If you use "class" / private members, you have to use methods to access the members. That, or a friend "class" containing static methods (a namespace won't do). Private methods are annoying because you're forced to expose implementation details, and are forced to duplicate function signatures in the code. Friend classes are annoying because you still have to name them in the class declaration, and having to use them with static methods leads to stylistically inconsistent code and you end up with a weird dummy class that isn't meant to ever be instanced.

AFAIK there isn't a nice way to deal with this other than simply not using private members and coding in a simple C like style. I don't think you've shown a way, either. I don't know what you mean by "protocol classes", but if you mean abstract classes with virtual methods that need to be overridden, those are a bad solution because they overhead of vtables without any technical need or benefits (unless you want runtime polymorphism and vtables are exactly the kind you want).