The difference between nil, nil, null and nsnull in OBJECTIVE-C

Nil: A null pointer to an Objective-c object.
(#define NIL ((ID) 0))

Nil represents a Objective-c object with a pointer to an empty

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

The first capital of nil and nil is a bit different, nil defines a pointer to an empty class, which is class, not an object.

#define NIL 0

null: A null pointer to anything else, was 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 are 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 was expecting an object. If They see Nil, they know the receiver is expecting a class. Readability.

If obj is nil, [obj message] would return NO, without nsexception
If obj is NSNull, [obj message would 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:

//There is an exception!
         NSObject *obj1 = [NSNull null];
         NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1, @"three" ,nil];
         For (NSString *str in arr1) {
             NSLog(@"array object: %@", [str lowercaseString]);
         }

         //modify
         NSObject *obj1 = [NSNull null];
         NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1, @"three" ,nil];
         For (NSString *str in arr1) {
             If (![str isEqual:[NSNull null]]){
                 NSLog(@"array object: %@", [str lowercaseString]);
             }
         } 

The difference between nil, nil, null and nsnull in OBJECTIVE-C