NAME

std::reverse - std::reverse

Synopsis

Defined in header <algorithm>
template< class BidirIt > (until C++20)
void reverse( BidirIt first, BidirIt last );
template< class BidirIt > (since C++20)
constexpr void reverse( BidirIt first, BidirIt last ); (1)
template< class ExecutionPolicy, class BidirIt >
void reverse( ExecutionPolicy&& policy, BidirIt first, BidirIt (2) (since C++17)
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, (last-i) -
1 for each non-negative i < (last-first)/2.
2) Same as (1), but executed according to policy. This overload does not participate
in overload resolution unless
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20) is true.

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 <vector>
#include <iostream>
#include <iterator>
#include <algorithm>

int main()
{
std::vector<int> v{1, 2, 3};
std::reverse(v.begin(), v.end());
for(auto e : v) std::cout << e;
std::cout << '\n';

int a[] = {4, 5, 6, 7};
std::reverse(std::begin(a), std::end(a));
for(auto e : a) std::cout << e;
}

Output:

321
7654