Question

I have been using NSError in a few places but I don’t really grasp the concept behind it. Especially why it is used using a double pointer like this:

NSError *err = nil;
NSString *s = nil;
s = [NSString stringWithContentsOfURL:url error:&err];

So why can’t we just pass the variable errto the method, which is passed as “by-reference” anyway as far as I can tell.

Thanks for some clarification

Answer 1

This is one of the best descriptions around about NSError.

Actually your are passing not an object but a pointer to a pointer. With this, you can set your NSError before you send a message to an object to nil and only if an error occurs, your NSError will “suddenly” hold an value so that you can do stuff like this:

NSError *err = nil;

[SomeClass doSomethingGreatWhichCanFailWithError:&err];

if(err) {
   NSLog("Something failed :-(");
   //TODO: Add better logging!
}

Otherwise you would have to create an instance of NSError before you send the message and than check for a property or something else if an error occured.