From: Logan Capaldo <logancapaldo@...>
Date: 2006-05-12T08:18:11+09:00
Subject: Re: final/closed classes in Ruby (was: Sharp knives and glue)


On May 11, 2006, at 5:27 PM, Berger, Daniel wrote:

> The 'Sharp knives and glue' thread inspired this.  It's probably been
> done before but here's my implementation of a final and/or closed  
> class:
>
> # Make a class unsubclassable
> class Class
>    def final(klass)
>       class << klass
>          def inherited(subclass)
>             error = "Illegal attempt to subclass #{self} with
> #{subclass}"
>             raise RuntimeError, error
>          end
>       end
>    end
> end
>
> class Foo
>    final(self) # No subclasses allowed
> end
>
> # Boom!
> class Bar < Foo
> end

Has potential, but why not just do:

class Class
   def final # or even final!
     class << self
       def inherited(subclass)
         error = "Illegal attempt to subclass #{self} with
#{subclass}"
         raise RuntimeError, error
       end
     end
   end
end

class Foo
   final
end

class Bar < Foo
end

RuntimeError: Illegal attempt to subclass Foo with
Bar
         from (irb):7:in `inherited'
         from (irb):17

Unfortunately, it raises an exception way too late.

try:

class Foo
   final
end

begin
class Bar < Foo
end
# Foo.inherited(Bar) pretty much happens here
rescue
end

b = Bar.new