# std::ranges::rend

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
Defined in header `<iterator>` |  |
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 < decltype( ranges::rbegin ( std::declval <T>()))> auto rend( T&& t ); |  | (since C++20)

Returns a sentinel indicating the end of a reversed range.

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.

### 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!

### 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 non-member `rend` found by ADL | removed such machinery

### See also

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

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