Our mental picture of a class is that there is one instance for each 
class. For example, we tend to think there is one instance out there 
representing the String class, and the browser helps feed this image.

In actuality, there two instances exist for every class: a Metaclass 
instance  and a Class instance. The Metaclass instance contains the 
class methods and class instance variable names for a class. When you 
click on the 'class' button in a browser, you are actually looking at 
the methods from the Metaclass instance. The Class instance contains 
the methods and variables that belong to the instances of the class. So 
there are three instances for the string 'abc': the String Metaclass, 
the String Class, and the instance containing the characters 'abc'.

Class and Metaclass are just like other classes, so each of them also 
have a Class instance and a Metaclass instance.

There is a little magic that goes on at this point because there is a 
chicken and egg problem: the 'Class' class can't exist without the 
Class instance and Class metaclass, yet how can you have a Class 
instance until the Class class exists? I haven't looked into how Squeak 
solves this, but 'The Art of Metaobject Protocol' goes into great 
detail on one way to solve the bootstrapping problem.

In a way there are three levels of definition for every instance in the 
environment:
- metaclass to hold class method/variable information
- class to hold instance method/variable information
- instance to hold instance data

When you are holding an instance and ask it to evaluate a selector, it 
determines the code to execute by looking one level higher. For 
example, '3 + 5' sends #+ to the instance '3'. The #+ selector is 
looked for one level higher at the class. For '3 class name', #class is 
sent to '3' which returns the class level (which is the Class instance 
for SmallInteger). This method happens to return a Class instance. So 
#name is sent to the next level up: the metaclass level.

You noted that:
    'Symbol class inspect' is a Metaclass
but
    Symbol class = Metaclass ----> false

By the same token, note that
    '3 class inspect' is a SmallInteger class
but
    3 class = SmallInteger class ----> false

So what shows on the top of the inspector is really one level higher. 
Try:
    Symbol class class = Metaclass ----> true

Hope this helps a little ...

Brian.

On Oct 13, 2004, at 4:32 PM, Michael van der Gulik wrote:

> Hi all.
>
> I'm getting confused. This is an actual problem I've come across in 
> DPON (my vapourware project).
>
> Now, in DPON, objects can be replicated. E.g. a Symbol (like #me) can 
> be replicated. To do so, you need to implement
>
> Symbol class>>replicationAlgorithm
>
> to return the replication algorithm. Classes themselves also need to 
> be replicated. In that case, I implement
>
> Symbol class>>replicationAlgorithm
>
> but... then Metaclass>>replicationAlgorithm gets called. Fair 
> enough... a Metaclass is the class of a Class. But, in Squeak 3.7:
>
> Symbol class = Metaclass (print it...)---> false.
>
> and...
>
> [Symbol class isKindOf: Class] ---> false.
> [Metaclass isKindOf: Class] ---> true.
>
> Hmm.. but if I inspect [Symbol class], I'm looking at Metaclass.
>
> Could somebody explain this to me please?
>
> Michael.
>
>
>
>
>
>