Last Updated: February 25, 2016
· blazeeboy

A Method swapping between objects in ruby (method swizzle)

objective C has a weired feature, it can swaps 2 methods between objects, so if you are trying to call a Method1 from Object1 it will call Method2 from Object2, i created a solution for that in ruby as there is no native implementation in our lovely language, but Matz Gave us a great dynamic language to hack in to.
first i aliased the two methods in the classes of the two objects then i reimplemented the two methods in objects to call the other method.
this way you can swap methods between objects and as Classes itself is an object it self we can swap the new method or any other class method :D

Gist :

#!/usr/bin/env ruby
# Author : Emad Elsaid (
def swap_methods(from_obj, from_method, to_obj, to_method)

  from_alias  = "#{from_method}_#{rand 1000}"
  to_alias    = "#{to_method}_#{rand 1000}"

  # alias methods in both objects
  from_obj.class.class_eval do
    alias_method from_alias, from_method

  to_obj.class.class_eval do
    alias_method to_alias, to_method

  # override methods and call aliases in both direction
  from_obj.define_singleton_method(from_method) do |*params, &block|
    to_obj.send(to_alias, *params, &block)

  to_obj.define_singleton_method(to_method) do |*params, &block|
    from_obj.send(from_alias, *params, &block)


# calling swap between two methods on two objects
# should swap them, so if you call obj1.method1 
# will execute obj2.method2 and vice versa
obj1 = "this is my first string object"
obj2 = "this is my second string object"
swap_methods obj1, :to_s, obj2, :to_s

# this should print the second string
puts obj1.to_s
#  and this should print the first one
puts obj2.to_s

# swapping String new method with
# other class new method, so whenever
# you create a new String an instance of
# the other class
class X
  attr_accessor :value
  def initialize(value)
    @value = value
swap_methods String, :new, X, :new
x_instance = "Heeeey"
puts x_instance.class

# this code will output the following lines:
# this is my second string object
# this is my first string object
# X
# it normally should be :
# this is my frist string object
# this is my second string object
# String