[libcxx-dev] std::ctype, std::numpunct base templates

[James Skene via libcxx-dev]

Hi Marshall,

Thanks for your prompt response.  I have some follow-ups below.  I hope you
don't object to me pursuing this a little bit.

On Tue, 4 Aug 2020 at 17:30, Marshall Clow <mclow.lists at gmail.com> wrote:

> On Aug 3, 2020, at 8:25 PM, James Skene via libcxx-dev <
> libcxx-dev at lists.llvm.org> wrote:
> >
> > Hi,
> >
> > I'm considering submitting a bug report against libc++, but thought I
> would check here first.
> >
> > The standard gives a synopsis for the locale facet std::ctype<CharT> in
> Section 25.4.1 (C++17, but it is also in C++11), and std::numpunct<CharT>
> in Section 25.4.3.  Under libc++ specialisations of these templates are
> defined for char and wchar_t (in header '__locale') but the base template
> definition is omitted.  This means in particular that I cannot define:
> >
> > class my_ctype : public std::ctype<char16_t> { ... };
> >
> > class my_numpunct : public std::numpunct<char16_t> { ... };
>
> Yes, I believe that this is correct.
>
> > This seems to go against the intent of these classes, which include
> abstract virtual function declarations.  Although clause 30.2.2 restricts
> the support for stream-based I/O that an implementation must provide to the
> types char and wchar_t, it is not clear to me that this means the
> definitions of the above templates can be omitted.
> >
> > I would appreciate advice as to: a. whether this is actually an issue or
> my misreading of the spec; and b. if so, if it is a known issue.
>
> https://wg21.link/locale.category lists the required specializations for
> numpunct and ctype.
>
> It doesn’t say anything about the general template.
>
> So, I would say this is not a bug.
>

Does it, anywhere in the specification, state that where required
specialisations are listed, the need to implement the general template is
removed?

There seems to be no such statement in Section 20.5.5.2 Conforming
Implementations.  On the other hand 20.5.2.2.1 states:

A C++ header shall provide the declarations and definitions that appear in
its synopsis.


> From an implementation point of view, the implementations of ctype<char>
> and type<wchar_t> are fairly different, and have a lot of system -specific
> knowledge baked into them. I don’t think I really know enough about
> charXX_t (8/16/32) on various systems to provide high-quality
> implementations (and in the case of char32_t, in a reasonable size of
> code/data). ICU does much of this, but it’s quite large.
>
>
I think there is a problem with the way the specification is written in
this instance.  The virtual functions in the base class should really be
pure virtual and the descriptions given of their (general) behaviour should
be requirements on subclasses or specialisations.  GCC and Visual Studio
both implement these functions by casting (or trying to narrow) their
character arguments to char.  GCCs inline documentation states
'implementations are provided for all the protected virtual functions, but
will likely not be useful'.  That said, bearing in mind that the ctype
functionality only applies to characters in the basic execution character
set, which are usually 7-bit ASCII (invariant) characters, this is probably
a reasonable choice.  Another choice, and perhaps no more nonstandard than
simply omitting the base class, would be to implement these functions as
assertion failures.

I agree that, notwithstanding the way the specification is written, it is
unreasonable to expect implementers to provide useful implementations for
more than the required specialisations.  But actually I do not want a
useful implementation, just the opportunity to override a virtual function.
As it happens my goal is specifically to extend ctype, numpunct, and the
other facets to implement the functionality using ICU.  I would prefer this
work to be portable to clang.

Thanks again,
James
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