std::ranges::rend
| Defined in header <ranges>
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| Defined in header <iterator>
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| inline namespace /* unspecified */ { inline constexpr /* unspecified */ rend = /* unspecified */; |
(since C++20) (customization point object) |
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| Call signature |
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| template< class T > requires /* see below */ |
(since C++20) | |
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:
- 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>()))>.
- Otherwise, decay-copy(rend(t))(until C++23)auto(rend(t))(since C++23), if
Tis 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 ofrendis established as if by performing argument-dependent lookup only. - 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"; }
Output:
found a 5 in array a!
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
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removed such machinery |
See also
| (C++20) |
returns a reverse end iterator to a read-only range (customization point object) |
| (C++20) |
returns a reverse iterator to a range (customization point object) |
| (C++14) |
returns a reverse end iterator for a container or array (function template) |