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 $h{pear} = 1; # "key pear stored in helem"
55 Magic is Perl's way of enhancing variables.
56 This mechanism lets the user add extra data to any variable and hook syntactical operations (such as access, assignment or destruction) that can be applied to it.
57 With this module, you can add your own magic to any variable without having to write a single line of XS.
59 You'll realize that these magic variables look a lot like tied variables.
60 It's not surprising, as tied variables are implemented as a special kind of magic, just like any 'irregular' Perl variable : scalars like C<$!>, 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...
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 It isn't copied on assignment.
71 You attach it to variables, not values (as for blessed references).
75 It doesn't replace the original semantics.
77 Magic callbacks usually get triggered before the original action takes place, and can't 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 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 It's mostly invisible at the Perl level.
91 Magical and non-magical variables cannot be distinguished with C<ref>, C<tied> or another trick.
97 Mainly because perl's way of handling magic is lighter by nature, and because there's 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 one is triggered each time the value of the variable changes.
118 It is called for array subscripts and slices, but never for hashes.
124 This magic is a little special : it is called when the 'size' or the 'length' of the variable has to be known by Perl.
125 Typically, 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>).
126 The callback has then to return the length 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 one can be considered as an object destructor.
140 It happens when the variable goes out of scope, but not when it is undefined.
146 This magic only applies to tied arrays and hashes.
147 It fires when you try to access or change their elements.
148 It is available on your perl iff C<MGf_COPY> is true.
154 Invoked when the variable is cloned across threads.
155 Currently not available.
161 When this magic is set on a variable, all subsequent localizations of the variable will trigger the callback.
162 It is available on your perl iff C<MGf_LOCAL> is true.
166 The following actions only apply to hashes and are available iff L</VMG_UVAR> is true.
167 They are referred to as C<uvar> magics.
175 This magic happens each time an element is fetched from the hash.
181 This one is called when an element is stored into the hash.
187 This magic fires when a key is tested for existence in the hash.
193 This last one triggers when a key is deleted in the hash, regardless of whether the key actually exists in it.
197 You can refer to the tests to have more insight of where the different magics are invoked.
205 XSLoader::load(__PACKAGE__, $VERSION);
212 get => sub { my ($ref, $data [, $op]) = @_; ... },
213 set => sub { my ($ref, $data [, $op]) = @_; ... },
215 my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen
217 clear => sub { my ($ref, $data [, $op]) = @_; ... },
218 free => sub { my ($ref, $data [, $op]) = @_, ... },
219 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
220 local => sub { my ($ref, $data [, $op]) = @_; ... },
221 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
222 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
223 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
224 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
226 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
229 This function creates a 'wizard', an opaque type that holds the magic information.
230 It takes a list of keys / values as argument, whose keys can be :
238 A code (or string) reference to a private data constructor.
239 It is called each time this magic is cast on a variable, and the scalar returned is used as private data storage for it.
240 C<$_[0]> is a reference to the magic object and C<@_[1 .. @_-1]> are all extra arguments that were passed to L</cast>.
244 C<get>, C<set>, C<len>, C<clear>, C<free>, C<copy>, C<local>, C<fetch>, C<store>, C<exists> and C<delete>
246 Code (or string) references to the corresponding magic callbacks.
247 You don't have to specify all of them : the magic associated with undefined entries simply won't be hooked.
248 In 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).
250 Moreover, 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<$num == VMG_OP_INFO_OBJECT>.
251 Both have a performance hit, but just getting the name is lighter than getting the op object.
253 Other arguments are specific to the magic hooked :
261 When the variable is an array or a scalar, C<$_[2]> contains the non-magical length.
262 The callback can return the new scalar or array length to use, or C<undef> to default to the normal length.
268 C<$_[2]> is a either a copy or an alias of the current key, which means that it is useless to try to change or cast magic on it.
269 C<$_[3]> is an alias to the current element (i.e. the value).
273 C<fetch>, C<store>, C<exists> and C<delete>
275 C<$_[2]> is an alias to the current key.
276 Nothing prevents you from changing it, but be aware that there lurk dangerous side effects.
277 For example, it may rightfully be readonly if the key was a bareword.
278 You can get a copy instead by passing C<< copy_key => 1 >> to L</wizard>, which allows you to safely assign to C<$_[2]> in order to e.g. redirect the action to another key.
279 This however has a little performance drawback because of the copy.
283 All the callbacks are expected to return an integer, which is passed straight to the perl magic API.
284 However, only the return value of the C<len> callback currently holds a meaning.
288 Each 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.
290 Note 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.
292 Here's a simple usage example :
294 # A simple scalar tracer
296 get => sub { print STDERR "got ${$_[0]}\n" },
297 set => sub { print STDERR "set to ${$_[0]}\n" },
298 free => sub { print STDERR "${$_[0]} was deleted\n" },
306 Carp::croak('Wrong number of arguments for wizard()');
311 my @keys = qw<op_info data get set len clear free copy dup>;
312 push @keys, 'local' if MGf_LOCAL;
313 push @keys, qw<fetch store exists delete copy_key> if VMG_UVAR;
318 $wiz = eval { _wizard(map $opts{$_}, @keys) };
322 $err =~ s/\sat\s+.*?\n//;
332 cast [$@%&*]var, $wiz, ...
334 This function associates C<$wiz> magic to the variable supplied, without overwriting any other kind of magic.
335 It returns true on success or when C<$wiz> magic is already present, and croaks on error.
336 All extra arguments specified after C<$wiz> are passed to the private data constructor in C<@_[1 .. @_-1]>.
337 If the variable isn't a hash, any C<uvar> callback of the wizard is safely ignored.
339 # Casts $wiz onto $x, and pass '1' to the data constructor.
