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

我遇到了这样的错误 -

I got an error like this -

由于未捕获的异常NSInvalidArgumentException"而终止应用程序，原因:-[NSCFDictionaryinitWithObjects:forKeys:count:]: 尝试在objects[0] (key: searched)'

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

然后我尝试了下面的代码

Then I tried the below code

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"];

我收到了 NSLog 消息，因为Obj 为零".

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

但是，当我在 Obj 是 NSString 并且我的应用程序因 nil 值而终止时使用上述相同的代码时，我收到了 NSLog 消息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] 有什么区别?

是否有不同类型的 Null 值?如果是，如何共同检查这些空值?

谢谢:-)

推荐答案

NULL 是原始 C 风格的值，它被赋予一个指针，该指针被认为没有指向明确的地址.它通常仅用于表示未设置"或作为哨兵值.

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 是适用于 Obj-C 对象引用的同一事物的 Objective-C 版本:

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，虽然您可以互换使用它们，但前者主要在 Obj-C 中用于传统指针(例如设置 void * 指向 NULL 的上下文指针)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.

之所以存在差异，是因为虽然 Obj-C 的语法恰好位于 C 之上，但该语言的设计是更高级别的，并且它的对象 ID 在概念上与指针"不同.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 旨在用于非常特定的情况(例如插入到集合中)，其中需要非值"值，但实际 nil 是不允许的，必须使用一个对象.

[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 字符串化非常相似:nil 字符串化为 (null) 和 NSNull 为 .

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

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