Public, protected and private methods in Ruby

public_protected_private.rb

# Note about `protected`:
#
# One use case for using `protected` is object comparator methods, where the method
# may need to call protected methods on `self` and another object of the same class.
# But 99% of the time, we don't write object comparator methods, that's why we almost
# Never use `protected`. See:
# See http://tenderlovemaking.com/2012/09/07/protected-methods-and-ruby-2-0.html

class MyClass
  # Internal Visibility / Implicit Receiver
  def implicit_receiver_calls
    puts public_method rescue puts "NG public"
    puts protected_method rescue puts "NG protected"
    puts private_method rescue puts "NG private"
  end

  # Internal Visibility / Explicit Receiver
  def explicit_receiver_calls
    puts self.public_method rescue puts "NG public"
    puts self.protected_method rescue puts "NG protected"
    puts self.private_method rescue puts "NG private"
  end

  def public_method; 'OK public' end

  protected
  def protected_method; 'OK protected' end

  private
  def private_method; 'OK private' end
end

# Internal Visibility
puts "\nInternal Visibility / Implicit Receiver"
puts "---------------------------------------"
MyClass.new.implicit_receiver_calls

puts "\nInternal Visibility / Explicit Receiver"
puts "---------------------------------------"
MyClass.new.explicit_receiver_calls

# External Visibility
puts "\nExternal Visibility"
puts "---------------------------------------"
my_instance = MyClass.new
puts my_instance.public_method rescue puts "NG public"
puts my_instance.protected_method rescue puts "NG protected"
puts my_instance.private_method rescue puts "NG private"

__END__

Outputs:

Internal Visibility / Implicit Receiver
---------------------------------------
OK public
OK protected
OK private

Internal Visibility / Explicit Receiver
---------------------------------------
OK public
OK protected
NG private

External Visibility
---------------------------------------
OK public
NG protected
NG private