Defined in header <ranges>
template<class T>
concept viewable_range =
  ranges::range<T> &&
  ((ranges::view<std::remove_cvref_t<T>> &&
    std::constructible_from<std::remove_cvref_t<T>, T>) ||
   (!ranges::view<std::remove_cvref_t<T>> &&
    (std::is_lvalue_reference_v<T> ||
     (std::movable<std::remove_reference_t<T>> && !/*is-initializer-list*/<T>))));
(since C++20)

The viewable_range concept is a refinement of range that describes a range that can be converted into a view through views::all.

The constant /*is-initializer-list*/<T> is true if and only if std::remove_cvref_t<T> is a specialization of std::initializer_list.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3481 C++20 viewable_range accepted an lvalue of a move-only view rejects
P2415R2 C++20 viewable_range only accepted non-view rvalues that are borrowed_range accepts more types

See also

a view that includes all elements of a range
(alias template) (range adaptor object)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.