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diff --git a/libstdc++-v3/doc/html/manual/bk01pt05ch13s03.html b/libstdc++-v3/doc/html/manual/bk01pt05ch13s03.html new file mode 100644 index 00000000000..c321667a729 --- /dev/null +++ b/libstdc++-v3/doc/html/manual/bk01pt05ch13s03.html @@ -0,0 +1,57 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Arbitrary Character Types</title><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="start" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="bk01pt05ch13.html" title="Chapter 13. String Classes" /><link rel="prev" href="bk01pt05ch13s02.html" title="Case Sensivitity" /><link rel="next" href="bk01pt05ch13s04.html" title="Tokenizing" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Arbitrary Character Types</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk01pt05ch13s02.html">Prev</a> </td><th width="60%" align="center">Chapter 13. String Classes</th><td width="20%" align="right"> <a accesskey="n" href="bk01pt05ch13s04.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="strings.string.character_types"></a>Arbitrary Character Types</h2></div></div></div><p> + </p><p>The <code class="code">std::basic_string</code> is tantalizingly general, in that + it is parameterized on the type of the characters which it holds. + In theory, you could whip up a Unicode character class and instantiate + <code class="code">std::basic_string<my_unicode_char></code>, or assuming + that integers are wider than characters on your platform, maybe just + declare variables of type <code class="code">std::basic_string<int></code>. + </p><p>That's the theory. Remember however that basic_string has additional + type parameters, which take default arguments based on the character + type (called <code class="code">CharT</code> here): + </p><pre class="programlisting"> + template <typename CharT, + typename Traits = char_traits<CharT>, + typename Alloc = allocator<CharT> > + class basic_string { .... };</pre><p>Now, <code class="code">allocator<CharT></code> will probably Do The Right + Thing by default, unless you need to implement your own allocator + for your characters. + </p><p>But <code class="code">char_traits</code> takes more work. The char_traits + template is <span class="emphasis"><em>declared</em></span> but not <span class="emphasis"><em>defined</em></span>. + That means there is only + </p><pre class="programlisting"> + template <typename CharT> + struct char_traits + { + static void foo (type1 x, type2 y); + ... + };</pre><p>and functions such as char_traits<CharT>::foo() are not + actually defined anywhere for the general case. The C++ standard + permits this, because writing such a definition to fit all possible + CharT's cannot be done. + </p><p>The C++ standard also requires that char_traits be specialized for + instantiations of <code class="code">char</code> and <code class="code">wchar_t</code>, and it + is these template specializations that permit entities like + <code class="code">basic_string<char,char_traits<char>></code> to work. + </p><p>If you want to use character types other than char and wchar_t, + such as <code class="code">unsigned char</code> and <code class="code">int</code>, you will + need suitable specializations for them. For a time, in earlier + versions of GCC, there was a mostly-correct implementation that + let programmers be lazy but it broke under many situations, so it + was removed. GCC 3.4 introduced a new implementation that mostly + works and can be specialized even for <code class="code">int</code> and other + built-in types. + </p><p>If you want to use your own special character class, then you have + <a class="ulink" href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00163.html" target="_top">a lot + of work to do</a>, especially if you with to use i18n features + (facets require traits information but don't have a traits argument). + </p><p>Another example of how to specialize char_traits was given <a class="ulink" href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00260.html" target="_top">on the + mailing list</a> and at a later date was put into the file <code class="code"> + include/ext/pod_char_traits.h</code>. We agree + that the way it's used with basic_string (scroll down to main()) + doesn't look nice, but that's because <a class="ulink" href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00236.html" target="_top">the + nice-looking first attempt</a> turned out to <a class="ulink" href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00242.html" target="_top">not + be conforming C++</a>, due to the rule that CharT must be a POD. + (See how tricky this is?) + </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt05ch13s02.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="bk01pt05ch13.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="bk01pt05ch13s04.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Case Sensivitity </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Tokenizing</td></tr></table></div></body></html> |