AutoLoader - load subroutines only on demand

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(See <a href="/5.8.6/perldata">perldata</a>.) The <b>AutoSplit</b> module can then be run manually to extract the definitions into individual files <i>auto/funcname.al</i>.</p> <p><b>AutoLoader</b> implements an AUTOLOAD subroutine. When an undefined subroutine in is called in a client module of <b>AutoLoader</b>, <b>AutoLoader</b>'s AUTOLOAD subroutine attempts to locate the subroutine in a file with a name related to the location of the file from which the client module was read. As an example, if <i>POSIX.pm</i> is located in <i>/usr/local/lib/perl5/POSIX.pm</i>, <b>AutoLoader</b> will look for perl subroutines <b>POSIX</b> in <i>/usr/local/lib/perl5/auto/POSIX/*.al</i>, where the <code>.al</code> file has the same name as the subroutine, sans package. If such a file exists, AUTOLOAD will read and evaluate it, thus (presumably) defining the needed subroutine. AUTOLOAD will then <code>goto</code> the newly defined subroutine.</p> <p>Once this process completes for a given function, it is defined, so future calls to the subroutine will bypass the AUTOLOAD mechanism.</p> <h2 id="Subroutine-Stubs"><a class="permalink" href="#Subroutine-Stubs">#</a><a id="Subroutine"></a>Subroutine Stubs</h2> <p>In order for object method lookup and/or prototype checking to operate correctly even when methods have not yet been defined it is necessary to "forward declare" each subroutine (as in <code>sub NAME;</code>). See <a href="/5.8.6/perlsub#SYNOPSIS">"SYNOPSIS" in perlsub</a>. Such forward declaration creates "subroutine stubs", which are place holders with no code.</p> <p>The AutoSplit and <b>AutoLoader</b> modules automate the creation of forward declarations. The AutoSplit module creates an 'index' file containing forward declarations of all the AutoSplit subroutines. When the AutoLoader module is 'use'd it loads these declarations into its callers package.</p> <p>Because of this mechanism it is important that <b>AutoLoader</b> is always <code>use</code>d and not <code>require</code>d.</p> <h2 id="Using-AutoLoader's-AUTOLOAD-Subroutine"><a class="permalink" href="#Using-AutoLoader's-AUTOLOAD-Subroutine">#</a><a id="Using"></a><a id="Using-AutoLoaders-AUTOLOAD-Subroutine"></a>Using <b>AutoLoader</b>'s AUTOLOAD Subroutine</h2> <p>In order to use <b>AutoLoader</b>'s AUTOLOAD subroutine you <i>must</i> explicitly import it:</p> <pre><code>use AutoLoader 'AUTOLOAD';</code></pre> <h2 id="Overriding-AutoLoader's-AUTOLOAD-Subroutine"><a class="permalink" href="#Overriding-AutoLoader's-AUTOLOAD-Subroutine">#</a><a id="Overriding"></a><a id="Overriding-AutoLoaders-AUTOLOAD-Subroutine"></a>Overriding <b>AutoLoader</b>'s AUTOLOAD Subroutine</h2> <p>Some modules, mainly extensions, provide their own AUTOLOAD subroutines. They typically need to check for some special cases (such as constants) and then fallback to <b>AutoLoader</b>'s AUTOLOAD for the rest.</p> <p>Such modules should <i>not</i> import <b>AutoLoader</b>'s AUTOLOAD subroutine. Instead, they should define their own AUTOLOAD subroutines along these lines:</p> <pre><code>use AutoLoader; use Carp; sub AUTOLOAD { my $sub = $AUTOLOAD; (my $constname = $sub) =~ s/.*:://; my $val = constant($constname, @_ ? $_[0] : 0); if ($! != 0) { if ($! =~ /Invalid/ || $!{EINVAL}) { $AutoLoader::AUTOLOAD = $sub; goto &AutoLoader::AUTOLOAD; } else { croak "Your vendor has not defined constant $constname"; } } *$sub = sub { $val }; # same as: eval "sub $sub { $val }"; goto &$sub; }</code></pre> <p>If any module's own AUTOLOAD subroutine has no need to fallback to the AutoLoader's AUTOLOAD subroutine (because it doesn't have any AutoSplit subroutines), then that module should not use <b>AutoLoader</b> at all.</p> <h2 id="Package-Lexicals"><a class="permalink" href="#Package-Lexicals">#</a><a id="Package"></a>Package Lexicals</h2> <p>Package lexicals declared with <code>my</code> in the main block of a package using <b>AutoLoader</b> will not be visible to auto-loaded subroutines, due to the fact that the given scope ends at the <code>__END__</code> marker. A module using such variables as package globals will not work properly under the <b>AutoLoader</b>.