std::ranges::rbegin(3)

NAME
std::ranges::rbegin - std::ranges::rbegin

Synopsis
   Defined in header <ranges>
   Defined in header <iterator>
   inline namespace /* unspecified */ {
       inline constexpr /* unspecified */ rbegin = /* (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 rbegin(
   T&& t );
   Returns an iterator to the last element of the argument.
   range-rbegin-rend.svg
   If T is an array type and std::remove_all_extents_t<std::remove_reference_t<T>> is
   incomplete, then the call to ranges::rbegin is ill-formed, no diagnostic required.
   If the argument is an lvalue or ranges::enable_borrowed_range<std::remove_cv_t<T>>
   is true, then a call to ranges::rbegin is expression-equivalent to:
    1. decay-copy(t.rbegin())
       (until C++23)
       auto(t.rbegin())
       (since C++23), if that expression is valid and its type models
       std::input_or_output_iterator.
    2. Otherwise,
       decay-copy(rbegin(t))
       (until C++23)
       auto(rbegin(t))
       (since C++23), if T is a class or enumeration type, that expression is valid and
       its type models std::input_or_output_iterator, where the meaning of rbegin is
       established as if by performing argument-dependent lookup only.
    3. Otherwise, std::make_reverse_iterator(ranges::end(t)) if both ranges::begin(t)
       and ranges::end(t) are valid expressions, have the same type, and that type
       models std::bidirectional_iterator.
   In all other cases, a call to ranges::rbegin is ill-formed, which can result in
   substitution failure when ranges::rbegin(t) appears in the immediate context of a
   template instantiation.

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::rbegin is ill-formed, which also results in substitution failure.
   The return type models std::input_or_output_iterator in all cases.
   The C++20 standard requires that if the underlying rbegin function call returns a
   prvalue, the return value is move-constructed from the materialized temporary
   object. All implementations directly return the prvalue instead. The requirement is
   corrected by the post-C++20 proposal P0849R8 to match the implementations.

Example
// Run this code
 #include <iostream>
 #include <ranges>
 #include <span>
 #include <vector>
 int main()
 {
     std::vector<int> v = {3, 1, 4};
     auto vi = std::ranges::rbegin(v);
     std::cout << *vi << '\n';
     *vi = 42; // OK
     int a[] = {-5, 10, 15};
     auto ai = std::ranges::rbegin(a);
     std::cout << *ai << '\n';
     *ai = 42; // OK
     // auto x_x = std::ranges::rbegin(std::vector{6, 6, 6});
     // ill-formed: the argument is an rvalue (see Notes ↑)
     auto si = std::ranges::rbegin(std::span{a}); // OK
     static_assert(std::ranges::enable_borrowed_range<
         std::remove_cv_t<decltype(std::span{a})>>);
     *si = 42; // OK
 }

Output:
 4
 15
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
   P2602R2 C++20 there's machinery to prohibit certain removed such machinery
                      non-member rbegin found by ADL

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