220 25075 <5db50906-b180-475d-9f4d-7453897be2b0@isocpp.org> article Path: news.gmane.org!not-for-mail From: Arthur O'Dwyer Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: new auto Date: Tue, 8 Mar 2016 19:16:01 -0800 (PST) Lines: 156 Approved: news@gmane.org Message-ID: <5db50906-b180-475d-9f4d-7453897be2b0@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_3535_914147474.1457493361379" X-Trace: ger.gmane.org 1457493379 19674 80.91.229.3 (9 Mar 2016 03:16:19 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 9 Mar 2016 03:16:19 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDLZJYWNDQIPFKX6WYCRUBB3XYVMG@isocpp.org Wed Mar 09 04:16:05 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ig0-f200.google.com ([209.85.213.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1adUbU-0002Yq-Bd for gclcip-std-proposals@m.gmane.org; Wed, 09 Mar 2016 04:16:04 +0100 Original-Received: by mail-ig0-f200.google.com with SMTP id rx16sf50164004igc.3 for ; Tue, 08 Mar 2016 19:16:03 -0800 (PST) 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 :x-spam-checked-in-group:list-post:list-help:list-archive:sender :list-subscribe:list-unsubscribe; bh=TVYd8RGsOuvBMejvfv4jzrgil25Aksw8li66NztvtwQ=; b=J/ZopfrTBdHPwerHwHnIMDyauf35J2/961upmws0/v215wkat+ZQoUsougxW8peORH 6txtJMhuuBPJQnVzGE6SkzHdw3D+gs1p/uKomDkSeLvZqlG2yGRT5XnF2uh0QYTBwDnt 7oD2YS9UXkSBf+5zXX7YXYShrS4u/BZdgyyH5UIxexJIGbie+y0Mgz7OzFfMI0MBcD+B jZUKxd9Qa296BdnRq/BBIM/0xbVEMUsjkQFmQHe3wWMPKunMkDvege4QyShmy/dZex1Y MZ1qgXxaV3hs9sqo+20I64NgqUvD2pyU+ok9Be458MQsQUwQbqUp7qn8ZlutBRBghOzy Salw== 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 :x-spam-checked-in-group:list-post:list-help:list-archive:sender :list-subscribe:list-unsubscribe; bh=TVYd8RGsOuvBMejvfv4jzrgil25Aksw8li66NztvtwQ=; b=l+q2iRevIvZAxDbOYxXdi/7TENrxMGNV5C4RiRkG82NBe+QBVugq0pSw6XZkcIjPLr 4/BjD0fW20eS5/zfBnZvvIWHGCi3pIn78op6H2biI8h6mHMU8Bg8avBdaQ3F4Z5lsaUq c5mIPhzqnhq1JaGLKPAgrxC/AQDLDH6/wqAtEarHMJ79MyEplhMUkCls599XMiNDr7Ks tXJeie7N+JnyLHwSjKBpvD5TC3K9GoeAKy1R94SdaKma2HPLIbAzpqn40P0RXpLZjL8S woqkWzwCopfl02mpYJgvK3kZu2cStt7kjcOK4KFbkOXqN99ofriiNoN5OOhF8j0BOGib IUZA== 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:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:sender:list-subscribe:list-unsubscribe; bh=TVYd8RGsOuvBMejvfv4jzrgil25Aksw8li66NztvtwQ=; b=BhxNw5h9Ha8jVhYk+xrIKMMlOqFqBdWOjcepixbrw3PzR7xz5oruG5Zll9mNhvi7Y8 sChz/3euMC9N3PrHMdYzwbkMg9MWCWAZ1lAONVv6ivsFen3aVOFM1wuX91yH0vR66toc MlUV4MuM/rgQkMK2sOQrSz+pD+SgXBnhjdoMCEniOZ+/hF4PBX98u4yTIC2znO8pAePD u7XGVK+BD/Y654pwysarv7bJht7U0iMNP7wsRNNrOW8/xChNkFKgFiiLlsqwo7dECrRY jLACsUfmSBpynTdegxy67ULJPPbF5BuCW4YvKBM959AdXtzq4hCdnncpCmeFAYn9eg3n yv9g== X-Gm-Message-State: AD7BkJLOjX53C5sh20pRlO8ddqTmXxQYIZpbwa3qhjdghoyYiNynzYCk9IDhmRqK/5/pbg== X-Received: by 10.182.65.135 with SMTP id x7mr24392319obs.13.1457493363370; Tue, 08 Mar 2016 19:16:03 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.164.38 with SMTP id n38ls242240ioe.80.gmail; Tue, 08 Mar 2016 19:16:02 -0800 (PST) X-Received: by 10.50.142.39 with SMTP id rt7mr423862igb.8.1457493362071; Tue, 08 Mar 2016 19:16:02 -0800 (PST) In-Reply-To: X-Original-Sender: arthur.j.odwyer@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: Original-Sender: std-proposals@isocpp.org List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:25075 Archived-At: ------=_Part_3535_914147474.1457493361379 Content-Type: multipart/alternative; boundary="----=_Part_3536_1831975150.1457493361379" ------=_Part_3536_1831975150.1457493361379 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Tuesday, March 8, 2016 at 10:34:05 AM UTC-8, Dale Weiler wrote: > > Exibit > class A { > A(); > T *a; > }; > > > Need to know type of *a* here to call new correctly: > A::A() : a(new T) { } > > Imagine the class member type changes but implementing code stays the > same, thus it allocates an incorrect amount of memory for the object and > calls the wrong constructor. How do we solve this?=20 > A::A() : a (new typename remove_reference::type) { } > > Now, now, it's not that bad. A::A() : a(new std::remove_reference_t) {} Anyway, you can't have the type of auto in new auto depend on the *context= =20 in which it's called*. That way lies Perl, and std::initializer_list, and= =20 various other madnesses. The "best practices" answer in this context is almost certainly *don't use= =20 naked new.* Use owning smart pointers, or don't use pointers at all, and=20 then you don't need to use new, and then all your problems related to new's= =20 weird syntax pretty much go away. my $.02, =E2=80=93Arthur --=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. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/5db50906-b180-475d-9f4d-7453897be2b0%40isocpp.or= g. ------=_Part_3536_1831975150.1457493361379 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Tuesday, March 8, 2016 at 10:34:05 AM UTC-8, Dale Weile= r wrote:
=
Exibit


class A {
=C2=A0 A();
=C2=
=A0 T *=
a;
<=
/span>};


Need to know type of a here to call new correctly:

=C2=A0A::<=
span style=3D"color:#000">A() : a(new T) { }

Imagine the class member type changes but implementing code stays the
same, thus it allocates an incorrect amount of memory for the object and
calls the wrong constructor. How do we solve this? 
A::=
A() : a (new typename=
 remove_reference<decltype(A::a<=
wbr>)>::type) { }<=
/span>

Now, now, i= t's not that bad.

A::A() : a(new std::remove_reference_t<<= span style=3D"color: #008;" class=3D"styled-by-prettify">decltype(*a)>) {}

Anyway, = you can't have the type of auto in new auto depend on th= e context in which it's called. That way lies Perl, and <= font face=3D"courier new, monospace">std::initializer_list, and vari= ous other madnesses.

The "best practices"= ; answer in this context is almost certainly don't use naked new. Use owning smart poin= ters, or don't use pointers at all, and then you don't need to use = new, and then all your problem= s related to new's weird s= yntax pretty much go away.

my $.02,
=E2= =80=93Arthur

--
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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/5db50906-b180-475d-9f4d-7453897be2b0= %40isocpp.org.
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