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, (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

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

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)