Why to construct virtual base in sub- derived classes?

Question

Why do I have to call PoweredDevice in SuperCopier's constructor? Copier's constructor already initialize when a Copier object is directly created.

Is not there any easy way to encapsulate Copier? (just use it's constructor and do not care it's bases.)

Below code is from http://www.learncpp.com/cpp-tutorial/128-virtual-base-classes/ with little modifications.

#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);
}

Output:

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

I dont want to leave a complex class hierarchy further developer's those will inherit their new classes from Copier. Of course my classes are more detailed ... — ataman
– ataman, Commented Dec 13, 2016 at 12:54
@Jarod42 construction is not the only case. Classes have interface functions. Then people will fight with mapping interface functions to Copiers' — ataman
– ataman, Commented Dec 13, 2016 at 12:59
For that matter, you could use the decorator pattern as well... — user2393256
– user2393256, Commented Dec 13, 2016 at 13:00

Jarod42 · Accepted Answer · 2016-12-13 13:00:16Z

With composition, you might do

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";
    }
};

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