New/delete - problem with memory.

This is a discussion on New/delete - problem with memory. within the C++ Programming forums, part of the General Programming Boards category; Hi, I have such code: Code: #include <cstdio> class n { public: int *num; n do_something(n& a, n& b) { ...

  1. #1
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    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.

  2. #2
    Rabble Rouser Slacker's Avatar
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    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.

  3. #3
    Registered User hk_mp5kpdw's Avatar
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    And the solution of course is to always create a custom copy constructor when dealing with a class that uses pointers/dynamic memory. You cannot rely on the default copy constructor to work in these cases for this very reason.
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
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  4. #4
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    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.

  5. #5
    Rabble Rouser Slacker's Avatar
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    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;
    }

  6. #6
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    Ok, I understand. Thanks.
    Regards,
    apacz

  7. #7
    C++ Witch laserlight's Avatar
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    hmm... there's no need to check for self-assignment in the copy constructor, is there? After all, the object being constructed cannot possibly be the object copied.
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  8. #8
    Rabble Rouser Slacker's Avatar
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    >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?

  9. #9
    Cat without Hat CornedBee's Avatar
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    Since you can implement copy construction in terms of copy assignment (default construct and assign), but not the other way round, it doesn't make sense to put the self-assignment test in the copy constructor, since only copy assignment needs it.
    All the buzzt!
    CornedBee

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  10. #10
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    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.

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