Revised 2024-12-22 at 11:35:55 UTC

Unresolved Issues

423⁽ⁱ⁾. Effects of negative streamsize in iostreams

Section: 31 [input.output] Status: Open Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2018-12-09

Priority: 3

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Discussion:

A third party test suite tries to exercise istream::ignore(N) with a negative value of N and expects that the implementation will treat N as if it were 0. Our implementation asserts that (N >= 0) holds and aborts the test.

I can't find anything in section 27 that prohibits such values but I don't see what the effects of such calls should be, either (this applies to a number of unformatted input functions as well as some member functions of the basic_streambuf template).

[ 2009-07 Frankfurt ]

This is related to LWG 255⁽ⁱ⁾.

Move to NAD Future.

[LEWG Kona 2017]

Recommend Open: We agree that we should require N >= 0 for the selected functions

[2018-12-04 Reflector prioritization]

Set Priority to 3

Proposed resolution:

I propose that we add to each function in clause 27 that takes an argument, say N, of type streamsize a Requires clause saying that "N >= 0." The intent is to allow negative streamsize values in calls to precision() and width() but disallow it in calls to streambuf::sgetn(), istream::ignore(), or ostream::write().

[Kona: The LWG agreed that this is probably what we want. However, we need a review to find all places where functions in clause 27 take arguments of type streamsize that shouldn't be allowed to go negative. Martin will do that review.]

484⁽ⁱ⁾. Convertible to T

Section: 24.3.5.3 [input.iterators] Status: Open Submitter: Chris Jefferson Opened: 2004-09-16 Last modified: 2023-06-25

Priority: 3

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Discussion:

From comp.std.c++:

I note that given an input iterator a for type T, then *a only has to be "convertable to T", not actually of type T.

Firstly, I can't seem to find an exact definition of "convertable to T". While I assume it is the obvious definition (an implicit conversion), I can't find an exact definition. Is there one?

Slightly more worryingly, there doesn't seem to be any restriction on the this type, other than it is "convertable to T". Consider two input iterators a and b. I would personally assume that most people would expect *a==*b would perform T(*a)==T(*b), however it doesn't seem that the standard requires that, and that whatever type *a is (call it U) could have == defined on it with totally different symantics and still be a valid inputer iterator.

Is this a correct reading? When using input iterators should I write T(*a) all over the place to be sure that the object I'm using is the class I expect?

This is especially a nuisance for operations that are defined to be "convertible to bool". (This is probably allowed so that implementations could return say an int and avoid an unnecessary conversion. However all implementations I have seen simply return a bool anyway. Typical implementations of STL algorithms just write things like while(a!=b && *a!=0). But strictly speaking, there are lots of types that are convertible to T but that also overload the appropriate operators so this doesn't behave as expected.

If we want to make code like this legal (which most people seem to expect), then we'll need to tighten up what we mean by "convertible to T".

[Lillehammer: The first part is NAD, since "convertible" is well-defined in core. The second part is basically about pathological overloads. It's a minor problem but a real one. So leave open for now, hope we solve it as part of iterator redesign.]

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

[ 2009-10 Santa Cruz: ]

Mark as NAD Future. We agree there's an issue, but there is no proposed solution at this time and this will be solved by concepts in the future.

[2017-02 in Kona, LEWG recommends NAD]

Has been clarified by 14. By design. Ranges might make it go away. Current wording for input iterators is more constrained.

[2017-06-02 Issues Telecon]

Move to Open. This is very similar to 2962⁽ⁱ⁾, possibly a duplicate.

Marshall to research

[2017-07 Toronto Thurs Issue Prioritization]

Priority 2; same as 2962⁽ⁱ⁾.

Previous resolution [SUPERSEDED]:

Rationale:

[ San Francisco: ]

Solved by N2758.

[2023-06; Varna]

During LWG discussion of this issue it was decided to reduce the priority to 3.

Furthermore, the still presented "Solved by" comment has been recognized as being no longer true, since the referred to pre-C++11 concept paper wording N2758 is no longer part of the working paper.

It also has been observed, that the "convertible to bool" part has since been resolved by P1964 and the follow-up paper P2167.

Also LWG 3105⁽ⁱ⁾ has a lot of overlap with this issue.

Proposed resolution:

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

Section: 28.6 [re] Status: Open Submitter: Eric Niebler Opened: 2005-07-01 Last modified: 2020-07-17

Priority: 4

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Discussion:

A problem with TR1 regex is currently being discussed on the Boost developers list. It involves the handling of case-insensitive matching of character ranges such as [Z-a]. The proper behavior (according to the ECMAScript standard) is unimplementable given the current specification of the TR1 regex_traits<> class template. John Maddock, the author of the TR1 regex proposal, agrees there is a problem. The full discussion can be found at http://lists.boost.org/boost/2005/06/28850.php (first message copied below). We don't have any recommendations as yet.

-- Begin original message --

The situation of interest is described in the ECMAScript specification (ECMA-262), section 15.10.2.15:

"Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case ASCII letters as well as the symbols [, \, ], ^, _, and `."

A more interesting case is what should happen when doing a case-insensitive match on a range such as [Z-a]. It should match z, Z, a, A and the symbols [, \, ], ^, _, and `. This is not what happens with Boost.Regex (it throws an exception from the regex constructor).

The tough pill to swallow is that, given the specification in TR1, I don't think there is any effective way to handle this situation. According to the spec, case-insensitivity is handled with regex_traits<>::translate_nocase(CharT) — two characters are equivalent if they compare equal after both are sent through the translate_nocase function. But I don't see any way of using this translation function to make character ranges case-insensitive. Consider the difficulty of detecting whether "z" is in the range [Z-a]. Applying the transformation to "z" has no effect (it is essentially std::tolower). And we're not allowed to apply the transformation to the ends of the range, because as ECMA-262 says, "the case of the two ends of a range is significant."

So AFAICT, TR1 regex is just broken, as is Boost.Regex. One possible fix is to redefine translate_nocase to return a string_type containing all the characters that should compare equal to the specified character. But this function is hard to implement for Unicode, and it doesn't play nice with the existing ctype facet. What a mess!

-- End original message --

[ John Maddock adds: ]

One small correction, I have since found that ICU's regex package does implement this correctly, using a similar mechanism to the current TR1.Regex.

Given an expression [c1-c2] that is compiled as case insensitive it:

Enumerates every character in the range c1 to c2 and converts it to it's case folded equivalent. That case folded character is then used a key to a table of equivalence classes, and each member of the class is added to the list of possible matches supported by the character-class. This second step isn't possible with our current traits class design, but isn't necessary if the input text is also converted to a case-folded equivalent on the fly.

ICU applies similar brute force mechanisms to character classes such as [[:lower:]] and [[:word:]], however these are at least cached, so the impact is less noticeable in this case.

Quick and dirty performance comparisons show that expressions such as "[X-\\x{fff0}]+" are indeed very slow to compile with ICU (about 200 times slower than a "normal" expression). For an application that uses a lot of regexes this could have a noticeable performance impact. ICU also has an advantage in that it knows the range of valid characters codes: code points outside that range are assumed not to require enumeration, as they can not be part of any equivalence class. I presume that if we want the TR1.Regex to work with arbitrarily large character sets enumeration really does become impractical.

Finally note that Unicode has:

Three cases (upper, lower and title). One to many, and many to one case transformations. Character that have context sensitive case translations - for example an uppercase sigma has two different lowercase forms - the form chosen depends on context(is it end of a word or not), a caseless match for an upper case sigma should match either of the lower case forms, which is why case folding is often approximated by tolower(toupper(c)).

Probably we need some way to enumerate character equivalence classes, including digraphs (either as a result or an input), and some way to tell whether the next character pair is a valid digraph in the current locale.

Hoping this doesn't make this even more complex that it was already,

[ Portland: Alisdair: Detect as invalid, throw an exception. Pete: Possible general problem with case insensitive ranges. ]

[ 2009-07 Frankfurt ]

We agree that this is a problem, but we do not know the answer.

We are going to declare this NAD until existing practice leads us in some direction.

No objection to NAD Future.

Move to NAD Future.

[LEWG Kona 2017]

Recommend Open: Tim Shen proposes: forbid use of case-insensitive ranges with regex traits other than std::regex_traits<{char, wchar_t, char16_t, char32_t}> when regex_constants::collate is specified.

[2020-07-17; Priority set to 4 in telecon]

Proposed resolution:

532⁽ⁱ⁾. Tuple comparison

Section: 22.4.9 [tuple.rel], 99 [tr.tuple.rel] Status: LEWG Submitter: David Abrahams Opened: 2005-11-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 348

Discussion:

Where possible, tuple comparison operators <,<=,=>, and > ought to be defined in terms of std::less rather than operator<, in order to support comparison of tuples of pointers.

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

[ 2009-10 Santa Cruz: ]

If we solve this for tuple we would have to solve it for pair algorithms, etc. It is too late to do that at this time. Move to NAD Future.

Proposed resolution:

change 6.1.3.5/5 from:

Returns: The result of a lexicographical comparison between t and u. The result is defined as: (bool)(get<0>(t) < get<0>(u)) || (!(bool)(get<0>(u) < get<0>(t)) && ttail < utail), where rtail for some tuple r is a tuple containing all but the first element of r. For any two zero-length tuples e and f, e < f returns false.

to:

Returns: The result of a lexicographical comparison between t and u. For any two zero-length tuples e and f, e < f returns false. Otherwise, the result is defined as: cmp( get<0>(t), get<0>(u)) || (!cmp(get<0>(u), get<0>(t)) && ttail < utail), where rtail for some tuple r is a tuple containing all but the first element of r, and cmp(x,y) is an unspecified function template defined as follows.

Where T is the type of x and U is the type of y:

if T and U are pointer types and T is convertible to U, returns less<U>()(x,y)

otherwise, if T and U are pointer types, returns less<T>()(x,y)

otherwise, returns (bool)(x < y)

[ Berlin: This issue is much bigger than just tuple (pair, containers, algorithms). Dietmar will survey and work up proposed wording. ]

Rationale:

Recommend NAD. This will be fixed with the next revision of concepts.

[ San Francisco: ]

Solved by N2770.

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

Section: 23.3.3 [array] Status: Open Submitter: Bo Persson Opened: 2006-12-30 Last modified: 2022-11-12

Priority: 3

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Discussion:

The <array> header is given under 23.3 [sequences]. 23.3.3 [array]/paragraph 3 says:

"Unless otherwise specified, all array operations are as described in 23.2 [container.requirements]".

However, array isn't mentioned at all in section 23.2 [container.requirements]. In particular, Table 82 "Sequence requirements" lists several operations (insert, erase, clear) that std::array does not have in 23.3.3 [array].

Also, Table 83 "Optional sequence operations" lists several operations that std::array does have, but array isn't mentioned.

[ 2009-07 Frankfurt ]

The real issue seems to be different than what is described here. Non-normative text says that std::array is a sequence container, but there is disagreement about what that really means. There are two possible interpretations:

1. a sequence container is one that satisfies all sequence container requirements
2. a sequence container is one that satisfies some of the sequence container requirements. Any operation that the container supports is specified by one or more sequence container requirements, unless that operation is specifically singled out and defined alongside the description of the container itself.

Move to Tentatively NAD.

[ 2009-07-15 Loïc Joly adds: ]

The section 23.2.4 [sequence.reqmts]/1 states that array is a sequence. 23.2.4 [sequence.reqmts]/3 introduces table 83, named Sequence container requirements. This seems to me to be defining the requirements for all sequences. However, array does not follow all of this requirements (this can be read in the array specific section, for the standard is currently inconsistent).

Proposed resolution 1 (minimal change):

Say that array is a container, that in addition follows only some of the sequence requirements, as described in the array section:

The library provides five three basic kinds of sequence containers: array, vector, forward_list, list, and deque. In addition, array and forward_list follows some of the requirements of sequences, as described in their respective sections.

Proposed resolution 2 (most descriptive description, no full wording provided):

Introduce the notion of a Fixed Size Sequence, with it requirement table that would be a subset of the current Sequence container. array would be the only Fixed Size Sequence (but dynarray is in the queue for TR2). Sequence requirements would now be requirements in addition to Fixed Size Sequence requirements (it is currently in addition to container).

[ 2009-07 Frankfurt: ]

Move to NAD Editorial

[ 2009 Santa Cruz: ]

This will require a lot of reorganization. Editor doesn't think this is really an issue, since the description of array can be considered as overriding what's specified about sequences. Move to NAD.

[2022-10-27; Hubert Tong comments and requests to reopen]

This issue appears to be unresolved (should not be NAD).

As noted in 23.3.3.1 [array.overview] paragraph 3, array does not meet 23.2.2.2 [container.reqmts] paragraph 10. This means that array does not meet the container requirements, never mind the requirements for sequence containers or contiguous containers.

However, there is wording that claims the opposite.

23.2.4 [sequence.reqmts] paragraph 1:

In addition, array is provided as a sequence container which provides limited sequence operations because it has a fixed number of elements.

(Perhaps the above should be worded with "except".)

23.3.1 [sequences.general] paragraph 1:

The headers <array> […] define class templates that meet the requirements for sequence containers.

23.3.3.1 [array.overview] paragraph 1:

[…] An array is a contiguous container (23.2.2 [container.requirements.general]).

In this comment, Casey suggests that the requirements be changed so that array does meet the requirements.

[Kona 2022-11-12; Set Priority to 3]

Proposed resolution:

936⁽ⁱ⁾. Mutex type overspecified

Section: 32.6.4 [thread.mutex.requirements] Status: LEWG Submitter: Pete Becker Opened: 2008-12-05 Last modified: 2017-03-01

Priority: Not Prioritized

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Duplicate of: 961

Discussion:

32.6.4 [thread.mutex.requirements] describes the requirements for a type to be a "Mutex type". A Mutex type can be used as the template argument for the Lock type that's passed to condition_variable_any::wait (although Lock seems like the wrong name here, since Lock is given a different formal meaning in 32.6.5 [thread.lock]) and, although the WD doesn't quite say so, as the template argument for lock_guard and unique_lock.

The requirements for a Mutex type include:

• m.lock() shall be well-formed and have [described] semantics, including a return type of void.
• m.try_lock() shall be well-formed and have [described] semantics, including a return type of bool.
• m.unlock() shall be well-formed and have [described] semantics, including a return type of void.

Also, a Mutex type "shall not be copyable nor movable".

The latter requirement seems completely irrelevant, and the three requirements on return types are tighter than they need to be. For example, there's no reason that lock_guard can't be instantiated with a type that's copyable. The rule is, in fact, that lock_guard, etc. won't try to copy objects of that type. That's a constraint on locks, not on mutexes. Similarly, the requirements for void return types are unnecessary; the rule is, in fact, that lock_guard, etc. won't use any returned value. And with the return type of bool, the requirement should be that the return type is convertible to bool.

[ Summit: ]

Move to open. Related to conceptualization and should probably be tackled as part of that.

• The intention is not only to place a constraint on what types such as lock_guard may do with mutex types, but on what any code, including user code, may do with mutex types. Thus the constraints as they are apply to the mutex types themselves, not the current users of mutex types in the standard.
• This is a low priority issue; the wording as it is may be overly restrictive but this may not be a real issue.

[ Post Summit Anthony adds: ]

Section 32.6.4 [thread.mutex.requirements] conflates the requirements on a generic Mutex type (including user-supplied mutexes) with the requirements placed on the standard-supplied mutex types in an attempt to group everything together and save space.

When applying concepts to chapter 30, I suggest that the concepts Lockable and TimedLockable embody the requirements for *use* of a mutex type as required by unique_lock/lock_guard/condition_variable_any. These should be relaxed as Pete describes in the issue. The existing words in 32.6.4 [thread.mutex.requirements] are requirements on all of std::mutex, std::timed_mutex, std::recursive_mutex and std::recursive_timed_mutex, and should be rephrased as such.

[2017-03-01, Kona]

SG1: Agreement that we need a paper.

Proposed resolution:

961⁽ⁱ⁾. Various threading bugs #11

Section: 32.6.4 [thread.mutex.requirements] Status: LEWG Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2017-03-01

Priority: Not Prioritized

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Duplicate of: 936

Discussion:

32.6.4 [thread.mutex.requirements] describes required member functions of mutex types, and requires that they throw exceptions under certain circumstances. This is overspecified. User-defined types can abort on such errors without affecting the operation of templates supplied by standard-library.

[ Summit: ]

Move to open. Related to conceptualization and should probably be tackled as part of that.

[ 2009-10 Santa Cruz: ]

Would be OK to leave it as is for time constraints, could loosen later.

Mark as NAD Future.

[2017-03-01, Kona]

SG1: Agreement that we need a paper.

Proposed resolution:

1102⁽ⁱ⁾. std::vector's reallocation policy still unclear

Section: 23.3.11.3 [vector.capacity] Status: Open Submitter: Daniel Krügler Opened: 2009-04-20 Last modified: 2020-07-17

Priority: 3

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Discussion:

I have the impression that even the wording of current draft N2857 does insufficiently express the intent of vector's reallocation strategy. This has produced not too old library implementations which release memory in the clear() function and even modern articles about C++ programming cultivate the belief that clear is allowed to do exactly this. A typical example is something like this:

const int buf_size = ...;
std::vector<T> buf(buf_size);
for (int i = 0; i < some_condition; ++i) {
  buf.resize(buf_size);
  write_or_read_data(buf.data());
  buf.clear(); // Ensure that the next round get's 'zeroed' elements
}

where still the myth is ubiquitous that buf might be allowed to reallocate it's memory inside the for loop.

IMO the problem is due to the fact, that

1. the actual memory-reallocation stability of std::vector is explained in 23.3.11.3 [vector.capacity]/3 and /6 which are describing just the effects of the reserve function, but in many examples (like above) there is no explicit call to reserve involved. Further-more 23.3.11.3 [vector.capacity]/6 does only mention insertions and never mentions the consequences of erasing elements.

2. the effects clause of std::vector's erase overloads in 23.3.11.5 [vector.modifiers]/4 is silent about capacity changes. This easily causes a misunderstanding, because the counter parting insert functions described in 23.3.11.5 [vector.modifiers]/2 explicitly say, that

   Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid.

   It requires a complex argumentation chain about four different places in the standard to provide the — possibly weak — proof that calling clear() also does never change the capacity of the std::vector container. Since std::vector is the de-facto replacement of C99's dynamic arrays this type is near to a built-in type and it's specification should be clear enough that usual programmers can trust their own reading.

[ Batavia (2009-05): ]

Bill believes paragraph 1 of the proposed resolution is unnecessary because it is already implied (even if tortuously) by the current wording.

Move to Review.

[ 2009-10 Santa Cruz: ]

Mark as NAD. Rationale: there is no consensus to clarify the standard, general consensus that the standard is correct as written.

[2020-05-08; Reopen after reflector discussions]

"correct as written" has been disputed.

[2020-07-17; Priority set to 3 in telecon]

Proposed resolution:

[ This is a minimum version. I also suggest that the wording explaining the allocation strategy of std::vector in 23.3.11.3 [vector.capacity]/3 and /6 is moved into a separate sub paragraph of 23.3.11.3 [vector.capacity] before any of the prototype's are discussed, but I cannot provide reasonable wording changes now. ]

1. Change 23.3.11.3 [vector.capacity]/6 as follows:

   It is guaranteed that no reallocation takes place during insertions or erasures that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the value of capacity().

2. Change 23.3.11.5 [vector.modifiers]/4 as follows:

   Effects: The capacity shall remain unchanged and no reallocation shall happen. Invalidates iterators and references at or after the point of the erase.

1175⁽ⁱ⁾. unordered complexity

Section: 23.2.8 [unord.req] Status: Open Submitter: Pablo Halpern Opened: 2009-07-17 Last modified: 2020-09-06

Priority: 3

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Discussion:

When I look at the unordered_* constructors, I think the complexity is poorly described and does not follow the style of the rest of the standard.

The complexity for the default constructor is specified as constant. Actually, it is proportional to n, but there are no invocations of value_type constructors or other value_type operations.

For the iterator-based constructor the complexity should be:

Complexity: exactly n calls to construct value_type from InputIterator::value_type (where n = distance(f,l)). The number of calls to key_equal::operator() is proportional to n in the average case and n*n in the worst case.

[ 2010 Rapperswil: ]

Concern that the current wording may require O(1) where that cannot be delivered. We need to look at both the clause 23 requirements tables and the constructor description of each unordered container to be sure.

Howard suggests NAD Editorial as we updated the container requirement tables since this issue was written.

Daniel offers to look deeper, and hopefully produce wording addressing any outstanding concerns at the next meeting.

Move to Open.

[2011-02-26: Daniel provides wording]

I strongly suggest to clean-up the differences between requirement tables and individual specifications. In the usual way, the most specific specifications wins, which is in this case the wrong one. In regard to the concern expressed about missing DefaultConstructible requirements of the value type I disagree: The function argument n is no size-control parameter, but only some effective capacity parameter: No elements will be value-initialized by these constructors. The necessary requirement for the value type, EmplaceConstructible into *this, is already listed in Table 103 — Unordered associative container requirements. Another part of the proposed resolution is the fact that there is an inconsistency of the complexity counting when both a range and a bucket count is involved compared to constructions where only bucket counts are provided: E.g. the construction X a(n); has a complexity of n bucket allocations, but this part of the work is omitted for X a(i, j, n);, even though it is considerable larger (in the average case) for n ≫ distance(i, j).

[2011-03-24 Madrid meeting]

Move to deferred

[ 2011 Bloomington ]

The proposed wording looks good. Move to Review.

[2012, Kona]

Fix up some presentation issues with the wording, combining the big-O expressions into single expressions rather than the sum of two separate big-Os.

Strike "constant or linear", prefer "linear in the number of buckets". This allows for number of buckets being larger than requested n as well.

Default n to "unspecified" rather than "implementation-defined". It seems an un-necessary burden asking vendors to document a quantity that is easily determined through the public API of these classes.

Replace distance(f,l) with "number of elements in the range [f,l)"

Retain in Review with the updated wording

[2012, Portland: Move to Open]

The wording still does not call out Pablo's original concern, that the element constructor is called no more than N times, and that the N squared term applies to moves during rehash.

Inconsistent use of O(n)+O(N) vs. O(n+N), with a preference for the former.

AJM to update wording with a reference to "no more than N element constructor calls".

Matt concerned that calling out the O(n) requirements is noise, and dangerous noise in suggesting a precision we do not mean. The cost of constructing a bucket is very different to constructing an element of user-supplied type.

AJM notes that if there are multiple rehashes, the 'n' complexity is probably not linear.

Matt suggests back to Open, Pablo suggests potentially NAD if we keep revisitting without achieving a resolution.

Matt suggests complexity we are concerned with is the number of operations, such as constructing elements, moving nodes, and comparing/hashing keys. We are less concerned with constructing buckets, which are generally noise in this bigger picture.

[2015-01-29 Telecon]

AM: essentially correct, but do we want to complicate the spec?

HH: Pablo has given us permission to NAD it

JM: when I look at the first change in the P/R I find it mildly disturbing that the existing wording says you have a constant time constructor with a single element even if your n is 10^6, so I think adding this change makes people aware there might be a large cost in initializing the hash table, even though it doesn't show up in user-visible constructions.

HH: one way to avoid that problem is make the default ctor noexcept. Then the container isn't allowed to create an arbitrarily large hash table

AM: but this is the constructor where the user provides n

MC: happy with the changes, except I agree with the editorial recommendation to keep the two 𝒪s separate.

JW: yes, the constant 'k' is different in 𝒪(n) and 𝒪(N)

GR: do we want to talk about buckets at all

JM: yes, good to highlight that bucket construction might be a significant cost

HH: suggest we take the suggestion to split 𝒪(n+N) to 𝒪(n)+𝒪(N) and move to Tentatively Ready

GR: 23.2.1p2 says all complexity requirements are stated solely in terms of the number of operations on the contained object, so we shouldn't be stating complexity in terms of the hash table initialization

HH: channeling Pete, there's an implicit "unless otherwise specified" everywhere.

VV: seem to be requesting modifications that render this not Tentatively Ready

GR: I think it can't be T/R

AM: make the editorial recommendation, consider fixing 23.2.1/3 to give us permission to state complexity in terms of bucket initialization

HH: only set it to Review after we get new wording to review

[2015-02 Cologne]

Update wording, revisit later.

Previous resolution [SUPERSEDED]:

1. Modify the following rows in Table 103 — Unordered associative container requirements to add the explicit bucket allocation overhead of some constructions. As editorial recommendation it is suggested not to shorten the sum 𝒪(n) + 𝒪(N) to 𝒪(n + N), because two different work units are involved.

   Table 103 — Unordered associative container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   …
   X(i, j, n, hf, eq) X a(i, j, n, hf, eq)	X	… Effects: Constructs an empty container with at least n buckets, using hf as the hash function and eq as the key equality predicate, and inserts elements from [i, j) into it.	Average case 𝒪(n + N) (N is distance(i, j)), worst case 𝒪(n) + 𝒪(N2)
   X(i, j, n, hf) X a(i, j, n, hf)	X	… Effects: Constructs an empty container with at least n buckets, using hf as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it.	Average case 𝒪(n + N) (N is distance(i, j)), worst case 𝒪(n + N2)
   X(i, j, n) X a(i, j, n)	X	… Effects: Constructs an empty container with at least n buckets, using hasher() as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it.	Average case 𝒪(n + N) (N is distance(i, j)), worst case 𝒪(n + N2)
   …

2. Modify 23.5.3.2 [unord.map.cnstr] p. 1-4 as indicated (The edits of p. 1 and p. 3 attempt to fix some editorial oversight.):

   explicit unordered_map(size_type n = see below,
                          const hasher& hf = hasher(),
                          const key_equal& eql = key_equal(),
                          const allocator_type& a = allocator_type());
   

   1 Effects: Constructs an empty unordered_map using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecifiedimpldefdefault number of buckets in unordered_map. max_load_factor() returns 1.0.

