std::copy_n
From cppreference.com
Defined in header <algorithm>
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template< class InputIt, class Size, class OutputIt > OutputIt copy_n( InputIt first, Size count, OutputIt result ); |
(1) | (since C++11) (constexpr since C++20) |
template< class ExecutionPolicy, class ForwardIt1, class Size, class ForwardIt2 > |
(2) | (since C++17) |
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) |
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. See execution policy for details. |
Type requirements | ||
-InputIt must meet the requirements of LegacyInputIterator.
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-OutputIt must meet the requirements of LegacyOutputIterator.
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-ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
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[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
ExecutionPolicy
is 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, ++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
[edit] See also
(C++11) |
copies a range of elements to a new location (function template) |
(C++20) |
copies a number of elements to a new location (algorithm function object) |