220 32541 <99350a9e-0c6b-468d-9761-f2b2b052275e@isocpp.org> 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 02:03:45 -0700 (PDT) Lines: 424 Approved: news@gmane.org Message-ID: <99350a9e-0c6b-468d-9761-f2b2b052275e@isocpp.org> References: <1b5ee8eb-53df-4e98-af2f-829c7bc2e5b2@isocpp.org> <9064929.QEV8q21eIZ@tjmaciei-mobl1> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2043_190528582.1495357425403" X-Trace: blaine.gmane.org 1495357427 24836 195.159.176.226 (21 May 2017 09:03:47 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 21 May 2017 09:03:47 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBC7YTI72XQOBB4VPQXEQKGQEDUYWU4A@isocpp.org Sun May 21 11:03:42 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f198.google.com ([209.85.161.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dCMm5-0006KS-Tt for gclcip-std-proposals@m.gmane.org; Sun, 21 May 2017 11:03:42 +0200 Original-Received: by mail-yw0-f198.google.com with SMTP id f204sf61022531ywc.15 for ; Sun, 21 May 2017 02:03:47 -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=PiVZBObZnayN+xHPJo/ouOr/EC5xWTXTT8b4/bajfQI=; b=uTkhZWFi5ECmv4Q19xWZWm2HSJwtkhfxySXXYISKr5xU2zzbp8Tg1p6nppyJoqyat0 tKOow2zU9FZu021QyJR9mbbSNZbLhk4Cwdtl72ls4Ms9eskVa7cBF8UjHbHZE5wwJPv0 44QmI+gi+1cXGzRIqLa93uB/u/DmkIbJV/g/s1hZQxV8SmGeuTMzakMIkYNDPmwcwFie bEs1Bixv9qVMEXka3nl3MKWlToRv3YRGiW/x4GcKxCHNGGIUkrQ9bSxh2c1SCvsqRsqG PHMYIf4gbG6WWeKEipu+8tSFJ7qlYOxlSU68HlTjryFZv786B9HbaWLR812YRGmwddMQ W3/Q== 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=PiVZBObZnayN+xHPJo/ouOr/EC5xWTXTT8b4/bajfQI=; b=GZdp21mCwLimqqH03IHRprM6UC7xadUNMQF3oL5g2E4L1tN8SaLbixYT9UJ9n5TKp7 5n7blU5k9cV5F8jH0FUcpHjU1lSckCBx38VXZf1xvNGF9ws80Y7b7PALEvzqToZu/5al 8Wsx/pCeGjT43nIe0QEWenrXWlk/m1RJVTyk8P/F6kRHmeJ+yGqHJGD9T9m2MUJOBb0W dU+AqTuh3mQPeGOoynv3dE9WcIZtqR1AYwLglaZIT9J5FV6EY8xupdscmaCJcyqNRozr rn5IOgDfvh75DxfHc4SF3rMxPh4qNtLxskKoVM0rJc5ZbE+Y2TMj2PeqZA5GcDyyFw+O N4Rw== X-Gm-Message-State: AODbwcBqvOmFj7fkeeewxkaDRL5XaGpRkf0ZSoLcHj5JdiFhdSA0qoCe FHvC8VKWOCyqynD5 X-Received: by 10.129.38.213 with SMTP id m204mr8751750ywm.114.1495357426954; Sun, 21 May 2017 02:03:46 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.11.145 with SMTP id 17ls1982024oth.36.gmail; Sun, 21 May 2017 02:03:46 -0700 (PDT) X-Received: by 10.157.13.170 with SMTP id 39mr370469ots.18.1495357426099; Sun, 21 May 2017 02:03:46 -0700 (PDT) In-Reply-To: <9064929.QEV8q21eIZ@tjmaciei-mobl1> 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:32541 Archived-At: ------=_Part_2043_190528582.1495357425403 Content-Type: multipart/alternative; boundary="----=_Part_2044_681404285.1495357425404" ------=_Part_2044_681404285.1495357425404 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable I revisited your statement, that this isn't possible, because the type=20 would be determined at runtime. I have to disagree, now. It is possible at= =20 compile time. Look at the following code: auto x =3D runtime_condition ? A{} : B{}; x.doit(); This is not going to work, as you stated. But you could write the following= =20 instead: auto f =3D [&](auto x) { x.doit(); }; runtime_condition ? f(A{}) : f(B{}); The code, following the ternary statement is moved into a generic lambda. auto x =3D runtime_condition ? A{} : B{}; The generic lambda is instantiated two times, for each branch. Depending on= =20 runtime_condition, only one instance is executed. The statement auto x =3D runtime_condition ? A{} : X{}; could introduce the following code inside the current scope twice, like the= =20 generic lambda did. Ones for each type. Depending on the runtime_condition, the code pointer is moved to the first= =20 or the second instantiaton with a jump. The following C++ code snippet describes roughly how the generated code=20 could look like. #include struct A{ void doit(){ std::cout << "A::doit" << std::endl; } }; struct B{ void doit(){ std::cout << "B::doit" << std::endl; } }; int main() { auto runtime_condition =3D true; if(runtime_condition) goto ternary_branch_1; else goto ternary_branch_2; ternary_branch_1: A __instance1__; __instance1__.doit(); return 0; ternary_branch_2: B __instance2__; __instance2__.doit(); return 0; } Am Samstag, 20. Mai 2017 21:29:56 UTC+2 schrieb Thiago Macieira: > > On s=C3=A1bado, 20 de maio de 2017 10:41:56 PDT ma.ka...@web.de =20 > wrote:=20 > > Hi Guys,=20 > >=20 > > I had the following situation:=20 > >=20 > > I have a Group of StaticSet classes with a compatible interface but=20 > > distinct in there concrete type. empty_set is not an intervall!=20 > > I tried the following:=20 > >=20 > >=20 > > auto dec_digit =3D make_interval('0', '0' + std::min(base, DECIMAL_BASE= ) -=20 > 1);=20 > > auto upper_digit =3D (base > DECIMAL_DIGIT) ? make_interval('A', 'A' += =20 > (base=20 > > - 1 - DECIMAL_BASE)) : empty_set;=20 > > auto lower_digit =3D (base > DECIMAL_DIGIT) ? make_interval('a', 'a' += =20 > (base=20 > > - 1 - DECIMAL_BASE)) : empty_set;=20 > >=20 > >=20 > > Wouldn't it be possible to allow such code, especially in conjunction= =20 > with=20 > > concepts?=20 > > No. lower_digit must have exactly one type, determined at compile-time.= =20 > What=20 > you're asking for would cause the type to be determined at runtime, which= =20 > is=20 > not possible in C++.=20 > > Alternatives:=20 > * convert empty_set to make_interval's type=20 > * convert either to a common type (which you need to be explicit about, s= o=20 > it=20 > won't work with auto)=20 > * use std::variant=20 > > --=20 > Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org=20 > Software Architect - Intel Open Source Technology Center=20 > > --=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/99350a9e-0c6b-468d-9761-f2b2b052275e%40isocpp.or= g. ------=_Part_2044_681404285.1495357425404 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
I revisited your statement, that this isn't possible, = because the type would be determined at runtime. I have to disagree, now.= =C2=A0 It is possible at compile time.

