Why this destructor affect my code and class objects but without it all work fine?

Question

So i created a class along with 2 constructor, a function for print and a destructor. I tryed to create 2 objects with different values and i encountered some issues with destructor.

I wrote 2 constructors so i can create objects in 2 different ways : like this

myclass o1;
o1=myclass(..parameters..)

or like this

myclass o1(..parametters..)

When i run the code with destructor in it, my first object o1 don't get created correctly, and in some cases when it does, after i create object o2, the first object change it's parameters values and get the same elements as the second one.

What is strange, when i delete the destructor (~myclass) , the code works fine.

Also, i noticed that when i create o1 with parametered constructor like o2, it works fine again.

Why is this happening and how can i solve it ? (sorry for my bad english). Here is the code:

#include <iostream>
using namespace std;
class myclass{
private:
    int a;
    int *b;


public:
    myclass()
    {
        a=0;
        b=NULL;
    }
    myclass(int vect[100], int m){
        int i;
        a=m;
        b=new int[a+1];
        for(i=1;i<=m;++i)
            b[i]=vect[i];

    }

    void print()
    {
        int i;
        for(i=1;i<=a;i++)
        cout << b[i] << " ";
        cout << '\n';

    }
    ~myclass()
    {
        delete[] b;
    }
};

int main()
{
    int i,vect[100],n;
    cin >> n;
    for(i=1;i<=n;++i)
        cin >> vect[i];
    myclass o1;
    o1=myclass(vect,n);

    o1.print();

    cin >> n;
    for(i=1;i<=n;++i)
        cin >> vect[i];
    myclass o2(vect,n);

    o1.print();

    return 0;
}

And here is my input:

4
5 6 7 8
6
10 11 12 13 14 15

and my output:

12058816 6 7 8

10 11 12 13

With the destructor - you are leaking memory, with it, due to the lack of properly defined copy-constructor, you are double-freeing the same memory. — Algirdas Preidžius, Mar 08 '21 at 13:48
Also, your constructor with two arguments and the `print()` function have an off-by-one error. Array indexing in C++ starts at zero, not one. — Peter, Mar 08 '21 at 13:53

score 1 · Accepted Answer · answered Mar 08 '21 at 13:58

The main issue you have is with:

myclass o1;
o1=myclass(vect,n);

Since your class doesn't respect the rule of three (or rule of five), when a temporary is created, it shares int *b;. Then, when the temporary is deleted, your b member is deleted.

From here, you an hack it away with:

myclass o1(vect,n); // no temporaries, less issues

Or, for the proper fix, learn about the rule of three (and five) and apply it.

Why this destructor affect my code and class objects but without it all work fine?

1 Answers1