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std::ranges::views::as_const, std::ranges::as_const_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) 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
(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>)
(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>)
(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>)
(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>)
(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>)
(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>)
(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>);
 (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 >
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.

Notes

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

Example

#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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