std::ranges::View, std::ranges::enable_view, std::ranges::view_base

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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http://en.cppreference.com/w/cpp/ranges/View