From: German Monfort <monfort.german@...>
Date: 2007-05-01T10:32:31+09:00
Subject: Re: Class Level Variables

El Lunes, 30 de Abril de 2007 19:15, Gary Wright escribi�:
> 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

OK, I now understand the difference.

Probably I'm too new to programming to understand the full power of class 
variables, and I found this concept a little bit dangerous because you can 
affect the whole behavior of a class by simply changing a class variable in 
any other subclass ... so I try not to use class variables for noiw.

I still need to read more and more :D

Thanks Gary