On Mon, Dec 18, 2023 at 3:11 PM Frederick Virchanza Gotham via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
On Mon, Dec 18, 2023 at 4:55 PM Thiago Macieira wrote:
>
> There shouldn't be unaligned versions. The parameters should be plain T
> pointers.

'std::unaligned' will need to relocate an object into (and back out
of) unaligned space, and therefore we need an void* or byte*.

At least in all the existing "relocation" proposals, the verb "relocate" specifically means ending the lifetime of an object of type T in address `src`, and constructing a similar-valued object of type T in address `dest`. There is no such thing as "an object of type T" in space that's misaligned for a T.
There are at least two mental models you can have of your "unaligned<T>" thing:
(1) It's an opaque type similar to `atomic<T>` that behaves roughly like a T but just happens to have an (inaccessible) object representation that occupies less-aligned storage. This is how I initially envisioned what Gašper and Lénárd were talking about.
(2) It's a buffer of bytes into which you can dehydrate an actual object of type T and from which you can rehydrate an actual object of type T, but which stores plain old bytes in between. Here you have to be really clear about whether the type is "linear" — e.g. can you rehydrate two copies of the object from a single buffer? — and the model in general feels more complicated; but, on the plus side, it seems to give you two really clear verbs for dehydration and rehydration. P1144 does not give you those verbs at all; but, a competent performance-minded programmer will quickly observe that if `is_trivially_relocatable_v<U>` then both dehydrate and rehydrate can be accomplished in practice via memcpy (as long as the intervening buffer is treated "linearly" in the sense above).  As I said in my previous message, there are some really interesting patterns being developed in this space.
...You can have either of those mental models; and maybe others. But you cannot have the mental model that "an object of type T" ever exists in misaligned space, or that you're "relocating" from an object of type T into misaligned space. That's simply physically impossible.

–Arthur