From: Jamis Buck <jgb3@...>
Date: 2004-11-05T01:26:03+09:00
Subject: Re: class variable and singleton

Mohammad Khan wrote:
> consider the following two classes:
> 
> class Foo1
>   attr_accessor :data
>   include Singleton
> end
> 
> class Foo2
>   def data
>     return @@data
>   end
> 
>   def data=(val)
>     @@data = val
>   end
> end
> 
> in irb screen:
> 
> [mkhan@localhost mkhan]$ irb
> irb(main):001:0> require 'singleton'
> => true
> irb(main):002:0> class Foo1
> irb(main):003:1>   attr_accessor :value
> irb(main):004:1>   include Singleton
> irb(main):005:1> end
> => Foo1
> irb(main):006:0> a = Foo1.instance
> => #<Foo1:0xf6fc4d4c>
> irb(main):007:0> b = Foo1.instance
> => #<Foo1:0xf6fc4d4c>
> irb(main):009:0> a.value = 100
> => 100
> irb(main):010:0> p b.value
> 100
> => nil
> irb(main):011:0> class Foo2
> irb(main):012:1>   def value
> irb(main):013:2>     return @@value
> irb(main):014:2>   end
> irb(main):015:1>   def value=(val)
> irb(main):016:2>     @@value = val
> irb(main):017:2>   end
> irb(main):018:1> end
> => nil
> irb(main):019:0> a = Foo2.new
> => #<Foo2:0xf6fe0cb8>
> irb(main):020:0> b = Foo2.new
> => #<Foo2:0xf6fdcf00>
> irb(main):022:0> a.value = 100
> => 100
> irb(main):023:0> p b.value
> 100
> => nil
> 
> in both class we will have single instance of 'data'.
> Which one is better and why?
> Can someone explain it, please.
> 
> Thanks in advance...   :-)

It is difficult (though not impossible) to do initialization code on a 
class. For instance, with the Singleton include, you can have a 
constructor that does one-time initialization on the instance:

   class Foo
     include Singleton
     def initialize
       ...
     end
   end

   f = Foo.instance

To do the same thing with a class-based singleton would require you to 
do some less-intuitive initialization magic.

- Jamis

-- 
Jamis Buck
jgb3@email.byu.edu
http://www.jamisbuck.org/jamis