What's the difference between [NSNull null] and nil?
Objective CCocoa TouchCocoaNullNsnullObjective C Problem Overview
Here's a context where I have seen that:
NSMutableArray *controllers = [[NSMutableArray alloc] init];
for (unsigned i = 0; i < kNumberOfPages; i++) {
[controllers addObject:[NSNull null]];
}
why not nil in that place?
Objective C Solutions
Solution 1 - Objective C
Directly from Apple: >The NSNull class defines a singleton object you use to represent null values in situations where nil is prohibited as a value (typically in a collection object such as an array or a dictionary).
So in your example, that's exactly what's happening, the programmer is choosing to put a null object into the controllers array, where nil is not allowed as a value.
Solution 2 - Objective C
You cannot add a nil value to an NSArray or NSMutableArray. If you need to store a nil value, you need to use the NSNull wrapper class, as shown in that snippet you have. This is specified in the documentation.
Solution 3 - Objective C
We all agree that [NSNull null] is useful as a placeholder where an object is required, as elaborated above. But unless it's explicitly used in assignment for your object, it should not be used in comparison, a mistake I have made in the past.
id a;
NSLog(@"Case 1");
if (a == nil) NSLog(@"a == nil");
if (a == Nil) NSLog(@"a == Nil");
if ([a isEqual:[NSNull null]]) NSLog(@"a isEqual:[NSNull null]");
NSLog(@"Case 2");
a = [NSNull null];
if (a == nil) NSLog(@"a == nil");
if (a == Nil) NSLog(@"a == Nil");
if ([a isEqual:[NSNull null]]) NSLog(@"a isEqual:[NSNull null]");
Output:
2014-01-31 10:57:11.179 MCDocsApp[13266:a0b] Case 1
2014-01-31 10:57:11.179 MCDocsApp[13266:a0b] a == nil
2014-01-31 10:57:11.179 MCDocsApp[13266:a0b] a == Nil
2014-01-31 10:57:11.180 MCDocsApp[13266:a0b] Case 2
2014-01-31 10:57:11.180 MCDocsApp[13266:a0b] a isEqual:[NSNull null]
Solution 4 - Objective C
Collection classes like NSArray and NSDictionary cannot contain nil values. NSNULL was created specifically as a placeholder for nil. It can be put into collection classes, and only takes up space.
NSNull defines a singleton object, which means that there's only ever a single instance of NSNull (which you create using [NSNull null]), but it can be used in as many places as you wish.
Solution 5 - Objective C
The NSNull class defines a singleton object you use to represent null values in situations where nil is prohibited as a value (typically in a collection object such as an array or a dictionary).
You cannot add a nil value to an NSArray or NSMutableArray. If you need to store a nil value, you need to use the NSNull wrapper class.
Collection classes like NSArray and NSDictionary cannot contain nil values. NSNULL was created specifically as a placeholder for nil. It can be put into collection classes, and only takes up space.
See the link for reference
Solution 6 - Objective C
nil marks the end of an array after an array of objects...