ch12_07.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

So far, these techniques for managing instance data have offered no
mechanism for "protection" from external access.  Anyone outside the
class can open up the object's black box and poke about inside--if they
don't mind voiding the warranty.  Enforced privacy tends to get in the
way of people trying to get their jobs done.  Perl's philosophy is that
it's better to encapsulate one's data with a sign that says:
<blockquote>
<pre class="programlisting">IN CASE OF FIRE
  BREAK GLASS</pre>
</blockquote>

You should respect such encapsulation when possible, but still have
easy access to the contents in an emergency situation, like for
debugging.</p>

<p>But if you do want to enforce privacy, Perl isn't about to get in your
way.  Perl offers low-level building blocks that you can use to surround
your class and its objects with an impenetrable privacy shield--one
stronger, in fact, than that found in many popular object-oriented
languages.  Lexical scopes and the lexical variables inside them are
the key components here, and closures play a pivotal role.</p>

<p>
<a name="INDEX-2560"></a>
In the section <a href="ch12_05.htm#ch12-sect-pm">Section 12.5.5, "Private Methods"</a> we saw how a class can use
closures to implement methods that are invisible outside the module
file.  Later we'll look at accessor methods that regulate class
data so private that not even the rest of the class has unrestricted
access.  Those are still fairly traditional uses of closures.  The
truly interesting approach is to use a closure as the very object
itself.  The object's instance variables are locked up inside a
scope to which the object alone--that is, the closure--has free
access.  This is a very strong form of encapsulation; not only is
it proof against external tampering, even other methods in the same
class must use the proper access methods to get at the object's
instance data.</p>

<p>Here's an example of how this might work.  We'll use closures
both for the objects themselves and for the generated accessors:
<blockquote>
<pre class="programlisting">package Person;
sub new {
    my $invocant  = shift;
    my $class = ref($invocant) || $invocant;
    my $data = {
       NAME     =&gt; "unnamed",
       RACE     =&gt; "unknown",
       ALIASES  =&gt; [],
    };
    my $self = sub {
       my $field = shift;
       #############################
       ### ACCESS CHECKS GO HERE ###
       #############################
       if (@_) { $data-&gt;{$field} = shift }
       return    $data-&gt;{$field};
    };
    bless($self, $class);
    return $self;
}
# generate method names
for my $field (qw(name race aliases)) {
    no strict "refs";  # for access to the symbol table
    *$field = sub {
        my $self = shift;
        return $self-&gt;(uc $field, @_);
    };
}</pre>
</blockquote>

The object created and returned by the <tt class="literal">new</tt> method
is no longer a hash, as it was in other constructors we've looked at.
It's a closure with unique access to the attribute data stored in the
hash referred to by <tt class="literal">$data</tt>.  Once the constructor
call is finished, the only access to <tt class="literal">$data</tt> (and
hence to the attributes) is via the closure.</p>

<p>In a call like <tt class="literal">$him-&gt;name("Bombadil")</tt>,
the invoking object stored in <tt class="literal">$self</tt> is the closure
that was blessed and returned by the constructor.  There's not a lot
one can do with a closure beyond calling it, so we do just that with
<tt class="literal">$self-&gt;(uc $field, @_)</tt>.  Don't be fooled by the
arrow; this is just a regular indirect function call, not a method
invocation.  The initial argument is the string
"<tt class="literal">name</tt>", and any remaining arguments are whatever
else was passed in.<a href="#FOOTNOTE-7">[7]</a> Once we're executing inside the
closure, the hash reference inside <tt class="literal">$data</tt> is again
accessible.  The closure is then free to permit or deny access to
whatever it pleases.</p>
<blockquote class="footnote">

<a name="FOOTNOTE-7"></a>
<p>[7] Sure, the double-function call is
slow, but if you wanted fast, would you really be using objects in the
first place?</p>

</blockquote>

<p>
<a name="INDEX-2561"></a><a name="INDEX-2562"></a>
No one outside the closure object has unmediated access to this very
private instance data, not even other methods in the class.  They could
try to call the closure the way the methods generated by the <tt class="literal">for</tt>
loop do, perhaps setting an instance variable the class never heard
of.  But this approach is easily blocked by inserting various bits of code in 
the constructor where you see the comment about access checks.  First, we
need a common preamble:
<blockquote>
<pre class="programlisting">use Carp;
local $Carp::CarpLevel = 1;  # Keeps croak messages short
my ($cpack, $cfile) = caller();</pre>
</blockquote>

Now for each of the checks.  The first one makes sure the specified
attribute name exists:
<blockquote>
<pre class="programlisting">croak "No valid field '$field' in object"
    unless exists $data-&gt;{$field};</pre>
</blockquote>

This one allows access only by callers from the same file:
<blockquote>
<pre class="programlisting">carp "Unmediated access denied to foreign file"
    unless $cfile eq __FILE__;</pre>
</blockquote>

