CPP Polymophism and Multiple Inheritance

Multiple Inheritance is possible in C++

• C++ supports the possibility to derive a class from more than one base class.
  A class model:
  
• We can say that a PaperBox is made of paper and has a form of a Cuboid.
  To specify this in a C++ file:

  class PaperBox : public Cuboid, public Paper

• Polymorphism can be used with Multiple Inheritance in the same manner as for a single Inheritance. (You should study the Constructors and Destructors calling sequence in example below as well).
  Multiple Inheritance example:

  #include <iostream>
  #include <string>
  
  enum Quality { BAD, GOOD, EXCELLENT};
  
  class Paper {
  public:
    Paper(Quality quality) {
      this->quality=quality;
      std::cout << "Paper constructor... \n";
    }
    virtual ~Paper() { std::cout << "Paper destructor... \n"; }
    virtual  void showInfo() {
      std::cout << "a ";
      std::cout << getQuality() ;
      std::cout << " paper Quality\n ";}
    void setQuality(Quality quality ) {
      this->quality=quality;  }
  private:
    Quality quality;
    std::string getQuality() {
      switch (quality) {
        case BAD : return "Bad";
        case GOOD : return "Good";
        case EXCELLENT : return "Excellent";
        default: return "Unknown";
      }
    }
  
  };
  
  class Cuboid {
  public:
    Cuboid(int width, int height, int length ) {
      std::cout << "Cuboid constructor... \n";
      this->width=width;
      this->height=height;
      this->length=length;
    }
    virtual ~Cuboid() { std::cout << "Cuboid destructor... \n"; }
    virtual  void showInfo() {
      std::cout << "the volume is ";
      std::cout << getVolume() << "\n"; }
    void setLength(int length) { this->length=length; }
    int getVolume() const { return length*width*length; }
  private:
    int width;
    int height;
    int length;
  };
  // PaperBox has Cuboid class and Paper class as Base classes;
  class PaperBox : public Cuboid, public Paper {
  public:
    PaperBox(int width, int height, int length ):
        Cuboid( width, height,length),Paper(BAD) {
          std::cout << "PaperBox constructor... \n";
        }
        ~PaperBox() { std::cout << "PaperBox destructor... \n"; }
        void showInfo() {
          std::cout << "I am a PaperBox and ";
          Cuboid::showInfo();
          std::cout << "with ";
          Paper::showInfo();
        }
  };
  
  int main()
  {
    Paper * box1= new PaperBox(4,5,6);
    std::cout << "\n" ;
    box1->showInfo();
    std::cout << "\n" ;
    box1->setQuality(EXCELLENT);
    // down cast to set length of the box;
    PaperBox * box1Casted = dynamic_cast<PaperBox*>(box1);
    box1Casted->setLength(7);
    box1Casted->showInfo();
    std::cout << "\n" ;
    delete box1;
    std::cout << "\n" ;
  
    Cuboid * box2= new PaperBox(2,3,4);
    std::cout << "\n" ;
    box2->showInfo();
    std::cout << "\n" ;
    box2->setLength(8);
    // down cast to set quality of the box;
    PaperBox * box2Casted = dynamic_cast<PaperBox*>(box2);
    box2Casted->setQuality(GOOD);
    box2Casted->showInfo();
    std::cout << "\n" ;
    delete box2;
    std::cout << "\n" ;
  
    return 0;
  }

  When we run this application, the result will be:

  Cuboid constructor...
  Paper constructor...
  PaperBox constructor...
  
  I am a PaperBox and the volume is 144
  with a Bad paper Quality
  
  I am a PaperBox and the volume is 196
  with a Excellent paper Quality
  
  PaperBox destructor...
  Paper destructor...
  Cuboid destructor...
  
  Cuboid constructor...
  Paper constructor...
  PaperBox constructor...
  
  I am a PaperBox and the volume is 32
  with a Bad paper Quality
  
  I am a PaperBox and the volume is 128
  with a Good paper Quality
  
  PaperBox destructor...
  Paper destructor...
  Cuboid destructor...

  You can download this example here (needed tools can be found in the right menu on this page).