From: Daniel Schierbeck <daniel.schierbeck@...>
Date: 2006-07-06T23:54:06+09:00
Subject: Re: newbie Q: conditional object creation

Paul Van Delst wrote:
> I'm want to be able to create objects, but I want the object creation to 
> only proceed if the values passed to the init method are in a valid 
> list. E.g.:
> 
> class MyObj
>   VALID_NAMES=['bananas','dog','water']
>   attr_reader :name, :value
>   def initialize(name,value)
>     i=VALID_NAMES.index(name)
>     if i then
>       @name,@value=name,value
>     else
>       @name,@value=nil
>     end
>   end
> end
> 
> This sorta does what I want, but looks clunky, ugly and quite 
> un-ruby-ish. Is there a better way (idiom?) to achieve this sort of 
> thing? I.e. is there a way to make the init method not create even an 
> "empty" object (as happens above) based on the valid names list?

   class MyObj
     VALID_NAMES = %w{bananas dog water} # array of strings

     # go into the metaclass
     class << self
       alias_method :__new__, :new

       def new(name, value)
         raise unless VALID_NAMES.include? name
         __new__(name, value)
       end
     end

     def initialize(name, value)
       @name, @value = name, value
     end
   end

This will raise an exception when MyObj.new is called with an invalid 
name. You should read up on metaclasses if you haven't already[1].

If you just want MyObj.new to return nil when supplied with an invalid 
argument, this will do the trick:

   def new(name, value)
     __new__(name, value) if VALID_NAMES.include? name
   end


Cheers,
Daniel

[1]  http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html