Dynamic templated virtuals - I still often want them

[Manu]

On Thu, Jul 23, 2020 at 12:40 PM Adam D. Ruppe via Digitalmars-d <
digitalmars-d at puremagic.com> wrote:

> You might know about D's template this parameters, which adapt to
> the static type of a `this` reference on a call, or with the
> curiously recurring template pattern, which passes a derived
> class to the base class so the base class can inspect the derived
> class.
>
> Both of these are useful at times, but neither quite do what I
> have in mind here.
>
> Imagine this:
>
> ---
> class Base {
>       virtual string serialize(this This)() {
>             return This.stringof; // just for example
>       }
> }
>
> class Derived : Base {
>
> }
>
> void main() {
>      Base b = new Derived();
>      assert(b.serialize() == "Derived");
> }
> ---
>
> That does NOT work today. For one, of course, D has no `virtual`
> keyword, but if you left that out, it would compile, but fail the
> assert because the static type of `b` passed to the template This
> is actually still `Base`.
>
> But imagine if `serialize` actually got a virtual table entry,
> just like if it wasn't a template at all, but each child class
> automatically got an instance of it made for itself.
>
> So it would be as if I wrote
>
> class Base {
>       string serialize() {
>             return Base.stringof;
>       }
> }
>
> class Derived : Base {
>       override string serialize() {
>             return Derived.stringof;
>       }
> }
>
> by hand. Of course, it is possible to do this kind of thing with
> mixin templates or the CRTP, but in both cases, the Derived class
> must actually write it in the child class, and if you don't
> there's no way to really tell; it will still compile and just use
> the base class implementation. Which might be OK but it isn't
> perfect.
>
> It would just be cool if the base class template was
> automatically instantiated again for the child class, while still
> working like a normal virtual call.
>

That's clever, and I wish I had have thought of it before!!
This pattern would have helped me in multiple scenarios I've encountered in
the past where I had to deploy awkward and ugly mixin tricks.
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