220 37496 <97df4987-b0ed-4080-b58d-fe48b532d1cf@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allowing std::complex's magic permission for other types Date: Tue, 27 Mar 2018 06:57:26 -0700 (PDT) Lines: 164 Approved: news@gmane.org Message-ID: <97df4987-b0ed-4080-b58d-fe48b532d1cf@isocpp.org> References: <2f3f1821-385e-4357-bcf4-9a87312853fe@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_18521_1404379364.1522159046543" X-Trace: blaine.gmane.org 1522158925 21595 195.159.176.226 (27 Mar 2018 13:55:25 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 27 Mar 2018 13:55:25 +0000 (UTC) Cc: florian.csdt@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBR435HKQKGQEQIPV73Q@isocpp.org Tue Mar 27 15:55:21 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ua0-f197.google.com ([209.85.217.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1f0p4K-0005Uj-NF for gclcip-std-proposals@m.gmane.org; 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Tue, 27 Mar 2018 06:57:28 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.31.67.2 with SMTP id q2ls700119vka.0.gmail; Tue, 27 Mar 2018 06:57:27 -0700 (PDT) X-Received: by 10.31.69.207 with SMTP id s198mr1642875vka.0.1522159046960; Tue, 27 Mar 2018 06:57:26 -0700 (PDT) In-Reply-To: <2f3f1821-385e-4357-bcf4-9a87312853fe@isocpp.org> X-Original-Sender: jmckesson@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:37496 Archived-At: ------=_Part_18521_1404379364.1522159046543 Content-Type: multipart/alternative; boundary="----=_Part_18522_882855502.1522159046543" ------=_Part_18522_882855502.1522159046543 Content-Type: text/plain; charset="UTF-8" On Tuesday, March 27, 2018 at 5:53:23 AM UTC-4, floria...@gmail.com wrote: > > I have the impression that people are missing the big picture here. > > I see one big problem that can be solved with overloadable > reinterpret_cast: smart pointers. > If you look closely at shared_pointer for instance, you will see: > > template > std::shared_ptr static_pointer_cast(const std::shared_ptr& r) > noexcept; > > template > std::shared_ptr dynamic_pointer_cast(const std::shared_ptr& r) > noexcept; > > template > std::shared_ptr const_pointer_cast(const std::shared_ptr& r) > noexcept; > > template > std::shared_ptr reinterpret_pointer_cast(const std::shared_ptr& r) > noexcept; > > If it were possible to overload dynamic_cast const_cast and > reinterpret_cast, this would have been unnecessary to have a different > interface for shared pointers than for regular pointers. > If it were impossible to overload the reference operator, we wouldn't need to use `std::addressof` in template programming. But we allowed that, and therefore we have to write 99% of our `&value` statements as `std::addressof(value)` just to dodge those few users who unwisely overload `operator&`. There are some operations where overloading on user-defined types is reasonable. Casting is not one of them (and neither is `operator&`, but that ship has sailed). If you need to execute specialized code for a cast, then it's not a *cast* at all; it's simply a function call. -- 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/97df4987-b0ed-4080-b58d-fe48b532d1cf%40isocpp.org. ------=_Part_18522_882855502.1522159046543 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Tuesday, March 27, 2018 at 5:53:23 AM UTC-4, floria...@= gmail.com wrote:
I have the impression that people are missing the big picture here.
<= br>I see one big problem that can be solved with overloadable reinterpret_c= ast: smart pointers.
If you look closely at shared_pointer for instance,= you will see:

template<= span style=3D"color:#000"> <class T, class U>
std::shared_ptr<T<= span style=3D"color:#660">> static_poi= nter_cast(const std::shared_ptr<U>& r) noexcept;

template <class T, class U>
std::shared_ptr<T> dynamic_pointer_c= ast(con= st std:= :shared_ptr<U>& r) noexcept;

template <class T, class U>
std::shared_ptr<T> const_pointer_cast(const= std::s= hared_ptr<U>& r) noexcept;

template <class T, class U>
std::shared_ptr<T> reinterpret_pointer_cast(const std::shared_ptr&= lt;U>= ;& r) noexcept;

If it were possible to overload dynamic_cast const_cast and reinterpre= t_cast, this would have been unnecessary to have a different interface for = shared pointers than for regular pointers.

I= f it were impossible to overload the reference operator, we wouldn't ne= ed to use `std::addressof` in template programming. But we allowed that, an= d therefore we have to write 99% of our `&value` statements as `std::ad= dressof(value)` just to dodge those few users who unwisely overload `operat= or&`.

There are some operations where overloading on user-define= d types is reasonable. Casting is not one of them (and neither is `operator= &`, but that ship has sailed). If you need to execute specialized code = for a cast, then it's not a cast at all; it's simply a funct= ion call.
=C2=A0

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