220 35420 article Path: news.gmane.org!.POSTED!not-for-mail From: Bo Persson Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allow out-of-class operator overloading for enum classes Date: Mon, 20 Nov 2017 22:27:13 +0100 Lines: 106 Approved: news@gmane.org Message-ID: References: <608409c3-7708-4024-8ca5-8ecd566ca4a9@isocpp.org> <990fd030-20de-4bec-b11f-b43f9338667c@isocpp.org> <40465af3-4752-44b9-9367-cf2d1c41a335@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1511213246 29635 195.159.176.226 (20 Nov 2017 21:27:26 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 20 Nov 2017 21:27:26 +0000 (UTC) User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCZ6RLW3UIKBBP4RZXIAKGQEWKRWCGA@isocpp.org Mon Nov 20 22:27:22 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f71.google.com ([209.85.215.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eGtb7-0007Sk-Hg for gclcip-std-proposals@m.gmane.org; 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Original-Received: from list by blaine.gmane.org with local (Exim 4.84_2) (envelope-from ) id 1eGtax-0006u3-IE for std-proposals@isocpp.org; Mon, 20 Nov 2017 22:27:11 +0100 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 85 Original-X-Complaints-To: usenet@blaine.gmane.org In-Reply-To: <40465af3-4752-44b9-9367-cf2d1c41a335@isocpp.org> Content-Language: sv X-Original-Sender: gclcip-std-proposals@m.gmane.org X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 195.159.176.226 is neither permitted nor denied by best guess record for domain of gclcip-std-proposals@m.gmane.org) smtp.mailfrom=gclcip-std-proposals@m.gmane.org 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:35420 Archived-At: On 2017-11-20 19:30, Nicol Bolas wrote: > On Monday, November 20, 2017 at 1:19:55 PM UTC-5, Bo Persson wrote: >=20 > On 2017-11-20 18:36, Aar=C3=B3n Bueno Villares wrote: > > > > > > On Monday, 20 November 2017 18:18:36 UTC+1, Nicol Bolas wrote: > > > > > > > > =C2=A0 =C2=A0 On Monday, November 20, 2017 at 11:59:03 AM UTC-5, A= ar=C3=B3n Bueno > > =C2=A0 =C2=A0 Villares wrote: > > > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 Enum classes doesn't allow, by default= , implicit > conversion, not > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 even to the underlying type: > > > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 | > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 enumclassA :int{zero,one }; > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 voidfoo(int){} > > > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 intmain(){foo(A::zero);} > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 | > > > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 and that is a good thing, but sometime= s, you need a enum > class > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 only to avoid name colissions, for exa= mple: > > > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 | > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 enumclasstable1_cols {id,name }; > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 enumclasstable2_cols {id,address }; > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 | > > > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 If table1_cols and table2_cols were ra= w enums, there > would be a > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 colission between both id named values= , since they have > global > > =C2=A0 =C2=A0 =C2=A0 =C2=A0 namespace scope, > > > > > > =C2=A0 =C2=A0 If they were unscoped enums, you could /still/ refer= to them as > > =C2=A0 =C2=A0 `table1_cols::id` and `table2_cols::id`. So just do = that. > > > > > > It gives you (g++ at least) a compiler error because `id` has been > > redeclared. > > >=20 > So disable that warning, or put the unscoped enum inside its own scop= e > (namespace or struct). >=20 > namespace table1_cols { enum {id,name}; }; >=20 >=20 > But then, it's difficult to use them as a typed quantity. That is,=20 > making a variable of type `table1_cols` isn't possible, since it's a=20 > namespace. You'd have to use `decltype` gymnastics. This was to solve one problem - implicit conversion to ints. If you want a type name for this, you can name the enum and use that as=20 a type namespace table1 { enum cols {id,name}; }; using table1_cols =3D table1::cols; >=20 > Personally, I try to design things so that either we're talking about an= =20 > integer or we aren't. The OP's problem only arises when you have an=20 > enumeration that you sometimes use as an enum and sometimes use as an=20 > integer. I don't think we should make language facilities to facilitate= =20 > that corner case; we should make it as painful as possible to=20 > /discourage/ design which leads to that. > Yes, it is a problem when you use enum class to try to make the type not=20 implicitly convertible to an integer, except sometimes. It is hard to=20 write the code for "implicitly convertible only when I want it to be". Bo Persson --=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/ouvhb9%24odv%241%40blaine.gmane.org. .