Question

I have the following code:

NSString *content = [[NSUserDefaults standardUserDefaults] stringForKey:@"mykey"];
NSLog(@"string is %@",content);
if ([content stringIsEmpty]){
   NSLog(@"empty string");
}else{
  NSLog(@"string is not empty");
}

stringIsEmpty is class category on NSString:

- (BOOL ) stringIsEmpty {
    if ((NSNull *) self == [NSNull null]) {
        return YES;
    }
    if (self == nil) {
        return YES;
    } else if ([self length] == 0) {
        return YES;
    }
    return NO;
}

The output is:

string is (null)
string is not empty

How could it be null and not empty at the same time?

Answer 1

What happens is that:

[content stringIsEmpty:YES]

will return false (NO), when content is nil. So your code will take the

 NSLog(@"string is not empty");

branch. This would be better:

if (content && [content stringIsEmpty:YES]){
    ...

A better way of doing this would be reversing the semantics of the method:

if ([content stringIsNotEmpty]) {

this would work finely because when content is nil it would return NO, when it is not nil, it would execute your method.

EDIT:

In Objective-C, sending a message to nil is legal and by definition will evaluate to nil. Google for “objective c sending message to nil”.

In another language (C++), your code would crash (actually undefined behaviour, but to make things simple).