From: Sam Sungshik Kong <ssk@...>
Date: 2004-06-23T01:33:13+09:00
Subject: Re: A question about Class and Object

I don't think it's a stupid explanation...:)
It's just above my intelligence.
I'll keep learning Ruby and someday I'll understand it.

Thanks.

Sam
"ts" <decoux@moulon.inra.fr> wrote in message
news:200406220934.i5M9Y7r02023@moulon.inra.fr...
> >>>>> "S" == Sam Sungshik Kong <ssk@chol.nospam.net> writes:
>
> S> I'm still confused with class and object and metaclass...
>
>  well, try to forget about metaclass.
>
>  I'll try to give you a *STUPID* explanation
>
>  ruby has only objects, this mean that a class is an object and like any
>  object it has a class (Class) where are defined its methods.
>
>  This mean that, for example, the method ::new is defined in Class,
>  something like this
>
>    class Class
>       def new(*args, &block)
>          obj = allocate
>          obj.send(:initialize, *args, &block)
>          obj
>       end
>    end
>
>  This work fine, but you see that it exist a problem. If it was easy to
>  define ::new, it will be more difficult to define ::allocate in the same
>  way, because Array::allocate is completely different from Hash::allocate
>  and you don't want to define a method Class#allocate with a big switch
and
>  change this method each time that you introduce a new class
>
>  Another problem if that it will be nice if you can define a method
>  File::open, which take a block. You can't define this method in Class
>  (i.e. Class#open) because this mean that you'll define also Array::open,
>  Hash::open and you don't want this.
>
>  To solve these 2 problems, ruby associate with each class an object and
>  when it search a class method it search first in this object then it look
>  in Class.
>
>  Now ruby can work, because
>    * the method ::allocate will be defined in this special object, and you
>      still have Class#new
>
>    * you can define in the object associated with File, the method ::open
>      and only this class will have this method
>
>  But a class is a little special because there is inheritence and when you
>  write
>
>     class A < Array
>     end
>
>  you want to re-use, for A, the method ::allocate which was defined in the
>  special object associated with Array
>
>  This mean that for this special object, associated with a class, you want
>    * the possibility to define method
>    * the possibility to use inheritence
>
>  an object is well adapted to do this, this is precisely a class and
>  because this class will always be associated with only *one* other object
>  it will called a "singleton" class
>
>  Finally this give this (where (A) is the singleton class associated with
>  A)
>
>     Class  <  Module  <  Object
>    (Class) < (Module) < (Object) < Class
>
>       A    <  Array  <  Object
>      (A)   < (Array) < (Object)
>
>  and you have the schema which is given in object.c
>
>  Now, at the beginning, I've said that a class is an object and I've
>  introduced the singleton class. This mean that you can associate a
>  singleton class with any object, and this will give methods specifics to
>  this object, for example
>
>    a = []
>    class << a
>       def tt
>       end
>    end
>
>  When ruby will search a method for `a', it will first search in its
>  singleton class then in its class.
>
>
>
> Guy Decoux
>
>