Ruby class variable

classvar.rb

class Foo
  class << self
    attr_reader :ivar
  end
  @ivar = 'Hello!'
end

class Bar < Foo
  @ivar = 'World'
end

class Baz < Foo
  @ivar = 'Underworld'
end

puts Baz.ivar
puts Bar.ivar
puts Foo.ivar