Question

While playing a bit with Ruby, I wrote the following code:

class A
end

A.singleton_class.instance_eval do
    undef_method :new
end

# or
# class << B
#   undef_method :new
# end

A.new

> NoMethodError: undefined method `new' for A:Class
>         from (irb):8
>         from /home/mmsequeira/.rvm/rubies/ruby-1.9.3-p327/bin/irb:16:in `<main>'

This is cool. But how can I know which methods have been undefined in a given class?

Answer 1

You can’t by default. Undefining a method removes it from existence. You could, however, record them as they’re removed. This can be done with method hooks to capture everything and avoid ugly alias method chaining:

class Module
  def method_undefined name
    (@undefined_methods ||= []) << name
  end

  def singleton_method_undefined name
    (@undefined_methods ||= []) << name
  end

  def undefined_methods
    @undefined_methods || []
  end
end

This will capture methods undefined via undef_method or undef:

class C
  def foo; end
  def bar; end

  undef foo
  undef_method :bar
end

C.undefined_methods  #=> [:foo, :bar]
C.singleton_class.instance_eval { undef new }
C.singleton_class.undefined_methods  #=> [:new]

Of course, you must include the hook methods in Module before anything can be captured.