如何从类方法对 C++ 中的类进行操作？

在伪代码领域，如果我想用 Java 执行某些操作，我可以去

class Dragon
{
   //some code here defining what a Dragon is
}

class Knight
{
   //some code here defining what a Knight is

   public void Attack(Dragon dragon)   // <----- specifically this
   {
      //define an attack
   }
}
   
class Main
{
    public static void main (String[] args)
    {
         Knight knight = new Knight;
         Dragon dragon1 = new Dragon;
         Dragon dragon2 = new Dragon;
         knight.Attack(dragon1);     // <----- specifically this
    }
}

我将如何在 C++ 中做到这一点？当我尝试使用以下代码时，有人告诉我error: unknown type name 'Dragon'

#include <iostream>
#include <list>
#include <string>

class IObserver {
 public:
  virtual ~IObserver(){};
  virtual void Update(const std::string &message_from_subject) = 0;
};

class ISubject {
 public:
  virtual ~ISubject(){};
  virtual void Attach(IObserver *observer) = 0;

  virtual void Notify() = 0;
};

class Knight : public ISubject {
 public:

  void Attach(IObserver *observer) override {
    list_observer_.push_back(observer);
  }


  void Notify() override {
    std::list<IObserver *>::iterator iterator = list_observer_.begin();
    while (iterator != list_observer_.end()) {
      (*iterator)->Update(message_);
      ++iterator;
    }
  }

  void CreateMessage(std::string message = "Empty") {
    this->message_ = message;
    Notify();
  }

  void Attack(Dragon dragon) {    //<-----------right here
    this->message_ = "I am attacking";
    Notify();
    std::cout << "todo\n";
  }

 private:
  std::list<IObserver *> list_observer_;
  std::string message_;
};

class Dragon : public ISubject {
 public:

  void Attach(IObserver *observer) override {
    list_observer_.push_back(observer);
  }


  void Notify() override {
    std::list<IObserver *>::iterator iterator = list_observer_.begin();
    while (iterator != list_observer_.end()) {
      (*iterator)->Update(message_);
      ++iterator;
    }
  }

  void CreateMessage(std::string message = "Empty") {
    this->message_ = message;
    Notify();
  }


  void CallForHelp() {
    this->message_ = "I'm under attack";
    Notify();
    std::cout << "todo\n";
  }

 private:
  std::list<IObserver *> list_observer_;
  std::string message_;
};

class GameManager : public IObserver {
 public:
  GameManager(Knight &subject) : subject_(subject) {
    this->subject_.Attach(this);
    std::cout << "GameManager " << ++GameManager::static_number_ << " online\n";
    this->number_ = GameManager::static_number_;
  }

  void Update(const std::string &message_from_subject) override {
    message_from_subject_ = message_from_subject;
    PrintInfo();
  }

  void PrintInfo() {
    std::cout << "GameManager " << this->number_ << ": a new message is available --> " << this->message_from_subject_ << "\n";
  }

 private:
  std::string message_from_subject_;
  Knight &subject_;
  static int static_number_;
  int number_;
};

int GameManager::static_number_ = 0;

void ClientCode() {
  Knight *knight = new Knight;
  GameManager *gameManager = new GameManager(*knight);

  knight->CreateMessage("I exist");
  knight->CreateMessage("Going to attack dragon1");
  knight->Attack();

}

int main() {
  ClientCode();
  return 0;
}

我一定错过了一些东西，但我觉得这应该有效。我对抽象类或 C++ 不是很了解，但是应该可以进行等效的操作，对吗？