Callback-Example/GraphicsObject.h

#ifndef GRAPHICSOBJECT_H
#define GRAPHICSOBJECT_H

class GraphicsObject
{
public:
	// 2nd arg: pointer to callback function that takes a reference to an int as an argument
	GraphicsObject(int set_x)
		{
			x = set_x;
			user_data = NULL;
			callback = NULL; // function pointer
		}
	GraphicsObject(void (*set_callback)(int&, void*), void *set_user_data)
		{
			x = 0;
			user_data = set_user_data;
			callback = set_callback; // function pointer
		}
	int X()
		{
			// If you are SUPPOSED to callback, ...
			if(callback)
				// ... do so to ask for the new X value
				callback(x, user_data);
			return x; // either this was just set by callback, or it was always set by constructor.
		}
private:
	int x;
	void *user_data;
	void (*callback)(int&, void*);
};

#endif