220 38908 article Path: news.gmane.org!.POSTED!not-for-mail From: Justin Bassett Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: 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 x2-v6sor3635262otj.28.2018.06.29.08.29.01 for (Google Transport Security); Fri, 29 Jun 2018 08:29:01 -0700 (PDT) Received-SPF: pass (google.com: domain of jbassett271@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:a9d:a66:: with SMTP id 93-v6mr8390380otg.177.1530286141277; Fri, 29 Jun 2018 08:29:01 -0700 (PDT) In-Reply-To: <975ab4cd-2ff0-4a09-9fac-7c37dc67cf16@isocpp.org> X-Original-Sender: jbassett271@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=ncjgssor; spf=pass (google.com: domain of jbassett271@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=jbassett271@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:38908 Archived-At: --000000000000db0aa7056fc98118 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable I would extend it and say you can forward declare an arbitrary type, like in this thread: https://groups.google.com/a/isocpp.org/forum/#!msg/std-proposals/fm-ThgXXjF= 4/fukkKkh8Xh0J;context-place=3Dforum/std-proposals As much as I want universal forward declarations, I remember seeing that it can't be done, since some pointers can have different representations / sizes. It mightve been something like void* or char* could be bigger and are on certain systems. I couldn't find a link to back this up, though On Fri, Jun 29, 2018, 1:48 AM wrote: > We have to see what is the situation after Modules. > > For now your best bet is to actually create a class > class FloatImage : public CudaImage { /*using =E2=80=A6*/ }; > > > On Friday, June 29, 2018 at 10:56:26 AM UTC+3, Bengt Gustafsson 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 =3D 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 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 template. >> >> While I can update the forward declaration to: >> // forward declaration in one header file. >> template class CudaImage; >> using FloatImage =3D 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/975ab4cd-2ff= 0-4a09-9fac-7c37dc67cf16%40isocpp.org > > . > --=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/CAPuuy5cZ8ak8M%3DujRtqgjSXoPh4%2BOt4eALaoy69hU8Z= J-EFtCQ%40mail.gmail.com. --000000000000db0aa7056fc98118 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
I would extend it and say you can forward declare an arbi= trary type, like in this thread:=C2=A0https://groups.google.com/a/isocpp.org/forum/#!ms= g/std-proposals/fm-ThgXXjF4/fukkKkh8Xh0J;context-place=3Dforum/std-proposal= s

As much as I want univer= sal forward declarations, I remember seeing that it can't be done, sinc= e some pointers can have different representations / sizes. It mightve been= something like void* or char* could be bigger and are on certain systems. = I couldn't find a link to back this up, though

On Fri, Jun 29, 2018, 1:48 AM <mihailnajdenov@gmail.com> wro= te:
We have t= o see what is the situation after Modules.=C2=A0

F= or now your best bet is to actually create a class=C2=A0
class FloatImage : public CudaImage<flo= at> { /*using =E2=80=A6*/ };
=

On Friday, June 29, 2018 at 10:56:26 AM UTC+3, = Bengt Gustafsson wrote:
A forward declaration of a class name is currently not allowed to be r= esolved by a type alias of a class template. I don't see the logic to t= his, why is this illegal:

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


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

using FloatImage =3D CudaImage<float>= ;


Go= dbolt:=C2=A0https://godbolt.org/g/mmTTeQ shows that all com= pilers reject this.

This has=C2=A0 created a fair = amount of trouble for me when trying to refactor what was previously a Floa= tImage 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<typename T> class CudaImage;
using FloatImage =3D CudaImage<float>;
<= /div>

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

so is there a technical rea= son preventing this from being allowed or is it just an oversight that coul= d be corrected?

--
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