ch11_02.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

<p>You can say <tt class="literal">BEGIN { $Exporter::Verbose=1 }</tt> to see
how the specifications are being processed and what is actually being
imported into your package.</p>

<p>The <tt class="literal">Exporter</tt> is itself a Perl module, and if you're
curious you can see the typeglob trickery it uses to export symbols
from one package into another.  Inside the <tt class="literal">Export</tt>
module, the key function is named <tt class="literal">import</tt>, which
performs the necessary aliasing to make a symbol in one package appear
to be in another.  In fact, a <tt class="literal">use Bestiary</tt><em class="replaceable">LIST</em> statement is exactly equivalent to:
<blockquote>
<pre class="programlisting">BEGIN {
    require Bestiary;
    import Bestiary <em class="replaceable">LIST</em>;
}</pre>
</blockquote>

This means that your modules don't have to use the
<tt class="literal">Exporter</tt>.  A module can do anything it jolly well
pleases when it's used, since <tt class="literal">use</tt> just calls the
ordinary <tt class="literal">import</tt> method for the module, and you can
define that method to do anything you like.</p>


<h3 class="sect3">11.2.1.1. Exporting without using Exporter's import method</h3>

<p>
<a name="INDEX-2314"></a><a name="INDEX-2315"></a><a name="INDEX-2316"></a>
The <tt class="literal">Exporter</tt> defines a method called
<tt class="literal">export_to_level</tt>, used for situations where (for
some reason) you can't directly call <tt class="literal">Exporter</tt>'s
<tt class="literal">import</tt> method. The
<tt class="literal">export_to_level</tt> method is invoked like this:
<blockquote>
<pre class="programlisting">
<em class="replaceable">MODULE</em>-&gt;export_to_level($where_to_export, @what_to_export);</pre>
</blockquote>

where <tt class="literal">$where_to_export</tt> is an integer indicating how
far up the calling stack to export your symbols, and
<tt class="literal">@what_to_export</tt> is an array listing the symbols to
export (usually <tt class="literal">@_</tt>).</p>

<p>For example, suppose our <tt class="literal">Bestiary</tt> had an
<tt class="literal">import</tt> function of its own:
<blockquote>
<pre class="programlisting">package Bestiary;
@ISA = qw(Exporter);
@EXPORT_OK = qw ($zoo);

sub import {
    $Bestiary::zoo = "menagerie";
}</pre>
</blockquote>

The presence of this <tt class="literal">import</tt> function prevents
<tt class="literal">Exporter</tt>'s <tt class="literal">import</tt> function from
being inherited.  If you want <tt class="literal">Bestiary</tt>'s
<tt class="literal">import</tt> function to behave just like
<tt class="literal">Exporter</tt>'s <tt class="literal">import</tt> function once
it sets <tt class="literal">$Bestiary::zoo</tt>, you'd define it as follows:
<blockquote>
<pre class="programlisting">sub import {
    $Bestiary::zoo = "menagerie";
    Bestiary-&gt;export_to_level(1, @_);
}</pre>
</blockquote>

This exports symbols to the package one level "above" the current
package.  That is, to whatever program or module is using the
<tt class="literal">Bestiary</tt>.</p>




<h3 class="sect3">11.2.1.2. Version checking</h3>

<p>
<a name="INDEX-2317"></a><a name="INDEX-2318"></a>
If your module defines a <tt class="literal">$VERSION</tt> variable, a
program using your module can ensure that the module is sufficiently
recent.  For example:
<blockquote>
<pre class="programlisting">use Bestiary 3.14;   # The Bestiary must be version 3.14 or later
use Bestiary v1.0.4; # The Bestiary must be version 1.0.4 or later</pre>
</blockquote>

These are converted into calls to
<tt class="literal">Bestiary-&gt;require_version</tt>, which your module
then inherits.</p>




<h3 class="sect3">11.2.1.3. Managing unknown symbols</h3>

<p>
<a name="INDEX-2319"></a><a name="INDEX-2320"></a>
In some situations, you may want to <em class="emphasis">prevent</em>
certain symbols from being exported. Typically, this applies to
modules that have functions or constants that might not make sense on
some systems.  You can prevent the <tt class="literal">Exporter</tt> from
exporting those symbols by placing them in the
<tt class="literal">@EXPORT_FAIL</tt> array.</p>

<p>
<a name="INDEX-2321"></a><a name="INDEX-2322"></a>
If a program attempts to import any of these symbols, the
<tt class="literal">Exporter</tt> gives the module an opportunity to handle
the situation before generating an error.  It does this by calling an
<tt class="literal">export_fail</tt> method with a list of the failed
symbols, which you might define as follows (assuming your module uses
the <tt class="literal">Carp</tt> module):
<blockquote>
<pre class="programlisting">sub export_fail {
    my $class = shift;
    carp "Sorry, these symbols are unavailable: @_";
    return @_;
}</pre>
</blockquote>

The <tt class="literal">Exporter</tt> provides a default
<tt class="literal">export_fail</tt> method, which simply returns the list
unchanged and makes the <tt class="literal">use</tt> fail with an exception
raised for each symbol.  If <tt class="literal">export_fail</tt> returns an
empty list, no error is recorded and all the requested symbols are
exported.</p>




<h3 class="sect3">11.2.1.4. Tag-handling utility functions</h3>

<p>
<a name="INDEX-2323"></a>
Since the symbols listed within <tt class="literal">%EXPORT_TAGS</tt> must
also appear in either <tt class="literal">@EXPORT</tt> or
<tt class="literal">@EXPORT_OK</tt>, the <tt class="literal">Exporter</tt>
provides two functions to let you add those tagged sets of symbols:
<blockquote>
<pre class="programlisting">%EXPORT_TAGS = (foo =&gt; [qw(aa bb cc)], bar =&gt; [qw(aa cc dd)]);

Exporter::export_tags('foo');     # add aa, bb and cc to @EXPORT
Exporter::export_ok_tags('bar');  # add aa, cc and dd to @EXPORT_OK</pre>
</blockquote>

Specifying names that are not tags is erroneous.</p>

<a name="INDEX-2324"></a><a name="INDEX-2325"></a><a name="INDEX-2326"></a><a name="INDEX-2327"></a>




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