220 29029 article Path: news.gmane.org!.POSTED!not-for-mail From: joseph.thomson@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: [propagate_const] Implicit conversion to T Date: Fri, 21 Oct 2016 10:22:19 -0700 (PDT) Lines: 131 Approved: news@gmane.org Message-ID: References: <1f43ffe8-b267-4c8c-8834-f3e60035f18d@isocpp.org> <7f61be69-4f4c-40a5-9199-805edc90b428@isocpp.org> <13a71b20-8fd3-4a29-9279-716a2c9082a6@isocpp.org> <7abb6203-6ca7-4e04-ab6a-dc6a60e223ca@isocpp.org> <60ae1a96-7a31-491d-bcf8-c81590544006@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1464_112836723.1477070539556" X-Trace: blaine.gmane.org 1477070551 32207 195.159.176.226 (21 Oct 2016 17:22:31 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 21 Oct 2016 17:22:31 +0000 (UTC) Cc: jbcoe@me.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDQM7AFVVUERBTE5VHAAKGQEPT2RFOI@isocpp.org Fri Oct 21 19:22:26 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qt0-f197.google.com ([209.85.216.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1bxdWO-0006xg-VF for gclcip-std-proposals@m.gmane.org; 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Fri, 21 Oct 2016 10:22:23 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.142.17 with SMTP id q17ls2620384iod.1.gmail; Fri, 21 Oct 2016 10:22:20 -0700 (PDT) X-Received: by 10.36.103.199 with SMTP id u190mr1436561itc.6.1477070540551; Fri, 21 Oct 2016 10:22:20 -0700 (PDT) In-Reply-To: X-Original-Sender: joseph.thomson@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: 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:29029 Archived-At: ------=_Part_1464_112836723.1477070539556 Content-Type: multipart/alternative; boundary="----=_Part_1465_729748463.1477070539557" ------=_Part_1465_729748463.1477070539557 Content-Type: text/plain; charset=UTF-8 On Friday, 21 October 2016 05:08:54 UTC+8, Jonathan Coe wrote: > > Adding `const_type` so that `const propagate_const` is implicitly > convertible to `const_type_t` seems pretty appealing to me. > > `get_underlying` is very much like a cast operation, it was added so the > user can discard const_propagation when required. > It's worth noting that with this new implicit conversion, the implementation of get_underlying pretty much becomes a call to const_cast: template T const& get_underlying(propagate_const const& pc) { return const_cast&>(pc); } template T& get_underlying(propagate_const& pc) { return pc; } -- 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/d348adb2-19b5-48f9-8836-adf9c3fa3300%40isocpp.org. ------=_Part_1465_729748463.1477070539557 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Friday, 21 October 2016 05:08:54 UTC+8, Jonathan Coe w= rote:
Adding `= const_type` so that `const propagate_const<T>` is implicitly converti= ble to `const_type_t<T>` seems pretty appealing to me.

=
`get_underlying` is very much like a cast operation, it was added so t= he user can discard const_propagation when required.

It's worth noting that with this new implicit conversion= , the implementation of get_underlying pretty much becomes a call to const_cast:

template <typename T>
T const& get_underlying(= propagate_const<T= > const& pc) {
=C2=A0 =C2=A0 return const_cast<pro= pagate_const<T>&>= (pc= );
}

template <typename T>
T&am= p; get_und= erlying(propagate_co= nst<T>& pc) {
=C2=A0 =C2=A0 return pc;
}

--
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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/d348adb2-19b5-48f9-8836-adf9c3fa3300= %40isocpp.org.
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