From: James Britt <james_b@...>
Date: 2006-05-21T13:34:38+09:00
Subject: Re: Class variables and class instance variables in Pickaxe 2nd

uncutstone wu wrote:
> These are two concepts in Pickaxe 2nd edtion.
> 
> Class Variables example is in chapter 3:
> 
> class Song
>   @@plays = 0
>   def play
>     @@plays += 1
>   end
> end

Note that the class variable is being used here by an instance method.

> 
> Class instance variables example is in chapter 24:
> 
> class Test
>   @cls_var = 123
>   def Test.inc
>     @cls_var += 1
>   end
> end

And note that the class instance variable is being used here by a class 
  method.

A class is also an object; it's an instance of Class.  Since it is an 
object it may have its own instance variables.    Class variables are 
available to the class and to instances of that class; class instance 
variables are only visible to the class itself (not to instances of that 
class).  (Yeah, it can be confusing at first.)


class Foo
   @@cls_var = 123
   @cls_instvar = 'abc'

   def test
     p @@cls_var
     p @cls_instvar

   end

   def self.test
     p @@cls_var
     p @cls_instvar
   end
end


puts( "Foo is a #{Foo.class}")

f = Foo.new
f.test
Foo.test




-- 
James Britt

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