Nearly all object-oriented languages have the idea of nil, the pointer to no object. In Objective-C, nil is the zero pointer (same as NULL, which was discussed in Chapter 8).

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 was designed, it was decided that sending a message to nil would be OK; it just wouldn’t do anything at all. Thus, this code is completely legal:

D​o​g​ ​*​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​]​;​

Important thing #1: If you are sending messages and nothing is happening, make sure you aren’t sending messages to a pointer that has been set to nil.

Important thing #2: If you send a message to nil, the return value doesn’t mean anything.

D​o​g​ ​*​f​i​d​o​ ​=​ ​n​i​l​;​
N​e​w​s​p​a​p​e​r​ ​*​d​a​i​l​y​ ​=​ ​[​f​i​d​o​ ​g​o​G​e​t​T​h​e​N​e​w​s​p​a​p​e​r​]​;​

In this case, daily will be zero. (In general, if you expect a number or a pointer as a result, sending a message to nil will return zero. However, for other types like structs, you will get strange and unexpected return values.)