std::ranges::crbegin(3)

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

Synopsis
   Defined in header <ranges>
   Defined in header <iterator>
   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 /* see below */ auto crbegin( T&& t );
   Returns an iterator to the first element of the const-qualified (until C++23)
   argument that is treated as a reversed sequence.
   Returns a constant iterator to the first element of the argument that (since C++23)
   is treated as a reversed sequence.
   range-rbegin-rend.svg
   Let CT be
     * const std::remove_reference_t<T>& if the argument is an lvalue (i.e. T is
       an lvalue reference type), (until
     * const T otherwise. C++23)
   A call to ranges::crbegin is expression-equivalent to
   ranges::rbegin(static_cast<CT&&>(t)).
   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:
     * std::const_iterator<decltype(U)>(U) for some expression U equivalent to (since
       ranges::rbegin(/*possibly-const-range*/(t)). C++23)
   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.
   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
// 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
   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)