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    std::stable_partition

    From cppreference.com
     
    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Execution policies (C++17)
    Non-modifying sequence operations
    Batch operations
    (C++17)
    Search operations
    (C++11)                (C++11)(C++11)

    Modifying sequence operations
    Copy operations
    (C++11)
    (C++11)
    (C++11)
    Swap operations
    Transformation operations
    Generation operations
    Removing operations
    Order-changing operations
    (until C++17)(C++11)
    (C++20)(C++20)
    Sampling operations
    (C++17)

    Sorting and related operations
    Partitioning operations
    Sorting operations
    Binary search operations
    (on partitioned ranges)
    Set operations (on sorted ranges)
    Merge operations (on sorted ranges)
    Heap operations
    Minimum/maximum operations
    (C++11)
    (C++17)
    Lexicographical comparison operations
    (C++20)
    Permutation operations
    C library
    Numeric operations
    Operations on uninitialized memory
     
    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:
    See More

    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:

    (until C++11)
    (since C++11)

    Parameters

    first, last - the pair of iterators defining 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 N as std::distance(first, last):

    1) Exactly N applications of p.
    O(N) swaps if there is enough extra memory, otherwise at most N⋅log2(N) swaps.
    2) O(N) applications of p.
    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

    		 divides a range of elements into two groups
    (function template) [edit]
    (C++20)
    		 divides elements into two groups while preserving their relative order
    (algorithm function object)[edit]