220 32563 article Path: news.gmane.org!.POSTED!not-for-mail From: ma.kalbfuss@web.de Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: relaxing rules for ternary operator. Allow incompatible types. Date: Sun, 21 May 2017 14:03:05 -0700 (PDT) Lines: 164 Approved: news@gmane.org Message-ID: References: <1b5ee8eb-53df-4e98-af2f-829c7bc2e5b2@isocpp.org> <9064929.QEV8q21eIZ@tjmaciei-mobl1> <99350a9e-0c6b-468d-9761-f2b2b052275e@isocpp.org> <27694795.c6R7qxKFE5@tjmaciei-mobl1> <734764c8-e502-4635-be76-a76cab6261a1@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2211_1635247880.1495400585179" X-Trace: blaine.gmane.org 1495400587 19482 195.159.176.226 (21 May 2017 21:03:07 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 21 May 2017 21:03:07 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBC7YTI72XQOBBCMBRDEQKGQE3X2ZKUI@isocpp.org Sun May 21 23:03:02 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f199.google.com ([209.85.192.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dCY0E-0004t1-6G for gclcip-std-proposals@m.gmane.org; Sun, 21 May 2017 23:03:02 +0200 Original-Received: by mail-pf0-f199.google.com with SMTP id a66sf102770513pfl.6 for ; Sun, 21 May 2017 14:03:07 -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 :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=+Ym6e4mv+Md7DJl+eXl+U5X4+3H8Vnr0suX0eycm440=; b=S2NyE3ii3vl8iODMJNU/zMniSkT8z53UX3czGFInt+ihYQgYhH/5chQ5AHRhxAsey7 9gqNdRMw+SIE2SpUxHclo5UsmBOrl0Ox+sy4f250bbr+HYJ+SBOiVXUxOD6F0RZjU5LD aYp50blLgaO92mebqoHSaKgpI1E9d7TaxVn6kPDl90QGFcFdDue16MFOfBkYftXTlHPC 5EsOyqXfbsAAuEKNPrOHfpioBI/G7ZDyzzPqdDvWT/39r1vqTbP1VQzkRmapIwIPKZQE Bw4CUwIkzQKXHnDJUTIV52JS7qCQX1RsXZiUClZrtnZUgmryfcemKHDeoOay7+GT92Bb ZXdQ== 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=+Ym6e4mv+Md7DJl+eXl+U5X4+3H8Vnr0suX0eycm440=; b=tt8DdpontntsQSujOL4VGcvjVQrzyQEJD7EtyOa3kbl56vDKeduCVI9zOyZyNUxgcC wia2c1E+JSE/q56xe5xyrZa8bGoleO9k/sDt7aywJ5eA1Nt3WQqo07otGY9+h2HGjSsu bBpfOAz1xFMVJeTzo92d6fPz/bqVRvTfgTLnfDJCxxkxeAHTL8MJcEbmrbH2P2gtxGEf sUPbkzofRBHKCI63syncV8rsViJDZPFjwImXf1Dddzb52ZZIKP+a/KtTpYb3COPA5FOe P7Mk+VmpzDrX/k9+L44eEmewxzTM/lE9r2BA+BSN/MCmC1t10eTttVyqegwrA8BtTu8q bLjw== X-Gm-Message-State: AODbwcDQ9js4C4ZlH55lpZCzfLqtbAal2Gp2uOo5MwTizA1Qz7NCf4fI 8rBNG+bfGFAcnHKz X-Received: by 10.99.47.130 with SMTP id v124mr9858390pgv.116.1495400586542; Sun, 21 May 2017 14:03:06 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.54.70 with SMTP id w64ls2606385otb.26.gmail; Sun, 21 May 2017 14:03:05 -0700 (PDT) X-Received: by 10.157.48.161 with SMTP id s33mr391259otc.1.1495400585714; Sun, 21 May 2017 14:03:05 -0700 (PDT) In-Reply-To: <734764c8-e502-4635-be76-a76cab6261a1@isocpp.org> X-Original-Sender: ma.kalbfuss@web.de Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: 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:32563 Archived-At: ------=_Part_2211_1635247880.1495400585179 Content-Type: multipart/alternative; boundary="----=_Part_2212_1983742866.1495400585180" ------=_Part_2212_1983742866.1495400585180 Content-Type: text/plain; charset="UTF-8" Mabe you're right. Another operator could be a better choice. But I'm still unsure about it. Another approach is to use an attribute to disallow genericity. auto x = runtime_condition [[non generic]] ? A{} : B{}; or vice versa auto x = runtime_condition [[generic]] ? A{} : B{}; It would be interesting to know, if there are other places where the same technique could be applied. Maybe for different return types of function calls, depeding on the passed arguments. I have to explore that. Am Sonntag, 21. Mai 2017 19:35:10 UTC+2 schrieb Nicol Bolas: > > On Sunday, May 21, 2017 at 1:08:21 PM UTC-4, Thiago Macieira wrote: >> >> On domingo, 21 de maio de 2017 02:03:45 PDT ma.ka...@web.de wrote: >> > The code, following the ternary statement is moved into a generic >> lambda. >> > auto x = runtime_condition ? A{} : B{}; >> > >> > The generic lambda is instantiated two times, for each branch. >> Depending on >> > runtime_condition, >> > only one instance is executed. The statement >> >> You do realise this grows exponentially, right? >> >> auto x = runtime_condition ? A{} : B{}; >> auto y = runtime_condition ? C{} : D{}; >> auto z = runtime_condition ? E{} : F{}; >> x.f() + y.f() + z.f(); >> >> There are 8 different branches there to make the above work. >> > > This is one of the reasons why I think, if we're going to do this, then it > *needs* to be a different operator from ?:. Something where you can > clearly see that you're invoking this implicit template stuff. > > And like I said elsewhere, I get the feeling that there is more to this > implicit template instantiation than there first appears. This seems almost > like pattern matching, but done implicitly via template instantiation, > rather than explicitly through some kind of switch statement. > -- 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/d23a44b3-3286-4e0b-b48e-c72b77f465a3%40isocpp.org. ------=_Part_2212_1983742866.1495400585180 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Mabe you're right. Another operator could be a better = choice. But I'm still unsure about it. Another approach is to use an at= tribute to disallow genericity.


