1SelfLoader(3pm)        Perl Programmers Reference Guide        SelfLoader(3pm)
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NAME

6       SelfLoader - load functions only on demand
7

SYNOPSIS

9           package FOOBAR;
10           use SelfLoader;
11
12           ... (initializing code)
13
14           __DATA__
15           sub {....
16

DESCRIPTION

18       This module tells its users that functions in the FOOBAR package are to
19       be autoloaded from after the "__DATA__" token.  See also "Autoloading"
20       in perlsub.
21
22   The __DATA__ token
23       The "__DATA__" token tells the perl compiler that the perl code for
24       compilation is finished. Everything after the "__DATA__" token is
25       available for reading via the filehandle FOOBAR::DATA, where FOOBAR is
26       the name of the current package when the "__DATA__" token is reached.
27       This works just the same as "__END__" does in package 'main', but for
28       other modules data after "__END__" is not automatically retrievable,
29       whereas data after "__DATA__" is.  The "__DATA__" token is not
30       recognized in versions of perl prior to 5.001m.
31
32       Note that it is possible to have "__DATA__" tokens in the same package
33       in multiple files, and that the last "__DATA__" token in a given
34       package that is encountered by the compiler is the one accessible by
35       the filehandle. This also applies to "__END__" and main, i.e. if the
36       'main' program has an "__END__", but a module 'require'd (_not_ 'use'd)
37       by that program has a 'package main;' declaration followed by an
38       '"__DATA__"', then the "DATA" filehandle is set to access the data
39       after the "__DATA__" in the module, _not_ the data after the "__END__"
40       token in the 'main' program, since the compiler encounters the
41       'require'd file later.
42
43   SelfLoader autoloading
44       The SelfLoader works by the user placing the "__DATA__" token after
45       perl code which needs to be compiled and run at 'require' time, but
46       before subroutine declarations that can be loaded in later - usually
47       because they may never be called.
48
49       The SelfLoader will read from the FOOBAR::DATA filehandle to load in
50       the data after "__DATA__", and load in any subroutine when it is
51       called. The costs are the one-time parsing of the data after
52       "__DATA__", and a load delay for the _first_ call of any autoloaded
53       function. The benefits (hopefully) are a speeded up compilation phase,
54       with no need to load functions which are never used.
55
56       The SelfLoader will stop reading from "__DATA__" if it encounters the
57       "__END__" token - just as you would expect.  If the "__END__" token is
58       present, and is followed by the token DATA, then the SelfLoader leaves
59       the FOOBAR::DATA filehandle open on the line after that token.
60
61       The SelfLoader exports the "AUTOLOAD" subroutine to the package using
62       the SelfLoader, and this loads the called subroutine when it is first
63       called.
64
65       There is no advantage to putting subroutines which will _always_ be
66       called after the "__DATA__" token.
67
68   Autoloading and package lexicals
69       A 'my $pack_lexical' statement makes the variable $pack_lexical local
70       _only_ to the file up to the "__DATA__" token. Subroutines declared
71       elsewhere _cannot_ see these types of variables, just as if you
72       declared subroutines in the package but in another file, they cannot
73       see these variables.
74
75       So specifically, autoloaded functions cannot see package lexicals (this
76       applies to both the SelfLoader and the Autoloader).  The "vars" pragma
77       provides an alternative to defining package-level globals that will be
78       visible to autoloaded routines. See the documentation on vars in the
79       pragma section of perlmod.
80
81   SelfLoader and AutoLoader
82       The SelfLoader can replace the AutoLoader - just change 'use
83       AutoLoader' to 'use SelfLoader' (though note that the SelfLoader
84       exports the AUTOLOAD function - but if you have your own AUTOLOAD and
85       are using the AutoLoader too, you probably know what you're doing), and
86       the "__END__" token to "__DATA__". You will need perl version 5.001m or
87       later to use this (version 5.001 with all patches up to patch m).
88
89       There is no need to inherit from the SelfLoader.
90
91       The SelfLoader works similarly to the AutoLoader, but picks up the subs
92       from after the "__DATA__" instead of in the 'lib/auto' directory.
93       There is a maintenance gain in not needing to run AutoSplit on the
94       module at installation, and a runtime gain in not needing to keep
95       opening and closing files to load subs. There is a runtime loss in
96       needing to parse the code after the "__DATA__". Details of the
97       AutoLoader and another view of these distinctions can be found in that
98       module's documentation.
99
100   __DATA__, __END__, and the FOOBAR::DATA filehandle.
101       This section is only relevant if you want to use the "FOOBAR::DATA"
102       together with the SelfLoader.
103
104       Data after the "__DATA__" token in a module is read using the
105       FOOBAR::DATA filehandle. "__END__" can still be used to denote the end
106       of the "__DATA__" section if followed by the token DATA - this is
107       supported by the SelfLoader. The "FOOBAR::DATA" filehandle is left open
108       if an "__END__" followed by a DATA is found, with the filehandle
109       positioned at the start of the line after the "__END__" token. If no
110       "__END__" token is present, or an "__END__" token with no DATA token on
111       the same line, then the filehandle is closed.
112
113       The SelfLoader reads from wherever the current position of the
114       "FOOBAR::DATA" filehandle is, until the EOF or "__END__". This means
115       that if you want to use that filehandle (and ONLY if you want to), you
116       should either
117
118       1. Put all your subroutine declarations immediately after the
119       "__DATA__" token and put your own data after those declarations, using
120       the "__END__" token to mark the end of subroutine declarations. You
121       must also ensure that the SelfLoader reads first by  calling
122       'SelfLoader->load_stubs();', or by using a function which is
123       selfloaded;
124
125       or
126
127       2. You should read the "FOOBAR::DATA" filehandle first, leaving the
128       handle open and positioned at the first line of subroutine
129       declarations.
130
131       You could conceivably do both.
132
133   Classes and inherited methods.
134       For modules which are not classes, this section is not relevant.  This
135       section is only relevant if you have methods which could be inherited.
136
137       A subroutine stub (or forward declaration) looks like
138
139         sub stub;
140
141       i.e. it is a subroutine declaration without the body of the subroutine.
142       For modules which are not classes, there is no real need for stubs as
143       far as autoloading is concerned.
144
145       For modules which ARE classes, and need to handle inherited methods,
146       stubs are needed to ensure that the method inheritance mechanism works
147       properly. You can load the stubs into the module at 'require' time, by
148       adding the statement 'SelfLoader->load_stubs();' to the module to do
149       this.
150
151       The alternative is to put the stubs in before the "__DATA__" token
152       BEFORE releasing the module, and for this purpose the
153       "Devel::SelfStubber" module is available.  However this does require
154       the extra step of ensuring that the stubs are in the module. If this is
155       done I strongly recommend that this is done BEFORE releasing the module
156       - it should NOT be done at install time in general.
157

