From: Matthias Lampert <ml@...>
Date: 2001-08-30T17:55:20+09:00
Subject: [ruby-talk:20571] To those who want abstract classes in Ruby


Hi, folks!

There are people who miss a feature like abstract/virtual 
classes in Ruby.  I was one of them, I admit.  

By the time I learned, though, that I could implement this
feature myself.  Just like here.


#########################################################################
class Abstract
	attr :attrib

	def initialize( i_attrib )
		@attrib = i_attrib
		check4ForbiddenInstantiating
	end


	def method_a
		crash4ViolationOfAbstractness("method_a")
	end

	def method_b
		crash4ViolationOfAbstractness("method_b")
	end
	

	def crash4ViolationOfAbstractness( sMethodName )
		raise "Ahem... You need to implement " + 
              "#{self.type.to_s}##{sMethodName} to use it!"
	end

	def check4ForbiddenInstantiating
		if self.type.to_s == "Abstract"
			raise "Sorry, lad! I'm abstract!"
		end
	end

end


class Concrete < Abstract
	def initialize( i_attrib )
		super(i_attrib)
	end

	def method_a
		puts "This instantiation is " + attrib.to_s 
		puts "Implemented Abstract#method_a"
	end 
end


# Abstract.new( "forbidden!")      # ==> ERROR!
obj = Concrete.new("allowed!")
obj.method_a
# obj.method_b                     # ==> ERROR!

#########################################################################


It's primitive, I know that, but it's a place to get started, and it did
a great job for me so far.  Maybe one of the gurus can make a module of it;
would be great to create an abstract class semi-automatically.  Perhaps I 
can do it myself, but I need to be a guru first!


CU,

-- 

Matthias Lampert, Hamburg
Just another XP and Ruby fan