Extending objects. : Consider one or more practical examples of Ruby's extend method--that is,
examples that serve a real need and are not contrived.  What facilities do other languages have for extending
objects?  Are they more or less powerful than Ruby's?  Also consider prototype-based languages such as Self, 
which don't even provide classes.

Introduction

One of the most fundamental advantages of using Object Orientation is to reuse code. Rather than coding what was already coded, we can simply extend it to suit the needs of the new class. This helps to develop applications rapidly. Many languages provide built in libraries which can be included in classes. The O-O languages also provide a way to add functionality to existing classes through extension.

Implementation of Code Reuse:

Inheritance is the word used in O-O for the process of inheriting the members of a class and extending the functionality. A class can inherit from a single class or multiple classes, the latter is called as Multiple Inheritance. While a child class inherits from the parent class, it tends to inherit all the members of the class.

Comparison of Extend methods in Java, C# and Ruby.

Java:

A class declaration can use the extend keyword on another class as follows:

Class Course extends College 
{
..............
}

When a class Child extends class Parent, Child automatically has all variables and methods defined in class Parent. If class Child defines a variable or method that has the same name in class Parent, class Child's definition overrides that of Parent's.

class Parent {
int x;
int y;
int get(int a, int b){
x=a; y=b; return(0);
}
void Show(){
System.out.println(x);
}
}
class Child extends Parent {
public static void main(String args[]){
Parent p = new Parent();
p.get(5,6);
p.Show();
}
void display(){
System.out.println("Child");
}
}

C#:

Extension methods enable the programmer to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type.

class ExtensionMethod    
{
   static void Main()
   {            
       int[] s = { 1, 55, 75, 93, 81, 96 };
       var r = s.OrderBy(g => g);
       foreach (var a in result)
       {
           System.Console.Write(a + " ");
       }           
   }        
}

 Output: 
 1       55          15       81            93          96

The above example shows how to call the standard query operator OrderBy method on an array of integers.

C++

Similar to Java, C++ also extends functionality of a base class to a derived class. The derived class can use the functions and variables of a base class as well as add their own functionality, thus making them a more specialized version of the base class. Similar to Java, overriding is also supported in C++. Here is an example of how a class is extended in C++.

class Base{
public:
int x;
int y;
int get(int a, int b){
x=a; 
y=b; 
return(0);
}
void display()
cout<<x<<y;
}
}
class derived:public base{
void Main(){
Base b;
b.get(5,6);
b.display();
void display(){
cout<<”Derived”;
}
}

Python:

To extend a class in Python, we simply call the parent class in the parenthesis while defining the child class. This will pass all the methods and variables defined in the parent class to the child class. An illustration of how classes are extended in python.

class Parent:
a=5
b=4
def add():
print(a+b)
class Child(Parent):
def difference():
print(a-b)
p = Parent()
c = Child()
c.add()
c.difference()

Ruby:

The include method effectively adds a module as a super class of self. It is used inside a class definition to make the instance methods in the module available to instances of the class. It is sometimes useful to add the instance methods to a particular object. This is done using Object#extend.


module Write
def pen
"#{self} says use a pen not a pencil for your exam!"
end
end
obj = " Proctor"
obj.extend Write
puts obj.pen

Output: Proctor says use a pen not pencil for your exam

If the extend key word is used within a class definition, the modules methods become class methods. This is because extend is equal to self.extend, so the methods are added to self, which in a class definition is the class itself.

module Write
def pen
"#{self} says use a pen not pencil for your exam"
end
end
class Proctor
extend Write 
end
puts Proctor.pen

Output: Proctor says use a pen not pencil for your exam

How Extending Objects in Ruby is more powerful than other languages

The one main disadvantage of inheriting from a class is that, we tend to inherit all of its members, most of which we may not use in our programs. Ruby however gives us the option of extending only what we need. Several modules can be created and these modules can be extended at runtime. The extend keyword makes the module available to all instances of the class. A class can simply extend the modules and uses it the way it wants on the move. This, however, does not change the objects original code. This is really a unique and powerful feature of Ruby.

Comparison of "Extend property " in Prototype Based languages to Class Based Languages.

Prototype based Languages in Brief

Prototype-based programming is a kind of object-oriented programming in which there are no classes, and behavior reuse is performed through the process of cloning (Cloning is the process of making of an exact copy of an object) existing objects that serve as prototypes. Languages like Self, Newton Script, Slate, Ioke, Io are examples of a prototype language. Most class-based Object Oriented languages are based on Classes which can be described as defining the basic qualities and behaviors of objects and object instances which can be described as being manifestations of classes. In prototype-based languages like Self, the duality between classes and object instances is not needed. There is no need for having an instance of an object that is based on a class. For example, in Self a copy of an existing object is made, and then changes are made to it. Basic objects that are used primarily to make copies are known as prototypes. When an existing object proves to be an insufficient, a programmer can create a modified object with the expected behavior, and use that instead of the previous one.

Cloning

In languages like Self, instead of having a class definition take up memory, and every instance take up memory, clones of objects generally take up a small amount memory. Every object just points to another object. So the fewer changes the programmer makes to a clone of an object, the fewer the amounts of new data need to be stored, and thus lesser amount of memory that a programmer needs to do a new clone. Prototype-based languages will help simplify the understanding of each object.

It is also a fact that languages like Self, make it easier to fix bugs in code. This is because prototype-based languages use the concept of slots. We are referring to a method as slot because the programmer can change the functionality of any object, by changing a method alone. When new code is placed into an object's slot the functionality of that object is changed. Another advantage is that the user interfaces also benefit from the use of prototype-based languages. User interfaces hold less amount of objects. Objects are created in interfaces for a specific purpose. Without creating many classes for many purposes, or creating just a few big ones, each of them with a number of methods for implementation, cloning objects is a proved easier solution.

When "extends" is more useful than Cloning

Just like any other style, Prototype-based programming has its own disadvantages. Consider the cases when repetitive types of data is used like the ones in a database. Here prototype-based programming would decrease efficiency of these applications. And also in this case, Class-based programming will be best and perhaps the only solution for these kinds of problems.

We saw that in Java, inheritance is achieved using extends keyword. In Java Script, a prototype based language, Prototype chaining is used to build new types of objects based on existing ones. The functionality is very similar to inheritance in a class based language. Constructor functions have a property known as prototype. Adding properties and methods to the prototype property will also adds the method or property to all objects created by the constructor function. This prototype property is a mere javascript object such that it is possible to create a function’s prototype using another constructor function. When this is done, all of the properties and methods from the constructor function’s prototype are automatically added to new the prototype object. This makes it easy to create a constructor function that builds objects that are just an extended version of an existing one.