220 19419 article Path: news.gmane.org!not-for-mail From: Matthew Woehlke Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Testing for supported features: Per-feature macros? Sentinel compilation? 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[80.91.229.3]) by mx.google.com with ESMTPS id la8si22307735lac.132.2015.07.29.12.26.36 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Wed, 29 Jul 2015 12:26:36 -0700 (PDT) Received-SPF: pass (google.com: domain of gclcip-std-proposals@m.gmane.org designates 80.91.229.3 as permitted sender) client-ip=80.91.229.3; Original-Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1ZKWzq-0004hm-3A for std-proposals@isocpp.org; Wed, 29 Jul 2015 21:26:34 +0200 Original-Received: from tripoint.kitware.com ([66.194.253.20]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Wed, 29 Jul 2015 21:26:34 +0200 Original-Received: from mwoehlke.floss by tripoint.kitware.com with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Wed, 29 Jul 2015 21:26:34 +0200 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 51 Original-X-Complaints-To: usenet@ger.gmane.org X-Gmane-NNTP-Posting-Host: tripoint.kitware.com User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 In-Reply-To: X-Original-Sender: mwoehlke.floss@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of gclcip-std-proposals@m.gmane.org designates 80.91.229.3 as permitted sender) smtp.mail=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-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:19419 Archived-At: On 2015-07-27 00:15, denis bider wrote: > On 2015-07-26 23:21, Thiago Macieira wrote: >> I'm skeptical how much the compiler could recover in case >> of a syntax it does not understand with your proposal. > > If it cannot parse something within the block, it would note that as a test > block error (not a compilation error); skip the current line, and skip any > further lines until the first one that's a preprocessor directive. > > The first line that's a preprocessor directive marks the end of the test > block. You do realize that by the time the compiler sees the code, the PP has already run? You would need something other than PP markup to denote the beginning and end of feature-test blocks. TBH I think the idea is interesting, but I just don't see how it would ever work except to have the compiler interrupt compiling, spin up a separate "process" (not necessarily in the POSIX sense) to attempt to compile some external, self-contained hunk of code, and act on the result. The problem there is that the standard has strongly resisted specifying anything about how the compiler interacts with the system, i.e. anything beyond it consuming text from an input stream. Ergo I think you would have a serious uphill battle to get such a feature accepted. (Similarly, there is a lot of resistance to any PP changes.) > If I have control over the build system, I don't have the problem to begin > with. In that case, I can also choose the compiler, and I know what > features it implements and what it doesn't. > > The problem mainly exists for libraries; especially cross-platform > libraries, and such a library can't rely on a single build system. I take it you are talking about the case of your library headers being used by some consumer? Is that even *wise* to be changing your build configuration at that point? In my experience, it's more typical that users are required to use a compiler at least as capable as the one used to build the library, and the library hard-codes what features were enabled when it was built. (Some may even attempt to force matching compiler abilities.) Incidentally, CMake-based projects certainly encourage consumers to also use CMake :-). (And CMake has feature tests. You can even arrange for requests to use your library - i.e. by the consumer's build - to re-run feature tests in the consumer's environment and propagate the results to anyone using your library.) -- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. .