CSC/ECE 517 Fall 2007/wiki1 4 ar

Private Method Invocation in Ruby

     Ruby's private methods can be called only on "self" (the calling object). They can't be applied to other objects of the same class, even from within the class definition. In class today, I opined that it is rarely if ever necessary to call a private method on another object of the same class. Try to find Java or C++ examples on the Web where a private method is called on another object. Decide if this is good design, or if it would be better performed by using accessor functions, from the standpoint of elegance or maintainability. If you cannot find any such examples (or even if you can), try to find arguments about why one should not invoke private methods on other objects. Analyze these arguments; can you find weaknesses?

Ruby’s Access Control Mechanisms

Ruby has three levels of access controls – Public, Protected and Private.

Public methods – can be accessed by anyone. In Ruby, public is default access control, except for initialize method which is private by default.
Protected methods – can be accessed only by the defining class and its subclasses. Access is kept within the family.
Private methods – can be accessed only by the objects of the defining class. The explicit receiver is always self (the current object), which means that it is impossible to invoke another object’s private methods and variables directly even if the object has the same class type as the caller.

Example -1:

   class Base
     private 
       def foo
         puts "foo"
       end
     public
       def test
         foo
       end
     end

   Output :
   irb(main):170:0> Base.new.test
   foo
   irb(main):171:0> Base.new.foo
   NoMethodError: private method `foo' called for #<Base:0x695e9d4>

Example -1 shows that private methods can be called only with an implicit receiver. Calling a private method using an explicit receiver throws NoMethodError. Base.new.foo is the explicit receiver which is trying to call the private method, hence throwing NoMethodError.

Private methods cannot be invoked using an explicit receiver. But we use "self" to invoke writer methods else the method will be interpreted as an assignment to a local variable.

Comparing the private method invocation in C++ and Ruby

The private methods in C++ can be invoked by another object of the same class whereas Ruby does not allow explicit receiver for private methods.

Example -2:

   class Base {
   private:
       void foo();
   public:
       void spam(Base *otherObject) {
           foo();              // this is allowed
           otherObject->foo(); // and so is this
       }
   };

Example -2 shows that a private method can be called by any instance of the same class. otherObject is the instance of the same class Base, which calls the private method foo successfully.

Example -3:

   class Base
       private
        def foo
            puts "foo"
        end
       public
        def test
            foo
            self.foo
        end
   end
   b = Base.new
   b.test

   Output:
       foo
       NoMethodError: private method `foo' called for #<Base:0x4ef2690>

Example -3 shows that when calling a private method a receiver cannot be specified in Ruby. Even calling the private method foo through self throws NoMethodError.

So it can be concluded that in C++, “private” defines “private to the class”, while in Ruby it means “private to this instance”.

Bypassing private method invocation in Ruby

Example -4:

   class Test
     private
       def print_hello
         puts "Hello everyone!"
       end
   end
   t = Test.new
   t.send( "print_hello" )

   Output: 
      Hello everyone!

Example -4 shows that it is possible to access private methods in Ruby through “send” function. In Ruby, declaring as private is only a guideline and not a strict rule. There are workarounds to access those private methods. The advantage of such workarounds allows testing of private methods but it is a bad practice since private methods are internals of a class.
Since private methods have a loophole in security, an alternative good practice would be to use accessor functions. In accessor functions, we implement the getter and setter methods for the private attributes, thereby providing the required security.

There are three accessor functions in Ruby –
attr_reader – creates a getter method.
attr_writer – creates a setter method.
attr_accessor – creates getter and setter methods.

Example -5:

   class Vehicle
     attr_reader :model
     attr_writer :color
     attr_accessor :price
         def initialize(model,color,price)
             @model = model
             @color = color
             @price = price
         end
   end
   v = Vehicle.new("Camry","black",20000)
   puts v.model
   puts v.color  #throws NoMethodError
   puts v.price
   v.price = 30000
   puts v.price

   Output:
       Camry
       NoMethodError: undefined method `color' for #<Vehicle:0x66604d0 @model="Camry", @price=40000, @color="black">
       20000
       30000

Example -5 shows the implementation of the accessor functions. The model attribute has only a getter method, hence we cannot set a value to it. The color attribute has only a setter method, hence we cannot read value from it. But price has both getter and setter methods, hence we can read and write value to it.

The advantages of using accessor functions are – Flexibility and Maintainability

Since the implementation is confined to the accessor functions, any future changes needs to be made at a single point.

Any type checking that needs to be done can be included within the accessor function. This prevents the object from getting into an inconsistent state.

In general, it is a good programming practice to use accessor functions instead of invoking private methods in Ruby.

References

1. Programming Ruby: The Pragmatic Programmers' Guide
2. http://www.rubycentral.com/pickaxe/win32.html
3. http://lylejohnson.name/blog/?p=5
4. http://www.ruby-forum.com/topic/120257#new