How to use c++11 decltype variadic template?

I am new in cpp, I want to pass parameters in a flexible way like this:

#include <iostream>
#include <vector>

struct Req
{
    void test()
    { std::cout << "req" << std::endl; }
};

struct Resp
{
    void test()
    { std::cout << "resp" << std::endl; }
};


template<typename Fn>
auto update(Fn &&fn_)
-> decltype(fn_(nullptr, nullptr), void())
{
    Req req;
    Resp resp;
    fn_(&req, &resp);
}

template<typename Fn>
auto update(Fn &&fn_)
-> decltype(fn_(nullptr), void())
{
    update([&fn_](Req *req, Resp *resp)
           {
               fn_(resp);
           });
}

template<typename Fn>
auto update(Fn &&fn_)
-> decltype(fn_(), void())
{
    update([&fn_](Req *req, Resp *resp)
           { fn_(); });
}

int main()
{
    update([](Req *req, Resp *resp)
             {
                 std::cout << "1" << std::endl;
                 req->test();
                 resp->test();
             });
    update([](Resp *resp)
             {
                 std::cout << "2" << std::endl;
             });
    update([]()
             {
                 std::cout << "3" << std::endl;
             });
}

But I want to add more parameters. Here I write it in a fixed way.

template<typename Fn>
auto update(Fn &&fn_)
-> decltype(fn_(nullptr, nullptr, 0, 0), void())
{
    Req req;
    Resp resp;
    fn_(&req, &resp, 1, 2);
}

// ...


update([](Req *req, Resp *resp, int i, int j)
       {
           std::cout << i << " : " << j << std::endl;
           req->test();
           resp->test();
       });

I want to write like this

[](int i, double j, float k ....)

I want to add some other parameters.

How can I use template magic to do it in a flexible way?

Thanks for any help !!!

I want to use c++11 to implement this.