nil，NULL和[NSNULL nil]之间有什么区别？ [英] What are the differences between nil, NULL and [NSNULL nil]?

问题描述

我收到这样的错误 - *由于未捕获的异常'NSInvalidArgumentException'终止应用程序，原因：** - [NSCFDictionary initWithObjects：forKeys：count：]：尝试在对象[0]处插入nil值（ key：搜索）'

I got an error like this - * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: **-[NSCFDictionary initWithObjects:forKeys:count:]: attempt to insert nil value at objects[0] (key: searched)'

然后我尝试了以下代码

    if (Obj == (id)[NSNull null])  //Obj is an id 
{
    NSLog(@" Obj is (id)[NSNull null]");    
}
else if (Obj == nil)
{
    NSLog(@"Obj is nil");   
} 
else if (Obj == NULL)
{
    NSLog(@"Obj is NULL");  
} 
else
{
    NSLog(@"Obj is something else");
}

searchedDict = [NSDictionary dictionaryWithObject:Obj forKey:@"searched"];

我得到的NSLog消息为'Obj is nil'。

And I got the NSLog message as 'Obj is nil'.

但当我使用相同的上述代码时，Obj是NSString并且我的应用程序由于nil值而被终止，我得到的NSLog消息为'Obj is（id）[ NSNull null]'。

But when I used the same above code when Obj was NSString and my app got terminated due to nil value, I got the NSLog message as 'Obj is (id)[NSNull null]'.

nil，NULL和[NSNULL nil]之间有什么区别？

是否有不同类型的Null值？如果是，如何通常检查这些空值？

谢谢： - ）

推荐答案

NULL 是指向指针的原始C样式值，该指针被认为指向没有显式地址。它通常仅用于表示未设置或哨兵价值。

NULL is the original C-style value given to a pointer which is deemed to point to no explicit address. It's generally just used to mean "unset" or as a sentinel value.

char * oldstring = NULL;

nil 是Objective-C版本的与应用于Obj-C对象引用相同的事情：

nil is the Objective-C version of the same thing as applied to Obj-C object references:

NSString * str = nil;
id foo = nil;

它们都被定义为等于零值，虽然你可以可以互换使用它们，前者主要在Obj-C中用于传统指针（比如将 void * 上下文指针设置为NULL）和 nil 用于所有Obj-C对象引用。

They are both defined to be equal to the value zero, and although you could use them interchangeably, the former is seen mainly within Obj-C for traditional pointers (like setting void * context pointers to NULL) and nil is used for all Obj-C object references.

存在差异是因为尽管Obj-C的语法恰好位于C的顶部，但设计该语言的级别较高，其对象ID为 aren'概念上相同的只是指针。因此为他们创建了一个特殊的哨兵。

The difference exists because although Obj-C's syntax happens to ride on top of C's, the design of the language is higher-level, and its object IDs aren't conceptually the same as just "pointers". Hence the creation of a special sentinel for them.

[NSNull null] 是一个对象旨在用作在某些情况下（如插入集合），需要非值值，但不允许实际的 nil ，并且必须使用对象。

[NSNull null] is an object that is intended to be uses in very certain cases (like inserting into collections) where an "non-value" value is desired, but actual nil is not allowed, and an object must be used.

请注意 nil 和 NSNull stringify非常相似： nil 字符串为（null）， NSNull 为< null> 。

Note that nil and NSNull stringify very similarly: nil stringifies as (null), and NSNull as <null>.

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