From: Daniel Brockman <daniel@...>
Date: 2005-07-28T09:52:04+09:00
Subject: Re: What's so special about operators, built-in classes and modules?

Pit Capitain <pit@capitain.de> writes:

> For me, the main difference between modules and classes is
> that classes "define" state (instance variables), not only
> behaviour (methods).

Yet there are many examples of modules whose modules set
instance variables, and there are many examples of classes
whose methods do not.

That alleged difference is not grounded in reality.

> One classic problem with multiple inheritence is the so
> called diamond:
>
>    class A
>      def initialize(val)
>        @var = val
>      end
>    end
>
>    class B
>      include A
>      def initialize
>        super(5)
>      end
>    end
>
>    class C
>      include A
>      def initialize
>        super(42)
>      end
>    end
>
>    class D
>      include B
>      include C
>    end
>
>    d = D.new

As has been pointed out countless times already, the diamond
inheritance problem already exists in Ruby:

   module A
     def initialize(val)
       @var = val
     end
   end

   module B
     include A
     def initialize
       super(5)
     end
   end

   module C
     include A
     def initialize
       super(42)
     end
   end

   class D
     include B
     include C
   end

> Now, how many instances of A are in d?

One (assuming you mean how many are in D's ancestor list).

-- 
Daniel Brockman <daniel@brockman.se>