Protected Inheritance

Protected inheritance is a variation on private inheritance. It uses the keyword protected when listing a base class:

class Student : protected std::string,
                protected std::valarray<double>
{...};

With protected inheritance, public and protected members of a base class become protected members of the derived class. As with private inheritance, the interface for the base class is available to the derived class but not to the outside world. The main difference between private and protected inheritance occurs when you derive another class from the derived class. With private inheritance, this third-generation class doesn’t get the internal use of the base-class interface. That’s because the public base-class methods become private in the derived class, and private members and methods can’t be directly accessed by the next level of derivation. With protected inheritance, public base-class methods become protected in the second generation and so are available internally to the next level of derivation.

Table 14.1 summarizes public, private, and protected inheritance. The term implicit upcasting means that you can have a base-class pointer or reference refer to a derived class object without using an explicit type cast.

Table 14.1. Varieties of Inheritance