220 29248 article Path: news.gmane.org!.POSTED!not-for-mail From: Daniel Frey Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: explicit conversion from enum class to its underlying type Date: Sun, 30 Oct 2016 02:40:19 +0200 Lines: 74 Approved: news@gmane.org Message-ID: References: <4afed494-4fc5-4b06-b3e7-255aa293068e@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 (Mac OS X Mail 10.1 \(3251\)) Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1477788054 20766 195.159.176.226 (30 Oct 2016 00:40:54 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 30 Oct 2016 00:40:54 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDYYLHN44AERB5MC2XAAKGQEEPV6JSY@isocpp.org Sun Oct 30 02:40:49 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wm0-f72.google.com ([74.125.82.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1c0eAd-0001IV-37 for gclcip-std-proposals@m.gmane.org; Sun, 30 Oct 2016 02:40:19 +0200 Original-Received: by mail-wm0-f72.google.com with SMTP id i128sf49734150wme.2 for ; Sat, 29 Oct 2016 17:40:22 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=from:content-transfer-encoding:mime-version:subject:date:references :to:in-reply-to:message-id: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=34kNE1vy9lGFL3NbmTq6enuTGTuAO4Yq5StsUGTIq7w=; b=Os9soYA+HYZ8z707LDh+xxxND+6jr7+vXhMawVfit/bM84ZRkehkXKi4K+3RdoKTdq pfLGaLJkBFpHDJo3oQhg8b8JDr5qGZzC8QcSS577/2/0F11OVbuoJfj1MhHBbRHlHUb9 qwLhSYoR7VO+ud6ZspjCrV8UVGQWkw8oP7zQdM2XIypC9fzNtdPBxmqeCXEDXcbV35Vl K3ZUc9/9QE27D9anZ9I3qQ6720wFPJZFcd+bY6ZZN0OmDfdEjSSrT2knzG9TlqdBYSEg 2/qTZ1QPSa740jhydpIWn8cMm9CQMdQgAPL+0iLfnVCE58y6In3Tx8Aks/spETksh/4y 5X6A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:content-transfer-encoding:mime-version :subject:date:references:to:in-reply-to:message-id: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=34kNE1vy9lGFL3NbmTq6enuTGTuAO4Yq5StsUGTIq7w=; b=RXIPZyZVGfogYiQZ7jPaRexfabnaxCaqOM9auCbP60tXvf2Y9QcT1keVEJqe23R/Gz crUY8SNRiviNTDvcZL8obR01j75thP26xE3b18EsSwvCdx2+zBvic2uRqjf/+HK/1JQi jZrd0sxoKULyokPfI02OE2q236Z99ctjNHa7xRnfOT90IrTOVFYgtY54oHQzrFxwuqaP ND/PvvEtOLqaUPZTid+UACEkXy9YRh+GCj48yy+okg4KJm//Lz/WZtEITKZrfvdK05n2 VC9HM5Qqmu3CQ+Iizktte/9hvUoVsYbPQyrsYQTKaJOCobX+UkLlSUT3KMFe/rCuugC/ qSQ X-Gm-Message-State: ABUngvcZHU2GTdITm0BQfYAUOg25v2+eLgy1HJzn5APlgxHTD3swZ6YU2tIE8utK9wBWmg== X-Received: by 10.28.198.10 with SMTP id w10mr368796wmf.7.1477788022004; Sat, 29 Oct 2016 17:40:22 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.28.48.195 with SMTP id w186ls984194wmw.24.gmail; Sat, 29 Oct 2016 17:40:21 -0700 (PDT) X-Received: by 10.194.116.225 with SMTP id jz1mr17167636wjb.224.1477788021036; Sat, 29 Oct 2016 17:40:21 -0700 (PDT) Original-Received: from mout.gmx.net (mout.gmx.net. [212.227.17.21]) by mx.google.com with ESMTPS id h4si22615233wjl.68.2016.10.29.17.40.20 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 29 Oct 2016 17:40:21 -0700 (PDT) Received-SPF: pass (google.com: domain of d.frey@gmx.de designates 212.227.17.21 as permitted sender) client-ip=212.227.17.21; Original-Received: from fiasko.fritz.box ([84.175.87.245]) by mail.gmx.com (mrgmx103) with ESMTPSA (Nemesis) id 0LjJCt-1cbN6J2MU3-00dXAs for ; Sun, 30 Oct 2016 02:40:20 +0200 In-Reply-To: <4afed494-4fc5-4b06-b3e7-255aa293068e@isocpp.org> X-Mailer: Apple Mail (2.3251) X-Provags-ID: V03:K0:dsnn3zirrIKC+O+eN7jQ2SdXprZHfvxx9LZQhrXPiZXCkZSTdcr R67HsMwz18nh2oHqB3STOwZcBE2vD7E5yWG37VkoSyPFZB1/4XftUaokhmtqH6BxWgYET+3 lhA1Q3Gr8STUrN3zpSq3qe7MVJ/bqEaYlK3f5kNH/gAGfD535sEgoAY1QCRHuZc6FZlnRTt H/wlVyokHiWJ1pRE/pFvQ== X-UI-Out-Filterresults: notjunk:1;V01:K0:fqM7+vpQkmU=:5W7XxxbgFqp/H9MtOCcxkK pfHWh6wm8pGJCsBFMu34Qc1bbIFtZQOvBdaPE1VcoaQp/SLG+nnwXO4fw6hn9Wok0fG6x1d44 Q0BoqyiNNEvCn3ukNrjkVBlMfYkBTaIrldZhlrcRrDAILTWD12Ls/P1Oacbq4vWdGIb1jPjIT 3mRt3780QWDgd5tzguTR0lBMdNLNEyq4OwPASvoHHXakfcTdtAkdmHccKqCgIQy/p9Qfpj62k RzqgdcBGhCqt4Xlt7YJkxrfzFdPHDX9mGj9voH6i7oi1JJYthdHRzfVa0Cs/Q+y6aWjwtX1Rz UuskmFH8nn2TI5abyrOQ8fslvAXlFZWxdPFDT0uaGVXr703aBds3G21K7P1L3tDEw4+QYScza PEhHOcjtBJ7a40szWOyhf7+BetTTpCkgcMJ6YbDBvdx7DXGMwh07MkD9UJQ8ouAdMHgdQtD1A bCe527mw043aUNaht4aGAdi/UuhuSuu1Ha0CDon0I7bEY5tzGCHl4KfT8gW8l3Vn9HgBxOjf2 MUtYlKE9yu3tw5RN4Ck6mzTRcDbGKYDTfYPWIOUJ8GkKDdCB+Cd2K9FyP1JTTAUXrr5uBcdhh JlgfkLzBkii1ZlOjZwrkwpSxv11H5vhejohSGUh40+6sIu77QkpYu+yY/Wr1auLyoMs8dytjF 3kOnzDMp53yc6Pi6kz9KsmtXc2j9VG91tj7CsxLlabcDjCgENDOzjAUb6nBxStce/1lVuRLfU BiBtiFCaoZ30OEmcCVYUquWFnaGAhmnQQdIuB8Vt7UUbdtScVC+1yXD/ueNt1dacH2XKBNnE X-Original-Sender: d.frey@gmx.de X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of d.frey@gmx.de designates 212.227.17.21 as permitted sender) smtp.mailfrom=d.frey@gmx.de 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:29248 Archived-At: > On 29 Oct 2016, at 23:51, Andrzej Krzemie=C5=84ski w= rote: >=20 > I do not know of any practical use of explicit conversion operators excep= t for the contextual conversion to bool. And it is only this conversion tha= t I am interested in. If all you need is boolean conversion, how about template constexpr std::enable_if_t,bool> is(E e) noexcept { return e =3D=3D E(true); } to be used like if(is(skipUnused)) { ... } Instead of is(), you could also use operators, e.g. +skipUnused or !skipUnused (the latter might actually be sensible, but inconsistent if skipUnused itse= lf is not convertible to bool) There is a caveat with using boolean enums, i.e., if the user writes: enum class SkipUnused { Yes, No }; (Note the order of the values). There is no way for us to detect this, unless we explicitly would expect a = value called Yes (or should it be YES or TRUE?) and change the above to template constexpr std::enable_if_t,bool> is(E e) noexcept { return e =3D=3D E::Yes; } > Do you think, it would be acceptable to add such explicit conversion to t= he underlying type, for enum classes? If anything, it should be explicit for a type, not implicit for all enums. = Maybe allowing something like: enum class SkipUnused { No, Yes }; explicit constexpr SkipUnused::operator bool() =3D default; or explicit constexpr SkipUnused::operator bool() noexcept { return *this = =3D=3D Yes; } where *this refers to the enum value and Yes is in the scope of SkipUnused,= hence no SkipUnused::Yes is needed. Something like a "member" of an enum c= lass. This might also allow other conversions (explicitly), making a possib= le proposal usable in more contexts. I'm not really sure it is worth it, th= ough. --=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/D44CA73E-022C-4803-8F52-F644679B996E%40gmx.de. .