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3355. The memory algorithms should support move-only input iterators introduced by P1207

Section: 27.11.5 [uninitialized.copy], 27.11.6 [uninitialized.move] Status: C++20 Submitter: Corentin Jabot Opened: 2019-11-12 Last modified: 2021-02-25

Priority: 2

View all other issues in [uninitialized.copy].

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Discussion:

P1207 introduced move-only input iterators but did not modify the specialized memory algorithms to support them.

[2020-01 Priority set to 2 after review on the reflector.]

[2020-02 Status to Immediate on Friday morning in Prague.]

Proposed resolution:

This wording is relative to N4842.

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

   namespace ranges {
     template<input_iterator I, sentinel_for<I> S1,
              no-throw-forward-iterator O, no-throw-sentinel<O> S2>
         requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
       uninitialized_copy_result<I, O>
         uninitialized_copy(I ifirst, S1 ilast, O ofirst, S2 olast);
     template<input_range IR, no-throw-forward-range OR>
         requires constructible_from<range_value_t<OR>, range_reference_t<IR>>
       uninitialized_copy_result<safe_iterator_t<IR>, safe_iterator_t<OR>>
         uninitialized_copy(IR&& in_range, OR&& out_range);
   }
   

   -4- Preconditions: [ofirst, olast) shall not overlap with [ifirst, ilast).

   -5- Effects: Equivalent to:

   for (; ifirst != ilast && ofirst != olast; ++ofirst, (void)++ifirst) {
     ::new (voidify(*ofirst)) remove_reference_t<iter_reference_t<O>>(*ifirst);
   }
   return {std::move(ifirst), ofirst};
   

   […]

   namespace ranges {
     template<input_iterator I, no-throw-forward-iterator O, no-throw-sentinel<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) shall not overlap with [ifirst, n).

   -10- Effects: Equivalent to:

   auto t = uninitialized_copy(counted_iterator(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, sentinel_for<I> S1,
              no-throw-forward-iterator O, no-throw-sentinel<O> S2>
         requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
       uninitialized_move_result<I, O>
         uninitialized_move(I ifirst, S1 ilast, O ofirst, S2 olast);
     template<input_range IR, no-throw-forward-range OR>
         requires constructible_from<range_value_t<OR>, range_rvalue_reference_t<IR>>
       uninitialized_move_result<safe_iterator_t<IR>, safe_iterator_t<OR>>
         uninitialized_move(IR&& in_range, OR&& out_range);
   }
   

   -3- Preconditions: [ofirst, olast) shall not overlap with [ifirst, ilast).

   -4- Effects: Equivalent to:

   for (; ifirst != ilast && ofirst != olast; ++ofirst, (void)++ifirst) {
     ::new (voidify(*ofirst)) 
       remove_reference_t<iter_reference_t<O>>(ranges::iter_move(*ifirst));
   }
   return {std::move(ifirst), ofirst};
   

   […]

   namespace ranges {
     template<input_iterator I, no-throw-forward-iterator O, no-throw-sentinel<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) shall not overlap with [ifirst, n).

   -9- Effects: Equivalent to:

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