std::copy_backward

template< class BidirIt1, class BidirIt2 >
BidirIt2 copy_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last );

Copies the elements from the source range [first, last) to the destination range ending at d_last, starting from last and proceeding to first (reverse order).

If d_last is in (first, last], the behavior is undefined.

Parameters

Return value

An iterator to the last copy-assigned element in the destination range, or d_last if no element is copied.

Complexity

Exactly std::distance(first, last) assignments.

Notes

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

Possible implementation

template<class BidirIt1, class BidirIt2>
BidirIt2 copy_backward(BidirIt1 first, BidirIt1 last, BidirIt2 d_last)
{
    while (first != last)
        *(--d_last) = *(--last);
    return d_last;
}

Example

#include <algorithm>
#include <iostream>
#include <numeric>
#include <vector>

int main()
{
    std::vector<int> source(4);
    std::iota(source.begin(), source.end(), 1); // fills with 1, 2, 3, 4
    
    std::vector<int> destination(6);
    
    std::copy_backward(source.begin(), source.end(), destination.end());
    
    std::cout << "destination contains: ";
    for (auto i: destination)
        std::cout << i << ' ';
    std::cout << '\n';
}

destination contains: 0 0 1 2 3 4

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also