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

Defined in header `<algorithm>`
Call signature
`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 );`	(1)	(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 );`	(2)	(since C++20)
Helper types
`template< class I, class O > using reverse_copy_result = ranges::in_out_result<I, O>;`	(3)	(since C++20)

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. Behaves as if by executing the assignment *(result + N - 1 - i) = *(first + i) once for each integer i in [0, N). The behavior is undefined if the source and destination ranges overlap.

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}.

Complexity

Exactly N assignments.

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::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';
}

Output:

12345 → 54321 → 12345

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

Docs

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