Question

I have an NSMutableDictionary called source, and then I create another this way:

NSMutableDictionary *new = [source copy];

But when I modify one of new‘s values, the value of source is also modified. Why is that?

Answer 1

The problem is that iOS (and OSX) use shallow copies. That means the new collection has the same objects in it as the original. As the @danDedo said in his answer, your copy is non-mutable – that it, its a dictionary not a mutable dictionary (so you could not add or delete objects). But the objects are the same. So if these objects are themselves mutable, then changes in one place will affect all references (that is, all the objects in the copies).

The solution to this is to write your own “deep copy” (if you need it) – you can for sure find examples searching for “deep copy” here.