From: Robert Klemme <shortcutter@...>
Date: 2007-12-16T21:40:00+09:00
Subject: Re: Abstract -- what's the Ruby way?

On 13.12.2007 07:58, Charles Oliver Nutter wrote:
> rbaldwin wrote:
>> 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.
>>>
>>> Thanks!
>> Here is one way. Its not very satisfying, as you have no way to access
>> the initialize method on any parent classes of the Animal class (such
>> as ActiveRecord)...
> 
> You could privatize new and define a new new for inheriting classes that 
> calls the old new:
> 
> class Animal
>    class << self
>      private :new
>      def inherited(cls)
>        cls.instance_eval "def new(*args); super; end"
>      end
>    end
> end
> 
> class Dog < Animal
> 
> puts Dog.new # => #<Dog:0x2506c>
> pust Animal.new => NoMethodError: private method 'new' called for Animal...
> 
> I use instance_eval here because I don't like the semantics of 
> define_method (or I dislike them more than eval magic); but it would 
> probably work about the same.
> 
> The downside of this approach is subclasses that want to also define 
> their own 'new' would have to know to call super. I'd expect that's 
> pretty rare.

This downside can be easily avoided: simply change #inherited to make 
new public again instead of redefining it.

irb(main):001:0> class Animal
irb(main):002:1>   class <<self
irb(main):003:2>     private :new
irb(main):004:2>     def inherited(cl)
irb(main):005:3>       class <<cl
irb(main):006:4>         public :new
irb(main):007:4>       end
irb(main):008:3>     end
irb(main):009:2>   end
irb(main):010:1> end
=> nil
irb(main):011:0> class Dog < Animal
irb(main):012:1> end
=> nil
irb(main):013:0> Animal.new
NoMethodError: private method `new' called for Animal:Class
         from (irb):13
         from :0
irb(main):014:0> Dog.new
=> #<Dog:0x7ff71960>
irb(main):015:0>

Kind regards

	robert