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Access Control in Ruby

27 Jul 2018

Ruby is an object-oriented language, which means that every object is an instance of a class and every object has certain behaviors (methods). Access control is a way in which Ruby protects the behaviors (methods) defined in a class.

Ruby provides three levels of access control:

  1. public : Public methods can be called by everyone - no access control is enforced. A class’s instance methods are public by default, which means anyone can call them.

  2. private : Private methods cannot be called with an explicit receiver - the receiver is always self. Private methods can be called only in the context of current object, you cannot invoke another objects private method. To make methods private, we use private keyword before writing a method.

  3. protected: Protected methods are like private methods. These methods can be invoked only by objects of the defining class and its subclasses. Access is kept within the family. protected has limited usage. To make methods protected, we use protected keyword before writing a method.

In the example below, we have class Album and a Subclass - CD

     class Album
       def song
         @name = 'Shape of you' # public method
       end
          
       # in method title I am accessing protected method - self.artist and private method - genre.  
       # Note: if I say self.genre in place of genre, ruby will throw an error. Reason? You cannot call private methods on explicit reciever
        
       def title
         "#{self.artist} - #{genre}"  
       end
          
       protected

       def artist
         @artist = 'Ed Shereen' #protected method
       end

       private

       def genre
         @genre = 'Pop' #private method
       end
     end
        
     album = Album.new
     => #<Album:0x007f8a6482bb58> 
        
     album.song
     => "Shape of you"  #=> accessing public method of class Album
        
     album.title
     => "Ed Shereen - Pop" #=> public method of class Album
        
     album.artist  #=> protected method of class Album
     NoMethodError: protected method `artist' called for #<Album:0x007f8a6482bb58 @name="Shape of you">
        
     album.genre  #=> private method of class Album
     NoMethodError: private method `genre' called for #<Album:0x007f8a6482bb58 @name="Shape of you">
        
     # creating a class CD which is a subclass of Album
        
     class CD < Album
        
       def name_sticker   
         self.artist         # name_sticker access the protected method of Album class
       end
     end
        
     cd = CD.new
     => #<CD:0x007f8a64143778>
     
     cd.name_sticker        # public method of class CD
     => "Ed Shereen"
      
     cd.title               # public method of class Album which accesses private method of Album class
     => "Ed Shereen - Pop"
      
     cd.song                # public method of class Album
     => "Shape of you"
      
     cd.artist              # protected method of class Album. It should be noted that you cannot access method artist from outside the class, but you can access it inside the subclass
     =>NoMethodError: protected method `artist' called for #<CD:0x007fa0fd9c0810>
      
     cd.genre               # private method of class Album.
     =>NoMethodError: private method `genre' called for #<CD:0x007fa0fd9c0810>

The table below might make the differences between the three access controls - public, protected and private a bit clearer:

Methods Inside Class Outside Class Inside Sub-Class
Public Yes (public_method_name) Yes (obj.public_method_name) Yes (public_method_name)
Protected Yes (self.protected_public_name) No Yes (protected_method_name)
Private Yes (But, Can’t say self.private_method_name No No
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