Re: [Boost-users] [smart_ptr] Getting segfaults with weak_ptr<T>::lock()

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-07-10 20:12:31

Buddha Buck wrote:
...
> My solution was:
>
> template <typename T>
> class wxObserver : public Observer {
>  private:
>    T* p;
>    wxObserver();
>  public:
>    wxObserver(T* p) : p(p);
>    void update(shared_ptr<Observed> o) { if (p) p->update(o); }
> }
>
> class MyWidget : public wxWidget {
>  private:
>    shared_ptr<wxObserver<MyWidget> > observer;
>  public:
>    MyWidget(...) : wxWidget(...), observer(this) { ...
> observed->register(observer); ... }
>    ...
> }
>
> The idea is that when a MyWidget is created, it creates a shared
> pointer to a wxObserver and can register that wxObserver with observed
> objects without a problem.  When wxWidgets chooses to delete the
> MyWidget, the shared pointer the MyWidget goes away, the wxObserver is
> deleted, and the weak pointers held by the observed objects expire.
> The implementation of Observed::notifyObservers() checks to make sure
> that the observers still exist before update()ing them, and only the
> live observers are updated.
>
> The problem is that sometimes, after wxWidgets destroys a MyWidget,
> calls to Observed::notifyObservers() crashes with a segfault within
> the call to i->lock().

I think I see one problem in your code, but it cannot explain a segfault 
within i->lock(). When MyWidget is destroyed, if at the exact same time an 
Observed locks its weak_ptr to the wxObserver, the T* p pointer will dangle.

Can you post a stack trace? The problem you describe sounds like a race 
between notifyObservers and ~Observed or register, is that possible?