220 41376 <057b85fb-1020-4119-a6a5-19eb251f7057@isocpp.org> article Path: news.gmane.org!.POSTED.ciao.gmane.org!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Destructive move, via forwarding and destructuring operations Date: Mon, 21 Jan 2019 10:32:28 -0800 (PST) Approved: news@gmane.org Message-ID: <057b85fb-1020-4119-a6a5-19eb251f7057@isocpp.org> References: <7b2204ba-51d0-481e-9974-10029547031c@isocpp.org> <20190120222650.5222481.91670.69069@gmail.com> Reply-To: std-proposals@isocpp.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2124_372852324.1548095548383" Injection-Info: ciao.gmane.org; posting-host="ciao.gmane.org:195.159.176.228"; logging-data="157791"; mail-complaints-to="usenet@ciao.gmane.org" Cc: dcrc2cpp@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBPNATDRAKGQEDDI4EGY@isocpp.org Mon Jan 21 20:00:09 2019 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb1-f200.google.com ([209.85.219.200]) by ciao.gmane.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.89) (envelope-from ) id 1gleno-000ez0-B9 for gclcip-std-proposals@m.gmane.org; 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Mon, 21 Jan 2019 10:32:30 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:a81:7d87:: with SMTP id y129ls2800516ywc.2.gmail; Mon, 21 Jan 2019 10:32:29 -0800 (PST) X-Received: by 2002:a81:27cd:: with SMTP id n196mr346606ywn.2.1548095549027; Mon, 21 Jan 2019 10:32:29 -0800 (PST) In-Reply-To: <20190120222650.5222481.91670.69069@gmail.com> X-Original-Sender: jmckesson@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:41376 Archived-At: ------=_Part_2124_372852324.1548095548383 Content-Type: multipart/alternative; boundary="----=_Part_2125_1693843354.1548095548384" ------=_Part_2125_1693843354.1548095548384 Content-Type: text/plain; charset="UTF-8" On Sunday, January 20, 2019 at 5:26:53 PM UTC-5, Tony V E wrote: > > Dangling references and destructive move are two things that could > possibly be solved by something called 'owning references'. > > I haven't read the paper, but the high cost of another reference could be > worth it if it solved these issues. > .... huh. A while back, I went exploring down the rabbit hole of dangling references. At some point, I realized that what I was about to define was de-facto another reference type (along with further subdividing xvalues into two new value categories). At which point I deemed the whole exploration to be insane and stopped. Would a third reference type be worth it if it solved both destructive move (at a language level) and dangling reference? I don't know. The big problem with third references is that you're adding greater complexity to a part of the language that already has a lot of complexity. Reference collapsing rules already have 4 cases; adding a new reference type makes 8 cases. You now have a third case where they get applied to `this`, but you also have to decide how that interacts with the other two `this` reference. It's hard enough to teach beginning C++ programmers how to use rvalue references correctly. The distinction between genuine rvalue references and forwarding references, when and how to use `move`, etc. This is why solving the dangling reference problem through a third reference makes a degree of sense: that's an area of the language that is already complex for C++ users; if adding a bit more complexity allows users to write working code, then it's a win. By contrast, solving destructive move alone is adding complexity without removing complexity. It (potentially) adds performance, but at the cost of complexity in a domain that C++ is already over-complicated within. So I guess I could say this: if you can solve both problems with the same mechanism, it might be worth doing. -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/057b85fb-1020-4119-a6a5-19eb251f7057%40isocpp.org. ------=_Part_2125_1693843354.1548095548384 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Sunday, January 20, 2019 at 5:26:53 PM UTC-5, Tony V E = wrote:
= Dangling references and destructive move are two = things that could possibly be solved by something called 'owning refere= nces'.

I haven't read the paper, but the high cost of another refer= ence could be worth it if it solved these issues.
<= div>
... huh.

A while back, I went e= xploring down the rabbit hole of dangling references. At some point, I real= ized that what I was about to define was de-facto another reference type (a= long with further subdividing xvalues into two new value categories). At wh= ich point I deemed the whole exploration to be insane and stopped.

Would a third reference type be worth it if it solved both= destructive move (at a language level) and dangling reference? I don't= know.

The big problem with third references is th= at you're adding greater complexity to a part of the language that alre= ady has a lot of complexity. Reference collapsing rules already have 4 case= s; adding a new reference type makes 8 cases. You now have a third case whe= re they get applied to `this`, but you also have to decide how that interac= ts with the other two `this` reference.

It'= ;s hard enough to teach beginning C++ programmers how to use rvalue referen= ces correctly. The distinction between genuine rvalue references and forwar= ding references, when and how to use `move`, etc.

= This is why solving the dangling reference problem through a third referenc= e makes a degree of sense: that's an area of the language that is alrea= dy complex for C++ users; if adding a bit more complexity allows users to w= rite working code, then it's a win.

By contras= t, solving destructive move alone is adding complexity without removing com= plexity. It (potentially) adds performance, but at the cost of complexity i= n a domain that C++ is already over-complicated within.

So I guess I could say this: if you can solve both problems with the = same mechanism, it might be worth doing.

--
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