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#