1package Variable::Magic; 2 3use 5.008; 4 5use strict; 6use warnings; 7 8use Carp qw/croak/; 9 10=head1 NAME 11 12Variable::Magic - Associate user-defined magic to variables from Perl. 13 14=head1 VERSION 15 16Version 0.43 17 18=cut 19 20our $VERSION; 21BEGIN { 22 $VERSION = '0.43'; 23} 24 25=head1 SYNOPSIS 26 27 use Variable::Magic qw/wizard cast VMG_OP_INFO_NAME/; 28 29 { # A variable tracer 30 my $wiz = wizard set => sub { print "now set to ${$_[0]}!\n" }, 31 free => sub { print "destroyed!\n" }; 32 33 my $a = 1; 34 cast $a, $wiz; 35 $a = 2; # "now set to 2!" 36 } # "destroyed!" 37 38 { # A hash with a default value 39 my $wiz = wizard data => sub { $_[1] }, 40 fetch => sub { $_[2] = $_[1] unless exists $_[0]->{$_[2]}; () }, 41 store => sub { print "key $_[2] stored in $_[-1]\n" }, 42 copy_key => 1, 43 op_info => VMG_OP_INFO_NAME; 44 45 my %h = (_default => 0, apple => 2); 46 cast %h, $wiz, '_default'; 47 print $h{banana}, "\n"; # "0", because the 'banana' key doesn't exist in %h 48 $h{pear} = 1; # "key pear stored in helem" 49 } 50 51=head1 DESCRIPTION 52 53Magic is Perl's way of enhancing variables. 54This mechanism lets the user add extra data to any variable and hook syntactical operations (such as access, assignment or destruction) that can be applied to it. 55With this module, you can add your own magic to any variable without having to write a single line of XS. 56 57You'll realize that these magic variables look a lot like tied variables. 58It's not surprising, as tied variables are implemented as a special kind of magic, just like any 'irregular' Perl variable : scalars like C<$!>, C<$(> or C<$^W>, the C<%ENV> and C<%SIG> hashes, the C<@ISA> array, C<vec()> and C<substr()> lvalues, L<threads::shared> variables... 59They all share the same underlying C API, and this module gives you direct access to it. 60 61Still, the magic made available by this module differs from tieing and overloading in several ways : 62 63=over 4 64 65=item * 66 67It isn't copied on assignment. 68 69You attach it to variables, not values (as for blessed references). 70 71=item * 72 73It doesn't replace the original semantics. 74 75Magic callbacks usually get triggered before the original action takes place, and can't prevent it from happening. 76This also makes catching individual events easier than with C<tie>, where you have to provide fallbacks methods for all actions by usually inheriting from the correct C<Tie::Std*> class and overriding individual methods in your own class. 77 78=item * 79 80It's type-agnostic. 81 82The same magic can be applied on scalars, arrays, hashes, subs or globs. 83But the same hook (see below for a list) may trigger differently depending on the the type of the variable. 84 85=item * 86 87It's mostly invisible at the Perl level. 88 89Magical and non-magical variables cannot be distinguished with C<ref>, C<tied> or another trick. 90 91=item * 92 93It's notably faster. 94 95Mainly because perl's way of handling magic is lighter by nature, and because there's no need for any method resolution. 96Also, since you don't have to reimplement all the variable semantics, you only pay for what you actually use. 97 98=back 99 100The operations that can be overloaded are : 101 102=over 4 103 104=item * 105 106C<get> 107 108This magic is invoked when the variable is evaluated. 109It is never called for arrays and hashes. 110 111=item * 112 113C<set> 114 115This one is triggered each time the value of the variable changes. 116It is called for array subscripts and slices, but never for hashes. 117 118=item * 119 120C<len> 121 122This magic is a little special : it is called when the 'size' or the 'length' of the variable has to be known by Perl. 123Typically, it's the magic involved when an array is evaluated in scalar context, but also on array assignment and loops (C<for>, C<map> or C<grep>). 124The callback has then to return the length as an integer. 125 126=item * 127 128C<clear> 129 130This magic is invoked when the variable is reset, such as when an array is emptied. 131Please note that this is different from undefining the variable, even though the magic is called when the clearing is a result of the undefine (e.g. for an array, but actually a bug prevent it to work before perl 5.9.5 - see the L<history|/PERL MAGIC HISTORY>). 