From: matz@... (Yukihiro Matsumoto)
Date: 2001-08-12T00:07:02+09:00
Subject: [ruby-talk:19535] Re: the way class variables work

Hi,

In message "[ruby-talk:19506] the way class variables work"
    on 01/08/11, David Alan Black <dblack@candle.superlink.net> writes:

|   class Vehicle
|     @@tires = 0
|     def tire_count
|       @@tires
|     end
|   end
|
|   class Car < Vehicle
|     @@tires = 4
|   end
|
|   class Bike < Vehicle
|     @@tires = 2
|   end
|
|   puts Car.new.tire_count  # => 2

Class variables are to share value among classes/modules and its
instances.  If you want to have per class information, you have class
instance variable, or constants.

class instance variables

   class Vehicle
     @tires = 0
     def Vehicle::tire_count
       @tires
     end
     def tire_count
       type::tire_count
     end
   end

   class Car < Vehicle
     @tires = 4
   end

   class Bike < Vehicle
     @tires = 2
   end

   puts Car.new.tire_count  # => 4

constants

   class Vehicle
     def tire_count
       type::TIRES
     end
   end

   class Car < Vehicle
     TIRES = 4
   end

   class Bike < Vehicle
     TIRES = 2
   end

   puts Car.new.tire_count  # => 4

class variables are global variables with limited scope (to specific
class hierarchy), so I do not recommend to abuse them.

							matz.