Deep copying an NSArray

As the Apple documentation about deep copies explicitly states:

If you only need a one-level-deep copy:

NSMutableArray *newArray = [[NSMutableArray alloc]                              initWithArray:oldArray copyItems:YES];

The above code creates a new array whose members are shallow copies of the members of the old array.

Note that if you need to deeply copy an entire nested data structure — what the linked Apple docs call a true deep copy — then this approach will not suffice. Please see the other answers here for that.

The only way I know to easily do this is to archive and then immediately unarchive your array. It feels like a bit of a hack, but is actually explicitly suggested in the Apple Documentation on copying collections, which states:

If you need a true deep copy, such as when you have an array of arrays, you can archive and then unarchive the collection, provided the contents all conform to the NSCoding protocol. An example of this technique is shown in Listing 3.

Listing 3 A true deep copy

NSArray* trueDeepCopyArray = [NSKeyedUnarchiver unarchiveObjectWithData:          [NSKeyedArchiver archivedDataWithRootObject:oldArray]];

The catch is that your object must support the NSCoding interface, since this will be used to store/load the data.

Swift 2 Version:

let trueDeepCopyArray = NSKeyedUnarchiver.unarchiveObjectWithData(    NSKeyedArchiver.archivedDataWithRootObject(oldArray))

Copy by default gives a shallow copy

That is because calling copy is the same as copyWithZone:NULL also known as copying with the default zone. The copy call does not result in a deep copy. In most cases it would give you a shallow copy, but in any case it depends on the class. For a thorough discussion I recommend the Collections Programming Topics on the Apple Developer site.

initWithArray:CopyItems: gives a one-level deep copy

NSArray *deepCopyArray = [[NSArray alloc] initWithArray:someArray copyItems:YES];

NSCoding is the Apple recommended way to provide a deep copy

For a true deep copy (Array of Arrays) you will need NSCoding and archive/unarchive the object:

NSArray *trueDeepCopyArray = [NSKeyedUnarchiver unarchiveObjectWithData:[NSKeyedArchiver archivedDataWithRootObject:oldArray]];