C++ std::async is being called synchronously when inside a switch statement

I am using c++, and want to do asynchronous programming. I tried the following code:

#include <thread>
#include <chrono>
#include <iostream>
#include <future>

void f(int id) {

  switch (id) {
    case 28: {
      std::this_thread::sleep_for(std::chrono::milliseconds(1000));
      break;
    }
    case 9: {
      std::async(f, 28); //I am using std::async instead of std::thread because I want to get a return value from it in my real code
      break;
    }
  }

  std::cout << "Test For " << id << std::endl;
}

int main() {
  f(9);
}

and this prints

Test For 28
Test For 9

(and the entire message gets printed after 1 second) What i want to happen is

Test For 9
Test For 28

(and I want the two messages to be printed 1 second from each other)

then I tried to do it wil 100000 milliseconds, but the same thing happens (except it takes much longer).

is there a reason for this not happening?

this also does not work

#include <thread>
#include <chrono>
#include <iostream>
#include <future>

int f(int id) {

  switch (id) {
    case 28: {
      std::this_thread::sleep_for(std::chrono::milliseconds(100000));
      break;
    }
    case 9: {
      std::future<int> a = std::async(f, 28);
      break;
    }
  }

  std::cout << "Test For " << id << std::endl;
  return 0;
}

int main() {
  f(9);
}

as it produced the same result