std::copy_n

 

C++

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Symbols index

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Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (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) (constexpr since C++20)
template< class ExecutionPolicy,           class ForwardIt1, class Size, class ForwardIt2 > ForwardIt2 copy_n( ExecutionPolicy&& policy,                    ForwardIt1 first, Size count, ForwardIt2 result );	(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)

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

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