Why constructor calling constructor creates weird results?

Question

I'm just testing a code shown below (learning oop in c++).

(system: linux, gcc compiler with -std=c++11)

#include <iostream>

class Point {
  private:
    double x, y;

  public:
    static unsigned int pCount;

    // constructor1 with no initial arguments
    Point() {
      x = 0.0f;
      y = 0.0f;
      std::cout << "Point()" << " x = " << x << " y = " << y
          << " point no: " << ++pCount << std::endl;
    }

    /*
    // constructor1 alternative: calling another constructor
    Point() {
      Point(0.0f, 0.0f);
      std::cout << "Point()" << std::endl;
    }
    */

    // constructor2 with initial arguments
    Point(double cx, double cy) {
      x = cx;
      y = cy;
      std::cout << "Point(double, double)" << " x = " << x << " y = " << y
          << " point no: " << ++pCount << std::endl;
    }

    void Show() {
      std::cout << "Show()" << " x = " << x << " y = " << y << std::endl;
    }

    virtual ~Point() {
      std::cout << "~Point()" << " x = " << x << " y = " << y 
          << " point no: " << pCount-- << std::endl;
    }
};

unsigned int Point::pCount = 0;

int main(int argc, char *argv[]) {
  Point p1(2.5, 7.6);
  Point p2;

  p2.Show();
  p1.Show();

  return (0);
}

The code above creates the output as:

Point(double, double) x = 2.5 y = 7.6 point no: 1
Point() x = 0 y = 0 point no: 2
Show() x = 0 y = 0
Show() x = 2.5 y = 7.6
~Point() x = 0 y = 0 point no: 2
~Point() x = 2.5 y = 7.6 point no: 1

However, when I comment-out the constructor1 and uncomment constructor1 alternative, the output created gets a little weird as below:

Point(double, double) x = 2.5 y = 7.6 point no: 1
Point(double, double) x = 0 y = 0 point no: 2
~Point() x = 0 y = 0 point no: 2
Point()
Show() x = 4.64944e-310 y = 9.88131e-324
Show() x = 2.5 y = 7.6
~Point() x = 4.64944e-310 y = 9.88131e-324 point no: 1
~Point() x = 2.5 y = 7.6 point no: 0

I think in the latter case (constructor1 alternative), a temporary object is created and then its pointer is passed to p2. Anyway, so far, so good... But I'm stuck with the values assigned to the variables x & y: The former shows x = 0, y = 0 as expected; however the latter spits out x = 4.63813e-310, y = 9.88131e-324, though values are very close to zero, it still seems weird to me.

Why the latter case does not assign the exact "zero" values?

https://stackoverflow.com/questions/308276/can-i-call-a-constructor-from-another-constructor-do-constructor-chaining-in-c — melpomene, Dec 01 '18 at 13:43

lubgr · Accepted Answer · 2018-12-01T13:48:18.007

I think in the latter case (constructor1 alternative), a temporary object is created and then its pointer is passed to p2

You're right, a temporary is indeed constructed, but it's destroyed right after construction. You might want to use a delegate constructor like this:

Point() : Point(0.0, 0.0) {}

This will call the constructor with two double arguments, and both data members are initialized. Note that the "weird" behavior you were referring to is nothing but uninitialized data - the data members contain garbage values, as they haven't been initialized.

Just as a side note (I realize that you said it's about learning OOP, so don't take that remark too seriously): it seems to me that the two coordinate data members of your Point class can vary independently. Your class hence doens't manage an invariant - in that case, you might want to consider to simplify the type to

struct Point { double x = 0.0; double y = 0.0; };

which allows for default construction

Point p; // both x and y are 0.0 now

as well as aggregate initialization

Point p{3.14, 42.0};

Additional functionality that is part of the interface of your class (printing, distance to another point etc.) can conveniently be defined within free functions.

Why constructor calling constructor creates weird results?

1 Answers1