Sams Teach Yourself Ruby in 21 Days

Class Variables

If we needed each philosopher to keep track of his or her creation (or graduation) date, it would be a fairly simple task.

class Philosopher
  def initialize
    @graduation_date = Time.now
  end
  attr_reader :graduation_date
end

But suppose we also thought that each philosopher should come stamped with a serial number. The initialize method would assign a value to an instance variable, say @serial, and that number would increment with each new graduate who walked out the door.

It turns out that we'll need a new kind of variable to get the job done, because somehow the academy always has to keep track of what the next serial number should be. Surely this isn't a job for an instance variable; we only want one copy of this information, ...