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<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 Sensitivity" /><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 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>
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