236 lines
8.5 KiB
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236 lines
8.5 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
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<HTML>
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<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<META NAME="AUTHOR" CONTENT="pme@sources.redhat.com (Phil Edwards)">
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<META NAME="KEYWORDS" CONTENT="HOWTO, libstdc++, GCC, g++, libg++, STL">
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<META NAME="DESCRIPTION" CONTENT="HOWTO for the libstdc++ chapter 22.">
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<META NAME="GENERATOR" CONTENT="vi and eight fingers">
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<TITLE>libstdc++-v3 HOWTO: Chapter 22</TITLE>
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<LINK REL=StyleSheet HREF="../lib3styles.css">
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<!-- $Id: howto.html,v 1.8 2000/12/03 23:47:47 jsm28 Exp $ -->
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</HEAD>
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<BODY>
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<H1 CLASS="centered"><A NAME="top">Chapter 22: Localization</A></H1>
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<P>Chapter 22 deals with the C++ localization facilities.
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</P>
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<!-- ####################################################### -->
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<HR>
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<H1>Contents</H1>
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<UL>
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<LI><A HREF="#1">Bjarne Stroustrup on Locales</A>
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<LI><A HREF="#2">Nathan Myers on Locales</A>
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<LI><A HREF="#3">class locale</A>
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<LI><A HREF="#4">class codecvt</A>
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<LI><A HREF="#5">class ctype</A>
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<LI><A HREF="#6">Correct Transformations</A>
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</UL>
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<HR>
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<H2><A NAME="1">Stroustrup on Locales</A></H2>
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<P>Dr. Bjarne Stroustrup has released a
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<A HREF="http://www.research.att.com/~bs/3rd_loc0.html">pointer</A>
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to Appendix D of his book,
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<A HREF="http://www.research.att.com/~bs/3rd.html">The C++
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Programming Language (3rd Edition)</A>. It is a detailed
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description of locales and how to use them.
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</P>
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<P>He also writes:
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<BLOCKQUOTE><EM>
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Please note that I still consider this detailed description of
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locales beyond the needs of most C++ programmers. It is written
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with experienced programmers in mind and novices will do best to
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avoid it.
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</EM></BLOCKQUOTE>
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</P>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<HR>
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<H2><A NAME="2">Nathan Myers on Locales</A></H2>
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<P> An article entitled "The Standard C++ Locale" was published in
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Dr. Dobb's Journal and can be found
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<A HREF="http://www.cantrip.org/locale.html">here</A>
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</P>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<HR>
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<H2><A NAME="5">class locale</A></H2>
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<P> Notes made during the implementation of locales can be found
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<A HREF="locale.html">here</A>.
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</P>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<HR>
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<H2><A NAME="4">class codecvt</A></H2>
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<P> Notes made during the implementation of codecvt can be found
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<A HREF="codecvt.html">here</A>.
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</P>
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<P> The following is the abstract from the implementation notes:
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<BLOCKQUOTE>
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The standard class codecvt attempts to address conversions
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between different character encoding schemes. In particular, the
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standard attempts to detail conversions between the
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implementation-defined wide characters (hereafter referred to as
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wchar_t) and the standard type char that is so beloved in classic
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"C" (which can now be referred to as narrow characters.)
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This document attempts to describe how the GNU libstdc++-v3
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implementation deals with the conversion between wide and narrow
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characters, and also presents a framework for dealing with the huge
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number of other encodings that iconv can convert, including Unicode
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and UTF8. Design issues and requirements are addressed, and examples
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of correct usage for both the required specializations for wide and
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narrow characters and the implementation-provided extended
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functionality are given.
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</BLOCKQUOTE>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<HR>
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<H2><A NAME="5">class ctype</A></H2>
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<P> Notes made during the implementation of ctype can be found
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<A HREF="ctype.html">here</A>.
