NAME¶

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 (1) (constexpr since C++20)
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 integer i in [0, 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 participates in overload resolution only if

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¶

// 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¶

copy copies a range of elements to a new location
copy_if (function template)
(C++11)
ranges::copy_n copies a number of elements to a new location
(C++20) (niebloid)