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Re: [std-proposals] Relocation in C++

From: Sébastien Bini <sebastien.bini_at_[hidden]>
Date: Fri, 30 Sep 2022 10:33:13 +0200
On Thu, Sep 29, 2022 at 6:06 PM Edward Catmur <ecatmur_at_[hidden]>

> On Thu, 29 Sept 2022 at 14:12, Sébastien Bini <sebastien.bini_at_[hidden]>
> wrote:
>> On Mon, Sep 26, 2022 at 7:36 PM Edward Catmur <ecatmur_at_[hidden]>
>> wrote:
>>> On Mon, 26 Sept 2022 at 15:20, Sébastien Bini <sebastien.bini_at_[hidden]>
>>> wrote:
>>>> I agree with the class invariant part.
>>>> But the no user-provided reloc, move and copy ctor clause was not
>>>> motivated because of the invariant, but because we wanted to be sure that
>>>> subobjects of a class could be independently relocated or destroyed. So at
>>>> minimum we must have a default destructor. And that's not enough.
>>>> struct PainterGuard
>>>> {
>>>> Painter* _p;
>>>> StateGuard(Painter& p) : _p{&p} { _p->save(); }
>>>> ~StateGuard() { _p->restore(); }
>>>> };
>>>> class PainterWithGuard
>>>> {
>>>> public:
>>>> PainterWithGuard(Painter p) : _p{reloc p}, _guard{_p} {}
>>>> PainterWithGuard() : _guard{_p} {}
>>>> PainterWithGuard(PainterWithGuard) /* reloc ctor */ { _guard._p =
>>>> &_p; }
>>>> public:
>>>> Painter _p;
>>>> private:
>>>> PainterGuard _guard;
>>>> };
>>>> This is a perfectly valid class. Its default destructor works fine.
>>>> Unfortunately, from outside the class (with no access to private
>>>> data-members), doing:
>>>> `auto [p] = std::decompose<&PainterWithGuard::_p>(reloc
>>>> painterWithGuard);`
>>>> Will call `restore()` on a destructed/relocated object.
>>>> Likewise doing (in the class implementation this time):
>>>> `auto [p, g] = std::decompose<&PainterWithGuard::_p,
>>>> &PainterWithGuard::_guard>(reloc painterWithGuard);`
>>>> Will construct a PainterGuard that points to invalid data. This one can
>>>> be incriminated to the class writer as it needs access to private data.
>>>> I agree this class has a bad design, but its safe to use otherwise.
>>>> std::decompose shouldn't cause crashes on badly designed classes.
>>>> IMO the no user-provided reloc, move and copy ctor clause gives us that
>>>> extra guarantee, that there are no relationships between subobjects and as
>>>> such it is safe to independently relocate or destroy them.
>>> Great example, and that's the sort of thing I was trying to describe
>>> with my talk of private inheritance. (PainterGuard could be a private base
>>> of PainterWithGuard, perhaps.)
>>> But I can still write a similar class that is currently safe, has no
>>> user-defined special member functions but would be unsafe to decompose:
>>> instead of writing or deleting the relocating constructor, have
>>> PainterGuard inherit from boost::noncopyable. So the existence of
>>> user-defined special member functions is insufficient to determine whether
>>> a class is safe to decompose.
>> Yes. But this example could also be viewed as to give us a hint that
>> there must be no user-provided default constructor as well.
> Still problematic:
> class Greeter : private boost::noncopyable
> {
> public:
> std::atomic<std::string> name;
> void run() { thrd = std::jthrd([&] { std::println("Hello {}",
> name.load()); }); }
> private:
> std::jthread thrd;
> };
> Modifying `name` from user code is fine, so it's OK (if slightly odd) for
> it to be public, but `decompose`ing it out is not, since then the jthread
> is referring to a destroyed object. Yet `Greeter` has no special member
> functions! So the real issue is that user code is affecting an object
> (Greeter::thrd) to which it has no access.

Hmm. It feels off to use access control as a guarantee of class safety. IMO
the real issue is that we are trying to decompose something that isn't

Unfortunately there is no is_decomposable trait, and implementing one seems
impossible. Even a C array could not be decomposable because of some
dependencies between its elements: `std::any arr[2];` good luck trying to
find out what's safe to do.

My main worry is that structured binding relocation is built on top of
structured binding. In `auto [x, y] = foo();`, what is to say that `x` and
`y` are structured bindings and not complete individual objects? For the
moment we stated the rules on how it would work but not what triggers it.

