nil: A null pointer to an Objective-C object.

( #define nil ((id)0)  )

Nil: A null pointer to an Objective-C class.

NULL: A null pointer to anything else,  is for C-style memory pointers.

( #define NULL ((void *)0)  )

NSNull: A class defines a singleton object used to represent null values in collection objects (which don't allow nil values).

[NSNull null]: The singleton instance of NSNull.

 

Technically they're all the same,,, but in practice they give someone reading your code some hints about what's going on; just like naming classes with a capital letter and instances with lowercase is recommended, but not required.

If someone sees you passing NULL, they know the receiver expects a C pointer. If they see nil, they know the receiver is expecting an object. If they see Nil, they know the receiver is expecting a class. Readability.

 

if obj is nil , [obj message] will return NO, without NSException

if obj is NSNull , [obj message will throw a NSException

Demo1:

[NSApp beginSheet:sheet              modalForWindow:mainWindow
              modalDelegate:nil       //pointing to an object               didEndSelector:NULL     //pointing to a non object/class               contextInfo:NULL];      //pointing to a non object/class

Demo2:

NSObject *obj1;        if (obj1 != nil) {            NSLog(@"object is not nil");        }else        {            NSLog(@"object is nil");        }                testClass *c1;        if (c1 != Nil) {            NSLog(@"class is not Nil");        }else        {            NSLog(@"class is Nil");        }                int *money;        if (money != NULL) {            NSLog(@"money is not NULL");        }else        {            NSLog(@"money is NULL");        }

Demo3:

NSObject *obj1 = [[NSObject alloc] init];        NSObject *obj2 = [NSNull null];        NSObject *obj3 = [NSObject new];        NSObject *obj4;        NSArray *arr1 = [NSArray arrayWithObjects:obj1, obj2, obj3, obj4, nil];        NSLog(@"arr1 count: %ld", [arr1 count]);    //arr1 count: 3        NSObject *obj1;        NSObject *obj2 = [[NSObject alloc] init];        NSObject *obj3 = [NSNull null];        NSObject *obj4 = [NSObject new];        NSArray *arr2 = [NSArray arrayWithObjects:obj1, obj2, obj3, obj4, nil];        NSLog(@"arr2 count: %ld", [arr2 count]);   //arr2 count: 0

Demo4:

//有异常！        NSObject *obj1 = [NSNull null];        NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1, @"three" ,nil];        for (NSString *str in arr1) {            NSLog(@"array object: %@", [str lowercaseString]);        }        //修改        NSObject *obj1 = [NSNull null];        NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1, @"three" ,nil];        for (NSString *str in arr1) {            if (str != [NSNull null]){                NSLog(@"array object: %@", [str lowercaseString]);            }        }