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<html><head><title>Managing Class Data (Programming Perl)</title><!-- STYLESHEET --><link rel="stylesheet" type="text/css" href="../style/style1.css"><!-- METADATA --><!--Dublin Core Metadata--><meta name="DC.Creator" content=""><meta name="DC.Date" content=""><meta name="DC.Format" content="text/xml" scheme="MIME"><meta name="DC.Generator" content="XSLT stylesheet, xt by James Clark"><meta name="DC.Identifier" content=""><meta name="DC.Language" content="en-US"><meta name="DC.Publisher" content="O'Reilly & Associates, Inc."><meta name="DC.Source" content="" scheme="ISBN"><meta name="DC.Subject.Keyword" content=""><meta name="DC.Title" content="Managing Class Data"><meta name="DC.Type" content="Text.Monograph"></head><body><!-- START OF BODY --><!-- TOP BANNER --><img src="gifs/smbanner.gif" usemap="#banner-map" border="0" alt="Book Home"><map name="banner-map"><AREA SHAPE="RECT" COORDS="0,0,466,71" HREF="index.htm" ALT="Programming Perl"><AREA SHAPE="RECT" COORDS="467,0,514,18" HREF="jobjects/fsearch.htm" ALT="Search this book"></map><!-- TOP NAV BAR --><div class="navbar"><table width="515" border="0"><tr><td align="left" valign="top" width="172"><a href="ch12_07.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="ch12_01.htm">Chapter 12: Objects</a></td><td align="right" valign="top" width="172"><a href="ch12_09.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr></table></div><hr width="515" align="left"><!-- SECTION BODY --><h2 class="sect1">12.8. Managing Class Data</h2><p><a name="INDEX-2584"></a><a name="INDEX-2585"></a><a name="INDEX-2586"></a><a name="INDEX-2587"></a>We've looked at several approaches to accessing per-object datavalues. Sometimes, though, you want some common state shared by allobjects of a class. Instead of being an attribute of just one instanceof the class, these variables are global to the entire class, no matterwhich class instance (object) you use to access them through. (C++programmers would think of these as static member data.) Here aresome situations where class variables might come in handy:</p><ul><li><p>To keep a count of all objects ever created, or how many arestill kicking around.</p></li><li><p>To keep a list of all objects over which you can iterate.</p></li><li><p>To store the name or file descriptor of a log file used bya class-wide debugging method.</p></li><li><p>To keep collective data, like the total amount of cash dispensed byall ATMs in a network in a given day.</p></li><li><p>To track the last object created by a class, or the most accessed object.</p></li><li><p>To keep a cache of in-memory objects that have already been reconstitutedfrom persistent memory.</p></li><li><p>To provide an inverted lookup table so you can find an object based onthe value one of its attributes.</p></li></ul><p><a name="INDEX-2588"></a>The question comes down to deciding where to store the state forthose shared attributes. Perl has no particular syntactic mechanismto declare class attributes, any more than it has for instanceattributes. Perl provides the developer with a broad set of powerfulbut flexible features that can be uniquely crafted to the particulardemands of the situation. You can then select the mechanismthat makes the most sense for the given situation instead of havingto live with someone else's design decisions. Alternatively, youcan live with the design decisions someone else has packaged upand put onto CPAN. Again, TMTOWTDI.</p><p>Like anything else pertaining to a class, class data shouldn't beaccessed directly, especially from outside the implementation of theclass itself. It doesn't say much for encapsulation to set upcarefully controlled accessor methods for instance variables but theninvite the public in to diddle your class variables directly, such asby setting <tt class="literal">$SomeClass::Debug = 1</tt>. To establish aclear firewall between interface and implementation, you can createaccessor methods to manipulate class data similar to those you use forinstance data.</p><p><a name="INDEX-2589"></a>Imagine we want to keep track of the total world population of<tt class="literal">Critter</tt> objects. We'll store that number in apackage variable, but provide a method called<tt class="literal">population</tt> so that users of the class don't have toknow about the implementation.</p><blockquote><pre class="programlisting">Critter->population() # Access via class name$gollum->population() # Access via instance</pre></blockquote><p>Since a class in Perl is just a package, the most natural place tostore class data is in a package variable. Here's a simpleimplementation of such a class. The <tt class="literal">population</tt>method ignores its invocant and just returns the current value of thepackage variable, <tt class="literal">$Population</tt>. (Some programmerslike to capitalize their globals.)