Re: Help sought from C++ expert: is this `value' a type or something else (what)?

[Alan Mackenzie <acm@muc.de>]

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).