From: ptkwt@...1.aracnet.com (Phil Tomson)
Date: 2002-03-14T03:20:20+09:00
Subject: Re: Method type 'abstract' (Module#abstract)

In article <a6lhp11tqu@enews4.newsguy.com>,
Phil Tomson <ptkwt@shell1.aracnet.com> wrote:
>In article <00e001c1c9e8$18a97940$0700000a@backpack>,
>Nat Pryce <nat.pryce@b13media.com> wrote:
>>From: "Peter Hickman" <peter@semantico.com>
>>> At present I have to insert a normal method in the class that raises an
>>> exception which will then cause the subclass to fall over if it did not
>>> override it. It works but it isn't clean, to my mind abstract is just as
>>> fundemental as public or private and therefore should be marked as such.
>>
>>Why not just *not* implement the method at all and let method_missing throw
>>the exception for you?
>>
>
>That would be a problem in classes where you define method_missing for 
>some other use.
>
>I'm content with throwing an exception in methods that are 'abstract' (ie. 
>methods that should be overridden by subclasses), but perhaps we can make 
>that easier by defining an 'abstract' method within Module, like:
>

The code I posted previously doesn't work, however I played with it and 
came up with the following code that does work:

class AbstractException < Exception
end

class Module
  def abstract(*methods)
    methods.each { |s|
      class_eval("def #{s}; raise AbstractException,'#{s} should be 
overriden!'; end")
    }
  end
end

class MyClass
  abstract :some_method
end

mc = MyClass.new
mc.some_method
#end

Now if you run this you'll get:

(eval):1:in `some_method': some_method should be overriden! 
(AbstractException) from abstract.rb:23

Now you can create a subclass of MyClass and override 'some_method', like:

class OtherClass < MyClass
  def some_method
    puts "OtherClass::some_method"
  end
end

oc = OtherClass.new
oc.some_method #=> "OtherClass::some_method"

Phil