220 33441 <6a2c32ed-99b5-4cc5-86a5-ee3cc07567a3@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: james.a.cooper@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Proposal: underlying_cast Date: Sun, 30 Jul 2017 04:50:41 -0700 (PDT) Lines: 487 Approved: news@gmane.org Message-ID: <6a2c32ed-99b5-4cc5-86a5-ee3cc07567a3@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_3238_804273539.1501415441388" X-Trace: blaine.gmane.org 1501415443 16830 195.159.176.226 (30 Jul 2017 11:50:43 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 30 Jul 2017 11:50:43 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCR6HLXQUAORBEUQ67FQKGQEFV4JC4I@isocpp.org Sun Jul 30 13:50:39 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f200.google.com ([209.85.220.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dbmk2-00045j-5r for gclcip-std-proposals@m.gmane.org; Sun, 30 Jul 2017 13:50:38 +0200 Original-Received: by mail-qk0-f200.google.com with SMTP id d136sf82619491qkg.11 for ; Sun, 30 Jul 2017 04:50:44 -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=68Ok3E/RXdo1ATumOJbmjT4589jnW1IIZ05zWMy5evA=; b=dAd2rmuqigMTy5mrr9ZqDiXUHB5o0SsP0fPrGtmKGyeA9fAjLemmpmLtoIgKFFSswV 3XoWApov+13jI7GGRfFvlKcl4326cN1yiiIM4x5L+JwxGKyUybYGZTNC7xVaXLGozcXy 2A4Y5ogeV8MNwd3In5TokBDh01B2xUgiBjZssl1XO4F66+Yflrz2QLhonUX4dkkcYHl2 zBaR6zPawCjYBy6/F94ppbQl6BbiDb1eXqPIudzct1dCJj41fkOGps15XeGFwzUo4kTy HN45nvyGFFNw6OQXvV6ATHWTO8AN1o5t6RkbIqPR0lq6YKmbqRwe43V1/vLObvYZ73sF 60ng== 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=68Ok3E/RXdo1ATumOJbmjT4589jnW1IIZ05zWMy5evA=; b=omBIDcx+ZZfnY8dfujkV599gg9Q3xMqZQzEFAvfoyfs61/gPp2gwM2S3C/2TistKrl O/Bp/kgLq1OA+GOWtbAwL3zChNsJYZSZvp5brJzwIIw/owmuXy8QKlcgG6iuPNtqpRbJ pXsEzwzT71sf77nNOtw+r45fhK8VllEzFeV/pessOVnFG6e+vHj4Q3DOYX0oTaUsePoy QhjvDhwaLpVaCKe9t/dzo4MMR+8ihUjXbFvX2k/2h1LthufAyptj2+1oJKkTMegiDK95 GwyxogKRMHP0Gqa53/QtDQsWyKpxleZNryKSX1eZGfUkoBTHUobRF0rEKbpzR8QusL4M cvBg== 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=68Ok3E/RXdo1ATumOJbmjT4589jnW1IIZ05zWMy5evA=; b=qU4zfqYeMpIv71z//8EcJicMvWNjBLMwmxDHMhMaThHFZqPEWcAK3gwSL1FQcvBgjV iof6XXUukertNHdGOLrs8/vmrjHJJ9SuK6ZeVbX18Rxgdo/M/LLK7K3LuxEvwfK4rwu/ i7ZtXIqlUKRhI7th6l9TQxBXZFruJ31yQmGPYwuRNb0c9yFp/j00mEo7u2bSTs65eZyc 1RDbnpd1uqxyIfF8Pad7OIvzQ8NV1XBKLxHitnjH1/6uzK1GflPOizQd6+OksFzXBPwc i3NOrQYcoZd/a+sBJOzjc1pL9AF3PSfS4yvG78ix0iKUFNtqjql0c0JP33suQlpkv/Fo e2yw== X-Gm-Message-State: AIVw113GGm70jpNwEbGj5NUxKCaS0WgzaczzHo9CkRH003M4cAIV9Ewy JPAuGTNy/hGyzoXh X-Received: by 10.237.62.245 with SMTP id o50mr9260034qtf.35.1501415443502; Sun, 30 Jul 2017 04:50:43 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.138.134 with SMTP id v128ls6692028itd.15.gmail; Sun, 30 Jul 2017 04:50:42 -0700 (PDT) X-Received: by 10.31.49.205 with SMTP id x196mr68389vkx.2.1501415441894; Sun, 30 Jul 2017 04:50:41 -0700 (PDT) X-Original-Sender: james.a.cooper@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:33441 Archived-At: ------=_Part_3238_804273539.1501415441388 Content-Type: multipart/alternative; boundary="----=_Part_3239_836813329.1501415441390" ------=_Part_3239_836813329.1501415441390 Content-Type: text/plain; charset="UTF-8" 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; template using EnumTypesOnly = typename std::enable_if::value, E>:: type; template using NotEnumTypesOnly = typename std::enable_if::value, T >::type; // For enums do the cast. template > constexpr UnderlyingType underlying_cast(E e) { return static_cast>(e); } // For other types do nothing. template > constexpr T underlying_cast(T t) { return t; } The reason this is required is that even when the enum type is explicitly set, it can be ignored during function overloading. For example: enum AnEnum : bool { A, B }; //void print(AnEnum) { std::cout << "print(AnEnum)\n"; } void print(int) { std::cout << "print(int)\n"; } void print(bool) { std::cout << "print(bool)\n"; } int main(int argc, char* argv[]) { print(A); // Compiler chooses print(int) overload. return 0; } If, in the above code, the print(AnEnum) overload is removed, the compiler will choose the print(int) function despite the compiler being told that the enum is a bool (additionally, if a 'C' value is added to the enum the compiler will complain). If this value is being serialised to JSON, for example, this is not the intended outcome. The following code behaves as the programmer intended enum AnEnum : bool { A, B }; void print(int) { std::cout << "print(int)\n"; } void print(bool) { std::cout << "print(bool)\n"; } int main(int argc, char* argv[]) { print(underlying_cast(A)); // Compiler chooses print(bool) overload. return 0; } -- 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/6a2c32ed-99b5-4cc5-86a5-ee3cc07567a3%40isocpp.org. ------=_Part_3239_836813329.1501415441390 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
C++ allows enums to have an underlying type specified. Whe= n serialising enum values it's important to use the correct type.
Th= is proposal is to add a utility function that casts an enum type to its und= erlying value. This would make it easier to handle
enum types in functio= n overloads for serialisation. For example:

