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    std::partition

    From cppreference.com
     
    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Execution policies (C++17)
    Non-modifying sequence operations
    Batch operations
    (C++17)
    Search operations
    (C++11)                (C++11)(C++11)

    Modifying sequence operations
    Copy operations
    (C++11)
    (C++11)
    (C++11)
    Swap operations
    Transformation operations
    Generation operations
    Removing operations
    Order-changing operations
    (until C++17)(C++11)
    (C++20)(C++20)
    Sampling operations
    (C++17)

    Sorting and related operations
    Partitioning operations
    Sorting operations
    Binary search operations
    (on partitioned ranges)
    Set operations (on sorted ranges)
    Merge operations (on sorted ranges)
    Heap operations
    Minimum/maximum operations
    (C++11)
    (C++17)
    Lexicographical comparison operations
    (C++20)
    Permutation operations
    C library
    Numeric operations
    Operations on uninitialized memory
     
    Defined in header <algorithm>
    template< class ForwardIt, class UnaryPred > ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPred p );
    		 (1) (constexpr since C++20)
    template< class ExecutionPolicy, class ForwardIt, class UnaryPred > ForwardIt partition( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPred p );
    		 (2) (since C++17)
    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 all following conditions are satisfied:

    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 pair of iterators defining the range of elements to reorder
    policy - the execution policy to use
    p - 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 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 N as std::distance(first, last):

    1) Exactly N applications of p.
    At most N/2 swaps if ForwardIt meets the requirements of LegacyBidirectionalIterator, and at most N swaps otherwise.
    2) O(N) applications of p.
    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
    last to be LegacyBidirectionalIterator     		only required to be
    LegacyForwardIterator
    LWG 2150 C++98 std::partition was only required to place one element
    satisfying p before one element not satisfying p     		corrected the
    requirement

    See also

    (C++11)
    		 determines if the range is partitioned by the given predicate
    (function template) [edit]
    		divides elements into two groups while preserving their relative order
    (function template) [edit]
    (C++20)
    		 divides a range of elements into two groups
    (algorithm function object)[edit]