Talbot, George wrote:
I thought that the current implementation of shared_ptr is lock-free and multithreaded in the sense that I can pass shared_ptr between threads, and the reference count is updated correctly, etc. (I'm using BOOST 1.33.) Am I incorrect in thinking this?
This is not the same as atomic. An atomic type allows you to modify a variable from thread A while, at the same time, thread B is also reading or writing it. shared_ptr doesn't offer this level of thread-safety. As an example, consider: shared_ptr<X> px; // thread A shared_ptr<X> px2( px ); // thread B px = px3; (you can also have thread B doing compare and swap, it doesn't matter.) Imagine that thread A reads the two pointers from px and is preempted right after that. Thread B overwrites the contents of px, and in doing so, causes its reference count to drop to zero; the object and the control block are destroyed. Thread A wakes up and attempts to increment the now non-existent reference count. Bad things happen. But maybe I'm misunderstanding the intent of your compare and swap primitive.