From: gwtmp01@...
Date: 2007-01-25T04:27:08+09:00
Subject: Re: Can Ruby do the Objective-C/Cocoa style alloc/init pattern?


On Jan 24, 2007, at 2:07 PM, Eric Hodel wrote:
> On Jan 24, 2007, at 01:50, Greg Hurrell wrote:
>> The "Class" is sent an "alloc" message (memory is allocated and a new
>> instance is actually created) and then the new instance is sent an
>> "init" message (allowing the object to prepare itself for use).
>
> new is written like:
>
> class Object
>   def self.new(*args, &block)
>     obj = allocate
>     obj.send(:initialize, *args, &block) # initialize is private
>     obj
>   end
> end

A less drastic approach than overriding new in the class is to simply
write your own specialized constructor, which can itself call new,
allocate, or #initialize, in whatever combination is needed.

class A
   def self.my_constructor(*a, &b)
     # whatever but good idea to ensure that
     # an instance of A is returned.
   end
end
a = A.my_constructor

I think that overriding A#new in such a way that it returns something
other than an instance of A should be avoided.  The general principle
is that a class should be a factory for its own instances.  If you want
a more general factory pattern, I'd use a module method that delegated
construction to an appropriate class:

class A;end
class B;end
module Factory
   def self.build(a)
     case a
     when 'A' then A.new
     when 'B' then B.new
     else raise ArgumentError, "'A' or 'B' expected"
     end
   end
end

Factory.build 'A'     # instance of A
Factory.build 'B'     # instance of B
Factory.build 'C'     # ArgumentError

Gary Wright