ch08_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,

<a name="FOOTNOTE-2"></a>
<p>[2]But even though there's only one anonymous
subroutine, there may be several copies of the lexical variables in
use by the subroutine, depending on when the subroutine reference was
generated.  These are discussed later in the section <a href="ch08_03.htm#ch08-sect-closure">Section 8.3.7, "Closures"</a>.</p>

</blockquote>

<a name="INDEX-1989"></a>







<h3 class="sect2">8.2.3. Object Constructors</h3>

<a name="INDEX-1990"></a><a name="INDEX-1991"></a><a name="INDEX-1992"></a><a name="INDEX-1993"></a><a name="INDEX-1994"></a>
<p>Subroutines can also return references.  That may sound trite, but
sometimes you are <em class="emphasis">supposed</em> to use a subroutine to create a
reference rather than creating the reference yourself.  In particular,
special subroutines called <em class="emphasis">constructors</em> create and return references to
objects.  An object is simply a special kind of reference that happens
to know which class it's associated with, and constructors know how to
create that association.  They do so by taking an ordinary referent
and turning it into an object with the <tt class="literal">bless</tt> operator, so we can
speak of an object as a blessed reference.  There's nothing religious
going on here; since a class acts as a user-defined type, blessing
a referent simply makes it a user-defined type in addition to a built-in
one.  Constructors are often named <tt class="literal">new</tt>--especially by C++
programmers--but they can be named anything in Perl.</p>

<p>Constructors can be called in any of these ways:
<blockquote>
<pre class="programlisting">$objref = Doggie::-&gt;new(Tail =&gt; 'short', Ears =&gt; 'long');  #1
$objref = new Doggie:: Tail =&gt; 'short', Ears =&gt; 'long';    #2
$objref = Doggie-&gt;new(Tail =&gt; 'short', Ears =&gt; 'long');    #3
$objref = new Doggie Tail =&gt; 'short', Ears =&gt; 'long';      #4</pre>
</blockquote>

The first and second invocations are the same.  They both call a
function named <tt class="literal">new</tt> that is supplied by the <tt class="literal">Doggie</tt> module. The
third and fourth invocations are the same as the first two, but are
slightly more ambiguous: the parser will get confused if you define
your own subroutine named <tt class="literal">Doggie</tt>.  (Which is why people typically
stick with lowercase names for subroutines and uppercase for modules.)
The fourth invocation can also get confused if you've defined your own
<tt class="literal">new</tt> subroutine and don't happen to have done either a <tt class="literal">require</tt> or
a <tt class="literal">use</tt> of the <tt class="literal">Doggie</tt> module, either of which has the effect of
declaring the module.  Always declare your modules if you want to use #4.
(And watch out for stray <tt class="literal">Doggie</tt> subroutines.)</p>

<p>See <a href="ch12_01.htm">Chapter 12, "Objects"</a> for a discussion of Perl objects.</p>





<a name="ch08-sect-hr"></a>
<h3 class="sect2">8.2.4. Handle References</h3>

<a name="INDEX-1995"></a><a name="INDEX-1996"></a><a name="INDEX-1997"></a><a name="INDEX-1998"></a>
<p>References to filehandles or directory handles can be created by
referencing the typeglob of the same name:
<blockquote>
<pre class="programlisting">splutter(\*STDOUT);

sub splutter {
    my $fh = shift;
    print $fh "her um well a hmmm\n";
}

$rec = get_rec(\*STDIN);
sub get_rec {
    my $fh = shift;
    return scalar &lt;$fh&gt;;
}</pre>
</blockquote>

If you're passing around filehandles, you can also use the bare
typeglob to do so: in the example above, you could have used
<tt class="literal">*STDOUT</tt> or <tt class="literal">*STDIN</tt> instead of <tt class="literal">\*STDOUT</tt> and <tt class="literal">\*STDIN</tt>.</p>

<p>
<a name="INDEX-1999"></a>
Although you can usually use typeglob and references to typeglobs
interchangeably, there are a few places where you can't.  Simple typeglobs
can't be <tt class="literal">bless</tt>ed into objectdom, and typeglob references can't
be passed back out of the scope of a localized typeglob.</p>

<p>When generating new filehandles, older code would often do something
like this to open a list of files:
<blockquote>
<pre class="programlisting">for $file (@names) {
    local *FH;
    open(*FH, $file) || next;
    $handle{$file} = *FH;
}</pre>
</blockquote>
</p>

<p>
<a name="INDEX-2000"></a><a name="INDEX-2001"></a>
That still works, but now it's just as easy to let an undefined
variable autovivify an anonymous typeglob:
<blockquote>
<pre class="programlisting">for $file (@names) {
    my $fh;
    open($fh, $file) || next;
    $handle{$file} = $fh;
}</pre>
</blockquote>

