Undoc'd Feature?

[ijabbott]

(07-12-2022 01:49 PM)Wes Loewer Wrote:  

(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 ++.

EDIT: IGNORE THE FOLLOWING - IT IS WRONG (see my post just above this one)

At some point 7 is stored in `i` and at some other point `i` is modified by adding 1 to it. Those two points are unsequenced relative to each other so the behaviour is undefined. Maybe `i` is incremented to 6 before 7 is stored in `i`, or maybe 7 is stored in `i` before `i` is incremented to 8, or (since the behaviour is undefined) maybe 7 is stored in `i` before 1 is added to the old value (5) of `i` and stored in `i`.