I am tying to understand the example in the apple docs for deep copying an array of dictionaries Here.
In Listing 3 A true deep copy
The example shows
NSArray* trueDeepCopyArray = [NSKeyedUnarchiver unarchiveObjectWithData:[NSKeyedArchiver archivedDataWithRootObject:oldArray]];
Notice there is no copy, mutable copy , or autorelease operators, so I am struggling to understand how this is a copy and how it gets released. In fact, if I release my implementation, it will crash. It does however work as expected, and there does not appear to be any abandoned memory using it as the example shows.
I looked the the NSUnarchiver class reference and it mentions that the unarchiveObjectWithData method creates a temporary copy of the object(s)?
Does this mean the proper implementation then is to allocate and init the new array rather than just assigning the pointer so it can be released?
Thanks in advance.
They are doing a deep copy by doing a full archival pass on the object graph. Using NSArchiver enables things like automatic cyclic reference management and the objects can choose not to encode things like their delegate or they can hook stuff back up on unarchival.
For all intents and purposes, it is archival by saving something to disk and then reading it back in as a new document. Only, instead of the disk, it is just stored in memory all the time.
It is slow, very expensive, and totally inappropriate for anything but when you very occasionally need to duplicate a complex object graph completely.
unarchiveObjectWithData: returns an auto-released object. If you using MRC, you need to retain it if you want to keep it. But, a better solution would be to move to ARC.