4 ms·

I don't see the name "thaumasiotes" at that link, nor do I see anything relevant to the code in the title. The behavior of "int a = 5; a = a++ + ++a;" is undef

by _kst_ 5mo ago

I don't see the name "thaumasiotes" at that link, nor do I see anything relevant to the code in the title. The behavior of "int a = 5; a = a++ + ++a;" is undefined. There is no guarantee of a numeric result, because there is no guarantee of anything.

susam 5mo ago
I believe they were referring to thaumasiotes's thread here: https://news.ycombinator.com/item?id=48141294 https://news.ycombinator.com/item?id=48141294 I think the objection thaumasiotes has raised there is valid and I have made an attempt to answer it as well in the same thread.
HarHarVeryFunny 5mo ago
It's only the order of evaluation that is undefined.
- afdbcreid 5mo ago
  Nope, there is no sequence point in the middle and modifying an object more than once between sequence points is undefined behavior.
- _kst_ 5mo ago
  No, the behavior is undefined. That means, quoting the ISO C standard, "behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this document imposes no requirements". A conforming implementation could reject it at compile time, or generate code that traps, or generate code that set a to 137, or, in principle, generate code that reformats your hard drive. Some of these behaviors are unlikely, but none are forbidden by the language standard.
  - HarHarVeryFunny 5mo ago
    I was wrong. I was looking at this: https://en.cppreference.com/cpp/language/eval_order https://en.cppreference.com/cpp/language/eval_order I'm not sure where precisely this sequencing exception to the default "eval order undefined" rule is given, but after the 24(!) sequencing rules they do give this "++i + i++" as an explicit example of undefined behavior. Interestingly that page says that since C++17 f(++i, ++i) is "unspecified" rather than "undefined", whatever that means, and presumably plus(++i, i++) would be too, which seems a bit inconsistent.

by _kst_ 5mo ago

susam 5mo ago

I believe they were referring to thaumasiotes's thread here: https://news.ycombinator.com/item?id=48141294 https://news.ycombinator.com/item?id=48141294 I think the objection thaumasiotes has raised there is valid and I have made an attempt to answer it as well in the same thread.

HarHarVeryFunny 5mo ago

It's only the order of evaluation that is undefined.

afdbcreid 5mo ago
Nope, there is no sequence point in the middle and modifying an object more than once between sequence points is undefined behavior.
_kst_ 5mo ago
No, the behavior is undefined. That means, quoting the ISO C standard, "behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this document imposes no requirements". A conforming implementation could reject it at compile time, or generate code that traps, or generate code that set a to 137, or, in principle, generate code that reformats your hard drive. Some of these behaviors are unlikely, but none are forbidden by the language standard.
- HarHarVeryFunny 5mo ago
  I was wrong. I was looking at this: https://en.cppreference.com/cpp/language/eval_order https://en.cppreference.com/cpp/language/eval_order I'm not sure where precisely this sequencing exception to the default "eval order undefined" rule is given, but after the 24(!) sequencing rules they do give this "++i + i++" as an explicit example of undefined behavior. Interestingly that page says that since C++17 f(++i, ++i) is "unspecified" rather than "undefined", whatever that means, and presumably plus(++i, i++) would be too, which seems a bit inconsistent.