132 133=item * 134 135C<free> 136 137This one can be considered as an object destructor. 138It happens when the variable goes out of scope, but not when it is undefined. 139 140=item * 141 142C<copy> 143 144This magic only applies to tied arrays and hashes. 145It fires when you try to access or change their elements. 146It is available on your perl iff C<MGf_COPY> is true. 147 148=item * 149 150C<dup> 151 152Invoked when the variable is cloned across threads. 153Currently not available. 154 155=item * 156 157C<local> 158 159When this magic is set on a variable, all subsequent localizations of the variable will trigger the callback. 160It is available on your perl iff C<MGf_LOCAL> is true. 161 162=back 163 164The following actions only apply to hashes and are available iff C<VMG_UVAR> is true. 165They are referred to as C<uvar> magics. 166 167=over 4 168 169=item * 170 171C<fetch> 172 173This magic happens each time an element is fetched from the hash. 174 175=item * 176 177C<store> 178 179This one is called when an element is stored into the hash. 180 181=item * 182 183C<exists> 184 185This magic fires when a key is tested for existence in the hash. 186 187=item * 188 189C<delete> 190 191This last one triggers when a key is deleted in the hash, regardless of whether the key actually exists in it. 192 193=back 194 195You can refer to the tests to have more insight of where the different magics are invoked. 196 197To prevent any clash between different magics defined with this module, an unique numerical signature is attached to each kind of magic (i.e. each set of callbacks for magic operations). 198At the C level, magic tokens owned by magic created by this module have their C<< mg->mg_private >> field set to C<0x3891> or C<0x3892>, so please don't use these magic (sic) numbers in other extensions. 199 200=head1 FUNCTIONS 201 202=cut 203 204BEGIN { 205 require XSLoader; 206 XSLoader::load(__PACKAGE__, $VERSION); 207} 208 209=head2 C<wizard> 210 211 wizard data => sub { ... }, 212 get => sub { my ($ref, $data [, $op]) = @_; ... }, 213 set => sub { my ($ref, $data [, $op]) = @_; ... }, 214 len => sub { my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen; }, 215 clear => sub { my ($ref, $data [, $op]) = @_; ... }, 216 free => sub { my ($ref, $data [, $op]) = @_, ... }, 217 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... }, 218 local => sub { my ($ref, $data [, $op]) = @_; ... }, 219 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... }, 220 store => sub { my ($ref, $data, $key [, $op]) = @_; ... }, 221 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... }, 222 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... }, 223 copy_key => $bool, 224 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ] 225 226This function creates a 'wizard', an opaque type that holds the magic information. 227It takes a list of keys / values as argument, whose keys can be : 228 229=over 4 230 231=item * 232 233C<data> 234 235A code (or string) reference to a private data constructor. 236It is called each time this magic is cast on a variable, and the scalar returned is used as private data storage for it. 237C<$_[0]> is a reference to the magic object and C<@_[1 .. @_-1]> are all extra arguments that were passed to L</cast>. 238 239=item * 240 241C<get>, C<set>, C<len>, C<clear>, C<free>, C<copy>, C<local>, C<fetch>, C<store>, C<exists> and C<delete> 242 243Code (or string) references to the corresponding magic callbacks. 244You don't have to specify all of them : the magic associated with undefined entries simply won't be hooked. 245In those callbacks, C<$_[0]> is always a reference to the magic object and C<$_[1]> is always the private data (or C<undef> when no private data constructor was supplied). 246 247Moreover, when you pass C<< op_info => $num >> to C<wizard>, the last element of C<@_> will be the current op name if C<$num == VMG_OP_INFO_NAME> and a C<B::OP> object representing the current op if C<$num == VMG_OP_INFO_OBJECT>. 248Both have a performance hit, but just getting the name is lighter than getting the op object. 