metaprogramming - How to dynamically alter inheritance in Ruby

Question

I would like to dynamically specify the parent class for a class in Ruby. Consider this code:

class Agent
  def self.hook_up(calling_class, desired_parent_class)
    # Do some magic here
  end
end

class Parent
  def bar
    puts "bar"
  end
end

class Child
  def foo
    puts "foo"
  end

  Agent.hook_up(self, Parent)
end

Child.new.bar

Neither the Parent nor the Child class definition specifies a parent class, so they both inherit from Object. My first question is: what would I need to do in Agent.hook_up in order to make Parent the superclass of Child (so for example Child objects can inherit the 'bar' method).

My second question is: do I need to pass the first argument to Agent.hook_up, or is there some way the hook_up method can programmatically determine the class from which it was called?

Answer 1

Perhaps you are looking for this

Child = Class.new Parent do
  def foo
    "foo"
  end
end

Child.ancestors   # => [Child, Parent, Object, Kernel]
Child.new.bar     # => "bar"
Child.new.foo     # => "foo"

Since parent is an argument to Class.new, you can swap it out with other classes.

I've used this technique before when writing certain kinds of tests. But I have difficulty thinking of many good excuses to do such a thing.

---

I suspect what you really want is a module.

class Agent
  def self.hook_up(calling_class, desired_parent_class)
    calling_class.send :include , desired_parent_class
  end
end

module Parent
  def bar
    "bar"
  end
end

class Child
  def foo
    "foo"
  end

  Agent.hook_up(self, Parent)
end

Child.ancestors   # => [Child, Parent, Object, Kernel]
Child.new.bar     # => "bar"
Child.new.foo     # => "foo"

---

Though, of course, there is no need for the Agent at all

module Parent
  def bar
    "bar"
  end
end

class Child
  def foo
    "foo"
  end

  include Parent
end

Child.ancestors   # => [Child, Parent, Object, Kernel]
Child.new.bar     # => "bar"
Child.new.foo     # => "foo"