If I don't actually access the dereferenced "object", is dereferencing the null pointer still undefined?
int* p = 0;
int& r = *p; // undefined?
int* q = &*p; // undefined?
A slightly more practical example: can I dereference the null pointer to distinguish between overloads?
void foo(Bar&);
void foo(Baz&);
foo(*(Bar*)0); // undefined?
Okay, the reference examples are definitely undefined behavior according to the standard:
a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the "object" obtained by dereferencing a null pointer, which causes undefined behavior.
Unfortunately, the emphasized part is ambiguous. Is it the binding part that causes undefined behavior, or is the dereferencing part sufficient?
Yes it is undefined behavior, because the spec says that an "lvalue designates an object or function" (at clause 3.10) and it says for the *-operator "the result [of dereferencing] is an lvalue referring to the object or function to which the expression points" (at clause 5.3.1).
That means there is no description for what happens when you dereference a null pointer. It's simply undefined behavior.