220 10106 article Path: news.gmane.org!not-for-mail From: Richard Smith Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Shouldn't the constexpr variables be treated as static by default? Date: Mon, 31 Mar 2014 00:47:23 -0700 Lines: 137 Approved: news@gmane.org Message-ID: References: <21e34045-b833-4468-a39e-dbf177938012@isocpp.org> <6838b332-f256-45e7-894b-7cba3593d789@isocpp.org> <4271540.MD85hg9ecn@tjmaciei-mobl4> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=089e013a1fcc19d76504f5e243de X-Trace: ger.gmane.org 1396252052 19115 80.91.229.3 (31 Mar 2014 07:47:32 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 31 Mar 2014 07:47:32 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDVNBJG4YAIBBDF34SMQKGQEB3MZROY@isocpp.org Mon Mar 31 09:47:27 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f200.google.com ([209.85.214.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1WUWwI-0003PD-Hc for gclcip-std-proposals@m.gmane.org; Mon, 31 Mar 2014 09:47:26 +0200 Original-Received: by mail-ob0-f200.google.com with SMTP id gq1sf27739669obb.7 for ; Mon, 31 Mar 2014 00:47:25 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:sender:in-reply-to:references:date :message-id:subject:from:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=fMJqlUgGR7C9SHFgBUFuSL40fWHO314MZ9UGpg/u2do=; b=QyQmgX51T3rUfrpGzSIto7AK0JGphRgPKGnsLILbgslAmJDfewFNQq6tJ50CHORDB+ fEngk1Mo6iwXAdBPWxPpijBiMiqPceKmm70QTh7m6Yf9ZptF1Yg7bXYTUkz/pYw+UdtE dpm0eOI3jdpq49RhXWQCBjRPKQQHJ92rIxgoW7INVpK6+ixYefQCTI/S8NkS5o54cMK/ BqByDnBKL9D96lSBNnRAkglctR0vhY4wVomJBYf4HChtCwah4Hnoz1Tmsr7/BnWL1i+N rASSVEN785/9PoKzYvtx7afZJzqW9mBCkRhXoNJqJT0V2xUvGYOCnS3FtmdEYaFNXmk8 x6Tg== X-Gm-Message-State: ALoCoQlDjopI2LAbgLvGYzG7l6i7Fc5auCwR9a6n/wb53mvs/Ch4LW/9EjvE1r+EcOTny2zcgy5I X-Received: by 10.43.161.202 with SMTP id mh10mr7742765icc.23.1396252045069; Mon, 31 Mar 2014 00:47:25 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.90.80 with SMTP id w74ls2229488qgd.95.gmail; Mon, 31 Mar 2014 00:47:24 -0700 (PDT) X-Received: by 10.52.128.231 with SMTP id nr7mr18516664vdb.17.1396252044417; Mon, 31 Mar 2014 00:47:24 -0700 (PDT) Original-Received: from mail-vc0-x22a.google.com (mail-vc0-x22a.google.com [2607:f8b0:400c:c03::22a]) by mx.google.com with ESMTPS id tz5si2833057vdc.133.2014.03.31.00.47.24 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Mon, 31 Mar 2014 00:47:24 -0700 (PDT) Received-SPF: pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:400c:c03::22a as permitted sender) client-ip=2607:f8b0:400c:c03::22a; Original-Received: by mail-vc0-f170.google.com with SMTP id hu19so8164797vcb.29 for ; Mon, 31 Mar 2014 00:47:24 -0700 (PDT) X-Received: by 10.52.191.100 with SMTP id gx4mr18635584vdc.4.1396252044021; Mon, 31 Mar 2014 00:47:24 -0700 (PDT) Original-Sender: metafoo@gmail.com Original-Received: by 10.221.12.203 with HTTP; Mon, 31 Mar 2014 00:47:23 -0700 (PDT) In-Reply-To: <4271540.MD85hg9ecn@tjmaciei-mobl4> X-Original-Sender: richard@metafoo.co.uk X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:400c:c03::22a as permitted sender) smtp.mail=metafoo@gmail.com; dkim=pass header.i=@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:10106 Archived-At: --089e013a1fcc19d76504f5e243de Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable On Sun, Mar 30, 2014 at 11:27 PM, Thiago Macieira wrot= e: > Em dom 30 mar 2014, =E0s 22:18:43, Geoffrey Romer escreveu: > > Ah, I hadn't noticed the addresses were varying in the static example. = I > > grant that this could theoretically be a breaking change, but how much > real > > code would be broken? The kinds of things you'd have to do to notice th= e > > difference seem like things that you'd normally never do with a constex= pr > > variable. > > I'd say that for anyone who caused the address to be taken, they really > want > the current behaviour. So it will break a lot of code. > Can you point to any code it would break? I'm having a hard time seeing why you would ever want an automatic storage duration constexpr variable. > What's the benefit you were looking for? It'd fix this weirdness: void f() { constexpr auto x =3D { 1, 2, 3 }; // ill-formed static constexpr auto y =3D { 1, 2, 3 }; // ok // or more obviously... constexpr auto &&a =3D 1; // ill-formed static constexpr auto &&b =3D 1; // ok } The problem in these cases is that a temporary is lifetime-extended to the lifetime of the variable (an array temporary in the first cases, and an int temporary in the last cases), and a constexpr reference can't bind to an automatic storage duration object. It'd also remove the unnecessary stack usage in common cases like: void g(const char*); void h() { constexpr char foo[] =3D "some string literal goes here"; g(foo); } Under the current language rules, the compiler is *required* to allocate stack space and copy that string literal onto the stack each time through this function (assuming it can't see the definition of 'g'). --=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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. --089e013a1fcc19d76504f5e243de Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable
On S= un, Mar 30, 2014 at 11:27 PM, Thiago Macieira <thiago@macieira.org&g= t; wrote:
Em dom 30 mar 2014, =E0s 22:18:43, Geoffrey = Romer escreveu:
> Ah, I hadn't noticed the addresses were varying in= the static example. I
> grant that this could theoretically be a breaking change, but how much= real
> code would be broken? The kinds of things you'd have to do to noti= ce the
> difference seem like things that you'd normally never do with a co= nstexpr
> variable.

I'd say that for anyone who caused the address to be taken, they = really want
the current behaviour. So it will break a lot of code.

Can you point to any code it would break? I'm having a = hard time seeing why you would ever want an automatic storage duration cons= texpr variable.
=A0
What's the benefit you were looking for?

It'd fix this weirdness:

void f() {
=A0 constexpr auto x =3D { 1, 2, 3 }; // ill-formed
=A0 sta= tic constexpr auto y =3D { 1, 2, 3 }; // ok

=A0 // or more obviously...
=A0 constexpr aut= o &&a =3D 1; // ill-formed
=A0 static constexpr auto &= ;&b =3D 1; // ok
}

The problem in th= ese cases is that a temporary is lifetime-extended to the lifetime of the v= ariable (an array temporary in the first cases, and an int temporary in the= last cases), and a constexpr reference can't bind to an automatic stor= age duration object.

It'd also remove the unnecessary stack usage in com= mon cases like:

void g(const char*);
voi= d h() {
=A0 constexpr char foo[] =3D "some string literal go= es here";
=A0 g(foo);
}

Under the current lan= guage rules, the compiler is *required* to allocate stack space and copy th= at string literal onto the stack each time through this function (assuming = it can't see the definition of 'g').

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