From: Mike Depot <mdepot@...>
Date: 2002-06-01T05:17:27+09:00
Subject: Singleton Waring - Mixins & Inheretance

When I run the test script below, I get the following warning:
(eval):4: warning: instance variable @__instance__ not initialized

After playing around with it a bit, I noticed that including Singleton in
MyDerivedClass eliminates the warning.  The code seems to function properly
either way.

If I mixin module A into class B, and then define class C as inherited from
class B, do I have to explicitly mixin A into class C?
Is this something to do with the way mixins get inhereted into subclasses,
or am I just doing something else wrong entirely?


#!/usr/bin/ruby -w

require 'singleton'

class MyBaseClass
    include Singleton
    attr_accessor :x
    def initialize
        @x = 'Hello'
    end;
end

class MyDerivedClass < MyBaseClass
    # include Singleton   # do i need this?
    attr_accessor :y
    def initialize
        super
        @y = 'World'
    end;
end

a = MyDerivedClass.instance;
b = MyDerivedClass.instance;

p a.x
p a.y
a.x = "Hi"
a.y = "There"

p a.x
p a.y
p b.x
p b.y