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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata reserve_hint (C++26)   empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

Range concepts
range borrowed_range    common_range	sized_range viewable_range    view	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	approximately_sized_range (C++26) constant_range (C++23)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while common_viewviews::common views::counted as_input_viewviews::as_input (C++26)(C++26)	ref_view drop_viewviews::drop drop_while_viewviews::drop_while lazy_split_viewviews::lazy_split split_viewviews::split join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   concat_viewviews::concat (C++26)(C++26) cache_latest_viewviews::cache_latest (C++26)(C++26)	owning_view reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

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

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

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_ ).

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

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