Ruby idioms : Avoid check class membership

Posted March 9, 2008
Filed under: Ruby, bad smell, code, ruby commom bugs, ruby idioms, smellcode |

Avoid use class, is_a? and kind_of? to check class membership. Replace these statement with a message to the object that respond to the message. The code below is an anti-pattern.

if mortgage.class==FixRateMortgage
   mortgage.change_rate_not_allowed
else
   mortgage.rate=rate
end

The code below is more idiomatic in Ruby.

class Mortgage
  attr_accessor :rate
end 

class FixedRateMortgage < Mortgage
  def initialize()
    @rate=5
  end
  def rate=(rate)
    change_rate_not_allowed
  end

  def change_rate_not_allowed
     puts "The rate in a fixed rate mortage can not be change"
  end
end

Sending the method rate= to either kind of classes will result in the appropriate behaviour.

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