Does the 'offsetof' macro from <stddef.h> invoke undefined behaviour?

Question

Example from MSVC's implementation:

#define offsetof(s,m) \
    (size_t)&reinterpret_cast<const volatile char&>((((s *)0)->m))
//                                                   ^^^^^^^^^^^

As can be seen, it dereferences a null pointer, which normally invokes undefined behaviour. Is this an exception to the rule or what is going on?

Answer 1

Where the language standard says "undefined behavior", any given compiler can define the behavior. Implementation code in the standard library typically relies on that. So there are two questions:

(1) Is the code UB with respect to the C++ standard?

That's a really hard question, because it's a well known almost-defect that the C++98/03 standard never says right out in normative text that in general it's UB to dereference a nullpointer. It is implied by the exception for typeid, where it's not UB.

What you can say decidedly is that it's UB to use offsetof with a non-POD type.

(2) Is the code UB with respect to the compiler that it's written for?

No, of course not.

A compiler vendor's code for a given compiler can use any feature of that compiler.

Cheers & hth.,

Answer 2

The notion of "undefined behavior" is not applicable to the implementation of the Standard Library, regardless of whether it is a macro, a function or anything else.

In general case, the Standard Library should not be seen as implemented in C++ (or C) language. That applies to standard header files as well. The Standard Library should conform to its external specification, but everything else is an implementation detail, exempt from all and any other requirements of the language. The Standard Library should be always thought of as implemented in some "internal" language, which might closely resemble C++ or C, but still is not C++ or C.

In other words, the macro you quoted does not produce undefined behavior, as long as it is specifically the offsetof macro defined in the Standard Library. But if you do exactly the same thing in your code (like define your own macro in the very same way), it will indeed result in undefined behavior. "Quod licet Jovi, non licet bovi".