# std::copy_n

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class Size, class OutputIt > OutputIt copy_n( InputIt first, Size count, OutputIt result ); | (1) | (since C++11) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class Size, class ForwardIt2 > ForwardIt2 copy_n( ExecutionPolicy&& policy, ForwardIt1 first, Size count, ForwardIt2 result ); | (2) | (since C++17)

1) Copies exactly count values from the range beginning at first to the range beginning at result. Formally, for each integer i in `[`​0​`,` count`)`, performs *(result + i) = *(first + i).
Overlap of ranges is formally permitted, but leads to unpredictable ordering of the results.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

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 | - | the beginning of the range of elements to copy from
count | - | number of the elements to copy
result | - | the beginning of the destination range
policy | - | the execution policy to use
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator.

### Return value

Iterator in the destination range, pointing past the last element copied if count > 0 or result otherwise.

### Complexity

Zero assignments if count < 0; count assignments otherwise.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports 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.

### Possible implementation

    template<class InputIt, class Size, class OutputIt>
    constexpr //< since C++20
    OutputIt copy_n(InputIt first, Size count, OutputIt result)
    {
        if (count > 0)
        {
            *result = *first;
            ++result;
            for (Size i = 1; i != count; ++i, ++result)
                *result = *++first;
        }

        return result;
    }

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <numeric>
    #include <string>
    #include <vector>

    int main()
    {
        std::string in {"1234567890"};
        std::string out;

        std::copy_n(in.begin(), 4, std::back_inserter(out));
        std::cout << out << '\n';

        std::vector<int> v_in(128);
        std::iota(v_in.begin(), v_in.end(), 1);
        std::vector<int> v_out(v_in.size());

        std::copy_n(v_in.cbegin(), 100, v_out.begin());
        std::cout << std::accumulate(v_out.begin(), v_out.end(), 0) << '\n';
    }

Output:

    1234
    5050

### See also

copycopy_if (C++11) | copies a range of elements to a new location (function template)
ranges::copy_n (C++20) | copies a number of elements to a new location (algorithm function object)

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