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std::ranges::rend

From cppreference.com
< cpp‎ | ranges
 
 
 
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.

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. std::forward<T>(t).rend(), if that expression is valid, and its return type models std::sentinel_for<decltype(ranges::rbegin(std::forward<T>(t)))>.
  2. Otherwise, rend(std::forward<T>(t)), if T is a class or enumeration type, the aforementioned unqualified call is valid, and its return type models std::sentinel_for<decltype(ranges::rbegin(std::forward<T>(t)))>, where the overload resolution is performed with the following candidates:
  3. Otherwise, std::make_reverse_iterator(ranges::begin(std::forward<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.

Contents

[edit] 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.

[edit] Customization point objects

The name ranges::rend denotes a customization point object, which is a const function object of a literal semiregular class type (denoted, for exposition purposes, as rend_ftor). All instances of rend_ftor are equal. 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_ftor will satisfy std::invocable<const rend_ftor&, Args...>. Otherwise, no function call operator of rend_ftor 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 modeling std::ranges::range.

[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] 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]