220 33234 article Path: news.gmane.org!.POSTED!not-for-mail From: Daemon Snake Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Proposal "is_constexpr" Date: Mon, 17 Jul 2017 17:13:42 +0200 Lines: 132 Approved: news@gmane.org Message-ID: References: <803a7b14-34c2-4ee6-8c5a-154532143a6e@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="001a113f73d863d2cb055484d982" X-Trace: blaine.gmane.org 1500304447 31968 195.159.176.226 (17 Jul 2017 15:14:07 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 17 Jul 2017 15:14:07 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCX2P6W5SEDBBPFIWPFQKGQE3IYLNYY@isocpp.org Mon Jul 17 17:14:02 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f199.google.com ([209.85.161.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dX7ih-0007ys-Vz for gclcip-std-proposals@m.gmane.org; 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[2607:f8b0:4002:c05::244]) by mx.google.com with ESMTPS id x5si3487234ybd.449.2017.07.17.08.14.03 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 17 Jul 2017 08:14:03 -0700 (PDT) Received-SPF: pass (google.com: domain of swac31@gmail.com designates 2607:f8b0:4002:c05::244 as permitted sender) client-ip=2607:f8b0:4002:c05::244; Original-Received: by mail-yw0-x244.google.com with SMTP id v193so2928553ywg.0 for ; Mon, 17 Jul 2017 08:14:03 -0700 (PDT) X-Received: by 10.129.94.198 with SMTP id s189mr17841303ywb.335.1500304443145; Mon, 17 Jul 2017 08:14:03 -0700 (PDT) Original-Received: by 10.37.87.4 with HTTP; Mon, 17 Jul 2017 08:13:42 -0700 (PDT) In-Reply-To: <803a7b14-34c2-4ee6-8c5a-154532143a6e@isocpp.org> X-Original-Sender: swac31@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.b=FRtHxml0; spf=pass (google.com: domain of swac31@gmail.com designates 2607:f8b0:4002:c05::244 as permitted sender) smtp.mailfrom=swac31@gmail.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=gmail.com 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:33234 Archived-At: --001a113f73d863d2cb055484d982 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Hi, is_constexp(s) is not possible, nor likely currently as function arguments are never constexpr. The subject is highly controversial (possible ODR violations, etc...). Read this discussion: https://groups.google.com/a/isocpp.org/forum/?fromgroups#!search/is_constex= pr/std-proposals/NErPRYCz3ZU/AOcjouU8BAAJ 2017-07-17 12:15 GMT+02:00 : > =E2=96=A1 Motivation > In C++, the same semantic functions sometimes have to write two different > versions, a constexpr version, an efficient general version. > For example: > constexpr auto StrLen(const char *s) {auto tp=3Ds;while(*s) s++;return s-= tp;} > auto StrLenSSE4(const char *s){...} > =E2=96=A1 Proposal > Gcc has a built-in function __builtin_constant_p (v) to determine whether > a value is a compile time constant, but it can only be used for macros, a= nd > v can only be one literal. > Can you provide a function of *is_constexpr*(v) to determine whether v is > a compile time constant? > In this way, the function above can use a unified interface. > constexpr auto StrLen(const char *s){ > if constexpr(*is_constexpr*(s)) { > auto tp=3Ds;while(*s) s++;return s-tp; > } > else return StrLenSSE4(s); > } > > -- > 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/803a7b14-34c2-4ee6- > 8c5a-154532143a6e%40isocpp.org > > . > --=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/CAAE_HdJWNqFYUJvCDGGXE4oYF6vhVd5iFY3atKPjtH9pNRr= Aqg%40mail.gmail.com. --001a113f73d863d2cb055484d982 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Hi,

is_constexp(s) is not possibl= e, nor likely currently as function arguments are never constexpr.
The = subject is highly controversial (possible ODR violations, etc...).
Read this discussion: https://groups.google.com/a/isocpp.org/forum/?fromgroups#!search/is_cons= texpr/std-proposals/NErPRYCz3ZU/AOcjouU8BAAJ

<= div>


2017-07-17 12:15 GMT+02:00 <dimeiyi2= 4@gmail.com>:
=E2=96=A1 Motivation
In C++, the same semantic functions sometimes have to write two dif= ferent versions, a constexpr version, an efficient general version.
For = example:
constexpr auto StrLen(const char *s) {auto tp=3Ds;while(*s) s++= ;return s-tp;}
auto StrLenSSE4(const char *s){...}
=E2=96=A1 Proposal
Gcc has a built-in function __builtin_co= nstant_p (v) to determine whether a value is a compile time constant, but i= t can only be used for macros, and v can only be one literal.
Can you pr= ovide a function of is_constexpr(v) to determine whether v is a compile time constant?
In this way,= the function above can use a unified interface.
constexpr auto StrLen(c= onst char *s){
=C2=A0 if constexpr(is_constexpr(s)) {=C2=A0=C2=A0=C2=A0 auto tp=3Ds;while(*s) s++;return s-tp;
=C2=A0 }
= =C2=A0 else return StrLenSSE4(s);
}
<= font color=3D"#888888">

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