Multiple packages and fully qualified subroutine names

159       Subroutines in multiple packages within the same file are supported -
160       but you should note that this requires exporting the
161       "SelfLoader::AUTOLOAD" to every package which requires it. This is done
162       automatically by the SelfLoader when it first loads the subs into the
163       cache, but you should really specify it in the initialization before
164       the "__DATA__" by putting a 'use SelfLoader' statement in each package.
165
166       Fully qualified subroutine names are also supported. For example,
167
168          __DATA__
169          sub foo::bar {23}
170          package baz;
171          sub dob {32}
172
173       will all be loaded correctly by the SelfLoader, and the SelfLoader will
174       ensure that the packages 'foo' and 'baz' correctly have the SelfLoader
175       "AUTOLOAD" method when the data after "__DATA__" is first parsed.
176

AUTHOR

178       "SelfLoader" is maintained by the perl5-porters. Please direct any
179       questions to the canonical mailing list. Anything that is applicable to
180       the CPAN release can be sent to its maintainer, though.
181
182       Author and Maintainer: The Perl5-Porters <perl5-porters@perl.org>
183
184       Maintainer of the CPAN release: Steffen Mueller <smueller@cpan.org>
185
187       This package has been part of the perl core since the first release of
188       perl5. It has been released separately to CPAN so older installations
189       can benefit from bug fixes.
190
191       This package has the same copyright and license as the perl core:
192
193       Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001,
194       2002, 2003, 2004, 2005, 2006 by Larry Wall and others
195
196       All rights reserved.
197
198       This program is free software; you can redistribute it and/or modify it
199       under the terms of either:
200
201       a)  the GNU General Public License as published by the Free Software
202           Foundation; either version 1, or (at your option) any later
203           version, or
204
205       b)  the "Artistic License" which comes with this Kit.
206
207       This program is distributed in the hope that it will be useful, but
208       WITHOUT ANY WARRANTY; without even the implied warranty of
209       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See either the
210       GNU General Public License or the Artistic License for more details.
211
212       You should have received a copy of the Artistic License with this Kit,
213       in the file named "Artistic".  If not, I'll be glad to provide one.
214
215       You should also have received a copy of the GNU General Public License
216       along with this program in the file named "Copying". If not, write to
217       the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
218       Boston, MA 02110-1301, USA or visit their web page on the internet at
219       http://www.gnu.org/copyleft/gpl.html.
220
221       For those of you that choose to use the GNU General Public License, my
222       interpretation of the GNU General Public License is that no Perl script
223       falls under the terms of the GPL unless you explicitly put said script
224       under the terms of the GPL yourself.  Furthermore, any object code
225       linked with perl does not automatically fall under the terms of the
226       GPL, provided such object code only adds definitions of subroutines and
227       variables, and does not otherwise impair the resulting interpreter from
228       executing any standard Perl script.  I consider linking in C
229       subroutines in this manner to be the moral equivalent of defining
230       subroutines in the Perl language itself.  You may sell such an object
231       file as proprietary provided that you provide or offer to provide the
232       Perl source, as specified by the GNU General Public License.  (This is
233       merely an alternate way of specifying input to the program.)  You may
234       also sell a binary produced by the dumping of a running Perl script
235       that belongs to you, provided that you provide or offer to provide the
236       Perl source as specified by the GPL.  (The fact that a Perl interpreter
237       and your code are in the same binary file is, in this case, a form of
238       mere aggregation.)  This is my interpretation of the GPL.  If you still
239       have concerns or difficulties understanding my intent, feel free to
240       contact me.  Of course, the Artistic License spells all this out for
241       your protection, so you may prefer to use that.
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245perl v5.26.3                      2018-03-01                   SelfLoader(3pm)
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