Consider the following code:
@@num_things = 0
The two @ characters define num_things to be a class variable.
The instance variables are preceded by a single @, like @name.
Whereas each new object or instance of a class assigns its own values to its own instance variables.
All objects derived from a specific class share the same class variables.
You can use the @@num_things class variable to keep a running total of the number of Thing objects.
The scope of a class variable is within the current class, as opposed to within specific objects of that class.
Class variables start with two @ symbols (@@) as opposed to the single @ symbol of object variables.
Class variables are useful for storing information relevant to all objects of a certain class.
For example, you could store the number of objects created so far in a certain class using a class variable like so:
class Square def initialize if defined?(@@number_of_squares) @@number_of_squares += 1 # w w w. jav a 2 s . c o m else @@number_of_squares = 1 end end def self.count @@number_of_squares end end a = Square.new b = Square.new puts Square.count
Because @@number_of_squares is a class variable, it's already defined each time you create a new object.