Look at the following code:
au= to x =3D runtime_condition ? A{} : B{};
x.doit();

This is not going to work, as you stated. But you could write = the following instead:

auto f =3D [&= ;](auto x) { x.doit(); = };<= span style=3D"color: #000;" class=3D"styled-by-prettify">
runtime_condit= ion ? f(A{}) : f(B= {});

The code, following the ternary statement is moved into a generi= c lambda.
auto x =3D runtime_condition ? A{} = : B{};

The generic lambd= a is instantiated two times, for each branch. Depending on runtime_conditio= n,
only one instance is executed. The statement

= auto x =3D runtime_condition ? A{} : X{}= ;

could introduce the following code inside the curr= ent scope twice, like the generic lambda did. Ones for each type.
Depend= ing on the runtime_condition, the code pointer is moved to the first or the= second instantiaton with a jump.

The following C++ co= de snippet describes roughly how the generated code could look like.
#includ= e <= span style=3D"color: #080;" class=3D"styled-by-prettify"><iostream>

struct<= span style=3D"color: #000;" class=3D"styled-by-prettify"> A{ void doit(){ std::c= out <&l= t; = "A::doit&quo= t; = << std::endl; } = };
struct<= span style=3D"color: #000;" class=3D"styled-by-prettify"> B{ void doit(){ std::c= out <&l= t; = "B::doit&quo= t; = << std::endl; } = };

= int= main()
{
=C2=A0 =C2=A0 auto runtime_condition =3D true;

=C2=A0 =C2=A0 if(runtime_condition)
=C2=A0 =C2=A0 =C2=A0 =C2=A0 goto ternary_branch_1;
=C2=A0 =C2=A0 else
=C2=A0 =C2=A0 =C2=A0 =C2=A0 goto ternary_branch_2;

ternary_branch_1:
=C2=A0 =C2=A0 A __instance1__;
=C2=A0 =C2=A0 __i= nstance1__= ..doit();
=C2=A0 =C2=A0 return<= span style=3D"color: #000;" class=3D"styled-by-prettify"> 0;
ternary_branch_2:
=C2=A0 =C2=A0 B __instance2__;
=C2=A0 =C2=A0 __instance= 2__.doit();
=C2=A0 =C2=A0 return 0;
}







Am= Samstag, 20. Mai 2017 21:29:56 UTC+2 schrieb Thiago Macieira:
On s=C3=A1bado, 20 de maio de 2017 10:41:56= PDT = ma.ka...@web.de wrote:
> Hi Guys,
>=20
> I had the following situation:
>=20
> I have a Group of StaticSet classes with a compatible interface bu= t
> distinct in there concrete type. empty_set is not an intervall!
> I tried the following:
>=20
>=20
> auto dec_digit =3D make_interval('0', '0' + std::m= in(base, DECIMAL_BASE) - 1);
> auto upper_digit =3D (base > DECIMAL_DIGIT) ? make_interval(= 9;A', 'A' + (base
> - 1 - DECIMAL_BASE)) : empty_set;
> auto lower_digit =3D (base > DECIMAL_DIGIT) ? make_interval(= 9;a', 'a' + (base
> - 1 - DECIMAL_BASE)) : empty_set;
>=20
>=20
> Wouldn't it be possible to allow such code, especially in conj= unction with
> concepts?

No. lower_digit must have exactly one type, determined at compile-time.= What=20
you're asking for would cause the type to be determined at runtime,= which is=20
not possible in C++.

Alternatives:
* convert empty_set to make_interval's type
* convert either to a common type (which you need to be explicit about,= so it=20
=C2=A0 =C2=A0won't work with auto)
* use std::variant<interval, decltype(empty_set)>

--=20
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
=C2=A0 =C2=A0Software Architect - Intel Open Source Technology Center

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