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std::stable_partition

Defined in header <algorithm>

Defined in header `<algorithm>`
template< class BidirIt, class UnaryPredicate > BidirIt stable_partition( BidirIt first, BidirIt last, UnaryPredicate p );	(1)
template< class ExecutionPolicy, class BidirIt, class UnaryPredicate > BidirIt stable_partition( ExecutionPolicy&& policy, BidirIt first, BidirIt last, UnaryPredicate p );	(2)	(since C++17)

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

(1)

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

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 signature of the predicate function should be equivalent to the following: `bool pred(const Type &a);` The signature does not need to have `const &`, but the function must not modify the objects passed to it. The type `Type` must be such that an object of type `BidirIt` can be dereferenced and then implicitly converted to `Type`.
Type requirements
-`BidirIt` must meet the requirements of `ValueSwappable` and `BidirectionalIterator`.
-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 = last - first.

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 three 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.

Example

#include <iostream>
#include <algorithm>
#include <vector>
 
int main()
{
    std::vector<int> v{0, 0, 3, 0, 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 0 0