ch13_03.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

are binary.  Here's how we could overload <tt class="literal">&gt;&gt;</tt>
to do something like <tt class="literal">chop</tt>:
<blockquote>
<pre class="programlisting">package ShiftString;

use overload
    '&gt;&gt;' =&gt; \&amp;right_shift,
    '""' =&gt; sub { ${ $_[0] } };

sub new {
    my $class = shift;
    my $value = shift;
    return bless \$value =&gt; $class;
}

sub right_shift {
    my ($x, $y) = @_;
    my $value = $$x;
    substr($value, -$y) = "";
    return bless \$value =&gt; ref($x);
}

$camel = ShiftString-&gt;new("Camel");
$ram = $camel &gt;&gt; 2;
print $ram;            # Cam</pre>
</blockquote>
</p>
</dd>


<dt>
<b><em class="emphasis">Assignment operators:</em> <tt class="literal">+=</tt><em class="emphasis">,</em> <tt class="literal">-=</tt><em class="emphasis">,</em> <tt class="literal">*=</tt><em class="emphasis">,</em> <tt class="literal">/=</tt><em class="emphasis">,</em> <tt class="literal">%=</tt><em class="emphasis">,</em> <tt class="literal">**=</tt><em class="emphasis">,</em> <tt class="literal">x=</tt><em class="emphasis">,</em> <tt class="literal">.=</tt><em class="emphasis">,</em> <tt class="literal">&lt;&lt;=</tt><em class="emphasis">,</em> <tt class="literal">&gt;&gt;=</tt><em class="emphasis">,</em> <tt class="literal">++</tt><em class="emphasis">,</em> <tt class="literal">--</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-"></a><a name="INDEX-"></a>
These assignment operators might change the value of their arguments
or leave them as is.  The result is assigned to the lefthand operand
only if the new value differs from the old one. This allows the same
handler to be used to overload both <tt class="literal">+=</tt> and
<tt class="literal">+</tt>.  Although this is permitted, it is seldom
recommended, since by the semantics described later under "When an
Overload Handler Is Missing (nomethod and fallback)", Perl will invoke
the handler for <tt class="literal">+</tt> anyway, assuming
<tt class="literal">+=</tt> hasn't been overloaded directly.</p>

<p>
<a name="INDEX-"></a>
Concatenation (<tt class="literal">.=</tt>) can be autogenerated using
stringification followed by ordinary string concatenation. The
<tt class="literal">++</tt> and <tt class="literal">--</tt> operators can
be autogenerated from <tt class="literal">+</tt> and <tt class="literal">-</tt>
(or <tt class="literal">+=</tt> and <tt class="literal">-=</tt>).</p>

<p>
<a name="INDEX-"></a><a name="INDEX-"></a>
Handlers implementing <tt class="literal">++</tt> and <tt class="literal">--</tt> are expected to <em class="emphasis">mutate</em> (alter)
their arguments.  If you wanted autodecrement to work on letters as
well as numbers, you could do that with a handler as follows:
<blockquote>
<pre class="programlisting">package MagicDec;

use overload
    q(--) =&gt; \&amp;decrement,
    q("") =&gt; sub { ${ $_[0] } };

sub new {
    my $class = shift;
    my $value = shift;
    bless \$value =&gt; $class;
}

sub decrement {
    my @string = reverse split(//, ${ $_[0] } );
    my $i;
    for ($i = 0; $i &lt; @string; $i++ ) {
        last unless $string[$i] =~ /a/i;
        $string[$i] = chr( ord($string[$i]) + 25 );
    }
    $string[$i] = chr( ord($string[$i]) - 1 );
    my $result = join('', reverse @string);
    $_[0] = bless \$result =&gt; ref($_[0]);
}

package main;

for $normal (qw/perl NZ Pa/) {
    $magic = MagicDec-&gt;new($normal);
    $magic--;
    print "$normal goes to $magic\n";
}</pre>
</blockquote>

That prints out:
<blockquote>
<pre class="programlisting">perl goes to perk
NZ goes to NY
Pa goes to Oz</pre>
</blockquote>

exactly reversing Perl's magical string autoincrement operator.
<a name="INDEX-"></a>
</p>

