What is the right way to check for a null string in Objective-C?

Question

I was using this in my iPhone app

if (title == nil) {
    // do something
}

but it throws some exception, and the console shows that the title is "(null)".

So I'm using this now:

if (title == nil || [title isKindOfClass:[NSNull class]]) {
    //do something
}

What is the difference, and what is the best way to determine whether a string is null?

Answer 1

If that kind of thing does not already exist, you can make an NSString category:

@interface NSString (TrucBiduleChoseAdditions)

- (BOOL)isEmpty;

@end

@implementation NSString (TrucBiduleChoseAdditions)

- (BOOL)isEmpty {
    return self == nil || [@"" isEqualToString:self];
}

@end

Answer 2

Refer to the following related articles on this site:

• http://stackoverflow.com/questions/535120/is-if-variable-the-same-as-if-variable-nil-in-objective-c h

I think your error is related to something else as you shouldn't need to do the extra checking.

Also see this related question: http://stackoverflow.com/questions/598396/proper-checking-of-nil-sqlite-text-column

Answer 3

If you want to test against all nil/empty objects (like empty strings or empty arrays/sets) you can use the following:

static inline BOOL IsEmpty(id object) {
    return object == nil
        || ([object respondsToSelector:@selector(length)]
        && [(NSData *) object length] == 0)
        || ([object respondsToSelector:@selector(count)]
        && [(NSArray *) object count] == 0);
}

Answer 4

it is just as simple as

if([object length] >0)
{
  // do something
}

remember that in objective C if object is null it returns o as the value.

This will get you both a null string and a 0 length string.

Answer 5

As others have pointed out, there are many kinds of "null" under Cocoa/Objective C. But one further thing to note is that [title isKindOfClass:[NSNull class]] is pointlessly complex since [NSNull null] is documented to be a singleton so you can just check for pointer equality. See Topics for Cocoa: Using Null.

So a good test might be:

if (title == [NSNull null] || title.length == 0 ) title = @"Something";

Note how you can use the fact that even if title is nil, title.length will return 0/nil/false, ie 0 in this case, so you do not have to special case it. This is something that people who are new to Objective C have trouble getting used to, especially coming form other languages where messages/method calls to nil crash.

Answer 6

After trying all the stuff mentioned above below solution worked for me

[text isEqualToString:@"(null)"]

Wondering if there is a full proof method