With indirect filehandles, it doesn't matter whether you use use
typeglobs, references to typeglobs, or one of the more exotic I/O
objects.  You just use a scalar that--one way or another--gets
interpreted as a filehandle.  For most purposes, you can use either a
typeglob or a typeglob reference almost indiscriminately.  As we
admitted earlier, there is some implicit dereferencing magic going
on here.</p>





<a name="ch08-sect-str"></a>
<h3 class="sect2">8.2.5. Symbol Table References</h3>

<p>
<a name="INDEX-2002"></a><a name="INDEX-2003"></a>
In unusual circumstances, you might not know what type of reference
you need when your program is written.  A reference can be created by
using a special syntax, affectionately known as 
the <tt class="literal">*foo{</tt><em class="replaceable">THING</em><tt class="literal">}</tt>
syntax.  <tt class="literal">*foo{</tt><em class="replaceable">THING</em><tt class="literal">}</tt> 
returns a reference to the <em class="replaceable">THING</em> slot in
<tt class="literal">*foo</tt>, which is the symbol table entry holding the values of <tt class="literal">$foo</tt>,
<tt class="literal">@foo</tt>, <tt class="literal">%foo</tt>, and friends.
<blockquote>
<pre class="programlisting">$scalarref = *foo{SCALAR};   # Same as \$foo
$arrayref  = *ARGV{ARRAY};   # Same as \@ARGV
$hashref   = *ENV{HASH};     # Same as \%ENV
$coderef   = *handler{CODE}; # Same as \&amp;handler
$globref   = *foo{GLOB};     # Same as \*foo
$ioref     = *STDIN{IO};     # Er...</pre>
</blockquote>
<a name="INDEX-2004"></a>
All of these are self-explanatory except for <tt class="literal">*STDIN{IO}</tt>.  It yields
the actual internal <tt class="literal">IO::Handle</tt> object that the typeglob contains, that
is, the part of the typeglob that the various I/O functions are
actually interested in.  For compatibility with previous versions of
Perl, <tt class="literal">*foo{FILEHANDLE}</tt> is a synonym for the hipper <tt class="literal">*foo{IO}</tt>
notation.</p>

<p>In theory, you can use a <tt class="literal">*</tt><em class="replaceable">HANDLE</em><tt class="literal">{IO}</tt> anywhere you'd use a <tt class="literal">*</tt><em class="replaceable">HANDLE</em> or a <tt class="literal">\*</tt><em class="replaceable">HANDLE</em>, such as for passing handles
into or out of subroutines, or storing them in larger data structures.
(In practice, there are still some wrinkles to be ironed out.) The
advantage of them is that they access only the real I/O object you
want, not the whole typeglob, so you run no risk of clobbering more
than you want to through a typeglob assignment (although if you always
assign to a scalar variable instead of to a typeglob, you'll be okay).
One disadvantage is that there's no way to autovivify one as of
yet.<a href="#FOOTNOTE-3">[3]</a>
<blockquote>
<pre class="programlisting">splutter(*STDOUT);
splutter(*STDOUT{IO});

sub splutter {
    my $fh = shift;
    print $fh "her um well a hmmm\n";
}</pre>
</blockquote>

Both invocations of <tt class="literal">splutter()</tt> print "<tt class="literal">her um well a hmmm</tt>".</p>
<blockquote class="footnote">

<a name="FOOTNOTE-3"></a>
<p>[3] Currently, <tt class="literal">open my $fh</tt> autovivifies a typeglob instead
of an <tt class="literal">IO::Handle</tt> object, but someday we may fix that, so you
shouldn't rely on the typeglobbedess of what <tt class="literal">open</tt> currently
autovivifies.</p>

</blockquote>

<p>The <tt class="literal">*foo{</tt><em class="replaceable">THING</em><tt class="literal">}</tt> thing returns <tt class="literal">undef</tt> if that particular
<em class="replaceable">THING</em> hasn't been seen by the compiler yet, except when <em class="replaceable">THING</em> is
<tt class="literal">SCALAR</tt>.  It so happens that <tt class="literal">*foo{SCALAR}</tt> returns a reference to
an anonymous scalar even if <tt class="literal">$foo</tt> hasn't been seen yet.  (Perl always
adds a scalar to any typeglob as an optimization to save a bit of code
elsewhere.  But don't depend on it to stay that way in future releases.)</p>






<h3 class="sect2">8.2.6. Implicit Creation of References</h3>

<p>A final method for creating references is not really a method at all.
References of an appropriate type simply spring into existence if you
dereference them in an lvalue context that assumes they exist.  This is
extremely useful, and is also What You Expect.  This topic is covered
later in this chapter, where we'll discuss how to dereference all of the
references we've created so far.</p>

<a name="INDEX-2005"></a>



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