220 30464 article Path: news.gmane.org!.POSTED!not-for-mail From: Peter Koch Larsen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: enum_cast proposal Date: Tue, 10 Jan 2017 18:01:45 +0100 Lines: 92 Approved: news@gmane.org Message-ID: References: <57258435-5d82-48e7-8ed7-3c38682418fb@isocpp.org> <58750288.5080004@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-Trace: blaine.gmane.org 1484067716 24104 195.159.176.226 (10 Jan 2017 17:01:56 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 10 Jan 2017 17:01:56 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDRIBXXKZQERB65G2TBQKGQEFWD4TKY@isocpp.org Tue Jan 10 18:01:52 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb0-f199.google.com ([209.85.213.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cQznr-000535-I9 for gclcip-std-proposals@m.gmane.org; Tue, 10 Jan 2017 18:01:43 +0100 Original-Received: by mail-yb0-f199.google.com with SMTP id l23sf406579226ybj.6 for ; Tue, 10 Jan 2017 09:01:48 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=mime-version:in-reply-to:references:from:date:message-id:subject: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=ue/x5jwy+JRnQIfFz0G9kAaTxgPPMw0YlKHaIQZIlf0=; b=1ck2G7ZVbpvgQLofs+AZZIsUYKlKjs7EG5fVF913z4PWW72Fxd1wMGePhEMK0ue3oC j17Uv1VANR2z7TztqwDts3Cz1ZnnUPvnJLyJLfTLeRDsppCxi2e8rzVXadffAENA5nth feZhK/774X7OVnps3jKvVbyCEaW0pfpm06tTIufZrk95HM7b8yrYRjRrtd5fnqDaIjN1 6jhL4yGgjlzWCBqomPE919AcuUI5ZBXMDYTfBXFvLK5XGRDe03Rwuk9Vy+hi1JlESJnt ODGGLNh+EcbINOaQT+FQwsy9EjJiwpmlxZVi+hkAv5QwVpEMDUoLX71s46bqy5jNp3nn fKrQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject: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=ue/x5jwy+JRnQIfFz0G9kAaTxgPPMw0YlKHaIQZIlf0=; b=WRMW4LEAvAGTDf2h7d0Li+ChQlCepCCyiWNqc04I9jSm3We/7NJTgOTdyl4awFFYED Bm463Ok6ttGThSpN1oggwR97PTnFX6Z+IA1oITSNZCMn0Ihx+nZIETjloKZlF5hIorON OLxb/yPoNj1LQnBvhXIYQdF8Ta35oe5qy0XEVJvPy+ia8Od5/5+5XyT2c/To+Fo9NN/Q JOR8MzONXaNQ9d8iWkD7VHeEH/phSKdAB7jhzuka3wUAgsSgZyctSizP7YfQ3XJTpDbi lONV7jvZpeRYcGXUPp/bJJ9ao8INQR3UdieizQVQtD1fS/bD1YptxY9QsBHAEWUT+Yoz Ygww== X-Gm-Message-State: AIkVDXI26CP6sgc3GtScCRuQjFn6LDZeXZXZyz2ASuW8xaMKmIUpdD7oTlIh69Qigz47fg== X-Received: by 10.129.175.36 with SMTP id n36mr245836ywh.30.1484067707867; Tue, 10 Jan 2017 09:01:47 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.17.154 with SMTP id v26ls1998650otf.0.gmail; Tue, 10 Jan 2017 09:01:47 -0800 (PST) X-Received: by 10.31.232.198 with SMTP id f189mr1861325vkh.85.1484067706997; Tue, 10 Jan 2017 09:01:46 -0800 (PST) Original-Received: from mail-vk0-x22d.google.com (mail-vk0-x22d.google.com. [2607:f8b0:400c:c05::22d]) by mx.google.com with ESMTPS id 91si655764uan.100.2017.01.10.09.01.46 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 10 Jan 2017 09:01:46 -0800 (PST) Received-SPF: pass (google.com: domain of peter.koch.larsen@gmail.com designates 2607:f8b0:400c:c05::22d as permitted sender) client-ip=2607:f8b0:400c:c05::22d; Original-Received: by mail-vk0-x22d.google.com with SMTP id t8so30707211vke.3 for ; Tue, 10 Jan 2017 09:01:46 -0800 (PST) X-Received: by 10.31.73.66 with SMTP id w63mr1928011vka.106.1484067706410; Tue, 10 Jan 2017 09:01:46 -0800 (PST) Original-Received: by 10.103.11.5 with HTTP; Tue, 10 Jan 2017 09:01:45 -0800 (PST) In-Reply-To: X-Original-Sender: peter.koch.larsen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of peter.koch.larsen@gmail.com designates 2607:f8b0:400c:c05::22d as permitted sender) smtp.mailfrom=peter.koch.larsen@gmail.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=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:30464 Archived-At: Nicols proposal is very similar to mine, except that I do not have an enum_cast_opt. And I do not really see a reason to have this option. I believe that noone would use that option. Why write std::optional e = enum_opt_cast(x); if (e) { normal processing } else { handle conversion failure } instead of if (is_enum(e)) { Enum e = to_enum(x); normal processing } else { handle conversion failure } ? Things to consider when implementing: What do you do if you have an enum with some same_valued elements (enum E { ea = 0, eb = 0})? My implementation forbids this. It could also be useful to have an ordinal position: enum E { ea = 1000, eb = -1000, ec = 32 }; static_assert(to_ordinal(ea) == 0,"??"); static_assert(to_ordinal(eb) == 1,"??"); static_assert(to_ordinal(ec) == 2,"??"); I have found uses for this in my code. /Peter On Tue, Jan 10, 2017 at 5:20 PM, Nicol Bolas wrote: > On Tuesday, January 10, 2017 at 10:49:34 AM UTC-5, Matthew Woehlke wrote: >> >> On 2017-01-10 10:31, m.ce...@gmail.com wrote: >> > For sure baseline API must be exception-free. >> > Do we really want to additionally support API that throws on error? >> >> I *really* hope we can use something like std::expected; this is a >> textbook use case for that, and makes it trivial to have a >> throw-on-error API (just grab the value from the expected without >> checking it first; this will turn around and throw if the value is not >> there). > > > But the fact that it's throwing the wrong exception makes it useless from an > exception handling perspective. If you want an API to throw, you want it to > throw a meaningful exception type, not the same kind of exception that 20 > other APIs throw. > > So yes, we really do need to support a throwing API. I say we should have 3 > interfaces: > > * `can_enum_cast`: specifies whether the value is in an enumeration. This is > where the compiler magic comes in, of course. > * `enum_cast`: Converts to the enumeration type, or throws if it cannot. > * `enum_cast_opt`: Returns an `optional`. > > The first allows you to build whatever tools you want. But we have default > tools for C++'s standard mechanisms: throwing and `optional`. > > -- > 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/e85f0d46-f244-42fe-a276-b4e18b3d627f%40isocpp.org. -- 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/CANPtknwzegDLrDFrMoc5U9Z1y52PBZAC2BtQ%3DiS6Eur2A6f7NQ%40mail.gmail.com. .