Is null reference possible?
Is this piece of code valid (and defined behavior)?
int &nullReference = *(int*)0;
Both g++ and clang++ compile it without any warning, even when using -Wall, -Wextra, -std=c++98, -pedantic, -Weffc++...
Of course the reference is not actually null, since it cannot be accessed (it would mean dereferencing a null pointer), but we could check whether it's null or not by checking its address:
if( & nullReference == 0 ) // null reference
Answer
References are not pointers.
8.3.2/1:
A reference shall be initialized to refer to a valid object or function. [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. As described in 9.6, a reference cannot be bound directly to a bit-field. ]
1.9/4:
Certain other operations are described in this International Standard as undefined (for example, the effect of dereferencing the null pointer)
As Johannes says in a deleted answer, there's some doubt whether "dereferencing a null pointer" should be categorically stated to be undefined behavior. But this isn't one of the cases that raise doubts, since a null pointer certainly does not point to a "valid object or function", and there is no desire within the standards committee to introduce null references.