From: Jeff Putsch <putsch@...>
Date: 2001-12-04T07:13:59+09:00
Subject: [ruby-talk:27379] Re: Selecting class based on external information

Thanks,

Although I didn't mention it, I had ruled out the brute force approach, and
was looking for something like the one Dave provided.

Thanks Dave.

Jeff.
On Tue, Dec 04, 2001 at 06:44:01AM +0900, Dave Thomas wrote:
> 3. MOre complex, but sexier:
> 
>    Have the Host class hierarchy manage itself, and generate the
>    factory method semi-automatically. For example:
> 
>        class Host
>           ##
>           # This is a simple factory system that lets us associate fragement
>           # types (a string) with a subclass of fragment
> 
>           TYPE_MAP = {}
> 
>           def Host.os_name(name)
>             TYPE_MAP[os_name] = self
>           end
> 
>           # Here's the factory method
>           def Host.for(os_name, *args)
>             klass =  TYPE_MAP[0s_name] || raise("Unknown host: #{os_name}")
>            return klass.new(*args)
>           end
> 
>           # .. rest of class Host
>         end
> 
>    Then, when you define your subclasses, do
> 
>         class SunOS < Host
>           os_name "SunOS"
>           # ...
>         end
> 
>         class Linus < Host
>           os_name "Linus"
>           # ...
>         end
> 
>    To get the right object given an os_name, you jst call
> 
>         my_host = Host.for(os_name)

Jeff.