std::ranges::reverse(3)

💅 std::ranges::reverse 💅

hey bestie, welcome to the std::ranges::reverse(3) page from the C++ Standard Libary manual ✨ this one is short and iconic: it flips a range around, no cap.

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🌸 NAME 🌸

std::ranges::reverse — std::ranges::reverse (yes, the name is literally the name, we love a self-referential moment 💖)

✨ 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. That's the whole budget, bestie.

🔥 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. That's the note, periodt.

🧚 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 👑

• 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 view in reverse order (C++20) (class template)
• views::reverse — reverses the order of elements in a range (C++20) (range adaptor object)
• reverse — reverses the order of elements in a range (function template)

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mwah 💋 — source/version: http://cppreference.com, date: 2024.06.10, page: std::ranges::reverse(3). Stay reversed, bestie 🫶