NAME

std::copy_backward - std::copy_backward

Synopsis

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

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 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 <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