220 31755 <470f86cc-36b7-4244-82dd-b795af107996@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allow constexpr static data members in a literal type. Date: Sun, 26 Mar 2017 07:35:44 -0700 (PDT) Lines: 118 Approved: news@gmane.org Message-ID: <470f86cc-36b7-4244-82dd-b795af107996@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_4619_1018717613.1490538944376" X-Trace: blaine.gmane.org 1490538948 26272 195.159.176.226 (26 Mar 2017 14:35:48 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 26 Mar 2017 14:35:48 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBQFD37DAKGQEXM34PNI@isocpp.org Sun Mar 26 16:35:42 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pg0-f72.google.com ([74.125.83.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cs9Gd-0005yw-VI for gclcip-std-proposals@m.gmane.org; Sun, 26 Mar 2017 16:35:40 +0200 Original-Received: by mail-pg0-f72.google.com with SMTP id v21sf29796233pgo.22 for ; Sun, 26 Mar 2017 07:35:46 -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=sQSEvGWzGSBR18IO+5hhMdq7I1iDfnkOgQ0zeRfcfws=; b=yShUvEbjiER1nn7FFm+yXvkLvyih1y9NhwagYjoqdT1cUCc7k+sHGoZTtMXS8Ngjfq qeYjpAapeARvYk2bUTwF2b/YJBOoLiTnJvy2epG9uS0XHVipXUomx08c8D7fXn0lyf+r YZD8WsG5Qqn+qNBE5KZKFor//wIapEOkQWbSiVqlmuC3W7bNE4lsje7GJlTu6ISdP5f4 XE4IaICai6G/cVTV/XJwR5TRd+nkrDhdc0Ywy4xrfgqaAtO+jCJzj7yseggou4bT1tHO 9oJG2IbIxlQWUupe4Ckqv3Jn4ri1ulr/roRaJSd73ZD3ux60Z8CGj6rhUi0FYhEHsLhJ 2SLw== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; 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=sQSEvGWzGSBR18IO+5hhMdq7I1iDfnkOgQ0zeRfcfws=; b=nubiXzAbaQxURO7AtbT5o6TOS/1/H6E8zfq4Q7mkDPUFKyJyLc1lld8rH4lnG19Fs2 cfkKAsFpLlhnuLnO3miifvrMSMyMWF3hNgiuuGhIJmXKVKs8JiJnaRLB2wXQsHfpAiKX JCoJ4lyKVuqVRsaXxnA83GbkfAKzJxSkkDdDRwYaPX0Tz+Xw9otfFOXTHAMt7w+JwvqM hf7OFJFU46holQYl0BvCJ70Di64vXlVQnREAkbOK2FoLLr8XQSOqR3gNYlLd0zcJbKcJ KIlUQ1gQqTu4o7DSh1oZGcHYI/cZ0bYX256WprR5uneBDmrIvAxY3NWBUsd4LdI17k9h 1tRw== 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=sQSEvGWzGSBR18IO+5hhMdq7I1iDfnkOgQ0zeRfcfws=; b=B+XrJRV5B6IrodPmAuXOX3XC0xuLhduFotsT8xR956aWnIxre9/3tOwKGThwXil1hI iWoU7DTBWCKXpvKps0z8B18kVb03gHTORl3jYtEYwdKBT430ErKyPYyT6QFSBQhvmF7q 03zkUwyi73oETxddrny+J/PrzMWxcCpBConP4J2tb2UcYlTSxI84wU509zkqZcmffazv 6yQGtzNYQfjNMN2WjGzSa4TX+42ZPoYS3/EIVPe1vxahTu92O7wprN0O18AavVWLYcPh Zkzv/bxi/fisCT9zhHM9eYNOr7iFUWYg1YZ5L5HZjN2mWnF4Beg2OfzjWLQUx2ggTR2O hiSQ== X-Gm-Message-State: AFeK/H1PijJbL+3xMcQT4p7ie5eKBBuuPZEQndt//91EdZp6Ddb03eCNgrtVNaz/7nPBiw== X-Received: by 10.99.124.27 with SMTP id x27mr6301880pgc.50.1490538945623; Sun, 26 Mar 2017 07:35:45 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.47.171 with SMTP id r40ls1068205otb.0.gmail; Sun, 26 Mar 2017 07:35:44 -0700 (PDT) X-Received: by 10.157.11.150 with SMTP id 22mr235830oth.18.1490538944826; Sun, 26 Mar 2017 07:35:44 -0700 (PDT) In-Reply-To: X-Original-Sender: jmckesson@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:31755 Archived-At: ------=_Part_4619_1018717613.1490538944376 Content-Type: multipart/alternative; boundary="----=_Part_4620_1380570845.1490538944376" ------=_Part_4620_1380570845.1490538944376 Content-Type: text/plain; charset=UTF-8 On Sunday, March 26, 2017 at 10:30:29 AM UTC-4, Avi Kivity wrote: > > Consider this type-safe bool class: > > > template > > class bool_class { > > bool _value; > > public: > > explicit constexpr bool_class(bool v) noexcept : _value(v) {} > > // more emulation of bool > > static constexpr bool_class yes = bool_class{true}; > > static constexpr bool_class no = bool_class{true}; > > }; > > > Compilation fails, I believe rightly, on clang, because by the time yes > and no are parsed, bool_class is an incomplete type, and therefore not a > literal type. It could be worked around by making yes and no functions. > > Should we make this legal? It seems reasonable for types to offer > constants scoped under their own name, and if they are literal types, > that these constants can be constexpr. > How does it seem reasonable to define variables of a type that has not yet been defined? What's wrong with defining them outside of the scope of the class? -- 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/470f86cc-36b7-4244-82dd-b795af107996%40isocpp.org. ------=_Part_4620_1380570845.1490538944376 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Sunday, March 26, 2017 at 10:30:29 AM UTC-4, Avi Kivity= wrote:
Consider this type-safe= bool class:


=C2=A0 =C2=A0template <typename Tag>

=C2=A0 =C2=A0class bool_class {

=C2=A0 =C2=A0 =C2=A0 =C2=A0bool _value;

=C2=A0 =C2=A0public:

=C2=A0 =C2=A0 =C2=A0 =C2=A0explicit constexpr bool_class(bool v) noexce= pt : _value(v) {}

=C2=A0 =C2=A0 =C2=A0 =C2=A0// more emulation of bool

=C2=A0 =C2=A0 =C2=A0 =C2=A0static constexpr bool_class yes =3D bool_cla= ss{true};

=C2=A0 =C2=A0 =C2=A0 =C2=A0static constexpr bool_class no =3D bool_clas= s{true};

=C2=A0 =C2=A0};


Compilation fails, I believe rightly, on clang, because by the time yes= =20
and no are parsed, bool_class is an incomplete type, and therefore not = a=20
literal type. =C2=A0It could be worked around by making yes and no func= tions.

Should we make this legal? =C2=A0It seems reasonable for types to offer= =20
constants scoped under their own name, and if they are literal types,= =20
that these constants can be constexpr.

How doe= s it seem reasonable to define variables of a type that has not yet been de= fined? What's wrong with defining them outside of the scope of the clas= s?

--
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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/470f86cc-36b7-4244-82dd-b795af107996= %40isocpp.org.
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