Objective-c 中 nil, Nil, NULL和NSNull的区别
2014-02-13 11:59
423 查看
nil: A null pointer to an Objective-C object.
( #define nil ((id)0) )
Nil: A null pointer to an Objective-C class.
NULL: A null pointer to anything else, is for C-style memory pointers.
( #define NULL ((void *)0) )
NSNull: A class defines a singleton object used to represent null values in collection objects (which don't allow nil values).
[NSNull null]: The singleton instance of NSNull.
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.
if obj is nil , [obj message] will return NO, without NSException
if obj is NSNull , [obj message will throw a NSException
Demo1:
Demo2:
Demo3:
Demo4:
( #define nil ((id)0) )
Nil: A null pointer to an Objective-C class.
NULL: A null pointer to anything else, is for C-style memory pointers.
( #define NULL ((void *)0) )
NSNull: A class defines a singleton object used to represent null values in collection objects (which don't allow nil values).
[NSNull null]: The singleton instance of NSNull.
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.
if obj is nil , [obj message] will return NO, without NSException
if obj is NSNull , [obj message will throw a NSException
Demo1:
[NSApp beginSheet:sheet modalForWindow:mainWindow modalDelegate:nil //pointing to an object didEndSelector:NULL //pointing to a non object/class contextInfo:NULL]; //pointing to a non object/class
Demo2:
NSObject *obj1; if (obj1 != nil) { NSLog(@"object is not nil"); }else { NSLog(@"object is nil"); } testClass *c1; if (c1 != Nil) { NSLog(@"class is not Nil"); }else { NSLog(@"class is Nil"); } int *money; if (money != NULL) { NSLog(@"money is not NULL"); }else { NSLog(@"money is NULL"); }
Demo3:
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
Demo4:
//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]); } }
相关文章推荐
- 源码推荐(1月20日):加载解析RSS的Objective-C框架 Pininterest 弹出菜单的方式
- java JSON使用之JSONObject、JSONArray与Object的转换
- Objective-c 中 nil, Nil, NULL和NSNull的区别
- FileSystemObject对象及常用方法
- Kobject相关
- ie6中 object doesn’t support this property or method
- Objective-C相关Category的收集,实用工具
- 源码推荐(2月12日):高效低内存音频播放引擎 基于NSXMLParser的Objective-C XML阅读器
- 【Objective-C】OC中对象归档(序列化)的基本概念和用法
- 【Objective-C】OC中对象归档(序列化)的基本概念和用法
- IOS json转对象 /json 转 Object / ios json to Object / ios json convert Object
- deleted object would be re-saved by cascade
- Object-C中的类-类的声明
- Object-C中的类-类的声明
- Object-C中的字符串对象1-不可变字符串
- Object-C中的字符串对象1-不可变字符串
- Object-C中的数字对象
- Object-C中的数字对象
- 1、Objective-C的@property属性和@synthesize属性
- Qt中 QObject类干了什么事情