From: Marko Schulz <in6x059@...>
Date: 2001-04-24T07:18:14+09:00
Subject: [ruby-talk:14112] Re: RCR: getClassFromString method

On Tue, Apr 24, 2001 at 03:41:23AM +0900, Phil Tomson wrote:
> In article <988034673.396094.26164.nullmailer@ev.netlab.zetabits.com>,
> Yukihiro Matsumoto <matz@zetabits.com> wrote:
> >
> > [ Should it be encouraged style of programming to retrieve a class
> >   by its name in a string? ]
> 
> Well, perhaps it shouldn't be encouraged, I don't know.  I'm not 
> necessarily an OO purist, I suppose.
> 
> I know that I use this feature in a certain case where I have a class 
> hierarchy like:
> 
> class Tool
>   #methods to handle generic stuff common to PC and UNIX
> end 
> 
> class Tool_PC < Tool
>   #methods do PC specific stuff
> end
> 
> class Tool_UNIX < Tool
>   #methods do UNIX related stuff
> end
> 
> Let's say that early on in the script I figure out what platform I'm 
> running on, say:
> $PLATFORM = "PC"  ($PLATFORM is global so I can instantiate 
> Tool_"$PLATFORM" objects anywhere later on in the script)
> 
> Then I can instantiate the correct class based on the value of PLATFORM
> 
> There are probably other ways to do this, but getting the Class from a 
> String seems pretty straitforward in this case - and it is how I've 
> implemented this.
> 
> So maybe it's not a good 'pure' thing to allow, but it does have its uses.

I would use a factory-method for this:

  class ToolFactory

     def ToolFactory.getTool
       case $PLATFORM
         when "Unix" then Tool_Unix
         when "PC"   then Tool_PC
         else             nil
       end
     end

  end


I would favor passing the platform as a parameter since I don't like
global variables.

-- 
marko schulz

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