Although Box::duplicate is being invoked at runtime (via virtual dispatch), and although Box::duplicate does override Shape::duplicate (covariantly), and although Box::duplicate does return a Box*, you’ll still get a Shape* pointer because you are calling duplicate() through a Shape* pointer, and Shape* is the return type of Shape::duplicate(), and the compiler only sees you calling Shape::duplicate, … Read more
You can either add a type parameter to your struct, as in Zernike’s answer, or use a trait object. Using the type parameter is better for performance because each value of T will create a specialized copy of the struct, which allows for static dispatch. A trait object uses dynamic dispatch so it lets you … Read more
This is just how protocols currently dispatch methods. A protocol witness table (see this WWDC talk for more info) is used in order to dynamically dispatch to implementations of protocol requirements upon being called on a protocol-typed instance. All it is, is really just a listing of the function implementations to call for each requirement … Read more