I forgot that I was accessing x publicly instead of calling the appropriate functions to set it ... which undermined the whole proof.

GraphicsObject.h

@@ -25,10 +25,9 @@ public:
 				callback(x, user_data);
 			return x; // either this was just set by callback, or it was always set by constructor.
 		}
-//private:
+private:
 	int x;
 	void *user_data;
-private:
 	void (*callback)(int&, void*);
 };