220 29175 article Path: news.gmane.org!.POSTED!not-for-mail From: Andrey Semashev Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: pragma once Date: Fri, 28 Oct 2016 02:06:21 +0300 Lines: 31 Approved: news@gmane.org Message-ID: References: <2c278558-0dbc-4568-9b6b-7f159c94707e@isocpp.org> <4610802.tOzma7na2T@tjmaciei-mobl1> 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 1477609590 28408 195.159.176.226 (27 Oct 2016 23:06:30 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 27 Oct 2016 23:06:30 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBCXY5TXHWYJRB3UQZLAAKGQETXXZ4GY@isocpp.org Fri Oct 28 01:06:26 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oi0-f69.google.com ([209.85.218.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1bztkb-0006IX-2C for gclcip-std-proposals@m.gmane.org; Fri, 28 Oct 2016 01:06:21 +0200 Original-Received: by mail-oi0-f69.google.com with SMTP id d185sf57459599oig.1 for ; Thu, 27 Oct 2016 16:06:24 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=mime-version:in-reply-to:references:from:date:message-id:subject: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=vhIzHgn5xqlJ3WPYsBFeLPWipy0bPdwx2WF4Z2T4FPY=; b=K6wqGkR0Njh2D7eZYDK7Z+PQw4E2v58YFdtiyzaP6/j+tLP9ZeWRiNcLGDZJfDVb74 XYBQu+m77+sMAVOJqm3bNXYtrL8vPs/588v7uk+VHf50npJAMQUFKg5Q73yhOu/cPZoJ PeUq253XPaLXX/vwZ82c+Y+QAQDiipdqfSdQPFVRwEGu4xhgodFCDQO6sv/N+49aCFyO tuAkXNxZLORUADVMi1CTzBDP1V2DVy3n7UXoS9tH6M9XLupMIsmN/z+3riJiE5HiSqN2 EEvK8LCU3V6U/faOxthcdwZUOEUYAtOet1d7F/PNw1dcL8VTCcsL9Itka/mB4wxtut6B eLKQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject: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=vhIzHgn5xqlJ3WPYsBFeLPWipy0bPdwx2WF4Z2T4FPY=; b=BqFf3lcSkDytYsSUbnZ8KBbiz3KoBhC5HTykNUy/71agRnpyFL8+FQhxCOgali3veg Xo9RF3GmBUws5r6daFXgOssb/yNijGxs0gaq4c/XwWSb/XROHFmf4yOQqmShWHUmPYw6 1/HzMwsUAB9e0n2nrHQucEuUJ6jLpv9ubdnNCmaWgS+HDKqougBYS8RjAZ2zGKfdLsaW sN2y8CUdEOKsN1QxNBvUXo0adS0tZKwdHNJKbrquSJLgt2+k8Zmq8NS4CsRsRsU4tlKg gcZhh0tY/lzLKsVHiPMeg2ssiIEKk+DkZP0E85INCLCLolXxt+bC7RCmJblf8wXi0vjv pFrA== X-Gm-Message-State: ABUngve1+tKW/X975XEe4XiCzcecqVO5Zgr3pS9Robh75hIkANLTQRPYIxsGtC3RImPsKw== X-Received: by 10.157.1.15 with SMTP id 15mr3700070otu.147.1477609583449; Thu, 27 Oct 2016 16:06:23 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.24.66 with SMTP id t2ls5464268ott.0.gmail; Thu, 27 Oct 2016 16:06:22 -0700 (PDT) X-Received: by 10.157.58.3 with SMTP id j3mr8853401otc.194.1477609582645; Thu, 27 Oct 2016 16:06:22 -0700 (PDT) Original-Received: from mail-oi0-x230.google.com (mail-oi0-x230.google.com. [2607:f8b0:4003:c06::230]) by mx.google.com with ESMTPS id s34si6354535ots.100.2016.10.27.16.06.22 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 27 Oct 2016 16:06:22 -0700 (PDT) Received-SPF: pass (google.com: domain of andrey.semashev@gmail.com designates 2607:f8b0:4003:c06::230 as permitted sender) client-ip=2607:f8b0:4003:c06::230; Original-Received: by mail-oi0-x230.google.com with SMTP id n202so74757888oig.3 for ; Thu, 27 Oct 2016 16:06:22 -0700 (PDT) X-Received: by 10.36.80.67 with SMTP id m64mr838285itb.59.1477609582111; Thu, 27 Oct 2016 16:06:22 -0700 (PDT) Original-Received: by 10.79.37.81 with HTTP; Thu, 27 Oct 2016 16:06:21 -0700 (PDT) In-Reply-To: X-Original-Sender: andrey.semashev@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of andrey.semashev@gmail.com designates 2607:f8b0:4003:c06::230 as permitted sender) smtp.mailfrom=andrey.semashev@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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:29175 Archived-At: On Fri, Oct 28, 2016 at 1:17 AM, Nevin Liber wrote: > > There are three possibilities: > > 1. It is the same file according to the OS (e.g.: inode and device match). > 2. It must have different contents according to the OS (e.g. the size doesn't > match). I think, the above two bullets cannot be implemented reliably. Event the size may not be available if the source is read from a pipe. > 3. The OS doesn't know if the files have the same content. I'm pretty sure > this would require an extra pass to compute the fingerprint hash, as you > shouldn't be processing the file until you are sure it is unique. You still > have to scan for include guards as that is just normal macro processing. As I shown earlier, basing the decision on the hash can result in incorrect result. IMO, content equivalence is not the right criteria for skipping file inclusion. I think a proposal similar to the previously mentioned "#once guard_phrase" has more potential. It does give the user a way to indicate file equivalence (through the guard name) and also is arguably less expensive than computing a hash value. -- 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. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CAEhD%2B6BXvsZ%2BNJgnbX3yiGQWQRzWODYz-NfG8p3U%2B%2BAu90cKjA%40mail.gmail.com. .