From: dblack@...
Date: 2006-11-17T01:24:36+09:00
Subject: Re: basic mistake?

Hi --

On Fri, 17 Nov 2006, Nia wrote:

>> Hal is correct, moving it into initialize solves the problem.
>>
>> It appears that declaring the instance variable at the class
>> level makes it an instance variable of the class, rather than
>> of the object of the class being created when initialize is called.
>>
>> In short, declare your instance attributes inside initialize.
>>
>> Cheers
>> Chris
>>
>
> hehe, i knew it was some basic mistake. thank you all very much. so
> is it wrong to declare a variable outside a method? cause i usually
> do that a lot,

It's wrong to initialize a variable in one scope and try to use it in
another :-)  Actually there are (at least) two things to keep in mind:

1. Local variables are visible in their local scope.  So here:

   class C
     x = 1
     def meth
       x = 2
     end
   end

the two x's are totally unrelated.

2. Instance variables are scoped to, or associated with, "self".  In
other words, an instance variable (@var) always belongs to whatever
object is "self" at that point.  Here:

   class C
     @x = 1             # (a)
     def meth
       @x = 2           # (b)
     end
   end

you're dealing with two selfs (selves?).  At (a), self is the class
object itself, C.  At (b), self will be whatever instance of C calls
the methods "meth".  So, since there are two selfs, the two @x's are
totally unrelated to each other.


David

-- 
                   David A. Black | dblack@rubypal.com
Author of "Ruby for Rails"   [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB's Weblog)        [2] | Co-director, Ruby Central, Inc.   [4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org