Value vs. Reference in Swift
This is a general concern of mine in Swift: how to actually know what has value vs. reference semantics.
Swift adds let and var, which fixes one problem. But there is no way to see what is a struct or a class, which adds another. It’s even more complicated in the context of the proposal to remove the “NS” prefix from Foundation class names.
As an example, the seemingly similar
SetandCountedSettypes produce different results from nearly identical code. Swift’s Set type has value semantics, so changes to a copy don’t affect the original Set instance […]CountedSet(néeNSCountedSet), on the other hand, uses reference semantics, so changes to a copy of an instance. This is true whether the copy is an explicit one made via assignment or the implicit copy made when calling a function withCountedSetas a parameter. This example shows how nearly code nearly identical to the above produces very different results […]
But even a struct can have reference semantics if it includes a class reference and stores its data in the class. Or it can have hybrid semantics: store some data within the struct and some data behind a class reference. You never really know what semantics a type has unless you look at its documentation or implementation.
Although in this case here it'd be a bit silly to have `Set` and `CountedSet` behave so differently: it breaks too many expectations. I understand why they want to remove the NS prefix, but there's plenty of cases (like this one) where it's a bad idea.