Is using reference to pointer that was casted with reinterpret_cast undefined behavior?

Question

Is the following code UB?

int   i  = 5;
void *p  = &i;

int* &r  = reinterpret_cast<int* &>(p);
int*  p2 = r;

Please note I do not dereference pointer.

Answer 1

Yes, it is UB.

reinterpret_cast<int* &>(p);

is equivalent to

*reinterpret_cast<int**>(&p);

reinterpret_cast of void** to int** is allowed, but the implicit dereference is UB because the type of data ( void* ) and the type it is being accessed as ( int* ) are not similar.

Answer 2

In this absolutely specific case without deferencing it should be okay I think. I verified pointer value. It's different story when sizeof(void*) and sizeof(int*) are different (although I do not know whether that is even possible).

By doing this, you are taking complete responsibility of very known scenario .

   int   i = 5;
   void *p = &i; //convert int* => void*


   int* &r = reinterpret_cast<int* &>(p); //convert void* which was int* to int*&
   int*  p2 = r; //**copy** address stays same