std::partition_copy(3)

NAME
std::partition_copy - std::partition_copy

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class OutputIt1,
             class OutputIt2, class UnaryPred >
   std::pair<OutputIt1, OutputIt2> (since C++11)
       partition_copy( InputIt first, InputIt last, (1) (constexpr since C++20)
                       OutputIt1 d_first_true, OutputIt2
   d_first_false,
                       UnaryPred p );
   template< class ExecutionPolicy, class ForwardIt1, class
   ForwardIt2,
             class ForwardIt3, class UnaryPred >
   std::pair<ForwardIt2, ForwardIt3>
       partition_copy( ExecutionPolicy&& policy, (2) (since C++17)
                       ForwardIt1 first, ForwardIt1 last,
                       ForwardIt2 d_first_true, ForwardIt3
   d_first_false,
                       UnaryPred 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.
   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)
   If *first is not writable to d_first_true or d_first_false, the program is
   ill-formed.
   Among the input range and the two output ranges, if any two ranges overlap, the
   behavior is undefined.

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.
   -
   OutputIt1, OutputIt2 must meet the requirements of LegacyOutputIterator.
   -
   ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of
   LegacyForwardIterator.
   -
   UnaryPred 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 std::distance(first, last) applications of p.
   For the overload (2), 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
                        partition_copy (1)
   template<class InputIt, class OutputIt1,
            class OutputIt2, class UnaryPred>
   constexpr //< since C++20
   std::pair<OutputIt1, OutputIt2>
       partition_copy(InputIt first, InputIt last,
                      OutputIt1 d_first_true, OutputIt2 d_first_false,
                      UnaryPred p)
   {
       for (; first != last; ++first)
       {
           if (p(*first))
           {
               *d_first_true = *first;
               ++d_first_true;
           }
           else
           {
               *d_first_false = *first;
               ++d_first_false;
           }
       }
       return std::pair<OutputIt1, OutputIt2>(d_first_true, d_first_false);
   }

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

Output:
 true_arr: 5 6 7 8 9
 false_arr: 0 1 2 3 4
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
                      1. the value type of InputIt 1. not required
   P0896R4 C++11 (C++11)/ForwardIt1 (C++17) 2. the behavior is
           C++17 was required to be CopyAssignable undefined in this
                      2. the two output ranges could overlap case

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)