This one allows access only by callers from the same package:
<blockquote>
<pre class="programlisting">carp "Unmediated access denied to foreign package ${cpack}::"
    unless $cpack eq __PACKAGE__;</pre>
</blockquote>

And this one allows access only by callers whose classes inherit ours:
<blockquote>
<pre class="programlisting">carp "Unmediated access denied to unfriendly class ${cpack}::"
    unless $cpack-&gt;isa(__PACKAGE__);</pre>
</blockquote>

All these checks block unmediated access only.  Users of the class who
politely use the class's designated methods are under no such
restriction.  Perl gives you the tools to be just as persnickety as you
want to be.  Fortunately, not many people want to be.</p>

<p>
<a name="INDEX-2563"></a><a name="INDEX-2564"></a><a name="INDEX-2565"></a><a name="INDEX-2566"></a>
But some people ought to be.  Persnickety is good when you're writing
flight control software.  If you either want or ought to be one of
those people, and you prefer using working code over reinventing
everything on your own, check out Damian Conway's
<tt class="literal">Tie::SecureHash</tt> module on CPAN.  It implements
restricted hashes with support for public, protected, and private
persnicketations.  It also copes with the inheritance issues that
we've ignored in the previous example.  Damian has also written an
even more ambitious module, <tt class="literal">Class::Contract</tt>, that
imposes a formal software engineering regimen over Perl's flexible
object system.  This module's feature list reads like a checklist from
a computer science 

professor's software engineering

textbook,<a href="#FOOTNOTE-8">[8]</a>including enforced encapsulation, static
inheritance, and
design-by-contract condition checking for object-oriented Perl, along
with a declarative syntax for attribute, method, constructor, and
destructor definitions at both the object and class level, and
preconditions, postconditions, and class invariants.  Whew!</p>
<blockquote class="footnote">

<a name="FOOTNOTE-8"></a>
<p>[8] Can you guess what Damian's job is?  By the
way, we highly recommend his book, <em class="emphasis">Object Oriented
Perl</em> (Manning Publications,
1999).</p>

</blockquote>

<a name="INDEX-2567"></a><a name="INDEX-2568"></a><a name="INDEX-2569"></a><a name="INDEX-2570"></a>






<h3 class="sect2">12.7.6. New Tricks</h3>

<p>
<a name="INDEX-2571"></a><a name="INDEX-2572"></a><a name="INDEX-2573"></a><a name="INDEX-2574"></a><a name="INDEX-2575"></a><a name="INDEX-2576"></a><a name="INDEX-2577"></a>
As of release 5.6 of Perl, you can also declare a method to indicate
that it returns an lvalue.  This is done with the 
lvalue subroutine attribute (not to be confused with object attributes).
This experimental feature allows you to treat the method as something
that would appear on the lefthand side of an equal sign:
<blockquote>
<pre class="programlisting">package Critter;

sub new {
    my $class = shift;
    my $self = { pups =&gt; 0, @_ };    # Override default.
    bless $self, $class;
}

sub pups : lvalue {                  # We'll assign to pups() later.
    my $self = shift;
    $self-&gt;{pups};
}

package main;
$varmint = Critter-&gt;new(pups =&gt; 4);
$varmint-&gt;pups *= 2;                 # Assign to $varmint-&gt;pups!
$varmint-&gt;pups =~ s/(.)/$1$1/;       # Modify $varmint-&gt;pups in place!
print $varmint-&gt;pups;                # Now we have 88 pups.</pre>
</blockquote>

This lets you pretend <tt class="literal">$varmint-&gt;pups</tt> is a variable while
still obeying encapsulation.  See the section <a href="ch06_05.htm#ch06-sect-lvalue">Section 12.5.2, "The lvalue Attribute"</a> in
<a href="ch06_01.htm">Chapter 6, "Subroutines"</a>.</p>

<p>
<a name="INDEX-2578"></a><a name="INDEX-2579"></a><a name="INDEX-2580"></a><a name="INDEX-2581"></a>
If you're running a threaded version of Perl and want to ensure that only one
thread can call a particular method on an object, you can use the
<tt class="literal">locked</tt> and <tt class="literal">method</tt> attributes to do that:
<blockquote>
<pre class="programlisting">sub pups : locked method {
    ...
}</pre>
</blockquote>

When any thread invokes the <tt class="literal">pups</tt> method on an
object, Perl locks the object before execution, preventing other
threads from doing the same.  See the section <a href="ch06_05.htm#ch06-sect-locked">Section 12.5.1, "The locked and method Attributes"</a> in <a href="ch06_01.htm">Chapter 6, "Subroutines"</a>.</p>

<a name="INDEX-2582"></a><a name="INDEX-2583"></a>



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