noway for virtuals

main.cpp

#include <iostream>

template <typename T> constexpr
void print_type() {
    std::cerr << __PRETTY_FUNCTION__ << std::endl;
}

enum ETypes {
    type1,
    type2
};

struct Type1 {
    static void func1() {
        std::cerr << "Type1 Func1" << std::endl;
    }
};

struct Type2 {
    static void func1() {
        std::cerr << "Type2 Func1" << std::endl;
    }
};

// interface for functions, available to call via dispatch
namespace details {
    template <typename T> struct Func1 {
        static decltype(&T::func1) get() { return &T::func1; }
    };
}

//TODO: abstact over enum and possible types via typelist?
struct SumType {
    ETypes discr;
    union {
        Type1 typ1;
        Type2 typ2;
    };

    // tag dispatching
    template<template <typename T> class GETTER, typename R = void, typename... ARGS> R dispatch(ARGS... args) {
        switch (discr) {
            //TODO: generate dispatch function via typelist, each type will contain static tag to be compared with?
            // foldexpression instead of typelist, C++17 only :(?
            case (ETypes::type1): {
                return (*GETTER<Type1>::get())();
            }
            case (ETypes::type2): {
                return (*GETTER<Type2>::get())();
            }
        }
    }
};

int main() {
    SumType obj { ETypes::type2, Type1{} };

    obj.dispatch<details::Func1>();
}