Ruby: Can someone elaborate this code? [duplicate]

This question already has answers here: Closed 11 years ago.

Possible Duplicate:

What does ||= mean in Ruby?

开发者_StackOverflow中文版  class Person
    attr_accessor :name

    class << self
      attr_accessor :instances

      def instances
        @instances ||= []
        @instances
      end
    end

    def initialize( name )
      Person.instances << self
      @name = name
    end

  end

  joe = Person.new( "Joe" )
  zach = Person.new( "Zach" )

  puts "You have #{Person.instances.size} people in existance!" POINT C

  class Paramedic < Person

    def initialize( name )
      super( name )
      Paramedic.instances << self
    end
 end

  sally = Paramedic.new( "Sally" )
  puts "You have #{Person.instances.size} people in existance!" #POINT B
  puts "You have #{Paramedic.instances.size} paramedics in existance!" POINT C

What do these lines do?

@instances ||= []
Person.instances << self
Paramedic.instances << self

---

There is class level variable @instances.

First line

@instances ||= []

initialise this variable by empty array if @instances is nil.

Then during initialisation of instance of class Person code Person.instances << self add this instance to array of all instances of class Person.

So if you call Person.instances you will give all instances of class Person.

Same situation with Paramedic.

---

This code essentially keeps track of instances of Person and Paramedic. Each time one of these is constructed, the created object is added to the list represented by the class variable instances. self refers to the instance itself in this case.

@instances ||= [] is a Ruby idiom that is short for @instances = @instances || []. Because in Ruby only false and nil evaluate to false, the snippet either returns @instances or an empty list if @instances does not yet exist.