How is stable_partition an adaptive algorithm?

Question

stable_partition is a function template present in algorithm header file of c++ STL. I read that it is an adaptive algorithm and its time complexity is O(n*logn) or O(n) dep

Answer 1

There are 2 implementations.

1. "slower" O(n*logn) implementation
2. "faster" O(n) implementation

The fast implementation however needs to use a lot of memory, which may not be available. So the stable_partition asks the operating system if there is enough memory available and then chooses between the two implementations.

Here is an example from the gcc 4.8.1 implementation with some of my comments:

template
    _ForwardIterator
    stable_partition(_ForwardIterator __first, _ForwardIterator __last,
             _Predicate __pred)
    {
       ...
       ...

       /// Try to allocate enough memory for the faster algorithm
      _Temporary_buffer<_ForwardIterator, _ValueType> __buf(__first, __last);

    if (__buf.size() > 0)  /// If there is enough memory
      return
        std::__stable_partition_adaptive(__first, __last, __pred,
                      _DistanceType(__buf.requested_size()),
                      __buf.begin(),
                      _DistanceType(__buf.size()));
    else  /// If there is not enough memory
      return
        std::__inplace_stable_partition(__first, __pred,
                     _DistanceType(__buf.requested_size()));
    }

here std::__stable_partition_adaptive is the fast algorithm and std::__inplace_stable_partition is the slow algorithm.