220 29275 article Path: news.gmane.org!.POSTED!not-for-mail From: Matthew Woehlke Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: pragma once Date: Wed, 26 Oct 2016 12:29:18 -0400 Lines: 98 Approved: news@gmane.org Message-ID: References: <5810C33B.9060503@gmail.com> <2908d573-f002-4b58-b0da-18420a816c16@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 1477920888 7742 195.159.176.226 (31 Oct 2016 13:34:48 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 31 Oct 2016 13:34:48 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC37LBFWUIFBBWEQ3XAAKGQEE4R6IAI@isocpp.org Mon Oct 31 14:34:41 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f197.google.com ([209.85.220.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1c1Cj8-0006wD-Cr for gclcip-std-proposals@m.gmane.org; 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Wed, 26 Oct 2016 09:30:08 -0700 (PDT) Original-Received: from blaine.gmane.org ([195.159.176.226]) by mx.google.com with ESMTPS id g40si1877478lfi.366.2016.10.26.09.30.08 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 26 Oct 2016 09:30:08 -0700 (PDT) Received-SPF: neutral (google.com: 195.159.176.226 is neither permitted nor denied by best guess record for domain of gclcip-std-proposals@m.gmane.org) client-ip=195.159.176.226; Original-Received: from list by blaine.gmane.org with local (Exim 4.84_2) (envelope-from ) id 1bzR55-0001he-Tw for std-proposals@isocpp.org; Wed, 26 Oct 2016 18:29:35 +0200 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 91 Original-X-Complaints-To: usenet@blaine.gmane.org In-Reply-To: <2908d573-f002-4b58-b0da-18420a816c16@isocpp.org> X-Original-Sender: mwoehlke.floss@gmail.com X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 195.159.176.226 is neither permitted nor denied by best guess record for domain of gclcip-std-proposals@m.gmane.org) smtp.mailfrom=gclcip-std-proposals@m.gmane.org; dmarc=fail (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:29275 Archived-At: On 2016-10-26 11:28, Hans Guijt wrote: > On Wednesday, October 26, 2016 at 4:52:45 PM UTC+2, Matthew Woehlke wrote: >> Please do your due diligence before making claims like this, or, for >> that matter, asking for "obvious" features. I can assure you, this is >> not the first time someone has asked to standardize #pragma once. > > So you are saying this is a common request from the community, which is not > being answered by the standards committee because ...? ....for the reasons I listed. (Which BTW are from memory. I apologize for any fudging of the reasons, but *you* are the one trying to propose the feature; I expect you to do the work identifying and addressing the reasons why this exact request has historically been rejected.) >> What should happen if you include the same header (same inode) via >> different names? What about if the compiler sees two identical include >> directives (same path, same <> vs. "") that resolve to different files >> with different contents? What about platforms with esoteric file >> systems, or *no* file system? > > This is an attempt at complication of a painfully simple feature. No. These are questions that *need to be answered*. > You don't want it, so you just ask meaningless questions that have > trivial answers in the hope of convincing others it is all incredibly > complicated, while it really isn't. I am probably neutral. I am not personally opposed to it (other than my awareness that it has problems), but I don't feel a particular need for it, either. Rather, I am trying to point out the reasons *given by others* that have previously caused this proposal to fail. What I see here is that you a) haven't done your homework, b) refuse to do your homework when others point this out, and c) resort to ad hominem attacks on said others instead. This strikes me as a good way to get people to ignore you entirely. > I don't see a problem with including a file twice if a symlink > exists; after all, you could just copy it and include it twice that > way as well. If you don't see a problem including a header twice, then obviously we don't need #pragma once. (Also see example below.) > I have some trouble imagining how the same header > name could resolve to two different files during the same compilation. // foo/foo.cpp #include // /usr/include/cow/cow.h #include // /usr/include/pork/pig.h #include #include "config.h" // foo/config.h // bar/bar.h #include "thirdparty/cow.h" // bar/thirdparty/cow/include/cow.h #include // -> /usr/include/pork/pig.h #include "config.h" // bar/config.h // foo/config.h ... // bar/config.h ...totally different content from foo/config.h... I've shown three corner cases here: - Different files, different include paths; content may or may not be the same - Different files, same include paths, different content - Same files, different include paths The first two are not especially contrived, and this is not necessarily an exhaustive list. It is well specified how traditional include guards will handle these cases. What about #pragma once? Hand-waving is not a sufficient answer. > The whole thing is completely trivial It isn't. Please *do your homework* (and don't expect us to do it for you), identify the issues that have previously prevented this from being standardized, and come back when you have solutions to those problems. Otherwise you are just wasting everyone's time rehashing an old proposal that has already been rejected. -- Matthew -- 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/nuqlkr%24sg7%241%40blaine.gmane.org. .