What's the difference between [NSNull null] and nil?

问题

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?

回答1:

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.

回答2:

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.

回答3:

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.

回答4:

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]

回答5:

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

https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/NumbersandValues/Articles/Null.html