Common interface with a wrapper around std::variant or unions - c++

Question

This question is related to Enforcing a common interface with std::variant without inheritance.
The difference between that question and this one, is that I wouldn't mind inheritance, I am simply looking for the following structs/classes...
struct Parent { virtual int get() = 0; };
struct A : public Parent { int get() { return 1; } };
struct B : public Parent { int get() { return 2; } };
struct C : public Parent { int get() { return 3; } };
... to be AUTOMATICALLY "assembled" into a template:
template<typename PARENT, typename... TYPES>
struct Multi
{
// magic happens here
}
// The type would accept assignment just like an std::variant would...
Multi<Parent, A, B, C> multiA = A();
Multi<Parent, A, B, C> multiB = B();
Multi<Parent, A, B, C> multiC = C();
// And it would also be able to handle virtual dispatch as if it were a Parent*
Multi<Parent, A, B, C> multiB = B();
multiB.get(); // returns 2
Is this possible? If so, how? I would like to avoid working with handling pointers, as the use of std::variant/unions is intended to make memory contiguous.

Answer 1

You can't automagically set this up to allow multiB.get(), but you can allow multiB->get() or (*multiB).get() and even implicit conversion, by providing operator overloads:
template<typename Base, typename... Types>
struct Multi : std::variant<Types...>
{
using std::variant<Types...>::variant;
operator Base&() { return getref<Base>(*this); }
Base& operator*() { return static_cast<Base&>(*this); }
Base* operator->() { return &static_cast<Base&>(*this); }
operator const Base&() const { return getref<const Base>(*this); }
const Base& operator*() const { return static_cast<const Base&>(*this); }
const Base* operator->() const { return &static_cast<const Base&>(*this); }
private:
template<typename T, typename M>
static T& getref(M& m) {
return std::visit([](auto&& x) -> T& { return x; }, m);
}
};
You've probably encountered this kind of thing before when using iterators from the standard library.
Example:
int main()
{
Multi<Parent, A, B, C> multiA = A();
Multi<Parent, A, B, C> multiB = B();
Multi<Parent, A, B, C> multiC = C();
// Dereference
std::cout << (*multiA).get();
std::cout << (*multiB).get();
std::cout << (*multiC).get();
// Indirection
std::cout << multiA->get();
std::cout << multiB->get();
std::cout << multiC->get();
// Implicit conversion
auto fn = [](Parent& p) { std::cout << p.get(); };
fn(multiA);
fn(multiB);
fn(multiC);
}
Output:
123123123