The Book of Ruby

Class Variables

A few other interesting things are going on in this program. Right at the top of the Thing class you will see this:

@@num_things = 0

The two @ characters at the start of this variable name, @@num_things, define this to be a class variable. The variables we’ve used inside classes up to now have been instance variables, 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. I have assigned 0 to the @@num_things variable to ensure that it has a meaningful value at the outset.

Here, the @@num_things class variable is used to keep a running total of the number of Thing objects in ...