Re: [Boost-users] [shared_ptr] Is reference counting thread safe in boost 1.38

Subject: Re: [Boost-users] [shared_ptr] Is reference counting thread safe in boost 1.38
From: Thomas Klimpel (Thomas.Klimpel_at_[hidden])
Date: 2009-02-21 07:09:23

> I am not able to figure out if the following case is handled:
>
> 1. Thread 1 has a shared_ptr and hence the reference is 1

Why do you conclude that the reference is 1? If both thread 1 and thread 2 have access to the underlying object, the reference should be at least 2.

> 2. Thread 1 is in the process of decrementing the reference and thread 2 is in the process of incrementing the the reference
> 3. If thread 1 succeeds in decrementing, the reference will become 0 and hence 'destroy' will get called.

So it looks like thread 1 gave thread 2 a reference to its shared_ptr instead of a copy of its shared_ptr. Later on, it "invalidated" the reference, by letting the shared_ptr go out of scope. So the "thread safety" problem seems to be in the code that uses shared_ptr, not in shared_ptr.

> 4. Can thread 2 increment after when thread 1 is in a call to destroy?

No, it can't. But this is not a bug of shared_ptr, as explained above.