From: Marnen Laibow-Koser <marnen@...>
Date: 2009-11-28T22:33:51+09:00
Subject: Re: Class variables, instance variables, singleton; Ruby v.

Ralph Shnelvar wrote:
[...]
> Let me flesh out my questions/comments about this paragraph so that you 
> and
> others have something specific to answer.  Let's call the classed  F &
> G.
> 
> (1) OK, classes are (i) objects;

Right.

> (ii) are executable code.

Wrong.  Class *definitions* are executable code.

> 
> 
> (2) If I do the following
>       class F
>       end
>       f = F.new
>       F.class  # class
>       f.class  # F
> this makes sense to me.
> 
> What (relevant and important conceptually) messages (other than "new")
> can I send to a class?  

See the documentation for class Class.

> But see (3), below.
> 
> 
> 
> (3)
>     class F
>       def sub1
>         @@x = 1
>       end
>     end
> 
>     class G
>       self.sub1
>         @@x=2
>       end
>     end
> 
>     # Why?  Didn't the interpreter "see" @@x in class F?
>     F.class_variables  # []

Probably not, since you haven't called the function yet!

> 
>     # makes sense
>     G.class_variables  # ["@@x"]

I'm actually rather surprised by this.

> 
>     f = F.new
> 
>     # Why?  Shouldn't the method inherit from the class?
>     f.class_variables  # undefined method

No!  class_variables is a class method of F.  Instances do not get 
access to their class' class methods, because they don't inherit from 
the Class object in the conventional sense.  The Class object is a 
little bit like a JavaScript prototype, if that helps.

If it doesn't...well, I don't know what to say.  If you're really as 
good a C++ programmer as you claim to be, then you should be having no 
trouble at all with the difference between class and instance methods.

> 
>     f.sub1  # 1
> 
>     # makes sense.  class variable now explicitly executed
>     F.class_variables # ["@@x"]
> 
>     # How to use class_variable_get if the method is private???
>     F.class_variable_get(:@@x)  # error private method!
> 

Just don't.  You're not meant to be getting at class variables from 
outside the class.  Use an accessor function if you need it.

(There is a way to call private methods from outside, but I will leave 
you to find it out on your own.  It's not generally a good thing, and 
I'm not going to hand you a dangerous tool until you understand when not 
to use it.)

> 
> 
> OK, that should do it for now.

Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen@marnen.org
-- 
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