220 33241 <85d371d4-0fa1-470a-9e02-559dd5415857@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: "'Michael Davies' via ISO C++ Standard - Future Proposals" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Lambda "Injection/Inheritance/Override" function idea proprosal. Date: Mon, 17 Jul 2017 16:08:13 -0700 (PDT) Lines: 285 Approved: news@gmane.org Message-ID: <85d371d4-0fa1-470a-9e02-559dd5415857@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_943_1160982901.1500332893513" X-Trace: blaine.gmane.org 1500332900 17272 195.159.176.226 (17 Jul 2017 23:08:20 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 17 Jul 2017 23:08:20 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCS45AVH5YKBBXUGWXFQKGQEXZCBTWQ@isocpp.org Tue Jul 18 01:08:16 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f200.google.com ([209.85.192.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dXF7a-000408-2l for gclcip-std-proposals@m.gmane.org; Tue, 18 Jul 2017 01:08:10 +0200 Original-Received: by mail-pf0-f200.google.com with SMTP id p1sf4464013pfl.2 for ; Mon, 17 Jul 2017 16:08:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=mnVGfYTZx2dgJQS+zKE2iVF+V4LMjsYXq/hzbsDac3Y=; b=NdEimvJhBExzLvHLfGWgU/oWCd+7hVMxQApuGNy4VKBYH5W2bcMuYk2AzfLlm93h0K wBZkmeWyifkIky9YNhrL1i65HBsfyDLA4mdXugXlvd+8sXfCcbXWKY8hlPCBz7zMuqov FPGmOHfmcMUChUHkz/WSTmxwBP1GLeVm9ThDjUBNyLAz9PIQE5PMMMz++L3ManqEkbiv YAe8a3KeOWEEyET3ZdAyTwCollKTWhYaVSjlEP/dJduWV8VvGSpzQ3+qouNHlkG9ufXz zxfvyBQQ+91JCZvYqH99PodPkRk3h3/CF9/4orTAJjqMxAI3K3os9FGJIrrWqcrtMm+b sdlA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=mnVGfYTZx2dgJQS+zKE2iVF+V4LMjsYXq/hzbsDac3Y=; b=Yqsyn7m01Xy5paH9Tev8xffYWVDhlP7LtkHFvXcE0OccspfK8Dvaz2p9rKswNJnTHV gpNXsB6jv20bdzJc84es/Zm1h2XTvv+np8WMCklj/+xSeSc1AVHAvkLEpBd4OphdvdGo WwcbxQLTVB5n4CG6ZUPWJ67A4cNot+yBkmoH3YyHkKLzuj93v3te1vEVnFhvc8XtncEI Kp8Inl780HI+vITuJZM/s4UJh/MT274mfC2Sw2+epZNClnVFC+3xzPrCeFOju8REWaYd 8e4yxFpkaZIGOtydJmlSyoTM7sb1hK5BuYCMfDn5aRK1fAlveGaeUJf+O5v3/oCsFimd Ekig== X-Gm-Message-State: AIVw113MfzRqOI/GBsj3czTxRtHNkA/rrhhNRZsJCFpbcn4jqjCsLn2x f9tcacRv+N/UOqRm X-Received: by 10.99.96.23 with SMTP id u23mr6391305pgb.17.1500332895342; Mon, 17 Jul 2017 16:08:15 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.122.208 with SMTP id a199ls1266789itc.5.gmail; Mon, 17 Jul 2017 16:08:14 -0700 (PDT) X-Received: by 10.31.77.2 with SMTP id a2mr90416vkb.5.1500332893948; Mon, 17 Jul 2017 16:08:13 -0700 (PDT) In-Reply-To: X-Original-Sender: MichaelDavies32@googlemail.com X-Original-From: Michael Davies Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: 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:33241 Archived-At: ------=_Part_943_1160982901.1500332893513 Content-Type: multipart/alternative; boundary="----=_Part_944_1060658338.1500332893513" ------=_Part_944_1060658338.1500332893513 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Hi Arthur, Thanks for the response! In regards to this, I was thinking more along the= =20 lines of treating the lambda function as if you were actually defining a=20 function definition within the class. So: [] () {=20 m_iWin +=3D1; std::cout << "Overloaded: " << m_iWin << '\n'; } Would be more equivalent to treating the lambda function as if it were a=20 direct function definition: TestOverride::winProcess(){ m_iWin +=3D 1; std::cout << "Overloaded: " << m_iWin << '\n'; } Perhaps there should be some way of defining the lambda as if it were=20 actually a function that was part of the class? Rather than those data=20 members being accessible by main() they are only accessible within the very= =20 lambda you create rather than accessible by main(). On Monday, July 17, 2017 at 11:52:39 PM UTC+1, Arthur O'Dwyer wrote: > > > > On Monday, July 17, 2017 at 6:09:25 AM UTC-7, Michael Davies wrote: >> >> Hey everyone, >> >> I would like to hear your thoughts on an idea I've been toiling away=20 >> over. Playing around with some code I was curious if I could try to crea= te=20 >> a small library filled with a few classes and override the default funct= ion=20 >> behaviour of those classes. This was for test purposes so I could have a= =20 >> similar sort of design where I work if it was useful. Through experiment= ing=20 >> I got something like this which is honestly not ideal: >> > > https://wandbox.org/permlink/B9u9orFV5Zd3L5Fi > > What would be more ideal, is if I could create a new function without=20 >> having to define and declare references to things within the class to ma= ke=20 >> them available to the lambda. >> > > You already can! The only problem with your existing code is that you're= =20 > trying to refer to private data members of your class from outside the=20 > class, which is fundamentally never allowed in C++ because it would break= =20 > encapsulation. If you want those data members to be accessible by main(),= =20 > then you need to make them *public*, or else make main() a *friend* of=20 > your class. Here's an example with "friend": > > https://wandbox.org/permlink/0KlqS0oL3H3BvaaQ > > > The relevant snippet is: > > TEST.OverrideWinProcess( > [&x =3D TEST]() { > x.m_iWin +=3D 1; > std::cout << "Overloaded: " << x.m_iWin << '\n';=20 > > } > ); > > If you're not a *friend* of the class, then of course you can't access=20 > its private members; that's by design and will never, ever change. > > HTH, > =E2=80=93Arthur > --=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/85d371d4-0fa1-470a-9e02-559dd5415857%40isocpp.or= g. ------=_Part_944_1060658338.1500332893513 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Hi Arthur,

