smartpointer_return_covariant

#include <memory>
#include <unordered_map>
#include <typeindex>
#include <iostream>

std::unordered_map<std::type_index, std::shared_ptr<void>(*)(void const*)> cloners;

template <typename Derived>
struct cloneable
{
    std::shared_ptr<Derived> clone() const {
        return std::static_pointer_cast<Derived>(cloners[typeid(*this)](this));
    }

protected:
    cloneable() {
        static int const init_dummy = []{
            cloners[typeid(Derived)] = [](void const* original) -> std::shared_ptr<void> {
                return std::make_shared<Derived>(*static_cast<Derived const*>(original));
            };
            return 0;
        }();
    }

    virtual ~cloneable() = default;
};

struct B : cloneable<B> {};
struct C : cloneable<C>, B {};
struct D : cloneable<D>, B {};

int main()
{
    std::shared_ptr<B> b1 = std::make_shared<D>();
    std::shared_ptr<B> b2 = b1->clone();
    std::cout << typeid(*b2).name(); // D
}