[ub] Canonical ordering
Herb Sutter
hsutter at microsoft.com
Fri Oct 18 19:22:56 CEST 2013
Thanks to everyone who is interested in this. Alas, I'm sure I'm not the only one who hasn't been able to keep up with the whole thread.
A request: Whenever this thread wraps up, it would be useful if a small paper (or just email to EWG) report would summarize the reasons why we should NOT require < on T* to have identical semantics to today's less<T*>. I think that summary would be helpful.
Thanks,
Herb
From: ub-bounces at open-std.org [mailto:ub-bounces at open-std.org] On Behalf Of Nevin Liber
Sent: Friday, October 18, 2013 10:13 AM
To: WG21 UB study group
Subject: Re: [ub] Canonical ordering
On 18 October 2013 01:46, Christopher Jefferson <chris at bubblescope.net<mailto:chris at bubblescope.net>> wrote:
Here is a suggestion. How about we make the result of comparing pointers from different allocations into implementation-defined behaviour?
While it gets it out of this group, I don't think that addresses the real problem.
Of course, if you think that:
bool isInArray = std::begin(a) <= p && p < std::end(a);
is an unreasonable piece of code, there are no problems. :-) If you think it is reasonable, I don't see how making it implementation-defined behavior stops the optimizer from rewriting that as:
bool isInArray = static_cast<bool>(p);
(possibly even 'isInArray = true;', but I'd have to study the rules for nullptrs to see if this is allowed)
And developers can't count on implementation-defined behavior in portable code. Look at all the pain people go through because C didn't nail down (for valid reasons at the time) the signness of char. And the implementation-defined behavior of the representation of char only has two possible outcomes...
--
Nevin ":-)" Liber <mailto:nevin at eviloverlord.com<mailto:nevin at eviloverlord.com>> (847) 691-1404<tel:%28847%29%20691-1404>
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