How to organize a class's member functions?

    rb_hash_has_key(VALUE hash, VALUE key)
    {
        if (!RHASH(hash)->ntbl)
            return Qfalse;
        if (st_lookup(RHASH(hash)->ntbl, key, 0)) {
            return Qtrue;
        }
        return Qfalse;
    }

    rb_hash_has_key(VALUE hash, VALUE key)
    {
        if (!RHASH(hash)->ntbl)
            return Qfalse;
        if (st_lookup(RHASH(hash)->ntbl, key, 0)) {
            return Qtrue;
        }
        return Qfalse;
    }

int deterministic_wrapper(double t, /* ... */ void * params) {
  void** voidPtrs = static_cast<void**>(params);
  return static_cast<sde*>(voidPtrs[1])->deterministic(t, /* ... */ params);
}

struct myParams
{
  model* _model;
  sde* _sde;
};

// ...

int deterministic_wrapper(double t, /* ... */ void * params) {
  return static_cast<myParams*>(params)->_sde->deterministic(t, /* ... */ params);
}