How would the standard allow indirection through a null pointer?

EDIT: This is related to active issue 232 on the C++ Standard Core Language Active Issues

So the real question is how would the standard allow indirection through a null pointer?

Consider the following code:

struct a { int x; };
struct b { int y; };

struct c: a, b { };

b *f(c *pointer_to_c) { return pointer_to_c; }

f(...) has to test if pointer_to_c is NULL and return NULL if it is. (A NULL pointer is always a NULL pointer, no matter how you cast it.)

Now, consider the following:

b *f_ref(c &reference_to_c) { return &reference_to_c; }

First question: does f_ref need to check if &reference_to_c is NULL ? Second question: if C++ eventually defines the behavior of indirection through a null pointer, does this mean f_ref has to check for NULL ? (I suppose the answer depends on what the standard allows, ie, if it says "casting a NULL reference to a base class is undefined for multiple inheritance", then the answer is clearly no.)

For one last puzzle, now consider this:

const b *f_const_ref(const c &reference_to_c) { return &reference_to_c; }

Does f_const_ref have to check &reference_to_c to NULL ? Incidentally, with VC++ 2010, all three functions got optimized to f , that is, all three functions test the input against NULL .

So given that you cannot pass a null reference in a well-defined program, does that mean that if the C++ standard eventually allows indirection through a null pointer it cannot result in an expression or subexpression that would otherwise bind to a null pointer and create a null reference? In other words, if the standard allows indirection through a null pointer, what are the various ways they could allow it?

---

De-referencing a NULL pointer is invalid (ie Undefined behavior).

Thus it is never possible to get a reference from a NULL (in valid code).

Thus any reference you get will never be NULL and thus you do not need to check for it.

First question: does f_ref need to check if &reference_to_c is NULL?

No.

Second question: if C++ eventually defines the behavior of indirection through a null pointer does this mean f_ref has to check for NULL?

It does. Its undefined behavior. Thus any further speculation is worthless.

For one last puzzle, Does f_const_ref have to check &reference_to_c to NULL?

No.

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If your reference in f_ref or f_const_ref is NULL, you are deep in the realm of undefined behavior.

This is undefined because the only way to get a NULL reference is to dereference a NULL pointer. So the below is undefined, though on g++ 4.5.2 it will output 0.

#include <iostream>
struct C {};
C* foo(C& r) {  return &r; }
int main() {
  C* c = NULL;

  std::cout << foo(*c) << std::endl;
  return 0;
}

---

C++ does not allow this. The reference to NULL is illegal and standards-wise will result in undefined behaviour.

Having said that, I've never seen a compiler do anything other than your expectation.

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