2195. Missing constructors for match_results

Section: 31.10 [re.results] Status: Open Submitter: Daniel Krügler Opened: 2012-10-06 Last modified: 2016-02-10

Priority: 3

View all other issues in [re.results].

View all issues with Open status.

Discussion:

The requirement expressed in 31.10 [re.results] p2

The class template match_results shall satisfy the requirements of an allocator-aware container and of a sequence container, as specified in 26.2.3 [sequence.reqmts], except that only operations defined for const-qualified sequence containers are supported.

can be read to require the existence of the described constructors from as well, but they do not exist in the synopsis.

The missing sequence constructors are:

match_results(initializer_list<value_type>);
match_results(size_type, const value_type&);
template<class InputIterator> match_results(InputIterator, InputIterator);

The missing allocator-aware container constructors are:

match_results(const match_results&, const Allocator&);
match_results(match_results&&, const Allocator&);

It should be clarified, whether (a) constructors are an exception of above mentioned operations or (b) whether at least some of them (like those accepting a match_results value and an allocator) should be added.

As visible in several places of the standard (including the core language), constructors seem usually to be considered as "operations" and they certainly can be invoked for const-qualified objects.

The below given proposed resolution applies only the minimum necessary fix, i.e. it excludes constructors from above requirement.

[2013-04-20, Bristol]

Check current implementations to see what they do and, possibly, write a paper.

[2013-09 Chicago]

Ask Daniel to update the proposed wording to include the allocator copy and move constructors.

[2014-01-18 Daniel changes proposed resolution]

Previous resolution from Daniel [SUPERSEDED]:

  1. Change 31.10 [re.results] p2 as indicated:

    The class template match_results shall satisfy the requirements of an allocator-aware container and of a sequence container, as specified in 26.2.3 [sequence.reqmts], except that only operations defined for const-qualified sequence containers that are not constructors are supported.

[2015-05-06 Lenexa]

MC passes important knowledge to EF.

VV, RP: Looks good.

TK: Second form should be conditionally noexcept

JY: Sequence constructors are not here, but mentioned in the issue writeup. Why?

TK: That would have been fixed by the superseded wording.

JW: How does this interact with Mike Spertus' allocator-aware regexes? [...] Perhaps it doesn't.

JW: Can't create match_results, want both old and new resolution.

JY: It's problematic that users can't create these, but not this issue.

VV: Why conditional noexcept?

MC: Allocator move might throw.

JW: Update superseded wording to "only non-constructor operations that are"?

MC: Only keep superseded, but append "and the means of constructing match_results are limited to [...]"?

JY: Bullet 4 paragraph 2 needs to address the allocator constructor.

Assigned to JW for drafting.

[2015-10, Kona Saturday afternoon]

STL: I want Mike Spertus to be aware of this issue.

Proposed resolution:

This wording is relative to N3936.

  1. Change 31.10 [re.results] p4, class template match_results synopsis, as indicated:

    […]
    // 28.10.1, construct/copy/destroy:
    explicit match_results(const Allocator& a = Allocator());
    match_results(const match_results& m);
    match_results(const match_results& m, const Allocator& a);
    match_results(match_results&& m) noexcept;
    match_results(match_results&& m, const Allocator& a) noexcept;
    […]
    
  2. Change 31.10.1 [re.results.const] as indicated: [Drafting note: Paragraph 6 as currently written, makes not much sense, because the noexcept does not allow any exception to propagate. Further-on, the allocator requirements do not allow for throwing move constructors. Deleting it seems to be near to editorial — end drafting note]

    match_results(const match_results& m);
    match_results(const match_results& m, const Allocator& a);
    

    -4- Effects: Constructs an object of class match_results, as a copy of m.

    match_results(match_results&& m) noexcept;
    match_results(match_results&& m, const Allocator& a) noexcept;
    

    -5- Effects: Move-constructs an object of class match_results from m satisfying the same postconditions as Table 142. AdditionallyFor the first form, the stored Allocator value is move constructed from m.get_allocator().

    -6- Throws: Nothing if the allocator's move constructor throws nothing.