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std::ranges::rend

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< cpp‎ | ranges
 
 
Ranges library
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(C++23)

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(until C++23)(C++23)


 
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.

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.

Contents

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.

[edit] 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.

[edit] 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!

[edit] 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

[edit] See also

returns a reverse end iterator to a read-only range
(customization point object)[edit]
returns a reverse iterator to a range
(customization point object)[edit]
(C++14)
returns a reverse end iterator for a container or array
(function template) [edit]