The root of the type hierarchy is the abstract class BaseType. This class, because it is abstract, is never instantiated itself. BaseType is used as the base class for all of the different types of variables, and contains common member functions used by all the other type classes. For simple variables such as Int32, only the abstract virtual functions in BaseType need to be added to complete the class definition. Compound types like Structure usually require more. A compound type contains one or more instances of BaseType, and requires methods to access, add and remove these member variables.