> On Oct 13, 2015, at 3:24 PM, 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 mod operator uses the value after the increment. That part is well-defined.
The compiler is complaining about the two stores to `ix`, one from the
preincrement and one from operator =. Storing to the same location twice in the
same statement is undefined in many cases: the language does not specify which
one wins. Look up "sequence point" for more details (although the current
language standards no longer use the "sequence point" terminology).
I thought that this particular form was allowed, but I'm not enough of a
language lawyer to know for sure. You might get better answers from a compiler
mailing list.
You can avoid confusion (human and compiler) by writing the statement without
the preincrement's side effect:
ix = (ix+1) % guess.count;
--
Greg Parker gpar...@apple.com Runtime Wrangler
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