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std::copy_n

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

Defined in header <algorithm>

template< class InputIt, class Size, class OutputIt >
OutputIt copy_n( InputIt first, Size count, OutputIt result );

(1)

(since C++11)

template< class ExecutionPolicy, class InputIt, class Size, class OutputIt >
OutputIt copy_n( ExecutionPolicy&& policy, InputIt first, Size count, OutputIt result );

(2)

(since C++17)

1) Copies exactly count values from the range beginning at first to the range beginning at result, if count>0. Does nothing otherwise.

2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless 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 `InputIterator`.
- `OutputIt` must meet the requirements of `OutputIterator`.

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, std::terminate is called.
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

#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:

copycopy_if (C++11)	copies a range of elements to a new location (function template)
std::experimental::parallel::copy_n (parallelism TS)	parallelized version of `std::copy_n` (function template)