Objectivec nil, Nil, NULL和NSNull

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]);
            }
        }