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.
151 This magic is invoked when the variable is cloned across threads.
152 It is currently not available.
158 When this magic is set on a variable, all subsequent localizations of the variable will trigger the callback.
159 It is available on your perl if and only if C<MGf_LOCAL> is true.
163 The following actions only apply to hashes and are available if and only if L</VMG_UVAR> is true.
164 They are referred to as I<uvar> magics.
172 This magic is invoked each time an element is fetched from the hash.
178 This one is called when an element is stored into the hash.
184 This magic fires when a key is tested for existence in the hash.
190 This magic is triggered when a key is deleted in the hash, regardless of whether the key actually exists in it.
194 You can refer to the tests to have more insight of where the different magics are invoked.
202 XSLoader::load(__PACKAGE__, $VERSION);
209 get => sub { my ($ref, $data [, $op]) = @_; ... },
210 set => sub { my ($ref, $data [, $op]) = @_; ... },
212 my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen
214 clear => sub { my ($ref, $data [, $op]) = @_; ... },
215 free => sub { my ($ref, $data [, $op]) = @_, ... },
216 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
217 local => sub { my ($ref, $data [, $op]) = @_; ... },
218 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
219 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
220 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
221 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
223 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
226 This function creates a 'wizard', an opaque object that holds the magic information.
227 It takes a list of keys / values as argument, whose keys can be :
235 A code (or string) reference to a private data constructor.
236 It is called in scalar context each time the magic is cast onto a variable, with C<$_[0]> being a reference to this variable and C<@_[1 .. @_-1]> being all extra arguments that were passed to L</cast>.
237 The scalar returned from this call is then attached to the variable and can be retrieved later with L</getdata>.
241 C<get>, C<set>, C<len>, C<clear>, C<free>, C<copy>, C<local>, C<fetch>, C<store>, C<exists> and C<delete>
243 Code (or string) references to the respective magic callbacks.
244 You don't have to specify all of them : the magic corresponding to undefined entries will simply not be hooked.
246 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).
247 Other arguments depend on which kind of magic is involved :
255 C<$_[2]> contains the natural, non-magical length of the variable (which can only be a scalar or an array as I<len> magic is only relevant for these types).
256 The callback is expected to return the new scalar or array length to use, or C<undef> to default to the normal length.
262 C<$_[2]> is a either an alias or a copy of the current key, and C<$_[3]> is an alias to the current element (i.e. the value).
263 Because C<$_[2]> might be a copy, it is useless to try to change it or cast magic on it.
267 I<fetch>, I<store>, I<exists> and I<delete>
269 C<$_[2]> is an alias to the current key.
270 Note that C<$_[2]> may rightfully be readonly if the key comes from a bareword, and as such it is unsafe to assign to it.
271 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.
275 Finally, if C<< op_info => $num >> is also passed to C<wizard>, then one extra element is appended to C<@_>.
276 Its nature depends on the value of C<$num> :
284 C<$_[-1]> is the current op name.
288 C<VMG_OP_INFO_OBJECT>
290 C<$_[-1]> is the C<B::OP> object for the current op.
294 Both result in a small performance hit, but just getting the name is lighter than getting the op object.
296 These callbacks are executed in scalar context and are expected to return an integer, which is then passed straight to the perl magic API.
297 However, only the return value of the I<len> magic callback currently holds a meaning.
301 Each callback can be specified as :
307 a code reference, which will be called as a subroutine.
311 a string reference, where the string denotes which subroutine is to be called when magic is triggered.
312 If the subroutine name is not fully qualified, then the current package at the time the magic is invoked will be used instead.
316 a reference to C<undef>, in which case a no-op magic callback is installed instead of the default one.
317 This may especially be helpful for I<local> magic, where an empty callback prevents magic from being copied during localization.
321 Note that I<free> magic is never called during global destruction, as there is no way to ensure that the wizard object and the callback were not destroyed before the variable.
