The Visitor Pattern implementation in Ruby from the Wikipedia example

visitor_pattern_example.rb

class CarElement
  def accept(visitor)
    raise NotImpelementedError.new
  end
end

module Visitable
  def accept(visitor)
    visitor.visit(self)
  end
end

class Wheel < CarElement
  include Visitable

  attr_reader :name
  def initialize(name)
    @name = name
  end
end

class Engine < CarElement
  include Visitable
end

class Body < CarElement
  include Visitable
end

class Car < CarElement
  def initialize
    @elements = []
    @elements << Wheel.new("front left")
    @elements << Wheel.new("front right")
    @elements << Wheel.new("back left")
    @elements << Wheel.new("back right")
    @elements << Body.new
    @elements << Engine.new
  end

  def accept(visitor)
    @elements.each do |element|
      element.accept(visitor)
    end
    visitor.visit(self)
  end
end

class CarElementPrintVisitor
  def visit(subject)
    puts "Visiting: %s" % subject.class.to_s
  end
end

class CarElementDoVisitor
  def visit(subject)
    puts "Do some other visitation: %s" % subject.class.to_s
  end
end

c = Car.new
c.accept(CarElementPrintVisitor.new)
c.accept(CarElementDoVisitor.new)

Thanks heaps for this example, just what I am looking for!

Thanks for this example!

Thanks, but this example does not quite accomplishes the visitor pattern. See this other post:
http://blog.bigbinary.com/2013/07/07/visitor-pattern-and-double-dispatch.html

And this example from Arel (used in Rails / ActiveRecrdr):
https://github.com/rails/arel/blob/master/lib/arel/visitors/visitor.rb

The key difference with ruby (from other languages like C++ and Java) is that we cannot overload methods with different signatures, we need to create new methods names that encodes the desired signature.