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 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.

[edit] Data members

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

[edit] 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>`) [edit]
cbegin (C++23)	returns a constant iterator to the beginning of the range (public member function of `std::ranges::view_interface<D>`) [edit]
cend (C++23)	returns a sentinel for the constant iterator of the range (public member function of `std::ranges::view_interface<D>`) [edit]
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>`) [edit]
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>`) [edit]
front	returns the first element in the derived view, provided if it satisfies `forward_range` (public member function of `std::ranges::view_interface<D>`) [edit]
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>`) [edit]
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>`) [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

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_ ).

[edit] Deduction guides

template< class R > as_const_view( R&& ) -> as_const_view<views::all_t<R>>;		(since C++23)

[edit] 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.

[edit] 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`

[edit] 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
}

[edit] See also

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

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