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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "AutoLoader 3" | |
132 | .TH AutoLoader 3 "2002-06-01" "perl v5.8.0" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | AutoLoader \- load subroutines only on demand | |
135 | .SH "SYNOPSIS" | |
136 | .IX Header "SYNOPSIS" | |
137 | .Vb 2 | |
138 | \& package Foo; | |
139 | \& use AutoLoader 'AUTOLOAD'; # import the default AUTOLOAD subroutine | |
140 | .Ve | |
141 | .PP | |
142 | .Vb 7 | |
143 | \& package Bar; | |
144 | \& use AutoLoader; # don't import AUTOLOAD, define our own | |
145 | \& sub AUTOLOAD { | |
146 | \& ... | |
147 | \& $AutoLoader::AUTOLOAD = "..."; | |
148 | \& goto &AutoLoader::AUTOLOAD; | |
149 | \& } | |
150 | .Ve | |
151 | .SH "DESCRIPTION" | |
152 | .IX Header "DESCRIPTION" | |
153 | The \fBAutoLoader\fR module works with the \fBAutoSplit\fR module and the | |
154 | \&\f(CW\*(C`_\|_END_\|_\*(C'\fR token to defer the loading of some subroutines until they are | |
155 | used rather than loading them all at once. | |
156 | .PP | |
157 | To use \fBAutoLoader\fR, the author of a module has to place the | |
158 | definitions of subroutines to be autoloaded after an \f(CW\*(C`_\|_END_\|_\*(C'\fR token. | |
159 | (See perldata.) The \fBAutoSplit\fR module can then be run manually to | |
160 | extract the definitions into individual files \fIauto/funcname.al\fR. | |
161 | .PP | |
162 | \&\fBAutoLoader\fR implements an \s-1AUTOLOAD\s0 subroutine. When an undefined | |
163 | subroutine in is called in a client module of \fBAutoLoader\fR, | |
164 | \&\fBAutoLoader\fR's \s-1AUTOLOAD\s0 subroutine attempts to locate the subroutine in a | |
165 | file with a name related to the location of the file from which the | |
166 | client module was read. As an example, if \fI\s-1POSIX\s0.pm\fR is located in | |
167 | \&\fI/usr/local/lib/perl5/POSIX.pm\fR, \fBAutoLoader\fR will look for perl | |
168 | subroutines \fB\s-1POSIX\s0\fR in \fI/usr/local/lib/perl5/auto/POSIX/*.al\fR, where | |
169 | the \f(CW\*(C`.al\*(C'\fR file has the same name as the subroutine, sans package. If | |
170 | such a file exists, \s-1AUTOLOAD\s0 will read and evaluate it, | |
171 | thus (presumably) defining the needed subroutine. \s-1AUTOLOAD\s0 will then | |
172 | \&\f(CW\*(C`goto\*(C'\fR the newly defined subroutine. | |
173 | .PP | |
174 | Once this process completes for a given function, it is defined, so | |
175 | future calls to the subroutine will bypass the \s-1AUTOLOAD\s0 mechanism. | |
176 | .Sh "Subroutine Stubs" | |
177 | .IX Subsection "Subroutine Stubs" | |
178 | In order for object method lookup and/or prototype checking to operate | |
179 | correctly even when methods have not yet been defined it is necessary to | |
180 | \&\*(L"forward declare\*(R" each subroutine (as in \f(CW\*(C`sub NAME;\*(C'\fR). See | |
181 | \&\*(L"\s-1SYNOPSIS\s0\*(R" in perlsub. Such forward declaration creates \*(L"subroutine | |
182 | stubs\*(R", which are place holders with no code. | |
183 | .PP | |
184 | The AutoSplit and \fBAutoLoader\fR modules automate the creation of forward | |
185 | declarations. The AutoSplit module creates an 'index' file containing | |
186 | forward declarations of all the AutoSplit subroutines. When the | |
187 | AutoLoader module is 'use'd it loads these declarations into its callers | |
188 | package. | |
189 | .PP | |
190 | Because of this mechanism it is important that \fBAutoLoader\fR is always | |
191 | \&\f(CW\*(C`use\*(C'\fRd and not \f(CW\*(C`require\*(C'\fRd. | |
192 | .Sh "Using \fBAutoLoader\fP's \s-1AUTOLOAD\s0 Subroutine" | |
193 | .IX Subsection "Using AutoLoader's AUTOLOAD Subroutine" | |
194 | In order to use \fBAutoLoader\fR's \s-1AUTOLOAD\s0 subroutine you \fImust\fR | |
195 | explicitly import it: | |
196 | .PP | |
197 | .Vb 1 | |
198 | \& use AutoLoader 'AUTOLOAD'; | |
199 | .Ve | |
200 | .Sh "Overriding \fBAutoLoader\fP's \s-1AUTOLOAD\s0 Subroutine" | |
201 | .IX Subsection "Overriding AutoLoader's AUTOLOAD Subroutine" | |
202 | Some modules, mainly extensions, provide their own \s-1AUTOLOAD\s0 subroutines. | |
203 | They typically need to check for some special cases (such as constants) | |
204 | and then fallback to \fBAutoLoader\fR's \s-1AUTOLOAD\s0 for the rest. | |
205 | .PP | |
206 | Such modules should \fInot\fR import \fBAutoLoader\fR's \s-1AUTOLOAD\s0 subroutine. | |
