std::stable_partition() algorithm

since C++26

// (1)
template< class BidirIt, class UnaryPredicate >
constexpr BidirIt stable_partition( BidirIt first, BidirIt last, UnaryPredicate p );

// (2)
template< class ExecutionPolicy, class BidirIt, class UnaryPredicate >
BidirIt stable_partition( ExecutionPolicy&& policy,
                          BidirIt first, BidirIt last, UnaryPredicate p );

since C++17

// (1)
template< class BidirIt, class UnaryPredicate >
BidirIt stable_partition( BidirIt first, BidirIt last, UnaryPredicate p );

// (2)
template< class ExecutionPolicy, class BidirIt, class UnaryPredicate >
BidirIt stable_partition( ExecutionPolicy&& policy,
                          BidirIt first, BidirIt last, UnaryPredicate p );

until C++17

// (1)
template< class BidirIt, class UnaryPredicate >
BidirIt stable_partition( BidirIt first, BidirIt last, UnaryPredicate 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 the elements for which predicate p returns false.

  Relative order of the elements is preserved.

• (2) Same as (1), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if 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 reorder.
policy	The execution policy to use. See execution policy for details.
p	Unary predicate which returns true for the required element. 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 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

BidirIt	LegacyBidirectionalIterator ValueSwappable
Dereferenced BidirIt	MoveAssignable MoveConstructible
UnaryPredicate	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 and O(N) swaps if there is enough extra memory. If memory is insufficient, at most N * log(N) swaps.

(2) O(N * log(N)) swaps and O(N) applications of p.

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 any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

stable_partition (1)

template<class ForwardIt, class UnaryPredicate>
ForwardIt stable_partition(ForwardIt first, ForwardIt last, UnaryPredicate 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;
}

Notes

This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.

Implementations in libc++ and libstdc++ also accept ranges denoted by LegacyForwardIterators as an extension.

Examples

Main.cpp

#include <algorithm>
#include <iostream>
#include <vector>

int main()
{
    std::vector<int> v {0, 0, 3, -1, 2, 4, 5, 0, 7};
    std::stable_partition(v.begin(), v.end(), [](int n) { return n > 0; });
    for (int n : v)
        std::cout << n << ' ';
    std::cout << '\n';
}

Output

3 2 4 5 7 0 0 -1 0