220 38912 article Path: news.gmane.org!.POSTED!not-for-mail From: Richard Hodges 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 189-v6sor3646753ioc.320.2018.06.29.09.43.05 for (Google Transport Security); Fri, 29 Jun 2018 09:43:05 -0700 (PDT) Received-SPF: pass (google.com: domain of hodges.r@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:a6b:c781:: with SMTP id x123-v6mr13883178iof.100.1530290585301; Fri, 29 Jun 2018 09:43:05 -0700 (PDT) In-Reply-To: X-Original-Sender: hodges.r@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=KPdb92Cn; spf=pass (google.com: domain of hodges.r@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=hodges.r@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:38912 Archived-At: --000000000000bd76c3056fca8a75 Content-Type: text/plain; charset="UTF-8" I am wondering if your motivation is an XY problem, please see inline. On Fri, 29 Jun 2018 at 09:56, 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. > It seems to me that you previously forward-declared the class FloatImage, which was later defined. The might have looked like this: class FloatImage; // we can reference the type before definition extern void foo(FloatImage& fi); Later, before using a FloatImage, you would define the class. The new code would need to look like this: template class CudaImage; using FloatImage = CudaImage; // we can *still* reference the type before definition extern void foo(FloatImage& fi); Later, but before actually using a FloatImage, you would define the template: template class CudaImage { }; > > 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. > I think the above solution resolves this leakage. > > so is there a technical reason preventing this from being allowed or is it > just an oversight that could be corrected? > I don't think in reality that it's needed, or that the lack of the feature is an oversight. > -- > 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 > > . > -- 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/CALvx3hbmq1NUgE3njDeLLrmVX%3DGwAVfsUK-T7spZZFmgk_vmUg%40mail.gmail.com. --000000000000bd76c3056fca8a75 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
I am wondering if your motivation is an XY problem, please= see inline.

On Fri, 29 = Jun 2018 at 09:56, Bengt Gustafsson <bengt.gustafsson@beamways.com> wrote:
A forward declaration of a class n= ame is currently not allowed to be resolved by a type alias of a class temp= late. I don't see the logic to this, why is this illegal:

// forward declaration in one header file.
class FloatImage;


// D= efinition in another header file.
template= <typename T> clas= s CudaImage {
<= span style=3D"color:#660" class=3D"m_-6556502269256549060styled-by-prettify= ">};

using FloatImage= =3D CudaImage<float>;


Godbolt:=C2=A0https://godbolt.org/g/m= mTTeQ shows that all compilers reject this.

Th= is has=C2=A0 created a fair amount of trouble for me when trying to refacto= r 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 templat= e.

It seems to me t= hat you previously forward-declared the class FloatImage, which was later d= efined.

The might have looked like this:

class FloatImage;
// we can reference the type before definition
= exte= rn void foo(FloatImage& = fi);


Later, before using a FloatImage, you would define the cla= ss.

The new code would need to look like this:

template<typen= ame T> class CudaImage;
using FloatImage =3D CudaImage<float>;
// we can *still* reference the type before definition
extern=C2=A0= void=C2=A0foo(FloatImage& fi);=


Later, b= ut before actually using a FloatImage, you would define the template:

template<class T> class CudaImage=
{
};

=C2=A0

While I can update = the forward declaration to:
// forward declaration in one header file.
= template<typename T>= ; class CudaImage;=
using FloatImage = =3D CudaImage<float>;=

This seems to be an unnecess= ary leakage of implementation detail.

I think the above solution resolves this leakage.<= /div>
=C2=A0
=

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

I don't thi= nk in reality that it's needed, or that the lack of the feature is an o= versight.
=C2=A0

--
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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.

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