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std::stable_partition

Defined in header `<algorithm>`
	(1)
template< class BidirIt, class UnaryPredicate > BidirIt stable_partition( BidirIt first, BidirIt last, UnaryPredicate p );		(until C++26)
template< class BidirIt, class UnaryPredicate > constexpr BidirIt stable_partition( BidirIt first, BidirIt last, UnaryPredicate p );		(since C++26)
template< class ExecutionPolicy, class BidirIt, class UnaryPredicate > BidirIt stable_partition( ExecutionPolicy&& policy, BidirIt first, BidirIt last, UnaryPredicate 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 the elements for which predicate p returns false. Relative order of the elements is preserved.

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>>` is `true`.	(until C++20)
`std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>` is `true`.	(since C++20)

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` 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 `BidirIt`, 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` must meet the requirements of ValueSwappable and LegacyBidirectionalIterator.
-The type of dereferenced `BidirIt` must meet the requirements of MoveAssignable and MoveConstructible.
-`UnaryPredicate` must meet the requirements of Predicate.

Return value

Iterator to the first element of the second group.

Complexity

Given N = std::distance(first, last),

1) Exactly N applications of the predicate 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 the predicate

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.

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.

Feature-test macro	Value	Comment
`__cpp_lib_constexpr_algorithms`	202306L	`constexpr` stable sorting

Example

#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

partition	divides a range of elements into two groups (function template)
ranges::stable_partition (C++20)	divides elements into two groups while preserving their relative order (niebloid)