This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++23 status.

3747. ranges::uninitialized_copy_n, ranges::uninitialized_move_n, and ranges::destroy_n should use std::move

Section: 27.11.5 [uninitialized.copy], 27.11.6 [uninitialized.move], 27.11.9 [specialized.destroy] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-28 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [uninitialized.copy].

View all issues with C++23 status.

Discussion:

Currently, ranges::uninitialized_copy_n has the following equivalent Effects:

auto t = uninitialized_copy(counted_iterator(ifirst, n),
                            default_sentinel, ofirst, olast);
return {std::move(t.in).base(), t.out};

Given that ifirst is just an input_iterator which is not guaranteed to be copyable, we should move it into counted_iterator. The same goes for ranges::uninitialized_move_n and ranges::destroy_n.

[2022-08-23; Reflector poll]

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

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 27.11.5 [uninitialized.copy] as indicated:

   namespace ranges {
     template<input_iterator I, nothrow-forward-iterator O, nothrow-sentinel-for<O> S>
       requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
       uninitialized_copy_n_result<I, O>
         uninitialized_copy_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast);
   }
   

   -9- Preconditions: [ofirst, olast) does not overlap with ifirst + [0, n) .

   -10- Effects: Equivalent to:

   auto t = uninitialized_copy(counted_iterator(std::move(ifirst), n),
                               default_sentinel, ofirst, olast);
   return {std::move(t.in).base(), t.out};
   

2. Modify 27.11.6 [uninitialized.move] as indicated:

   namespace ranges {
     template<input_iterator I, nothrow-forward-iterator O, nothrow-sentinel-for<O> S>
       requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
       uninitialized_move_n_result<I, O>
         uninitialized_move_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast);
   }
   

   -8- Preconditions: [ofirst, olast) does not overlap with ifirst + [0, n) .

   -9- Effects: Equivalent to:

   auto t = uninitialized_move(counted_iterator(std::move(ifirst), n),
                               default_sentinel, ofirst, olast);
   return {std::move(t.in).base(), t.out};
   

3. Modify 27.11.9 [specialized.destroy] as indicated:

   namespace ranges {
     template<nothrow-input-iterator I>
       requires destructible<iter_value_t<I>>
       constexpr I destroy_n(I first, iter_difference_t<I> n) noexcept;
   }
   

   -5- Effects: Equivalent to:

   return destroy(counted_iterator(std::move(first), n), default_sentinel).base();