NAME¶

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

Synopsis¶

Defined in header <ranges>
template< ranges::view V >

requires ranges::input_range<V> (1) (since C++23)
class as_const_view

: public ranges::view_interface<as_const_view<V>>
namespace views {

inline constexpr /* unspecified */ as_const = /* unspecified (2) (since C++23)
*/;

}
Call signature
template< ranges::viewable_range R >

requires /* see below */ (since C++23)

constexpr ranges::view auto as_const( R&& r );

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:
* 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.

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)
Inherited from std::ranges::view_interface
empty returns whether the derived view is empty. Provided if it satisfies
(C++20) sized_range or forward_range.
(public member function of std::ranges::view_interface<D>)
cbegin returns a constant iterator to the beginning of the range.
(C++23) (public member function of std::ranges::view_interface<D>)
cend returns a sentinel for the constant iterator of the range.
(C++23) (public member function of std::ranges::view_interface<D>)
operator bool returns whether the derived view is not empty. Provided if
(C++20) 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 if its iterator type
(C++20) satisfies contiguous_iterator.
(public member function of std::ranges::view_interface<D>)
front returns the first element in the derived view. Provided if it
(C++20) satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
back returns the last element in the derived view. Provided if it satisfies
(C++20) 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 if it satisfies
(C++20) 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 > (since C++23)
as_const_view( R&& ) -> as_const_view<views::all_t<R>>;

Helper templates

template< class T >

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

std::ranges::enable_borrowed_range<T>;

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¶

// 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¶

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