CSC/ECE 517 Fall 2010/ch1 2c jp

Prototype-based object oriented programming is the method in which object behavior is defined by existing objects (or prototypes), not classes. In prototype-based programming (also called instance-based programming) prototypes are cloned to create extension objects.[1][4] Programmers then define the differences between the default behavior inherited from the prototype to create additional functionality for the extension object.[4]

All extension objects contain two parts: a list of pointers to their prototype objects, and a set of instructions unique to itself. When a method invocation is sent to an extension object, the object first looks to see if the method is defined in its own set of instructions. If the method is not found within the objects own code, it then looks for the method among the object's prototype methods, and so on. This system of method invocation is called delegation. Programming languages without delegation cannot implement prototype-based programming.[4] The examples in this article will use the Ruby programming language.

First, we create a class called FairyTaleCharacter with one method defined to show the character's morality. We assume all fairy tale characters are good people:

 class FairyTaleCharacter
   def morality
     puts "I'm a good person!"
   end
 end

Next we create two instances of class FairyTaleCharacter:

 pinocchio = FairyTaleCharacter.new
 peterPan = FairyTaleCharacter.new

We want to create a third fairy tale character, Captain Hook, but he is not a good person like we originally assumed. Instead of modifying our original class we can alter only the instance for Captain Hook by using the extend method:

 captainHook = FairyTaleCharacter.new
 module Evil
   def morality
     puts "I'm evil!"
   end
 end
 captainHook.extend(Evil)

 peterPan.morality #=> "I'm a good person!"
 captainHook.morality #=> "I'm evil!"

Next we wish to create a new fairy tale character that also has the ability to fly. Instead of creating a new instance of class FairyTaleCharacter and extending it with the Flying module, we can create a new instance using the object peterPan as our prototype.

wickedWitch = tinkerBell.clone() wickedWitch.extend(Evil)

pinocchio.morality robinHood.morality tinkerBell.morality wickedWitch.morality

tinkerBell.fly wickedWitch.fly

In this example, we created three instances of class FairyTaleCharacter. We then extending the object tinkerBell with the ability to fly. At this point, we wish to create a fourth fairy tale character with the ability to flyThen we create a forth instance using the object tinkerBell as the prototype