From: Joel VanderWerf <vjoel@...>
Date: 2006-12-03T10:27:29+09:00
Subject: Re: Factory pattern, abstract base class

Daniel Berger wrote:
> Hi all,
> 
> I'm trying to setup an abstract base class as a factory for its
> subclasses.  The below code fails, as it always returns an instance of
> Foo::Bar instead of one of its subclasses.
> 
> module Foo
>    class Bar
>       def initialize(arg)
>          return if self.class != Foo::Bar
>          case arg.downcase
>             when 'baz'
>                return Baz.new(arg)
>             when 'zap'
>                return Zap.new(arg)
>          end
>       end
>    end
> 
>    class Baz < Bar
>    end
> 
>    class Zap < Bar
>    end
> end
> 
> fb = Foo::Bar.new('baz')
> p fb # Returns Foo::Bar, but I want Foo::Baz
> 
> Perhaps my whole approach is wrong.  What is the proper way to achieve
> what I'm after (without resorting to making Bar a module)?

module Foo
    class Bar
       def self.new(arg)
          return super() if self != Foo::Bar
          case arg.downcase
             when 'baz'
                return Baz.new(arg)
             when 'zap'
                return Zap.new(arg)
          end
       end
    end

    class Baz < Bar
    end

    class Zap < Bar
    end
end

fb = Foo::Bar.new('baz')
p fb # ==> #<Foo::Baz:0xb7d66c3c>


-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407