<p>The <tt class="literal">++$a</tt> operation can be autogenerated using
<tt class="literal">$a += 1</tt> or <tt class="literal">$a = $a + 1</tt>, and
<tt class="literal">$a--</tt> using <tt class="literal">$a -= 1</tt> or
<tt class="literal">$a = $a - 1</tt>.  However, this does not trigger the
copying behavior that a real <tt class="literal">++</tt> operator would.
See "The Copy Constructor" later in this chapter.</p>
</dd>


<dt>
<b><em class="emphasis">Comparison operators:</em>
<tt class="literal">==</tt><em class="emphasis">,</em>
<tt class="literal">&lt;</tt><em class="emphasis">,</em>
<tt class="literal">&lt;=</tt><em class="emphasis">,</em>
<tt class="literal">&gt;</tt><em class="emphasis">,</em>
<tt class="literal">&gt;=</tt><em class="emphasis">,</em>
<tt class="literal">!=</tt><em class="emphasis">,</em>
<tt class="literal">&lt;=&gt;</tt><em class="emphasis">,</em>
<tt class="literal">lt</tt><em class="emphasis">,</em>
<tt class="literal">le</tt><em class="emphasis">,</em>
<tt class="literal">gt</tt><em class="emphasis">,</em>
<tt class="literal">ge</tt><em class="emphasis">,</em>
<tt class="literal">eq</tt><em class="emphasis">,</em>
<tt class="literal">ne</tt><em class="emphasis">,</em> <tt class="literal">cmp</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-"></a><a name="INDEX-"></a>
If <tt class="literal">&lt;=&gt;</tt> is overloaded, it can be used to
autogenerate behaviors for <tt class="literal">&lt;</tt>,
<tt class="literal">&lt;=</tt>, <tt class="literal">&gt;</tt>,
<tt class="literal">&gt;=</tt>, <tt class="literal">==</tt>, and
<tt class="literal">!=</tt>.  Similarly, if <tt class="literal">cmp</tt> is
overloaded, it can be used to autogenerate behaviors for
<tt class="literal">lt</tt>, <tt class="literal">le</tt>, <tt class="literal">gt</tt>,
<tt class="literal">ge</tt>, <tt class="literal">eq</tt>, and
<tt class="literal">ne</tt>.</p>

<p>
<a name="INDEX-"></a><a name="INDEX-"></a>
Note that overloading <tt class="literal">cmp</tt> won't let you sort
objects as easily as you'd like, because what will be compared are
the stringified versions of the objects instead of the objects
themselves.  If that was your
goal, you'd want to overload <tt class="literal">""</tt> as well.</p>
</dd>


<dt>
<b><em class="emphasis">Mathematical functions:</em> <tt class="literal">atan2</tt><em class="emphasis">,</em> <tt class="literal">cos</tt><em class="emphasis">,</em> <tt class="literal">sin</tt><em class="emphasis">,</em> <tt class="literal">exp</tt><em class="emphasis">,</em> <tt class="literal">abs</tt><em class="emphasis">,</em> <tt class="literal">log</tt><em class="emphasis">,</em> <tt class="literal">sqrt</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-"></a><a name="INDEX-"></a>
If <tt class="literal">abs</tt> is unavailable, it can be autogenerated from
<tt class="literal">&lt;</tt> or <tt class="literal">&lt;=&gt;</tt> combined with
either unary minus or subtraction.</p>

<p>An overloaded <tt class="literal">-</tt> can be used to autogenerate missing
handlers for unary minus or for the <tt class="literal">abs</tt> function,
which may also be separately overloaded.  (Yes, we know that
<tt class="literal">abs</tt> looks like a function, whereas unary minus
looks like an operator, but they aren't all that different as far as
Perl's concerned.)</p>
</dd>