249 250Other arguments are specific to the magic hooked : 251 252=over 8 253 254=item * 255 256C<len> 257 258When the variable is an array or a scalar, C<$_[2]> contains the non-magical length. 259The callback can return the new scalar or array length to use, or C<undef> to default to the normal length. 260 261=item * 262 263C<copy> 264 265C<$_[2]> is a either a copy or an alias of the current key, which means that it is useless to try to change or cast magic on it. 266C<$_[3]> is an alias to the current element (i.e. the value). 267 268=item * 269 270C<fetch>, C<store>, C<exists> and C<delete> 271 272C<$_[2]> is an alias to the current key. 273Nothing prevents you from changing it, but be aware that there lurk dangerous side effects. 274For example, it may rightfully be readonly if the key was a bareword. 275You can get a copy instead by passing C<< copy_key => 1 >> to L</wizard>, which allows you to safely assign to C<$_[2]> in order to e.g. redirect the action to another key. 276This however has a little performance drawback because of the copy. 277 278=back 279 280All the callbacks are expected to return an integer, which is passed straight to the perl magic API. 281However, only the return value of the C<len> callback currently holds a meaning. 282 283=back 284 285Each callback can be specified as a code or a string reference, in which case the function denoted by the string will be used as the callback. 286 287Note that C<free> callbacks are I<never> called during global destruction, as there's no way to ensure that the wizard and the C<free> callback weren't destroyed before the variable. 288 289Here's a simple usage example : 290 291 # A simple scalar tracer 292 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" }, 293 set => sub { print STDERR "set to ${$_[0]}\n" }, 294 free => sub { print STDERR "${$_[0]} was deleted\n" } 295 296=cut 297 298sub wizard { 299 croak 'Wrong number of arguments for wizard()' if @_ % 2; 300 my %opts = @_; 301 my @keys = qw/data op_info get set len clear free/; 302 push @keys, 'copy' if MGf_COPY; 303 push @keys, 'dup' if MGf_DUP; 304 push @keys, 'local' if MGf_LOCAL; 305 push @keys, qw/fetch store exists delete copy_key/ if VMG_UVAR; 306 my $ret = eval { _wizard(map $opts{$_}, @keys) }; 307 if (my $err = $@) { 308 $err =~ s/\sat\s+.*?\n//; 309 croak $err; 310 } 311 return $ret; 312} 313 314=head2 C<cast> 315 316 cast [$@%&*]var, $wiz, ... 317 318This function associates C<$wiz> magic to the variable supplied, without overwriting any other kind of magic. 319It returns true on success or when C<$wiz> magic is already present, and croaks on error. 320All extra arguments specified after C<$wiz> are passed to the private data constructor in C<@_[1 .. @_-1]>. 321If the variable isn't a hash, any C<uvar> callback of the wizard is safely ignored. 322 323 # Casts $wiz onto $x, and pass '1' to the data constructor. 324 my $x; 325 cast $x, $wiz, 1; 326 327The C<var> argument can be an array or hash value. 328Magic for those behaves like for any other scalar, except that it is dispelled when the entry is deleted from the container. 329For example, if you want to call C<POSIX::tzset> each time the C<'TZ'> environment variable is changed in C<%ENV>, you can use : 330 331 use POSIX; 332 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () }; 333 334If you want to overcome the possible deletion of the C<'TZ'> entry, you have no choice but to rely on C<store> uvar magic. 335 336=head2 C<getdata> 337 338 getdata [$@%&*]var, $wiz 339 340This accessor fetches the private data associated with the magic C<$wiz> in the variable. 341It croaks when C<$wiz> do not represent a valid magic object, and returns an empty list if no such magic is attached to the variable or when the wizard has no data constructor. 342 343 # Get the attached data, or undef if the wizard does not attach any. 344 my $data = getdata $x, $wiz; 345 346=head2 C<dispell> 347 348 dispell [$@%&*]variable, $wiz 349 350The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable. 351This function returns true on success, C<0> when no magic represented by C<$wiz> could be found in the variable, and croaks if the supplied wizard is invalid. 