The class hierarchy diagram shown in the Class Libraries Reference exposes the some of the interrelationships between the iostream classes. There are further “membership” relationships between the ios family of classes and the streambuf family.
The hierarchy diagram is useful mainly in conjunction with Part 3, “iostream Class Reference.” Use it for locating the base classes that provide inherited member functions for their derived classes. You don't need to understand all the interclass relationships in order to work through the examples in this tutorial.