GNU Info

(libc.info)Using Wide Char Classes

---

Next: Wide Character Case Conversion Prev: Classification of Wide Characters Up: Character Handling

---

Enter node , (file) or (file)node

Notes on using the wide character classes
=========================================

   The first note is probably not astonishing but still occasionally a
cause of problems.  The `iswXXX' 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 `wctype.h' header is included
the macros will be used.  This is the same as the `char' type versions
of these functions.

   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.

     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;
     }

   Now, with the `wctype' and `iswctype' you can avoid the `if'
cascades, but rewriting the code as follows is wrong:

     int
     is_in_class (int c, const char *class)
     {
       wctype_t desc = wctype (class);
       return desc ? iswctype ((wint_t) c, desc) : 0;
     }

   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:

     int
     is_in_class (int c, const char *class)
     {
       wctype_t desc = wctype (class);
       return desc ? iswctype (btowc (c), desc) : 0;
     }

   Note: Converting a Character, for more information on `btowc'.
Note that this change probably does not improve the performance of the
program a lot since the `wctype' function still has to make the string
comparisons.  It gets really interesting if the `is_in_class' function
is called more than once for the same class name.  In this case the
variable DESC could be computed once and reused for all the calls.
Therefore the above form of the function is probably not the final one.