Does iOS 5 have garbage collection?

Question

Do I no longer have to worry about memory management iOS 5 onwards? Also, will all programs written for iOS 4 and earlier versions have to be rewritten to allow iOS to manage the memory for you?

Answer 1

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.

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.

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.

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.