Define subpackages for variadic templates in C++

Question

I have something like that based on variadic templates:

class MyClass {
  public:
    template <typename... OtherTs>
    typename std::enable_if<sizeof...(OtherTs) == 0>::type Setup() {}

    template <typename T, typename...OtherTs>
    void Setup() {
      DoSomeStuff(new T());
      Setup<OtherTs>();
    }
};

It's working well and I can setup an instantiation of MyClass based on other classes like that myClass.Setup<A,B,C>(). What I would like to do now is to define subpackages of classes like Default = A, B such that myClass.Setup<Default, C>() or even myClass.Setup<MyClass::Default, C>() will be interpreted as myClass.Setup<A,B,C>().

I know I can define #define Default A, B but is there a similar way that does not rely on macro and could be defined in a namespace/class?

Thanks for reading and have a nice day. :)

Answer 1

You might use recursion when tuple or any specific type (tag here) is given:

template <typename...> struct tag{};

class MyClass {
private:
    template <typename T>
    void SetupImpl(tag<T>)
    {
        // Here, individual setup
        DoSomeStuff(new T());
    }
    template <typename ...Ts>
    void SetupImpl(tag<tag<Ts...>>)
    {
        Setup<Ts...>();
    }

public:
    template <typename ... Ts>
    void Setup() {
      (SetupImpl(tag<Ts>{}), ...); // C++17 fold expression
    }
};

using Default = tag<A, B>;

Demo