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.
84 You can safely apply different kinds of magics to the same variable, and each of them will be invoked successively.
88 Magic is type-agnostic.
90 The same magic can be applied on scalars, arrays, hashes, subs or globs.
91 But the same hook (see below for a list) may trigger differently depending on the the type of the variable.
95 Magic is invisible at Perl level.
97 Magical and non-magical variables cannot be distinguished with C<ref>, C<tied> or another trick.
101 Magic is notably faster.
103 Mainly because perl's way of handling magic is lighter by nature, and because there is no need for any method resolution.
104 Also, since you don't have to reimplement all the variable semantics, you only pay for what you actually use.
108 The operations that can be overloaded are :
116 This magic is invoked when the variable is evaluated.
117 It is never called for arrays and hashes.
123 This magic is called each time the value of the variable changes.
124 It is called for array subscripts and slices, but never for hashes.
130 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.
131 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>).
132 The length is returned from the callback as an integer.
138 This magic is invoked when the variable is reset, such as when an array is emptied.
139 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>).
145 This magic is called when a variable is destroyed as the result of going out of scope (but not when it is undefined).
146 It behaves roughly like Perl object destructors (i.e. C<DESTROY> methods), except that exceptions thrown from inside a I<free> callback will always be propagated to the surrounding code.
152 This magic only applies to tied arrays and hashes, and fires when you try to access or change their elements.
158 This magic is invoked when the variable is cloned across threads.
159 It is currently not available.
165 When this magic is set on a variable, all subsequent localizations of the variable will trigger the callback.
166 It is available on your perl if and only if C<MGf_LOCAL> is true.
170 The following actions only apply to hashes and are available if and only if L</VMG_UVAR> is true.
171 They are referred to as I<uvar> magics.
179 This magic is invoked each time an element is fetched from the hash.
185 This one is called when an element is stored into the hash.
191 This magic fires when a key is tested for existence in the hash.
197 This magic is triggered when a key is deleted in the hash, regardless of whether the key actually exists in it.
201 You can refer to the tests to have more insight of where the different magics are invoked.
209 XSLoader::load(__PACKAGE__, $VERSION);
216 get => sub { my ($ref, $data [, $op]) = @_; ... },
217 set => sub { my ($ref, $data [, $op]) = @_; ... },
219 my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen
221 clear => sub { my ($ref, $data [, $op]) = @_; ... },
222 free => sub { my ($ref, $data [, $op]) = @_, ... },
223 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
224 local => sub { my ($ref, $data [, $op]) = @_; ... },
225 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
226 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
227 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
228 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
230 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
233 This function creates a 'wizard', an opaque object that holds the magic information.
234 It takes a list of keys / values as argument, whose keys can be :
242 A code (or string) reference to a private data constructor.
243 It is called in scalar context each time the magic is cast onto a variable, with C<$_[0]> being a reference to this variable and C<@_[1 .. @_-1]> being all extra arguments that were passed to L</cast>.
244 The scalar returned from this call is then attached to the variable and can be retrieved later with L</getdata>.
248 C<get>, C<set>, C<len>, C<clear>, C<free>, C<copy>, C<local>, C<fetch>, C<store>, C<exists> and C<delete>
250 Code (or string) references to the respective magic callbacks.
251 You don't have to specify all of them : the magic corresponding to undefined entries will simply not be hooked.
253 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).
254 Other arguments depend on which kind of magic is involved :
262 C<$_[2]> contains the natural, non-magical length of the variable (which can only be a scalar or an array as I<len> magic is only relevant for these types).
263 The callback is expected to return the new scalar or array length to use, or C<undef> to default to the normal length.
269 C<$_[2]> is a either an alias or a copy of the current key, and C<$_[3]> is an alias to the current element (i.e. the value).
270 Because C<$_[2]> might be a copy, it is useless to try to change it or cast magic on it.
274 I<fetch>, I<store>, I<exists> and I<delete>
276 C<$_[2]> is an alias to the current key.
277 Note that C<$_[2]> may rightfully be readonly if the key comes from a bareword, and as such it is unsafe to assign to it.
278 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.
282 Finally, if C<< op_info => $num >> is also passed to C<wizard>, then one extra element is appended to C<@_>.
283 Its nature depends on the value of C<$num> :
291 C<$_[-1]> is the current op name.
295 C<VMG_OP_INFO_OBJECT>
297 C<$_[-1]> is the C<B::OP> object for the current op.
301 Both result in a small performance hit, but just getting the name is lighter than getting the op object.
303 These callbacks are executed in scalar context and are expected to return an integer, which is then passed straight to the perl magic API.
