"Peter Dimov" <pdimov@mmltd.net> wrote in message news:00e901c6f613$06ed8a00$6607a8c0@pdimov2...
Chris Thomasson wrote:
You have two pointers, global and local; what levels of thread safety do these offer?
ptr::global = strong
Is global the atomic pointer and local the "as safe as an int" equivalent? Or is local to be kept local to a thread?
ptr::local should only have one 1 thread loading or storing into it at any one time; local does not have atomic pointer swaps. It makes use of impl_base::swap_naked(...). No reason to have atomic pointer swaps for a ptr::local. However, the count updates are still atomic.
So ptr::local = basic.
Yup.
- You can contain ptr::local<foo> in a shared collection that is protected by a mutex. If the collection was not protected by a mutex then it would have to contain ptr::global<foo> instead.
I don't see how this can work; the collection itself needs to be atomic, not its elements (it could demand element atomicity, I guess, but it still needs to be atomic on top of that)...
Are you referring to the case in which a collection was not protected by a mutex? If so, I was basically referring to some sort of lock-free, or mostly lock-free collection. You can use ptr::global in a lock-free collection, it isn't very practical because of the spinlock, but it is compatible. Its was just a simple example to show some of the algorithms flexibility...
Having two separate pointer types is a legitimate option but (as you might've deduced by now) I've been exploring having only one and exposing the 'strong thread safety' as separate member functions. The reason I haven't added copy/replace to shared_ptr is that I'm not sure whether it won't be better to generalize the idea even further and implement something along the lines of
atomic_cell< shared_ptr<X> > px;
which would be useful for adding atomicity on top of any (thread-safe as int) type, not just shared_ptr.
What do you think of that?
Interesting...
Is a "mostly lock-free" atomic_cell possible?
Probably, but it might be more expensive that a more direct solution... Humm... Need to put my thinking cap back on...