# std::copy_backward

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class BidirIt1, class BidirIt2 > BidirIt2 copy_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last ); |  | (until C++20)
template< class BidirIt1, class BidirIt2 > constexpr BidirIt2 copy_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last ); |  | (since C++20)

Copies the elements from the range, defined by `[first, last)`, to another range ending at `d_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]`. std::copy must be used instead of `std::copy_backward` in that case.

### Parameters

first, last | - | the range of the elements to copy
d_last | - | end of the destination range..
Type requirements
- `BidirIt` must meet the requirements of LegacyBidirectionalIterator.

### Return value

iterator to the last element copied.

### Complexity

Exactly `last - first` 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

Run this code

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

    int main()
    {
        std::vector<int> from_vector;
        for (int i = 0; i < 10; i++) {
            from_vector.push_back(i);
        }

        std::vector<int> to_vector(15);

        std::copy_backward(from_vector.begin(), from_vector.end(), to_vector.end());

        std::cout << "to_vector contains: ";
        for (auto i: to_vector) {
            std::cout << i << " ";
        }
     }

Output:

    to_vector contains: 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9

### See also

copy copy_if (C++11) | copies a range of elements to a new location (function template)

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