Adding a class instance variable and attr_reader to Ruby class at runtime?

Question

How do I add a class instance variable, the data for it and a attr_reader at runtime?

class Module
  def additional_data member, data
    self.class.send(:define_method, member)  {
      p "Added method #{member} to #{name}"
    }
  end
end

For example, given this class

class Test
  additional_data :status, 55
end

So that now I can call:

p Test.status # => prints 55

Answer 1

How about this?

class Object
  def self.additional_data(name, value)
    ivar_name = "@#{name}"

    instance_variable_set(ivar_name, value)

    self.class.send(:define_method, name) do
      instance_variable_get(ivar_name)
    end

    self.class.send(:define_method, "#{name}=") do |new_value|
      instance_variable_set(ivar_name, new_value)
    end
  end
end

class Foo
  additional_data :bar, 'baz'
end

puts Foo.bar # => 'baz'
Foo.bar = 'quux'
puts Foo.bar # => 'quux'

It's pretty self-explanatory, but let me know if you have any questions.

Answer 2

Module#class_eval is what you want:

def add_status(cls)
  cls.class_eval do
    attr_reader :status
  end
end

add_status(Test)