On Mon, 30 May 2022 at 11:25, Arthur O'Dwyer <arthur.j.odwyer@gmail.com> wrote:
On Mon, May 30, 2022 at 12:53 PM Edward Catmur <ecatmur@googlemail.com> wrote:
On Mon, 30 May 2022 at 10:23, Arthur O'Dwyer <arthur.j.odwyer@gmail.com> wrote:
On Mon, May 30, 2022 at 11:37 AM Edward Catmur via Std-Proposals <std-proposals@lists.isocpp.org> wrote:

We expect there to exist types that are immovable and [non-trivially] relocatable.

FWIW, I (in P1144) consider that an oxymoron. A type that is immobile/immovable cannot be relocatable, because it's not movable. Immobility is for types that cannot be moved from place to place — like `std::mutex`, for example.

That's the strongest version of immovability; types that must occupy the same storage throughout their lifetime. But there are weaker versions where a class type has no copy or move constructor but is nevertheless relocatable: [e.g.]
- types that have invariants that preclude having an empty state (gsl::non_null)

Perhaps it would help to introduce terminology distinguishing the strong case from the weaker case? Say, "immobile" for the strong case and "immovable" for the weaker case? Or, just "non-semiregular" for the weaker case, if you'd prefer.

I strongly discourage making any Memmi/Derrida–style subtle distinctions between the words "immobile" and "immovable."

I suggest phrases like "types that have an `operator reloc` but no move constructor" (describing the physical situation) or "types that are conceptually movable but lack any moved-from state" (describing the conceptual situation).

Well, yes, but that's a bit of a mouthful, so people will come up with a more concise terminology, or at best an initialism.

If you want to permit "types that are conceptually movable but lack any moved-from state," then you need to flesh out the semantics of operations like assignment and swapping. I've seen a lot in this thread about relocating-into-a-constructor, but not much IIRC about relocating-into-an-assignment or swapping-by-relocating.

    gsl::non_null<int*> p1, p2, p3 = ...;
    gsl::non_null<int*> q1 = reloc p1;  // OK in your world
    q1 = reloc p2;  // OK??
    std::swap(q1, p3);  // OK??

Conceptually, all of these operations should be legal, right? None of them leave a gsl::non_null<int*> object in a moved-from state. Can your `operator reloc` actually handle all of them, though?  (This would go in the paper.)

1. Yes.
2. Yes.
3. Yes, if gsl::non_null writes a swap overload, but also std::swap could destroy and recreate its arguments:

template<class T> requires std::is_noexcept_relocatable_v<T>
void swap(T& lhs, T& rhs) {
    T temp = relocate_at(&lhs);
    new (&lhs) T(relocate_at(&rhs));
    new (&rhs) T(relocate_at(&temp));
}

(relocate_at is the magic library function that (destroys and) relocates its argument into a returned prvalue.)
 
The advantage of a syntax is that you can express trivial relocation simply by defaulting it, and it Does The Right Thing if any member becomes non-trivial.

Pedantic terminology nit: Just like with any other special member, `=default` on your `operator reloc` wouldn't mean it was trivial; it would just mean it was defaulted, i.e., memberwise.
struct A { std::string s; operator reloc(A&&) = default; };  // memberwise and trivial
struct B { std::any a; operator reloc(B&&) = default; };  // memberwise and non-trivial

Absolutely, yes; and that's an advantage, since it means that a memberwise (defaulted) relocation operation is trivial precisely when each subobject is trivially relocatable; you won't end up making mistakes when changing data members or if another programmer changes the semantics of a type that you use.

Aside: is std::string necessarily trivially relocatable? I seem to remember in the old days some SSO strings that used self-reference, but maybe no library implementor has gone for that strategy.