Hi WT, >> If I understand you correctly, what you're saying about your variable >> "__block SomeObjType foo" is not true. Regardless of the __block qualifier, >> foo is effectively a stack-based variable, so two simultaneously-executing >> threads have their own independent instance of that variable. There can be >> no "cross-talk" between the variables in separate threads - all you are >> doing is enabling the block that a given thread executes to have access to >> that specific thread's stack variable. > > I'm afraid you're incorrect there. According to the documentation (or, more > precisely, my understanding of the documentation), a variable qualified with > __block starts in the stack but is moved to the heap, precisely because it's > shared by all blocks capturing it and by all function scopes where it's > declared. Therefore, I do not believe "two simultaneously-executing threads > have their own independent instance of that variable" to be true.
I tested this out before replying as I wasn't 100% certain. It may be that we have misunderstood each other somehow, but the following code (in a clean new project) was what I used to confirm to myself that two concurrently-executing threads have independent instances of the variable: http://www.dur.ac.uk/j.m.taylor/block_test_code.m >> However I don't think you should need to do this anyway. I would change your >> code to something like this: >> >> - (SomeObjType) foo >> { >> dispatch_sync(queue, >> ^{ >> // Code in this block ensures bah is valid >> if (nil == bah) >> { >> // Code to compute and store bah goes here >> } >> }); >> >> return bah; >> } > > I don't see how that could possibly work as is because bah is out of scope by > the time the return statement is executed. Perhaps you meant that bah is an > ivar or property declared in the same class as the accessor. I'm suddenly > drawing a blank here because I considered this before and came to the > conclusion that it wouldn't work but now I can't remember why I thought that > way. I need to think this through a bit more. OK, well since you do not declare bah locally, and from what you are doing it is presumably meant to be persistent, I assumed that it was indeed a property (or a global?). If you are doing something else with it (though I'm not quite sure what...) then obviously what I wrote may be wrong... Jonny_______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
