Why does changing a mutable array contained in a dictionary not require updating the dictionary?

Question

I was trying a piece of code from CS193P course (Objective-C). I noticed something in the way that the compiler works. An `NSMutableArray` called `photos` was added to an `NSMutableDictionary`, `photosByPhotographer`. Later on, a change was made to `photos` without any changes to `photosByPhotographer`. When I logged `photosByPhotographer`, the change was automatically applied to it and it did not need any extra lines of code! I wonder how the compiler makes this work? Any materials to read from? The code is as follows: ``` - (void)updatePhotosByPhotographer { NSMutableDictionary *photosByPhotographer = [NSMutableDictionary dictionary]; for (NSDictionary *photo in self.photos) { NSString *photographer = [photo objectForKey:FLICKR_PHOTO_OWNER]; NSMutableArray *photos = [photosByPhotographer objectForKey:photographer]; if (!photos) { photos = [NSMutableArray array]; [photosByPhotographer setObject:photos forKey:photographer]; NSLog(@"photosByPhotographer in if: %@", photosByPhotographer…

Accepted Answer

That's because in Cocoa, you are using objects and pass by reference.

Imagine this code:

```
NSMutableArray *a, *b;

a  = [NSMutableArray new];
[a addObject:@"A"]; // a contains @"A"
b = a; // b is assigned to the same object
[b addObject:@"B"]; // a and b contain @"A", @"B"

NSLog(@"a = %p and b = %p", a, b); // same values
```

The variables `a` and `b` point to the same object. You can also see that by comparing the pointer values.

However, if we do the following:

```
NSMutableArray *a, *b;

a  = [NSMutableArray new];
[a addObject:@"A"]; // a contains @"A"
b = [[a mutableCopy] autorelease]; // b is assigned to a COPY of the object
[b addObject:@"B"]; // b contains @"A", @"B", while a still contains @"A"

NSLog(@"a = %p and b = %p", a, b); // NOT the same values
```

`b` is assigned to a copy of `a` (not the original), so `b` points to an other object. You can check by comparing the addresses of the objects.

Why does changing a mutable array contained in a dictionary not require updating the dictionary?

Solution

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