Objective-c nil, Nil, NULL和NSNull的區別

在OC中可能經常會遇到 nil，Nil，NULL和NSNull，下面分析一下之間的區別：

Symbol	Value	Meaning
NULL	(void *)0	literal null value for C pointers
nil	(id)0	literal null value for Objective-C objects
Nil	(Class)0	literal null value for Objective-C classes
NSNull	[NSNull null]	singleton object used to represent null

一、nil：物件為空

定義某一例項物件為空值。例如：

NSObject* obj = nil;
        if (nil == obj) {
            NSLog(@"obj is nil");
        }
        else {
            NSLog(@"obj is not nil");
        }

二、Nil：類為空

定義某一類為空。例如：

Class someClass = Nil;
        Class anotherClass = [NSString class];

三、NULL：基本資料物件指標為空

用於c語言的各種資料型別的指標為空。例如：

int *pointerToInt = NULL; 
char *pointerToChar = NULL; 
struct TreeNode *rootNode = NULL;

四、NSNull

集合物件無法包含 nil 作為其具體值，如NSArray、NSSet和NSDictionary。相應地，nil 值用一個特定的物件 NSNull 來表示。NSNull 提供了一個單一例項用於表示物件屬性中的的nil值。

@interface NSNull : NSObject <NSCopying, NSSecureCoding>

+ (NSNull *)null;

@end

在NSNull單例類中，提供了唯一的方法null：Returns the singleton instance of NSNull.

例如：

 NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionary];
        mutableDictionary[@"someKey"] = [NSNull null]; // Sets value of NSNull singleton for `someKey`
        NSLog(@"Keys: %@", [mutableDictionary allKeys]); // @[@"someKey"]

五、說明：

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.

六、注

下面附帶幾個有趣的例子：

（1）

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

為啥第一個陣列元素有三個，而第二個陣列元素為0.先看看：

NSObject* obj;
        if (nil == obj) {
            NSLog(@"obj is nil");
        }
        else {
            NSLog(@"obj is not nil");
        }

這個輸出：obj is nil。而NSArray是以nil結尾的。所以知道原因了吧！

（2）

//有異常！
        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 isEqual:[NSNull null]]){
                NSLog(@"array object: %@", [str lowercaseString]);
            }
        }