missing ptr = NULL in destructor causing error, works fine with ptr set to NULL. Why?

Question

If i comment the line arr = 0; in destructor definition, the program terminates with an error. If i uncomment that line i.e., arr is set to 0, then the program executes without any error. Why is it so as there is no need of setting a pointer to NULL. The pointer itself is destroyed after the execution of destructor.

Screenshot of the error

Below is my code.

Array.h

#ifndef ARRAY_H
#define ARRAY_H
class Array
{
    int* arr;
    int size;
public:
    Array(int size = 10);
    Array(const Array& arr);
    ~Array();
    void display () const;
};
#endif

Array.cpp

#include "Array.h"
#include <iostream>
using namespace std;

Array::Array(int size)
{
    arr = new int[size];
    this->size = size;
    for (int i = 0; i < size; i++)
        arr[i] = i;
}

Array::Array(const Array& a)
{
    arr = new int[a.size];
    for (int i = 0; i < a.size; i++)
        arr[i] = a.arr[i];
    size = a.size;
}

Array::~Array()
{
    delete[] arr;
    arr = 0;
}

void Array::display() const
{
    cout << endl;
    for (int i = 0; i < size; i++)
        cout << arr[i] << " ";
    cout << endl;
}

Main.cpp

#include <iostream>
#include "Array.h"
using namespace std;

int main()
{
    Array arr(4);
    Array a1 = arr;
    a1.display();
    arr.~Array();
    a1.display();

    return 0;
}

juanchopanza · Accepted Answer · 2013-11-23 14:15:55Z

4

You shouldn't be calling the destructor here:

arr.~Array();

When arr goes out of scope, the destructor is called a second time. That leads to undefined behaviour when delete[] is called on the pointer data member arr. When you set that to 0, you side-step the problem, but the fact remains that you shouldn't be invoking the destructor like that.

Here's an example showing how the lifetime of an automatic storage object is bound to its scope:

#include <iostream>

struct Foo
{
  ~Foo() { std::cout << "Foo destructor\n";
};

int main()
{
  std::cout << in main()\n";

  {
    Foo f;
  }  // f gets destroyed here

  std::cout << "Exiting main()\n";
};

edited Nov 23, 2013 at 14:15

answered Nov 23, 2013 at 13:53

juanchopanza

228k35 gold badges421 silver badges493 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Umar Chaudhary Over a year ago

When the destructor is called second time? How a destructor is being called again on an already destroyed object that no more exists.

juanchopanza Over a year ago

@UmarShabbir the object exists until main is exited. This is when the destructor gets automatically called.

josefx Over a year ago

@UmarShabbir in c++ the compiler ensures that every object on the stack will be automatically destroyed at the end of the scope it is defined in. While you can destroy the object yourself you have to ensure to create a new object at the same place (placement new) before the compiler generated code calls the destructor on it a second time. Generally you wont find explicit destructor calls or placement new outside of container classes (std::vector) and unless you have a very good reason you should avoid calling them yourself.

Rabbiya Shahid · Accepted Answer · 2013-11-23 13:59:37Z

0

You don't need to explicitly call destructors. They will execute once the program terminates. http://www.learncpp.com/cpp-tutorial/86-destructors/ look at the article under the heading constructor and destructor timing

The destructors are written to deallocate the memory assigned to pointers. Otherwise the pointer is deleted but its memory remains dangling. When the program terminates, all the variables will be lost from the memory, all resources will be deallocated, and during that time the destructor will execute itself.

answered Nov 23, 2013 at 13:59

Rabbiya Shahid

4224 silver badges14 bronze badges

1 Comment

juanchopanza Over a year ago

In this case, the destructor is called when the scope of main is left, not at program termination.

Collectives™ on Stack Overflow

missing ptr = NULL in destructor causing error, works fine with ptr set to NULL. Why?

2 Answers 2

3 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related