NSError * vs NSError **

Question

I realize that this is similar to an existing post here, What's the Point of (NSError**)error?, but my question is a little different. I understand how the double pointer works, and how this is the common iOS API error pattern. My question is more around the single pointer, and why this code doesn't work:

- (BOOL)someMethodWithError:(NSError *)error
{
  ...
  if( errorOccured )
  {
    NSError *e = [[[NSError alloc] initWithDomain:@"" code:1 userInfo:nil] autorelease];
    error = e;
    return NO;
  }

  return YES;
}

implemented using:

NSError *error = nil;
if( ![someObj someMethodWithError:error] )
{
  NSLog(@"Error: %@", [error localizedDescription]);
}

Why doesn't the assignment in the method implementation reassign the pointer to the new NSError object?

Answer 1

I find that it helps to think of a pointer as just an integer. (That's what it is.)

Look at your example with an int.

-(BOOL)someMethodWithError:(int)error
{
     error =100;
     return NO;
}

That integer is pass by value. after that function is called error will not change.

   error  = 123;
  [self someMethodWithError:error];
  //error is = 123;

The pointer is the same thing. It's pass by value.

  NSError * error; //this assigns this to a value in memory.
  NSLog(@"%p",error);
  [self someMethodWithError:error];
  NSLog(@"%p",error); // the value of error will not have changed.

if you want the pointer to change you need to send in a pointer to that pointer and change it. It is confusing but draw yourself a diagram of memory and think about it. :)