From: "Berger, Daniel" <Daniel.Berger@...>
Date: 2006-05-12T06:27:13+09:00
Subject: final/closed classes in Ruby (was: Sharp knives and glue)

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

# Close a class (no method definitions or redefinitions)

class Module
   def close(klass)
      class << self
         def method_added(method)
            error = "cannot add or change method #{method} to closed"
            error += " class #{self}"
            raise RuntimeError, error
         end
      end
   end
end

class Foo
   def test
      puts "hello"
   end
   close(self) # No method additions/redefinitions allowed
end

# Boom!
class Foo
   def test2
      puts "eh?"
   end
end

This could probably be combined into a single method, and perhaps put in
the Kernel module as well.

Anyway, there you have it.

Regards,

Dan


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