CF_IMPLICIT_BRIDGING_ENABLED non-functional in xcode5? - ios

Question

The WWDC 2013 lecture slides contain a section that in some cases the explicit bridging casts can be omitted. I wrote this in Xcode 5 (using 10.8 though, not 10.9), and the compiler complains I need a bridging cast. Am I just totally not understanding the concept?
#import <Foundation/Foundation.h>
CF_IMPLICIT_BRIDGING_ENABLED
CFStringRef MyCreateStringFromNothing();
CF_IMPLICIT_BRIDGING_DISABLED
void SomeFunction() {
// compiler requires bridging cast here...
NSString* x = MyCreateStringFromNothing();
}
The reason I ask is I wanted to write:
NSString* s = CFUUIDCreateString(NULL, uuid);
and thought that the new work on implicit bridging should simply enable me to write this. But this also needs a bridging cast.

Answer 1

This appears to be sort of black magic, but from my experiments functions with "Create" in them don't work with implicit bridging.
When I compiled this:
CF_IMPLICIT_BRIDGING_ENABLED
CFStringRef MyCreateStringFromNothing();
CFStringRef MyGetStringFromNothing();
CF_IMPLICIT_BRIDGING_DISABLED
void SomeFunction() {
NSString *const fails1 = MyCreateStringFromNothing();
NSString *const fails2 = CFStringCreateCopy(kCFAllocatorDefault, CFSTR("FOO"));
NSString *const works = MyGetStringFromNothing();
}
The first two fail but the third one "works."
Note that implicit bridging apparently STILL does NOT actually keep the compiler from warning you about converting CFStringRef to NSString *, it just stops requiring you to insert a __bridge as well.
You'll notice that in slide at WWDC2013 in session 404 at 50:30, he uses CFDictionaryGetValue(), which returns a "void *", so it's one of the few functions that won't raise a warning. Any of the CF functions that return an actual type (e.g. CFStringRef, or CFDictionary) are still going to throw warnings, so, meh.

Answer 2

Check the compiler settings on the project and/or target. If this is an older project, you might have a stray option there causing you to use an older compiler. (This tripped me up a few times when I moved to Xcode 5 and wanted to use the latest Obj-C newness.)
These days, I always choose "Default compiler" in my build options.