A lightweight module for class extensions in D

[Janderson]

Gregor Richards wrote:
> I ran into a situation where I needed (essentially) the visitor pattern, but the visitor pattern sucks, so I wanted to make something like class extensions instead (that is, methods added to a class outside of the class definition).
> 
> Of course, it's not possible to do this particularly cleanly, but I made a system that works (albeit using gross string mixins). Essentially, if you have a class A, class B : A, class C : B, you could do something like this:
> 
> mixin(extensions("A", "void", "doFoo", "", ""));
> 
> mixin(extend("A", "doFoo"));
> void A_doFoo(A pthis) {
>     /* method for A */
> }
> 
> mixin(extend("B", "doFoo"));
> void B_doFoo(B pthis) {
>     /* method for B */
> }
> 
> Then if you call doFoo(new A()) the call will become A_doFoo(new A()), if you call doFoo(new B()) the call will become B_doFoo(new B()), if you call doFoo(new C()) the call will become B_doFoo(new C()).
> 
> If anybody has some improvements, that'd be cool. Maybe you can get rid of the dependence on string mixins ... but I don't think templates quite cut it.
> 
>  - Gregor Richards
> 

What about using a delegate, function pointer or a functor for your 
visitor pattern?

-Joel