std::copy_n
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C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Algorithm library
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
(C++17)
(C++17) (C++17)(C++17)(C++20)
(C++17)
(C++11)
(until C++17)(C++11)
(C++20)(C++20)
(C++17)
(C++17)
(C++17)
(C++17)
Defined in header
<algorithm>
template< class InputIt, class Size, class OutputIt >
OutputIt copy_n( InputIt first, Size count, OutputIt result );
(1)
(since C++11) OutputIt copy_n( InputIt first, Size count, OutputIt result );
(constexpr since C++20)
template< class ExecutionPolicy,
(2)
(since C++17)
class ForwardIt1, class Size, class ForwardIt2 >
ForwardIt2 copy_n( ExecutionPolicy&& policy,
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 all following conditions are satisfied:
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)Contents
[edit] 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
Type requirements
-
InputIt must meet the requirements of LegacyInputIterator.
-
OutputIt must meet the requirements of LegacyOutputIterator.
-
ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
[edit] Return value
Iterator in the destination range, pointing past the last element copied if count > 0 or result otherwise.
[edit] Complexity
Zero assignments if count < 0; count assignments otherwise.
[edit] 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
ExecutionPolicyis one of the standard policies, std::terminate is called. For any otherExecutionPolicy, the behavior is implementation-defined. - If the algorithm fails to allocate memory, std::bad_alloc is thrown.
[edit] 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, (void)++result) *result = *++first; } return result; }
[edit] 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