std::ranges::copy_backward, std::ranges::copy_backward_result

Defined in header <algorithm>
Call signature
template< std::bidirectional_iterator I1, std::sentinel_for<I1> S1,
          std::bidirectional_iterator I2 >
requires  std::indirectly_copyable<I1, I2>
constexpr copy_backward_result<I1, I2>
    copy_backward( I1 first, S1 last, I2 result );
(1) (since C++20)
template< ranges::bidirectional_range R, std::bidirectional_iterator I >
requires  std::indirectly_copyable<ranges::iterator_t<R>, I>
constexpr copy_backward_result<ranges::borrowed_iterator_t<R>, I>
    copy_backward( R&& r, I result );
(2) (since C++20)
Helper types
template< class I1, class I2 >
using copy_backward_result = ranges::in_out_result<I1, I2>;
(3) (since C++20)
1) Copies the elements from the range, defined by [firstlast), to another range [result - Nresult), 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 result is within (first, last]. In such a case std::ranges::copy can be used instead.
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:

In practice, they may be implemented as function objects, or with special compiler extensions.


first - the beginning of the range of elements to copy from
last - the end of the range of elements to copy from
r - the range of the elements to copy from
result - the end of the destination range

Return value

{last, result - N}.


Exactly N assignments.


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).

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 result) const
        I1 last1 {ranges::next(first, std::move(last))};
        for (I1 i {last1}; i != first;)
            *--result = *--i;
        return {std::move(last1), std::move(result)};
    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 result) const
        return (*this)(ranges::begin(r), ranges::end(r), std::move(result));
inline constexpr copy_backward_fn copy_backward{};


#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);
        << "(in - src.begin) == " << std::distance(src.begin(), in) << '\n'
        << "(out - dst.begin) == " << std::distance(dst.begin(), out) << '\n';


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

See also

copies a range of elements to a new location
copies a number of elements to a new location
copies a range of elements omitting those that satisfy specific criteria
copies a range, replacing elements satisfying specific criteria with another value
creates a copy of a range that is reversed
copies and rotate a range of elements
creates a copy of some range of elements that contains no consecutive duplicates
moves a range of elements to a new location
moves a range of elements to a new location in backwards order
copies a range of elements in backwards order
(function template)

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