A virtual member function is a member fn preceded by the keyword 'virtual'.
It has the effect of allowing derived classes to replace the implementation of the fn. Furthermore the replacement is always called whenever the object in question is actually of the derived class. The impact is that algorithms in the base class can be replaced in the derived class without effecting the operation of the base class. The replacement can be either full or partial, since the derived class operation can invoke the base class version if desired. This is discussed further below.
