Testing for null in objective-c

Question

I have the following code in a loop

   NSArray * images = [definitionDict objectForKey:@"Images"];
    NSLog(@"images in definitionDict %@", images);
    if (!images )
        NSLog(@"NULL");
    else
        NSLog(@"NOTNULL");

which gives the following outputs

images in definitionDict (
    "/some/brol/brol.jpg"
)
NOTNULL
images in definitionDict <null>
NOTNULL

I do not understand the second case, where the images array is null. Why is this not detected correctly in my test ? How can I debug such a problem ?

Answer 1

<null> is not nil. nil will print (null) when printed. What you have is an NSNull. NSNull IS an object, it just doesn't respond to much. Its available as a placeholder for you to use.

To test for NSNull you can use if ([images isEqual:[NSNull null]])

See the docs for more info on NSNull

Answer 2

If you want to print out the memory address of an Objective-C object, or any other pointer, you should use the flag %p not %@. The flag %@, expects a string.

However if the argument isn't a string, NSLog will automatically call -description on the passed object. And when the method returns an NSNullobject, the -descriptionon that object returns the string <null>

NSObject *o = nil;
NSLog(@"%p", o);

Output: 0x00000000

NSObject *o = [[NSObject alloc] init];
NSLog(@"%p", o);
[o release];

Output: something like 0x12345678

Mind:

NSNull *n = [NSNull null];
NSLog(@"%p", n);

Output: a memory address that always will be the same, but will differ from 0x00000000

The correct way to test if their are objects in the array is like this.

NSArray *myArray = [someObject array];
if([myArray isEqual:[NSNull null]]) {
    NSLog(@"No objects");
} else {
    NSLog(@"%d objects.", (int)[myArray length];
}