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| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "AutoLoader 3" |
| 132 | .TH AutoLoader 3 "2001-09-21" "perl v5.8.8" "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. |