std::reverse(3)

NAME
std::reverse - std::reverse

Synopsis
   Defined in header <algorithm>
   template< class BidirIt > (1) (constexpr since C++20)
   void reverse( BidirIt first, BidirIt last );
   template< class ExecutionPolicy, class BidirIt >
   void reverse( ExecutionPolicy&& policy, BidirIt first, (2) (since C++17)
   BidirIt 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 and (last
   - i) - 1 for each integer i in [0, std::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.

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.

Complexity
   Exactly std::distance(first, last) / 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.

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);
   }

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.

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

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