如何通过 class 方法对 C++ 中的 class 执行操作？

Question

在伪代码领域，如果我想对 Java 中的内容采取行动，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
    }
}

我如何在 C++ 中执行此操作？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;
}

我一定是遗漏了什么，但我觉得这应该有用。我对抽象类或 C++ 不是很了解，但应该可以执行等效的操作，对吗？

Answer 1

在 C++ 中，当在下面声明 class 或任何其他标识符时，就编译器所知，它是未声明的。你需要在龙下面声明骑士。

还要注意按值、引用或指针将 class 作为参数传递的区别。在大多数情况下，您会希望通过引用传递 class，例如 Dragon& dragon，而不是 Dragon dragon 等值。这样可以防止不必要的复制。

Answer 2

C 和 C++ 代码与 Java 不同，它们是一次性编译的，因此 classes 在定义之前不能被引用。当您在定义 class Dragon 之前引用 class Dragon 时，编译器无法识别它。在使用它之前，您必须在代码的前面定义 class Dragon;。

如何通过 class 方法对 C++ 中的 class 执行操作？

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