ch12_04.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

$shelob = Spider-&gt;spawn;</pre>
</blockquote>

And each object would be of the proper class.
This even works indirectly, as in:
<blockquote>
<pre class="programlisting">$type  = "Spider";
$shelob = $type-&gt;spawn;         # same as "Spider"-&gt;spawn</pre>
</blockquote>

That's still a class method, not an instance method, because its
invocant holds a string and not a reference.</p>

<p>
<a name="INDEX-2421"></a>
If <tt class="literal">$type</tt> were an object instead of a class name, the previous
constructor definition wouldn't have worked, because <tt class="literal">bless</tt> needs a
class name.  But for many classes, it makes sense to use an existing
object as the template from which to create another.  In these cases,
you can design your constructors so that they work with either objects
or class names:
<blockquote>
<pre class="programlisting">sub spawn {
    my $invocant = shift;
    my $class    = ref($invocant) || $invocant;  # Object or class name
    my $self     = { };
    bless($self, $class);
    return $self;
}</pre>
</blockquote>
</p>






<h3 class="sect2">12.4.2. Initializers</h3>

<p>
<a name="INDEX-2422"></a><a name="INDEX-2423"></a><a name="INDEX-2424"></a><a name="INDEX-2425"></a><a name="INDEX-2426"></a><a name="INDEX-2427"></a><a name="INDEX-2428"></a><a name="INDEX-2429"></a>
Most objects maintain internal information that is indirectly
manipulated by the object's methods.  All our constructors so far have
created empty hashes, but there's no reason to leave them empty.  For
instance, we could have the constructor accept extra arguments to
store into the hash as key/value pairs.  The OO literature often
refers to such data as <em class="emphasis">properties</em>, <em class="emphasis">attributes</em>, <em class="emphasis">accessors</em>,
<em class="emphasis">member data</em>, <em class="emphasis">instance data</em>, or <em class="emphasis">instance variables</em>.  The
section "Instance Variables" later in this chapter discusses attributes in more detail.</p>

<p>Imagine a <tt class="literal">Horse</tt> class with instance attributes like "name" and
"color":
<blockquote>
<pre class="programlisting">$steed = Horse-&gt;new(name =&gt; "Shadowfax", color =&gt; "white");</pre>
</blockquote>

If the object is implemented as a hash reference, the key/value pairs
can be interpolated directly into the hash once the invocant is
removed from the argument list:
<blockquote>
<pre class="programlisting">sub new {
    my $invocant = shift;
    my $class = ref($invocant) || $invocant;
    my $self = { @_ };          # Remaining args become attributes
    bless($self, $class);       # Bestow objecthood
    return $self;
}</pre>
</blockquote>
<a name="INDEX-2430"></a><a name="INDEX-2431"></a>
This time we used a method named <tt class="literal">new</tt> for the
class's constructor, which just might lull C++ programmers into
thinking they know what's going on.  But Perl doesn't consider
"<tt class="literal">new</tt>" to be anything special; you may name your
constructors whatever you like.  Any method that happens to create and
return an object is a de facto constructor.  In general, we recommend
that you name your constructors whatever makes sense in the context of
the problem you're solving.  For example, constructors in the
<tt class="literal">Tk</tt> module are named after the widgets they create.
In the <tt class="literal">DBI</tt> module, a constructor named
<tt class="literal">connect</tt> returns a database handle object, and
another constructor named <tt class="literal">prepare</tt> is invoked as an
instance method and returns a statement handle object.  But if there
is no suitable context-specific constructor name,
<tt class="literal">new</tt> is perhaps not a terrible choice.  Then again,
maybe it's not such a bad thing to pick a random name to force people
to read the interface contract (meaning the class documentation)
before they use its constructors.</p>

<p>Elaborating further, you can set up your constructor with default
key/value pairs, which the user could later override by supplying them
as arguments:
<blockquote>
<pre class="programlisting">sub new {
    my $invocant = shift;
    my $class   = ref($invocant) || $invocant;
    my $self = {
        color  =&gt; "bay",
        legs   =&gt; 4,
        owner  =&gt; undef,
        @_,                 # Override previous attributes
    };
    return bless $self, $class;
}

$ed       = Horse-&gt;new;                    # A 4-legged bay horse
$stallion = Horse-&gt;new(color =&gt; "black");  # A 4-legged black horse</pre>
</blockquote>

This <tt class="literal">Horse</tt> constructor ignores its invocant's existing attributes
when used as an instance method.  You could create a second
constructor designed to be called as an instance method, and if
designed properly, you could use the values from the invoking object as
defaults for the new one:
<blockquote>
<pre class="programlisting">$steed  = Horse-&gt;new(color =&gt; "dun");
$foal   = $steed-&gt;clone(owner =&gt; "EquuGen Guild, Ltd.");

sub clone {
    my $model = shift;
    my $self  = $model-&gt;new(%$model, @_);
    return $self;     # Previously blessed by -&gt;new
}</pre>
</blockquote>

(You could also have rolled this functionality directly into <tt class="literal">new</tt>,
but then the name wouldn't quite fit the function.)</p>

<p>Notice how even in the <tt class="literal">clone</tt> constructor, we don't hardcode the
name of the <tt class="literal">Horse</tt> class.  We have the original object invoke its
own <tt class="literal">new</tt> method, whatever that may be.  If we had written that as
<tt class="literal">Horse-&gt;new</tt> instead of <tt class="literal">$model-&gt;new</tt>, then the class
wouldn't have facilitated inheritance by a <tt class="literal">Zebra</tt> or <tt class="literal">Unicorn</tt>
class.  You wouldn't want to clone Pegasus and suddenly find yourself
with a horse of a different color.</p>

<p>
<a name="INDEX-2432"></a>
Sometimes, however, you have the opposite problem: rather than
trying to share one constructor among several classes, you're trying to
have several constructors share one class's object.  This happens
whenever a constructor wants to call a base class's constructor to do
part of the construction work.  Perl doesn't do hierarchical
construction for you.  That is, Perl does not automatically call the
constructors (or the destructors) for any base classes of the class
requested, so your constructor will have to do that itself and then add
any additional attributes the derived class needs.  So the situation is
not unlike the <tt class="literal">clone</tt> routine, except that instead of copying an
existing object into the new object, you want to call your base class's
constructor and then transmogrify the new base object into your new
derived object.</p>

<a name="INDEX-2433"></a><a name="INDEX-2434"></a>



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