Ichiro Koshida; "shift-JIS" changed to "JIS".]
Section: 27.6 [lib.iostream.format] Status: Open Submitter: Martin Sebor Date: 19 Mar 2001
The descriptions of the constructors of basic_istream<>::sentry (27.6.1.1.2 [lib.istream::sentry]) and basic_ostream<>::sentry (27.6.2.3 [lib.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 27.6.1.1.2 [lib.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 27.6.2.3 [lib.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).
Proposed resolution:
Add the following paragraph immediately after 27.6.1.1.2 [lib.istream::sentry], p5
If an exception is thrown during the preparation then ios::badbit is turned on* in is's error state.
[Footnote: This is done without causing an ios::failure to be thrown. --- end footnote]
If (is.exceptions() & ios_base::badbit)!= 0 then the exception is rethrown.
And strike the following sentence from 27.6.1.1.2 [lib.istream::sentry], p5
During preparation, the constructor may call setstate(failbit) (which may throw ios_base::failure (lib.iostate.flags))
Add the following paragraph immediately after 27.6.2.3 [lib.ostream::sentry], p3
If an exception is thrown during the preparation then ios::badbit is turned on* in os's error state.
[Footnote: This is done without causing an ios::failure to be thrown. --- end footnote]
If (os.exceptions() & ios_base::badbit)!= 0 then the exception is rethrown.
And strike the following sentence from 27.6.2.3 [lib.ostream::sentry], p3
During preparation, the constructor may call setstate(failbit) (which may throw ios_base::failure (lib.iostate.flags))
(Note that the removal of the two sentences means that the ctors will not be able to report the failure of any implementation-dependent operations referred to in footnotes 280 and 293, unless such operations throw an exception.)
[ Copenhagen: It was agreed that there was an issue here, but there was disagreement about the resolution. Some LWG members argued 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. ]
Section: 23.2.2.1 [lib.list.cons] Status: Ready Submitter: Howard Hinnant Date: 17 May 2001
Section 23.2.2.1, paragraphs 6-8 specify that list assign (both forms) have the "effects" of a call to erase followed by a call to insert.
I would like to document that implementers have the freedom to implement assign by other methods, as long as the end result is the same and the exception guarantee is as good or better than the basic guarantee.
The motivation for this is to use T's assignment operator to recycle existing nodes in the list instead of erasing them and reallocating them with new values. It is also worth noting that, with careful coding, most common cases of assign (everything but assignment with true input iterators) can elevate the exception safety to strong if T's assignment has a nothrow guarantee (with no extra memory cost). Metrowerks does this. However I do not propose that this subtlety be standardized. It is a QoI issue.
Existing practise: Metrowerks and SGI recycle nodes, Dinkumware and Rogue Wave don't.
Proposed resolution:
Change 23.2.2.1/7 from:
Effects:
erase(begin(), end()); insert(begin(), first, last);
to:
Effects: Replaces the contents of the list with the range [first, last).
In 23.1.1 [lib.sequence.reqmts], in Table 67 (sequence requirements), add two new rows:
a.assign(i,j) void pre: i,j are not iterators into a.
Replaces elements in a with a copy
of [i, j).
a.assign(n,t) void pre: t is not a reference into a.
Replaces elements in a with n copies
of t.
Change 23.2.2.1/8 from:
Effects:
erase(begin(), end()); insert(begin(), n, t);
to:
Effects: Replaces the contents of the list with n copies of t.
[Redmond: Proposed resolution was changed slightly. Previous version made explicit statement about exception safety, which wasn't consistent with the way exception safety is expressed elsewhere. Also, the change in the sequence requirements is new. Without that change, the proposed resolution would have required that assignment of a subrange would have to work. That too would have been overspecification; it would effectively mandate that assignment use a temporary. Howard provided wording. ]
[Curaçao: Made editorial improvement in wording; changed "Replaces elements in a with copies of elements in [i, j)." with "Replaces the elements of a with a copy of [i, j)." Changes not deemed serious enough to requre rereview.]
Section: 26.5 [lib.c.math] Status: Open Submitter: Dave Abrahams Date: 4 June 2001
Currently the standard mandates the following overloads of abs():
abs(long), abs(int) in <cstdlib>
abs(float), abs(double), abs(long double) in <cmath>
template<class T> T abs(const complex<T>&) in <complex>
template<class T> valarray<T> abs(const valarray<T>&); in <valarray>
The problem is that having only some overloads visible of a function that works on "implicitly inter-convertible" types is dangerous in practice. The headers that get included at any point in a translation unit can change unpredictably during program development/maintenance. The wrong overload might be unintentionally selected.
Currently, there is nothing that mandates the simultaneous visibility of these overloads. Indeed, some vendors have begun fastidiously reducing dependencies among their (public) headers as a QOI issue: it helps people to write portable code by refusing to compile unless all the correct headers are #included.
The same issue may exist for other functions in the library.
Redmond: PJP reports that C99 adds two new kinds of abs: comples, and int_max_abs.
Related issue: 343.
Proposed resolution:
[Redmond: General agreement that the current situation is somewhat fragile. No consensus on whether it's more fragile than any number of other things, or whether there's any good way to fix it. Walter suggests that abs should be defined for all built-in types in both <cmath> and <cstdlib>, but that no effort should be made to put all overloads for class types in one place. Beman suggests closing this issue as "NAD Future", and adding a <all> header as an extension. The <all> header would solve a more general problem: users who can't remember which names are defined in which headers. (See issue 343)]
Section: 24.1.2 [lib.output.iterators] Status: Ready Submitter: Dave Abrahams Date: 7 June 2001
Table 73 suggests that output iterators have value types. It requires the expression "*a = t". Additionally, although Table 73 never lists "a = t" or "X(a) = t" in the "expressions" column, it contains a note saying that "a = t" and "X(a) = t" have equivalent (but nowhere specified!) semantics.
According to 24.1/9, t is supposed to be "a value of value type T":
In the following sections, a and b denote values of X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T.
Two other parts of the standard that are relevant to whether output iterators have value types:
The first of these passages suggests that "*i" is supposed to return a useful value, which contradicts the note in 24.1.2/2 saying that the only valid use of "*i" for output iterators is in an expression of the form "*i = t". The second of these passages appears to contradict Table 73, because it suggests that "*i"'s return value should be void. The second passage is also broken in the case of a an iterator type, like non-const pointers, that satisfies both the output iterator requirements and the forward iterator requirements.
What should the standard say about *i's return value when i is an output iterator, and what should it say about that t is in the expression "*i = t"? Finally, should the standard say anything about output iterators' pointer and reference types?
Proposed resolution:
24.1 p1, change
All iterators i support the expression *i, resulting in a value of some class, enumeration, or built-in type T, called the value type of the iterator.
to
All input iterators i support the expression *i, resulting in a value of some class, enumeration, or built-in type T, called the value type of the iterator. All output iterators support the expression *i = o where o is a value of some type that is in the set of types that are writable to the particular iterator type of i.
24.1 p9, add
o denotes a value of some type that is writable to the output iterator.
Table 73, change
*a = t
to
*r = o
and change
*r++ = t
to
*r++ = o
[post-Redmond: Jeremy provided wording]
Rationale:
The LWG considered two options: change all of the language that seems to imply that output iterators have value types, thus making it clear that output iterators have no value types, or else define value types for output iterator consistently. The LWG chose the former option, because it seems clear that output iterators were never intended to have value types. This was a deliberate design decision, and any language suggesting otherwise is simply a mistake.
A future revision of the standard may wish to revisit this design decision.
Section: 22.2.6.3.2 [lib.locale.moneypunct.virtuals] Status: Ready Submitter: Martin Sebor Date: 02 Jul 2001
The Returns clause in 22.2.6.3.2, p3 says about moneypunct<charT>::do_grouping()
Returns: A pattern defined identically as the result of numpunct<charT>::do_grouping().241)
Footnote 241 then reads
This is most commonly the value "\003" (not "3").
