std::ranges::rend(3)

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

Synopsis
   Defined in header <ranges>
   inline namespace /*unspecified*/ {
                        (since C++20)
   inline constexpr /*unspecified*/ rend = /*unspecified*/; (customization
                        point object)
   }
   Call signature
   template< class T >
   requires /* see below */
   constexpr (since C++20)
   std::sentinel_for<decltype(ranges::rbegin(std::declval<T>()))> auto
   rend( T&& t );
   Returns a sentinel indicating the end of a reversed range.
   range-rbegin-rend.svg
   Let t be an object of type T. 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. t.rend() converted to its decayed type, if that expression with conversion is
       valid, and its converted type models
       std::sentinel_for<decltype(ranges::rbegin(std::declval<T>()))>.
    2. Otherwise, rend(t) converted to its decayed type, if T is a class or enumeration
       type, the aforementioned unqualified call with conversion is valid, and its
       converted type models
       std::sentinel_for<decltype(ranges::rbegin(std::declval<T>()))>, where the
       overload resolution is performed with the following candidates:
          * void rend(auto&) = delete;
          * void rend(const auto&) = delete;
          * any declarations of rend found by argument-dependent lookup.
    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.
  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::rend 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 __rend_fn.
   All instances of __rend_fn are equal. The effects of invoking different instances of
   type __rend_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::rend 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::rend above, __rend_fn models
     * std::invocable<__rend_fn, Args...>,
     * std::invocable<const __rend_fn, Args...>,
     * std::invocable<__rend_fn&, Args...>, and
     * std::invocable<const __rend_fn&, Args...>.
   Otherwise, no function call operator of __rend_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, 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`!

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)