From: Robert Klemme <bob.news@...>
Date: 2005-04-17T19:49:39+09:00
Subject: Re: self


"Trans" <transfire@gmail.com> schrieb im Newsbeitrag 
news:1113672669.627263.226310@f14g2000cwb.googlegroups.com...
> robert,
>
> i have one for you. tell me how to create a class whose subclasses DO
> NOT inherit some particular state from. In other words I need:
>
>  class Base
>     def self.state ; true ; end  # or what have you
>  end
>
>  class Derived < Base
>  end
>
>  Derived.state  #=> nil  (was NOT inherited)
>
> how do i get something like that to work?

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.

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

Cheers

    robert