Objective-C 中不同类型的“空值”

以 NSDictionary 的 objectForKey 方法为例，如果字典中不存在指定的 key，返回值为 nil，如果指定的 key 存在，但是值为空，返回值则是 [NSNull null]。

There are three possible interpretations of "null" NSString:

someStringPtr == nil
(id)someStringPtr == [NSNull null]
someStringPtr.length == 0
If you may have the possibility of all 3, the 3rd check subsumes the first, but I don't know of a simple check for all three.

In general, JSON will return [NSNull null] for a null JSON value, but some kits may return @"" (length == 0) instead. nil will never be used in iOS since it can't be placed in arrays/dictionaries.

被这个问题坑了很久。