Question

I know there is no concept of an abstract class in Ruby. But if it needs to be implemented, how do I go about it? I tried something like this:

class A   def self.new     raise 'Doh! You are trying to write Java in Ruby!'   end end  class B < A   ...   ... end 

But, when I try to instantiate B, it is internally going to call A.new which is going to raise the exception.

Also, modules cannot be instantiated, but they cannot be inherited too. Making the new method private will also not work.

Does anyone have any pointers?

Answer 1

I don’t like using abstract classes in Ruby (there’s almost always a better way). If you really think it’s the best technique for the situation though, you can use the following snippet to be more declarative about which methods are abstract:

module Abstract   def abstract_methods(*args)     args.each do |name|       class_eval(<<-END, __FILE__, __LINE__)         def #{name}(*args)           raise NotImplementedError.new('You must implement #{name}.')         end       END       # important that this END is capitalized, since it marks the end of <<-END     end   end end  require 'rubygems' require 'rspec'  describe 'abstract methods' do   before(:each) do     @klass = Class.new do       extend Abstract        abstract_methods :foo, :bar     end   end    it 'raises NoMethodError' do     proc {       @klass.new.foo     }.should raise_error(NoMethodError)   end    it 'can be overridden' do     subclass = Class.new(@klass) do       def foo         :overridden       end     end      subclass.new.foo.should == :overridden   end end 

Basically, you just call abstract_methods with the list of methods that are abstract, and when they get called by an instance of the abstract class, a NotImplementedError exception will be raised.