From: "David A. Black" <dblack@...>
Date: 2005-10-26T03:23:59+09:00
Subject: Re: having reference problems

Hi --

On Wed, 26 Oct 2005, shalperin wrote:

> Here is a code snippet:
>
> class Foo
>       @a
>       @b
>       @reference
>             def initialize()
>           @reference = [@a, @b]
>       end
>
>       attr_writer :a, :b, :reference
>       attr_reader :a, :b, :reference
> end
>
> f = Foo.new()
> f.reference[0] = 1
> puts f.a  #nil  WTF!
>
> What I am trying to do is refer to an instance variable both by its index in 
> some array, and also directly.
>
> Any help would be greatly apreciated.

OK, you need to clear some cobwebs :-)

First of all, this:

   @a

is just the value of @a.  It doesn't do anything; it's essentially a
waste of electrons.

Second, there's a rule about instance variables that will help you
not only here but generally: *Whenever* you see an instance variable
(@a or whatever), that instance variable belongs to whatever object
is, at that moment, "self" (the current object).

If you peek inside a class definition, you'll find that the current
object is a class object:

   class Foo
     p self  #  Foo
   end

If you peek inside an instance method definition, you'll find that
self is the instance:

   class Foo
     def talk
       p self
     end
   end

   Foo.new.talk  #  #<Foo:0xbf510d18>

That means that you're dealing with two completely different scopes,
for instance variable (and most other) purposes:

   class Foo
     @a = 1   # this @a belongs to the Class object called Foo
     def initialize
       @a = 1  # *this* @a will belong to a newly-minted Foo instance
     end
   end

No two objects share instance variables -- even if one of the objects
happens to be an instance of the other object.  Like about 3/4 of all
Ruby problems, this one comes down to: classes are objects too :-)

Third....  when you do this:

   foo.reference[0] = 1

you are *replacing* the first element of foo.reference with 1.  At
this point, foo.reference will consist of:

   [1,nil]  # the new 1 value, plus the default value of @b (nil)

Both @a and @b will be nil, because you have not assigned to either of
them.


David

-- 
David A. Black
dblack@wobblini.net