   2 Complexity: ConstantLinear in the number of buckets.

   template <class InputIterator>
   unordered_map(InputIterator f, InputIterator l,
                 size_type n = see below,
                 const hasher& hf = hasher(),
                 const key_equal& eql = key_equal(),
                 const allocator_type& a = allocator_type());
   

   3 Effects: Constructs an empty unordered_map using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecifiedimpldefdefault number of buckets in unordered_map. Then inserts elements from the range [f, l). max_load_factor() returns 1.0.

   4 Complexity: Average case linear, worst case quadraticLinear in the number of buckets. In the average case linear in N and in the worst case quadratic in N to insert the elements, where N is equal to number of elements in the range [f,l).

3. Modify 23.5.4.2 [unord.multimap.cnstr] p. 1-4 as indicated (The edits of p. 1 and p. 3 attempt to fix some editorial oversight.):

   explicit unordered_multimap(size_type n = see below,
                               const hasher& hf = hasher(),
                               const key_equal& eql = key_equal(),
                               const allocator_type& a = allocator_type());
   

   1 Effects: Constructs an empty unordered_multimap using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecifiedimpldefdefault number of buckets in unordered_multimap. max_load_factor() returns 1.0.

   2 Complexity: ConstantLinear in the number of buckets.

   template <class InputIterator>
   unordered_multimap(InputIterator f, InputIterator l,
                      size_type n = see below,
                      const hasher& hf = hasher(),
                      const key_equal& eql = key_equal(),
                      const allocator_type& a = allocator_type());
   

   3 Effects: Constructs an empty unordered_multimap using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecifiedimpldefdefault number of buckets in unordered_multimap. Then inserts elements from the range [f, l). max_load_factor() returns 1.0.

   4 Complexity: Average case linear, worst case quadraticLinear in the number of buckets. In the average case linear in N and in the worst case quadratic in N to insert the elements, where N is equal to number of elements in the range [f,l).

4. Modify 23.5.6.2 [unord.set.cnstr] p. 1-4 as indicated (The edits of p. 1 and p. 3 attempt to fix some editorial oversight.):

   explicit unordered_set(size_type n = see below,
                          const hasher& hf = hasher(),
                          const key_equal& eql = key_equal(),
                          const allocator_type& a = allocator_type());
   

   1 Effects: Constructs an empty unordered_set using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecifiedimpldefdefault number of buckets in unordered_set. max_load_factor() returns 1.0.

   2 Complexity: ConstantLinear in the number of buckets.

   template <class InputIterator>
   unordered_set(InputIterator f, InputIterator l,
                 size_type n = see below,
                 const hasher& hf = hasher(),
                 const key_equal& eql = key_equal(),
                 const allocator_type& a = allocator_type());
   

   3 Effects: Constructs an empty unordered_set using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecifiedimpldefdefault number of buckets in unordered_set. Then inserts elements from the range [f, l). max_load_factor() returns 1.0.

   4 Complexity: Average case linear, worst case quadraticLinear in the number of buckets. In the average case linear in N and in the worst case quadratic in N to insert the elements, where N is equal to number of elements in the range [f,l).

5. Modify 23.5.7.2 [unord.multiset.cnstr] p. 1-4 as indicated (The edits of p. 1 and p. 3 attempt to fix some editorial oversight.):

   explicit unordered_multiset(size_type n = see below,
                               const hasher& hf = hasher(),
                               const key_equal& eql = key_equal(),
                               const allocator_type& a = allocator_type());
   

   1 Effects: Constructs an empty unordered_multiset using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecifiedimpldefdefault number of buckets in unordered_multiset. max_load_factor() returns 1.0.

   2 Complexity: ConstantLinear in the number of buckets.

   template <class InputIterator>
   unordered_multiset(InputIterator f, InputIterator l,
                      size_type n = see below,
                      const hasher& hf = hasher(),
                      const key_equal& eql = key_equal(),
                      const allocator_type& a = allocator_type());
   

   3 Effects: Constructs an empty unordered_multiset using the specified hash function, key equality function, and allocator, and using at least n buckets. If n is not provided, the number of buckets is unspecifiedimpldefdefault number of buckets in unordered_multiset. Then inserts elements from the range [f, l). max_load_factor() returns 1.0.

   4 Complexity: Average case linear, worst case quadraticLinear in the number of buckets. In the average case linear in N and in the worst case quadratic in N to insert the elements, where N is equal to number of elements in the range [f,l).

[2019-03-17; Daniel comments and provides revised wording]

The updated wording ensures that we can now specify complexity requirements for containers even when they are not expressed in terms of the number on the contained objects by an exception of the rule. This allows us to say that 𝒪(n) describes the complexity in terms of bucket initialization instead.

Proposed resolution:

This wording is relative to N4810.

1. Modify 23.2.2 [container.requirements.general] as indicated:

   -2- Unless otherwise specified,All of the complexity requirements in this Clause are stated solely in terms of the number of operations on the contained objects. [Example: The copy constructor of type vector<vector<int>> has linear complexity, even though the complexity of copying each contained vector<int> is itself linear. — end example]

2. Modify 23.2.8 [unord.req] as indicated:

   -11- In Table 70:

   1. (11.1) — […]

   2. […]

   3. (11.23) — […]

   4. (11.?) — Notwithstanding the complexity requirements restrictions of 23.2.2 [container.requirements.general], the complexity form 𝒪(n) describes the number of operations on buckets.

3. Modify the following rows in Table 70 — "Unordered associative container requirements" to add the explicit bucket allocation overhead of some constructions.

   [Drafting note: It is kindly suggested to the Project Editor not to shorten the sum 𝒪(n) + 𝒪(N) to 𝒪(n + N), because two different work units are involved. — end drafting note]

   Table 70 — Unordered associative container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   …
   X() X a;	X	Expects: […] Effects: Constructs an empty container with an unspecified number n of buckets, using hasher() as the hash function and key_equal() as the key equality predicate.	constant𝒪(n)
   X(i, j, n, hf, eq) X a(i, j, n, hf, eq)	X	Expects: […] Effects: Constructs an empty container with at least n buckets, using hf as the hash function and eq as the key equality predicate, and inserts elements from [i, j) into it.	Average case 𝒪(n) + 𝒪(N) (N is distance(i, j)), worst case 𝒪(n) + 𝒪(N2)
   X(i, j, n, hf) X a(i, j, n, hf)	X	Expects: […] Effects: Constructs an empty container with at least n buckets, using hf as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it.	Average case 𝒪(n) + 𝒪(N) (N is distance(i, j)), worst case 𝒪(n) + 𝒪(N2)
   X(i, j, n) X a(i, j, n)	X	Expects: […] Effects: Constructs an empty container with at least n buckets, using hasher() as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it.	Average case 𝒪(n) + 𝒪(N) (N is distance(i, j)), worst case 𝒪(n) + 𝒪(N2)
   X(i, j) X a(i, j)	X	Expects: […] Effects: Constructs an empty container with an unspecified number n of buckets, using hasher() as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it.	Average case 𝒪(n) + 𝒪(N) (N is distance(i, j)), worst case 𝒪(n) + 𝒪(N2)
   …

4. Modify 23.5.3.1 [unord.map.overview], class template unordered_map, as indicated:

   // 23.5.3.2 [unord.map.cnstr], construct/copy/destroy
   […]
   template <class InputIterator>
     unordered_map(InputIterator f, InputIterator l,
                   size_type n = see belowunspecified,
                   const hasher& hf = hasher(),
                   const key_equal& eql = key_equal(),
                   const allocator_type& a = allocator_type());
   […]
   unordered_map(initializer_list<value_type> il,
                 size_type n = see belowunspecified,
                 const hasher& hf = hasher(),
                 const key_equal& eql = key_equal(),
                 const allocator_type& a = allocator_type());
   […]
   

5. Modify 23.5.3.2 [unord.map.cnstr] as indicated:

   unordered_map() : unordered_map(size_type(see belowunspecified)) { }
   explicit unordered_map(size_type n,
                          const hasher& hf = hasher(),
                          const key_equal& eql = key_equal(),
                          const allocator_type& a = allocator_type());
   

   -1- Effects: Constructs an empty unordered_map using the specified hash function, key equality predicate, and allocator, and using at least n buckets. For the default constructor, the number of buckets is implementation-defined. max_load_factor() returns 1.0.

   -?- Ensures: max_load_factor() == 1.0

   -2- Complexity: ConstantLinear in the number of buckets.

   template <class InputIterator>
   unordered_map(InputIterator f, InputIterator l,
                 size_type n = see belowunspecified,
                 const hasher& hf = hasher(),
                 const key_equal& eql = key_equal(),
                 const allocator_type& a = allocator_type());
   unordered_map(initializer_list<value_type> il,
                 size_type n = see belowunspecified,
                 const hasher& hf = hasher(),
                 const key_equal& eql = key_equal(),
                 const allocator_type& a = allocator_type());
   

   -3- Effects: Constructs an empty unordered_map using the specified hash function, key equality predicate, and allocator, and using at least n buckets. If n is not provided, the number of buckets is implementation-defined. Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form. max_load_factor() returns 1.0.

   -?- Ensures: max_load_factor() == 1.0

   -4- Complexity: Average case linear, worst case quadraticLinear in the number of buckets, plus 𝒪(N) (average case) or 𝒪(N2) (worst case) where N is the number of insertions.

6. Modify 23.5.4.1 [unord.multimap.overview], class template unordered_multimap, as indicated:

   // 23.5.4.2 [unord.multimap.cnstr], construct/copy/destroy
   […]
   template <class InputIterator>
     unordered_multimap(InputIterator f, InputIterator l,
                        size_type n = see belowunspecified,
                        const hasher& hf = hasher(),
                        const key_equal& eql = key_equal(),
                        const allocator_type& a = allocator_type());
   […]
   unordered_multimap(initializer_list<value_type> il,
                      size_type n = see belowunspecified,
                      const hasher& hf = hasher(),
                      const key_equal& eql = key_equal(),
                      const allocator_type& a = allocator_type());
   […]
   

7. Modify 23.5.4.2 [unord.multimap.cnstr] as indicated:

   unordered_multimap() : unordered_multimap(size_type(see belowunspecified)) { }
   explicit unordered_multimap(size_type n,
                               const hasher& hf = hasher(),
                               const key_equal& eql = key_equal(),
                               const allocator_type& a = allocator_type());
   

   -1- Effects: Constructs an empty unordered_multimap using the specified hash function, key equality predicate, and allocator, and using at least n buckets. For the default constructor, the number of buckets is implementation-defined. max_load_factor() returns 1.0.

   -?- Ensures: max_load_factor() == 1.0

   -2- Complexity: ConstantLinear in the number of buckets.

   template <class InputIterator>
   unordered_multimap(InputIterator f, InputIterator l,
                      size_type n = see belowunspecified,
                      const hasher& hf = hasher(),
                      const key_equal& eql = key_equal(),
                      const allocator_type& a = allocator_type());
   unordered_multimap(initializer_list<value_type> il,
                      size_type n = see belowunspecified,
                      const hasher& hf = hasher(),
                      const key_equal& eql = key_equal(),
                      const allocator_type& a = allocator_type());
   

   -3- Effects: Constructs an empty unordered_multimap using the specified hash function, key equality predicate, and allocator, and using at least n buckets. If n is not provided, the number of buckets is implementation-defined. Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form. max_load_factor() returns 1.0.

   -?- Ensures: max_load_factor() == 1.0

   -4- Complexity: Average case linear, worst case quadraticLinear in the number of buckets, plus 𝒪(N) (average case) or 𝒪(N2) (worst case) where N is the number of insertions.

8. Modify 23.5.6.1 [unord.set.overview], class template unordered_set, as indicated:

   // 23.5.6.2 [unord.set.cnstr], construct/copy/destroy
   […]
   template <class InputIterator>
     unordered_set(InputIterator f, InputIterator l,
                   size_type n = see belowunspecified,
                   const hasher& hf = hasher(),
                   const key_equal& eql = key_equal(),
                   const allocator_type& a = allocator_type());
   […]
   unordered_set(initializer_list<value_type> il,
                 size_type n = see belowunspecified,
                 const hasher& hf = hasher(),
                 const key_equal& eql = key_equal(),
                 const allocator_type& a = allocator_type());              
   […]
   

9. Modify 23.5.6.2 [unord.set.cnstr] as indicated:

   unordered_set() : unordered_set(size_type(see belowunspecified)) { }
   explicit unordered_set(size_type n,
                          const hasher& hf = hasher(),
                          const key_equal& eql = key_equal(),
                          const allocator_type& a = allocator_type());
   

   -1- Effects: Constructs an empty unordered_set using the specified hash function, key equality predicate, and allocator, and using at least n buckets. For the default constructor, the number of buckets is implementation-defined. max_load_factor() returns 1.0.

   -?- Ensures: max_load_factor() == 1.0

   -2- Complexity: ConstantLinear in the number of buckets.

   template <class InputIterator>
   unordered_set(InputIterator f, InputIterator l,
                 size_type n = see belowunspecified,
                 const hasher& hf = hasher(),
                 const key_equal& eql = key_equal(),
                 const allocator_type& a = allocator_type());
   unordered_set(initializer_list<value_type> il,
                 size_type n = see belowunspecified,
                 const hasher& hf = hasher(),
                 const key_equal& eql = key_equal(),
                 const allocator_type& a = allocator_type());              
   

   -3- Effects: Constructs an empty unordered_set using the specified hash function, key equality predicate, and allocator, and using at least n buckets. If n is not provided, the number of buckets is implementation-defined. Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form. max_load_factor() returns 1.0.

   -?- Ensures: max_load_factor() == 1.0

   -4- Complexity: Average case linear, worst case quadraticLinear in the number of buckets, plus 𝒪(N) (average case) or 𝒪(N2) (worst case) where N is the number of insertions.

10. Modify 23.5.6.1 [unord.set.overview], class template unordered_multiset, as indicated:

    // 23.5.7.2 [unord.multiset.cnstr], construct/copy/destroy
    […]
    template <class InputIterator>
      unordered_multiset(InputIterator f, InputIterator l,
                         size_type n = see belowunspecified,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    […]
    unordered_multiset(initializer_list<value_type> il,
                       size_type n = see belowunspecified,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());                   
    […]
    

11. Modify 23.5.7.2 [unord.multiset.cnstr] as indicated:

    unordered_multiset() : unordered_multiset(size_type(see belowunspecified)) { }
    explicit unordered_multiset(size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    

    -1- Effects: Constructs an empty unordered_multiset using the specified hash function, key equality predicate, and allocator, and using at least n buckets. For the default constructor, the number of buckets is implementation-defined. max_load_factor() returns 1.0.

    -?- Ensures: max_load_factor() == 1.0

    -2- Complexity: ConstantLinear in the number of buckets.

    template <class InputIterator>
    unordered_multiset(InputIterator f, InputIterator l,
                       size_type n = see belowunspecified,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_multiset(initializer_list<value_type> il,
                       size_type n = see belowunspecified,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());                   
    

    -3- Effects: Constructs an empty unordered_multiset using the specified hash function, key equality predicate, and allocator, and using at least n buckets. If n is not provided, the number of buckets is implementation-defined. Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form. max_load_factor() returns 1.0.

    -?- Ensures: max_load_factor() == 1.0

    -4- Complexity: Average case linear, worst case quadraticLinear in the number of buckets, plus 𝒪(N) (average case) or 𝒪(N2) (worst case) where N is the number of insertions.

1213⁽ⁱ⁾. Meaning of valid and singular iterator underspecified

Section: 24.3 [iterator.requirements] Status: Open Submitter: Daniel Krügler Opened: 2009-09-19 Last modified: 2016-01-28

Priority: 4

View all other issues in [iterator.requirements].

View all issues with Open status.

Discussion:

The terms valid iterator and singular aren't properly defined. The fuzziness of those terms became even worse after the resolution of 208⁽ⁱ⁾ (including further updates by 278⁽ⁱ⁾). In 24.3 [iterator.requirements] as of N2723 the standard says now:

5 - These values are called past-the-end values. Values of an iterator i for which the expression *i is defined are called dereferenceable. The library never assumes that past-the-end values are dereferenceable. Iterators can also have singular values that are not associated with any container. [...] Results of most expressions are undefined for singular values; the only exceptions are destroying an iterator that holds a singular value and the assignment of a non-singular value to an iterator that holds a singular value. [...] Dereferenceable values are always non-singular.

10 - An invalid iterator is an iterator that may be singular.

First, issue 208⁽ⁱ⁾ intentionally removed the earlier constraint that past-the-end values are always non-singular. The reason for this was to support null pointers as past-the-end iterators of e.g. empty sequences. But there seem to exist different views on what a singular (iterator) value is. E.g. according to the SGI definition a null pointer is not a singular value:

Dereferenceable iterators are always nonsingular, but the converse is not true. For example, a null pointer is nonsingular (there are well defined operations involving null pointers) even thought it is not dereferenceable.

and proceeds:

An iterator is valid if it is dereferenceable or past-the-end.

Even if the standard prefers a different meaning of singular here, the change was incomplete, because by restricting feasible expressions of singular iterators to destruction and assignment isn't sufficient for a past-the-end iterator: Of-course it must still be equality-comparable and in general be a readable value.

Second, the standard doesn't clearly say whether a past-the-end value is a valid iterator or not. E.g. 26.11 [specialized.algorithms]/1 says:

In all of the following algorithms, the formal template parameter ForwardIterator is required to satisfy the requirements of a forward iterator (24.1.3) [..], and is required to have the property that no exceptions are thrown from [..], or dereference of valid iterators.

The standard should make better clear what "singular pointer" and "valid iterator" means. The fact that the meaning of a valid value has a core language meaning doesn't imply that for an iterator concept the term "valid iterator" has the same meaning.

Let me add a final example: In 99 [allocator.concepts.members] of N2914 we find:

pointer X::allocate(size_type n);

11 Returns: a pointer to the allocated memory. [Note: if n == 0, the return value is unspecified. —end note]

[..]

void X::deallocate(pointer p, size_type n);

Preconditions: p shall be a non-singular pointer value obtained from a call to allocate() on this allocator or one that compares equal to it.

If singular pointer value would include null pointers this make the preconditions unclear if the pointer value is a result of allocate(0): Since the return value is unspecified, it could be a null pointer. Does that mean that programmers need to check the pointer value for a null value before calling deallocate?

[ 2010-11-09 Daniel comments: ]

A later paper is in preparation.

[ 2010 Batavia: ]

Doesn't need to be resolved for Ox

[2014-02-20 Re-open Deferred issues as Priority 4]

Consider to await the paper.

Proposed resolution:

1238⁽ⁱ⁾. Defining algorithms taking iterator for range

Section: 26 [algorithms] Status: Open Submitter: Alisdair Meredith Opened: 2009-10-15 Last modified: 2020-09-06

Priority: 3

View other active issues in [algorithms].

View all other issues in [algorithms].

View all issues with Open status.

Discussion:

The library has many algorithms that take a source range represented by a pair of iterators, and the start of some second sequence given by a single iterator. Internally, these algorithms will produce undefined behaviour if the second 'range' is not as large as the input range, but none of the algorithms spell this out in Requires clauses, and there is no catch-all wording to cover this in clause 17 or the front matter of 25.

There was an attempt to provide such wording in paper n2944 but this seems incidental to the focus of the paper, and getting the wording of this issue right seems substantially more difficult than the simple approach taken in that paper. Such wording will be removed from an updated paper, and hopefully tracked via the LWG issues list instead.

It seems there are several classes of problems here and finding wording to solve all in one paragraph could be too much. I suspect we need several overlapping requirements that should cover the desired range of behaviours.

Motivating examples:

A good initial example is the swap_ranges algorithm. Here there is a clear requirement that first2 refers to the start of a valid range at least as long as the range [first1, last1). n2944 tries to solve this by positing a hypothetical last2 iterator that is implied by the signature, and requires distance(first2,last2) < distance(first1,last1). This mostly works, although I am uncomfortable assuming that last2 is clearly defined and well known without any description of how to obtain it (and I have no idea how to write that).

A second motivating example might be the copy algorithm. Specifically, let us image a call like:

copy(istream_iterator<int>(is),istream_iterator(),ostream_iterator<int>(os));

In this case, our input iterators are literally simple InputIterators, and the destination is a simple OutputIterator. In neither case am I happy referring to std::distance, in fact it is not possible for the ostream_iterator at all as it does not meet the requirements. However, any wording we provide must cover both cases. Perhaps we might deduce last2 == ostream_iterator<int>{}, but that might not always be valid for user-defined iterator types. I can well imagine an 'infinite range' that writes to /dev/null and has no meaningful last2.

The motivating example in n2944 is std::equal, and that seems to fall somewhere between the two.

Outlying examples might be partition_copy that takes two output iterators, and the _n algorithms where a range is specified by a specific number of iterations, rather than traditional iterator pair. We should also not accidentally apply inappropriate constraints to std::rotate which takes a third iterator that is not intended to be a separate range at all.

I suspect we want some wording similar to:

For algorithms that operate on ranges where the end iterator of the second range is not specified, the second range shall contain at least as many elements as the first.

I don't think this quite captures the intent yet though. I am not sure if 'range' is the right term here rather than sequence. More awkwardly, I am not convinced we can describe an Output sequence such as produce by an ostream_iterator as "containing elements", at least not as a precondition to the call before they have been written.

Another idea was to describe require that the trailing iterator support at least distance(input range) applications of operator++ and may be written through the same number of times if a mutable/output iterator.

We might also consider handling the case of an output range vs. an input range in separate paragraphs, if that simplifies how we describe some of these constraints.

[ 2009-11-03 Howard adds: ]

Moved to Tentatively NAD Future after 5 positive votes on c++std-lib.

[LEWG Kona 2017]

Recommend Open: The design is clear here; we just need wording

[2019-01-20 Reflector prioritization]

Set Priority to 3

Rationale:

Does not have sufficient support at this time. May wish to reconsider for a future standard.

Proposed resolution:

1422⁽ⁱ⁾. vector<bool> iterators are not random access

Section: 23.3.12 [vector.bool] Status: Open Submitter: BSI Opened: 2010-08-25 Last modified: 2020-09-06

Priority: 3

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Discussion:

Addresses GB-118

vector<bool> iterators are not random access iterators because their reference type is a special class, and not bool &. All standard libary operations taking iterators should treat this iterator as if it was a random access iterator, rather than a simple input iterator.

[ Resolution proposed in ballot comment ]

Either revise the iterator requirements to support proxy iterators (restoring functionality that was lost when the Concept facility was removed) or add an extra paragraph to the vector<bool> specification requiring the library to treat vector<bool> iterators as-if they were random access iterators, despite having the wrong reference type.

[ Rapperswil Review ]

The consensus at Rapperswil is that it is too late for full support for proxy iterators, but requiring the library to respect vector<bool> iterators as-if they were random access would be preferable to flagging this container as deliberately incompatible with standard library algorithms.

Alisdair to write the note, which may become normative Remark depending on the preferences of the project editor.

[ Post-Rapperswil Alisdair provides wording ]

Initial wording is supplied, deliberately using Note in preference to Remark although the author notes his preference for Remark. The issue of whether iterator_traits<vector<bool>>::iterator_category is permitted to report random_access_iterator_tag or must report input_iterator_tag is not addressed.

[ Old Proposed Resolution: ]

Insert a new paragraph into 23.3.12 [vector.bool] between p4 and p5:

[Note All functions in the library that take a pair of iterators to denote a range shall treat vector<bool> iterators as-if they were random access iterators, even though the reference type is not a true reference.-- end note]

[ 2010-11 Batavia: ]

Closed as NAD Future, because the current iterator categories cannot correctly describe vector<bool>::iterator. But saying that they are Random Access Iterators is also incorrect, because it is not too hard to create a corresponding test that fails. We should deal with the more general proxy iterator problem in the future, and see no benefit to take a partial workaround specific to vector<bool> now.

[2017-02 in Kona, LEWG recommends NAD]

D0022 Proxy Iterators for the Ranges Extensions - as much a fix as we’re going to get for vector<bool>.

[2017-06-02 Issues Telecon]

P0022 is exploring a resolution. We consider this to be fairly important issue

Move to Open, set priority to 3

Proposed resolution:

Rationale:

No consensus to make this change at this time.

1459⁽ⁱ⁾. Overlapping evaluations are allowed

Section: 32.5.4 [atomics.order] Status: LEWG Submitter: Canada Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 1458

Discussion:

Addresses CA-21, GB-131

32.5.5 [atomics.lockfree] p.8 states:

An atomic store shall only store a value that has been computed from constants and program input values by a finite sequence of program evaluations, such that each evaluation observes the values of variables as computed by the last prior assignment in the sequence.

... but 6.9.1 [intro.execution] p.13 states:

If A is not sequenced before B and B is not sequenced before A, then A and B are unsequenced. [ Note: The execution of unsequenced evaluations can overlap. — end note ]

Overlapping executions can make it impossible to construct the sequence described in 32.5.5 [atomics.lockfree] p.8. We are not sure of the intention here and do not offer a suggestion for change, but note that 32.5.5 [atomics.lockfree] p.8 is the condition that prevents out-of-thin-air reads.

For an example, suppose we have a function invocation f(e1,e2). The evaluations of e1 and e2 can overlap. Suppose that the evaluation of e1 writes y and reads x whereas the evaluation of e2 reads y and writes x, with reads-from edges as below (all this is within a single thread).

 e1           e2
Wrlx y--   --Wrlx x
      rf\ /rf
         X
        / \
Rrlx x<-   ->Rrlx y

This seems like it should be allowed, but there seems to be no way to produce a sequence of evaluations with the property above.

In more detail, here the two evaluations, e1 and e2, are being executed as the arguments of a function and are consequently not sequenced-before each other. In practice we'd expect that they could overlap (as allowed by 6.9.1 [intro.execution] p.13), with the two writes taking effect before the two reads. However, if we have to construct a linear order of evaluations, as in 32.5.5 [atomics.lockfree] p.8, then the execution above is not permited. Is that really intended?

[ Resolution proposed by ballot comment ]

Please clarify.

[2011-03-09 Hans comments:]

I'm not proud of 32.5.4 [atomics.order] p9 (formerly p8), and I agree with the comments that this isn't entirely satisfactory. 32.5.4 [atomics.order] p9 was designed to preclude out-of-thin-air results for races among memory_order_relaxed atomics, in spite of the fact that Java experience has shown we don't really know how to do that adequately. In the long run, we probably want to revisit this.

