SelfLoader - load functions only on demand
package FOOBAR;use SelfLoader;... (initializing code)__DATA__sub {....This module tells its users that functions in the FOOBAR package are to be autoloaded from after the__DATA__ token. See also"Autoloading" in perlsub.
The__DATA__ token tells the perl compiler that the perl code for compilation is finished. Everything after the__DATA__ token is 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 in package 'main', but for other modules data after__END__ is not automatically retrievable, whereas data after__DATA__ is. The__DATA__ token is not recognized in versions of perl prior to 5.001m.
Note that it is possible to have__DATA__ tokens in the same package in multiple files, and that the last__DATA__ token in a given package that is encountered by the compiler is the one accessible by the filehandle. This also applies to__END__ and main, i.e. if the '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 theDATA 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.
TheSelfLoader works by the user placing the__DATA__ tokenafter perl code which needs to be compiled and run at 'require' time, butbefore subroutine declarations that can be loaded in later - usually because they may never be called.
TheSelfLoader will read from the FOOBAR::DATA filehandle to load in the data after__DATA__, and load in any subroutine when it is called. The costs are the one-time parsing of the data 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 load functions which are never used.
TheSelfLoader will stop reading from__DATA__ if it encounters the__END__ token - just as you would expect. If the__END__ token is present, and is followed by the token DATA, then theSelfLoader leaves the FOOBAR::DATA filehandle open on the line after that token.
TheSelfLoader exports theAUTOLOAD subroutine to the package using theSelfLoader, and this loads the called subroutine when it is first called.
There is no advantage to putting subroutines which will _always_ be called after the__DATA__ token.
A 'my $pack_lexical' statement makes the variable $pack_lexical local _only_ to the file up to the__DATA__ token. Subroutines declared elsewhere _cannot_ see these types of variables, just as if you declared subroutines in the package but in another file, they cannot see these variables.
So specifically, autoloaded functions cannot see package lexicals (this applies to both theSelfLoader and the Autoloader). Thevars pragma provides an alternative to defining package-level globals that will be visible to autoloaded routines. See the documentation onvars in the pragma section ofperlmod.
TheSelfLoader can replace the AutoLoader - just change 'use AutoLoader' to 'use SelfLoader' (though note that theSelfLoader exports the AUTOLOAD function - but if you have your own AUTOLOAD and are using the AutoLoader too, you probably know what you're doing), and the__END__ token to__DATA__. You will need perl version 5.001m or later to use this (version 5.001 with all patches up to patch m).
There is no need to inherit from theSelfLoader.
TheSelfLoader works similarly to the AutoLoader, but picks up the subs from after the__DATA__ instead of in the 'lib/auto' directory. There is a maintenance gain in not needing to run AutoSplit on the module at installation, and a runtime gain in not needing to keep opening and closing files to load subs. There is a runtime loss in needing to parse the code after the__DATA__. Details of theAutoLoader and another view of these distinctions can be found in that module's documentation.
This section is only relevant if you want to use theFOOBAR::DATA together with theSelfLoader.
Data after the__DATA__ token in a module is read using the FOOBAR::DATA filehandle.__END__ can still be used to denote the end of the__DATA__ section if followed by the token DATA - this is supported by theSelfLoader. TheFOOBAR::DATA filehandle is left open if an__END__ followed by a DATA is found, with the filehandle positioned at the start of the line after the__END__ token. If no__END__ token is present, or an__END__ token with no DATA token on the same line, then the filehandle is closed.
TheSelfLoader reads from wherever the current position of theFOOBAR::DATA filehandle is, until the EOF or__END__. This means that if you want to use that filehandle (and ONLY if you want to), you should either
1. Put all your subroutine declarations immediately after the__DATA__ token and put your own data after those declarations, using the__END__ token to mark the end of subroutine declarations. You must also ensure that theSelfLoader reads first by calling 'SelfLoader->load_stubs();', or by using a function which is selfloaded;
or
2. You should read theFOOBAR::DATA filehandle first, leaving the handle open and positioned at the first line of subroutine declarations.
You could conceivably do both.
For modules which are not classes, this section is not relevant. This section is only relevant if you have methods which could be inherited.
A subroutine stub (or forward declaration) looks like
sub stub;i.e. it is a subroutine declaration without the body of the subroutine. For modules which are not classes, there is no real need 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 works properly. You can load the stubs into the module at 'require' time, by adding the statement 'SelfLoader->load_stubs();' to the module to do this.
The alternative is to put the stubs in before the__DATA__ token BEFORE releasing the module, and for this purpose theDevel::SelfStubber module is available. However this does require the extra step of ensuring that the stubs are in the module. If this is done I strongly recommend that this is done BEFORE releasing the module - it should NOT be done at install time in general.
Subroutines in multiple packages within the same file are supported - but you should note that this requires exporting theSelfLoader::AUTOLOAD to every package which requires it. This is done automatically by theSelfLoader when it first loads the subs into the cache, but you should really specify it in the initialization before the__DATA__ by putting a '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 theSelfLoader, and theSelfLoader will ensure that the packages 'foo' and 'baz' correctly have theSelfLoaderAUTOLOAD method when the data after__DATA__ is first parsed.
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