mingw/html/lib/AutoLoader.html

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  13 <big><strong><span class="block">&nbsp;AutoLoader - load subroutines only on demand</span></strong></big>
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  17 <p><a name="__index__"></a></p>
  18 <!-- INDEX BEGIN -->
  19
  20 <ul>
  21
  22         <li><a href="#name">NAME</a></li>
  23         <li><a href="#synopsis">SYNOPSIS</a></li>
  24         <li><a href="#description">DESCRIPTION</a></li>
  25         <ul>
  26
  27                 <li><a href="#subroutine_stubs">Subroutine Stubs</a></li>
  28                 <li><a href="#using_autoloader_s_autoload_subroutine">Using <strong>AutoLoader</strong>'s AUTOLOAD Subroutine</a></li>
  29                 <li><a href="#overriding_autoloader_s_autoload_subroutine">Overriding <strong>AutoLoader</strong>'s AUTOLOAD Subroutine</a></li>
  30                 <li><a href="#package_lexicals">Package Lexicals</a></li>
  31                 <li><a href="#not_using_autoloader">Not Using AutoLoader</a></li>
  32                 <li><a href="#autoloader_vs__selfloader"><strong>AutoLoader</strong> vs. <strong>SelfLoader</strong></a></li>
  33         </ul>
  34
  35         <li><a href="#caveats">CAVEATS</a></li>
  36         <li><a href="#see_also">SEE ALSO</a></li>
  37 </ul>
  38 <!-- INDEX END -->
  39
  40 <hr />
  41 <p>
  42 </p>
  43 <h1><a name="name">NAME</a></h1>
  44 <p>AutoLoader - load subroutines only on demand</p>
  45 <p>
  46 </p>
  47 <hr />
  48 <h1><a name="synopsis">SYNOPSIS</a></h1>
  49 <pre>
  50     package Foo;
  51     use AutoLoader 'AUTOLOAD';   # import the default AUTOLOAD subroutine</pre>
  52 <pre>
  53     package Bar;
  54     use AutoLoader;              # don't import AUTOLOAD, define our own
  55     sub AUTOLOAD {
  56         ...
  57         $AutoLoader::AUTOLOAD = &quot;...&quot;;
  58         goto &amp;AutoLoader::AUTOLOAD;
  59     }</pre>
  60 <p>
  61 </p>
  62 <hr />
  63 <h1><a name="description">DESCRIPTION</a></h1>
  64 <p>The <strong>AutoLoader</strong> module works with the <strong>AutoSplit</strong> module and the
  65 <code>__END__</code> token to defer the loading of some subroutines until they are
  66 used rather than loading them all at once.</p>
  67 <p>To use <strong>AutoLoader</strong>, the author of a module has to place the
  68 definitions of subroutines to be autoloaded after an <code>__END__</code> token.
