NAME¶

std::copy_backward - std::copy_backward

Synopsis¶

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

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 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¶

// 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
1. the behavior was well-defined if d_last
LWG 1206 C++98 == last 1. made undefined
2. the behavior was undefined if d_last == 2. made well-defined
first

See also¶

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