Must I use __bridge or __bridge_retained if I'm bridging an autoreleased object to Core Foundation?

问题

The ARC Migration Tool is having trouble with this:

NSURL *fileURL = [NSURL fileURLWithPath:path];
AudioFileOpenURL((CFURLRef)fileURL, kAudioFileReadPermission, 0, &fileID);

In particular, it isn't sure about if it should do a __bridge or __bridge_retained. And I'm either.

-fileURLWithPath returns an autoreleased object, and in this place I'm not the owner of fileURL. But at the same time, the object has a retain count of at least +1.

I'd bet this has to be done with __bridge only.

回答1:

You want to use the regular __bridge cast only for this. You would use __bridge_retained only if you want to manage the lifecycle of a cast CF object. For example:

CFStringRef cf_string = (__bridge_retained CFStringRef)someNSString;
// some long time later, perhaps in another method etc
CFRelease(cf_string);

So the __bridge_retained is really telling the compiler that you had an ARC object and now you want to basically turn it into a CF object that you're going to manage directly.

来源：https://stackoverflow.com/questions/8842953/must-i-use-bridge-or-bridge-retained-if-im-bridging-an-autoreleased-object