C++ variadic macro and template

Question

I am playing with variadic macro and template. Is there any simple way to achieve the following?

It's like std::make_tuple. How to implement make_my_class? I saw I probably need "std::decay" but I don't quite understand that. Thanks a lot in advance.

template <typename... Args>
class my_class
{
public:
   my_class(Args... args)
      : items_(args...)
   {
   }

private:
   std::tuple<Args...> items_;
};

// How to fix this line?
#define CREATE_MY_CLASS(var_name, args...) my_class<decltype(args...)> var_name(args...);

// Usage:
// CREATE_MY_CLASS(xyz, 1, 2, 3)

// Target:
// my_class<decltype(1), decltype(2), decltype(3)> xyz(1, 2, 3);

Answer 1

What's wrong with using a function template?

    template <typename ... Args>
    my_class<Args...> get_my_class(Args&&...args)
    {
        return my_class<Args...>(std::forward<Args>(args)...);
    }

    auto x = get_my_class(1,2,3,4,5,6);

To use the variadic arguments of a macro requires __VA_ARGS__ and I don't know how or if you can pass that to decltype() since you need to do something that will amount to decltype(args)... to get a list of types of args . The function template bypasses all of that with its implicit template deduction and the auto keyword means you don't have to know what types are being passed to my_class to declare a variable of the appropriate type.

Answer 2

You can do it like this:

#include <iostream>
#include <tuple>
#include <utility>

// my_class declaration
// ...


template <typename... Args>
my_class<Args...> make_my_class(Args&&... args) {
   return my_class<Args...>( std::forward<Args>(args)... );
}

int main() {
   auto f = make_my_class(1,2);
}

Little explaination: This works same way as std::make_pair . make_my_class is receiving rvalue refs and forwarding into my_class constructor. Return value from make_my_class should be rvalue by default too. I think there should be zero overhead in such constructing method.