Importance of virtual functions

Private virtual functions are useful when you expect a particular method to be overridden, but do not wish the override to be called from outside of the base class. That is, the base class implementation and its overrides remain private to the base class.

Private virtual methods are particularly useful in implementing template method patterns, where certain algorithmic steps need to be deferred to subclasses, but where those steps need not be exposed to those subclasses. In many cases the private virtual methods will be declared pure-virtual, thus rendering the base class an abstract base class.