From: Ryan Pavlik <rpav@...>
Date: 2003-08-22T14:19:12+09:00
Subject: Re: Class variables - a surprising result

On Fri, 22 Aug 2003 13:59:54 +0900
Patrick Bennett <patrick.bennett@inin.com> wrote:

> Dan Doel wrote:
<snip>
> > Foo.print # => 6
> > Foo.new.print # => nil 
> 
> Geesh - sneaky stuff..  This kind of thing *definitely* violates the 
> P.O.L.S. imo.  For one, having the 'same' variable actually be two 
> different 'instances' (one, a class variable effectively,
> and another, an instance variable) is very confusing.  I understand it 
> now (although I have to think about it for a sec, and personally, I 
> think that's a bad sign).
> I think the above @var=6 assignment outside of a method decl should be 
> illegal.  It should have to be a @@ reference.


Definitely not.  One of the coolest things about ruby is that when you
say "class" or "module", it doesn't open a declarative section of code,
but rather an new executive section.   This helps greatly in eliminating
special syntaxes.  For instance:

    class A
        attr_accessor :a, :b

        private

        def foo; ... end
    end

In this case, "attr_accessor" and "private", which would regularly be
syntax, are actually method calls.  This is Very Cool:

    class Module
      def def_abstract(*args)
        for sym in args
          module_eval <<-CODE
          def #{sym}(*args); raise SubclassResponsibility; end
          CODE
        end
      end #m:def_abstract
    end #c:Module

Now I can do:

    class B
       def_abstract :foo, :bar
    end

Nifty!  In fact, I've used this to define new attr_* functions, and a
few other things, to make life easier:


    class C
        attr_accessor_typed String, :title, :text
    end

The fact that a class or module body is just code along with everything
else gives great flexibility.

As for @instance variables of Class being confusing... it's not at all.
It's like this:

    #<Class D:0x1234 @a=42>  <--- Over here we have a Class object

       over here we have a D object ----->  #<D:0x4321 @x=0, @y=0>

If you think of everything being an object, the proper place to put your
"static" variables is in the Class object itself.  It's a matter of
understanding the proper way to think about Ruby, rather than trying to
translate it into something else.

-- 
Ryan Pavlik <rpav@users.sf.net>

"Let super-dimensional physics take its course!" - 8BT