[yati]

I kind of understand what you describe, but I still don't understand why the type needs to be private. You can return opaque objects of a public type just as well for this job, can't you?

    class Token {
      virtual ~Token();
      protected:
        friend class MyClass;
        ...
        /* all your state */
    };

> what is a callback lambda but private state?

You can (and do) program to the callback's interface -- say you ask for an std::function<void()>, and the user gives you that. What is behind the function (private state or not) is not your concern, you simply perform an public operation (calling it) on a public interface (i.e., via std::function).

[gumby]

What is the difference to you if I pass a private object or a public object all of whose instance variables and methods are private? Either way the point is to say “this is not your object to manipulate”. But making it a type you cannot even express makes that far more explicit.

[yati]

I see, I guess then that's where we disagree. It is very confusing for people from my limited experience, for little gain. Plus you make it harder to do things like put these tokens in e.g., a vector without resorting to decltype.

Going back to the original point i was making with this example, I still think this was probably not a capability consciously designed in the language.