CSC/ECE 517 Fall 2007/wiki1b 2 22

Introduction - method_missing

The method_missing is a method that called whenever someone tries to call a method in your object that doesn't exist. It is a Ruby kernel method. It used as a method of last resort. When you send a message to a Ruby object, Ruby looks for a method to invoke with the same name as the message you sent. First it looks in the current self object’s own instance methods. Then it looks in the list of instance methods that all objects of that class share, and then in each of the included modules of that class, in reverse order of inclusion. Then it looks in that class’s superclass, and then in the superclass’s included modules, all the way up until it reaches the class Object. If it still can’t find a method, the very last place it looks is in the Kernel module, included in the class Object.

Examples

1. Object composition

class SimpleCallLogger
 def initialize(o)
   @obj = o
 end
 def method_missing(methodname, *args)
   puts "called: #{methodname}(#{args})"
   a = @obj.send(methodname, *args)
   puts "\t-> returned: #{a}"
   return a
 end
end

This object "intercepts" all method calls made to it, prints out a message and forwards on the method call to an internal object using the 'send' method, without knowing anything about the object passed to it. It can be used to debug some code without littering it with print statements. [1]

2. Factorial

Let's create a Computer class that contains a factorial method (you know the famous n! thing).

 class Computer
   def factorial n
     raise ArgumentError if n < 0
     f = 1
     n.downto(1) do |i|
       f = f * i
     end
     f
   end
 end

 computer = Computer.new
 puts computer.factorial(4)

We would like to use some notation close to the usual notation:

 computer = Computer.new
 puts computer._4!

Obviously, we cannot create methods for every integer, to do it we use the method_missing.

 def method_missing(meth, *args)
   meth.to_s =~ /_([0-9]*)!/
   return super if ! $1
   factorial($1.to_i)
 end

If we use the special notation (_<digits>!) the method_missing implementation extracts the number, from the method name, and calls the factorial method to get the result. Each time and for any method the same processing happens. [2]

Advantages

Allow you to catch problem at runtime.