Non-public Inheritance in C++

Parent classes are usually listed using the public keyword. You may be wondering if a parent can be private or protected. In fact, it can, though neither is as common as public.

If you don't specify any access specifier for the parent, then it is private inheritance for a class, and public inheritance for a struct.

Declaring the relationship with the parent to be protected means that all public and protected member functions and data members from the base class become protected in the context of the derived class. Similarly, specifying private inheritance means that all public and protected member functions and data members of the base class become private in the derived class. (Private members in the parent class remain parents and thus inaccessible.)

There are a handful of reasons why you might want to uniformly degrade the access level of the parent in this way, but most reasons imply flaws in the design of the hierarchy. Some programmers abuse this language feature, often in combination with multiple inheritance, to implement components of a class. Instead of making an Airplane class that contains an Engine data member and a Fuselage data member, they make an Airplane class that is a protected Engine and a protected Fuselage. In this way, the Airplane doesn't look like an engine or a fuselage to client code (because everything is protected), but it is able to use all of that functionality internally.