220 38914 article Path: news.gmane.org!.POSTED!not-for-mail From: Brian Bi Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allow forward class declarations to match a type alias. 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[209.85.220.41]) by mx.google.com with SMTPS id x4-v6sor3779740oif.91.2018.06.29.14.45.11 for (Google Transport Security); Fri, 29 Jun 2018 14:45:11 -0700 (PDT) Received-SPF: pass (google.com: domain of bbi5291@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:aca:66db:: with SMTP id m88-v6mr9444100oik.141.1530308710863; Fri, 29 Jun 2018 14:45:10 -0700 (PDT) Original-Received: by 2002:a4a:6f4d:0:0:0:0:0 with HTTP; Fri, 29 Jun 2018 14:45:10 -0700 (PDT) In-Reply-To: X-Original-Sender: bbi5291@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=pcZfGQj8; spf=pass (google.com: domain of bbi5291@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=bbi5291@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE 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:38914 Archived-At: --0000000000001b98b9056fcec3d8 Content-Type: text/plain; charset="UTF-8" It's not an oversight. It's a design decision that "class FloatImage;" forward-declares a class whose *identity* is known; its true name is FloatImage (in the same namespace). It is already known not to be identical with CudaImage. Saying that it is, later, is contradicting what you told the compiler earlier. As Nicol Bolas says, you need a third type of declaration to do what you want to do. On Fri, Jun 29, 2018 at 12:56 AM, Bengt Gustafsson < bengt.gustafsson@beamways.com> wrote: > 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 > > . > -- *Brian Bi* -- 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/CAMmfjbNwyrk7D3wFqe7xxrOwED0AVMLz5_P9m1%3DS2WunjM1Lqg%40mail.gmail.com. --0000000000001b98b9056fcec3d8 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
It's not an oversight. It's a design decision= that "class FloatImage;" forward-declares a class whose ident= ity is known; its true name is FloatImage (in the same namespace). It i= s already known not to be identical with CudaImage<float>. Saying tha= t it is, later, is contradicting what you told the compiler earlier.
As Nicol Bolas says, you need a third type of declaration to do what= you want to do.

On Fri, Jun 29, 2018 at 12:56 AM, Bengt Gustafsson <= bengt.gustafsson@beamways.com> wrote:
A forward declaration of a class name is curren= tly not allowed to be resolved by a type alias of a class template. I don&#= 39;t see the logic to this, why is this illegal:

// forward d= eclaration in one header file.
class FloatImage;


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

using FloatImage = =3D CudaImage<float>;


Godbolt:=C2=A0https://godbolt.org/g/mmTTeQ<= /a> shows that all compilers reject this.


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

so is there a technical reason preventing this fr= om being allowed or is it just an oversight that could be corrected?
<= span class=3D"HOEnZb">

=

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--
<= div>
Brian Bi

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