Objective-c nil, Nil, NULL和NSNull的区别
在OC中可能经常会遇到 nil,Nil,NULL和NSNull,下面分析一下之间的区别:
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: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");
- }
这个输出:obj is nil。而NSArray是以nil结尾的。所以知道原因了吧!
(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]);
- }
- }
最新文章
- git工作区、暂存区、版本库之间的关系
- SpringMVC学习系列(9) 之 实现注解式权限验证
- http://www.cnblogs.com/xqin/p/4862849.html
- VI下删除所有内容
- Struts2_搭建环境及HelloWorld
- JavaScript 现状:方言篇
- Hadoop 安装(1) CENTOS 安装与配置
- 所谓编码--泛谈ASCII、Unicode、UTF8、UTF16、UCS-2等编码格式
- Cocos2D:塔防游戏制作之旅(十八)
- mysql 错误2002
- 开源防火墙(pfSense)的安装部署与配置
- AngularJS入门基础——表达式
- windows下,java环境变量的设置,设置点击startup.bat启动tomcat
- A request has been denied as a potential CSRF attack错误解决方法
- dp合集 广场铺砖问题&;&;硬木地板
- 20172321 2018-2019《Java软件结构与数据结构》第三周学习总结
- mcake活动维护,检查 ★ ★
- 基于iscroll实现下拉和上拉刷新
- Python函数 __import__()
- linux网络编程中的基本概念