From: Jeremy Henty <jeremy@...>
Date: 2006-02-02T06:58:31+09:00
Subject: Re: question about class variables and instance variables

On 2006-01-31, Eric Hodel <drbrain@segment7.net> wrote:

> class variables are shared between a class, its instances, its
> subclasses and its subclass instances.

Is this really the whole story?  Because until now I always thought I
didn't understand class variables but if this is so then now I do.
Wheeeee!

But there's at least one gotcha; a class variable shadows any class
variable of the same name that you subsequently create in an ancestor
class:

    $ irb
    irb(main):001:0> class A ; end ; class B < A ; end
    => nil
    irb(main):002:0> class B ; @@x = 1 ; end ; class A ; @@x = 0 ; end
    => 0
    irb(main):003:0> class A ; puts @@x ; end ; class B ; puts @@x ; end
    0
    1
    => nil
    irb(main):004:0> exit

.... but if you create the class variable in the ancestor class first
then the descendant class inherits it instead of creating a new one:

    $ irb
    irb(main):001:0> class A ; end ; class B < A ; end
    => nil
    irb(main):002:0> class A ; @@x = 0 ; end ; class B ; @@x = 1 ; end
    => 1
    irb(main):003:0> class A ; puts @@x ; end ; class B ; puts @@x ; end
    1
    1
    => nil
    irb(main):004:0> exit

Any other subtleties?

Cheers, 

Jeremy Henty