From: Robert Klemme <shortcutter@...>
Date: 2006-07-06T23:55:39+09:00
Subject: Re: newbie Q: conditional object creation

Daniel Schierbeck wrote:
> 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

The same effect can be achieved in two other way that do not involve 
meta classes:

1. make initialize throw an exception.  Although the instance will be 
created, nobody ever gets to see it (unless initialize stores self 
somewhere).

class MyClass
   VALID_NAMES=['bananas','dog','water']

   attr_reader :name, :value

   def initialize(name, value)
     raise ArgumentError, "Invalid name: #{name}" unless
        VALID_NAMES.include? name
     @name = name
     @value = value
   end
end


2. add a class method instead of aliasing new.

class MyClass
   VALID_NAMES=['bananas','dog','water']

   attr_reader :name, :value

   def self.create(name, value)
     raise ArgumentError, "Invalid name: #{name}" unless
        VALID_NAMES.include? name
     new name, value
   end

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


I'd probably favor approach 1 as it is more robust (you cannot forget to 
call "create" as in the second approach).

Kind regards

	robert