304 However, only the return value of the I<len> magic callback currently holds a meaning.
308 Each callback can be specified as :
314 a code reference, which will be called as a subroutine.
318 a string reference, where the string denotes which subroutine is to be called when magic is triggered.
319 If the subroutine name is not fully qualified, then the current package at the time the magic is invoked will be used instead.
323 a reference to C<undef>, in which case a no-op magic callback is installed instead of the default one.
324 This may especially be helpful for I<local> magic, where an empty callback prevents magic from being copied during localization.
328 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.
330 Here is a simple usage example :
332 # A simple scalar tracer
334 get => sub { print STDERR "got ${$_[0]}\n" },
335 set => sub { print STDERR "set to ${$_[0]}\n" },
336 free => sub { print STDERR "${$_[0]} was deleted\n" },
344 Carp::croak('Wrong number of arguments for wizard()');
349 my @keys = qw<op_info data get set len clear free copy dup>;
350 push @keys, 'local' if MGf_LOCAL;
351 push @keys, qw<fetch store exists delete copy_key> if VMG_UVAR;
356 $wiz = eval { _wizard(map $opts{$_}, @keys) };
360 $err =~ s/\sat\s+.*?\n//;
370 cast [$@%&*]var, $wiz, @args
372 This function associates C<$wiz> magic to the supplied variable, without overwriting any other kind of magic.
373 It returns true on success or when C<$wiz> magic is already attached, and croaks on error.
374 When C<$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]>.
375 Otherwise, C<@args> is ignored.
377 # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
381 The C<var> argument can be an array or hash value.
382 Magic for these scalars behaves like for any other, except that it is dispelled when the entry is deleted from the container.
383 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 :
386 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
388 If you want to handle the possible deletion of the C<'TZ'> entry, you must also specify I<store> magic.
392 getdata [$@%&*]var, $wiz
394 This accessor fetches the private data associated with the magic C<$wiz> in the variable.
395 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.
397 # Get the data attached to $wiz in $x, or undef if $wiz
398 # did not attach any.
399 my $data = getdata $x, $wiz;
403 dispell [$@%&*]variable, $wiz
405 The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable.
406 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.
409 die 'no such magic in $x' unless dispell $x, $wiz;
415 Evaluates to true if and only if the I<copy> magic is available.
416 This is the case for perl 5.7.3 and greater, which is ensured by the requirements of this module.
420 Evaluates to true if and only if the I<dup> magic is available.
421 This is the case for perl 5.7.3 and greater, which is ensured by the requirements of this module.
425 Evaluates to true if and only if the I<local> magic is available.
426 This is the case for perl 5.9.3 and greater.
430 When this constant is true, you can use the I<fetch>, I<store>, I<exists> and I<delete> magics on hashes.
431 Initial L</VMG_UVAR> capability was introduced in perl 5.9.5, with a fully functional implementation shipped with perl 5.10.0.
