Question

In a project I’m creating, I have various classes. One of my classes has an instance of NSMutableArray that holds objects of another one of my classes. I thought I had a firm understanding on this topic, but somehow it got jumbled up in my mind again.

When initializing an instance of this class, I have this initilize method:

- (MMShowMovement *) initWithMovementName: (NSString *) name andNumber: (NSInteger) number {

 if( [super init] ) {

  [self setMovementTitle: name];
  [self setMovementNumber: number];

  [self setDotArray: [[NSMutableArray alloc] init]];

 }

 return self;

}

Later on, after an instance of this class is created, I can add objects to the NSMutableArray “dotArray.” Here is that method.

- (void) addDot: (MMDot *) dot {

 [dotArray addObject: dot]; 

}

(I know, its simple) I’m wondering, when I use “dotArray” in this method, I am accessing the dotArray object for the instance of the class for which this method has been invoked, correct? Or should I use the self keyword here also?

- (void) addDot: (MMDot *) dot {

 [[self dotArray] addObject: dot]; 

}

Honestly, I’m not really sure. I believe it is the former, but I’m unsure as to why. And it is not necessary to use the self keyword in this method, why do I have to use it in the initializer to access the object?

Answer 1

Usually we’ll write

self = [super init]  

Because super‘s initialization method might return an object that’s not the same as the one that was allocated. So that’s why we use self in initxxx method and we don’t need to use self in other kinds of methods.

I learned this stuff clearly from the book Learn Objective-C on the Mac, Chap 10 Object Initialization, Mark Dalrymple and Scott Knaster, Apress.

A excerpt from the book:
instance variables are found at a memory location that’s a fixed distance from the hidden self parameter. If a new object is returned from an init method, we need to update self so that any subsequent instance variable references affect the right places in memory.