std::ranges:: crbegin

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. For exposition purposes, the cv-unqualified version of its type is denoted as __crbegin_fn .

All instances of __crbegin_fn are equal. The effects of invoking different instances of type __crbegin_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::crbegin can be copied freely and its copies can be used interchangeably.

Given a set of types Args... , if std:: declval < Args > ( ) ... meet the requirements for arguments to ranges::crbegin above, __crbegin_fn models

• std:: invocable < __crbegin_fn, Args... > ,
• std:: invocable < const __crbegin_fn, Args... > ,
• std:: invocable < __crbegin_fn & , Args... > , and
• std:: invocable < const __crbegin_fn & , Args... > .
  

Otherwise, no function call operator of __crbegin_fn participates in overload resolution.

Example

#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