cast a string value in NSDictionary to long make app crash

2
votes

I have a NSDictionary object get from server API. data is like this,

"filesize_mp3":"60488"

when I use following code to retrieve file size, app crashes. obj.filesize is in long type.

obj.filesize = [[dict objectForKey:@"filesize_mp3"] longValue],

lldb console,

2014-07-22 12:50:14.643 YouVoiceNews[2019:60b] -[__NSCFString longValue]: unrecognized selector sent to instance 0x9bf8e10
2014-07-22 12:50:14.644 YouVoiceNews[2019:60b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString longValue]: unrecognized selector sent to instance 0x9bf8e10'

but when I use 

obj.filesize = (long)[dict objectForKey:@"filesize_mp3"];

everything is fine.

so my question is what's the differences between above 2 ways. why first one cause app crash?

Thanks in advance.

1
iosobjective-cnsstring
Read the error. NSString doesn't have a method named longValue. See the docs for NSString. – rmaddy
one doesn't work with warning and one doesn't work without warning. – Bryan Chen
Your second line of code does not do what you think it does. It is completely wrong despite the lack of a crash. – rmaddy
Thanks a lot. Xcode's intelligent sense shows a longValue method. I should pay more attention on doc. change to longLongValue or integerValue is fine. – LetBulletFlies

1 Answers

7
votes

Answer to question 1: what's the differences between above 2 ways?

  1. The first approach attempts to send the message -longValue to the object stored at key @"filesize_mp3"
  2. The second approach attempts to cast the pointer to the object stored at key @"filesize_mp3"

Answer to question 2: why does the first one cause the app to crash?

  1. The object stored at key @"filesize_mp3" is a NSString, and -longValue is an invalid selector. The reason the code compiled in the first place is because [dict objectForKey:@"filesize_mp3"] is not typed (it's an id), and for convenience id will accept any selector at compilation time. A slightly more conservative approach is provided below ; casting the object to a NSString will not allow [filesizeStr longValue].
  2. The second approach does not crash because you are simply casting a pointer into a long. While it does not give you anything of interest, it is a valid operation.

Answer to the question you did not ask: What should I have done?

Your dictionary contains NSString, not NSNumber nor int or long. A dictionary is a collection ofNSObject. Thus you must convert these strings into int, long or whatever format you see fit.

NSString * filesizeStr = [dict objectForKey:@"filesize_mp3"];
obj.filesize = [filesizeStr longLongValue];