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- <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
- <meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL" />
- <meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 22." />
- <meta name="GENERATOR" content="vi and eight fingers" />
- <title>libstdc++-v3 HOWTO: Chapter 22</title>
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-
-<h1 class="centered"><a name="top">Chapter 22: Localization</a></h1>
-
-<p>Chapter 22 deals with the C++ localization facilities.
-</p>
-<!-- I wanted to write that sentence in something requiring an exotic font,
- like Cyrllic or Kanji. Probably more work than such cuteness is worth,
- but I still think it'd be funny.
- -->
-
-
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-<hr />
-<h1>Contents</h1>
-<ul>
- <li><a href="#1">class locale</a></li>
- <li><a href="#2">class codecvt</a></li>
- <li><a href="#3">class ctype</a></li>
- <li><a href="#4">class messages</a></li>
- <li><a href="#5">Bjarne Stroustrup on Locales</a></li>
- <li><a href="#6">Nathan Myers on Locales</a></li>
- <li><a href="#7">Correct Transformations</a></li>
-</ul>
-
-<!-- ####################################################### -->
-
-<hr />
-<h2><a name="1">class locale</a></h2>
- <p>Notes made during the implementation of locales can be found
- <a href="locale.html">here</a>.
- </p>
-
-<hr />
-<h2><a name="2">class codecvt</a></h2>
- <p>Notes made during the implementation of codecvt can be found
- <a href="codecvt.html">here</a>.
- </p>
-
- <p>The following is the abstract from the implementation notes:
- </p>
- <blockquote>
- 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.
- </blockquote>
-
-<hr />
-<h2><a name="3">class ctype</a></h2>
- <p>Notes made during the implementation of ctype can be found
- <a href="ctype.html">here</a>.
- </p>
-
-<hr />
-<h2><a name="4">class messages</a></h2>
- <p>Notes made during the implementation of messages can be found
- <a href="messages.html">here</a>.
- </p>
-
-<hr />
-<h2><a name="5">Stroustrup on Locales</a></h2>
- <p>Dr. Bjarne Stroustrup has released a
- <a href="http://www.research.att.com/~bs/3rd_loc0.html">pointer</a>
- to Appendix D of his book,
- <a href="http://www.research.att.com/~bs/3rd.html">The C++
- Programming Language (3rd Edition)</a>. It is a detailed
- description of locales and how to use them.
- </p>
- <p>He also writes:
- </p>
- <blockquote><em>
- 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.
- </em></blockquote>
-
-<hr />
-<h2><a name="6">Nathan Myers on Locales</a></h2>
- <p>An article entitled "The Standard C++ Locale" was
- published in Dr. Dobb's Journal and can be found
- <a href="http://www.cantrip.org/locale.html">here</a>.
- </p>
-
-<hr />
-<h2><a name="7">Correct Transformations</a></h2>
- <!-- Jumping directly to here from chapter 21. -->
- <p>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?"
- </p>
- <p>(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...)
- </p>
- <p>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
- <code> extern "Danish" int tolower (int); </code>.
- </p>
- <p>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:
- </p>
- <pre>
- namespace std {
- template <class charT>
- charT
- toupper (charT c, const locale& loc) const;
- template <class charT>
- charT
- tolower (charT c, const locale& loc) const;
- }</pre>
- <p>
- This function extracts the appropriate "facet" from the
- locale <em>loc</em> and calls the appropriate member function of that
- facet, passing <em>c</em> as its argument. The resulting character
- is returned.
- </p>
- <p>For the C/POSIX locale, the results are the same as calling the
- classic C <code>toupper/tolower</code> function that was used in previous
- examples. For other locales, the code should Do The Right Thing.
- </p>
- <p>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.
- </p>
- <p>The next-to-final version of the code started in Chapter 21 looks like:
- </p>
- <pre>
- #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");
- std::locale loc_c("C");
- ToUpper up(loc_c);
- ToLower down(loc_c);
-
- // 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);
- }</pre>
- <p>The <code>ToUpper</code> and <code>ToLower</code> structs can be
- generalized for other character types by making <code>operator()</code>
- a member function template.
- </p>
- <p>The final version of the code uses <code>bind2nd</code> 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.
- </p>
-
-
-<!-- ####################################################### -->
-
-<hr />
-<p class="fineprint"><em>
-See <a href="../17_intro/license.html">license.html</a> for copying conditions.
-Comments and suggestions are welcome, and may be sent to
-<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
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