Yes it is thread-safe, that's the whole point of the system:

shared_ptr is implemented (in boost or std implementations) with an atomic counter used to sync the moment when the object need to be destroyed.

Once the object is destroyed, all wp.lock() calls will return null shared_ptr. Just lock the shared_ptr from any thread (by copy) and check if it's null or not, 

or use wp.expired() if you don't want to aquire the object but just check if it's still alive.

It is also good to know that there is some kind of synchronization on shared_ptr copy/construction that might 

in some cases, not be wanted for single-threaded applications where the cost isn't acceptable.

Joel Lamotte