However, in the short term, I'm still inclined to declare this NAD, for two separate reasons:

1. 6.9.1 [intro.execution] p15 states: "If a side effect on a scalar object is unsequenced relative to either another side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined." I think the examples presented here have undefined behavior as a result. It's not completely clear to me whether examples can be constructed that exhibit this problem, and don't have undefined behavior.

2. This comment seems to be using a different meaning of "evaluation" from what is used elsewhere in the standard. The sequence of evaluations here doesn't have to consist of full expression evaluations. They can be evaluations of operations like lvalue to rvalue conversion, or individual assignments. In particular, the reads and writes executed by e1 and e2 in the example could be treated as separate evaluations for purposes of producing the sequence. The definition of "sequenced before" in 6.9.1 [intro.execution] makes little sense if the term "evaluation" is restricted to any notion of complete expression. Perhaps we should add yet another note to clarify this? 32.5.4 [atomics.order] p10 probably leads to the wrong impression here.

   An alternative resolution would be to simply delete our flakey attempt at preventing out-of-thin-air reads, by removing 32.5.4 [atomics.order] p9-11, possibly adding a note that explains that we technically allow, but strongly discourage them. If we were starting this from scratch now, that would probably be my preference. But it seems like too drastic a resolution at this stage.

[2011-03-24 Madrid]

Moved to NAD Future

Proposed resolution:

1484⁽ⁱ⁾. Need a way to join a thread with a timeout

Section: 32.4.3 [thread.thread.class] Status: LEWG Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-03-01

Priority: Not Prioritized

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Discussion:

Addresses US-183

There is no way to join a thread with a timeout.

[ Resolution proposed by ballot comment: ]

Add join_for and join_until. Or decide one should never join a thread with a timeout since pthread_join doesn't have a timeout version.

[ 2010 Batavia ]

The concurrency working group deemed this an extension beyond the scope of C++0x.

Rationale:

The LWG does not wish to make a change at this time.

[2017-03-01, Kona]

SG1 recommends: Close as NAD

There has not been much demand for it, and it would usually be difficult to deal with thread_local destructor races. It can be approximated with a condition variable wait followed by an unconditional join. Adding it would create implementation issues on Posix. As always, this may be revisited if we have a paper exploring the issues in detail.

Proposed resolution:

1488⁽ⁱ⁾. Improve interoperability between the C++0x and C1x threads APIs

Section: 32.6 [thread.mutex] Status: LEWG Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-03-01

Priority: Not Prioritized

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Discussion:

Addresses US-185

Cooperate with WG14 to improve interoperability between the C++0x and C1x threads APIs. In particular, C1x mutexes should be conveniently usable with a C++0x lock_guard. Performance overheads for this combination should be considered.

[ Resolution proposed by ballot comment: ]

Remove C++0x timed_mutex and timed_recursive_mutex if that facilitates development of more compatible APIs.

[ 2010 Batavia ]

The concurrency sub-group reviewed the options, and decided that closer harmony should wait until both standards are published.

Rationale:

The LWG does not wish to make any change at this time.

[2017-03-01, Kona]

SG1 recommends: Close as NAD

Papers about C compatibility are welcome, but there may be more pressing issues. C threads are not consistently available at this point, so there seems to be little demand to fix this particular problem.

Proposed resolution:

1493⁽ⁱ⁾. Add mutex, recursive_mutex, is_locked function

Section: 32.6.4 [thread.mutex.requirements] Status: LEWG Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-03-01

Priority: Not Prioritized

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Discussion:

Addresses US-189

mutex and recursive_mutex should have an is_locked() member function. is_locked allows a user to test a lock without acquiring it and can be used to implement a lightweight try_try_lock.

[ Resolution proposed by ballot comment: ]

Add a member function:

bool is_locked() const;

to std::mutex and std::recursive_mutex. These functions return true if the current thread would not be able to obtain a mutex. These functions do not synchronize with anything (and, thus, can avoid a memory fence).

[ 2010 Batavia ]

The Concurrency subgroup reviewed this issue and deemed it to be an extension to be handled after publishing C++0x.

Rationale:

The LWG does not wish to make a change at this time.

[2017-03-01, Kona]

SG1 recommends: Close as NAD

Several participants voiced strong objections, based on either memory model issues or lock elision. No support. It is already possible to write a wrapper that explicitly tracks ownership for testing in the owning thread, which may have been part of the intent here.

Proposed resolution:

1521⁽ⁱ⁾. Requirements on internal pointer representations in containers

Section: 23.2.2 [container.requirements.general] Status: Open Submitter: Mike Spertus Opened: 2010-10-16 Last modified: 2019-01-20

Priority: 3

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Discussion:

Addresses US-104, US-141

The standard doesn't say that containers should use abstract pointer types internally. Both Howard and Pablo agree that this is the intent. Further, it is necessary for containers to be stored, for example, in shared memory with an interprocess allocator (the type of scenario that allocators are intended to support).

In spite of the (possible) agreement on intent, it is necessary to make this explicit:

An implementations may like to store the result of dereferencing the pointer (which is a raw reference) as an optimization, but that prevents the data structure from being put in shared memory, etc. In fact, a container could store raw references to the allocator, which would be a little weird but conforming as long as it has one by-value copy. Furthermore, pointers to locales, ctypes, etc. may be there, which also prevents the data structure from being put in shared memory, so we should make explicit that a container does not store raw pointers or references at all.

[ Pre-batavia ]

This issue is being opened as part of the response to NB comments US-104/141. See paper N3171 in the pre-Batavia mailing.

[2011-03-23 Madrid meeting]

Deferred

[ 2011 Batavia ]

This may be an issue, but it is not clear. We want to gain a few years experience with the C++11 allocator model to see if this is already implied by the existing specification.

[LEWG Kona 2017]

Status to Open: Acknowledged, need wording: (N4618 numbering) 23.2.1 container.requirements.general p8 first sentence. Replace non-normative note with requirement.

See discussion on LEWG Wiki

[2019-01-20 Reflector prioritization]

Set Priority to 3

Proposed resolution:

Add to the end of 23.2.2 [container.requirements.general] p. 8:

[..] In all container types defined in this Clause, the member get_allocator() returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement. The container may not store internal objects whose types are of the form T * or T & except insofar as they are part of the item type or members.

2035⁽ⁱ⁾. Output iterator requirements are broken

Section: 24.3.5.4 [output.iterators] Status: Open Submitter: Daniel Krügler Opened: 2011-02-27 Last modified: 2016-01-28

Priority: 3

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Discussion:

During the Pittsburgh meeting the proposal N3066 became accepted because it fixed several severe issues related to the iterator specification. But the current working draft (N3225) does not reflect all these changes. Since I'm unaware whether every correction can be done editorial, this issue is submitted to take care of that. To give one example: All expressions of Table 108 — "Output iterator requirements" have a post-condition that the iterator is incrementable. This is impossible, because it would exclude any finite sequence that is accessed by an output iterator, such as a pointer to a C array. The N3066 wording changes did not have these effects.

[2011-03-01: Daniel comments:]

This issue has some overlap with the issue 2038⁽ⁱ⁾ and I would prefer if we could solve both at one location. I suggest the following approach:

1. The terms dereferencable and incrementable could be defined in a more general way not restricted to iterators (similar to the concepts HasDereference and HasPreincrement from working draft N2914). But on the other hand, all current usages of dereferencable and incrementable are involved with types that satisfy iterator requirements. Thus, I believe that it is sufficient for C++0x to add corresponding definitions to 24.3.1 [iterator.requirements.general] and to let all previous usages of these terms refer to this sub-clause. Since the same problem occurs with the past-the-end iterator, this proposal suggest providing similar references to usages that precede its definition as well.

2. We also need to ensure that all iterator expressions get either an operational semantics in terms of others or we need to add missing pre- and post-conditions. E.g. we have the following ones without semantics:

   *r++ = o // output iterator
   *r--     // bidirectional iterator
   

   According to the SGI specification these correspond to

   { *r = o; ++r; }                         // output iterator
   { reference tmp = *r; --r; return tmp; } // bidirectional iterator
   

   respectively. Please note especially the latter expression for bidirectional iterator. It fixes a problem that we have for forward iterator as well: Both these iterator categories provide stronger guarantees than input iterator, because the result of the dereference operation is reference, and not only convertible to the value type (The exact form from the SGI documentation does not correctly refer to reference).

[2011-03-14: Daniel comments and updates the suggested wording]

In addition to the before mentioned necessary changes there is another one need, which became obvious due to issue 2042⁽ⁱ⁾: forward_list<>::before_begin() returns an iterator value which is not dereferencable, but obviously the intention is that it should be incrementable. This leads to the conclusion that imposing dereferencable as a requirement for the expressions ++r is wrong: We only need the iterator to be incrementable. A similar conclusion applies to the expression --r of bidirectional iterators.

[ 2011 Bloomington ]

Consensus this is the correct direction, but there are (potentially) missing incrementable preconditions on some table rows, and the Remarks on when an output iterator becomes dereferencable are probably better handled outside the table, in a manner similar to the way we word for input iterators.

There was some concern about redundant pre-conditions when the operational semantic is defined in terms of operations that have preconditions, and a similar level of concern over dropping such redundancies vs. applying a consistent level of redundant specification in all the iterator tables. Wording clean-up in either direction would be welcome.

[2011-08-18: Daniel adapts the proposed resolution to honor the Bloomington request]

There is only a small number of further changes suggested to get rid of superfluous requirements and essentially non-normative assertions. Operations should not have extra pre-conditions, if defined by "in-terms-of" semantics, see e.g. a != b or a->m for Table 107. Further, some remarks, that do not impose anything or say nothing new have been removed, because I could not find anything helpful they provide. E.g. consider the remarks for Table 108 for the operations dereference-assignment and preincrement: They don't provide additional information say nothing surprising. With the new pre-conditions and post-conditions it is implied what the remarks intend to say.

[ 2011-11-03: Some observations from Alexander Stepanov via c++std-lib-31405 ]

The following sentence is dropped from the standard section on OutputIterators:

"In particular, the following two conditions should hold: first, any iterator value should be assigned through before it is incremented (this is, for an output iterator i, i++; i++; is not a valid code sequence); second, any value of an output iterator may have at most one active copy at any given time (for example, i = j; *++i = a; *j = b; is not a valid code sequence)."

[ 2011-11-04: Daniel comments and improves the wording ]

In regard to the first part of the comment, the intention of the newly proposed wording was to make clear that for the expression

*r = o

we have the precondition dereferenceable and the post-condition incrementable. And for the expression

++r

we have the precondition incrementable and the post-condition dereferenceable or past-the-end. This should not allow for a sequence like i++; i++; but I agree that it doesn't exactly say that.

In regard to the second point: To make this point clearer, I suggest to add a similar additional wording as we already have for input iterator to the "Assertion/note" column of the expression ++r:

"Post: any copies of the previous value of r are no longer required to be dereferenceable or incrementable."

The proposed has been updated to honor the observations of Alexander Stepanov.

[2015-02 Cologne]

The matter is complicated, Daniel volunteers to write a paper.

Proposed resolution:

1. Add a reference to 24.3.1 [iterator.requirements.general] to the following parts of the library preceding Clause 24 Iterators library: (I stopped from 23.2.8 [unord.req] on, because the remaining references are the concrete containers)

   1. 16.4.4.3 [swappable.requirements] p5:

      -5- A type X satisfying any of the iterator requirements (24.2) is ValueSwappable if, for any dereferenceable (24.3.1 [iterator.requirements.general]) object x of type X, *x is swappable.

   2. 16.4.4.6 [allocator.requirements], Table 27 — "Descriptive variable definitions", row with the expression c:

      a dereferenceable (24.3.1 [iterator.requirements.general]) pointer of type C*

   3. 20.2.3.3 [pointer.traits.functions]:

      Returns: The first template function returns a dereferenceable (24.3.1 [iterator.requirements.general]) pointer to r obtained by calling Ptr::pointer_to(r); […]

   4. 27.4.3.4 [string.iterators] p. 2:

      Returns: An iterator which is the past-the-end value (24.3.1 [iterator.requirements.general]).

   5. 28.3.4.6.2.3 [locale.time.get.virtuals] p. 11:

      iter_type do_get(iter_type s, iter_type end, ios_base& f,
        ios_base::iostate& err, tm *t, char format, char modifier) const;
      

      Requires: t shall be dereferenceable (24.3.1 [iterator.requirements.general]).

   6. 23.2.2 [container.requirements.general] p. 6:

      […] end() returns an iterator which is the past-the-end (24.3.1 [iterator.requirements.general]) value for the container. […]

   7. 23.2.4 [sequence.reqmts] p. 3:

      […] q denotes a valid dereferenceable (24.3.1 [iterator.requirements.general]) const iterator to a, […]

   8. 23.2.7 [associative.reqmts] p. 8 (I omit intentionally one further reference in the same sub-clause):

      […] q denotes a valid dereferenceable (24.3.1 [iterator.requirements.general]) const iterator to a, […]

   9. 23.2.8 [unord.req] p. 10 (I omit intentionally one further reference in the same sub-clause):

      […] q and q1 are valid dereferenceable (24.3.1 [iterator.requirements.general]) const iterators to a, […]

2. Edit 24.3.1 [iterator.requirements.general] p. 5 as indicated (The intent is to properly define incrementable and to ensure some further library guarantee related to past-the-end iterator values):

   -5- Just as a regular pointer to an array guarantees that there is a pointer value pointing past the last element of the array, so for any iterator type there is an iterator value that points past the last element of a corresponding sequence. These values are called past-the-end values. Values of an iterator i for which the expression *i is defined are called dereferenceable. Values of an iterator i for which the expression ++i is defined are called incrementable. The library never assumes that past-the-end values are dereferenceable or incrementable. Iterators can also have singular values that are not associated with any sequence. […]

3. Modify the column contents of Table 106 — "Iterator requirements", 24.3.5.2 [iterator.iterators], as indicated:

   Table 106 — Iterator requirements

   Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
   *r	reference		pre: r is dereferenceable.
   ++r	X&		pre: r is incrementable.

4. Modify the column contents of Table 107 — "Input iterator requirements", 24.3.5.3 [input.iterators], as indicated [Rationale: The wording changes attempt to define a minimal "independent" set of operations, namely *a and ++r, and to specify the semantics of the remaining ones. This approach seems to be in agreement with the original SGI specification — end rationale]:

   Table 107 — Input iterator requirements (in addition to Iterator)

   Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
   a != b	contextually convertible to bool	!(a == b)	pre: (a, b) is in the domain of ==.
   *a	convertible to T		pre: a is dereferenceable. The expression (void)*a, *a is equivalent to *a. If a == b and (a,b) is in the domain of == then *a is equivalent to *b.
   a->m		(*a).m	pre: a is dereferenceable.
   ++r	X&		pre: r is dereferenceableincrementable. post: r is dereferenceable or r is past-the-end. post: any copies of the previous value of r are no longer required either to be dereferenceable, incrementable, or to be in the domain of ==.
   (void)r++		(void)++r	equivalent to (void)++r
   *r++	convertible to T	{ T tmp = *r; ++r; return tmp; }

5. Modify the column contents of Table 108 — "Output iterator requirements", 24.3.5.4 [output.iterators], as indicated [Rationale: The wording changes attempt to define a minimal "independent" set of operations, namely *r = o and ++r, and to specify the semantics of the remaining ones. This approach seems to be in agreement with the original SGI specification — end rationale]:

   Table 108 — Output iterator requirements (in addition to Iterator)

   Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
   *r = o	result is not used		pre: r is dereferenceable. Remark: After this operation r is not required to be dereferenceable and any copies of the previous value of r are no longer required to be dereferenceable or incrementable. post: r is incrementable.
   ++r	X&		pre: r is incrementable. &r == &++r. Remark: After this operation r is not required to be dereferenceable. Remark: After this operation r is not required to be incrementable and any copies of the previous value of r are no longer required to be dereferenceable or incrementable. post: r is dereferenceable or r is past-the-endincrementable.
   r++	convertible to const X&	{ X tmp = r; ++r; return tmp; }	Remark: After this operation r is not required to be dereferenceable. post: r is incrementable.
   *r++ = o	result is not used	{ *r = o; ++r; }	Remark: After this operation r is not required to be dereferenceable. post: r is incrementable.

6. Modify the column contents of Table 109 — "Forward iterator requirements", 24.3.5.5 [forward.iterators], as indicated [Rationale: Since the return type of the expression *r++ is now guaranteed to be type reference, the implied operational semantics from input iterator based on value copies is wrong — end rationale]

   Table 109 — Forward iterator requirements (in addition to input iterator)

   Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
   r++	convertible to const X&	{ X tmp = r; ++r; return tmp; }
   *r++	reference	{ reference tmp = *r; ++r; return tmp; }

7. Modify the column contents of Table 110 — "Bidirectional iterator requirements", 24.3.5.6 [bidirectional.iterators], as indicated:

   Table 110 — Bidirectional iterator requirements (in addition to forward iterator)

   Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
   --r	X&		pre: there exists s such that r == ++s. post: r is dereferenceableincrementable. --(++r) == r. --r == --s implies r == s. &r == &--r.
   r--	convertible to const X&	{ X tmp = r; --r; return tmp; }
   *r--	reference	{ reference tmp = *r; --r; return tmp; }

2038⁽ⁱ⁾. Missing definition for incrementable iterator

Section: 24.3.5.4 [output.iterators] Status: Open Submitter: Pete Becker Opened: 2011-02-27 Last modified: 2016-01-28

Priority: 3

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Discussion:

In comp.lang.c++, Vicente Botet raises the following questions:

"In "24.2.4 Output iterators" there are 3 uses of incrementable. I've not found the definition. Could some one point me where it is defined?

Something similar occurs with dereferenceable. While the definition is given in "24.2.1 In general" it is used several times before.

Shouldn't these definitions be moved to some previous section?"

He's right: both terms are used without being properly defined.

There is no definition of "incrementable".

While there is a definition of "dereferenceable", it is, in fact, a definition of "dereferenceable iterator". "dereferenceable" is used throughout Clause 23 (Containers) before its definition in Clause 24. In almost all cases it's referring to iterators, but in 16.4.4.3 [swappable.requirements] there is a mention of "dereferenceable object"; in 16.4.4.6 [allocator.requirements] the table of Descriptive variable definitions refers to a "dereferenceable pointer"; 20.2.3.3 [pointer.traits.functions] refers to a "dereferenceable pointer"; in 28.3.4.6.2.3 [locale.time.get.virtuals]/11 (do_get) there is a requirement that a pointer "shall be dereferenceable". In those specific cases it is not defined.

[2011-03-02: Daniel comments:]

I believe that the currently proposed resolution of issue 2035⁽ⁱ⁾ solves this issue as well.

[ 2011 Bloomington ]

Agree with Daniel, this will be handled by the resolution of 2035⁽ⁱ⁾.

Proposed resolution:

2077⁽ⁱ⁾. Further incomplete constraints for type traits

Section: 21.3.5.4 [meta.unary.prop] Status: Open Submitter: Daniel Krügler Opened: 2011-08-20 Last modified: 2016-01-28

Priority: 3

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Discussion:

The currently agreed on proposed wording for 2015⁽ⁱ⁾ using remove_all_extents<T>::type instead of the "an array of unknown bound" terminology in the precondition should be extended to some further entries especially in Table 49, notably the is_*constructible, is_*assignable, and is_*destructible entries. To prevent ODR violations, incomplete element types of arrays must be excluded for value-initialization and destruction for example. Construction and assignment has to be honored, when we have array-to-pointer conversions or pointer conversions of incomplete pointees in effect.

[2012, Kona]

The issue is that in three type traits, we are accidentally saying that in certain circumstances the type must give a specified answer when given an incomplete type. (Specifically: an array of unknown bound of incomplete type.) The issue asserts that there's an ODR violation, since the trait returns false in that case but might return a different version when the trait is completed.

Howard argues: no, there is no risk of an ODR violation. is_constructible<A[]> must return false regardless of whether A is complete, so there's no reason to forbid an array of unknown bound of incomplete types. Same argument applies to is_assignable. General agreement with Howard's reasoning.

There may be a real issue for is_destructible. None of us are sure what is_destructible is supposed to mean for an array of unknown bound (regardless of whether its type is complete), and the standard doesn't make it clear. The middle column doesn't say what it's supposed to do for incomplete types.

In at least one implementation, is_destructible<A[]> does return true if A is complete, which would result in ODR violation unless we forbid it for incomplete types.

Move to open. We believe there is no issue for is_constructible or is_assignable, but that there is a real issue for is_destructible.

Proposed resolution:

2095⁽ⁱ⁾. promise and packaged_task missing constructors needed for uses-allocator construction

Section: 32.10.6 [futures.promise], 32.10.10 [futures.task] Status: LEWG Submitter: Jonathan Wakely Opened: 2011-11-01 Last modified: 2019-06-03

Priority: 4

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Discussion:

This example is ill-formed according to C++11 because uses_allocator<promise<R>, A>::value is true, but is_constructible<promise<R>, A, promise<R>&&>::value is false. Similarly for packaged_task.

#include <future>
#include <memory>
#include <tuple>

using namespace std;

typedef packaged_task<void()> task;
typedef promise<void> prom;
allocator<task> a;

tuple<task, prom> t1{ allocator_arg, a };
tuple<task, prom> t2{ allocator_arg, a, task{}, prom{} };

[2012, Portland]

This is an allocator issue, and should be dealt with directly by LWG.

[2013-03-06]

Jonathan suggests to make the new constructors non-explicit and makes some representational improvements.

[2013-09 Chicago]

Move to deferred.

This issue has much in common with similar problems with std::function that are being addressed by the polymorphic allocators proposal currently under evaluation in LEWG. Defer further discussion on this topic until the final outcome of that paper and its proposed resolution is known.

[2014-02-20 Re-open Deferred issues as Priority 4]

[2016-08 Chicago]

Fri PM: Send to LEWG - and this also applies to function in LFTS.

[2019-06-03 Jonathan Wakely provides updated wording]

Jonathan updates wording post-2976⁽ⁱ⁾ and observes that this resolution conflicts with 3003⁽ⁱ⁾.

Previous resolution [SUPERSEDED]:

[This wording is relative to the FDIS.]

1. Add to 32.10.6 [futures.promise], class template promise synopsis, as indicated:

   namespace std {
     template <class R>
     class promise {
     public:
       promise();
       template <class Allocator>
       promise(allocator_arg_t, const Allocator& a);
       template <class Allocator>
       promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;
       promise(promise&& rhs) noexcept;
       promise(const promise& rhs) = delete;
       ~promise();	
       […]
     };
     […]
   }
   

2. Change 32.10.6 [futures.promise] as indicated:

   promise(promise&& rhs) noexcept;
   template <class Allocator>
   promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;
   

   -5- Effects: constructs a new promise object and transfers ownership of the shared state of rhs (if any) to the newly-constructed object.

   -6- Postcondition: rhs has no shared state.

   -?- [Note: a is not used — end note]

3. Add to 32.10.10 [futures.task], class template packaged_task synopsis, as indicated:

   namespace std {
     template<class> class packaged_task; // undefined
   
     template<class R, class... ArgTypes>
     class packaged_task<R(ArgTypes...)> {
     public:
       // construction and destruction
       packaged_task() noexcept;
       template <class Allocator>
         packaged_task(allocator_arg_t, const Allocator& a) noexcept;
       template <class F>
         explicit packaged_task(F&& f);
       template <class F, class Allocator>
         explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);
       ~packaged_task();
   	
       // no copy
       packaged_task(const packaged_task&) = delete;
       template<class Allocator>
         packaged_task(allocator_arg_t, const Allocator& a, const packaged_task&) = delete;
       packaged_task& operator=(const packaged_task&) = delete;
       
       // move support
       packaged_task(packaged_task&& rhs) noexcept;
       template <class Allocator>
         packaged_task(allocator_arg_t, const Allocator& a, packaged_task&& rhs) noexcept;
       packaged_task& operator=(packaged_task&& rhs) noexcept;
       void swap(packaged_task& other) noexcept;
       […]
     };
     […]
   }
   

4. Change 32.10.10.2 [futures.task.members] as indicated:

   packaged_task() noexcept;
   template <class Allocator>
     packaged_task(allocator_arg_t, const Allocator& a) noexcept;
   

   -1- Effects: constructs a packaged_task object with no shared state and no stored task.

   -?- [Note: a is not used — end note]

   […]

   packaged_task(packaged_task&& rhs) noexcept;
   template <class Allocator>
     packaged_task(allocator_arg_t, const Allocator& a, packaged_task&& rhs) noexcept;
   

   -5- Effects: constructs a new packaged_task object and transfers ownership of rhs's shared state to *this, leaving rhs with no shared state. Moves the stored task from rhs to *this.

   -6- Postcondition: rhs has no shared state.

   -?- [Note: a is not used — end note]

Proposed resolution:

[This wording is relative to N4810.]

1. Add to 32.10.6 [futures.promise], class template promise synopsis, as indicated:

   namespace std {
     template <class R>
     class promise {
     public:
       promise();
       template <class Allocator>
         promise(allocator_arg_t, const Allocator& a);
       template <class Allocator>
         promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;
       promise(promise&& rhs) noexcept;
       promise(const promise& rhs) = delete;
       ~promise();
       […]
     };
     […]
   }
   

2. Change 32.10.6 [futures.promise] as indicated:

   promise(promise&& rhs) noexcept;
   template <class Allocator>
     promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;
   

   -5- Effects: constructs a new promise object and transfers ownership of the shared state of rhs (if any) to the newly-constructed object.

   -6- Postcondition: rhs has no shared state.

   -?- [Note: a is not used — end note]

2115⁽ⁱ⁾. Undefined behaviour for valarray assignments with mask_array index?

Section: 29.6.8 [template.mask.array] Status: Open Submitter: Thomas Plum Opened: 2011-12-10 Last modified: 2016-01-28

Priority: 4

View all issues with Open status.

