Undoc'd Feature?

[Wes Loewer]

(07-11-2022 07:49 PM)ijabbott Wrote:  Not really. For `i = i++ + 1;`, the `i++` part says that the old value of `i` plus 1 will be be stored in `i`, and the `i = i + 1` part also says that the old value of `i` plus 1 will be stored in `i`, so there is no conflict (in C++17, but undefined in C17).

So are you saying that (i = i++ + 2) would also be undefined since it is ambiguous whether i gets incremented by 1 or by 2?

My (perhaps flawed) understanding was that this is defined to follow the following sequence. Say i=5 to start.
1) i is evaluated to 5
2) 2 is added to 5 to get 7
3) i is incremented to 6
4) the value of 7 is stored in i

Or maybe steps 2 and 3 might be reversed, but in either case, i ends up equaling 7, so compilers are free to ignore the ++.

(07-11-2022 07:49 PM)ijabbott Wrote:  (The other reason is that the storage to `i` by `i++` is unsequenced relative to the evaluation of `i` in the `... + i`.)

I'm certainly no expert in these matters, but my understanding was that this is the only reason that (i = i++ + i) was undefined.