220 38909 article Path: news.gmane.org!.POSTED!not-for-mail From: =?UTF-8?B?R2HFoXBlciBBxb5tYW4=?= 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 g88-v6sor1219989ljg.64.2018.06.29.08.31.26 for (Google Transport Security); Fri, 29 Jun 2018 08:31:26 -0700 (PDT) Received-SPF: pass (google.com: domain of gasper.azman@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:a2e:2b0e:: with SMTP id q14-v6mr9028717lje.147.1530286286352; Fri, 29 Jun 2018 08:31:26 -0700 (PDT) Original-Received: by 2002:ab3:404:0:0:0:0:0 with HTTP; Fri, 29 Jun 2018 08:31:05 -0700 (PDT) In-Reply-To: X-Original-Sender: gasper.azman@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=F8ccPboB; spf=pass (google.com: domain of gasper.azman@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=gasper.azman@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:38909 Archived-At: --00000000000080b5f3056fc98aa0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable AFAIK it's not possible because a using declaration is not a linkable symbol - you need to know the actual type name in order to mangle things. Then again perhaps you can skirt this by just not using it anywhere you'd actually need a symbol. G On Fri, Jun 29, 2018 at 4:28 PM, Justin Bassett wrote: > 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-ThgXXjF4/fukkKkh8Xh0J;context-place=3Dforum/std-proposal= s > > 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 >>> 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 =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 Group= s >> "ISO C++ Standard - Future Proposals" group. >> To unsubscribe from this group and stop receiving emails from it, send a= n >> 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-2ff0-4a09- >> 9fac-7c37dc67cf16%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/CAPuuy5cZ8ak8M%3DujRtqgjSXoPh4% > 2BOt4eALaoy69hU8ZJ-EFtCQ%40mail.gmail.com > > . > --=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/CAANG%3DkXw2S5myinkSRf%3DjVCN9%3D2U3905%2BTNtMD6= JMj3Ge3mMbg%40mail.gmail.com. --00000000000080b5f3056fc98aa0 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
AFAIK it's not possible because a using declaration is= not a linkable symbol - you need to know the actual type name in order to = mangle things.

Then again perhaps you can skirt this by = just not using it anywhere you'd actually need a symbol.

=
G

On Fri, Jun 29, 2018 at 4:28 PM, Justin Bassett &l= t;jbassett271@gm= ail.com> wrote:
I would extend it and say you can forward declare an arbitrary type,= like in this thread:=C2=A0https://groups.google.com/a/isocpp.or= g/forum/#!msg/std-proposals/fm-ThgXXjF4/fukkKkh8Xh0J;context-plac= e=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 representatio= ns / sizes. It mightve been something like void* or char* could be bigger a= nd are on certain systems. I couldn't find a link to back this up, thou= gh

On Fri, Jun 29, 2018, 1:48 AM <mihailnajdenov@gmail.com> wrote:
=
We have to see w= hat is the situation after Modules.=C2=A0

For now = your best bet is to actually create a class=C2=A0
class FloatImage : public CudaImage<float>= { /*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 resolve= d by a type alias of a class template. I don't see the logic to this, w= hy is this illegal:

//= forward declaration in one header file.<= br>class FloatImage;


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

using FloatImage<= span style=3D"color:#000"> =3D CudaImage<= span style=3D"color:#080"><float>;<= /span>


Godb= olt:=C2=A0https://godbolt.org/g/mmTTeQ shows that all = compilers reject this.

This has=C2=A0 created a fa= ir amount of trouble for me when trying to refactor what was previously a F= loatImage 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 clas= s, 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>;

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 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/975ab4cd-2ff0-4a09-9fac-7c37dc67cf16%40isocpp.org.

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To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/= std-proposals/CAPuuy5cZ8ak8M%3DujRtqgjSXoPh4%2BOt4eALaoy69hU= 8ZJ-EFtCQ%40mail.gmail.com.

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To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CAANG%3Dk= Xw2S5myinkSRf%3DjVCN9%3D2U3905%2BTNtMD6JMj3Ge3mMbg%40mail.gmail.com. --00000000000080b5f3056fc98aa0-- .