Flexo Flexo - 22 days ago 7
C++ Question

"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};
saved.delayed_dispatch();
}


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?

Answer

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

template<int ...>
struct seq { };

template<int N, int ...S>
struct gens : gens<N-1, N-1, S...> { };

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


// ...
  void delayed_dispatch() {
     callFunc(typename gens<sizeof...(Args)>::type());
  }

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