220 24881 <0b4a0bab-fa39-4214-b6dd-51ffdfb078e8@isocpp.org> article Path: news.gmane.org!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Go-like embedding Date: Wed, 2 Mar 2016 11:25:16 -0800 (PST) Lines: 250 Approved: news@gmane.org Message-ID: <0b4a0bab-fa39-4214-b6dd-51ffdfb078e8@isocpp.org> References: <245e65c5-c96d-4366-b616-814606d4ebe9@isocpp.org> <2bbe1861-b3b8-49cf-8405-ab840bd7b5fd@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_24_708486682.1456946716143" X-Trace: ger.gmane.org 1456946720 25096 80.91.229.3 (2 Mar 2016 19:25:20 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 2 Mar 2016 19:25:20 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBHP43S3AKGQE7F3PVLI@isocpp.org Wed Mar 02 20:25:20 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ig0-f199.google.com ([209.85.213.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1abCOd-0003Gm-K3 for gclcip-std-proposals@m.gmane.org; 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Wed, 02 Mar 2016 11:25:18 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.112.42 with SMTP id in10ls657121igb.13.canary; Wed, 02 Mar 2016 11:25:17 -0800 (PST) X-Received: by 10.50.41.106 with SMTP id e10mr72977igl.2.1456946717238; Wed, 02 Mar 2016 11:25:17 -0800 (PST) In-Reply-To: <2bbe1861-b3b8-49cf-8405-ab840bd7b5fd@isocpp.org> X-Original-Sender: jmckesson@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:24881 Archived-At: ------=_Part_24_708486682.1456946716143 Content-Type: multipart/alternative; boundary="----=_Part_25_454662279.1456946716143" ------=_Part_25_454662279.1456946716143 Content-Type: text/plain; charset=UTF-8 On Wednesday, March 2, 2016 at 8:59:35 AM UTC-5, NeoCode wrote: > > On Wednesday, March 2, 2016 at 4:46:29 PM UTC+3, 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. >> > > Nothing wrong with inheritance. It's just an additional opportunity. > "An additional opportunity" to do what, exactly? If we already have a way to do what you're trying to do, then we don't need to add a new feature to the language. And if there's something you're trying to do that *cannot* be done currently, then that should be used as justification for the addition. As well as some reason to believe that lacking this functionality is a problem of significant importance that it requires a language feature to solve. "Because has that feature" is not a good enough reason to add the feature to C++. -- 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/0b4a0bab-fa39-4214-b6dd-51ffdfb078e8%40isocpp.org. ------=_Part_25_454662279.1456946716143 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Wednesday, March 2, 2016 at 8:59:35 AM UTC-5, NeoCode w= rote:
On Wedne= sday, March 2, 2016 at 4:46:29 PM UTC+3, 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.

No= thing wrong with inheritance. It's just an additional opportunity.

"An additional opportunity" to do wh= at, exactly? If we already have a way to do what you're trying to do, t= hen we don't need to add a new feature to the language. And if there= 9;s something you're trying to do that cannot be done currently,= then that should be used as justification for the addition. As well as som= e reason to believe that lacking this functionality is a problem of signifi= cant importance that it requires a language feature to solve.

"= Because <insert language here> has that feature" is not a good e= nough reason to add the feature to C++.

--
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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/0b4a0bab-fa39-4214-b6dd-51ffdfb078e8= %40isocpp.org.
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