220 41147 article Path: news.gmane.org!.POSTED!not-for-mail From: Phil Miller 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 a13-v6sor3558591ljj.25.2018.11.30.05.49.03 for (Google Transport Security); Fri, 30 Nov 2018 05:49:03 -0800 (PST) Received-SPF: pass (google.com: domain of unmobile@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:a2e:88cf:: with SMTP id a15-v6mr3863630ljk.76.1543585742419; Fri, 30 Nov 2018 05:49:02 -0800 (PST) In-Reply-To: X-Original-Sender: unmobile@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b="eS5Pve/d"; spf=pass (google.com: domain of unmobile@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=unmobile@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:41147 Archived-At: --000000000000db7bba057be20fc2 Content-Type: text/plain; charset="UTF-8" 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 reference 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 avoided. On Fri, Nov 30, 2018, 7:38 AM Vu Pham While thinking about my (ahem) streaming proposal > , > I think I found a missing feature in C++. Since this group is highly > populated with people speaking 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&, > apparently there is no way to make it work the way I want. Here are a few > alternatives I tried: > > - Make f1() returns A*, then cast the returned pointer. This doesn't look > really nice. > - Change the return type of f1() to something like decltype(*this) or > auto, none of this seems to work. > > This issue may get worse if A and B are templated, or in a more > complicated 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 child class of B. > > Since C++ allows multiple inheritence, and in the same light with the > *concept* idea that are coming up in new C++, should we coin a new > annotation to indicate that a function returns an instance of *this* > class? The actual returned 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 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/CAFhKpXFOV9DSmmrC3HdGF7Fr44fRzpq%3Dn2JTkUcHHE6Qbux-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/isocpp.org/d/msgid/std-proposals/CAMDbWJH0JuEzBgDrbRVd6jxNtQ2FMX377iJRuyuRFT3tos93iA%40mail.gmail.com. --000000000000db7bba057be20fc2 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
I believe you could accomplish what you want with CRTP. T= emplate the base class over its child class type, and have the base class m= ethod return a reference to that child type. Without having tried it, I thi= nk you may need to add a hidden extra base-of-base class to make it work, b= ut that may be avoided.

On Fri, Nov 30, 2018, 7:38 AM Vu Pham <phvu225@gmail.com wrote:
=
While thinking about my (ahem) streaming proposal, I think I found a missin= g feature in C++. Since this group is highly populated with people speaking= C++ fluently, I will ask this here instead.

Assum= e I have the following classes:

class A {
public:
=C2=A0=C2=A0=C2=A0 A&= amp; f1(float& x) {
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 x++;<= br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return *this;
=C2=A0=C2=A0= =C2=A0 }
};

class B: public A {
public:
=C2=A0=C2=A0=C2=A0 = B& f2(float& x) {
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 x--= ;
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return *this;
=C2=A0=C2= =A0=C2=A0 }
};

int main()
{
=C2=A0=C2=A0=C2=A0 float x(100.= 0f);
=C2=A0=C2=A0=C2=A0 B b;
=C2=A0=C2=A0=C2=A0 static_cast<B&g= t;(b.f1(x)).f2(x);=C2=A0=C2=A0 // this doesn't compile
=C2=A0=C2= =A0=C2=A0 return 0;
}

The idea is I = want B::f1() to return an instance of B (it should, given the implementatio= n of f1), but since the return type of f1() is A&, apparently there is = no way to make it work the way I want. Here are a few alternatives I tried:=

- Make f1() returns A*, then cast the returned po= inter. This doesn't look really nice.
- Change the return typ= e of f1() to something like decltype(*this) or auto, none of this seems to = work.

This issue may get worse if A and B are temp= lated, or in a more complicated scenario where A::f1() returns a child clas= s 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 child class of B.

Since C= ++ allows multiple inheritence, and in the same light with the concept idea that are coming up in new C++, should we coin a new annotation to i= ndicate that a function returns an instance of this class? The actua= l returned type will change depending on the actual type of the class that = mix those functions in.

What do you think?
Vu

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