The returns clause seems to imply that the two member functions must return an identical value which in reality may or may not be true, since the facets are usually implemented in terms of struct std::lconv and return the value of the grouping and mon_grouping, respectively. The footnote also implies that the member function of the moneypunct facet (rather than the overridden virtual functions in moneypunct_byname) most commonly return "\003", which contradicts the C standard which specifies the value of "" for the (most common) C locale.
Proposed resolution:
Replace the text in Returns clause in 22.2.6.3.2, p3 with the following:
Returns: A pattern defined identically as, but not necessarily equal to, the result of numpunct<charT>::do_grouping().241)
and replace the text in Footnote 241 with the following:
To specify grouping by 3s the value is "\003", not "3".
Rationale:
The fundamental problem is that the description of the locale facet virtuals serves two purposes: describing the behavior of the base class, and describing the meaning of and constraints on the behavior in arbitrary derived classes. The new wording makes that separation a little bit clearer. The footnote (which is nonnormative) is not supposed to say what the grouping is in the "C" locale or in any other locale. It is just a reminder that the values are interpreted as small integers, not ASCII characters.
Section: 23.2.4.2 [lib.vector.capacity], 23.2.4.3 [lib.vector.modifiers] Status: Ready Submitter: Anthony Williams Date: 13 Jul 2001
There is an apparent contradiction about which circumstances can cause a reallocation of a vector in Section 23.2.4.2 [lib.vector.capacity] and section 23.2.4.3 [lib.vector.modifiers].
23.2.4.2p5 says:
Notes: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the size specified in the most recent call to reserve().
Which implies if I do
std::vector<int> vec; vec.reserve(23); vec.reserve(0); vec.insert(vec.end(),1);
then the implementation may reallocate the vector for the insert, as the size specified in the previous call to reserve was zero.
However, the previous paragraphs (23.2.4.2, p1-2) state:
(capacity) Returns: The total number of elements the vector can hold without requiring reallocation
...After reserve(), capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise...
This implies that vec.capacity() is still 23, and so the insert() should not require a reallocation, as vec.size() is 0. This is backed up by 23.2.4.3p1:
(insert) Notes: Causes reallocation if the new size is greater than the old capacity.
Though this doesn't rule out reallocation if the new size is less than the old capacity, I think the intent is clear.
Proposed resolution:
Change the wording of 23.2.4.2 [lib.vector.capacity] paragraph 5 to:
Notes: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the value of capacity().
[Redmond: original proposed resolution was modified slightly. In the original, the guarantee was that there would be no reallocation until the size would be greater than the value of capacity() after the most recent call to reserve(). The LWG did not believe that the "after the most recent call to reserve()" added any useful information.]
Rationale:
There was general agreement that, when reserve() is called twice in succession and the argument to the second invocation is smaller than the argument to the first, the intent was for the second invocation to have no effect. Wording implying that such cases have an effect on reallocation guarantees was inadvertant.
Section: 27.6.2.7 [lib.ostream.manip] Status: Ready Submitter: PremAnand M. Rao Date: 27 Aug 2001
A footnote in 27.6.2.7 [lib.ostream.manip] states:
[Footnote: The effect of executing cout << endl is to insert a newline character in the output sequence controlled by cout, then synchronize it with any external file with which it might be associated. --- end foonote]
Does the term "file" here refer to the external device? This leads to some implementation ambiguity on systems with fully buffered files where a newline does not cause a flush to the device.
Choosing to sync with the device leads to significant performance penalties for each call to endl, while not sync-ing leads to errors under special circumstances.
I could not find any other statement that explicitly defined the behavior one way or the other.
Proposed resolution:
Remove footnote 300 from section 27.6.2.7 [lib.ostream.manip].
Rationale:
We already have normative text saying what endl does: it inserts a newline character and calls flush. This footnote is at best redundant, at worst (as this issue says) misleading, because it appears to make promises about what flush does.
Section: 23.3.1.2 [lib.map.access] Status: Ready Submitter: Andrea Griffini Date: 02 Sep 2001
The current standard describes map::operator[] using a code example. That code example is however quite inefficient because it requires several useless copies of both the passed key_type value and of default constructed mapped_type instances. My opinion is that was not meant by the comitee to require all those temporary copies.
Currently map::operator[] behaviour is specified as:
Returns:
(*((insert(make_pair(x, T()))).first)).second.
This specification however uses make_pair that is a template function of which parameters in this case will be deduced being of type const key_type& and const T&. This will create a pair<key_type,T> that isn't the correct type expected by map::insert so another copy will be required using the template conversion constructor available in pair to build the required pair<const key_type,T> instance.
If we consider calling of key_type copy constructor and mapped_type default constructor and copy constructor as observable behaviour (as I think we should) then the standard is in this place requiring two copies of a key_type element plus a default construction and two copy construction of a mapped_type (supposing the addressed element is already present in the map; otherwise at least another copy construction for each type).
A simple (half) solution would be replacing the description with:
Returns:
(*((insert(value_type(x, T()))).first)).second.
This will remove the wrong typed pair construction that requires one extra copy of both key and value.
However still the using of map::insert requires temporary objects while the operation, from a logical point of view, doesn't require any.
I think that a better solution would be leaving free an implementer to use a different approach than map::insert that, because of its interface, forces default constructed temporaries and copies in this case. The best solution in my opinion would be just requiring map::operator[] to return a reference to the mapped_type part of the contained element creating a default element with the specified key if no such an element is already present in the container. Also a logarithmic complexity requirement should be specified for the operation.
This would allow library implementers to write alternative implementations not using map::insert and reaching optimal performance in both cases of the addressed element being present or absent from the map (no temporaries at all and just the creation of a new pair inside the container if the element isn't present). Some implementer has already taken this option but I think that the current wording of the standard rules that as non-conforming.
Proposed resolution:
Replace 23.3.1.2 [lib.map.access] paragraph 1 with
-1- Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map.
-2- Returns: A reference to the mapped_type corresponding to x in *this.
-3- Complexity: logarithmic.
[This is the second option mentioned above. Howard provided wording. We may also wish to have a blanket statement somewhere in clause 17 saying that we do not intend the semantics of sample code fragments to be interpreted as specifing exactly how many copies are made. See issue 98 for a similar problem.]
Rationale:
This is the second solution described above; as noted, it is consistent with existing practice.
Note that we now need to specify the complexity explicitly, because we are no longer defining operator[] in terms of insert.
Section: 17 [lib.library] Status: Open Submitter: Detlef Vollmann Date: 05 Sep 2001
From c++std-edit-873:
17.4.1.2 [lib.headers], Table 11. In this table, the header <strstream> is missing.
This shows a general problem: The whole clause 17 refers quite often to clauses 18 through 27, but D.7 is also a part of the standard library (though a deprecated one).
Proposed resolution:
[Redmond: The LWG agrees that <strstream> should be added to table 11. A review is needed to determine whether there are any other places in clause 17 where clause D material should be referred to. Beman will review clause 17.]
[Curaçao: Beman emailed wording to Matt, but not in time for the pre-meeting mailing.]
Section: 22.2 [lib.locale.categories] Status: Ready Submitter: Martin Sebor Date: 17 Sep 2001
From Stage 2 processing in 22.2.2.1.2 [lib.facet.num.get.virtuals], p8 and 9 (the original text or the text corrected by the proposed resolution of issue 221) it seems clear that no whitespace is allowed within a number, but 22.2.3.1 [lib.locale.numpunct], p2, which gives the format for integer and floating point values, says that whitespace is optional between a plusminus and a sign.
The text needs to be clarified to either consistently allow or disallow whitespace between a plusminus and a sign. It might be worthwhile to consider the fact that the C library stdio facility does not permit whitespace embedded in numbers and neither does the C or C++ core language (the syntax of integer-literals is given in 2.13.1 [lex.icon], that of floating-point-literals in 2.13.3 [lex.fcon] of the C++ standard).
