std::reverse

 

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is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
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iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
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uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Defined in header <algorithm>
	(1)
template< class BidirIt > void reverse( BidirIt first, BidirIt last );			(until C++20)
template< class BidirIt > constexpr void reverse( BidirIt first, BidirIt last );			(since C++20)
template< class ExecutionPolicy, class BidirIt > void reverse( ExecutionPolicy&& policy, BidirIt first, BidirIt last );		(2)	(since C++17)

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 non-negative i < (last - first) / 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)

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 ValueSwappable and LegacyBidirectionalIterator.

Return value

(none)

Complexity

Exactly (last - first) / 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::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

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.

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>::iterator_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); }

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 retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 223	C++98	swap was applied to each pair of iterators	applies iter_swap instead

See also

reverse_copy	creates a copy of a range that is reversed (function template)
ranges::reverse (C++20)	reverses the order of elements in a range (niebloid)