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std::ranges::rbegin(3) C++ Standard Libary std::ranges::rbegin(3) NAME std::ranges::rbegin - std::ranges::rbegin Synopsis Defined in header <ranges> Defined in header <iterator> inline namespace /* unspecified */ { inline constexpr /* unspecified */ rbegin = /* (since C++20) unspecified */; (customiza- tion point object) } Call signature template< class T > requires /* see below */ (since C++20) constexpr std::input_or_output_iterator auto rbegin( T&& t ); Returns an iterator to the last element of the argument. range-rbegin-rend.svg If T is an array type and std::remove_all_extents_t<std::remove_ref- erence_t<T>> is incomplete, then the call to ranges::rbegin is ill-formed, no diag- nostic required. If the argument is an lvalue or ranges::enable_bor- rowed_range<std::remove_cv_t<T>> is true, then a call to ranges::rbegin is expression-equivalent to: 1. decay-copy(t.rbegin()) (until C++23) auto(t.rbegin()) (since C++23), if that expression is valid and its type models std::input_or_output_iterator. 2. Otherwise, decay-copy(rbegin(t)) (until C++23) auto(rbegin(t)) (since C++23), if T is a class or enumeration type, that expres- sion is valid and its type models std::input_or_output_iterator, where the meaning of rbegin is established as if by performing argument-dependent lookup only. 3. Otherwise, std::make_reverse_iterator(ranges::end(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::rbegin is ill-formed, which can result in substitution failure when ranges::rbegin(t) appears in the immediate context of a template instantiation. 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, the call to ranges::rbegin is ill-formed, which also results in substitution failure. The return type models std::input_or_output_iterator in all cases. The C++20 standard requires that if the underlying rbegin 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 <iostream> #include <ranges> #include <span> #include <vector> int main() { std::vector<int> v = {3, 1, 4}; auto vi = std::ranges::rbegin(v); std::cout << *vi << '\n'; *vi = 42; // OK int a[] = {-5, 10, 15}; auto ai = std::ranges::rbegin(a); std::cout << *ai << '\n'; *ai = 42; // OK // auto x_x = std::ranges::rbegin(std::vector{6, 6, 6}); // ill-formed: the argument is an rvalue (see Notes ) auto si = std::ranges::rbegin(std::span{a}); // OK static_assert(std::ranges::enable_borrowed_range< std::remove_cv_t<decltype(std::span{a})>>); *si = 42; // OK } Output: 4 15 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior P2602R2 C++20 there's machinery to prohibit certain re- moved such machinery non-member rbegin found by ADL See also ranges::crbegin returns a reverse iterator to a read-only range (C++20) (customization point object) rbegin returns a reverse iterator to the beginning of a container or array crbegin (function template) (C++14) http://cppreference.com 2024.06.10 std::ranges::rbegin(3)
NAME | Synopsis | Notes | Example | Output: | See also
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