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]);
}
}