# std::ranges::rend

< cpp‎ | ranges
 Defined in header  inline namespace /*unspecified*/ {     inline constexpr /*unspecified*/ rend = /*unspecified*/; } (since C++20) (customization point object) Call signature template< class T >     requires /* see below */ constexpr std::sentinel_for()))> auto     rend( T&& t ); (since C++20)

Returns a sentinel indicating the end of a reversed range.

Let t be an object of type T. If the argument is an lvalue or 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, if both 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.

## Contents

### 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

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 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

#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!