From: Calvin Bornhofen <calvin.bornhofen@...>
Date: 2012-09-21T22:02:01+09:00
Subject: Re: Class variables

> --Quote
> @@suits = ['C', 'D', 'H', 'S']
>
> isn't what you would expect, so no one uses them.  Rubyists use class
> instance variables instead, e.g
> <snip>
> --End quote
>
> >From what I can see online
> @@suits = ['C', 'D', 'H', 'S'] creates the array only once no matter how
> many instances you have of the object. Is that not a good thing?
> I don't understand why nobody is using this?
>
> isn't what you would expect, so no one uses them.  Rubyists use class
> instance variables instead, e.g.
The @@ makes the variable a static-like variable as you know it from 
other OO-languages. The thing is, that this variable is not a static 
member of the class in the usual sense. It is consistent over the whole 
class hierarchy. So, if you change the value in one subclass, you change 
it for all classes. Sometimes that is what you want to have, but usually 
it is not. But, Ruby has an alternative: Since classes are also objects, 
you can declare a variable on the class level by using @ instead of @@. 
This makes the variable available to your class, and all subclasses, 
_but_ makes it a different variable for each class. Next question is, 
how do you access them? The usual attr_* methods don't work, since they 
add the methods to the instance level. So, what we could do, is to write 
the accessors on our own ( by defining methods like self.myvariable 
and/or self.myvariable= ). But Ruby wouldn't be Ruby if that wouldn't be 
easier:

   class Foo
     class << self; attr_accessor :suits end
     @suits = ['C', 'D', 'H', 'S']
   end

The construct "class << self; [...] end" makes all it's contents apply 
on the class level, not the instance level ( you may have seen it in 
tutorials you read ). That's all the magic.
Of course, you could use @@ as you are doing it currently, but you 
should be aware of the implications of that.

FYI: I think this page ( 
http://railstips.org/blog/archives/2006/11/18/class-and-instance-variables-in-ruby/ 
) gives a really nice explanation of the topic.