MAN page from Old RedHat 5.X perl-5.004-4.i386.rpm
SelfLoader
Section: Perl Programmers Reference Guide (3)
Updated: perl 5.004, patch 04
Index NAME
SelfLoader - load functions only on demand
SYNOPSIS
package FOOBAR; use SelfLoader; ... (initializing code) __DATA__ sub {.... DESCRIPTION
This module tells its users that functions in the FOOBAR package are to beautoloaded from after the
__DATA__ token. See alsothe section on
Autoloading in the
perlsub manpage.
The __DATA__ token
The
__DATA__ token tells the perl compiler that the perl codefor compilation is finished. Everything after the
__DATA__ tokenis available for reading via the filehandle
FOOBAR::DATA,where
FOOBAR is the name of the current package when the
__DATA__token is reached. This works just the same as
__END__ does inpackage `main', but for other modules data after
__END__ is notautomatically retreivable , whereas data after
__DATA__ is.The
__DATA__ token is not recognized in versions of perl prior to5.001m.
Note that it is possible to have __DATA__ tokens in the same packagein multiple files, and that the last __DATA__ token in a givenpackage that is encountered by the compiler is the one accessibleby the filehandle. This also applies to __END__ and main, i.e. ifthe `main' program has an __END__, but a module `require'd (_not_ `use'd)by that program has a `package main;' declaration followed by an `__DATA__',then the DATA filehandle is set to access the data after the __DATA__in the module, _not_ the data after the __END__ token in the `main'program, since the compiler encounters the `require'd file later.
SelfLoader autoloading
The
SelfLoader works by the user placing the
__DATA__token
after perl code which needs to be compiled andrun at `require' time, but
before subroutine declarationsthat can be loaded in later - usually because they may neverbe called.
The SelfLoader will read from the FOOBAR::DATA filehandle toload in the data after __DATA__, and load in any subroutinewhen it is called. The costs are the one-time parsing of thedata after __DATA__, and a load delay for the _first_call of any autoloaded function. The benefits (hopefully)are a speeded up compilation phase, with no need to loadfunctions which are never used.
The SelfLoader will stop reading from __DATA__ ifit encounters the __END__ token - just as you would expect.If the __END__ token is present, and is followed by thetoken DATA, then the SelfLoader leaves the FOOBAR::DATAfilehandle open on the line after that token.
The SelfLoader exports the AUTOLOAD subroutine to thepackage using the SelfLoader, and this loads the calledsubroutine when it is first called.
There is no advantage to putting subroutines which will _always_be called after the __DATA__ token.
Autoloading and package lexicals
A `my
$pack_lexical' statement makes the variable
$pack_lexicallocal _only_ to the file up to the
__DATA__ token. Subroutinesdeclared elsewhere _cannot_ see these types of variables,just as if you declared subroutines in the package but in anotherfile, they cannot see these variables.
So specifically, autoloaded functions cannot see packagelexicals (this applies to both the SelfLoader and the Autoloader).The vars pragma provides an alternative to defining package-levelglobals that will be visible to autoloaded routines. See the documentationon vars in the pragma section of the perlmod manpage.
SelfLoader and AutoLoader
The
SelfLoader can replace the AutoLoader - just change `use AutoLoader'to `use SelfLoader' (though note that the
SelfLoader exportsthe
AUTOLOAD function - but if you have your own
AUTOLOAD andare using the AutoLoader too, you probably know what you're doing),and the
__END__ token to
__DATA__. You will need perl version 5.001mor later to use this (version 5.001 with all patches up to patch m).
There is no need to inherit from the SelfLoader.
The SelfLoader works similarly to the AutoLoader, but picks up thesubs from after the __DATA__ instead of in the `lib/auto' directory.There is a maintainance gain in not needing to run AutoSplit on the moduleat installation, and a runtime gain in not needing to keep opening andclosing files to load subs. There is a runtime loss in needingto parse the code after the __DATA__. Details of the AutoLoader andanother view of these distinctions can be found in that module'sdocumentation.
__DATA__, __END__, and the FOOBAR::DATA filehandle.
This section is only relevant if you want to usethe
FOOBAR::DATA together with the
SelfLoader.
Data after the __DATA__ token in a module is read using theFOOBAR::DATA filehandle. __END__ can still be used to denote the endof the __DATA__ section if followed by the token DATA - this is supportedby the SelfLoader. The FOOBAR::DATA filehandle is left open if an__END__ followed by a DATA is found, with the filehandle positioned atthe start of the line after the __END__ token. If no __END__ token ispresent, or an __END__ token with no DATA token on the same line, thenthe filehandle is closed.
The SelfLoader reads from wherever the currentposition of the FOOBAR::DATA filehandle is, until theEOF or __END__. This means that if you want to usethat filehandle (and ONLY if you want to), you should either
1. Put all your subroutine declarations immediately afterthe __DATA__ token and put your own data after thosedeclarations, using the __END__ token to mark the endof subroutine declarations. You must also ensure that the SelfLoaderreads first by calling `SelfLoader->load_stubs();', or by using afunction which is selfloaded;
or
2. You should read the FOOBAR::DATA filehandle first, leavingthe handle open and positioned at the first line of subroutinedeclarations.
You could conceivably do both.
Classes and inherited methods.
For modules which are not classes, this section is not relevant.This section is only relevant if you have methods which couldbe inherited.
A subroutine stub (or forward declaration) looks like
sub stub;
i.e. it is a subroutine declaration without the body of thesubroutine. For modules which are not classes, there is no realneed for stubs as far as autoloading is concerned.
For modules which ARE classes, and need to handle inherited methods,stubs are needed to ensure that the method inheritance mechanism worksproperly. You can load the stubs into the module at `require' time, byadding the statement `SelfLoader->load_stubs();' to the module to dothis.
The alternative is to put the stubs in before the __DATA__ token BEFOREreleasing the module, and for this purpose the Devel::SelfStubbermodule is available. However this does require the extra step of ensuringthat the stubs are in the module. If this is done I strongly recommendthat this is done BEFORE releasing the module - it should NOT be doneat install time in general.
Multiple packages and fully qualified subroutine names
Subroutines in multiple packages within the same file are supported - but youshould note that this requires exporting the
SelfLoader::AUTOLOAD toevery package which requires it. This is done automatically by the
SelfLoader when it first loads the subs into the cache, but you shouldreally specify it in the initialization before the
__DATA__ by puttinga `use SelfLoader' statement in each package.
Fully qualified subroutine names are also supported. For example,
__DATA__ sub foo::bar {23} package baz; sub dob {32}will all be loaded correctly by the
SelfLoader, and the
SelfLoaderwill ensure that the packages `foo' and `baz' correctly have the
SelfLoader AUTOLOAD method when the data after
__DATA__ is firstparsed.
Index
- NAME
- SYNOPSIS
- DESCRIPTION
- The __DATA__ token
- SelfLoader autoloading
- Autoloading and package lexicals
- SelfLoader and AutoLoader
- __DATA__, __END__, and the FOOBAR::DATA filehandle.
- Classes and inherited methods.
- Multiple packages and fully qualified subroutine names
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