220 27199 article Path: news.gmane.org!not-for-mail From: Derek Ross Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Generalized "using" member aliases. Date: Thu, 21 Jul 2016 19:23:07 -0700 (PDT) Lines: 342 Approved: news@gmane.org Message-ID: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_91_839745895.1469154187556" X-Trace: ger.gmane.org 1469154203 31512 80.91.229.3 (22 Jul 2016 02:23:23 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 22 Jul 2016 02:23:23 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDJJ7WHE3YKRBDEHY26AKGQEPLRWQHQ@isocpp.org Fri Jul 22 04:23:17 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-io0-f199.google.com ([209.85.223.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bQQ7N-0008Da-Og for gclcip-std-proposals@m.gmane.org; Fri, 22 Jul 2016 04:23:14 +0200 Original-Received: by mail-io0-f199.google.com with SMTP id m101sf201595633ioi.0 for ; Thu, 21 Jul 2016 19:23:13 -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:x-spam-checked-in-group :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=lI7sFcEsBXVJ7M3THBF92jv+9rOtSEP1XyqGlDDU1Oo=; b=czg9++HOrCZ1zfAvCkidmN0YQx2Te4ZrIBXZjBwWhzctt3fgB6B2cqjZzVnBZ0HzMW OX3v1gkElS5Mqzs8BIxkUmSTyDdMmvy0zkkl+ZmgPrS4SvTLQZYuJkHIhAYVlqiZwD0i TltgUbJmoNDGGhl3RZpWibP8yMuwxErnY5ubdFpt5BllYh5yUDIQ2ecNj7+PCpV+HdYW OnZBV9K7Twi6JWfheZZ078f8KXgFsnICcx6qEV/iV9FmisIwg5vqv/EBNhmjXl08Cxhd njY25GLf9rwx/JKMx9Yd0x4pDaHBWJUSK3Ug/vhgmNho9ZL39FCBr2P/1dfLSUffdti/ BwwA== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=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=lI7sFcEsBXVJ7M3THBF92jv+9rOtSEP1XyqGlDDU1Oo=; b=iZoygeVXIFFv/I9F4OdnOQngOufLs7dmFPE8xxqbNX6wftFS057aP49XKYdKWTZtcJ kiQM6m+mjMKVBAA7evRzv642JEl4JDXLiMZBK4MkX2jpvd3Bn+8pTbRRNCEn8CgDCPpT C522jzNQ/WoXF9UU9zaG75NK6OLvN4O/KtZP32UUe7ayyx2pYZqYnbT1nCtWoXsT+dZh 7NnLbK8p5i47mKdwHm3AWpfPO+6lcnyb1dmxN0Y51qbz29YUVQEoAQqHR9NuKpFjWexb Eo27HrONmuHBEmETLny+aRBkoQIZdpEZ+3Ol8Vdb136tnbK1V+dPdBKrWoGP7opHhmYX DU5w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; 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=lI7sFcEsBXVJ7M3THBF92jv+9rOtSEP1XyqGlDDU1Oo=; b=IM7sESpkMshZFbhwAhScq9QvGIiDplrKOzj0e/cQNrm8kolKz4ilN7IGYWd9/J9ba0 IvLotW3ricJtDaM1Ny9UInaKHSP0qZVTxwYumqlz3MnbGfe1uQY5tf35Kf/DAB+5cVKv AIIFV5hSMLR3571cAI75e69M0IaYILciktgN59yJb15eGZCwlZiDwp3Smq8O3zpWu5l8 +yjx1jxR4ZQBbn6aUnTI2UsFfJ6L9zRkB6d+zSaBSGPaTQpoWzdlUDGlQGiMWRkavK08 JfDgOPCWdoyuLqg1vElmg+GTOhj5mR1cXkdv98hgoSMJ8HiEldkEpGVQkp+L1a/FfbFe 9okQ== X-Gm-Message-State: ALyK8tJ5GFV3L2BetAUBg8o4nby+ML+vW+rZOEy04i58OkT5rdE+urOHps6sWHYsnDeQwg== X-Received: by 10.36.110.210 with SMTP id w201mr10924060itc.7.1469154192676; Thu, 21 Jul 2016 19:23:12 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.214.71 with SMTP id o68ls1066866itg.8.gmail; Thu, 21 Jul 2016 19:23:08 -0700 (PDT) X-Received: by 10.36.217.2 with SMTP id p2mr69739itg.5.1469154188558; Thu, 21 Jul 2016 19:23:08 -0700 (PDT) X-Original-Sender: antiquarktv@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:27199 Archived-At: ------=_Part_91_839745895.1469154187556 Content-Type: multipart/alternative; boundary="----=_Part_92_2098299631.1469154187557" ------=_Part_92_2098299631.1469154187557 Content-Type: text/plain; charset=UTF-8 It would be useful if "using" aliases could be expanded beyond namespaces and types. Why not provide aliases for class members? This would be useful if you wanted to derive a class from a standard type such as pair or tuple, but wanted more meaningful member names than the built-in .first, .second, etc, names. In effect, a way to easily "rename" a member. The member alias syntax would look like: class X { ... using NewName = ExistingName; where ExistingName is an existing member variable or member function in a class. The implemention would treat NewName like a macro, and substitute ExistingName wherever NewName appears. ExistingName could also specify a function, e.g., "ExistingName()", in which case NewName becomes a property-like element. *Examples:* You could derive from a std::pair, then alias different names to the .first and .second members of the pair. struct Gps_Pos : pair { using latitude = first; using longitude = second; }; Gps_Pos gps; gps.latitude = 12.34; //sets gps.first Another possibility could be to alias names to certain elements of an array. Thus, an N-dimensional array could provide names for each dimension. struct Vec3d{ double data[3]; using x = data[0]; using y = data[1]; using z = data[2]; }; Vec3d v; v.z = 123; // sets data[2] Member aliases could also be used to shorten inconveniently long names: struct My_Database{ int GetNumberOfCustomersCurrentlyActive(){ ... } using custs = GetNumberOfCustomersCurrentlyActive; }; My_Database db; cout << db.custs() ; // calls db.GetNumberOfCustomersCurrentlyActive() Finally, member aliases could be used to access elements of a tuple through meaningful names. This brings tuples one step closer to being a struct replacement, with all the extra features that a tuple provides. struct Person : tuple{ string& get0(){return std::get<0>(*this);} int& get1(){return std::get<1>(*this);} using name = get0(); using age = get1(); }; Person p; p.name = "Bob"; //sets via get<0>(p) ---- Cheers, Derek -- 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/d0eb7c1b-19b5-4ab4-97c3-e3f1a88a7fe3%40isocpp.org. ------=_Part_92_2098299631.1469154187557 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
It would be useful if "using" aliases could be e= xpanded beyond namespaces and types. Why not provide aliases for class memb= ers? This would be useful if you wanted to derive a class from a standard t= ype such as pair or tuple, but wanted more meaningful member names than the= built-in .first, .second, etc, names. In effect, a way to easily "ren= ame" a member.