Proposed resolution:
Change the first part of 22.2.3.1 [lib.locale.numpunct] paragraph 2 from:
The syntax for number formats is as follows, where digit represents the radix set specified by the fmtflags argument value, whitespace is as determined by the facet ctype<charT> (22.2.1.1), and thousands-sep and decimal-point are the results of corresponding numpunct<charT> members. Integer values have the format:
integer ::= [sign] units sign ::= plusminus [whitespace] plusminus ::= '+' | '-' units ::= digits [thousands-sep units] digits ::= digit [digits]
to:
The syntax for number formats is as follows, where digit represents the radix set specified by the fmtflags argument value, and thousands-sep and decimal-point are the results of corresponding numpunct<charT> members. Integer values have the format:
integer ::= [sign] units sign ::= plusminus plusminus ::= '+' | '-' units ::= digits [thousands-sep units] digits ::= digit [digits]
Rationale:
It's not clear whether the format described in 22.2.3.1 [lib.locale.numpunct] paragraph 2 has any normative weight: nothing in the standard says how, or whether, it's used. However, there's no reason for it to differ gratuitously from the very specific description of numeric processing in 22.2.2.1.2 [lib.facet.num.get.virtuals]. The proposed resolution removes all mention of "whitespace" from that format.
Section: 22.2.1 [lib.category.ctype], 17.3.2.1.2 [lib.bitmask.types] Status: Ready Submitter: Martin Sebor Date: 17 September 2001
The ctype_category::mask type is declared to be an enum in 22.2.1 [lib.category.ctype] with p1 then stating that it is a bitmask type, most likely referring to the definition of bitmask type in 17.3.2.1.2 [lib.bitmask.types], p1. However, the said definition only applies to clause 27, making the reference in 22.2.1 somewhat dubious.
Proposed resolution:
Clarify 17.3.2.1.2, p1 by changing the current text from
Several types defined in clause 27 are bitmask types. Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset (23.3.5 [lib.template.bitset]).
to read
Several types defined in clauses lib.language.support through lib.input.output and Annex D are bitmask types. Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset (lib.template.bitset).
Additionally, change the definition in 22.2.1 to adopt the same convention as in clause 27 by replacing the existing text with the following (note, in particluar, the cross-reference to 17.3.2.1.2 in 22.2.1, p1):
22.2.1 The ctype category [lib.category.ctype]
namespace std { class ctype_base { public: typedef T mask; // numeric values are for exposition only. static const mask space = 1 << 0; static const mask print = 1 << 1; static const mask cntrl = 1 << 2; static const mask upper = 1 << 3; static const mask lower = 1 << 4; static const mask alpha = 1 << 5; static const mask digit = 1 << 6; static const mask punct = 1 << 7; static const mask xdigit = 1 << 8; static const mask alnum = alpha | digit; static const mask graph = alnum | punct; }; }The type mask is a bitmask type (17.3.2.1.2 [lib.bitmask.types]).
[Curaçao: The LWG notes that T above should be bold-italics to be consistent with the rest of the standard.]
Section: 22.1.1.1.1 [lib.locale.category] Status: Ready Submitter: Martin Sebor Date: 18 Sep 2001
It's unclear whether 22.1.1.1.1, p3 says that has_facet<Facet>(loc) returns true for any Facet from Table 51 or whether it includes Table 52 as well:
For any locale loc either constructed, or returned by locale::classic(), and any facet Facet that is a member of a standard category, has_facet<Facet>(loc) is true. Each locale member function which takes a locale::category argument operates on the corresponding set of facets.
It seems that it comes down to which facets are considered to be members of a standard category. Intuitively, I would classify all the facets in Table 52 as members of their respective standard categories, but there are an unbounded set of them...
The paragraph implies that, for instance, has_facet<num_put<C, OutputIterator> >(loc) must always return true. I don't think that's possible. If it were, then use_facet<num_put<C, OutputIterator> >(loc) would have to return a reference to a distinct object for each valid specialization of num_put<C, OutputIteratory>, which is clearly impossible.
On the other hand, if none of the facets in Table 52 is a member of a standard category then none of the locale member functions that operate on entire categories of facets will work properly.
It seems that what p3 should mention that it's required (permitted?) to hold only for specializations of Facet from Table 52 on C from the set { char, wchar_t }, and InputIterator and OutputIterator from the set of { {i,o}streambuf_iterator<{char,wchar_t}> }.
Proposed resolution:
In 22.1.1.1.1 [lib.locale.category], paragraph 3, change "that is a member of a standard category" to "shown in Table 51".
Rationale:
The facets in Table 52 are an unbounded set. Locales should not be required to contain an infinite number of facets.
It's not necessary to talk about which values of InputIterator and OutputIterator must be supported. Table 51 already contains a complete list of the ones we need.
Section: 23.2.4.2 [lib.vector.capacity] Status: Ready Submitter: Anthony Williams Date: 27 Sep 2001
It is a common idiom to reduce the capacity of a vector by swapping it with an empty one:
std::vector<SomeType> vec; // fill vec with data std::vector<SomeType>().swap(vec); // vec is now empty, with minimal capacity
However, the wording of 23.2.4.2 [lib.vector.capacity]paragraph 5 prevents the capacity of a vector being reduced, following a call to reserve(). This invalidates the idiom, as swap() is thus prevented from reducing the capacity. The proposed wording for issue 329 does not affect this. Consequently, the example above requires the temporary to be expanded to cater for the contents of vec, and the contents be copied across. This is a linear-time operation.
However, the container requirements state that swap must have constant complexity (23.1 [lib.container.requirements] note to table 65).
This is an important issue, as reallocation affects the validity of references and iterators.
If the wording of 23.2.4.2p5 is taken to be the desired intent, then references and iterators remain valid after a call to swap, if they refer to an element before the new end() of the vector into which they originally pointed, in which case they refer to the element at the same index position. Iterators and references that referred to an element whose index position was beyond the new end of the vector are invalidated.
If the note to table 65 is taken as the desired intent, then there are two possibilities with regard to iterators and references:
Proposed resolution:
Add a new paragraph after 23.2.4.2 [lib.vector.capacity] paragraph 5:
void swap(vector<T,Allocator>& x);Effects: Exchanges the contents and capacity() of *this with that of x.
Complexity: Constant time.
[This solves the problem reported for this issue. We may also have a problem with a circular definition of swap() for other containers.]
Rationale:
swap should be constant time. The clear intent is that it should just do pointer twiddling, and that it should exchange all properties of the two vectors, including their reallocation guarantees.
Section: 27.6.1.3 [lib.istream.unformatted] Status: Open Submitter: Howard Hinnant Date: 09 Oct 201
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 [lib.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.
Dietmar: nothing should depend on eofbit. Eofbit should only be examined by the user to determine why something failed.
[Taken from c++std-lib-8873, c++std-lib-8874, c++std-lib-8876]
Proposed resolution:
[Howard will do a survey to find out if there are any other places where we have a problem, where the difference between fail() and !good() is important.]
Section: 22.1.1.1.1 [lib.locale.category] Status: Open Submitter: P.J. Plauger, Nathan Myers Date: 23 Oct 2001
In 22.1.1.1.1 [lib.locale.category] paragraph 1, the category members are described as bitmask elements. In fact, the bitmask requirements in 17.3.2.1.2 [lib.bitmask.types] don't seem quite right: none and all are bitmask constants, not bitmask elements.
In particular, the requirements for none interact poorly with the requirement that the LC_* constants from the C library must be recognizable as C++ locale category constants. LC_* values should not be mixed with these values to make category values.
We have two options for the proposed resolution. Informally: option 1 removes the requirement that LC_* values be recognized as category arguments. Option 2 changes the category type so that this requirement is implementable, by allowing none to be some value such as 0x1000 instead of 0.
Nathan writes: "I believe my proposed resolution [Option 2] merely re-expresses the status quo more clearly, without introducing any changes beyond resolving the DR.