<dt>
<b><em class="emphasis">Iterative operator:</em> <tt class="literal">&lt;&gt;</tt></b>
</dt>
<dd>
<a name="INDEX-"></a><a name="INDEX-"></a>
<p>The <tt class="literal">&lt;&gt;</tt> handler can be triggered by using
either <tt class="literal">readline</tt> (when it reads from a filehandle,
as in <tt class="literal">while (&lt;FH&gt;)</tt>) or
<tt class="literal">glob</tt> (when it is used for fileglobbing, as in
<tt class="literal">@files = &lt;*.*&gt;</tt>).
<blockquote>
<pre class="programlisting">package LuckyDraw;
 
use overload
    '&lt;&gt;' =&gt; sub {
        my $self = shift;
        return splice @$self, rand @$self, 1;
    };
 
 
sub new {
    my $class = shift;
    return bless [@_] =&gt; $class;
}
 
package main;
 
$lotto = new LuckyDraw 1 .. 51;
 
for (qw(1st 2nd 3rd 4th 5th 6th)) {
    $lucky_number = &lt;$lotto&gt;;
    print "The $_ lucky number is: $lucky_number.\n";
}
 
$lucky_number = &lt;$lotto&gt;;
print "\nAnd the bonus number is: $lucky_number.\n";</pre>
</blockquote>

In California, this prints:
<blockquote>
<pre class="programlisting">The 1st lucky number is: 18
The 2nd lucky number is: 11
The 3rd lucky number is: 40

The 4th lucky number is: 7
The 5th lucky number is: 51
The 6th lucky number is: 33
 
And the bonus number is: 5</pre>
</blockquote>
</p>
</dd>


<dt>
<b><em class="emphasis">Dereference operators:</em> <tt class="literal">${}</tt><em class="emphasis">,</em> <tt class="literal">@{}</tt><em class="emphasis">,</em> <tt class="literal">%{}</tt><em class="emphasis">,</em> <tt class="literal">&amp;{}</tt><em class="emphasis">,</em> <tt class="literal">*{}</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-"></a><a name="INDEX-"></a>
Attempts to dereference scalar, array, hash, subroutine, and glob
references can be intercepted by overloading these five symbols.</p>

<p>
<a name="INDEX-"></a>
The online Perl documentation for <tt class="literal">overload</tt> demonstrates how you can
use this operator to simulate your own pseudohashes.  Here's a simpler example
that implements an object as an anonymous array but permits hash
referencing.  Don't try to treat it as a real hash; you won't be able
to <tt class="literal">delete</tt> key/value pairs from the object.  If you want to combine
array and hash notations, use a real pseudohash (as it were).
<blockquote>
<pre class="programlisting">package PsychoHash;

use overload '%{}' =&gt; \&amp;as_hash;

sub as_hash {
    my ($x) = shift;
    return { @$x };
}

sub new {
    my $class = shift;
    return bless [ @_ ] =&gt; $class;
}

$critter = new PsychoHash( height =&gt; 72, weight =&gt; 365, type =&gt; "camel" );

print $critter-&gt;{weight};   # prints 365</pre>
</blockquote>

Also see <a href="ch14_01.htm">Chapter 14, "Tied Variables"</a>, for a mechanism to let you redefine basic
operations on hashes, arrays, and scalars.</p>
</dd>

</dl>

<p>
</p>

<p>
<a name="INDEX-2622"></a><a name="INDEX-2623"></a><a name="INDEX-2624"></a><a name="INDEX-2625"></a>
When overloading an operator, try not to create objects with references
to themselves.  For instance,
<blockquote>
<pre class="programlisting">use overload '+' =&gt; sub { bless [ \$_[0], \$_[1] ] };</pre>
</blockquote>

This is asking for trouble, since if you say <tt class="literal">$animal +=
$vegetable</tt>, the result will make 
<tt class="literal">$animal</tt>
a reference to a blessed array reference whose first element is
<tt class="literal">$animal</tt>.  This is a <em class="emphasis">circular
reference</em>, which means that even if you destroy
<tt class="literal">$animal</tt>, its memory won't be freed until your
process (or interpreter) terminates.  See "Garbage Collection,
Circular References, and Weak References" in <a href="ch08_01.htm">Chapter 8, "References"</a>.</p>

<a name="INDEX-2626"></a><a name="INDEX-2627"></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="ch13_02.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="ch13_04.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td>
</tr>
<tr>
<td align="left" valign="top" width="172">13.2. Overload Handlers</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">13.4. The Copy Constructor (=)</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>