std::partition(3)

NAME
std::partition - std::partition

Synopsis
   Defined in header <algorithm>
   template< class ForwardIt, class UnaryPred >
   ForwardIt partition( ForwardIt first, ForwardIt last, (1) (constexpr since C++20)
   UnaryPred p );
   template< class ExecutionPolicy, class ForwardIt, class
   UnaryPred >
   ForwardIt partition( ExecutionPolicy&& policy, (2) (since C++17)
                        ForwardIt first, ForwardIt last,
   UnaryPred p );
   1) Reorders the elements in the range [first, last) in such a way that all elements
   for which the predicate p returns true precede all elements for which predicate p
   returns false. Relative order of the elements is not preserved.
   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
   the type of *first is not Swappable
   (until C++11)
   ForwardIt is not ValueSwappable
   (since C++11), the behavior is undefined.

Parameters
   first, last - the range of elements to reorder
   policy - the execution policy to use. See execution policy for details.
                 unary predicate which returns true if the element should be ordered
                 before other elements.
                 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 ForwardIt,
                 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
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.
   -
   UnaryPred must meet the requirements of Predicate.

Return value
   Iterator to the first element of the second group.

Complexity
   Given \(\scriptsize N\)N as std::distance(first, last):
   1) Exactly \(\scriptsize N\)N applications of p.
   At most \(\scriptsize N/2\)N/2 swaps if ForwardIt meets the requirements of
   LegacyBidirectionalIterator, and at most \(\scriptsize N\)N swaps otherwise.
   2) \(\scriptsize O(N)\)O(N) applications of p.
   \(\scriptsize O(N \cdot log(N))\)O(N·log(N)) swaps.

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
   Implements overload (1) preserving C++11 compatibility.
   template<class ForwardIt, class UnaryPred>
   ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPred p)
   {
       first = std::find_if_not(first, last, p);
       if (first == last)
           return first;
       for (auto i = std::next(first); i != last; ++i)
           if (p(*i))
           {
               std::iter_swap(i, first);
               ++first;
           }
       return first;
   }

Example
// Run this code
 #include <algorithm>
 #include <forward_list>
 #include <iostream>
 #include <iterator>
 #include <vector>
 template<class ForwardIt>
 void quicksort(ForwardIt first, ForwardIt last)
 {
     if (first == last)
         return;
     auto pivot = *std::next(first, std::distance(first, last) / 2);
     auto middle1 = std::partition(first, last, [pivot](const auto& em)
     {
         return em < pivot;
     });
     auto middle2 = std::partition(middle1, last, [pivot](const auto& em)
     {
         return !(pivot < em);
     });
     quicksort(first, middle1);
     quicksort(middle2, last);
 }
 int main()
 {
     std::vector<int> v{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
     std::cout << "Original vector: ";
     for (int elem : v)
         std::cout << elem << ' ';
     auto it = std::partition(v.begin(), v.end(), [](int i) {return i % 2 == 0;});
     std::cout << "\nPartitioned vector: ";
     std::copy(std::begin(v), it, std::ostream_iterator<int>(std::cout, " "));
     std::cout << "* ";
     std::copy(it, std::end(v), std::ostream_iterator<int>(std::cout, " "));
     std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
     std::cout << "\nUnsorted list: ";
     for (int n : fl)
         std::cout << n << ' ';
     quicksort(std::begin(fl), std::end(fl));
     std::cout << "\nSorted using quicksort: ";
     for (int fi : fl)
         std::cout << fi << ' ';
     std::cout << '\n';
 }

Possible output:
 Original vector: 0 1 2 3 4 5 6 7 8 9
 Partitioned vector: 0 8 2 6 4 * 5 3 7 1 9
 Unsorted list: 1 30 -4 3 5 -4 1 6 -8 2 -5 64 1 92
 Sorted using quicksort: -8 -5 -4 -4 1 1 1 2 3 5 6 30 64 92
   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 498 C++98 std::partition required first and only required to be
                       last to be LegacyBidirectionalIterator LegacyForwardIterator
                       std::partition was only required to place
   LWG 2150 C++98 one element corrected the
                       satisfying p before one element not requirement
                       satisfying p

See also
   is_partitioned determines if the range is partitioned by the given predicate
   (C++11) (function template)
                     divides elements into two groups while preserving their relative
   stable_partition order
                     (function template)
   ranges::partition divides a range of elements into two groups
   (C++20) (niebloid)