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

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

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Technical Specifications

 

Ranges library

Range access
begincbegin endcend	rbegincrbegin rendcrend	size empty datacdata
Range primitives
iterator_t	sentinel_t
Dangling iterator handling
dangling	safe_iterator_t	safe_subrange_t
Range concepts
Range SizedRange View	InputRange OutputRange ForwardRange	BidirectionalRange RandomAccessRange ContiguousRange	CommonRange ViewableRange
Views
view_interface	subrange
Semiregular wrapper
Factories
empty_viewview::empty	single_viewview::single	iota_viewview::iota	view::counted
Adaptors
all_viewview::all ref_view filter_viewview::filter	transform_viewview::transform                                take_viewview::take	join_viewview::join split_viewview::split	common_viewview::common reverse_viewview::reverse

 

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.