a = a++ + ++a with a=5 in C/C++ produces 11, 12, or 13 depending on compiler due to two distinct undefined behaviors around evaluation order.
Key Takeaways
Two UBs combine: unknown fetch order for operands, and whether post-increment result survives or is overwritten by the assignment.
The four UB combinations map to results 11, 12, 12, 13 – with 12 reachable two ways, explaining its frequency in empirical results.
Empirical table across gcc (2.95-4.6), clang, MSVC, Intel C++, Keil, SDCC, PHP, Java, and C# shows real divergence: gcc clusters at 12/13, clang at 11/12.
Java and C# are not affected: both specs mandate strict left-to-right evaluation, making the result deterministic (11 for a = a++ + ++a).
Overloaded operator results in MSVC and g++ differ from their non-overloaded counterparts, adding another axis of divergence.
Hacker News Comment Review
Strong consensus that invoking UB means any output is valid – including segfaults – not just the three values the post enumerates; the post’s framing of “three correct answers” is itself incorrect.
Commenters with Indian CS backgrounds note this class of question persisted in university exams and textbooks for years, taught as having a definitive answer.
The historical reason C left this undefined is compiler and architecture diversity: evaluation order was left flexible so compilers could match calling conventions and hardware without extra coordination.
Notable Comments
@Boxxed: “if you invoke undefined behavior any result at all is possible” – the post’s three-answer framing understates actual UB scope.
@bombcar: proposes a middle category of “unspecified behavior” where a compiler must pick one value but is not constrained to which.
@HelloNurse: “The final value of a is that if you write this you are fired.”
