From: Trans <transfire@...>
Date: 2005-04-17T22:59:39+09:00
Subject: Re: self


Robert Klemme wrote:
> Hey, that's an easy one:
>
> class Base
>   class << self
>     attr :state
>   end
>   instance_variable_set "@state", true
> end
>
> class Derived < Base
> end
>
> ?> Base.state
> => true
> >> Derived.state
> => nil
>
> :-)
>
> State is never inherited.  The reason why your code didn't do what
you
> wanted was that the implementation of the method was inherited.

Ah, of course! --Until this thread I never really gave much thought to
setting instance varaible in the class scope like this. Your simple
example makes it quite plain. That'll be helpful, now that I see it
clearly. Thank you.

> Btw, I would find something like this useful in the std lib:
>
> class Class
>   def properties(*h)
>     h = h[0] if h.size == 1
>     initial = {}
>
>     h.each do |name, val|
>       attr_accessor name
>       initial[name]=val unless val.nil?
>     end
>
>     unless initial.empty?
>       alias :new_plain :new
>       class<<self;self;end.instance_eval do
>         define_method(:new) do |*a|
>           obj = allocate
>           initial.each {|name, val| obj.send("#{name}=".to_sym, val)}
>           obj.instance_eval { initialize(*a) }
>           obj
>         end
>       end
>     end
>
>     h
>   end
> end
>
> >> class Foo
> >>   properties( :name => "foo", :age => 10 )
> >> end
> => {:age=>10, :name=>"foo"}
> >> Foo.new.name
> => "foo"
> >> class Bar
> >>   properties :name, :age
> >> end
> => [:name, :age]
> >> Bar.new.name
> => nil

:-) Ara sent me something much like this recently, too. And something
like them is in route to Ruby Carats for sure. Thanks for helpful code.

T.