Exercises in this lecture  previous -- Keyboard shortcut: 'p'        Go to the slide, where this exercise belongs -- Keyboard shortcut: 'u'  

Exercise 4.3
Point destruction - now with a problematic point copy constructor

We continue our work with class Point, using a dynamically allocated point representation. It may be beneficial to review the two version of the classes (together with client programs) on the accompanying slide.

In the version below we add a copy constructor to class Point. This constructor turns out to be problematic.

// Redoing the example - same header file as the previous version.

class Point {
private: 
  double *point_representation;

public:
  Point();                 
  Point(double, double);    
  Point(const Point&);           // A new copy constructor
  ~Point();                // Destructor

  double getx () const;
  double gety () const;
  void move(double dx, double dy);
};

std::ostream& operator<<(std::ostream&, const Point&);

The class is implemented as follows:

#include <cmath>
#include <iostream>
#include "point.h"

Point::Point() : point_representation{new double[2]{0.0, 0.0}} {
}

Point::Point(double x_coord, double y_coord) : point_representation{new double[2]{x_coord, y_coord}}{
}

Point::Point(const Point& p): point_representation(p.point_representation){   // ATTEMPTING copy construction.
}

Point::~Point(){
  std::cout << "Deleting point" << "(" << getx() << "," << gety() << ")" << std::endl;
  delete[] point_representation;
}


double Point::getx () const{
  return point_representation[0];
}

double Point::gety () const{
  return point_representation[1];
}

void Point::move(double dx, double dy){
  point_representation[0] += dx;
  point_representation[1] += dy;
}

std::ostream& operator<<(std::ostream& s, const Point& p){
  return s << "(" << p.getx() << "," << p.gety() << ")" ;
}

There are problems in the following program: (1) When point r is moved (using the move member function) s is also moved. This is not as intended. In addition, the program aborts on exit from the function f.

#include <iostream>
#include "point.h"

using namespace std;

int f(){

  Point p, q,                         
        r(11.0, 12.0);

  cout << "Point p: "      << p << endl;         // (0,0)
  cout << "Point q: "      << q << endl;         // (0,0)
  cout << "Point r: "      << r << endl;         // (11,12)

  Point s(r);                                    // s is intended to be a copy of r.
  cout << "Point s: "      << s << endl << endl; // (11,12)

  r.move(1.0, 2.0);                              //  Moves r.

  cout << "Point s: "      << s << endl;         // (12,14)   !! s is ALSO MOVED - why?
  cout << "Point r: "      << r << endl << endl; // (12,14)

  // PROGRAM ABORTS ON EXIT FROM F - why?
}

int main(){
  f();
}

Explain both of the problems, and make a correct version of class Point.

Solution