Smalltalk and Object Orientation: an Introduction - Free

32. The Metaclass Framework
32.1 Introduction
The metaclass concept is probably one of the most confusing parts of the whole of Smalltalk. This is
partly due to the very confusing names used (for example, Class class and Metaclass class)
but also because almost all of it is hidden from the developer. You are therefore only vaguely aware of
it (if you are aware of it at all) during development. This means that developers do not need to
understand it in detail and most people avoid trying to understand what appear s to be a difficult concept
if they don’t have to. It also means that developers can only grasp what is happening abstractly, with
little chance to see the mechanics of what is going on.
However, contrary to popular myth, the metaclass concept is actually not that difficult (if you can
get around the terminology), although it can be confusing especially when its recursive definitions are
encountered. We will however leave that aspect of metaclasses until right at the end. This means that
you should hopefully gain at least an appreciation of what metaclasses about, even if the fine details of
the implementation pass you by. In many ways, it is only an appreciation that you need (if you need it at
all) to understand what you observe in the development environment.
As the basic concept being considered in this chapter is the concept of a class, Section 2 provides a
review of what is meant by a class etc.
Metaclasses have not always been present in Smalltalk, they were only introduced into Smalltalk
with the dev elopment of the Smalltalk -80 version of Smalltalk. This is essentially the version of
Smalltalk which is used as the basis of commercial versions of the language (such as VisualWorks).
Prior to that a number of (simpler) approaches were used. However, each of these provided fundamental
problems which were overcome with the introduction of metaclasses.
Section 3 discusses the metaclass mechanism and how it works. As was stated before, it does this
without going into the complexities of the implementation of metaclasses. This allows the casual reader
to understand where metaclasses fit in and what they do without needing to understand the intricacies of
the full metaclass inheritance hierarchy.
The classes ClassDescription and Behavior are introduced in Secti on 4. These classes
encapsulate what it means to be a class and how classes do what they do. Section 5 considers the
relationship between metaclasses, classes and the Metaclass class. This is not actually that complex, but
can appear confusing at first sight.
A point to note before reading this chapter are the conventions used for Smalltalk, classes and
instances. Remember:
• All Smalltalk code appears in a courier font, e.g. Collection new.
• All classes start with a capital letter e.g. Set, Object, Array.
• All instance names start with a lower case letter, e.g. aSet, anObject, anEmployee.
These conventions are adhered to rigidly below.
32.2 What are classes?
All classes are ultimately subclasses of the class Object (with the exception of Object itself). That
is, a class inherits properties form its superclass (which in turn inherits properties form its superclass)
up to Object. In turn every thing in Smalltalk is an object and every object is an instance of a class.
On page 269 of the Purple Book [Goldberg and Robson 1989] the following statement is made:
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