ch12_05.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

We use the <tt class="literal">%seen</tt> hash to keep track of which subroutines we've
already called, so we can avoid calling the same subroutine more than
once.  This could happen if several parent classes shared a common
ancestor.</p>

<p>Methods that would trigger an <tt class="literal">AUTOLOAD</tt> (described in the next section) will
not be accurately reported unless the package has declared (but not
defined) the subroutines it wishes to have autoloaded.</p>
</dd>


<dt>
<b><em class="replaceable">INVOCANT</em><tt class="literal">-&gt;VERSION(</tt><em class="replaceable">NEED</em><tt class="literal">)</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-2478"></a><a name="INDEX-2479"></a>
The <tt class="literal">VERSION</tt> method returns the version number of
<em class="replaceable">INVOCANT</em>'s class, as stored in the
package's <tt class="literal">$VERSION</tt> variable.  If the
<em class="replaceable">NEED</em> argument is provided, it verifies that
the current version isn't less than <em class="replaceable">NEED</em>
and raises an exception if it is.  This is the method that
<tt class="literal">use</tt> invokes to determine whether a module is
sufficiently recent.
<blockquote>
<pre class="programlisting">use Thread 1.0;   # calls Thread-&gt;VERSION(1.0)
print "Running version ", Thread-&gt;VERSION, " of Thread.\n";</pre>
</blockquote>

You may supply your own <tt class="literal">VERSION</tt> method to override the method in
<tt class="literal">UNIVERSAL</tt>.  However, this will cause any classes derived from your class to use the overridden method, too. If you don't want that to happen, you
should design your method to delegate other classes' version requests
back up to <tt class="literal">UNIVERSAL</tt>.</p>
</dd>

</dl>

<p>The methods in <tt class="literal">UNIVERSAL</tt> are built-in Perl subroutines, which you
may call if you fully qualify them and pass two arguments, as in
<tt class="literal">UNIVERSAL::isa($formobj, "HASH")</tt>.  (This is not recommended,
though, because <tt class="literal">can</tt> usually has the answer you're really looking for.)</p>

<p>
<a name="INDEX-2480"></a><a name="INDEX-2481"></a>
You're free to add your own methods to class
<tt class="literal">UNIVERSAL</tt>.  (You should be careful, of course; you
could really mess someone up who is expecting <em class="emphasis">not</em>
to find the method name you're defining, perhaps so that they can
autoload it from somewhere else.)  Here we create a
<tt class="literal">copy</tt> method that objects of all classes can use if
they've not defined their own.  (We fail spectacularly if invoked on a
class instead of an object.)
<blockquote>
<pre class="programlisting">use Data::Dumper;   
use Carp;
sub UNIVERSAL::copy {
    my $self = shift;
    if (ref $self) {
        return eval Dumper($self);  # no CODE refs
    } else {
        confess "UNIVERSAL::copy can't copy class $self";
    }
}</pre>
</blockquote>
 This <tt class="literal">Data::Dumper</tt> strategy doesn't
work if the object contains any references to subroutines, because
they cannot be properly reproduced.  Even if the source were
available, the lexical bindings would be lost.</p>

<a name="INDEX-2482"></a><a name="INDEX-2483"></a>






<h3 class="sect2">12.5.4. Method Autoloading</h3>

<a name="INDEX-2484"></a><a name="INDEX-2485"></a><a name="INDEX-2486"></a><a name="INDEX-2487"></a><a name="INDEX-2488"></a>
<p>
<a name="INDEX-2489"></a>
Normally, when you call an undefined subroutine in a package that
defines an <tt class="literal">AUTOLOAD</tt> subroutine, the
<tt class="literal">AUTOLOAD</tt> subroutine is called in lieu of raising an
exception (see the section <a href="ch10_02.htm#ch10-sect-auto">Section 12.2, "Autoloading"</a> in <a href="ch10_01.htm">Chapter 10, "Packages"</a>).  With methods, this works a little
differently.  If the regular method search (through the class, its
ancestors, and finally <tt class="literal">UNIVERSAL</tt>) fails to find a
match, the same sequence is run again, this time looking for an
<tt class="literal">AUTOLOAD</tt> subroutine.  If found, this subroutine is
called as a method, with the package's <tt class="literal">$AUTOLOAD</tt>
variable set to the fully qualified name of the subroutine on whose
behalf <tt class="literal">AUTOLOAD</tt> was called.</p>

