From: Gary Wright <gwtmp01@...>
Date: 2007-05-01T04:15:50+09:00
Subject: Re: Class Level Variables


On Apr 30, 2007, at 1:52 PM, German Monfort wrote:

>> 2) Class variables are associated with a class *and* its subclasses
>> and the order of initialization is important.
>>
>> class C
>>    @@foo = 42        # shared by C and its decendents!
>>    def foo
>>      @@foo           # shared with C, D, and E
>>    end
>>    def bar
>>      @@bar           # this is C's @@bar
>>    end
>> end
>>
>> class D < C
>> end
>>
>> class E < C
>>    @@foo = 43        # @@foo shared with C, D, and E
>>    @@bar = 44        # @@bar only defined for E
>>    def bar
>>      @@bar           # this is E's @@bar
>>    end
>> end
>>
>> puts D.new.foo       # 43
>>
>> puts E.new.bar       # 44
>> puts C.new.bar       # undefined!
>>
>> class C
>>    @@bar = 45
>> end
>>
>> puts C.new.bar       # 45
>> puts E.new.bar       # still 44
>>
>> Gary Wright
> Shouldn't be
> puts D.new.foo       # 42 ?
>
> Because D < C and has no foo method defined (so it uses C' foo) and  
> also has
> no @@foo defined (so it uses C's @@foo) which is set to 42 not 43.
> I'm confused here.

I was illustrating the fact that C and E don't share the same @@bar  
due to
initialization order and the location of the definitions of C.bar and  
E.bar.

If you called D.new.foo you would get 43 because @@foo is shared by  
C, D, and E
and was updated to reference 43 in the class block that defined E.

If you called D.new.bar you would get 45 because the call to 'bar'  
would be
implemented by the definition in C where @@bar refers to C's @@bar.   
This is
a good example of how class variables are not relative to self!

Gary Wright