# std::copy, std::copy_if

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class OutputIt > OutputIt copy( InputIt first, InputIt last, OutputIt d_first ); | (1) |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > ForwardIt2 copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first ); | (2) | (since C++17)
template< class InputIt, class OutputIt, class UnaryPredicate > OutputIt copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPredicate pred ); | (3) | (since C++11)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryPredicate > ForwardIt2 copy_if( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, UnaryPredicate pred ); | (4) | (since C++17)

Copies the elements in the range, defined by `[first, last)`, to another range beginning at `d_first`.

1) Copies all elements in the range `[first, last)`. The behavior is undefined if `d_first` is within the range `[first, last)`. In this case, std::copy_backward may be used instead.
3) Only copies the elements for which the predicate `pred` returns true. The order of the elements that are not removed is preserved. The behavior is undefined if the source and the destination ranges overlap.
2,4) Same as (1,3), but executed according to `policy`. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

### Parameters

first, last | - | the range of elements to copy
d_first | - | the beginning of the destination range.
policy | - | the execution policy to use. See execution policy for details.

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`.
- `ForwardIt1, ForwardIt2` must meet the requirements of `ForwardIterator`.
- `UnaryPredicate` must meet the requirements of `Predicate`.

### Return value

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

### Complexity

1-2) Exactly `last - first` assignments

3-4) Exactly `last - first` applications of the predicate

For the overloads with an ExecutionPolicy, there may be a performance cost if `ForwardIt1`'s value_type is not `MoveConstructible`.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the three standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### 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

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;
    }

### 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

### See also

copy_backward | copies a range of elements in backwards order (function template)
reverse_copy | creates a copy of a range that is reversed (function template)
copy_n (C++11) | copies a number of elements to a new location (function template)
fill | copy-assigns the given value to every element in a range (function template)
remove_copy remove_copy_if | copies a range of elements omitting those that satisfy specific criteria (function template)

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