Why is `&` (ampersand) put in front of some method parameters?

Question

I'ver wondered, why is it that in front of an NSError, such as below, do we put: &error and not error?

E.g.

NSArray *result = [managedObjectContext executeFetchRequest:fetchRequest error:&error]; 

Hope you can explain, also is this always the way or only in certain situations this is needed? Thanks.

Answer 1

You need to take the address of error because the function needs to modify it. error is passed by pointer, so you need the "take address" operator & for it.

C and Objective-C pass parameters by value. If you pass error without an ampersand and the method that you call modifies it, your function that made the call would not see any changes, because the method would operate on its local copy of NSError*.

You know that you need an ampersand in front of the corresponding parameter if you look at the signature of the method and see ** there:

- (NSArray *)executeFetchRequest:(NSFetchRequest *)request error:(NSError **)error                                             //   ^ one                    ^^ two 

Answer 2

The error parameter's type is (NSError **), that is a pointer to a pointer to an NSError. The error variable that you use as the argument is probably declared as NSError *, so in order to get the types to match properly, you have to use the address of operator to get a pointer to a pointer (&error). The reason the method needs a pointer to a pointer in the first place is so that it can modify the value of error and have that new value be available to you, the caller of the method.