Smalltalk and Object Orientation: an Introduction - Free

Behaviour
Behaviour
class
ClassDescription
ClassDescription
class
Class
Class class
Object
Object class
Collection
Collection
class
Set
Set class
aDictionary
Dictionary
Dictionary
class
Figure 32.4: The metaclasses for Class, ClassDescription and Behavior
ClassDescription then inherits from a class known as Behavior. The class Behavior
defines how classes should behave (for example how to create new instances). In particular it defines
the minimum state necessary for objects that have instances.
Together these three classes define what a class is, how it should behave, how new inst ances should
be created and what a class should look like. Figure 32.3 illustrates the metaclass inheritance hierarchy
up to the class Behavior.
You may have noticed from the above class names, that Class, ClassDescription and
Behavior are not metaclasses (i.e. they do not have class after their names). They are in fact classes,
thus at the top of the metaclass hierarchy we find (not surprisingly) that metaclasses are classes after all.
This raises two questions, firstly “do Class, ClassDescription and Behavior have metaclasses?” and “If
metaclass are really classes, shouldn’t they be instances of something?”. In this section we will answer
the first question, the second question is deferred to the next section.
As Class, ClassDescription and Behavior are classes, they are all instances of their
metaclasses. This is illustrated in Figure 32.4. The metaclass of Class is of course Class class
(the terminology can and will get worse!), the metaclass of ClassDescription is
ClassDescription class and the metaclass of Behavior is Behavior class.
If you are comfortable with the description so far you have learnt most (if not all) of what you need
to know to exploit the metaclass structure and t he presence of Class, ClassDescription and
Behavior. If you find that you become lost in the next section don’t worry, much of what is described
there is implementation level detail and overly convoluted.
32.5 The metaclass concept goes recursive!
32.5.1 What are metaclasses?
A question which was raised in the previous section (but not answered) was “If metaclass are really
classes, shouldn’t they be instances of something?”. The answer to this question is “yes”, just like any
class, they are in fact an object and objects are all instances of some class. The next question then is, “of
what class is a metaclass an instance?”. The answer this time, is the class Metaclass (note the
capitalization and font).
That is, all metaclasses (such as Collection class, Object class and Set class) are
instances of the class Metaclass. This is because all metaclasses have the same behavior. They
define the structure, definition and behavior of a class. Each subsequent class may have a different
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