220 33230 <803a7b14-34c2-4ee6-8c5a-154532143a6e@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: dimeiyi24@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Proposal "is_constexpr" Date: Mon, 17 Jul 2017 03:15:53 -0700 (PDT) Lines: 82 Approved: news@gmane.org Message-ID: <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/mixed; boundary="----=_Part_1105_515843791.1500286553806" X-Trace: blaine.gmane.org 1500286562 3296 195.159.176.226 (17 Jul 2017 10:16:02 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 17 Jul 2017 10:16:02 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCQLF3GW5QGBBWU4WLFQKGQEWEON64Q@isocpp.org Mon Jul 17 12:15:53 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f197.google.com ([209.85.161.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dX34A-0000GK-I0 for gclcip-std-proposals@m.gmane.org; Mon, 17 Jul 2017 12:15:50 +0200 Original-Received: by mail-yw0-f197.google.com with SMTP id x125sf105738562ywa.0 for ; Mon, 17 Jul 2017 03:15:56 -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:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=Swm3dt1qNmDgDLclW+bo3Ik7u3pUNK1N0uTx+KyWueE=; b=WVFILNLQn9qi02wvVDSTjXpA0mpOceYSJ1aKaF4gagwdS7ohqF7ldO2nNBnoVUkhd7 XPT5g+83ixFPeeeoMrPWoPLMoQDVa0ZFX6jEPagEEJkS9yiy0H7dKn7Y07G5tYwTC3uz +2BztrJX425vtIApVngPPGQzlTMuW7y1bIMNBDIFYfIzjHSDtjuEFbpKt+qucQ+WHWJj IblzExKL8XyLzu9SxkHAx1nEJm3+pc19NpeHXakYGvVDuIuUO7N7q4VQMcD/ZAHY8cBN GyPP3KJFicURj7AzZ0ev7sb3aogjp+2BukxJ0M4fS63lyi9wSl0epzM42v5R3SUWNFMX vVLA== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=Swm3dt1qNmDgDLclW+bo3Ik7u3pUNK1N0uTx+KyWueE=; b=rsXsFcmXnZe20qxocJ5phGguFWb9isW4/XoNoAvpM2voG5+k24QnMdHxhOUQUy6kjq cyEzYcgbc/+9gEjh8nvMOtrnCReWUokljqlI5kNJ44gkbib8Rj0unqzmmI8M4sTIUXAL 3mLg8hvx6ZzujuI0qhaPo0HkS3SFN3lz36xV2s/kCVF0JR1ml3G3wiVVqOuVTmhUASow xs16dFWPvdFeKjbVtq0CAe1TeXsTOwAxc7d4GF8sMh2K73s9KQf+R/+9r+ziTM1U91wn W9EuZBI4nm7Swm5WYCe3P4/fA3Q8XJjyPCTl8FtoEN2K2z92IJWD2aslDyad8SS5L2pt zd7w== 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: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=Swm3dt1qNmDgDLclW+bo3Ik7u3pUNK1N0uTx+KyWueE=; b=P7iBXech6jAdfP6t8lakFp1fblZf+rmUkw9qT4vyOTIU8Ow444k3owQ26AwYNSjPsO qsvxQvrMzDqDdeGEe+nxojXL/sdeMwzxVEX1JlolqnxrSqxx+p14TkSiSOe54i/gOwHr RIfQzQwmucYq/CXpPrkvv0qN4QLYwYcRwaLs/4hXsQzIOAlMwgLkRvEtgDgnNdJ+YU2E HxSxcu8xFJV77eZntT6gtgNGOjyqQGbt9rVrPLQmIUgD60ZvEXoEntlgwrgQbJs0shGA Sh9HIVGHa2+Fqos91sT/MJOoo9R0oXfZrJjZG/T8OUjDh7UNAjUHK/NURw2L+ALtUPFa smbw== X-Gm-Message-State: AIVw113BFI7N/7qybjQJNM/TIDwaXz8/6B4qlU7nlkWU6SxAysOn4U5i kw44tvqoCqTig745 X-Received: by 10.13.220.68 with SMTP id f65mr14421684ywe.18.1500286555720; Mon, 17 Jul 2017 03:15:55 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.16.148 with SMTP id 142ls196709ity.18.gmail; Mon, 17 Jul 2017 03:15:54 -0700 (PDT) X-Received: by 10.31.165.196 with SMTP id o187mr49554vke.0.1500286554352; Mon, 17 Jul 2017 03:15:54 -0700 (PDT) X-Original-Sender: dimeiyi24@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:33230 Archived-At: ------=_Part_1105_515843791.1500286553806 Content-Type: multipart/alternative; boundary="----=_Part_1106_530145505.1500286553806" ------=_Part_1106_530145505.1500286553806 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable =E2=96=A1 Motivation In C++, the same semantic functions sometimes have to write two different= =20 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= =20 value is a compile time constant, but it can only be used for macros, and v= =20 can only be one literal. Can you provide a function of *is_constexpr*(v) to determine whether v is a= =20 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); } --=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/803a7b14-34c2-4ee6-8c5a-154532143a6e%40isocpp.or= g. ------=_Part_1106_530145505.1500286553806 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
=E2=96=A1 = Motivation
In C++, the same semantic functions sometimes have to = write two different versions, a constexpr version, an efficient general ver= sion.
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 co= nstant, but it can only be used for macros, and v can only be one literal.<= br>Can you provide a function of is_constex= pr(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){
=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);
}

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