class Testable
{
private:
 Thing *mData;

public:
 typedef void (xOGModelRefPtr::*bool_type)() const;
 void type_does_not_support_comparisons() const {}
 operator bool_type() const
  {
    return (mData && mData->IsValid()) ? Testable::type_does_not_support_comparisons : NULL;
  }

 Testable() : mData(NULL) {}
 operator Thing*() const { return mData->mPtr; }
};

void fn(Thing *);
void main()
{
  Testable t;
  if(t) // crash on access of null mData->
    fn(t);
}