Thanks for the response! In = regards to this, I was thinking more along the lines of treating the lambda= function as if you were actually defining a function definition within the= class. So:

[] = ()<= span style=3D"color: #000;" class=3D"styled-by-prettify"> {
m_iWin +=3D1;
std::cout << "Overloaded: " << m_iWin << '\n';
}=

Would be more equivalent to treating the lamb= da=C2=A0function as if it were a direct function definition:

=
TestOverride::winProcess(){
=C2=A0 =C2=A0 m_iWin +=3D 1;
=C2=A0 =C2=A0 std::cout << &quo= t;Overloaded: " << = m_iWin <= ;< '\n';
}

Perhaps there should be some way of defining the lambda as if it were= actually a function that was part of the class? Rather than those data mem= bers being accessible by main() they are only accessible within the very la= mbda you create rather than accessible by main().

On Monday, July 17= , 2017 at 11:52:39 PM UTC+1, Arthur O'Dwyer wrote:


On Monday, July 17, 2017 a= t 6:09:25 AM UTC-7, Michael Davies wrote:
Hey everyone,

I would like to hear your thoug= hts on an idea I've been toiling away over. Playing around with some co= de I was curious if I could try to create a small library filled with a few= classes and override the default function behaviour of those classes. This= was for test purposes so I could have a similar sort of design where I wor= k if it was useful. Through experimenting I got something like this which i= s honestly not ideal:


What would be more ideal, is if I could create a = new function without having to define and declare references to things with= in the class to make them available to the lambda.
<= br>
You already can! The only problem with your existing code is = that you're trying to refer to private data members of your class from = outside the class, which is fundamentally never allowed in C++ because it w= ould break encapsulation. If you want those data members to be accessible b= y main(), then you need to make them public, or else make mai= n() a friend of your class. Here's an example with "= friend":



TEST.OverrideWinProcess(
=C2=A0 =C2=A0=C2=A0[&x =3D TEST]() {
=C2=A0 = =C2=A0 =C2=A0 =C2=A0=C2=A0x.m_iWin +=3D 1;
=C2=A0 =C2=A0 =C2=A0 =C2=A0=C2=A0std::cout << "Overloa= ded: " << x.m_iWin << '\n';
=C2=A0 =C2=A0= =C2=A0}
);

If you're not a friend of the class, then of cou= rse you can't access its private members; that's by design and will= never, ever change.

HTH,
=E2=80=93Arthu= r

--
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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/85d371d4-0fa1-470a-9e02-559dd5415857= %40isocpp.org.
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