220 40168 <9fd9c42a-916b-44de-8ab4-82d77df09859@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: "Ivan G." Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Class templates curiously parametrized by their own pointer-to-members Date: Sun, 16 Sep 2018 15:01:47 -0700 (PDT) Lines: 203 Approved: news@gmane.org Message-ID: <9fd9c42a-916b-44de-8ab4-82d77df09859@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1411_1233671835.1537135307886" X-Trace: blaine.gmane.org 1537135187 10100 195.159.176.226 (16 Sep 2018 21:59:47 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 16 Sep 2018 21:59:47 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBD26JFH7QQHRBTFF7POAKGQEJCYVVAI@isocpp.org Sun Sep 16 23:59:43 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb1-f200.google.com ([209.85.219.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1g1f4u-0002UA-5l for gclcip-std-proposals@m.gmane.org; Sun, 16 Sep 2018 23:59:40 +0200 Original-Received: by mail-yb1-f200.google.com with SMTP id v12-v6sf9643903ybe.23 for ; Sun, 16 Sep 2018 15:01:51 -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:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=eTqH4rvXMD5DD4zHcnPdHp9ut0rA2Vi3NIU9qm/Gr28=; b=QllVZy2nDuenTkpIw5Nyw6kcTf+yMhItZcSbSdf5I9YA10NRZe39Y/ut/heo5N2RZt sm/XoVrDLAswXYZlBRm6LGaDn7OEG+H1IQZ0gYxATJH6q5WS+S9KAnEnABTQVFsomjta Td/MofLJcSeWQi6KmVoZ7X8Zhl29d2yj30zdQKUwl8YJKz06Cc3t1hRhZmdrzcJt+89U /hkPNMS1JI17f4W+p/tQuZZYCTAjxpf2PfXnWXoo9qAIBGqmWnNrVnZygdTHJEbavsQ/ WMV2aihRHt5DtRy1+CSHwESIjGUmzY1+/evK85WmEd7vyZ2/exHJPMGpaYILvlp+dXKr 54Sg== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=eTqH4rvXMD5DD4zHcnPdHp9ut0rA2Vi3NIU9qm/Gr28=; b=Tr5h68nuOz86xqZyEG5oEmWPJPrNpO+4RAtfQM2nsiknj3DsHyZgaEDuFXEQDS9cZH VUHAgiwgHK7c3YHCmIemHX5T9tcLn8MyDgCCOqfYheoJXb5wbG/Dl/IehRDE41Dt2bJO NKsvxOw0bap/PbksfB833nbiBKAavmITKGZA3lNcinybT0MnLb4dW4MVSl8AJtPA+5eP t6paL2YxY4EzIzVXaVZqeB8wqFV2YDDgJmcWk3wvwunslcxL+SjM8wnsW/ZV2Q9FNsr4 12oCXPLoX0bWrNheMLq0+jDRl2aHPtM794n52kef3cO0malmKFXGSKI/UdkNSwxUKFaR IEFw== 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: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=eTqH4rvXMD5DD4zHcnPdHp9ut0rA2Vi3NIU9qm/Gr28=; b=qVCQexF/SVlOK3T3L9bLcJiH71crJfkLepKo7L+ur9LKoM4yBx6Bh/d5Wpl00kQ7k8 eVy6H71JpleIxaDBrDLisnYaMZ9rx45j+TBxT46Ck1oDEZJMBy+nAlO7V/xMgQTNgzy5 BnZ4GKzQ/R1E0x/RxcF+O2XwAFjQZRLuE58vFLQQsPORJ89OAO/2q93gkcCY2B0KXDJb AhU0HLLQUu1W72ktfPrukUqULWs+ODfJvnGGsbLE0XvxcE2dpig3bdWal90ZDGZ9JuKq +K92OgP7VPzyepd9/OVd+yI4wq0b1pElN9rj8IKkqPSsPwnWs3OqKzCOTc20uUBFsJ51 H3Qw== X-Gm-Message-State: APzg51Cr7GitvJf9wQEKu0fuD3N1JOk1pcfJGkTp3Y3f+DLEnG5e3HkD T8IH2edal5iuqc4ZEOaCQ7woBA== X-Google-Smtp-Source: ANB0Vda8IylhOun5KFvo2WvrWUoMkP/UVc2aOg9EH5h18esAQqGS1ehPhKVmosjmQ/4k4V8a7j3fyw== X-Received: by 2002:a0d:e3c4:: with SMTP id m187-v6mr3540215ywe.50.1537135310282; Sun, 16 Sep 2018 15:01:50 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:a25:ef4b:: with SMTP id w11-v6ls2869917ybm.16.gmail; Sun, 16 Sep 2018 15:01:48 -0700 (PDT) X-Received: by 2002:a25:c048:: with SMTP id c69-v6mr157838ybf.4.1537135308676; Sun, 16 Sep 2018 15:01:48 -0700 (PDT) X-Original-Sender: nekotekina@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:40168 Archived-At: ------=_Part_1411_1233671835.1537135307886 Content-Type: multipart/alternative; boundary="----=_Part_1412_119249185.1537135307886" ------=_Part_1412_119249185.1537135307886 Content-Type: text/plain; charset="UTF-8" Hello, this is another simple attempt on properties, mixins, and adjacent problems. It probably doesn't contain anything new, maybe just another point of view. The idea is to have special template parameter with special syntax, of pointer-to-member type, which turns the class template into a special one which cannot be instantiated outside of the context of another class. It can strictly become only a class member. template struct deeper { int foo() { return z; // Ok: z belongs to property } int bar() { return static_cast(this)->y.fu(); // Ok: can reach higher levels by casting pointer } }; template struct property { int z; deeper w; int fu() { // return this->z; // Error: bar doesn't have z return (this->*This).z; // Ok: 'this' points to bar } }; struct bar { property<> x; // 'This' is equal to &bar::x property<> y; // 'This' is equal to &bar::y // deeper<> inside the property should have zero side, just like empty base optimization static_assert(sizeof(x) == sizeof(int)); }; auto this::* is "magical" pointer-to-member, which is normally impossible and seems absurd at the first glance. However, I believe it's just a minor technical issue and can be implemented. Because every member's type is unique, I believe that pointers to such members can be simply upcasted with static_cast. Objects of such types obviously cannot be constructed. Outer class is directly visible from inside of them, but I cannot decide to what extent, and how it should interact with inner classes' own members. -- 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/9fd9c42a-916b-44de-8ab4-82d77df09859%40isocpp.org. ------=_Part_1412_119249185.1537135307886 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Hello, this is another simple attempt on properties, mixin= s, and adjacent problems. It probably doesn't contain anything new, may= be just another point of view.

