220 32842 <20170622082445.GA5007@noemi.bahnhof.se> article Path: news.gmane.org!.POSTED!not-for-mail From: Magnus Fromreide Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: The Proxies - A Language Feature Decoupling Implementations from Requirements of Polymorphism Date: Thu, 22 Jun 2017 10:24:45 +0200 Lines: 65 Approved: news@gmane.org Message-ID: <20170622082445.GA5007@noemi.bahnhof.se> References: <8d79a013-fe0e-4fb6-8278-d438ae9b4c64@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" X-Trace: blaine.gmane.org 1498119890 27297 195.159.176.226 (22 Jun 2017 08:24:50 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 22 Jun 2017 08:24:50 +0000 (UTC) User-Agent: Mutt/1.8.0 (2017-02-23) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDELLREETMBRBUH5VXFAKGQEXPIBQ7Y@isocpp.org Thu Jun 22 10:24:45 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f71.google.com ([209.85.215.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dNxPx-0006m3-JR for gclcip-std-proposals@m.gmane.org; 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[2001:6b0:17:f0a0::3]) by mx.google.com with ESMTPS id 6si422851ljf.333.2017.06.22.01.24.48 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 22 Jun 2017 01:24:48 -0700 (PDT) Received-SPF: pass (google.com: domain of magfr@lysator.liu.se designates 2001:6b0:17:f0a0::3 as permitted sender) client-ip=2001:6b0:17:f0a0::3; Original-Received: from mail.lysator.liu.se (localhost [127.0.0.1]) by mail.lysator.liu.se (Postfix) with ESMTP id 6D21640011 for ; Thu, 22 Jun 2017 10:24:47 +0200 (CEST) Original-Received: from noemi.bahnhof.se (h-155-4-132-137.NA.cust.bahnhof.se [155.4.132.137]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.lysator.liu.se (Postfix) with ESMTPSA id 477034000F for ; Thu, 22 Jun 2017 10:24:47 +0200 (CEST) Mail-Followup-To: std-proposals@isocpp.org Content-Disposition: inline In-Reply-To: X-Virus-Scanned: ClamAV using ClamSMTP X-Original-Sender: magfr@lysator.liu.se X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of magfr@lysator.liu.se designates 2001:6b0:17:f0a0::3 as permitted sender) smtp.mailfrom=magfr@lysator.liu.se 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:32842 Archived-At: On Wed, Jun 21, 2017 at 08:02:35PM -0700, Mingxin Wang wrote: > I designed another general wrapper type yesterday, which is intended for > small and trivial types, as is shown below: > > template > class TrivialWrapper { > public: > /* Constructors */ > template > TrivialWrapper(T&& data) requires > !std::is_same>, > TrivialWrapper>::value && > std::is_trivial>>::value > && > (sizeof(std::remove_cv_t>) <= SIZE) { Why remove_cv_t here? > memcpy(data_.get(), &data, > sizeof(std::remove_cv_t>)); > } > TrivialWrapper() = default; > TrivialWrapper(const TrivialWrapper&) = default; > TrivialWrapper(TrivialWrapper&&) = default; > > TrivialWrapper& operator=(const TrivialWrapper& rhs) = default; > TrivialWrapper& operator=(TrivialWrapper&&) = default; This is generally broken. Consider the folloving trivial data structure: struct c_list { struct c_list *next, *prev; int data; }; It is a trivial data structure but if you start moving (or copying) it around with your wrapper things will break badly. > /* Meets the Wrapper requirements */ > void* get() { > // The address of the wrapped object is calculated with a constant > offset > return data_.get(); > } > > private: > MemoryBlock data_; > }; > > > I am wondering if these design for wrappers (DeferredWrapper, DeepWrapper, > SharedWrapper and TrivialWrapper) are adequate to be added to the standard > (discard of naming). I think you need to put in a lot of more work on why they are generally useful. /MF -- 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. 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