From: Eric Jacoboni <jaco@...>
Date: 2001-06-26T08:50:33+09:00
Subject: [ruby-talk:16868] Something strange with Ruby's inheritance mechanism

As Ruby beginner, i try some "canonical" OO scripts. Doing so, I've
noticed some strange behaviour in inheritance mechanism (from my point
of vue). Considering a base class Employee :

class Employee
  attr_reader :name, :age, :salary
  attr_writer :name, :age, :salary
  def initialize(name, age, salary)
    @name   = name
    @age    = age
    @salary = salary
  end
  def to_s
    "Name: #{@name}, #{@age}, #{@salary}"
  end
end

Considering Manager, an Employee's subclass

class Manager < Employee
  attr_reader :grade
  attr_writer :grade
  def initialize(name, age, salary, grade) 
     super
     @grade = grade
  end
  def to_s
   super + ", #{@grade}"
  end
end

A Manager "is a" Employee, reverse is not true (that's the classical
assertion...) 

In Ruby, it's legal to assign an Manager to an Employee with

anEmployee = aManager

anEmployee is now a Manager... I know Ruby doesn't care about type
checking and i'm aware of the fact that both names are references,
but, when it comes to classes, it hurts my inheritance conception. Is
there a way to avoid this behaviour or is it a "feature" ?
-- 
�ric Jacoboni, n� il y a 1296955303 secondes