# std::ranges::reverse

From cppreference.com
< cpp‎ | algorithm‎ | ranges

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 algorithm function objects (informally known as 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.

### 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 (C++20) | creates a copy of a range that is reversed (algorithm function object)
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)

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