220 37373 <8751b79c-2f93-4916-bd0e-51b8e3eb910f@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: "T. C." Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Pointer overload for || Date: Fri, 16 Mar 2018 15:12:40 -0700 (PDT) Lines: 237 Approved: news@gmane.org Message-ID: <8751b79c-2f93-4916-bd0e-51b8e3eb910f@isocpp.org> References: <1fea07df-46a6-4df2-865a-849be8f8d129@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2769_1776788060.1521238360549" X-Trace: blaine.gmane.org 1521238240 6178 195.159.176.226 (16 Mar 2018 22:10:40 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 16 Mar 2018 22:10:40 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCQ43G7NQIIRBWMCWHKQKGQEKAR3PUQ@isocpp.org Fri Mar 16 23:10:36 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vk0-f70.google.com ([209.85.213.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1ewxYZ-0001VB-F6 for gclcip-std-proposals@m.gmane.org; Fri, 16 Mar 2018 23:10:35 +0100 Original-Received: by mail-vk0-f70.google.com with SMTP id w68sf7169241vkd.16 for ; Fri, 16 Mar 2018 15:12:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=6L9cXsepTztVkveBYdJtFTGkCW2GPEXMcYdGObsXYCc=; b=vJbB4gvqyuMAPdWWR3zzYBcPjQ+vMcQ4vPk1e/VNAU/aPfXpSAgJ9TPlbXxSjDO5lQ L2xXXFUoOiW0M9DT6xm/5/9rHqar08eIuYqRULYWDrisVxLAm/1XkYl2IuYf+ParxUdr a2DvU/Y0SgPkfPQFF0BsDqob9fOPicODzlHPz4sru9ucbuF5jxwVo8JQPECk7wLlfcd+ sviMpV5xX+oX1eKy8J0yi1UnKGXzTBrOk+xe8o8nFO2tOCdk4199rzv/eD4QWlC3uXAF kO1iLgdK7qKsRMufm75SXKQVfHmIdQpG8c4lmkooK4b9Jq+gH2d9Fx/D09aTP9R7SBZ8 IWkA== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=6L9cXsepTztVkveBYdJtFTGkCW2GPEXMcYdGObsXYCc=; b=DjSUB481fLEs5mFaGh4e2bfMnCXku+looqOjp7D2UTR5P44K66PD25x8ebJelLS4ik D51w8TGXv3jGSJ+UMJEaPFDWTTMClLKLgxVD1wRu163UAJXfLgHw8PwZwEv6DsDWzkY6 538YSmp8Q9MQKj5Qs2/zrf3DPFkQ+ywBc/5/FPpvlMC3C75Pec5M2Mqhco1JKndqWoDu arwLEfYCsUD4PybE9zgwlAsEhBzhB95TWusWjXvXD0a0d2VLlceu3g6w+xtRbIgbCZtT FzKL5z/nGrDvSVh8aomCub3Pt47j4C9cciR0OnRnIRCf18apWjGsqQ/WYkZokBSFtb9Z WRRw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=6L9cXsepTztVkveBYdJtFTGkCW2GPEXMcYdGObsXYCc=; b=UPUA+rZI7t28opiEkdXWfy1dpAOEMRPEGbcSSfCo3j7tZfK+htj3fCFhjmxFSIdL0g 0pkvutn3qpankrjSP+/v7Y/KQIFjIA4xhrfIJtp9xOLEE1cv5AB0kZpVSPbXFn8mHpE+ m/MMq4SLTNq5AwK5qKB8CaWtktSPXTR3tatrg36oAwmHnsLa2jDtiflnCDodr3M0W8W/ cng26uD8bhES0n6LM2JanBXpwwLx1BwI8483/xXAoBXevUmKTdqAuZI4v9Uv9gYtIf1P Wb5w5qChcbU56QqBRIS6APJLLygPkdGdqe/2h1E39LUxr2XVMD4yD/FjIIE8RFWeXnu0 CkhA== X-Gm-Message-State: AElRT7E60cnKsWaj49rHPAzt0KxNEUusgdhCTykT5gz0DA49432GGAlr q1q6rTJSa026ViHGwvSxUGiCEg== X-Google-Smtp-Source: AG47ELt3AIYe27mxk216iXj/gKpm1YJ+FdIbmvoJ+Z8xsXf5F0sd1gINvAwvqZcl6xvY2dO1W3op3w== X-Received: by 10.176.80.248 with SMTP id d53mr1556448uaa.43.1521238362576; Fri, 16 Mar 2018 15:12:42 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.31.120.135 with SMTP id t129ls4384655vkc.15.gmail; Fri, 16 Mar 2018 15:12:41 -0700 (PDT) X-Received: by 10.31.151.87 with SMTP id z84mr398356vkd.5.1521238361066; Fri, 16 Mar 2018 15:12:41 -0700 (PDT) In-Reply-To: X-Original-Sender: rs2740@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:37373 Archived-At: ------=_Part_2769_1776788060.1521238360549 Content-Type: multipart/alternative; boundary="----=_Part_2770_1313924794.1521238360550" ------=_Part_2770_1313924794.1521238360550 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable This was proposed in N4120 and rejected by EWG. On Tuesday, March 13, 2018 at 7:26:46 AM UTC-4, Ga=C5=A1per A=C5=BEman wrot= e: > > But my point was we *already have* an operator with the exact semantics o= f=20 > the macro above - it's just a gnu extension. We could just propose that! > > https://gcc.gnu.org/onlinedocs/gcc/Conditionals.html > > x =3D x?:new int; > > It's unambiguous, it's obvious it's a conditional, it's a one-liner, and= =20 > more importantly, it can be used for way more than just pointers. Oh, and= =20 > we have implementations and knowledge of how it behaves, and it's also no= t=20 > a weird new form of || or |. > > It also fails to compile if "x" and the expression if x is falsy don't=20 > have a common type, which is *also* important. > > So, I'd encourage the author of this proposal to instead propose the gnu= =20 > extension as an actual feature, instead of trying to fit a "don't evaluat= e=20 > this" model as a special case for just certain types - making user-define= d=20 > types less like built-in types is an explicit no-no these days, and this= =20 > proposal is just trying to special-case pointers. > > G > > > On Mon, Mar 12, 2018 at 11:41 AM, Miguel Ojeda > wrote: > >> On Mon, Mar 12, 2018 at 11:42 AM, Richard Hodges > > wrote: >> > >> > Consider: >> > >> > p |=3D new_thing(); >> > >> > In the above code, do we mean that we wish to replace p with a new=20 >> thing if >> > empty, or does it mean that we wish to merge a new thing into the obje= ct >> > referenced by p? >> >> For me ||=3D would be the replace. But since we don't have ||=3D, I just >> picked |=3D, which indeed looks fishy. >> >> Now, what would be a nice experiment would be ||, &&, ||=3D and &&=3D wi= th >> short-circuit semantics, which could be implemented if we had the >> "expression E" stuff: >> >> template >> operator||=3D(T & t, E) >> { >> if (t) >> return; >> t =3D E; >> } >> >> Then: >> >> x ||=3D new int; >> >> would have the expected results. >> >> Cheers, >> Miguel >> >> -- >> You received this message because you are subscribed to the Google Group= s=20 >> "ISO C++ Standard - Future Proposals" group. >> To unsubscribe from this group and stop receiving emails from it, send a= n=20 >> email to std-proposal...@isocpp.org . >> To post to this group, send email to std-pr...@isocpp.org . >> To view this discussion on the web visit=20 >> https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CANiq72kNst= xnyLTCOBfNorEiyudTijCAUHwH4DsoT14WqcNs4w%40mail.gmail.com >> . >> > > --=20 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 e= mail 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/8751b79c-2f93-4916-bd0e-51b8e3eb910f%40isocpp.or= g. ------=_Part_2770_1313924794.1521238360550 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
This was proposed in=C2=A0N4120 and rejected by EWG.
On Tuesday, March 13, 2018 at 7:26:46 AM UTC-4, Ga=C5=A1per A=C5=BEman wr= ote:
But my po= int was we *already have* an operator with the exact semantics of the macro= above - it's just a gnu extension. We could just propose that!

