C++ standard: dereferencing NULL pointer to get a reference? [duplicate]

Question

I'm wondering about what the C++ standard says about code like this:

int* ptr = NULL; int& ref = *ptr; int* ptr2 = &ref; 

In practice the result is that ptr2 is NULL but I'm wondering, is this just an implementation detail or is this well defined in the standard?
Under different circumstances a dereferencing of a NULL pointer should result in a crash but here I'm dereferencing it to get a reference which is implemented by the compiler as a pointer so there's really no actual dereferencing of NULL.

Answer 1

Dereferencing a NULL pointer is undefined behavior.

In fact the standard calls this exact situation out in a note (8.3.2/4 "References"):

Note: in particular, 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.

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As an aside: The one time I'm aware of that a NULL pointer can be "dereferenced" in a well-defined way is as the operand to the sizeof operator, because the operand to sizeof isn't actually evaluated (so the dereference never actually occurs).