サブ派生クラスで仮想ベースを構築する理由

Question

スーパーコンピュータのコンストラクタでPoweredDeviceを呼び出す必要があるのはなぜですか？ Copierのコンストラクタは、Copierオブジェクトが直接作成されたときに既に初期化されています。

コピー機を簡単にカプセル化する方法はありませんか？ （ちょうどそのコンストラクタを使用し、その基底を気にしないでください）

以下のコードは少し修正を加えてhttp://www.learncpp.com/cpp-tutorial/128-virtual-base-classes/です。

#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); 
} 

出力：

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

Answer 1

組成物を用いて、あなたが

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"; 
    } 
};