220 26555 <7E3409C2-A2AC-4562-BB06-6D5D5ACE8AC8@gmail.com> article Path: news.gmane.org!not-for-mail From: Jonathan Coe Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: auto type deduction for *_cast return argument Date: Tue, 5 Jul 2016 08:59:03 +0100 Lines: 163 Approved: news@gmane.org Message-ID: <7E3409C2-A2AC-4562-BB06-6D5D5ACE8AC8@gmail.com> References: <58140e46-2adc-4eac-8f51-72d3685cdf0b@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (1.0) Content-Type: multipart/alternative; boundary=Apple-Mail-F1A4DED3-8A03-4DCE-AA0E-20125957F19A Content-Transfer-Encoding: 7bit X-Trace: ger.gmane.org 1467705552 10719 80.91.229.3 (5 Jul 2016 07:59:12 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 5 Jul 2016 07:59:12 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC2JVFPBRAHBBSOR5W5QKGQEVUA4KSA@isocpp.org Tue Jul 05 09:59:08 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wm0-f71.google.com ([74.125.82.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bKLG6-0004UA-Ij for gclcip-std-proposals@m.gmane.org; Tue, 05 Jul 2016 09:59:06 +0200 Original-Received: by mail-wm0-f71.google.com with SMTP id r190sf81182405wmr.0 for ; Tue, 05 Jul 2016 00:59:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=from:content-transfer-encoding:mime-version:subject:message-id:date :references:in-reply-to:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=6CQG6sURny/najvKvLDN//HJDYPdGXnaSMwaMn6fyjQ=; b=1/cCrh4SjnVZQ85opUC8S9E4HLCBb3InEuFzFAk138JDm1UNQh6blVCtqIBUKxfQzl eiZcNFErTCSH2TQwe7SYcSejXoTBIQqBHg9hhm5Q8oWYZtA0wgHXHf8VUh/IOapTEbQI gcnw6uwhfNX4LmvgaUf5ttr8ZR4HYVhhEjgySmuXB2nonuhu9PoH2oaMbwDOhKmLldnh VFkSLHOZVPk7bMWO/juDSV8AwQqdlRcOYImS6zq6iiSGpDBwmH5P+uHaRu3qm1bN1vIs +PDrzoxpEDgzry35W3V6y1+7ZelsED2GNJzg24nEymusTrqtuTnBzkX3tNNDdG8HJth1 p2cg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:content-transfer-encoding:mime-version :subject:message-id:date:references:in-reply-to:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=6CQG6sURny/najvKvLDN//HJDYPdGXnaSMwaMn6fyjQ=; b=Klx5q9rZfXFpoaZpK/cjkUu+MNQAEsYRiTYvgmWNx69JQu/d9Cn+zOESafo9tr5FUJ w70y+to8WJ/51Y5Sho9sQ1SlCgLVnPiXzX5/sI0Z6oEElq8RIKftOnRdXXwIQPXKx1ID bltz16mZojKLWvfT9VNEp/LmNjgTiymb0Py0LhJPCo2NR6oZpm29+GDXnxb8w67dD3IZ JhdsQLcEI3WdkXfg3HbE+o1o7UQPlxkSEyG4R2F1loJgF5+40/93L3uGA1ICqGBif2PA ZAilR2IFk2FDuAUM1JT7/OIPFnNXWaFyQGrHKrho3qu2TvKTu8rnooD6jFpDYxMlNuoh R7a X-Gm-Message-State: ALyK8tIrPNvpY3Sot6Xm3lL/4/0qNkhC9l9o7fN3YtpXnRSXL4ZXXN79lpO8JKKuW/Gytg== X-Received: by 10.194.22.169 with SMTP id e9mr6478271wjf.6.1467705545871; Tue, 05 Jul 2016 00:59:05 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.28.134.13 with SMTP id i13ls354696wmd.15.canary; Tue, 05 Jul 2016 00:59:04 -0700 (PDT) X-Received: by 10.28.165.5 with SMTP id o5mr14720466wme.87.1467705544780; Tue, 05 Jul 2016 00:59:04 -0700 (PDT) Original-Received: from mail-wm0-x234.google.com (mail-wm0-x234.google.com. [2a00:1450:400c:c09::234]) by mx.google.com with ESMTPS id k135si52480wmg.61.2016.07.05.00.59.04 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 05 Jul 2016 00:59:04 -0700 (PDT) Received-SPF: pass (google.com: domain of jonathanbcoe@gmail.com designates 2a00:1450:400c:c09::234 as permitted sender) client-ip=2a00:1450:400c:c09::234; Original-Received: by mail-wm0-x234.google.com with SMTP id r201so140883996wme.1 for ; Tue, 05 Jul 2016 00:59:04 -0700 (PDT) X-Received: by 10.28.187.134 with SMTP id l128mr13569674wmf.51.1467705544391; Tue, 05 Jul 2016 00:59:04 -0700 (PDT) Original-Received: from [192.168.1.71] (host86-163-90-41.range86-163.btcentralplus.com. [86.163.90.41]) by smtp.gmail.com with ESMTPSA id s67sm1914844wmf.3.2016.07.05.00.59.03 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Tue, 05 Jul 2016 00:59:03 -0700 (PDT) In-Reply-To: <58140e46-2adc-4eac-8f51-72d3685cdf0b@isocpp.org> X-Mailer: iPhone Mail (13F69) X-Original-Sender: jonathanbcoe@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of jonathanbcoe@gmail.com designates 2a00:1450:400c:c09::234 as permitted sender) smtp.mailfrom=jonathanbcoe@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=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:26555 Archived-At: --Apple-Mail-F1A4DED3-8A03-4DCE-AA0E-20125957F19A Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Thanks for suggesting this, I'm afraid I don't think I like this idea. It saves a bit of typing but would make it less clear what's going on and i= n cases where a function signature changes would change the meaning of exis= ting code without any edits being made f(A a); Changed to f(B b); Would change the meaning of=20 f(dynamic_cast(x)); In a rather non-obvious way. Regards Jon > On 5 Jul 2016, at 08:47, xavinaar@gmail.com wrote: >=20 > When using reinterpret_cast, dynamic_cast, static_cast and const_cast, yo= u always have to specify type explicitly, yet it almost always can be deduc= ed from the context. >=20 > Consider an example: >=20 > void someFunc(Derived *d); >=20 > // ... >=20 > const Base *cbase =3D new Base; > const Derived *cderived =3D dynamic_cast(cbase); > Derived *derived =3D const_cast(cderived); > someFunc(derived); >=20 > Having knowledge of desired argument type (such as explicitly declared va= riable type or function argument type) we can auto-deduce desired return ty= pes. My suggestion is to make type specialization for cast operations optio= nal in such cases which will result in much cleaner code: >=20 > const Base *cbase =3D new Base; > someFunc(dynamic_cast(const_cast(cbase))); >=20 > Furthermore, as soon as const_cast is mostly used to cast const pointers/= references to mutable ones (backwards conversion is implicit), I also sugge= st to define default behavior of const_cast as to return non-const pointer/= reference to argument type, such as: >=20 > const Base *cbase =3D new Base; > auto base =3D const_cast(cbase); // type of base is "Base" >=20 > This rule can also be generalized to any templated function: >=20 > template > T someFunc(); >=20 > // ... >=20 > int i =3D someFunc(); // will invoke someFunc(); > --=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= 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/isoc= pp.org/d/msgid/std-proposals/58140e46-2adc-4eac-8f51-72d3685cdf0b%40isocpp.= org. --=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/7E3409C2-A2AC-4562-BB06-6D5D5ACE8AC8%40gmail.com= .. --Apple-Mail-F1A4DED3-8A03-4DCE-AA0E-20125957F19A Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Thanks for suggesting t= his, I'm afraid I don't think I like this idea.

