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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 [firstlast).
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:

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

#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

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

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https://en.cppreference.com/w/cpp/algorithm/ranges/reverse