220 26554 <58140e46-2adc-4eac-8f51-72d3685cdf0b@isocpp.org> article Path: news.gmane.org!not-for-mail From: xavinaar@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: auto type deduction for *_cast return argument Date: Tue, 5 Jul 2016 00:47:02 -0700 (PDT) Lines: 102 Approved: news@gmane.org Message-ID: <58140e46-2adc-4eac-8f51-72d3685cdf0b@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1397_473365688.1467704822121" X-Trace: ger.gmane.org 1467704826 31920 80.91.229.3 (5 Jul 2016 07:47:06 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 5 Jul 2016 07:47:06 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCX4LEEDVMDRB56L5W5QKGQERQFCXTY@isocpp.org Tue Jul 05 09:47:06 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f200.google.com ([209.85.161.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bKL4T-0006LC-4p for gclcip-std-proposals@m.gmane.org; Tue, 05 Jul 2016 09:47:05 +0200 Original-Received: by mail-yw0-f200.google.com with SMTP id i12sf137079756ywa.0 for ; Tue, 05 Jul 2016 00:47:04 -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=vJgK22uHxZi582hEIBB2mczqgz9A6wklH1Go8pECXPY=; b=aVpD/kk0qC0Iq72ThhL1giR53a+MfcLV53FU1k38v+rIZHlVBUoaJGMeiucsGBqIhq IOYARoJ0T6WHLhoFa7DARBA8oahTOdEMjN0GJt//RGATshKf43rTjW5cuxs2drGHQM3I gJo4W0KX/A+fmauIh7feCgo07O0N7p4KdzoGC22KL9UBN5buGBpWjjt7XafxhUt9X1mG ve7MAAykuzoQjm/c6LIlsRA6BHwwUZDF/bu+p0BFzjohikn+bdxvVb5hTpy2JwcTTkan Qk7llS5/YnsS9hDrwx3pBzaJIso0dSFBfbJK8oKJuNWdUBWK8KWZ6W0XVIfqc0j8SIgx DAdQ== 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=vJgK22uHxZi582hEIBB2mczqgz9A6wklH1Go8pECXPY=; b=uZVECnjcWnM076feTeONmllA6Ut9LgwmcJC6lCqNecd/qtBD5DrDmMicHfV5whWBNU TOajKO/RtH197+bLLnray0WDoxGkCkKBsu3icAz1+JVKL73QvYO5gXaRjF6jrkhiU0tY jY5Nk4zAksRYXW7EKmfBCTVy7SJrTXrplXEnBxEuUgRYf+OMKjkXy95XPlAiMGCX/toD EeQPspL6v1kp58YVpM136C2Pc4CP1lqzhby9gpkMguRC888AHx/9cToV5PP6+hDEv0Sr ZrJySEBOV/kzVLhd530Tu6/H1GrpuJPLL5R8fZNiipzHnkhAc8KNiQT1hM8DiLYCIOvW 9sgw== 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=vJgK22uHxZi582hEIBB2mczqgz9A6wklH1Go8pECXPY=; b=JPx8i0nsVYmliGBcPCZYf7U2K9duKwBddpJ+MaLg5X9n0sGbD0i3nIIdo6D3O0ilH4 tSgrpHdm5YZ69wVGW2FYcORBXMJxAd15OybdimC+VGOJgY3WZeXQ7q+h0kV3R7nXGk5f IHDnXn0WTGxR5m2CDUQe4ohhIMP066fDUTM75EIMDc9K3+8mLONLKqh//LQotVPWif5N 7puzxRibAxkvLiCZGUObmYZKRcVIUSA1Ti5WIPpi2pmimKMztVgaL7ilU5VABYYa14f1 qbJUEYbxNv9Ven5f/W0O2xAO4jKOC5PATA83CvBzWiRMj0y+uhwN60LDJeTTL/pYLrKk JGkg== X-Gm-Message-State: ALyK8tI10wyXtt/9EJwOiP+vBfA8VqXpyHbjaEKRH+805LVsDg74V/yXgRDDsYHA14Cs4g== X-Received: by 10.129.75.139 with SMTP id y133mr34717635ywa.44.1467704824206; Tue, 05 Jul 2016 00:47:04 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.202.1 with SMTP id a1ls1938198iog.49.gmail; Tue, 05 Jul 2016 00:47:03 -0700 (PDT) X-Received: by 10.36.17.15 with SMTP id 15mr337919itf.10.1467704823152; Tue, 05 Jul 2016 00:47:03 -0700 (PDT) X-Original-Sender: Xavinaar@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:26554 Archived-At: ------=_Part_1397_473365688.1467704822121 Content-Type: multipart/alternative; boundary="----=_Part_1398_1831151153.1467704822121" ------=_Part_1398_1831151153.1467704822121 Content-Type: text/plain; charset=UTF-8 When using* reinterpret_cast,* *dynamic_cast*, *static_cast* and *const_cast*, you always have to specify type explicitly, yet it almost always can be deduced from the context. Consider an example: void someFunc(Derived *d); // ... const Base *cbase = new Base; const Derived *cderived = dynamic_cast(cbase); Derived *derived = const_cast(cderived); someFunc(derived); Having knowledge of desired argument type (such as explicitly declared variable type or function argument type) we can auto-deduce desired return types. My suggestion is to make type specialization for cast operations optional in such cases which will result in much cleaner code: const Base *cbase = new Base; someFunc(dynamic_cast(const_cast(cbase))); Furthermore, as soon as *const_cast* is mostly used to cast const pointers/references to mutable ones (backwards conversion is implicit), I also suggest to define default behavior of *const_cast* as to return non-const pointer/reference to argument type, such as: const Base *cbase = new Base; auto base = const_cast(cbase); // type of base is "Base" This rule can also be generalized to any templated function: template T someFunc(); // ... int i = someFunc(); // will invoke someFunc(); -- 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/58140e46-2adc-4eac-8f51-72d3685cdf0b%40isocpp.org. ------=_Part_1398_1831151153.1467704822121 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
When using reinterpret_cast, dynamic_cast, <= b>static_cast and const_cast, you always have to specify type ex= plicitly, yet it almost always can be deduced from the context.

