Smalltalk and Object Orientation: an Introduction - Free

“There are two kinds of objects in the system, ones that can create instances of
themselves (classes) and ones that can not.”
This is a very important point, it highlights the fact that everything in Smalltalk is an object. That is,
everything, including classes, are objects and can be sent messages (this of course means that classes
are also instances of something, but what this is we will leave until later). However classes are special,
they can create new objec ts called instances. Instances of classes however cannot create other instances
(they must ask a class to do it for them).
In fact classes have only a very few roles in the system and these are essentially limited to:
• creating new instances,
• defining what those instances will do,
• holding class information (such as class variables).
Whereas instances hold application data and perform the operations defined by the (instance)
methods held by the class. That is, the instance holds the data, but when it is se nt a message it looks in
the instance methods of its class for a method which implements that message. If no method is found in
its class, then its superclass is searched for such a method. This search continues until the class Object
is reached. If no met hod is found then the message doesNotUnderstand: is sent to the original
receiver.
32.3 Classes and metaclasses
32.3.1 Metaclasses
As was indicated in the last section each class is an instance of a class. However we want each class to
be able to behave in a different manner to any other class. Therefore each class must be an instance of a
different class (as all instances of the same class must behave in the same manner). In Smalltalk -80 the
concept of a metaclass was introduced. A metaclass is a special class, whose instance is a class. In
general a metaclass only possesses a single instance (a class) and a class will be an instance of a single
metaclass.
That is, a class is an instance of a metaclass and a metaclass defines how a class behaves. This
means that it is actually in the metaclass that the class methods are defined and held. This means that
when a class is sent a message, it looks in its metaclass for the method which implements the message.
In effect, when you click on the class radio button in one of the various browsers in VisualWorks you
are actually examining the classes’ metaclass 16 .
If you are confused by the terminology try to think of it in this way:
“A metaclass defines information about a class (hence the term meta)”.
Metaclasses are not actuall y named directly, instead their names are a combination of the name of
the class they define and “ class”. Thus the metaclass of Collection is Collection class, the
metaclass of Object is Object class and the metaclass of a class called Employee would be
Employee class. It is at this point that people begin to be confused by the terminology. We shall
therefore attempt to recap.
• A class is an object which can create instances.
• A metaclass is a class which defines a class.
• A metaclass has only one instance (the class it defines).
• A metaclass is named after the class it defines concatenated with the word class.
16
In fact if you examine the Browser class you will find references to self meta. This is a reference to whether the user is
currently examining the instance side (meta = false) or the class side (meta = true).
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