220 38903 <975ab4cd-2ff0-4a09-9fac-7c37dc67cf16@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: mihailnajdenov@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allow forward class declarations to match a type alias. 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I don't see the logic to=20 > 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=20 > this. > > This has created a fair amount of trouble for me when trying to refactor= =20 > what was previously a FloatImage class to a generic pixel type template. = I=20 > don't see the logic to making this > illegal as FloatImage is still the name of a class, i.e. an instance of= =20 > 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 i= t=20 > just an oversight that could be corrected? > > --=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/975ab4cd-2ff0-4a09-9fac-7c37dc67cf16%40isocpp.or= g. ------=_Part_11176_1127489972.1530262106688 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
We have to see what 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*/ };<= /div>


On Friday, = June 29, 2018 at 10:56:26 AM UTC+3, Bengt Gustafsson wrote:
A forward declaration of a cl= ass 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<typename T> class CudaImage {
};

<= span style=3D"color:#008">using FloatImage =3D <= span style=3D"color:#606">CudaImage<fl= oat>;


Godbolt:=C2=A0https://godbolt.or= g/g/mmTTeQ shows that all compilers reject this.

This has=C2=A0 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 Flo= atImage is still the name of a class, i.e. an instance of the CudaImage cla= ss template.

While I can update the forward declar= ation to:
// forward declaration in one header= file.
template<typename T> class CudaImage;
using FloatImage =3D CudaImage<float>;<= span style=3D"color:#000">

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

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

<= /div>

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