</p> <p>The <code>vars</code> pragma (see <a href="/5.8.6/perlmod#vars">"vars" in perlmod</a>) may be used in such situations as an alternative to explicitly qualifying all globals with the package namespace. Variables pre-declared with this pragma will be visible to any autoloaded routines (but will not be invisible outside the package, unfortunately).</p> <h2 id="Not-Using-AutoLoader"><a class="permalink" href="#Not-Using-AutoLoader">#</a><a id="Not"></a>Not Using AutoLoader</h2> <p>You can stop using AutoLoader by simply</p> <pre><code>no AutoLoader;</code></pre> <h2 id="AutoLoader-vs.-SelfLoader"><a class="permalink" href="#AutoLoader-vs.-SelfLoader">#</a><a id="AutoLoader"></a><b>AutoLoader</b> vs. <b>SelfLoader</b></h2> <p>The <b>AutoLoader</b> is similar in purpose to <b>SelfLoader</b>: both delay the loading of subroutines.</p> <p><b>SelfLoader</b> uses the <code>__DATA__</code> marker rather than <code>__END__</code>. While this avoids the use of a hierarchy of disk files and the associated open/close for each routine loaded, <b>SelfLoader</b> suffers a startup speed disadvantage in the one-time parsing of the lines after <code>__DATA__</code>, after which routines are cached. <b>SelfLoader</b> can also handle multiple packages in a file.</p> <p><b>AutoLoader</b> only reads code as it is requested, and in many cases should be faster, but requires a mechanism like <b>AutoSplit</b> be used to create the individual files. <a href="/5.8.6/ExtUtils::MakeMaker">ExtUtils::MakeMaker</a> will invoke <b>AutoSplit</b> automatically if <b>AutoLoader</b> is used in a module source file.</p> <h1 id="CAVEATS"><a class="permalink" href="#CAVEATS">#</a>CAVEATS</h1> <p>AutoLoaders prior to Perl 5.002 had a slightly different interface. Any old modules which use <b>AutoLoader</b> should be changed to the new calling style. Typically this just means changing a require to a use, adding the explicit <code>'AUTOLOAD'</code> import if needed, and removing <b>AutoLoader</b> from <code>@ISA</code>.</p> <p>On systems with restrictions on file name length, the file corresponding to a subroutine may have a shorter name that the routine itself. This can lead to conflicting file names. The <i>AutoSplit</i> package warns of these potential conflicts when used to split a module.</p> <p>AutoLoader may fail to find the autosplit files (or even find the wrong ones) in cases where <code>@INC</code> contains relative paths, <b>and</b> the program does <code>chdir</code>.</p> <h1 id="SEE-ALSO"><a class="permalink" href="#SEE-ALSO">#</a><a id="SEE"></a>SEE ALSO</h1> <p><a href="/5.8.6/SelfLoader">SelfLoader</a> - an autoloader that doesn't use external files.</p> </div> <div id="footer"> <p>Perldoc Browser is maintained by Dan Book (<a href="https://metacpan.org/author/DBOOK">DBOOK</a>). Please contact him via the <a href="https://github.com/Grinnz/perldoc-browser/issues">GitHub issue tracker</a> or <a href="mailto:dbook@cpan.org">email</a> regarding any issues with the site itself, search, or rendering of documentation.</p> <p>The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. Please contact them via the <a href="https://github.com/Perl/perl5/issues">Perl issue tracker</a>, the <a href="https://lists.perl.org/list/perl5-porters.html">mailing list</a>, or <a href="https://kiwiirc.com/client/irc.perl.org/p5p">IRC</a> to report any issues with the contents or format of the documentation.</p> </div> </div> <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.slim.min.js" integrity="sha512-/DXTXr6nQodMUiq+IUJYCt2PPOUjrHJ9wFrqpJ3XkgPNOZVfMok7cRw6CSxyCQxXn6ozlESsSh1/sMCTF1rL/g==" crossorigin="anonymous"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.1/umd/popper.min.js" integrity="sha512-ubuT8Z88WxezgSqf3RLuNi5lmjstiJcyezx34yIU2gAHonIi27Na7atqzUZCOoY4CExaoFumzOsFQ2Ch+I/HCw==" crossorigin="anonymous"></script> <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/js/bootstrap.min.js" integrity="sha384-B4gt1jrGC7Jh4AgTPSdUtOBvfO8shuf57BaghqFfPlYxofvL8/KUEfYiJOMMV+rV" crossorigin="anonymous"></script> <script src="/js/highlight.pack.js"></script> <script>hljs.highlightAll();</script> </body> </html>

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AutoLoader - load subroutines only on demand - Perldoc Browser