Object Inheritance

Does anyone know of any languages which support inheritance/polymorphism from a specific instance of a class at runtime? The idea is something analogous to cloning the object (shallow or deep) to capture internal state, and deriving from it at the same time. There are a few variations on this idea. The first is inheritance and replacement - inherit from an instance override what you need, and replace said instance. Another is an extension of the PrototypePattern where you create an instance but need to do something such as add new functionality, while maintaining polymorphism AND internal state.

A codebase can be reworked to solve these problems in most cases, but a language feature this has a certain interest. Anyone know of anything? --BrianMcCallister

All inheritance in SelfLanguage is based on delegation to prototypes (see PrototypeBasedProgramming).

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Vaguely related:

• In SmalltalkLanguage an object can become: another object so that all references of the original object now point to this object.
• CommonLisp allows to change classes at runtime and to adapt their instances in a user-definable way.

Also see RuntimeTypeMutability.

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JavaScript is also very interesting PrototypeBasedLanguage. And you can implement inheritance using proto copy of the super class. Here is an example

 /**
 * Vector class
 */
 function Vector(){
   //constructor
   this.current = 0;
   this.elements = new Array();

   // member functions
   this.add = function(element) { elements.add(element); ++current };
 }

 /**
 *Stack class inherits vector
 */
 function Stack() {
   //proto typical copy of base
   this.base = Vector;
   this.base();

   //member functions
   this.push = function(element) { this.add(element); //call to super };
 }

-SeshKumar

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Ruby allows you to do that (and I think SmallTalk as well, if I remember correctly). Here's a Ruby code snippet from ProgrammingRuby. It's pretty self explanatory, the built-in String::to_s method is overridden for object a only.

 a = "hello"
 b = a.dup  
 def a.to_s  
   "The value is '#{self}'"  
 end  
 def a.twoTimes  
   self + self  
 end  

 a.to_s  » "The value is 'hello'"  
 a.twoTimes  » "hellohello"  
 b.to_s  » "hello"  

--AndrewQueisser