220 24315 <56BC51E9.10200@cern.ch> article Path: news.gmane.org!not-for-mail From: Axel Naumann Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: variant construction/assignment from braced value Date: Thu, 11 Feb 2016 10:18:33 +0100 Lines: 168 Approved: news@gmane.org Message-ID: <56BC51E9.10200@cern.ch> References: <9A41B088-08C0-4222-8558-FA734F61BDE5@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="------------000606080509080901030803" X-Trace: ger.gmane.org 1455182259 12503 80.91.229.3 (11 Feb 2016 09:17:39 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 11 Feb 2016 09:17:39 +0000 (UTC) Cc: ISO C++ Standard - Future Proposals To: David Krauss , =?UTF-8?Q?Agust=c3=adn_K-ballo_Berg=c3=a9?= Original-X-From: std-proposals+bncBCBIFNO6TUPRBH5D6G2QKGQERDEZW2A@isocpp.org Thu Feb 11 10:17:22 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f72.google.com ([209.85.220.72]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1aTnNJ-0005y1-IO for gclcip-std-proposals@m.gmane.org; Thu, 11 Feb 2016 10:17:21 +0100 Original-Received: by mail-pa0-f72.google.com with SMTP id fl4sf88111366pad.2 for ; Thu, 11 Feb 2016 01:17:21 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=subject:to:references:cc:from:message-id:date:user-agent :mime-version:in-reply-to:content-type:spamdiagnosticoutput :spamdiagnosticmetadata: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=yVL/yOaWHg5VrTj6lA7i2M4NCGjLsEeQHaOXxYuifus=; b=fNMmD8cbOaciN2dbWcAihOEgvZ21k2HMpj8Re4FM1nF4lIQvL9K1iTV9KALXSVlgkf DdYxD4M4AWoieJ3TBTO56U5sNoVEA8bMiTqT7Dqs1+FjVdKgJeXBi/CrVdk0f9uI0Wmg DIlh/W+H5gCw6jeSSK6waEm9CWdMJYzojzyvpABrKarAqYClazrDJexVBerbjTeXPPR+ PkCuj5ok8NRrr+sqEmARNhHuYja8Wbp4sP1XU5lHxpG7gbCMChgcN4EOASX3jhmjDwYQ HNK40lBDhXYsA6MSUoAuJBNcKUAvsP 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:cc:from:message-id:date :user-agent:mime-version:in-reply-to:content-type :spamdiagnosticoutput:spamdiagnosticmetadata: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=yVL/yOaWHg5VrTj6lA7i2M4NCGjLsEeQHaOXxYuifus=; b=abLFK+inpCetXCRmEWCb22c7OUtdflSC+5FgZ6PjK9rFO093ldbgezzLk805Ix0v4n ZwgfH+tewaQh7CC6UtVi38dtTO2+Umuigqg4sTbQhKTX+dmmFsJL0fmDmH/1a90yudno 4+uPoeSSUVy4CMJDki+MOlbI4lh7YwG0l6QVvtzJXwBIX2oFV2jCU+OIJFsLYbiqjHXe M1xBbj7jUH7LBwH3JO6b+XIEHp1J3ow66JFW5PxcIwuN9+KSTbCHD0bcBv5p0aJWB8xj uI2IwgE09GOS480JCHI9nYK7Bh X-Gm-Message-State: AG10YORbfQnerMyomWK1GrOcCBImXFBz6oL15E5XxXVLJbaktYjIz77+lZDA4ORO+J1aRA== X-Received: by 10.98.68.194 with SMTP id m63mr3007012pfi.7.1455182240476; Thu, 11 Feb 2016 01:17:20 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.45.161 with SMTP id o1ls491413obm.27.gmail; Thu, 11 Feb 2016 01:17:18 -0800 (PST) X-Received: by 10.60.67.166 with SMTP id o6mr43220325oet.77.1455182238970; Thu, 11 Feb 2016 01:17:18 -0800 (PST) Original-Received: from emea01-am1-obe.outbound.protection.outlook.com (mail-am1on0097.outbound.protection.outlook.com. 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spf=pass (google.com: domain of Axel.Naumann@cern.ch designates 157.56.112.97 as permitted sender) smtp.mailfrom=Axel.Naumann@cern.ch; dkim=pass header.i=@cern.onmicrosoft.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:24315 Archived-At: --------------000606080509080901030803 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Hi David, Thank you, that's a good argument. Would you mind writing a paper on this? I don't want to halt the LWG process for this. The current variant will hopefully end up in a TS at some point; before we vote that out we can have your improvement in. I'd rather not stop the current variant by going back to LEWG and re-opening the design discussion at this stage. I'd prefer it to arrive in the TS, even at the cost of a only partially perfect (-ly forwarding) variant. Cheers, Axel. On 11/02/16 09:56, David Krauss wrote: > Hi Axel and Agust=C3=ADn, > > The latest variant proposal and implementation provide catch-all > constructor and assignment operator templates. These implement > overload resolution to determine which (if any) alternative type is > most suitable to receive the given value, then forward to the > appropriate delegate. > > However, perfect forwarding isn=E2=80=99t completely perfect. It doesn=E2= =80=99t > support braced init lists. > > variant< char, string > q =3D { { 999 } }; // error: can=E2=80=99t deduce= T. > variant< char, string > r =3D { 999 }; // valid, but overflows. > variant< char, string > s =3D { 'a', 'b', 'c' }; // error. > > Wouldn=E2=80=99t it be better to provide overload sets of non-template > constructors and assignment operators? This might present a different > set of implementation challenges, but the wrapping would be thinner > and the compile-time complexity may ultimately be lower. The > specification would certainly be simpler. > > - Cheers, > D > --=20 ---=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. Visit this group at https://groups.google.com/a/isocpp.org/group/std-propos= als/. --------------000606080509080901030803 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Hi David,

Thank you, that's a good argument. Would you mind writing a paper on this? I don't want to halt the LWG process for this. The current variant will hopefully end up in a TS at some point; before we vote that out we can have your improvement in.

I'd rather not stop the current variant by going back to LEWG and re-opening the design discussion at this stage. I'd prefer it to arrive in the TS, even at the cost of a only partially perfect (-ly forwarding) variant.

Cheers, Axel.

On 11/02/16 09:56, David Krauss wrote:
Hi Axel and Agust=C3=ADn,

The latest variant proposal and implementation provide catch-all constructor and assignment operator templates.=C2=A0These implement overload resolution to determine which (if any) alternative type is most suitable to receive the given value, then forward to the appropriate delegate.

However, perfect forwarding isn=E2=80=99t completely perfect. It doesn=E2=80=99t support braced init lists.

variant< char, string > q =3D { { 999 } }; // error: can=E2=80=99t deduce T.<= /font>
variant< char, string > r =3D { 999 }; // valid, but overflows.
variant< char, string > s =3D {=C2=A0'a',=C2=A0<= /font>'b'= ,=C2=A0'c'= =C2=A0}; // error.

Wouldn=E2=80=99t it be better to provide overload set= s of non-template constructors and assignment operators? This might present a different set of implementation challenges, but the wrapping would be thinner and the compile-time complexity may ultimately be lower. The specification would certainly be simpler.

- Cheers,
D


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