Discussion:

Recently I received a Service Request (SR) alleging that one of our testcases causes an undefined behavior. The complaint is that 29.6.8 [template.mask.array] in C++11 (and the corresponding subclause in C++03) are interpreted by some people to require that in an assignment "a[mask] = b", the subscript mask and the rhs b must have the same number of elements.

IMHO, if that is the intended requirement, it should be stated explicitly.

In any event, there is a tiny editorial cleanup that could be made:

In C++11, 29.6.8.1 [template.mask.array.overview] para 2 mentions

"the expression a[mask] = b;"

but the semicolon cannot be part of an expression. The correction could omit the semicolon, or change the word "expression" to "assignment" or "statement".

Here is the text of the SR, slightly modified for publication:

Subject: SR01174 LVS _26322Y31 has undefined behavior [open]

[Client:]
The test case t263.dir/_26322Y31.cpp seems to be illegal as it has an undefined behaviour. I searched into the SRs but found SRs were not related to the topic explained in this mail (SR00324, SR00595, SR00838).

const char vl[] = {"abcdefghijklmnopqrstuvwxyz"};
const char vu[] = {"ABCDEFGHIJKLMNOPQRSTUVWXYZ"};
const std::valarray<char> v0(vl, 27), vm5(vu, 5), vm6(vu, 6);
std::valarray<char> x = v0;
[…]
const bool vb[] = {false, false, true, true, false, true};
const std::valarray<bool> vmask(vb, 6);
x = v0;
x[vmask] = vm5;      // ***** HERE....
steq(&x[0], "abABeCghijklmnopqrstuvwxyz");
x2 = x[vmask];       // ***** ....AND HERE
[…]

This problem has already been discussed between [experts]: See thread http://gcc.gnu.org/ml/libstdc++/2009-11/threads.html#00051 Conclusion http://gcc.gnu.org/ml/libstdc++/2009-11/msg00099.html

[Plum Hall:]
Before I log this as an SR, I need to check one detail with you.

I did read the email thread you mentioned, and I did find a citation (see INCITS ISO/IEC 14882-2003 Section 26.3.2.6 on valarray computed assignments):

Quote: "If the array and the argument array do not have the same length, the behavior is undefined",

But this applies to computed assignment (*=, +=, etc), not to simple assignment. Here is the C++03 citation re simple assignment:

26.3.2.2 valarray assignment [lib.valarray.assign]

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

1 Each element of the *this array is assigned the value of the corresponding element of the argument array. The resulting behavior is undefined if the length of the argument array is not equal to the length of the *this array.

In the new C++11 (N3291), we find ...

26.6.2.3 valarray assignment [valarray.assign]

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

1 Each element of the *this array is assigned the value of the corresponding element of the argument array. If the length of v is not equal to the length of *this, resizes *this to make the two arrays the same length, as if by calling resize(v.size()), before performing the assignment.

So it looks like the testcase might be valid for C++11 but not for C++03; what do you think?

[Client:]
I quite agree with you but the two problems I mentioned:

x[vmask] = vm5;      // ***** HERE....
[…]
x2 = x[vmask];       // ***** ....AND HERE

refer to mask_array assignment hence target the C++03 26.3.8 paragraph. Correct?

[Plum Hall:]
I mentioned the contrast between C++03 26.3.2.2 para 1 versus C++11 26.6.2.3 para 1.

But in C++03 26.3.8, I don't find any corresponding restriction. Could you quote the specific requirement you're writing about?

[Client:]
I do notice the difference between c++03 26.3.2.2 and c++11 26.6.2.3 about assignments between different sized valarray and I perfectly agree with you.

But, as already stated, this is not a simple valarray assignment but a mask_array assignment (c++03 26.3.8 / c++11 26.6.8). See c++11 quote below:

26.6.8 Class template mask_array
26.6.8.1 Class template mask_array overview
[....]

1. This template is a helper template used by the mask subscript operator: mask_array<T> valarray<T>::operator[](const valarray<bool>&).

2. It has reference semantics to a subset of an array specified by a boolean mask. Thus, the expression a[mask] = b; has the effect of assigning the elements of b to the masked elements in a (those for which the corresponding element in mask is true.)

26.6.8.2 mask_array assignment

void operator=(const valarray<T>&) const;
const mask_array& operator=(const mask_array&) const;

1 These assignment operators have reference semantics, assigning the values of the argument array elements to selected elements of the valarray<T> object to which it refers.

In particular, [one of the WG21 experts] insisted on the piece "the elements of b".

That is why I reported the test t263.dir/_26322Y31.cpp having an undefined behaviour.

[Plum Hall:]
OK, I can see that I will have to ask WG21; I will file an appropriate issue with the Library subgroup. In the meantime, I will mark this testcase as "DISPUTED" so that it is not required for conformance testing, until we get a definitive opinion.

[2012, Kona]

Moved to Open.

There appears to be a real need for clarification in the standard, and implementations differ in their current interpretation. This will need some research by implementers and a proposed resolution before further discussion is likely to be fruitful.

Proposed resolution:

2116⁽ⁱ⁾. is_nothrow_constructible and destructors

Section: 21.3.5.4 [meta.unary.prop] Status: Open Submitter: Dave Abrahams Opened: 2011-12-09 Last modified: 2023-05-25

Priority: 3

View other active issues in [meta.unary.prop].

View all other issues in [meta.unary.prop].

View all issues with Open status.

Discussion:

IMO if we specified is_[nothrow_]constructible in terms of a variable declaration whose validity requires destructibility, it is clearly a bug in our specification and a failure to realize the actual original intent. The specification should have been in terms of placement-new.

Daniel:
At the time of the specification this was intended and the solution is not done by removing the destruction semantics of is_constructible.

The design of is_constructible was also impacted by the previous Constructible concept that explicitly contained destruction semantics, because during conceptification of the library it turned out to simplify the constraints in the library because you did not need to add Destructible all the time. It often was implied but never spoken out in C++03.

Pure construction semantics was considered as useful as well, so HasConstructor did also exist and would surely be useful as trait as well.

Another example that is often overlooked: This also affects wrapper types like pair, tuple, array that contain potentially more than one type: This is easy to understand if you think of T1 having a deleted destructor and T2 having a constructor that may throw: Obviously the compiler has potentially need to use the destructor of T1 in the constructor of std::pair<T1, T2> to ensure that the core language requirements are satisfied (All previous fully constructed sub-objects must be destructed).

The core language also honors this fact in [class.copy] p11:

A defaulted copy/move constructor for a class X is defined as deleted (9.5.3 [dcl.fct.def.delete]) if X has:
[…]
— any direct or virtual base class or non-static data member of a type with a destructor that is deleted or inaccessible from the defaulted constructor,
[…]

Dave:
This is about is_nothrow_constructible in particular. The fact that it is foiled by not having a noexcept dtor is a defect.

[2012, Kona]

Move to Open.

is_nothrow_constructible is defined in terms of is_constructible, which is defined by looking at a hypothetical variable and asking whether the variable definition is known not to throw exceptions. The issue claims that this also examines the type's destructor, given the context, and thus will return false if the destructor can potentially throw. At least one implementation (Howard's) does return false if the constructor is noexcept(true) and the destructor is noexcept(false). So that's not a strained interpretation. The issue is asking for this to be defined in terms of placement new, instead of in terms of a temporary object, to make it clearer that is_nothrow_constructible looks at the noexcept status of only the constructor, and not the destructor.

Sketch of what the wording would look like:

require is_constructible, and then also require that a placement new operation does not throw. (Remembering the title of this issue... What does this imply for swap?

If we accept this resolution, do we need any changes to swap?

STL argues: no, because you are already forbidden from passing anything with a throwing destructor to swap.

Dietmar argues: no, not true. Maybe statically the destructor can conceivably throw for some values, but maybe there are some values known not to throw. In that case, it's correct to pass those values to swap.

[2017-01-27 Telecon]

Gave the issue a better title

This issue interacts with 2827⁽ⁱ⁾

Ville would like "an evolution group" to take a look at this issue.

[2020-08; LWG reflector]

A poll was taken to close the issue as NAD, but only gained three votes in favour (and one vote against, which was subsequently withdrawn).

[2022-03; LWG reflector]

A poll was taken to close the issue as NAD, with six votes in favour. (and one vote against, subsequently withdrawn).

"Write a paper if you want something else. These traits have well established meaning now." "Minimizing requirements is not as important a concern for standard library concepts as as minimizing the number of concepts. Requirements like 'I need to construct but not destroy an object' are niche enough that we don't need to support them."

[2022-11-30; LWG telecon]

Alisdair intends to write a paper for this.

[2023-05-25; May 2023 mailing]

Alisdair provided P2842R0.

Proposed resolution:

2117⁽ⁱ⁾. ios_base manipulators should have showgrouping/noshowgrouping

Section: 28.3.4.3.3.3 [facet.num.put.virtuals], 31.5.2.2.2 [ios.fmtflags], 31.5.5.1 [fmtflags.manip] Status: Open Submitter: Benjamin Kosnik Opened: 2011-12-15 Last modified: 2023-02-07

Priority: 3

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

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

View all issues with Open status.

Discussion:

Iostreams should include a manipulator to toggle grouping on/off for locales that support grouped digits. This has come up repeatedly and been deferred. See LWG 826⁽ⁱ⁾ for the previous attempt.

If one is using a locale that supports grouped digits, then output will always include the generated grouping characters. However, very plausible scenarios exist where one might want to output the number, un-grouped. This is similar to existing manipulators that toggle on/off the decimal point, numeric base, or positive sign.

See some user commentary here.

[21012, Kona]

Move to Open.

This is a feature request.

Walter is slightly uncomfortable with processing feature requests through the issues lists.

Alisdair says this is far from the first feature request that has come in from the issues list.

STL: The fact that you can turn off grouping on hex output is compelling.

Marshall: if we add this flag, we'll need to update tables 87-91 as well.

STL: If it has been implemented somewhere, and it works, we'd be glad to add it.

Howard: We need to say what the default is.

Alisdair sumarizes:

(1) We want clear wording that says what the effect is of turning the flag off;

(2) what the default values are, and

(3) how this fits into tables 87-90. (and 128)

[Issaquah 2014-02-10-12: Move to LEWG]

Since this issue was filed, we have grown a new working group that is better placed to handle feature requests.

We will track such issues with an LEWG status until we get feedback from the Library Evolution Working Group.

[Issaquah 2014-02-12: LEWG discussion]

Think about the ABI break for adding a flag. But this could be mitigated by putting the data into an iword instead of a flag.

This needs to change Stage 2 in [facet.num.put.virtuals].

Previous resolution, which needs the above corrections:

This wording is relative to the FDIS.

1. Insert in 28.3.4.3.3.3 [facet.num.put.virtuals] paragraph 5:

   Stage 1: The first action of stage 1 is to determine a conversion specifier. The tables that describe this determination use the following local variables

   fmtflags flags = str.flags() ;
   fmtflags basefield = (flags & (ios_base::basefield));
   fmtflags uppercase = (flags & (ios_base::uppercase));
   fmtflags floatfield = (flags & (ios_base::floatfield));
   fmtflags showpos = (flags & (ios_base::showpos));
   fmtflags showbase = (flags & (ios_base::showbase));
   fmtflags showgrouping = (flags & (ios_base::showgrouping));
   

2. Change header <ios> synopsis, [iostreams.base.overview] as indicated:

   #include <iosfwd>
   
   namespace std {
     […]
     // 27.5.6, manipulators:
     […]
     ios_base& showpoint     (ios_base& str);
     ios_base& noshowpoint   (ios_base& str);
     ios_base& showgrouping  (ios_base& str);
     ios_base& noshowgrouping(ios_base& str);
     ios_base& showpos       (ios_base& str);
     ios_base& noshowpos     (ios_base& str);
     […]
   }
   

3. Change class ios_base synopsis, 31.5.2 [ios.base] as indicated:

   namespace std {
     class ios_base {
     public:
     class failure;
       // 27.5.3.1.2 fmtflags
       typedef T1 fmtflags;
       […]
       static constexpr fmtflags showpoint = unspecified ;
       static constexpr fmtflags showgrouping = unspecified ;
       static constexpr fmtflags showpos = unspecified ;
       […]
     };
   }
   

4. Add a new entry to Table 122 — "fmtflags effects" as indicated:

   Table 122 — fmtflags effects

   Element	Effect(s) if set
   […]
   showpoint	generates a decimal-point character unconditionally in generated floatingpoint output
   showgrouping	generates grouping characters unconditionally in generated output
   […]

5. After [ios::fmtflags] p12 insert the following:

   ios_base& showgrouping(ios_base& str);
   

   -?- Effects: Calls str.setf(ios_base::showgrouping).

   -?- Returns: str.

   ios_base& noshowgrouping(ios_base& str);
   

   -?- Effects: Calls str.unsetf(ios_base::showgrouping).

   -?- Returns: str.

Proposed resolution:

2136⁽ⁱ⁾. Postconditions vs. exceptions

Section: 16.3.2 [structure] Status: Open Submitter: Jens Maurer Opened: 2012-03-08 Last modified: 2024-10-05

Priority: 3

View all issues with Open status.

Discussion:

The front matter in clause 17 should clarify that postconditions will not hold if a standard library function exits via an exception. Postconditions or guarantees that apply when an exception is thrown (beyond the basic guarantee) are described in an "Exception safety" section.

[ 2012-10 Portland: Move to Open ]

Consensus that we do not clearly say this, and that we probably should. A likely location to describe the guarantees of postconditions could well be a new sub-clause following 99 [res.on.required] which serves the same purpose for requires clauses. However, we need such wording before we can make progress.

Also, see 2137⁽ⁱ⁾ for a suggestion that we want to see a paper resolving both issues together.

[2015-05-06 Lenexa: EricWF to write paper addressing 2136 and 2137]

MC: Idea is to replace all such "If no exception" postconditions with "Exception safety" sections.

[2021-06-20; Daniel comments]

An informal editorial change suggestion has recently been made whose editorial implementation would promote the idea that the default assumption is that Postconditions: are only met if the function doesn't exit with an exception.

After analyzing all current existing Postconditions: elements the following seems to hold: Affected by this issue are only non-noexcept functions and mostly non-constructor functions (unless the Postconditions: element says something about the value of its arguments). Most existing Postconditions seem to be intended to apply only in non-exceptional cases. I found some where this is presumably not intended, namely those of the expressions os << x and is >> v in Tables [tab:rand.req.eng] and [tab:rand.req.dist], maybe also 30.11.2.4 [time.zone.db.remote] p4.

Nonetheless, the editorial change seems to be applicable even without having this issue resolved, because it doesn't actually change the normative state by itself.

[2024-10-03; Jonathan adds wording]

Proposed resolution:

This wording is relative to N4988.

1. Change 16.3.2.4 [structure.specifications] as indicated:

   (3.6) — Postconditions: the conditions (sometimes termed observable results) established by the function when a call to it returns normally.

2137⁽ⁱ⁾. Misleadingly constrained post-condition in the presence of exceptions

Section: 28.6.7.3 [re.regex.assign] Status: Open Submitter: Jonathan Wakely Opened: 2012-03-08 Last modified: 2024-10-03

Priority: 3

View all other issues in [re.regex.assign].

View all issues with Open status.

Discussion:

The post-conditions of basic_regex<>::assign 28.6.7.3 [re.regex.assign] p16 say:

If no exception is thrown, flags() returns f and mark_count() returns the number of marked sub-expressions within the expression.

The default expectation in the library is that post-conditions only hold, if there is no failure (see also 2136⁽ⁱ⁾), therefore the initial condition should be removed to prevent any misunderstanding.

[ 2012-10 Portland: Move to Open ]

A favorable resolution clearly depends on a favorable resolution to 2136⁽ⁱ⁾. There is also a concern that this is just one example of where we would want to apply such a wording clean-up, and which is really needed to resolve both this issue and 2136⁽ⁱ⁾ is a paper providing the clause 17 wording that gives the guarantee for postcondition paragraphs, and then reviews clauses 18-30 to apply that guarantee consistently. We do not want to pick up these issues piecemeal, as we risk opening many issues in an ongoing process.

[2015-05-06 Lenexa: EricWF to write paper addressing 2136 and 2137]

[2024-10-03; Jonathan comments]

I could find no other cases in the entire standard where we say something like this in a Postconditions: element. 31.6.3.5.4 [streambuf.virt.pback] p2 says "Postconditions: On return, the constraints of [...]" which is probably redundant (postconditions are always "on return").

Proposed resolution:

This wording is relative to N3376.

template <class string_traits, class A>
  basic_regex& assign(const basic_string<charT, string_traits, A>& s,
    flag_type f = regex_constants::ECMAScript);

[…]

-15- Effects: Assigns the regular expression contained in the string s, interpreted according the flags specified in f. If an exception is thrown, *this is unchanged.

-16- Postconditions: If no exception is thrown, flags() returns f and mark_count() returns the number of marked sub-expressions within the expression.

2146⁽ⁱ⁾. Are reference types CopyConstructible/MoveConstructible/CopyAssignable/MoveAssignable/Destructible?

Section: 16.4.4.2 [utility.arg.requirements] Status: Open Submitter: Nikolay Ivchenkov Opened: 2012-03-23 Last modified: 2024-12-04

Priority: 3

View all other issues in [utility.arg.requirements].

View all issues with Open status.

Discussion:

According to 16.4.4.2 [utility.arg.requirements] p1

The template definitions in the C++ standard library refer to various named requirements whose details are set out in tables 17-24. In these tables, T is an object or reference type to be supplied by a C++ program instantiating a template; a, b, and c are values of type (possibly const) T; s and t are modifiable lvalues of type T; u denotes an identifier; rv is an rvalue of type T; and v is an lvalue of type (possibly const) T or an rvalue of type const T.

Is it really intended that T may be a reference type? If so, what should a, b, c, s, t, u, rv, and v mean? For example, are "int &" and "int &&" MoveConstructible?

As far as I understand, we can explicitly specify template arguments for std::swap and std::for_each. Can we use reference types there?

1. #include <iostream>
   #include <utility>
   
   int main()
   {
     int x = 1;
     int y = 2;
     std::swap<int &&>(x, y); // undefined?
     std::cout << x << " " << y << std::endl;
   }
   

2. #include <algorithm>
   #include <iostream>
   #include <iterator>
   #include <utility>
   
   struct F
   {
     void operator()(int n)
     {
       std::cout << n << std::endl;
       ++count;
     }
     int count;
   } f;
   
   int main()
   {
     int arr[] = { 1, 2, 3 };
     auto&& result = std::for_each<int *, F &&>( // undefined?
       std::begin(arr),
       std::end(arr),
       std::move(f));
     std::cout << "count: " << result.count << std::endl;
   }
   

Are these forms of usage well-defined?

Let's also consider the following constructor of std::thread:

template <class F, class ...Args>
explicit thread(F&& f, Args&&... args);

Requires: F and each Ti in Args shall satisfy the MoveConstructible requirements.

When the first argument of this constructor is an lvalue (e.g. a name of a global function), template argument for F is deduced to be lvalue reference type. What should "MoveConstructible" mean with regard to an lvalue reference type? Maybe the wording should say that std::decay<F>::type and each std::decay<Ti>::type (where Ti is an arbitrary item in Args) shall satisfy the MoveConstructible requirements?

[2013-03-15 Issues Teleconference]

Moved to Open.

The questions raised by the issue are real, and should have a clear answer.

[2015-10, Kona Saturday afternoon]

STL: std::thread needs to be fixed, and anything behaving like it needs to be fixed, rather than reference types. std::bind gets this right. We need to survey this. GR: That doesn't sound small to me. STL: Seach for CopyConstructible etc. It may be a long change, but not a hard one.

MC: It seems that we don't have a PR. Does anyone have one? Is anyone interested in doing a survey?

[2016-03, Jacksonville]

Casey volunteers to make a survey

[2016-06, Oulu]

During an independent survey performed by Daniel as part of the analysis of LWG 2716⁽ⁱ⁾, some overlap was found between these two issues. Daniel suggested to take responsibility for surveying LWG 2146⁽ⁱ⁾ and opined that the P/R of LWG 2716⁽ⁱ⁾ should restrict to forwarding references, where the deduction to lvalue references can happen without providing an explicit template argument just by providing an lvalue function argument.

[2018-06, Rapperwsil]

Jonathan says that this will be covered by his Omnibus requirements paper.

[2019 Cologne Wednesday night]

Daniel will start working on this again; Marshall to provide rationale why some of the examples are not well-formed.

[2020-10-02; Issue processing telecon: change from P2 to P3]

For the examples given in the original report, the for_each case is now banned, because 26.2 [algorithms.requirements] p15 forbids explicit template argument lists. std::thread's constructor has also been fixed to describe the requirements on decay_t<T> instead of T.

We believe we're more careful these days about using remove_cvref or decay as needed, but there are still places where we incorrectly state requirements in terms of types that might be references. The swap case still needs solving. Still need a survey.

[2024-03-15; LWG 4047⁽ⁱ⁾ addresses the swap part]

[2024-12-04; Daniel comments]

The mentioned requirement sets have been renamed a while ago to:

• Cpp17CopyConstructible

• Cpp17MoveConstructible

• Cpp17CopyAssignable

• Cpp17MoveAssignable

• Cpp17Destructible

Proposed resolution:

2152⁽ⁱ⁾. Instances of standard container types are not swappable

Section: 16.4.4.3 [swappable.requirements], 23.2.2 [container.requirements.general] Status: LEWG Submitter: Robert Shearer Opened: 2012-04-13 Last modified: 2020-09-06

Priority: 3

View all other issues in [swappable.requirements].

View all issues with LEWG status.

Discussion:

Sub-clause 16.4.4.3 [swappable.requirements] defines two notions of swappability: a binary version defining when two objects are swappable with one another, and a unary notion defining whether an object is swappable (without qualification), with the latter definition requiring that the object satisfy the former with respect to all values of the same type.

Let T be a container type based on a non-propagating allocator whose instances do not necessarily compare equal. Then sub-clause 23.2.2 [container.requirements.general] p7 implies that no object t of type T is swappable (by the unary definition).

Throughout the standard it is the unary definition of "swappable" that is listed as a requirement (with the exceptions of 22.2.2 [utility.swap] p4, 22.3.2 [pairs.pair] p31, 22.4.4.4 [tuple.swap] p2, 26.7.3 [alg.swap] p2, and 26.7.3 [alg.swap] p6, which use the binary definition). This renders many of the mutating sequence algorithms of sub-clause 26.7 [alg.modifying.operations], for example, inapplicable to sequences of standard container types, even where every element of the sequence is swappable with every other.

Note that this concern extends beyond standard containers to all future allocator-based types.

Resolution proposal:

I see two distinct straightforward solutions:

1. Modify the requirements of algorithms from sub-clause 26.7 [alg.modifying.operations], and all other places that reference the unary "swappable" definition, to instead use the binary "swappable with" definition (over a domain appropriate to the context). The unary definition of "swappable" could then be removed from the standard.
2. Modify sub-clause 23.2.2 [container.requirements.general] such that objects of standard container types are "swappable" by the unary definition.

I favor the latter solution, for reasons detailed in the following issue.

[ 2012-10 Portland: Move to Open ]

The issue is broader than containers with stateful allocotors, although they are the most obvious example contained within the standard itself. The basic problem is that once you have a stateful allocator, that does not propagate_on_swap, then whether two objects of this type can be swapped with well defined behavior is a run-time property (the allocators compare equal) rather than a simple compile-time property that can be deduced from the type. Strictly speaking, any type where the nature of swap is a runtime property does not meet the swappable requirements of C++11, although typical sequences of such types are going to have elements that are all swappable with any other element in the sequence (using our other term of art for specifying requirements) as the common case is a container of elements who all share the same allocator.

The heart of the problem is that the swappable requirments demand that any two objects of the same type be swappable with each other, so if any two such objects would not be swappable with each other, then the whole type is never swappable. Many algorithms in clause 25 are specified in terms of swappable which is essentially an overspecification as all they actually need is that any element in the sequence is swappable with any other element in the sequence.

At this point Howard joins the discussion and points out that the intent of introducing the two swap-related terms was to support vector<bool>::reference types, and we are reading something into the wording that was never intended. Consuses is that regardless of the intent, that is what the words today say.

There is some support to see a paper reviewing the whole of clause 25 for this issue, and other select clauses as may be necessary.

There was some consideration to introducing a note into the front of clause 25 to indicate swappable requirements in the clause should be interpreted to allow such awkward types, but ultimately no real enthusiasm for introducing a swappable for clause 25 requirement term, especially if it confusingly had the same name as a term used with a subtly different meaning through the rest of the standard.

There was no enthusiasm for the alternate resolution of requiring containers with unequal allocators that do not propagate provide a well-defined swap behavior, as it is not believed to be possible without giving swap linear complexity for such values, and even then would require adding the constraint that the container element types are CopyConstructible.

Final conclusion: move to open pending a paper from a party with a strong interest in stateful allocators.

[2016-03 Jacksonville]

Alisdair says that his paper P0178 addresses this.

[2016-06 Oulu]

P0178 reviewed, and sent back to LEWG for confirmation.

Thursday Morning: A joint LWG/LEWG meeting declined to adopt P0178.

[2017-02 in Kona, LEWG responds]

Note in the issue that this is tracked here

[2017-06-02 Issues Telecon]

Leave as LEWG; priority 3

Proposed resolution:

Apply P0178.

2153⁽ⁱ⁾. Narrowing of the non-member swap contract

Section: 22.2.2 [utility.swap], 16.4.4.3 [swappable.requirements], 23.2.2 [container.requirements.general] Status: LEWG Submitter: Robert Shearer Opened: 2012-04-13 Last modified: 2020-10-02

Priority: 2

View all other issues in [utility.swap].

View all issues with LEWG status.

Discussion:

Sub-clause 22.2.2 [utility.swap] defines a non-member 'swap' function with defined behavior for all MoveConstructible and MoveAssignable types. It does not guarantee constant-time complexity or noexcept in general, however this definition does render all objects of MoveConstructible and MoveAssignable type swappable (by the unary definition of sub-clause 16.4.4.3 [swappable.requirements]) in the absence of specializations or overloads.

