FAQ 37.01 What are private inheritance and protected inheritance?

Has-a, not is-a. From the user's perspective, private and protected inheritance are semantically similar to composition, but they are very different from normal public inheritance. Thus private and protected inheritance are a lot more like "has-a" than "is-a" (more precisely, than "is-substitutable-for"). Here are the ways they are like "has-a".

Like normal composition, private and protected inheritance cause an inner object to be contained inside the outer object. With normal composition, this inner object is called a member object. The syntax for doing this with private inheritance is different than the syntax for normal composition, but the idea is the same. With private and protected inheritance, the inner object is called the base class subobject. The important thing to note is that the outer object contains the inner object in both cases.

Like normal composition, private and protected inheritance prevent normal users from directly accessing the inner object. With normal composition, this is normally done by declaring the member object in the private: or protected: part of the outer object. The syntax for doing this with private inheritance is different than the syntax for normal composition, but the idea is the same. With private and protected inheritance, this encapsulation is done automagically: normal users are not allowed to convert a derived pointer to its private or protected base class pointer (this is different than the normal is-a conversion of public inheritance; see FAQ 2.24).

Like normal composition, private and protected inheritance allow the outer object to select specific features of the inner object that users can access, and users are prevented from accessing any other features than the ones explicitly allowed by the outer object. With normal composition, the outer object grants normal users access to specific features of the inner object by call-through member functions. A call-through member function is often a one-line function that simply calls the corresponding member function on the inner object. The syntax for doing this with private inheritance is different than that for normal composition, but the idea is the same. With private and protected inheritance, there are two options: either the derived class defines a call-through member function that calls the corresponding member function of the private or protected base class subobject, or the using syntax can be used to make a base class member function public: (see FAQ 37.06).