class-variables.rb

#! /usr/bin/env ruby

class Foo
  DEFAULT_VAR = 'I am the default'

  class << self
    attr_accessor :var
  end

  def self.var
    @var || DEFAULT_VAR
  end

  attr_accessor :var

  def var
    @var || self.class.var
  end
end

class Bar < Foo
end

puts "Foo.var => #{ Foo.var.inspect }"
puts "Bar.var => #{ Bar.var.inspect }"
puts "Foo.new.var => #{ Foo.new.var.inspect }"
puts "Bar.new.var => #{ Bar.new.var.inspect }"

puts "\nChanging Bar.var to 5"
Bar.var = 5

puts "Foo.var => #{ Foo.var.inspect }"
puts "Bar.var => #{ Bar.var.inspect }"
puts "Foo.new.var => #{ Foo.new.var.inspect }"
puts "Bar.new.var => #{ Bar.new.var.inspect }"

puts "\nChanging an instance of Bar to 10"
instance_of_bar = Bar.new
instance_of_bar.var = 10
puts "instance_of_bar.var => #{ instance_of_bar.var.inspect }"
puts "Foo.var => #{ Foo.var.inspect }"
puts "Bar.var => #{ Bar.var.inspect }"
puts "Foo.new.var => #{ Foo.new.var.inspect }"
puts "Bar.new.var => #{ Bar.new.var.inspect }"