Swift Programming: The Big Nerd Ranch Guide

Failable Initializers

Sometimes it is useful to define a type whose initialization can fail. In these cases, you need a way to report to the caller that you were not able to initialize the instance. You use failable initializers to handle these scenarios.

There are a number of reasons why you might want initialization to fail. A type’s initializer may be given invalid parameters. For example, you might want to have an initializer fail if someone tries to initialize an instance of Town with a negative population. Or perhaps a type’s initialization depends on an external resource that is not available, as in let image = UIImage(named: "nonexisting-image"). This code would fail to create a UIImage instance because the image resource does ...