The overload of the non-member swap function defined in Table 96, however, defines semantics incompatible with the generic non-member swap function, since it is defined to call a member swap function whose semantics are undefined for some values of MoveConstructible and MoveAssignable types.

The obvious (perhaps naive) interpretation of sub-clause 16.4.4.3 [swappable.requirements] is as a guide to the "right" semantics to provide for a non-member swap function (called in the context defined by 16.4.4.3 [swappable.requirements] p3) in order to provide interoperable user-defined types for generic programming. The standard container types don't follow these guidelines.

More generally, the design in the standard represents a classic example of "contract narrowing". It is entirely reasonable for the contract of a particular swap overload to provide more guarantees, such as constant-time execution and noexcept, than are provided by the swap that is provided for any MoveConstructible and MoveAssignable types, but it is not reasonable for such an overload to fail to live up to the guarantees it provides for general types when it is applied to more specific types. Such an overload or specialization in generic programming is akin to an override of an inherited virtual function in OO programming: violating a superclass contract in a subclass may be legal from the point of view of the language, but it is poor design and can easily lead to errors. While we cannot prevent user code from providing overloads that violate the more general swap contract, we can avoid doing so within the library itself.

My proposed resolution is to draw a sharp distinction between member swap functions, which provide optimal performance but idiosyncratic contracts, and non-member swap functions, which should always fulfill at least the contract of 22.2.2 [utility.swap] and thus render objects swappable. The member swap for containers with non-propagating allocators, for example, would offer constant-time guarantees and noexcept but would only offer defined behavior for values with allocators that compare equal; non-member swap would test allocator equality and then dispatch to either member swap or std::swap depending on the result, providing defined behavior for all values (and rendering the type "swappable"), but offering neither the constant-time nor the noexcept guarantees.

[2013-03-15 Issues Teleconference]

Moved to Open.

This topic deserves more attention than can be given in the telecon, and there is no proposed resolution.

[2016-03 Jacksonville]

Alisdair says that his paper P0178 addresses this.

[2016-08 Chicago]

Send to LEWG

[2016-06 Oulu]

P0178 reviewed, and sent back to LEWG for confirmation.

Thursday Morning: A joint LWG/LEWG meeting declined to adopt P0178.

[2020-10-02; remove P0178 as Proposed Resolution]

Proposed resolution:

2157⁽ⁱ⁾. How does std::array<T,0> initialization work when T is not default-constructible?

Section: 23.3.3.5 [array.zero] Status: Open Submitter: Daryle Walker Opened: 2012-05-08 Last modified: 2021-03-14

Priority: 3

View all other issues in [array.zero].

View all issues with Open status.

Discussion:

Objects of std::array<T, N> are supposed to be initialized with aggregate initialization (when not the destination of a copy or move). This clearly works when N is positive. What happens when N is zero? To continue using an (inner) set of braces for initialization, a std::array<T, 0> implementation must have an array member of at least one element, and let default initialization take care of those secret elements. This cannot work when T has a set of constructors and the default constructor is deleted from that set. Solution: Add a new paragraph in 23.3.3.5 [array.zero]:

The unspecified internal structure of array for this case shall allow initializations like:

array<T, 0> a = { };

and said initializations must be valid even when T is not default-constructible.

[2012, Portland: Move to Open]

Some discussion to understand the issue, which is that implementations currently have freedom to implement an empty array by holding a dummy element, and so might not support value initialization, which is surprising when trying to construct an empty container. However, this is not mandated, it is an unspecified implementation detail.

Jeffrey points out that the implication of 23.3.3.1 [array.overview] is that this initialization syntax must be supported by empty array objects already. This is a surprising inference that was not obvious to the room, but consensus is that the reading is accurate, so the proposed resolution is not necessary, although the increased clarity may be useful.

Further observation is that the same clause effectively implies that T must always be DefaultConstructible, regardless of N for the same reasons - as an initializer-list may not supply enough values, and the remaining elements must all be value initialized.

Concern that we are dancing angels on the head of pin, and that relying on such subtle implications in wording is not helpful. We need a clarification of the text in this area, and await wording.

[2015-02 Cologne]

DK: What was the outcome of Portland? AM: Initially we thought we already had the intended behaviour. We concluded that T must always be DefaultConstructible, but I'm not sure why. GR: It's p2 in std::array, "up to N". AM: That wording already implies that "{}" has to work when N is zero. But the wording of p2 needs to be fixed to make clear that it does not imply that T must be DefaultConstructible.

Conclusion: Update wording, revisit later.

[2015-10, Kona Saturday afternoon]

MC: How important is this? Can you not just use default construction for empty arrays?

TK: It needs to degenerate properly from a pack. STL agrees.

JW: Yes, this is important, and we have to make it work.

MC: I hate the words "initialization like".

JW: I'll reword this.

WEB: Can I ask that once JW has reworded this we move it to Review rather than Open?

MC: We'll try to review it in a telecon and hopefully get it to tentatively ready.

STL: Double braces must also work: array<T, 0> a = {{}};.

Jonathan to reword.

[2018-03-14 Wednesday evening issues processing]

Jens suggested that we remove the requirement that begin() == end() == unique-value, specifically the unique value part.

Previous resolution [SUPERSEDED]:

This wording is relative to N3376.

Add the following new paragraph between the current 23.3.3.5 [array.zero] p1 and p2:

-1- array shall provide support for the special case N == 0.

-?- The unspecified internal structure of array for this case shall allow initializations like:

array<T, 0> a = { };

and said initializations must be valid even when T is not default-constructible.

-2- In the case that N == 0, begin() == end() == unique value. The return value of data() is unspecified.

-3- The effect of calling front() or back() for a zero-sized array is undefined.

-4- Member function swap() shall have a noexcept-specification which is equivalent to noexcept(true).

[2018-06-14, Jonathan Wakely comments and provides revised wording]

The new wording does not address the 2018-03-14 suggestion from Jens to remove the unique value. It wasn't clear to me that there was consensus to make that change, and it would be a change in behaviour not just a clarification of the existing wording.

Previous resolution [SUPERSEDED]:

This wording is relative to N4750.

Modify 23.3.3.5 [array.zero] as indicated:

-1- array shall provides support for the special case of a zero-sized array that is always empty, i.e. N == 0, with the properties described in this subclause.

-?- A zero-sized array type is an aggregate that meets the DefaultConstructible (Table 22) and CopyConstructible (Table 24) requirements. There is a single element of the aggregate, of an unspecified DefaultConstructible type. [Note: This allows initialization of the form array<T, 0> a = {{}};. There is no requirement for T to be DefaultConstructible. — end note]

-2- In the case that N == 0, begin() == end() == unique valuebegin() and end() return non-dereferenceable iterators such that begin() == end() and a.begin() != b.begin() where a and b are distinct objects of the same zero-sized array type. The return value of data() is unspecified.

-3- The effect of calling front() or back() for a zero-sized array is undefined.

-4- Member function swap() shall havehas constant complexity and a non-throwing exception specification.

[2018-08-30, Jonathan revises wording following feedback from Daniel Kruegler and Tim Song.]

Daniel noted that it's undefined to compare iterators from different containers, so a.begin() != b.begin() can't be used. That means whether the iterators from different containers are unique is unobservable anyway. We can say they don't share the same underlying sequence, which tells users they can't compare them and tells implementors they can't return value-initialized iterators.
Tim noted that it's not sufficient to say the unspecified type in a zero-sized array is DefaultConstructible, it also needs to be constructible from = {}. Also, a zero-sized array should be CopyAssignable.

Previous resolution [SUPERSEDED]:

This wording is relative to N4762.

Modify 23.3.3.5 [array.zero] as indicated:

-1- array shall provides support for the special case of a zero-sized array that is always empty, i.e. N == 0, with the properties described in this subclause.

-?- A zero-sized array type is an aggregate that meets the Cpp17DefaultConstructible (Table 24) and Cpp17CopyConstructible (Table 26) and Cpp17CopyAssignable (Table 28) requirements. There is a single element of the aggregate, of an unspecified Cpp17DefaultConstructible type that is copy-list-initializable from an empty list. [Note: This allows initialization of the form array<T, 0> a = {{}};. There is no requirement for T to be Cpp17DefaultConstructible. — end note]

-2- In the case that N == 0, begin() == end() == unique valuebegin() and end() return non-dereferenceable iterators such that begin() == end(). When a and b are distinct objects of the same zero-sized array type, a.begin() and b.begin() are not iterators over the same underlying sequence. [Note: Therefore begin() does not return a value-initialized iterator — end note]. The return value of data() is unspecified.

-3- The effect of calling front() or back() for a zero-sized array is undefined.

-4- Member function swap() shall havehas constant complexity and a non-throwing exception specification.

[2021-03-14; Johel Ernesto Guerrero Peña comments and provides improved wording]

The currently proposed wording specifies:

There is a single element of the aggregate, of an unspecified Cpp17DefaultConstructible type that is copy-list-initializable from an empty list.

This doesn't specify which expressions involving zero-sized array specializations are constant expressions. 23.3.3.1 [array.overview] p4 specifies array<T, 0> to be a structural type when T is a structural type. This requires that its single element, let's call it single-element, be a structural type. But that says nothing about which of the special member functions of single-element are constant expressions. By being a structural type, single-element is permitted to be implemented as a literal class type. To meet this requirement, single-element can be implemented to have one constexpr constructor that is not a copy or move constructor (6.8.1 [basic.types.general] p10), so its default constructor needn't be constexpr. This is unlike non-zero-sized array specializations, which inherit these properties from T. Furthermore, this permits an implementation of single-element whose default constructor stores the result of std::source_location::current() in a data member (as exemplified in the specification for current). Cpp17DefaultConstructible doesn't require the default constructor to produce equal values. The simplest way to solve these issues and any other that might arise from future changes and oversights would be to specify single-element as an empty aggregate type. Then the wording from 23.3.3.2 [array.cons] p1 makes it clear that all the special member functions are constant expressions. It would also mean that the default constructor produces template-argument-equivalent values.

Proposed resolution:

This wording is relative to N4878.

Modify 23.3.3.5 [array.zero] as indicated:

1. -1- array shall provides support for the special case of a zero-sized array that is always empty, i.e. N == 0, with the properties described in this subclause.

   -?- A zero-sized array type is an aggregate that meets the Cpp17DefaultConstructible (Table 29 [tab:cpp17.defaultconstructible]) and Cpp17CopyConstructible (Table 31 [tab:cpp17.copyconstructible]) and Cpp17CopyAssignable (Table 33 [tab:cpp17.copyassignable]) requirements. There is a single element of the aggregate, of an unspecified empty aggregate type. [Note: This allows initialization of the form array<T, 0> a = {{}};. There is no requirement for T to be Cpp17DefaultConstructible. — end note]

   -2- In the case that N == 0, begin() == end() == unique valuebegin() and end() return non-dereferenceable iterators such that begin() == end(). When a and b are distinct objects of the same zero-sized array type, a.begin() and b.begin() are not iterators over the same underlying sequence. [Note: Therefore begin() does not return a value-initialized iterator — end note].. The return value of data() is unspecified.

   -3- The effect of calling front() or back() for a zero-sized array is undefined.

   -4- Member function swap() shall havehas constant complexity and a non-throwing exception specification.

2158⁽ⁱ⁾. Conditional copy/move in std::vector

Section: 23.3.11.3 [vector.capacity] Status: Open Submitter: Nikolay Ivchenkov Opened: 2012-05-08 Last modified: 2022-11-06

Priority: 3

View other active issues in [vector.capacity].

View all other issues in [vector.capacity].

View all issues with Open status.

Discussion:

There are various operations on std::vector that can cause elements of the vector to be moved from one location to another. A move operation can use either rvalue or const lvalue as argument; the choice depends on the value of !is_nothrow_move_constructible<T>::value && is_copy_constructible<T>::value, where T is the element type. Thus, some operations on std::vector (e.g. 'resize' with single parameter, 'reserve', 'emplace_back') should have conditional requirements. For example, let's consider the requirement for 'reserve' in N3376 – 23.3.11.3 [vector.capacity]/2:

Requires: T shall be MoveInsertable into *this.

This requirement is not sufficient if an implementation is free to select copy constructor when !is_nothrow_move_constructible<T>::value && is_copy_constructible<T>::value evaluates to true. Unfortunately, is_copy_constructible cannot reliably determine whether T is really copy-constructible. A class may contain public non-deleted copy constructor whose definition does not exist or cannot be instantiated successfully (e.g., std::vector<std::unique_ptr<int>> has copy constructor, but this type is not copy-constructible). Thus, the actual requirements should be:

• if !is_nothrow_move_constructible<T>::value && is_copy_constructible<T>::value then T shall be CopyInsertable into *this;

• otherwise T shall be MoveInsertable into *this.

Maybe it would be useful to introduce a new name for such conditional requirement (in addition to "CopyInsertable" and "MoveInsertable").

[2016-08 Chicago]

The problem does not appear to be as severe as described. The MoveInsertable requirements are consistently correct, but an issue may arise on the exception-safety guarantees when we check for is_copy_constructible_v<T>. The problem, as described, is typically for templates that appear to have a copy constructor, but one that fails to compile once instantiated, and so gives a misleading result for the trait.

In general, users should not provide such types, and the standard would not serve users well by trying to address support for such types. However, the standard should not be providing such types either, such as vector<unique_ptr<T>>. A possible resolution would be to tighten the constraints in Table 80 — Container Requirements, so that if the Requirements for the copy constructor/assingment operator of a container are not satisfied, that operation shall be deleted.

A futher problem highlighted by this approach is that there are no constraints on the copy-assignment operator, so that vector<unique_ptr<T>> should be CopyAssignable! However, we can lift the equivalent constraints from the Allocator-aware container requirements.

[08-2016, Chicago]

Fri PM: Move to Open

[2017-11 Albuquerque Saturday issues processing]

There's a bunch of uses of "shall" here that are incorrect. Also, CopyInsertable contains some semantic requirements, which can't be checked at compile time, so 'ill-formed' is not possible for detecting that.

[2018-06 Rapperswil Wednesday issues processing]

Daniel to provide updated wording.

[2018-06-12, Daniel provides revised wording]

Previous resolution [SUPERSEDED]:

This wording is relative to N4606.

23.2.2 [container.requirements.general] Table 80 — Container requirements

Expression	Return type	Operational semantics	Assertion/note/pre-/post-condition	Complexity
X(a)			Requires: T is CopyInsertable into X (see below)., otherwise this expression shall be ill-formed. post: a == X(a).	linear
X u(a) X u = a;			Requires: T is CopyInsertable into X (see below)., otherwise this expression shall be ill-formed. post: u == a.	linear
...	...	...	...	...
r = a	X&		Requires: T is CopyInsertable into X and CopyAssignable, otherwise this expression shall be ill-formed. post: r == a.	linear

23.2.2 [container.requirements.general] Table 83 — Allocator-aware container requirements

Expression	Return type	Operational semantics	Assertion/note/pre-/post-condition	Complexity
a = t	X&		Requires: T is CopyInsertable into X and CopyAssignable., otherwise this expression shall be ill-formed post: r == a.	linear

[2018-08-23 Batavia Issues processing. Priority to 3]

Changed CopyInsertable -> Cpp17CopyInsertable in the resolution.

Tim says that the wording is not quite right — it imposes additional requirements.

[2022-11-06; Daniel comments]

This issue has considerable overlap with LWG 3758⁽ⁱ⁾.

Proposed resolution:

This wording is relative to N4750.

The revised wording below uses the new Mandates: element introduced by adopting P0788R3 at the Rapperswil meeting 2018 and which will become a new term of art with Jonathan's omnibus paper throughout the Standard Library.

2173⁽ⁱ⁾. The meaning of operator + in the description of the algorithms

Section: 26 [algorithms] Status: Open Submitter: Nikolay Ivchenkov Opened: 2012-08-01 Last modified: 2018-06-12

Priority: 4

View other active issues in [algorithms].

View all other issues in [algorithms].

View all issues with Open status.

Discussion:

According to 26.1 [algorithms.general]/12,

In the description of the algorithms operators + and - are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics of a+n is the same as that of

X tmp = a;
advance(tmp, n);
return tmp;

There are several places where such operator + is applied to an output iterator — for example, see the description of std::copy:

template<class InputIterator, class OutputIterator>
OutputIterator copy(InputIterator first, InputIterator last,
                    OutputIterator result);

-1- Effects: Copies elements in the range [first,last) into the range [result,result + (last - first)) starting from first and proceeding to last. For each non-negative integer n < (last - first), performs *(result + n) = *(first + n).

std::advance is not supposed to be applicable to output iterators, so we need a different method of description.

See also message c++std-lib-32908.

[2014-06-07 Daniel comments and provides wording]

The specification for output iterators is somewhat tricky, because here a sequence of increments is required to be combined with intervening assignments to the dereferenced iterator. I tried to respect this fact by using a conceptual assignment operation as part of the specification.

Another problem in the provided as-if-code is the question which requirements are imposed on n. Unfortunately, the corresponding function advance is completely underspecified in this regard, so I couldn't borrow wording from it. We cannot even assume here that n is the difference type of the iterator, because for output iterators there is no requirements for this associated type to be defined. The presented wording attempts to minimize assumptions, but still can be considered as controversial.

[2018-06 Rapperswil Wednesday issues processing]

Status to Open

Proposed resolution:

This wording is relative to N4606.

1. Change 26.1 [algorithms.general] around p12 as indicated:

   -12- In the description of the algorithms operators + and - are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics of a+n is the same as that of

   X tmp = a;
   advance(tmp, n);
   return tmp;
   

   when X meets the input iterator requirements (24.3.5.3 [input.iterators]), otherwise it is the same as that of

   X tmp = a;
   for (auto i = n; i; ++tmp, (void) --i) 
     *tmp = Expr(i); 
   return tmp;
   

   where Expr(i) denotes the (n-i)^th expression that is assigned to for the corresponding algorithm; and that of b-a is the same as of

   return distance(a, b);
   

2189⁽ⁱ⁾. Throwing swap breaks unordered containers' state

Section: 23.2.8.2 [unord.req.except] Status: Open Submitter: Alisdair Meredith Opened: 2012-09-23 Last modified: 2019-07-22

Priority: 3

View all issues with Open status.

Discussion:

The hash functor and key-comparison functor of unordered containers are allowed to throw on swap.

23.2.8.2 [unord.req.except]p3 "For unordered associative containers, no swap function throws an exception unless that exception is thrown by the swap of the container's Hash or Pred object (if any)."

In such a case we must offer the basic exception safety guarantee, where both objects are left in valid but unspecified states, and no resources are leaked. This yields a corrupt, un-usable container if the first swap succeeds, but the second fails by throwing, as the functors form a matched pair.

So our basic scenario is first, swap the allocators if the allocators propagate on swap, according to allocator_traits. Next we swap the pointers to our internal hash table data structures, so that they match the allocators that allocated them. (Typically, this operation cannot throw). Now our containers are back in a safely destructible state if an exception follows.

Next, let's say we swap the hash functor, and that throws. We have a corrupt data structure, in that the buckets are not correctly indexed by the correct functors, lookups will give unpredicatable results etc. We can safely restore a usable state by forcibly clearing each container - which does not leak resources and leaves us with two (empty but) usable containers.

Now let us assume that the hasher swap succeeds. Next we swap the equality comparator functor, and this too could throw. The important point to bear in mind is that these two functors form an important pairing - two objects that compare equal by the equality functor must also hash to the same value. If we swap one without the other, we most likely leave the container in an unusable state, even if we clear out all elements.

1. A colleague pointed out that the solution for this is to dynamically allocate the two functors, and then we need only swap pointers, which is not a throwing operation. And if we don't want to allocate on default construction (a common QoI request), we might consider moving to a dynamically allocated functors whenever swap is called, or on first insertion. Of course, allocating memory in swap is a whole new can of worms, but this does not really sound like the design we had intended.

2. The simplest option is to say that we do not support hasher or equality functors that throw on ADL swap. Note that the requirement is simply to not throw, rather than to be explicitly marked as noexcept. Throwing functors are allowed, so long as we never use values that would actually manifest a throw when used in an unordered container.

Pablo went on to give me several more options, to be sure we have a full set to consider:

3. Disallow one or the other functor from throwing. In that case, the possibly-throwing functor must be swapped first, then the other functor, the allocator, and the data pointer(s) afterwards (in any order -- there was a TC that allocator assignment and swap may not throw if the corresponding propagation trait is true.). Of course, the question becomes: which functor is allowed to throw and which one is not?

4. Require that any successful functor swap be reliably reversible. This is very inventive. I know of no other place in the standard where such a requirement is stated, though I have occasionally wanted such a guarantee.

5. Allow a failed swap to leave the containers in a state where future insertions may fail for reasons other than is currently allowed. Specifically, if the hash and equality functors are out of sync, all insertions will fail. Presumably some "incompletely swapped" exception would be thrown. This is "slightly" inventive, although people have been discussing "radioactive" states for a while.

[2013-03-15 Issues Teleconference]

Moved to Open.

[2019 Cologne Wednesday night]

Billy to write resolution for option #2. This may require a paper.

Proposed resolution:

2198⁽ⁱ⁾. max_load_factor(z) makes no strong guarantees, but bans useful behavior

Section: 23.2.8 [unord.req] Status: Open Submitter: Alisdair Meredith Opened: 2012-10-09 Last modified: 2016-12-10

Priority: 3

View other active issues in [unord.req].

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Discussion:

The user cannot specify a max_load_factor for their unordered container at construction, it must be supplied after the event, when the container is potentially not empty. The contract for this method is deliberately vague, not guaranteeing to use the value supplied by the user, and any value actually used will be used as a ceiling that the container will attempt to respect.

The only guarantee we have is that, if user requests a max_load_factor that is less than the current load_factor, then the operation will take constant time, thus outlawing an implementation that chooses to rehash and so preserve as a class invariant that load_factor < max_load_factor.

Reasonable options conforming to the standard include ignoring the user's request if the requested value is too low, or deferring the rehash to the next insert operation and allowing the container to have a strange state (wrt max_load_factor) until then - and there is still the question of rehashing if the next insert is for a duplicate key in a unique container.

Given the deliberate vagueness of the current wording, to support a range of reasonable (but not perfect) behaviors, it is not clear why the equally reasonable rehash to restore the constraint should be outlawed. It is not thought that this is a performance critical operation, where users will be repeatedly setting low load factors on populated containers, in a tight or (less unlikely) an instant response scenario.

[2013-03-15 Issues Teleconference]

Moved to Open.

Alisdair to provide wording.

[2016-11-12, Issaquah]

Sat PM: Howard to provide wording

[2016-11-17 Howard provided wording.]

The provided wording is consistent with LWG discussion in Issaquah. An implementation of the proposed wording would be setting max_load_factor() to max(z, load_factor()). This preserves the container invariant:

load_factor() <= max_load_factor()

And it preserves the existing behavior that no rehash is done by this operation.

If it is desired to change the max_load_factor() to something smaller than the current load_factor() that can be done by first reducing the current load_factor() by either increasing bucket_count() (via rehash or reserve), or decreasing size() (e.g. erase), and then changing max_load_factor().

This resolution reaffirms that load_factor() <= max_load_factor() is a container invariant which can never be violated.

[2016-11-27, Nico comments]

Current implementations behave differently.

In regard to the sentence

"The only guarantee we have is that, if user requests a max_load_factor that is less than the current load_factor, then the operation will take constant time, thus outlawing an implementation that chooses to rehash and so preserve as a class invariant that load_factor < max_load_factor."

This issue is related to LWG 2199⁽ⁱ⁾.

Proposed resolution:

Modify Table 87 as follows:

2202⁽ⁱ⁾. Missing allocator support by async

Section: 32.10.9 [futures.async] Status: Deferred Submitter: Detlef Vollmann Opened: 2012-10-19 Last modified: 2016-01-28

Priority: 4

View other active issues in [futures.async].

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Discussion:

promise, packaged_task, and async are the only places where a shared state is actually supposed to be allocated. Accordingly, promise and packaged_task are "allocator-aware". But function template async provides no way to provide an allocator.

[2013-09 Chicago]

Matt: deprecate async

Nico: read my paper

Alisdair: defer issues to wait for polymorphic allocators

Alisdair: defer, active topic of research Deferred

[2014-02-20 Re-open Deferred issues as Priority 4]

[2015-05 Lenexa, SG1 response]

We want whatever status approximates: "will not fix; we're working on a replacement facility and don't want to add features to a broken one"

Proposed resolution:

2206⁽ⁱ⁾. Inaccuracy in initializer_list constructor requirements

Section: 23.2.4 [sequence.reqmts], 23.2.7 [associative.reqmts], 23.2.8 [unord.req], 29.5.3.2 [rand.req.seedseq] Status: Open Submitter: Jeffrey Yasskin Opened: 2012-10-21 Last modified: 2020-09-06

Priority: 3

View other active issues in [sequence.reqmts].

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Discussion:

In 23.2.4 [sequence.reqmts] p3, we have "il designates an object of type initializer_list<value_type>", and then several functions that take 'il' as an argument. However, an expression like {1, 2, 'a'} is not an object of type initializer_list<int> unless it's used to initialize an explicitly-typed variable of that type. I believe we want:

std::vector<int> v;
v = {1, 2, 'a'};

to compile portably, so we should say something different when defining 'il'. The same phrasing happens in 23.2.7 [associative.reqmts], 23.2.8 [unord.req], and 29.5.3.2 [rand.req.seedseq].

This may just be an editorial issue because the actual class synopses declare the functions to take initializer_list<exact_type>.

[2013-03-15 Issues Teleconference]

Moved to Open.

This is definitely not NAD

Should copy the suggested wording as the proposed resolution.

[2019-03-26; Daniel comments and provides wording]

The 2013-03-15 comment is confusing, since it recommends to "copy the suggested wording as the proposed resolution". I couldn't find such wording in the issue nor in the associated wiki, so I provided that wording out of myself. The tricky part is to define which kind of braced-init-list we want to allow. As Tim Song pointed out, we still need the existing support for std::initializer_list<value_type> as well, because otherwise existing semantics based on expressions such as li.begin() won't work anymore. The below suggested wording restricts supported braced-init-lists to every initializer list that can be used to copy-list-initialize an object of type std::initializer_list<value_type> by saying:

"bil designates any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list])"

As a drive-by fix, the provided wording adds another initialization "expression" that makes the construction of the form

std::vector<int> v = {1, 2, 'a'};

valid (We just miss a copy-initialization case).