<blockquote><pre class="programlisting">package Critter;our $Population = 0;sub population { return $Population; }sub DESTROY { $Population-- }sub spawn { my $invocant = shift; my $class = ref($invocant) || $invocant; $Population++; return bless { name => shift || "anon" }, $class;}sub name { my $self = shift; $self->{name} = shift if @_; return $self->{name};}</pre></blockquote><a name="INDEX-2590"></a><a name="INDEX-2591"></a>If you want to make class data methods that work like accessorsfor instance data, do this:<blockquote><pre class="programlisting">our $Debugging = 0; # class datumsub debug { shift; # intentionally ignore invocant $Debugging = shift if @_; return $Debugging;}</pre></blockquote><a name="INDEX-2592"></a>Now you can set the overall debug level through the class or throughany of its instances.</p><p><a name="INDEX-2593"></a>Because it's a package variable, <tt class="literal">$Debugging</tt> isglobally accessible. But if you change the <tt class="literal">our</tt>variable to <tt class="literal">my</tt>, then only code later in that samefile can see it. You can go still further--you can restrictunfettered access to class attributes even from the rest of classitself. Wrap the variable declaration in a block scope:<blockquote><pre class="programlisting">{ my $Debugging = 0; # lexically scoped class datum sub debug { shift; # intentionally ignore invocant $Debugging = shift if @_; return $Debugging; }}</pre></blockquote>Now no one is allowed to read or write the class attributes withoutusing the accessor method, since only that subroutine is in thesame scope as the variable and has access to it.</p><p><a name="INDEX-2594"></a>If a derived class inherits these class accessors, then these stillaccess the original data, no matter whether the variables weredeclared with <tt class="literal">our</tt> or <tt class="literal">my</tt>. The data isn't package-relative.You might look at it as methods executing in the class in whichthey were originally defined, not in the class that invoked them.</p><p>For some kinds of class data, this approach works fine, but forothers, it doesn't. Suppose we create a <tt class="literal">Warg</tt> subclass of<tt class="literal">Critter</tt>. If we want to keep our populations separate, <tt class="literal">Warg</tt>can't inherit <tt class="literal">Critter</tt>'s population method, because that methodas written always returns the value of <tt class="literal">$Critter::Population</tt>.</p><p>You'll have to decide on a case-by-case basis whether it makes anysense for class attributes to be package relative. If you wantpackage-relative attributes, use the invocant's class to locate the package holding the class data:<blockquote><pre class="programlisting">sub debug { my $invocant = shift; my $class = ref($invocant) || $invocant; my $varname = $class . "::Debugging"; no strict "refs"; # to access package data symbolically $$varname = shift if @_; return $$varname;}</pre></blockquote>We temporarily rescind strict references because otherwisewe couldn't use the fully qualified symbolic name for the packageglobal. This is perfectly reasonable: since all package variablesby definition live in a package, there's nothing wrong with accessingthem via that package's symbol table.</p><p><a name="INDEX-2595"></a>Another approach is to make everything an object needs--even itsglobal class data--available via that object (or passed in asparameters). To do this, you'll often have to make a dedicatedconstructor for each class, or at least have a dedicated initializationroutine to be called by the constructor. In the constructor orinitializer, you store references to any class data directly in theobject itself, so nothing ever has to go looking for it. Theaccessor methods use the object to find a reference to the data.</p><p>Rather than put the complexity of locating the class data in eachmethod, just let the object tell the method where the data is located.This approach works well only when the class data accessor methods are invoked asinstance methods, because the class data could be in unreachablelexicals you couldn't get at using a package name.</p><p><a name="INDEX-2596"></a>No matter how you roll it, package-relative class data is always abit awkward. It's really a lot cleaner if, when you inherit a classdata accessor method, you effectively inherit the state data thatit's accessing as well. See the <em class="emphasis">perltootc</em> manpage for numerous, moreelaborate approaches to management of class data.</p><a name="INDEX-2597"></a><a name="INDEX-2598"></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="ch12_07.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="ch12_09.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr><tr><td align="left" valign="top" width="172">12.7. Managing Instance Data</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">12.9. Summary</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 © 2001</a> O'Reilly & 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>⌨️ 快捷键说明
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