Proposed resolution:
Option 1:
Replace the first two paragraphs of 22.1.1.1 [lib.locale.types] with:
typedef int category;Valid category values include the locale member bitmask elements collate, ctype, monetary, numeric, time, and messages, each of which represents a single locale category. In addition, locale member bitmask constant none is defined as zero and represents no category. And locale member bitmask constant all is defined such that the expression
(collate | ctype | monetary | numeric | time | messages | all) == allis true, and represents the union of all categories. Further the expression (X | Y), where X and Y each represent a single category, represents the union of the two categories.
locale member functions expecting a category argument require one of the category values defined above, or the union of two or more such values. Such a category argument identifies a set of locale categories. Each locale category, in turn, identifies a set of locale facets, including at least those shown in Table 51:
Option 2:
Replace the first paragraph of 22.1.1.1 [lib.locale.types] with:
Valid category values include the enumerated values. In addition, the result of applying commutative operators | and & to any two valid values is valid, and results in the setwise union and intersection, respectively, of the argument categories. The values all and none are defined such that for any valid value cat, the expressions (cat | all == all), (cat & all == cat), (cat | none == cat) and (cat & none == none) are true. For non-equal values cat1 and cat2 of the remaining enumerated values, (cat1 & cat2 == none) is true. For any valid categories cat1 and cat2, the result of (cat1 & ~cat2) is valid, and equals the setwise union of those categories found in cat1 but not found in cat2. [Footnote: it is not required that all equal the setwise union of the other enumerated values; implementations may add extra categories.]
[Curaçao: need input from locale experts.]
Section: 20.2.2 [lib.pairs] Status: Open Submitter: Andy Sawyer Date: 23 Oct 2001
The current wording of 20.2.2 [lib.pairs] p6 precludes the use of operator< on any pair type which contains a pointer.
Proposed resolution:
In 20.2.2 [lib.pairs] paragraph 6, replace:
Returns: x.first < y.first || (!(y.first < x.first) && x.second <
y.second).
With:
Returns: std::less<T1>()( x.first, y.first ) ||
(!std::less<T1>()( y.first, x.first) &&
std::less<T2>()( x.second, y.second ) )
[Curaçao: LWG leaning toward NAD. In favor of the PR is that it removes a trap for users. Concerns: 1) will break some small amount of existing code (which define less and operator < with different behavior), 2) don't have any indication of rationale for current design (and unwilling to change without knowing rationale), 3) consistency; pairs of ptrs would behave differenly from individual pointers.]
Section: 24.5.2 [lib.ostream.iterator] Status: Ready Submitter: Andy Sawyer Date: 24 Oct 2001
24.5.2 [lib.ostream.iterator] states:
[...]
private:
// basic_ostream<charT,traits>* out_stream; exposition only
// const char* delim; exposition only
Whilst it's clearly marked "exposition only", I suspect 'delim' should be of type 'const charT*'.
Proposed resolution:
In 24.5.2 [lib.ostream.iterator], replace const char* delim with const charT* delim.
Section: 20.4.1.1 [lib.allocator.members], 20.1.5 [lib.allocator.requirements], 17.4.1.1 [lib.contents] Status: Open Submitter: Nathan Myers Date: 25 Oct 2001
See c++std-lib-9006 and c++std-lib-9007. This issue is taken verbatim from -9007.
The core language feature allowing definition of operator&() applied to any non-builtin type makes that operator often unsafe to use in implementing libraries, including the Standard Library. The result is that many library facilities fail for legal user code, such as the fragment
class A { private: A* operator&(); };
std::vector<A> aa;
class B { };
B* operator&(B&) { return 0; }
std::vector<B> ba;
In particular, the requirements table for Allocator (Table 32) specifies no semantics at all for member address(), and allocator<>::address is defined in terms of unadorned operator &.
[Curaçao: The LWG believes both examples are ill-formed. The contained type is required to be CopyConstructible (20.1.3), and that includes the requirement that &t return the usual types and values. Since the CopyConstructible requirements appear to have been written to deal with the concerns of this issue, the LWG feels it is NAD unless someone can come up with a well-formed example exhibiting a problem.]
Proposed resolution:
In 20.4.1.1, Change the definition of allocator<>::address from:
Returns: &x
to:
Returns: The value that the built in operator&(x) would return if not overloaded.
In 20.1.5, Table 32, add to the Notes column of the a.address(r) and a.address(s) lines, respectively:
allocator<T>::address(r) allocator<T>::address(s)
In addition, in clause 17.4.1.1, add a statement:
The Standard Library does not apply operator& to any type for which operator& may be overloaded.
[Curaçao: If the issues isn't NAD, suggest changing "if not overloaded" to "ignoring all overloads".]
Rationale:
The obvious implementations for std::allocator<>::address are
T* reinterpret_cast<T*>(&static_cast<char&>(o));
and
T const* reinterpret_cast<T const*>(&static_cast<char const&>(o));
but to define them formally in terms of reinterpret_cast<> seems to introduce semantic difficulties best avoided. Using a.address() should not introduce unspecified or implementation-defined semantics into a user program.
Section: 21.1.2 [lib.char.traits.typedefs] Status: Open Submitter: Martin Sebor Date: 2 Dec 2001
(1) There are no requirements on the stateT template parameter of fpos listed in 27.4.3. The interface appears to require that the type be at least Assignable and CopyConstructible (27.4.3.1, p1), and I think also DefaultConstructible (to implement the operations in Table 88).
21.1.2, p3, however, only requires that char_traits<charT>::state_type meet the requirements of CopyConstructible types.
(2) Additionally, the stateT template argument has no corresponding typedef in fpos which might make it difficult to use in generic code.
Proposed resolution:
Modify 21.1.2, p4 from
Requires: state_type shall meet the requirements of CopyConstructible types (20.1.3).
Requires: state_type shall meet the requirements of Assignable (23.1, p4), CopyConstructible (20.1.3), and DefaultConstructible (20.1.4) types.
Add to the definition of the fpos class template the following member:
typedef stateT state_type;
and add to 27.4.3.1 a paragraph with the following text:
typedef stateT state_type;
Requires: state_type shall meet the requirements of Assignable (23.1, p4), CopyConstructible (20.1.3), and DefaultConstructible (20.1.4) types.
[Curaçao: The LWG feels this is two issues, as indicated above. The first is a defect; more I/O experts need to review the PR. The second is questionable; who would use it? Unless motivation is provided, the second should be considered NAD.]
Section: 23.1.2 [lib.associative.reqmts] Status: Ready Submitter: Hans Aberg Date: 17 Dec 2001
Discussions in the thread "Associative container lower/upper bound requirements" on comp.std.c++ suggests that there is a defect in the C++ standard, Table 69 of section 23.1.2, "Associative containers", [lib.associative.reqmts]. It currently says:
a.find(k): returns an iterator pointing to an element with the key equivalent to k, or a.end() if such an element is not found.
a.lower_bound(k): returns an iterator pointing to the first element with key not less than k.
a.upper_bound(k): returns an iterator pointing to the first element with key greater than k.
We have "or a.end() if such an element is not found" for find, but not for upper_bound or lower_bound. As the text stands, one would be forced to insert a new element into the container and return an iterator to that in case the sought iterator does not exist, which does not seem to be the intention (and not possible with the "const" versions).
Proposed resolution:
Change Table 69 of section 23.1.2 [lib.associative.reqmts] indicated entries to:
a.lower_bound(k): returns an iterator pointing to the first element with key not less than k, or a.end() if such an element is not found.
a.upper_bound(k): returns an iterator pointing to the first element with key greater than k, or a.end() if such an element is not found.
[Curaçao: LWG reviewed PR.]
Section: 23.1.1 [lib.sequence.reqmts] Status: Review Submitter: Yaroslav Mironov Date: 23 Jan 2002
Table 68 "Optional Sequence Operations" in 23.1.1/12 specifies operational semantics for "a.back()" as "*--a.end()", which may be ill-formed [because calling operator-- on a temporary (the return) of a built-in type is ill-formed], provided a.end() returns a simple pointer rvalue (this is almost always the case for std::vector::end(), for example). Thus, the specification is not only incorrect, it demonstrates a dangerous construct: "--a.end()" may successfully compile and run as intended, but after changing the type of the container or the mode of compilation it may produce compile-time error.
