Checking if a Cocoa object is empty is a little harder then in other languages, say C++, (but easier in some ways). Because every object in Objective-C is actually a pointer to an object, there are two ways, obj, can be empty.
obj = {}
obj points to an object that is empty. Say an array with 0 items, or the string "", etc..
obj = nil
obj, the pointer obj, is NULL, nil, 0, or whatever you want to call it. You might argue that obj isn’t really an object, but it is empty, because there’s nothing in it.
Bug:
When I first started writing Objective-C, I made the mistake of writing code like: if([name isEqualToString:@""]){ ... }, to test for empty strings. And this code would work for a while until I used it in a situation where name was nil, and then, because sending any method called on nil “returns” NO, I would have mysterious errors. (Worse then a crash, because it’s harder to track down.)
Bug:
It’s tempting to avoid the previous bug, by explicitly testing for nil and {}. Say with code like:
if (email == nil || ![email isEqualTo:@""] ) email = @"An email address is required";
But generally this is a bad idea. It means more code, which means more places for a bug. I know it’s only one trivial test, but I’m serious, when I say it’s asking for a bug — like the bug in the example above, which sets email to @"An email address is required", whenever it is not the empty string, rather then when it is empty. (Values have been changed tho protect the innocent but it’s a bug I’ve seen.)
Solutions:
Wil Shipley suggests using the global function:
static inline BOOL IsEmpty(id thing) {
return thing == nil
|| ([thing respondsToSelector:@selector(length)]
&& [(NSData *)thing length] == 0)
|| ([thing respondsToSelector:@selector(count)]
&& [(NSArray *)thing count] == 0);
}
I’ve been using his IsEmpty() for about a year. I’ve had zero problems with it, while it’s made my code more readable and concise.
Another solution is to take advantage of what happens when you send a message to nil. (To over-simplify, you get back 0 or NO.) So you can just say “if ([obj count] == 0) then obj is empty.” This often means reversing your thinking, and testing “IsNotEmpty()” instead of “IsEmpty()”. I don’t think it’s as clear is IsEmpty() in general, but in cases where it is, there you have it.
C Comment Trivia
― Vincent Gable on December 8, 2008
//I'll bet \
you \
didn't \
know \
you \
could \
comment \
like \
this \
in \
C/C++/Objective-C
Well, you can. Finding out was a lot of fun, believe me.
At some point, I’m not sure when, I unknowingly put a ‘\’ at the end of a block of
//comments. That commented out the method-call the block was referring to. As you might guess from the fact that it had a big comment talking about why it was there, this was an important method call. I spent more time then I want to admit debugging and staring at my code trying to see the problem. Finally I noticed “hey, the color of that code is like a comment, even though it’s not in a comment”.