220 24296 <20160210161034.4898896.57740.4502@gmail.com> article Path: news.gmane.org!not-for-mail From: Tony V E Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Choosing value categories using a specialized traits class Date: Wed, 10 Feb 2016 11:10:34 -0500 Lines: 703 Approved: news@gmane.org Message-ID: <20160210161034.4898896.57740.4502@gmail.com> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Trace: ger.gmane.org 1455120649 3476 80.91.229.3 (10 Feb 2016 16:10:49 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 10 Feb 2016 16:10:49 +0000 (UTC) To: =?utf-8?b?QW5kcsOp?= , ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCUZ5QWKNQIP3QPNWUCRUBDRU3VGA@isocpp.org Wed Feb 10 17:10:41 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wm0-f72.google.com ([74.125.82.72]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1aTXLk-0004F0-B8 for gclcip-std-proposals@m.gmane.org; Wed, 10 Feb 2016 17:10:40 +0100 Original-Received: by mail-wm0-f72.google.com with SMTP id 128sf11043969wmz.0 for ; Wed, 10 Feb 2016 08:10:39 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=content-type:mime-version:content-transfer-encoding:message-id:date :subject:from:in-reply-to:references:to: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=4HLh4TR3u+z92Xcp9u59n9mqnZFuCiJHjeEeMUudmZY=; b=1xaJE3bBuEylxZu0TsJ8uRwpalEAYZCuYxK7cMSz03zysn44uLFtVHeqwkNRzJil2H JHhpxgVCfFpMQ1dtJ/LvbAU5DeMfckYHg2kuvi/HFsn7YWcFS52TEPvlEwx7zW9qbmcv xMGocgEwGdLT660mw2j/7eFYqwE+Y0sduVY9+1SKEMClJUBHg9XiOs+s/dvUpIi9Yy52 HWYUcmshLBdvoxGnspbiWvas0L3/5wJIifsPSKzfMZ6tNyBpc59YXO+OeiFDV7OIoABH tWAsE0BVxuVhcfva7mZOGQ6SxosN9L7m14ctYavFMbU1kKjDqgKfOE76RpYQ7sWM7i9E X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:content-type:mime-version :content-transfer-encoding:message-id:date:subject:from:in-reply-to :references:to: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=4HLh4TR3u+z92Xcp9u59n9mqnZFuCiJHjeEeMUudmZY=; b=cR0mItiA5p6KCB5/ifxdjViXycW5bUmDFgD/YcidsuBgj2EfWY+x8q4C581za9cVWm IChF8Dz1PqhknnRZhNj/i+61VQeyukwCzYTfKvsBFPKsjdI6IZUZ3tt0Kdw89etiLLql u4acBiSp7l234kmXJvhL+mmS1leFf1GNV2BgN1+MOwJe3tANPSvVaEIHBAnfhqNnCoL/ IuTUVWmaCQ3PY4UEbqfAYOuOPcWM417UP5cJhlWdYy50TLDVWkcnT6duxqgBekHBXmWc Bl2I8fkYeXPMmDPSyxyCwGnQt+vTJu4WXJsL/3CjaKbmU/37YEArMP9PCp1P5h4UyCep X-Gm-Message-State: AG10YOQLKvGcixJmuwzHIznl6a8z1rP/tSPo8h521FPIjIdRfUngli/QBokSdAJew3Y47g== X-Received: by 10.112.77.169 with SMTP id t9mr4861134lbw.8.1455120639706; Wed, 10 Feb 2016 08:10:39 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.23.229 with SMTP id 92ls409780qgp.65.gmail; Wed, 10 Feb 2016 08:10:37 -0800 (PST) X-Received: by 10.55.26.136 with SMTP id l8mr38493060qkh.103.1455120637644; Wed, 10 Feb 2016 08:10:37 -0800 (PST) Original-Received: from mail-qg0-x232.google.com (mail-qg0-x232.google.com. [2607:f8b0:400d:c04::232]) by mx.google.com with ESMTPS id 188si4297840qhj.38.2016.02.10.08.10.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 10 Feb 2016 08:10:37 -0800 (PST) Received-SPF: pass (google.com: domain of tvaneerd@gmail.com designates 2607:f8b0:400d:c04::232 as permitted sender) client-ip=2607:f8b0:400d:c04::232; Original-Received: by mail-qg0-x232.google.com with SMTP id b35so16965551qge.0 for ; Wed, 10 Feb 2016 08:10:37 -0800 (PST) X-Received: by 10.140.157.7 with SMTP id d7mr28846669qhd.17.1455120637433; Wed, 10 Feb 2016 08:10:37 -0800 (PST) Original-Received: from [127.0.0.1] ([24.114.71.79]) by smtp.gmail.com with ESMTPSA id f5sm1548313qkb.30.2016.02.10.08.10.34 for (version=TLSv1/SSLv3 cipher=OTHER); Wed, 10 Feb 2016 08:10:35 -0800 (PST) X-Mailer: BlackBerry Email (10.3.2.2876) In-Reply-To: X-Original-Sender: tvaneerd@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of tvaneerd@gmail.com designates 2607:f8b0:400d:c04::232 as permitted sender) smtp.mailfrom=tvaneerd@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-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:24296 Archived-At: =
Not a bad idea. 

But I'd rather see = some kind of language feature for param passing. A "pass by copy if cheap"= =E2=80=8E syntax. Also a pass by copy-if-write (for cases where the functio= n body I available to determine that). 

Vaguely = speaking.
= <= div style=3D"width: 100%; font-size: initial; font-family: Calibri, 'Slate = Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial;= background-color: rgb(255, 255, 255);">
= =
S= ent from my BlackBerry portable Babbage Devic= e
= =
From: Andr=C3=A9
Sent: Wednesday, Februar= y 10, 2016 10:37 AM
To: ISO C++ Standard - Future Proposal= s
Reply To: std-proposals@isocpp.org
Subject:= [std-proposals] Choosing value categories using a specialized traits c= lass

Hello= everyone,

This idea for a proposal comes from discussio= ns such as this one. There= are several situations in STL's API in which integral values are passed by= const reference, such as vector's insert for template parameters o= f integral types. Consider the following code:

std::vector<int> v;
int a;

= ...

v.push_back(a);

I'm not sure= to at what extent do compilers optimize this (if at all) by passing "a" by= copy instead of using a constant reference. In any case, this idea would s= olve this problem at the framework level, which seems optimal to me. Consid= er push_back's signature:

= 
void push_back (const value_type& val);

This proposal consists in providing= a template traits class that implements a set of typedefs and = specializes them to provider cleaner and optimal definitions, regarding mul= tiple situations. Here's a simplified example:

template <class T> class traits

{

public:

   

  &= nbsp;typedef T value_type;

   

   <= /span>typedef value_type &am= p; referen= ce;=

   typedef value_type * pointer;

  =  

   typ= edef const= = value_type & const_reference;

   typedef const value_type * const_pointer;

};


In this case, it doesn't do much other than d= efining some pretty standard stuff. These definitions can be found immensel= y in STL. The next step would be to provide some more exotic definitions. S= ome examples:

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