From: Dave Thomas <Dave@...>
Date: 2001-12-04T06:44:01+09:00
Subject: [ruby-talk:27377] Re: Selecting class based on external information

Jeff Putsch <putsch@hf.mxim.com> writes:

> Howdy,
> 
> I'm trying to select the class for an instance variable based on external
> information and cannot figure out how. For example:
> 
>    class Host
>       # variables and routines common to all hosts, default values for
>       # OS specific things
>    end
>    
>    class SunOS < Host
>       # overrides for OS specific things
>    end 
>    
>    # main routine would do this...
>    
>    OSname = `/bin/uname -s`.chomp
>    
>    myHost = OSname.new # on a Solaris system this should
>                        # be equivalent to doing "myHost = SunOS.new"
> 
> The initialization of "myHost" is where I run into trouble. I can not figure
> out how to do this. I'm sure Ruby has some "magic" to get this done, but
> it's beyond me ... 

Well, there are a number of ways:

1. Brute force...

   Add a factory method somewhere that has

      def HostFactory(osname)
        case osname
        when /SunOS/
          return SunOS.new
        when /Linux/
          return Linux.new
        etc..

2. Dangerous, but

   Extract the name of the OS from uname into some cannonical form,
   and use that as the classname

       os_name = extract_name_from(`uname -s`)
       myHost = eval(os_name)

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)



Regards


Dave