x =3D x?:new int;

It's unambiguous, it's obvious it's a conditional, it'= ;s a one-liner, and more importantly, it can be used for way more than just= pointers. Oh, and we have implementations and knowledge of how it behaves,= and it's also not a weird new form of || or |.

It also fails to compile if "x" and the expression if x is fals= y don't have a common type, which is *also* important.

So, I'd encourage the author of this proposal to instead propo= se the gnu extension as an actual feature, instead of trying to fit a "= ;don't evaluate this" model as a special case for just certain typ= es - making user-defined types less like built-in types is an explicit no-n= o these days, and this proposal is just trying to special-case pointers.

G


On Mon, Mar 12, 2018 at 11:41 AM, Miguel Ojeda <miguel.oje...@gmail.com> wrote:
On Mon, Mar 12, 2018 at 11:42 AM, Richard Hodges <hodg...@gm= ail.com> wrote:
>
> Consider:
>
> p |=3D new_thing();
>
> In the above code, do we mean that we wish to replace p with a new thi= ng if
> empty, or does it mean that we wish to merge a new thing into the obje= ct
> referenced by p?

For me ||=3D would be the replace. But since we don't have ||=3D= , I just
picked |=3D, which indeed looks fishy.

Now, what would be a nice experiment would be ||, &&, ||=3D and &am= p;&=3D with
short-circuit semantics, which could be implemented if we had the
"expression E" stuff:

=C2=A0 =C2=A0 template <typename T, expression E>
=C2=A0 =C2=A0 operator||=3D(T & t, E)
=C2=A0 =C2=A0 {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 if (t)
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return;
=C2=A0 =C2=A0 =C2=A0 =C2=A0 t =3D E;
=C2=A0 =C2=A0 }

Then:

=C2=A0 =C2=A0 x ||=3D new int;

would have the expected results.

Cheers,
Miguel

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--
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