220 33458 article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Proposal: underlying_cast Date: Sun, 30 Jul 2017 17:43:29 -0700 (PDT) Lines: 134 Approved: news@gmane.org Message-ID: References: <6a2c32ed-99b5-4cc5-86a5-ee3cc07567a3@isocpp.org> <20170731003821.GA7900@noemi.bahnhof.se> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_3649_911313278.1501461809434" X-Trace: blaine.gmane.org 1501461815 30546 195.159.176.226 (31 Jul 2017 00:43:35 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 31 Jul 2017 00:43:35 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBMX27HFQKGQETODYYDQ@isocpp.org Mon Jul 31 02:43:27 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-it0-f72.google.com ([209.85.214.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dbynu-0007Pq-FH for gclcip-std-proposals@m.gmane.org; Mon, 31 Jul 2017 02:43:26 +0200 Original-Received: by mail-it0-f72.google.com with SMTP id 7sf250542098ita.0 for ; Sun, 30 Jul 2017 17:43:31 -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 :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=CdRRp0a02jDsBEf8f4leTVnOp44+rf/OPquXT/XCVI8=; b=Ne50wR1DcUtua5Yn1Q/w6v7EtlCLwG1y+t+mlMl/Eho0h4dHaPtEAzxtx/vcc2IBTS WxnDbb07qZo77YuMtlqA5Y8XMqQGEQSGURy0qj9gTPxS92S13lqnT4epAxBXoyIMGg2d 4Ld6xkAWHeo3X/E/X+vKI+xoeU95Ux6hyCroFPzmuiWw/No+mqZaw9RkM3tj3tV8d5M2 VuNPkntPE+b3HP1LWT58M1R3svdaTyd4Z0K3xqBmdFD+Cwuv5fQi6eMsx11/RWYdmJId 8sdwTqsAZIIoBV7FU0sYeF2bK9rFvpmTNfedqTxiE589g/al4/vA01AdGZkpysvLxvvg 5/dg== 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 :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=CdRRp0a02jDsBEf8f4leTVnOp44+rf/OPquXT/XCVI8=; b=cE/Y+/t8As0CxfPYjtmegDhFywAiKABoS804g02slO9T5QaqvzhXXPD4ps0pWVpBGX 59WbGYyT2XS7dju/djl/97NHnGWbwlUq4/M/dgMxLlkatSQwLnrhQEFRlFpzAJjzuvvM 9Sl0foOcnY+FEf5Vke+/8Vu5qLZrrR2vKVkblflgRxx0F45ZJSZB4WbMQibPVM8X4HBZ DvvpYBZQ2fjFQrngt8gglSCNPxhx1mHxHkcLbgqNjSXXuFMJ7Vsoae3RVT2rFEiZ7oBw OtUY4V60UzUuhI7iavPMswJ5ehYryTdSalEvxfqGgRr5CkoffsZOkmaX0eV49qTgzvf/ SHaw== 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=CdRRp0a02jDsBEf8f4leTVnOp44+rf/OPquXT/XCVI8=; b=TmERPva1HQwTkmwppn+kJGOiqIeC10bBEJBKkPkYqmGP1YlYp+uODaMmVntFFr5R2k wuhV073q2ngy94Yatu5eaiAFlVbWDqT5141AHtekv+W/bCCLYKCRw23KIgQyTiM15guh j/ZD64t3Kw+0WbdQgcgu8i7yl1BsTO34cgpBX0dHGS/nY8u9MTTdzYV4n1UsbOkLQSnv Sf7WvBEhIbY1x3aqdHs343Ni92ceO3BzfzWOgkpE91p16hI7p6s+TI5/HsgDTx97nk1I 8kAVu0hJNlbltt/jg4ybOsre8+S3yEx0sdKBGv4hJABb+GcYOXus7IxowxA7U6Ow5SUL UZ4g== X-Gm-Message-State: AIVw112Y+84WM1hFdBCBniOii/F69A4oxrWs5CohcXqUq1vhwtuhohrO U547D/hoIzI8CUFw X-Received: by 10.107.18.225 with SMTP id 94mr10508510ios.102.1501461811271; Sun, 30 Jul 2017 17:43:31 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.23.207 with SMTP id 198ls7340820ith.18.gmail; Sun, 30 Jul 2017 17:43:29 -0700 (PDT) X-Received: by 10.31.179.145 with SMTP id c139mr76734vkf.24.1501461809896; Sun, 30 Jul 2017 17:43:29 -0700 (PDT) In-Reply-To: <20170731003821.GA7900@noemi.bahnhof.se> 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:33458 Archived-At: ------=_Part_3649_911313278.1501461809434 Content-Type: multipart/alternative; boundary="----=_Part_3650_846813795.1501461809434" ------=_Part_3650_846813795.1501461809434 Content-Type: text/plain; charset="UTF-8" On Sunday, July 30, 2017 at 8:38:28 PM UTC-4, Magnus Fromreide wrote: > > On Sun, Jul 30, 2017 at 04:50:41AM -0700, james.a...@gmail.com > wrote: > > C++ allows enums to have an underlying type specified. When serialising > > enum values it's important to use the correct type. > > This proposal is to add a utility function that casts an enum type to > its > > underlying value. This would make it easier to handle > > enum types in function overloads for serialisation. For example: > > > > template > > std::ostream& write(std::ostream& os, const T& v) > > { > > os << std::underlying_cast(v); > > return os; > > } > > > > > > This function would, in the case of enums cast the value to the > underlying > > type, and in the case of any other type it would do nothing. > > > > One possible implementation: > > > > template > > using UnderlyingType = typename std::underlying_type::type; > > What about recursive enums? > > enum foo : int { value1 = 4096 }; > enum bar : foo { value2 = 4095 }; > > What does std::underlying_type::type resolve to? > Nothing, because `bar` is ill-formed: > The type-specifier-seq of an enum-base shall name an integral type; any cv-qualification is ignored Enumeration types are not "integral type"s. > > /MF > -- 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/b7df0078-2395-4def-bd73-fd708083969d%40isocpp.org. ------=_Part_3650_846813795.1501461809434 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Sunday, July 30, 2017 at 8:38:28 PM UTC-4, Magn= us Fromreide wrote:
On Sun, Jul= 30, 2017 at 04:50:41AM -0700, james.a...@gmail.com wrote:
> C++ allows enums to have an underlying type specified. When serial= ising=20
> enum values it's important to use the correct type.
> This proposal is to add a utility function that casts an enum type= to its=20
> underlying value. This would make it easier to handle
> enum types in function overloads for serialisation. For example:
>=20
> template <typename T>
> std::ostream& write(std::ostream& os, const T& v)
> {
> =C2=A0 =C2=A0 os << std::underlying_cast(v);
> =C2=A0 =C2=A0 return os;
> }
>=20
>=20
> This function would, in the case of enums cast the value to the un= derlying=20
> type, and in the case of any other type it would do nothing.
>=20
> One possible implementation:
>=20
> template <typename E>
> using UnderlyingType =3D typename std::underlying_type<E>::t= ype;

What about recursive enums?

enum foo : int { value1 =3D 4096 };
enum bar : foo { value2 =3D 4095 };

What does std::underlying_type<bar>::type resolve to?

Nothing, because `bar` is ill-formed:

>= The type-specifier-seq of an enum-base shall name an integral type; any cv= -qualification is ignored

Enumeration types are not "integral t= ype"s.
=C2=A0

/MF

--
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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/b7df0078-2395-4def-bd73-fd708083969d= %40isocpp.org.
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