Forward referencing templates..

[Bradley Smith]

James Dean Palmer wrote:
> I am interested in expressing some data structure relationships like the 
> half-edge data structure using templates.  For example, consider these 
> classes with these type dependencies:
> 
> class edge(TFace, TVector) {
>  ...
> }
> 
> class face(TSurface, TEdge) {
>  ...
> }
> 
> class surface(TFace) {
>  ...
> }
> 
> class vector(TNumeric) {
>  ...
> }
> 
> The idea being any one of these classes could be subclassed - for 
> example to represent weighted edges, weighted faces, etc.. Or a 
> different kind of basic numeric could be used. Now I would like to say 
> this..
> 
> alias vector!(double) vector3D;
> alias edge!(face3D, vector3D) edge3D;
> alias face!(surface3D, edge3D) face3D;
> alias surface!(face3D) surface3D;
> 
> But I get an error about a forward reference when trying to do it like 
> this.  C++ has difficulty expressing the same kind of relationship 
> because it doesn't forward reference templates.  I believe that Java can 
> represent this kind of relationship though.
> 
> Does anyone know if this can't be done in D? Or is there a better "D" 
> way to do it?  Thanks!


If you substitute surface3D into the alias for face3D, you get

alias face!(surface!(face3D), edge3D) face3D;

This shows that the alias for face3D is being defined by face3D. Isn't 
that an infinitely recursive definition?

How would you construct a similar relationship in Java?

   Bradley