According to the Swift language guide Strings, Arrays, and Dictionaries are all copied on assignment to a variable. So wouldn't the author's primary assumption (that let a and var b refer to the same array) be false? And this whole issue is null?

https://developer.apple.com/library/prerelease/mac/documenta...

Assignment and Copy Behavior for Strings, Arrays, and Dictionaries

Swift’s String, Array, and Dictionary types are implemented as structures. This means that strings, arrays, and dictionaries are copied when they are assigned to a new constant or variable, or when they are passed to a function or method.

This behavior is different from NSString, NSArray, and NSDictionary in Foundation, which are implemented as classes, not structures. NSString, NSArray, and NSDictionary instances are always assigned and passed around as a reference to an existing instance, rather than as a copy.

NOTE The description above refers to the “copying” of strings, arrays, and dictionaries. The behavior you see in your code will always be as if a copy took place. However, Swift only performs an actual copy behind the scenes when it is absolutely necessary to do so. Swift manages all value copying to ensure optimal performance, and you should not avoid assignment to try to preempt this optimization.