How to explain undefined behavior to know-it-all newbies?

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Undefined means explicitly unreliable. Software should be reliable. You shouldn’t have to say much else.

A frozen pond is a good example of an undefined walking surface. Just because you make it across once doesn’t mean you should add the shortcut to your paper route, especially if you’re planning for the four seasons.

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  5. Optimizing away a “while(1);” in C++0x
  6. Does initialization entail lvalue-to-rvalue conversion? Is `int x = x;` UB?
  7. Is `reinterpret_cast`ing between hardware SIMD vector pointer and the corresponding type an undefined behavior?
  8. What’s a proper way of type-punning a float to an int and vice-versa?
  9. Does “Undefined Behavior” really permit *anything* to happen? [duplicate]
  10. How could pairing new[] with delete possibly lead to memory leak only?
  11. Where exactly does C++ standard say dereferencing an uninitialized pointer is undefined behavior?
  12. Is passing a C++ object into its own constructor legal?
  13. Why is it undefined behavior to delete[] an array of derived objects via a base pointer?
  14. Are multiple mutations within initializer lists undefined behavior?
  15. A C++ implementation that detects undefined behavior?
  16. Why does the enhanced GCC 6 optimizer break practical C++ code?
  17. Why is it that we can write outside of bounds in C?
  18. Why is f(i = -1, i = -1) undefined behavior?
  19. Does “undefined behaviour” extend to compile-time?
  20. Is infinite loop still undefined behavior in C++ if it calls shared library?
  21. Why is locking a std::mutex twice ‘Undefined Behaviour’?
  22. What is a dangling reference? [duplicate]
  23. Mixing class and struct
  24. Is there a platform or situation where dereferencing (but not using) a null pointer to make a null reference will behave badly?
  25. Is it allowed to cast away const on a const-defined object as long as it is not actually modified?
  26. Ramifications of C++20 requiring two’s complement
  27. Can branches with undefined behavior be assumed unreachable and optimized as dead code?
  28. How disastrous is integer overflow in C++?
  29. behavior of const_cast in C++ [duplicate]
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