How to write a concept for a move input iterator?

Question

I want to write a function that moves the data of a container into another container using an iterator pair.

I have the following concept:

template <typename Iter, typename T>
concept move_input_iterator = std::input_iterator<Iter> && std::same_as<std::iter_value_t<Iter>, T&&>;

And I use it like so:

template <move_input_iterator<sound_sample> Iter>
sound create_sound(const sound_details& details, Iter begin, Iter end)
{
    return sound{
        .details = details,
        .samples = {begin,end}
    };
}

sounds[sound_key] = create_sound(
    {
        .pitch_min = short(sound_info->pitch_min),
        .pitch_max = short(sound_info->pitch_max),
        .volume_min = short(sound_info->volume_min),
        .volume_max = short(sound_info->volume_max),
        .is_looped = /*TODO:*/ true,
        .radius = /*TODO:*/ 0,
    },
    std::make_move_iterator(samples.begin()),
    std::make_move_iterator(samples.end()));

(sound::samples is a std::vector<sound_sample> )

I receive the following error:

Game/src/common/sound.h:33:8: note: candidate template ignored: constraints not satisfied [with Iter = move_iterator<sound_sample *>]
   33 |         sound create_sound(const sound_details& details, Iter begin, Iter end)
      |               ^
Game/src/common/sound.h:32:12: note: because 'move_input_iterator<std::move_iterator<raid::sound_sample *>, sound_sample>' evaluated to false
   32 |         template <move_input_iterator<sound_sample> Iter>
      |                   ^
Game/src/common/iterator.h:12:61: note: because 'std::same_as<std::iter_value_t<move_iterator<sound_sample *> >, raid::sound_sample &&>' evaluated to false
   12 |         concept move_input_iterator = std::input_iterator<Iter> && std::same_as<std::iter_value_t<Iter>, T&&>;
      |                                                                    ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/15.2.1/../../../../include/c++/15.2.1/concepts:65:9: note: because '__detail::__same_as<raid::sound_sample, raid::sound_sample &&>' evaluated to false
   65 |       = __detail::__same_as<_Tp, _Up> && __detail::__same_as<_Up, _Tp>;
      |         ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/15.2.1/../../../../include/c++/15.2.1/concepts:59:27: note: because 'std::is_same_v<raid::sound_sample, raid::sound_sample &&>' evaluated to false
   59 |       concept __same_as = std::is_same_v<_Tp, _Up>;

I understand the error itself, but I do not understand why std::is_same_v fails, when std::make_move_iterator is supposed to make a move iterator (which should result in a sound_sample&&?)

康桓瑋 · Accepted Answer · 2025-10-24 08:42:21Z

8

iter_value_t always produces a cv-unqualified value type. What you need is iter_reference_t, which checks the return type of *it：

template <typename Iter, typename T>
concept move_input_iterator = 
  std::input_iterator<Iter> && 
  std::same_as<std::iter_reference_t<Iter>, T&&>;

Note that move_iterator::operator*() will return ranges::iter_move(it), where it is the underlying iterator. This return value can be a prvalue in addition to an rvalue reference. So you may want to check:

template <typename Iter, typename T>
concept move_input_iterator = 
  std::input_iterator<Iter> && 
  std::same_as<std::iter_value_t<Iter>, T> &&
  std::same_as<std::iter_rvalue_reference_t<Iter>, std::iter_reference_t<Iter>>;

edited Oct 24 at 8:42

answered Oct 24 at 8:34

康桓瑋

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Comments

Raunak Mishra · Accepted Answer · 2025-10-24 08:58:16Z

0

std::make_move_iterator creates an iterator whose value type is T&&.Your old concept required T to match exactly, so sound_sample ≠ sound_sample&&.Using std::remove_cvref_t normalizes both, treating them as sound_sample.

#include <iterator>
#include <type_traits>

template<typename Iter, typename T>
concept move_input_iterator =
    std::input_iterator<Iter> &&
    std::same_as<std::remove_cvref_t<std::iter_value_t<Iter>>, std::remove_cvref_t<T>>;

This code usess std::remove_cvref_t to ignore references and const, so both T and T&& are treated as the same underlying type.

answered Oct 24 at 8:58

Raunak Mishra

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1 Comment

Raildex Oct 24 at 9:04

This will match regular iterators, which is not what I want godbolt.org/z/b5cKvY7M1

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