Alexander Terekhov wrote:
John Torjo wrote: [...]
Whenever a thread (other than main) wants to access this window, it will query the weak_pointer. The weak_pointer needs to know the LATEST reference count in order to know if the weak pointer is still valid. Thus, atomic_read that simply returns the value is not enough.
No barriers needed for its lock() call (and shared_ptr-from-weak_ptr ctor). It can't see 'false' zero unless you violate the "basic" thread- safety contract. Value consistency is ensured by conditional increment
which is?
using 'naked' (w/o any barriers) CAS (LL/LR-SC aside for a moment) IFF the observed 'old' value is not zero.
(after about 3 hours of reading different threads on google) I still think I'm right ;) Here's my scenario: (remember - we're talking about: http://www.pdimov.com/cpp/shared_count_x86_exp2.hpp) class test { ... }; [thread1] shared_ptr<test> p1( new test); // refcount = 1 ---------------- thread switch [thread2] shared_ptr<test> p2(p1); // refcount = 2 weak_ptr<test> weak(p2); p2 = shared_ptr<test>(); // refcount = 1 ------------------thread switch [thread1] p1 = shared_ptr<test>(); // refcount = 0 - object gets destroyed ---------------- thread switch [thread2] // here - you need InterlockedCompareExchange // to realize that refcount is 0 (zero) shared_ptr<test> p3 = weak.lock(); Also, see my other post to Peter Dimov. Best, John -- John Torjo -- john@torjo.com Contributing editor, C/C++ Users Journal -- "Win32 GUI Generics" -- generics & GUI do mix, after all -- http://www.torjo.com/win32gui/ -- v1.4 - save_dlg - true binding of your data to UI controls! + easily add validation rules (win32gui/examples/smart_dlg)