# std::copy_n

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class InputIt, class Size, class OutputIt > OutputIt copy_n( InputIt first, Size count, OutputIt result ); |  | (since C++11) (until C++20)
template< class InputIt, class Size, class OutputIt > constexpr OutputIt copy_n( InputIt first, Size count, OutputIt result ); |  | (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 non-negative integer `i < n`, performs *(result + i) = *(first + i). Overlap of ranges is not permitted.
2) Same as (1), 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 | - | 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. See execution policy for details.
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

Exactly `count` assignments, if `count>0`.

### 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>
    OutputIt copy_n(InputIt first, Size count, OutputIt result)
    {
        if (count > 0) {
            *result++ = *first;
            for (Size i = 1; i < count; ++i) {
                *result++ = *++first;
            }
        }
        return result;
    }

### Example

Run this code

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

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

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

Output:

    1234

### See also

copycopy_if (C++11) | copies a range of elements to a new location (function template)

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