1 package Variable::Magic;
10 Variable::Magic - Associate user-defined magic to variables from Perl.
25 use Variable::Magic qw<wizard cast VMG_OP_INFO_NAME>;
29 set => sub { print "now set to ${$_[0]}!\n" },
30 free => sub { print "destroyed!\n" },
35 $a = 2; # "now set to 2!"
38 { # A hash with a default value
40 data => sub { $_[1] },
41 fetch => sub { $_[2] = $_[1] unless exists $_[0]->{$_[2]}; () },
42 store => sub { print "key $_[2] stored in $_[-1]\n" },
44 op_info => VMG_OP_INFO_NAME,
47 my %h = (_default => 0, apple => 2);
48 cast %h, $wiz, '_default';
49 print $h{banana}, "\n"; # "0" (there is no 'banana' key in %h)
50 $h{pear} = 1; # "key pear stored in helem"
55 Magic is Perl's way of enhancing variables.
56 This mechanism lets the user add extra data to any variable and hook syntactical operations (such as access, assignment or destruction) that can be applied to it.
57 With this module, you can add your own magic to any variable without having to write a single line of XS.
59 You'll realize that these magic variables look a lot like tied variables.
60 It is not surprising, as tied variables are implemented as a special kind of magic, just like any 'irregular' Perl variable : scalars like C<$!>, 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...
61 They all share the same underlying C API, and this module gives you direct access to it.
63 Still, the magic made available by this module differs from tieing and overloading in several ways :
69 Magic is not copied on assignment.
71 You attach it to variables, not values (as for blessed references).
75 Magic does not replace the original semantics.
77 Magic callbacks usually get triggered before the original action takes place, and cannot prevent it from happening.
78 This 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.
82 Magic is type-agnostic.
84 The same magic can be applied on scalars, arrays, hashes, subs or globs.
85 But the same hook (see below for a list) may trigger differently depending on the the type of the variable.
89 Magic is invisible at the Perl level.
91 Magical and non-magical variables cannot be distinguished with C<ref>, C<tied> or another trick.
95 Magic is notably faster.
97 Mainly because perl's way of handling magic is lighter by nature, and because there is no need for any method resolution.
98 Also, since you don't have to reimplement all the variable semantics, you only pay for what you actually use.
102 The operations that can be overloaded are :
110 This magic is invoked when the variable is evaluated.
111 It is never called for arrays and hashes.
117 This magic is called each time the value of the variable changes.
118 It is called for array subscripts and slices, but never for hashes.
124 This magic only applies to scalars and arrays, and is triggered when the 'size' or the 'length' of the variable has to be known by Perl.
125 This is typically 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>).
126 The length is returned from the callback as an integer.
132 This magic is invoked when the variable is reset, such as when an array is emptied.
133 Please 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>).
139 This magic is called when an object is destroyed as the result of the variable going out of scope (but not when the variable is undefined).
145 This magic only applies to tied arrays and hashes, and fires when you try to access or change their elements.
146 It is available on your perl if and only if C<MGf_COPY> is true.
152 This magic is invoked when the variable is cloned across threads.
153 It is currently not available.
159 When this magic is set on a variable, all subsequent localizations of the variable will trigger the callback.
160 It is available on your perl if and only if C<MGf_LOCAL> is true.
164 The following actions only apply to hashes and are available if and only if L</VMG_UVAR> is true.
165 They are referred to as C<uvar> magics.
173 This magic is invoked each time an element is fetched from the hash.
179 This one is called when an element is stored into the hash.
185 This magic fires when a key is tested for existence in the hash.
191 This magic is triggered when a key is deleted in the hash, regardless of whether the key actually exists in it.
195 You can refer to the tests to have more insight of where the different magics are invoked.
203 XSLoader::load(__PACKAGE__, $VERSION);
210 get => sub { my ($ref, $data [, $op]) = @_; ... },
211 set => sub { my ($ref, $data [, $op]) = @_; ... },
213 my ($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 [, $op]) = @_; ... },
224 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
227 This function creates a 'wizard', an opaque object that holds the magic information.
228 It takes a list of keys / values as argument, whose keys can be :
236 A code (or string) reference to a private data constructor.
237 It is called in scalar context each time the magic is cast onto a variable, with C<$_[0]> being a reference to this variable and C<@_[1 .. @_-1]> being all extra arguments that were passed to L</cast>.
