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std::ranges:: crbegin

From cppreference.net
C++
Ranges library
Range adaptors
Defined in header <ranges>
Defined in header <iterator>
inline namespace /* unspecified */ {

inline constexpr /* unspecified */ crbegin = /* unspecified */ ;

}
(since C++20)
(customization point object)
Call signature
template < class T >

requires /* see below */

constexpr /* see below */ auto crbegin ( T && t ) ;
(since C++20)

Returns an iterator to the first element of the const-qualified argument that is treated as a reversed sequence.

(until C++23)

Returns a constant iterator to the first element of the argument that is treated as a reversed sequence.

(since C++23)

range-rbegin-rend.svg

Let CT be

A call to ranges::crbegin is expression-equivalent to ranges:: rbegin ( static_cast < CT && > ( t ) ) .

(until C++23)

If the argument is an lvalue or ranges:: enable_borrowed_range < std:: remove_cv_t < T >> is true , then a call to ranges::crbegin is expression-equivalent to:

In all other cases, a call to ranges::crbegin is ill-formed, which can result in substitution failure when the call appears in the immediate context of a template instantiation.

(since C++23)

The return type models std::input_or_output_iterator and constant-iterator (since C++23) in all cases.

Customization point objects

The name ranges::crbegin denotes a customization point object , which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

Example

Run this code 
#include <cassert>
#include <iterator>
#include <span>
#include <vector>
int main()
{
    std::vector<int> v{3, 1, 4};
    auto vi = std::ranges::crbegin(v);
    assert(*vi == 4);
    ++vi; // OK, iterator object is mutable
    assert(*vi == 1);
    // *vi = 13; // Error: underlying element is read-only
    int a[]{-5, 10, 15};
    auto ai = std::ranges::crbegin(a);
    assert(*ai == 15);
    // auto x_x = std::ranges::crbegin(std::vector<int>{6, 6, 6});
    // ill-formed: the argument is an rvalue (see Notes ↑)
    auto si = std::ranges::crbegin(std::span{a}); // OK
    assert(*si == 15);
    static_assert
    (
        std::ranges::enable_borrowed_range<std::remove_cv_t<decltype(std::span{a})>>
    );
}

See also

(C++20)
returns a reverse iterator to a range
(customization point object)
(C++14)
returns a reverse iterator to the beginning of a container or array
(function template)