433 =head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN>
435 True for perls that don't call I<len> magic when taking the C<length> of a magical scalar.
437 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN>
439 True for perls that don't call I<len> magic when you push an element in a magical array.
440 Starting from perl 5.11.0, this only refers to pushes in non-void context and hence is false.
442 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID>
444 True for perls that don't call I<len> magic when you push in void context an element in a magical array.
446 =head2 C<VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID>
448 True for perls that don't call I<len> magic when you unshift in void context an element in a magical array.
450 =head2 C<VMG_COMPAT_ARRAY_UNDEF_CLEAR>
452 True for perls that call I<clear> magic when undefining magical arrays.
454 =head2 C<VMG_COMPAT_HASH_DELETE_NOUVAR_VOID>
456 True for perls that don't call I<delete> magic when you delete an element from a hash in void context.
458 =head2 C<VMG_COMPAT_GLOB_GET>
460 True for perls that call I<get> magic for operations on globs.
462 =head2 C<VMG_PERL_PATCHLEVEL>
464 The perl patchlevel this module was built with, or C<0> for non-debugging perls.
466 =head2 C<VMG_THREADSAFE>
468 True if and only if this module could have been built with thread-safety features enabled.
470 =head2 C<VMG_FORKSAFE>
472 True if and only if this module could have been built with fork-safety features enabled.
473 This is always true except on Windows where it is false for perl 5.10.0 and below.
475 =head2 C<VMG_OP_INFO_NAME>
477 Value to pass with C<op_info> to get the current op name in the magic callbacks.
479 =head2 C<VMG_OP_INFO_OBJECT>
481 Value to pass with C<op_info> to get a C<B::OP> object representing the current op in the magic callbacks.
485 =head2 Associate an object to any perl variable
487 This technique can be useful for passing user data through limited APIs.
488 It is similar to using inside-out objects, but without the drawback of having to implement a complex destructor.
491 package Magical::UserData;
493 use Variable::Magic qw<wizard cast getdata>;
495 my $wiz = wizard data => sub { \$_[1] };
497 sub ud (\[$@%*&]) : lvalue {
499 my $data = &getdata($var, $wiz);
500 unless (defined $data) {
502 &cast($var, $wiz, $slot)
503 or die "Couldn't cast UserData magic onto the variable";
510 BEGIN { *ud = \&Magical::UserData::ud }
513 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
516 $cb->(); # Hello, world!
519 =head2 Recursively cast magic on datastructures
521 C<cast> can be called from any magical callback, and in particular from C<data>.
522 This allows you to recursively cast magic on datastructures :
525 $wiz = wizard data => sub {
526 my ($var, $depth) = @_;
530 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
531 } elsif ($r eq 'HASH') {
532 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
537 my ($var, $depth) = @_;
539 print "free $r at depth $depth\n";
551 When C<%h> goes out of scope, this prints something among the lines of :
555 free SCALAR at depth 2
556 free ARRAY at depth 1
557 free SCALAR at depth 3
558 free SCALAR at depth 3
560 Of course, this example does nothing with the values that are added after the C<cast>.
562 =head1 PERL MAGIC HISTORY
564 The places where magic is invoked have changed a bit through perl history.
565 Here is a little list of the most recent ones.
573 I<p14416> : I<copy> and I<dup> magic.
579 I<p28160> : Integration of I<p25854> (see below).
581 I<p32542> : Integration of I<p31473> (see below).
587 I<p25854> : I<len> magic is no longer called when pushing an element into a magic array.
589 I<p26569> : I<local> magic.
595 I<p31064> : Meaningful I<uvar> magic.
597 I<p31473> : I<clear> magic was not invoked when undefining an array.
598 The bug is fixed as of this version.
604 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.
610 I<p32969> : I<len> magic is no longer invoked when calling C<length> with a magical scalar.
612 I<p34908> : I<len> magic is no longer called when pushing / unshifting an element into a magical array in void context.
613 The C<push> part was already covered by I<p25854>.
615 I<g9cdcb38b> : I<len> magic is called again when pushing into a magical array in non-void context.
621 The functions L</wizard>, L</cast>, L</getdata> and L</dispell> are only exported on request.
622 All of them are exported by the tags C<':funcs'> and C<':all'>.
624 All the constants are also only exported on request, either individually or by the tags C<':consts'> and C<':all'>.
628 use base qw<Exporter>;
632 'funcs' => [ qw<wizard cast getdata dispell> ],
634 MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR
635 VMG_COMPAT_SCALAR_LENGTH_NOLEN
636 VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID
637 VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID
638 VMG_COMPAT_ARRAY_UNDEF_CLEAR
639 VMG_COMPAT_HASH_DELETE_NOUVAR_VOID
642 VMG_THREADSAFE VMG_FORKSAFE
643 VMG_OP_INFO_NAME VMG_OP_INFO_OBJECT
646 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
647 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
651 In order to hook hash operations with magic, you need at least perl 5.10.0 (see L</VMG_UVAR>).
653 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.
654 You can work around this gotcha by storing a reference to the magic object instead.
656 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.
663 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.
665 L<Carp> (core since perl 5), L<XSLoader> (since 5.006).
667 Copy tests need L<Tie::Array> (core since perl 5.005) and L<Tie::Hash> (since 5.002).
668 Some uvar tests need L<Hash::Util::FieldHash> (since 5.009004).
669 Glob tests need L<Symbol> (since 5.002).
670 Threads tests need L<threads> and L<threads::shared> (both since 5.007003).
674 L<perlguts> and L<perlapi> for internal information about magic.
676 L<perltie> and L<overload> for other ways of enhancing objects.
680 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
682 You can contact me by mail or on C<irc.perl.org> (vincent).
686 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>.
687 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
691 You can find documentation for this module with the perldoc command.
693 perldoc Variable::Magic
695 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>.
697 =head1 COPYRIGHT & LICENSE
699 Copyright 2007,2008,2009,2010,2011,2012 Vincent Pit, all rights reserved.
701 This program is free software; you can redistribute it and/or modify it
702 under the same terms as Perl itself.
706 1; # End of Variable::Magic