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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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