From: Sam Sungshik Kong <ssk@...>
Date: 2004-06-23T13:58:11+09:00
Subject: Re: A question about Class and Object

Who are you?
I would have thought that you were "matz" if I hadn't seen your name.

No, no...
Don't expect me to fully understand what you said.
I'm just a beginner.

I think what bothers me is terminologies.
Class is an object and Object is class.
It's pretty recursive.
It would be better if we have different terms for each.

Thank you very much.

Sam
"Mehr, Assaph (Assaph)" <assaph@avaya.com> wrote in message
news:338366A6D2E2CA4C9DAEAE652E12A1DE166E1C@au3010avexu1.global.avaya.com...
>
> > All other classes are an instance of Class (Object is an instance of
> Class).
> > What about Class?
> > It's an instance of itself, right?
>
> "Class" is an object, whose class is Class. :-)
>
> > To test this strange thing, I tried to subclass Class but it failed.
> > class MyClass < Class
> > end
> > =>TypeError: can't make subclass of Class
>
> That is because you're trying to extend the concept of Class-ness, not
> of a particular class. That will be too hard to maintain, so Ruby
> forbids it.
>
> > Now, I created an instance of Class.
> > aClass = Class.new
> > Is aClass a class like Object or just an object like obj (obj =
> Object.new)?
> > It seems like a class.
>
> You will see that: aClass.class gives Class, so aClass is an object
> whose class is Class.
>
> > aaClass = aClass.new #->Works!
>
> Now aaClass is an object, whose class is aClass. You will not see this
> in irb because you have not named aClass. Naming is done by binding it
> to a constant (beginning with a capital):
>
> irb(main):001:0> Fred = Class.new
> => Fred
> irb(main):002:0> aFred = Fred.new
> => #<Fred:0x2b436e0>
> irb(main):003:0> Fred.class
> => Class
> irb(main):004:0> aFred.class
> => Fred
>
> > So Class is actually a class maker.
> > It's instances are classes.
> > That means that to make a class, I don't have to define a class.
> > If I just create an instance of Class, it's a class.
>
> Correct. For another example, Struct#new is also a "class maker":
>
> irb(main):005:0> struct = Struct.new "MyStructClass", :foo, :bar
> => Struct::MyStructClass
> irb(main):006:0> struct.class
> => Class
> irb(main):007:0> st1 = struct.new
> => #<struct Struct::MyStructClass foo=nil, bar=nil>
> irb(main):009:0> st1.foo = 1
> => 1
>
> We have added a new class to the runtime, called MyStructClass.
>
> > When I think of Class and Object, the "chicken and egg" problem comes
> to my
> > mind.
> > Object is an instance of Class and Class inherits from Object.
> > Which one is first? (Which one should exist first?)
>
> irb(main):010:0> Object.id
> => 20796620
> irb(main):011:0> Class.id
> => 20796596
>
> So it seems that the Class class-object is created before the Object
> class-object :-)
>
>
> Try these (make sure you understand each one before going to the next):
>
> * Every object has a singleton-class associated with it.
> * The singleton-class is unique for that object and holds all the
>   methods descriptions etc for that object.
> * All objects are created from a boiler-plate class. Until they are
>   modified the singleton-classes for them will be identical to the
>   class-object they were created from.
> * Concepts are represented as classes.
> * Instances of concepts are represented as objects.
> * "Object" is an identifier that represents the concept for "thing".
> Being
>   a concept its singelton-class is Class.
> * "Class" is the identifier to represent the concept of "class", or the
> actual
>   representation of the concept of concepts. Therefore its
> singelton-class is
>   also Class.
> * Singleton-classes in themselves are concepts. Therefore they are
> represented as
>   _objects_ of class _Class_.
> * Inheritance is used to signify a lot of things (specification,
> extension etc).
>   In practice it means take the specification of the parent and
>   modify it.
> * The minimal information you have of something is that it is a "thing".
>   All "things" have a low common denominator.
>   Therefore everything inherits from Object.
> * Class is a more specific concept than "thing".
>   Therefore "Class" inherits from Object.
> * Object is a concept. Therefore it is of the class of things that are
> concepts.
>   Therefore Object's class is Class.
> * The order of the creation of the objects is the Ruby (or any other
> runtime for
>   that matter) bootstrap process. It is as immaterial a question as
> "what
>   happened before the big bang": There is a magic that creates the
> memory
>   allocations before you have access to the runtime.
>
> HTH. If not try books by Wittgenstein, Godel and Chomsky.
>
>
>