220 7022 article Path: news.gmane.org!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: More converting constructors for Optional Date: Sat, 5 Oct 2013 20:33:01 +0300 Lines: 106 Approved: news@gmane.org Message-ID: References: <417ebcd9-ff0b-49d8-b055-bd098e97dddc@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=001a11c379ec89752804e801cfef X-Trace: ger.gmane.org 1380994382 23685 80.91.229.3 (5 Oct 2013 17:33:02 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 5 Oct 2013 17:33:02 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC5JHI7A7ALRBTU2YGJAKGQE37NV2HY@isocpp.org Sat Oct 05 19:33:06 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-we0-f200.google.com ([74.125.82.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VSVix-0003Di-9m for gclcip-std-proposals@m.gmane.org; Sat, 05 Oct 2013 19:33:03 +0200 Original-Received: by mail-we0-f200.google.com with SMTP id u57sf4474243wes.7 for ; Sat, 05 Oct 2013 10:33:03 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=cAva3ClaFmbmffum93flJA87xx8oEcckofjPcixrSZY=; b=UfxPhORwpyfBfNYOP1jRBFn2U070QABHP0brA/Z/Y4LpDPlnbiPhpeBhgz0FkMf/TH M66TJoOkiIMsFVRMDClceJ7ShpJI82eUUvPKjIrcQWKvRdauhgNK90gw2fQxi7SqAB6p lB61SklJZkix+nrbJKj0nZw8QzxJQm6YURUnIfRsPA0t+sTvBEyAmOG576EnAyXJfU0T mUE0aphbt/9NMyYlD11l6/zOnBQgC/w0ZY8RJdhtQBuJeOe0Ckyf+SFpOjRwwyDnq5F1 tdVGX1ZrIs1klMvWGX/PVC3iDusUC7OyINNIi8n3NI/FqEzzNYdveknUe7V2tCkX5IZl zXxg== X-Received: by 10.180.14.194 with SMTP id r2mr5732506wic.0.1380994383011; Sat, 05 Oct 2013 10:33:03 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.205.6 with SMTP id lc6ls602385wic.25.gmail; Sat, 05 Oct 2013 10:33:01 -0700 (PDT) X-Received: by 10.204.168.197 with SMTP id v5mr18658803bky.24.1380994381774; Sat, 05 Oct 2013 10:33:01 -0700 (PDT) Original-Received: from mail-lb0-x230.google.com (mail-lb0-x230.google.com [2a00:1450:4010:c04::230]) by mx.google.com with ESMTPS id lk6si3136118bkb.213.1969.12.31.16.00.00 (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sat, 05 Oct 2013 10:33:01 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2a00:1450:4010:c04::230 as permitted sender) client-ip=2a00:1450:4010:c04::230; Original-Received: by mail-lb0-f176.google.com with SMTP id y6so4311055lbh.35 for ; Sat, 05 Oct 2013 10:33:01 -0700 (PDT) X-Received: by 10.112.89.100 with SMTP id bn4mr17203635lbb.16.1380994381343; Sat, 05 Oct 2013 10:33:01 -0700 (PDT) Original-Received: by 10.112.89.229 with HTTP; Sat, 5 Oct 2013 10:33:01 -0700 (PDT) In-Reply-To: <417ebcd9-ff0b-49d8-b055-bd098e97dddc@isocpp.org> X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2a00:1450:4010:c04::230 as permitted sender) smtp.mail=ville.voutilainen@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=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:7022 Archived-At: --001a11c379ec89752804e801cfef Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 5 October 2013 20:14, Andrzej Krzemie=C5=84ski wrot= e: > > (because this would turn an explicit conversion into an implicit one). > Such "conditionally explicit" converting constructor is doable, but I run > on another problem. The additional constructors would look like: > > * template > * > * optional::optional(U&& u); > > * > * template > * > * optional::optional(optional const& ou);* > > Suppose you have the following class: > > * struct Port > { > * > * Port (optional oi); > }; > * > * > * > * optional oi; > * > * optional op =3D oi;* > > which of the two constructors would the user expect to be called? I guess > the former, but he would be surprised. I do not know how to solve this > problem. Perhaps someone on the list does. > > Depends on the user. Advanced users will know that the universal-reference ctor is a better match since there's no const involved. Perhaps you could SFINAE it out in case the incoming type is optional? --=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 http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. --001a11c379ec89752804e801cfef Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable



On 5 October 2013 20:14, Andrzej Krzemie=C5=84ski <akrzemi1@gmail= ..com> wrote:

(b= ecause this would turn an explicit conversion into an implicit one). Such=20 "conditionally explicit" converting constructor is doable, but I = run on=20 another problem. The additional constructors would look like:

=
=C2=A0 template <class U>
=C2=A0 optiona= l::optional(U&& u);

=C2=A0 template <cl= ass U>
=C2=A0 optional::optional(optional<U> const& ou= );

Suppose you have the following class:
=C2=A0 struct Port
=C2=A0 {
=C2= =A0=C2=A0=C2=A0 Port (optional<int> oi);
=C2=A0 };=C2=A0

=C2=A0 optional= <int> oi;
=C2=A0 optional<Port> op =3D oi;<= br>
which of the two constructors would the user expect to be called? I guess the former, but he would be surprised. I do not know how to solve this=20 problem. Perhaps someone on the list does.

<= br>
Depends on the user. Advanced users will know that the univer= sal-reference ctor is a better
match since there's no const involved= .. Perhaps you could SFINAE it out in case the incoming
type is optional<U>?

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