std::ranges::rend

 

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Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range primitives
iterator_tsentinel_t	range_size_trange_difference_t	range_reference_trange_rvalue_reference_t	range_value_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	basic_istream_viewviews::istream
Adaptors
views::all_tviews::all ref_view owning_view common_viewviews::common reverse_viewviews::reverse views::counted zip_viewviews::zip (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

Defined in header <ranges>
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);

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 ranges::enable_borrowed_range<std::remove_cv_t<T>> is true, then a call to ranges::rend is expression-equivalent to:

1. std::forward<T>(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(std::forward<T>(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(std::forward<T>(t))) if both ranges::begin(std::forward<T>(t)) and ranges::end(std::forward<T>(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, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

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 modeling 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 (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)