Objective-c nil, Nil, NULL和NSNull的区别
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一、nil:对象为空
定义某一实例对象为空值。例如:
- NSObject* obj = nil;
- if (nil == obj) {
- NSLog(@"obj is nil");
- }
- else {
- NSLog(@"obj is not nil");
- }
二、Nil:类为空
定义某一类为空。例如:
- Class someClass = Nil;
- Class anotherClass = [NSString class];
三、NULL:基本数据对象指针为空
用于c语言的各种数据类型的指针为空。例如:
- intint *pointerToInt = NULL;
- charchar *pointerToChar = NULL;
- struct TreeNode *rootNode = NULL;
四、NSNull
集合对象无法包含 nil 作为其具体值,如NSArray、NSSet和NSDictionary。相应地,nil 值用一个特定的对象 NSNull 来表示。NSNull 提供了一个单一实例用于表示对象属性中的的nil值。
- @interface NSNull : NSObject <NSCopying, NSSecureCoding>
- + (NSNull *)null;
- @end
在NSNull单例类中,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、说明:
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
下面附带几个有趣的例子:
(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 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 nil];
- NSLog(@"arr2 count: %ld", [arr2 count]); //arr2 count: 0
为啥第一个数组元素有三个,而第二个数组元素为0.先看看:
- NSObject* obj;
- if (nil == obj) {
- NSLog(@"obj is nil");
- }
- else {
- NSLog(@"obj is not nil");
- }
(2)
- //有异常!
- 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]);
- }
- }
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c nil, Nil, NULL和NSNull的区别
- Objective-c nil, Nil, NULL和NSNull的区别
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- Objective-c nil, Nil, NULL和NSNull的区别
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