323 Here is a simple usage example :
325 # A simple scalar tracer
327 get => sub { print STDERR "got ${$_[0]}\n" },
328 set => sub { print STDERR "set to ${$_[0]}\n" },
329 free => sub { print STDERR "${$_[0]} was deleted\n" },
337 Carp::croak('Wrong number of arguments for wizard()');
342 my @keys = qw<op_info data get set len clear free copy dup>;
343 push @keys, 'local' if MGf_LOCAL;
344 push @keys, qw<fetch store exists delete copy_key> if VMG_UVAR;
349 $wiz = eval { _wizard(map $opts{$_}, @keys) };
353 $err =~ s/\sat\s+.*?\n//;
363 cast [$@%&*]var, $wiz, @args
365 This function associates C<$wiz> magic to the supplied variable, without overwriting any other kind of magic.
366 It returns true on success or when C<$wiz> magic is already attached, and croaks on error.
367 When C<$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]>.
368 Otherwise, C<@args> is ignored.
370 # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
374 The C<var> argument can be an array or hash value.
375 Magic for these scalars behaves like for any other, except that it is dispelled when the entry is deleted from the container.
376 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 :
379 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
381 If you want to handle the possible deletion of the C<'TZ'> entry, you must also specify I<store> magic.
385 getdata [$@%&*]var, $wiz
387 This accessor fetches the private data associated with the magic C<$wiz> in the variable.
388 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.
390 # Get the data attached to $wiz in $x, or undef if $wiz
391 # did not attach any.
392 my $data = getdata $x, $wiz;
396 dispell [$@%&*]variable, $wiz
398 The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable.
399 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.
402 die 'no such magic in $x' unless dispell $x, $wiz;
408 Evaluates to true if and only if the I<copy> magic is available.
409 This is the case for perl 5.7.3 and greater, which is ensured by the requirements of this module.
413 Evaluates to true if and only if the I<dup> magic is available.
414 This is the case for perl 5.7.3 and greater, which is ensured by the requirements of this module.
418 Evaluates to true if and only if the I<local> magic is available.
419 This is the case for perl 5.9.3 and greater.
423 When this constant is true, you can use the I<fetch>, I<store>, I<exists> and I<delete> magics on hashes.
424 Initial L</VMG_UVAR> capability was introduced in perl 5.9.5, with a fully functional implementation shipped with perl 5.10.0.
426 =head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN>
428 True for perls that don't call I<len> magic when taking the C<length> of a magical scalar.
430 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN>
432 True for perls that don't call I<len> magic when you push an element in a magical array.
433 Starting from perl 5.11.0, this only refers to pushes in non-void context and hence is false.
435 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID>
437 True for perls that don't call I<len> magic when you push in void context an element in a magical array.
439 =head2 C<VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID>
441 True for perls that don't call I<len> magic when you unshift in void context an element in a magical array.
443 =head2 C<VMG_COMPAT_ARRAY_UNDEF_CLEAR>
445 True for perls that call I<clear> magic when undefining magical arrays.
447 =head2 C<VMG_COMPAT_HASH_DELETE_NOUVAR_VOID>
449 True for perls that don't call I<delete> magic when you delete an element from a hash in void context.
451 =head2 C<VMG_COMPAT_GLOB_GET>
453 True for perls that call I<get> magic for operations on globs.
455 =head2 C<VMG_PERL_PATCHLEVEL>
457 The perl patchlevel this module was built with, or C<0> for non-debugging perls.
459 =head2 C<VMG_THREADSAFE>
461 True if and only if this module could have been built with thread-safety features enabled.
463 =head2 C<VMG_FORKSAFE>
465 True if and only if this module could have been built with fork-safety features enabled.
466 This is always true except on Windows where it is false for perl 5.10.0 and below.
468 =head2 C<VMG_OP_INFO_NAME>
470 Value to pass with C<op_info> to get the current op name in the magic callbacks.
472 =head2 C<VMG_OP_INFO_OBJECT>
474 Value to pass with C<op_info> to get a C<B::OP> object representing the current op in the magic callbacks.
478 =head2 Associate an object to any perl variable
480 This technique can be useful for passing user data through limited APIs.
481 It is similar to using inside-out objects, but without the drawback of having to implement a complex destructor.
484 package Magical::UserData;
486 use Variable::Magic qw<wizard cast getdata>;
488 my $wiz = wizard data => sub { \$_[1] };
490 sub ud (\[$@%*&]) : lvalue {
492 my $data = &getdata($var, $wiz);
493 unless (defined $data) {
495 &cast($var, $wiz, $slot)
496 or die "Couldn't cast UserData magic onto the variable";
503 BEGIN { *ud = \&Magical::UserData::ud }
506 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
509 $cb->(); # Hello, world!
