template<class Sender>
class Event {
private:
    Sender *s;
    friend Sender;

protected:
    event(Sender *s) : s(s) {}
    void operator() { }

public:
    // register a function for the event
    void operator +=(void (*f)(Sender*)) { }

    // deregister a function from the event
    void operator -=(void (*f)(Sender*)) { }
}

class InternetConnectionListener {
public:
    Event<InternetConnectionListener> onConnectionUp;
    Event<InternetConnectionListener> onConnectionDown;

private:
    void someMethodThatGetsCalledWhenNetworkStateChanges() {
        if (networkIsUp) {
            // only this class is allowed to execute the () operator
            onConnectionUp();
        } else {
            onConnectionDown();
        }
    }
}