Class Instance Variable example

class_instance_variable_example.rb

class A
  @obj ||= Object.new

  def self.obj
    @obj
  end

  def self.inherited(subclass)
    puts 'Hello inherited'
    puts "self: #{self}"
    puts "self obj_id: #{@obj.object_id}"
    puts "subclass: #{subclass}"
    subclass.instance_exec(@obj) do |obj| 
      puts "Hello object_id: #{obj.object_id}"
      @obj = obj
    end
  end
end

class B < A; end

A.obj == B.obj
A.obj.equal? B.obj

B.instance_eval { @obj = Object.new }

A.obj == B.obj
A.obj.equal? B.obj