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Is null reference possible?
4 answers
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.
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.
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).
Dereferencing a NULL pointer is explicitly undefined behaviour in the C++ standard, so what you see is implementation specific.
Copying from 1.9.4 in the C++0x draft standard (similar to previous standards in this respect):
Certain other operations are described
in this International Standard as
undefined (for example, the effect of
dereferencing the null pointer).
[Note: this International Standard
imposes no requirements on the
behavior of programs that contain
undefined behavior. - end note]
Dereferencing a NULL pointer is undefined behaviour. You should check if a value is NULL before dereferencing it.
For completeness, this: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#232 talks specifically about this issue.
int& ref = *ptr;
The above statement doesn't actually dereference anything. So there's no problem until you use the ref (which is invalid).
