Is (--i == i++) an Undefined Behavior?

Question

this question is related to my previous problem. The answer I got was \"It is an Undefined behavior.\"

Please anyone explain:

• What is an undef

Answer 1

Your previous question was tagged [C], so I'm answering based on C, even though the code in your current question doesn't look like C.

The definition of undefined behavior in C99 says (§3.4.3):
1 undefined behavior
behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements

2 NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).

Appendix J.2 of the C standard has a (long -- several pages) list of undefined behavior, though even that still isn't exhaustive. For the most part, undefined behavior means you broke the rules, so the way to know it is to know the rules.