The idea is to have speci= al template parameter with special syntax, of pointer-to-member type, which= turns the class template into a special one which cannot be instantiated o= utside of the context of another class. It can strictly become only a class= member.


template <auto Prop, = auto this::* This>
struct deeper
{
=C2=A0 =C2=A0int<= span style=3D"color: #000000;"> foo<= /span>()
=C2=A0 =C2=A0{
=C2=A0 =C2=A0 =C2=A0 =C2=A0ret= urn z; // Ok: z belongs to property
=C2=A0 =C2=A0}

=C2=A0 =C2=A0 int bar()
=C2=A0 =C2=A0{
=C2=A0 =C2=A0 =C2=A0 =C2=A0return<= /span> static_cast<struct <= span style=3D"color: #267f99;">bar*&= gt;(this)->y.fu(); // Ok: can reach higher levels by castin= g pointer
=C2=A0 =C2=A0}<= /span>
};

template <auto this::* This>= ;
struct property
{
=C2=A0 =C2=A0i= nt z;

=C2=A0 =C2=A0 deeper<This> w;
=

=C2=A0 =C2=A0 int = fu()=
=C2=A0 =C2=A0{
=C2=A0 =C2=A0 =C2=A0 =C2=A0= // return this->z; // Error: bar doesn&#= 39;t have z

=C2=A0 =C2= =A0 =C2=A0 =C2=A0 return (this->*This).z; // Ok: 'this' points to bar
<= span style=3D"color: #000000;"> =C2=A0 =C2=A0}
};

struct bar
= {
=C2=A0=C2=A0=C2=A0=C2=A0= property<> x; // 'Thi= s' is equal to &bar::x
=C2=A0=C2=A0=C2=A0=C2=A0property<> y; // 'This' is equal to &bar::y

<= div>=C2=A0=C2=A0=C2=A0=C2=A0// deeper<> inside the property should have z= ero side, just like empty base optimization
=C2=A0=C2=A0=C2=A0=C2=A0static_assert(sizeof(x) =3D=3D sizeof(int));
};

auto this::* is "magical" pointer-to-member, which is norma= lly impossible and seems absurd at the first glance. However, I believe it&= #39;s just a minor technical issue and can be implemented.

Because every member's type is unique, I believe that pointers= to such members can be simply upcasted with static_cast.

Objects of such types obviously cannot be constructed.
Ou= ter class is directly visible from inside of them, but I cannot decide to w= hat extent, and how it should interact with inner classes' own members.=

--
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/9fd9c42a-916b-44de-8ab4-82d77df09859= %40isocpp.org.
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