How to make an instance property only visible to subclass?

Access control along inheritance lines doesn’t really fit with the design philosophies behind Swift and Cocoa:

When designing access control levels in Swift, we considered two main use cases:

• keep private details of a class hidden from the rest of the app
• keep internal details of a framework hidden from the client app

These correspond to private and internal levels of access, respectively.

In contrast, protected conflates access with inheritance, adding an entirely new control axis to reason about. It doesn’t actually offer any real protection, since a subclass can always expose “protected” API through a new public method or property. It doesn’t offer additional optimization opportunities either, since new overrides can come from anywhere. And it’s unnecessarily restrictive — it allows subclasses, but not any of the subclass’s helpers, to access something.

There’s further explanation on Apple’s Swift blog.