Ruby/Class/abstract class

class Class
  def abstract(*args)
    args.each do |method_name|
      define_method(method_name) do |*args|
        if method_name == :initialize
          msg = "#{self.class.name} is an abstract class."
        else
          msg = "#{self.class.name}##{method_name} is an abstract method."
        end
        raise NotImplementedError.new(msg)
     end
    end
  end
end
class Animal
  abstract :initialize, :move
end
 
class Dog < Animal
  def initialize
    @type = :Dog
  end
end
 
dog = Dog.new
dog.move
class Cat < Animal
  def initialize
    @type = :Cat
  end
  def move
    "Running!"
  end
end
puts Cat.new.move
class Dog
  def move
    "walk!"
  end
end
puts dog.move

class Shape2D
  def initialize
    raise NotImplementedError.
      new("#{self.class.name} is an abstract class.")
  end
  def area
    raise NotImplementedError.
      new("#{self.class.name}#area is an abstract method.")
  end
end
 
class Square < Shape2D
  def initialize(length)
    @length = length
  end
  def area
    @length ** 2
  end
end
Square.new(10).area                              # => 100
# NotImplementedError: Shape2D is an abstract class.