使用動態投地返回變量中派生類

Question

不同的值，試圖找到一個辦法讓我的動態轉換工作，但我不斷收到運行時錯誤。它在打印出來時跳轉到else語句塊的值（當它應該是一個if塊的值，但是當我在派生類中調用它時甚至沒有被使用，所以它顯示錯誤的值並且根本不會在。計算這是爲什麼感謝您的幫助

class Package 
{ 
protected: 
    string name_and_address = "?"; 

    double cost = 0.0; 
    double discount = 0.0; 
    double discount_rate = 0.0; 

    bool overnight_delivery = false; 
    bool insured = false; 

    string package_contents = "?"; 

    double shipcost = 0.0; 

public: 
    Package() {}; 
    ~Package() {}; 

protected: 
    virtual double calculate_cost() = 0; 

    class Video_Games {}; //defined the classes 
    class Genius_Phone {}; 
    class Sausage {}; 
    class Albums {}; 

    // here I'm trying to change "shipcost" inside each of the derived classes 
    // to their respective and appropriate dollar values. 
    // However, I am getting a runtime error wherein it will jump DIRECTLY 
    // to the else statement 50.00 value for Video_Games 
    // and not even calculate the value as 50.00. SO it's doubly useless. 
    // It just skips to the else value and doesn't even factor 
    // that into the Video_Games calculation when I try to test it. 
    virtual double shipping_cost() { 
     if (dynamic_cast<Video_Games*>(this)) 
      return 4.99; 
     else if (dynamic_cast<Genius_Phone*>(this)) 
      return 25.00; 
     else if (dynamic_cast<Sausage*>(this)) 
      return 9.00; 
     else 
     { 
      // should be assigned to class Albums, 
      // but is somehow being triggered by class Video_Games. 
      // Not sure why this is. 
      return 50.00; 
     } 
    } 
}; 

class Video_Games :public Package 
{ 
private: 
    int num_games = 0; 

public: 
    Video_Games(string location, int number_of_games, bool express, bool insurance) 
    { 
     num_games = number_of_games; 
     name_and_address = location; 
     overnight_delivery = express; 
     insured = insurance; 

     package_contents = to_string(num_games) + " Video Game(s)"; 

     cost = calculate_cost(); 
     discount = calculate_discount(); 
     shipcost = shipping_cost(); 
    } 

    ~Video_Games() {}; 

protected: 

    double calculate_cost() 
    { 
     cost = num_games * 19.99; 
     // this is where the magic should happen. 
     // shipcost here should be 4.99, but instead it's not even being used here. 
     // In fact - it's empty. I'm not sure why shipcost 
     // when set equal shipping_cost() is not returning that appropriate value. 
     // Very baffling. 
     if (overnight_delivery) { cost += shipcost; } 
     if (insured) { cost *= 1.06; } 

     return cost; 
    } 
}; 

Answer 1

的問題是你是如何定義的其他類：。

class Package 
{ 
    class Video_Games {}; //defined the classes 
    class Genius_Phone {}; 
    class Sausage {}; 
    class Albums {}; 

這些嵌套類型，裏面Package您已經定義了一個類Package::Video_Games其中沒有基類和沒有成員dynamic_cast表達式使用這些種類型，這是不能從Package衍生自：

 if (dynamic_cast<Video_Games*>(this)) 
      return 4.99; 

的Package::shipping_cost()函數內名稱查找找到Package::Video_Games類型，這是一個沒有基類或成員的空類。對象是從未該類型，因爲Package::Video_Games不從Package衍生所以this指針Package*類型的不能指向Package::Video_Games類型的對象。

後來你定義一個新的類型名稱相同，即是從Package得出：

class Video_Games :public Package 
{ 

但是，這是一個新類型。 Video_Games與Package::Video_Games不同。定義派生包對象時，它將使用此類型，而不是演員使用的類型。所以你的演員永遠不會成功。

您需要刪除嵌套類型的定義裏面Package和移動身體的Package::shipping_cost()功能的類外，其他所有類型被定義之後（所以編譯器已見過那些其他類型的定義和認識它們來源於Package）。

class Package 
{ 
    virtual double shipping_cost(); 
    // ... 
}; 

class Video_Games : public Package 
{ 
    // ... 
}; 

// Now you can define the virtual function: 
double Package::shipping_cost() 
{ 
    if (dynamic_cast<Video_Games*>(this)) 
    // ... 
} 

（但正如評論說，這是一個非常糟糕的方式來定義一個多態接口 - 它違背了虛函數的整個目的）

Answer 2

諷刺的是，鑑於您的用戶名，你錯過了使用多態性來提供正確的實現。相反，對類型切換，你可以調用虛方法，這將拿起正確的版本：

class Package { 
public: 
    virtual ~Package(); 
    virtual double shipping_cost() const; 
}; 
class Video_Games : public Package { 
public: 
    double shipping_cost() const override; 
}; 
class Genius_Phone : public Package { 
public: 
    double shipping_cost() const override; 
}; 
class Sausage : public Package { 
public: 
    double shipping_cost() const override; 
}; 

Package::~Package() {} 

double Package::shipping_cost() const 
{ 
    // Default implementation - may be overridden in subclasses 
    return 50.00; 
} 

double Video_Games::shipping_cost() const 
{ 
    return 4.99; 
} 

double Genius_Phone::shipping_cost() const 
{ 
    return 25.99; 
} 

double Sausage::shipping_cost() const 
{ 
    return 9.00; 
} 

我也警告說，使用貨幣值的浮點類型會導致你陷入困境 - 搜索堆棧溢出應找到解釋原因的答案。