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Subject: CWG710 and races during object construction
From: Giuseppe D'Angelo (giuseppe.dangelo_at_[hidden])
Date: 2021-02-11 11:56:07


Hi,

CWG710 deals with races during object construction (and similarly
destruction):

> http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#710

The concern raised was about races on the virtual table itself, for
instance by placing calls to virtual functions during an object's
construction or destruction from another thread than the one
constructing/destroying the object.

The resolution for it added this paragraph to basic.life:

> https://eel.is/c++draft/basic.life#11

> In this subclause, “before” and “after” refer to the “happens before” relation ([intro.multithread]).
> [Note: Therefore, undefined behavior results if an object that is being constructed in one thread is referenced from a different thread without adequate synchronization. —end note]

It's not however clear to me how this resolution makes behaviour
undefined in something like:

> struct B {
> B() {
> std::thread t([&]() { typeid(*this); });
> #if DETACH
> t.detach(); // 1
> #else
> t.join(); // 2
> #endif
> }
> virtual ~B();
> };
>
> struct D : B {
> ~D();
> };
>
> D d;

Is 1) UB, and 2) well formed (yielding B's type info)? "In practice"
they are, but I'm just failing to find the right wording in the Standard
that would express this.

As far as I can see, this isn't a data race: the vtable and all the
related RTTI information are just magical "metadata" [1] and thus
doesn't participate in data races (which by definition involve memory
locations [2][3], and memory locations are objects or bitfields [4], not
the "metadata").

[1] https://eel.is/c++draft/basic.memobj#intro.object-1.sentence-9
[2] https://eel.is/c++draft/intro.multithread#intro.races-21.sentence-2
[3] https://eel.is/c++draft/intro.multithread#intro.races-2.sentence-1
[4] https://eel.is/c++draft/intro.memory#3.sentence-1

I planned to raise a language defect, but then I stumbled into CWG710
and thus wanted to ask for some guidance. What am I missing?

Thanks,

-- 
Giuseppe D'Angelo | giuseppe.dangelo_at_[hidden] | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
KDAB - The Qt, C++ and OpenGL Experts



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