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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]);
            }
        }