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:
        //...
    }
}