Objective-C Programming: The Big Nerd Ranch Guide

NSError

As you might imagine, all sorts of things can go wrong when you try to write a string to a file. For example, the user may not have write-access to the directory where the file would go. Or the directory may not exist at all. Or the filesystem may be full. For situations like these, where an operation may be impossible to complete, the method needs a way to return a description of what went wrong in addition to the boolean value for success or failure.

Recall from Chapter 10 that when you need a function to return something in addition to its return value, you can use pass-by-reference. You pass the function (or method) a reference to a variable where it can directly store or manipulate a value. The reference is the memory ...