std::ranges::reverse() algorithm

od C++20

Simplified

// (1)
constexpr I reverse( I first, S last );

// (2)
constexpr ranges::borrowed_iterator_t<R> reverse( R&& r );

The type of arguments are generic and have following constraints:

• I - std::bidirectional_iterator
• S - std::sentinel_for<I>
• R - ranges::bidirectional_range

Additionally, each overload has the following constraints:

• (1) - std::permutable<I>
• (2) - std::permutable<ranges::iterator_t<R>>

Detailed

// (1)
template<
	std::bidirectional_iterator I,
	std::sentinel_for<I> S
>
	requires std::permutable<I>
constexpr I reverse( I first, S last );

// (2)
template<
	ranges::bidirectional_range R
>
	requires std::permutable<ranges::iterator_t<R>>
constexpr ranges::borrowed_iterator_t<R> reverse( R&& r );

Reverses the order of elements.

• (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.

Parameters

first last	The range of elements to reverse (iterator, sentinel pair).
r	The range of elements to reverse (ranges).

Return value

An iterator equal to last.

Complexity

Exactly (last - first) / 2 swaps.

Exceptions

(none)

Possible implementation

reverse(1) and reverse(2)

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 {};

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.

Examples

Main.cpp

#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