Alexander Terekhov <terekhov@web.de> wrote:
Ben Hutchings wrote: [...]
If atomic_read returns 0 that means the use count has dropped to 0 [*] and can never increase again, so it must be the latest version. Otherwise weak_ptr makes a second test that is properly protected. So it can never use an old value.
[*] I think this is right but I'm not certain that it can't return a 0 that the processor read during creation of an object that uses enable_shared_from_this.
What makes you uncertain?
Simply not having thought enough about it. Now that I have done, I realise that it's impossible for a thread to use a weak_ptr to an object without either (1) constructing a shared_ptr to it, setting the use count to be non-zero, or (2) synchronising with another thread that constructs the shared_ptr and/or weak_ptr, which should provide an appropriate memory barrier that will prevent the use of any invalid pre-fetched value of the memory containing the use count.
Having said all that, I feel the naming of functions may give a false sense of generality and encapsulation when they actually only work in the way they are being used currently.
You mean that
bool expired() { return lock(); }
is better?
Not really. I will expand in my mail to Peter Dimov.