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

From cppreference.com
< cpp‎ | ranges
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
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

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

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)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/ranges/as_const_view&oldid=172506