ch18_07.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

during run time.  Once a <tt class="literal">BEGIN</tt> block has run, it is immediately
undefined and any code it used is returned to Perl's memory pool.
You couldn't call a <tt class="literal">BEGIN</tt> block as a subroutine even if you tried,
because by the time it's there, it's already gone.</p>

<p>
<a name="INDEX-3314"></a>
Similar to <tt class="literal">BEGIN</tt> blocks, <tt class="literal">INIT</tt>
blocks are run just before the Perl run time begins execution, in
"first in, first out" (FIFO) order.  For example, the code generators
documented in <em class="emphasis">perlcc</em> make use of
<tt class="literal">INIT</tt> blocks to initialize and resolve pointers to
XSUBs. <tt class="literal">INIT</tt> blocks are really just like
<tt class="literal">BEGIN</tt> blocks, except they let the programmer
distinguish construction that must happen at compile phase from
construction that must happen at run phase.  When you're running a
script directly, that's not terribly important because the compiler
gets invoked every time anyway; but when compilation is separate from
execution, the distinction can be crucial.  The compiler may only be
invoked once, and the resulting executable may be invoked many times.</p>

<p>Similar to <tt class="literal">END</tt> blocks, <tt class="literal">CHECK</tt>
blocks are run just after the Perl compile phase ends but before run phase begins, in LIFO order.  <tt class="literal">CHECK</tt> blocks are
useful for "winding down" the compiler just as <tt class="literal">END</tt>
blocks are useful for winding down your program.  In particular, the backends all use <tt class="literal">CHECK</tt> blocks as the hook from
which to invoke their respective code generators.  All they need to do
is put a <tt class="literal">CHECK</tt> block into their own module, and it
will run at the right time, so you don't have to install a
<tt class="literal">CHECK</tt> into your program.  For this reason, you'll
rarely write a <tt class="literal">CHECK</tt> block yourself, unless you're
writing such a module.</p>

<p>
<a name="INDEX-3315"></a><a name="INDEX-3316"></a>
Putting it all together, <a href="ch18_07.htm#perl3-tab-annotations">Table 18-1</a> lists various constructs with details on
when they compile and when they run the code represented by "<tt class="literal">...</tt>".</p>


<a name="perl3-tab-annotations"></a>
<h4 class="objtitle">Table 18.1. What Happens When</h4>
<table border="1">

<tr>
<th>
<p>Block</p>
</th>
<th>
<p>Compiles</p>
</th>
<th>
<p>Traps</p>
</th>
<th>
<p>Runs</p>
</th>
<th>
<p>Traps</p>
</th>
<th>
<p>Call</p>
</th>
</tr>

<tr>
<th>
<p>or</p>
</th>
<th>
<p>During</p>
</th>
<th>
<p>Compile</p>
</th>
<th>
<p>During</p>
</th>
<th>
<p>Run</p>
</th>
<th>
<p>Trigger</p>
</th>
</tr>

<tr>
<th>
<p>Expression</p>
</th>
<th>
<p>Phase</p>
</th>
<th>
<p>Errors</p>
</th>
<th>
<p>Phase</p>
</th>
<th>
<p>Errors</p>
</th>
<th>
<p>Policy</p>
</th>
</tr>




<tr>
<td><tt class="literal">use ...</tt></td>
<td>C</td>
<td>No</td>
<td>C</td>
<td>No</td>
<td>Now</td>
</tr>

<tr>
<td><tt class="literal">no ...</tt></td>
<td>C</td>
<td>No</td>
<td>C</td>
<td>No</td>
<td>Now</td>
</tr>

<tr>
<td><tt class="literal">BEGIN {...}</tt></td>
<td>C</td>
<td>No</td>
<td>C</td>
<td>No</td>
<td>Now</td>
</tr>

<tr>
<td><tt class="literal">CHECK {...}</tt></td>
<td>C</td>
<td>No</td>
<td>C</td>
<td>No</td>
<td>Late</td>
</tr>

<tr>
<td><tt class="literal">INIT {...}</tt></td>
<td>C</td>
<td>No</td>
<td>R</td>
<td>No</td>
<td>Early</td>
</tr>

<tr>
<td><tt class="literal">END {...}</tt></td>
<td>C</td>
<td>No</td>
<td>R</td>
<td>No</td>
<td>Late</td>
</tr>

<tr>
<td><tt class="literal">eval {...}</tt></td>
<td>C</td>
<td>No</td>
<td>R</td>
<td>Yes</td>
<td>Inline</td>
</tr>

<tr>
<td><tt class="literal">eval "..."</tt></td>
<td>R</td>
<td>Yes</td>
<td>R</td>
<td>Yes</td>
<td>Inline</td>
</tr>

<tr>
<td><tt class="literal">foo(...)</tt></td>
<td>C</td>
<td>No</td>
<td>R</td>
<td>No</td>
<td>Inline</td>
</tr>

<tr>
<td><tt class="literal">sub foo {...}</tt></td>
<td>C</td>
<td>No</td>
<td>R</td>
<td>No</td>
<td>Call anytime</td>
</tr>

<tr>
<td><tt class="literal">eval "sub {...}"</tt></td>
<td>R</td>
<td>Yes</td>
<td>R</td>
<td>No</td>
<td>Call later</td>
</tr>

<tr>
<td><tt class="literal">s/pat/.../e</tt></td>
<td>C</td>
<td>No</td>
<td>R</td>
<td>No</td>
<td>Inline</td>
</tr>

<tr>
<td><tt class="literal">s/pat/"..."/ee</tt></td>
<td>R</td>
<td>Yes</td>
<td>R</td>
<td>Yes</td>
<td>Inline</td>
</tr>


</table>

<p>
Now that you know the score, we hope you'll be able to compose and
perform your Perl pieces with greater confidence.</p>

<a name="INDEX-3317"></a><a name="INDEX-3318"></a>


<!-- BOTTOM NAV BAR -->

<hr width="515" align="left">
<div class="navbar">
<table width="515" border="0">
<tr>
<td align="left" valign="top" width="172"><a href="ch18_06.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0"></a></td><td align="right" valign="top" width="172"><a href="ch19_01.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td>
</tr>
<tr>
<td align="left" valign="top" width="172">18.6. Code Development Tools</td><td align="center" valign="top" width="171"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0"></a></td><td align="right" valign="top" width="172">19. The Command-Line Interface</td>
</tr>
</table>
</div>
<hr width="515" align="left">

<!-- LIBRARY NAV BAR -->

<img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p>
<font size="-1"><a href="copyrght.htm">Copyright &copy; 2001</a> O'Reilly &amp; Associates. All rights reserved.</font>
</p>
<map name="library-map"> <area shape="rect" coords="2,-1,79,99" href="../index.htm"><area shape="rect" coords="84,1,157,108" href="../perlnut/index.htm"><area shape="rect" coords="162,2,248,125" href="../prog/index.htm"><area shape="rect" coords="253,2,326,130" href="../advprog/index.htm"><area shape="rect" coords="332,1,407,112" href="../cookbook/index.htm"><area shape="rect" coords="414,2,523,103" href="../sysadmin/index.htm">
</map>

<!-- END OF BODY -->

</body>
</html>