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

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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

be decltype ( ( e ) ) , and let

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 X & > ( e. base ( ) ) ) if U denotes ranges:: ref_view < X > for some type X and const X models constant_range ;
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.

Data members

Member	Description
`V` `base_` (private)	the underlying view ( exposition-only member object* )

Member functions

(constructor)	constructs an `as_const_view` (public member function)
base	returns the underlying view `V` (public member function)
begin	returns the beginning iterator of the `as_const_view` (public member function)
end	returns the end iterator of the `as_const_view` (public member function)
size	returns the size of the view if it is bounded (public member function)
reserve_hint (C++26)	returns the approximate size of the underlying `approximately_sized_range` (public member function)
Inherited from std::ranges::view_interface
empty	returns whether the derived view is empty, provided only if it satisfies `sized_range` or `forward_range` (public member function of `std::ranges::view_interface<D>` )
cbegin (C++23)	returns a constant iterator to the beginning of the range (public member function of `std::ranges::view_interface<D>` )
cend (C++23)	returns a sentinel for the constant iterator of the range (public member function of `std::ranges::view_interface<D>` )
operator bool	returns whether the derived view is not empty, provided only if ranges::empty is applicable to it (public member function of `std::ranges::view_interface<D>` )
data	gets the address of derived view's data, provided only if its iterator type satisfies `contiguous_iterator` (public member function of `std::ranges::view_interface<D>` )
front	returns the first element in the derived view, provided if it satisfies `forward_range` (public member function of `std::ranges::view_interface<D>` )
back	returns the last element in the derived view, provided only if it satisfies `bidirectional_range` and `common_range` (public member function of `std::ranges::view_interface<D>` )
operator[]	returns the `n` ^th element in the derived view, provided only 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. 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. 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. Equivalent to return ranges:: size ( base_ ) ; .

std::ranges::as_rvalue_view:: reserve_hint

constexpr auto reserve_hint ( ) requires ranges :: approximately_sized_range < V > ;	(1)	(since C++26)
constexpr auto reserve_hint ( ) const requires ranges :: approximately_sized_range < const V > ;	(2)	(since C++26)

Returns ranges :: reserve_hint ( 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 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
`__cpp_lib_ranges_reserve_hint`	`202502L`	(C++26)	ranges::approximately_sized_range and `reserve_hint`

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
}

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External libraries

ranges::as_rvalue_view views::as_rvalue (C++23)	a `view` of a sequence that casts each element to an rvalue (class template) (range adaptor object)
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)
as_const (C++17)	obtains a reference to const to its argument (function template)
basic_const_iterator (C++23)	iterator adaptor that converts an iterator into a constant iterator (class template)

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