Hi all,

I almost want to propose a magic library function that takes a prvalue, a list of immediate bases and pointers to data members, and returns those subobjects as a destructurable class type. Something like:

    auto [b, x, arr] = std::decompose<B, &D::x_, &D::arr_>(reloc obj);

It would check that the returned bases and data members are direct, distinct, non-overlapping and relocatable, and destroy any subobjects not mentioned. Then you would be able to return that `std::decompose` result directly, or perform further processing it.

This looks good, but how are you able to mix type (B) and non-type (&D::x) parameters in the function declaration?

Oh, cheating. Actually, it looks like there's some renewed momentum on P1985 Universal Template Parameters, so that would help here. Otherwise the non-type parameters could be passed by function argument, although that's less elegant.

One annoying thing about this is that there's no proof that the caller has the right to access base B of D. We could mandate that by fiat, but it'd be nicer if there were a way to denote a specific base-of-derived, e.g. with the same syntax as a pointer to data member. That would be a lot more work, though, so just mandating that the base be accessible and unambiguous in the context of the caller might have to do.

I know what you mean and I don't know if that might do. I've seen and written code that passes the pointer to some data member as a template parameter to some static free function in the same TU. This is a good way to factorise code that performs the same set of operations on different data members. For instance:

template <auto Member> static void Do(D& self);
void D::do_a() { Do<&D::_a>(*this); }
void D::do_b() { Do<&D::_b>(*this); }

My point is that `Do` cannot access data members of T under normal circumstances, but since the address is supplied as a template parameter, it works. Now back to std::decompose. If done in `Do` then it could not access the base class of `D`, and there is no similar way to grant it access.

That may have to do as I cannot think of something better. Besides, if at some point the language supports pointers to base members, we can still put them into std::decompose.

> If `operator T()` were prvalue qualified that would help prevent bugs, using deducing this on the object parameter:
>
>     T relocating_wrapper<T>::operator T(this relocating_wrapper self) { return self.opt_.pop(); }
>
> Users would have to go to considerable lengths to misuse it and the resulting code would have a manifest use-after-move bug. Yes, there's still a double relocation but that should be elidable.

That code is already okay w/t C++23 right?

> Well, this is a good argument for `reloc` to DTRT on references: that is, `reloc r` should behave on reference typed variables r as static_cast<decltype(r)>(r) while ending the lifetime of the reference and removing it from scope.

Indeed that looks better.

> Fair enough, though I think it'd be worth mentioning in a "further directions" section.

You make me doubt. How large would that chunk of the proposal be? At minimum, I guess all template parameter pack functions that capture its parameter pack with a forwarding reference will need to be reconsidered... to be eventually replaced by the decltype(auto)... syntax, plus changing std::forward to reloc. That would also open the door to the non-variadic syntax: vector<T>::push_back(decltype(auto) x). And what about decltype(Concept)... syntax?

Also, another technicality now that I am in the writing of the proposal, is it okay to consider reloc as a new unary operator, or does it need to be a keyword? I have heard here and there that new keywords are difficult to be approved, that's why I am asking. Personally I don't see why reloc would need to be a keyword.