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 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 arrays (though it used to also apply to scalars), 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.
134 Starting from perl 5.12, this magic is no longer called by the C<length> keyword, and starting from perl 5.17.4 it is also no longer called for scalars in any situation, making this magic only meaningful on arrays.
135 You can use the constants L</VMG_COMPAT_SCALAR_LENGTH_NOLEN> and L</VMG_COMPAT_SCALAR_NOLEN> to see if this magic is available for scalars or not.
141 This magic is invoked when the variable is reset, such as when an array is emptied.
142 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>).
148 This magic is called when a variable is destroyed as the result of going out of scope (but not when it is undefined).
149 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.
155 This magic only applies to tied arrays and hashes, and fires when you try to access or change their elements.
161 This magic is invoked when the variable is cloned across threads.
162 It is currently not available.
168 When this magic is set on a variable, all subsequent localizations of the variable will trigger the callback.
169 It is available on your perl if and only if C<MGf_LOCAL> is true.
173 The following actions only apply to hashes and are available if and only if L</VMG_UVAR> is true.
174 They are referred to as I<uvar> magics.
182 This magic is invoked each time an element is fetched from the hash.
188 This one is called when an element is stored into the hash.
194 This magic fires when a key is tested for existence in the hash.
200 This magic is triggered when a key is deleted in the hash, regardless of whether the key actually exists in it.
204 You can refer to the tests to have more insight of where the different magics are invoked.
212 XSLoader::load(__PACKAGE__, $VERSION);
219 get => sub { my ($ref, $data [, $op]) = @_; ... },
220 set => sub { my ($ref, $data [, $op]) = @_; ... },
222 my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen
224 clear => sub { my ($ref, $data [, $op]) = @_; ... },
225 free => sub { my ($ref, $data [, $op]) = @_, ... },
226 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
227 local => sub { my ($ref, $data [, $op]) = @_; ... },
228 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
229 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
230 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
231 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
233 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
236 This function creates a 'wizard', an opaque object that holds the magic information.
237 It takes a list of keys / values as argument, whose keys can be :
245 A code (or string) reference to a private data constructor.
246 It is called in scalar context each time the magic is cast onto a variable, with C<$_[0]> being a reference to this variable and C<@_[1 .. @_-1]> being all extra arguments that were passed to L</cast>.
247 The scalar returned from this call is then attached to the variable and can be retrieved later with L</getdata>.
251 C<get>, C<set>, C<len>, C<clear>, C<free>, C<copy>, C<local>, C<fetch>, C<store>, C<exists> and C<delete>
253 Code (or string) references to the respective magic callbacks.
254 You don't have to specify all of them : the magic corresponding to undefined entries will simply not be hooked.
256 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).
257 Other arguments depend on which kind of magic is involved :
265 C<$_[2]> contains the natural, non-magical length of the variable (which can only be a scalar or an array as I<len> magic is only relevant for these types).
266 The callback is expected to return the new scalar or array length to use, or C<undef> to default to the normal length.
272 C<$_[2]> is a either an alias or a copy of the current key, and C<$_[3]> is an alias to the current element (i.e. the value).
273 Because C<$_[2]> might be a copy, it is useless to try to change it or cast magic on it.
277 I<fetch>, I<store>, I<exists> and I<delete>
279 C<$_[2]> is an alias to the current key.
280 Note that C<$_[2]> may rightfully be readonly if the key comes from a bareword, and as such it is unsafe to assign to it.
281 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.
285 Finally, if C<< op_info => $num >> is also passed to C<wizard>, then one extra element is appended to C<@_>.
286 Its nature depends on the value of C<$num> :
294 C<$_[-1]> is the current op name.
298 C<VMG_OP_INFO_OBJECT>
300 C<$_[-1]> is the C<B::OP> object for the current op.
304 Both result in a small performance hit, but just getting the name is lighter than getting the op object.
306 These callbacks are executed in scalar context and are expected to return an integer, which is then passed straight to the perl magic API.
307 However, only the return value of the I<len> magic callback currently holds a meaning.
311 Each callback can be specified as :
317 a code reference, which will be called as a subroutine.
321 a string reference, where the string denotes which subroutine is to be called when magic is triggered.
322 If the subroutine name is not fully qualified, then the current package at the time the magic is invoked will be used instead.
326 a reference to C<undef>, in which case a no-op magic callback is installed instead of the default one.
327 This may especially be helpful for I<local> magic, where an empty callback prevents magic from being copied during localization.
