Question

So, I saw this:

error:(NSError **)error 

in the apple doc’s. Why two stars? What is the significance?

Answer 1

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.

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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.