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

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
template<class T> concept view = ranges::range<T> && std::movable<T> && ranges::enable_view<T>; | (1) | (since C++20)
template<class T> inline constexpr bool enable_view = std::derived_from<T, view_base> || /*is-derived-from-view-interface*/<T>; | (2) | (since C++20)
struct view_base { }; | (3) | (since C++20)

1) The `view` concept specifies the requirements of a `range` type that has suitable semantic properties for use in constructing range adaptor pipelines.
2) The `enable_view` variable template is used to indicate whether a `range` is a `view`. /*is-derived-from-view-interface*/<T> is true if and only if `T` has exactly one public base class ranges::view_interface<U> for some type `U`, and `T` has no base classes of type ranges::view_interface<V> for any other type `V`.
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`.

### Semantic requirements

1) `T` models `view` only if:

- 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.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P2325R3 | C++20 | `view` required `default_initializable` | does not require
LWG 3549 | C++20 | `enable_view` did not detect inheritance from `view_interface` | detects
P2415R2 | C++20 | the restriction on the time complexity of destruction was too strict | relaxed

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