238 The scalar returned from this call is then attached to the variable and can be retrieved later with L</getdata>.
242 C<get>, C<set>, C<len>, C<clear>, C<free>, C<copy>, C<local>, C<fetch>, C<store>, C<exists> and C<delete>
244 Code (or string) references to the respective magic callbacks.
245 You don't have to specify all of them : the magic corresponding to undefined entries will simply not be hooked.
247 When those callbacks are executed, C<$_[0]> is a reference to the magic variable and C<$_[1]> is the associated private data (or C<undef> when no private data constructor is supplied with the wizard).
248 Other arguments depend on which kind of magic is involved :
256 C<$_[2]> contains the natural, non-magical length of the variable (which can only be a scalar or an array as len magic is only relevant for these types).
257 The callback is expected to return the new scalar or array length to use, or C<undef> to default to the normal length.
263 C<$_[2]> is a either an alias or a copy of the current key, and C<$_[3]> is an alias to the current element (i.e. the value).
264 Because C<$_[2]> might be a copy, it is useless to try to change it or cast magic on it.
268 C<fetch>, C<store>, C<exists> and C<delete>
270 C<$_[2]> is an alias to the current key.
271 Note that C<$_[2]> may rightfully be readonly if the key comes from a bareword, and as such it is unsafe to assign to it.
272 You can ask for a copy instead by passing C<< copy_key => 1 >> to L</wizard> which, at the price of a small performance hit, allows you to safely assign to C<$_[2]> in order to e.g. redirect the action to another key.
276 Finally, if C<< op_info => $num >> is also passed to C<wizard>, then one extra element is appended to C<@_>.
277 Its nature depends on the value of C<$num> :
285 C<$_[-1]> is the current op name.
289 C<VMG_OP_INFO_OBJECT>
291 C<$_[-1]> is the C<B::OP> object for the current op.
295 Both result in a small performance hit, but just getting the name is lighter than getting the op object.
297 These callbacks are executed in scalar context and are expected to return an integer, which is then passed straight to the perl magic API.
298 However, only the return value of the C<len> callback currently holds a meaning.
302 Each callback can be specified as :
308 a code reference, which will be called as a subroutine.
312 a string reference, where the string denotes which subroutine is to be called when magic is triggered.
313 If the subroutine name is not fully qualified, then the current package at the time the magic is invoked will be used instead.
317 a reference to C<undef>, in which case a no-op magic callback is installed instead of the default one.
318 This may especially be helpful for 'local' magic, where an empty callback prevents magic from being copied during localization.
322 Note that C<free> callbacks are I<never> called during global destruction, as there is no way to ensure that the wizard object and the C<free> callback were not destroyed before the variable.
324 Here is a simple usage example :
326 # A simple scalar tracer
328 get => sub { print STDERR "got ${$_[0]}\n" },
329 set => sub { print STDERR "set to ${$_[0]}\n" },
330 free => sub { print STDERR "${$_[0]} was deleted\n" },
338 Carp::croak('Wrong number of arguments for wizard()');
343 my @keys = qw<op_info data get set len clear free copy dup>;
344 push @keys, 'local' if MGf_LOCAL;
345 push @keys, qw<fetch store exists delete copy_key> if VMG_UVAR;
350 $wiz = eval { _wizard(map $opts{$_}, @keys) };
354 $err =~ s/\sat\s+.*?\n//;
364 cast [$@%&*]var, $wiz, @args
366 This function associates C<$wiz> magic to the supplied variable, without overwriting any other kind of magic.
367 It returns true on success or when C<$wiz> magic is already attached, and croaks on error.
368 When C<$wiz> provides a data constructor, it is called just before magic is cast onto the variable, and it receives a reference to the target variable in C<$_[0]> and the content of C<@args> in C<@_[1 .. @args]>.
