Smalltalk and Object Orientation: an Introduction - Free

Object
Object class
Collection
Collection
class
Set
Set class
aDictionary
Dictionary
Dictionary
class
Figure 32.2: Class and instance relationships
32.4 ClassDescription and Behavior
One point we ignored above was “what happens when we get to Object?”. That is, Object is the
root of the class hierarchy in Smalltalk and therefore Object does not inherit from any superclass.
This is not a problem for the class hierarchy as Object encapsulates all the concepts associated with
being an object. However, what happens in the metaclass hierarchy? One possibility is that Object’s
metaclass, Object class, encapsulates all the concepts associated with being a class. However, in
Smalltalk-80 a different and more sophisticated approach was adopted. (The reasons for adopting this
approach only really become clear later when we discuss the recursive nature of metaclass, for the
moment just accept that the following approach was taken).
In Smalltalk-80, Object class , the metaclass of Object inherits from a class called Class
(See Figure 32.3). This allows Object class to define how the class Object should behave, while
the concepts associated with being a class are encapsulated within Class. In particular, Class defines
what instances of a class should look like and how they should behave. It also provides facilities which
enable class variable names and pool (shared) variables.
Behaviour
ClassDescription
Class
Object class
Collection
class
Figure 32.3: The metaclass hierarchy up to Behavior
In turn, Class inherits from ClassDescription. This class provides facilities for naming
classes, commenting classes (and this is why we defined commentTemplateString here) and
naming instance variables. ClassDescription was provided as a super class of Class so that
another class called Metaclass (which we will consider in the next section) could also inherit these
facilities.
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