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4553. Wording for FR-025-246 25.7.18.2 Add a reserve_hint function to concat_view

Section: 25.7.18.2 [range.concat.view] Status: Immediate Submitter: Hui Xie Opened: 2026-03-24 Last modified: 2026-03-25

Priority: Not Prioritized

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Discussion:

Addresses FR-025-246

LEWG approved the NB comment "FR-025-246 25.7.18.2 Add a reserve_hint function to concat_view" without wording. This issue proposes the wording.

[Croydon 2026-03-24; move to Immediate]

Proposed resolution:

This wording is relative to N5032.

  1. Modify 25.7.18.2 [range.concat.view] as indicated:

    namespace std::ranges {
  […]
  template<input_range... Views>
    requires (view<Views> && ...) && (sizeof...(Views) > 0) &&
              concatable<Views...>
  class concat_view : public view_interface<concat_view<Views...>> {
    […]
    constexpr auto size() requires (sized_range<Views> && ...);
    constexpr auto size() const requires (sized_range<const Views> && ...);
    constexpr auto reserve_hint() requires (approximately_sized_range<Views> && ...);
    constexpr auto reserve_hint() const requires (approximately_sized_range<const Views> && ...);
  };
}
[…]

constexpr auto size() requires (sized_range<Views> && ...);
constexpr auto size() const requires (sized_range<const Views> && ...);

-8- Effects: Equivalent to:

    
return apply(
  [](auto... sizes) {
    using CT = make-unsigned-like-t<common_type_t<decltype(sizes)...>>;
    return (CT(sizes) + ...);
  },
  tuple-transform(ranges::size, views_));

    


constexpr auto reserve_hint() requires (approximately_sized_range<Views> && ...);
constexpr auto reserve_hint() const requires (approximately_sized_range<const Views> && ...);

-9- Effects: Equivalent to:

    
return apply(
  [](auto... sizes) {
    using CT = make-unsigned-like-t<common_type_t<decltype(sizes)...>>;
    return (CT(sizes) + ...);
  },
  tuple-transform(ranges::reserve_hint, views_));