369 Otherwise, C<@args> is ignored.
371 # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
375 The C<var> argument can be an array or hash value.
376 Magic for these scalars behaves like for any other, except that it is dispelled when the entry is deleted from the container.
377 For example, if you want to call C<POSIX::tzset> each time the C<'TZ'> environment variable is changed in C<%ENV>, you can use :
380 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
382 If you want to handle the possible deletion of the C<'TZ'> entry, you must also specify C<store> uvar magic.
386 getdata [$@%&*]var, $wiz
388 This accessor fetches the private data associated with the magic C<$wiz> in the variable.
389 It croaks when C<$wiz> does 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.
391 # Get the data attached to $wiz in $x, or undef if $wiz
392 # did not attach any.
393 my $data = getdata $x, $wiz;
397 dispell [$@%&*]variable, $wiz
399 The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable.
400 This 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.
403 die 'no such magic in $x' unless dispell $x, $wiz;
409 Evaluates to true if and only if the 'copy' magic is available.
413 Evaluates to true if and only if the 'dup' magic is available.
417 Evaluates to true if and only if the 'local' magic is available.
421 When this constant is true, you can use the C<fetch,store,exists,delete> callbacks on hashes.
422 Initial VMG_UVAR capability was introduced in perl 5.9.5, with a fully functional implementation shipped with perl 5.10.0.
424 =head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN>
426 True for perls that don't call 'len' magic when taking the C<length> of a magical scalar.
428 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN>
430 True for perls that don't call 'len' magic when you push an element in a magical array.
431 Starting from perl 5.11.0, this only refers to pushes in non-void context and hence is false.
433 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID>
435 True for perls that don't call 'len' magic when you push in void context an element in a magical array.
437 =head2 C<VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID>
439 True for perls that don't call 'len' magic when you unshift in void context an element in a magical array.
441 =head2 C<VMG_COMPAT_ARRAY_UNDEF_CLEAR>
443 True for perls that call 'clear' magic when undefining magical arrays.
445 =head2 C<VMG_COMPAT_HASH_DELETE_NOUVAR_VOID>
447 True for perls that don't call 'delete' uvar magic when you delete an element from a hash in void context.
449 =head2 C<VMG_COMPAT_GLOB_GET>
451 True for perls that call 'get' magic for operations on globs.
453 =head2 C<VMG_PERL_PATCHLEVEL>
455 The perl patchlevel this module was built with, or C<0> for non-debugging perls.
457 =head2 C<VMG_THREADSAFE>
459 True if and only if this module could have been built with thread-safety features enabled.
461 =head2 C<VMG_FORKSAFE>
463 True if and only if this module could have been built with fork-safety features enabled.
464 This is always true except on Windows where it is false for perl 5.10.0 and below.
466 =head2 C<VMG_OP_INFO_NAME>
468 Value to pass with C<op_info> to get the current op name in the magic callbacks.
470 =head2 C<VMG_OP_INFO_OBJECT>
472 Value to pass with C<op_info> to get a C<B::OP> object representing the current op in the magic callbacks.
476 =head2 Associate an object to any perl variable
478 This technique can be useful for passing user data through limited APIs.
479 It is similar to using inside-out objects, but without the drawback of having to implement a complex destructor.
482 package Magical::UserData;
484 use Variable::Magic qw<wizard cast getdata>;
486 my $wiz = wizard data => sub { \$_[1] };
488 sub ud (\[$@%*&]) : lvalue {
490 my $data = &getdata($var, $wiz);
491 unless (defined $data) {
493 &cast($var, $wiz, $slot)
494 or die "Couldn't cast UserData magic onto the variable";
501 BEGIN { *ud = \&Magical::UserData::ud }
504 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
507 $cb->(); # Hello, world!
