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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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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]

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> {
   >     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 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 (16.4.4.3 [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>`.