< cpp‎ | algorithm
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
Sampling operations

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
Defined in header <algorithm>
template< class BidirIt >
void reverse( BidirIt first, BidirIt last );
(1) (constexpr 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 [firstlast).
Behaves as if applying std::iter_swap to every pair of iterators first + i and (last - i) - 1 for each integer i in [0std::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.


[edit] 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.

[edit] Complexity

Exactly std::distance(first, last) / 2 swaps.

[edit] 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.

[edit] 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)
        for (--last; first < last; (void)++first, --last)
            std::iter_swap(first, last);
        while (first != last && first != --last)
            std::iter_swap(first++, last);

[edit] 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.

[edit] 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);


after reverse, v = 3 2 1
after reverse, a = 7 6 5 4

[edit] 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 std::swap was applied to each pair of iterators applies std::iter_swap instead
LWG 2039 C++98 std::iter_swap was also applied when i
equals std::distance(first, last) / 2
not applied

[edit] See also

creates a copy of a range that is reversed
(function template) [edit]
reverses the order of elements in a range