爲什麼要在子派生類中構建虛擬基礎？

Question

爲什麼我必須在SuperCopier的構造函數中調用PoweredDevice？直接創建複印機對象時，複印機的構造函數已經初始化。

是否有任何簡單的方法來封裝複印機？ （只是使用它的構造函數，並不在乎它的基地。）

下面的代碼是從http://www.learncpp.com/cpp-tutorial/128-virtual-base-classes/幾乎沒有任何修改。

#include <iostream> 

class PoweredDevice 
{ 
public: 
    PoweredDevice(int power) 
    { 
     std::cout << "PoweredDevice: " << power << '\n'; 
    } 
}; 

class Scanner: virtual public PoweredDevice // note: PoweredDevice is now a virtual base class 
{ 
public: 
    Scanner(int scanner, int power) 
     : PoweredDevice(power) // this line is required to create Scanner objects, but ignored in this case 
    { 
     std::cout << "Scanner: " << scanner << '\n'; 
    } 
}; 

class Printer: virtual public PoweredDevice // note: PoweredDevice is now a virtual base class 
{ 
public: 
    Printer(int printer, int power) 
     : PoweredDevice(power) // this line is required to create Printer objects, but ignored in this case 
    { 
     std::cout << "Printer: " << printer << '\n'; 
    } 
}; 

class Copier: public Scanner, public Printer 
{ 
public: 
    Copier(int scanner, int printer, int power) 
     : PoweredDevice(power), // PoweredDevice is constructed here 
      Scanner(scanner, power), Printer(printer, power) 

    { 
    } 
}; 

class SuperCopier: public Copier 
{ 
public: 
    SuperCopier(int scanner, int printer, int power) 
     : PoweredDevice(power),//This is the line I want to get rid of 
       Copier(scanner, printer, power) 
    { 
     std::cout<<"Super copier constructed\n"; 
    } 
}; 

int main() 
{ 
    Copier copier(1,2,3); 
    SuperCopier superCopier(4, 5, 6); 
} 

輸出：

PoweredDevice: 3 
Scanner: 1 
Printer: 2 
PoweredDevice: 6 
Scanner: 4 
Printer: 5 
Super copier constructed 

Answer 1

隨着組成，你可以做

class PoweredDevice 
{ 
public: 
    explicit PoweredDevice(int power) 
    { 
     std::cout << "PoweredDevice: " << power << '\n'; 
    } 
}; 

class Scanner 
{ 
public: 
    Scanner(PoweredDevice& powerDevice, int scanner) 
     : poweredDevice(powerDevice) 
    { 
     std::cout << "Scanner: " << scanner << '\n'; 
    } 
private: 
    PoweredDevice poweredDevice 
}; 

class Printer 
{ 
public: 
    Printer(PoweredDevice& powerDevice, int printer) 
     : poweredDevice(powerDevice) 
    { 
     std::cout << "Printer: " << printer << '\n'; 
    } 
private: 
    PoweredDevice poweredDevice 
}; 

class Copier: public PoweredDevice, public Scanner, public Printer 
{ 
public: 
    Copier(int scanner, int printer, int power) 
     : PoweredDevice(power), 
      Scanner(*this, scanner), 
      Printer(*this, printer) 
    { 
    } 
}; 

class SuperCopier: public Copier 
{ 
public: 
    SuperCopier(int scanner, int printer, int power) 
     : Copier(scanner, printer, power) 
    { 
     std::cout<<"Super copier constructed\n"; 
    } 
};