Hidden Instance Variables

In this chapter, you will learn about inspecting an object, customizing the inspect message and visibility of instance variables.

Inspecting an Object

We can print the car object to inspect it.

class Car
  def initialize(color)
    @color = color
  end

  def drive
    'driving'
  end
end

car = Car.new('red')
p car

This prints:

#<Car:0xcafb @color="red">

This shows that the car instance has the color instance variable with the value red and the memory location #CAFB of the car object.

Customize Inspecting an Object

We can provide a custom implementation of to_s method.

class Car
  def initialize(color)
    @color = color
  end

  def drive
    'driving'
  end

  def to_s
    "I am a #{@color} car"
  end
end

car = Car.new('red')
puts car

The to_s provides string representation of the car object. This prints:

I am a red car

Visibility of Instance Variable

The instance variables are not visible from the outside of an object.

Let's see what happens when we access the color instance variable.

class Car
  def initialize(color)
    @color = color
  end

  def drive
    'driving'
  end
end

car = Car.new('red')
p car.color

This gives an error:

NoMethodError: undefined method ‘color’ for #<Car:0x007fbc0 @color="red">

We can define a color method that returns the color instance variable.

class Car
  def initialize(color)
    @color = color
  end

  def color
    @color
  end
end

car = Car.new('red')
p car.color

This prints:

red

Ruby has accessors that expose variables to the outside. So, instead of defining our own method, we can use attr_reader.

class Car
  attr_reader :color

  def initialize(color)
    @color = color
  end
end

car = Car.new('red')
p car.color

This prints:

red

• By default, instance variables are hidden from the outside.
• We can expose an instance variable via an accessor.
• You can over-ride the to_s method to customize the inspect message.

Summary

In this chapter, we inspected the color instance variable of the car object. We also discussed the concept of accessing the instance variables via accessors.