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. Next we create three instances of class FairyTaleCharacter:

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

 pinocchio = FairyTaleCharacter.new
 robinHood = FairyTaleCharacter.new
 tinkerBell = FairyTaleCharacter.new

/code

module Flying

 def fly
   puts "I can fly!"
 end

end

tinkerBell.extend(Flying)

module Evil

 def morality
   puts "I'm evil!"
 end

end

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