Proposed resolution:

This wording is relative to N4810.

[Drafting note: We need to special-case the "expression" X u = bil; below, because for empty braced-init-list the effects are those of calling the default constructor. — end drafting note]

1. Modify 23.2.4 [sequence.reqmts] as indicated:

   -3- In Tables 66 and 67, […] il designates an objectvalue of type initializer_list<value_type>, bil designates any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list]), […]

2. Modify Table 66 — "Sequence container requirements (in addition to container)" as indicated:

   Table 66 — Sequence container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition
   […]
   X(il) X u = il;		Equivalent to X(il.begin(), il.end()) or X u(il.begin(), il.end());, respectively
   X(bil)		Equivalent to X(initializer_list<value_type>(bil))
   X u = bil;		If bil is empty, equivalent to X u;, otherwise equivalent to X u = initializer_list<value_type>(bil);
   a = il	X&	[…]
   a = bil	X&	Equivalent to a = initializer_list<value_type>(bil)
   […]
   a.insert(p, il)	iterator	[…]
   a.insert(p, bil)	iterator	Equivalent to a.insert(p, initializer_list<value_type>(bil))
   […]
   a.assign(il)	void	[…]
   a.assign(bil)	void	Equivalent to a.assign(initializer_list<value_type>(bil))
   […]

3. Modify 23.2.7 [associative.reqmts] as indicated:

   -8- In Table 69, […] il designates an objectvalue of type initializer_list<value_type>, bil designates any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list]), […]

4. Modify Table 69 — "Associative container requirements (in addition to container)" as indicated:

   Table 69 — Associative container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   […]
   X(il) X u = il;		same as X(il.begin(), il.end()) or X u(il.begin(), il.end());, respectively	same as X(il.begin(), il.end()) or X u(il.begin(), il.end());, respectively
   X(bil)		Equivalent to X(initializer_list<value_type>(bil))
   X u = bil;		If bil is empty, equivalent to X u;, otherwise equivalent to X u = initializer_list<value_type>(bil);
   X(il,c)		same as X(il.begin(), il.end(), c)	same as X(il.begin(), il.end(), c)
   X(bil, c)		Equivalent to X(initializer_list<value_type>(bil), c)
   a = il	X&	[…]	[…]
   a = bil	X&	Equivalent to a = initializer_list<value_type>(bil)
   […]
   a.insert(il)	void	equivalent to a.insert(il.begin(), il.end())
   a.insert(bil)	void	Equivalent to a.insert(initializer_list<value_type>(bil))
   […]
   a.assign(il)	void	[…]
   a.assign(bil)	void	Equivalent to a.assign(initializer_list<value_type>(bil))
   […]

5. Modify 23.2.8 [unord.req] p11's bullet list as indicated:

   -11- In Table 70:

   1. (11.1) — […]

   2. […]

   3. (11.14) — il denotes a value of type initializer_list<value_type>,

   4. (11.?) — bil denotes any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list]),

   5. […]

6. Modify Table 70 — "Unordered associative container requirements (in addition to container)" as indicated:

   [Drafting note: There is a preexisting issue with Table 70, that there is no symbol u specified ("u denotes the name of a variable being declared"), so existing initialization forms with a named variable are currently always written as "X a[…]" where a is defined as "a denotes a value of type X", the wording below follows this existing practice but the author of this wording would like to kindly ask the Project Editor to introduce said symbol u and apply it to all existing and new such named initialization forms instead. — end drafting note]

   Table 70 — Unordered associative container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   […]
   X(il) X a = il;	X	Same as X(il.begin(), il.end()) or X a(il.begin(), il.end());, respectively	Same as X(il.begin(), il.end()) or X a(il.begin(), il.end());, respectively
   X(bil)	X	Equivalent to X(initializer_list<value_type>(bil))
   X a = bil;	X	If bil is empty, equivalent to X a;, otherwise equivalent to X a = initializer_list<value_type>(bil);
   X(il, n)	X	Same as X(il.begin(), il.end(), n)	Same as X(il.begin(), il.end(), n)
   X(bil, n)	X	Equivalent to X(initializer_list<value_type>(bil), n)
   X(il, n, hf)	X	Same as X(il.begin(), il.end(), n, hf)	Same as X(il.begin(), il.end(), n, hf)
   X(bil, n, hf)	X	Equivalent to X(initializer_list<value_type>(bil), n, hf)
   X(il, n, hf, eq)	X	Same as X(il.begin(), il.end(), n, hf, eq)	Same as X(il.begin(), il.end(), n, hf, eq)
   X(bil, n, hf, eq)	X	Equivalent to X(initializer_list<value_type>(bil), n, hf, eq)
   […]
   a = il	X&	[…]	[…]
   a = bil	X&	Equivalent to a = initializer_list<value_type>(bil)
   […]
   a.insert(il)	void	Same as a.insert(il.begin(), il.end()).	Same as a.insert(il.begin(), il.end()).
   a.insert(bil)	void	Equivalent to a.insert(initializer_list<value_type>(bil))
   […]

7. Modify 29.5.3.2 [rand.req.seedseq] p2's bullet list as indicated:

   -2- A class S satisfies the requirements of a seed sequence if the expressions shown in Table 82 are valid and have the indicated semantics, and […] In that Table and throughout this subclause:

   1. (2.1) — […]

   2. (2.?) — u denotes the name of a variable being declared,

   3. […]

   4. (2.6) — il is a value of initializer_list<T>.;

   5. (2.?) — bil denotes any braced-init-list suitable to copy-list-initialize an object of type initializer_list<T> (9.4.5 [dcl.init.list]).

8. Modify Table 82 — "Seed sequence requirements" as indicated:

   Table 82 — Seed sequence requirements

   Expression	Return type	Pre/post-condition	Complexity
   […]
   S(il) S u = il;		Same as S(il.begin(), il.end()) or S u(il.begin(), il.end());, respectively	same as S(il.begin(), il.end()) or S u(il.begin(), il.end());, respectively
   S(bil)		Equivalent to S(initializer_list<T>(bil))
   S u = bil;		If bil is empty, equivalent to S u;, otherwise equivalent to S u = initializer_list<T>(bil);
   […]

2214⁽ⁱ⁾. Clarify basic_ios::init call restrictions

Section: 31.5.4.2 [basic.ios.cons] Status: Open Submitter: Andrey Semashev Opened: 2012-11-09 Last modified: 2021-07-31

Priority: 4

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Discussion:

There is an ambiguity in how std::basic_ios::init method (31.5.4.2 [basic.ios.cons]) can be used in the derived class. The Standard only specify the state of the basic_ios object after the call completes. However, in basic_ios default constructor description (31.5.4.2 [basic.ios.cons]) there is this sentence:

Effects: Constructs an object of class basic_ios (31.5.2.8 [ios.base.cons]) leaving its member objects uninitialized. The object shall be initialized by calling basic_ios::init before its first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.

This restriction hints that basic_ios::init should be called exactly once before the object can be used or destroyed, because basic_ios::init may not know whether it was called before or not (i.e. whether its members are actually uninitialized or are initialized by the previous call to basic_ios::init). There is no such restriction in the basic_ios::init preconditions so it is not clear whether it is allowed to call basic_ios::init multiple times or not.

This problem has already affected publicly available implementations. For example, Microsoft Visual C++ STL introduces a memory leak if basic_ios::init is called multiple times, while GCC 4.7 and STLPort reinitialize the basic_ios object correctly without memory leak or any other undesired effects. There was a discussion of this issue on Boost developers mailing list, and there is a test case that reproduces the problem. The test case is actually a bug report for my Boost.Log library, which attempts to cache basic_ostream-derived objects internally to avoid expensive construction and destruction. My stream objects allowed resetting the stream buffer pointers the stream is attached to, without requiring to destroy and construct the stream.

My personal view of the problem and proposed resolution follows.

While apparently the intent of basic_ios::init is to provide a way to initialize basic_ios after default construction, I see no reason to forbid it from being called multiple times to reinitialize the stream. Furthermore, it is possible to implement a conforming basic_ios that does not have this restriction.

The quoted above section of the Standard that describes the effects of the default constructor is misleading. The Standard does not mandate any data members of basic_ios or ios_base (31.5.2 [ios.base]), which it derives from. This means that the implementation is allowed to use non-POD data members with default constructors that initialize the members with particular default values. For example, in the case of Microsoft Visual C++ STL the leaked memory is an std::locale instance that is dynamically allocated during basic_ios::init, a raw pointer to which is stored within ios_base. It is possible to store e.g. an unique_ptr instead of a raw pointer as a member of ios_base, the smart pointer will default initialize the underlying raw pointer on default construction and automatically destroy the allocated object upon being reset or destroyed, which would eliminate the leak and allow basic_ios::init to be called multiple times. This leads to conclusion that the default constructor of basic_ios cannot leave "its member objects uninitialized" but instead performs default initialization of the member objects, which would mean the same thing in case of POD types.

However, I feel that restricting ios_base and basic_ios members to non-POD types is not acceptable. Since multiple calls to basic_ios::init are not forbidden by the Standard, I propose to correct the basic_ios default constructor description so that it is allowed to destroy basic_ios object without calling basic_ios::init. This would imply that any raw members of basic_ios and ios_base should be initialized to values suitable for destruction (essentially, this means only initializing raw pointers to NULL). The new wording could look like this:

Effects: Constructs an object of class basic_ios (31.5.2.8 [ios.base.cons]) initializing its member objects to unspecified state, only suitable for basic_ios destruction. The object shall be initialized by calling basic_ios::init before its first use; otherwise the behavior is undefined.

This would remove the hint that basic_ios::init must be called exactly once. Also, this would remove the requirement for basic_ios::init to be called at all before the destruction. This is also an important issue because the derived stream constructor may throw an exception before it manages to call basic_ios::init (for example, if the streambuf constructor throws), and in this case the basic_ios destructor has undefined behavior.

To my mind, the described modification is sufficient to resolve the issue. But to emphasize the possibility to call basic_ios::init multiple times, a remark or a footnote for basic_ios::init postconditions could be added to explicitly state the semantics of calling it multiple times. The note could read as follows:

The function can be called multiple times during the object lifetime. Each subsequent call reinitializes the object to the described in postconditions initial state.

[2013-04-20, Bristol]

Alisdair: The current wording is unclear but the proposed resolution is wrong

Solution: Clarify that init must be called once and only once. Move then to review.

[2021-07-29 Tim comments]

The requirement that "init must be called once and only once" conflicts with the disposition of LWG 135⁽ⁱ⁾.

Proposed resolution:

This wording is relative to N3485.

1. Edit 31.5.4.2 [basic.ios.cons] as indicated:

   basic_ios();
   

   -2- Effects: Constructs an object of class basic_ios (31.5.2.8 [ios.base.cons]) leaving its member objects uninitializedinitializing its member objects to unspecified state, only suitable for basic_ios destruction. The object shall be initialized by calling basic_ios::init before its first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.

   void init(basic_streambuf<charT,traits>* sb);
   

   Postconditions: The postconditions of this function are indicated in Table 128.

   -?- Remarks: The function can be called multiple times during the object lifetime. Each subsequent call reinitializes the object to the described in postconditions initial state.

2215⁽ⁱ⁾. (unordered) associative container functors should be CopyConstructible

Section: 23.2.7 [associative.reqmts], 23.2.8 [unord.req] Status: Open Submitter: Alisdair Meredith Opened: 2012-11-14 Last modified: 2015-10-22

Priority: 3

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Discussion:

The requirements on the functors used to arrange elements in the various associative and unordered containers are given by a set of expressions in tables 102 — Associative container requirements, and 103 — Unordered associative container requirements. In keeping with Library convention these expressions make the minimal requirements necessary on their types. For example, we have the following 3 row extracts for the unordered containers:

X(n, hf, eq)
X a(n, hf, eq)

X(n, hf)
X a(n, hf)

X(n)
X a(n)

However, the signature for each class template requires that the functors must effectively be CopyConstructible for each of these expressions:

template <class Key,
          class T,
          class Hash  = hash<Key>,
          class Pred  = std::equal_to<Key>,
          class Allocator = std::allocator<std::pair<const Key, T> > >
class unordered_map
{
  ...

  // construct/destroy/copy
  explicit unordered_map(size_type n = see below,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());

  ...
}

The letter of the standard can be honored as long as implementors recognize their freedom to split this one signature into multiple overloads, so that the documented default arguments (requiring a CopyConstructible functor) are not actually passed as default arguments.

As we look into the requirements for the copy constructor and copy-assignment operator, the requirements are even more vague, as the explicit requirements on the functors are not called out, other than saying that the functors are copied.

Must the functors be CopyAssignable? Or is CopyConstructible sufficient in this case? Do we require that the functors be Swappable so that the copy-swap idiom can be deployed here? Note that a type that is both CopyConstructible and CopyAssignable is still not guaranteed to be Swappable as the user may delete the swap function for their type in their own namespace, which would be found via ADL.

Some clean-up of the requirements table looks necessary, to at least document the assignment behavior. In addition, we should have clear guidance on whether these functors should always be CopyConstructible, as suggested by the class template definitions, or if the requirement tables are correct and we should explicitly split up the constructors in the (unordered) associative containers to no longer use default (function) arguments to obtain their defaulted functors.

I recommend the simplest solution would be to always require that the functors for (unordered) associative containers be CopyConstructible, above the requirements tables themselves, so that the issue need not be addressed within the tables. I suggest that the assignment operators for these containers add the requirement that the functors be Swappable, rather than forwarding the corresponding Assignable requirement.

[2013-03-15 Issues Teleconference]

Moved to Open.

Alisdair to propose wording.

[2014-06-08, Daniel comments]

The area of this issue partially overlaps what LWG 2227⁽ⁱ⁾ addresses.

[2015-10-20, Daniel comments]

The revised resolution of LWG 2227⁽ⁱ⁾ should resolve this issue as well. It follows the recommendations of the submitter to require CopyConstructible requirements for the function objects owned by containers, but it does not impose any further fundamental requirements.

Proposed resolution:

See the resolution of LWG 2227⁽ⁱ⁾.

2216⁽ⁱ⁾. regex_replace(basic_string) allocator handling

Section: 28.6.10.4 [re.alg.replace] Status: New Submitter: Jeffrey Yasskin Opened: 2012-11-26 Last modified: 2016-01-28

Priority: 3

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Discussion:

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

and friends are documented as

Constructs an empty string result of type basic_string<charT, ST, SA> and calls regex_replace(back_inserter(result), s.begin(), s.end(), e, fmt, flags).

This appears to require the result to have a default-constructed allocator, which isn't even possible for all allocator types. I suspect the allocator should be copied from 's' instead. Possibly there should be an additional defaulted argument to override the allocator of the result.

Proposed resolution:

2220⁽ⁱ⁾. Under-specification of operator== for regex_token_iterator

Section: 28.6.11.2.3 [re.tokiter.comp] Status: Open Submitter: Pete Becker Opened: 2012-11-21 Last modified: 2024-10-03

Priority: 3

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Discussion:

Consider the following example:

std::string str0("x");
std::regex rg0("a");
std::regex_token_iterator it0(str0.begin(), str0.end(), rg0, -1); // points at "x" in str0
std::string str1("x");
std::regex rg1("b");
std::regex_token_iterator it1(str1.begin(), str1.end(), rg1, -1); // points at "x" in str1

28.6.11.2.3 [re.tokiter.comp] p1 says that it0.operator==(it1) returns true "if *this and right are both suffix iterators and suffix == right.suffix"; both conditions are satisfied in this example. It does not say that they must both be iterators into the same sequence, nor does it say (as general iterator requirements do) that they must both be in the domain of == in order for the comparison to be meaningful. It's a simple statement: they're equal if the strings they point at compare equal. Given this being a valid comparison, the obtained result of "true" looks odd.

The problem is that for iterator values prior to the suffix iterator, equality means the same regular expression and the same matched sequence (both uses of "same" refer to identity, not equality); for the suffix iterator, equality means that the matched sequences compare equal.

[2014-02-10]

Priority set to 2

[2018-08-20 Casey adds a proposed resolution]

Priority changed to 3.

Previous resolution [SUPERSEDED]:

This wording is relative to N4762.

• Modify 28.6.11.2.3 [re.tokiter.comp] as follows:

  bool operator==(const regex_token_iterator& right) const;
  

  -?- Expects: *this and right are both end-of-sequence iterators or both have the same underlying sequence.

  -1- Returns: true if *this and right are both end-of-sequence iterators, or if […]

  bool operator!=(const regex_token_iterator& right) const;
  

  -?- Expects: *this and right are both end-of-sequence iterators or both have the same underlying sequence.

  -2- Returns: !(*this == right).

[2018-08-23 Casey revises the P/R in response to LWG feedback]

Previous resolution [SUPERSEDED]:

This wording is relative to N4762.

• Modify 28.6.11.2.3 [re.tokiter.comp] as follows:

  bool operator==(const regex_token_iterator& right) const;
  

  -?- Expects: At least one of *this and right is an end-of-sequence iterator, or both *this and right have the same underlying sequence.

  -1- Returns: true if *this and right are both end-of-sequence iterators, or if […]

  bool operator!=(const regex_token_iterator& right) const;
  

  -?- Expects: At least one of *this and right is an end-of-sequence iterator, or both *this and right have the same underlying sequence.

  -2- Returns: !(*this == right).

[2024-10-03; Jonathan rebases the wording on the latest WP]

Proposed resolution:

This wording is relative to N4988.

• Modify 28.6.11.2.3 [re.tokiter.comp] as follows:

  bool operator==(const regex_token_iterator& right) const;
  

  -?- Preconditions: At least one of *this and right is an end-of-sequence iterator, or *this and right have the same underlying sequence.

  -1- Returns: true if *this and right are both end-of-sequence iterators, or if *this and right are both suffix iterators and suffix == right.suffix; otherwise returns false if *this or right is an end-of-sequence iterator or a suffix iterator. Otherwise returns true if position == right.position, N == right.N, and subs == right.subs. Otherwise returns false.

2227⁽ⁱ⁾. Stateful comparison objects in associative containers

Section: 23.2.7 [associative.reqmts] Status: Open Submitter: Juan Soulie Opened: 2012-12-19 Last modified: 2019-04-23

Priority: 3

View other active issues in [associative.reqmts].

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Discussion:

Table 102 in 23.2.7 [associative.reqmts]/8 states on expression a.key_comp() that it "returns the comparison object out of which a was constructed". At the same time, 23.2.2 [container.requirements.general]/8 states (starting in the third line) that "...Any Compare, Pred, or Hash objects belonging to a and b shall be swappable and shall be exchanged by unqualified calls to non-member swap...". This is problematic for any compliant implementation, since once swapped the container cannot return the comparison object out of which it was constructed unless incurring in storing an otherwise needless object.

The simple solution is to correct that statement in Table 102, but I believe this is part of a larger problem of underspecified behavior: The new standard has made an effort in regards to allocators and now fully specifies what happens to stateful allocator objects. It has even specified what happens to stateful hasher and key_equal members of unordered containers (they propagate), but it says nothing about stateful comparison objects of (ordered) associative containers, except for the statement in 23.2.2 [container.requirements.general]/8 referred above and only related to swap.

For example, it is unclear to me what is specified to happen on an assignment: should the comparison object be copied/moved along with the elements, or should the left-hand side object keep its own? Maybe this has been intentionally left unspecified with the purpose of compatibility with C++98, which I understand it specified that comparison objects were kept for the entire life of the container (like allocators) — an unfortunate choice. But anyway, the segment of 23.2.2 [container.requirements.general] quoted above seems to break any possible backwards compatibility with C++98 in this regard.

Therefore, taking into consideration consistency with how this is dealed with for unordered associative containers, I propose that Table 102 is modified as follows:

• The row for expression a.key_comp() is changed so that its "assertion/note pre-/post-condition" reads "Returns a's comparison object."

• A new row is added at the appropriate location (which I believe would be after "X(il)" row), with:

  Table 102 — Associative container requirements (in addition to container)

  Expression	Return type	Assertion/note pre-/post-condition	Complexity
  X(b) X a(b)	X	Copy constructor. In addition to the requirements of Table 96, copies the comparison object.	Linear in b.size()
  a = b	X&	Copy assignment operator. In addition to the requirements of Table 96, copies the comparison object.	Linear in a.size() and b.size()

[2013-03-15 Issues Teleconference]

Moved to Review.

[2013-04-18, Bristol]

STL: can't believe we don't specify this already. this is totally necessary

Alisdair: how does it do this? copy construction? assignment?

Also need it for move.

STL: we already specify this for constructing from a comparator, not during copy construction though.

Jonathan: don't like wording, should say "key_compare is CopyConstructible. Uses b.key_comp() as a comparison object."

STL: we get it right for unordered!

Jonathan: can't wordsmith this now, but I think implementations do the right thing.

Alisdair: not sure what right thing is for moves. Also we say nothing about propagating allocators to functors.

Moved to Open.

[2015-02 Cologne]

TK: There's no need for fine-grained propagate/not-propagate control. If you don't want to propagate the predicate, you can simply construct or insert from an iterator range.

VV: libstdc++ already implements the resolution of this issue.

GR: There are a couple of other problems. We don't specify move constructor and move assignment for maps. Those are just general.

TK: General container requirements already describe the semantics for {copy,move}-{construction,assignment}, so it doesn't seem that there's room for choice in std::map assignments. unordered_map is different, though.

[Note: Check what general container requirements say about container equality.]

DK will draft wording. The decision is to unambiguously make all {copy,move}-{construction,assignment} operations endow the LHS with the exact state of the RHS, including all predicates and hash function states.

Conclusion: Update wording, revisit later.

[2015-05-06 Lenexa: Waiting for updated wording]

Previous resolution [SUPERSEDED]:

This wording is relative to N3485.

1. Change Table 102 as indicated:

   Table 102 — Associative container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   …
   X(il)		Same as X(il.begin(), il.end()).	same as X(il.begin(), il.end()).
   X(b) X a(b)		Copy constructor. In addition to the requirements of Table 96, copies the comparison object.	Linear in b.size()
   a = b	X&	Copy assignment operator. In addition to the requirements of Table 96, copies the comparison object.	Linear in a.size() and b.size()
   …
   a.key_comp()	X::key_compare	rReturns thea's comparison object out of which a was constructed.	constant

[2015-10-19 Daniel comments and provides alternative wording]

The current standard is especially unclear in regard to what effects move operations of unordered/associative containers should have. We have one example that is standardized exactly in this way by looking at 23.6.4.3 [priqueue.cons.alloc] p7:

template <class Alloc> priority_queue(priority_queue&& q, const Alloc& a);

-7- Effects: Initializes c with std::move(q.c) as the first argument and a as the second argument, and initializes comp with std::move(q.comp)

A similarly comparable example are the move-operations of std::unique_ptr in regard to the deleter (when this is no a reference), which also respect move-capabilities of that function object.

We have wording from C++98 for associative containers (but not for unordered containers!) that was never adjusted to C++11 move-semantics in 23.2.7 [associative.reqmts] p12:

When an associative container is constructed by passing a comparison object the container shall not store a pointer or reference to the passed object, even if that object is passed by reference. When an associative container is copied, either through a copy constructor or an assignment operator, the target container shall then use the comparison object from the container being copied, as if that comparison object had been passed to the target container in its constructor.

The second sentence of this wording is problematic for several reasons:

1. It only talks about copy operations, not about move operations, except that the term "assignment" without leading "copy" is a bit ambigious (albeit it seems clear in the complete context).

2. It is not really clear how to interpret "as if that comparison object had been passed to the target container in its constructor" for an assignment operation. A possible but not conclusive interpretation could be that this is wording supporting a "copy-via-swap" idiom.

3. There does not exist similar wording for unordered containers, except that Table 102 provides entries for copy construction and copy assignment of the containers whose wording just talks of "copies" in either case.

Existing implementations differ already:

1. Visual Studio 2015 uses copy construction and copy assignment for the two copy operations but uses swap operations for the move operations.

2. GCC's libstdc++ performs copy construction and copy assignment for the two copy operations and for the two move operations, respectively

3. clang++'s libc++ performs copy/move construction and copy/move assignment for the corresponding four copy/move operations

The alternative wording provided below attempts to clarify that container copy/move operations perform the corresponding copy/move operations on the owned function objects.

In addition the wording also resolves LWG 2215⁽ⁱ⁾: I believe that the current wording should require that container function objects should meet the CopyConstructible requirements. Adding this general requirement also fixes the underspecified requirements of the accessor functions key_comp() and value_comp().

I don't think that a general requirement for Swappable is needed, only the member swap function currently requires this. Nonetheless the wording below does support stateful functors that are also moveable or move-assignable, therefore the specified semantics in terms of move operations.

I should add the following warning, though: If this proposed wording would be accepted, there is a little chance of code breakage, because the current wording can be read that in general there is no requirement that the container functors are CopyConstructible. The following code example is accepted by gcc + libstd++:

#include <map>
#include <utility>
#include <iostream>

struct Cmp {
  Cmp() = default;
  Cmp(const Cmp&) = delete;
  Cmp(Cmp&&) = delete;
  Cmp& operator=(const Cmp&) = delete;
  Cmp& operator=(Cmp&&) = delete;
  template<class T>
  bool operator()(const T& x, const T& y) const
  {
    return x < y;
  }
};

typedef std::map<int, int, Cmp> MyMap;

int main() {
  MyMap m;
  std::cout << (m.find(12) == m.end()) << std::endl;
}

Previous resolution [SUPERSEDED]:

This wording is relative to N4527.

