# std::copy, std::copy_if

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class InputIt, class OutputIt > OutputIt copy( InputIt first, InputIt last, OutputIt d_first ); |  | (until C++20)
template< class InputIt, class OutputIt > constexpr OutputIt copy( InputIt first, InputIt last, OutputIt d_first ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > ForwardIt2 copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first ); | (2) | (since C++17)
| (3) |
template< class InputIt, class OutputIt, class UnaryPredicate > OutputIt copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPredicate pred ); |  | (since C++11) (until C++20)
template< class InputIt, class OutputIt, class UnaryPredicate > constexpr OutputIt copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPredicate pred ); |  | (since C++20)
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`)` starting from first and proceeding to last - 1. 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 relative order of the elements that are copied 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. These overloads do not participate in overload resolution unless

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### 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 expression pred(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of VT&is not allowed, nor is VT unless for `VT` a move is equivalent to a copy (since C++11). ​
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator.
- `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, between ​0​ and (last - first) assignments (assignment for every element for which predicate is equal to true, dependent on predicate and input data).

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 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 and the iterator types satisfy LegacyContiguousIterator.

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

copy (1)
    template<class InputIt, class OutputIt>
    OutputIt copy(InputIt first, InputIt last,
                  OutputIt d_first)
    {
        for (; first != last; (void)++first, (void)++d_first)
            *d_first = *first;

        return d_first;
    }
copy_if (3)
    template<class InputIt, class OutputIt, class UnaryPredicate>
    OutputIt copy_if(InputIt first, InputIt last,
                     OutputIt d_first, UnaryPredicate pred)
    {
        for (; first != last; ++first)
        {
            if (pred(*first))
            {
                *d_first = *first;
                ++d_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 <iterator>
    #include <numeric>
    #include <vector>

    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';

        std::cout << "odd numbers in to_vector are: ";

        std::copy_if(to_vector.begin(), to_vector.end(),
                     std::ostream_iterator<int>(std::cout, " "),
                     [](int x) { return x % 2 != 0; });
        std::cout << '\n';

        std::cout << "to_vector contains these multiples of 3: ";

        to_vector.clear();
        std::copy_if(from_vector.begin(), from_vector.end(),
                     std::back_inserter(to_vector),
                     [](int x) { return x % 3 == 0; });

        for (int x : to_vector)
            std::cout << x << ' ';
        std::cout << '\n';
    }

Possible output:

    to_vector contains: 0 1 2 3 4 5 6 7 8 9
    odd numbers in to_vector are: 1 3 5 7 9
    to_vector contains these multiples of 3: 0 3 6 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)
ranges::copy ranges::copy_if (C++20)(C++20) | copies a range of elements to a new location (niebloid)

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