Yeah your first approach to runtime dispatch is just dumb (don't listen to those silly people), you want the second in all cases, even with a closure implementation of
classes. Objects should be implemented as in the second case, or more concretely, as:
struct MyObj {
vtable: MyInterface*
data: void*
}
Which is how dynamic traits are implemented in Rust. The C++ approach of a pointer to:
struct Obj {
vtable: ...
field1: T1
field2: T2
field3: T3
}
Pretty much means you need to define all the interfaces you implemente on class declaration, which is not nearly as nice as doing it independently (as with traits, protocols, typeclasses, go interfaces, etc.)
My point is that MyInterface is just a vtable, a class is a function that fills in that vtable, an object is a pair of a filled in vtable and data. These mechanism are restricted to the compiler for no good reason, which is why you end up with "metaclasses" and "mixins" and all that other nonsense. Just expose the mechanism itself and give it some nice syntax.
Classes mix too much stuff together: inheritance, subtyping, access control, data specifications, interface specifications, etc. We've figured out that's not really a good thing which is why Go and Rust rightfully split them up and use separate mechanisms.
I'm saying go even further and don't hide the runtime representation, let me create these things at runtime if I so wish. That gives metaclasses and mixins and what not for free (with a constexpr-like mechanism they can even be done at compile time).
You can still keep traits as the way of defining bundles of methods, they're also useful as generic concepts, that goes into the ergonomics of it.
This is a similar discussion to Zig generics. Zig has no idea what generics are, a "generic" is just a function that given a type produces another type. Works pretty well.
> A class is any construct that can:
That's just your definition. Your points describe an abstract datatype, a concept that predates classes. We could call that a class these days, but that's not reflective of the programming language construct called "class" as used in language like C++, Java, C# and friends.