How to create a simple version of std::function class

Suppose that I want to create a simple version ofstd::function, which has following behaviors:

1. function(){} -> A void constructor
2. function(_ReturnType, Args... vlist) -> To convert function pointer to a funcion object
3. _ReturnType operator() (_ArgTypes... vlist) -> To call function by fn(Args...)

I have already tried to write down a version, but it seems to fail during compiling... I design it like this:

template <typename ReType, typename... _ArgTypes>
    class function
    {
    protected:
        ReType(*fn) = NULL;

    public:
        function() {}
        function(ReType R, _ArgTypes... vlist) { fn = R; }

        ReType operator()(_ArgTypes... vlist)
        {
            return fn(vlist...);
        }
    };

With compiler error:

In file included from main.cpp:11:
functional.h: In instantiation of 'class nuts::function<double(double)>':
main.cpp:102:27:   required from here
functional.h:16:16: error: function returning a function
         ReType operator()(_ArgTypes... vlist)
                ^~~~~~~~
main.cpp: In function 'int main()':
main.cpp:103:19: error: no match for call to '(nuts::function<double(double)>) (double)'
  std::cout << fn(2.0) << std::endl;
                   ^