std::ranges::crbegin

inline namespace /* unspecified */ {
    inline constexpr /* unspecified */ crbegin = /* unspecified */;
}

template< class T >
    requires /* see below */
constexpr std::input_or_output_iterator auto crbegin( T&& t );

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

Let CT be.

• const std::remove_reference_t<T>& if the argument is an lvalue (i.e. T is an lvalue reference type),
• const T otherwise.

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

The return type models std::input_or_output_iterator in both 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.

Notes

If the argument is an rvalue (i.e. T is an object type) and ranges::enable_borrowed_range<std::remove_cv_t<T>> is false, the call to ranges::crbegin is ill-formed, which also results in substitution failure.

The return type models std::input_or_output_iterator in all cases.

Example

#include <iostream>
#include <iterator>
#include <span>
#include <vector>
 
int main()
{
    std::vector<int> v = { 3, 1, 4 };
    auto vi = std::ranges::crbegin(v);
    std::cout << *vi << '\n';
 
    int a[] = { -5, 10, 15 };
    auto ai = std::ranges::crbegin(a);
    std::cout << *ai << '\n';
 
    // 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:
    static_assert(std::ranges::enable_borrowed_range<
        std::remove_cv_t<decltype(std::span{a})>>);
    std::cout << *si << '\n';
}

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See also