From: Mohammad Khan <mkhan@...>
Date: 2004-11-05T01:20:05+09:00
Subject: class variable and singleton

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...   :-)

--
Mohammad