// Define + and = for class ThreeD
  #include<iostream>
  using namespace std;

  class ThreeD  { 
    public:
      ThreeD();                      // default constructor
      ThreeD(int i, int j, int k);   // explicit constructor  
      ~ThreeD() {}                   // destructor

      ThreeD operator+(ThreeD op2);  // op1 is implied
      ThreeD operator=(ThreeD op2);  // op1 is implied

      void show();

    private:
      int x, y, z;  // 3-d coordinates
    };

  int main()
  {
    ThreeD a(1, 2, 3), b(10, 10, 10), c;

    cout << "Original value of a:  ";
    a.show();
    cout << "Original value of b:  ";
    b.show();
    cout << "Original value of c:  ";
    c.show();
    cout << endl;

    c = a + b;  // add a and b
    cout << "Value of c after c = a + b:  ";
    c.show();
    cout << endl;

    c = b = a;  // demonstrate multiple assignment
    cout << "Value of a after c = b = a:  ";
    a.show();
    cout << "Value of b after c = b = a:  ";
    b.show();
    cout << "Value of c after c = b = a:  ";
    c.show();

    return 0;
  }

  ThreeD::ThreeD()  // default constructor
  {
    x = y = z = 0;
  }

  ThreeD::ThreeD(int i, int j, int k)  // explicit constructor  
  {
    if(i < 0 || j < 0 || k < 0)  {
      cout << "Negative coordinate - program terminated\n";
      exit (1);
    }

    x = i;
    y = j;
    z = k;
  }

  // overload +
  ThreeD ThreeD::operator+(ThreeD op2)
  {
    ThreeD temp;

    // integer additions in the standard sense
    temp.x = x + op2.x;  // temp.x = this->x + op2.x;  
    temp.y = y + op2.y;  // temp.y = this->y + op2.y;  
    temp.z = z + op2.z;  // temp.z = this->z + op2.z;  
    return temp;         // return the new object
  }

  // overload =
  ThreeD ThreeD::operator=(ThreeD op2)
  {
    // integer assignments in the standard sense
    x = op2.x;  // this->x = op2.x;
    y = op2.y;  // this->y = op2.y;
    z = op2.z;  // this->z = op2.z;

    // dereference the this pointer 
    // and return the modified object
    return *this;  
  }

  // show X, Y, and Z coordinates
  void ThreeD::show()
  {
    cout << x << ", ";
    cout << y << ", ";
    cout << z << endl;
  }