220 13943 <997390c5-072b-407a-bce9-22a29fc5e228@isocpp.org> article Path: news.gmane.org!not-for-mail From: Edward Catmur Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Default values for n4191 fold expressions Date: Wed, 15 Oct 2014 16:53:46 -0700 (PDT) Lines: 41 Approved: news@gmane.org Message-ID: <997390c5-072b-407a-bce9-22a29fc5e228@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Trace: ger.gmane.org 1413417235 22643 80.91.229.3 (15 Oct 2014 23:53:55 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 15 Oct 2014 23:53:55 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDZLZTXF7UJBBCUS7SQQKGQELPI73KQ@isocpp.org Thu Oct 16 01:53:48 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pd0-f199.google.com ([209.85.192.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1XeYO4-0003jT-Kn for gclcip-std-proposals@m.gmane.org; Thu, 16 Oct 2014 01:53:48 +0200 Original-Received: by mail-pd0-f199.google.com with SMTP id g10sf11759846pdj.10 for ; Wed, 15 Oct 2014 16:53:47 -0700 (PDT) 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: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:content-transfer-encoding; bh=63a0qAZ4wd3opBhOLzSNJq956E/4NCVWSt7CEQ3oaQM=; b=eN/imoOK0cOVFNHfuXJRT89yG/hqBXpSXx2YJ49Q0qw41P6Zeid5obuJkYs/bUhbuX nl6UB4XTtJivx6Oj1v7yM7GbNijLs6rTk+CVEBb0ip9zyt75teWYT3ZpiXQnA06wj7p/ DX34XOrTqiwzgUmlr3di7P25+Bd3QlWSO3saTy+1NIyaFqrd1Hu4ggXlTMfp+Rm0p1mt JtOh9d6JjSqaZvlUimEq9h0gmTEzjAyJNbAgZ8tBxQjaSIMuF2K50J7jHWNVMNKyhgAp qOVrA9SIEz6eNolPZSOw7qtWvN/kudXyvisqLJl/om88+3etykGzybtugRPDwq9IDEIL a4Kg== X-Gm-Message-State: ALoCoQl318QLP4nvKr1PM4LrD2BKSTtiDBtosyNPxFeQKChj11ln6cHYCM5nb8DjSveTxkhHd5mZ X-Received: by 10.66.139.167 with SMTP id qz7mr10793057pab.23.1413417227567; Wed, 15 Oct 2014 16:53:47 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.18.143 with SMTP id 15ls478523qgf.70.gmail; Wed, 15 Oct 2014 16:53:46 -0700 (PDT) X-Received: by 10.140.19.1 with SMTP id 1mr64424qgg.19.1413417226819; Wed, 15 Oct 2014 16:53:46 -0700 (PDT) X-Original-Sender: ed@catmur.co.uk 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:13943 Archived-At: N4191 provides that the fold of an empty parameter pack is defined for the = binary operators + * & | && || , to 0 1 0 -1 false true void() respectively= .. I am concerned that this may result in confusing or undefined behavior, e= specially for types that overload these operators. Consider string s =3D (to_string(args) + ...); if the 0 is the literal and = so can be interpreted as a null pointer constant this results in undefined = behavior, while if it is a prvalue int this gives a string of length 1 cont= aining a nul character. Other types give different errors; std::chrono::dur= ation fails as its constructor from int is explicit.=20 For binary * the library type valarray is unlikely to behave as desired; si= milarly a square matrix type could give incorrect results. The binary bitwi= se operators present a similar issue, and even for primitive types -1 is li= kely to be of the wrong length and/or signedness (what happens when 32-bit = -1 is promoted to uint64_t?) I would therefore argue to disallow empty parameter pack fold expansions fo= r the arithmetic and bitwise operators. The binary boolean operators and co= mma operator can stay; they are far less often overloaded and the default v= alues are simultaneously more obviously correct and unique, more difficult = to remember, and require more typing than for the arithmetic and bitwise op= erators. Regards, Ed=20 PS. If we are doing bitwise operators, what about ^? PPS. Is ~ really supposed to be in there?=20 --=20 ---=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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. .