C standard expression

Question

How come that the C standard says that the expreassion
j=i++ * i++;
is undefined, where as the expression
j=i++ && i++;
is perfectly legal

Answer 1

HonFu first :P

`&&` is a "sequence point", and `*` is not.

You can't have more than one side effect within a single sequence (at least you need to be careful), and `i++ * i++` has two.

Since there is no sequence point between `i++` and `i++`, we can't say when exactly which will happen (before multiplying? after?)

So it is undefined behaviour.

The sequence point in `i++ && i++` makes sure that the side effect of the left `i++` happens before we move to the right side. So it's not undefined.

I believe `&&` is a sequence point to allow "short-circuiting". For example, `false && x` will just be `false`, and you don't even have to look at what `x` is.

To fully get it you'll have to read up on sequence point rules I think.

Answer 2

Schindlabua does this concept of sequence points also apply to other programming languages like Php? I guess it does. I have seen at least one Php Sololearn challenge question which asks about the result of a "multiplication with increment" operation (which I can't locate right now but is something like the result of $i++ * $i++) for which I now think that the answer would be undefined.

Answer 3

Sonic No that's strictly C only! C++ has something similar (sequenced relations).

Most other languages I know do the operations left-to-right and so everything is defined always.

C does it like that so it can optimize code better. And it never really matters in practice because nobody writes statements like `i++ * i++` :P

Answer 4

+2 for Schindlabua !
HonFu boooo! 8D

https://en.m.wikipedia.org/wiki/Sequence_point

Answer 5

Another case for dr. UB - C++ Soldier (Babak)! 😉

Answer 6

Yeh Schindlabua that's right

Answer 7

Thanks for that resource C++ Soldier (Babak)