Self-referencing template parameters

[Oskar Linde]

Erik Rasmussen skrev:
> One way that I have found java's generics to be useful is in doing 
> something like this:
> 
> ---
> public abstract class Animal<T extends Animal<T>>
> {
>   public abstract T[] procreate(T mate);
> }
> 
> public class Monkey extends Animal<Monkey>
> {
>   public Monkey[] procreate(Monkey mate)
>   {
>      // make sweet monkey love
>   }
> }
> ---
> 
> Although you're not strictly forced to by the compiler, if you make it a 
> policy to always make your template variable the same as your concrete 
> subclass, then you have forced any class that extends Animal to provide 
> a way to "procreate" with its own kind.
> 
> I seem to be unable to implement this pattern in D.  The following 
> doesn't work:
> 
> ---
> abstract class Animal(T : Animal!(T))
> {
>   abstract T[] procreate(T mate);
> }
> ---
> 
> It doesn't like that template definition on the first line.  It says, 
> "template instance does not match any template declaration".
> 
> Any ideas?

Try:

template Animal(T) {
   class Animal {
     public abstract T[] procreate(T mate);
   }
}

class Monkey : Animal!(Monkey) {
   Monkey[] procreate(Monkey mate) {

   }
}

/Oskar