why use double pointer when single pointer can do the work in Objective-C?

Question

I understand the benefit of using pointer but double pointer is what I really don't understand. I was told that double pointer is used when you want the method to modify the value of param you pass in. Given the example below, 5 will be printed in log. Then why should we use double pointers in void changeInt(int **i)?

int main (int argc, const char * argv[])
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    int *a = 1;

    changeInt(&a);
    NSLog(@"%i", a);
    [pool drain];
    return 0;
}

void changeInt(int *i)
{
    *i = 5;
}

Answer 1

There's no need for a double pointer with a scalar (int). Obviously all you need is a pointer.

But an object already is a pointer. So to do the same thing with an object, you need a double pointer. The typical use case is NSError**:

+ (id)stringWithContentsOfFile:(NSString *)path
                      encoding:(NSStringEncoding)enc
                         error:(NSError **)error