From: Brian Marick <marick@...>
Date: 2007-12-13T11:49:41+09:00
Subject: Re: Abstract -- what's the Ruby way?


> On Dec 12, 5:10 pm, "Tim Rowe" <n...@digitig.co.uk> wrote:
>> What's the Ruby way to define a class that shouldn't be  
>> instantiated, but
>> with descendants that can be? Other than a comment in the  
>> documentation, of
>> course.
>>

I missed the beginning, so hope this response is to the point. See the  
code below. (What I do is name the class AbstractSomething and trust  
that's warning enough.)

=================================

class Abstract

   attr_reader :abstract_class_initialize_args

   def initialize(*args)
     raise "Initialized abstract class #{self.class}" if self.class ==  
Abstract
     @abstract_class_initialize_args = args
   end
end

class SubclassWithInitializer < Abstract

   attr_reader :subclass_initialize_arg

   def initialize(arg)
     super
     @subclass_initialize_arg = arg
   end

end

class SubclassWithoutInitializer < Abstract
end


require 'test/unit'

class TestCase < Test::Unit::TestCase

   def test_abstract_class_raises_exception
     assert_raises(RuntimeError) { Abstract.new }
   end

   def test_a_subclass_can_call_super
     sub = SubclassWithInitializer.new('arg')
     assert_equal(['arg'], sub.abstract_class_initialize_args)
     assert_equal('arg', sub.subclass_initialize_arg)
   end

   def test_a_subclass_need_not_define_its_own_initialize
     sub = SubclassWithoutInitializer.new('arg')
     assert_equal(['arg'], sub.abstract_class_initialize_args)
   end
end



-----
Brian Marick, independent consultant
Mostly on agile methods with a testing slant
www.exampler.com, www.exampler.com/blog, www.twitter.com/marick