std::ranges::crbegin(3)

NAME
std::ranges::crbegin - std::ranges::crbegin

Synopsis
   Defined in header <ranges>
   inline namespace /*unspecified*/ {
   inline constexpr /*unspecified*/ crbegin = (since C++20)
   /*unspecified*/; (customization point object)
   }
   Call signature
   template< class T >
   requires /* see below */ (since C++20)
   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.
   range-rbegin-rend.svg
   Let CT be
    1. const std::remove_reference_t<T>& if the argument is a lvalue (i.e. T is an
       lvalue reference type),
    2. 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.
  Expression-equivalent
   Expression e is expression-equivalent to expression f, if
     * e and f have the same effects, and
     * either both are constant subexpressions or else neither is a constant
       subexpression, and
     * either both are potentially-throwing or else neither is potentially-throwing
       (i.e. noexcept(e) == noexcept(f)).
  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
// Run this code
 #include <iostream>
 #include <vector>
 #include <iterator>
 #include <span>
 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';
 }

Output:
 4
 15
 15

See also
   ranges::rbegin returns a reverse iterator to a range
   (C++20) (customization point object)
   rbegin returns a reverse iterator to the beginning of a container or array
   crbegin (function template)
   (C++14)