  69 (See <a href="file://C|\msysgit\mingw\html/pod/perldata.html">the perldata manpage</a>.)  The <strong>AutoSplit</strong> module can then be run manually to
  70 extract the definitions into individual files <em>auto/funcname.al</em>.</p>
  71 <p><strong>AutoLoader</strong> implements an AUTOLOAD subroutine.  When an undefined
  72 subroutine in is called in a client module of <strong>AutoLoader</strong>,
  73 <strong>AutoLoader</strong>'s AUTOLOAD subroutine attempts to locate the subroutine in a
  74 file with a name related to the location of the file from which the
  75 client module was read.  As an example, if <em>POSIX.pm</em> is located in
  76 <em>/usr/local/lib/perl5/POSIX.pm</em>, <strong>AutoLoader</strong> will look for perl
  77 subroutines <strong>POSIX</strong> in <em>/usr/local/lib/perl5/auto/POSIX/*.al</em>, where
  78 the <code>.al</code> file has the same name as the subroutine, sans package.  If
  79 such a file exists, AUTOLOAD will read and evaluate it,
  80 thus (presumably) defining the needed subroutine.  AUTOLOAD will then
  81 <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_goto"><code>goto</code></a> the newly defined subroutine.</p>
  82 <p>Once this process completes for a given function, it is defined, so
  83 future calls to the subroutine will bypass the AUTOLOAD mechanism.</p>
  84 <p>
  85 </p>
  86 <h2><a name="subroutine_stubs">Subroutine Stubs</a></h2>
  87 <p>In order for object method lookup and/or prototype checking to operate
  88 correctly even when methods have not yet been defined it is necessary to
  89 ``forward declare'' each subroutine (as in <code>sub NAME;</code>).  See
  90 <a href="file://C|\msysgit\mingw\html/pod/perlsub.html#synopsis">SYNOPSIS in the perlsub manpage</a>.  Such forward declaration creates ``subroutine
  91 stubs'', which are place holders with no code.</p>
  92 <p>The AutoSplit and <strong>AutoLoader</strong> modules automate the creation of forward
  93 declarations.  The AutoSplit module creates an 'index' file containing
  94 forward declarations of all the AutoSplit subroutines.  When the
  95 AutoLoader module is 'use'd it loads these declarations into its callers
  96 package.</p>
  97 <p>Because of this mechanism it is important that <strong>AutoLoader</strong> is always
  98 <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_use"><code>use</code></a>d and not <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_require"><code>require</code></a>d.</p>
  99 <p>
 100 </p>
 101 <h2><a name="using_autoloader_s_autoload_subroutine">Using <strong>AutoLoader</strong>'s AUTOLOAD Subroutine</a></h2>
 102 <p>In order to use <strong>AutoLoader</strong>'s AUTOLOAD subroutine you <em>must</em>
 103 explicitly import it:</p>
 104 <pre>
 105     use AutoLoader 'AUTOLOAD';</pre>
 106 <p>
 107 </p>
 108 <h2><a name="overriding_autoloader_s_autoload_subroutine">Overriding <strong>AutoLoader</strong>'s AUTOLOAD Subroutine</a></h2>
 109 <p>Some modules, mainly extensions, provide their own AUTOLOAD subroutines.
 110 They typically need to check for some special cases (such as constants)
 111 and then fallback to <strong>AutoLoader</strong>'s AUTOLOAD for the rest.</p>
 112 <p>Such modules should <em>not</em> import <strong>AutoLoader</strong>'s AUTOLOAD subroutine.
 113 Instead, they should define their own AUTOLOAD subroutines along these
 114 lines:</p>
 115 <pre>
 116     use AutoLoader;
 117     use Carp;</pre>
 118 <pre>
 119     sub AUTOLOAD {
 120         my $sub = $AUTOLOAD;
 121         (my $constname = $sub) =~ s/.*:://;
 122         my $val = constant($constname, @_ ? $_[0] : 0);
 123         if ($! != 0) {
 124             if ($! =~ /Invalid/ || $!{EINVAL}) {
 125                 $AutoLoader::AUTOLOAD = $sub;
 126                 goto &amp;AutoLoader::AUTOLOAD;
 127             }
 128             else {
 129                 croak &quot;Your vendor has not defined constant $constname&quot;;
 130             }
 131         }
 132         *$sub = sub { $val }; # same as: eval &quot;sub $sub { $val }&quot;;
 133         goto &amp;$sub;
 134     }</pre>
 135 <p>If any module's own AUTOLOAD subroutine has no need to fallback to the
