A cheap abstract class in ruby

Posted: 03/29/2009

When I create a class that I don’t want to allow anyone to instantiate, I use the following pattern:

  class MakeMeAbstract
    def initialize
      raise "Don't instantiate me!" if abstract_class?
      puts "Instantiated!"
    end
    private
    def abstract_class?
      self.class == MakeMeAbstract
    end
  end



It can’t be instantiated:



  MakeMeAbstract.new rescue puts "I'm abstract"   # => "I'm abstract"





But it can be subclassed, and the subclass can be instantiated:



  class Child < MakeMeAbstract
  end
  Child.new   # => "Instantiated!"





As a full disclaimer, this is a learning experience for me.  When subclassing an abstract class I wonder if it would be better to use a mixin instead: I could overwrite mixed in methods just as easily as methods in a parent class.  The structure changes from this:




  class Bar
    def status
      print "is full"
    end
  end
  class WetBar < Bar
    def status
      print "is wet, "
      super
    end
  end
  wet_bar = WetBar.new
  wet_bar.status    # => "is wet, is full"





To this:




  module Bar
    def status
      print "is full"
    end
  end
  class WetBar
    include Bar
    def status
      print "is wet, "
      super
    end
  end
  wet_bar = WetBar.new
  wet_bar.status    # => "is wet, is full"





Input is welcome in the comments.



        
     


     

        
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