c++ decorator pattern, static polymorphism with templates and registering callback methods

Question

I am attempting to use static polymorphism to create a decorator pattern. As to why I do not use dynamic polymorphism, please see this QA. Basically, I could not dynamic_c

Answer 1

You started using mixins, so use them to the end.
It follows a minimal, working example:

#include<iostream>

struct I {
    virtual void et() = 0;
};

template<typename... T>
struct S: I, private T... {
    S(): T{}... {}

    void et() override {
        int arr[] = { (T::et(), 0)..., 0 };
        (void)arr;
        std::cout << "S" << std::endl;
    }
};

struct A {
    void et() {
        std::cout << "A" << std::endl;
    }
};

struct B {
    void et() {
        std::cout << "B" << std::endl;
    }
};

int main() {
    I *ptr = new S<A,B>{};
    ptr->et();
    delete ptr;
}

As in the original code, there is an interface I that offers the virtual methods to be called.
S implements that interface and erases a bunch of types passed as a parameter pack.
Whenever you invoke et on a specialization of S, it invokes the same method on each type used to specialize it.

I guess the example is quite clear and can serve as a good base for the final code.
If I've understood correctly the real problem, this could be a suitable design for your classes.

EDIT

I'm trying to reply to some comments to this answer that ask for more details.

A specialization of S is all the (sub)objects with which it is built.
In the example above, S<A, B> is both an A and a B.
This means that S can extend one or more classes to provide common data and can be used as in the following example to push around those data and the other subobjects:

#include<iostream>

struct I {
    virtual void et() = 0;
};

struct Data {
    int foo;
    double bar;
};

template<typename... T>
struct S: I, Data, private T... {
    S(): Data{}, T{}... {}

    void et() override {
        int arr[] = { (T::et(*this), 0)..., 0 };
        (void)arr;
        std::cout << "S" << std::endl;
    }
};

struct A {
    void et(Data &) {
        std::cout << "A" << std::endl;
    }
};

struct B {
    void et(A &) {
        std::cout << "B" << std::endl;
    }
};

int main() {
    I *ptr = new S<A,B>{};
    ptr->et();
    delete ptr;
}