Class Destructor

Question

In a class if i declare a destructor and a operator like below, then a destructor is called. For e.g.

#include <iostream>
using namespace std;

class CRectangle {
    int width, height;
  public:
    static int n;
    CRectangle (int,int);
    ~CRectangle ();
    int area () {return (width * height);}
    CRectangle operator + (CRectangle);
};
CRectangle CRectangle::operator+ (CRectangle param){
    x+=param.x;
    y+=param.y;
}
CRectangle::CRectangle (int a, int b) {
  width = a;
  height = b;
  n++;
}

CRectangle::~CRectangle () {
    n--;
}
CRectangle::n=0;
int main () {
  CRectangle rect (3,4), rectb (5,6);
  cout << "rect area: " << rect.area() << endl;
  cout << "rectb area: " << rectb.area() << endl;
  rect=rect+rectb;
  return 0;
}

Why does the destructor called when i am doing the operation +?? the final value of n is coming -1 after the program terminates....

destructor does not delete an instance of class, it is piece of code that is ran before an instance is deleted. — Öö Tiib, Feb 19 '13 at 20:57
does it assign width and height to 0 so as to the area would now be zero? i am just interested in the functioning.. i m not using it — Akshat Aggarwal, Feb 19 '13 at 20:57
It does assign 0 to both. But both `width` and `height`, as well as the whole `CRectangle` object, cease to exist by the time the destructor is done. So there is no point in doing this. — juanchopanza, Feb 19 '13 at 21:03
this looks like an xy problem. op thinks this is a solution to something despite the fact that it makes no sense. — thang, Feb 19 '13 at 21:27
@AkshatAggarwal: It is unclear what you really want. You can set the members to 0, but the object will be destroyed anyway on completion of the destructor, the values of the members are irrelevant at that point. — David Rodríguez - dribeas, Feb 19 '13 at 21:44
well actually i was considering it as a static member of the class and thinking of modifying it in whenever an object is created or destroyed.. got the point thanx :) — Akshat Aggarwal, Feb 20 '13 at 11:04
In the future, please start with that. It will make your questions clearer for everyone. — Michael Kristofik, Feb 21 '13 at 17:20

score 2 · Accepted Answer · edited May 23 '17 at 12:12

A destructor is just like a normal void function in the sense that you can technically do whatever you want. That includes modifying member variables that are about to be destroyed (pointless). Normally you use this function to clean up dynamically allocated memory or to free resources held by the object.

The destructor is normally called for you when an object goes out of scope. Forcing the width and height of your object to 0 would make the area 0, but you wouldn't be able to call area() on it anyway. It's already gone.

Update:

Upon seeing more code, I see why your final value of n is wrong. You're missing a copy constructor. The compiler generates one for you if you don't provide it, and it doesn't know to increment n. Your operator+ doesn't look right either (copy and paste error?), but I can only assume there's some copying going on somewhere in there or else it wouldn't compile. Try adding this:

CRectangle::CRectangle(const CRectangle &rhs) : width(rhs.width), height(rhs.height)
{
    ++n;
}

This is a classic example of the Rule of Three. A class that needs a copy constructor, assignment, or destructor usually needs all three. And if you're using C++11, it becomes the rule of Three, Four, or Five.

why should i increment n in a copy constructor? i am not deleting anything while copying, then what exactly is happening? — Akshat Aggarwal, Feb 28 '13 at 11:16
Because `n` counts how many Rectangles there are. The copy constructor creates a new Rectangle, so you should count it. — Michael Kristofik, Feb 28 '13 at 15:18

score 0 · Answer 2 · edited Feb 19 '13 at 22:39

width=height=0;

This sets the value of width and height to 0 but is useless after the destructor is done because you shouldn't be using the object any more, or trying to access its internal variables. Perhaps you could use those variables somewhere else in the desctructor but I am not sure what you're trying to do but based on the example. It is not obvious why you're trying this or what the 'real' problem you want to solve is.

If you will need those values later on have some globals that the destructor sets. Then each object's destructor will alter the globals.

You can set the variable values, but after the destructor is done you shouldn't be using them anyway so this example is of little practical use.

Class Destructor

2 Answers2