Class variables and class instance variables in Ruby

class_variables.rb

class Superclass
  @@var1 = "var 1 Superclass"
  @var2 = "var 2 Superclass"

  def self.var1
    @@var1
  end

  def self.var2
    @var2
  end
end

class Subclass < Superclass
  @@var1 = "var 1 Subclass"
  @var2 = "var 2 Subclass"
end


Superclass.class_variable_get(:@@var1)  # "var 1 Subclass"
Superclass.instance_variable_get(:@var2)  # "var 2 Superclass"

Subclass.class_variable_get(:@@var1)  # "var 1 Subclass"
Subclass.instance_variable_get(:@var2)  # "var 2 Subclass"



# Another example
class A
  def self.var
    @@var ||= begin
      puts "defining @@var"
      Time.now
    end
  end
end

class B < A
end

2.0.0p353 :014 > A.class_variable_defined?(:@@var)
false
2.0.0p353 :015 > B.class_variable_defined?(:@@var)
false
2.0.0p353 :016 > A.var
defining @@var
2014-02-19 11:26:17 -0300
2.0.0p353 :017 > A.var
2014-02-19 11:26:17 -0300
2.0.0p353 :018 > A.class_variable_defined?(:@@var)
true
2.0.0p353 :019 > B.class_variable_defined?(:@@var)
true
2.0.0p353 :020 > B.var
2014-02-19 11:26:17 -0300