Thread: New/delete - problem with memory.

Hi,
I have such code:

Code:

#include <cstdio>

class n
{
public:
        int     *num;
        n do_something(n& a, n& b)
        {
                n       lala=a;
                return  lala;
        }
        n(int i =1)
        {
                num=new int[i];
        }
        ~n()
        {
                delete[] num;
        }
};

int main()
{
        n       a(5),
                b(3);
        n       c=a.do_something(a,b);
        return 0;
}

maybe it makes no sense but I have a similar situation in my program so I just simplified it a little bit.
and I receive a warning:
a.out in free(): warning: chunk is already free
And I can't see any situation where the hell some pieace of memory is freed twice. Can anyone explain me that ?
Regards.
apacz.

The problem is here:

Code:

n       lala=a;

You're using the default copy constructor to initialize lala, which copies the pointer but not the dynamic memory. So you have two objects which alias the same block of memory, and when the destructor is called for both, you're freeing the memory twice.

Thank you for replies. I got it now. I have modified my code a little bit:

Code:

#include <cstdio>

class n
{
public:
        int     *num,
                size;
        n& operator=(const n& a)
        {
                printf("hm\n");
                size=a.size;
                num=new int[size];
                for(int i = 0; i < size; i++)
                        num[i]=a.num[i];
                return  *this;
        }
        n do_something(n& a, n& b)
        {
                n       lala=a;
                return  lala;
        }
        n(int i =1)
        {
                size=i;
                num=new int[i];
                for(int k = 0; k < size; k++)
                        num[k]=0;
        }
        ~n()
        {
                delete[] num;
        }
};

int main()
{
        n       a(5),
                b(3);
        n       c=a.do_something(a,b);
        return 0;
}

As the way as it should be corrected I hope. But I don't now if I have overloaded the operator = properly ? Becouse it seems not to be called.
Regards.

It's not called because you're using the copy constructor, not the copy assignment operator. This should work better, though it's a good idea to have both a copy constructor and a copy assignment operator if you have one. It's also possible to define the copy constructor in terms of the copy assignment operator, but I'll leave that to you.

Code:

#include <cstdio>

class n
{
public:
  int     *num,
    size;
  n do_something(n& a, n& b)
  {
    n       lala=a;
    return  lala;
  }
  n(int i =1)
  {
    size=i;
    num=new int[i];
    for(int k = 0; k < size; k++)
      num[k]=0;
  }
  n(const n& a)
  {
    // Protect against self assignment
    if (this == &a) return;
    printf("hm\n");
    size=a.size;
    num=new int[size];
    for(int i = 0; i < size; i++)
      num[i]=a.num[i];
  }
  ~n()
  {
    delete[] num;
  }
};

int main()
{
  n       a(5),
    b(3);
  n       c=a.do_something(a,b);
  return 0;
}

Ok, I understand. Thanks.
Regards,
apacz

>After all, the object being constructed cannot possibly be the object copied.
If you're too lazy to write both and self assignment is an important issue for one that's not related to the problem, what would you do?

The copy assignment operator must check for self assignment, and it should also delete the existing pointer. The copy constructor doesn't have to do either.

Also, in all constructors you should make sure num is set to 0 if you don't call new. This isn't a problem with the current code, but it could become a problem if you forget to add it to additional constructors.