I wish Swift focused on reference semantics. One of the big problems of value types in C++ is that you have to be a language lawyer to not accidentally make wasteful copies of everything, and the same is true of Swift:

http://rosslebeau.com/2016/swift-copy-write-psa-mutating-dic...

I thought Objective-C had already solved this problem quite nicely with the explicit NSFoo/NSMutableFoo class pairs. I don't see why this needed to be fixed again, in a less explicit way.