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Re: Help sought from C++ expert: is this `value' a type or something els
From: |
Alan Mackenzie |
Subject: |
Re: Help sought from C++ expert: is this `value' a type or something else (what)? |
Date: |
Sat, 21 Jan 2023 14:34:58 +0000 |
Hello, Michael.
On Thu, Jan 19, 2023 at 10:41:15 -0500, Michael Welsh Duggan wrote:
> Alan Mackenzie <acm@muc.de> writes:
> > In the following test file, templates-9.cc from the CC Mode test suite,
> > on the indicated line, what is the syntactic significance of `value'?
> Here `value' is a value, specifically a boolean struct member. This is
> boost's implementation of what became, in C++11, `std::is_same', which
> can be found documented here:
> https://en.cppreference.com/w/cpp/types/is_same
> > Traditionally, it has fontified with the default face, but recently after
> > some (not yet committed) changes in CC Mode, it has started getting
> > font-lock-type-face.
> > I don't understand the file at all, hardly, and can't work out whether
> > the old (non-) fontification was correct, or whether the new
> > fontification of f-l-type-face is correct. Help would be appreciated.
> Given a name `Foo', the compiler doesn't necessarily know if `Foo::bar'
> is a type or a value, because it doesn't necessarily know what type Foo
> is - it could be a template argument, for example, and won't be known
> until an instance is referenced. The assumption is that it is a value.
> If it was a type, this is pointed out to the compiler by calling it
> `typename Foo::bar'. For more on this, see:
> https://en.cppreference.com/w/cpp/language/dependent_name
Thanks, I've glanced over that page, now.
> >From that page, "In a declaration or a definition of a template,
> including alias templates, a name that is not a member of the current
> instantiation and is dependent on a template parameter is not considered
> to be a type unless the keyword `typename' is used or unless it was
> already established as a type name, e.g. with a typedef declaration or
> by being used to name a base class."
That's helpful.
> > Just as a matter of interest, in c++-ts-mode a treesit-query-error gets
> > thrown. It isn't clear to me whether the file is still valid C++.
> It looks valid. The `template' before the `then' is, like `typename' a
> disambiguator for the dependent name `then', stating that it is a
> template name.
Thanks. I can now read the file semi-fluently, whereas before it
appeared to me just as a random assemblage of C++ keywords and
operators. ;-(
> > /////////////////////////////////////////////////////////////////////////
> > typedef std::pair<int
> > , std::pair<long
> > , std::pair<char*
> > , std::pair<nil,nil>
> > >
> > >
> > > list_of_types;
> >
> > typedef std::pair<
> > int, std::pair<
> > long, std::pair<
> > char*, std::pair<nil,nil>
> > >
> > >
> >> list_of_types;
> >
> > typedef typename if_true
> > <
> > boost::is_same
> > <
> > boost::add_pointer<X>
> > , int*
> > >::value // <===================================================
> >>::template then
> > <
> > boost::remove_reference<X>
> > // else
> > , X
> >>::type modified_X;
> > /////////////////////////////////////////////////////////////////////////
> >
> > Thanks!
> --
> Michael Welsh Duggan
> (md5i@md5i.com)
--
Alan Mackenzie (Nuremberg, Germany).
Re: Help sought from C++ expert: is this `value' a type or something else (what)?, Michael Welsh Duggan, 2023/01/19
- Re: Help sought from C++ expert: is this `value' a type or something else (what)?,
Alan Mackenzie <=
Re: Help sought from C++ expert: is this `value' a type or something else (what)?, Óscar Fuentes, 2023/01/20