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3595. Exposition-only classes proxy and postfix-proxy for common_iterator should be fully constexpr

Section: 24.5.5.4 [common.iter.access], 24.5.5.5 [common.iter.nav] Status: C++23 Submitter: Hewill Kang Opened: 2021-09-18 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [common.iter.access].

View all issues with C++23 status.

Discussion:

LWG 3574⁽ⁱ⁾ added constexpr to all member functions and friends of common_iterator to make it fully constexpr, but accidentally omitted the exposition-only classes proxy and postfix-proxy defined for some member functions. We should make these two classes fully constexpr, too.

[2021-09-24; Reflector poll]

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

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 24.5.5.4 [common.iter.access] as indicated:

   decltype(auto) operator->() const
     requires see below;
   

   -3- […]

   -4- […]

   -5- Effects:

   1. (5.1) — […]

   2. (5.2) — […]

   3. (5.3) — Otherwise, equivalent to: return proxy(*get<I>(v_)); where proxy is the exposition-only class:

      class proxy {
        iter_value_t<I> keep_;
        constexpr proxy(iter_reference_t<I>&& x)
          : keep_(std::move(x)) {}
      public:
        constexpr const iter_value_t<I>* operator->() const noexcept {
          return addressof(keep_);
        }
      };
      

2. Modify 24.5.5.5 [common.iter.nav] as indicated:

   decltype(auto) operator++(int);
   

   -4- […]

   -5- Effects: If I models forward_iterator, equivalent to:

   […]

   Otherwise, if […]

   […]

   Otherwise, equivalent to:

   postfix-proxy p(**this);
   ++*this;
   return p;
   

   where postfix-proxy is the exposition-only class:

   class postfix-proxy {
     iter_value_t<I> keep_;
     constexpr postfix-proxy(iter_reference_t<I>&& x)
       : keep_(std::forward<iter_reference_t<I>>(x)) {}
   public:
     constexpr const iter_value_t<I>& operator*() const noexcept {
       return keep_;
     }
   };