Exactly. You spell auto or an interface or base class whose derived class cannot be named. I think both are ok. In the latter case you can name the interface in

Exactly. You spell auto or an interface or base class whose derived class cannot be named. I think both are ok. In the latter case you can name the interface instead of auto. Yes, auto is a perfectly valid return type, usually quite useful in generic code. However, in generic code, starting at C++20, I would recommend to use concepts to name a return type that can be many different thing but that is compliant with the same "compile-time" interface. You could do: forward_range auto f(auto && input); You do not spell the return type but you are saying that type conforms to forward_range. Note that the auto type returned could be one of many unrelated types and it is computed at compile-time.