220 35111 article Path: news.gmane.org!.POSTED!not-for-mail From: Marc Mutz Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Marking more std::string_view members noexcept Date: Thu, 26 Oct 2017 10:55:04 +0200 Organization: KDAB Lines: 67 Approved: news@gmane.org Message-ID: References: <3d03cd7d-2177-98fd-408d-f74b1f6a8552@gmail.com> <5005582.pCRZNEiGFO@tjmaciei-mobl1> <3446274.dRsjijLxpX@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"; format=flowed X-Trace: blaine.gmane.org 1509008119 30361 195.159.176.226 (26 Oct 2017 08:55:19 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 26 Oct 2017 08:55:19 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCRPXDOU7ECRB4OFY3HQKGQEB6KNNZI@isocpp.org Thu Oct 26 10:55:15 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wm0-f69.google.com ([74.125.82.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1e7dwQ-0006ST-IZ for gclcip-std-proposals@m.gmane.org; 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[176.9.126.58]) by mx.google.com with ESMTPS id d197si710999wme.168.2017.10.26.01.55.12 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 26 Oct 2017 01:55:12 -0700 (PDT) Received-SPF: pass (google.com: domain of marc.mutz@kdab.com designates 176.9.126.58 as permitted sender) client-ip=176.9.126.58; X-Virus-Scanned: amavisd-new at kdab.com In-Reply-To: <3446274.dRsjijLxpX@tjmaciei-mobl1> X-Sender: marc.mutz@kdab.com X-Original-Sender: marc.mutz@kdab.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of marc.mutz@kdab.com designates 176.9.126.58 as permitted sender) smtp.mailfrom=marc.mutz@kdab.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:35111 Archived-At: On 2017-10-26 05:04, Thiago Macieira wrote: > 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. In QStringView, it's just an optimisation, because... > Calculating the length of char16_t(&)[N] is easy: N - 1. .... this is C++11-constexpr (which we still need to care for, in Qt) while ... > Calculating the length of char16_t* can't be done in a constexpr way. .... is only C++14-constexpr (well, if you want to support reasonably long strings, otherwise you could come up with a recursive implementation). We should probably document that the QStringView array ctor has the precondition that it contains no embedded NULs, but it seems qdoc is too clever for it's own good: https://doc-snapshots.qt.io/qt5-5.10/qstringview.html doesn't have the docs for the array ctor. >> 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); > } Because ordinary functions are preferred over templates. You need constrained templates to overload on pointer vs. array: https://www.kdab.com/qstringview-diaries-masters-overloads/ (can't seem to link to a section, C-F for 'third'). Thanks, Marc -- 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/a82f7b048d91673203c75fa04e98f481%40kdab.com. .