std::ranges::views::as_const, std::ranges::as_const_view

 

C++

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Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty iota_viewviews::iota	single_viewviews::single basic_istream_viewviews::istream	repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse zip_viewviews::zip (C++23)(C++23) as_const_viewviews::as_const (C++23)(C++23) as_rvalue_viewviews::as_rvalue (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   cartesian_product_viewviews::cartesian_product (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split join_with_viewviews::join_with (C++23)(C++23) chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

Defined in header <ranges>
template< ranges::view V >     requires ranges::input_range<V> class as_const_view : public ranges::view_interface<as_const_view<V>>	(1)	(since C++23)
namespace views {     inline constexpr /* unspecified */ as_const = /* unspecified */; }	(2)	(since C++23)
Call signature
template< ranges::viewable_range R >     requires /* see below */ constexpr ranges::view auto as_const( R&& r );		(since C++23)

1) A range adaptor that represents a view of underlying view that is also a constant_range. An as_const_view always has read-only elements (if not empty).

2) Range adaptor object. Let e be a subexpression, let T be decltype((e)), and let U be std::remove_cvref_t<T>. Then the expression views::as_const(e) is expression-equivalent to:

• views::all(e), if it is a well-formed expression and views::all_t<T> models constant_range;
• otherwise, std::span<const X, Extent>(e) for some type X and some extent Extent if U denotes std::span<X, Extent>;
• otherwise, ranges::ref_view(static_cast<const U&>(e)) if e is an lvalue, const U models constant_range, and U does not model view.
• otherwise, as_const_view{e}.

as_const_view always models constant_range, and it models the contiguous_range, random_access_range, bidirectional_range, forward_range, borrowed_range, common_range, and sized_range when the underlying view V models respective concepts.

Expression-equivalent

Expression e is expression-equivalent to expression f, if

• e and f have the same effects, and
• either both are constant subexpressions or else neither is a constant subexpression, and
• either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

Data members

Typical implementations of as_const_view only hold one non-static data member:

• the underlying view of type V (shown here as base_, the name is exposition only).

Member functions

(constructor) (C++23)	constructs an as_const_view (public member function)
base (C++23)	returns the underlying view V (public member function)
begin (C++23)	returns the beginning iterator of the as_const_view (public member function)
end (C++23)	returns the end iterator of the as_const_view (public member function)
size (C++23)	returns the size of the view if it is bounded (public member function)
Inherited from std::ranges::view_interface
empty (C++20)	Returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range. (public member function of std::ranges::view_interface<D>)
operator bool (C++20)	Returns whether the derived view is not empty. Provided if ranges::empty is applicable to it. (public member function of std::ranges::view_interface<D>)
data (C++20)	Gets the address of derived view's data. Provided if its iterator type satisfies contiguous_iterator. (public member function of std::ranges::view_interface<D>)
front (C++20)	Returns the first element in the derived view. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>)
back (C++20)	Returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range. (public member function of std::ranges::view_interface<D>)
operator[] (C++20)	Returns the nth element in the derived view. Provided if it satisfies random_access_range. (public member function of std::ranges::view_interface<D>)

std::ranges::as_const_view::as_const_view

as_const_view() requires std::default_initializable<V> = default;	(1)	(since C++23)
constexpr explicit as_const_view(V base);	(2)	(since C++23)

1) Value-initializes base_ via its default member initializer (= V()).

2) Initializes base_ with std::move(base).

Parameters

base

-

a view

std::ranges::as_const_view::base

constexpr V base() const& requires std::copy_constructible<V>;	(1)	(since C++23)
constexpr V base() &&;	(2)	(since C++23)

Returns the underlying view.

1) Copy-constructs the result from the underlying view. Equivalent to return base_;.

2) Move-constructs the result from the underlying view. Equivalent to return std::move(base_);.

std::ranges::as_const_view::begin

constexpr auto begin() requires (!/*simple-view*/<V>) {   return ranges::cbegin(base_); }	(1)	(since C++23)
constexpr auto begin() const requires ranges::range<const V> {   return ranges::cbegin(base_); }	(2)	(since C++23)

Returns the constant iterator of the view.

std::ranges::as_const_view::end

constexpr auto end() requires (!/*simple-view*/<V>) {   return ranges::cend(base_); }	(1)	(since C++23)
constexpr auto end() const requires ranges::range<const V> {   return ranges::cend(base_); }	(2)	(since C++23)

Returns the constant sentinel of the view.

std::ranges::as_const_view::size

constexpr auto size() requires ranges::sized_range<V> {     return ranges::size(base_); }	(1)	(since C++23)
constexpr auto size() const requires ranges::sized_range<const V> {     return ranges::size(base_); }	(2)	(since C++23)

Returns the size of the view if the view is bounded.

Deduction guides

template<class R> as_const_view(R&&) -> as_const_view<views::all_t<R>>;		(since C++23)

Helper templates

template<class T> inline constexpr bool enable_borrowed_range<std::ranges::as_const_view<T>> =     std::ranges::enable_borrowed_range<T>;		(since C++23)

This specialization of std::ranges::enable_borrowed_range makes as_const_view satisfy borrowed_range when the underlying view satisfies it.

See also

ranges::cbegin (C++20)	returns an iterator to the beginning of a read-only range (customization point object)
ranges::cend (C++20)	returns a sentinel indicating the end of a read-only range (customization point object)