std::ranges...copy_result(3)

💖 std::ranges::reverse_copy,std::ranges::reverse_copy_result 💖

okay so bestie, welcome to the std::ranges...copy_result(3) page from the C++ Standard Library manual — this one is literally about copying a range in reverse and it's serving constexpr main character energy. 🦋✨ vibe check: passed.

🌸 NAME 🌸

std::ranges::reverse_copy, std::ranges::reverse_copy_result — the reverse-copy duo. That's the name, periodt. 💅

✨ Synopsis ✨

Defined in header <algorithm>.

🌟 Call signature

(1) (since C++20)

template< std::bidirectional_iterator I, std::sentinel_for<I> S,
          std::weakly_incrementable O >
    requires std::indirectly_copyable<I, O>
constexpr reverse_copy_result<I, O>
    reverse_copy( I first, S last, O result );

(2) (since C++20)

template< ranges::bidirectional_range R, std::weakly_incrementable O >
    requires std::indirectly_copyable<ranges::iterator_t<R>, O>
constexpr reverse_copy_result<ranges::borrowed_iterator_t<R>, O>
    reverse_copy( R&& r, O result );

🧚 Helper types

(3) (since C++20)

template< class I, class O >
using reverse_copy_result = ranges::in_out_result<I, O>;

okay so overload (1) copies the elements from the source range [first, last) to the destination range [result, result + N), where N is ranges::distance(first, last), in such a way that the elements in the new range are in reverse order. It behaves as if by executing the assignment *(result + N - 1 - i) = *(first + i) once for each integer i in [0, N). Undefined behavior alert 🚨: the behavior is undefined if the source and destination ranges overlap.

fr fr, do NOT let those ranges overlap. It's giving chaos and the standard says undefined behavior. 💀

(2) Same as (1), but uses r as the source 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 copy.
• r — the range of elements to copy.
• result — the beginning of the destination range.

💖 Return value 💖

{last, result + N}. That's the whole return value, bestie. ✨

🦋 Complexity 🦋

Exactly N assignments. No extra sprinkle, no cap. 💅

📝 Notes 📝

Implementations (e.g. MSVC STL) may enable vectorization when the both iterator types model contiguous_iterator and have the same value type, and the value type is TriviallyCopyable. Lowkey iconic optimization behavior. 🔥

🧚 Possible implementation 🧚

See also the implementations in MSVC STL and libstdc++.

struct reverse_copy_fn
{
    template<std::bidirectional_iterator I, std::sentinel_for<I> S,
             std::weakly_incrementable O>
    requires std::indirectly_copyable<I, O>
    constexpr ranges::reverse_copy_result<I, O>
        operator()(I first, S last, O result) const
    {
        auto ret = ranges::next(first, last);
        for (; last != first; *result = *--last, ++result);
        return {std::move(ret), std::move(result)};
    }

    template<ranges::bidirectional_range R, std::weakly_incrementable O>
    requires std::indirectly_copyable<ranges::iterator_t<R>, O>
    constexpr
ranges::reverse_copy_result<ranges::borrowed_iterator_t<R>, O>
        operator()(R&& r, O result) const
    {
        return (*this)(ranges::begin(r), ranges::end(r),
std::move(result));
    }
};

inline constexpr reverse_copy_fn reverse_copy {};

🌈 Example 🌈

// Run this code

#include <algorithm>
#include <iostream>
#include <string>

int main()
{
    std::string x {"12345"}, y(x.size(), ' ');
    std::cout << x << " → ";
    std::ranges::reverse_copy(x.begin(), x.end(), y.begin());
    std::cout << y << " → ";
    std::ranges::reverse_copy(y, x.begin());
    std::cout << x << '\n';
}

⭐ Output ⭐

12345 → 54321 → 12345

👑 See also 👑

• ranges::reverse — reverses the order of elements in a range (C++20) (niebloid)
• reverse_copy — creates a copy of a range that is reversed (function template)

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cute sign-off: source http://cppreference.com, version/date 2024.06.10, manual page std::ranges...copy_result(3) — bye bestie, slay your ranges! 🫶💖