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std::ranges::views::as_const, std::ranges::as_const_view - cppreference.com

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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 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.

Data members

Member Description
V base_ (private) the underlying view
(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)
returns the approximate size of the underlying approximately_sized_range
(public member function)
Inherited from ranges::view_interface
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>) [edit]

(C++23)

 returns a constant iterator to the beginning of the range
(public member function of std::ranges::view_interface<D>) [edit]

(C++23)

 returns a sentinel for the constant iterator of the range
(public member function of std::ranges::view_interface<D>) [edit]
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>) [edit]
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>) [edit]
returns the first element in the derived view, provided if it satisfies forward_range
(public member function of std::ranges::view_interface<D>) [edit]
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>) [edit]
returns the nth element in the derived view, provided only if it satisfies random_access_range
(public member function of std::ranges::view_interface<D>) [edit]

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

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

#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