CODEWITHSUNDEEP
objective-cpointers
Besi
Besi

Reputation: 22939

Concept behind NSError

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

Upvotes: 3

Views: 543

Answers (2)

yan.kun
yan.kun

Reputation: 6908

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.

Upvotes: 3

Dennis Bliefernicht
Dennis Bliefernicht

Reputation: 5157

Passing an object by reference would require you to allocate and initialize an NSError object that can then be manipulated by the called method to suit the needs. This also requires to have all funcitonality either directly in NSError or require the caller to know of and use specific subclasses of NSError.

Using this method the caller does not have to create an NSError object at all. In most cases no error should occur and there is no creation of useless error objects. Additionally any class could feature a subclass of NSError to give further information. If the caller does not care about this, it interprets the returned object as simple NSError and everything is fine. If the caller is interested in details, it can cast to the subclass and get further information.

Upvotes: 2

Related Questions