510 =head2 Recursively cast magic on datastructures
512 C<cast> can be called from any magical callback, and in particular from C<data>.
513 This allows you to recursively cast magic on datastructures :
516 $wiz = wizard data => sub {
517 my ($var, $depth) = @_;
521 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
522 } elsif ($r eq 'HASH') {
523 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
528 my ($var, $depth) = @_;
530 print "free $r at depth $depth\n";
542 When C<%h> goes out of scope, this prints something among the lines of :
546 free SCALAR at depth 2
547 free ARRAY at depth 1
548 free SCALAR at depth 3
549 free SCALAR at depth 3
551 Of course, this example does nothing with the values that are added after the C<cast>.
553 =head1 PERL MAGIC HISTORY
555 The places where magic is invoked have changed a bit through perl history.
556 Here is a little list of the most recent ones.
564 I<p14416> : 'copy' and 'dup' magic.
570 I<p28160> : Integration of I<p25854> (see below).
572 I<p32542> : Integration of I<p31473> (see below).
578 I<p25854> : 'len' magic is no longer called when pushing an element into a magic array.
580 I<p26569> : 'local' magic.
586 I<p31064> : Meaningful 'uvar' magic.
588 I<p31473> : 'clear' magic was not invoked when undefining an array.
589 The bug is fixed as of this version.
595 Since 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.
601 I<p32969> : 'len' magic is no longer invoked when calling C<length> with a magical scalar.
603 I<p34908> : 'len' magic is no longer called when pushing / unshifting an element into a magical array in void context.
604 The C<push> part was already covered by I<p25854>.
606 I<g9cdcb38b> : 'len' magic is called again when pushing into a magical array in non-void context.
612 The functions L</wizard>, L</cast>, L</getdata> and L</dispell> are only exported on request.
613 All of them are exported by the tags C<':funcs'> and C<':all'>.
615 All the constants are also only exported on request, either individually or by the tags C<':consts'> and C<':all'>.
619 use base qw<Exporter>;
623 'funcs' => [ qw<wizard cast getdata dispell> ],
625 MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR
626 VMG_COMPAT_SCALAR_LENGTH_NOLEN
627 VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID
628 VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID
629 VMG_COMPAT_ARRAY_UNDEF_CLEAR
630 VMG_COMPAT_HASH_DELETE_NOUVAR_VOID
633 VMG_THREADSAFE VMG_FORKSAFE
634 VMG_OP_INFO_NAME VMG_OP_INFO_OBJECT
637 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
638 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
642 In order to hook hash operations with magic, you need at least perl 5.10.0 (see L</VMG_UVAR>).
644 If you want to store a magic object in the private data slot, you will not be able to recover the magic with L</getdata>, since magic is not copied by assignment.
645 You can work around this gotcha by storing a reference to the magic object instead.
647 If you define a wizard with a C<free> callback and cast it on itself, it results in a memory cycle, so this destructor will not be called when the wizard is freed.
654 This module may happen to build with a C++ compiler as well, but don't rely on it, as no guarantee is made in this regard.
656 L<Carp> (core since perl 5), L<XSLoader> (since 5.006).
658 Copy tests need L<Tie::Array> (core since perl 5.005) and L<Tie::Hash> (since 5.002).
659 Some uvar tests need L<Hash::Util::FieldHash> (since 5.009004).
660 Glob tests need L<Symbol> (since 5.002).
661 Threads tests need L<threads> and L<threads::shared> (both since 5.007003).
665 L<perlguts> and L<perlapi> for internal information about magic.
667 L<perltie> and L<overload> for other ways of enhancing objects.
671 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
673 You can contact me by mail or on C<irc.perl.org> (vincent).
677 Please 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>.
678 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
682 You can find documentation for this module with the perldoc command.
684 perldoc Variable::Magic
686 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>.
688 =head1 COPYRIGHT & LICENSE
690 Copyright 2007,2008,2009,2010,2011,2012 Vincent Pit, all rights reserved.
692 This program is free software; you can redistribute it and/or modify it
693 under the same terms as Perl itself.
697 1; # End of Variable::Magic