Question

I wish to set an NSError pointer from within a block in a project using
automatic reference counting. What follows is a simplified version of my code:

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

    __block Frobnicator *blockSelf = self;
    [items enumerateObjectsUsingBlock:^(id item, NSUInteger idx, BOOL *stop) {
        [blockSelf doSomethingWithItem:item error:error];
    }];
}

This compiles but given error may be modified by
doSomethingWithItem I tried creating a local NSError for the block to
modify, which would then be used to set the original error after
the enumeration (which I haven’t shown):

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

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

This fails to compile with the following error:

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

Googling for this error only returns results from the Clang source code itself.

One solution that seems to work but is a bit ugly is to have an inner and outer error pointer:

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

    __block Frobnicator *blockSelf = self;
    __block NSError *outerError = nil;
    [items enumerateObjectsUsingBlock:^(id item, NSUInteger idx, BOOL *stop) {
        NSError *innerError = nil;
        [blockSelf doSomethingWithItem:item error:&innerError];
        outerError = innerError;
    }];
}

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

Answer 1

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. 🙂