Re: [boost] [C++0x] Emulate C++0x char16_t, char32_t, std::u16string, and std::u32string

Subject: Re: [boost] [C++0x] Emulate C++0x char16_t, char32_t, std::u16string, and std::u32string
From: Artyom Beilis (artyomtnk_at_[hidden])
Date: 2011-07-21 01:54:16

--- On Thu, 7/21/11, Beman Dawes <bdawes_at_[hidden]> wrote:

> From: Beman Dawes <bdawes_at_[hidden]>
 
> > You can't create working
> >
> >  std::basic_ostringstream<char16_t> stream;
> >
> > Because stream << 1245 would not work due to
> lack of std::locale facets.
> >
> > You can't create requires facets as for example they
> are specialized
> > in many standard libraries.
> 
> Emulation via simple uint16_t and uint32_t typedefs doesn't
> work for
> all use cases. So only use it when it does work.
> 

And Boost.Filesystem.v3 is one of these specific use cases.

Boost.Filesystem uses codecvt facet? You can't create them, at
least with gcc/linux because existing codecvt facets are specialized
for each type of character - and you have to derive from it.

For example this code would not link with Linux/gcc-4.6

#include <locale>

class real_facet : 
    public std::codecvt<char16_t,char,std::mbstate_t>
{
};

class simulated_facet :
    public std::codecvt<unsigned short,char,std::mbstate_t>
{
};

int main()
{
    std::locale l1(std::locale::classic(),new real_facet());
    std::locale l2(std::locale::classic(),new simulated_facet());
}

With errors like:

undefined reference to `std::codecvt<char16_t, char, __mbstate_t>::do_in(__mbstate_t&, char const*, char const*, char const*&, char16_t*, char16_t*, char16_t*&) const'
---
: undefined reference to `std::codecvt<unsigned short, char, __mbstate_t>::do_max_length() const'
> > Even existing Microsoft's VC2010 does not work if you
> compile application
> > with /MD or /MDd
> 
> I'll retest just to be sure, but I'm fairly sure that some
> of my tests
> have used those switches.
> 
MSVC2010 does not provide specialization for std::locale::id
for these new facets and they are undefined in the dll.
If you want I'll provide a fault case for MSVC10 as well.
And all these with compilers that **declare** support 
of charXX_t.
So you may try to put an effort in implementing some emulation
but at these points char16_t/char32_t are broken with modern
compilers.
Artyom Beilis
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