220 7039 article Path: news.gmane.org!not-for-mail From: =?UTF-8?Q?Andrzej_Krzemie=C5=84ski?= Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: More converting constructors for Optional Date: Sat, 5 Oct 2013 12:05:42 -0700 (PDT) Lines: 154 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="----=_Part_261_11738981.1380999942602" X-Trace: ger.gmane.org 1380999942 13314 80.91.229.3 (5 Oct 2013 19:05:42 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 5 Oct 2013 19:05:42 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDT2DGOJ34DBBB6GYGJAKGQE2KGB65I@isocpp.org Sat Oct 05 21:05:45 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qa0-f70.google.com ([209.85.216.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VSXAe-0004EM-Oe for gclcip-std-proposals@m.gmane.org; Sat, 05 Oct 2013 21:05:45 +0200 Original-Received: by mail-qa0-f70.google.com with SMTP id i13sf5559475qae.5 for ; Sat, 05 Oct 2013 12:05:43 -0700 (PDT) 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 :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=zU43tK9kIJ5iQR/ufh/P0kPgiGH3kcWJmezGBRTx4nw=; b=p6OFmNrxdMTkoCPhke/e+FzLXKoq7WmxbFHvn7XJxluhPB9h+9kJSAhf/fuaKH+pEj OI40In4v0SA+Dt6DXY7CADPtbQ3AGNnJ9rMJ3KgHj/8wwAAVBOTobsTnZZBQVR9yeV4w Yyax45ccb+KHhfm+nA9eyc+jhf47yQ5hblK+cN+Kl72rOefTRvQAD3yC7XxnNH4GAfcu /huy008fl4Ln+TfjRIPfSyyMZA0M+g7wKRS7vrcfc9RgY22GH0efDkI7d4HCFgsuK5Oq hYuZo9sGEsJtOSr77bPlqXjaqtDGlYnMcDzHWfmCvUVZzzVX3d4lJWIQ/p8jVtDagtSF pNEQ== X-Received: by 10.236.227.165 with SMTP id d35mr17744194yhq.51.1380999943636; Sat, 05 Oct 2013 12:05:43 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.12.103 with SMTP id x7ls704239qeb.14.gmail; Sat, 05 Oct 2013 12:05:43 -0700 (PDT) X-Received: by 10.49.75.168 with SMTP id d8mr836933qew.3.1380999943160; Sat, 05 Oct 2013 12:05:43 -0700 (PDT) In-Reply-To: X-Original-Sender: akrzemi1@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:7039 Archived-At: ------=_Part_261_11738981.1380999942602 Content-Type: text/plain; charset=ISO-8859-2 Content-Transfer-Encoding: quoted-printable W dniu sobota, 5 pa=BCdziernika 2013 19:33:01 UTC+2 u=BFytkownik Ville=20 Voutilainen napisa=B3: > > > On 5 October 2013 20:14, Andrzej Krzemie=F1ski > > wrote: > >> >> (because this would turn an explicit conversion into an implicit one).= =20 >> Such "conditionally explicit" converting constructor is doable, but I ru= n=20 >> 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); >> }; =20 >> * >> * >> * >> * optional oi; >> * >> * optional op =3D oi;* >> >> which of the two constructors would the user expect to be called? I gues= s=20 >> the former, but he would be surprised. I do not know how to solve this= =20 >> problem. Perhaps someone on the list does. >> >> > Depends on the user. Advanced users will know that the universal-referenc= e=20 > ctor is a better > match since there's no const involved. Perhaps you could SFINAE it out in= =20 > case the incoming > type is optional?=20 > Oh, you spotted a different problem than I did. Mine was that (even if I=20 fixed yours) it is not clear how to handle a case where *T* has a=20 constructor taking another optional. Consider a more tricky type: * struct Port { * * Port (optional oi); Port (int); };=20 * And now the code that uses it: * optional oi; ** optional op =3D oi;* Now, forget how optional is or might be implemented. As a user, which=20 constructor of *Port *would you expect to be called? --=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/. ------=_Part_261_11738981.1380999942602 Content-Type: text/html; charset=ISO-8859-2 Content-Transfer-Encoding: quoted-printable


W dniu sobota, 5 pa=BCdziernika 2013 19:33:01 UTC+= 2 u=BFytkownik Ville Voutilainen napisa=B3:

On 5 Octo= ber 2013 20:14, Andrzej Krzemie=F1ski <akrz...@g= mail.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:

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

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

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

  optional= <int> oi;
  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. Pe= rhaps you could SFINAE it out in case the incoming
type is optional<U>?

Oh, you spotted a different problem than I did. Mine was that = (even if I fixed yours) it is not clear how to handle a case where T= has a constructor taking another optional. Consider a more tricky type:
  struct Port
  {
    Po= rt (optional<int> oi);
    Port (int);
  };

And now the code that uses it:

 = optional<int> oi;
  optional<Port> op =3D oi;

Now, forget how optional is or might be implemented. As a user, w= hich constructor of Port would you expect to be called?


<= br>

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