Pointers

[Bango]

Hi there,

I have a question about pointers. I have two pointers: pnArray and pnPtr. The former points to a dynamically allocated array of integers and the latter points to the former. This is my code:

#include <iostream>

void print_array(int *pnArray, int nCount)
{
    using namespace std;

    for(int i=0; i<nCount; ++i)
        cout << *(pnArray+i) << ", ";

    cout << endl;
}

int main()
{
    int *pnArray = new int[5];

    for(int i=0; i<5; ++i)
        pnArray[i] = i+i;

    print_array(pnArray, 5);

    int *pnPtr;
    pnPtr = pnArray;
    print_array(pnPtr, 5);

    delete[] pnArray;
    pnArray = 0;

    print_array(pnPtr, 5);

    return 0;
}

After freeing the allocated memory which is being pointed to by pnArray, I try to print what is pnPtr pointing to. I was expecting a crash but to my surprise, pnPtr actually did print what pnArray was pointing to!

How is this happening? Apologies if the answer is obvious, I'm still learning C++.

Thanks!

[ZuK]

You were lucky the freed array was still in memory at the same location, propably because you never new'ed any new data after the delete. Using a deleted object gives undefined behavour. Anything can happen.

Kurt

[Elysia]

Trying to use freed memory is undefined behaviour. The standard doesn't say what happens.
For a more realistic explanation, most operating systems don't always block access to freed memory due to performance reasons.

[Bango]

Oh I see. Thanks for the quick response!