type casting in objective-c, (NSInteger) VS. integerValue

Question

I don't very understand about the diference between (NSInteger)aNumberValue and [aNumberValue integerValue], then do some test. For example, here is a response data from server:

You can see it's an int but the value is hold by NSNumber. I retrieve the data by writting NSInteger count = (NSInteger) dic[@"count"];, and in Xcode debug area, saw this:

it's really a strange value but when I run po count and saw this:

anyway, the value is correct, but another strange thing is:

the number 2 is not less than 100!

Then I try NSInteger a = [dic[@"count"] integerValue] and saw the normal value in Xcode debug area:

and:

So, I am a little bit confused, what's the deference between (NSInteger)aNumberValue and [aNumberValue integerValue]?

Answer 1

NSNumber is a class; NSInteger is just a typedef of long, which is a primitive type.

dic[@"count"] is a pointer, which means that dic[@"count"] holds an address that points to the NSNumber instance. NSNumber has a method called integerValue which returns an NSInteger as the underlying value that the NSNumber instance represents. So you can conclude that [dic[@"count"] integerValue] gets you a long, and that's how you retrieve the value out of NSNumber.

You don't retrieve the value of NSNumber by casting it to NSInteger. That's because dic[@"count"], as I said, is a pointer. So by writing

NSInteger count = (NSInteger) dic[@"count"];

you are actually casting the pointer itself to an NSInteger, which has nothing to do with the actual represented value. The value 402008592 you see is just a decimal representation of the value of the pointer, which is an address.

The command po is used for printing objects, so lldb will actually try to print out the object at the address of count. That's why you get 2 back using po. You can try p count and you'll get 402008592.

About po count < 100: The expression (count < 100) is evaluated first; since count is really just an NSInteger of 402008592, it will evaluate to false.