repos_ecs/其他2015后/日志库/log4cplus-1.1.3-rc6/docs/unicode.txt

UNICODE
-------

Log4cplus uses the expression "UNICODE" in at least two not so equal
meanings:

1. the Unicode standard as defined by the Unicode Consortium

2. compiler's and/or C++ standard library's support for strings of
   wchar_ts and their manipulation


WCHAR_T SUPPORT
---------------

Log4cplus is aimed to be portable and to have as little 3rd party
dependencies as possible. To fulfill this goal it has to use
facilities offered by the operating systems and standard libraries it
runs on. To offer the best possible level of support of national
character, it has to support usage of wchar_t and it has to use
wchar_t support (especially on Windows) provided by operating system
and standard C and C++ libraries.

This approach to portability has some limittations. One of the
limittations is lacking support for C++ locales in various operating
systems and standard C++ libraries. Some standard C++ libraries do not
support other than the "C" and "POSIX" locales. This usually means
that wchar_t <-> char conversion using codecvt<> facet is
impossible. On such deficient platforms, log4cplus can use either
standard C locale support or iconv() (through libiconv or built-in).


UNICODE AND FILE APPENDERS
--------------------------

Another limitation related to Unicode support is then inability to
write wchar_t messages that contain national characters that do not
map to any code point in single byte code page to log files using
FileAppender. This is a problem mainly on Windows. Linux and other
*NIX systems can avoid it because they do not need to use wchar_t
interfaces to have Unicode aware applications. They usually (as of
year 2012) use UTF-8 based locales. With proper C++ locale setup in
client applications, national characters can come through into log
files unharmed. But if they choose to use wchar_t strings, they face
the problem as well.


*NIX
----

To support output of non-ASCII characters in wchar_t message on *NIX
platforms, it is necessary to use UTF-8 based locale (e.g.,
en_US.UTF-8) and to set up global locale with std::codecvt facet or
imbue individual FileAppenders with that facet. The following code can
be used to get such std::locale instance and to set it into global
locale:

    std::locale::global (     // set global locale
        std::locale (         // using std::locale constructed from
            std::locale (),   // global locale
                              // and codecvt facet from user locale
            new std::codecvt_byname<wchar_t, char, std::mbstate_t>("")));


WINDOWS
-------

Windows do not support UTF-8 based locales. The above approach will
yield a std::locale instance converting wchar_ts to current process'
code page. Such locale will not be able to convert Unicode code points
outside the process' code page. This is true at least with the
std::codecvt facet implemented in Visual Studio 2010. Instead, with
Visual Studio 2010 and later, it is possible to use std::codecvt_utf8
facet:

    std::locale::global (     // set global locale
        std::locale (         // using std::locale constructed from
            std::locale (),   // global locale
                              // and codecvt_utf8 facet
            new std::codecvt_utf8<tchar, 0x10FFFF,
                static_cast<std::codecvt_mode>(std::consume_header
                    | std::little_endian)>));