c++20 coroutine minidemo

coro.h

#include <concepts>
#include <experimental/coroutine>
#include <iostream>
#include <unistd.h>
#include <thread>
#include <vector>



struct awaitable {
  std::experimental::coroutine_handle<> _h;
  bool await_ready() { return false; }
  void await_suspend(std::experimental::coroutine_handle<> h) {
    _h = h;
  }
  int val = 0;
  int await_resume() { return val;}
};

struct task
{
  struct promise_type
  {
    task get_return_object() { return {}; }
    std::experimental::suspend_never initial_suspend() { printf("initial\n"); return {}; }
    std::experimental::suspend_never final_suspend() noexcept { printf("final\n"); return {}; }
    void return_void() { printf("return\n");}
    void unhandled_exception() {}
  };
};

std::vector<awaitable*> waiters;


task run(int n) {
  awaitable a;
  waiters.push_back(&a);
  printf("start %d\n", n);
  auto x = co_await a;
  printf("done %d %d\n", n, x);
};

int main() {

  printf("s1\n");
  for (int i=0; i<5; i++) run(i);
  printf("s2\n");

  std::thread t( []{
    sleep(1);
    for (auto w: waiters) {
      w->val = 100;
      w->_h.resume();
      sleep(1);
    }
  });
  t.join();
}