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3204. sub_match::swap only swaps the base class

Section: 30.8 [re.submatch] Status: New Submitter: Jonathan Wakely Opened: 2019-05-07 Last modified: 2020-09-06

Priority: 3

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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;

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]

Proposed resolution:

This wording is relative to N4861.

  1. Modify 30.8 [re.submatch], class template sub_match synopsis, as indicated:

    template<class BidirectionalIterator>
    class sub_match : public pair<BidirectionalIterator, BidirectionalIterator> {
      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 30.8.2 [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 to satisfy the bidirectional iterator requirements. — end drafting note]

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

    -?- Effects: Equivalent to:

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

    -?- Remarks: The expression inside noexcept is equivalent to is_nothrow_swappable_v<BidirectionalIterator>.