Impl of clonable pattern https://godbolt.org/z/ZI0py6

clonable.cpp

#include <memory>

using namespace std;

// --- library ---

template<class T>
class clonable {
public:
    using Base = T;
    virtual shared_ptr<T> clone() const {
        return make_shared<T>(*static_cast<const T*>(this));
    }
};

template<class Super, class T>
class subclonable : public Super {
public:
    using Base = typename Super::Base;
    shared_ptr<Base> clone() const override {
        return make_shared<T>(*static_cast<const T*>(this));
    }
};

// --- usage ---

class B : public clonable<B> { };
class C : public subclonable<B, C> { };
class D : public subclonable<C, D> { };
class E : public subclonable<D, E> { };

int main() {
    shared_ptr<B> c = make_shared<C>();
    shared_ptr<B> d = make_shared<D>();
    shared_ptr<B> e = make_shared<E>();
    shared_ptr<B> c2 = c->clone();
    shared_ptr<B> d2 = d->clone();
    shared_ptr<B> e2 = e->clone();
    printf("c2=%s\n", typeid(*c2).name());
    printf("d2=%s\n", typeid(*d2).name());
    printf("e2=%s\n", typeid(*e2).name());
}