Private vs Protected in Ruby

Do you know the difference between private and protected methods before and after ruby 2.7?

Let’s get straight to the point by setting up the scenario for the private method first and then for the protected method as below,

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Private

Consider a scenario where we have Product class with some public methods and print_message as private method.

class Product
  def public_method1
   print_message # direct call without self
  end
# Don't miss special NOTE for this method below  
  def public_method2
   self.print_message # direct call with self
  end
  def public_method3
   Product.new.print_message # call with new/other instance 
  end
  private
  def print_message
   puts "In private method"
  end
end
product = Product.new
product.print_message
# => private method `print_message' called for #<Product:0x0000564957f70688> (NoMethodError)
product.public_method1 
# => "In private method"
product.public_method2 
# => "In private method"
product.public_method3 
# => private method `print_message' called for #<Product:0x000055e762dac3d0> (NoMethodError)

Observations:

1. The private methods can be called in the following cases,
   ✔ when called (with or without self) within the public method of the same class and then the public method can be called from outside of the class
   product.public_method1 # => "In private method"
2. The private methods cannot be called in the following cases,
   ✘ when called directly from outside of class it is defined
   product.print_message # => (NoMethodError)
   ✘ when called with a different instance object within the public method
   Product.public_method3 # => (NoMethodError)

Note: Before ruby 2.7 calling private method from public method with self (ex.self.print_message) use to give error as shown below,
product.public_method2 # => (NoMethodError)

Protected

Keeping the above scenario same, let’s replace private by protected so as to make print_message method protected.

class Product
  def public_method1
   print_message # direct call without self
  end
  
  def public_method2
   self.print_message # direct call with self
  end
  def public_method3
   Product.new.print_message # call with new/other instance
  end
  protected
  def print_message
   puts "In protected method"
  end
end
product = Product.new
product.print_message
# => protected method `print_message' called for #<Product:0x00005563e23881c8> (NoMethodError)
product.public_method1 
# => "In protected method"
product.public_method2 
# => "In protected method"
product.public_method3 
# => "In protected method"

Observations:

1. The protected methods can be called in the following cases,
   ✔ when called within the public method of the same class and then the public method can be called from outside of the class product.public_method1 # => “In protected method”
   ✔ when called with a different instance object within the public method
   Product.public_method3 # => “In protected method”
2. The protected methods cannot be called in the following cases,
   ✘ when called directly from outside of the class in which it is defined
   product.print_message # => (NoMethodError)

Note: The protected methods have slower response time than the private methods. So it is adviced to use private methods more than protected methods.