== cobalt/io/system_timer.hpp

The system_timer is a simple wrapper around an `asio::system_timer`.

NOTE: If the timer is already expired, the `co_await t.wait()` does not suspend.

[source,cpp]
----
struct system_timer
{
  /// The clock type.
  typedef std::chrono::system_clock clock_type;

  /// The duration type of the clock.
  typedef typename clock_type::duration duration;

  /// The time point type of the clock.
  typedef typename clock_type::time_point time_point;

  system_timer(const cobalt::executor & executor = this_thread::get_executor());
  system_timer(const time_point& expiry_time, const cobalt::executor & executor = this_thread::get_executor());
  system_timer(const duration& expiry_time,   const cobalt::executor & executor = this_thread::get_executor());

  // cancel the timer. This is safe to call from another thread
  void cancel();

  // The current expiration time.
  time_point expiry() const;
  // Rest the expiry time, either with an absolute time or a duration.
  void reset(const time_point& expiry_time);
  void reset(const duration& expiry_time);

  // Check if the timer is already expired.
  bool expired() const;
  // Get the wait operation. 
  [[nodiscard]] wait_op wait();


};
----