220 29246 <8da39f5b-a120-4be6-b9fd-35b9e9a28889@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Tim Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: explicit conversion from enum class to its underlying type Date: Sat, 29 Oct 2016 16:55:09 -0700 (PDT) Lines: 153 Approved: news@gmane.org Message-ID: <8da39f5b-a120-4be6-b9fd-35b9e9a28889@isocpp.org> References: <4afed494-4fc5-4b06-b3e7-255aa293068e@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1999_1551261694.1477785309285" X-Trace: blaine.gmane.org 1477785326 22249 195.159.176.226 (29 Oct 2016 23:55:26 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 29 Oct 2016 23:55:26 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBD2NX2O34YLRBXXN2TAAKGQEXRPPPZI@isocpp.org Sun Oct 30 01:55:22 2016 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 1c0dSu-0003N3-3g for gclcip-std-proposals@m.gmane.org; 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Sat, 29 Oct 2016 16:55:10 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.36.51 with SMTP id p48ls9632856ota.22.gmail; Sat, 29 Oct 2016 16:55:09 -0700 (PDT) X-Received: by 10.157.15.143 with SMTP id d15mr2409354otd.2.1477785309841; Sat, 29 Oct 2016 16:55:09 -0700 (PDT) In-Reply-To: <4afed494-4fc5-4b06-b3e7-255aa293068e@isocpp.org> X-Original-Sender: thaines.astro@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:29246 Archived-At: ------=_Part_1999_1551261694.1477785309285 Content-Type: multipart/alternative; boundary="----=_Part_2000_1717817825.1477785309286" ------=_Part_2000_1717817825.1477785309286 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable We already have explicit conversions for scoped enums (they aren't pretty,= =20 though) #include enum class SkipUnused : bool {No, Yes}; void process(SkipUnused skipUnused) { if (static_cast::type>(skipUnused)){} } On Saturday, October 29, 2016 at 4:51:12 PM UTC-5, Andrzej Krzemie=C5=84ski= =20 wrote: > > Hi All, > > One of the nice safety features of enum classes is that they cannot be=20 > implicitly converted to their underlying type. However, do you think we= =20 > would lose this safety feature if we allowed an explicit conversion to th= e=20 > underlying type? > > I do not know of any practical use of explicit conversion operators excep= t=20 > for the contextual conversion to bool. And it is only this conversion tha= t=20 > I am interested in. > > It would be beneficial to have a unique Boolean type that would not be=20 > convertible to another unique Boolean type. One solution is to use an enu= m=20 > class with `bool` as underlying type: > > enum class SkipUnused : bool { No, Yes }; > enum class SkipNeative : bool { No, Yes }; > > void process(SkipUnused skipUnused) > { > if (skipUnused) // doesn't compile > {} > } > > Unfortunately, such enum-based type does not work with boolean=20 > expressions. It would we had an explicit conversion. > > Do you think, it would be acceptable to add such explicit conversion to= =20 > the underlying type, for enum classes? > > Regards, > &rzej; > --=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. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/8da39f5b-a120-4be6-b9fd-35b9e9a28889%40isocpp.or= g. ------=_Part_2000_1717817825.1477785309286 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
We already have explicit conversions for scoped enums (the= y aren't pretty, though)

#include <type_trai= ts>

enum class SkipUnused : bool {No, Yes};
=

void process(SkipUnused skipUnused) {
<= span class=3D"Apple-tab-span" style=3D"white-space:pre"> if (static_= cast<std::underlying_type<SkipUnused>::type>(skipUnused)){}
}


On Saturday, October 29, 2016 a= t 4:51:12 PM UTC-5, Andrzej Krzemie=C5=84ski wrote:
Hi All,

One of the nice safety= features of enum classes is that they cannot be implicitly converted to th= eir underlying type. However, do you think we would lose this safety featur= e if we allowed an explicit conversion to the underlying type?

I do = not know of any practical use of explicit conversion operators except for t= he contextual conversion to bool. And it is only this conversion that I am = interested in.

It would be beneficial to have a unique Boolean type = that would not be convertible to another unique Boolean type. One solution = is to use an enum class with `bool` as underlying type:

enum class SkipUnused= : bool= { No, Yes };
enum class SkipNeative : bool { N= o, Yes = };

void proces= s(SkipU= nused skipUnused)
{
=C2=A0 if (skipUnused) // doesn't compile
=C2=A0 = =C2=A0 {}
}

Unfo= rtunately, such enum-based type does not work with boolean expressions. It = would we had an explicit conversion.

Do you think, it would be accep= table to add such explicit conversion to the underlying type, for enum clas= ses?

Regards,
&rzej;

--
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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/8da39f5b-a120-4be6-b9fd-35b9e9a28889= %40isocpp.org.
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