Amusement Park

Instance Variables

Objects can hold their own state by setting instance variables. These variables live within an object and are normally used to store some internal state.

Instance variables always start with an @. You can create a new instance variable by setting a variable starting with an @. For example:

class Repeater
  def hear(something)
    # Save what's been heard into a new instance variable called @heard
    @heard = something
  end

  def repeat
    # Return the value of the instance variable
    @heard
  end
end

repeater = Repeater.new
repeater.hear("Hello, friend!")
print repeater.repeat # Prints "Hello, friend!"

Objects usually set their initial state in an initialize method, which is automatically called when calling new on a class.

class Airplane
  def initialize
    @wings = 2
  end

  def wings
    @wings
  end
end

plane = Airplane.new
plane.wings # returns 2

The initialize method may also take arguments, so that each instance can start with a custom state:

class Suitcase
  def initialize(locked)
    @locked = locked
  end
end

It is good practice to consider instance variables to be private from external read and writes. We instead define methods to get and set their values. For example:

class Suitcase
  def initialize
    @locked = false
  end

  def locked? # Query methods should be named with a trailing `?`
    @locked
  end

  def lock! # Methods which mutate state should have trailing `!`
    @locked = true
  end
end

suitcase = Suitcase.new
suitcase.locked? # false
suitcase.lock!
suitcase.locked? # true

Nil

Nil is an English word meaning "nothing" or "zero". In Ruby, nil is used to express the absence of an object. In other programming languages, null or none values may play a similar role.

# I do not have a favorite color
favorite_color = nil

Ruby gives any instance variable the default value of nil when it is first encountered, until it is otherwise given a value.

print @favourite_color # prints nil
@favourite_color = "blue"
print @favourite_color # prints "blue"