passing parameters in iOS

Question

I am new to iOS and passing parameters by value or reference ( I am from a Java/.NEt background ) is confusing to me. you see I have a sort of utility / Helper method that I pass to it an NSMutableDictionary, and then a file location, and ask for it yo unrchive data form the file to the dictionary I sent it. this is the helper method :

 - (void)loadData:(NSMutableDictionary *)dictionary fromFile:(NSString *)fname  {
     dictionary = [ NSKeyedUnarchiver unarchiveObjectWithFile : [ self getFilePath:fname ] ] ;
    if ( [dictionary.allKeys count] < 1 ) {
       NSLog(@"Couldn't load file %@ ", fname);
    }  else  {
      NSLog(@"Loaded data from file %@ successfully", fname );
  }
}

now I call this method in the following line of code

      [ loadData: dataDict fromFile:@"data.archive"];

Now the problem is that by the end of the helper method , I have a variable called dictionary that does have values, but its not the original dictionary I passed from my calling line. What Am I Doing wrong?

Answer 1

As explained here - Passing arguments by value or by reference in objective C - in objective c parameters are passed by value and so a modification made to the parameter is modifying a value and not the actual value of the reference being passed in.

If you want to modify your dictionary you should prefix it with an ** to pass the value of the reference and then access with *.

- (void)loadData:(NSMutableDictionary**)dictionary fromFile:(NSString*)fname  {
{
    *dictionary = [NSKeyedUnarchiver unarchiveObjectWithFile:[self getFilePath:fname]];
    if ([*dictionary.allKeys count] < 1) {
        NSLog(@"Couldn't load file %@ ", fname);
    }  
    else  {
        NSLog(@"Loaded data from file %@ successfully", fname);
    }
}

Note: This sort of pointer dereferencing is a little funky and should probably restricted to **Error passing. It would probably make more sense to create a new dictionary copy content and return it.