Inheritance in COM does not mean code reuse. Because no implementations are associated with interfaces, interface inheritance does not mean code inheritance. It means only that the contract associated with an interface is inherited in a C++ pure-virtual base-class fashion and modified — either by adding new methods or by further qualifying the allowed usage of methods. There is no selective inheritance in COM: If one interface inherits from another, it includes all the methods that the other interface defines.
Inheritance is used sparingly in the predefined COM interfaces. All predefined interfaces (and any custom interfaces you define) inherit their definitions from the important interface IUnknown, which contains three vital methods: QueryInterface, AddRef, and Release. All COM objects must implement the IUnknown interface, because it provides the means to move freely between the different interfaces that an object supports with QueryInterface, and to manage its lifetime with AddRef and Release. More on these methods is discussed later in this chapter.
In creating an object that supports aggregation, you would need to implement one set of IUnknown functions for all interfaces as well as a stand-alone IUnknown interface. This is described in the following section. In any case, any object implementor will implement IUnknown methods. Refer to the section Using and Implementing IUnknown for more information.
While there are a few interfaces that inherit their definitions from a second interface, in addition to IUnknown, the majority are simply the IUnknown interface methods plus the methods defined in the interface. This makes most interfaces relatively compact and easy to encapsulate.