Chapter 22:  Localization
Chapter 22 deals with the C++ localization facilities.
Contents
#1
class locale
#2
class codecvt
#3
class ctype
#4
class messages
#5
Bjarne Stroustrup on Locales
#6
Nathan Myers on Locales
#7
Correct Transformations
class locale
Notes made during the implementation of locales can be found
locale.html
here
.
class codecvt
Notes made during the implementation of codecvt can be found
codecvt.html
here
.
The following is the abstract from the implementation notes:
The standard class codecvt attempts to address conversions between
different character encoding schemes. In particular, the standard
attempts to detail conversions between the implementation-defined
wide characters (hereafter referred to as wchar_t) and the standard
type char that is so beloved in classic "C" (which can
now be referred to as narrow characters.)  This document attempts
to describe how the GNU libstdc++-v3 implementation deals with the
conversion between wide and narrow characters, and also presents a
framework for dealing with the huge number of other encodings that
iconv can convert, including Unicode and UTF8. Design issues and
requirements are addressed, and examples of correct usage for both
the required specializations for wide and narrow characters and the
implementation-provided extended functionality are given.
class ctype
Notes made during the implementation of ctype can be found
ctype.html
here
.
class messages
Notes made during the implementation of messages can be found
messages.html
here
.
Bjarne Stroustrup on Locales
Dr. Bjarne Stroustrup has released a
http://www.research.att.com/~bs/3rd_loc0.html
pointer
to Appendix D of his book,
http://www.research.att.com/~bs/3rd.html
The C++
Programming Language (3rd Edition)
.  It is a detailed
description of locales and how to use them.
He also writes:
Please note that I still consider this detailed description of
locales beyond the needs of most C++ programmers.  It is written
with experienced programmers in mind and novices will do best to
avoid it.
Nathan Myers on Locales
An article entitled "The Standard C++ Locale" was
published in Dr. Dobb's Journal and can be found
http://www.cantrip.org/locale.html
here
.
Correct Transformations
A very common question on newsgroups and mailing lists is, "How
do I do <foo> to a character string?" where <foo> is
a task such as changing all the letters to uppercase, to lowercase,
testing for digits, etc.  A skilled and conscientious programmer
will follow the question with another, "And how do I make the
code portable?"
(Poor innocent programmer, you have no idea the depths of trouble
you are getting yourself into.  'Twould be best for your sanity if
you dropped the whole idea and took up basket weaving instead.  No?
Fine, you asked for it...)
The task of changing the case of a letter or classifying a character
as numeric, graphical, etc, all depends on the cultural context of the
program at runtime.  So, first you must take the portability question
into account.  Once you have localized the program to a particular
natural language, only then can you perform the specific task.
Unfortunately, specializing a function for a human language is not
as simple as declaring
extern "Danish" int tolower (int);
.
The C++ code to do all this proceeds in the same way.  First, a locale
is created.  Then member functions of that locale are called to
perform minor tasks.  Continuing the example from Chapter 21, we wish
to use the following convenience functions:
namespace std {
template <class charT>
charT
toupper (charT c, const locale& loc) const;
template <class charT>
charT
tolower (charT c, const locale& loc) const;
}
This function extracts the appropriate "facet" from the
locale
loc
and calls the appropriate member function of that
facet, passing
c
as its argument.  The resulting character
is returned.
For the C/POSIX locale, the results are the same as calling the
classic C
toupper/tolower
function that was used in previous
examples.  For other locales, the code should Do The Right Thing.
Of course, these functions take a second argument, and the
transformation algorithm's operator argument can only take a single
parameter.  So we write simple wrapper structs to handle that.
The next-to-final version of the code started in Chapter 21 looks like:
#include <iterator>    // for back_inserter
#include <locale>
#include <string>
#include <algorithm>
#include <cctype>      // old <ctype.h>
struct ToUpper
{
ToUpper(std::locale const& l) : loc(l) {;}
char operator() (char c) const  { return std::toupper(c,loc); }
private:
std::locale const& loc;
};
struct ToLower
{
ToLower(std::locale const& l) : loc(l) {;}
char operator() (char c) const  { return std::tolower(c,loc); }
private:
std::locale const& loc;
};
int main ()
{
std::string  s("Some Kind Of Initial Input Goes Here");
ToUpper      up(std::locale::classic());
ToLower      down(std::locale::classic());
// Change everything into upper case.
std::transform(s.begin(), s.end(), s.begin(), up);
// Change everything into lower case.
std::transform(s.begin(), s.end(), s.begin(), down);
// Change everything back into upper case, but store the
// result in a different string.
std::string  capital_s;
std::transform(s.begin(), s.end(), std::back_inserter(capital_s), up);
}
The
ToUpper
and
ToLower
structs can be
generalized for other character types by making
operator()
a member function template.
The final version of the code uses
bind2nd
to eliminate
the wrapper structs, but the resulting code is tricky.  I have not
shown it here because no compilers currently available to me will
handle it.
See
../17_intro/license.html
license.html
for copying conditions.
Comments and suggestions are welcome, and may be sent to
mailto:libstdc++@gcc.gnu.org
the libstdc++ mailing list
.
