220 35108 <3446274.dRsjijLxpX@tjmaciei-mobl1> article Path: news.gmane.org!.POSTED!not-for-mail From: Thiago Macieira Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Marking more std::string_view members noexcept Date: Wed, 25 Oct 2017 20:04:40 -0700 Lines: 70 Approved: news@gmane.org Message-ID: <3446274.dRsjijLxpX@tjmaciei-mobl1> References: <3d03cd7d-2177-98fd-408d-f74b1f6a8552@gmail.com> <5005582.pCRZNEiGFO@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 1508987091 30757 195.159.176.226 (26 Oct 2017 03:04:51 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 26 Oct 2017 03:04:51 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCB4TK757YBRBTFBYXHQKGQEPLR4BEQ@isocpp.org Thu Oct 26 05:04:44 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr0-f197.google.com ([209.85.128.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1e7YTG-0006YG-GN for gclcip-std-proposals@m.gmane.org; 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[2a01:4f8:d13:f81:21c:14ff:fe01:12a3]) by mx.google.com with ESMTP id o1si255213wmf.41.2017.10.25.20.04.44 for ; Wed, 25 Oct 2017 20:04:44 -0700 (PDT) Received-SPF: pass (google.com: domain of thiago@macieira.org designates 2a01:4f8:d13:f81:21c:14ff:fe01:12a3 as permitted sender) client-ip=2a01:4f8:d13:f81:21c:14ff:fe01:12a3; Original-Received: from tjmaciei-mobl1.localnet (unknown [IPv6:2601:1c0:4501:5f9f:d45e:bf18:2219:b394]) by gondolin.macieira.info (Postfix) with ESMTPSA id 1F24111B5A6 for ; Wed, 25 Oct 2017 20:04:44 -0700 (PDT) In-Reply-To: X-Original-Sender: thiago@macieira.org X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of thiago@macieira.org designates 2a01:4f8:d13:f81:21c:14ff:fe01:12a3 as permitted sender) smtp.mailfrom=thiago@macieira.org 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:35108 Archived-At: On Wednesday, 25 October 2017 19:30:21 PDT Nevin Liber wrote: > > QStringView has both a pointer and an array constructor. That means: > > const char16_t str[] = u"Hello\0World"; > > > > QStringView(str).size() == 11 // constexpr > > QStringView(+str).size() == 5 // not constexpr > > I'm not sure what constexpr has to do with it. Isn't it array vs. pointer > that matters? Just a dig at the inability to overload on constexprness. Calculating the length of char16_t(&)[N] is easy: N - 1. Calculating the length of char16_t* can't be done in a constexpr way. > It also means > > QStringView(b ? u"Hello\0World" : u"Howdy\0World") has subtly different > behavior than QStringView(b ? u"Hello\0World" : u"Goodbye\0World"), doesn't > it? Uh... I don't know. godbolting to the rescue... Why does this call the pointer version? template void f(const char (&)[N]); void f(const char *); void g(bool b) { const char str[] = "World"; f(str); } > > If we had added the constructor at the moment the class was introduced, > > there > > would have been no legacy to deal with. Now there will be, so it needs to > > be > > done with great care. > > Again, I think matching an array is the wrong solution to the question of > "is this a string literal"? Indeed. > A better solution might be a magic type that, in template contexts, matches > better than an array but easily decays to an array. OTOH, it might not be > (I haven't really thought about it much). And it still wouldn't address > the ternary operator problem I mentioned above. libstdc++ has a GCC-specific solution for string_view: __builtin_constant_p. Using that, they can determine whether it is a string literal and they can even do a constexpr strlen() for those. They do stop at a null, but that's by design. See the tricks here: https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/ char_traits.h#L220-L252 -- Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org Software Architect - Intel Open Source Technology Center -- 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/3446274.dRsjijLxpX%40tjmaciei-mobl1. .