Why does Ruby have both private and protected methods?

The difference

• Anyone can call your public methods.
• You can call your protected methods, or another member of your class (or a descendant class) can call your protected methods from the outside. Nobody else can.
• Only you can call your private methods, because they can only be called with an implicit receiver of self. Even you cannot call self.some_private_method; you must call private_method with self implied.
  • iGEL points out: "There is one exception, however. If you have a private method age=, you can (and have to) call it with self to separate it from local variables."
  • Since Ruby 2.7 the self receiver can be explicit, self.some_private_method is allowed. (Any other explicit receiver is still disallowed, even if the runtime value is the same as self.)

In Ruby, these distinctions are just advice from one programmer to another. Non-public methods are a way of saying "I reserve the right to change this; don't depend on it." But you still get the sharp scissors of send and can call any method you like.

A brief tutorial

# dwarf.rb
class Dwarf
  include Comparable

  def initialize(name, age, beard_strength)
    @name           = name
    @age            = age
    @beard_strength = beard_strength
  end

  attr_reader :name, :age, :beard_strength
  public    :name
  private   :age
  protected :beard_strength

  # Comparable module will use this comparison method for >, <, ==, etc.
  def <=>(other_dwarf)
    # One dwarf is allowed to call this method on another
    beard_strength <=> other_dwarf.beard_strength
  end

  def greet
    "Lo, I am #{name}, and have mined these #{age} years.\
       My beard is #{beard_strength} strong!"
  end

  def blurt
    # Not allowed to do this: private methods can't have an explicit receiver
    "My age is #{self.age}!"
  end
end

require 'irb'; IRB.start

Then you can run ruby dwarf.rb and do this:

gloin = Dwarf.new('Gloin', 253, 7)
gimli = Dwarf.new('Gimli', 62,  9)

gloin > gimli         # false
gimli > gloin         # true

gimli.name            # 'Gimli'
gimli.age             # NoMethodError: private method `age'
                         called for #<Dwarf:0x007ff552140128>

gimli.beard_strength # NoMethodError: protected method `beard_strength'
                        called for #<Dwarf:0x007ff552140128>

gimli.greet          # "Lo, I am Gimli, and have mined these 62 years.\
                           My beard is 9 strong!"

gimli.blurt          # private method `age' called for #<Dwarf:0x007ff552140128>

protected methods can be called by any instance of the defining class or its subclasses.

private methods can be called only from within the calling object. You cannot access another instance's private methods directly.

Here is a quick practical example:

def compare_to(x)
 self.some_method <=> x.some_method
end

some_method cannot be private here. It must be protected because you need it to support explicit receivers. Your typical internal helper methods can usually be private since they never need to be called like this.

It is important to note that this is different from the way Java or C++ works. private in Ruby is similar to protected in Java/C++ in that subclasses have access to the method. In Ruby, there is no way to restrict access to a method from its subclasses like you can with private in Java.

Visibility in Ruby is largely a "recommendation" anyways since you can always gain access to a method using send:

irb(main):001:0> class A
irb(main):002:1>   private
irb(main):003:1>   def not_so_private_method
irb(main):004:2>     puts "Hello World"
irb(main):005:2>   end
irb(main):006:1> end
=> nil

irb(main):007:0> foo = A.new
=> #<A:0x31688f>

irb(main):009:0> foo.send :not_so_private_method
Hello World
=> nil

Private methods in Ruby:

If a method is private in Ruby, then it cannot be called by an explicit receiver (object). It can only be call implicitly. It can be called implicitly by the class in which it has been described in as well as by the subclasses of this class.

The following examples will illustrate it better:

1) A Animal class with private method class_name

class Animal
  def intro_animal
    class_name
  end
  private
  def class_name
    "I am a #{self.class}"
  end
end

In this case:

n = Animal.new
n.intro_animal #=>I am a Animal
n.class_name #=>error: private method `class_name' called

2) A subclass of Animal called Amphibian:

class Amphibian < Animal
  def intro_amphibian
    class_name
  end 
end 

In this case:

  n= Amphibian.new
  n.intro_amphibian #=>I am a Amphibian
  n.class_name #=>error: private method `class_name' called

As you can see, private methods can be called only implicitly. They cannot be called by explicit receivers. For the same reason, private methods cannot be called outside the hierarchy of the defining class.

Protected Methods in Ruby:

If a method is protected in Ruby, then it can be called implicitly by both the defining class and its subclasses. Additionally they can also be called by an explicit receiver as long as the receiver is self or of same class as that of self:

1) A Animal class with protected method protect_me

class Animal
  def animal_call
    protect_me
  end
  protected
  def protect_me
    p "protect_me called from #{self.class}"
  end  
end

In this case:

n= Animal.new
n.animal_call #=> protect_me called from Animal
n.protect_me #=>error: protected method `protect_me' called

2) A mammal class which is inherited from animal class

class Mammal < Animal
  def mammal_call
    protect_me
  end
end 

In this case

n= Mammal.new
n.mammal_call #=> protect_me called from Mammal

3) A amphibian class inherited from Animal class (same as mammal class)

class Amphibian < Animal
  def amphi_call
    Mammal.new.protect_me #Receiver same as self
    self.protect_me  #Receiver is self
  end   
end

In this case

n= Amphibian.new
n.amphi_call #=> protect_me called from Mammal
             #=> protect_me called from Amphibian  

4) A class called Tree

class Tree
  def tree_call
    Mammal.new.protect_me #Receiver is not same as self
  end
end

In this case:

n= Tree.new
n.tree_call #=>error: protected method `protect_me' called for #<Mammal:0x13410c0>