nil, Nil, NULL, NSNull

Middle PRO

Four kinds of "nothing"

The first three are null pointers that differ only in type and usage convention; the fourth is a real object.

• nil — (id)0, the null pointer to an object. Messaging nil is safe and returns zero.
• Nil — (Class)0, the null pointer to a class (Class cls = Nil).
• NULL — ((void *)0), the null C pointer: char *, int *, blocks in C APIs, an NSError ** argument (when you do not need the error). Numerically all three are zero and the compiler does not distinguish them, but mixing them is bad style.
• NSNull — the singleton [NSNull null], a placeholder object meaning "value absent" for collections: NSArray/NSDictionary cannot hold nil (inserting it crashes with NSInvalidArgumentException), while NSNull is an ordinary object you can store and compare by pointer.

#import <Foundation/Foundation.h>

static void demo(void) {
    id obj = nil;
    Class cls = Nil;
    char *cstr = NULL;
    NSArray *row = @[@"Ann", [NSNull null], @42];
    for (id value in row) {
        if (value == [NSNull null]) {   // singleton — pointer comparison
            NSLog(@"missing");
        }
    }
    NSLog(@"%@ %@ %s", obj, cls, cstr ? cstr : "");
}

Trap: [NSNull null] is a live object, and [[NSNull null] length] crashes with "unrecognized selector" rather than returning 0 as it would with nil. So values from JSON must be checked for NSNull, not nil.