Smalltalk and Object Orientation: an Introduction - Free

In addition a metaclass cannot be accessed directly. It can only be access by sending the message
class to a class. For example:
Collection class.
If you evaluate this and print the result, you will get the name of the Collection classes’ metaclass,
which is of course Collection class (which confusingly looks exactly like the expression which
accessed the metaclass).
32.3.2 The Metaclass hierarchy
Figure 32.1: The metaclass hierarchy for the Collection classes
As you are aware, the classes in the Smalltalk system form a hierarchy rooted on the class Object.
Thus the class Dictionary inherits from Set which in turn inherits from the class Collection
which ultimately inherits from the class Object. This means that when instances of these class are
sent messages, the system can search up through the class hierarchy for methods which implement that
message.
Metaclasses also possesses a metaclass hierarchy. In this hierarchy one metaclass can inherit from
another metaclass. In Smalltalk -80 the metaclass hierarchy was constrained to mirror the class
hierarchy. This means that if some class side behavior is defined in Collection, then it will be
inherited by Set and Dictionary via their metaclasses. Figure 32.1 presents part of the metaclass
hierarchy for the Collection classes. The net result is that if a class is sent a message, t he system begins
the search for a corresponding method in its metaclass. If it does not find it there, it then looks in the
superclass of its metaclass. This process continues until either a suitable method is found or until the
doesNotUnderstand: message is triggered.
Figure 32.2 illustrates the relationships between the two hierarchies. Note that dashed arrows
represent instance relationships, whereas solid arrows represent inheritance. Please note the case used
for each label. As can be seen from this figure, this means that each class is an instance of its metaclass
and that the metaclass hierarchy exactly mirrors the class hierarchy. In the figure, the instance of the
class Dictionary, labeled as aDictionary, is pres ented in a rounded box, because it is an object which
cannot create new instances. Therefore it is a different kind of instance. To make this distinction clear a
different box style is used.
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