Question

(4.1/1) If the object to which the lvalue refers is not an object of
type T and is not an object of a type derived from T, or if the object
is uninitialized, a program that necessitates this conversion has
undefined behavior.

From this, I assume

struct B {
     int x; };

B *p;
*p; //undefined behavior

*p is a lvalue which refers to an uninitialized object. How can it refer to an object which is not the type of ‘B’ or its derived type? Am i misunderstanding something?

Answer 1

Easy:

int n = 5;
double * p = reinterpret_cast<double*>(&n);

*p += 1.0; // undefined behaviour: p points to an int, not a double

++*reinterpret_cast<int*>(p);  // legal; pointer can be cast back-and-forth

Less easy:

union bogus { int * a; double * b; } B;
int n;
B.a = &n;
*B.b += 1.0; // undefined behaviour

Common C pitfall (produces no diagnostic!):

void do_a(void * p) { ++*reinterpret_cast<int*>(p); }
void do_b(void * p) { *reinterpret_cast<double*>(p) += 1.0; }

int main() { int n = 4; do_b(&n); /* eek */ }