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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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| <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> |
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| |
| <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">Bjarne 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"); |
| 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); |
| }</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> |
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