How do you free a wrapped C++ object when associated Javascript object is garbage collected in V8?

void MakeWeak(void* parameters, WeakReferenceCallback callback);

typedef void (*WeakReferenceCallback)(Persistent<Object> object,
                                      void* parameter);

void CleanupV8Point(Persistent<Object> object, void*)
{
    // do whatever cleanup on object that you're looking for
    object.destroyCppObjects();
}

Parameter<ObjectTemplate> my_obj(ObjectTemplate::New());

// when the Javascript part of my_obj is about to be collected
// we'll have V8 call CleanupV8Point(my_obj)
my_obj.MakeWeak(NULL, &CleanupV8Point);