auto x =3D runtime_condition [[non generic]] ? A{} : B{};

or vice versa
aut= o x =3D runtime_condition [[generic]] ? A{} := B<= span style=3D"color: #660;" class=3D"styled-by-prettify">{};



It would be interesting to know, if there are other = places where the same technique could be applied. Maybe for different retur= n types of function calls, depeding on the passed arguments. I have to expl= ore that.

Am Sonntag, 21. Mai 2017 19:35:10 UTC+2 schrieb Nicol Bola= s:
On Sunday, = May 21, 2017 at 1:08:21 PM UTC-4, Thiago Macieira wrote:
On domingo, 21 de maio de 2017 02:03:45 PDT ma.ka...@web.de wrote:
> The code, following the ternary statement is moved into a generic = lambda.
> auto x =3D runtime_condition ? A{} : B{};
>=20
> The generic lambda is instantiated two times, for each branch. Dep= ending on
> runtime_condition,
> only one instance is executed. The statement

You do realise this grows exponentially, right?

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0auto x =3D runtime_cond= ition ? A{} : B{};
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0auto y =3D runtime_cond= ition ? C{} : D{};
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0auto z =3D runtime_cond= ition ? E{} : F{};
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0x.f() + y.f() + z.f();

There are 8 different branches there to make the above work.

This is one of the reasons why I think, if we'= ;re going to do this, then it needs to be a different operator from = ?:. Something where you can clearly see that you're invoking this impli= cit template stuff.

And like I said elsewhere, I get the feeling tha= t there is more to this implicit template instantiation than there first ap= pears. This seems almost like pattern matching, but done implicitly via tem= plate instantiation, rather than explicitly through some kind of switch sta= tement.

--
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