Setting NSError within a block, using ARC

What is the correct way to set an NSError from within a block?

As seen on "What's new in LLVM?" @ 14:55, there are two techniques to address the issue with the NSError which is implicitly autoreleasing.

Easiest fix is to use __strong

- (BOOL)frobnicateReturningError:(NSError *__strong *)error
{
    NSArray *items = [NSArray arrayWithObjects:@"One", @"Two", @"Three", nil];

    __block Frobnicator *blockSelf = self;

    [items enumerateObjectsUsingBlock:^(id item, NSUInteger idx, BOOL *stop) {
        NSError *innerError = nil;
        [blockSelf doSomethingWithItem:item error:&innerError];
        if(innerError && error) {
          *error = [NSError errorWithDomain:...];
        }
    }];
}

Second fix is to use __block

- (BOOL)frobnicateReturningError:(NSError **)error
{
        NSArray *items = [NSArray arrayWithObjects:@"One", @"Two", @"Three", nil];
    
        __block Frobnicator *blockSelf = self;
        __block NSError *strongError = nil;

        [items enumerateObjectsUsingBlock:^(id item, NSUInteger idx, BOOL *stop) {
            NSError *innerError = nil;
            [blockSelf doSomethingWithItem:item error:&innerError];
            if(innerError) {
              strongError = [NSError errorWithDomain:...];
            }
        }];
        if (error) *error = strongError;
    }

Note: As of 2020, this workaround is no longer necessary.

---

Try this:

// ...
__block NSError *blockError = nil;
[items enumerateObjectsUsingBlock:^(id item, NSUInteger idx, BOOL *stop) {
    NSError *localError = nil;
    if (![blockSelf doSomethingWithItem:item error:&localError]) {
        blockError = localError;
    }
}];
// ...

As for exactly why this is necessary, I'm still trying to get a grasp on that myself. I'll update this answer when I do. :)