> On Oct 13, 2015, at 15:50 , Greg Parker <gpar...@apple.com> wrote: > > >> 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;
Actually, yeah, this is a much better alternative. The document I linked in my other answer (http://en.cppreference.com/w/c/language/eval_order), under "Rules," specifies where the sequence points are. As of C11, rule 10 applies: the value of the right-hand-side is sequenced before the assignment, but not the side effects of the right-hand side. So, it's possible that the assignment to ix could happen first, and then the increment. So, it would compute ix+1, use that value in evaluating the right hand side, assign that result to ix, then store the old ix+1 into ix. -- Rick Mann rm...@latencyzero.com _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
