如何通过类方法对C++中的类进行操作？

Question

在伪代码领域，如果我想在Java中执行某些操作，我可以这样做

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
    }
}

我该如何在c++中做到这一点？当我尝试使用下面的代码时，我被告知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;
}

我一定是漏掉了什么，但我觉得这应该行得通。我对抽象类或C++不是很了解，但应该有一个等价的操作可以执行，对吧？

Answer 1

在C++中，当一个类或任何其他标识符在下面声明时，只要编译器知道它是未声明的。你需要在龙的下面声明骑士。

还要注意通过值、引用或指针将类作为参数传递之间的区别。在大多数情况下，您可能希望通过引用传递类，比如Dragon& dragon，而不是通过值传递，比如Dragon dragon。这可以防止不必要的副本。

Answer 2

与Java语言不同的是，C和C++代码是在一次遍历中编译的，因此类只有在定义好之后才能被引用。当您在定义class Dragon之前引用类Dragon时，编译器无法识别它。在使用class Dragon;之前，必须先在代码中定义它。