512 =head2 Recursively cast magic on datastructures
514 C<cast> can be called from any magical callback, and in particular from C<data>.
515 This allows you to recursively cast magic on datastructures :
518 $wiz = wizard data => sub {
519 my ($var, $depth) = @_;
523 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
524 } elsif ($r eq 'HASH') {
525 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
530 my ($var, $depth) = @_;
532 print "free $r at depth $depth\n";
544 When C<%h> goes out of scope, this prints something among the lines of :
548 free SCALAR at depth 2
549 free ARRAY at depth 1
550 free SCALAR at depth 3
551 free SCALAR at depth 3
553 Of course, this example does nothing with the values that are added after the C<cast>.
555 =head1 PERL MAGIC HISTORY
557 The places where magic is invoked have changed a bit through perl history.
558 Here is a little list of the most recent ones.
566 I<p14416> : I<copy> and I<dup> magic.
572 I<p28160> : Integration of I<p25854> (see below).
574 I<p32542> : Integration of I<p31473> (see below).
580 I<p25854> : I<len> magic is no longer called when pushing an element into a magic array.
582 I<p26569> : I<local> magic.
588 I<p31064> : Meaningful I<uvar> magic.
590 I<p31473> : I<clear> magic was not invoked when undefining an array.
591 The bug is fixed as of this version.
597 Since C<PERL_MAGIC_uvar> is uppercased, C<hv_magic_check()> triggers I<copy> magic on hash stores for (non-tied) hashes that also have I<uvar> magic.
603 I<p32969> : I<len> magic is no longer invoked when calling C<length> with a magical scalar.
605 I<p34908> : I<len> magic is no longer called when pushing / unshifting an element into a magical array in void context.
606 The C<push> part was already covered by I<p25854>.
608 I<g9cdcb38b> : I<len> magic is called again when pushing into a magical array in non-void context.
614 The functions L</wizard>, L</cast>, L</getdata> and L</dispell> are only exported on request.
615 All of them are exported by the tags C<':funcs'> and C<':all'>.
617 All the constants are also only exported on request, either individually or by the tags C<':consts'> and C<':all'>.
621 use base qw<Exporter>;
625 'funcs' => [ qw<wizard cast getdata dispell> ],
627 MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR
628 VMG_COMPAT_SCALAR_LENGTH_NOLEN
629 VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID
630 VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID
631 VMG_COMPAT_ARRAY_UNDEF_CLEAR
632 VMG_COMPAT_HASH_DELETE_NOUVAR_VOID
635 VMG_THREADSAFE VMG_FORKSAFE
636 VMG_OP_INFO_NAME VMG_OP_INFO_OBJECT
639 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
640 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
644 In order to hook hash operations with magic, you need at least perl 5.10.0 (see L</VMG_UVAR>).
646 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.
647 You can work around this gotcha by storing a reference to the magic object instead.
649 If you define a wizard with I<free> magic and cast it on itself, it results in a memory cycle, so this destructor will not be called when the wizard is freed.
656 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.
658 L<Carp> (core since perl 5), L<XSLoader> (since 5.006).
660 Copy tests need L<Tie::Array> (core since perl 5.005) and L<Tie::Hash> (since 5.002).
661 Some uvar tests need L<Hash::Util::FieldHash> (since 5.009004).
662 Glob tests need L<Symbol> (since 5.002).
663 Threads tests need L<threads> and L<threads::shared> (both since 5.007003).
667 L<perlguts> and L<perlapi> for internal information about magic.
669 L<perltie> and L<overload> for other ways of enhancing objects.
673 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
675 You can contact me by mail or on C<irc.perl.org> (vincent).
679 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>.
680 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
684 You can find documentation for this module with the perldoc command.
686 perldoc Variable::Magic
688 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>.
690 =head1 COPYRIGHT & LICENSE
692 Copyright 2007,2008,2009,2010,2011,2012 Vincent Pit, all rights reserved.
694 This program is free software; you can redistribute it and/or modify it
695 under the same terms as Perl itself.
699 1; # End of Variable::Magic