virtual function in private or protected inheritance

Question

It\'s easy to understand the virtual function in public inheritance. So what\'s the point for virtual function in private or protected inheritance?

For example:

Answer 1

An example would be:

/// Thread body interface
class runnable
{
public:

    virtual ~runnable() {}
    virtual void run() =0;
};

/// Starts OS thread, calls p->run() in new thread
thread_id start_thread( runnable* p );

/// Has a private thread
class actor: private runnable, private noncopyable
{
private:

    thread_id tid; /// private thread

public:

    actor() { tid = start_thread( this ); } // here this IS-A runnable
    // ...
    virtual ~actor() { stop_thread( tid ); }

private:

    virtual void run() { /* work */ }
};