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 using 'naked' (w/o any barriers) CAS (LL/LR-SC aside for a moment) IFF the observed 'old' value is not zero. Also, < Forward Inline > -------- Original Message -------- Message-ID: <415446A0.F2BD31CF@web.de> Newsgroups: comp.lang.c++.moderated Subject: Re: Vindicated? Sutter and COW Strings References: ... <slrncl63k0.pk.ben-public-nospam@decadentplace.org.uk> Ben Hutchings wrote: [...]
The odd thing is that there is no obvious way to read with an acquire or any other kind of memory barrier. It is possible to do this using InterlockedCompareExchange with the exchange and comparand set to the same value, preferably an unlikely one to avoid invalidating cache lines. (John Torjo suggested this on the Boost mailing list.) This provides a full memory barrier; recent versions of Windows have variants of the function that provide a acquire or release barrier.
Take a closer look at MS docu. See also: http://google.com/groups?threadm=XIRDc.192766%24Ly.83841%40attbi_s01 (Subject: Re: DCSI - thread safe singleton) I mean followups too. regards, alexander.