From: "Wolfgang Nádasi-Donner" <wonado@...>
Date: 2005-03-15T22:57:53+09:00
Subject: Re: [newby] class variables and class instance variable?

--
Wolfgang N�dasi-Donner
wonado@donnerweb.de
"Lionel Thiry" <lthiryidontwantspam@skynetnospam.be> schrieb im Newsbeitrag
news:4232ff57$0$30162$ba620e4c@news.skynet.be...
> Hello.
>
> class Test
>    @a = "value"
>
>    def self.a
>      @a
>    end
>
>    def initialize
>      @@a = "value2"
>    end
>
>    def a
>      @@a
>    end
> end
>
> puts Test.a # output: value
> puts Test.new.a # output: value2
>
> I don't understand (and I'm quite surprised), what is the difference in
terms of
> OO design between class variables, the @@a in the example above, and class
> instance variables, the @a in the example?
>
> Thanks in advance,
> Lionel Thiry

The class variable ("@@") belongs to the class object, while the instance
variable (@) belongs to the instanciated object. A short example. May be you
want to count how many objects are created based on a class. If you want to
avoid global variables ($), which is a very good decision, you can use class
variables. Using instance variables will not work.

>>>>> Example >>>>>
class Mytest
 @@n_of_Mytest = 0
 def initialize
  @@n_of_Mytest += 1
 end
 def Mytest.n_of_Mytest
  @@n_of_Mytest
 end
end

puts Mytest.n_of_Mytest
a = Mytest.new
puts Mytest.n_of_Mytest
b = Mytest.new
c = Mytest.new
puts Mytest.n_of_Mytest
d = Mytest.new
e = Mytest.new
f = Mytest.new
puts Mytest.n_of_Mytest
>>>>> Output >>>>>
0
1
3
6
>>>>> End of Example >>>>>

O.K.?