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Re: Help sought from C++ expert: is this `value' a type or something els
Michael Welsh Duggan
Re: Help sought from C++ expert: is this `value' a type or something else (what)?
Thu, 19 Jan 2023 10:41:15 -0500
Gnus/5.13 (Gnus v5.13)
Alan Mackenzie <email@example.com> 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:
> Traditionally, it has fontified with the default face, but recently after
> some (not yet committed) changes in CC Mode, it has started getting
> 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:
>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
> 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>
> typedef typename if_true
> , int*
> >::value // <===================================================
> // else
> , X
Michael Welsh Duggan
Re: Help sought from C++ expert: is this `value' a type or something else (what)?,
Michael Welsh Duggan <=
Re: Help sought from C++ expert: is this `value' a type or something else (what)?, Óscar Fuentes, 2023/01/20