Smalltalk and Object Orientation: an Introduction - Free

ehavior, but from the point of view of th e metaclass, all classes are constructed in the same way, are
structured in the same way (i.e. they have class methods, class instance variables and class variables)
and all require the same set of operations (i.e. they must be able to construct instances).
32.5.2 What is metaclass an instance of?
Metaclass class
(metaclass)
Metaclass
Array class
(metaclass)
Array
#(1 2 3 4) #($a $b $c)
Figure 32.5: Instance relationships between instances, classes and metaclasses for array
Metaclass, being a class, must also be an instance of somet hing. Following the standards laid down
about it is therefore an instance of its metaclass. This metaclass, like all other metaclasses, has its name
derived from the class, it is therefore called Metaclass class . However, every metaclass is an
instance of Metaclass, thus the Metaclass class is an instance of Metaclass. This is the
first point of circularity in Smalltalk and it is worth summarizing:
• All metaclasses are instances of Metaclass.
• The metaclass of Metaclass is Metaclass class.
• Metaclass class is a metaclass, it is therefore an instance of Metaclass.
• Therefore the metaclass of Metaclass is an instance of Metaclass.
This is illustrated in Figure 32.5. This figure shows that an object such as an array is an ins tance of the
class Array. In turn Array is an instance of its metaclass Array class . This metaclass is an
instance of the class Metaclass, which is an instance of its own metaclass Metaclass class .
And as has been said, Metaclass class is an instance of Metaclass (see what I mean about the
terminology!).
32.5.3 Instance relationships for classes and metaclasses
Behaviour
Behaviour
class
ClassDescript
ion
ClassDescript
ion class
Class
Class class
Object
Object class
Collection
Collection
class
Metaclass
Metaclass
class
Set
Set class
aDictionary
Dictionary
Dictionary
class
Figure 32.6: Instance relationships
To complete the explanation being given, consid er Figure 32.6. This illustrates the instance
relationships between classes, their metaclasses, the Metaclass and the Metaclass class. Note
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