Proposed resolution:
Change the specification in table 68 "Optional Sequence Operations" in 23.1.1/12 for "a.back()" from
*--a.end()
to
*a.rbegin()
and the specification for "a.pop_back()" from
a.erase(--a.end())
to
a.erase(rbegin())
[Curaçao: LWG changed PR from "{ X::iterator tmp = a.end(); return *--tmp; }" to "*a.rbegin()", and from "{ X::iterator tmp = a.end(); a.erase(--tmp); }" to "a.erase(rbegin())".]
[There is a second possible defect; table 68 "Optional Sequence Operations" in the "Operational Semantics" column uses operations present only in the "Reversible Container" requirements, yet there is no stated dependency between these separate requirements tables. Ask in Santa Cruz if the LWG would like a new issue opened.]
Section: 22.2.1 [lib.category.ctype] Status: New Submitter: Matt Austern Date: 23 Jan 2002
What should the following program print?
#include <locale>
#include <iostream>
class my_ctype : public std::ctype<char>
{
typedef std::ctype<char> base;
public:
my_ctype(std::size_t refs = 0) : base(my_table, false, refs)
{
std::copy(base::classic_table(), base::classic_table() + base::table_size,
my_table);
my_table[(unsigned char) '_'] = (base::mask) (base::print | base::space);
}
private:
mask my_table[base::table_size];
};
int main()
{
my_ctype ct;
std::cout << "isspace: " << ct.is(std::ctype_base::space, '_') << " "
<< "isalpha: " << ct.is(std::ctype_base::alpha, '_') << std::endl;
}
The goal is to create a facet where '_' is treated as whitespace.
On gcc 3.0, this program prints "isspace: 1 isalpha: 0". On Microsoft C++ it prints "isspace: 1 isalpha: 1".
I believe that both implementations are legal, and the standard does not give enough guidance for users to be able to use std::ctype's protected interface portably.
The above program assumes that ctype_base::mask enumerators like space and print are disjoint, and that the way to say that a character is both a space and a printing character is to or those two enumerators together. This is suggested by the "exposition only" values in 22.2.1 [lib.category.ctype], but it is nowhere specified in normative text. An alternative interpretation is that the more specific categories subsume the less specific. The above program gives the results it does on the Microsoft compiler because, on that compiler, print has all the bits set for each specific printing character class.
From the point of view of std::ctype's public interface, there's no important difference between these two techniques. From the point of view of the protected interface, there is. If I'm defining a facet that inherits from std::ctype<char>, I'm the one who defines the value that table()['a'] returns. I need to know what combination of mask values I should use. This isn't so very esoteric: it's exactly why std::ctype has a protected interface. If we care about users being able to write their own ctype facets, we have to give them a portable way to do it.
Related reflector messages: lib-9224, lib-9226, lib-9229, lib-9270, lib-9272, lib-9273, lib-9274, lib-9277, lib-9279.
Issue 339 is related, but not identical. The proposed resolution if issue 339 says that ctype_base::mask must be a bitmask type. It does not say that the ctype_base::mask elements are bitmask elements, so it doesn't directly affect this issue.
Proposed resolution:
Informally, we have three choices:
Either of the first two options is just as good from the standpoint of portability. Either one will require some implementations to change.
Section: 26.5 [lib.c.math] Status: Open Submitter: Ray Lischner Date: 26 Feb 2002
The float versions of the math functions have no meaningful value to return for a range error. The long double versions have a value they can return, but it isn't necessarily the most reasonable value.
Section 26.5 [lib.c.math], paragraph 5, says that C++ "adds float and long double overloaded versions of these functions, with the same semantics," referring to the math functions from the C90 standard.
The C90 standard, in section 7.5.1, paragraph 3, says that functions return "the value of the macro HUGE_VAL" when they encounter a range error. Section 7.5, paragraph 2, defines HUGE_VAL as a macro that "expands to a positive double expression, not necessarily representable as a float."
Therefore, the float versions of the math functions have no way to signal a range error. [Curaçao: The LWG notes that this isn't strictly correct, since errno is set.] The semantics require that they return HUGE_VAL, but they cannot because HUGE_VAL might not be representable as a float.
The problem with long double functions is less severe because HUGE_VAL is representable as a long double. On the other hand, it might not be a "huge" long double value, and might fall well within the range of normal return values for a long double function. Therefore, it does not make sense for a long double function to return a double (HUGE_VAL) for a range error.
Proposed resolution:
Curaçao: C99 was faced with a similar problem, which they fixed by adding HUGE_VALF and HUGE_VALL in addition to HUGE_VAL.
C++ must also fix, but it should be done in the context of the general C99 based changes to C++, not via DR. Thus the LWG in Curaçao felt the resolution should be NAD, FUTURE, but the issue is being held open for one more meeting to ensure LWG members not present during the discussion concur.
Section: 22.2.2.1.2 [lib.facet.num.get.virtuals] Status: New Submitter: Martin Sebor Date: 12 Mar 2002
I don't think thousands_sep is being treated correctly after decimal_point has been seen. Since grouping applies only to the integral part of the number, the first such occurrence should, IMO, terminate Stage 2. (If it does not terminate it, then 22.2.2.1.2, p12 and 22.2.3.1.2, p3 need to explain how thousands_sep is to be interpreted in the fractional part of a number.)
The easiest change I can think of that resolves this issue would be something like below.
Proposed resolution:
Change the first sentence of 22.2.2.1.2, p9 from
If discard is true then the position of the character is remembered, but the character is otherwise ignored. If it is not discarded, then a check is made to determine if c is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.
to
If discard is true, then if '.' has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if '.' has already been accumulated, the character is discarded and Stage 2 terminates. ...
Section: 22.2.2.2.1 [lib.facet.num.put.members] Status: New Submitter: Martin Sebor Date: 12 Mar 2002
22.2.2.2.1, p1:
iter_type put (iter_type out, ios_base& str, char_type fill,
bool val) const;
...
1 Returns: do_put (out, str, fill, val).
AFAICS, the behavior of do_put (..., bool) is not documented anywhere, however, 22.2.2.2.2, p23:
iter_type put (iter_type out, ios_base& str, char_type fill, bool val) const;Effects: If (str.flags() & ios_base::boolalpha) == 0 then do out = do_put(out, str, fill, (int)val) Otherwise dostring_type s = val ? use_facet<ctype<charT> >(loc).truename() : use_facet<ctype<charT> >(loc).falsename();and then insert the characters of s into out. out.
This means that the bool overload of do_put() will never be called, which contradicts the first paragraph. Perhaps the declaration should read do_put(), and not put()?
Note also that there is no Returns clause for this function, which should probably be corrected, just as should the second occurrence of "out." in the text.
I think the least invasive change to fix it would be something like the following:
Proposed resolution:
In 22.2.2.2.2, p23, make the following changes
Replace put() with do_put() in the declaration of the member function.
Change the Effects clause to a Returns clause (to avoid the requirement to call do_put(..., int) from do_put (..., bool)) like so:
23 Returns: If (str.flags() &
ios_base::boolalpha) == 0 then
do_put (out, str, fill, (int)val)
Otherwise the function obtains a string s as if by
string_type s =
val ? use_facet<ctype<charT> >(loc).truename()
: use_facet<ctype<charT> >(loc).falsename();
and then inserts each character c of s into out via
*out++ = c
and returns out.
Section: 22.1.1 [lib.locale] Status: New Submitter: Martin Sebor Date: 12 Mar 2002
22.1.1, p7 (copied below) allows iostream formatters and extractors to make assumptions about the values returned from facet members. However, such assumptions are apparently not guaranteed to hold in other cases (e.g., when the facet members are being called directly rather than as a result of iostream calls, or between successive calls to the same iostream functions with no interevening calls to imbue(), or even when the facet member functions are called from other member functions of other facets). This restriction prevents locale from being implemented efficiently.
