nil、Nil、NULL、NSNull、[NSNull null] in Objective-C

很多初學者對nil，Nil，NULL，NSNull，[NSNull null]這五個概念很混淆!先借用網上的解釋：

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

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

NULL: A null pointer to anything else. (primitive type or absence of data)
 

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

[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;
nil和NULL從字面意思來理解比較簡單，nil是一個對象，而NULL是一個值，我的理解為nil是將對象設定為空白，而NULL是將基本類型設定為空白的。而且我們對於nil調用方法，不會產生crash或者拋出異常。
看一下用法
NSURL *url = nil；
Class  class = Nil；
int *pointerInt = NULL；
nil是一個對象指標為空白，Nil是一個類指標為空白，NULL是基礎資料型別 (Elementary Data Type)為空白。

 這五個概念很容易混淆，在一些場合下他們具有同樣的用法。首先解釋一下“null 指標”意思，null 指標是指該指標未指向一塊有意義的記憶體區。如int *p; int *p = NULL;

        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");        }

        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]);    //count: 3 因為obj=nil，在加入obj4時就結束添加對象                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]);   //count: 0，因為obj1=nil，所以後面的對象沒有添加進去

[NSNull null]通常作為預留位置作用，如下：

        NSObject *obj1 = [NSNull null];        NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1,@"three",nil];        for (NSString *str in arr1) {            NSLog(@"array object: %@", str);        } //result：One、Two、<NULL>、three        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);            }        }//result：One、Two、three