The member alias syntax would look like:
class X { ...
=C2=A0using NewName =3D ExistingName<= /span>;

where ExistingName is an existing member variable or me= mber function in a class. The implemention would treat NewName like a macro= , and substitute ExistingName wherever NewName appears. ExistingName could = also specify a function, e.g., "ExistingName()", in which case Ne= wName becomes a property-like element.

Examples:

You = could derive from a std::pair, then alias different names to the .first and= .second members of the pair.

struct Gps_Pos = : pair<double,double> { =C2=A0 =C2=A0
=C2=A0 =C2=A0 using latitude =3D first;
=C2=A0 =C2=A0 using longitude =3D second;};=

Gps_Pos gps;
gps.latitude =3D 12.34; //sets gps.fi= rst

Another possibility could be to alias name= s to certain elements of an array. Thus, an N-dimensional array could provi= de names for each dimension.
=C2=A0
struct Vec3d{
=C2=A0 =C2=A0 double data[3]; =C2=A0 =C2=A0
=C2= =A0 =C2=A0 using x <= /span>=3D<= span style=3D"color: #000;" class=3D"styled-by-prettify"> data[0];
=C2=A0 =C2=A0 using y =3D data[1];
=C2=A0 =C2= =A0 using<= /span> z <= span style=3D"color: #660;" class=3D"styled-by-prettify">=3D data[2];
};

Vec3d= v;=
v.z =3D 123; // se= ts data[2]
=C2=A0
Member aliases could also be = used to shorten inconveniently long names:
=C2=A0
struct My_Database{
=C2=A0 =C2=A0 int GetNumberOfCustomersCurrentlyActive(){
=C2=A0 =C2=A0 =C2=A0...
=C2=A0 =C2=A0 } =C2=A0 =C2=A0
=C2=A0 =C2=A0 using<= span style=3D"color: #000;" class=3D"styled-by-prettify"> custs =3D GetNumberOfCustomersCurrentlyActive;
};
=C2=A0
My_Database db;
cout << db.custs() = ; // calls db.GetNumberOfCusto= mersCurrentlyActive()

Finally, member aliases = could be used to access elements of a tuple through meaningful names. This = brings tuples one step closer to being a struct replacement, with all the e= xtra features that a tuple provides.

struct Person : t= uple<string,int>{
=C2=A0 =C2=A0 string& get0(){return std::get<0>(*this);}
=C2=A0 =C2=A0 int& =C2=A0 =C2=A0get1(){return std::get<1>(*= this);}
=C2=A0 = =C2=A0
=C2=A0 =C2=A0 using name =3D = get0();
=C2=A0 = =C2=A0 usi= ng age =C2= =A0=3D get1();
};

Person p;
p.na= me =3D "Bob"<= span style=3D"color: #660;" class=3D"styled-by-prettify">; //sets via get<0>(p)

----
Cheers,
Derek

--
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