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std::copy_n

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 integer 0 ≤ i < 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 does 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	-	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

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

#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

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 (niebloid)