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)