blocks|key|271765|text|nil和NULL实际上都等于零，所以在实践中它们是可以互换的。但是您说得对，为了保持一致性，TouchJSON的文档应该使用theDeserializer.nullObject+=+nil而不是NULL。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|271766|现在，当您这样做时，您的第二个谓词实际上工作得很好：|271767|if+(![jsonDataDict+objectForKey:someKey])|code-block|syntax|javascript|271768|因为当您将TouchJSON设置为nil+(或NULL)时，nullObject将省略字典中的键。当字典中不存在该键时，NSDictionary返回nil，nil为零，因此if条件按预期工作。|271769|如果不将nullObject指定为nil，则可以像这样检查null：|271770|if+([jsonDataDict+objectForKey:someKey]+==+[NSNull+null])|271771|entityMap^0|0|3|4|4|1Q|W|2P|4|0|0|0|H|3|N|4|U|A|0|4|A|H|3|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|15|8|@$9|16|A|17|B|C]|$9|18|A|19|B|C]|$9|1A|A|1B|B|C]]|D|@]|E|$]]|$1|O|3|P|5|6|7|1C|8|@$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|J|7|1H|8|@]|D|@]|E|$K|L]]|$1|S|3|-4|5|6|7|1I|8|@]|D|@]|E|$]]]|T|$]]

<code>nil</code> and <code>NULL</code> are actually both equal to zero, so they are, in practice, interchangeable. But you're right, for consistency, the documentation for TouchJSON should have used <code>theDeserializer.nullObject = nil</code> instead of <code>NULL</code>.

Now, when you do that, your second predicate actually works fine:

<pre><code>if (![jsonDataDict objectForKey:someKey])
</code></pre>

because TouchJSON omits the key from the dictionary when you have <code>nullObject</code> set to <code>nil</code> (or <code>NULL</code>). When the key doesn't exist in the dictionary, NSDictionary returns nil, which is zero so your if condition works as you expect.

If you don't specify <code>nullObject</code> as <code>nil</code>, you can instead check for null like so:

<pre><code>if ([jsonDataDict objectForKey:someKey] == [NSNull null])
</code></pre>

blocks|key|3186521|text|对于另一个JSON库，但有相同的问题，我在NSDictionary上创建了以下类别：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3186522|@implementation+NSDictionary+(Utility)

//+in+case+of+[NSNull+null]+values+a+nil+is+returned+...
-+(id)objectForKeyNotNull:(id)key+{
+++id+object+=+[self+objectForKey:key];
+++if+(object+==+[NSNull+null])
++++++return+nil;

+++return+object;
}

@end|code-block|syntax|javascript|3186523|每当我处理上述库中的JSON数据时，我都会像这样检索值：|3186524|NSString+*someString+=+[jsonDictionary+objectForKeyNotNull:@"SomeString"];|3186525|通过这种方式，我的项目中的代码变得更加整洁，同时我不必考虑处理NSNull、空值等问题。|3186526|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

For another JSON library, but with the same issues, I've created the following category on NSDictionary:

<pre><code>@implementation NSDictionary (Utility)

// in case of [NSNull null] values a nil is returned ...
- (id)objectForKeyNotNull:(id)key {
 id object = [self objectForKey:key];
 if (object == [NSNull null])
 return nil;

 return object;
}

@end
</code></pre>

Whenever I deal with JSON data from said library, I retrieve values like this:

<pre><code>NSString *someString = [jsonDictionary objectForKeyNotNull:@"SomeString"];
</code></pre>

This way the code in my projects become a lot cleaner and at the same time I don't have to think about dealing with [NSNull null] values and the like.

blocks|key|3186559|text|有一些库可以处理它。其中一个是Swift中的SwiftyJSON，另一个是Objective-C中的NSTEasyJSON。有了这个库(NSTEasyJSON)，就可以很容易地处理这些问题。在您的情况下，您可以只检查您需要的值：|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|3186560|NSTEasyJSON+*JSON+=+[NSTEasyJSON+withData:JSONData];
NSString+*someValue+=+JSON[someKey].string;|code-block|syntax|javascript|3186561|这个值将是NSString或nil，您不应该检查它是否为NSNull，否则您自己为空。|CODE|3186562|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/SwiftyJSON/SwiftyJSON|1|https://github.com/bernikowich/NSTEasyJSON^0|F|5|11|B|M|A|0|1E|B|1|0|0|5|8|E|3|0^^$0|@$1|2|3|4|5|6|7|X|8|@$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@$9|12|A|13|1|14]|$9|15|A|16|1|17]]|E|$]]|$1|F|3|G|5|H|7|18|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|19|8|@$9|1A|A|1B|B|M]|$9|1C|A|1D|B|M]]|D|@]|E|$]]|$1|N|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]]|O|$P|$5|Q|R|S|E|$T|U]]|V|$5|Q|R|S|E|$T|W]]]]

There are libraries which deal with it. One of them is <a href="https://github.com/SwiftyJSON/SwiftyJSON" rel="nofollow noreferrer">SwiftyJSON</a> in Swift, another one is <a href="https://github.com/bernikowich/NSTEasyJSON" rel="nofollow noreferrer">NSTEasyJSON</a> in Objective-C.
With this library (NSTEasyJSON) it will be easy to deal with such problems. In your case you can just check values you need:

<pre><code>NSTEasyJSON *JSON = [NSTEasyJSON withData:JSONData];
NSString *someValue = JSON[someKey].string;
</code></pre>

This value will be <code>NSString</code> or <code>nil</code> and you should not check it for NSNull, NULL yourself.

Hi
I am using TouchJSON to deserialize some JSON. I have been using it in the past and on those occasions I dealt with occurrences of NSNull manually. I would think the author had to deal with this as well, so me doing that again would just be overhead. I then found this in the documentation:

<blockquote>
 Avoiding NSNull values in output.
</blockquote>

<pre><code>NSData *theJSONData = /* some JSON data */
CJSONDeserializer *theDeserializer = [CJSONDeserializer deserializer];
theDeserializer.nullObject = NULL;
NSError *theError = nil;
id theObject = [theDeserializer deserialize:theJSONData error:&amp;theError];}
</code></pre>

The way I understand it the user of the class can pass a C-style null pointer to the deserializer and when it encounters a NSNull it will insert the values (NULL) passed to it. So later on when I use the values I won't get NSNull, but NULL.

This seems strange, the return value is an NSDictionary which can only contain Objects, shouldn't the value default to 'nil' instead?

If it is NULL can I check the values like this?

<pre><code>if([jsonDataDict objectForKey:someKey] == NULL)
</code></pre>

It would seem more logically to be able to do this:

<pre><code>if(![jsonDataDict objectForKey:someKey])
</code></pre>

No to mention all the cases where passing nil is allowed but passing NULL causes a crash.

Or can I just pass 'nil' to the deserializer?

Much of this stems from me still struggling with nil, NULL, [NSNULL null], maybe I am failing to see the potential caveats in using nil.

TouchJSON, dealing with NSNull

Objective-C

Swift

JSON

您好，我正在使用TouchJSON来反序列化一些JSON。我过去一直在使用它，在这些情况下，我手动处理了NSNull的出现。我认为作者也必须处理这个问题，所以我再做一次只会增加开销。然后，我在文档中找到了以下内容：避免输出中的NSNull值。NSData *theJSONData = /* some JSON data...

问TouchJSON，处理NSNull
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