warning: operation on 'zero' may be undefined

Dave Korn dk@artimi.com
Fri Oct 1 09:11:00 GMT 2004


> -----Original Message-----
> From: gcc-owner On Behalf Of Paul Brook
> Sent: 01 October 2004 04:12

> On Friday 01 October 2004 03:57, Mathieu Malaterre wrote:
> > Hello,
> >
> >  I am trying gcc4, and turning -Wall reveal a strange warning:
> >
> > int main()
> > {
> >    int zero = 125;
> >    zero = (++zero)%3;
> > }
> >
> > g++ -Wall foo.cxx
> >
> > foo.cxx: In function `int main()':
> > foo.cxx:4: warning: operation on 'zero' may be undefined
> >
> > What does this mean ?
> 
> You incrementing and assigning to the same variable in one 
> statement, which 
> invokes undefined behaviour.
> 
> The code you wrote can be interpreted as
> zero = zero%3;
> or
> zero = (zero % 3) + 1
> or possibly even something entirely different.

  I'm nitpicking, but considering it's a pre-increment, I'd have said

zero = (zero + 1) % 3;
or
zero = zero + 1;

were the interpretations.  And the problem is the lack of a sequence point between
them.

    cheers, 
      DaveK
-- 
Can't think of a witty .sigline today....



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