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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; }]; }

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