C++ standard: dereferencing NULL pointer to get a

Question

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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.

Answer 1

int& ref = *ptr;

The above statement doesn't actually dereference anything. So there's no problem until you use the ref (which is invalid).

Answer 2

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).

Answer 3

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]

Answer 4

For completeness, this: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#232 talks specifically about this issue.

Answer 5

Dereferencing a NULL pointer is undefined behaviour. You should check if a value is NULL before dereferencing it.