Abusing pointer casts to store any function and call it with a parameter pack at runtime

fn-recasting.cc

#include <iostream>
#include <string>
#include <functional>
#include <tuple>

using namespace std;

template <typename... TArgs>
void cast_apply(function<void()> fn, tuple<TArgs...> tup) {
	function<void(TArgs...)> realFn = *reinterpret_cast<function<void(TArgs...)>*>(&fn);
	apply(realFn, tup);
}

int main() {
	int foo = 10;
	function<void(int, int, string)> fn = [&foo](int a, int b, string c) {
		cout << "foo=" << foo << endl;
		cout << "a=" << a << endl;
		cout << "b=" << b << endl;
		cout << "c=" << c << endl;
	};
	fn(1, 2, "first");
	cout << "---" << endl;

	function<void()> opaque = *reinterpret_cast<function<void()>*>(&fn);
	auto tup = make_tuple(42, 69, string("second"));
	foo = 9001;
	cast_apply(opaque, tup);
	return 0;
}