From: Phrogz <gavin@...>
Date: 2006-10-26T13:50:06+09:00
Subject: Popularity Contest: Initializing Class-Level Instance Variables

Subclassing a class that uses class-level instance variables requires
that you initialize them for subclasses. Which of the following three
techniques do you prefer?


TECHNIQUE 1 - EXPLICIT INITIALIZATION
-------------------------------------
class Alpha
  @foo = []
  def self.m1; @foo << rand; end
  def self.m2; @foo.map{ rand }; end
end

class Bravo < Alpha
  @foo = []
end



TECHNIQUE 2 - ON-DEMAND TEST/INITIALIZATION
-------------------------------------------
class Alpha
  def self.m1; (@foo||=[]) << rand; end
  def self.m2; (@foo||=[]).map{ rand }; end
end

class Bravo < Alpha
  # Nothing special here
end



TECHNIQUE 3 - PARENT CLASS HANDLES FOR SUB-CLASSES
--------------------------------------------------
class Alpha
  @foo = []
  def self.m1; @foo << rand; end
  def self.m2; @foo.map{ rand }; end
  def self.inherited(subklass)
    subklass.instance_eval{ @foo=[] }
  end
end

class Bravo < Alpha
  # Nothing special here
end