# 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 `[`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 from
d_last | - | the 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 <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';
    }

Output:

    destination contains: 0 0 1 2 3 4

### Defect reports

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

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 1206 | C++98 | 1. the behavior was well-defined if d_last == last 2. the behavior was undefined if d_last == first | 1. made undefined 2. made well-defined

### See also

copy copy_if (C++11) | copies a range of elements to a new location (function template)
ranges::copy_backward (C++20) | copies a range of elements in backwards order (niebloid)

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