352 353 # Dispell now. 354 die 'no such magic in $x' unless dispell $x, $wiz; 355 356=head1 CONSTANTS 357 358=head2 C<MGf_COPY> 359 360Evaluates to true iff the 'copy' magic is available. 361 362=head2 C<MGf_DUP> 363 364Evaluates to true iff the 'dup' magic is available. 365 366=head2 C<MGf_LOCAL> 367 368Evaluates to true iff the 'local' magic is available. 369 370=head2 C<VMG_UVAR> 371 372When this constant is true, you can use the C<fetch,store,exists,delete> callbacks on hashes. 373 374=head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN> 375 376True for perls that don't call 'len' magic when you push an element in a magical array. 377Starting from perl 5.11.0, this only refers to pushes in non-void context and hence is false. 378 379=head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID> 380 381True for perls that don't call 'len' magic when you push in void context an element in a magical array. 382 383=head2 C<VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID> 384 385True for perls that don't call 'len' magic when you unshift in void context an element in a magical array. 386 387=head2 C<VMG_COMPAT_ARRAY_UNDEF_CLEAR> 388 389True for perls that call 'clear' magic when undefining magical arrays. 390 391=head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN> 392 393True for perls that don't call 'len' magic when taking the C<length> of a magical scalar. 394 395=head2 C<VMG_COMPAT_GLOB_GET> 396 397True for perls that call 'get' magic for operations on globs. 398 399=head2 C<VMG_PERL_PATCHLEVEL> 400 401The perl patchlevel this module was built with, or C<0> for non-debugging perls. 402 403=head2 C<VMG_THREADSAFE> 404 405True iff this module could have been built with thread-safety features enabled. 406 407=head2 C<VMG_FORKSAFE> 408 409True iff this module could have been built with fork-safety features enabled. 410This will always be true except on Windows where it's false for perl 5.10.0 and below . 411 412=head2 C<VMG_OP_INFO_NAME> 413 414Value to pass with C<op_info> to get the current op name in the magic callbacks. 415 416=head2 C<VMG_OP_INFO_OBJECT> 417 418Value to pass with C<op_info> to get a C<B::OP> object representing the current op in the magic callbacks. 419 420=head1 COOKBOOK 421 422=head2 Associate an object to any perl variable 423 424This technique can be useful for passing user data through limited APIs. 425It is similar to using inside-out objects, but without the drawback of having to implement a complex destructor. 426 427 { 428 package Magical::UserData; 429 430 use Variable::Magic qw/wizard cast getdata/; 431 432 my $wiz = wizard data => sub { \$_[1] }; 433 434 sub ud (\[$@%*&]) : lvalue { 435 my ($var) = @_; 436 my $data = &getdata($var, $wiz); 437 unless (defined $data) { 438 $data = \(my $slot); 439 &cast($var, $wiz, $slot) 440 or die "Couldn't cast UserData magic onto the variable"; 441 } 442 $$data; 443 } 444 } 445 446 { 447 BEGIN { *ud = \&Magical::UserData::ud } 448 449 my $cb; 450 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" }; 451 452 ud(&$cb) = 'world'; 453 $cb->(); # Hello, world! 454 } 455 456=head2 Recursively cast magic on datastructures 457 458C<cast> can be called from any magical callback, and in particular from C<data>. 459This allows you to recursively cast magic on datastructures : 460 461 my $wiz; 462 $wiz = wizard data => sub { 463 my ($var, $depth) = @_; 464 $depth ||= 0; 465 my $r = ref $var; 466 if ($r eq 'ARRAY') { 467 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var; 468 } elsif ($r eq 'HASH') { 469 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var; 470 } 471 return $depth; 472 }, 473 free => sub { 474 my ($var, $depth) = @_; 475 my $r = ref $var; 476 print "free $r at depth $depth\n"; 477 (); 478 }; 479 480 { 481 my %h = ( 482 a => [ 1, 2 ], 483 b => { c => 3 } 484 ); 485 cast %h, $wiz; 486 } 487 488When C<%h> goes out of scope, this will print something among the lines of : 489 490 free HASH at depth 0 491 free HASH at depth 1 492 free SCALAR at depth 2 493 free ARRAY at depth 1 494 free SCALAR at depth 3 495 free SCALAR at depth 3 496 497Of course, this example does nothing with the values that are added after the C<cast>. 