# std::stable_partition

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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)

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

Run this code

    #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

### See also

partition | divides a range of elements into two groups (function template)

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