The document discusses the pros and cons of using Ruby's method_missing method. It outlines four common pitfalls when using method_missing: 1) the Ghost House pitfall where methods are not actually defined, 2) the Liar Object pitfall where respond_to? returns incorrectly, 3) the Fake Ghost pitfall where methods are redefined incorrectly, and 4) the Snail Ghost pitfall which can cause stack overflow errors. In general, the document recommends avoiding method_missing when possible and instead using define_method to define delegated methods explicitly to avoid these pitfalls.

1. The revenge of method_missing()
Paolo “Nusco” Perrotta

2. Should I use method_missing()
to remove duplication from
my code, or should I avoid it
like the plague?
Paolo “Nusco” Perrotta

4. disclaimer

5. Person.find_by_user_name_and_password name, pwd

6. class NilClass
# ...
def method_missing(method, *args, &block)
if klass = METHOD_CLASS_MAP[method]
raise_nil_warning_for klass, method, caller
else
super
end
end
end

7. end of disclaimer

8. class InfoDesk
def flights
# ...
end
def trains
# ...
end
def hotels
# ...
end
# ...
end

9. class Sign
def initialize
@desk = InfoDesk.new
end
def flights
return “Out for lunch” if Clock.lunch_time?
@desk.flights
end
def trains
return “Out for lunch” if Clock.lunch_time?
@desk.local_transports
end
def hotels
return “Out for lunch” if Clock.lunch_time?
@desk.local_transports
end
# ...
end

12. class Sign
def initialize
@desk = InfoDesk.new
end
def method_missing(name, *args)
return “Out for lunch” if Clock.lunch_time?
@desk.send(name, *args)
end
end
# At 12:30...
Sign.new.flights # => “Out for lunch”

13. Ghost Methods

14. class Sign
def initialize
@desk = InfoDesk.new
end
InfoDesk.public_instance_methods.each do |m|
define_method(m) do
return “Out for lunch” if Clock.lunch_time?
@desk.send(m)
end
end
end

15. Ghost Methods
vs.
Dynamic Methods

16. the four pitfalls
of method_missing()

17. class Sign
def initialize
@desk = InfoDesk.new
end
def method_missing(name, *args)
return “Out for lunch” if Clock.lunch_time?
@desk.send(name, *args)
end
end

18. we have a problem

19. class Sign
def initialize
@desk = InfoDesk.new
end
def is_it_lunch_time_yet?
Clock.lunch_time?
end
def method_missing(name, *args)
return “Out for lunch” if is_it_lunchtime_yet?
@desk.send(name, *args)
end
end
# At 12:30...
Sign.new.flights # => ?

20. class Sign
def initialize
@desk = InfoDesk.new
end
def is_it_lunch_time_yet?
Clock.lunch_time?
end
def method_missing(name, *args)
return “Out for lunch” if is_it_lunchtime_yet?
@desk.send(name, *args)
end
end
# At 12:30...
Sign.new.flights # => ?

21. class Sign
def initialize
@desk = InfoDesk.new
end
def is_it_lunch_time_yet?
Clock.lunch_time?
end
def method_missing(name, *args)
return “Out for lunch” if is_it_lunchtime_yet?
@desk.send(name, *args)
end
end
# At 12:30...
Sign.new.flights # => SystemStackError: stack level too deep

22. the first pitfall:
the Ghost House

23. class Sign
def initialize
@desk = InfoDesk.new
end
def method_missing(name, *args)
return super unless @desk.respond_to? name
return “Out for lunch” if Clock.lunch_time?
@desk.send(name, *args)
end
end

24. we have a problem

25. Sign.new.respond_to? :flights # => false

26. the second pitfall:
the Liar Object

27. class Sign
def initialize
@desk = ::InfoDesk.new
end
def method_missing(name, *args)
return super unless @desk.respond_to? name
return “Out for lunch” if Clock.lunch_time?
@desk.send(name, *args)
end
def respond_to?(method)
@desk.respond_to?(method) || super
end
end
Sign.new.respond_to? :flights # => true

28. we have a problem

29. class InfoDesk
def flights
# ...
end
def local_transports
# ...
end
# ...
def display
# ...
end
end

30. Sign.new.display # => #<Sign:0x000001008451b0>

33. the third pitfall:
the Fake Ghost

34. class Sign
instance_methods.each do |m|
undef_method m unless m.to_s =~ /^__|object_id/
end
def initialize
@desk = InfoDesk.new
end
def method_missing(name, *args)
return super unless @desk.respond_to? name
return "Out for lunch" if Clock.lunch_time?
@desk.send(name, *args)
end
def respond_to?(method)
@desk.respond_to? method || super
end
end
Sign.new.display # => InfoDesk#display() called

35. class Sign < BasicObject
def initialize
@desk = ::InfoDesk.new
end
def method_missing(name, *args)
return super unless @desk.respond_to? name
return “Out for lunch” if ::Clock.lunch_time?
@desk.send(name, *args)
end
def respond_to?(method)
@desk.respond_to? method || super
end
end
Sign.new.display # => InfoDesk#display() called

36. we have a problem
(this is getting old already)

38. the fourth pitfall:
the Snail Ghost

39. class Sign < BasicObject
def initialize
@desk = ::InfoDesk.new
end
def method_missing(name, *args)
return super unless @desk.respond_to? name
singleton_class.send :define_method, name do
return “Out for lunch” if ::Clock.lunch_time?
@desk.send name
end
send name
end
def respond_to?(method)
@desk.respond_to? method || super
end
end
# At 12:30...
Sign.new.flights # => ?

40. class Sign < BasicObject
def initialize
@desk = ::InfoDesk.new
end
def method_missing(name, *args)
return super unless @desk.respond_to? name
singleton_class.send :define_method, name do
return “Out for lunch” if ::Clock.lunch_time?
@desk.send name
end
send name
end
def respond_to?(method)
@desk.respond_to? method || super
end
end
# At 12:30...
Sign.new.flights # => SystemStackError: stack level too deep

41. class Sign < BasicObject
def initialize
@desk = ::InfoDesk.new
end
def method_missing(name, *args)
return super unless @desk.respond_to? name
singleton_class.send :define_method, name do
return “Out for lunch” if ::Clock.lunch_time?
@desk.send name
end
send name
end
def respond_to?(method)
@desk.respond_to? method || super
end
end
# At 12:30...
Sign.new.flights # => SystemStackError: stack level too deep

44. should I use method_missing()
to remove duplication in
my code?

45. not if you can use
define_method() instead

46. just a rule of thumb

47. remember the four pitfalls:
the Ghost House
the Liar Object
the Fake Ghost
the Snail Ghost

48. thank you.