220 32802 <05407ce7-891e-4355-a5de-d58a7f8f947e@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: virtual class Date: Tue, 20 Jun 2017 08:54:07 -0700 (PDT) Lines: 88 Approved: news@gmane.org Message-ID: <05407ce7-891e-4355-a5de-d58a7f8f947e@isocpp.org> References: <24b0f800-5cea-47fe-b0ac-13333003b959@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2133_1985377505.1497974047951" X-Trace: blaine.gmane.org 1497974054 24094 195.159.176.226 (20 Jun 2017 15:54:14 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 20 Jun 2017 15:54:14 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBIEKUXFAKGQEDWSDCPQ@isocpp.org Tue Jun 20 17:54:08 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f198.google.com ([209.85.192.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dNLTj-0005iW-4X for gclcip-std-proposals@m.gmane.org; Tue, 20 Jun 2017 17:54:08 +0200 Original-Received: by mail-pf0-f198.google.com with SMTP id a82sf132982572pfc.8 for ; Tue, 20 Jun 2017 08:54:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; 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; bh=wpioKUWrnIGaxyOrHtlCqqFtMOzMiUE+ilY2F0V3R/w=; b=ZVFsKAA0bES5r+H9hbPG2/EuhpmP+wKrMP0dcQJzCHTSmIb5wnUQ5Nap4qu/r/8+1S n9KZmKyIozITmnCMZD7Lky39TIkeO1PBbfBkAspBqfKKYI2zd/t7QeuzN924id3B4AHG h/xoYheBjUDAVSVFtYsMy858oQzfY484dV8NJRUyxjNqhmm/7R/PGO7jcFEajXdeehmv IiFZ2PxKlRQT8/v5agvGBIPH1XG1GA21mUShKeW/Frz5UnCHRtQqAOpcoUmX8EkMrvfg FXonRql59W6TPoCnWfNUMz7xvVFFw9QXy801Y9a2NfBis52n5dD86FoJ5s2SrUQSsDXb lYSw== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; 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; bh=wpioKUWrnIGaxyOrHtlCqqFtMOzMiUE+ilY2F0V3R/w=; b=l4vlcSBFh6U80zOPhy25W7zzPhevJyCGNbL5BbGQkApLJpVSgD4Nie7bVxXNYaSzVF kXxcixt0MUgzasJlSE2eeouKinGw9AcChgNnDFAd5jTSlIkRem1X4Ydefti2R6BRdwUI 6oLFgSk61s/5xT4Y6SGmKFxUAPvAF5pMp9qIEKInCRzVQe+bTX+x5cJ1dbZVAwCvu5NB EUuOerRC/7zR+p8JM7UlfQnkI0Xde5ZrKezMrKSuqvprzuY7v0q07svk581yonmTzTv9 jpDg4gvN2sZ+vtUhGUMCYrVKuZ548ShAbH+2zSIpWbNrQg5UBBR6kOKMF3yaF9N+pFc8 djUg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=wpioKUWrnIGaxyOrHtlCqqFtMOzMiUE+ilY2F0V3R/w=; b=BALwliA5ReLMzGCjhoCaKuqvEwti+YNzvmmRbXINWnsNajYRw+kxMqxu7Y4Aor8zd5 R+jTNboNF8+SLmqUM82Bb5RUOa9719p77SiiOfhH388WGjROn76ZetR9NuC3VymbOHCg VEbTYLu9aqLcuA22snLV0bPlb/nALRoykji8jadI/+2UBXHkQHl7v5unFUE1ha7/cAJx nd+JplssPOO7Wug2CXeVx5gW1hwgdUDHjvxd4j3TE4TXMeBxTXE3EOdHVrDMoG9YwoLi KEPlPBguKfuuTAuLkjlY/Y9+gEvIM0VlPeRFssjn9SnWb729nsgTqW1nW8WNSb7R6h+v YrSA== X-Gm-Message-State: AKS2vOzdE6uj4fbi1W7MhxmsF3OX8GwIDniefjdMApuYYZnkEzmifGeT IOrIG/YIqtwLHIFu X-Received: by 10.101.88.203 with SMTP id e11mr16606975pgu.126.1497974049563; Tue, 20 Jun 2017 08:54:09 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.47.194 with SMTP id b2ls1914606otd.5.gmail; Tue, 20 Jun 2017 08:54:08 -0700 (PDT) X-Received: by 10.157.47.207 with SMTP id b15mr929786otd.20.1497974048557; Tue, 20 Jun 2017 08:54:08 -0700 (PDT) In-Reply-To: X-Original-Sender: jmckesson@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:32802 Archived-At: ------=_Part_2133_1985377505.1497974047951 Content-Type: multipart/alternative; boundary="----=_Part_2134_210507740.1497974047951" ------=_Part_2134_210507740.1497974047951 Content-Type: text/plain; charset="UTF-8" On Tuesday, June 20, 2017 at 11:36:57 AM UTC-4, Ricardo Andrade wrote: > > What to say about final and override then? > Do they lead to more use of bad dynamic polymorphism or just make it less > error prone? > Any idea that prevents mistakes should be welcome. > `override` merely prevents mistakes. The cases where `override` would cause a compile error are cases where the user's declared intent does not match the function signature they have written. You intend to override a function, but there is no matching signature to be override. That's clearly a mistake. This notion of `virtual` "prevents mistakes" only if you start from the belief that all non-virtual inheritance is a mistake. But this belief is decidedly nonsensical. Please provide a demonstration of a class that *must* be virtually inherited in all places where it gets used as a base class. Where *all* non-virtual inheritance of such a class would be as clear a mistake as those caught by `override`. You could suggest a "lighter" version of this `virtual class` notion, where you only get a compile error if there are multiple, non-virtual inheritances of the same base class. However, I would again like to see a base class where, by the nature of that base class, this would *always* be a problem. -- 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/05407ce7-891e-4355-a5de-d58a7f8f947e%40isocpp.org. ------=_Part_2134_210507740.1497974047951 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Tuesday, June 20, 2017 at 11:36:57 AM UTC-4, Ricardo An= drade wrote:
W= hat to say about final and override then?
Do they lead to more use of b= ad dynamic polymorphism or just make it less error prone?
Any ide= a that prevents mistakes should be welcome.

`override` merely prevents mistakes. The cases where `override` would= cause a compile error are cases where the user's declared intent does = not match the function signature they have written. You intend to override = a function, but there is no matching signature to be override. That's c= learly a mistake.

This notion of `virtual` "prevents mistakes&q= uot; only if you start from the belief that all non-virtual inheritance is = a mistake. But this belief is decidedly nonsensical.

Please provide = a demonstration of a class that must be virtually inherited in all p= laces where it gets used as a base class. Where all non-virtual inhe= ritance of such a class would be as clear a mistake as those caught by `ove= rride`.

You could suggest a "lighter" version of this `vir= tual class` notion, where you only get a compile error if there are multipl= e, non-virtual inheritances of the same base class. However, I would again = like to see a base class where, by the nature of that base class, this woul= d always be a problem.

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