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   4     </p><p>The <code class="code">std::basic_string</code> is tantalizingly general, in that
   5       it is parameterized on the type of the characters which it holds.
   6       In theory, you could whip up a Unicode character class and instantiate
   7       <code class="code">std::basic_string&lt;my_unicode_char&gt;</code>, or assuming
   8       that integers are wider than characters on your platform, maybe just
   9       declare variables of type <code class="code">std::basic_string&lt;int&gt;</code>.
  10    </p><p>That's the theory.  Remember however that basic_string has additional
  11       type parameters, which take default arguments based on the character
  12       type (called <code class="code">CharT</code> here):
  13    </p><pre class="programlisting">
  14       template &lt;typename CharT,
  15                 typename Traits = char_traits&lt;CharT&gt;,
  16                 typename Alloc = allocator&lt;CharT&gt; &gt;
  17       class basic_string { .... };</pre><p>Now, <code class="code">allocator&lt;CharT&gt;</code> will probably Do The Right
  18       Thing by default, unless you need to implement your own allocator
  19       for your characters.
  20    </p><p>But <code class="code">char_traits</code> takes more work.  The char_traits
  21       template is <span class="emphasis"><em>declared</em></span> but not <span class="emphasis"><em>defined</em></span>.
  22       That means there is only
  23    </p><pre class="programlisting">
  24       template &lt;typename CharT&gt;
  25         struct char_traits
  26         {
  27             static void foo (type1 x, type2 y);
  28             ...
  29         };</pre><p>and functions such as char_traits&lt;CharT&gt;::foo() are not
  30       actually defined anywhere for the general case.  The C++ standard
  31       permits this, because writing such a definition to fit all possible
  32       CharT's cannot be done.
  33    </p><p>The C++ standard also requires that char_traits be specialized for
  34       instantiations of <code class="code">char</code> and <code class="code">wchar_t</code>, and it
  35       is these template specializations that permit entities like
  36       <code class="code">basic_string&lt;char,char_traits&lt;char&gt;&gt;</code> to work.
  37    </p><p>If you want to use character types other than char and wchar_t,
  38       such as <code class="code">unsigned char</code> and <code class="code">int</code>, you will
  39       need suitable specializations for them.  For a time, in earlier
  40       versions of GCC, there was a mostly-correct implementation that
  41       let programmers be lazy but it broke under many situations, so it
  42       was removed.  GCC 3.4 introduced a new implementation that mostly
  43       works and can be specialized even for <code class="code">int</code> and other
  44       built-in types.
  45    </p><p>If you want to use your own special character class, then you have
  46       <a class="ulink" href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00163.html" target="_top">a lot
  47       of work to do</a>, especially if you with to use i18n features
  48       (facets require traits information but don't have a traits argument).
  49    </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
  50       mailing list</a> and at a later date was put into the file <code class="code">
  51       include/ext/pod_char_traits.h</code>.  We agree
  52       that the way it's used with basic_string (scroll down to main())
  53       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
  54       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
  55       be conforming C++</a>, due to the rule that CharT must be a POD.
  56       (See how tricky this is?)
  57    </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 Sensitivity </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>