220 13946 <92b49cce-e224-484d-9513-73a921e8369c@isocpp.org> article Path: news.gmane.org!not-for-mail From: TONGARI J Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Reconstruct function idea Date: Wed, 15 Oct 2014 19:51:56 -0700 (PDT) Lines: 401 Approved: news@gmane.org Message-ID: <92b49cce-e224-484d-9513-73a921e8369c@isocpp.org> References: <8a74f08b-7453-4d1a-9f20-bc1d3866617c@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_4471_1540226158.1413427916083" X-Trace: ger.gmane.org 1413427926 16610 80.91.229.3 (16 Oct 2014 02:52:06 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 16 Oct 2014 02:52:06 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCC3NA775MLBBTPF7SQQKGQEGUTOC5Y@isocpp.org Thu Oct 16 04:51:59 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qa0-f69.google.com ([209.85.216.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1XebAV-0001PB-8x for gclcip-std-proposals@m.gmane.org; Thu, 16 Oct 2014 04:51:59 +0200 Original-Received: by mail-qa0-f69.google.com with SMTP id k15sf4542581qaq.4 for ; Wed, 15 Oct 2014 19:51:58 -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=HjMtHq+ChJ1G++qVmiY8x08yc/5GKN+ddYPux6dMAww=; b=D49waR3/+/2AEgQoZUJH6yUuRRTQkARFyhknZON3IsmiOeLWMB3fJfKYg+DHSKg6U4 PkYNidd9Fvh/BUsIIsDdL1YJiV0bWHJV6Zi0SVje/JTGlcUo5yMEs92mbG3mAf7iu9VO aV2sHj1rtB1mQIi1YExweqBcrwsmLUMr99jUlNa4ConPuLmPtHrj9PIdwaWUcCiFeyUN aeTzgtQHtiDjhhTbmF7bUIZHV5KbqUIidILYoc48Rz4eNEr/jmFeJAl2YXnPaj4hVLMK GN/KD3gbBNaOQwiCnwQnl/QoKIVXXC7vici3ub0se5d0we/M2RbpFV0Uwpf5N8r9G02a t6BA== 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=HjMtHq+ChJ1G++qVmiY8x08yc/5GKN+ddYPux6dMAww=; b=ITsYwUzWwf+jmrwLVzZ0t75Mo5As17666/FCgF8d4P0buJ/36o37SXGo/HhpGYW+SZ PloR8QmF36x1lPpxhKvGTzv8IO2PHUGOiP0Mds7vVzOkuBjXJF1Z3M9suzuOSryrrY7m iFOv9zD2TeAevoSwQPuSc/4iL9X6vKcdjSGcw4mmLfbBdwX+WSL7Q6gzrttuxzDJZ9yy J9otVr+FR+BY4INKWY6DAPOyZNBxwd/YfRlyPBlIeAZrfoenqieks7COpYv2zxgHEM94 dLGea6inH/cgnS091dSOjo0Ujb6zMDt9lZwQFp2S1CiAA7MwzKYDSIPou2lgIRZv46N/ 1fnA== X-Gm-Message-State: ALoCoQn/YC+NYZLYyPZWCYbt9B4DAkxRvKBvx3WzLicf6N4BT6dqO7muPv8lcnb84KUHhtYhUFxq X-Received: by 10.236.110.2 with SMTP id t2mr4364403yhg.58.1413427918377; Wed, 15 Oct 2014 19:51:58 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.131.199 with SMTP id oo7ls460981igb.14.gmail; Wed, 15 Oct 2014 19:51:57 -0700 (PDT) X-Received: by 10.50.134.164 with SMTP id pl4mr206196igb.0.1413427917800; Wed, 15 Oct 2014 19:51:57 -0700 (PDT) In-Reply-To: X-Original-Sender: tongari95@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:13946 Archived-At: ------=_Part_4471_1540226158.1413427916083 Content-Type: text/plain; charset=UTF-8 On Thursday, October 16, 2014 5:48:11 AM UTC+8, Nevin ":-)" Liber wrote: > > > On 15 October 2014 16:17, Myriachan > > wrote: > >> While that is a nice feature, it's not the same as needing to do this to >> types that weren't created as std::optional. >> > > It is also *extremely* unsafe to do it with any class that can throw an > exception during construction or destruction. > How about this implementation? Is it unsafe? template struct priority : priority {}; template<> struct priority<0> {}; template std::enable_if_t::value && std:: is_nothrow_destructible::value> reconstruct_impl(priority<2>, T& t, As&&... as) noexcept { t.~T(); new(&t) T(std::forward(as)...); } template std::enable_if_t::value && std:: is_nothrow_destructible::value> reconstruct_impl(priority<1>, T& t, As&&... as) { T tmp(std::forward(as)...); t.~T(); new(&t) T(std::move(tmp)); } template void reconstruct_impl(priority<0>, T& t, As&&... as) { using std::swap; T tmp(std::forward(as)...); swap(t, tmp); } template void reconstruct(T& t, As&&... as) noexcept(std::is_nothrow_constructible::value && std::is_nothrow_destructible::value) { reconstruct_impl(priority<2>{}, t, std::forward(as)...); } The worst case fallbacks to copy-and-swap. -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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On Thursday, October 16, 2014 5:48:11 AM UTC+8, Nevin ":-)= " Liber wrote:


template<unsigned N>
stru= ct priorit= y := priority<= span style=3D"color: #660;" class=3D"styled-by-prettify"><N - 1> {};

temp= late<&g= t;
struct<= span style=3D"color: #000;" class=3D"styled-by-prettify"> priority<0> {};


template<class T= , <= span style=3D"color: #008;" class=3D"styled-by-prettify">class... As>
std::enable_if_t<std::is_noth= row_constructible<T,= As...>::value && std::is_nothrow_destructible<T>::value>
reconstruct_impl(priority<2>, T& t, As&&... as) noexce= pt
{
  &n= bsp; t.~T();
    new(&t) T(std::forwar= d<As>(as)...);
}


template<class T, class<= span style=3D"color: #660;" class=3D"styled-by-prettify">... As>
std::enable_if_t<std::is_n= othrow_move_constructible<T= >::valu= e &&am= p; std::is_nothrow_destructibl= e<T>::value>
reconstruct_impl(priority<1>, T& t, As&&... as)
{
    T tmp(std::forwa= rd<As>(as)...);
    t.~T();
    new(&t) T(std::move(tmp));
}


template<cla= ss T, class... As>
= void recon= struct_impl(priority= <0>, T& t, As= &&... as)
{
    using std::swap;
    T tmp(std::forward<As>(<= span style=3D"color: #008;" class=3D"styled-by-prettify">as)...);
    swap(t, tmp);
}


template<cla= ss T, class... As>
= void recon= struct(T& t, As&&... as) noexce= pt(= std::is_nothrow_constructible<= span style=3D"color: #660;" class=3D"styled-by-prettify"><T, As...>::value && std::is_= nothrow_destructible<T>:= :value)
{
    reconstruct_impl(priority<2>{}, t, std::forward<As>(<= span style=3D"color: #008;" class=3D"styled-by-prettify">as)...);
}

<= div>The worst case fallbacks to copy-and-swap.

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