NSNull, Nil, nil, NULL区别（OC）
直接给个表吧
| Symbol | Value | Meaning |
|---|---|---|
| NULL | (void *)0 | literal null value for C pointers |
| nil | (id)0 | literal null value for Objective-C objects |
| Nil | (Class)0 | literal null value for Objective-C classes |
| NSNull | [NSNull null] | singleton object used to represent null |
