How to access class instance variables from a subclass

Question

Below is my code with class MyClass and class OtherClass, which inherits MyClass. I have a class variable @@my_new_var and a class instance variable @my_var.

class MyClass @@my_new_var = "test" @my_var = 1 def self.read; @my_var end def ins_method; @my_var = 2 end end class OtherClass < MyClass def self.read_another; @@my_new_var end def self.test; @my_var end end

We can access class variables from a subclass:

OtherClass.read_another # => test

but we can't access class instance variables from a subclass:

MyClass.read # => 1 MyClass.new.ins_method # => 2 OtherClass.read # => nil OtherClass.test # => nil

Why is that? What is the scope of class instance variables?

The @my_var that appears in def ins_method; @my_var = 2 end is not a class instance variable. — sawa
– sawa, Commented Sep 28, 2014 at 13:41
yeah I knew that, class instance variables can be accessed in class methods only if i'm not wrong. — Kranthi
– Kranthi, Commented Sep 28, 2014 at 13:45
Class instance variables can be accessed by instances using Object#instance_variable_get; e.g., class Klass; @cat = "meow"; def cat; self.class.instance_variable_get(:@cat); end; end. Then Klass.new.cat => "meow". — Cary Swoveland
– Cary Swoveland, Commented Sep 28, 2014 at 22:00

sawa · Accepted Answer · 2014-09-28 13:32:03Z

2

Q1: Because they are defined so.

Q2: It is the class (which is an instance of the Class class).

answered Sep 28, 2014 at 13:32

sawa

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