220 28008 <1f2b9290-581d-1fec-a98d-5efe4f5f8fc9@scylladb.com> article Path: news.gmane.org!.POSTED!not-for-mail From: Avi Kivity Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Make (strongly typed) enums castable to char const * const Date: Sun, 28 Aug 2016 19:35:27 +0300 Organization: ScyllaDB Lines: 246 Approved: news@gmane.org Message-ID: <1f2b9290-581d-1fec-a98d-5efe4f5f8fc9@scylladb.com> References: <1472132849.17129.9.camel@bsdforen.de> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="------------86C3882CFC8E2D41499A0DDB" X-Trace: blaine.gmane.org 1472402139 31134 195.159.176.226 (28 Aug 2016 16:35:39 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 28 Aug 2016 16:35:39 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDHJFOWD7YCRBUVFRS7AKGQEE3OBYZQ@isocpp.org Sun Aug 28 18:35:34 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f70.google.com ([209.85.215.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1be33S-0007EB-Ly for gclcip-std-proposals@m.gmane.org; Sun, 28 Aug 2016 18:35:30 +0200 Original-Received: by mail-lf0-f70.google.com with SMTP id p85sf83339584lfg.3 for ; Sun, 28 Aug 2016 09:35:32 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=subject:to:references:from:organization:message-id:date:user-agent :mime-version:in-reply-to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=MegUZaQqP+Z+mNgI0xq36cxm7P5v8q8y7DI1q/wy1Ck=; b=tvrMeHTFsur2M9Yt1XGObaIpVAevXuvz6RwTcm1++flp3J0UnhflSs/aT+udC3Lihw FGQ7aN7YVJJ57Yj6mmBt6OHH8IrFSJqswEZLYttW+itE2t9YRoqgG95Y5kUUyVNv05eu LdVfww/kk9W9/ZY5+7/Zp+oAYGfRk7vQeGaW38E+Ez+Sp6PeQUN2kb01Mj4n1y1UsqSA B6sLr3EZzp99DHI1IgWJRmovjj5vO9sIJ+1CHAJF1rBAWAXC9yxb/p4cbpo6vzomrad8 GP+qaRcLrQQqs1eDwb9QFYcSMgHovbaUjdLhO8HEQSj5+hwo5sxkEl3SQ04KbHdpWau3 RwUg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:subject:to:references:from:organization :message-id:date:user-agent:mime-version:in-reply-to :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=MegUZaQqP+Z+mNgI0xq36cxm7P5v8q8y7DI1q/wy1Ck=; b=RJIw9QdZbfWrNFeIuMZ+k886ckcUb2UxUFZS44XFcH7YOsLxlM3CnUw/cTzfthrASI fkvBlS4e3gFnA6d6Qq6P/nN+6WTf1gUE/CgWj9Mbcs1mY9w06wO8En+f20Q09UjzszXq Xw7Sawf9ub7hfyGqyAd5y6vNKd/7lGihPUewyNb8aNsdvU1z9HeVNwIcEZx3+z78Y7XY VOy7yGlGtM5eQNgA+H5VTEdb+KYA/ae7EdJ4MnE04ZBGfypz8AHgt8MqapqGpSxAZIX7 /3nuP+4aEOPhoCIyyDe7YqS4kzQTx2zjB/g5ipqb9tPL/TLQeYp+jn5E+Zh0N8nKe1x6 LXsw= X-Gm-Message-State: AE9vXwNVkDTZ6MD1Mjf0r23/x/oD30y5p9LbLzT87muILPRCMVBq92f9+EyIhh12Up9t7w== X-Received: by 10.46.69.10 with SMTP id s10mr1732209lja.1.1472402131878; Sun, 28 Aug 2016 09:35:31 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.28.207.197 with SMTP id f188ls591812wmg.5.canary; Sun, 28 Aug 2016 09:35:30 -0700 (PDT) X-Received: by 10.194.54.137 with SMTP id j9mr13773086wjp.60.1472402130295; Sun, 28 Aug 2016 09:35:30 -0700 (PDT) Original-Received: from mail-wm0-x233.google.com (mail-wm0-x233.google.com. [2a00:1450:400c:c09::233]) by mx.google.com with ESMTPS id t73si8336900wme.10.2016.08.28.09.35.30 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 28 Aug 2016 09:35:30 -0700 (PDT) Received-SPF: pass (google.com: domain of avi@cloudius-systems.com designates 2a00:1450:400c:c09::233 as permitted sender) client-ip=2a00:1450:400c:c09::233; Original-Received: by mail-wm0-x233.google.com with SMTP id f65so50320647wmi.0 for ; Sun, 28 Aug 2016 09:35:30 -0700 (PDT) X-Received: by 10.194.88.41 with SMTP id bd9mr9527568wjb.75.1472402129865; Sun, 28 Aug 2016 09:35:29 -0700 (PDT) Original-Received: from avi.cloudius-systems.com ([37.142.229.250]) by smtp.gmail.com with ESMTPSA id x6sm30113736wjk.26.2016.08.28.09.35.28 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 28 Aug 2016 09:35:29 -0700 (PDT) In-Reply-To: X-Original-Sender: avi@scylladb.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@scylladb-com.20150623.gappssmtp.com; spf=pass (google.com: domain of avi@cloudius-systems.com designates 2a00:1450:400c:c09::233 as permitted sender) smtp.mailfrom=avi@cloudius-systems.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:28008 Archived-At: This is a multi-part message in MIME format. --------------86C3882CFC8E2D41499A0DDB Content-Type: text/plain; charset=UTF-8; format=flowed On 08/25/2016 06:52 PM, 'Jeffrey Yasskin' via ISO C++ Standard - Future Proposals wrote: > On Thu, Aug 25, 2016 at 8:41 AM, Larry Evans > > wrote: > > On 08/25/2016 09:12 AM, Ville Voutilainen wrote: > > On 25 August 2016 at 16:47, Dominic Fandrey > > wrote: > > I searched this list for similar suggestions, but of > course I may > have overlooked something. > > I frequently use the following pattern: > > enum class FooBar { KEKS, DOSE }; > char const * const FooBarStr[]{"KEKS", "DOSE"}; > > I use the strings to in error messages, verbose output, etc. > > FooBarStr[static_cast(FooBar::KEKS)] // "KEKS" > > This has some disadvantages, e.g. enums with explicitly > stated values > may have gaps, negative values and may be defined out of > order. > Also, changing the enum also means changing the strings > manually. > > What I'd like to see: > > static_cast(FooBar::KEKS) // "KEKS" > > > > See > http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0194r1.html > > > Could you be more specific. I grepped for enumName, for char const*, > and scanned briefly for other strings that might show where > conversion from enum to char const* might be, without success :( > > > In that proposal, the operation you want is spelled > get_name_v. > > Likely, he wants to index the array with a non-constexpr value. So he'd need code to create a static array (or unordered_map, if the enum is sparse) for a dynamic lookup. Something like const char* as_string(FooBar v) { if constexpr (is_sparse_enum_v) { return as_string_sparse(v); } else { return as_string_dense(b); } } template const char* as_string_dense(Enum v) static const char* tab[] = { magic code to populate the table with reflexpr(Enum)... }; return tab[static_cast>(v)]; } template const char* as_string_sparse(Enum v) { static std::unordered_map, const char*> = { more magic code to populate the table with reflexpr(Enum)... }; return tab[static_cast>(v)]; } this is complex enough to merit a library, and common enough to be standardized. -- 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/1f2b9290-581d-1fec-a98d-5efe4f5f8fc9%40scylladb.com. --------------86C3882CFC8E2D41499A0DDB Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 08/25/2016 06:52 PM, 'Jeffrey Yasskin' via ISO C++ Standard - Future Proposals wrote:
On Thu, Aug 25, 2016 at 8:41 AM, Larry Evans <cppljevans@suddenlink.net> wrote:
On 08/25/2016 09:12 AM, Ville Voutilainen wrote:
On 25 August 2016 at 16:47, Dominic Fandrey <kamikaze@bsdforen.de> wrote:
I searched this list for similar suggestions, but of course I may
have overlooked something.