We must be sure that statements that work fine today because they use
structured bindings will not break because they silently got turned into
individual complete objects, but their original type was not decomposable.
For instance, in C++17 `auto [x, y] = foo();` the hidden object of type E
is introduced, but E may not be decomposable, albeit safe to create from
`foo`. We must be sure that this statement does not break silently with
this proposal.

We could simply state that if reloc is used on one of the identifiers
introduced in the structured binding syntax, and that said identifier is
not ref-qualified, then we use the new APIs to construct individual
complete objects instead of structured bindings. I don't know if that's
reasonable as it implies searching in the rest of the function body to know
how to interpret that first statement.

Another way would be to have a new statement: `auto reloc [x, y] = foo();`
(or `auto &< [x, y] = foo();`...) to clearly mark that we want to split
into individual objects. This best expresses the intent, and would not
break existing code.

> Instead, consider: `std::decompose` is accessing (on behalf of its caller)
>>> each direct subobject (base and data member) that is returned. But it is
>>> also accessing the *other* direct subobjects that it does not return, in
>>> order to destroy them. So let's say that to call std::decompose, you must
>>> have access to each direct subobject, including those that you don't
>>> request. (Plus their relocators or destructors, respectively.)
>>> Then `auto [p] = std::decompose<&PainterWithGuard::_p>(reloc
>>> painterWithGuard);` would be ill-formed because in that context,
>>> `painterWithGuard._guard` is ill-formed.
>>> Do you think this would work?
>> But if _guard were declared as public by mistake then all those safeties
>> are bypassed and std::decompose will cause trouble.
> In that case, aggregate-style structured binding would likely also work.
> And that's definitely the class author's fault.

Yes but it would not break things the way std::decompose does. (type may be
movable but not decomposable).

> This also limits the use of std::decompose to the class implementation and
>> friends. It is not necessarily a bad thing as it requires the owner to know
>> of the class internals (or at least to be in the position of knowing...).
>> Besides I expect the main use of std::decompose to be from the new special
>> get_bindings function.
> Yes, and that customization point (or some `get_all`,
> `get<index_sequence<I...>>`, etc.) would be expected to be a member or
> friend.

That could work. BTW I am more inclined to give it another name than `get`
as we could have a tuple that contains an index_sequence as parameter, and
then we don't know what std::get would refer to.

> In addition, now that I think of it, the no user-provided special ctors
>> and dtor clause is required to allow structure binding relocation from a
>> class-type. In auto [x, y] = reloc data, data must be a C array (no problem
>> there), a class-type with all data-members coming from the class-type or
>> the same base, or provide a get_binding function that applies recursively.
>> The end result of get_binding must fall into the C array case or the
>> class-type case. For the class-type case, we must be allowed to split the
>> type into individual subobjects, and have again that sort of guarantee that
>> subobjects can be considered individually. Hence this no user-provided
>> special ctors and dtor clause must apply to the class-type. This is the
>> case for whatever std::decompose returns, but it is worth mentioning. And
>> that bit makes me think that this clause should also apply to
>> std::decompose.
> Remember, the motivation is for libraries (not necessarily the Standard
> Library) to be able to write decompositions for their types; in particular,
> for tuple types. For example, Boost.Tuple should be able to support
> relocating decomposition without any change to its current ABI or API. So
> user-provided special member functions must be allowed.

Fair point.

> I can't feel satisfied with this solution. We end up with two ways of
>> decomposing: std::decompose and the get_binding function. get_binding is
>> still necessary as you can't access nested subobjects from std::decompose,
>> or even generate objects on demand when using structured bindings. And
>> other times get_bindings can be bypassed if the user performs structured
>> binding by calling std::decompose directly. I find this dual approach a bit
>> confusing.
> Well, the `get` protocol is for public use, and `std::decompose` for
> privileged access - although it can be used on aggregate types. Also it'd
> be useful for prvalue qualified member functions, etc. where previously the
> "union hack" would have been necessary, e.g.:
> template<class T>
> T* unique_ptr<T>::release(this unique_ptr self) {
> auto const [ptr] = std::decompose<&unique_ptr::ptr_>(reloc self);
> return ptr;
> }

Yes, I agree with that. I worry about mistaken uses of std::decompose for
classes where access control is badly designed.

Received on 2022-09-30 08:33:27