331 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.
333 Here is a simple usage example :
335 # A simple scalar tracer
337 get => sub { print STDERR "got ${$_[0]}\n" },
338 set => sub { print STDERR "set to ${$_[0]}\n" },
339 free => sub { print STDERR "${$_[0]} was deleted\n" },
347 Carp::croak('Wrong number of arguments for wizard()');
352 my @keys = qw<op_info data get set len clear free copy dup>;
353 push @keys, 'local' if MGf_LOCAL;
354 push @keys, qw<fetch store exists delete copy_key> if VMG_UVAR;
359 $wiz = eval { _wizard(map $opts{$_}, @keys) };
363 $err =~ s/\sat\s+.*?\n//;
373 cast [$@%&*]var, $wiz, @args
375 This function associates C<$wiz> magic to the supplied variable, without overwriting any other kind of magic.
376 It returns true on success or when C<$wiz> magic is already attached, and croaks on error.
377 When C<$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]>.
378 Otherwise, C<@args> is ignored.
380 # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
384 The C<var> argument can be an array or hash value.
385 Magic for these scalars behaves like for any other, except that it is dispelled when the entry is deleted from the container.
386 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 :
389 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
391 If you want to handle the possible deletion of the C<'TZ'> entry, you must also specify I<store> magic.
395 getdata [$@%&*]var, $wiz
397 This accessor fetches the private data associated with the magic C<$wiz> in the variable.
398 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.
400 # Get the data attached to $wiz in $x, or undef if $wiz
401 # did not attach any.
402 my $data = getdata $x, $wiz;
406 dispell [$@%&*]variable, $wiz
408 The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable.
409 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.
412 die 'no such magic in $x' unless dispell $x, $wiz;
418 Evaluates to true if and only if the I<copy> magic is available.
419 This is the case for perl 5.7.3 and greater, which is ensured by the requirements of this module.
423 Evaluates to true if and only if the I<dup> magic is available.
424 This is the case for perl 5.7.3 and greater, which is ensured by the requirements of this module.
428 Evaluates to true if and only if the I<local> magic is available.
429 This is the case for perl 5.9.3 and greater.
433 When this constant is true, you can use the I<fetch>, I<store>, I<exists> and I<delete> magics on hashes.
434 Initial L</VMG_UVAR> capability was introduced in perl 5.9.5, with a fully functional implementation shipped with perl 5.10.0.
436 =head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN>
438 True for perls that don't call I<len> magic when taking the C<length> of a magical scalar.
440 =head2 C<VMG_COMPAT_SCALAR_NOLEN>
442 True for perls that don't call I<len> magic on scalars.
443 Implies L</VMG_COMPAT_SCALAR_LENGTH_NOLEN>.
445 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN>
447 True for perls that don't call I<len> magic when you push an element in a magical array.
448 Starting from perl 5.11.0, this only refers to pushes in non-void context and hence is false.
450 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID>
452 True for perls that don't call I<len> magic when you push in void context an element in a magical array.
454 =head2 C<VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID>
456 True for perls that don't call I<len> magic when you unshift in void context an element in a magical array.
458 =head2 C<VMG_COMPAT_ARRAY_UNDEF_CLEAR>
460 True for perls that call I<clear> magic when undefining magical arrays.
462 =head2 C<VMG_COMPAT_HASH_DELETE_NOUVAR_VOID>
464 True for perls that don't call I<delete> magic when you delete an element from a hash in void context.
466 =head2 C<VMG_COMPAT_GLOB_GET>
468 True for perls that call I<get> magic for operations on globs.
470 =head2 C<VMG_PERL_PATCHLEVEL>
472 The perl patchlevel this module was built with, or C<0> for non-debugging perls.
474 =head2 C<VMG_THREADSAFE>
476 True if and only if this module could have been built with thread-safety features enabled.
478 =head2 C<VMG_FORKSAFE>
480 True if and only if this module could have been built with fork-safety features enabled.
481 This is always true except on Windows where it is false for perl 5.10.0 and below.
483 =head2 C<VMG_OP_INFO_NAME>
485 Value to pass with C<op_info> to get the current op name in the magic callbacks.
487 =head2 C<VMG_OP_INFO_OBJECT>
489 Value to pass with C<op_info> to get a C<B::OP> object representing the current op in the magic callbacks.
493 =head2 Associate an object to any perl variable
495 This technique can be useful for passing user data through limited APIs.
496 It is similar to using inside-out objects, but without the drawback of having to implement a complex destructor.
499 package Magical::UserData;
501 use Variable::Magic qw<wizard cast getdata>;
503 my $wiz = wizard data => sub { \$_[1] };
505 sub ud (\[$@%*&]) : lvalue {
507 my $data = &getdata($var, $wiz);
508 unless (defined $data) {
510 &cast($var, $wiz, $slot)
511 or die "Couldn't cast UserData magic onto the variable";
518 BEGIN { *ud = \&Magical::UserData::ud }
521 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
524 $cb->(); # Hello, world!