1. Change 23.2.7 [associative.reqmts] p8 as indicated:

   -8- In Table 101, X denotes an associative container class, a denotes a value of type X, b denotes a possibly const value of type X, rv denotes a non-const rvalue of type X, u denotes the name of a variable being declared, […]

2. Change Table 101 as indicated:

   Table 101 — Associative container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   …
   X::key_compare	Compare	Requires: Compare is CopyConstructible. defaults to less<key_type>	compile time
   X(c) X u(c);		Requires: key_compare is CopyConstructible. Effects: Constructs an empty container. Uses a copy of c as a comparison object.	[…]
   …
   X(i,j,c) X u(i,j,c);		Requires: key_compare is CopyConstructible. value_type is EmplaceConstructible into X from *i. Effects: Constructs an empty container and inserts elements from the range [i, j) into it; uses c as a comparison object.	[…]
   …
   X(il)		Same as X(il.begin(), il.end()).	same as X(il.begin(), il.end()).
   X(b) X a(b)		(In addition to the requirements of Table 95) Effects: Copy constructs the comparison object of a from the comparison object of b.	Linear in b.size()
   X(rv) X a(rv)		(In addition to the requirements of Table 95 and Table 98) Effects: Move constructs the comparison object of a from the comparison object of rv.	constant
   a = b	X&	(In addition to the requirements of Table 95 and Table 98) Requires: key_compare is CopyAssignable. Effects: Copy assigns the comparison object of b to the comparison object of a.	Linear in a.size() and b.size()
   a = rv	X&	(In addition to the requirements of Table 95 and Table 98) Requires: key_compare is MoveAssignable. Effects: Move assigns from the comparison object of rv to the comparison object of a.	Linear
   …
   a.key_comp()	X::key_compare	rReturns thea's comparison object out of which a was constructed.	constant

3. Change 23.2.7 [associative.reqmts] p12 as indicated:

   -12- When an associative container is constructed by passing a comparison object the container shall not store a pointer or reference to the passed object, even if that object is passed by reference. When an associative container is copied, either through a copy constructor or an assignment operator, the target container shall then use the comparison object from the container being copied, as if that comparison object had been passed to the target container in its constructor.

4. Change 23.2.8 [unord.req] p11 as indicated:

   -11- In Table 102: X denotes an unordered associative container class, a denotes a value of type X, b denotes a possibly const value of type X, rv denotes a non-const rvalue of type X, […]

5. Change Table 102 as indicated:

   Table 102 — Unordered associative container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   …
   X::hasher	Hash	Requires: Hash is CopyConstructible. Hash shall be a unary function object type such that the expression hf(k) has type std::size_t.	compile time
   X::key_equal	Pred	Requires: Pred is CopyConstructible. Pred shall be a binary predicate that takes two arguments of type Key. Pred is an equivalence relation.	compile time
   …
   X(n, hf, eq) X a(n, hf, eq)	X	Requires: hasher and key_equal are CopyConstructible. Effects: […]	[…]
   X(n, hf) X a(n, hf)	X	Requires: hasher is CopyConstructible and key_equal is DefaultConstructible. Effects: […]	[…]
   …
   X(i, j, n, hf, eq) X a(i, j, n, hf, eq)	X	Requires: hasher and key_equal are CopyConstructible. value_type is EmplaceConstructible into X from *i. Effects: […]	[…]
   X(i, j, n, hf) X a(i, j, n, hf)	X	Requires: hasher is CopyConstructible and key_equal is DefaultConstructible. value_type is EmplaceConstructible into X from *i. Effects: […]	[…]
   …
   X(b) X a(b)	X	Copy constructor. In addition to the requirements of Table 95, copies the hash function, predicate, and maximum load factor. (In addition to the requirements of Table 95) Effects: Copy constructs the hash function, predicate, and maximum load factor of a from the corresponding objects of b.	Average case linear in b.size(), worst case quadratic.
   X(rv) X a(rv)	X	(In addition to the requirements of Table 95 and Table 98) Effects: Move constructs the hash function, predicate, and maximum load factor of a from the corresponding objects of rv.	constant
   a = b	X&	Copy assignment operator. In addition to the requirements of Table 95, copies the hash function, predicate, and maximum load factor. (In addition to the requirements of Table 95 and Table 98) Requires: hasher and key_equal are CopyAssignable. Effects: Copy assigns the hash function, predicate, and maximum load factor of b to the corresponding objects of a.	Average case linear in b.size(), worst case quadratic.
   a = rv	X&	(In addition to the requirements of Table 95 and Table 98) Requires: hasher and key_equal are MoveAssignable. Effects: Move assigns the hash function, predicate, and maximum load factor from rv to the corresponding objects of a.	Linear
   …

[2016-08-07]

Daniel removes the previously proposed wording to work on revised wording.

[2019-04-22, Billy comments]

In addition to the Cpp17CopyConstructible discussion going on there, I think we need to require that calling the comparison function when Compare itself is const needs to produce the same answer as if Compare is non-const.

Proposed resolution:

2236⁽ⁱ⁾. kill_dependency unconditionally noexcept

Section: 32.5.2 [atomics.syn], 32.5.4 [atomics.order] Status: SG1 Submitter: Daniel Krügler Opened: 2013-01-19 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The "magic" kill_dependency function is a function without any constraints on the template parameter T and is specified as

template <class T>
T kill_dependency(T y) noexcept;

-14- Effects: The argument does not carry a dependency to the return value (1.10).

-15- Returns: y.

I wonder whether the unconditional noexcept is really intended here: Assume we have some type U that has a potentially throwing move constructor (or it has a potentially throwing copy constructor and no move constructor), for any "normal" function template with the same signature and the same effects (modulo the dependency magic) this would mean that it cannot safely be declared noexcept because of the return statement being part of the complete function call affected by noexcept (The by-value function argument is irrelevant in this context). In other words it seems that a function call such as

struct S {
  ...
  S(const S& r) { if(some condition) throw Something(); }
  ...
};

int main() {
  S s1 = ...;
  S s2 = std::kill_dependency(s1);
}

would be required to call std::terminate if the copy constructor of S throws during the return of std::kill_dependency.

To require copy elision for this already magic function would look like a low-hanging fruit to solve this problem, but this case is not covered by current copy elision rules see 12.8 p31 b1:

"— in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) with the same cv-unqualified type as the function return type, the copy/move operation can be omitted by constructing the automatic object directly into the function's return value".

Some options come into my mind:

1. Make the exception-specification a constrained one in regard via std::is_nothrow_move_constructible:

   template <class T>
   T kill_dependency(T y) noexcept(see below);
   

   This is similar to the approach taken for function templates such as std::swap.

2. Use perfect forwarding (This needs further wording to correct the effects):

   template <class T>
   T&& kill_dependency(T&& y) noexcept;
   

3. Impose constraints on the template arguments in regard to throwing exceptions while copying/moving.

4. Keep the state as it is but possibly add a note about a call of std::terminate in above scenario.

A second problem is that the current wording is not clear whether it is well-defined to call the function with types that are reference types, such as in the following example:

#include <atomic>

int main()
{
  int a = 12;
  int& b = std::kill_dependency<int&>(a);
}

It is unclear what kind of dependency is killed here. This is presumably a core language problem, but could affect the possible resolutions of the problem.

[2014-11 Urbana]

Recommend using a revised example:

int lookup(class D* p) 
{
  class E* q = p->a.load(memory_order_consume);
  int y = std::kill_dependency(q->y);
}

[2015-02 Cologne]

Handed over to SG1.

Proposed resolution:

2237⁽ⁱ⁾. <cuchar> macros

Section: 27.5 [c.strings] Status: New Submitter: Jason Merrill Opened: 2013-01-29 Last modified: 2016-01-28

Priority: 4

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Discussion:

Apparently C1X changes __STDC_UTF_16__ and __STDC_UTF_32__ from macros defined in uchar.h (and reflected in C++ by Table 79) to be predefined by the compiler. Do we want to do the same?

Proposed resolution:

2238⁽ⁱ⁾. Problematic iterator-pair constructor of containers

Section: 27.5 [c.strings] Status: Open Submitter: Johannes Schaub Opened: 2013-02-02 Last modified: 2016-08-09

Priority: 3

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Discussion:

The non-explicit nature of the iterator-pair constructor of containers, such a

template <class InputIterator>
vector(InputIterator first, InputIterator last, const Allocator& = Allocator());

can be selected in unexpected situations, leading to a hard runtime error, as demonstrated by the following example:

#include <vector>

void f(std::vector<char> v){ /* ... */}

int main() {
  f({"A", "B"});
}

The actually intended initializer-list constructor isn't feasible here, so the best match is the constructor template

template <class InputIterator>
vector(InputIterator first, InputIterator last, const Allocator& = Allocator());

This compiles, but will result in code running amok. The potential trap (that cannot be easily detected by the library implementation) could be reduced by making this constructor explicit. It would still have the effect to be selected here, but the code would be ill-formed, so the programmer gets a clear message here.

[2014-06 Rapperswil]

JW: can't fix this, don't want to touch this, Do The Right Thing clause has been a source of tricky issues. only really happens with string literals, that's the only way to create an array that isn't obviously an array

GR: want to see paper

AM: is it only string literals, or also UDLs?

STL: maybe, but we don't need to deal with that. This is only a problem in a very specific case

Leave as Open.

Proposed resolution:

2248⁽ⁱ⁾. numeric_limits::is_iec559 misnamed

Section: 17.3.5 [numeric.limits] Status: New Submitter: Pete Becker Opened: 2013-03-08 Last modified: 2018-11-08

Priority: 4

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Discussion:

This member should probably be named "is_ieee754". Or at least the standard should explain that IEC-559 no longer exists, and that it's been superseded by IEEE-754.

[2016-06, Oulu]

The ISO version of the standard is ISO/IEC/IEEE 60559:2011, which C11 Annex F refers to as IEC 60559 (although C still refers to it as IEC 559 in the __STDC_IEC_559__ macro).

Proposed resolution:

2262⁽ⁱ⁾. Requirement for unique_ptr<T>::get_deleter()(p) to be able to destroy the unique_ptr

Section: 20.3.1.3 [unique.ptr.single] Status: Open Submitter: Rob Desbois Opened: 2013-05-15 Last modified: 2017-03-21

Priority: 3

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Discussion:

N3337 20.3.1.3.6 [unique.ptr.single.modifiers] contains 2 non-normative notes stating:

[para 4]: "The order of these operations is significant because the call to get_deleter() may destroy *this."

[para 5]: "The postcondition does not hold if the call to get_deleter() destroys *this since this->get() is no longer a valid expression."

It seems this wording was created to resolve 998⁽ⁱ⁾ due to the possibility that a unique_ptr may be destroyed through deletion of its stored pointer where that directly or indirectly refers to the same unique_ptr. If unique_ptr is required to support circular references then it seems this must be normative text: an implementation is currently allowed to operate on *this after the assignment and deletion specified in para 4, since this is only 'disallowed' by the non-normative note.

I propose the following draft rewording:

[para 4]: Effects: assigns p to the stored pointer, and then if the old value of the stored pointer, old_p, was not equal to nullptr, calls get_deleter()(old_p). No operation shall be performed after the call to get_deleter()(old_p) that requires *this to be valid, because the deletion may destroy *this if it is referred to directly or indirectly by the stored pointer. [Note: The order of these operations is significant because the call to get_deleter() may destroy *this. — end note]

[para 5]: Postconditions: If the call get_deleter()(old_p) destroyed *this, none. Otherwise, get() == p. [Note: The postcondition does not hold if the call to get_deleter() destroys *this since this->get() is no longer a valid expression. — end note]

I expect it will also be necessary to amend the requirements for a deleter, so in addition:

20.3.1.3 [unique.ptr.single] [para 1]: The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function object type (20.10), lvalue-reference to function, or lvalue-reference to function object type for which, given a value d of type D and a value ptr of type unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter. Where D is not an lvalue reference type, d(ptr) shall be valid if ptr refers directly or indirectly to the invoking unique_ptr object.

[2013-10-05, Stephan T. Lavavej comments and provides alternative wording]

In Chicago, we determined that the original proposed change to 20.3.1.3 [unique.ptr.single]/1 was insufficient, because d might be a reference to a deleter functor that's destroyed during self-destruction.

We believed that 20.3.1.3.6 [unique.ptr.single.modifiers]/4 was already sufficiently clear. The Standard occasionally prevents implementations of X from doing various things, through the principle of "nothing allows X to fail in that situation". For example, v.push_back(v[0]) is required to work for non-empty vectors because nothing allows that to fail. In this case, the intent to allow self-destruction is already clear.

Additionally, we did not believe that 20.3.1.3.6 [unique.ptr.single.modifiers]/5 had to be changed. The current note is slightly squirrely but it does not lead to confusion for implementers or users.

Previous resolution from Rob Desbois:

1. Edit 20.3.1.3 [unique.ptr.single] p1 as indicated:

   The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function object type (20.10), lvalue-reference to function, or lvalue-reference to function object type for which, given a value d of type D and a value ptr of type unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter. Where D is not an lvalue reference type, d(ptr) shall be valid if ptr refers directly or indirectly to the invoking unique_ptr object.

2. Edit 20.3.1.3.6 [unique.ptr.single.modifiers] p4+5 as indicated:

   void reset(pointer p = pointer()) noexcept;
   

   -3- Requires: The expression get_deleter()(get()) shall be well formed, shall have well-defined behavior, and shall not throw exceptions.

   -4- Effects: assigns p to the stored pointer, and then if the old value of the stored pointer, old_p, was not equal to nullptr, calls get_deleter()(old_p). No operation shall be performed after the call to get_deleter()(old_p) that requires *this to be valid, because the deletion may destroy *this if it is referred to directly or indirectly by the stored pointer. [Note: The order of these operations is significant because the call to get_deleter() may destroy *this. — end note]

   -5- Postconditions: If the call get_deleter()(old_p) destroyed *this, none. Otherwise, get() == p. [Note: The postcondition does not hold if the call to get_deleter() destroys *this since this->get() is no longer a valid expression. — end note]

Previous resolution [SUPERSEDED]:

This wording is relative to N3691.

1. Edit 20.3.1.3 [unique.ptr.single] p1 as indicated:

   The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function object type (20.10), lvalue-reference to function, or lvalue-reference to function object type for which, given a value d of type D and a value ptr of type unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter. d(ptr) shall be valid even if it triggers the destruction of d or (if D is an lvalue reference to function object type) the function object that d refers to.

[2015-05, Lenexa]

After some discussion in Lenexa there was some wavering on if the added sentence is necessary. Here is example code that demonstrates why the extra sentence is necessary. In this example the call to d(ptr) is valid, however the deleter references *this after destructing its element:

#include <cassert>
#include <memory>
#include <iostream>

class Deleter
{
    int state_ = 0;

    enum
    {
        destructed            = -4,
        self_move_assigned    = -3,
        move_assigned_from    = -2,
        move_constructed_from = -1
    };
public:
    ~Deleter() {state_ = destructed;}

    Deleter() = default;
    Deleter(Deleter const&) = default;
    Deleter& operator=(Deleter const&) = default;

    Deleter(Deleter&& a) noexcept
        : state_(a.state_)
    {a.state_ = move_constructed_from;}

    Deleter& operator=(Deleter&& a) noexcept
    {
        if (this == &a)
            state_ = self_move_assigned;
        else
        {
            state_ = a.state_;
            a.state_ = move_assigned_from;
        }
        return *this;
    }

    Deleter(int state)
        : state_(state)
    {
        assert(state >= 0);
    }

    template <class T>
    void
    operator()(T* t) const
    {
        std::cout << "Deleter beginning operator()(T*)\n";
        std::cout << "The deleter = " << *this << '\n';
        std::cout << "Deleter about to destruct the X.\n";
        delete t;
        std::cout << "Deleter has destructed the X.\n";
        std::cout << "The deleter = " << *this << '\n';
        std::cout << "Deleter ending operator()(T*)\n";
    }

    friend
    std::ostream&
    operator<<(std::ostream& os, const Deleter& a)
    {
        switch (a.state_)
        {
        case destructed:
            os << "**destructed**";
            break;
        case self_move_assigned:
            os << "self_move_assigned";
            break;
        case move_assigned_from:
            os << "move_assigned_from";
            break;
        case move_constructed_from:
            os << "move_constructed_from";
            break;
        default:
            os << a.state_;
            break;
        }
        return os;
    }
};

struct X
{
    Deleter deleter_{1};
};

int main()
{
    auto xp = new X;
    {
        std::unique_ptr<X, Deleter&> p(xp, xp->deleter_);
        std::cout << "unique_ptr is constructed.\n";
        std::cout << "The deleter = " << p.get_deleter() << '\n';
        std::cout << "Destructing unique_ptr...\n";
    }
    std::cout << "unique_ptr is destructed.\n";
}

Which outputs:

unique_ptr is constructed.
The deleter = 1
Destructing unique_ptr...
Deleter beginning operator()(T*)
The deleter = 1
Deleter about to destruct the X.
Deleter has destructed the X.
The deleter = **destructed**
Deleter ending operator()(T*)
unique_ptr is destructed.

The line "The deleter = **destructed**" represents the deleter referencing itself after it has been destructed by the d(ptr) expression, but prior to that call returning.

Suggested alternative to the current proposed wording:

The expression d(ptr) shall not refer to the object d after it executes ptr->~T().

[2015-07, Telecon]

Geoffrey: Deleter may or may not execute ~T().
Alisdair: After the destructor after the element has run. Say it in words instead of code.
Howard will provide updated wording. Perhaps need both normative and non-normative wording.

[2015-08-03, Howard updates P/R per telecon discussion.]

[2017-03-04, Kona]

This is related to 2751⁽ⁱ⁾, which has been suggested NAD.

STL wants "Effects equivalent to" here - say it in code. Marshall to research.

Proposed resolution:

This wording is relative to N4431.

1. Edit 20.3.1.3 [unique.ptr.single] p1 as indicated:

   The default type for the template parameter D is default_delete. A client-supplied template argument D shall be a function object type (20.9), lvalue-reference to function, or lvalue-reference to function object type for which, given a value d of type D and a value ptr of type unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter. The expression d(ptr), if it destructs the object referred to by ptr, shall not refer to the object d after it destructs *ptr. [Note: The object being destructed may control the lifetime of d. — end note]

2265⁽ⁱ⁾. 29.3p9 appears to rule out some acceptable executions

Section: 32.5.4 [atomics.order] Status: Open Submitter: Brian Demsky Opened: 2013-06-17 Last modified: 2016-01-28

Priority: 4

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Discussion:

I believe that the following variation on IRIW should admit executions in which c1 = d1 = 5 and c2 = d2 = 0. If this is allowed, then what is sequence of program evaluations for 32.5.4 [atomics.order] p9 that justifies the store to z? It seems that 32.5.4 [atomics.order] p9 should not allow this execution because one of the stores to x or y has to appear earlier in the sequence, each of the fetch_adds reads the previous load in the thread (and thus must appear later in the sequence), and 32.5.4 [atomics.order] p9 states that each load must read from the last prior assignment in the sequence.

atomic_int x;
atomic_int y;
atomic_int z;
int c1, c2, d1, d2;

static void a(void* obj)
{
  atomic_store_explicit(&x, 5, memory_order_relaxed); 
}

static void b(void* obj)
{
  atomic_store_explicit(&y, 5, memory_order_relaxed); 
}

static void c(void* obj)
{
  c1 = atomic_load_explicit(&x, memory_order_relaxed);
  // this could also be an atomic load if the address depends on c1:
  c2 = atomic_fetch_add_explicit(&y, c1, memory_order_relaxed);  
}

static void d(void* obj)
{
  d1 = atomic_load_explicit(&y, memory_order_relaxed);
  d2 = atomic_fetch_add_explicit(&x, d1, memory_order_relaxed); 
}

int user_main(int argc, char** argv)
{
  thrd_t t1, t2, t3, t4;

  atomic_init(&x, 0);
  atomic_init(&y, 0);

  printf("Main thread: creating 4 threads\n");
  thrd_create(&t1, (thrd_start_t)&a, NULL);
  thrd_create(&t2, (thrd_start_t)&b, NULL);
  thrd_create(&t3, (thrd_start_t)&c, NULL);
  thrd_create(&t4, (thrd_start_t)&d, NULL);

  thrd_join(t1);
  thrd_join(t2);
  thrd_join(t3);
  thrd_join(t4);
  printf("c1=%d c2=%d\n",c1,c2);
  printf("d1=%d d2=%d\n",d1,d2);

  // Can this store write 1000 (i.e., c1=d1=5, c2=d2=0)?
  atomic_store(&z, (c1+d1)*100+c2+d2);

  printf("Main thread is finished\n");

  return 0;
}

It seems that the easiest fix is to allow a load in 32.5.4 [atomics.order] p9 to read from any prior store in the evaluation order.

That said, I would personally advocate the following: It seems to me that C/C++ atomics are in a bit of different situation than Java because:

1. People are expected to use relaxed C++ atomics in potentially racy situations, so it isn't clear that semantics as complicated as the JMM's causality would be sane.

2. People who use C/C++ atomics are likely to be experts and use them in a very controlled fashion. I would be really surprised if compilers would find any real wins by optimizing the use of atomics.

Why not do something like:

There is satisfaction DAG of all program evaluations. Each evaluation observes the values of variables as computed by some prior assignment in the DAG.

There is an edge x->y between two evaluations x and y if:

1. the evaluation y observes a value computed by the evaluation x or

2. the evaluation y is an atomic store, the evaluation x is an atomic load, and there is a condition branch c that may depend (intrathread dependence) on x and x-sb->c and c-sb->y.

This seems to allow reordering of relaxed atomics that processors do without extra fence instructions, allows most reorderings by the compiler, and gets rid of satisfaction cycles.

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

This was partially addressed (weasel-worded) in C++14 (See N3786). The remainder is an open research problem. N3710 outlines a "solution" that doesn't have a consensus behind it because it costs performance. We have no better solution at the moment.

Proposed resolution:

2267⁽ⁱ⁾. partial_sort_copy underspecified for ranges of two different types

Section: 26.8.2.4 [partial.sort.copy] Status: New Submitter: Matt Austern Opened: 2013-06-26 Last modified: 2016-01-28

Priority: 3

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Discussion:

The signature of this function is:

template<class InputIterator, class RandomAccessIterator>
RandomAccessIterator
partial_sort_copy(InputIterator first, InputIterator last,
                  RandomAccessIterator result_first,
                  RandomAccessIterator result_last);

(and the usual overload for an explicitly provided comparison function). The standard says nothing about requirements in the case where the input type (iterator_traits<InputIterator>::value_type) and the output type (iterator_traits<RandomAccessIterator>::value_type) are different.

Presumably the input type must be convertible to the output type. What's less clear is what the requirements are on the comparison operator. Does the algorithm only perform comparisons on two values of the output type, or does it also perform comparisons on values of the input type, or might it even perform heterogeneous comparisons?

Proposed resolution:

2269⁽ⁱ⁾. Container iterators and argument-dependent lookup

Section: 23.2.2 [container.requirements.general] Status: New Submitter: Matt Austern Opened: 2013-06-26 Last modified: 2016-01-28

Priority: 4

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Discussion:

Consider the following code snippet:

#include <vector>
#include <algorithm>

int main() {
  std::vector<int> v1(100, 3);
  std::vector<int> v2(100);
  copy(v1.begin(), v1.end(), v2.begin());
}

It compiles without error on my desktop. Is it required to? I can't find evidence from the standard that it is. In my test std::copy was found by argument-dependent lookup because the implementation I used made std::vector<int>::iterator a user-defined type defined in namespace std. But the standard only requires std::vector<int>::iterator to be an implementation specified random access iterator type. I can't find anything requiring it to be a user-defined type at all (and in fact there are reasonable implementation where it isn't), let alone a user defined type defined in a specific namespace.

Since the defining namespace of container iterators is visible to users, should the standard say anything about what that namespace is?

Proposed resolution:

2286⁽ⁱ⁾. stringbuf::underflow() underspecified

Section: 31.8.2.5 [stringbuf.virtuals] Status: Open Submitter: Sergey Zubkov Opened: 2013-08-29 Last modified: 2018-06-12

Priority: 4

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Discussion:

In 31.8.2.5 [stringbuf.virtuals]/1, basic_stringbuf::underflow() is specified to unconditionally return traits::eof() when a read position is not available.

The semantics of basic_stringbuf require, and existing libraries implement it so that this function makes a read position available if possible to do so, e.g. if some characters were inserted into the stream since the last call to overflow(), resulting in pptr() > egptr(). Compare to the conceptually similar 99 [depr.strstreambuf.virtuals]/15.

[2018-06-06, Billy argues for NAD]

The existing "Any character in the underlying buffer which has been initialized is considered to be part of the input sequence." sentence already describes what the stringbuf is supposed to do to the get area. The specific mechanism that the stringbuf uses to alter the get area is unspecified because the mechanism by which the stringbuf remembers the "high water mark" is unspecified.

Consider the following:

stringstream s;
s << "Hello";
s.seekp(0);
string x;
s >> x;

Before this P/R, this will store Hello in x, because the characters Hello are initialized. After this P/R, the "written put area" is empty, so it will store the empty string in x.

Saying that the initialized part of the string is used already describes what needs to happen here.

[2018-06 Rapperswil Wednesday issues processing]

Billy to provide rationale for closing as NAD.

Proposed resolution:

This wording is relative to N3691.

1. Change 31.8.2.5 [stringbuf.virtuals] as indicated:

   int_type underflow();
   

   -1- Returns: If the input sequence has a read position available or the function makes a read position available (as described below), returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof(). Any character in the underlying buffer which has been initialized is considered to be part of the input sequence.

   -?- The function can make a read position available only if (mode & ios_base::in) != 0 and if the write next pointer pptr() is not null and is greater than the current read end pointer egptr(). To make a read position available, the function alters the read end pointer egptr() to equal pptr().

2289⁽ⁱ⁾. constexpr guarantees of defaulted functions still insufficient

Section: 22.3.2 [pairs.pair], 22.4.4.2 [tuple.cnstr], 30.5 [time.duration] Status: Open Submitter: Daniel Krügler Opened: 2013-09-09 Last modified: 2020-06-13

Priority: 3

View other active issues in [pairs.pair].

