> On May 2, 2016, at 11:30 AM, Steve Mills <sjmi...@mac.com> wrote: > > On May 02, 2016, at 12:58 PM, John McCall <rjmcc...@apple.com> wrote: > > > You don't have to declare something with __block just to use it in a block. > > __block specifically means that the variable will be captured *by > > reference* in the block, meaning that changes to the variable within the > > block are visible in the original scope and vice-versa. Without this, the > > block literal will capture the current value of the variable, and you will > > not be able to assign to it within the block. > > Ah yes, good point. My __block decoration on rezes might've been done in an > earlier iteration of the code, where it really *was* allocated and assigned > inside the block. > > > When blocks capture a value of ObjC pointer type like that, they retain it. > > That's not always desired, either for efficiency reasons or because it > > introduces retain cycles, and so __block is sometimes used in non-ARC code > > as a (very inefficient) way to prevent that retain. That doesn't work in > > ARC, because assigning to a __strong __block variable retains the new > > value, just like assigning to any other __strong variable. So the migrator > > tries to recognize __block variables that are being used that way (i.e. > > __block variables that are never modified) and instead declare them as > > __unsafe_unretained. > > So is the compiler/migrator recognizing here that rezes get added to the root > object, then later used in various blocks (there are more than one, but I > didn't show them) before finally going out of scope, so it's just being more > efficient, rather than just letting its retain count get incremented when > it's added to root, then decremented when rezes goes out of scope?
No. It's just recognizing that putting __block on rezes is otherwise pointless because you never assign to it, so it guesses that you might be trying to suppress a retain. Now, maybe that's giving the programmer too much credit. :) But I think it's more reasonable to add __unsafe_unretained than to silently do nothing at the risk of introducing a retain cycle — for one, it's preserving the behavior of your code, and for another, it's much easier for programmers to statically see the explicit __unsafe_unretained and try to figure out if it's reasonable than for them to dynamically track down yet another retain cycle. Migrating to ARC is definitely *not* fire-and-forget! You really do need to look over the results and see whether they make sense. John. _______________________________________________ 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: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
