objective-c - What's the difference between (null) vs ?

Question

I was getting random crashes in my app until I narrowed it down to a particular method. In that method I expect an NSString as a parameter. This NSString can sometimes be nil in which case the method ends and no harm is done.

When I run my method's parameter through NSLog(@"%@", myString) I found that I get one of these:

1. The contents of an actual NSString
2. (null)
3. <null>

The first two are expected and handled by my method. The third one, <null>, crashes my app with -[NSNull length]: unrecognized selector sent to instance 0x1b2ace8.

I have found a way around the problem by checking for nil or isKindOfClass, but my question is what is the difference between (null) and <null>?.

Answer 1

(null)是当您使用带值NSLog()的格式说明符时打印的字符串。是发送到单例（您可以通过 访问）的结果。%@nil<null>descriptionNSNull[NSNull null]

NSNullNSArray在 Cocoa 集合 (和)中用作“无对象”占位符，NSDictionary因为它们不能包含nil.

这两个描述字符串令人困惑地相似，有人可能会争辩说NSNull应该针对它提交一个错误以使其更加清晰。