220 38901 article Path: news.gmane.org!.POSTED!not-for-mail From: Bengt Gustafsson Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Allow forward class declarations to match a type alias. Date: Fri, 29 Jun 2018 00:56:26 -0700 (PDT) Lines: 170 Approved: news@gmane.org Message-ID: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_10954_67878902.1530258986119" X-Trace: blaine.gmane.org 1530258862 31570 195.159.176.226 (29 Jun 2018 07:54:22 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 29 Jun 2018 07:54:22 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCRIRSPDTQIRBK6M27MQKGQEVAIXGBI@isocpp.org Fri Jun 29 09:54:17 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb0-f197.google.com ([209.85.213.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fYoES-00086P-TI for gclcip-std-proposals@m.gmane.org; Fri, 29 Jun 2018 09:54:17 +0200 Original-Received: by mail-yb0-f197.google.com with SMTP id s16-v6sf6038420ybp.12 for ; Fri, 29 Jun 2018 00:56:28 -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:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=42J8WypY/Bzx10JVfn/HKtAI379Gwv5Un6EU2ELFgnE=; b=svVNxYnNzgR8ecZ6WSDiu8cucHISfSmowHyPvT2uvVmZ7cJyeicB6oJ5O4Nq6ZThSv XjtI722nGiDrRq5kq0HOANOJLgj5wmqYL9YFwfSUeALD9ayqNRKLNfX6gdYeK6ZjpcQ4 Qk53xJ70lWQ/KvR4F6Y8aO91r00zeWqI/7mTOucsgW2L4e4xjlnftnYiT+hStCDlrrNY xLtzydgw9S1HLkHEvU2IUSJAEOXaNQbqnN0f5lReIjfW+xzaFENUe/76nXfk+8++8usy SSqxaw+THbW44pPd6lQlV4O+P+RsUoYw9V/y8BARsxyDDyIuJZ8pAxfXfTZBf6xebi3Y 3aBg== 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: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=42J8WypY/Bzx10JVfn/HKtAI379Gwv5Un6EU2ELFgnE=; b=DEkZR6XcOi3Jwt1O0CAeIzBkIfWGH6EaKX09gc/GWa1zcb4COgPKughow3oWBYUkL1 Hyn3SAhydOVjII30CAU6o9v7GDUGnTNiGcGJ4AKn9iJQ1+uf/dYusm0s9p2yii7dAFeb +A+TBg9PeMt7uEDs71ygvh4eXQ0kz10cwjC5a9EFIHm+mu49clSN6rLjeNBnryA0jww+ 6iIjiTj1vyuH0KvRcfdssDlIbtCi/IfuFo68lre965qUUCIC9D1TWV2fUT4+SoTBU5P2 rbZvXbDnpKs6Lvqhwe92kRGY5h5LKPpQBsEQlcZwXnBe7BCfSTknR4ofbzfSF1SgNY8v Jg9Q== X-Gm-Message-State: APt69E0/d2WbIwAaD8i5TsdiN0BctxYBqz8i5H5btH13Kf0g9N6o5ZTO IWPyBfVwYMy3jze/McqAiQEenw== X-Google-Smtp-Source: AAOMgpcSB/VN4KMTNRxtuTLhIbG54jKqYXUGMRomPFRngVBWQZ8tZGdBPyKUTUPTKJ5zJdcLxOkpAg== X-Received: by 2002:a0d:e583:: with SMTP id o125-v6mr3809831ywe.50.1530258988281; Fri, 29 Jun 2018 00:56:28 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:a25:ba4d:: with SMTP id z13-v6ls2825801ybj.8.gmail; Fri, 29 Jun 2018 00:56:26 -0700 (PDT) X-Received: by 2002:a25:8448:: with SMTP id r8-v6mr1298620ybm.2.1530258986719; Fri, 29 Jun 2018 00:56:26 -0700 (PDT) X-Original-Sender: bengt.gustafsson@beamways.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:38901 Archived-At: ------=_Part_10954_67878902.1530258986119 Content-Type: multipart/alternative; boundary="----=_Part_10955_1719410585.1530258986120" ------=_Part_10955_1719410585.1530258986120 Content-Type: text/plain; charset="UTF-8" A forward declaration of a class name is currently not allowed to be resolved by a type alias of a class template. I don't see the logic to this, why is this illegal: // forward declaration in one header file. class FloatImage; // Definition in another header file. template class CudaImage { }; using FloatImage = CudaImage; Godbolt: https://godbolt.org/g/mmTTeQ shows that all compilers reject this. This has created a fair amount of trouble for me when trying to refactor what was previously a FloatImage class to a generic pixel type template. I don't see the logic to making this illegal as FloatImage is still the name of a class, i.e. an instance of the CudaImage class template. While I can update the forward declaration to: // forward declaration in one header file. template class CudaImage; using FloatImage = CudaImage; This seems to be an unnecessary leakage of implementation detail. so is there a technical reason preventing this from being allowed or is it just an oversight that could be corrected? -- 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/b2fb570b-aadb-423e-9821-025cb447cf60%40isocpp.org. ------=_Part_10955_1719410585.1530258986120 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
A forward declaration of a class name is currently not all= owed to be resolved by a type alias of a class template. I don't see th= e logic to this, why is this illegal:

// forward declarat= ion in one header file.
class FloatIm= age;


// Definition = in another header file.
template<t= ypename T<= /span>>= class CudaImage {
};

= using FloatImage =3D CudaImage<float>;


Godbol= t:=C2=A0https://godbolt.org/g/mmTTeQ shows that all compilers reject this.<= /div>

This has=C2=A0 created a fair amount of trouble f= or me when trying to refactor what was previously a FloatImage class to a g= eneric pixel type template. I don't see the logic to making this
<= div liberation=3D"" mono",=3D"" courier,=3D"" monospace;font-weight:=3D"" n= ormal;font-size:=3D"" 14px;line-height:=3D"" 19px;white-space:=3D"" pre;"= =3D"">illegal as FloatImage is still the name of a class, i.e. an instance = of the CudaImage class template.

While I can updat= e the forward declaration to:
// forward declaration in one header file.
template<typename T> class CudaImage;
using<= span style=3D"color: #000;" class=3D"styled-by-prettify"> FloatImage =3D CudaImage<float>;

This seems to= be an unnecessary leakage of implementation detail.

=
so is there a technical reason preventing this from being allowed or = is it just an oversight that could be corrected?

=

--
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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/b2fb570b-aadb-423e-9821-025cb447cf60= %40isocpp.org.
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