CODEWITHSUNDEEP
c++arraysdelete-operator
Vikram Singh
Vikram Singh

Reputation: 435

Getting "heap corruption detected" while calling delete[] on couple of arrays in my class - C++

I have the following code, where I am allocating memory for three int* in the constructor of the class. And deleting it later, in the destructor of the class. I am having an issue deleting two of the int*, in the destructor (I have put the comments in the code below, where I am having the issue):

#define CAP 3
class SetOfStacks
{
private:
    int* a1;
    int* a2;
    int* a3;
    int index = -1;
public:
    void push(int data);
    SetOfStacks()
    {
        a1 = new int[CAP];
        a2 = new int[CAP];
        a3 = new int[CAP];
    }
    ~SetOfStacks()
    {
        delete [] a1; //This works just fine
        delete [] a2; //heap corruption here
        delete [] a3; //heap corruption here
    }
};

void SetOfStacks::push(int data)
{
    if (index >= 3 * CAP)
    {
        cout << "stack overflow" << endl;
        return;
    }
    if(index>=-1 && index<=1)
    {
        index++;
        a1[index] = data;
    }
    else if (index >1  && index<=4)
    {
        index++;
        a2[index] = data;
    }
    else if (index >4 && index<=7)
    {
        index++;
        a3[index] = data;
    }
}


int main()
{
    SetOfStacks s;
    s.push(10);
    s.push(20);
    s.push(30);;
    s.push(40);
    s.push(50);
    s.push(60);
    s.push(70);
    s.push(80);
    s.push(90);

    return 0;
}

I debugged the code many times, but, I am not sure why I am getting HEAP CORRUPTION when I delete[] a2 and when I delete[] a3. Performing delete[] a1 works just fine. I am getting the following error on delete[] a2 and delete[] a3:

heap corruption detected

Do you know why I am getting this error, and what is causing it (I am confused why deleting a1 works fine but, deleting a2 and a3 is giving this error - though they go almost through the same logic in code)?

Upvotes: 1

Views: 805

Answers (2)

elldur
elldur

Reputation: 2103

index is out of your array bounds:

if(index>=-1 && index<=1)
{
    index++;
    a1[index] = data; // index can be 0, 1 or 2 (fine)
}
else if (index >1  && index<=4)
{
    index++;
    a2[index] = data; // index can be 3, 4 or 5 (all out of range)
    // This fixes it:
    // a2[index - 3] = data; // index can now only be 0, 1 or 2
}
else if (index >4 && index<=7)
{
    index++;
    a3[index] = data; // index can be 6, 7 or 8 (all out of range)
    // This fixes it:
    // a3[index - 6] = data; // index can now only be 0, 1 or 2
}

I would also recommend making the conditions more representative of the actual indices being used so this kind of error is easier to spot:

if(0 <= index && index < 3) {
    a1[index++] = data;
} else if (index < 6) {
    a2[index++ - 3] = data
} else if (index < 9) {
    a3[index++ - 6] = data;
}

Upvotes: 1

Jan Schultke
Jan Schultke

Reputation: 39959

All of your arrays have a length of CAP, which is 3 (judging by your if-statements), but you don't relativize your indices when writing to the arrays.

    else if (index >4 && index<=7)
    {
        index++;
        a3[index] = data;
    }

In these lines, you could be writing an index of up to 8 to the array. This is why you are getting the error:

CRT detected that the application wrote to memory after end of heap buffer.

You can fix this by using the modulo operator when writing to the indices using index % CAP. For this to work properly with any CAP, your if-statements should also use >= N * CAP as boundaries.

   else if (index >= 2 * CAP) // no upper check needed in an if-else chain
    {
        a3[index++ % CAP] = data; // you can also do index++ after
    }

Upvotes: 2

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