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### 2529. Assigning to `enable_shared_from_this::__weak_this` twice

Section: 20.11.6 [util.smartptr.enab] Status: Resolved Submitter: Jonathan Wakely Opened: 2015-08-26 Last modified: 2020-09-06

Priority: 3

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Discussion:

It is unclear what should happen if a pointer to an object with an `enable_shared_from_this` base is passed to two different `shared_ptr` constructors.

>     #include <memory>

>     using namespace std;

>     int main()
>     {
>       struct X : public enable_shared_from_this<X> { };
>       auto xraw = new X;
>       shared_ptr<X> xp1(xraw);  // #1
>       {
>         shared_ptr<X> xp2(xraw, [](void*) { });  // #2
>       }
>       xraw->shared_from_this();  // #3
>     }

This is similar to LWG 2179, but involves no undefined behaviour due to the no-op deleter, and the question is not whether the second `shared_ptr` should share ownership with the first, but which `shared_ptr` shares ownership with the `enable_shared_from_this::__weak_this` member.

With all three of the major `std::shared_ptr` implementations the `xp2` constructor modifies the `__weak_this` member so the last line of the program throws `bad_weak_ptr`, even though all the requirements on the `shared_from_this()` function are met (20.11.6 [util.smartptr.enab])/7:

> Requires: `enable_shared_from_this<T>` shall be an accessible base class of `T`. `*this` shall be a subobject of an object `t` of type `T`. There shall be at least one `shared_ptr` instance `p` that owns `&t`.

Boost doesn't update `__weak_this`, leaving it sharing with `xp1`, so the program doesn't throw. That change was made to `boost::enable_shared_from_this` because someone reported exactly this issue as a bug, see Boost issue 2584.

On the reflector Peter Dimov explained that there are real-world use cases that rely on the Boost behaviour, and none which rely on the behaviour of the current `std::shared_ptr` implementations. We should specify the behaviour of `enable_shared_from_this` more precisely, and resolve this issue one way or another.

[2016-03-16, Alisdair comments]

This issues should be closed as `Resolved` by paper p0033r1 at Jacksonville.

Proposed resolution: