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