<p>You need to be a bit cautious when autoloading methods.  First, the
<tt class="literal">AUTOLOAD</tt> subroutine should return immediately if
it's being called on behalf of a method named
<tt class="literal">DESTROY</tt>, unless your goal was to simulate
<tt class="literal">DESTROY</tt>, which has a special meaning to Perl
described in the section <a href="ch12_06.htm#ch10-sect-destruct">Section 12.6, "Instance Destructors"</a> later in this chapter.
<blockquote>
<pre class="programlisting">sub AUTOLOAD {
    return if our $AUTOLOAD =~ /::DESTROY$/;
    ...
}</pre>
</blockquote>
Second, if the class is providing an <tt class="literal">AUTOLOAD</tt>
safety net, you won't be able to use <tt class="literal">UNIVERAL::can</tt>
on a method name to check whether it's safe to invoke. You have to
check for <tt class="literal">AUTOLOAD</tt> separately:
<blockquote>
<pre class="programlisting">if ($obj-&gt;can("methname") || $obj-&gt;can("AUTOLOAD")) {
    $obj-&gt;methname();
}</pre>
</blockquote>
Finally, under multiple inheritance, if a class inherits from two or
more classes each of which has an <tt class="literal">AUTOLOAD</tt>, only
the leftmost will ever be triggered, since Perl stops as soon as it
finds the first <tt class="literal">AUTOLOAD</tt>.
<a name="INDEX-2490"></a>
</p>

<p>
<a name="INDEX-2491"></a><a name="INDEX-2492"></a>
The last two quirks are easily circumvented by declaring the
subroutines in the package whose <tt class="literal">AUTOLOAD</tt> is
supposed to manage those methods.  You can do this either with
individual declarations:
<blockquote>
<pre class="programlisting">package Goblin;
sub kick;
sub bite;
sub scratch;</pre>
</blockquote>
<a name="INDEX-2493"></a>
or with the <tt class="literal">use subs</tt> pragma, which is more convenient if you have
many methods to declare:
<blockquote>
<pre class="programlisting">package Goblin;
use subs qw(kick bite scratch);</pre>
</blockquote>

Even though you've only declared these subroutines and not defined them, this is enough for the system to think they're real.  They
show up in a <tt class="literal">UNIVERAL::can</tt> check, and, more importantly, they show up
in step 2 of the search for a method, which will never progress to step 3,
let alone step 4.</p>

<p>"But, but," you exclaim, "they invoke <tt class="literal">AUTOLOAD</tt>, don't they?"  Well,
yes, they do eventually, but the mechanism is different.  Having found
the method stub via step 2, Perl tries to call it.  When it is
discovered that the method isn't all it was cracked up to be, the
<tt class="literal">AUTOLOAD</tt> method search kicks in again, but this time, it starts its
search in the class containing the stub, which restricts the method
search to that class and its ancestors (and <tt class="literal">UNIVERSAL</tt>).  That's how Perl finds the correct <tt class="literal">AUTOLOAD</tt> to run and knows 
to ignore <tt class="literal">AUTOLOAD</tt>s from
the wrong part of the original inheritance tree.</p>





<a name="ch12-sect-pm"></a>
<h3 class="sect2">12.5.5. Private Methods</h3>

<p>
<a name="INDEX-2494"></a><a name="INDEX-2495"></a><a name="INDEX-2496"></a>
There is one way to invoke a method so that Perl ignores inheritance
altogether.  If instead of a literal method name, you specify a simple
scalar variable containing a reference to a subroutine, then the
subroutine is called immediately.  In the description of 
<tt class="literal">UNIVERSAL-&gt;can</tt> in the previous section, the last example invokes all overridden
methods using the subroutine's reference, not its name.</p>

<p>
<a name="INDEX-2497"></a><a name="INDEX-2498"></a>
An intriguing aspect of this behavior is that it can be used to
implement private method calls.  If you put your class in a module,
you can make use of the file's lexical scope for privacy.  First,
store an anonymous subroutine in a file-scoped lexical:
<blockquote>
<pre class="programlisting"># declare private method
my $secret_door = sub {
    my $self = shift;
    ...
};</pre>
</blockquote>

Later on in the file, you can use that variable as though it held a
method name.  The closure will be called directly, without regard to
inheritance.  As with any other method, the invocant is passed as an
extra argument.
<blockquote>
<pre class="programlisting">sub knock {
    my $self = shift;
    if ($self-&gt;{knocked}++ &gt; 5) {
        $self-&gt;$secret_door();
    }
}</pre>
</blockquote>

This enables the file's own subroutines (the class methods) to invoke
a method that code outside that lexical scope cannot access.</p>

<a name="INDEX-2499"></a><a name="INDEX-2500"></a><a name="INDEX-2501"></a>



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