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

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>
    reverse_copy( I first, S last, O d_first );

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>
    reverse_copy( R&& r, O d_first );

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

Copies elements from the source range [first, last) or r to the destination range beginning at d_first in reverse order.

If the source and destination ranges overlap, the behavior is undefined.

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

Return value

A ranges::reverse_copy_result object where:

• The data member in holds the past-the-end iterator of the source range.
• The data member out holds the past-the-end iterator of the destination range.

Complexity

Given \(\scriptsize N\)N as ranges::distance(first, last) or ranges::distance(r):

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.

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::next(ranges::begin(r), ranges::end(r)),
                       std::move(result));
    }
};

inline constexpr reverse_copy_fn reverse_copy{};

Example

#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';
}

12345  54321  12345

See also