OpenMP and functors

Hi,

I am currently trying to parallelise an image transformation loop. The transformations are arbitrary and performed consecutively, so they currently are kept in a vector of functor pointers. A much simplified version looks like this (syntax errors and typos notwithstanding):

class FunctorBase
{
  virtual void get_coord(double& x, double&y) = 0;
};

class Transform
{
  public:
    void transform()
    {
#pragma omp parallel for
      for (size_t j = 0; j < lines; ++j)
      {
        for (size_t i = 0; i < lines; ++i)
        {
          ...
          get_coord(x, y);
          ...
        }
      }
    }

  private:
    void get_coord(double& x, double&)
    {
      for (size_t i = 0; i < n_func_; ++i)
      {
        func_[i]->get_coord(x, y);
      }
    }

  private:
    std::vector<FunctorBase*> func_;
    size_t n_func_;
};

If I allow more than one thread, the result is garbled (for OMP_NUM_THREADS==1, everything is fine), though I do not really understand why. The transformation functors do not use ugly stuff like static variables or similar, and I always thought it was save to call a method in the same object from different threads, _provided_ one does not change the object state (i.e., all member variables are untouched).

So: what do I do? I can wrap the transformation list in a separate class and implement a copy constructor for it, which would enable me to put a thread-private instance of it on the stack. But this is quite a lot of overhead, so I was hoping that maybe I am not seeing the wood for the trees here (i.e., overlooking the obvious).

Regards,
Robert