220 29258 article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: explicit conversion from enum class to its underlying type Date: Sun, 30 Oct 2016 07:23:08 -0700 (PDT) Lines: 148 Approved: news@gmane.org Message-ID: References: <4afed494-4fc5-4b06-b3e7-255aa293068e@isocpp.org> <1c81b80e-33b8-4de3-9bf7-94656e8b98f5@isocpp.org> <74d02121-9633-b6d1-1187-2a9ab660089d@wanadoo.fr> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1520_927821942.1477837388206" X-Trace: blaine.gmane.org 1477837411 30744 195.159.176.226 (30 Oct 2016 14:23:31 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 30 Oct 2016 14:23:31 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBTEE3DAAKGQEQOZVS6I@isocpp.org Sun Oct 30 15:23:26 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oi0-f71.google.com ([209.85.218.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1c0r0s-000510-Np for gclcip-std-proposals@m.gmane.org; Sun, 30 Oct 2016 15:23:06 +0100 Original-Received: by mail-oi0-f71.google.com with SMTP id y2sf54252588oie.3 for ; Sun, 30 Oct 2016 07:23:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=s8mge/9R4ShGpeeouMMb1RbfB9M4yvyLsEe005NBRis=; b=Q4fvty1e/oFStx/BQeSqkCAQdfWgwY4o4neuAtXP6qd6xycTUCDIIebZL/u1EuLfMC xYDHlx3QR2qRu9SQU8/rZaFH90+j8myMPB2mrPyNczEjM9nMx1l7dnUsIYV8LcZALxr4 z6hxOe54g4h5jWqNCJt+GCZha1abQ/Vy307VBwhcXgwt7fgjEpBr26ISts2waDflIi5g a08dfDK0Pn/aUrgHgLjXjRxr2bm256XAzp5bdNdBafvfj1y3qhTUbnKqsWU5avfItP81 KjKz21Ilv/GubEbqkSNDTNmiPFxO0MMjOZkbChTHpNbX7gsy1UvfCwjRvekCpfY8hXje iLqg== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=s8mge/9R4ShGpeeouMMb1RbfB9M4yvyLsEe005NBRis=; b=ok4mfpZU66Yn7n5Zt1OpSjmHiPzPG+USHUIbErubjWZcQt1mTm+IXgKtXmXjLKVYWf jN7sDUgDFaWKkKX9NzVcQtZy7bwAhbKdk+vaNxdJ9mr/rGXzIjZswJSQmsdT3rMGnUSG hHNyRbjwBcRif2QcaZGrrVXdwM7+5xvvj7b7BNptsxpXxY2Nl7NnO/0qrDHk2Ufjc7oB m3yK1YMy8CdxZ17RtsyTmeJ29l4nME4kSh8GNgRMD8+xFMpd2ROU1nAO2loI+JHTPOcu dU2Bz+q0UcIgL7JVcR0So3Gcqnfyv7NZfaarifwHBMInJwkcYFrn5e+BFB2fF6HZncOj EC4Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=s8mge/9R4ShGpeeouMMb1RbfB9M4yvyLsEe005NBRis=; b=Be3Ic5TknB0Tk+iILCFJCHaSTyYBUDQan2gL/ZQKZYr1sHFefE2N7sectZ8wFSJ/Ym dzwQSk1Mdy/gBdlisN8kIaIEFUWk9XaaTNxHeENvmOM0juadYFoXwuAN2WhwMtHssKDM gMuhBozhmDIClyYbjO5ay9aLI5Da9BtWZcjtvN01MW7OXhoSHFWRsoxJGDRQmAn0i0dx Sg+tk9CLYPk3CQyj0xRcfAP6qxQid+x284L8BUIzVSRkQTa/+qQRKS9xb0uG1Ok5fkuV GOtJG34tiRS01rTQ7Jo35Iks0fsOwogdywTxDCqXajC9J6mqXhaXeo2i9Ii+aiQsUWrG x+ZA== X-Gm-Message-State: ABUngvdAX+GesLSSwwXmd+4tVz5U8Ww5Xd3A7Fvj0cjNkXthR54od8xD1SnY9N8cfuOkvA== X-Received: by 10.157.13.163 with SMTP id 32mr7963709ots.11.1477837389246; Sun, 30 Oct 2016 07:23:09 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.4.101 with SMTP id 92ls6207181otc.28.gmail; Sun, 30 Oct 2016 07:23:08 -0700 (PDT) X-Received: by 10.157.17.167 with SMTP id v36mr297158otf.12.1477837388551; Sun, 30 Oct 2016 07:23:08 -0700 (PDT) In-Reply-To: <74d02121-9633-b6d1-1187-2a9ab660089d@wanadoo.fr> X-Original-Sender: jmckesson@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:29258 Archived-At: ------=_Part_1520_927821942.1477837388206 Content-Type: multipart/alternative; boundary="----=_Part_1521_1776317284.1477837388206" ------=_Part_1521_1776317284.1477837388206 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Sunday, October 30, 2016 at 9:53:41 AM UTC-4, Vicente J. Botet Escriba= =20 wrote: > > Le 30/10/2016 =C3=A0 13:20, Nicol Bolas a =C3=A9crit : > > > > On Sunday, October 30, 2016 at 3:50:18 AM UTC-4, Vicente J. Botet Escriba= =20 > wrote:=20 >> >> Le 29/10/2016 =C3=A0 23:51, Andrzej Krzemie=C5=84ski a =C3=A9crit : >> >> 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 t= he=20 >> underlying type? >> >> I do not know of any practical use of explicit conversion operators=20 >> except for the contextual conversion to bool. And it is only this=20 >> conversion that I am interested in. >> >> >> This works already in a non-contextual conversion, isn't it? >> >> bool(e) >> > > No, it doesn't. The only way to convert a strongly typed enum to its=20 > underlying type is with a `static_cast`. > > > It works for clang and gcc > So why doesn't it work when you use direct initialization of a variable=20 (like `bool b{e}`)? Direct initialization and=20 creating a temporary ought to work identically. So if this is an issue, then I would say that the issue is the asymmetry=20 between direct initialization of enum classes and creating prvalues of them= .. --=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/dc9396b6-00c6-4249-8988-a36f41c52519%40isocpp.or= g. ------=_Part_1521_1776317284.1477837388206 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Sunday, October 30, 2016 at 9:53:41 AM UTC-4, Vicente J= .. Botet Escriba wrote:
=20 =20 =20
Le 30/10/2016 =C3=A0 13:20, Nicol Bolas a =C3=A9crit=C2=A0:


On Sunday, October 30, 2016 at 3:50:18 AM UTC-4, Vicente J. Botet Escriba wrote:
Le 29/10/2016 =C3=A0 23:51, Andrzej Krzemie=C5=84ski a =C3= =A9crit=C2=A0:
Hi All,

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

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

This works already in a non-contextual conversion, isn't it= ?

=C2=A0=C2=A0=C2=A0 bool(e)

No, it doesn't. The only way to convert a strongly typed enum to its underlying type is with a `static_cast`.

It works for clang and gcc

So why doesn&= #39;t it work when you use direct init= ialization of a variable (like `bool b{e}`)? Direct initialization and = creating a temporary ought to work identically.

So if this is an iss= ue, then I would say that the issue is the asymmetry between direct initial= ization of enum classes and creating prvalues of them.

--
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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/dc9396b6-00c6-4249-8988-a36f41c52519= %40isocpp.org.
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