Re: [boost] [lexical_cast] locale dependent result?

Subject: Re: [boost] [lexical_cast] locale dependent result?
From: Artyom (artyomtnk_at_[hidden])
Date: 2011-03-02 11:09:34

> > If you want to do casting  without locale context (which is desirable)
> > 
> >      std::ostringstream ss;
> >     ss.imbue(std::locale::classic());  //  C locale
> >     ss << 65535;
> >      return ss.str();
> 
> Thanks!  So std::locale::classic effectively means  'turn locale awareness
> off'?

It installs default so called "C" or "POSIX" locale that is what you need.

> 
> > Unfortunately this problem is very  common and causes 
> > many bugs, especially in C with setlocale and  printf.
> 
> I can imagine!  It's so subtle and unexpected.  In  fact, I used
> lexical_cast precisely because I thought it wouldn't do anything  funky with
> the conversion.
> 

It is not about lexical cast only it is about anything that uses iostream
and *printf/*scanf/ato* and many other library functions.

I recently seen how C (and C++) SQL libraries being broken because
of global locale.

> At least that's how I interpreted the  documentation: "For more involved
> conversions, such as where precision or  formatting need tighter control
> than is offered by the default behavior of  lexical_cast, the conventional
> stringstream approach is  recommended."
> 

The best is unfortunatly is avoid installing std::locale global using standard
C++ localization, in fact because of this problem in Boost.Locale the default is
to use C locale. i.e.

    ss << 65536;
    ss.str() == "65536";

And if you need localized numbers:

    ss << boost::locale::as::number
    ss.str() == "65,536"

Because too many libraries become broken by this "feature" as

   std::locale::global(std::locale("de_DE.UTF-8"));
   std::ostringstream ss;
   ss << 12345.345;

Would create "12.345,345" and

   printf("%f\n",12345.345);

Would generate

   12345,345   // <-- note "," and not "."

So imagine how many libraries and programs do not
work in such setups :-)

Artyom