498 499=head1 PERL MAGIC HISTORY 500 501The places where magic is invoked have changed a bit through perl history. 502Here's a little list of the most recent ones. 503 504=over 4 505 506=item * 507 508B<5.6.x> 509 510I<p14416> : 'copy' and 'dup' magic. 511 512=item * 513 514B<5.8.9> 515 516I<p28160> : Integration of I<p25854> (see below). 517 518I<p32542> : Integration of I<p31473> (see below). 519 520=item * 521 522B<5.9.3> 523 524I<p25854> : 'len' magic is no longer called when pushing an element into a magic array. 525 526I<p26569> : 'local' magic. 527 528=item * 529 530B<5.9.5> 531 532I<p31064> : Meaningful 'uvar' magic. 533 534I<p31473> : 'clear' magic wasn't invoked when undefining an array. 535The bug is fixed as of this version. 536 537=item * 538 539B<5.10.0> 540 541Since C<PERL_MAGIC_uvar> is uppercased, C<hv_magic_check()> triggers 'copy' magic on hash stores for (non-tied) hashes that also have 'uvar' magic. 542 543=item * 544 545B<5.11.x> 546 547I<p32969> : 'len' magic is no longer invoked when calling C<length> with a magical scalar. 548 549I<p34908> : 'len' magic is no longer called when pushing / unshifting an element into a magical array in void context. 550The C<push> part was already covered by I<p25854>. 551 552I<g9cdcb38b> : 'len' magic is called again when pushing into a magical array in non-void context. 553 554=back 555 556=head1 EXPORT 557 558The functions L</wizard>, L</cast>, L</getdata> and L</dispell> are only exported on request. 559All of them are exported by the tags C<':funcs'> and C<':all'>. 560 561All the constants are also only exported on request, either individually or by the tags C<':consts'> and C<':all'>. 562 563=cut 564 565use base qw/Exporter/; 566 567our @EXPORT = (); 568our %EXPORT_TAGS = ( 569 'funcs' => [ qw/wizard cast getdata dispell/ ], 570 'consts' => [ qw/ 571 MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR 572 VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID 573 VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID 574 VMG_COMPAT_ARRAY_UNDEF_CLEAR 575 VMG_COMPAT_SCALAR_LENGTH_NOLEN 576 VMG_COMPAT_GLOB_GET 577 VMG_PERL_PATCHLEVEL 578 VMG_THREADSAFE VMG_FORKSAFE 579 VMG_OP_INFO_NAME VMG_OP_INFO_OBJECT 580 / ], 581); 582our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS; 583$EXPORT_TAGS{'all'} = [ @EXPORT_OK ]; 584 585=head1 CAVEATS 586 587If you store a magic object in the private data slot, the magic won't be accessible by L</getdata> since it's not copied by assignment. 588The only way to address this would be to return a reference. 589 590If you define a wizard with a C<free> callback and cast it on itself, this destructor won't be called because the wizard will be destroyed first. 591 592=head1 DEPENDENCIES 593 594L<perl> 5.8. 595 596L<Carp> (standard since perl 5), L<XSLoader> (standard since perl 5.006). 597 598Copy tests need L<Tie::Array> (standard since perl 5.005) and L<Tie::Hash> (since 5.002). 599 600Some uvar tests need L<Hash::Util::FieldHash> (standard since perl 5.009004). 601 602Glob tests need L<Symbol> (standard since perl 5.002). 603 604Threads tests need L<threads> and L<threads::shared>. 605 606=head1 SEE ALSO 607 608L<perlguts> and L<perlapi> for internal information about magic. 609 610L<perltie> and L<overload> for other ways of enhancing objects. 611 612=head1 AUTHOR 613 614Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>. 615 616You can contact me by mail or on C<irc.perl.org> (vincent). 617 618=head1 BUGS 619 620Please report any bugs or feature requests to C<bug-variable-magic at rt.cpan.org>, or through the web interface at L<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes. 621 622=head1 SUPPORT 623 624You can find documentation for this module with the perldoc command. 625 626 perldoc Variable::Magic 627 628Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>. 629 630=head1 COPYRIGHT & LICENSE 631 632Copyright 2007,2008,2009,2010 Vincent Pit, all rights reserved. 633 634This program is free software; you can redistribute it and/or modify it 635under the same terms as Perl itself. 636 637=cut 638 6391; # End of Variable::Magic 640