Initializing Instance Variables Defined by a Module

Credit: Phil Tomson

Problem

You have a mixin module that defines some instance variables. Given a class that mixes in the module, you want to initialize the instance variables whenever an instance of the class is created.

Solution

Define an initialize method in the module, and call super in your class's constructor. Here's a Timeable module that tracks when objects are created and how old they are:

	module Timeable
	 attr_reader :time_created

	 def initialize
	 @time_created = Time.now
	 end

	 def age #in seconds
	 Time.now - @time_created
	 end
	end

Timeable has an instance variable time_created, and an initialize method that assigns Time.now (the current time) to the instance variable. Now let's mix Timeable into another class that also defines an initialize method:

	class Character
	 include Timeable
	 attr_reader :name
	 def initialize( name )
	 @name = name
	 super() #calls Timeable's initialize
	 end
	end
	c = Character.new "Fred"

	c.time_created
	# => Mon Mar 27 18:34:31 EST 2006

Discussion

You can define and access instance variables within a module's instance methods, but you can't actually instantiate a module. A module's instance variables only exist within objects of a class that includes the module. However, classes don't usually need to know about the instance variables defined by the modules they include. That sort of information should be initialized and maintained by the module itself.

The Character#initialize method overrides the Timeable#initialize method, but you can use super to call the Timeable constructor from within the Character constructor. When a module is included in a class, that module becomes an ancestor of the class. We can test this in the context of the example above by calling the Module#ancestors on the Character class:

	Character.ancestors # => [Character, Timeable, Object, Kernel]

When you call super from within a method (such as initialize), Ruby finds every ancestor that defines a method with the same name, and calls it too.