Smalltalk and Object Orientation: an Introduction - Free

that no distinction is made between a class being an instance of another class
instance of the Dictionary class.
and a user generated
32.5.4 Class inheritance
A point we have still not covered is what does the class Metaclass inherit from? Remember all
classes (except Object) have a superclass and Metaclass is no exception. Metaclass inherits
from ClassDescription, this is because the structure of classes and metaclasses are very similar.
For example instances of both Class and Metaclass can have methods. In turn, the metaclass of
Metaclass, (i.e. Metaclass class ) follows the standard laid d own and it inherits from the
ClassDescription class. This is illustrated in Figure 32.7.
As ClassDescription is a subclass of Behavior, it also means that both Class and
Metaclass inherit from the facilities provided b y Behavior. This means that instances of both
Metaclass and Class can create new instances etc. (Remember Behavior provides all the protocol for
making new instances so this is not surprising).
Behaviour
Behaviour
class
ClassDescription
ClassDescription
class
Class
Class class
Object
Object class
Collection
Collection
class
Metaclass
Metaclass
class
Set
Set class
Dictionary
Dictionary
class
Figure 32.7: Class inheritance
We are now left with the class Behavior and what it inherits from. All other classes have been
accounted for. What then does Behavior inherit from? It inherits from the class Object. This closes
the loop and ensures that all things in the whole of Smalltalk are objects (i.e. at some point their class
inherits from Object) whether they are class, metaclasses or ordinary instances.
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