ch12_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,

call:
<blockquote>
<pre class="programlisting">$mage = summon Wizard "Gandalf";
$nemesis = summon Balrog home =&gt; "Moria", weapon =&gt; "whip";
move $nemesis "bridge";
speak $mage "You cannot pass";
break $staff;               # safer to use: break $staff ();</pre>
</blockquote>

The list operator syntax should be familiar to you; it's the same style
used for passing filehandles to <tt class="literal">print</tt> or <tt class="literal">printf</tt>:
<blockquote>
<pre class="programlisting">print STDERR "help!!!\n";</pre>
</blockquote>

It's also similar to English sentences like "Give Gollum the
Preciousss", so we call it the <em class="emphasis">indirect object</em> form.  The invocant
is expected in the <em class="emphasis">indirect object slot</em>.  When you read about
passing a built-in function like <tt class="literal">system</tt> or <tt class="literal">exec</tt> something in its
"indirect object slot", this means that you're supplying this extra,
comma-less argument in the same place you would when you invoke a method
using the indirect object syntax.</p>

<p>The indirect object form even permits you to specify the <em class="replaceable">INVOCANT</em>
as a <em class="replaceable">BLOCK</em> that evaluates to an object (reference) or class
(package).  This lets you combine those two invocations into one
statement this way:
<blockquote>
<pre class="programlisting">speak { summon Wizard "Gandalf" } "friend";</pre>
</blockquote>
</p>






<h3 class="sect2">12.3.3. Syntactic Snafus with Indirect Objects</h3>

<p>
<a name="INDEX-2384"></a>
One syntax will often be more readable than the other.  The indirect
object syntax is less cluttered, but suffers from several forms of
syntactic ambiguity.  The first is that the <em class="replaceable">LIST</em> part of an
indirect object invocation is parsed the same as any other list
operator.  Thus, the parentheses of:
<blockquote>
<pre class="programlisting">enchant $sword ($pips + 2) * $cost;</pre>
</blockquote>

are assumed to surround all the arguments, regardless of what comes
afterward.  It would therefore be be equivalent to this:
<blockquote>
<pre class="programlisting">($sword-&gt;enchant($pips + 2)) * $cost;</pre>
</blockquote>
<a name="INDEX-2385"></a><a name="INDEX-2386"></a><a name="INDEX-2387"></a><a name="INDEX-2388"></a><a name="INDEX-2389"></a><a name="INDEX-2390"></a><a name="INDEX-2391"></a>
That's unlikely to do what you want: <tt class="literal">enchant</tt> is only being
called with <tt class="literal">$pips + 2</tt>, and the method's return value is then
multiplied by <tt class="literal">$cost</tt>.  As with other list operators,
you must also be careful of the precedence of <tt class="literal">&amp;&amp;</tt> and <tt class="literal">||</tt>
versus <tt class="literal">and</tt> and <tt class="literal">or</tt>.</p>

<p>For example, this:
<blockquote>
<pre class="programlisting">name $sword $oldname || "Glamdring";   # can't use "or" here!</pre>
</blockquote>

becomes the intended:
<blockquote>
<pre class="programlisting">$sword-&gt;name($oldname || "Glamdring");</pre>
</blockquote>

but this:
<blockquote>
<pre class="programlisting">speak $mage "friend" &amp;&amp; enter();        # should've been "and" here!</pre>
</blockquote>

becomes the dubious:
<blockquote>
<pre class="programlisting">$mage-&gt;speak("friend" &amp;&amp; enter());</pre>
</blockquote>

which could be fixed by rewriting into one of these equivalent forms:
<blockquote>
<pre class="programlisting">enter() if $mage-&gt;speak("friend");
$mage-&gt;speak("friend") &amp;&amp; enter();
speak $mage "friend" and enter();</pre>
</blockquote>

The second syntactic infelicity of the indirect object form is that
its <em class="replaceable">INVOCANT</em> is limited to a name, an unsubscripted scalar
variable, or a block.<a href="#FOOTNOTE-4">[4]</a> As soon as the
parser sees one of these, it has its <em class="replaceable">INVOCANT</em>, so it starts looking
for its <em class="replaceable">LIST</em>.  So these invocations:
<blockquote>
<pre class="programlisting">move $party-&gt;{LEADER};               # probably wrong!
move $riders[$i];                    # probably wrong!</pre>
</blockquote>

actually parse as these:
<blockquote>
<pre class="programlisting">$party-&gt;move-&gt;{LEADER};
$riders-&gt;move([$i]);</pre>
</blockquote>

rather than what you probably wanted:
<blockquote>
<pre class="programlisting">$party-&gt;{LEADER}-&gt;move;
$riders[$i]-&gt;move;</pre>
</blockquote>

