std::ranges::reverse(3)

NAME
std::ranges::reverse - std::ranges::reverse

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::bidirectional_iterator I, std::sentinel_for<I> S >
   requires std::permutable<I> (1) (since C++20)
   constexpr I
       reverse( I first, S last );
   template< ranges::bidirectional_range R >
   requires std::permutable<ranges::iterator_t<R>> (2) (since C++20)
   constexpr ranges::borrowed_iterator_t<R>
       reverse( R&& r );
   1) Reverses the order of the elements in the range [first, last).
   Behaves as if applying ranges::iter_swap to every pair of iterators first + i, last
   - i - 1 for each integer i, where 0 ≤ i < (last - first) / 2.
   2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first and
   ranges::end(r) as last.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first, last - the range of elements to reverse
   r - the range of elements to reverse

Return value
   An iterator equal to last.

Complexity
   Exactly (last - first) / 2 swaps.

Notes
   Implementations (e.g. MSVC STL) may enable vectorization when the iterator type
   models contiguous_iterator and swapping its value type calls neither non-trivial
   special member function nor ADL-found swap.

Possible implementation
   See also implementations in libstdc++ and MSVC STL.
   struct reverse_fn
   {
       template<std::bidirectional_iterator I, std::sentinel_for<I> S>
       requires std::permutable<I>
       constexpr I operator()(I first, S last) const
       {
           auto last2 {ranges::next(first, last)};
           for (auto tail {last2}; !(first == tail or first == --tail); ++first)
               ranges::iter_swap(first, tail);
           return last2;
       }
       template<ranges::bidirectional_range R>
       requires std::permutable<ranges::iterator_t<R>>
       constexpr ranges::borrowed_iterator_t<R>
           operator()(R&& r) const
       {
           return (*this)(ranges::begin(r), ranges::end(r));
       }
   };
   inline constexpr reverse_fn reverse {};

Example
// Run this code
 #include <algorithm>
 #include <array>
 #include <iostream>
 #include <string>
 int main()
 {
     std::string s {"ABCDEF"};
     std::cout << s << " → ";
     std::ranges::reverse(s.begin(), s.end());
     std::cout << s << " → ";
     std::ranges::reverse(s);
     std::cout << s << " │ ";
     std::array a {1, 2, 3, 4, 5};
     for (auto e : a)
         std::cout << e << ' ';
     std::cout << "→ ";
     std::ranges::reverse(a);
     for (auto e : a)
         std::cout << e << ' ';
     std::cout << '\n';
 }

Output:
 ABCDEF → FEDCBA → ABCDEF │ 1 2 3 4 5 → 5 4 3 2 1

See also
   ranges::reverse_copy creates a copy of a range that is reversed
   (C++20) (niebloid)
   ranges::reverse_view a view that iterates over the elements of another bidirectional
   views::reverse view in reverse order
   (C++20) (class template) (range adaptor object)
   reverse reverses the order of elements in a range
                        (function template)