Limited member function templates?

[Christopher Wright]

Yigal Chripun Wrote:

> Christopher Wright wrote:
> > Janice Caron wrote:
> >> On 11/10/07, Janice Caron <caron800 at googlemail.com> wrote:
> >>> On 11/10/07, Bill Baxter <dnewsgroup at billbaxter.com> wrote:
> >>>> The only thing you can't have is _virtual_ member function
> >>>> templates. Is making them virtual what you are talking about?
> >>> All member functions in D are virtual (unless you explicitly use
> >>> the "final" keyword). So obviously, yes.
> >> 
> >> When we get the common calling convention whereby f(x) and x.f()
> >> are interchangable, then adding non-polymorphic template functions
> >> will be a piece of cake.
> >> 
> >> Just declare as f(T)(X x, T t) { ... } and call as x.f(t).
> >> 
> >> Maybe that will be enough. But yeah - I intended to mean "normal" 
> >> functions. Polymorphic. Virtual.
> > 
> > I was just about to feature-request virtual templated member
> > functions. I can't mock them, which is one of two major problems in
> > my mocks library. You also can't have a templated method in an
> > interface:
> > 
> > interface Collection (T) { void addAll(U : T) (Collection!(U) toAdd);
> > // fail }
> > 
> > Which leads to annoyances: "why can't I add all my Bicycles to my 
> > Vehicles collection?"
> 
> I may be mistaken, but isn't it enough to use the following:
> ----
> interface Collection (T) {
>     void addAll(T) (Collection!(T) toAdd);
> }
> ----
> a bicycle should conform to a is-a relationship
> (a bicycle is a vehicle). so you can in fact add them all due to 
> inheritance. or am I missing something here?

A Collection!(Bicycle) is not a Collection!(Vehicle). You cannot add arbitrary Vehicles to a Collection!(Bicycle), nor can you read arbitrary elements of a Collection!(Vehicle) as Bicycles. You can't cast between the two.

So let's say I have Set!(int) called integers and a Set!(long) called longs. In order to add all of the set integers to the set longs, what do I have to do?

My fictional Set class defines:
void add(T)(T element);
void addAll(T) (Set!(T) other);

I have to do:
foreach (i; integers)
   longs.add(i);

> templates are only useful at compile time. in run time we need to rely 
> on polymorphism (inheritance).

But Set!(int) doesn't inherit from Set!(long). Set!(Object) and Set!(MyClass) are entirely unrelated. There's no polymorphism here.

> Maybe you could give a better example for the case when you need to have 
> template (virtual) member functions?
> 
> Yigal

I'm not sure I ever *need* them, but they'd be quite convenient. Of course, for me, I want them for my mocks library: it's a large collection of final methods, and I don't currently have a way of mocking final methods.