The parser only looks a little ways ahead to find the invocant for an
indirect object, not even as far as it would look for a unary
operator.  This oddity does not arise with the first
notation, so you might wish to stick with the arrow as your weapon of
choice.</p>
<blockquote class="footnote">

<a name="FOOTNOTE-4"></a>
<p>[4] Attentive readers will recall that this
is precisely the same list of syntactic items that are allowed after a
funny character to indicate a variable dereference--for example,
<tt class="literal">@ary</tt>, <tt class="literal">@$aryref</tt>, or <tt class="literal">@{$aryref}</tt>.</p>

</blockquote>

<p>Even English has a similar issue here.  Think about the command, "Throw
your cat out the window a toy mouse to play with."  If you parse that
sentence too quickly, you'll end up throwing the cat, not the mouse
(unless you notice that the cat is already out the window).  Like Perl,
English has two different syntaxes for expressing the agent: "Throw
your cat the mouse" and "Throw the mouse to your cat."  Sometimes the
longer form is clearer and more natural, and sometimes the shorter one
is.  At least in Perl, you're required to use braces around any complicated
indirect object.</p>






<h3 class="sect2">12.3.4. Package-Quoted Classes</h3>

<a name="INDEX-2392"></a><a name="INDEX-2393"></a><a name="INDEX-2394"></a><a name="INDEX-2395"></a>
<p>The final syntactic ambiguity with the indirect object style of
method invocation is that it may not be parsed as a method call at
all, because the current package may have a subroutine of the same
name as the method.  When using a class method with a literal package
name as the invocant, there is a way to resolve this ambiguity
while still keeping the indirect object syntax: package-quote the
classname by appending a double colon to it.
<blockquote>
<pre class="programlisting">$obj = method <em class="replaceable">CLASS</em>::;   # forced to be "<em class="replaceable">CLASS</em>"-&gt;method</pre>
</blockquote>

This is important because the commonly seen notation:
<blockquote>
<pre class="programlisting">$obj = new <em class="replaceable">CLASS</em>;        # might not parse as method</pre>
</blockquote>

will not always behave properly if the current package has a
subroutine named <tt class="literal">new</tt> or <em class="replaceable">CLASS</em>.  Even if you studiously use
the arrow form instead of the indirect object form to invoke methods,
this can, on rare occasion, still be a problem.  At the cost of
extra punctuation noise, the <em class="replaceable">CLASS</em><tt class="literal">::</tt> notation guarantees how
Perl will parse your method invocation.  The first two examples below do
not always parse the same way, but the second two do:
<blockquote>
<pre class="programlisting">$obj = new ElvenRing;               # could be new("ElvenRing")
                                    # or even new(ElvenRing())
$obj = ElvenRing-&gt;new;              # could be ElvenRing()-&gt;new()

$obj = new ElvenRing::;             # always "ElvenRing"-&gt;new()
$obj = ElvenRing::-&gt;new;            # always "ElvenRing"-&gt;new()</pre>
</blockquote>

This package-quoting notation can be made prettier with some creative
alignment:
<blockquote>
<pre class="programlisting">$obj = new ElvenRing::
            name    =&gt; "Narya",
            owner   =&gt; "Gandalf",
            domain  =&gt; "fire",
            stone   =&gt; "ruby";</pre>
</blockquote>
<a name="INDEX-2396"></a>
Still, you may say, "Oh, ugh!" at that double colon, so we'll tell you
that you can almost always get away with a bare class name, provided
two things are true.  First, there is no subroutine of the same
name as the class.  (If you follow the convention that subroutine names like
<tt class="literal">new</tt> start lowercase and class names like <tt class="literal">ElvenRing</tt> start
uppercase, this is never a problem.)  Second, the class has
been loaded with one of:
<blockquote>
<pre class="programlisting">use ElvenRing;
require ElvenRing;</pre>
</blockquote>

Either of these declarations ensures that Perl knows <tt class="literal">ElvenRing</tt> is a
module name, which forces any bare name like <tt class="literal">new</tt> before the class
name <tt class="literal">ElvenRing</tt> to be interpreted as a method call, even if you
happen to have declared a <tt class="literal">new</tt> subroutine of your own in the current
package.  People don't generally get into trouble with indirect objects
unless they start cramming multiple classes into the same file, in
which case Perl might not know that a particular package name was
supposed to be a class name.  People who name subroutines with names
that look like <tt class="literal">ModuleNames</tt> also come to grief eventually.
<a name="INDEX-2397"></a><a name="INDEX-2398"></a>
</p>

<a name="INDEX-2399"></a><a name="INDEX-2400"></a>



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