Cons= ider an example:

=C2=A0=C2=A0=C2=A0 void someFunc(Derived *d);
=C2=A0=C2=A0=C2=A0 // ...

=C2=A0=C2=A0=C2=A0 const Base *cbase =3D= new Base;
=C2=A0=C2=A0=C2=A0 const Derived *cderived =3D dynamic_cast&l= t;const Derived *>(cbase);
=C2=A0=C2=A0=C2=A0 Derived *derived =3D co= nst_cast<Derived *>(cderived);
=C2=A0=C2=A0=C2=A0 someFunc(derived= );

Having knowledge of desired argument type (such as explicitly dec= lared variable type or function argument type) we can auto-deduce desired r= eturn types. My suggestion is to make type specialization for cast operatio= ns optional in such cases which will result in much cleaner code:

= =C2=A0=C2=A0=C2=A0 const Base *cbase =3D new Base;
=C2=A0=C2=A0=C2=A0 so= meFunc(dynamic_cast(const_cast(cbase)));

Furthermore, as soon as = const_cast is mostly used to cast const pointers/references to mutable = ones (backwards conversion is implicit), I also suggest to define default b= ehavior of const_cast as to return non-const pointer/reference to ar= gument type, such as:

=C2=A0=C2=A0=C2=A0 const Base *cbase =3D new B= ase;
=C2=A0=C2=A0=C2=A0 auto base =3D const_cast(cbase); // type of base= is "Base"

This rule can also be generalized to any templa= ted function:

=C2=A0=C2=A0=C2=A0 template <typename T>
=C2= =A0=C2=A0=C2=A0 T someFunc();

=C2=A0=C2=A0=C2=A0 // ...

=C2= =A0=C2=A0=C2=A0 int i =3D someFunc(); // will invoke someFunc<int>();=

--
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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/58140e46-2adc-4eac-8f51-72d3685cdf0b= %40isocpp.org.
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