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Subject: Re: [ub] An update on signed integers
From: Arthur O'Dwyer (arthur.j.odwyer_at_[hidden])
Date: 2018-03-16 13:04:24


P0907r1 proposes this addition relative to the WD:

> If overflow caused by an operation which would require representing an
integer which cannot be represented by the type, the behavior is undefined.

However, it simultaneously proposes these deletions relative to the WD:

> [*Note:* Operators can be regrouped according to the usual mathematical
rules only where the operators really are associative or commutative ...
and
> an operation that would have undefined behavior as specified in Clause 4
through 19 of this document [*Note*: including, for example, signed integer
overflow, certain pointer arithmetic, division by zero, or certain shift
operations —*end note*]

The removals are all non-normative, but they seem to be aimed at
eliminating references to "signed overflow is UB", even though signed
overflow *is* still UB. Was there a sense in the room that we wanted to
downplay the importance of *teaching* signed UB in C++, but not actually
eliminate the UB itself? Or what's the point of re-wording these existing
notes?

Also, separately and less importantly, I'd love to hear someone's rationale
for making (1<<31) UB in C++2a when it's IDB in C++17. The straw poll
result in P0907r1 sounds unambiguous, but I don't understand what rationale
could exist for taking this construct from IDB into UB. Is the assumption
that people haven't yet had a chance to write any programs whose
correctness depends on the value of (1<<31), so if we change it back to UB
fast enough, nobody will notice?

–Arthur

On Fri, Mar 16, 2018 at 8:56 AM, JF Bastien <cxx_at_[hidden]> wrote:

> Hello EWG,
>
> SG6 and SG12 discussed wg21.link/P0907r0 Signed Integers are Two’s
> Complement and provided extensive feedback.
>
> I've attached an updated paper listing polls and addressing most feedback
> (except some wording fiddle) to the EWG wiki for this afternoon's
> discussion:
>
> http://wiki.edg.com/pub/Wg21jacksonville2018/
> EvolutionWorkingGroup/D0907r1.html
>
>
> Thanks,
>
> JF
>
>
> _______________________________________________
> ub mailing list
> ub_at_[hidden]
> http://www.open-std.org/mailman/listinfo/ub
>
>



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