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Demonstrating the semantics of __weak objects in Objective-C. See http://clang.llvm.org/docs/AutomaticReferenceCounting.html#semantics for more information.
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| // The weak pointer is read, and retained until the message send completes its synchronous work. | |
| [weakValue doAThing]; |
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| - (void)doAThing { | |
| // Within here, 'self' is __strong. | |
| __unsafe_unretained selfCopy = self; | |
| dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ | |
| // 'selfCopy' may be garbage here. | |
| [selfCopy doAnotherThing]; | |
| }); | |
| } |
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Second one is obviously bad, but what about this:
__weak selfCopy = self; dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ [selfCopy doAnotherThing]; // selfCopy may point to nil here, never to garbage, right? [selfCopy doSomethingElse]; });