Question

I use JSON for implementing Facebook in an app and I’m just making my code ARC-friendly. However, when I make lines such as this one

CFStringAppendCharacters((CFMutableStringRef)json, &uc, 1);

become

CFStringAppendCharacters((__bridge CFMutableStringRef)json, &uc, 1);

my app is no longer able to pull my photo albums (I allow the user to log into Facebook and I then display his albums in order for him/her to get a picture to later use in the app).

This is the entire code that is not appreciated by ARC – (could anyone give me a hint how to bridge it please?)

NSString* escaped_value = (NSString *)CFURLCreateStringByAddingPercentEscapes(
                              NULL, /* allocator */
                              (CFStringRef)value,
                              NULL, /* charactersToLeaveUnescaped */
                              (CFStringRef)@"!*'();:@&=+$,/?%#[]",
                              kCFStringEncodingUTF8);

Does anyone know how I could port the JSON framework for ARC use?

Answer 1

I see in your comment that you just decided to go with NSJSONSerialization, that will definitely work. But to actually answer your question.

Using the __bridge cast is easy when you think about the memory management involved. __bridge just casts without doing any memory management operations for you; __bridge_transfer also casts, but it decrements the retain count of the object being casted. So with that in mind, your function call can be broken down like so:

CFStringRef originalValue = (__bridge CFStringRef)value;// Only __bridge required because ownership not changing
CFStringRef escapeChars = (CFStringRef)@"!*'();:@&=+$,/?%#[]";// __bridge not required for string literal
CFStringRef escaped_CFString = CFURLCreateStringByAddingPercentEscapes(NULL, originalValue, NULL, escapeChars, kCFStringEncodingUTF8);// returns a CFStringRef that YOU own.
NSString *escaped_value = (__bridge_transfer NSString *)escaped_CFString; // __bridge_transfer tells the compiler to send a release call to escaped_CFString.

Now that you see what’s happening you can safely stack the calls like this:

NSString* escaped_value = (__bridge_transfer NSString *)CFURLCreateStringByAddingPercentEscapes(NULL,(__bridge CFStringRef)value,NULL,(CFStringRef)@"!*'();:@&=+$,/?%#[]",kCFStringEncodingUTF8);

Note that this method is still not that reliable. A more complete solution can be found in Dave DeLong’s answer to this question.