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?

The problem is that this line contains a recursive instantiation:
abstract class Animal(T : Animal!(T))

Animal!(T) has to be instantiated before the compiler knows how to what 
type it is.

But if you place a static assert in a place outside what the compiler 
needs to deduce the type:

class Animal(T) {
	this() {
		static assert(is(T : Animal));
	}
	public abstract T[] procreate(T mate);
}

Things work as they should.


/Oskar