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Subject: Re: PointerWrapper Trait vs croe::ops::Deref
From:   Finn Behrens <me@kloenk.de>
In-Reply-To: <CANiq72=8M3ALxrmdYvHNfDVX9F4Se0=a-EbzrZPgVv1xsh932Q@mail.gmail.com>
Date:   Mon, 10 May 2021 16:04:54 +0200
Cc:     Wedson Almeida Filho <wedsonaf@google.com>,
        rust-for-linux <rust-for-linux@vger.kernel.org>
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 <CANiq72=8M3ALxrmdYvHNfDVX9F4Se0=a-EbzrZPgVv1xsh932Q@mail.gmail.com>
To:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
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Howdy,

> On 9. May 2021, at 16:21, Miguel Ojeda =
<miguel.ojeda.sandonis@gmail.com> wrote:
>=20
> On Sun, May 9, 2021 at 12:25 PM Finn Behrens <me@kloenk.de> wrote:
>>=20
>> Maybe we could also remove the result, as it would possible be an =
invariant, and never happen as the pointer should already be valid.
>=20
> If it can fail, we cannot use `Deref` to begin with. So yeah, to make
> the examples equivalent, the first one should not return a `Result`.
>=20
> Now, assuming it cannot fail, what I mentioned in the meeting is that
> with `Deref` all this compiles:
>=20
> wrapper.deref().number
> (*wrapper).number
> wrapper.number
>=20
> The question boils down to: do we want this shortcut or not?

I would not like to decide that on my own :-).
After thinking about it I think we don=E2=80=99t want that shortcut, as =
the other way give the same behaviour, but it is explicit.

> Having said that, I am not sure I understand the traits shown here.
> Isn't `PointerWrapper` meant only to save a `Box<T>` and similar types
> (which already implement `Deref` themselves etc.) as a pointer in a C
> data structure? (which is why `into_pointer()` consumes the object).
>=20

All Pointers I=E2=80=99m wrapping are got from the C side. Either =
because there is a special malloc flag and some initialisation to the C =
struct, or as this is in a function call, and therefor the object is =
constructed out of my reach.
I usually do not consume the pointer, as I have to give the pointer to =
multiple function on the C side.

> In the examples shown, the new `get_pointer()` does not consume
> `self`; and `main()` creates a `Wrapper` using `from_pointer()`. But
> the idea is that you already have an object (constructed via other
> means), transform it into a raw pointer, and then later on
> re-construct the original object back from that pointer.
>=20
> So `PointerWrapper` trait is like a `SavedAsPointer` trait, if you =
will.

Renaming to `SavedAsPointer` sounds great.

>=20
> Cheers,
> Miguel

CU,
Finn