Private inheritance

Example from Meyers "Effective C++"

class Timer {
public:
explicit Timer(int tickFrequency);
virtual void onTick() const; // Called automatically for each tic, 
// onTick() must be redefined to do things
...
};


class Widget: private Timer { // private inheritance
private:
virtual void onTick() const; // redefined to make job done
... 
};

Now clients of Widget get interface untouched and required job is done

Example of protecting method from redefinition in derived classes:

class Widget {
private:
    class WidgetTimer: public Timer {
    public:
        virtual void onTick() const;
        ...
    };
    WidgetTimer timer;
    ...
};

Classes derived from Widget unable to redefine onClick. Analogue of final in Java and sealed in C#