std::copy_n

Defined in header <algorithm>

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)

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 do 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 \(\scriptsize \max(\mathtt{count},0)\)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

#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

ranges::copy_n (C++20)	copies a number of elements to a new location (algorithm function object)
copycopy_if (C++11)	copies a range of elements to a new location (function template & algorithm function object)
ranges::copyranges::copy_if (C++20)(C++20)