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

Ball.h

@@ -12,14 +12,16 @@ public:
 			instance_num = ++instance_counter;
 			// NOTE(dev): don't go above 100, please, for the sake of the example.
 		}
-	void GetXFromPhysicsSimulation()
+	int GetXFromPhysicsSimulation()
 		{
-			x = instance_num + ((rand() % 10) * 100); // move around by ten, but keep single digit
+			// NOTE(dev): move around by 100 to show that the number is different each time,
+			//             but keep the instance number to show you're on the same one.
+			return instance_num + ((rand() % 10) * 100);
 		}
 	static void SetX(int &x, void *instance)
 		{
 			Ball *ball = static_cast<Ball*>(instance);
-			ball->GetXFromPhysicsSimulation();
+			x = ball->GetXFromPhysicsSimulation();
 			return;
 		}
 

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*);
 };
 

PhysicsObject.h

@@ -1,16 +0,0 @@
-#ifndef PHYSICSOBJECT_H
-#define PHYSICSOBJECT_H
-
-#include "IPhysics.h"
-
-class PhysicsObject
-	: public IPhysics
-{
-public:
-	PhysicsObject() : IPhysics(&x) {}
-	int GetX() { Update(); return x; }
-private:
-	int x;
-};
-
-#endif