Private Classes in Ruby

The concept of private, protected and public methods in Ruby is a bit different than it other languages like Java. In Ruby, it is all about which class the person is calling, as classes are objects in ruby.

Private Class

When a constant is declared private in Ruby, it means this constant can never be called with an explicit receiver, a private constant will be called only with an implicit receiver. So, in ruby private classes can be defined inside a class as a sub-class and declaring them into private constants, here this private class can be only accessed through the outer-class. Syntax:

 private_constant: class

Below is the example of Public class: 

Ruby

# by default public
class Marvel

  # by default public
  class Guardians
      def Quill
          puts "Legendary outlaw"
        end
      def Groot
          puts "I am Groot"
        end
    end

  # by default public
  class Avengers
      def Tony
        puts "I am Iron-man"
      end
    end
end
Marvel::Avengers.new.Tony
Marvel::Guardians.new.Quill

Output:

I am Iron-man
Legendary outlaw

In the above example as the sub-classes Guardians and Avengers are public, both implicit and explicit users have access to it.

• Check out the private class Example :

Ruby

# Ruby program of accessing private class
# public
class Marvel

  # Private
  class Guardians
      def Quill
          puts "Legendary outlaw"
        end
      def Groot
          puts "I am Groot"
        end
    end

  # public
  class Avengers
      def Tony
        puts "I am Iron-man"
      end
    end

# making Guardians class private
private_constant :Guardians
end

Marvel::Avengers.new.Tony

# throws an error(NameError)
Marvel::Guardians.new.Quill

Output:

I am Iron-man
main.rb:20:in `': private constant Marvel::Guardians referenced (NameError)

• So, here if we look at the code they aren’t any access-specifier keywords to make a class as private or public, but they do exist in Ruby, but cannot deal with classes. To make a class as private we need to take the help of private_constant, here this makes any object, class or method as private which will be unavailable to access for explicit users.
• how can a private class can be accessed through the outer-class.

Ruby

# Ruby program a private class accessed through the outer-class.
# public
class Marvel

  # private
  class Guardians
      def Quill
          puts "Legendary outlaw"
        end
      def Groot
          puts "I am Groot"
        end
    end

  # private
  class Avengers
      def Tony
        puts "I am Iron-man"
      end
    end

# outer-class method accessing private classes
def fury
    Guardians.new.Groot
    Avengers.new.Tony
end
private_constant :Guardians
private_constant :Avengers
end

# calls fury method in Marvel call.
Marvel.new.fury

# throws error as it is explicit accessing.
Marvel::Avengers.new.Tony

# throws error as it is explicit accessing.
Marvel::Guardians.new.Quill

Output:

I am Groot
I am Iron-man
main.rb:26:in `': private constant Marvel::Avengers referenced (NameError)

• The only way to access the private class(inner-class) is by its outer-class. And private classes can only exist in the form of inner-class only. As we can see in the above code we accessed Groot and Tony (private class methods) with fury (outer class method), by creating Guardians.new and Avengers.new (private class objects) and calling the respective methods with the respective objects Guardians.new.Groot and Avengers.new.Tony (calling the methods) from fury (outer-class method). if the outer-class is private it will be inaccessible for both implicit and explicit users. Because creating an object for private class outside its class is not possible. Therefore no access.

Note :Outer-class can never be private.