What is ARC and how does it work?
If you remember, the idea of reference counting covers the actual deletion of objects from the memory. With reference counting, Objective-C takes care of the actual object destruction. Owner objects are only responsible for releasing their claim of ownership on the object. So, logically the idea that appeared next was to make everything completely automatic as it was done in languages such as Java and C#. This idea was developed in the Garbage collection branch and Automatic Reference Counting.
Garbage collection is only available for Mac OS X, starting with version 10.5. Also, note that iOS applications can't use Garbage collection; since it relies on the power of the device, it will take some time to process, forcing the user to wait the process end, thus producing a bad user experience. It is also deprecated since OS X Version 10.8 is in favor of ARC and is scheduled to be removed in the forthcoming versions of OS X.
ARC is a new and innovative way that contains many of the Garbage collection's advantages, yet different from Garbage collection. ARC does not have any process in the background to make the object's deallocation, which gives ARC a big advantage against Garbage collection when comparing their performance.
However, before explaining how ARC does this, it's important to understand what ARC does not do:
- ARC does not impose a runtime memory model as Garbage collection does. Code compiled under ARC uses the same memory model as plain C or non-ARC Objective-C code, and can be linked to the same libraries.
- ARC only makes automatic memory management possible for Objective-C objects, inherited from
NSObject(note that in Objective-C, blocks also happen to be objects under the covers though). - Memory allocated in any other way is not touched and must still be managed manually. The same goes for other resources such as file handles and sockets, such as streams.