> On Oct 13, 2015, at 15:24 , Graham Cox <graham....@bigpond.com> wrote:
>
> In a piece of code I haven’t worked on in a while, I’m suddenly getting a new
> warning which I’ve never seen before:
>
> “Multiple unsequenced modifications to ‘ix’”
>
> Code is:
>
>
> ix = ++ix % guess.count;
>
> where ix is a NSUInteger.
>
> Is this telling me that the order of the preincrement and the mod operation
> is undefined? Surely a preincrement is defined to happen first, that’s why
> it’s called a PREincrement? Or does the warning refer to something else? I’ve
> used this form of expression for years without any issues, why is it suddenly
> one?
The answer is here. It seems ridiculous, but there it is:
http://en.cppreference.com/w/c/language/eval_order#Undefined_behavior
You can read the entire page a few times to try to understand why. I still
don't, and think this is counterintuitive at best, and plain wrong at worst. At
least I prefer to make those kinds of things explicit anyway, and am not likely
to write that code.
ix += 1;
ix %= guess.count;
hth,
--
Rick Mann
rm...@latencyzero.com
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