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 *r         


        

Answer 1

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.