An explanation of iOS/Mac OS bridging casts

bridging casts

Using Core Foundation (CF) objects in ARC


ARC makes most aspects of memory management easier, but managing CF objects is still up to you, and converting 
between NSObjects and CFObjects is actually more complicated.

If you create a CF object that follows the "Create Rule" (you own an object that is created with a method with 
"create" or "copy" its name) you have to release it.

In ARC, that's still true.

Where it gets confusing is when you need to cast objects between CF objects and NSObjects.

There are 3 "bridging cast" operators that let you deal with converting between CF types and NSObject types.

__bridge simply converts between CF and Objective C objects. It works in both directions. It does not adjust 
retain counts for you. 

__bridge_transfer (or the macro CFBridgingRelease() ) converts a CF object to an NSObject. It is used for 
"toll-free bridged" CF objects. It lets you pass a CF object that you own to it's Objective C equivalent. 
The CF object is released in the process, and ARC takes ownership of the equivalent NSObject.

__bridge_retain (or the macro CFBridgingRetain() ) does the reverse of __bridge_transfer. It lets you convert 
an autoreleased NSObject to it's CF equivalent. The bridge CFRetains the resulting CF object, so you have to 
CFRelease it when you're done with it.

For example, say you ask the address book for a string using , ABRecordCopyValue . In that case, you are 
converting a CFString, which is toll-free bridged to NSString, to it's NSString equivalent. The 
ABRecordCopyValue method has copy in it's name, so it follows the "Create rule" for CF objects. You own it 
and need to release it. Thus, you 
should use this line:


NSString *fname = (NSString*) CFBridgingRelease(ABRecordCopyValue(person, kABPersonFirstNameProperty));


After that line, fname holds a "strong" reference to the converted CFString, and will release it when it 
goes out of scope. Thus you can forget about the CF object and treat it as an NSObject from then on.