Does the ‘offsetof’ macro from invoke undefined behaviour?

by

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.,

More Related Contents:

  1. Undefined behavior and sequence points
  2. What are all the common undefined behaviours that a C++ programmer should know about? [closed]
  3. Accessing inactive union member and undefined behavior?
  4. Optimizing away a “while(1);” in C++0x
  5. Does initialization entail lvalue-to-rvalue conversion? Is `int x = x;` UB?
  6. Does “Undefined Behavior” really permit *anything* to happen? [duplicate]
  7. How could pairing new[] with delete possibly lead to memory leak only?
  8. Where exactly does C++ standard say dereferencing an uninitialized pointer is undefined behavior?
  9. Is right shift undefined behavior if the count is larger than the width of the type?
  10. May I take the address of the one-past-the-end element of an array? [duplicate]
  11. Are memory leaks “undefined behavior” class problem in C++?
  12. Are multiple mutations within initializer lists undefined behavior?
  13. A C++ implementation that detects undefined behavior?
  14. Why does the enhanced GCC 6 optimizer break practical C++ code?
  15. Why is it that we can write outside of bounds in C?
  16. Why is f(i = -1, i = -1) undefined behavior?
  17. Does “undefined behaviour” extend to compile-time?
  18. Is infinite loop still undefined behavior in C++ if it calls shared library?
  19. Why is locking a std::mutex twice ‘Undefined Behaviour’?
  20. What is a dangling reference? [duplicate]
  21. Mixing class and struct
  22. Is there a platform or situation where dereferencing (but not using) a null pointer to make a null reference will behave badly?
  23. Is it allowed to cast away const on a const-defined object as long as it is not actually modified?
  24. Ramifications of C++20 requiring two’s complement
  25. Can branches with undefined behavior be assumed unreachable and optimized as dead code?
  26. How disastrous is integer overflow in C++?
  27. behavior of const_cast in C++ [duplicate]
  28. reinterpret_cast from object to first member
  29. In which versions of the C++ standard does “(i+=10)+=10” have undefined behaviour?
  30. Fun with uninitialized variables and compiler (GCC)

Leave a Comment