std::ranges:: rend

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

#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";
}

found a 5 in array a!

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also