template <typename T>
std::ostream&a= mp; write<= /span>(std::ostream& os, const T&= ; v= )
{
=C2=A0 =C2=A0 os << std::underlying_cast(v);
=C2=A0 =C2=A0 return os;}


This function would, in the case of enums cast the value to the underlying=20 type, and in the case of any other type it would do nothing.
One possibl= e implementation:

template <typename E>
using UnderlyingType =3D typename std::underlying_type<E>= ::type;

template <typename E>
using En= umTypesOnly =3D typename std::enable_if<std::is_enum<E>:= :value, E>::type;

template <typename T>=
using NotEnumTypesOnly =3D typename std::enable_if<!std:= :is_enum<<= span class=3D"styled-by-prettify" style=3D"color: #000;">T>::value, T>::type;


// For enums do the cast.
template <typ= ename E, typename =3D EnumTypesOnly<E>>
co= nstexpr UnderlyingTy= pe<E> underlying_cast(E e)
{
=C2=A0 =C2=A0return static_cast<UnderlyingType<>>= ;(e= );
}

// For other types do nothing.
template <typename T, typename=3D NotEnumTypesOnly= <T>>
constexpr T underlying_cast(T t)
{
=C2=A0 =C2=A0return t;<= /span>
= }



The reason this is required is that even when the enum type= is explicitly set, it can be ignored during function overloading.
For e= xample:

enum AnEnum : bool { A, B = };<= span class=3D"styled-by-prettify" style=3D"color: #000;">

//void print(AnEnum)= { std::cout << "print(AnEnum)\n"; }
void print(int { std::cout << "print(int)\n"; }
void print= (bool) { std::cout &= lt;< "print(= bool)\n"; }

int main(int argc, = char* argv[])
{
=C2=A0 =C2=A0 print(A); // Compiler chooses print(int) overload.
=C2=A0 =C2=A0 return 0;
}

If, in the above code, the print(AnEnum) overload is removed, the compiler=20 will choose the print(int) function despite the compiler being told thatthe enum is a bool (additionally, if a 'C' value is added to the e= num the compiler will complain).
If this value is being serialised to JSON, for example, this is not the=20 intended outcome. The following code behaves as the programmer intended
=
enum = AnEnum : bool { A B };

void print(int) { std::cout << "print(int)\n"; }
void <= /span>print(bool) { std::cout &= lt;< "print(= bool)\n"; }

int main(int argc, = char* argv[])
{
=C2=A0 =C2=A0 print(underlying_cast(A)); // Compiler chooses print(bool) overload.
= =C2=A0 =C2=A0 return= 0<= span class=3D"styled-by-prettify" style=3D"color: #660;">;
}

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
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/6a2c32ed-99b5-4cc5-86a5-ee3cc07567a3= %40isocpp.org.
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