Dangling references and undefined behavior

by

Supposing that x was initialized with a valid object, which was then destroyed, §3.8/6 applies:

Similarly, before the lifetime of an object has started but after the storage which the object will occupy has been allocated or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any glvalue that refers to the original object may be used but only in limited ways. For an object under construction or destruction, see 12.7. Otherwise, such a glvalue refers to allocated storage (3.7.4.2), and using the properties of the glvalue that do not depend on its value is well-defined. The program has undefined behavior if:

— an lvalue-to-rvalue conversion (4.1) is applied to such a glvalue,

— the glvalue is used to access a non-static data member or call a non-static member function of the
object, or

— the glvalue is bound to a reference to a virtual base class (8.5.3), or

— the glvalue is used as the operand of a dynamic_cast (5.2.7) or as the operand of typeid.

So, simply taking the address is well-defined, and (referring to the neighboring paragraphs) can even be productively used to create a new object in place of the old one.

As for not taking the address and just writing x, that really does absolutely nothing, and it is a proper subexpression of &x. So it’s also OK.

More Related Contents:

  1. Accessing inactive union member and undefined behavior?
  2. When does invoking a member function on a null instance result in undefined behavior?
  3. Has C++ standard changed with respect to the use of indeterminate values and undefined behavior in C++14?
  4. Optimizing away a “while(1);” in C++0x
  5. Does initialization entail lvalue-to-rvalue conversion? Is `int x = x;` UB?
  6. Do I need to cast to unsigned char before calling toupper(), tolower(), et al.?
  7. Is `reinterpret_cast`ing between hardware SIMD vector pointer and the corresponding type an undefined behavior?
  8. Why does left shift operation invoke Undefined Behaviour when the left side operand has negative value?
  9. Does “Undefined Behavior” really permit *anything* to happen? [duplicate]
  10. Where exactly does C++ standard say dereferencing an uninitialized pointer is undefined behavior?
  11. Is null reference possible?
  12. Difference between Undefined Behavior and Ill-formed, no diagnostic message required
  13. Is passing a C++ object into its own constructor legal?
  14. Is it safe to push_back an element from the same vector?
  15. Are multiple mutations of the same variable within initializer lists undefined behavior pre C++11
  16. Does not evaluating the expression to which sizeof is applied make it legal to dereference a null or invalid pointer inside sizeof in C++?
  17. In C++11, does `i += ++i + 1` exhibit undefined behavior?
  18. So why is i = ++i + 1 well-defined in C++11?
  19. Why is f(i = -1, i = -1) undefined behavior?
  20. Implementing a std::vector like container without undefined behavior
  21. What is a dangling reference? [duplicate]
  22. Are pointer variables just integers with some operators or are they “symbolic”?
  23. The behaviour of floating point division by zero
  24. Is there a platform or situation where dereferencing (but not using) a null pointer to make a null reference will behave badly?
  25. Can branches with undefined behavior be assumed unreachable and optimized as dead code?
  26. In which versions of the C++ standard does “(i+=10)+=10” have undefined behaviour?
  27. Observable behavior and undefined behavior — What happens if I don’t call a destructor?
  28. Rvalues, lvalues and formal definitions
  29. Why ‘&’ reference in not required in classes in C++?
  30. Overload resolution with ref-qualifiers

Leave a Comment