Reference variable to a pointer

Question

int main() {

    int x=10;
    int *p=&x;
    int &y =*p;

    cout<< x << endl << &x <<endl;
    cout<< *p << endl << p <<endl;
    cout<< y << endl << &y <<endl;

    p++;
   *p = 20;

   cout<< x << endl << &x <<endl;
   cout<< *p << endl << p <<endl;
   cout<< y <<endl<< &y <<endl;

   return 0;
}

Above is the code which would best explain my question. Normally, a variable reference (&) takes the address of a variable and starts referring to the same. I have tried doing the same through a pointer. I have defined a variable, pointer p points to x and a reference variable y refers to *p. Would this mean that y refers to the same variable x now? As the next step, I stopped pointing to x through *p, what happens to reference variable y now? What would it hold. In the code above, cout<

Can someone help explain the behavior here.

Answer 1

[note: this answer is only about C++. In C, your code shouldn't compile.]

Yes, after you initialize y, it refers to x. Incrementing p doesn't change that -- y still refers to x.

Unfortunately, when you do the p++; *p=20;, you've modified p so it no longer refers to any allocated storage. When you write to it, you get undefined behavior. That means it's perfectly reasonable for the program to halt (or do essentially anything else).

Note, however, that incrementing p is entirely allowable--in this respect, x acts like an array of one element, and forming a pointer to one past the last element of an array is explicitly allowed. It's only when you write to the address that pointer refers to that you get the undefined behavior. (That is: p++; is fine; *p=20; is not).