std::copy_if() algorithm
- od C++20
- od C++17
- od C++11
// (1)
template< class InputIt, class OutputIt, class UnaryPredicate >
constexpr OutputIt copy_if( InputIt first, InputIt last,
OutputIt d_first,
UnaryPredicate pred );
// (2)
template< class ExecutionPolicy,
class ForwardIt1, class ForwardIt2, class UnaryPredicate >
ForwardIt2 copy_if( ExecutionPolicy&& policy,
ForwardIt1 first, ForwardIt1 last,
ForwardIt2 d_first,
UnaryPredicate pred );
// (1)
template< class InputIt, class OutputIt, class UnaryPredicate >
OutputIt copy_if( InputIt first, InputIt last,
OutputIt d_first,
UnaryPredicate pred );
// (2)
template< class ExecutionPolicy,
class ForwardIt1, class ForwardIt2, class UnaryPredicate >
ForwardIt2 copy_if( ExecutionPolicy&& policy,
ForwardIt1 first, ForwardIt1 last,
ForwardIt2 d_first,
UnaryPredicate pred );
// (1)
template< class InputIt, class OutputIt, class UnaryPredicate >
OutputIt copy_if( InputIt first, InputIt last,
OutputIt d_first,
UnaryPredicate pred );
Copies the elements in the range, defined by [first
; last
), to another range beginning at d_first
.
-
(1) Only copies the elements for which the predicate
pred
returnstrue
.Undefined BehaviourThe behavior is undefined if
d_first
is within the range [first
;last
). -
(2) Same as (1), but executed according to
policy
.
These overloads participate in overload resolution only if
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(do C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(od C++20) is true
.
Parameters
first second | 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 The expression |
Type requirements
InputIt | LegacyInputIterator |
OutputIt | LegacyOutputIterator |
ForwardIt1 ForwardIt2 | LegacyForwardIterator |
UnaryPredicate | Predicate |
Return value
Output iterator to the element in the destination range, one past the last element copied.
Complexity
Exactly last - first
applications of the predicate, between 0
and last - first
assignments (assignment for every element for which predicate is equal to true).
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 none otherExecutionPolicy
, the behavior is implementation-defined. - If the algorithm fails to allocate memory,
std::bad_alloc
is thrown.
Possible implementation
copy_if (1)
template<class InputIt, class OutputIt, class UnaryPredicate>
OutputIt copy_if(InputIt first, InputIt last,
OutputIt d_first, UnaryPredicate pred)
{
for (; first != last; ++first)
{
if (pred(*first))
{
*d_first = *first;
++d_first;
}
}
return d_first;
}
Notes
In practice, implementations of std::copy_if
avoid multiple assignments and use bulk copy_if functions such as std::memmove
if the value type is TriviallyCopyable
and the iterator types satisfy LegacyContiguousIterator
.
When copy_ifing overlapping ranges, std::copy_if
is appropriate when copy_ifing to the left (beginning of the destination range is outside the source range),
while std::copy_if_backward
is appropriate when copy_ifing to the right (end of the destination range is outside the source range).
Examples
#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_if(from_vector.begin(), from_vector.end(),
std::back_inserter(to_vector));
// or, alternatively,
// std::vector<int> to_vector(from_vector.size());
// std::copy_if(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_if(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_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_if(from_vector.begin(), from_vector.end(),
std::back_inserter(to_vector),
[](int x) { return x % 3 == 0; });
for (int x : to_vector)
std::cout << x << ' ';
std::cout << '\n';
}
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
Hover to see the original license.