220 35114 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 11:55:08 +0200 Organization: KDAB Lines: 53 Approved: news@gmane.org Message-ID: References: <3d03cd7d-2177-98fd-408d-f74b1f6a8552@gmail.com> <5005582.pCRZNEiGFO@tjmaciei-mobl1> <3446274.dRsjijLxpX@tjmaciei-mobl1> <545c118e-8d6d-456b-a640-0e3c0784f583@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"; format=flowed X-Trace: blaine.gmane.org 1509011734 27760 195.159.176.226 (26 Oct 2017 09:55:34 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 26 Oct 2017 09:55:34 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCRPXDOU7ECRBBHCY3HQKGQE6WVGTXA@isocpp.org Thu Oct 26 11:55:27 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr0-f198.google.com ([209.85.128.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1e7esc-0004cj-Fp for gclcip-std-proposals@m.gmane.org; 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[176.9.126.58]) by mx.google.com with ESMTPS id o64si686446wmo.121.2017.10.26.02.55.16 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 26 Oct 2017 02:55:16 -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: <545c118e-8d6d-456b-a640-0e3c0784f583@isocpp.org> 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:35114 Archived-At: On 2017-10-26 11:30, pavel.kryukov@phystech.edu wrote: >> This one still has the precondition that [Char, N) is a valid range. > It >> might not be: > >> char bug[512]; >> auto s = string_view(bug); > > Should std::string_view have such precondition, isn't it just a > wrapper around pointer and size which may dangle? Yes, it should, IMHO. Consider; f(std::string); // wide contract: all strings are acceptable f(const char *s, size_t n); // narrow contract: [s,n) must be valid range f(std::string_view); // wide or narrow contract? By putting the precondition on the string_view ctor, we gain wide contracts for any string_view function. This means they can be noexcept, and that is much more valuable than having a noexcept string_view ctor (which is inline), because string_view-taking functions are often out-of-line, so marking them noexcept (where no additional preconditions prevent it) actually helps the compiler. > You may construct a nullptr string_view using argument-less ctor (it > is even noexcept) Yes, and? > or get a valid string_view from a valid string, but > then break it. > > auto tmp = new std::string("Hello World"); > std::string_view sv(tmp.c_str()); > delete tmp; Yes, this is a problem. But I'm not sure that it actually can be formulated with the term 'precondition'. The closest I can come up with is an external class invariant, which is a bit of an oxymoron to start with, because classes exist to establish and maintain their invariants. But it's not really different from std::ref, say. 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/e989bea8667eb2c1c36f370e259e00c2%40kdab.com. .