Dynamic memory and undeclared variables

Why does this code work?

Code:

---

#include <iostream>
using namespace std;

int main()
{
        int * p;
        p = new int;

        p[0] = 3;
        p[1] = 7;
        p[2] = 4;
        cout << p[0] << endl;
        cout << p[1] << endl;
        cout << p[2] << endl;

        return 0;
}

---

It has access to p[1 and 2] even though I never declared

Code:

---

p = new int [3];

---

If it's valid code, whats the point of declaring how many new you want?

In general, it doesn't work. You just lucked out. If you compile or run it again under different conditions, it will eventually crash.

Just to hammer the point home, try this:

Code:

---

#include <iostream>
using namespace std;

int main()
{
        int * p;
        p = new int;

        for (int i=0; i<1000000; i++) {
          p[i] = i;
          cout << p[i] << endl;
        }

        return 0;
}

---

and see how far it runs before you get a segmentation fault. In my case it got up to 33789. In general it depends on the exact conditions when you run it, but there's no guarantee it will get any farther than exactly what is allocated, which is just the first element.

> Why does this code work?

Because it is undefined behavior accessing memory outside the boundaries of the variable. It's not a guaranteed crash.

In most cases it will overwrite memory used by the next new. And crash will occure during delete of that memory for example...

Thanks for the responses, I understand it now.