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Discussion:

During the acceptance of N3471 and some similar constexpr papers, specific wording was added to pair, tuple, and other templates that were intended to impose implementation constraints that ensure that the observable constexpr "character" of a defaulted function template is solely determined by the required expressions of the user-provided types when instantiated, for example:

The defaulted move and copy constructor, respectively, of pair shall be a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function.

This wording doesn't require enough, especially since the core language via CWG 1358 does now support constexpr function template instantiations, even if such function cannot appear in a constant expression (as specified in 7.7 [expr.const]) or as a constant initializer of that object (as specified in [basic.start.init]). The wording should be improved and should require valid uses in constant expressions and as constant initializers instead.

[Lenexa 2015-05-05]

STL : notice order of move/copy and copy/move with "respectively".

General word-smithing; ask for updated wording

Are we happy with this with changes we are suggesting?

unanimous

[2016-12-14, Daniel comments]

LWG 2833⁽ⁱ⁾ overlaps considerably and both should be resolved together.

This wording is relative to N3691.

1. Change 22.3.2 [pairs.pair] p2 as indicated:

   -2- The defaulted move and copy constructor, respectively, of pair shall be a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr functionAn invocation of the move or copy constructor of pair shall be a constant expression (7.7 [expr.const]) if all required element-wise initializations would be constant expressions. An invocation of the move or copy constructor of pair shall be a constant initializer for that pair object ( [basic.start.init]) if all required element-wise initializations would be constant initializers for the respective subobjects.

2. Change 22.4.4.2 [tuple.cnstr] p2 as indicated:

   -2- The defaulted move and copy constructor, respectively, of tuple shall be a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function. The defaulted move and copy constructor of tuple<> shall be constexpr functionsAn invocation of the move or copy constructor of tuple shall be a constant expression (7.7 [expr.const]) if all required element-wise initializations would be constant expressions. An invocation of the move or copy constructor of tuple shall be a constant initializer for that tuple object ( [basic.start.init]) if all required element-wise initializations would be constant initializers for the respective subobjects. An invocation of the move or copy constructor of tuple<> shall be a constant expression, or a constant initializer for that tuple<> object, respectively, if the function argument would be constant expression.

3. Change 30.5 [time.duration] p7 as indicated:

   -7- Remarks: The defaulted copy constructor of duration shall be a constexpr function if and only if the required initialization of the member rep_ for copy and move, respectively, would satisfy the requirements for a constexpr function.An invocation of the copy constructor of duration shall be a constant expression (7.7 [expr.const]) if the required initialization of the member rep_ would be a constant expression. An invocation of the copy constructor of duration shall be a constant initializer for that duration object ( [basic.start.init]) if the required initialization of the member rep_ would be constant initializers for this subobject.

[2020-06-08 Nina Dinka Ranns comments]

The revised wording provided by LWG 2833⁽ⁱ⁾ should resolve this issue as well.

Proposed resolution:

2290⁽ⁱ⁾. Top-level "SFINAE"-based constraints should get a separate definition in Clause 17

Section: 21 [meta] Status: Open Submitter: Daniel Krügler Opened: 2013-09-02 Last modified: 2016-01-28

Priority: 3

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Discussion:

The current library specification uses at several places wording that is intended to refer to core language template deduction failure at the top-level of expressions (aka "SFINAE"), for example:

The expression declval<T>() = declval<U>() is well-formed when treated as an unevaluated operand (Clause 5). Access checking is performed as if in a context unrelated to T and U. Only the validity of the immediate context of the assignment expression is considered. [Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]

Similar wording can be found in the specification of result_of, is_constructible, and is_convertible, being added to resolve an NB comment by LWG 1390⁽ⁱ⁾ and 1391⁽ⁱ⁾ through N3142.

This wording is necessary to limit speculative compilations needed to implement these traits, but it is also lengthy and repetitive.

[2014-05-19, Daniel suggests a descriptive term]

constrictedly well-formed expression:

An expression e depending on a set of types A1, ..., An which is well-formed when treated as an unevaluated operand (Clause 5). Access checking is performed as if in a context unrelated to A1, ..., An. Only the validity of the immediate context of e is considered. [Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]

[2014-05-20, Richard and Jonathan suggest better terms]

Richard suggested "locally well-formed"

Jonathan suggested "contextually well-formed" and then "The expression ... is valid in a contrived argument deduction context"

[2014-06-07, Daniel comments and improves wording]

The 2014-05-19 suggestion did only apply to expressions, but there are two important examples that are not expressions, but instead are involving an object definition (std::is_constructible) and a function definition (std::is_convertible), respectively, instead. Therefore I suggest to rephrase the usage of "expression" into "program construct" in the definition of Jonathan's suggestion of "valid in a contrived argument deduction context".

I would like to point out that given the new definition of "valid in a contrived argument deduction context", there are several other places of the Library specification that could take advantage of this wording to improve the existing specification, such as 22.10.17.3 [func.wrap.func] p2, most functions in 20.2.9.3 [allocator.traits.members], and the **Insertable, EmplaceConstructible, and Erasable definitions in 23.2.2 [container.requirements.general], but given that these are not fully described in terms of the aforementioned wording yet, I would recommend to fix them by a separate issue once the committee has agreed on following the suggestion presented by this issue.

[2015-05-05 Lenexa: Move to Open]

...

MC: I think we like the direction but it isn't quite right: it needs some work

JW: I'm prepared to volunteer to move that further, hopefully with the help of Daniel

Roger Orr: should this be Core wording because it doesn't really have anything to do with libraries - the term could then just be used here

AM: Core has nothing to deal with that, though

HT: it seems there is nothing to imply that allows dropping out with an error - maybe that's a separate issue

MC: I'm not getting what you are getting at: could you write an issue? - any objection to move to Open?

...

Proposed resolution:

This wording is relative to N3936.

1. Add the following new definition to [definitions] as indicated:

   valid in a contrived argument deduction context [defns.valid.contr.context]

   A program construct c depending on a set of types A1, ..., An, and treated as an unevaluated operand (Clause 5) when c is an expression, which is well-formed. Access checking is performed as if in a context unrelated to A1, ..., An. Only the validity of the immediate context (13.10.3 [temp.deduct]) of c is considered. [Note: The compilation of c can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note].

2. Change Table 49 ("Type property predicates") as indicated:

   Table 49 — Type property predicates

   Template	Condition	Preconditions
   …
   template <class T, class U> struct is_assignable;	The expression declval<T>() = declval<U>() is valid in a contrived argument deduction context ([defns.valid.contr.context]) for types T and U. well-formed when treated as an unevaluated operand (Clause 5). Access checking is performed as if in a context unrelated to T and U. Only the validity of the immediate context of the assignment expression is considered. [Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]	[…]
   …

3. Change 21.3.5.4 [meta.unary.prop] p7 as indicated:

   -7- Given the following function prototype:

   template <class T>
     add_rvalue_reference_t<T> create() noexcept;
   

   the predicate condition for a template specialization is_constructible<T, Args...> shall be satisfied if and only if the following variable definition would be well-formed for some invented variable t would be valid in a contrived argument deduction context ([defns.valid.contr.context]) for types T and Args...:

   T t(create<Args>()...);
   

   [Note: These tokens are never interpreted as a function declaration. — end note] Access checking is performed as if in a context unrelated to T and any of the Args. Only the validity of the immediate context of the variable initialization is considered. [Note: The evaluation of the initialization can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]

4. Change Table 57 ("Other transformations") as indicated:

   Table 57 — Other transformations

   Template	Condition	Comments
   …
   template <class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)>;	[…]	If the expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) is valid in a contrived argument deduction context ([defns.valid.contr.context]) for types Fn and ArgTypes... well formed when treated as an unevaluated operand (Clause 5), the member typedef type shall name the type decltype(INVOKE(declval<Fn>(), declval<ArgTypes>()...)); otherwise, there shall be no member type. Access checking is performed as if in a context unrelated to Fn and ArgTypes. Only the validity of the immediate context of the expression is considered. [Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]
   …

5. Change 21.3.7 [meta.rel] p4 as indicated:

   -4- Given the following function prototype:

   template <class T>
     add_rvalue_reference_t<T> create() noexcept;
   

   the predicate condition for a template specialization is_convertible<From, To> shall be satisfied if and only if the return expression in the following code would be well-formedvalid in a contrived argument deduction context ([defns.valid.contr.context]) for types To and From, including any implicit conversions to the return type of the function:

   To test() {
     return create<From>();
   }
   

   [Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note] Access checking is performed as if in a context unrelated to To and From. Only the validity of the immediate context of the expression of the return-statement (including conversions to the return type) is considered. [Note: The evaluation of the conversion can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]

2303⁽ⁱ⁾. Explicit instantiation of std::vector<UserType> broken?

Section: 17.6.3.4 [new.delete.placement] Status: New Submitter: Daniel Krügler Opened: 2013-09-18 Last modified: 2016-01-28

Priority: 3

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Discussion:

The library gives explicit permission in 16.4.5.2.1 [namespace.std] p2 that user code may explicitly instantiate a library template provided that the instantiations depend on at least one user-defined type:

A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type and the instantiation meets the standard library requirements for the original template.

But it seems that the C++11 library is not specified in a way that guarantees such an instantiation to be well-formed if the minimum requirements of the library is not satisfied.

For example, in general, the first template parameter of std::vector is not required to be DefaultConstructible in general, but due to the split of the single C++03 member function with default argument

void resize(size_type sz, T c = T());

into

void resize(size_type sz);
void resize(size_type sz, const T& c);

the effect is now that for a type ND that is not DefaultConstructible, such as

struct NP { 
  NP(int); 
};

the explicit instantiation of std::vector<ND> is no longer well-formed, because the attempt to instantiate the single-argument overload of resize cannot not succeed, because this function imposes the DefaultInsertable requirements and given the default allocator this effectively requires DefaultConstructible.

But DefaultConstructible is not the only point, what about CopyConstructible versus MoveConstructible alone? It turns out that currently the second resize overload would fail during an explicit instantiation for a type like

struct MO { 
  MO() = default; 
  MO(MO&&) = default; 
};

because it imposes CopyInsertable requirements that end up being equivalent to the CopyConstructible requirements for the default allocator.

Technically a library can solve these issues: For special member functions by defining them in some base class, for others by transforming them effectively into a function template due to the great feature of default template arguments for function templates (At the very moment the validity of the latter approach depends on a resolution of core language issue CWG 1635, though). E.g. the here mentioned resize functions of std::vector could be prevented from instantiation by defining them like this with an implementation:

template<class = void>
void resize(size_type sz) { […] }
template<class = void>
void resize(size_type sz, const T& c) { […] }

In this case, these functions could also be defined in a base class, but the latter approach won't work in all cases.

Basically such an implementation is required to constrain all member functions that are not covered by the general requirements imposed on the actual library template parameters. I tested three different C++11 library implementations and but none could instantiate for example std::list, std::vector, or std::deque with value types that are not DefaultConstructible or only MoveConstructible.

This issue is raised to clarify the current situation in regard to the actual requirements imposed on user-provided types that are used to explicitly instantiate Library-provided templates. For example, the current Container requirements impose very little requirements on the actual value type and it is unclear to which extend library implementations have to respect that.

The minimum solution of this issue should be to at least realize that there is no fundamental requirement on DefaultConstructible for value types of library containers, because we have since C++03 the general statement of 16.4.4.2 [utility.arg.requirements] ("In general, a default constructor is not required."). It is unclear whether CopyConstructible should be required for an explicit instantiation request, but given the careful introduction of move operations in the library it would seem astonishing that a MoveConstructible type wouldn't suffice for value types of the container types.

In any case I can envision at least two approaches to solve this issue:

1. As indicated in LWG 2292⁽ⁱ⁾, those function could get an explicit "Template Constraints:" element, albeit this promises more than needed to solve this issue.

2. The library could introduce a completely new element form, such as "Instantiation Constraints:" that would handle this situation for explicit instantiation situations. This would allow for simpler techniques to solve the issue when explicit instantiation is required compared to the first bullet, because it would not (necessarily) guarantee SFINAE-friendly expression-wellformedness, such as inspecting the expression std::declval<std::vector<ND>&>.resize(0) in an unevaluated context.

It should be noted that the 2013-08-27 comment to LWG 2193⁽ⁱ⁾ could be resolved by a similar solution as indicated in this issue here.

Proposed resolution:

2307⁽ⁱ⁾. Should the Standard Library use explicit only when necessary?

Section: 23 [containers] Status: LEWG Submitter: Zhihao Yuan Opened: 2013-09-26 Last modified: 2018-11-12

Priority: 2

View other active issues in [containers].

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Discussion:

LWG 2193⁽ⁱ⁾ yields explicit for default ctors to allow {}, but not for all cases of uniform initialization. For example:

explicit vector(size_type count, const Allocator& alloc = Allocator());

This prevents {n, alloc()}. Although this use is relatively rare, but the behavior is inconsistent with that of

vector(size_type count, const T& value, const Allocator& alloc = Allocator());

[Urbana 2014-11-07: Move to Open]

[2018-08 Batavia Monday issue discussion]

This really needs a paper; splitting a lot of constructors. Nevin to write paper.

[2018-11 San Diego Thursday night issue processing]

LEWG has rejected Nevin's paper, so they need to formulate a policy.

Proposed resolution:

2321⁽ⁱ⁾. Moving containers should (usually) be required to preserve iterators

Section: 23.2.2 [container.requirements.general] Status: Open Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2023-01-20

Priority: 3

View other active issues in [container.requirements.general].

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Discussion:

23.2.2 [container.requirements.general]/10 says that unless otherwise specified, "no swap() function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. [Note: The end() iterator does not refer to any element, so it may be invalidated. — end note]". However, move constructors and move assignment operators aren't given similar invalidation guarantees. The guarantees need several exceptions, so I do not believe that blanket language like /11 "Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container." is applicable.

[2014-02-13 Issaquah]

General agreeement on intent, several wording nits and additional paragraphs to hit.

STL to provide updated wording. Move to Open.

[2015-02 Cologne]

AM: in the proposed wording, I'd like to mention that the iterators now refer to elements of a different container. I think we're saying something like this somewhere. JY: There's some wording like that for swap I think. TK: It's also in list::splice(). DK to JY: 23.2.1p9.

VV: The issue says that STL was going to propose new wording. Has he done that? AM: I believe we're looking at that. GR: The request touches on multiple paragraphs, and this PR has only one new paragraph, so this looks like it's not up-to-date. MC: This was last updated a year ago in Issaquah.

Conclusion: Skip, not up to date.

[2015-06, Telecon]

Still waiting for updated wording

[2015-08 Chicago]

Still waiting for updated wording

[2018-08-23 Batavia Issues processing]

Priority to 3

[2023-01-20; std-proposals post]

Emile Cormier observed that the proposed resolution of this issue contradicts with changes made by LWG 2839⁽ⁱ⁾. Specifially, the current draft does not require container elements to be preserved on self-move-assignment. If this issue is accepted, it would either need to allow iterator invalidation on self-move-assignment or remove the "If a and rv do not refer to the same object" changes added to the container requirements by LWG 2839⁽ⁱ⁾.

Proposed resolution:

This wording is relative to N3691.

1. In 23.2.2 [container.requirements.general]/10 change as indicated:

   -10- Unless otherwise specified (see 23.2.4.1, 23.2.5.1, 23.3.3.4, and 23.3.7.5) all container types defined in this Clause meet the following additional requirements:

   • […]

   • no copy constructor or assignment operator of a returned iterator throws an exception.

   • no move constructor (or move assignment operator when allocator_traits<allocator_type>::propagate_on_container_move_assignment::value is true) of a container (except for array) invalidates any references, pointers, or iterators referring to the elements of the source container. [Note: The end() iterator does not refer to any element, so it may be invalidated. — end note]

   • no swap() function throws an exception.

   • no swap() function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. [Note: The end() iterator does not refer to any element, so it may be invalidated. — end note]

2331⁽ⁱ⁾. regex_constants::collate's effects are inaccurately summarized

Section: 28.6.4.2 [re.synopt] Status: Open Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2016-01-28

Priority: 3

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Discussion:

The table in 28.6.4.2 [re.synopt]/1 says that regex_constants::collate "Specifies that character ranges of the form "[a-b]" shall be locale sensitive.", but 28.6.12 [re.grammar]/14 says that it affects individual character comparisons too.

[2012-02-12 Issaquah : recategorize as P3]

Marshall Clow: 28.13/14 only applies to ECMAScript

All: we're unsure

Jonathan Wakely: we should ask John Maddock

Move to P3

[2014-5-14, John Maddock response]

The original intention was the original wording: namely that collate only made character ranges locale sensitive. To be frank it's a feature that's probably hardly ever used (though I have no real hard data on that), and is a leftover from early POSIX standards which required locale sensitive collation for character ranges, and then later changed to implementation defined if I remember correctly (basically nobody implemented locale-dependent collation).

So I guess the question is do we gain anything by requiring all character-comparisons to go through the locale when this bit is set? Certainly it adds a great deal to the implementation effort (it's not what Boost.Regex has ever done). I guess the question is are differing code-points that collate identically an important use case? I guess there might be a few Unicode code points that do that, but I don't know how to go about verifying that.

STL:

If this was unintentional, then 28.6.4.2 [re.synopt]/1's table should be left alone, while 28.6.12 [re.grammar]/14 should be changed instead.

Jeffrey Yasskin:

This page mentions that [V] in Swedish should match "W" in a perfect world.

However, the most recent version of TR18 retracts both language-specific loose matches and language-specific ranges because "for most full-featured regular expression engines, it is quite difficult to match under code point equivalences that are not 1:1" and "tailored ranges can be quite difficult to implement properly, and can have very unexpected results in practice. For example, languages may also vary whether they consider lowercase below uppercase or the reverse. This can have some surprising results: [a-Z] may not match anything if Z < a in that locale."

ECMAScript doesn't include collation at all.

IMO, +1 to changing 28.13 instead of 28.5.1. It seems like we'd be on fairly solid ground if we wanted to remove regex_constants::collate entirely, in favor of named character classes, but of course that's not for this issue.

Proposed resolution:

This wording is relative to N3691.

1. In 28.6.4.2 [re.synopt]/1, Table 138 — "syntax_option_type effects", change as indicated:

   Table 138 — syntax_option_type effects

   Element	Effect(s) if set
   …
   collate	Specifies that character ranges of the form "[a-b]"comparisons and character range comparisons shall be locale sensitive.
   …

2338⁽ⁱ⁾. §[re.traits]/7 expects of locale facets something not guaranteed by [locale.facet]/4

Section: 28.6.6 [re.traits], 28.3.3.1.2.2 [locale.facet] Status: Open Submitter: Sergey Zubkov Opened: 2013-10-15 Last modified: 2016-02-01

Priority: 3

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Discussion:

28.6.6 [re.traits]/7, begins with "if typeid(use_facet<collate<charT> >) == typeid(collate_byname<charT>)", which appears to be pseudocode with the intention to convey that the collate facet has not been replaced by the user. Cf. the wording in N1429 "there is no portable way to implement transform_primary in terms of std::locale, since even if the sort key format returned by std::collate_byname<>::transform is known and can be converted into a primary sort key, the user can still install their own custom std::collate implementation into the locale object used, and that can use any sort key format they see fit.".

Taken literally, 28.6.6 [re.traits]/7 appears to imply that named locales are required to hold their collate facets with dynamic type std::collate_byname<charT>, which is in fact true in some implementations (e.g libc++), but not others (e.g. libstdc++). This does not follow from the description of _byname in 28.3.3.1.2.2 [locale.facet]/4, which is only required to provide equivalent semantics, to the named locale's facet, not to actually be one.

[2015-05-06 Lenexa: Move to Open]

MC, RP: Consequence of failing to follow the rule is UB.

MC: Tightening of requirements.

RP: It should be this way, we just didn't impose it before.

MC: Second change is a bug fix, original code didn't work.

TK: Doesn't seem to make things worse.

Bring up in larger group tomorrow.

JW arrives.

JW: libstdc++ violates this due to two std::string ABIs.

JW: This prevents installing a type derived from Facet_byname, constrains the implementor from using a smarter derived class version.

JW: Can't look at facet id to detect replacement, because replacements have the same id.

RP: Can you give it multiple ids through multiple inheritance?

JW: No, the facet mechanism wouldn't like that.

JW: We should also ask Martin Sebor, he's implemented this stuff recently.

MC: Sounds like this resolution doesn't work, need a better solution.

JW: Write in words "if the facet has not been replaced by the user", the implementation knows how to detect that, but not like this.

RP: User RE traits need to detect this too.

JW: =(

Move to Open, JW will invite Martin Sebor to join LWG for discussion.

Later ...

JW: This is not needed for user specializations after all.

MC: Agree, [re.traits]/7 only applies to the stdlib traits.

NM: Effects: doesn't make sense.

JW, NM, Martin Sebor to come up with new wording.

Proposed resolution:

This wording is relative to N3691.

1. Modify 28.3.3.1.2.2 [locale.facet]/4 as indicated:

   For some standard facets a standard "..._byname" class, derived from it, implements the virtual function semantics equivalent toprovided by that facet of the locale constructed by locale(const char*) with the same name. Each such facet provides a constructor that takes a const char* argument, which names the locale, and a refs argument, which is passed to the base class constructor. Each such facet also provides a constructor that takes a string argument str and a refs argument, which has the same effect as calling the first constructor with the two arguments str.c_str() and refs. If there is no "..._byname" version of a facet, the base class implements named locale semantics itself by reference to other facets. For any locale loc constructed by locale(const char*) and facet Facet that has a corresponding standard Facet_byname class, typeid(use_facet<Facet>(loc)) == typeid(Facet_byname).

2. Modify 28.6.6 [re.traits]/7 as indicated:

   template <class ForwardIterator>
     string_type transform_primary(ForwardIterator first, ForwardIterator last) const;
   

   -7- Effects: if typeid(use_facet<collate<charT> >(getloc())) == typeid(collate_byname<charT>) and the form of the sort key returned by collate_byname<charT>::transform(first, last) is known and can be converted into a primary sort key then returns that key, otherwise returns an empty string.

2342⁽ⁱ⁾. User conversion to wchar_t const* or to wchar_t not invoked for operator<<

Section: 31.7.6.2 [ostream] Status: New Submitter: Alf P. Steinbach Opened: 2013-10-29 Last modified: 2016-01-28

Priority: 4

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Discussion:

For wide streams argument types wchar_t const* and wchar_t are supported only as template parameters. User defined conversions are not considered for template parameter matching. Hence inappropriate overloads of operator<< are selected when an implicit conversion is required for the argument, which is inconsistent with the behavior for char const* and char, is unexpected, and is a useless result.

Demonstration:

#include <iostream>

struct Byte_string
{ 
  operator char const*() const { return "Hurray, it works!"; } 
};

struct Wide_string
{ 
  operator wchar_t const*() const { return L"Hurray, it works!"; } 
};

struct Byte_ch
{ 
  operator char() const { return 'X'; } 
};

struct Wide_ch
{ 
  operator wchar_t() const { return L'X'; } 
};

auto main() -> int
{
  using namespace std;
  wcout << "'X' as char value   : " << Byte_ch() << endl;
  wcout << "'X' as wchar_t value: " << Wide_ch() << endl;
  wcout << "Byte string pointer : " << Byte_string() << endl;
  wcout << "Wide string pointer : " << Wide_string() << endl;
}

Example output:

'X' as char value   : X
'X' as wchar_t value: 88
Byte string pointer : Hurray, it works!
Wide string pointer : 000803C8

Proposed resolution:

This wording is relative to N3797.

1. Modify 31.7.6.2 [ostream], class template basic_ostream synopsis, as indicated:

   namespace std {
   […]
   
   // 27.7.3.6.4 character inserters
   template<class charT, class traits>
     basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
                                             charT);
   template<class charT, class traits>
     basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
                                             char);
   template<class traits>
     basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>&,
                                            char);
   template<class traits>
     basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>&,
                                               wchar_t);
   […]
   
   template<class charT, class traits>
     basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
                                             const charT*);
   template<class charT, class traits>
     basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
                                             const char*);
   template<class traits>
     basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>&,
                                            const char*);
   template<class traits>
     basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>&,
                                               const wchar_t*);
   […]
   }
   
   

2. Modify 31.7.6.3.4 [ostream.inserters.character] as indicated: [Drafting note: The replacement of os by out in p1 and the insertion of "out." in p4 just fix two obvious typos — end drafting note]

   template<class charT, class traits>
     basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                             charT c);
   template<class charT, class traits>
     basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                             char c);
   // specialization
   template<class traits>
     basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                            char c);
   template<class traits>
     basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>& out,
                                               wchar_t c);
   
   // signed and unsigned
   template<class traits>
     basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                             signed char c);
   template<class traits>
     basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                             unsigned char c);
   

   -1- Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of out. Constructs a character sequence seq. If c has type char and the character type of the stream is not char, then seq consists of out.widen(c); otherwise seq consists of c. Determines padding for seq as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Inserts seq into out. Calls osout.width(0).

   -2- Returns: out.

   template<class charT, class traits>
     basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                             const charT* s);
   template<class charT, class traits>
     basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                             const char* s);
   template<class traits>
     basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                            const char* s);
   template<class traits>
     basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>& out,
                                               const wchar_t* s);
   											
   template<class traits>
     basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                            const signed char* s);
   template<class traits>
     basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                            const unsigned char* s);
   

   -3- Requires: s shall not be a null pointer.

   -4- Effects: Behaves like a formatted inserter (as described in 31.7.6.3.1 [ostream.formatted.reqmts]) of out. Creates a character sequence seq of n characters starting at s, each widened using out.widen() (27.5.5.3), where n is the number that would be computed as if by:

   • traits::length(s) for the following overloads:

     • where the first argument is of type basic_ostream<charT, traits>& and the second is of type const charT*,

     • and also for the overload where the first argument is of type basic_ostream<char, traits>& and the second is of type const char*,

     • where the first argument is of type basic_ostream<wchar_t, traits>& and the second is of type const wchar_t*,

   • std::char_traits<char>::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const char*,

   • traits::length(reinterpret_cast<const char*>(s)) for the other two overloads.

   Determines padding for seq as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Inserts seq into out. Calls out.width(0).

   -5- Returns: out.