Value types versus reference types

As a refresher, let's quickly cover the difference between value and reference types. Value types hold values and copy content on assignment or when they are passed to methods as a parameter.

let avg1: Double = 98.1 
var avg2 = avg1 
avg2 += 1.2   // -> 99.3 

In the preceding example, avg2 copies the value of avg1, and is free to change its value without affecting the value of avg1. Reference types, on the other hand, share content by default. Changing one of the variables will change the underlying value that each reference shares.

 
let dateComponents1 = NSDateComponents() 
dateComponents1.setValue(10, forComponent: .Day) 
dateComponents1.day // => 10 
var dateComponents2 = dateComponents1 
dateComponents2.setValue(2, forComponent: .Day) 
dateComponents1.day // => 2 
dateComponents2.day // => 2 

In this example, we use the reference type NSDateComponents to create dateComponents1 and set...