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

James Skene via libcxx-dev libcxx-dev at lists.llvm.org
Wed Aug 5 13:30:01 PDT 2020

On Thu, 6 Aug 2020 at 03:13, Marshall Clow <mclow.lists at gmail.com> wrote:

> On Aug 4, 2020, at 8:45 PM, James Skene <j.skene at gmail.com> wrote:
> On Wed, 5 Aug 2020 at 13:35, Marshall Clow <mclow.lists at gmail.com> wrote:
>> On Aug 4, 2020, at 4:33 PM, James Skene <j.skene at gmail.com> wrote:
>> On Wed, 5 Aug 2020 at 10:18, Marshall Clow <mclow.lists at gmail.com> wrote:
>>> On Aug 4, 2020, at 2:41 PM, James Skene <j.skene at gmail.com> wrote:
>>> The general templates would be declared as per the spec.  The member
>>> functions would be defined something like this:
>>> Sure, they *could* be defined something like that.
>>> But my point is that a conforming implementation need not have
>>> `numpunct<char16_t>` at all.
>>> — Marshall
>> And that comes down to the interpretation of the specification.  Your
>> position, as I understand it, is that the specification does not require
>> the general template to be defined.
>> My position is that if you attempt to instantiate `numpunct<char16_t>`,
>> that is not required to succeed.
>> But I don't see the justification for that in the letter of the
>> specification.  And neither is this the position that GCC or Visual Studio
>> take.  So what am I failing to understand?
>> What makes you think that the Visual Studio maintainers (or the libtdc++
>> maintainers) believe that those are required?
>> I see that:
>> 1) The standard requires that some specializations exist.
>> 2) libstdc++ and Visual Studio provide those, and more.
>> They’re allowed to do that.
>> That’s different from believing that they are required.
>> — Marshall
> Well, I have already more or less explicitly given three reasons why I
> think they are required, and they are good enough reasons that I expect the
> maintainers of VS and libstdc++ probably agree with me:
> 1. It looks to me that the standard says they are required (
> and nowhere does it say they are not required;
> 2. Other implementers provide them;
> 3. Implementing them actually *doesn't* require any major effort of
> implementation or the introduction of platform or locale dependent code
> into the library, so we can't assume that the standard means for them to be
> excluded on that basis.
> To this list I will add:
> 4. The classes of the locale and Input/Output library are everywhere class
> templates parameterised by character type, and the possibility of extending
> support for other character types is explicitly extended to library
> implementers at least; nor is it explicitly denied to user code, and
> furthermore the standard library anticipates user extensions, and gives
> rules for how the standard library can be extended in user code.  But if
> those two class templates, ctype and numpunct are omitted, then the
> formatted output operators for std::basic_ostream cannot be reused for
> char16_t and char32_t (or any other primitive type) by user code, because
> user code cannot define and add those facets for the different character
> type to their locale.  User code is explicitly prevented from defining just
> those facets by the injunction ( 'A program may add a template
> specialization for any standard library template to namespace std only if
> the declaration depends on a user-defined type and the specialization meets
> the  library requirements for the original template and is not explicitly
> prohibited.'  The specialisations std::ctype<char16_t>,
> std::numpunct<char16_t>, std::ctype<char32_t>, std::numpunct<char32_t> do
> not depend on any user-defined type and so cannot be defined in user code.
> Agreed.
> So did the designers of the C++ language and standard library, a group
> which you may count yourself a member of, Marshall, really provide their
> users with a library that seems extensible, and character types that seem
> perfect to use to extend it, but with the intent that this should not in
> fact be done and that nobody should be silly enough to try?  And to make
> sure that nobody tries, they make the effort impossible due to a
> technicality involving two obscure aspects of the localisation library, and
> don't even even bother to include a note to that effect in the
> specification?
> Yes. Part of this is historical; the iostreams classes were designed (back
> in the late 1980s/early 1990s) long before char8/16/32_t were added to the
> language. They were specified to work with char/wchar_t. There have been
> several proposals made to update iostreams to support these additional
> character types, but the committee has chosen to work on other things
> instead.
No. I still don't believe the possibility of future extension was
deliberately sandbagged via the localisation library.

> Also, leaving aside the question of whether excluding these templates is
> an error, you should want to include them anyway because doing so
> significantly improves the utility of your implementation.  Under GNU and
> VS the following works:
> std::basic_stringstream<char16_t> s;
> s << u"Hello world";
> The same code fails to compile with libc++ because 'Implicit instantiation
> of undefined template 'std::__1::ctype<char16_t>'.  Note that neither the
> VS or GNU implementations depend on any explicitly specialised templates or
> char16_t-specific code to make the above work, so they are not actually
> providing anything beyond the specification - at least if you believe, as I
> do, that the base template of ctype should be included in the
> implementation.
> And both of those behaviors are perfectly fine w.r.t the wording in the
> standard.
> An implementation is neither required to nor prohibited from providing an
> implementation of `ctype<char16_t>`, and that’s what is happening here.
That isn't what I wrote though.  I wrote 'leaving aside the question of
whether excluding these templates is an error, you should want to include
them anyway'.  Including them mitigates quite a bit of the harm that the
limitation of iostreams to char/wchar_t implies.  Not including them, for
the sake of saving having to write a couple of hundred lines of code, most
of which is already provided in the specification, makes a few people's
lives easier at the expense of making a lot of people's lives more
difficult, which is more or less exactly the opposite of what libraries are
intended to achieve.

> — Marshall
> P.S. I checked your claim that "this the position that GCC or Visual
> Studio take” with the libstdc++ maintainer. He says “I do not believe that.
> If we happen to provide that …. well we just happen to provide it”
But you did not address my point that the way the standard is written does
not seem to support leaving out any of the definitions that it provides.
It is possible that both you and he are wrong.

Well, that is all I have to say on this issue and I should really get on
with improving my own code rather than trying to improve yours.  Thanks

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