I’m fairly new to Objective-C, and am currently reading up on memory management. I’d like to use ARC for our shared libraries, but in the Advanced Memory Management documentation there’s a slightly cryptic warning about doing so:
If you plan on writing code for iOS, you must use explicit memory management (the subject of this guide). Further, if you plan on writing library routines, plug-ins, or shared code—code that might be loaded into either a garbage-collection or non-garbage-collection process—you want to write your code using the memory-management techniques described throughout this guide.
I assume the first sentence is out of date as ARC appears to be supported on iOS 4 and later, but the second sentence appears to imply that for shared libraries we should be using MRR rather than ARC.
I’m not clear why it would be a problem to do so, as I would have thought that once compiled there should be no difference to calling code, as the reference counting calls would have been inserted by the compiler. Is there really a problem with using ARC for shared libraries?
That quoted section is awkwardly phrased and was probably not changed from when that guide merely differentiated between manual retain-release and garbage collection.
iOS does not and probably never will support garbage collection. As you noted, though, iOS 4 completely supports ARC. Additionally, ARC is designed such that there are no real issues mixing ARC code with non-ARC code (as opposed to garbage-collected code, which has no such luxury). ARC is the recommended way to write all new code going forward, including shared library code, as long as you don’t need to target versions of iOS before iOS 4.
In a sense, ARC is explicit memory management, merely not manual memory management. Like I said, though, that phrasing is awkward and written before ARC existed.