Negative indexed arrays.

In first describing how rediculous C and C++ was to my class when I was in my first C++ programming course, I remember him saying that you can even create negative indexed arrays. Now that I am a bit wiser in the ways of C++ and memory management, I actually know how to do it!

The following small C++ program illustrates:

#include <iostream>
using namespace std;
int main() {
    const char** array = new const char*[2];
    array[0] = "Hello";
    array[1] = "World!";
    cout << (++array)[-1] << endl;
    delete --array;
}

When executed, it gives us this:

$ g++ -Wall NegArray.cpp -o NegArray && ./NegArray
Hello

The reason this happens is that since arrays in C and C++ are linear in memory, you can do math on them (the ++array and --array).
So if the array looks like this:

 0xFADC  0xFADD  <-- This is the address
-----------------
| Hello | World |
-----------------

If you printed your array (without an index), you would see 0xFADC... the address of the first element.
Therefore if you add one to that, your array's element 0 would point at memory address 0xFADD, and it's element -1 would point at memory address 0xFADC.

REMEMBER: Don't try to delete an array like this before restoring it to the way it originally was or you will get a segfault ^_^.

A shorter way of doing it that doesn't involve using new is:

#include <iostream>
#include <string>
using namespace std;
int main() {
    const string array[2] = { "Hello", "World!" };
    cout << ((const string*) &array + 1)[-1] << endl;
}