It= saves a bit of typing but would make it less clear what's going on and in = cases where a function signature changes would change the meaning of existi= ng code without any edits being made

f(A a);
=

Changed to

f(B b);
<= br>
Would change the meaning of 

f(= dynamic_cast(x));

In a rather non-obvious way.

Regards

Jon

On= 5 Jul 2016, at 08:47, xavinaar@gmail= ..com wrote:

When using reinterpret_cast, dynamic_cast, static_cast and const_cast, you always have to specify type explicitly, yet i= t almost always can be deduced from the context.

Consider an example= :

    void someFunc(Derived *d);

  &= nbsp; // ...

    const Base *cbase =3D new Base;
&= nbsp;   const Derived *cderived =3D dynamic_cast<const Derived= *>(cbase);
    Derived *derived =3D const_cast<Der= ived *>(cderived);
    someFunc(derived);

Havin= g 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 =3D new Base;
    someFunc(dynamic_ca= st(const_cast(cbase)));

Furthermore, as soon as const_cast is= mostly used to cast const pointers/references to mutable ones (backwards c= onversion is implicit), I also suggest to define default behavior of con= st_cast as to return non-const pointer/reference to argument type, such= as:

    const Base *cbase =3D new Base;
 &nb= sp;  auto base =3D const_cast(cbase); // type of base is "Base"
This rule can also be generalized to any templated function:

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

    // ...

    int i =3D some= Func(); // will invoke someFunc<int>();

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