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operation on ‘numsels’ may be undefined

Kevin Ushey 
I don't see what's so surprising here.

That statement is identical to writing:

    if (arrMask[i] == 1) {
        numsels = ++numsels;
    } else {
        numsels = numsels;
    }

and

    numsels = ++numsels;

has two statements modifying the value of numsels (= and prefix-++) in
a single sequence point. (Do we increment then assign, or assign then
increment? The C / C++ standards leave this undefined.)

Imagine writing the operations out as functions: we have the `=`
function, and the `prefix-++` function -- both of these 'modify' (one
of) their arguments. Do we evaluate it as `=`(a, `prefix-++`(a)) or
`prefix-++`(`=`(a, a))? The C standard leaves this undefined, so
compilers are free to do what they wish (and the nice ones warn you
when there is such an ambiguity). I guess the net result of the
operation is the same in each case _here_, but this is of course not
the case for other functions that modify the value of their
operand(s). And, in truth, this is _undefined behaviour_ and so the
compiler could still rightly make demons fly out of your nose if it
wanted to upon program execution.

I highly recommend reading the slides at
http://www.slideshare.net/olvemaudal/deep-c, especially the bit on
sequence points.

Cheers,
Kevin

On Mon, Jun 23, 2014 at 9:22 PM, Kasper Daniel Hansen
<kasperdanielhansen at gmail.com> wrote:

Thread (14 messages)

cstrato operation on ‘numsels’ may be undefined Jun 23 Romain Francois operation on ‘numsels’ may be undefined Jun 23 cstrato operation on ‘numsels’ may be undefined Jun 23 Romain Francois operation on ‘numsels’ may be undefined Jun 23 cstrato operation on ‘numsels’ may be undefined Jun 23 Kasper Daniel Hansen operation on ‘numsels’ may be undefined Jun 23 cstrato operation on ‘numsels’ may be undefined Jun 23 Hervé Pagès operation on ‘numsels’ may be undefined Jun 23 Hervé Pagès operation on ‘numsels’ may be undefined Jun 23 Kasper Daniel Hansen operation on ‘numsels’ may be undefined Jun 23 Kevin Ushey operation on ‘numsels’ may be undefined Jun 23 cstrato operation on ‘numsels’ may be undefined Jun 24 Hervé Pagès operation on ‘numsels’ may be undefined Jun 24 Kevin Ushey operation on ‘numsels’ may be undefined Jun 24