IOS 空值 nil Nil NULL NSNull kCFNull
2017-05-12 11:16
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nil: Defines the id of a null instance,指向一个(实例)对象的空指针 例如: NSString *msg = nil; NSDate *date =nil;
Nil: Defines the id of a null class,指向一个类的空指针 例如: Class class = Nil;
NULL:定义其他类型(基本类型、C类型)的空指针 char *p = NULL;
NSNull:数组中元素的占位符,数据中的元素不能为nil(可以为空,也就是NSNull), 原因:nil 是数组的结束标志 如果用nil,就会变成 NSArray *array = [NSArray arrayWithObjects: [[NSObject alloc] init], nil, [[NSObject alloc] init], [[NSObject alloc] init], nil];, 那么数组到第二个位置就会结束。打印[array count]的话会显示1而不是5
kCFNull: NSNull的单例 CoreFoundation 中有一段对 kCFNull的定义,实际上就是 NSNull 的单例 typedef const struct CF_BRIDGED_TYPE(NSNull) __CFNull * CFNullRef; CF_EXPORT CFTypeID CFNullGetTypeID(void); CF_EXPORT const CFNullRef kCFNull; // the singleton null instance NSNull *null1 = (id)kCFNull; NSNull *null2 = [NSNull null];
Talking is cheap ,show me the code!
NSLog(@"nil is %p",nil); NSLog(@"Nil is %p",Nil); NSLog(@"Null is %p",NULL); NSLog(@"nil is %@",nil); NSLog(@"NSNULL is %@",kCFNull); nil is 0x0** Nil is 0x0** Null is 0x0** nil is (null)** NSNULL is <null>**
主要的区别就在 nil 系 和 NSNull系 的区别
nil : 作为对象的空指针和数组的结束标志
NSNull:作为数组中的空值占位符
写这篇文章我修改了好多次,随着学习的深入,有些早期的认识是错误的啊!!!
例如这段代码
Class class = nil; Class class1 = Nil; char *p =nil; char *p1 = NULL; NSString *str = NULL; NSLog(@"nil is : %d",class==nil); NSLog(@"nil is : %d",class==Nil); NSLog(@"Nil is : %d",class1==nil); NSLog(@"Nil is : %d",class1==Nil); NSLog(@"integer is nil : %d",num == nil); NSLog(@"integer is NULL : %d",num == NULL); NSLog(@"integer is Nil : %d",num == Nil); NSLog(@"nil equals Nil: %d",nil == Nil); NSLog(@"Nil equals NULL:%d",p == p1); NSLog(@"nil equals NULL: %d",nil == NULL); 结果全是1,所以这样看,本质上 nil , Nil 和 NULL 是一样的 -->聪明的你能发现上面的错误码??? 看下下面这段说明 In Objective-C, it's important that you distinguish between objects and primitive types. An **object** is always stored as a pointer, which is the object's location in memory. A pointer is just a number. With NSLog, you can use %p to see this value. You can display it in the debugger too, like this: print myObject. A pointer is displayed as a hexadecimal number, with a 0x prefix. nil is essentially location zero (0x0000). When you allocate any kind of object, you'll get a pointer which isn't zero. When you assign an object to a variable, you are simply copying the memory address, not duplicating the object. With NSLog, you can use %@ to print out an object's description . In the debugger, like this: print-object myObject . **Primitive types** like NSInteger aren't objects. Instead of storing a pointer, usually you just store the value. When you assign an NSInteger variable, you make a copy of the value. You can see the value in the debugger using print . Or like this: NSLog("%ld", (long)currentRow) . When you assign a primitive, you copy its value. Don't use %@ or print-object with primitives — they expect objects.
参考:
1.http://blog.sina.com.cn/s/blog_5fb39f910101akm1.html
2.http://xiongzenghuidegithub.github.io/blog/2014/02/23/nil-nsnull-null-kcfnull/
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