3918. std::uninitialized_move/_n and guaranteed copy elision

Section: 27.11.6 [uninitialized.move] Status: LEWG Submitter: Jiang An Opened: 2023-04-04 Last modified: 2023-06-01 14:25:26 UTC

Priority: 3

View all issues with LEWG status.


Currently std::move is unconditionally used in std::uninitialized_move and std::uninitialized_move_n, which may involve unnecessary move construction if dereferencing the input iterator yields a prvalue.

The status quo was mentioned in paper issue #975, but no further process is done since then.

[2023-06-01; Reflector poll]

Set priority to 3 after reflector poll. Send to LEWG.

"P2283 wants to remove guaranteed elision here." "Poorly motivated, not clear anybody is using these algos with proxy iterators." "Consider using iter_move in the move algos."

Proposed resolution:

This wording is relative to N4944.

  1. Modify 27.11.1 [specialized.algorithms.general] as indicated:

    -3- Some algorithms specified in 27.11 [specialized.algorithms] make use of the following exposition-only functions voidify:

    template<class T>
      constexpr void* voidify(T& obj) noexcept {
        return addressof(obj);
    template<class I>
      decltype(auto) deref-move(const I& it) {
        if constexpr (is_lvalue_reference_v<decltype(*it)>)
          return std::move(*it);
          return *it;
  2. Modify 27.11.6 [uninitialized.move] as indicated:

    template<class InputIterator, class NoThrowForwardIterator>
      NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last,
                                                NoThrowForwardIterator result);

    -1- Preconditions: result + [0, (last - first)) does not overlap with [first, last).

    -2- Effects: Equivalent to:

    for (; first != last; (void)++result, ++first)
      ::new (voidify(*result))
        typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first));
    return result;
    template<class InputIterator, class Size, class NoThrowForwardIterator>
      pair<InputIterator, NoThrowForwardIterator>
        uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result);

    -6- Preconditions: result + [0, n) does not overlap with first + [0, n).

    -7- Effects: Equivalent to:

    for (; n > 0; ++result,(void) ++first, --n)
      ::new (voidify(*result))
        typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first));
    return {first, result};