527 =head2 Recursively cast magic on datastructures
529 C<cast> can be called from any magical callback, and in particular from C<data>.
530 This allows you to recursively cast magic on datastructures :
533 $wiz = wizard data => sub {
534 my ($var, $depth) = @_;
538 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
539 } elsif ($r eq 'HASH') {
540 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
545 my ($var, $depth) = @_;
547 print "free $r at depth $depth\n";
559 When C<%h> goes out of scope, this prints something among the lines of :
563 free SCALAR at depth 2
564 free ARRAY at depth 1
565 free SCALAR at depth 3
566 free SCALAR at depth 3
568 Of course, this example does nothing with the values that are added after the C<cast>.
570 =head1 PERL MAGIC HISTORY
572 The places where magic is invoked have changed a bit through perl history.
573 Here is a little list of the most recent ones.
581 I<p14416> : I<copy> and I<dup> magic.
587 I<p28160> : Integration of I<p25854> (see below).
589 I<p32542> : Integration of I<p31473> (see below).
595 I<p25854> : I<len> magic is no longer called when pushing an element into a magic array.
597 I<p26569> : I<local> magic.
603 I<p31064> : Meaningful I<uvar> magic.
605 I<p31473> : I<clear> magic was not invoked when undefining an array.
606 The bug is fixed as of this version.
612 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.
618 I<p32969> : I<len> magic is no longer invoked when calling C<length> with a magical scalar.
620 I<p34908> : I<len> magic is no longer called when pushing / unshifting an element into a magical array in void context.
621 The C<push> part was already covered by I<p25854>.
623 I<g9cdcb38b> : I<len> magic is called again when pushing into a magical array in non-void context.
629 The functions L</wizard>, L</cast>, L</getdata> and L</dispell> are only exported on request.
630 All of them are exported by the tags C<':funcs'> and C<':all'>.
632 All the constants are also only exported on request, either individually or by the tags C<':consts'> and C<':all'>.
636 use base qw<Exporter>;
640 'funcs' => [ qw<wizard cast getdata dispell> ],
642 MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR
643 VMG_COMPAT_SCALAR_LENGTH_NOLEN
644 VMG_COMPAT_SCALAR_NOLEN
645 VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID
646 VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID
647 VMG_COMPAT_ARRAY_UNDEF_CLEAR
648 VMG_COMPAT_HASH_DELETE_NOUVAR_VOID
651 VMG_THREADSAFE VMG_FORKSAFE
652 VMG_OP_INFO_NAME VMG_OP_INFO_OBJECT
655 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
656 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
660 In order to hook hash operations with magic, you need at least perl 5.10.0 (see L</VMG_UVAR>).
662 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.
663 You can work around this gotcha by storing a reference to the magic object instead.
665 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.
672 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.
674 L<Carp> (core since perl 5), L<XSLoader> (since 5.006).
676 Copy tests need L<Tie::Array> (core since perl 5.005) and L<Tie::Hash> (since 5.002).
677 Some uvar tests need L<Hash::Util::FieldHash> (since 5.009004).
678 Glob tests need L<Symbol> (since 5.002).
679 Threads tests need L<threads> and L<threads::shared> (both since 5.007003).
683 L<perlguts> and L<perlapi> for internal information about magic.
685 L<perltie> and L<overload> for other ways of enhancing objects.
689 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
691 You can contact me by mail or on C<irc.perl.org> (vincent).
695 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>.
696 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
700 You can find documentation for this module with the perldoc command.
702 perldoc Variable::Magic
704 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>.
706 =head1 COPYRIGHT & LICENSE
708 Copyright 2007,2008,2009,2010,2011,2012 Vincent Pit, all rights reserved.
710 This program is free software; you can redistribute it and/or modify it
711 under the same terms as Perl itself.
715 1; # End of Variable::Magic