std::copy

template< class InputIt, class OutputIt >
OutputIt copy( InputIt first, InputIt last, OutputIt d_first );

template< class InputIt, class OutputIt, class UnaryPredicate >
OutputIt copy_if( InputIt first, InputIt last,
                  OutputIt d_first,
                  UnaryPredicate pred );

Copies the elements in the range, defined by [first, last), to another range beginning at d_first. The second function only copies the elements for which the predicate pred returns true. The order of the elements that are not removed is preserved.

For std::copy, the behavior is undefined if d_first is within the range [first, last). In this case, std::copy_backward may be used instead.

For std::copy_if, the behavior is undefined if the source and the destination ranges overlap.

Parameters

Return value

Output iterator to the element in the destination range, one past the last element copied.

Complexity

1) Exactly last - first assignments.

2) Exactly last - first applications of the predicate.

Notes

In practice, implementations of std::copy avoid multiple assignments and use bulk copy functions such as std::memmove if the value type is TriviallyCopyable.

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 InputIt, class OutputIt>
OutputIt copy(InputIt first, InputIt last, 
              OutputIt d_first)
{
    while (first != last) {
        *d_first++ = *first++;
    }
    return d_first;
}

template<class InputIt, class OutputIt, class UnaryPredicate>
OutputIt copy_if(InputIt first, InputIt last, 
                 OutputIt d_first, UnaryPredicate pred)
{
    while (first != last) {
        if (pred(*first))
            *d_first++ = *first;
        first++;
    }
    return d_first;
}

Example

#include <algorithm>
#include <iostream>
#include <vector>
#include <iterator>
#include <numeric>
 
int main()
{
    std::vector<int> from_vector(10);
    std::iota(from_vector.begin(), from_vector.end(), 0);
 
    std::vector<int> to_vector;
    std::copy(from_vector.begin(), from_vector.end(),
              std::back_inserter(to_vector));
// or, alternatively,
//  std::vector<int> to_vector(from_vector.size());
//  std::copy(from_vector.begin(), from_vector.end(), to_vector.begin());
// either way is equivalent to
//  std::vector<int> to_vector = from_vector;
 
    std::cout << "to_vector contains: ";
 
    std::copy(to_vector.begin(), to_vector.end(),
              std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
}

to_vector contains: 0 1 2 3 4 5 6 7 8 9

See also