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std::ranges::rend(3) C++ Standard Libary std::ranges::rend(3) 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 poten- tially-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 dif- ferent 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 inter- changeably. 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 implemen- tations. 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 ar- ray crend (function template) (C++14) http://cppreference.com 2022.07.31 std::ranges::rend(3)
NAME | Synopsis | Notes | Example | Output: | See also
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