c++ - "unpacking" a tuple to call a matching function pointer

I'm trying to store in a std::tuple a varying number of values, which will later be used as arguments for a call to a function pointer which matches the stored types.

I've created a simplified example showing the problem I'm struggling to solve:

#include 
#include 


void f(int a, double b, void* c) {
  std::cout << a << ":" << b << ":" << c << std::endl;
}

template 
struct save_it_for_later {
  std::tuple params;
  void (*func)(Args...);

  void delayed_dispatch() {

     // How can I "unpack" params to call func?
     func(std::get<0>(params), std::get<1>(params), std::get<2>(params));
     // But I *really* don't want to write 20 versions of dispatch so I'd rather 
     // write something like:
     func(params...); // Not legal
  }
};

int main() {
  int a=666;

  double b = -1.234;
  void *c = NULL;

  save_it_for_later saved = {
                                 std::tuple(a,b,c), f};
  saved.delayed_dispatch();
}

Normally for problems involving std::tuple or variadic templates I'd write another template like template to recursively evaluate all of the types one by one, but I can't see a way of doing that for dispatching a function call.

The real motivation for this is somewhat more complex and it's mostly just a learning exercise anyway. You can assume that I'm handed the tuple by contract from another interface, so can't be changed but that the desire to unpack it into a function call is mine. This rules out using std::bind as a cheap way to sidestep the underlying problem.

What's a clean way of dispatching the call using the std::tuple, or an alternative better way of achieving the same net result of storing/forwarding some values and a function pointer until an arbitrary future point?

Answer

You need to build a parameter pack of numbers and unpack them

template
struct seq { };


template
struct gens : gens { };

template
struct gens<0, S...> {
  typedef seq type;
};


// ...

  void delayed_dispatch() {
     callFunc(typename gens::type());
  }

  template
  void callFunc(seq) {
     func(std::get(params) ...);
  }
// ...