220 24880 <357de2ac-1b24-48bb-a99d-1b9ede35d142@isocpp.org> article Path: news.gmane.org!not-for-mail From: Giovanni Piero Deretta Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Go-like embedding Date: Wed, 2 Mar 2016 07:11:34 -0800 (PST) Lines: 242 Approved: news@gmane.org Message-ID: <357de2ac-1b24-48bb-a99d-1b9ede35d142@isocpp.org> References: <245e65c5-c96d-4366-b616-814606d4ebe9@isocpp.org> <0ee10bc5-b45e-430e-bb08-69a80863cecb@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_6327_290501325.1456931494651" X-Trace: ger.gmane.org 1456931507 17891 80.91.229.3 (2 Mar 2016 15:11:47 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 2 Mar 2016 15:11:47 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDEP3I7TGEIKPBO4WYCRUBAUHELZA@isocpp.org Wed Mar 02 16:11:44 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yk0-f197.google.com ([209.85.160.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ab8RC-00062R-EU for gclcip-std-proposals@m.gmane.org; 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Wed, 02 Mar 2016 07:11:36 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.25.200 with SMTP id 191ls2183382ioz.19.gmail; Wed, 02 Mar 2016 07:11:35 -0800 (PST) X-Received: by 10.50.43.194 with SMTP id y2mr17445igl.9.1456931495233; Wed, 02 Mar 2016 07:11:35 -0800 (PST) In-Reply-To: <0ee10bc5-b45e-430e-bb08-69a80863cecb@isocpp.org> X-Original-Sender: gpderetta@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:24880 Archived-At: ------=_Part_6327_290501325.1456931494651 Content-Type: multipart/alternative; boundary="----=_Part_6328_1560416818.1456931494652" ------=_Part_6328_1560416818.1456931494652 Content-Type: text/plain; charset=UTF-8 On Wednesday, March 2, 2016 at 2:57:51 PM UTC, TONGARI J wrote: > > On Wednesday, March 2, 2016 at 9:46:29 PM UTC+8, Nicol Bolas wrote: >> >> On Wednesday, March 2, 2016 at 8:27:30 AM UTC-5, NeoCode wrote: >>> >>> In Go programming language, there is a feature named "emedding", which >>> is used instead of classic c++-like inheritance. >>> For example: >>> >>> type SmallBox struct { >>> a, b, c int >>> } >>> type BigBox struct { >>> Timestamp time.Time >>> SmallBox // <<< embedding >>> Name []string >>> } >>> >>> This is very simple and low-level extension of the concept of >>> inheritance; the programmer explicitly specifies the location of the base >>> object in the successor. It is strange that this simple and obvious >>> opportunity not appeared in the C language. >>> >>> I propose enhanced (compared with Go) feature of such embedding. We can >>> use the existing "inline" keyword (or some other keyword, eg. "embed") to >>> indicate the embedded fields. >>> >>> struct SmallBox { >>> int a, b, c; >>> }; >>> >>> struct BigBox { >>> time_t Timestamp; >>> inline SmallBox inlinedBox; >>> std::string Name; >>> }; >>> >>> As a result, we can access to the fields of SmallBox both directly and >>> through an intermediate name "inlinedBox": >>> >>> BigBox box; >>> box.a = 10; //directly, like inheritance >>> box.inlinedBox.b = 10; // using an intermediate name, like aggregation >>> >>> >>> >> ... what's wrong with inheritance? I know people tend to believe that >> using inheritance for something like this is wrong, but these cases are >> *exactly* why C++ has robust support for multiple inheritance, unlike >> most languages. >> > > I think it's for object layout. Using inheritance would force you to > layout `SmallBox` before `Timestamp` in the example (unless you make > `Timestamp` another base). > > Also in many cases will prevent the object from being a standard layout type. -- gpd -- 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/357de2ac-1b24-48bb-a99d-1b9ede35d142%40isocpp.org. ------=_Part_6328_1560416818.1456931494652 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Wednesday, March 2, 2016 at 2:57:51 PM UTC, TONGARI J w= rote:
On Wedne= sday, March 2, 2016 at 9:46:29 PM UTC+8, Nicol Bolas wrote:
On Wednesday, March 2, 2016 at 8:2= 7:30 AM UTC-5, NeoCode wrote:
In Go programming language,=C2=A0there is a feature named "= emedding",=C2=A0which is used instead of classic c++-like inheritance.=
For example:

type SmallBox struct<= span style=3D"color:rgb(51,51,51);font-family:Menlo,Monaco,'Courier New= ',monospace;font-size:12px;line-height:22.3999996185303px;white-space:p= re-wrap;background-color:rgb(248,248,248)"> {
=C2=A0 =C2=A0 = =C2=A0 =C2=A0 a, b, c int
}
type BigBox struct {=
=C2=A0 =C2=A0 =C2=A0 =C2=A0 Timestamp time.Time
=C2=A0 =C2=A0 = =C2=A0 =C2=A0 SmallBox =C2=A0// <<< em= bedding
=C2=A0 =C2=A0 =C2=A0 =C2=A0 Name []string
}
=

This is very simple and low-level extension of the concept= of inheritance;=C2=A0the programmer explicitly specifies the location of t= he base object in the successor.=C2=A0It is strange that this simple and ob= vious opportunity not appeared in the C language.

I propose enhanced (compared with Go) feature of such embedding.=C2= =A0We can use the existing "inline" keyword (or some other keywor= d, eg. "embed") to indicate the embedded fields.
struct SmallBox {
=C2= =A0 int= a, b, c;
<= span style=3D"color:#660">};

struct BigBox {
=C2=A0t= ime_t Timestamp;
=C2=A0inline SmallBox inlinedBox;
=C2=A0std::string<= /span> Name;
};

As a result, we can access to the fields of= =C2=A0SmallBox=C2=A0both directly and through an intermediate name "in= linedBox":

<= code>
BigBox<= span style=3D"color:#000"> box;
box.a =3D 10; //directly, like inheritance
box= .inline= dBox.b = =3D 10;= // using an intermediate name, lik= e aggregation



... what's wrong with inheritance? I know people tend to b= elieve that using inheritance for something like this is wrong, but these c= ases are exactly why C++ has robust support for multiple inheritance= , unlike most languages.

I think it's for o= bject layout. Using inheritance would force you to layout `SmallBox` before= `Timestamp` in the example (unless you make `Timestamp` another base).


Also in many cases will prevent the object from be= ing a standard layout type.

-- gpd

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/357de2ac-1b24-48bb-a99d-1b9ede35d142= %40isocpp.org.
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