std::ranges::rend(3)

NAME
std::ranges::rend - std::ranges::rend

Synopsis
   Defined in header <ranges>
   Defined in header <iterator>
   inline namespace /* unspecified */ {
                        (since C++20)
       inline constexpr /* unspecified */ rend = /* unspecified (customization point
   */; object)
   }
   Call signature
   template< class T >
       requires /* see below */
   constexpr std::sentinel_for< (since C++20)
       decltype(ranges::rbegin(std::declval<T>()))> auto rend(
   T&& t );
   Returns a sentinel indicating the end of a reversed range.
   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::rend 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::rend is expression-equivalent to:
    1. decay-copy(t.rend())
       (until C++23)
       auto(t.rend())
       (since C++23), if that expression is valid and its type models
       std::sentinel_for<decltype(ranges::rbegin(std::declval<T>()))>.
    2. Otherwise,
       decay-copy(rend(t))
       (until C++23)
       auto(rend(t))
       (since C++23), if T is a class or enumeration type, that expression is valid and
       its type models std::sentinel_for<decltype(ranges::rbegin(std::declval<T>()))>,
       where the meaning of rend is established as if by performing argument-dependent
       lookup only.
    3. Otherwise, std::make_reverse_iterator(ranges::begin(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::rend is ill-formed, which can result in
   substitution failure when ranges::rend(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, or if it is of an array
   type of unknown bound, the call to ranges::rend is ill-formed, which also results in
   substitution failure.
   If ranges::rend(std::forward<T>(t)) is valid, then
   decltype(ranges::rend(std::forward<T>(t))) and
   decltype(ranges::begin(std::forward<T>(t))) model std::sentinel_for in all cases,
   while T models std::ranges::range.
   The C++20 standard requires that if the underlying rend 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 <algorithm>
 #include <iostream>
 #include <ranges>
 #include <vector>
 int main()
 {
     std::vector<int> v = {3, 1, 4};
     namespace ranges = std::ranges;
     if (ranges::find(ranges::rbegin(v), ranges::rend(v), 5) != ranges::rend(v))
         std::cout << "found a 5 in vector v!\n";
     int a[] = {5, 10, 15};
     if (ranges::find(ranges::rbegin(a), ranges::rend(a), 5) != ranges::rend(a))
         std::cout << "found a 5 in array a!\n";
 }

Output:
 found a 5 in array a!
   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 rend found by ADL

See also
   ranges::crend returns a reverse end iterator to a read-only range
   (C++20) (customization point object)
   ranges::rbegin returns a reverse iterator to a range
   (C++20) (customization point object)
   rend returns a reverse end iterator for a container or array
   crend (function template)
   (C++14)