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std::ranges::view, std::ranges::enable_view, std::ranges::view_base

From cppreference.com
< cpp‎ | ranges
 
 
 
Defined in header <ranges>
template<class T>
concept view = ranges::range<T> && std::semiregular<T> && ranges::enable_view<T>;
(1)
template<class T>
inline constexpr bool enable_view = std::derived_from<T, view_base>;
(2)
struct view_base { };
(3)
1) The view concept specifies the requirements of a range type that has constant time copy, move, and assignment operations (e.g. a pair of iterators, or a generator Range that creates its elements on-demand. Notably, the standard library containers are ranges, but not views)
2) The enable_view variable template is used to indicate that whether a range is a view. By default, a type is considered a view if it is publicly and unambiguously derived from view_base.
Users may specialize enable_view to true for cv-unqualified program-defined types which model view, and false for types which do not. Such specializations must be usable in constant expressions and have type const bool.
3) Deriving from view_base enables range types to model view.