Question

This may be simple but it confuses me.

int x;
int *p = NULL;
int *q = &x;

What happens when

 q = p;   // Address where q points to equals NULL  .
 &x = q;  // I don't think this is possible  .
 *q = 7;  // Value of memory where q is pointing to is 7?  
 *q = &x  // That's just placing the address of x into memory where q points to right?  
 x = NULL;

Answer 1

q = p;

Yes. q now points to NULL, just like p.

&x = q;

Not legal. You cannot reassign the address of a variable.

*q = 7;

Yes, sets the memory of the address where q is pointing to 7. If q points to NULL then this will cause an error.

*q = &x;

Not legal, q points to an integer, so you cannot assign an address to it. This is legal, as there is an implicit cast from int* (&x) to int (*q), but not very safe. In C++, it is just a plain error. You are right in saying that it places the address of x (cast to an int) into the memory pointed to by q.