std::copy, std::copy_if

 

C++

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Ranges library (C++20)

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Technical specifications

Symbols index

External libraries

 

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 OutputIt > OutputIt copy( InputIt first, InputIt last,                OutputIt d_first );	(1)	(constexpr since C++20)
template< class ExecutionPolicy,           class ForwardIt1, class ForwardIt2 > ForwardIt2 copy( ExecutionPolicy&& policy,                  ForwardIt1 first, ForwardIt1 last,                  ForwardIt2 d_first );	(2)	(since C++17)
template< class InputIt, class OutputIt, class UnaryPred > OutputIt copy_if( InputIt first, InputIt last,                   OutputIt d_first, UnaryPred pred );	(3)	(since C++11) (constexpr since C++20)
template< class ExecutionPolicy,           class ForwardIt1, class ForwardIt2, class UnaryPred > ForwardIt2 copy_if( ExecutionPolicy&& policy,                     ForwardIt1 first, ForwardIt1 last,                     ForwardIt2 d_first, UnaryPred pred );	(4)	(since C++17)

Copies the elements in the range, defined by [first, last), to another range beginning at d_first (copy destination range).

1) Copies all elements in the range [first, last) starting from first and proceeding to last.

If d_first is in [first, last), the behavior is undefined. In this case, std::copy_backward may be used instead.

2) Copies the elements, 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)

If [first, last) and the copy destination range overlaps, the behavior is undefined.

3) Only copies the elements for which the predicate pred returns true. This copy algorithm is stable: the relative order of the elements that are copied is preserved.

If [first, last) and the copy destination range overlaps, the behavior is undefined.

4) Same as (3), 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, last	-	the range of elements to copy
d_first	-	the beginning of the destination range
policy	-	the execution policy to use. See execution policy for details.
pred	-	unary predicate which returns ​true for the required elements. The expression pred(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of InputIt, regardless of value category, and must not modify v. Thus, a parameter type of VT&is not allowed, nor is VT unless for VT a move is equivalent to a copy(since C++11). ​
Type requirements
- InputIt must meet the requirements of LegacyInputIterator.
- OutputIt must meet the requirements of LegacyOutputIterator.
- ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
- UnaryPred must meet the requirements of Predicate.

Return value

Output iterator to the element in the destination range, one past the last element copied.

Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize N\)N assignments.

3,4) Exactly \(\scriptsize N\)N applications of the predicate pred, and at most \(\scriptsize N\)N assignments.

For the overloads with an ExecutionPolicy, there may be a performance cost if ForwardIt1's value type is not MoveConstructible.

Exceptions

The overloads with a template parameter named ExecutionPolicy report 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

copy (1)
template<class InputIt, class OutputIt> OutputIt copy(InputIt first, InputIt last, OutputIt d_first) { for (; first != last; (void)++first, (void)++d_first) *d_first = *first;   return d_first; }
copy_if (3)
template<class InputIt, class OutputIt, class UnaryPred> OutputIt copy_if(InputIt first, InputIt last, OutputIt d_first, UnaryPred pred) { for (; first != last; ++first) if (pred(*first)) { *d_first = *first; ++d_first; }   return d_first; }

Notes

In practice, implementations of std::copy avoid multiple assignments and use bulk copy functions such as std::memmove if the value type is TriviallyCopyable and the iterator types satisfy LegacyContiguousIterator.

When copying overlapping ranges, std::copy is appropriate when copying to the left (beginning of the destination range is outside the source range) while std::copy_backward is appropriate when copying to the right (end of the destination range is outside the source range).

Example

The following code uses std::copy to both copy the contents of one std::vector to another and to display the resulting std::vector.

Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>
 
int main()
{
    std::vector<int> from_vector(10);
    std::iota(from_vector.begin(), from_vector.end(), 0);
 
    std::vector<int> to_vector;
    std::copy(from_vector.begin(), from_vector.end(),
              std::back_inserter(to_vector));
// or, alternatively,
//  std::vector<int> to_vector(from_vector.size());
//  std::copy(from_vector.begin(), from_vector.end(), to_vector.begin());
// either way is equivalent to
//  std::vector<int> to_vector = from_vector;
 
    std::cout << "to_vector contains: ";
 
    std::copy(to_vector.begin(), to_vector.end(),
              std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
 
    std::cout << "odd numbers in to_vector are: ";
 
    std::copy_if(to_vector.begin(), to_vector.end(),
                 std::ostream_iterator<int>(std::cout, " "),
                 [](int x) { return x % 2 != 0; });
    std::cout << '\n';
 
    std::cout << "to_vector contains these multiples of 3: ";
 
    to_vector.clear();
    std::copy_if(from_vector.begin(), from_vector.end(),
                 std::back_inserter(to_vector),
                 [](int x) { return x % 3 == 0; });
 
    for (const int x : to_vector)
        std::cout << x << ' ';
    std::cout << '\n';
}

Possible output:

to_vector contains: 0 1 2 3 4 5 6 7 8 9
odd numbers in to_vector are: 1 3 5 7 9
to_vector contains these multiples of 3: 0 3 6 9

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2039	C++11	the return value of std::copy_if was not specified	specified
LWG 2044	C++11	the stability of std::copy_if was not defined	defined

See also

copy_backward	copies a range of elements in backwards order (function template)
reverse_copy	creates a copy of a range that is reversed (function template)
copy_n (C++11)	copies a number of elements to a new location (function template)
fill	copy-assigns the given value to every element in a range (function template)
remove_copyremove_copy_if	copies a range of elements omitting those that satisfy specific criteria (function template)
ranges::copyranges::copy_if (C++20)(C++20)	copies a range of elements to a new location (niebloid)