How does the new automatic reference counting mechanism work?
Can someone briefly explain to me how ARC works? I know it's different from Garbage Collection, but I was just wondering exactly how it worked.
Also, if ARC does what GC does without hindering performance, then why does Java use GC? Why doesn't it use ARC as well?
Answer
Every new developer who comes to Objective-C has to learn the rigid rules of when to retain, release, and autorelease objects. These rules even specify naming conventions that imply the retain count of objects returned from methods. Memory management in Objective-C becomes second nature once you take these rules to heart and apply them consistently, but even the most experienced Cocoa developers slip up from time to time.
With the Clang Static Analyzer, the LLVM developers realized that these rules were reliable enough that they could build a tool to point out memory leaks and overreleases within the paths that your code takes.
Automatic reference counting (ARC) is the next logical step. If the compiler can recognize where you should be retaining and releasing objects, why not have it insert that code for you? Rigid, repetitive tasks are what compilers and their brethren are great at. Humans forget things and make mistakes, but computers are much more consistent.
However, this doesn't completely free you from worrying about memory management on these platforms. I describe the primary issue to watch out for (retain cycles) in my answer here, which may require a little thought on your part to mark weak pointers. However, that's minor when compared to what you're gaining in ARC.
When compared to manual memory management and garbage collection, ARC gives you the best of both worlds by cutting out the need to write retain / release code, yet not having the halting and sawtooth memory profiles seen in a garbage collected environment. About the only advantages garbage collection has over this are its ability to deal with retain cycles and the fact that atomic property assignments are inexpensive (as discussed here). I know I'm replacing all of my existing Mac GC code with ARC implementations.
As to whether this could be extended to other languages, it seems geared around the reference counting system in Objective-C. It might be difficult to apply this to Java or other languages, but I don't know enough about the low-level compiler details to make a definitive statement there. Given that Apple is the one pushing this effort in LLVM, Objective-C will come first unless another party commits significant resources of their own to this.
The unveiling of this shocked developers at WWDC, so people weren't aware that something like this could be done. It may appear on other platforms over time, but for now it's exclusive to LLVM and Objective-C.