Objective-C Programming: The Big Nerd Ranch Guide

Sending messages to nil

Nearly all object-oriented languages have the idea of nil, the pointer to no object. In Objective-C, we use nil instead of NULL, which was discussed in Chapter 9. They really are the same thing: the zero pointer. By convention, though, we use nil when referring to the value of an empty pointer declared as pointing to an Objective-C object type, and NULL when referring to any other pointer, such as to a struct.

In most object-oriented languages, sending a message to nil is not allowed. As a result, you have to check for non-nil-ness before accessing an object. So you see this sort of thing a lot:

i​f​ ​(​f​i​d​o​ ​!​=​ ​n​i​l​)​ ​{​
 ​ ​ ​ ​[​f​i​d​o​ ​g​o​G​e​t​T​h​e​N​e​w​s​p​a​p​e​r​]​;​
}​

When Objective-C ...