# 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 = Range<T> && Semiregular<T> && enable_view<T>; | (1) |
template<class T> inline constexpr bool enable_view = /*see description*/ ; | (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, as follows:

For a type T, the default value of enable_view<T> is:

- If DerivedFrom<T, view_base> is true, enable_view<T> is true.
- Otherwise, if T is a specialization of class template std::initializer_list, std::set, std::multiset, std::unordered_set, std::unordered_multiset, std::match_results, enable_view<T> is false
- Otherwise, if both T and const T model `Range` and iter_reference_t<iterator_t<T>> is not the same type as iter_reference_t<iterator_t<const T>>, enable_view<T> is false. (in other words, deep constness implies element ownership, shallow constness implies reference (view) semantics)
- Otherwise, enable_view<T> is true.

3) Deriving from view_base enables `Range` types to model View.

### Notes

Users may derive from view_base or specialize enable_view to true for types which model `View`, and specialize enable_view to false for types which do not.

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