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

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C++
Ranges library
Range adaptors
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) RangeAdaptorObject . 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:

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.

Data members

Member object Definition
base_ (private) The underlying view of type V .
( exposition-only member object* )

Member functions

constructs an as_const_view
(public member function)
returns the underlying view V
(public member function)
returns the beginning iterator of the as_const_view
(public member function)
returns the end iterator of the as_const_view
(public member function)
returns the size of the view if it is bounded
(public member function)
Inherited from std::ranges::view_interface
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> )
(C++23)
returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D> )
(C++23)
returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D> )
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> )
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> )
returns the first element in the derived view. Provided if it satisfies forward_range .
(public member function of std::ranges::view_interface<D> )
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> )
returns the n th 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 > ) ;
(1) (since C++23)
constexpr auto begin ( ) const requires ranges:: range < const V > ;
(2) (since C++23)

Returns the constant iterator of the view.

1,2) Equivalent to return ranges:: cbegin ( base_ ) ;

std::ranges::as_const_view:: end

constexpr auto end ( ) requires ( ! __simple_view < V > ) ;
(1) (since C++23)
constexpr auto end ( ) const requires ranges:: range < const V > ;
(2) (since C++23)

Returns the constant sentinel of the view.

1,2) Equivalent to return ranges:: cend ( base_ ) ;

std::ranges::as_const_view:: size

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

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

1,2) Equivalent to return ranges:: size ( base_ ) ;

Deduction guides

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

Helper templates

template < class T >

constexpr bool enable_borrowed_range < std :: ranges :: as_const_view < T >> =

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.

Notes

Feature-test macro Value Std Feature
__cpp_lib_ranges_as_const 202207L (C++23) ranges::as_const_view , std:: const_iterator

Example

Run this code 
#include <cassert>
#include <ranges>
 
int main()
{
    int x[]{1, 2, 3, 4, 5};
 
    auto v1 = x | std::views::drop(2);
    assert(v1.back() == 5);
    v1[0]++; // OK, can modify non-const element
 
    auto v2 = x | std::views::drop(2) | std::views::as_const;
    assert(v2.back() == 5);
    // v2[0]++; // Compile-time error, cannot modify read-only element
}

See also

(C++23)
a view of a sequence that casts each element to an rvalue
(class template) (range adaptor object)
(C++20)
returns an iterator to the beginning of a read-only range
(customization point object)
(C++20)
returns a sentinel indicating the end of a read-only range
(customization point object)
(C++17)
obtains a reference to const to its argument
(function template)
(C++23)
iterator adaptor that converts an iterator into a constant iterator
(class template)
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