We are using shared_ptr extensively in our application, and seem to be having some threading issues. In particular, we have an Envelope class that is used to hold messages received over the network. We use Spirit to parse the message string and construct the Envelope object, then we pass is on to the EnvelopeController, which holds a thread pool to handle all of the Envelopes. These Envelopes are passed around as shared_ptr<Envelope> between these different components. In essense, we have one thread that is receiving the message from the network stack, parsing it, and passing it to the thread_pool, which is then using one (or more) additional threads to actually deal with the contents of the envelope. On a Uni-processor box, everything works great. All of our envelopes are being destroyed properly. On a multi-processor machine, none of the envelopes are getting destroyed. We are in a situation where two shared_ptrs to the same underlying object may go out of scope and need to decrement the ref count at the same time. Is the in the realm of "undefined behavior?" Am I using the shared_ptr incorrectly? Thanks for any advice. Thread 1 --------- { shared_ptr<Envelope> envelope(new Envelope(...)); ... [pass envelope to Thread 2] ... [envelope goes out of scope] } Thread 2 -------- void process(shared_ptr<Envelope> envelope) { [do work on envelope] [envelope goes out of scope] }