From: dblack@...
Date: 2006-02-21T11:48:12+09:00
Subject: Re: Class variables and Constants

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Hi --

On Tue, 21 Feb 2006, David Vallner wrote:

> D=C5=88a Pondelok 20 Febru=C3=A1r 2006 14:50 dblack@wobblini.net nap=C3=
=ADsal:
>
>> I think Matz is planning to change class variables in 2.0 so that they
>> are class/module scoped rather than hierarchy scoped.  Meanwhile,
>> here's another fun variant:
>>
>> class A
>> end
>>
>> class B < A
>>    @@var =3D 10
>>    def self.var
>>      @@var
>>    end
>> end
>>
>> class A
>>    @@var =3D 20
>>    def self.var
>>      @@var
>>    end
>> end
>>
>> p A.var
>> p B.var
>>
>
> Oh dear. I do hope the change gets into 2.0, I can't wait for Java trolls=
 to
> ramble against it...
>
> (Now don't tell anyone, but I have a hunch class instance variables actua=
lly
> work like you'd expect in this case. I think.)

Basically a "class instance variable" is no different from any other
instance variable -- it just happens to belong to a class object.  The
phrase has become commonplace, I guess, because it's sort of unclear
otherwise, when you talk about, say, "a class's instance variables",
whether you mean the class object's or the ones it is responsible for
providing for its own instances.

Anyway, instance variables of class objects do, indeed, provide
per-class (i.e., per-object-that-happens-to-be-a-class) state, which
class variables don't.  I think the 2.0 change is going to be that
class variables will be like instance variables of a class, except
that they will be visible inside the class's instance methods.


David

--=20
David A. Black (dblack@wobblini.net)
Ruby Power and Light (http://www.rubypowerandlight.com)

"Ruby for Rails" chapters now available
from Manning Early Access Program! http://www.manning.com/books/black
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