Вольная вариация на тему std::latch, но для ситуации, когда значение счетчика заранее неизвестно.

sketch.cpp

class meeting_room_t {
   std::mutex lock_;
   std::condition_variable wakeup_cv_;

   unsigned attenders_{};

public:
   meeting_room_t() = default;

   void enter() {
      std::lock_guard<std::mutex> lock{lock_};
      ++attenders_;
   }

   void leave() {
      std::lock_guard<std::mutex> lock{lock_};
      --attenders_;
      if(!attenders_)
         wakeup_cv_.notify_all();
   }

   void wait_for_emptiness() {
      std::unique_lock<std::mutex> lock{lock_};
      if(attenders_)
      {
         wakeup_cv_.wait(lock, [this]{ return 0u == attenders_; });
      }
   }
};

class auto_enter_leave_t {
   meeting_room_t & room_;

public:
   auto_enter_leave_t(meeting_room_t & room) : room_{room} {
      room_.enter();
   }
   ~auto_enter_leave_t() {
      room_.leave();
   }
};