Start by picturing a traditional Objective-C source code file written by an expert Cocoa programmer. The retain, release, and autorelease messages are sent in all the right places and are in perfect balance.

Now, imagine editing the source code file, removing every instance of the retain, release, and autorelease messages, and changing a single build setting in Xcode that instructs the compiler to put all the suitable memory management calls back into your program when the source code is compiled. That's ARC. It's just what the name suggests—traditional Cocoa reference counting, being automatically done.

At its core, ARC is not a runtime service; it doesn't work on program execution, as Garbage collection does. On the other hand, the new Clang, the compiler frontend for C, C++, Objective-C, and Objective-C++, provides it as a two-part phase (we will call these phases "cycles"). In the following diagram, you can see these two phases. At the cycle...