Can a D library have some types determined by the client program?

[Liam McGillivray]

On Thursday, 7 March 2024 at 22:18:40 UTC, Richard (Rikki) Andrew 
Cattermole wrote:
> There are two ways to do this.
>
> 1. Use templates. 
> https://tour.dlang.org/tour/en/basics/templates
> 2. Use a factory function. 
> https://tour.dlang.org/tour/en/basics/delegates
>
> ```d
> class Map(ATile : Tile) {
> 	ATile[] tiles;
> }
> ```

Thank you. Is this first example you gave the template? Is the 
syntax `(ATile : Tile)` saying that ATile must be a derived class 
of Tile? If this isn't worse in any way than your second example, 
then I don't know why I wouldn't choose this one.

I suppose that if I do this, then the derived class `Mission` 
would be declared like `class Mission : Map(GridTile)`, right?

When I tried adding that parameter to the Map class, on 
attempting to build it it complained that references to Map in 
other classes were incomplete, as they didn't include a 
parameter. I suppose I must make my other classes templates too.

Something strange that I just realized is that (without doing any 
of the changes you suggested to me), I have a reference to a Map 
object as one of the class variables in Unit, yet it has allowed 
me to place a Mission object in it's place. It no longer allows 
me if I change it to a pointer. Why is it sometimes possible to 
put a derived class in a place meant for the class it inherits 
from?