Ruby inheritance -Access modifiers and misconceptions behind them

Aditya Tiwari

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Jun 18, 2018

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How does Public, Private and Protected behave in child class of inheritance?

In Ruby Private and Protected access modifiers behave differently in case of inheritance as compared to other languages like Java. Public behaves pretty much same.

Let’s not waste the time and discuss about them one by one with examples.

Public:

All public methods of parent class can be accessed in child class, it’s that simple!

class Car
  def mileage
    "I returns mileage of a #{self.class}"
  end
endclass Porsche < Car; end;Car.new.mileage
Porsche.new.mileageirb(main):001:0> Car.new.mileage
=> "I returns mileage of a Car"irb(main):002:0> Porsche.new.mileage
=> "I returns mileage of a Porsche"

Private:

Just like public methods, All private methods of parent class is also available as private methods in child class. This behaviour is different than Java, In Java you can’t access private methods of parent class in child class.

class Car
  def call_car_mileage
    mileage
  end  private  def mileage
    "I returns mileage of a #{self.class}"
  end
endclass Porsche < Car
  def call_porsche_mileage
    mileage
  end
endirb(main):103:0> Car.new.call_car_mileage
=> "I returns mileage of a Car"
irb(main):104:0> Porsche.new.call_porsche_mileage
=> "I returns mileage of a Porsche"

Protected:

Protected is most misunderstood access modifier in Ruby, be with me, I can try to explain.

Protected methods can be accessed inside the child class just like private methods. For example:

class Car
  def call_car_mileage
    mileage
  end  protected  def mileage
    "I returns mileage of a #{self.class}"
  end
endclass Porsche < Car
  def call_porsche_mileage
    mileage
  end
endirb(main):103:0> Car.new.call_car_mileage
=> "I returns mileage of a Car"
irb(main):104:0> Porsche.new.call_porsche_mileage
=> "I returns mileage of a Porsche"

So, what’s the difference between Protected and Private?

Let’s try calling Protected method mileage directly on object.

irb(main):111:0> Porsche.new.mileage
NoMethodError: protected method `mileage' called for #<Porsche:0x007fc9922c97b0>

As you can see mileage is not available directly on object of class, now let’s try to call mileage on object of Porsche class from the inside the context where mileage is defined.

class Car
  def call_milage
    Porsche.new.mileage
  end
  
  protected  def mileage
    "I returns mileage of a #{self.class}"
  end
endclass Porsche < Car; end;irb(main):001:0> Car.new.call_milage
=> "I returns mileage of a Porsche"

As you can see Porsche.new.mileage is not working from outside but it is working when called in the context of class Car.

So, we can say that Protected methods behaves as Private when called from outside the context of defined class and behaves as Public when called from inside the context of defined class.

In my next post, I will talk more about the use case of Protected methods, meanwhile you can checkout my other similar posts.