std::copy, std::copy_if

Defined in header `<algorithm>`
template< class InputIt, class OutputIt > OutputIt copy( InputIt first, InputIt last, OutputIt d_first );	(1)
template< class InputIt, class OutputIt, class UnaryPredicate > OutputIt copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPredicate pred );	(2)	(since C++11)

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 function d_first shall not be in the range [first, last). std::copy_backward must be used in that case.

For std::copy_if source and destination ranges shall not overlap.

first, last	-	the range of elements to copy
d_first	-	the beginning of the destination range.
pred	-	unary predicate which returns true for the required elements. The signature of the predicate function should be equivalent to the following: bool pred(const Type &a); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type.
Type requirements
- `InputIt` must meet the requirements of `InputIterator`.
- `OutputIt` must meet the requirements of `OutputIterator`.
- `UnaryPredicate` must meet the requirements of `Predicate`.

[edit] Return value

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

[edit] Complexity

1) Exactly last - first assignments

2) Exactly last - first applications of the predicate

[edit] 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).

[edit] Possible implementation

First version

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

Second version

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;
}

If you do not have C++11, an equivalent to std::copy_if is to use std::remove_copy_if with the negated predicate.

template<class InputIt, class OutputIt, class UnaryPredicate>
OutputIt copy_if(InputIt first, InputIt last, 
                 OutputIt d_first, UnaryPredicate pred)
{
    return std::remove_copy_if(first, last, d_first, std::not1(pred));
}

[edit] Example

The following code uses copy to both copy the contents of one vector to another and to display the resulting vector:

Run this code

#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';
}

Output:

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

[edit] See also

copy_backward	copies a range of elements in backwards order (function template)
copy_n (C++11)	copies a number of elements to a new location (function template)
remove_copyremove_copy_if	copies a range of elements omitting those that satisfy specific criteria (function template)

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)