207 | Instead, they should define their own \s-1AUTOLOAD\s0 subroutines along these | |
208 | lines: | |
209 | .PP | |
210 | .Vb 2 | |
211 | \& use AutoLoader; | |
212 | \& use Carp; | |
213 | .Ve | |
214 | .PP | |
215 | .Vb 16 | |
216 | \& sub AUTOLOAD { | |
217 | \& my $sub = $AUTOLOAD; | |
218 | \& (my $constname = $sub) =~ s/.*:://; | |
219 | \& my $val = constant($constname, @_ ? $_[0] : 0); | |
220 | \& if ($! != 0) { | |
221 | \& if ($! =~ /Invalid/ || $!{EINVAL}) { | |
222 | \& $AutoLoader::AUTOLOAD = $sub; | |
223 | \& goto &AutoLoader::AUTOLOAD; | |
224 | \& } | |
225 | \& else { | |
226 | \& croak "Your vendor has not defined constant $constname"; | |
227 | \& } | |
228 | \& } | |
229 | \& *$sub = sub { $val }; # same as: eval "sub $sub { $val }"; | |
230 | \& goto &$sub; | |
231 | \& } | |
232 | .Ve | |
233 | .PP | |
234 | If any module's own \s-1AUTOLOAD\s0 subroutine has no need to fallback to the | |
235 | AutoLoader's \s-1AUTOLOAD\s0 subroutine (because it doesn't have any AutoSplit | |
236 | subroutines), then that module should not use \fBAutoLoader\fR at all. | |
237 | .Sh "Package Lexicals" | |
238 | .IX Subsection "Package Lexicals" | |
239 | Package lexicals declared with \f(CW\*(C`my\*(C'\fR in the main block of a package | |
240 | using \fBAutoLoader\fR will not be visible to auto-loaded subroutines, due to | |
241 | the fact that the given scope ends at the \f(CW\*(C`_\|_END_\|_\*(C'\fR marker. A module | |
242 | using such variables as package globals will not work properly under the | |
243 | \&\fBAutoLoader\fR. | |
244 | .PP | |
245 | The \f(CW\*(C`vars\*(C'\fR pragma (see \*(L"vars\*(R" in perlmod) may be used in such | |
246 | situations as an alternative to explicitly qualifying all globals with | |
247 | the package namespace. Variables pre-declared with this pragma will be | |
248 | visible to any autoloaded routines (but will not be invisible outside | |
249 | the package, unfortunately). | |
250 | .Sh "Not Using AutoLoader" | |
251 | .IX Subsection "Not Using AutoLoader" | |
252 | You can stop using AutoLoader by simply | |
253 | .PP | |
254 | .Vb 1 | |
255 | \& no AutoLoader; | |
256 | .Ve | |
257 | .Sh "\fBAutoLoader\fP vs. \fBSelfLoader\fP" | |
258 | .IX Subsection "AutoLoader vs. SelfLoader" | |
259 | The \fBAutoLoader\fR is similar in purpose to \fBSelfLoader\fR: both delay the | |
260 | loading of subroutines. | |
261 | .PP | |
262 | \&\fBSelfLoader\fR uses the \f(CW\*(C`_\|_DATA_\|_\*(C'\fR marker rather than \f(CW\*(C`_\|_END_\|_\*(C'\fR. | |
263 | While this avoids the use of a hierarchy of disk files and the | |
264 | associated open/close for each routine loaded, \fBSelfLoader\fR suffers a | |
265 | startup speed disadvantage in the one-time parsing of the lines after | |
266 | \&\f(CW\*(C`_\|_DATA_\|_\*(C'\fR, after which routines are cached. \fBSelfLoader\fR can also | |
267 | handle multiple packages in a file. | |
268 | .PP | |
269 | \&\fBAutoLoader\fR only reads code as it is requested, and in many cases | |
270 | should be faster, but requires a mechanism like \fBAutoSplit\fR be used to | |
271 | create the individual files. ExtUtils::MakeMaker will invoke | |
272 | \&\fBAutoSplit\fR automatically if \fBAutoLoader\fR is used in a module source | |
273 | file. | |
274 | .SH "CAVEATS" | |
275 | .IX Header "CAVEATS" | |
276 | AutoLoaders prior to Perl 5.002 had a slightly different interface. Any | |
277 | old modules which use \fBAutoLoader\fR should be changed to the new calling | |
278 | style. Typically this just means changing a require to a use, adding | |
279 | the explicit \f(CW'AUTOLOAD'\fR import if needed, and removing \fBAutoLoader\fR | |
280 | from \f(CW@ISA\fR. | |
281 | .PP | |
282 | On systems with restrictions on file name length, the file corresponding | |
283 | to a subroutine may have a shorter name that the routine itself. This | |
284 | can lead to conflicting file names. The \fIAutoSplit\fR package warns of | |
285 | these potential conflicts when used to split a module. | |
286 | .PP | |
287 | AutoLoader may fail to find the autosplit files (or even find the wrong | |
288 | ones) in cases where \f(CW@INC\fR contains relative paths, \fBand\fR the program | |
289 | does \f(CW\*(C`chdir\*(C'\fR. | |
290 | .SH "SEE ALSO" | |
291 | .IX Header "SEE ALSO" | |
292 | SelfLoader \- an autoloader that doesn't use external files. |