I frequently use the following pattern:

=C2=A0 =C2=A0 enum class FooBar { KEKS, DOSE };
=C2=A0 =C2=A0 char const * const FooBarStr[]{"KEKS", "D= OSE"};

I use the strings to in error messages, verbose output, etc.

=C2=A0 =C2=A0 FooBarStr[static_cast<int>(Foo= Bar::KEKS)] // "KEKS"

This has some disadvantages, e.g. enums with explicitly stated values
may have gaps, negative values and may be defined out of order.
Also, changing the enum also means changing the strings manually.

What I'd like to see:

=C2=A0 =C2=A0 static_cast<char const *>(FooBar::K= EKS) // "KEKS"


See h= ttp://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0194r1.html

 Could you be more specific.=C2=A0 I grepped for enumName, for char const*,
and scanned briefly for other strings that might show where
conversion from enum to char const* might be, without success :(

In that proposal, the operation you want is spelled get_name_v<reflexpr(FooBar::KEKS)>.



Likely, he wants to index the array with a non-constexpr value.=C2=A0 S= o he'd need code to create a static array (or unordered_map, if the enum is sparse) for a dynamic lookup.

Something like

const char* as_string(FooBar v) {
=C2=A0=C2=A0=C2=A0 if constexpr (is_sparse_enum_v<FooBar>) {
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return as_string_sparse(v);<= br> =C2=A0=C2=A0=C2=A0 } else {
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return as_string_dense(b); =C2=A0=C2=A0=C2=A0 }
}

template <typename Enum>
const char* as_string_dense(Enum v)
=C2=A0=C2=A0=C2=A0 static const char* tab[] =3D { magic code to populat= e the table with reflexpr(Enum)... };
=C2=A0=C2=A0=C2=A0 return tab[static_cast<std::underlying_type_t<Enum>>(v)];
}

template <typename Enum>
const char* as_string_sparse(Enum v) {
=C2=A0=C2=A0=C2=A0 static std::unordered_map<std::underlying_type_t&= lt;Enum>, const char*> =3D { more magic code to populate the table with reflexpr(Enum)... };
=C2=A0=C2=A0=C2=A0 return tab[static_cast<std::underlying_type_t<Enum>>(v)];
}

this is complex enough to merit a library, and common enough to be standardized.

--
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