"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 <iostream>
#include <tuple>

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

template <typename ...Args>
struct save_it_for_later {
  std::tuple<Args...> 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<int,double,void*> saved = {
                                 std::tuple<int,double,void*>(a,b,c), f};

Normally for problems involving std::tuple or variadic templates I'd write another template like template <typename Head, typename ...Tail> 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?


  • The C++17 solution is simply to use std::apply:

    auto f = [](int a, double b, std::string c) { std::cout<<a<<" "<<b<<" "<<c<< std::endl; };
    auto params = std::make_tuple(1,2.0,"Hello");
    std::apply(f, params);

    Just felt that should be stated once in an answer in this thread (after it already appeared in one of the comments).

    The basic C++14 solution is still missing in this thread. EDIT: No, it's actually there in the answer of Walter.

    This function is given:

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

    Call it with the following snippet:

    template<typename Function, typename Tuple, size_t ... I>
    auto call(Function f, Tuple t, std::index_sequence<I ...>)
         return f(std::get<I>(t) ...);
    template<typename Function, typename Tuple>
    auto call(Function f, Tuple t)
        static constexpr auto size = std::tuple_size<Tuple>::value;
        return call(f, t, std::make_index_sequence<size>{});


    int main()
        std::tuple<int, double, int*> t;
        //or std::array<int, 3> t;
        //or std::pair<int, double> t;
        call(f, t);