CSC/ECE 517 Fall 2012/ch1 1w17 ir

Extending Objects

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The ‘extend’ feature in computer programming languages allow the programmer to dynamically extend the functionality of an object at runtime, as opposed to extending functionality at compile time. This feature can be found in Object Oriented Programming (OOP) Languages.

Introduction

The programming paradigm OOP uses 'objects' for designing applications. These objects consist of an encapsulated set of data fields and methods, and are usually an instance of a class. These objects can communicate with each other through messages (or 'methods') and process data in object oriented programming. But in order to increase an object's functionality, it is necessary to add the features of one object to another. This can be done dynamically at run time, without repeating the code, by extending the features of one class to another. This can be done by the use of the 'extend' keyword. Many OOP oriented languages support the 'extend' feature.

An object is an entity that serves as a container for data and also controls access to the data. Associated with an object is a set of attributes, which are essentially no more than variables belonging to that object. Also associated with an object is a set of functions that provide an interface to the functionality of the object, called methods. - Hal Fulton[2]

Languages Using 'extend' Functionality

Ruby

Ruby is a OOP language. It differs from the previous two OOP languages mentioned above in how it interprets what 'object-oriented' means and the different terminologies for the concepts it employs.

A class hierarchy in Ruby [1]

In Ruby, everything is an object and all objects are instances of a class. Even classes are instances of class objects. So when there is a class, .new is called to get an instance of the class and when there is an instance, .class is called to obtain its class [3]. So each class has a superclass and if it doesn't specify any class from which it inherits, then it inherits directly from the Object class.

Example Code Snippet

Here is an example that shows how Object.extend(module) adds the methods defined in a module at the object's level.

 module RubyExample
   def helloWorld
       "Hello user! Welcome to Ruby!"
   end
 end

Now, we can extend this module's functionality (i.e, the Greet method) to any object we instantiate. Below we can see that the module's methods have been mixed in to the object.

 obj = Object.new
 obj.extend(RubyExample)
 obj.helloWorld #=> "Hello user! Welcome to Ruby!"

As mentioned earlier, everything in Ruby is an object. The following confirms this fact:

 >> RubyExample.object_id
 => 45097
 >> RubyExample.class
 => Module
 >> RubyExample.respond_to?(:extend)
 => true

Now this is not the only way to extend object functionality in Ruby. We can also do the same thing by

 RubyExample = Module.new do
   def helloWorld
       "Hello user! Welcome to Ruby!"
   end
 end

 RubyExample.extend(RubyExample)

 RubyExample.helloWorld #=> "Hello user! Welcome to Ruby!"

So, the above alternative method clarifies the fact that RubyWorld is just an instance of the class Module. We can also do the same thing by including the extend() call within the module definition.

 module RubyExample
    extend RubyExample
    def helloWorld
        "Hello user! Welcome to Ruby!"
    end
 end

 RubyExample.helloWorld #=> "Hello user! Welcome to Ruby!"

In Ruby, the keyword 'self' refers to the current or default object itself. So another way of implementing the above is as follows

 module RubyExample
    extend self
    def helloWorld
        "Hello user! Welcome to Ruby!"
    end
 end

 RubyExample.helloWorld #=> "Hello user! Welcome to Ruby!"

References

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