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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@gcc.gnu.org (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: Localization</title>
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<link rel="Start" href="../documentation.html" type="text/html"
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title="GNU C++ Standard Library" />
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<link rel="Prev" href="../21_strings/howto.html" type="text/html"
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title="Strings" />
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<link rel="Next" href="../23_containers/howto.html" type="text/html"
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title="Containers" />
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<link rel="Bookmark" href="locale.html" type="text/html" title="class locale" />
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<link rel="Bookmark" href="codecvt.html" type="text/html" title="class codecvt" />
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<link rel="Bookmark" href="ctype.html" type="text/html" title="class ctype" />
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<link rel="Bookmark" href="messages.html" type="text/html" title="class messages" />
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<link rel="Bookmark" href="http://www.research.att.com/~bs/3rd_loc0.html" type="text/html" title="Bjarne Stroustrup on Locales" />
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<link rel="Bookmark" href="http://www.cantrip.org/locale.html" type="text/html" title="Nathan Myers on Locales" />
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<link rel="Copyright" href="../17_intro/license.html" type="text/html" />
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<link rel="Help" href="../faq/index.html" type="text/html" title="F.A.Q." />
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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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<!-- I wanted to write that sentence in something requiring an exotic font,
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like Cyrllic or Kanji. Probably more work than such cuteness is worth,
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but I still think it'd be funny.
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-->
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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">class locale</a></li>
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<li><a href="#2">class codecvt</a></li>
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<li><a href="#3">class ctype</a></li>
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<li><a href="#4">class messages</a></li>
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<li><a href="#5">Bjarne Stroustrup on Locales</a></li>
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<li><a href="#6">Nathan Myers on Locales</a></li>
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<li><a href="#7">Correct Transformations</a></li>
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</ul>
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<!-- ####################################################### -->
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<hr />
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<h2><a name="1">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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<hr />
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<h2><a name="2">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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</p>
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<blockquote>
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The standard class codecvt attempts to address conversions between
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different character encoding schemes. In particular, the standard
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attempts to detail conversions between the implementation-defined
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wide characters (hereafter referred to as wchar_t) and the standard
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type char that is so beloved in classic "C" (which can
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now be referred to as narrow characters.) This document attempts
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to describe how the GNU libstdc++-v3 implementation deals with the
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conversion between wide and narrow characters, and also presents a
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framework for dealing with the huge number of other encodings that
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iconv can convert, including Unicode and UTF8. Design issues and
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requirements are addressed, and examples of correct usage for both
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the required specializations for wide and narrow characters and the
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implementation-provided extended functionality are given.
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</blockquote>
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<hr />
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<h2><a name="3">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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<hr />
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<h2><a name="4">class messages</a></h2>
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<p>Notes made during the implementation of messages can be found
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<a href="messages.html">here</a>.
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</p>
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<hr />
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<h2><a name="5">Bjarne 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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</p>
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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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<hr />
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<h2><a name="6">Nathan Myers on Locales</a></h2>
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<p>An article entitled "The Standard C++ Locale" was
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published in 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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<hr />
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<h2><a name="7">Correct Transformations</a></h2>
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<!-- Jumping directly to 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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<code> extern "Danish" int tolower (int); </code>.
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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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</p>
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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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<p>
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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 <code>toupper/tolower</code> 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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</p>
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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) const { 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) const { 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::classic());
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ToLower down(std::locale::classic());
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// Change everything into upper case.
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std::transform(s.begin(), s.end(), s.begin(), up);
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// Change everything into lower case.
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std::transform(s.begin(), s.end(), s.begin(), down);
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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), up);
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}</pre>
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<p>The <code>ToUpper</code> and <code>ToLower</code> structs can be
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generalized for other character types by making <code>operator()</code>
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a member function template.
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</p>
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<p>The final version of the code uses <code>bind2nd</code> 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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<!-- ####################################################### -->
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<hr />
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<p class="fineprint"><em>
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See <a href="../17_intro/license.html">license.html</a> for copying conditions.
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Comments and suggestions are welcome, and may be sent to
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<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
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</em></p>
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</body>
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</html>
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