Re: [boost] Weak pointer to object not managed by a shared_ptr

Subject: Re: [boost] Weak pointer to object not managed by a shared_ptr
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-09-03 12:27:51

Mateusz Loskot wrote:
>
> simple testes using GCC 4.3 and Visual C++ 9.0 show that destruction
> occurs in different point than we expect:

Perhaps it is different than you expected.

> 1) I added verbose destructor to X
>
> ~X() { cout << "X dtor\n"; }
>
> 2) Compiled and executed the following use case of X:
>
> shared_ptr<X> spx;
> {
>     X x1(7);
>     spx = shared_ptr<X>(x1.get_weak_ptr());
> } // *** x1 is being destructed
> cout << spx->get_i() << std::endl;
>
> mloskot_at_dog:~$ g++ -Wall -pedantic weak_ptr_local.cpp
> mloskot_at_dog:~$ ./a.out
> X dtor
> 7
> deleter
> mloskot_at_dog:~$
>
> So, x1 is destructed but its value of 7 is still accessible.

x1 went out of scope.  A shared_ptr referencing its address does nothing to extend it's lifetime.

> I assume I'm observing UB here. I understand nature of UB and that
> it can give unexpectedly 'correct' results.

As Overmind showed, the stack has yet to be overwritten by that point, so the bits exist.  What is undefined is accessing what used to be an object after its destructor ran.  That you can see the value is due to X::~X() not overwriting it and of the data member type not having a destructor that likewise does something destructive.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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