std::copy_backward() algorithm

od C++20

// (1)
template< class BidirIt1, class BidirIt2 >
constexpr BidirIt2 copy_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last );

do C++20

// (1)
template< class BidirIt1, class BidirIt2 >
BidirIt2 copy_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last );

• (1) 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.

  Undefined Behaviour

  The behavior is undefined if d_first is within [first; last). In this case, std::copy_backward may be used instead.

Parameters

first last	The range of elements to copy.
d_first	The beginning of the destination range.

Type requirements

BidirIt

LegacyBidirectionalIterator

Return value

Iterator to the last element copied.

Complexity

Exactly last - first assignments.

Exceptions

(none)

Possible implementation

copy_backward (1)

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

    return d_last;
}

Notes

When copying overlapping ranges:

• std::copy is appropriate when copying to the left (beginning of the destination range is outside the source range).
• std::copy_backward is appropriate when copying to the right (end of the destination range is outside the source range).

Examples

Main.cpp

#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