From: dblack@...
Date: 2006-08-25T01:11:31+09:00
Subject: Re: Call functions of superclass

Hi --

On Thu, 24 Aug 2006, Nathan Smith wrote:

> Hello,
>
> On Thu, 24 Aug 2006 dblack@wobblini.net wrote:
>
>> No; it will point to the superclass of the class of self :-)
>>
>>    class C
>>    end
>>    class D < C
>>    end
>>    D.new.class.superclass   # C
>
>
> I think we are using different terminology. Take a look at this code:
>
> class A
>  def A.hi
>    puts "A.hi"
>  end
>
>  def hi
>    puts "hi"
>  end
> end
>
> class B < A
>  def hi
>    self.class.superclass.hi
>  end
> end
>
> b = B.new
> b.hi
>
>
> This will output
>
> A.hi

Yes, because B.new.class.superclass is A, and A responds to "hi".  The
method A.hi is defined in A's singleton class (or "metaclass") -- but
the object to which you are sending the message "hi" is A, not A's
singleton class.

> In "Programming Ruby", they define the "metaclass" as the object which
> contains the class-wide objects of the class. IE, the metaclass of A would
> contain A.hi, and not A#hi (I could be getting the notation of A.hi and
> A#hi backwards, or completely wrong -- A.hi is the class method, and A#hi
> is the instance method.)
>
> Therefore, in B#hi, self.class.superclass does not refer to the superclass
> of B -- rather it refers to the metaclass of the super of B. If it pointed
> to the superclass of B, then calling
>
> self.class.superclass.hi
>
> would print "hi", instead of "A.hi"

No, because A is the superclass of the class of "self" in your
example.  A is an object in its own right; it responds to "hi".

I think you're adding an extra level of reference or indirection or
something.  Look at this:

irb(main):001:0> class C; object_id; end
=> 1615634
irb(main):002:0> C.new.class.object_id
=> 1615634
irb(main):003:0> C.new.class.superclass.object_id
=> 1683084
irb(main):004:0> C.new.class.superclass
=> Object
irb(main):005:0> Object.object_id
=> 1683084
irb(main):006:0> class << C; object_id; end
=> 1615554

As you can see, the singleton class of C is not the same object as
either C or the class of C.

("Singleton class" is the most general term; the Pickaxe uses
"metaclass" to mean singleton class of a Class object.  I tend to just
use "singleton".)


David

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