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Showing content from https://en.cppreference.com/w/cpp/algorithm/../symbol_index/../algorithm/ranges/copy_backward.html below:

std::ranges::copy_backward, std::ranges::copy_backward_result - cppreference.com

Call signature

(1) (since C++20) (2) (since C++20)

Helper types

(3) (since C++20) 1)

Copies the elements from the range, defined by

[first, last)

, to another range

[d_last - N, d_last)

, where

N = ranges::distance(first, last)

. The elements are copied in reverse order (the last element is copied first), but their relative order is preserved. The behavior is undefined if

d_last

is within

(first, last]

. In such a case

std::ranges::copy

can be used instead.

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.

[edit] Parameters first, last - the iterator-sentinel pair defining the range of elements to copy from r - the range of the elements to copy from d_last - the end of the destination range [edit] Return value

{last, d_last - N}

[edit] Complexity

Exactly N assignments.

[edit] Notes

When copying overlapping ranges, ranges::copy is appropriate when copying to the left (beginning of the destination range is outside the source range) while ranges::copy_backward is appropriate when copying to the right (end of the destination range is outside the source range).

[edit] Possible implementation

struct copy_backward_fn
{
    template<std::bidirectional_iterator I1, std::sentinel_for<I1> S1,
             std::bidirectional_iterator I2>
    requires std::indirectly_copyable<I1, I2>
    constexpr ranges::copy_backward_result<I1, I2>
        operator()(I1 first, S1 last, I2 d_last) const
    {
        I1 last1 {ranges::next(first, std::move(last))};
        for (I1 i {last1}; i != first;)
            *--d_last = *--i;
        return {std::move(last1), std::move(d_last)};
    }
 
    template<ranges::bidirectional_range R, std::bidirectional_iterator I>
    requires std::indirectly_copyable<ranges::iterator_t<R>, I>
    constexpr ranges::copy_backward_result<ranges::borrowed_iterator_t<R>, I>
        operator()(R&& r, I d_last) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(d_last));
    }
};
 
inline constexpr copy_backward_fn copy_backward{};

[edit] Example

#include <algorithm>
#include <iostream>
#include <ranges>
#include <string_view>
#include <vector>
 
void print(std::string_view rem, std::ranges::forward_range auto const& r)
{
    for (std::cout << rem << ": "; auto const& elem : r)
        std::cout << elem << ' ';
    std::cout << '\n';
}
 
int main()
{
    const auto src = {1, 2, 3, 4};
    print("src", src);
 
    std::vector<int> dst(src.size() + 2);
    std::ranges::copy_backward(src, dst.end());
    print("dst", dst);
 
    std::ranges::fill(dst, 0);
    const auto [in, out] =
        std::ranges::copy_backward(src.begin(), src.end() - 2, dst.end());
    print("dst", dst);
 
    std::cout
        << "(in - src.begin) == " << std::distance(src.begin(), in) << '\n'
        << "(out - dst.begin) == " << std::distance(dst.begin(), out) << '\n';
}

Output:

src: 1 2 3 4
dst: 0 0 1 2 3 4
dst: 0 0 0 0 1 2
(in - src.begin) == 2
(out - dst.begin) == 4

[edit] See also

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