std::ranges::reverse

Defined in header `<algorithm>`
Call signature
template< std::bidirectional_iterator I, std::sentinel_for<I> S > requires std::permutable<I> constexpr I reverse( I first, S last );	(1)	(since C++20)
template< ranges::bidirectional_range R > requires std::permutable<ranges::iterator_t<R>> constexpr ranges::borrowed_iterator_t<R> reverse( R&& r );	(2)	(since C++20)

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 may not be specified when calling any of them.
None of them is visible to argument-dependent lookup.
When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

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

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

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