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std::reverse

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 does not participate in overload resolution unless

`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

#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

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)