220 17486 <15891333.nZL6fDpIhZ@lastique-pc> article Path: news.gmane.org!not-for-mail From: Andrey Semashev Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: constexpr function arguments Date: Fri, 24 Apr 2015 01:15:27 +0300 Lines: 63 Approved: news@gmane.org Message-ID: <15891333.nZL6fDpIhZ@lastique-pc> References: <1799313.qMEWgUf3zn@lastique-pc> <8221738.H9gkvWvTUl@lastique-pc> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-Trace: ger.gmane.org 1429827343 5120 80.91.229.3 (23 Apr 2015 22:15:43 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 23 Apr 2015 22:15:43 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCXY5TXHWYJRBAW64WUQKGQEBZ2THIY@isocpp.org Fri Apr 24 00:15:37 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-la0-f71.google.com ([209.85.215.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1YlPPF-0004Pv-7k for gclcip-std-proposals@m.gmane.org; Fri, 24 Apr 2015 00:15:37 +0200 Original-Received: by laat2 with SMTP id t2sf8028406laa.2 for ; Thu, 23 Apr 2015 15:15:31 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:to:subject:date:message-id:user-agent :in-reply-to:references:mime-version:content-type:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=kuSwRgNX68goDE0lnmuZ6/XHrNiz/0tWJKFL760Z+sE=; b=imEPO+G9ZRENN3Xr3NTPU2QoQjulFg3o1arcFz7qn+QClN9URvWjW9hbmQ/t76IGXG sE8jR3jR+Oc9qWdKe/IA0neDvafj1y/yqkY2HMjXTr6UgM5cBSpN3aiQA7YLG7+YxVJp a2T9OR435K5iAvYUnQWRGMv5mlXbM+ebD5gW7hkZiYWNcHfCUWsOOPgzSF1TkzoqjF39 sZb2Jgu7f5/9lb+EVSr7gZ0B/QNlZpssDCAsQk9p/oFp5HRgPjECYvrgUpLkyBbahGVp wyyvwAInsZtQB8zyN9Bwq/Tmu1uy3aEnBBEkWw10BiFyAX/BWoMteYRzdn+YgmWvBp7m JWWg== X-Gm-Message-State: ALoCoQm61adHnX7vNnuqn86+dgEaTBQK3rDIBUoG+FKi+uW9kDZPmIfFlPbAxSNyOTh0eq4FrzUv X-Received: by 10.194.249.1 with SMTP id yq1mr2460629wjc.2.1429827331672; Thu, 23 Apr 2015 15:15:31 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.152.207.65 with SMTP id lu1ls381419lac.104.gmail; Thu, 23 Apr 2015 15:15:30 -0700 (PDT) X-Received: by 10.112.134.167 with SMTP id pl7mr4336641lbb.50.1429827330093; Thu, 23 Apr 2015 15:15:30 -0700 (PDT) Original-Received: from mail-la0-x22b.google.com (mail-la0-x22b.google.com. [2a00:1450:4010:c03::22b]) by mx.google.com with ESMTPS id es16si6876861lbc.122.2015.04.23.15.15.30 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 23 Apr 2015 15:15:30 -0700 (PDT) Received-SPF: pass (google.com: domain of andrey.semashev@gmail.com designates 2a00:1450:4010:c03::22b as permitted sender) client-ip=2a00:1450:4010:c03::22b; Original-Received: by lagv1 with SMTP id v1so22691514lag.3 for ; Thu, 23 Apr 2015 15:15:29 -0700 (PDT) X-Received: by 10.112.78.105 with SMTP id a9mr4323174lbx.70.1429827329737; Thu, 23 Apr 2015 15:15:29 -0700 (PDT) Original-Received: from lastique-pc.localnet (broadband-178-140-129-233.nationalcablenetworks.ru. [178.140.129.233]) by mx.google.com with ESMTPSA id z2sm2144830lae.20.2015.04.23.15.15.27 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 23 Apr 2015 15:15:28 -0700 (PDT) User-Agent: KMail/4.14.1 (Linux/3.16.0-34-generic; KDE/4.14.1; x86_64; ; ) In-Reply-To: X-Original-Sender: andrey.semashev@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of andrey.semashev@gmail.com designates 2a00:1450:4010:c03::22b as permitted sender) smtp.mail=andrey.semashev@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=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:17486 Archived-At: On Thursday 23 April 2015 14:30:26 'Matt Calabrese' via ISO C++ Standard - Future Proposals wrote: > On Thu, Apr 23, 2015 at 2:09 PM, Andrey Semashev > wrote: > > It would be great if constexpr-ness just transparently propagated into the > > function body, but then the function would likely need an intrinsic to > > detect > > if a certain argument (or more generically speaking, an expression) is a > > constant expression. Say, an is_constexpr() operator, which returns a > > constexpr bool. Do you think such addition would be more feasible? [snip] > As for whether constexpr-ness could truly transparently propagate, I think > it wouldn't be quite so simple to make it actually transparent as it likely > would involve making changes to the type system (unless people see other > routes forward). Isn't it possible to just describe the rules under which constexpr-ness should propagate without changing the type system? Something like "if the function argument is initialized with a constant expression, and the function definition is visible at the call site then that argument is a constexpr within that function body in the context of that particular call." Function- local static variables would still have to be shared between all constexpr or not calls to such function but I think it's not a problem. The address operator would return a pointer to the function that takes runtime arguments. All in all it would appear as if for some calls the function has shadow specializations which are not detectable other than by calling them. It could result in code bloat though. So it's probably better to mark such arguments with constexpr so that such special rules are applied only where needed. It wouldn't affect overload resolution because the function is the same (as constexpr would not participate in the function type). template< typename T > void atomic::store(T val, constexpr memory_order order) { // ... } atomic a; memory_order o = memory_order_relaxed; a.store(10, o); // order is not constexpr within store(); the fully runtime // version of store() is called a.store(10, memory_order_relaxed); // order is constexpr within store(); // a shadow version of store() is called auto p = &atomic::store; // decltype(p) is // void (atomic::*)(T, memory_order); // p points to the fully runtime version // of store() -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. .