220 40780 <2314717.GH8Ag49nql@tjmaciei-mobl1> article Path: news.gmane.org!.POSTED!not-for-mail From: Thiago Macieira Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: a string type for APIs Date: Fri, 26 Oct 2018 10:59:05 -0700 Lines: 74 Approved: news@gmane.org Message-ID: <2314717.GH8Ag49nql@tjmaciei-mobl1> References: <13400688.pJn7XIzHAo@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 1540576628 9983 195.159.176.226 (26 Oct 2018 17:57:08 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 26 Oct 2018 17:57:08 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCB4TK757YBRB4NLZXPAKGQEV2AW3GI@isocpp.org Fri Oct 26 19:57:04 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qt1-f200.google.com ([209.85.160.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1gG6M4-0002UW-0j for gclcip-std-proposals@m.gmane.org; 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[209.222.10.136]) by mx.google.com with ESMTPS id q11si53868qvr.54.2018.10.26.10.59.11 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 26 Oct 2018 10:59:11 -0700 (PDT) Received-SPF: pass (google.com: domain of thiago@macieira.org designates 209.222.10.136 as permitted sender) client-ip=209.222.10.136; Original-Received: from tjmaciei-mobl1.localnet (jfdmzpr05-ext.jf.intel.com [134.134.139.74]) by smtp.macieira.info (Postfix) with ESMTPSA id 429401B497 for ; Fri, 26 Oct 2018 10:59:10 -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 209.222.10.136 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-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:40780 Archived-At: On Friday, 26 October 2018 09:41:02 PDT Roberto Hinz wrote: > It's possible to implement a function that instantiates an api_string with > an > alternative allocator. But I would leave such function in a separate header. > A copy of allocator object would not be stored as member variable of > api_string, but inside the allocated memory, together with the reference > counters. So you need an extra memory allocation to use this class? Please account for that when discussing the benefits and disadvantages of your API. > > But I don't see compilation time as an argument. It is most definitely > > QoI: > > for example, a compiler can react to #include by inserting a pre- > > compiled implementation into the AST. > > I think only a compiler implementer can tell us how feasible is that. > For example, how could a clang user switch from libc++ to libstdc++ > if std::string is pre-compiled ? If they didn't solve the problem yet, > probably it's because it's not easy. They didn't implement it because they didn't want it. The switch you mentioned is part of their objectives, as is the decoupling between the compiler and the standard library. Implementing a non-file #include mechanism would go against that. What I am saying is that a compiler could choose to do that, under different circumstances. > > So, can you describe the benefits your API would have or problems it would > > solve, without referring to build times and ABI? > > 1) Its copy constructor is always fast and never throws ( because it only > needs to increment the reference count ). This makes it ideal to be > stored inside exceptions I personally like it this way. As you may be aware, QString is reference- counted, as are most Qt container types. But I am definitely not in the majority. The view of the majority of committee members and others in this list is that reference counting is bad, especially if (or is it "only if"?) it's coupled with Copy-on-Write. > 2) If you want to create a api_string from a raw string that has static > storage duration - like a string literal, which is very common - then you > can skip the memory allocation and copy, by using the api_string_ref() > function. This creates an api_string in "reference mode", that only > references the char array without ever deleting it. QString has that too, called fromRawData(), which allocates a memory block for the refcount and tracks that, but doesn't own the character data itself. I'm changing it for Qt 6: instead, if the control block pointer is null, there's no reference counting needed. This could lead to further code merges between QString and QStringView, though prototypes need to be written still. > In sum, I would say api_string aims to be ideal return string type. It's worse than std::string_view, which has a trivial copy constructor. But it does support returning an allocated string and passing on its lifetime, which std::string_view can't do. -- 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/2314717.GH8Ag49nql%40tjmaciei-mobl1. .