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)

Atomic operations library (C++11)

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Technical Specifications

 

Ranges library

Range access
begincbegin endcend	rbegincrbegin rendcrend	size empty datacdata
Range primitives
iterator_tsentinel_t	range_difference_trange_value_t	range_reference_trange_rvalue_reference_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Semiregular wrapper
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	views::counted
Adaptors
all_viewviews::all join_viewviews::join take_viewviews::take basic_istream_viewistream_view	ref_view                                split_viewviews::split take_while_viewviews::take_while elements_viewviews::elements	common_viewviews::common transform_viewviews::transform drop_viewviews::drop keys_viewviews::keys	reverse_viewviews::reverse filter_viewviews::filter drop_while_viewviews::drop_while values_viewviews::values

 

Defined in header <ranges>
template<class T> concept view = ranges::range<T> && std::semiregular<T> && ranges::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 std::derived_from<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 ranges::range_reference_t<T> is not the same type as ranges::range_reference_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 specialize enable_view to true for cv-unqualified program-defined types which model view, and false for types which do not. Such specializations shall be usable in constant expressions and have type const bool.