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[Page 678]

12.6. public, protected and private Inheritance

When deriving a class from a base class, the base class may be inherited through public, protected or private inheritance. Use of protected and private inheritance is rare, and each should be used only with great care; we normally use public inheritance in this book. (Chapter 21 demonstrates private inheritance as an alternative to composition.) Figure 12.27 summarizes for each type of inheritance the accessibility of base-class members in a derived class. The first column contains the base-class access specifiers.

Figure 12.27. Summary of base-class member accessibility in a derived class.
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When deriving a class from a public base class, public members of the base class become public members of the derived class and protected members of the base class become protected members of the derived class. A base class's private members are never accessible directly from a derived class, but can be accessed through calls to the public and protected members of the base class.

When deriving from a protected base class, public and protected members of the base class become protected members of the derived class. When deriving from a private base class, public and protected members of the base class become private members (e.g., the functions become utility functions) of the derived class. Private and protected inheritance are not is-a relationships.


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