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    std::copy_n

    From cppreference.com
     
    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Non-modifying sequence operations    
    Batch operations
    (C++17)
    Search operations
    Modifying sequence operations
    Copy operations
    (C++11)
    (C++11)
    (C++11)
    Swap operations
    Transformation operations
    Generation operations
    Removing operations
    Order-changing operations
    (until C++17)(C++11)
    (C++20)(C++20)
    Sampling operations
    (C++17)

    Sorting and related operations
    Partitioning operations
    (C++11)    
    (C++11)

    Sorting operations
    Binary search operations
    (on partitioned ranges)
    Set operations (on sorted ranges)
    Merge operations (on sorted ranges)
    Heap operations
    Minimum/maximum operations
    (C++11)
    (C++17)
    Lexicographical comparison operations
    (C++20)
    Permutation operations


     
    Defined in header <algorithm> 
    template< class InputIt, class Size, class OutputIt >
OutputIt copy_n( InputIt first, Size count, OutputIt d_first );
    		(1) (since C++11)
    (constexpr since C++20)    		 
    template< class ExecutionPolicy,
          class ForwardIt1, class Size, class ForwardIt2 >
ForwardIt2 copy_n( ExecutionPolicy&& policy,
                   ForwardIt1 first, Size count, ForwardIt2 d_first );
    		(2) (since C++17)
    1) If count is positive, copies all elements in the source range [first, std::next(first, count)) to the destination range [d_first, std::next(d_first, count)). Otherwise does nothing.
    The source and destination ranges can overlap, but leads to unpredictable ordering of the results.
    2) Same as (1), but executed according to policy.
    This overload participates in overload resolution only if the value of the following expression is true:

    std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>

    		(until C++20)

    std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>

    		(since C++20)

    Parameters

    first - the beginning of the source range
    count - number of the elements to copy
    d_first - the beginning of the destination range
    policy - the execution policy to use
    Type requirements
    -
    InputIt must meet the requirements of LegacyInputIterator.
    -
    OutputIt must meet the requirements of LegacyOutputIterator.
    -
    ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
    -
    Size must be convertible to an integral type.

    Return value

    The past-the-end iterator of the destination range.

    Complexity

    Exactly max(count,0) assignments.

    Exceptions

    2) During the execution process:
    • If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
    • If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

    Possible implementation

    template<class InputIt, class Size, class OutputIt>
constexpr //< since C++20
OutputIt copy_n(InputIt first, Size count, OutputIt d_first)
{
    if (count > 0)
    {
        *d_first = *first;
        ++d_first;
        for (Size i = 1; i != count; ++i, (void)++d_first)
            *d_first = *++first;
    }
    
    return d_first;
}

    Example

    Run this code
    #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

    See also

    (C++20)
    		 copies a number of elements to a new location
    (algorithm function object)[edit]
    (C++11)
    		 copies a range of elements to a new location
    (function template & algorithm function object)[edit]
    (C++20)(C++20)