IRIX Base Documentation 2002 November/SGI IRIX Base Documentation 2002 November.iso/usr/share/catman/p_man/cat3/perl5/AutoLoader.z/AutoLoader

Text File | 2002-10-03 | 10.3 KB | 265 lines

AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr((((3333))))                                                    AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr((((3333))))



NNNNAAAAMMMMEEEE
     AutoLoader - load subroutines only on demand

SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS
         package Foo;
         use AutoLoader 'AUTOLOAD';   # import the default AUTOLOAD subroutine

         package Bar;
         use AutoLoader;              # don't import AUTOLOAD, define our own
         sub AUTOLOAD {
             ...
             $AutoLoader::AUTOLOAD = "...";
             goto &AutoLoader::AUTOLOAD;
         }


DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
     The AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr module works with the AAAAuuuuttttooooSSSSpppplllliiiitttt module and the __END__
     token to defer the loading of some subroutines until they are used rather
     than loading them all at once.

     To use AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr, the author of a module has to place the definitions of
     subroutines to be autoloaded after an __END__ token.  (See the _p_e_r_l_d_a_t_a
     manpage.)  The AAAAuuuuttttooooSSSSpppplllliiiitttt module can then be run manually to extract the
     definitions into individual files _a_u_t_o/_f_u_n_c_n_a_m_e._a_l.

     AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr implements an AUTOLOAD subroutine.  When an undefined
     subroutine in is called in a client module of AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr, AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr's
     AUTOLOAD subroutine attempts to locate the subroutine in a file with a
     name related to the location of the file from which the client module was
     read.  As an example, if _P_O_S_I_X._p_m is located in
     /_u_s_r/_l_o_c_a_l/_l_i_b/_p_e_r_l_5/_P_O_S_I_X._p_m, AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr will look for perl subroutines
     PPPPOOOOSSSSIIIIXXXX in /_u_s_r/_l_o_c_a_l/_l_i_b/_p_e_r_l_5/_a_u_t_o/_P_O_S_I_X/*._a_l, where the .al file has the
     same name as the subroutine, sans package.  If such a file exists,
     AUTOLOAD will read and evaluate it, thus (presumably) defining the needed
     subroutine.  AUTOLOAD will then goto the newly defined subroutine.

     Once this process completes for a given function, it is defined, so
     future calls to the subroutine will bypass the AUTOLOAD mechanism.

     SSSSuuuubbbbrrrroooouuuuttttiiiinnnneeee SSSSttttuuuubbbbssss

     In order for object method lookup and/or prototype checking to operate
     correctly even when methods have not yet been defined it is necessary to
     "forward declare" each subroutine (as in sub NAME;).  See the section on
     _S_Y_N_O_P_S_I_S in the _p_e_r_l_s_u_b manpage.  Such forward declaration creates
     "subroutine stubs", which are place holders with no code.

     The AutoSplit and AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr modules automate the creation of forward
     declarations.  The AutoSplit module creates an 'index' file containing
     forward declarations of all the AutoSplit subroutines.  When the
     AutoLoader module is 'use'd it loads these declarations into its callers



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AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr((((3333))))                                                    AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr((((3333))))



     package.

     Because of this mechanism it is important that AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr is always used
     and not required.

     UUUUssssiiiinnnngggg AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr's AUTOLOAD Subroutine

     In order to use AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr's AUTOLOAD subroutine you _m_u_s_t explicitly
     import it:

         use AutoLoader 'AUTOLOAD';


     OOOOvvvveeeerrrrrrrriiiiddddiiiinnnngggg AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr's AUTOLOAD Subroutine

     Some modules, mainly extensions, provide their own AUTOLOAD subroutines.
     They typically need to check for some special cases (such as constants)
     and then fallback to AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr's AUTOLOAD for the rest.

     Such modules should _n_o_t import AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr's AUTOLOAD subroutine.
     Instead, they should define their own AUTOLOAD subroutines along these
     lines:

         use AutoLoader;
         use Carp;

         sub AUTOLOAD {
             my $sub = $AUTOLOAD;
             (my $constname = $sub) =~ s/.*:://;
             my $val = constant($constname, @_ ? $_[0] : 0);
             if ($! != 0) {
                 if ($! =~ /Invalid/) {
                     $AutoLoader::AUTOLOAD = $sub;
                     goto &AutoLoader::AUTOLOAD;
                 }
                 else {
                     croak "Your vendor has not defined constant $constname";
                 }
             }
             *$sub = sub { $val }; # same as: eval "sub $sub { $val }";
             goto &$sub;
         }

     If any module's own AUTOLOAD subroutine has no need to fallback to the
     AutoLoader's AUTOLOAD subroutine (because it doesn't have any AutoSplit
     subroutines), then that module should not use AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr at all.

     PPPPaaaacccckkkkaaaaggggeeee LLLLeeeexxxxiiiiccccaaaallllssss

     Package lexicals declared with my in the main block of a package using
     AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr will not be visible to auto-loaded subroutines, due to the
     fact that the given scope ends at the __END__ marker.  A module using



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AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr((((3333))))                                                    AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr((((3333))))



     such variables as package globals will not work properly under the
     AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr.

     The vars pragma (see the section on _v_a_r_s in the _p_e_r_l_m_o_d manpage) may be
     used in such situations as an alternative to explicitly qualifying all
     globals with the package namespace.  Variables pre-declared with this
     pragma will be visible to any autoloaded routines (but will not be
     invisible outside the package, unfortunately).

     AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr vs. SSSSeeeellllffffLLLLooooaaaaddddeeeerrrr

     The AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr is similar in purpose to SSSSeeeellllffffLLLLooooaaaaddddeeeerrrr: both delay the
     loading of subroutines.

     SSSSeeeellllffffLLLLooooaaaaddddeeeerrrr uses the __DATA__ marker rather than __END__.  While this
     avoids the use of a hierarchy of disk files and the associated open/close
     for each routine loaded, SSSSeeeellllffffLLLLooooaaaaddddeeeerrrr suffers a startup speed disadvantage
     in the one-time parsing of the lines after __DATA__, after which routines
     are cached.  SSSSeeeellllffffLLLLooooaaaaddddeeeerrrr can also handle multiple packages in a file.

     AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr only reads code as it is requested, and in many cases should
     be faster, but requires a mechanism like AAAAuuuuttttooooSSSSpppplllliiiitttt be used to create the
     individual files.  the _E_x_t_U_t_i_l_s::_M_a_k_e_M_a_k_e_r manpage will invoke AAAAuuuuttttooooSSSSpppplllliiiitttt
     automatically if AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr is used in a module source file.

CCCCAAAAVVVVEEEEAAAATTTTSSSS
     AutoLoaders prior to Perl 5.002 had a slightly different interface.  Any
     old modules which use AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr should be changed to the new calling
     style.  Typically this just means changing a require to a use, adding the
     explicit 'AUTOLOAD' import if needed, and removing AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr from @ISA.

     On systems with restrictions on file name length, the file corresponding
     to a subroutine may have a shorter name that the routine itself.  This
     can lead to conflicting file names.  The _A_u_t_o_S_p_l_i_t package warns of these
     potential conflicts when used to split a module.

     AutoLoader may fail to find the autosplit files (or even find the wrong
     ones) in cases where @INC contains relative paths, aaaannnndddd the program does
     chdir.

SSSSEEEEEEEE AAAALLLLSSSSOOOO
     the _S_e_l_f_L_o_a_d_e_r manpage - an autoloader that doesn't use external files.













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AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr((((3333))))                                                    AAAAuuuuttttooooLLLLooooaaaaddddeeeerrrr((((3333))))























































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