The Book of Ruby

Class Variables

Class methods may remind you of the class variables you used previously (that is, variables whose names begin with @@). You may recall that you used class variables in a simple adventure game (see 2adventure.rb in Attribute Readers and Writers) to keep a tally of the total number of objects in the game; each time a new Thing object was created, 1 was added to the @@num_things class variable:

class Thing
   @@num_things = 0

   def initialize( aName, aDescription )
      @@num_things +=1
   end

end

Unlike an instance variable (that is, a variable that belongs to a specific object created from a class), a class variable must be given a value when it is first declared:

@@classvar = 1000     # class variables must be initialized

Initialization of either instance ...