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<h3 class="section">4.4 Notes on using the wide character classes</h3>

<p>The first note is probably not astonishing but still occasionally a
cause of problems.  The <code>isw</code><var>XXX</var> functions can be implemented
using macros and in fact, the GNU C library does this.  They are still
available as real functions but when the <samp><span class="file">wctype.h</span></samp> header is
included the macros will be used.  This is the same as the
<code>char</code> type versions of these functions.

   <p>The second note covers something new.  It can be best illustrated by a
(real-world) example.  The first piece of code is an excerpt from the
original code.  It is truncated a bit but the intention should be clear.

<pre class="smallexample">     int
     is_in_class (int c, const char *class)
     {
       if (strcmp (class, "alnum") == 0)
         return isalnum (c);
       if (strcmp (class, "alpha") == 0)
         return isalpha (c);
       if (strcmp (class, "cntrl") == 0)
         return iscntrl (c);
       ...
       return 0;
     }
</pre>
   <p>Now, with the <code>wctype</code> and <code>iswctype</code> you can avoid the
<code>if</code> cascades, but rewriting the code as follows is wrong:

<pre class="smallexample">     int
     is_in_class (int c, const char *class)
     {
       wctype_t desc = wctype (class);
       return desc ? iswctype ((wint_t) c, desc) : 0;
     }
</pre>
   <p>The problem is that it is not guaranteed that the wide character
representation of a single-byte character can be found using casting. 
In fact, usually this fails miserably.  The correct solution to this
problem is to write the code as follows:

<pre class="smallexample">     int
     is_in_class (int c, const char *class)
     {
       wctype_t desc = wctype (class);
       return desc ? iswctype (btowc (c), desc) : 0;
     }
</pre>
   <p>See <a href="Converting-a-Character.html#Converting-a-Character">Converting a Character</a>, for more information on <code>btowc</code>. 
Note that this change probably does not improve the performance
of the program a lot since the <code>wctype</code> function still has to make
the string comparisons.  It gets really interesting if the
<code>is_in_class</code> function is called more than once for the
same class name.  In this case the variable <var>desc</var> could be computed
once and reused for all the calls.  Therefore the above form of the
function is probably not the final one.

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