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; }
See Converting Single Characters, 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.