The derived classes in this chapter all specify the keyword public in front of the base class's name. This specifies public derivation, which means that the public members of the base class are public members of the derived class, and protected members of the base class are protected members of the derived class.
You can also specify the keyword private in front of the base class's name. This specifies private derivation, which means that both the public and protected members of the base class are private members of the derived class. Those members are accessible to the derived class's member functions, but not to anyone using the derived class.
Private derivation is rarely used. Someone using the class cannot implicitly convert a pointer to a derived class object into a pointer to a base class object, or use polymorphism. (However, within the member functions of the derived class, you can perform such conversions and use polymorphism). Private derivation is very similar to defining a member object of another class; it's a method of using another class, but it's not appropriate for indicating that one class is a subtype of another.