Actually, the iPhone Obj-C runtime has undergone very little changes since the launch of the iPhone. Apple had already debuted Obj-C 2.0 (which was the most sig

Actually, the iPhone Obj-C runtime has undergone very little changes since the launch of the iPhone. Apple had already debuted Obj-C 2.0 (which was the most significant change) in OS X 10.5 , before the iPhone launch. Apple's clang/llvm transition was also already underway, but this had more to do with building more integrated developer tools and not having to deal with gcc, not as much the runtime. ARC, perhaps the other big new thing you were thinking of, is again just a developer tool that was enabled by the transition to clang. Fundamentally, it is simply the compiler now being smart enough to inject retains/releases automatically when compiling your code instead of you doing it manually. Again, the impact of this is for developers, not end user (runtime) experience.