What is double star (eg. NSError **)?

A "double star" is a pointer to a pointer. So NSError ** is a pointer to a pointer to an object of type NSError. It basically allows you to return an error object from the function. You can create a pointer to an NSError object in your function (call it *myError), and then do something like this:

*error = myError;

to "return" that error to the caller.

In reply to a comment posted below:

You can't simply use an NSError * because in C, function parameters are passed by value—that is, the values are copied when passed to a function. To illustrate, consider this snippet of C code:

void f(int x){    x = 4;}void g(void){    int y = 10;    f(y);    printf("%d\n", y);    // Will output "10"}

The reassignment of x in f() does not affect the argument's value outside of f() (in g(), for example).

Likewise, when a pointer is passed into a function, its value is copied, and re-assigning will not affect the value outside of the function.

void f(int *x){    x = 10;}void g(void){    int y = 10;    int *z = &y;    printf("%p\n", z);    // Will print the value of z, which is the address of y    f(z);    printf("%p\n", z);    // The value of z has not changed!}

Of course, we know that we can change the value of what z points to fairly easily:

void f(int *x){    *x = 20;}void g(void){    int y = 10;    int *z = &y;    printf("%d\n", y);    // Will print "10"    f(z);    printf("%d\n", y);    // Will print "20"}

So it stands to reason that, to change the value of what an NSError * points to, we also have to pass a pointer to the pointer.

In C everything is pass by value. If you want to change the value of something you pass the address of it (which passes the value of the memory address). If you want to change where a pointer points you pass the the addres of the pointer.

Take a look here for a simple explanation.

In C, a double star is a pointer to a pointer. There are a couple of reasons to do this. First is that the pointer might be to an array of pointers. Another reason would be to pass a pointer to a function, where the function modifies the pointer (similar to an "out" parameter in other languages).