Question

I am writing a c++ program implementing the Merge Sort iteratively. The main code is shown below and I cannot understand why I am having the “Access violation writing location 0xXXXXXXXX” error, even though I allocated much more memory (1 gb) in the same way in another program.

void main()
{
    //int a[size];
    int* a =  new int(size); //initialising an int array dynamically contains 16777216 el
    srand(time(NULL));
    for(int i = 0 ; i < size; i++)
    {
        a[i]= 1 + rand() % 10;
    }

    for(int i = 0;  (size / 2) / pow((double)2, i)>= 1; i++)
    {
        int n = pow((double)2, i);
        int offset = 0;
        for(int j = 0; j < (size / 2) / pow((double)2, i); j++)
        {
            int* tmp = new int(n);
            merge(a + offset, n, a + offset + n, n, tmp);
            memcpy(a + offset, tmp, n*2 * sizeof(int));
            offset += pow((double)2, i+1);
        }
    }

    for(int i = 0; i < size; i++)
    {
        cout<<a[i]<<" ";
        //printf("%d ", a[i]);
    }
    cout<<endl;
    system("PAUSE");
}

Answer 1

You allocate only memory enough for 1 integer: new int(size) and assign it as size. So you don’t have 1GB of memory pointed by a. Accessing it beyond 1 integer is undefined behavior, which can give you an access violation.

Change new int(size) to square brackets: new int[size].

And better still, use a std::vector<int>.