Class Instance Variables

Use Class instance Variables

We all know that class variables such as @@ are dangerous to use due to their leaky inheritance.

	class Presentaion < Document
	@@default_font = :ariel
	def self.default_font=(font)
	@@default_font=(font)
	end
	def default_font
	@@default_font
	end
	end
	class Resume < Document
	@@default_font = :nimbus
	def self.default_font=(font)
	@@default_font=(font)
	end
	def default_font
	@@default_font
	end
	end
	class Document
	@@default_font = :times ##LEAKY INTHERITANCE HERE
	end
	require 'document'
	require 'resume' # Rewrite class variable
	require 'presentation' # Rewrite class viable
	##THE FIX
	# As an alternative we can use...
	# 1. Use class variables with this in mind
	# 2. Use CONSTANTS
	# 3. Open up the Singleton Class
	class Presentaion < Document
	@default_font = :ariel
	class << self
	attr_accessor :default_font
	end
	end
	class Resume < Document
	@default_font = :nimbus
	class << self
	attr_accessor :default_font
	end
	end
	class Document
	@default_font = :times
	class << self
	attr_accessor :default_font
	end
	end

view raw gistfile1.rb hosted with ❤ by GitHub

<script src="https://gist.github.com/surgentt/9464842.js"></script>class Presentaion < Document


WHAT IS GOING ON HERE

 The class << self syntax opens up a classes singleton class.
  This allows you to specialize the behavior of methods called on that specific object.

http://stackoverflow.com/questions/2505067/class-self-idiom-in-ruby

Object
    ^
     |
  <Singleton Class>
    ^
    |
Instance

Singleton Method is a method that is defined for exactly one object instance.


The <Singleton Class> is only invoked when it is filled with something.

  - An example of this is a method on an instance variable.

hand_built_stub_printer = Object.new def hand_built_stub_printer.avaiable? true end