220 41148 article Path: news.gmane.org!.POSTED!not-for-mail From: =?UTF-8?Q?Micha=C5=82_Gawron?= Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Return this or *this in overridden method in child class? 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[209.85.220.41]) by mx.google.com with SMTPS id r1-v6sor887336ywc.170.2018.11.30.06.10.27 for (Google Transport Security); Fri, 30 Nov 2018 06:10:27 -0800 (PST) Received-SPF: pass (google.com: domain of mcvsama@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:a0d:c644:: with SMTP id i65-v6mr5694315ywd.495.1543587026515; Fri, 30 Nov 2018 06:10:26 -0800 (PST) In-Reply-To: X-Original-Sender: mcvsama@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=GIAE5Fkj; spf=pass (google.com: domain of mcvsama@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=mcvsama@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE 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:41148 Archived-At: Vu, I wonder what you mean by saying "an instance", is that a separate object (a copy) or a reference? In this concrete example, change static_cast to B& instead of B, it will compile and work. Of course it's far from ideal when you have to static_cast to derived class= es. pt., 30 lis 2018 o 14:49 Phil Miller napisa=C5=82(a): > > I believe you could accomplish what you want with CRTP. Template the base= class over its child class type, and have the base class method return a r= eference to that child type. Without having tried it, I think you may need = to add a hidden extra base-of-base class to make it work, but that may be a= voided. > > On Fri, Nov 30, 2018, 7:38 AM Vu Pham > >> While thinking about my (ahem) streaming proposal, I think I found a mis= sing feature in C++. Since this group is highly populated with people speak= ing C++ fluently, I will ask this here instead. >> >> Assume I have the following classes: >> >> class A { >> public: >> A& f1(float& x) { >> x++; >> return *this; >> } >> }; >> >> class B: public A { >> public: >> B& f2(float& x) { >> x--; >> return *this; >> } >> }; >> >> int main() >> { >> float x(100.0f); >> B b; >> static_cast(b.f1(x)).f2(x); // this doesn't compile >> return 0; >> } >> >> The idea is I want B::f1() to return an instance of B (it should, given = the implementation of f1), but since the return type of f1() is A&, apparen= tly there is no way to make it work the way I want. Here are a few alternat= ives I tried: >> >> - Make f1() returns A*, then cast the returned pointer. This doesn't loo= k really nice. >> - Change the return type of f1() to something like decltype(*this) or au= to, none of this seems to work. >> >> This issue may get worse if A and B are templated, or in a more complica= ted scenario where A::f1() returns a child class C of A, while B is a child= of A but not C, and I want to override B::f1() to return D, which is a chi= ld class of B. >> >> Since C++ allows multiple inheritence, and in the same light with the co= ncept idea that are coming up in new C++, should we coin a new annotation t= o indicate that a function returns an instance of this class? The actual re= turned type will change depending on the actual type of the class that mix = those functions in. >> >> What do you think? >> Vu >> >> -- >> You received this message because you are subscribed to the Google Group= s "ISO C++ Standard - Future Proposals" group. >> To unsubscribe from this group and stop receiving emails from it, send a= n 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/iso= cpp.org/d/msgid/std-proposals/CAFhKpXFOV9DSmmrC3HdGF7Fr44fRzpq%3Dn2JTkUcHHE= 6Qbux-PA%40mail.gmail.com. > > -- > 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/isoc= pp.org/d/msgid/std-proposals/CAMDbWJH0JuEzBgDrbRVd6jxNtQ2FMX377iJRuyuRFT3to= s93iA%40mail.gmail.com. --=20 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 e= mail 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/CAB%2BhmstdjiuT7Z8%2B_pyRjCNZJ4p7o7gyHD%2BKhB8yY= RuZEiKRUA%40mail.gmail.com. .