220 15458 <683eba24-3371-4c96-960d-361e34b7e1e6@isocpp.org> article Path: news.gmane.org!not-for-mail From: R.HL@gmx.net Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Adding a remove_cv_reference type trait Date: Sun, 4 Jan 2015 15:15:37 -0800 (PST) Lines: 176 Approved: news@gmane.org Message-ID: <683eba24-3371-4c96-960d-361e34b7e1e6@isocpp.org> References: <5880acc5-8236-40a0-bd5d-dfa6d9284572@isocpp.org> <7c1ddf8b-051b-4fa8-9c00-8fe239890edc@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_350_1423860656.1420413337827" X-Trace: ger.gmane.org 1420413345 5801 80.91.229.3 (4 Jan 2015 23:15:45 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 4 Jan 2015 23:15:45 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDRN5RPSR4MRBGUTU6SQKGQESHPHYEQ@isocpp.org Mon Jan 05 00:15:40 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f198.google.com ([209.85.214.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1Y7uOa-00036w-6G for gclcip-std-proposals@m.gmane.org; Mon, 05 Jan 2015 00:15:40 +0100 Original-Received: by mail-ob0-f198.google.com with SMTP id vb8sf508064164obc.1 for ; Sun, 04 Jan 2015 15:15:39 -0800 (PST) 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:in-reply-to:references :subject:mime-version:content-type:x-original-sender:reply-to :precedence:mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=50BSIrq9R88yzHPPuYrnz5X/jcC2rNkE/NJdLyHbo/w=; b=WMlhTo94uko45+DTvtnEEzbF8eHldgeM/IqHPxVuQuQQUXhmokBuTT23UAyWc7LNJD feNmKqQVJ04znB9zp3N6iL2crUeZD9ohMKiEOSKQM8VPaHCRqJceQ52qNrdZanzrWVCO UyahwN8K5OX90vIMqjnjNWCdnhZBdx+I7jyLd8OSGKyODZ1xwt7HAiRLTT98nbmn6rWZ gEp9oMHlnY0txMEetWsVdng9MsBzrPN2zVyZXIseJEAVicRlWJyZwaAbdi/uwua2CdNo O3Wbyd9QGMsK4WGTGA1I6Df2DqYTPPb+DE7CytIf8UhQimTOxomsvpnKEl3N4LckZOKi kISg== X-Gm-Message-State: ALoCoQmWzHN8isLF4E/b6OvIupIpl/QjV2agFlfSdxEklZ/zC4plp3qE0R0Smn2JXdlpdMP3ylXL X-Received: by 10.42.75.196 with SMTP id b4mr68218806ick.7.1420413338843; Sun, 04 Jan 2015 15:15:38 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.92.22 with SMTP id a22ls648256qge.83.gmail; Sun, 04 Jan 2015 15:15:38 -0800 (PST) X-Received: by 10.140.27.197 with SMTP id 63mr85763qgx.6.1420413338067; Sun, 04 Jan 2015 15:15:38 -0800 (PST) In-Reply-To: X-Original-Sender: R.HL@gmx.net 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:15458 Archived-At: ------=_Part_350_1423860656.1420413337827 Content-Type: multipart/alternative; boundary="----=_Part_351_1415766871.1420413337827" ------=_Part_351_1415766871.1420413337827 Content-Type: text/plain; charset=UTF-8 Unfortunately I don't have many. Recently I came across an application of SFINAE in conjunction with reference parameters: If you want to restrict the type set you have to get the underlying type of the reference, excluding cv-qualifiers. template std::enable_if_t>, MyType>{}> foo(T&& t) {/* Some operations */ bar(std::forward(t));} (Possibly with is_base_of instead of is_same) Of course expression SFINAE could be applied but would have a different effect on overload resolution as it would take anything that the parameter is initialize-able with, whereas this overly choosy function template only takes MyTypes (or subclasses, if desired). I then thought of simplifying the above by splitting up the enable_if-part: template using enable_if_compatible = std::enable_if_t>, MyType>{}> I wondered why there is no trait to get the underlying, cv-unqualified type of a reference. This is how I ended up proposing it in here. I don't know whether it is often used but it is a lot more idiomatic to write remove_cv_reference instead of remove_cv>, which also seems to confuse people .. -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. ------=_Part_351_1415766871.1420413337827 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Unfortunately I don't have many. Recently I came across an= application of SFINAE in conjunction with reference parameters: If you wan= t to restrict the type set you have to get the underlying type of the refer= ence, excluding cv-qualifiers.

template <typename T= >
st= d::= enable_if_t<std::is_same<std::remove_cv_t<std::remo= ve_reference_t<T<= /span>>>, MyType>{}> foo(T&& t)
{/* Som= e operations */ bar(= std= ::forward<T>(t));}
(Possibly with = is_base_of instead of&nb= sp;is_same)
Of cour= se expression SFINAE could be applied but would have a different effect on = overload resolution as it would take anything that the parameter is initial= ize-able with, whereas this overly choosy function template only takes MyTypes (or subclasses, if desired).

I then thought of simplifying the above by spli= tting up the enable_if-part:

template <typename T, typename U>=
using enable_if_compatible= =3D std::enable_if_t<std::is_same<std::remove_cv_t<st= d::= remove_reference_= t<T>>, MyType>{}>

I wondered why there is n= o trait to get the underlying, cv-unqualified type of a reference. This is = how I ended up proposing it in here. I don't know whether it is often used = but it is a lot more idiomatic to write remove_cv_reference instead of remove_cv<remove_reference<>>, which also seems to confuse people.

--

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