std::partition_copy(3)

NAME
std::partition_copy - std::partition_copy

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class OutputIt1,
   class OutputIt2, class UnaryPredicate >
   std::pair<OutputIt1, OutputIt2> (since C++11)
   partition_copy( InputIt first, InputIt last, (until C++20)
   OutputIt1 d_first_true, OutputIt2 d_first_false,
   UnaryPredicate p );
   template< class InputIt, class OutputIt1,
   class OutputIt2, class UnaryPredicate >
   constexpr std::pair<OutputIt1, OutputIt2> (since C++20)
   partition_copy( InputIt first, InputIt last, (1)
   OutputIt1 d_first_true, OutputIt2 d_first_false,
   UnaryPredicate p );
   template< class ExecutionPolicy, class ForwardIt1, class
   ForwardIt2,
   class ForwardIt3, class UnaryPredicate >
   std::pair<ForwardIt2, ForwardIt3> (2) (since C++17)
   partition_copy( ExecutionPolicy&& policy, ForwardIt1 first,
   ForwardIt1 last,
   ForwardIt2 d_first_true, ForwardIt3 d_first_false,
   UnaryPredicate p );
   1) Copies the elements from the range [first, last) to two different ranges
   depending on the value returned by the predicate p. The elements that satisfy the
   predicate p are copied to the range beginning at d_first_true. The rest of the
   elements are copied to the range beginning at d_first_false.
   The behavior is undefined if the input range overlaps either of the output ranges.
   2) Same as (1), but executed according to policy. This overload does not participate
   in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.

Parameters
   first, last - the range of elements to copy from
   d_first_true - the beginning of the output range for the elements that satisfy p
   d_first_false - the beginning of the output range for the elements that do not
                   satisfy p
   policy - the execution policy to use. See execution policy for details.
                   unary predicate which returns true if the element should be
                   placed in d_first_true.
                   The expression p(v) must be convertible to bool for every argument v
   p - 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.
   -
   The type of dereferenced InputIt must meet the requirements of CopyAssignable.
   -
   OutputIt1, OutputIt2 must meet the requirements of LegacyOutputIterator.
   -
   ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of
   LegacyForwardIterator. ForwardIt1's value type must be CopyAssignable, writable to
   ForwardIt2 and ForwardIt3, and convertible to UnaryPredicate's argument type
   -
   UnaryPredicate must meet the requirements of Predicate.

Return value
   A std::pair constructed from the iterator to the end of the d_first_true range and
   the iterator to the end of the d_first_false range.

Complexity
   Exactly distance(first, last) applications of p.
   For the overload with an ExecutionPolicy, there may be a performance cost if
   ForwardIt's value type is not CopyConstructible.

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 OutputIt1,
            class OutputIt2, class UnaryPredicate>
   std::pair<OutputIt1, OutputIt2>
       partition_copy(InputIt first, InputIt last,
                      OutputIt1 d_first_true, OutputIt2 d_first_false,
                      UnaryPredicate p)
   {
       while (first != last) {
           if (p(*first)) {
               *d_first_true = *first;
               ++d_first_true;
           } else {
               *d_first_false = *first;
               ++d_first_false;
           }
           ++first;
       }
       return std::pair<OutputIt1, OutputIt2>(d_first_true, d_first_false);
   }

Example
// Run this code
 #include <iostream>
 #include <algorithm>
 #include <utility>
 int main()
 {
     int arr [10] = {1,2,3,4,5,6,7,8,9,10};
     int true_arr [5] = {0};
     int false_arr [5] = {0};
     std::partition_copy(std::begin(arr), std::end(arr), std::begin(true_arr),std::begin(false_arr),
                        [] (int i) {return i > 5;});
     std::cout << "true_arr: ";
     for (int x : true_arr) {
         std::cout << x << ' ';
     }
     std::cout << '\n';
     std::cout << "false_arr: ";
     for (int x : false_arr) {
         std::cout << x << ' ';
     }
     std::cout << '\n';
     return 0;
 }

Output:
 true_arr: 6 7 8 9 10
 false_arr: 1 2 3 4 5

See also
   partition divides a range of elements into two groups
                        (function template)
                        divides elements into two groups while preserving their
   stable_partition relative order
                        (function template)
   copy copies a range of elements to a new location
   copy_if (function template)
   (C++11)
   remove_copy copies a range of elements omitting those that satisfy
   remove_copy_if specific criteria
                        (function template)
   ranges::partition_copy copies a range dividing the elements into two groups
   (C++20) (niebloid)