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

[Michael Welsh Duggan]

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

>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."


> 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.

> /////////////////////////////////////////////////////////////////////////
> 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)