Question

Possible Duplicate:
What does ||= mean in Ruby?

  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

Answer 1

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.