Object-Oriented Ruby

Virtual Constructor

You can run into trouble inheriting from a class containing a method that creates a new instance of the class. This section describes a solution to this problem.

Let's say you want to create a Number class with an overloaded addition operator

class Number
    attr_reader :x
    def initialize(x) @x = x end
    def +(o) Number.new(@x + o.x) end
    def to_s() "the Number #{@x}" end
end

If you inherit from it

class FancyNumber < Number
    def to_s() "the FancyNumber #{@x}" end
end

the addition operator won't return a FancyNumber, but a Number. It can be solved with this trick

class Number
    attr_reader :x
    def initialize(x) @x = x end
    def +(o) self.class.new(@x + o.x) end  <- create a new instance of 
    def to_s() "the Number #{@x}" end         'this class'
end

Mixins

This is just a small example of a mixin, the Ruby answer to multiple inheritance / interfaces

module Info # mixin
    def showHp() puts "* HIT POINTS: #{@hp}" end
end
class Creature
    include Info
    def initialize(hp) @hp = hp end
end
class Hero
    include Info
    def initialize(str, hp) @str, @hp = str, hp end
end

orc = Creature.new(100)
knight = Hero.new(40, 70)
orc.showHp()
knight.showHp()

it outputs

* HIT POINTS: 100
* HIT POINTS: 70