# std::stable_partition

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class BidirIt, class UnaryPred > BidirIt stable_partition( BidirIt first, BidirIt last, UnaryPred p ); | (1) | (constexpr since C++26)
template< class ExecutionPolicy, class BidirIt, class UnaryPred > BidirIt stable_partition( ExecutionPolicy&& policy, BidirIt first, BidirIt 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 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 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 any of the following conditions is satisfied, the behavior is undefined:

- The type of *first is not Swappable.

(until C++11)

- `BidirIt` is not ValueSwappable.
- The type of *first is not MoveConstructible.
- The type of *first is not MoveAssignable.

(since C++11)

### Parameters

first, last | - | 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 `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 LegacyBidirectionalIterator.
- `UnaryPred` must meet the requirements of Predicate.

### Return value

Iterator to the first element of the second group.

### Complexity

Given \(\scriptsize N\)N as std::distance (first, last):

1) Exactly \(\scriptsize N\)N applications of p.
\(\scriptsize O(N)\)O(N) swaps if there is enough extra memory, otherwise at most \(\scriptsize N \cdot log_{2}(N)\)N⋅log2(N) swaps.
2) \(\scriptsize O(N)\)O(N) applications of p.
\(\scriptsize N \cdot log(N)\)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.

### 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 | Std | Feature
--- | --- | --- | ---
`__cpp_lib_constexpr_algorithms` | `202306L` | (C++26) | constexpr stable sorting (1)

### Example

Run this code

    #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

### 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 2150 | C++98 | `std::stable_partition` was only required to place one element satisfying p before one element not satisfying p | corrected the requirement

### See also

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 (algorithm function object)

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