std::copy, std::copy_if

 

C++

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Technical Specifications

 

Algorithm library

Functions
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each countcount_if mismatch equal	findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) fill fill_n transform generate generate_n removeremove_if remove_copyremove_copy_if	replacereplace_if replace_copyreplace_copy_if swap swap_ranges iter_swap reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) unique unique_copy
Partitioning operations
is_partitioned (C++11) partition partition_copy (C++11)	stable_partition partition_point (C++11)
Sorting operations (on sorted ranges)
is_sorted (C++11) is_sorted_until (C++11) sort	partial_sort partial_sort_copy stable_sort nth_element
Binary search operations (on sorted ranges)
lower_bound upper_bound	binary_search equal_range
Set operations (on sorted ranges)
merge inplace_merge includes	set_difference set_intersection set_symmetric_difference set_union
Heap operations
is_heap (C++11) is_heap_until (C++11) make_heap	push_heap pop_heap sort_heap
Minimum/maximum operations
max max_element min min_element minmax (C++11) minmax_element (C++11)	lexicographical_compare is_permutation (C++11) next_permutation prev_permutation
Numeric operations
iota (C++11) accumulate inner_product	adjacent_difference partial_sum
C library
qsort	bsearch

 

Defined in header <algorithm>
template< class InputIt, class OutputIt > OutputIt copy( InputIt first, InputIt last, OutputIt d_first );	(1)
template< class InputIt, class OutputIt, class UnaryPredicate > OutputIt copy_if( InputIt first, InputIt last,                   OutputIt d_first,                   UnaryPredicate pred );	(2)	(since C++11)

Copies the elements in the range, defined by [first, last), to another range beginning at d_first. The second function only copies the elements for which the predicate pred returns true. The order of the elements that are not removed is preserved.

For std::copy, the behavior is undefined if d_first is within the range [first, last). In this case, std::copy_backward may be used instead.

For std::copy_if, the behavior is undefined if the source and the destination ranges overlap.

Parameters

first, last	-	the range of elements to copy
d_first	-	the beginning of the destination range.
pred	-	unary predicate which returns ​true for the required elements. The signature of the predicate function should be equivalent to the following:  bool pred(const Type &a); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type. ​
Type requirements
- InputIt must meet the requirements of InputIterator.
- OutputIt must meet the requirements of OutputIterator.
- UnaryPredicate must meet the requirements of Predicate.

Return value

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

Complexity

1) Exactly last - first assignments

2) Exactly last - first applications of the predicate

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

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).

Possible implementation

First version
template<class InputIt, class OutputIt> OutputIt copy(InputIt first, InputIt last, OutputIt d_first) { while (first != last) { *d_first++ = *first++; } return d_first; }
Second version
template<class InputIt, class OutputIt, class UnaryPredicate> OutputIt copy_if(InputIt first, InputIt last, OutputIt d_first, UnaryPredicate pred) { while (first != last) { if (pred(*first)) *d_first++ = *first; first++; } return d_first; }

Example

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

Run this code

#include <algorithm>
#include <iostream>
#include <vector>
#include <iterator>
#include <numeric>
 
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';
}

Output:

to_vector contains: 0 1 2 3 4 5 6 7 8 9

See also

copy_backward	copies a range of elements in backwards order (function template)
copy_n (C++11)	copies a number of elements to a new location (function template)
fill	assigns a range of elements a certain value (function template)
remove_copyremove_copy_if	copies a range of elements omitting those that satisfy specific criteria (function template)
std::experimental::parallel::copy (parallelism TS)	parallelized version of std::copy (function template)
std::experimental::parallel::copy_if (parallelism TS)	parallelized version of std::copy_if (function template)