Beginning meta-programming question

Ruby
1

I’m pretty new to ruby, and just playing around. I can’t work out why
the
following won’t work, though:

  1. module TrackInheritors
  2. def self.extended(parent)
  3.  class << parent

  4.    puts self

  5.    @@inheritors = []

  7.    def self.inheritors

  8.      @@inheritors

  9.    end

  11.    def self.inherited(child)

  12.      @@inheritors << child

  13.    end

  14.  end
  15. end
  16. end
  18. class Parent
  19. extend TrackInheritors
  20. end
  22. class Child1 < Parent
  23. end
  25. class Child2 < Parent
  26. end
  28. puts Parent.inheritors

  30. -:28: undefined method `inheritors’ for Parent:Class

(NoMethodError)

2

I’m a noob myself so sorry if I give incorrect information but I think
you have a going out of scope issue. The inheirtors method is defined
within the class that starts on line 3. However, the class it’s put in
is inside the extended method on line 2, so what I think is going on is
that by the time you’re trying to the inheirtors method, its class has
already gone out of scope since it’s called after the extended method
has ended.

3

On Mon, Jul 26, 2010 at 6:48 AM, Andrew W. [email protected]
wrote:

  1.      @@inheritors
  2. extend TrackInheritors

  3. -:28: undefined method `inheritors’ for Parent:Class

(NoMethodError)

When you open the singleton class of parent, you don’t need to define
the methods for “self.”, just define them directly, cause you are
already on the singleton class:

module TrackInheritors
def self.extended(parent)
class << parent
puts self
@@inheritors = []

  def inheritors
    @@inheritors
  end

  def inherited(child)
    @@inheritors << child
  end
end

end
end

class Parent
extend TrackInheritors
end

class Child1 < Parent
end

class Child2 < Parent
end

puts Parent.inheritors

$ ruby inheritors.rb
#Class:Parent
Child1
Child2

I’m not really sure where were you defining those methods, in the
singleton class of the singleton class?

Jesus.