Proposed resolution:
Change the first sentence in 22.1.1, p7 from
In successive calls to a locale facet member function during a call to an iostream inserter or extractor or a streambuf member function, the returned result shall be identical. [Note: This implies that such results may safely be reused without calling the locale facet member function again, and that member functions of iostream classes cannot safely call imbue() themselves, except as specified elsewhere. --end note]
to
In successive calls to a locale facet member function on a facet object installed in the same locale, the returned result shall be identical. ...
Section: 22.2.2.2.2 [lib.facet.num.put.virtuals] Status: New Submitter: Martin Sebor Date: 12 Mar 2002
22.2.2.2.2, p12 specifies that thousands_sep is to be inserted only for integral types (issue 282 suggests that this should be done for all arithmetic types).
22.2.2.1.2, p12 requires that grouping be checked for all extractors including that for void*.
I don't think that's right. void* values should not be checked for grouping, should they? (Although if they should, then num_put needs to write them out, otherwise their extraction will fail.)
Proposed resolution:
Change the first sentence of 22.2.2.2.2, p12 from
Digit grouping is checked. That is, the positions of discarded separators is examined for consistency with use_facet<numpunct<charT> >(loc).grouping(). If they are not consistent then ios_base::failbit is assigned to err.
to
Except for conversions to void*, digit grouping is checked...
Section: 20.3.6.2 [lib.bind.1st] Status: New Submitter: Andrew Demkin Date: 26 Apr 2002
The definition of bind1st() (20.3.6.2 [lib.bind.1st]) can result in the construction of an unsafe binding between incompatible pointer types. For example, given a function whose first parameter type is 'pointer to T', it's possible without error to bind an argument of type 'pointer to U' when U does not derive from T:
foo(T*, int);
struct T {};
struct U {};
U u;
int* p;
int* q;
for_each(p, q, bind1st(ptr_fun(foo), &u)); // unsafe binding
The definition of bind1st() includes a functional-style conversion to map its argument to the expected argument type of the bound function (see below):
typename Operation::first_argument_type(x)
A functional-style conversion (5.2.3 [expr.type.conv]) is defined to be semantically equivalent to an explicit cast expression (5.4 [expr.cast]), which may (according to 5.4, paragraph 5) be interpreted as a reinterpret_cast, thus masking the error.
The problem and proposed change also apply to 20.3.6.4 [lib.bind.2nd].
Proposed resolution:
The simplest and most localized change to prevent such errors is to require bind1st() use a static_cast expression rather than the functional-style conversion; that is, have bind1st() return:
binder1st<Operation>( op,
static_cast<typename Operation::first_argument_type>(x)).
A more agressive solution is to change the semantics of functional-style conversions to not permit a reinterpret_cast. For contexts that require the semantics of reinterpret_cast, the language may want to require the use of an explicit cast expression such as '(T) x' or 'reinterpret_cast<T>(x)' and limit the behavior of the functional notation to match statically-checked and standard conversions (as defined by 5.2.9 and 4.10, etc.). Although changing the semantics of functional-style conversions may seem drastic and does have language-wide ramifications, it has the benefit of better unifying the conversion rules for user defined types and built-in types, which can be especially important for generic template programming.
Section: 27.4.2.1.1 [lib.ios::failure] Status: New Submitter: Walter Brown Date: 20 May 2002
The destructor of ios_base::failure should have an empty throw specification, because the destructor of its base class, exception, is declared in this way.
Proposed resolution:
Change the destructor to
virtual ~failure() throw();
Section: 27.5.2.4.2 [lib.streambuf.virt.buffer] Status: New Submitter: Walter Brown Date: 10 May 2002
27.5.2.4.2 [lib.streambuf.virt.buffer] paragraph 1 is inconsistent with the Effects clause for seekoff.
Proposed resolution:
Make this paragraph, the Effects clause for setbuf, consistent in wording with the Effects clause for seekoff in paragraph 3 by amending paragraph 1 to indicate the purpose of setbuf:
Original text:
1 Effects: Performs an operation that is defined separately for each class derived from basic_streambuf in this clause (27.7.1.3, 27.8.1.4).
Proposed text:
1 Effects: Influences stream buffering in a way that is defined separately for each class derived from basic_streambuf in this clause (27.7.1.3, 27.8.1.4).
Section: 27 [lib.input.output] Status: New Submitter: Walter Brown Date: 10 May 2002
None of the following member functions are declared const, but we believe each should be. See document N1360 for details and rationale.
Proposed resolution:
In 27.5.2 and 27.5.2.2.3
Replace
streamsize in_avail();
with
streamsize in_avail() const;
In 27.5.2 and 27.5.2.4.3, and 27.8.1.1 and 27.8.1.4
Replace
virtual streamsize showmanyc();
with
virtual streamsize showmanyc() const;
In 27.6.1.1 and 27.6.1.3
Replace
pos_type tellg();
with
pos_type tellg() const;
This requires additional change, because paragraph 37 describes the return value in terms of calls to non-const member functions. Either of the two following solutions would allow tellg to be declared const.
Option 1: Implementers may cast away const-ness, to allow calling the non-const rdbuf.
In paragraph 37, replace:
.... rdbuf() ->pubseekoff(0, cur, in).
by
.... const_cast<basic_istream<charT, traits>*>(this)->rdbuf() ->pubseekoff(0, cur, in).
Option 2: Provide const member functions to do the job. The proposals in a later section (specifically, the modifications concerning rdbuf throughout the iostream library) meet most of this need; we would also need the following addition to basic_streambuf:
basic_streambuf<charT,traits>::pos_type basic_streambuf<charT,traits>::position(ios_base::openmode mode = ios_base::in|ios_base::out) const;Effects: same as calling basic_streambuf::pubseekoff(0, ios base::cur, mode)
In 27.6.2.1 and 27.6.2.4
Replace
pos_type tellp();
with
pos_type tell() const;
This requires additional change; see the discussion for tellg() above.
In 27.8.1.5, 27.8.1.7, 27.8.1.8, 27.8.1.10, 27.8.1.11, and 27.8.1.13
Replace
bool is_open();
with
bool is_open() const;
Section: 27 [lib.input.output] Status: New Submitter: Walter Brown Date: 10 May 2002
The following member functions are declared const, yet return non-const pointers. We believe they are should be changed, because they allow code that may surprise the user. See document N1360 for details and rationale.
Proposed resolution:
In 27.4.4 and 27.4.4.2
Replace
basic_ostream<charT,traits>* tie() const;
with
basic_ostream<charT,traits>* tie(); const basic_ostream<charT,traits>* tie() const;
and replace
basic_streambuf<charT,traits>* rdbuf() const;
with
basic_streambuf<charT,traits>* rdbuf(); const basic_streambuf<charT,traits>* rdbuf() const;
In 27.5.2 and 27.5.2.3.1
Replace
char_type* eback() const;
with
char_type* eback(); const char_type* eback() const;
Replace
char_type gptr() const;
with
char_type* gptr(); const char_type* gptr() const;
Replace
char_type* egptr() const;
with
char_type* egptr(); const char_type* egptr() const;
In 27.5.2 and 27.5.2.3.2
Replace
char_type* pbase() const;
with
char_type* pbase(); const char_type* pbase() const;
Replace
char_type* pptr() const;
with
char_type* pptr(); const char_type* pptr() const;
Replace
char_type* epptr() const;
with
char_type* epptr(); const char_type* epptr() const;
In 27.7.2, 27.7.2.2, 27.7.3 27.7.3.2, 27.7.4, and 27.7.6
Replace
basic_stringbuf<charT,traits,Allocator>* rdbuf() const;
with
basic_stringbuf<charT,traits,Allocator>* rdbuf(); const basic_stringbuf<charT,traits,Allocator>* rdbuf() const;
In 27.8.1.5, 27.8.1.7, 27.8.1.8, 27.8.1.10, 27.8.1.11, and 27.8.1.13
Replace
basic_filebuf<charT,traits>* rdbuf() const;
with
basic_filebuf<charT,traits>* rdbuf(); const basic_filebuf<charT,traits>* rdbuf() const;
Section: 25.2.7 [lib.alg.remove] Status: New Submitter: Anthony Williams Date: 13 May 2002
remove_copy and remove_copy_if (25.2.7 [lib.alg.remove]) permit their input range to be marked with Input Iterators. However, since two operations are required against the elements to copy (comparison and assigment), when the input range uses Input Iterators, a temporary copy must be taken to avoid dereferencing the iterator twice. This therefore requires the value type of the InputIterator to be CopyConstructible. If the iterators are at least Forward Iterators, then the iterator can be dereferenced twice, or a reference to the result maintained, so the temporary is not required.
