std::copy_backward (3) - Linux Manuals

std::copy_backward: std::copy_backward

NAME

std::copy_backward - std::copy_backward

Synopsis

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