220 38910 <94a61631-75a1-4972-b30b-095e30360f55@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas 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 > this, why is this illegal: > > // forward declaration in one header file. > class FloatImage; > > > // Definition in another header file. > template class CudaImage { > }; > > using FloatImage = CudaImage; > > OK, so you want to say that `FloatImage` is a class, then you want to say it's a type alias. Does that not seem... incoherent to you? Is `is_class_v` supposed to be true or false? After all, you could have done `using FloatImage = int;` later on, which would change the result. Or you could do `using FloatImage = SomeUnionType;`, which again changes the result. Remember: incomplete types can be used to instantiate a lot of the type traits. `class FloatImage` is not the same thing as "hey compiler, there's a typename called `FloatImage`, but you can't use it for anything right now." If you want to be able to say that, then you need a new kind of type name. Right now, types can be complete (meaning the full definition is available) or incomplete (meaning that the full definition is not available, but we do know *something* about it). You want a third category, where we no nothing more than that a name is a type. -- 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/94a61631-75a1-4972-b30b-095e30360f55%40isocpp.org. ------=_Part_13433_1843940901.1530286870502 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Friday, June 29, 2018 at 3:56:26 AM UTC-4, Beng= t 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<typename<= span style=3D"color:#000"> T> class CudaImage= {
};

using= FloatImage =3D CudaImage<float>;


OK, so you want to sa= y that `FloatImage` is a class, then you want to say it's a type alias.= Does that not seem... incoherent to you?

Is `is_c= lass_v<FloatImage>` supposed to be true or false? After all, you coul= d have done `using FloatImage =3D int;` later on, which would change the re= sult. Or you could do `using FloatImage =3D SomeUnionType;`, which again ch= anges the result.

Remember: incomplete types can b= e used to instantiate a lot of the type traits. `class FloatImage` is not t= he same thing as "hey compiler, there's a typename called `FloatIm= age`, but you can't use it for anything right now." If you want to= be able to say that, then you need a new kind of type name. Right now, typ= es can be complete (meaning the full definition is available) or incomplete= (meaning that the full definition is not available, but we do know some= thing about it). You want a third category, where we no nothing more th= an that a name is a type.

--
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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/94a61631-75a1-4972-b30b-095e30360f55= %40isocpp.org.
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