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From: johnson@cs.uiuc.EDU (Ralph Johnson)
Newsgroups: comp.lang.smalltalk
Subject: Re: Smalltalk MetaClass questions.
Message-ID: <1991Mar12.214143.10980@m.cs.uiuc.edu>
Date: 12 Mar 91 21:41:43 GMT
References: <9103111620.AA01484@sun.soe.clarkson.edu>
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Reply-To: johnson@cs.uiuc.EDU (Ralph Johnson)
Organization: University of Illinois
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Here are the rules that I use to understand the subclasses of Behavior.

1) Everything in Smalltalk is an object.
2) Every object is a class.

Obviously, classes must be objects, too.  Therefore, they must have
classes.  So, what is the class of Array, or View, or Object?

There are two possibilities: all classes can be instances of a single
class Class, or different classes can have different subclasses.
In order to reduce confusion, I'll call the class of a class a metaclass.
Thus, I'll rephrase this as: there is only one metaclass, or different
classes can have different metaclasses.

It is a lot simpler if all classes had the same class, i.e. if there
was only one metaclass, but that is not the way Smalltalk did it.
If it had, Class would be the only metaclass, and in fact it wouldn't
even be worth calling it a metaclass.  If it had, Smalltalk wouldn't
have class methods, and class methods are useful (though not necessary).

3) Every (nonmetaclass) class has its own metaclass.

Smalltalk not only lets every class have its own metaclass, it forces
each class to have its own metaclass.  Metaclasses don't have their
own names, thus the name of the class of Array is 'Array class'.
All metaclasses are instances of Metaclass, thus 'Array class class'
is Metaclass.  Pretty boring.

Thus, class can be divided into regular classes, which each have
their own metaclasses, and metaclasses, which each are instances
of Metaclass.

4) The metaclass of a superclass is the superclass of a metaclass.

This seems pretty confusing when you say it in English, but it makes
a lot of sense in a picture.  Basically, it means that since
Set is a subclass of Collection, Set class is a subclass of Collection class.

There is a general rule that 'If X is a Y then the class of X is a 
subclass of the class of Y or one of its subclasses'.  Thus, since 
Array is a class then  the class of Array is a subclass of Class or one of
its subclasses.  By the same logic, the class of Object is a subclass of 
Class or one of its subclasses.

We know that Object has no superclasses.  However, we just said that
Object class is a subclass of Class.  Thus, we might as well make
Object class be a direct subclass of Class.  This results in some
funny code, since the methods you see in the 'instance' side of the
browser on Class get inherited by the 'class' side of the browser on
all classes (including Class!).

One of the interesting things about the class/metaclass notion is that
it is completely invisible to the interpreter.  As far as the interpreter
is concerned, only Behavior (the common superclass of Class and Metaclass)
is real.  The only thing the interpreter does with classes is look up
methods in their method dictionary, and any kind of Behavior does that
just fine.  Thus, the class/metaclass notion is defined entirely by
the programming environment of Smalltalk, and not the interpreter.  Thus,
if you don't like it, you could change it!  (Good luck!!!!)

Ralph Johnson -- University of Illinois at Urbana-Champaign