Re: [boost] Usefulness of weak_ptr

From: Peter Dimov (pdimov_at_[hidden])
Date: 2002-02-09 06:27:16

From: "Joe Gottman" <joegottman_at_[hidden]>
>     I can see this, but there seem to be a few dangers.  For instance, it
> would be very dangerous to keep a set of weak_ptr's.  If the underlying
> object of a weak_ptr were deleted, then the weak_ptr's get() suddenly
> returns a 0.  This will almost certainly mess up the set's invariant that
> its members are stored in some sorted order.

Well spotted, thanks. I'll fix it.

>    Also, I think weak_ptr is inherently thread-unsafe.

Yes, you are right. Keeping a weak_ptr in thread A while all shared_ptr
owners are in thread B is asking for trouble. This is an inherent property
of weak_ptr (and any non-owning pointer in general, starting with raw
pointers) and cannot be fixed.

> For instance, its
> get() function is implemented as follows:
>
>     T * get() const // never throws
>     {
>         return use_count() == 0? 0: px;
>     }

No matter how you implement get(), it will still have the same problem.

weak_ptr is just a tiny bit above a raw pointer in the food chain.