Std::copy_n (3) Linux Manual Page

std::copy_n – std::copy_n

Synopsis

Defined in header<algorithm>

    template <class InputIt, class Size, class OutputIt>
    (since C++ 11)

        OutputIt copy_n(InputIt first, Size count, OutputIt result);
(until C++ 20)

    template <class InputIt, class Size, class OutputIt>
    (1)(since C++ 20)

        constexpr OutputIt copy_n(InputIt first, Size count, OutputIt result);

template <class ExecutionPolicy, class ForwardIt1, class Size, class ForwardIt2>
(2)(since C++ 17)

    ForwardIt2 copy_n(ExecutionPolicy &&policy, ForwardIt1 first, Size count, ForwardIt2 result);
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

std::copy_n (3) Linux Manual Page