std::thread task_thread;
std::atomic<bool> task_cancelled(false);

void
gui_thread()
{
  while (true) {
    event_data event = get_event();
    if (event.type == quit)
      break;
    process(event);
  }
}

void
task()
{
  while (!task_complete() && !task_cancelled) {
    do_next_operation();
  }
  if (task_cancelled) {
    perform_cleanup();
  }
  else {
    post_gui_event(task_complete);
  }
}

void
process(event_data const& event)
{
  switch (event.type) {
  case start_task:
    task_cancelled = false;
    task_thread    = std::thread(task);
    break;
  case stop_task:
    task_cancelled = true;
    task_thread.join();
    break;
  case task_complete:
    task_thread.join();
    display_results();
    break;
  default:
    //...
  }
}