220 24879 <0ee10bc5-b45e-430e-bb08-69a80863cecb@isocpp.org> article Path: news.gmane.org!not-for-mail From: TONGARI J Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Go-like embedding Date: Wed, 2 Mar 2016 06:57:51 -0800 (PST) Lines: 284 Approved: news@gmane.org Message-ID: <0ee10bc5-b45e-430e-bb08-69a80863cecb@isocpp.org> References: <245e65c5-c96d-4366-b616-814606d4ebe9@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_4710_755491452.1456930671711" X-Trace: ger.gmane.org 1456930681 3925 80.91.229.3 (2 Mar 2016 14:58:01 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 2 Mar 2016 14:58:01 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCC3NA775MLBB4H63O3AKGQEXEWTASI@isocpp.org Wed Mar 02 15:58:00 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qg0-f69.google.com ([209.85.192.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ab8Dv-0006Wi-K6 for gclcip-std-proposals@m.gmane.org; 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Wed, 02 Mar 2016 06:57:53 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.43.162 with SMTP id x2ls560187igl.34.gmail; Wed, 02 Mar 2016 06:57:52 -0800 (PST) X-Received: by 10.50.98.70 with SMTP id eg6mr15207igb.4.1456930672212; Wed, 02 Mar 2016 06:57:52 -0800 (PST) In-Reply-To: <245e65c5-c96d-4366-b616-814606d4ebe9@isocpp.org> X-Original-Sender: tongari95@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:24879 Archived-At: ------=_Part_4710_755491452.1456930671711 Content-Type: multipart/alternative; boundary="----=_Part_4711_2083531829.1456930671712" ------=_Part_4711_2083531829.1456930671712 Content-Type: text/plain; charset=UTF-8 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). I'm not familiar with Go, but if this is the OP's intent, I'd suggest taking a step further to make the feature help data packing, e.g. struct A { uint32_t a1; char a2; }; struct B { uint32_t b1; inline A; char b2; }; may allow the size of B to be 12 instead of 16. -- 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/0ee10bc5-b45e-430e-bb08-69a80863cecb%40isocpp.org. ------=_Part_4711_2083531829.1456930671712 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Wednesday, March 2, 2016 at 9:46:29 PM UTC+8, Nicol Bol= as wrote:
On W= ednesday, March 2, 2016 at 8:27:30 AM UTC-5, NeoCode wrote:
In Go programming language,=C2=A0t= here is a feature named "emedding",=C2=A0which is used instead of= classic c++-like inheritance.
For example:

type SmallBox struct {
=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.Ti= me
=C2=A0 =C2=A0 =C2=A0 =C2=A0 SmallBox =C2=A0// <<< embedding
= =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 explicit= ly specifies the location of the base object in the successor.=C2=A0It is s= trange that this simple and obvious opportunity not appeared in the C langu= age.

I propose enhanced (compared with Go) f= eature of such embedding.=C2=A0We can use the existing "inline" k= eyword (or some other keyword, eg. "embed") to indicate the embed= ded fields.

struct SmallBox {
=C2=A0 int<= span style=3D"color:#000"> a, b, c;
};

struct BigBox {
=C2=A0time_t Timest= amp; =C2=A0inline SmallBox inlinedBox;
=C2=A0std::string Name;
};

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

=
BigBox box;
box.a =3D 10; //directly, like inheritance
box.inlinedBox.<= span style=3D"color:#000">b =3D 10; // u= sing an intermediate name, = <= span style=3D"color:#800">like aggregation
=


... what's wrong with inherit= ance? I know people tend to believe that using inheritance for something li= ke this is wrong, but these cases are exactly why C++ has robust sup= port for multiple inheritance, unlike most languages.

I think it's for object layout. Using inheritance would for= ce you to layout `SmallBox` before `Timestamp` in the example (unless you m= ake `Timestamp` another base).

I'm not familiar with Go, but if this is the = OP's intent, I'd suggest taking a step further to make the feature = help data packing, e.g.

struct A
{
=C2=A0 =C2=A0 uint32_t a1;
=C2=A0 =C2=A0 char a2;
};

struct B
{
=C2= =A0 =C2=A0 uint32_t b1;
=C2=A0 =C2=A0 inline A;=
=C2=A0= =C2=A0 ch= ar b2;
};
=

may allow the size of B to be 12 instead of 16.=C2= =A0

--
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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/0ee10bc5-b45e-430e-bb08-69a80863cecb= %40isocpp.org.
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