Smalltalk and Object Orientation: an Introduction - Free

32.5.5 The whole enchilada
Behaviour
Behaviour
class
ClassDescription
ClassDescription
class
Class
Class class
Object
Object class
Collection
Collection
class
Metaclass
Metaclass
class
Set
Set class
aDictionary
Dictionary
Dictionary
class
Figure 32.8: Combining both the instance and class relationships
The last two sections have considered the instance relationship and the inheritance relationship between
classes in Smalltalk respectively. Each of these in isolation isn’t too complicate d, however when you
put the two together you obtain a diagram such as that illustrated in Figure 32.8.
This figure illustrates the instance and inheritance relationships between classes and metaclasses. As
can be seen f rom this diagram the overall structure is quite complex. However, if you study it closely
you should find that it does make sense. Even when there appears to be the potential for excessive
circularity it can be seen that this is not the case. There is one point of circularity which should be
noted, that is an Object is an instance of Object class which (eventually) inherits from Behavior
which inherits from Object.
32.6 Further reading
See Chapters 5 and 16 in the Purple Book [Goldberg and Robson 1989] for an in troduction to
metaclasses and a detailed description of the class Class, Metaclass, ClassDescription and
Behavior. However, the actual description of the metaclass concept and the diagrams presented with
it are a little confusing. Also see Chapter 26 of [ Hopkins and Horan 1995] for an excellent introduction
to metaclasses.
Part Seven
The Future
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