220 6059 article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Templates usage with *this Date: Sat, 31 Aug 2013 19:26:42 -0700 (PDT) Lines: 172 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_1511_23485993.1378002403000" X-Trace: ger.gmane.org 1378002407 32464 80.91.229.3 (1 Sep 2013 02:26:47 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 1 Sep 2013 02:26:47 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBZGLRKIQKGQE32MEXJA@isocpp.org Sun Sep 01 04:26:49 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ie0-f197.google.com ([209.85.223.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VFxNG-0001on-Bb for gclcip-std-proposals@m.gmane.org; Sun, 01 Sep 2013 04:26:46 +0200 Original-Received: by mail-ie0-f197.google.com with SMTP id c11sf1299243ieb.4 for ; Sat, 31 Aug 2013 19:26:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=VxDen5u57U0zVnYAtRLyWWldY1eNJB9+dNy7Doz4/Qs=; b=gzCHAzJw0/4mi5JoPk/d+JByv+KMjOQp+eOARad+SynQViOR/SLltMMIqKvJLQ9u3a BfwCEatcDInR6Gbm5PJAabzGzeWOyHAspBr3SOzLQU9bTkG/Y7MGTjrQfUTefcxTGv8P 3l82w1ssyb2+aB2+2Gf8UqmQHqmxfsU1pcHuPYS0g2tCM25XxPW7hJLWFCNIFG6oCqhO 2Y2VPo/BMYLJfsGbgADysrMNEDxcIWh/3rsWQH7TMhslAfjktDMLXRRRcjpG5Yvs3f2F ZfK4V6uIOOr0mH39u8TuYIl1YzQVYNlbv+bWz3rm2J/d4GHCo9EXwKDPOiFzYtx7ZiTI fLDw== X-Received: by 10.50.36.42 with SMTP id n10mr10284815igj.0.1378002404739; Sat, 31 Aug 2013 19:26:44 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.126.10 with SMTP id mu10ls902517igb.37.gmail; Sat, 31 Aug 2013 19:26:44 -0700 (PDT) X-Received: by 10.50.6.98 with SMTP id z2mr393056igz.0.1378002404181; Sat, 31 Aug 2013 19:26:44 -0700 (PDT) In-Reply-To: X-Original-Sender: potswa@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:6059 Archived-At: ------=_Part_1511_23485993.1378002403000 Content-Type: text/plain; charset=ISO-8859-1 I must admit I didn't read your entire post. You can't get the actual type of the argument which initialized *this; as noted only templates can represent simultaneous types and otherwise you have all the combinations of cv-qualification and value category. What you can do is forgo a member function and use ADL and a non-member function template with perfect forwarding regulated by SFINAE. This gets you not only cv-qualification and value category but also the static type from the call site. struct s { int x; template< typename self > friend typename std::enable_if< std::is_base_of< s, typename std::decay< self >::type >::value, std::decltype( std::declval< self >().x ) >::type get_x( self && o ) { return std::forward< self >( o ).x; } }; If I were you, though, I'd just elaborate the accessor functions as before and delegate from the rvalue overload to the lvalue overload with an explicit move. As you mentioned, const rvalue isn't actually used for anything, so the const overload need not be ref-qualified. So the new change doesn't effectively double the complexity, but in usual practice only one new case is added, which essentially just forwards to an existing case. (Standard disclaimer: accessor functions are evil. C++ has a perfectly good model for accessing members; do not roll your own unless you legitimately have a custom object model. It looks like OP here might.) -- --- 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_1511_23485993.1378002403000 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable
I must admit I didn't read your entire post.

You ca= n't get the actual type of the argument which initialized *this; as noted o= nly templates can represent simultaneous types and otherwise you have all t= he combinations of cv-qualification and value category.

What you can= do is forgo a member function and use ADL and a non-member function templa= te with perfect forwarding regulated by SFINAE. This gets you not only cv-q= ualification and value category but also the static type from the call site= ..

str= uct s {
    int x;

    template< typename self >
    friend
    typename std::e= nable_if&l= t; std::is_base_of< s, typename std::decay= < self >::type >::value,
        std::decltype( std::declval< se= lf = >().x ) >::type
    get_x( self && o )
&nb= sp;       { ret= urn std::forward< self >( o ).x; }
};

If I were yo= u, though, I'd just elaborate the accessor functions as before and delegate= from the rvalue overload to the lvalue overload with an explicit move. As you mentioned, c= onst rvalue isn't actually used for anything, so the const overload need not be ref-qualifi= ed. So the new change doesn't effectively double the complexity, but in usu= al practice only one new case is added, which essentially just forwards to = an existing case.

(Standard disclaimer: accessor functions are evil.= C++ has a perfectly good model for accessing members; do not roll your own= unless you legitimately have a custom object model. It looks like OP here = might.)

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