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.