Why destructors are not called in reverse order for array of objects?

Question

Destructors are called in reverse order of object creation in C++ but I do not understand why it is not maintained for array of objects.

#include <iostream>
using namespace std;
class test {
    int nmbr;
    static int c;
public:
    test(int j)
    {
        cout<<"constructor is called for object no :"<<j<<endl;
        nmbr=j;
    };
    ~test()
    {
        c++;
        cout<<"destructor is called for object no :"<<c<<endl;
    };
};

int test::c=0;

int main()
{
  test ob[]={test(1),test(2),test(3)};
  return 0;
}

The above program outputs

constructor is called for object no :1
constructor is called for object no :2
constructor is called for object no :3
destructor is called for object no :1
destructor is called for object no :2
destructor is called for object no :3

But why destructors are not called in reverse order?

Try `test ob[]={test(97),test(1043),test(-12)};` and see what happens. — molbdnilo, Jul 06 '15 at 12:50
or set test::c=5 or any random number other than 0 . you will understand — Madhu Kumar Dadi, Jul 06 '15 at 13:02

Anon · Accepted Answer · 2015-07-06T12:50:32.037

It is called in reverse order. You are printing variable c. Look at my comment in this program - "I changed here". You were printing a count variable where you should have printed the object that was being destroyed.

You can read more here: https://isocpp.org/wiki/faq/dtors#order-dtors-for-arrays

#include <iostream>
using namespace std;
class test {
    int nmbr;
    static int c;
public:
    test(int j)
    {
        cout<<"constructor is called for object no :"<<j<<endl;
        nmbr=j;
    };
    ~test()
    {
        c++;
        // I changed here
        cout<<"destructor is called for object no :"<<nmbr<<endl;
    };
};

int test::c=0;

int main()
{
  test ob[]={test(1),test(2),test(3)};
  return 0;
}

Silly mistake.Thanks, now should i delete this post as there is a very little probability that this question will help anyone in future? — Farsan Rashid, Jul 06 '15 at 13:23

score 3 · Answer 2 · answered Jul 06 '15 at 12:45

3

They are, the error is with your test. In calling the destructor you are accessing a number you are setting yourself. Change the destructor output to show the actual value within the class and you will see for yourself

cout<<"destructor is called for object no :"<<nmbr<<endl;

answered Jul 06 '15 at 12:45

Modred

249
3
15

1

Nope, writing at the same time, happens. Can't see any reason for the downvote for that – Modred Jul 06 '15 at 12:51

score 1 · Answer 3 · answered Jul 06 '15 at 13:02

The destructors are called in reverse order of the constructor calls.

It is your test that is producing and printing values incorrectly.

Try this code, and you will see the expected results.

#include <iostream>

class test {
    int this_instance;
    static int number_of_instances;
public:
    test() : this_instance(number_of_instances++)
    {
        std::cout << "constructor invoked for object :"<< this_instance << '\n';
    };
    ~test()
    {
        std::cout << "destructor invoked for object :" << this_instance << '\n';
    };
};

int test::number_of_instances = 0;

int main()
{
  test first_batch[4];

  return 0;
}

Why destructors are not called in reverse order for array of objects?

3 Answers3

Linked