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    <p>You appear to be talking about Automatic Reference Counting, mentioned in other answers. ARC is a kind of GC in that it automates memory freeing, but has a number of differences from a good garbage collector.</p> <p>Firstly, it's mainly a compiler technology. The compiler knows about Cocoa's reference-counting guidelines, so it inserts retains and releases where they should be according to the rules. It works just like if you'd written the retains and releases yourself — it simply inserts them for you. Normal garbage collectors keep track of your program's memory while it is running.</p> <p>Second, since it is just like retain and release, it can't catch retain cycles (if Object A retains Object B and Object B retains Object A, and nothing else references either of them, they both become immortal). You need to take the same precautions to prevent them.</p> <p>It also uses resources differently from an automatic garbage collector. The garbage collectors used with Objective-C have to scan for unreferenced memory and collect it — which is expensive, and can lead to "stuttering" on slower systems — but they only have to do this occasionally, and in theory can even fine-tune their collection cycles to match how a program actually uses its memory. In general, a GC program will use more memory than a non-GC program and will slow down significantly when the GC decides to collect. ARC, on the other hand, moves the "scanning" to compile-time and frees memory as soon as it's available, but it has to constantly update object reference counts instead of waiting for garbage to build up like a collector.</p>
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    1. PODoes iOS 5 have garbage collection?
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    1. COGreat stuff, thanks, @Chuck. "You need to take the same precautions to prevent them" as what? Meaning you need to avoid using "retain" properties on delegates, etc.? Also: will one declare properties as retain or assign at all? What about rewriting setters for a retain property, does the dev use `retain` then?
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      1. USDan Rosenstark
    2. CO@Yar: Yes, avoiding retains on delegates is a big one. Rather than `assign`, though, you'll want to use `weak` for unretained properties. As for using `retain` in a setter: ARC mode makes it illegal to explicitly send `retain`, `release` or `autorelease`. You just assign and the compiler will insert retains and releases where it thinks they're necessary.
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    3. COThanks @Chuck, helps a lot. So is this available with XCode or it's tied to the iOS 5 SDK? In other words, can you compile like this for iOS 4.x? I imagine not.
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