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

Question

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?

Answer 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.

Answer 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.