std:: copy_n

Parameters

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';
}

1234
5050

See also