std::ranges::views::as_const, std::ranges::as_const_view
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C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Ranges library
(exposition-only member object*)
returns the approximate size of the underlying
(public member function)
returns a constant iterator to the beginning of the range
(public member function of
returns a sentinel for the constant iterator of the range
(public member function of
returns a sentinel indicating the end of a read-only range
(customization point object)[edit]
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++26)(C++26)
as_const_viewviews::as_const
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)
(C++23)(C++23)
(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)
Defined in header
<ranges>
template< ranges::view V >
(1)
(since C++23)
requires ranges::input_range <V>
class as_const_view
namespace views {
(2)
(since C++23)
inline constexpr /* unspecified */ as_const = /* unspecified */;
Call signature
template< ranges::viewable_range R >
(since C++23)
requires /* see below */
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
Xand some extentExtentifUdenotes std::span <X, Extent>; - otherwise, ranges::ref_view (static_cast<const X&>(e.base())) if
Udenotes ranges::ref_view <X> for some typeXand const X modelsconstant_range; - otherwise, ranges::ref_view (static_cast<const U&>(e)) if
eis an lvalue, const U modelsconstant_range, andUdoes not modelview. - 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.
Contents
[edit] Data members
Member
Description
V base_ (private)
the underlying view(exposition-only member object*)
[edit] Member functions
(C++26)
approximately_sized_range (public member function)
Inherited from std::ranges::view_interface
returns whether the derived view is empty, provided only if it satisfies
(public member function of
sized_range or forward_range (public member function of
std::ranges::view_interface<D>) [edit]
(C++23)
(public member function of
std::ranges::view_interface<D>) [edit]
(C++23)
(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
(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
(public member function of
contiguous_iterator (public member function of
std::ranges::view_interface<D>) [edit]
returns the first element in the derived view, provided if it satisfies
(public member function of
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
(public member function of
bidirectional_range and common_range (public member function of
std::ranges::view_interface<D>) [edit]
returns the
(public member function of
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)
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)
requires ranges::approximately_sized_range<V>;
constexpr auto reserve_hint() const
requires ranges::approximately_sized_range<const V>;
(2)
(since C++26)
requires ranges::approximately_sized_range<const V>;
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)
as_const_view( R&& ) -> as_const_view<views::all_t <R>>;
[edit] Helper templates
template< class T >
(since C++23)
constexpr bool enable_borrowed_range<std::ranges::as_const_view<T>> =
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
a
(class template) (range adaptor object)[edit]
view of a sequence that casts each element to an rvalue(class template) (range adaptor object)[edit]
(C++20)
(customization point object)[edit]