On Wed, 30 Apr 2025 at 16:33, Hans Åberg via Std-Proposals
<std-proposals_at_[hidden]> wrote:
> > I don't even know what polymorphic operator new means, so this doesn't
> > seem like a very common scenario. Who would benefit from it being in
> > the standard?
>
> The polymorphic operator new is also called a clone operator or a virtual copy operator, given such names rather than “new”. As I also use it for move, I call them polymorphic operator new. As “new” is a reserved word, it cannot be used without additions to the language.
>
> >> As for the val<A> type, I think the GC references are generally used just as a hack for polymorphic values:
> >>
> >> One can use reference counting, like in std::shared_ptr, or say the Boehm GC. But then one will have to remember when to copy an object, and there one gets errors in the implementation.
> >
> > I'm still not sure what you're actually proposing for the standard.
>
> Suppose:
> class A {
> virtual A* new_p(void* p) const& { return new A(*this); }
> };
>
> class B : A {
> virtual B* new_p(void* p) const& { return new B(*this); }
> };
>
> class C : A {
> // No new_p
> };
>
> Then:
> A* bp = new B, cp = new C;
> bp->new_p(); // Gets a copy of *bp
> cp->new_p(); // Gets a copy of A(*cp)
> In the last copy, the object gets truncated to A, not the wanted full C object.
>
> So one must add by hand new_p to every new class created that is derived from A.
>
> An addition might be that one can write
> class A {
> virtual A* new_p(void* p) const& = default;
> virtual A* new_p(void* p) && = default;
> };
> with the implication that every derived class D of A, as well as A, gets
> class D : A
> virtual D* new_p(void* p) const& { return new D(*this); }
> virtual D* new_p(void* p) && { return new D(std::move(*this)); }
> };

So, use https://eel.is/c++draft/polymorphic