Re: [boost] boost::shared_ptr<>::compare_and_swap() -- Am Iinsaneforwanting this?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-11-10 09:53:21

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.