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</P>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<HR>
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<H2><A NAME="6">Correct Transformations</A></H2>
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<!-- Jumping directly here from chapter 21. -->
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<P>A very common question on newsgroups and mailing lists is, "How
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do I do <foo> to a character string?" where <foo> is
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a task such as changing all the letters to uppercase, to lowercase,
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testing for digits, etc. A skilled and conscientious programmer
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will follow the question with another, "And how do I make the
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code portable?"
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</P>
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<P>(Poor innocent programmer, you have no idea the depths of trouble
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you are getting yourself into. 'Twould be best for your sanity if
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you dropped the whole idea and took up basket weaving instead. No?
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Fine, you asked for it...)
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</P>
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<P>The task of changing the case of a letter or classifying a character
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as numeric, graphical, etc, all depends on the cultural context of the
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program at runtime. So, first you must take the portability question
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into account. Once you have localized the program to a particular
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natural language, only then can you perform the specific task.
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Unfortunately, specializing a function for a human language is not
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as simple as declaring
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<TT> extern "Danish" int tolower (int); </TT>.
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</P>
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<P>The C++ code to do all this proceeds in the same way. First, a locale
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is created. Then member functions of that locale are called to
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perform minor tasks. Continuing the example from Chapter 21, we wish
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to use the following convenience functions:
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<PRE>
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namespace std {
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template <class charT>
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charT
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toupper (charT c, const locale& loc) const;
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template <class charT>
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charT
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tolower (charT c, const locale& loc) const;
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}</PRE>
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This function extracts the appropriate "facet" from the
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locale <EM>loc</EM> and calls the appropriate member function of that
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facet, passing <EM>c</EM> as its argument. The resulting character
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is returned.
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</P>
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<P>For the C/POSIX locale, the results are the same as calling the
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classic C <TT>toupper/tolower</TT> function that was used in previous
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examples. For other locales, the code should Do The Right Thing.
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</P>
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<P>Of course, these functions take a second argument, and the
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transformation algorithm's operator argument can only take a single
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parameter. So we write simple wrapper structs to handle that.
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</P>
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<P>The next-to-final version of the code started in Chapter 21 looks like:
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<PRE>
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#include <iterator> // for back_inserter
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#include <locale>
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#include <string>
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#include <algorithm>
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#include <cctype> // old <ctype.h>
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struct Toupper
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{
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Toupper (std::locale const& l) : loc(l) {;}
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char operator() (char c) { return std::toupper(c,loc); }
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private:
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std::locale const& loc;
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};
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struct Tolower
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{
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Tolower (std::locale const& l) : loc(l) {;}
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char operator() (char c) { return std::tolower(c,loc); }
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private:
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std::locale const& loc;
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};
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int main ()
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{
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std::string s ("Some Kind Of Initial Input Goes Here");
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Toupper up ( std::locale("C") );
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Tolower down ( std::locale("C") );
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// Change everything into upper case
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std::transform (s.begin(), s.end(), s.begin(),
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up
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);
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// Change everything into lower case
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std::transform (s.begin(), s.end(), s.begin(),
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down
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);
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// Change everything back into upper case, but store the
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// result in a different string
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std::string capital_s;
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std::transform (s.begin(), s.end(), std::back_inserter(capital_s),
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up
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);
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}</PRE>
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</P>
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<P>The final version of the code uses <TT>bind2nd</TT> to eliminate
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the wrapper structs, but the resulting code is tricky. I have not
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shown it here because no compilers currently available to me will
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handle it.
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</P>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<!-- ####################################################### -->
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<HR>
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<P CLASS="fineprint"><EM>
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Comments and suggestions are welcome, and may be sent to
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<A HREF="mailto:pme@sources.redhat.com">Phil Edwards</A> or
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<A HREF="mailto:gdr@gcc.gnu.org">Gabriel Dos Reis</A>.
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<BR> $Id: howto.html,v 1.8 2000/12/03 23:47:47 jsm28 Exp $
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</EM></P>
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</BODY>
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</HTML>
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