Question

I’m converting some Objective-C code to ARC that makes heavy use of the GCC “Statements and Declarations in Expressions” extension ({}).

The GCC extension is being used in preprocessor macros to create Objective-C objects. The actual macros are fairly complex, so here is a contrived example to simplify things. Assume for the purposes of my example that the _x parameter is always NSString *:

#define f(_x) ({ NSString *x = (_x); [NSString stringWithString: x]; })

This macro would of course be called like so:

void foo() { NSString *s = f(@"foo"); }

In the pre-ARC environment this works just fine. The object created by the stringWithString: call is an autoreleased object and the GCC extension assigns it to s.

After converting to ARC (Xcode 4.4.1) however the object is released immediately when the statement block exits (I confirmed this by instantiating an object that NSLog()s it’s dealloc).

I’ve tried modifying the macro with typecasts like:

#define f(_x) ({ NSString *x = (_x); (__autoreleasing id) [NSString stringWithString: x]; })

and

#define f(_x) ({ NSString *x = (_x); (NSString __autoreleasing *) [NSString stringWithString: x]; })

but both forms result in a compilation error:

Explicit ownership qualifier on cast has no effect

It appears then that ARC and this GCC extension are incompatible.

Is this so or am I missing something simple that tells ARC not to immediately release the object?

I do have some options, the most attractive of which at the moment seems to be to convert all of these macros to __inline__ functions but I have quite a bit of code to deal with. I’m really hoping for a quick fix so any ideas are appreciated.

Answer 1

I’ve been debugging this for a few hours now and found a solution so I’ll answer my own question. FWIW I agree with CRD and think it’s a bug–I’ll file a report with Apple.

It happens that the “contrived example” I made up for my question actually works (doh). As I mentioned originally the macros I’m working with are much more complicated and they don’t. I’ve discovered though that they will if I return the created object through a temporary as the last statement in the statement block. When written like that ARC doesn’t release the object. I assume there’s some very narrow RVO-type code in clang and it’s colliding with the ARC parser.

At any rate, the workaround looks this (scroll to the very end of the line):

#define f(_x) ({ NSString *x = (_x); NSString *y = [NSString stringWithString: x]; y; })