Proposed resolution:
Add "If InputIterator does not meet the requirements of forward iterator, then the value type of InputIterator must be copy constructible. Otherwise copy constructible is not required." to 25.2.7 [lib.alg.remove] paragraph 6.
Section: 21.3.5.6 [lib.string::replace] Status: New Submitter: Beman Dawes Date: 3 Jun 2002
21.3.5.6 [lib.string::replace] basic_string::replace, second signature, given in paragraph 1, has two "Throws" paragraphs (3 and 5).
In addition, the second "Throws" paragraph (5) includes specification (beginning with "Otherwise, the function replaces ...") that should be part of the "Effects" paragraph.
Proposed resolution:
Section: 27.3 [lib.iostream.objects] Status: New Submitter: Ruslan Abdikeev Date: 8 Jul 2002
Is it safe to use standard iostream objects from constructors of static objects? Are standard iostream objects constructed and are their associations established at that time?
Surpisingly enough, Standard does NOT require that.
27.3/2 [lib.iostream.objects] guarantees that standard iostream objects are constructed and their associations are established before the body of main() begins execution. It also refers to ios_base::Init class as the panacea for constructors of static objects.
However, there's nothing in 27.3 [lib.iostream.objects], in 27.4.2 [lib.ios.base], and in 27.4.2.1.6 [lib.ios::Init], that would require implementations to allow access to standard iostream objects from constructors of static objects.
Details:
Core text refers to some magic object ios_base::Init, which will be discussed below:
"The [standard iostream] objects are constructed, and their associations are established at some time prior to or during first time an object of class basic_ios<charT,traits>::Init is constructed, and in any case before the body of main begins execution." (27.3/2 [lib.iostream.objects])
The first non-normative footnote encourages implementations to initialize standard iostream objects earlier than required.
However, the second non-normative footnote makes an explicit and unsupported claim:
"Constructors and destructors for static objects can access these [standard iostream] objects to read input from stdin or write output to stdout or stderr." (27.3/2 footnote 265 [lib.iostream.objects])
The only bit of magic is related to that ios_base::Init class. AFAIK, the rationale behind ios_base::Init was to bring an instance of this class to each translation unit which #included <iostream> or related header. Such an inclusion would support the claim of footnote quoted above, because in order to use some standard iostream object it is necessary to #include <iostream>.
However, while Standard explicitly describes ios_base::Init as an appropriate class for doing the trick, I failed to found a mention of an _instance_ of ios_base::Init in Standard.
Proposed resolution:
At the end of header <iostream> synopsis in 27.3 [lib.iostream.objects]
namespace std
{
... extern istream cin; ...
add the following lines
namespace
{
ios_base::Init <some_implementation_defined_name>;
}
}
Section: 27.6.1.3 [lib.istream.unformatted] Status: New Submitter: Ray Lischner Date: 15 Jul 2002
Defect report for description of basic_istream::get (section 27.6.1.3 [lib.istream.unformatted]), paragraph 15. The description for the get function with the following signature:
basic_istream<charT,traits>& get(basic_streambuf<char_type,traits>& sb);
is incorrect. It reads
Effects: Calls get(s,n,widen('\n'))
which I believe should be:
Effects: Calls get(sb,widen('\n'))
Proposed resolution:
Change the Effects paragraph to:
Effects: Calls get(sb,widen('\n'))
Section: 23.1 [lib.container.requirements] Status: New Submitter: Frank Compagner Date: 20 Jul 2002
The requirements for multiset and multimap containers (23.1 [lib.containers.requirements], 23.1.2 [lib.associative.reqmnts], 23.3.2 [lib.multimap] and 23.3.4 [lib.multiset]) make no mention of the stability of the required (mutating) member functions. It appears the standard allows these functions to reorder equivalent elements of the container at will, yet the pervasive red-black tree implementation appears to provide stable behaviour.
This is of most concern when considering the behaviour of erase(). A stability requirement would guarantee the correct working of the following 'idiom' that removes elements based on a certain predicate function.
multimap<int, int> m;
multimap<int, int>::iterator i = m.begin();
while (i != m.end()) {
if (pred(i))
m.erase (i++);
else
++i;
}
Although clause 23.1.2/8 guarantees that i remains a valid iterator througout this loop, absence of the stability requirement could potentially result in elements being skipped. This would make this code incorrect, and, furthermore, means that there is no way of erasing these elements without iterating first over the entire container, and second over the elements to be erased. This would be unfortunate, and have a negative impact on both performance and code simplicity.
If the stability requirement is intended, it should be made explicit (probably through an extra paragraph in clause 23.1.2).
If it turns out stability cannot be guaranteed, i'd argue that a remark or footnote is called for (also somewhere in clause 23.1.2) to warn against relying on stable behaviour (as demonstrated by the code above). If most implementations will display stable behaviour, any problems emerging on an implementation without stable behaviour will be hard to track down by users. This would also make the need for an erase_if() member function that much greater.
This issue is somewhat related to LWG issue 130.
Proposed resolution:
Section: 17.4.4.8 [lib.res.on.exception.handling], 18.6.1 [lib.exception], Status: New Submitter: Randy Maddox Date: 22 Jul 2002
Paragraph 3 under clause 17.4.4.8 [lib.res.on.exception.handling], Restrictions on Exception Handling, states that "Any other functions defined in the C++ Standard Library that do not have an exception-specification may throw implementation-defined exceptions unless otherwise specified." This statement is followed by a reference to footnote 178 at the bottom of that page which states, apparently in reference to the C++ Standard Library, that "Library implementations are encouraged (but not required) to report errors by throwing exceptions from (or derived from) the standard exceptions."
These statements appear to be in direct contradiction to clause 18.6.1 [lib.exception], which states "The class exception defines the base class for the types of objects thrown as exceptions by the C++ Standard library components ...".
Is this inconsistent?
Proposed resolution:
Section: 27.6.1.2.1 [lib.istream.formatted.reqmts], 27.6.2.5.1 [lib.ostream.formatted.reqmts] Status: New Submitter: Keith Baker Date: 23 Jul 2002
In 27.6.1.2.1 [lib.istream.formatted.reqmts] and 27.6.2.5.1 [lib.ostream.formatted.reqmts] (exception()&badbit) != 0 is used in testing for rethrow, yet exception() is the constructor to class std::exception in 18.6.1 [lib.exception] that has no return type. Should member function exceptions() found in 27.4.4 [lib.ios] be used instead?
Proposed resolution:
Section: 22.2.6.3.1 [lib.locale.moneypunct.members], 22.2.6.3.2 [lib.locale.moneypunct.virtuals] Status: New Submitter: Ray Lischner Date: 8 Aug 2002
In section 22.2.6.3.1 [lib.locale.moneypunct.members], frac_digits() returns type "int". This implies that frac_digits() might return a negative value, but a negative value is nonsensical. It should return "unsigned".
Similarly, in section 22.2.6.3.2 [lib.locale.moneypunct.virtuals], do_frac_digits() should return "unsigned".
Proposed resolution:
Section: 27.7.1.3 [lib.stringbuf.virtuals] Status: New Submitter: Ray Lischner Date: 14 Aug 2002
In Section 27.7.1.3 [lib.stringbuf.virtuals]: Table 90, Table 91, and paragraph 14 all contain references to "basic_ios::" which should be "ios_base::".
