220 26556 article Path: news.gmane.org!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: auto type deduction for *_cast return argument Date: Tue, 5 Jul 2016 11:08:37 +0300 Lines: 41 Approved: news@gmane.org Message-ID: References: <58140e46-2adc-4eac-8f51-72d3685cdf0b@isocpp.org> <7E3409C2-A2AC-4562-BB06-6D5D5ACE8AC8@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-Trace: ger.gmane.org 1467706132 19116 80.91.229.3 (5 Jul 2016 08:08:52 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 5 Jul 2016 08:08:52 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBC5JHI7A7ALRBBWW5W5QKGQEFSBCTZQ@isocpp.org Tue Jul 05 10:08:48 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f199.google.com ([209.85.192.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bKLPT-0001Co-6c for gclcip-std-proposals@m.gmane.org; Tue, 05 Jul 2016 10:08:47 +0200 Original-Received: by mail-pf0-f199.google.com with SMTP id g62sf434964404pfb.3 for ; Tue, 05 Jul 2016 01:08:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=SEknLSt60wlhrAKf0wA8Tq1iJcWzLU/C1TRdGnU+Vtw=; b=ac7uh7Td7i6AL2psAWC3fzFtymfAyXw9Mr62S8mLtqyLNRazTiuZsL7A7TMcoLyY2V 8jG2UGP2zfG4ArMVtz3Eddptf8Cw1xqsEdnNYsAm5QQZBZJOmy0RGIwUvNLWsBsLNbP9 YUhXgqlJRcmnMNYUyffiOTnn7bbGRdPLFWzGicXeltfNmNlHRxExOlHHJa48Y56WY4sL TKBR2wHtShBe8NsEgy4X6Lnk2HbwZGczlNZePAOqCUPqu6xRroVA15xNinfsgD0KYmVM VlqonJbyanAue/cCheoJx1Jl1VatPpkt3kCLswGzkVQtxoe8sM73U0H+AhEqN7CfTsDL PkfA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=SEknLSt60wlhrAKf0wA8Tq1iJcWzLU/C1TRdGnU+Vtw=; b=B2HKSVbGBQZvnBYkOKZ3Ra4BFYwUmoqnyE8REf6+Ypr6Quo9P5RYhi/5ZA+ld9BsOk FDR3X+RCNUwtrQzu7tbpfrAtEYCKUjgdw1DI7ZSDg8K1FXS1NN+xv0u3+lZbXOyW63Zm RATPwvKrKLrBNQFzAnzAhElIqdhnLvKfeuR9aBDPW+7jm7cfZPdBaksAnWhChG6lL6dE peVl0sdnN9VGgMD//N3WDRsNXtouC/LzyvOYO4RTYac41BVfNFVAe1eGiaPXnY/SZtuX djl40Y+4sUGZUWqgxW4n9HyJf+9HYRstLdndYSxvpucoXe7vWpDDAE5hxlnsy9+K8QOP 1dVA== X-Gm-Message-State: ALyK8tLPji/C7ouMpAVx+DWDzpMYc0Rj4I4na8/hpenBVxmsKUnNEypfRHAjcfuNEx97SQ== X-Received: by 10.66.7.165 with SMTP id k5mr9277921paa.10.1467706119022; Tue, 05 Jul 2016 01:08:39 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.19.205 with SMTP id 74ls1803061iot.6.gmail; Tue, 05 Jul 2016 01:08:38 -0700 (PDT) X-Received: by 10.159.54.201 with SMTP id p67mr5625523uap.59.1467706117961; Tue, 05 Jul 2016 01:08:37 -0700 (PDT) Original-Received: from mail-vk0-x230.google.com (mail-vk0-x230.google.com. [2607:f8b0:400c:c05::230]) by mx.google.com with ESMTPS id q189si2395476vkb.96.2016.07.05.01.08.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 05 Jul 2016 01:08:37 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400c:c05::230 as permitted sender) client-ip=2607:f8b0:400c:c05::230; Original-Received: by mail-vk0-x230.google.com with SMTP id k68so168652879vkb.0 for ; Tue, 05 Jul 2016 01:08:37 -0700 (PDT) X-Received: by 10.31.85.3 with SMTP id j3mr5345781vkb.130.1467706117558; Tue, 05 Jul 2016 01:08:37 -0700 (PDT) Original-Received: by 10.103.64.83 with HTTP; Tue, 5 Jul 2016 01:08:37 -0700 (PDT) In-Reply-To: <7E3409C2-A2AC-4562-BB06-6D5D5ACE8AC8@gmail.com> X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400c:c05::230 as permitted sender) smtp.mailfrom=ville.voutilainen@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=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:26556 Archived-At: On 5 July 2016 at 10:59, Jonathan Coe wrote: > Thanks for suggesting this, I'm afraid I don't think I like this idea. > > It saves a bit of typing but would make it less clear what's going on and in > cases where a function signature changes would change the meaning of > existing code without any edits being made > > f(A a); > > Changed to > > f(B b); > > Would change the meaning of > > f(dynamic_cast(x)); > > In a rather non-obvious way. I don't think the behaviour of existing casts should be changed, but presumably, with template argument deduction for constructors, we can define new wrapper types that take the wrapped type as an argument of a constructor template and have a conversion operator template that does the right kind of cast before returning the result. Then we can do something like f(auto_dyn_cast(x)); There are of course cases where the target type can't be deduced from the context, like when used in an init-capture of a lambda, or with an auto-typed variable declaration, or in a deduced template argument. -- 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/CAFk2RUaaP4XC9H7d9e1GBDi8JA4F2dSf2i%3DqB7mqRUXj1Lui5Q%40mail.gmail.com. .