From: James Edward Gray II <james@...>
Date: 2007-07-12T22:32:47+09:00
Subject: Re: Abstract methods in ruby ?

On Jul 12, 2007, at 7:05 AM, Robert Dober wrote:

> On 7/12/07, James Edward Gray II <james@grayproductions.net> wrote:
>> On Jul 11, 2007, at 5:40 PM, cypher.dp@gmail.com wrote:
>>
>> > How do you implement something like the "abstract" keyword in  
>> Java ?
>> > Or: Is there a technique to force a subclass to implement a special
>> > method ?
>>
>> One way is:
>>
>> class Parent
>>    def abstract_method
>>      raise NotImplementedError
>>    end
>> end
>>
>> You may just want to skip this step altogether though, since an
>> exception toss is the default behavior.
>
> Hmm, I would say no, you will be caught by #method_missing one day.
> It would be interesting why you want to do this?

If someone adds a method_missing() implementation to a child class  
that accepts messages it shouldn't, there is a bug in that  
implementation and it needs to be fixed.  Your tests are going to  
catch that for you.

> For documentation purpose?

That is about the best reason to add the stub, I think.

> If you are looking for warnings or errors for subclasses not
> implementing the abstract method you will be disappointed at Compile
> Time [which arguabley does not even exist in 1.8]

class AbtractClass
   REQUIRED_METHODS = %w[one two three]

   def self.inherited(subclass)
     REQUIRED_METHODS.each do |m|
       raise "An implementation of #{m} is required by #{self.class}" \
         unless subclass.instance_methods.include? m
     end
   end
end

class Broken < AbtractClass
   def one; end
   def two; end
end
# ~> -:7:in `inherited': An implementation of one is required by  
Class (RuntimeError)
# ~> 	from -:5:in `each'
# ~> 	from -:5:in `inherited'
# ~> 	from -:12

But don't do that.  ;)

James Edward Gray II