Syntax of ** in objective C

Question

Simple question. What is the meaning of the double asterisk in the interface below? It's not an NSError pointer, but what?

- (BOOL)checkResourceIsReachableAndReturnError:(NSError **)error

Answer 1

It's a pointer to a pointer to an NSError. It's used as an "out parameter" -- or you may want to think of it as a pointer to an NSError instance, considering an NSObject instance is always a pointer.

You use it like this:

NSError * outError = nil; << reserve place and a name for the out parameter

// pass the error so the client may return it to you:
BOOL ret  = [obj checkResourceIsReachableAndReturnError:&outError]; 

if (nil != outError) { << the error was set in checkResourceIsReachableAndReturnError:
  NSLog(@"Error: %@", outError); << log it
}
…

On the implementation side, it looks like this:

- (BOOL)checkResourceIsReachableAndReturnError:(NSError**)outError
{
  if (self.isThingReachable) {
    // okay - the caller may not be interested in the error, test it:
    if (0 != outError) {
      // they are interested this time!
      *outError = [NSError errorWithBlahBlahBlah];
    }
    return NO;
  }

  return YES;
}

Answer 2

You call it by sending a reference to an NSError*, like so:

NSError *myError = NULL;
[foo checkResourceIsReachableAndReturnError:&myError];
if (myError) {
    /* look through info, handle error, etc. */
}

The -checkResourceIsReachableAndReturnError: method can modify the pointer myError, including allocating space for a new NSError. This allows you to easily check if there was an error, by simply checking if myError is non-NULL.

Answer 3

Check out this blog post: "Using NSError To Great Effect" and pay particular attention to the section "Passing Pointers to Pointers."

Also the wiki page on pointers has a section on "Multiple Indirection"