From: dblack@...
Date: 2006-11-03T23:46:50+09:00
Subject: Re: Output not clear

Hi --

On Fri, 3 Nov 2006, Learning Ruby wrote:

> On 11/3/06, Jon Egil Strand <jes@luretanker.no> wrote:
>> 
>> In your withdraw-method it's not really fitting to use @bal as a class
>> variable. It's not needed outside the scope of a single withdraw, and as

@bal is an instance variable, not a class variable.

> Thanks. This is a much better code. One more related thing that confuses me
> is that I thought all instance variables of a class are private to the
> class. Then are the private instance variables inherited by a sub-class? If
> yes, then in my Savings class withdraw method, can I write @balance -= amt
> instead of self.balance -= amt ?

The terminology is getting a bit convoluted here.  Instance variables
are per-object, not per-class, and they're not inherited.  But if a
method uses one, and that method is available to subclasses, then it
will still use the variable -- but "the variable" in the sense of one
per object:

   class C
     def initialize
       @n = 100
     end

     def increase_n
       @n *= 20
     end

   end

   class D < C
     def show_n
       puts "n is #{@n}"
     end
   end

   d = D.new
   d.increase_n
   d.show_n

The output is:

   n is 2000

The @n in D's methods is the same (for each instance) as the one in C.

As for the @balance -= thing: you can do that, but sometimes there are
good reasons not to.  You might have a balance or balance= method that
does more than just get and set the instance variable.  Imagine
something like:

   def last_name=(n)
     @last_name = n.downcase.capitalize
   end

If you now say:  @last_name = "bLaCk", it won't get normalized.  (And
yes, I know that there's no one rule that governs all last names :-)
In such a case, it's better to go through the method: self.last_name =
"bLaCk".


David

-- 
                   David A. Black | dblack@wobblini.net
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