220 11753 article Path: news.gmane.org!not-for-mail From: tomaszkam@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: N4078: Rvalue reference overloads for value() method returns object by value Date: Wed, 9 Jul 2014 12:51:24 -0700 (PDT) Lines: 92 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_490_4862776.1404935484685" X-Trace: ger.gmane.org 1404935495 14245 80.91.229.3 (9 Jul 2014 19:51:35 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 9 Jul 2014 19:51:35 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDNPVXXG6IGBBPV262OQKGQEROLGAFA@isocpp.org Wed Jul 09 21:51:28 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oa0-f72.google.com ([209.85.219.72]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X4xtn-0004yW-Kr for gclcip-std-proposals@m.gmane.org; Wed, 09 Jul 2014 21:51:27 +0200 Original-Received: by mail-oa0-f72.google.com with SMTP id l6sf3572980oag.11 for ; Wed, 09 Jul 2014 12:51:26 -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=vdISjUtaq/D6MIaHtYM7i7DYAeap3RXGb+6Yw1PUquI=; b=Q3EQBjkR24JrXuGc7OLi8pWMq+wR9S8zhrHu9c8ZM0SL3527UJ2coh1gednwuIrace BLB2ySZJeIXld9BL3ToZLDgZ2OPZrxEA7hGnBIpNue5b6Lle7oUvmDnAhD3U914RAm0G KHiuH6d4z39QMuAt26MxplJ36a9WLo43xuN6dcUO56zEpjW6s5/vuP5YrZNFcQBOOLml ZHOqr1VTmc3egPksE5k9diohdQ5bmjmEtGyIl+XVfVOKrG0ck/U5Tl+Slx+apTXTqzMv 7deTfO9wA3ixeYwp9NIUrsgqqNI90Fd0/S49tuCq/i6hkXUwVVPXOubspishM9XAYWA7 BVpg== 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:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=vdISjUtaq/D6MIaHtYM7i7DYAeap3RXGb+6Yw1PUquI=; b=KOEYxR8fxS7lw/86NGxUM+x4eIPDhEa4EfI6ya2n2E4igYnBKo2nngxYXuWld5sckw 1kY0vGu2lqWrY0jD6LAT+Dd2QNP+DXyf6qOvv7futOYb0zkNdvCXBa2gYl2JOC/zhN8k XFApNHBit3wT19RPOnAYA2MgY4MK+9z6PxQMuIVCB+D4pDdWk/lj0JJ/BKTWL3mAsLXN KT55wKPxT4P2dXD1CaTvQsMex2GcU0JTXOPHebCF8pXqqFYVkCZqcbOdBrySpGFomW7L GIQ75AcbGtoUAFaTAotw33MU/VNzcCrhboRKuqe5XaRMF3COJHx3kR8CQkSdfAkUxx4v ocZQ== X-Gm-Message-State: ALoCoQmSgJIEKARgT13wghjNDXRPnAHAlYaDrc77MNvbMEZcf3X44sXGBBfNUzxFxeJnAMA32hZX X-Received: by 10.182.130.169 with SMTP id of9mr22217303obb.27.1404935486742; Wed, 09 Jul 2014 12:51:26 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.42.195 with SMTP id q3ls1331509obl.97.gmail; Wed, 09 Jul 2014 12:51:25 -0700 (PDT) X-Received: by 10.182.24.1 with SMTP id q1mr725obf.42.1404935485903; Wed, 09 Jul 2014 12:51:25 -0700 (PDT) In-Reply-To: X-Original-Sender: tomaszkam@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:11753 Archived-At: ------=_Part_490_4862776.1404935484685 Content-Type: text/plain; charset=UTF-8 After some thinking of the idea, I am not sure why it was choosen to introduce a additional construction of the temporary to the .value() method of the optional, even if we want to just: a) read the value passing opt.value() to function that takes the value type by contt reference: optional g(); std::cout << g.value() << std::endl; * Ville I would like you to add this to the gut report, because constructing a temporary to simply print a value may be overkill especially if used in generic context with forward:* template //optional, smart_ptr void (Nullable&& nullable) { //do something f(*std::forward(nullable)); //does it make a temporary here, or not?? } b) construct another value from it: optional g(); auto opt = g(); To make the following code to work correclty: auto&& opt = g().value(); When it may be fixed by writing: auto opt = g().value(); and this will still invoke the same amount of construcotr as the above one, because: auto&& opt = g().value(); is construction T from T&& and binding it to reference auto opt = g(); is constructing T from T&& on stack I start to think if the this auto&& everywhere recommendation that seems to pop up form time to time is actually a good design guide. -- --- 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 email 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-proposals/. ------=_Part_490_4862776.1404935484685 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
After some thinking of the idea, I am not sure why it was = choosen to introduce a additional construction of the temporary to the .val= ue() method of the optional, even if we want to just:
 a) read the = value passing opt.value() to function that takes the value type by contt re= ference:
      optional<T> g();
 =      std::cout << g.value() << std::endl;  Ville I would like you to add this to the gut report, because c= onstructing a temporary to simply print a value may be overkill especially = if used in generic context with forward:
  template<typename= Nullable> //optional, smart_ptr
  void (Nullable&& null= able)
  {
     //do something
  = ;   f(*std::forward<Nullable>(nullable)); //does it make a = temporary here, or not??
  }
  b) construct another value f= rom it:
      optional<T> g();
 &= nbsp;    auto opt =3D g();
To make the following code to = work correclty:
     auto&& opt =3D g().valu= e();
When it may be fixed by writing:
     auto o= pt =3D g().value();
and this will still invoke the same amount of constr= ucotr as the above one, because:
    auto&& opt = =3D g().value(); is construction T from T&& and binding it to refer= ence
    auto opt =3D g(); is constructing T from T&&= amp; on stack

I start to think if the this auto&& everywhere= recommendation that seems to pop up form time to time is actually a good d= esign guide.
  

--

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