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

template<class T>
concept view = ranges::range<T> && std::movable<T> && ranges::enable_view<T>;

template<class T>
inline constexpr bool enable_view =
    std::derived_from<T, view_base> || /*is-derived-from-view-interface*/<T>;

struct view_base { };

Semantic requirements

• move construction of T has constant time complexity, and
• if \(\scriptsize N\)N copies and/or moves are made from a T object holding \(\scriptsize M\)M elements, then these \(\scriptsize N\)N objects have \(\scriptsize \mathcal{O}{(N+M)}\)𝓞(N+M) destruction (which implies that a moved-from view object has \(\scriptsize \mathcal{O}{(1)}\)𝓞(1) destruction), and
• either std::copy_constructible<T> is false, or copy construction of T has constant time complexity, and
• either std::copyable<T> is false, or copy assignment of T has no more time complexity than destruction followed by copy construction.

Notes

Examples of view types are:

• A range type that wraps a pair of iterators, e.g., std::ranges::subrange<I>.
• A range type that holds its elements by std::shared_ptr and shares ownership with all its copies.
• A range type that generates its elements on demand, e.g., std::ranges::iota_view.

A copyable container such as std::vector<std::string> generally does not meet the semantic requirements of view since copying the container copies all of the elements, which cannot be done in constant time.

While views were originally described as cheaply copyable and non-owning ranges, a type is not required to be copyable or non-owning for it to model view. However, it must still be cheap to copy (if it is copyable), move, assign, and destroy, so that range adaptors will not have unexpected complexity.

By default, a type modeling movable and range is considered a view if it is publicly and unambiguously derived from view_base, or exactly one specialization of std::ranges::view_interface.

Defect reports

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