 136 AutoLoader's AUTOLOAD subroutine (because it doesn't have any AutoSplit
 137 subroutines), then that module should not use <strong>AutoLoader</strong> at all.</p>
 138 <p>
 139 </p>
 140 <h2><a name="package_lexicals">Package Lexicals</a></h2>
 141 <p>Package lexicals declared with <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_my"><code>my</code></a> in the main block of a package
 142 using <strong>AutoLoader</strong> will not be visible to auto-loaded subroutines, due to
 143 the fact that the given scope ends at the <code>__END__</code> marker.  A module
 144 using such variables as package globals will not work properly under the
 145 <strong>AutoLoader</strong>.</p>
 146 <p>The <code>vars</code> pragma (see <a href="file://C|\msysgit\mingw\html/pod/perlmod.html#vars">vars in the perlmod manpage</a>) may be used in such
 147 situations as an alternative to explicitly qualifying all globals with
 148 the package namespace.  Variables pre-declared with this pragma will be
 149 visible to any autoloaded routines (but will not be invisible outside
 150 the package, unfortunately).</p>
 151 <p>
 152 </p>
 153 <h2><a name="not_using_autoloader">Not Using AutoLoader</a></h2>
 154 <p>You can stop using AutoLoader by simply</p>
 155 <pre>
 156         no AutoLoader;</pre>
 157 <p>
 158 </p>
 159 <h2><a name="autoloader_vs__selfloader"><strong>AutoLoader</strong> vs. <strong>SelfLoader</strong></a></h2>
 160 <p>The <strong>AutoLoader</strong> is similar in purpose to <strong>SelfLoader</strong>: both delay the
 161 loading of subroutines.</p>
 162 <p><strong>SelfLoader</strong> uses the <code>__DATA__</code> marker rather than <code>__END__</code>.
 163 While this avoids the use of a hierarchy of disk files and the
 164 associated open/close for each routine loaded, <strong>SelfLoader</strong> suffers a
 165 startup speed disadvantage in the one-time parsing of the lines after
 166 <code>__DATA__</code>, after which routines are cached.  <strong>SelfLoader</strong> can also
 167 handle multiple packages in a file.</p>
 168 <p><strong>AutoLoader</strong> only reads code as it is requested, and in many cases
 169 should be faster, but requires a mechanism like <strong>AutoSplit</strong> be used to
 170 create the individual files.  <a href="file://C|\msysgit\mingw\html/lib/ExtUtils/MakeMaker.html">the ExtUtils::MakeMaker manpage</a> will invoke
 171 <strong>AutoSplit</strong> automatically if <strong>AutoLoader</strong> is used in a module source
 172 file.</p>
 173 <p>
 174 </p>
 175 <hr />
 176 <h1><a name="caveats">CAVEATS</a></h1>
 177 <p>AutoLoaders prior to Perl 5.002 had a slightly different interface.  Any
 178 old modules which use <strong>AutoLoader</strong> should be changed to the new calling
 179 style.  Typically this just means changing a require to a use, adding
 180 the explicit <code>'AUTOLOAD'</code> import if needed, and removing <strong>AutoLoader</strong>
 181 from <code>@ISA</code>.</p>
 182 <p>On systems with restrictions on file name length, the file corresponding
 183 to a subroutine may have a shorter name that the routine itself.  This
 184 can lead to conflicting file names.  The <em>AutoSplit</em> package warns of
 185 these potential conflicts when used to split a module.</p>
 186 <p>AutoLoader may fail to find the autosplit files (or even find the wrong
 187 ones) in cases where <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item__inc"><code>@INC</code></a> contains relative paths, <strong>and</strong> the program
 188 does <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_chdir"><code>chdir</code></a>.</p>
 189 <p>
 190 </p>
 191 <hr />
 192 <h1><a name="see_also">SEE ALSO</a></h1>
 193 <p><a href="file://C|\msysgit\mingw\html/lib/SelfLoader.html">the SelfLoader manpage</a> - an autoloader that doesn't use external files.</p>
 194 <table border="0" width="100%" cellspacing="0" cellpadding="3">
 195 <tr><td class="block" style="background-color: #cccccc" valign="middle">
 196 <big><strong><span class="block">&nbsp;AutoLoader - load subroutines only on demand</span></strong></big>
 197 </td></tr>
 198 </table>
 199
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