Common interface with a wrapper around std::variant or unions

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.

paddy · Accepted Answer · 2022-08-04T02:26:12.730

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

Common interface with a wrapper around std::variant or unions

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