Why does NSError need double indirection? (pointer to a pointer)

Question

This concept seems to trouble me. Why does an NSError object need its pointer passed to a method that is modifying the object? For instance, wouldn\'t just passing a referen

Answer 1

An old question, but still I think its worth putting this here -

The actual culprit is NSError. If you look at its class reference, there are no setter methods for any of its attributes, i.e. domain, code or userInfo. So there is no way, you can just alloc and initialize a NSError, pass it to the method and then populate information on the passed NSError object. (Had there been a setter method, we could have just passed a NSError * and done something like error.code = 1 in the method.)

So in case there is an error, you have to generate a new NSError object in the method and if you are doing so the only way to pass it back to the caller is by having a NSError ** argument. (For the reason explained in the above answers.)