Using output parameters with ARC

Question

So I have read this question, which seems to be exactly the kind of problem I am having, but the answer in that post does not solve my problem. I am attempting to write a data serialization subclass of NSMutableData. The problematic function header looks like this:

-(void)readString:(__autoreleasing NSString **)str

I do some data manipulation in the function to get the particular bytes the correspond to the next string in the data stream, and then I call this line:

*str = [[NSString alloc] initWithData:strData encoding:NSUTF8StringEncoding];

No errors in this code. But when I try to call the function like so:

+(id) deserialize:(SerializableData *)data
{
    Program *newProgram = [[Program alloc] init];

    [data readString:&(newProgram->programName)];

On the line where I actually call the function, I get the following error:

Passing address of non-local object to __autoreleasing parameter for write-back

I have tried placing the __autoreleasing in front of the NSString declaration, in front of the first *, and between the two *'s, but all configurations generate the error.

Did I just miss something when reading the other question, or has something in the ARC compiler changed since the time of that post?

EDIT:

It seems that the problem is coming from the way I am trying to access the string. I can work around it by doing something like this:

NSString* temp;
[data readString&(temp)];
newProgram.programName = temp;

but I would rather have direct access to the ivar

Answer 1

You can't. You might gain insight from LLVM's document Automatic Reference Counting, specifically section 4.3.4. "Passing to an out parameter by writeback". However, there really isn't that much extra detail other than you can't do that (specifically, this isn't listed in the "legal forms"), which you've already figured out. Though maybe you'll find the rationale interesting.