Ruby: Initializing class instance variables with default values for all subclasses

gistfile1.rb

class ParentClass
  @foo = "bar"
  
  class << self
    attr_accessor :foo
    
    def inherited(subclass)
      subclass.instance_variable_set :@foo, "bar"
      super
    end
  end
end

class ChildClass < ParentClass
end

gistfile2.rb

ParentClass.foo
#=> "bar"

ChildClass.foo
#=> "bar"

# Change the value for ChildClass only
ChildClass.foo = "look at me"
ChildClass.foo
#=> "look at me"

ParentClass.foo
#=> "bar"