Private virtual method in C++

Question

What is the advantage of making a private method virtual in C++?

I have noticed this in an open source C++ project:

class HTMLDocument : public Document, public CachedResourceClient {
private:
    virtual bool childAllowed(Node*);
    virtual PassRefPtr<Element> createElement(const AtomicString& tagName, ExceptionCode&);
};

Answer 1

Herb Sutter has very nicely explained it here.

Guideline #2: Prefer to make virtual functions private.

This lets the derived classes override the function to customize the behavior as needed, without further exposing the virtual functions directly by making them callable by derived classes (as would be possible if the functions were just protected). The point is that virtual functions exist to allow customization; unless they also need to be invoked directly from within derived classes' code, there's no need to ever make them anything but private

Answer 2

If the method is virtual it can be overridden by derived classes, even if it's private. When the virtual method is called, the overridden version will be invoked.

(Opposed to Herb Sutter quoted by Prasoon Saurav in his answer, the C++ FAQ Lite recommends against private virtuals, mostly because it often confuses people.)

Answer 3

Virtuality deals on this topic very well.