Proposed resolution:
Change all references to "basic_ios" in Table 90, Table 91, and paragraph 14 to "ios_base".
Section: 27.7.1.3 [lib.stringbuf.virtuals] Status: New Submitter: Ray Lischner Date: 14 Aug 2002
In Section 27.7.1.3 [lib.stringbuf.virtuals], Table 90, the implication is that the four conditions should be mutually exclusive, but they are not. The first two cases, as written, are subcases of the third. I think it would be clearer if the conditions were rewritten as follows:
(which & (ios_base::in|ios_base::out)) == ios_base::in
(which & (ios_base::in|ios_base::out)) == ios_base::out
(which & (ios_base::in|ios_base::out)) == (ios_base::in|ios_base::out) and way == either ios_base::beg or ios_base::end
Otherwise
As written, it is unclear what should be the result if cases 1 & 2 are true, but case 3 is false, e.g.,
seekoff(0, ios_base::cur, ios_base::in | ios_base::out)
Proposed resolution:
Section: 21.3.5.4 [lib.string::insert] Status: New Submitter: Ray Lischner Date: 16 Aug 2002
Section 21.3.5.4 [lib.string::insert], paragraph 4, contains the following, "Then throws length_error if size() >= npos - rlen."
Related to DR 83, this sentence should probably be removed.
Proposed resolution:
Section: 22.1.1 [lib.locale] Status: New Submitter: Martin Sebor Date: 6 Sep 2002
I think there is a problem with 22.1.1, p6 which says that
-6- An instance of locale is immutable; once a facet reference
is obtained from it, that reference remains usable as long
as the locale value itself exists.
and 22.1.1.2, p4:
const locale& operator=(const locale& other) throw();
-4- Effects: Creates a copy of other, replacing the current value.
How can a reference to a facet obtained from a locale object remain valid after an assignment that clearly must replace all the facets in the locale object? Imagine a program such as this
std::locale loc ("de_DE");
const std::ctype<char> &r0 = std::use_facet<std::ctype<char> >(loc);
loc = std::locale ("en_US");
const std::ctype<char> &r1 = std::use_facet<std::ctype<char> >(loc);
Is r0 really supposed to be preserved and destroyed only when loc goes out of scope?
Proposed resolution:
Suggest to replace 22.1.1 [lib.locale], p6 with
-6- Unless assigned a new value, locale objects are immutable;
once a facet reference is obtained from it, that reference
remains usable as long as the locale object itself exists
or until the locale object is assigned the value of another,
distinct locale object.
Section: 22.2.1.1.2 [lib.locale.ctype.virtuals] Status: New Submitter: Martin Sebor Date: 6 Sep 2002
The last sentence in 22.2.1.1.2, p11 below doesn't seem to make sense.
charT do_widen (char c) const;
-11- Effects: Applies the simplest reasonable transformation from
a char value or sequence of char values to the corresponding
charT value or values. The only characters for which unique
transformations are required are those in the basic source
character set (2.2). For any named ctype category with a
ctype<charT> facet ctw and valid ctype_base::mask value
M (is(M, c) || !ctw.is(M, do_widen(c))) is true.
Shouldn't the last sentence instead read
For any named ctype category with a ctype<char> facet ctc
and valid ctype_base::mask value M
(ctc.is(M, c) || !is(M, do_widen(c))) is true.
I.e., if the narrow character c is not a member of a class of characters then neither is the widened form of c. (To paraphrase footnote 224.)
Proposed resolution:
Replace the last sentence of 22.2.1.1.2 [lib.locale.ctype.virtuals], p11 with the following text:
For any named ctype category with a ctype<char> facet ctc
and valid ctype_base::mask value M
(ctc.is(M, c) || !is(M, do_widen(c))) is true.
Section: 22.2.1.5.2 [lib.locale.codecvt.virtuals] Status: New Submitter: Martin Sebor Date: 6 Sep 2002
Tables 53 and 54 in 22.2.1.5.2 [lib.locale.codecvt.virtuals] are both titled "convert result values," when surely "do_in/do_out result values" must have been intended for Table 53 and "do_unshift result values" for Table 54.
Table 54, row 3 says that the meaning of partial is "more characters needed to be supplied to complete termination." The function is not supplied any characters, it is given a buffer which it fills with characters or, more precisely, destination elements (i.e., an escape sequence). So partial means that space for more than (to_limit - to) destination elements was needed to terminate a sequence given the value of state.
Proposed resolution:
Change the title of Table 53 to "do_in/do_out result values" and the title of Table 54 to "do_unshift result values."
Change the text in Table 54, row 3, under the heading Meaning to "space for more than (to_limit - to) destination elements was needed to terminate a sequence given the value of state."
Section: 22.2.1.5.2 [lib.locale.codecvt.virtuals] Status: New Submitter: Martin Sebor Date: 6 Sep 2002
All but one codecvt member functions that take a state_type argument list as one of their preconditions that the state_type argument have a valid value. However, according to 22.2.1.5.2, p6, codecvt::do_unshift() is the only codecvt member that is supposed to return error if the state_type object is invalid.
It seems to me that the treatment of state_type by all codecvt member functions should be the same and the current requirements should be changed. Since the detection of invalid state_type values may be difficult in general or computationally expensive in some specific cases, I propose the following:
Proposed resolution:
Add a new paragraph before 22.2.1.5.2, p5, and after the function declaration below
result do_unshift(stateT& state,
externT* to, externT* to_limit, externT*& to_next) const;
as follows:
Requires: (to <= to_end) well defined and true; state initialized,
if at the beginning of a sequence, or else equal to the result of
converting the preceding characters in the sequence.
and change the text in Table 54, row 4, under the heading Meaning from
state has invalid value
to
an unspecified error has occurred
The return value of error should allow implementers to detect and report invalid state values but shouldn't require it, hence the word "unspecified" in the new wording.
Section: 22.2.1.5 [lib.locale.codecvt] Status: New Submitter: Martin Sebor Date: 30 Aug 2002
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 22.2.1.5.2 [lib.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 22.2.1.5.2 [lib.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)?
Proposed resolution:
To address these issues, I propose that paragraphs 2, 3, and 4 be rewritten as follows. The proposal incorporates the accepted resolution of lwg issue 19.
-2- Effects: Converts characters in the range of source elements
[from, from_end), placing the results in sequential positions
starting at destination to. Converts no more than (from_endfrom)
source elements, and stores no more than (to_limitto)
destination elements.
Stops if it encounters a sequence of source elements it cannot
convert to a valid destination character. It always leaves the
from_next and to_next pointers pointing one beyond the last
element successfully converted.
[Note: If returns noconv, internT and externT are the same type
and the converted sequence is identical to the input sequence
[from, from_next). to_next is set equal to to, the value of
state is unchanged, and there are no changes to the values in
[to, to_limit). --end note]
-3- Notes: Its operations on state are unspecified.
[Note: This argument can be used, for example, to maintain shift
state, to specify conversion options (such as count only), or to
identify a cache of seek offsets. --end note]
-4- Returns: An enumeration value, as summarized in Table 53:
Table 53 -- do_in/do_out result values
Value Meaning
+---------+----------------------------------------------------+
| ok | successfully completed the conversion of all |
| | complete characters in the source range |
+---------+----------------------------------------------------+
| partial | the characters in the source range would, after |
| | conversion, require space greater than that |
| | available in the destination range |
+---------+----------------------------------------------------+
| error | encountered either a sequence of elements in the |
| | source range forming a valid source character that |
| | could not be converted to a destination character, |
| | or a sequence of elements in the source range that |
| | could not possibly form a valid source character |
+---------+----------------------------------------------------+
| noconv | internT and externT are the same type, and input |
| | sequence is identical to converted sequence |
+---------+----------------------------------------------------+
A return value of partial, i.e., 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 character
can be produced.
----- End of document -----