Question

I have an NSMutableArray that was downloaded via FTP. The elements in the array are CFFTPStream resource constants that are of type CFStringRef.

I would like to create a String from the “kCFFTPResourceName” constant. However being new to Objective C and iphone development I am struggling.

Everything that I have done has resulted in ARC throwing a fit or errors like:

2013-01-03 15:31:44.874 Street Light Reporter[1382:11603] -[__NSCFDictionary objectAtIndex:]: unrecognized selector sent to instance 0x6e1e930
2013-01-03 15:31:44.875 Street Light Reporter[1382:11603] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFDictionary objectAtIndex:]: unrecognized selector sent to instance 0x6e1e930'

My most recent attempt is:
CFStringRef *c = [ar objectAtIndex:4];
which doesnt work for the following two reasons:

Incompatible pointer types initializing 'CFStringRef *' (aka 'const struct __CFString **') with an expression of type 'id'

AND

Implicit conversion of an Objective-C pointer to 'CFStringRef *' (aka 'const struct __CFString **') is disallowed with ARC

I have tried all sorts of typecasting and messing around with (__bridge) and whatnot and I have had no luck.

Can anybody help me out here? Any help will be appreciated.

Answer 1

You have two errors here: the first and the most severe one is that your ar object is a NSDictionary not a NSArray. That’s why performing

CFStringRef *c = [ar objectAtIndex:4];

you are getting a NSInvalidArgumentException. objectAtIndex: is a method of NSArray that you are sending to a NSdictionary instance.

The second error is the cast. As Fernando already pointed out, you need to cast it using the __bridge keyword like follows.

CFStringRef c = (__bridge CFStringRef)([ar objectAtIndex:4]);

so that ARC will know that you are now treating that object as a C pointer.

Note also that CFStringRef is defined as

typedef const struct __CFString * CFStringRef;

so it’s already a pointer and you have to get rid of the *.