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3204. sub_match::swap only swaps the base class

Section: 28.6.8 [re.submatch] Status: C++23 Submitter: Jonathan Wakely Opened: 2019-05-07 Last modified: 2023-11-22

Priority: 3

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Discussion:

sub_match<I> derives publicly from pair<I,I>, and so inherits pair::swap(pair&). This means that the following program fails:

#include <regex>
#include <cassert>

int main()
{
  std::sub_match<const char*> a, b;
  a.matched = true;
  a.swap(b);
  assert(b.matched);
}

The pair::swap(pair&) member should be hidden by a sub_match::swap(sub_match&) member that does the right thing.

[2019-06-12 Priority set to 3 after reflector discussion]

[2020-05-01; Daniel adjusts wording to recent working draft]

[2022-04-25; Daniel adjusts wording to recent working draft]

In addition the revised wording uses the new standard phrase "The exception specification is equivalent to"

Previous resolution [SUPERSEDED]:

This wording is relative to N4910.

1. Modify 28.6.8 [re.submatch], class template sub_match synopsis, as indicated:

   template<class BidirectionalIterator>
   class sub_match : public pair<BidirectionalIterator, BidirectionalIterator> {
   public:
     […]
     int compare(const sub_match& s) const;
     int compare(const string_type& s) const;
     int compare(const value_type* s) const;
     
     void swap(sub_match& s) noexcept(see below);
   };
   

2. Modify 28.6.8.2 [re.submatch.members] as indicated:

   int compare(const value_type* s) const;
   

   […]

   void swap(sub_match& s) noexcept(see below);
   

   [Drafting note: The swappable requirement should really be unnecessary because Cpp17Iterator requires it, but there is no wording that requires BidirectionalIterator in Clause 28.6 [re] in general meets the bidirectional iterator requirements. Note that the definition found in 26.2 [algorithms.requirements] does not extend to 28.6 [re] normatively. — end drafting note]

   -?- Preconditions: Lvalues of type BidirectionalIterator are swappable (16.4.4.3 [swappable.requirements]).

   -?- Effects: Equivalent to:

   this->pair<BidirectionalIterator, BidirectionalIterator>::swap(s);
   std::swap(matched, s.matched);
   

   -?- Remarks: The exception specification is equivalent to is_nothrow_swappable_v<BidirectionalIterator>.

[2022-11-06; Daniel comments and improves wording]

With the informal acceptance of P2696R0 by LWG during a pre-Kona telecon, we should use the new requirement set Cpp17Swappable instead of the "LValues are swappable" requirements.

[2022-11-30; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917 and assumes the acceptance of P2696R0.

1. Modify 28.6.8 [re.submatch], class template sub_match synopsis, as indicated:

   template<class BidirectionalIterator>
   class sub_match : public pair<BidirectionalIterator, BidirectionalIterator> {
   public:
     […]
     int compare(const sub_match& s) const;
     int compare(const string_type& s) const;
     int compare(const value_type* s) const;
     
     void swap(sub_match& s) noexcept(see below);
   };
   

2. Modify 28.6.8.2 [re.submatch.members] as indicated:

   int compare(const value_type* s) const;
   

   […]

   void swap(sub_match& s) noexcept(see below);
   

   [Drafting note: The Cpp17Swappable requirement should really be unnecessary because Cpp17Iterator requires it, but there is no wording that requires BidirectionalIterator in Clause 28.6 [re] in general meets the bidirectional iterator requirements. Note that the definition found in 26.2 [algorithms.requirements] does not extend to 28.6 [re] normatively. — end drafting note]

   -?- Preconditions: BidirectionalIterator meets the Cpp17Swappable requirements (16.4.4.3 [swappable.requirements]).

   -?- Effects: Equivalent to:

   this->pair<BidirectionalIterator, BidirectionalIterator>::swap(s);
   std::swap(matched, s.matched);
   

   -?- Remarks: The exception specification is equivalent to is_nothrow_swappable_v<BidirectionalIterator>.