The Revenge of method_missing()

The revenge of method_missing() Paolo “Nusco” Perrotta

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

disclaimer

Person.find_by_user_name_and_password name, pwd

Person.find :user_name => name, :password => pwd

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

end of disclaimer

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

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

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) endend# At 12:30...Sign.new.flights # => “Out for lunch”

Ghost Methods

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 endend

Ghost Methods vs.Dynamic Methods

the four pitfallsof method_missing()

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) endend

we have a problem

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) endend# At 12:30...Sign.new.flights # => ?

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) endend# At 12:30...Sign.new.flights # => SystemStackError: stack level too deep

the first pitfall: the Ghost House

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

we have a problem

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

the second pitfall: the Liar Object

class Sign def initialize @desk = ::InfoDesk.new end def method_missing(name, *args) 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 endendSign.new.respond_to? :flights # => true

we have a problem

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

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

the third pitfall: the Fake Ghost

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) 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 endendSign.new.display # => InfoDesk#display() called

class Sign < BasicObject def initialize @desk = ::InfoDesk.new end def method_missing(name, *args) 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 endendSign.new.display # => InfoDesk#display() called

we have a problem (this is getting old already)

the fourth pitfall: the Snail Ghost

class Sign < BasicObject def initialize @desk = ::InfoDesk.new end def method_missing(name, *args) 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 endend# At 12:30...Sign.new.flights # => ?

class Sign < BasicObject def initialize @desk = ::InfoDesk.new end def method_missing(name, *args) 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 endend# At 12:30...Sign.new.flights # => SystemStackError: stack level too deep

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

not if you can usedefine_method() instead

just a rule of thumb

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

thank you.

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The Revenge of method_missing()

by paoloperrotta on May 30, 2011

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