Does using the ++ or -- operator on different variables in the same expression invoke UB?

Question

I wanted to know whether using the increment(++) or the decrement(--) operator on different variables in the same expression undefined behavior; Such as

int i=1,j=2;
int k=i++ + j++;

In the above code,the value of k is 3 in clang,GCC and in vc.

why write such code? BTW - This "popular" question has been asked many times. Just do one thing at a time and you cannot go far wrong — Ed Heal, Jan 03 '15 at 09:48
@EdHeal This question is asked many times but people just don't know to look on 'sequence points' as term when looking up this question — hetepeperfan, Jan 03 '15 at 10:01
@hetepeperfan One can find all the explanations one needs by searching “increment undefined behavior”, all words that the OP definitely knows. — Pascal Cuoq, Jan 03 '15 at 12:04

haccks · Accepted Answer · 2015-01-03T10:11:51.433

5

Does using the ++ or — operator on different variables in the same expression invoke UB?

No. It will not invoke undefined behavior. You can use ++ or -- any number of times in a expression on different objects. In this case each variable is modifying only once within two sequence points.

edited Jan 03 '15 at 10:11

answered Jan 03 '15 at 09:36

haccks

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But isn't the answser yes then, the OP uses two variables, thus `k` becomes three? – hetepeperfan Jan 03 '15 at 10:06
So long as it is a different variable `int a=0, *p=&a; b=++a+(*p)--;` gives undefined behavior because `*p` and `a` are the same object. – Jasen Jan 03 '15 at 10:07
@hetepeperfan; Yes. Value of `k` will be 3. I answered about UB. – haccks Jan 03 '15 at 10:15

Does using the ++ or -- operator on different variables in the same expression invoke UB?

1 Answers1