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std::reverse(3) C++ Standard Libary std::reverse(3) NAME std::reverse - std::reverse Synopsis Defined in header <algorithm> template< class BidirIt > (1) (const- expr since C++20) void reverse( BidirIt first, BidirIt last ); template< class ExecutionPolicy, class BidirIt > void reverse( ExecutionPolicy&& policy, BidirIt first, (2) (since C++17) BidirIt last ); 1) Reverses the order of the elements in the range [first, last). Behaves as if applying std::iter_swap to every pair of iterators first + i and (last - i) - 1 for each integer i in [0, std::distance(first, last) / 2). 2) Same as (1), but executed according to policy. This overload participates in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since C++20) If *first is not Swappable (until C++20) BidirIt is not ValueSwappable (since C++20), the behavior is undefined. Parameters first, last - the range of elements to reverse policy - the execution policy to use. See execution policy for details. Type requirements - BidirIt must meet the requirements of LegacyBidirectionalIterator. Complexity Exactly std::distance(first, last) / 2 swaps. Exceptions The overload with a template parameter named ExecutionPolicy reports errors as follows: * If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::termi- nate is called. For any other ExecutionPolicy, the behavior is implementation- defined. * If the algorithm fails to allocate memory, std::bad_alloc is thrown. Possible implementation See also the implementations in libstdc++, libc++, and MSVC STL. template<class BidirIt> constexpr // since C++20 void reverse(BidirIt first, BidirIt last) { using iter_cat = typename std::iterator_traits<BidirIt>::itera- tor_category; // Tag dispatch, e.g. calling reverse_impl(first, last, iter_cat()), // can be used in C++14 and earlier modes. if constexpr (std::is_base_of_v<std::random_access_iterator_tag, iter_cat>) { if (first == last) return; for (--last; first < last; (void)++first, --last) std::iter_swap(first, last); } else while (first != last && first != --last) std::iter_swap(first++, last); } Notes Implementations (e.g. MSVC STL) may enable vectorization when the iterator type satisfies LegacyContiguousIterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap. Example // Run this code #include <algorithm> #include <iostream> #include <iterator> #include <vector> void println(auto rem, auto const& v) { for (std::cout << rem; auto e : v) std::cout << e << ' '; std::cout << '\n'; } int main() { std::vector<int> v {1, 2, 3}; std::reverse(v.begin(), v.end()); println("after reverse, v = ", v); int a[] = {4, 5, 6, 7}; std::reverse(std::begin(a), std::end(a)); println("after reverse, a = ", a); } Output: after reverse, v = 3 2 1 after reverse, a = 7 6 5 4 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 LWG 223 C++98 std::swap was applied to each pair of ap- plies std::iter_swap iterators in- stead LWG 2039 C++98 std::iter_swap was also applied when i not applied equals std::distance(first, last) / 2 See also reverse_copy creates a copy of a range that is reversed (function template) ranges::reverse reverses the order of elements in a range (C++20) (niebloid) http://cppreference.com 2024.06.10 std::reverse(3)
NAME | Synopsis | Parameters | Type requirements | Complexity | Exceptions | Possible implementation | Notes | Example | Output: | See also
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