Class instances, both in Swift and in Objective-C, are reference types (see “Value Types and Reference Types”). Behind the scenes, Swift and Objective-C memory management for reference types works essentially the same way. Such memory management, as I pointed out in Chapter 5, can be a tricky business.
Fortunately, Swift uses ARC (automatic reference counting), so you don’t have to manage the memory for every reference type object explicitly and individually, as was once necessary in Objective-C. Thanks to ARC, you are far less likely to make a memory management mistake, and more of your time is liberated to concentrate on what your app actually does instead of dealing with memory management concerns.
Still, even in Swift, even with ARC, it is possible to make a memory management mistake, or to be caught unawares by Cocoa’s memory management behavior. A memory management mistake can lead to runaway excessive memory usage, crashes, or mysterious misbehavior of your app. Cocoa memory management can be surprising in individual cases, and you need to understand, and prepare for, what Cocoa is going to do.
The reason why reference type memory must be managed at all is that references to reference type objects are merely pointers. The real object pointed to occupies a hunk of memory that must be explicitly set aside when the object is brought into existence and that must be explicitly freed up when the object goes ...