343 The C<var> argument can be an array or hash value.
344 Magic for those behaves like for any other scalar, except that it is dispelled when the entry is deleted from the container.
345 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 :
348 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
350 If you want to overcome the possible deletion of the C<'TZ'> entry, you have no choice but to rely on C<store> uvar magic.
354 getdata [$@%&*]var, $wiz
356 This accessor fetches the private data associated with the magic C<$wiz> in the variable.
357 It 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.
359 # Get the attached data, or undef if the wizard does not attach any.
360 my $data = getdata $x, $wiz;
364 dispell [$@%&*]variable, $wiz
366 The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable.
367 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.
370 die 'no such magic in $x' unless dispell $x, $wiz;
376 Evaluates to true iff the 'copy' magic is available.
380 Evaluates to true iff the 'dup' magic is available.
384 Evaluates to true iff the 'local' magic is available.
388 When this constant is true, you can use the C<fetch,store,exists,delete> callbacks on hashes.
389 Initial VMG_UVAR capability was introduced in perl 5.9.5, with a fully functional implementation
390 shipped with perl 5.10.0.
392 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN>
394 True for perls that don't call 'len' magic when you push an element in a magical array.
395 Starting from perl 5.11.0, this only refers to pushes in non-void context and hence is false.
397 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID>
399 True for perls that don't call 'len' magic when you push in void context an element in a magical array.
401 =head2 C<VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID>
403 True for perls that don't call 'len' magic when you unshift in void context an element in a magical array.
405 =head2 C<VMG_COMPAT_ARRAY_UNDEF_CLEAR>
407 True for perls that call 'clear' magic when undefining magical arrays.
409 =head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN>
411 True for perls that don't call 'len' magic when taking the C<length> of a magical scalar.
413 =head2 C<VMG_COMPAT_GLOB_GET>
415 True for perls that call 'get' magic for operations on globs.
417 =head2 C<VMG_PERL_PATCHLEVEL>
419 The perl patchlevel this module was built with, or C<0> for non-debugging perls.
421 =head2 C<VMG_THREADSAFE>
423 True iff this module could have been built with thread-safety features enabled.
425 =head2 C<VMG_FORKSAFE>
427 True iff this module could have been built with fork-safety features enabled.
428 This will always be true except on Windows where it's false for perl 5.10.0 and below .
430 =head2 C<VMG_OP_INFO_NAME>
432 Value to pass with C<op_info> to get the current op name in the magic callbacks.
434 =head2 C<VMG_OP_INFO_OBJECT>
436 Value to pass with C<op_info> to get a C<B::OP> object representing the current op in the magic callbacks.
440 =head2 Associate an object to any perl variable
442 This technique can be useful for passing user data through limited APIs.
443 It is similar to using inside-out objects, but without the drawback of having to implement a complex destructor.
446 package Magical::UserData;
448 use Variable::Magic qw<wizard cast getdata>;
450 my $wiz = wizard data => sub { \$_[1] };
452 sub ud (\[$@%*&]) : lvalue {
454 my $data = &getdata($var, $wiz);
455 unless (defined $data) {
457 &cast($var, $wiz, $slot)
458 or die "Couldn't cast UserData magic onto the variable";
465 BEGIN { *ud = \&Magical::UserData::ud }
468 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
471 $cb->(); # Hello, world!
474 =head2 Recursively cast magic on datastructures
476 C<cast> can be called from any magical callback, and in particular from C<data>.
477 This allows you to recursively cast magic on datastructures :
480 $wiz = wizard data => sub {
481 my ($var, $depth) = @_;
485 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
486 } elsif ($r eq 'HASH') {
487 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
492 my ($var, $depth) = @_;
494 print "free $r at depth $depth\n";
506 When C<%h> goes out of scope, this will print something among the lines of :
510 free SCALAR at depth 2
511 free ARRAY at depth 1
512 free SCALAR at depth 3
513 free SCALAR at depth 3
515 Of course, this example does nothing with the values that are added after the C<cast>.
517 =head1 PERL MAGIC HISTORY
519 The places where magic is invoked have changed a bit through perl history.
520 Here's a little list of the most recent ones.
528 I<p14416> : 'copy' and 'dup' magic.
534 I<p28160> : Integration of I<p25854> (see below).
536 I<p32542> : Integration of I<p31473> (see below).
542 I<p25854> : 'len' magic is no longer called when pushing an element into a magic array.
544 I<p26569> : 'local' magic.
550 I<p31064> : Meaningful 'uvar' magic.
552 I<p31473> : 'clear' magic wasn't invoked when undefining an array.
553 The bug is fixed as of this version.
559 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.
565 I<p32969> : 'len' magic is no longer invoked when calling C<length> with a magical scalar.
567 I<p34908> : 'len' magic is no longer called when pushing / unshifting an element into a magical array in void context.
568 The C<push> part was already covered by I<p25854>.
570 I<g9cdcb38b> : 'len' magic is called again when pushing into a magical array in non-void context.
576 The functions L</wizard>, L</cast>, L</getdata> and L</dispell> are only exported on request.
577 All of them are exported by the tags C<':funcs'> and C<':all'>.
579 All the constants are also only exported on request, either individually or by the tags C<':consts'> and C<':all'>.
583 use base qw<Exporter>;
587 'funcs' => [ qw<wizard cast getdata dispell> ],
589 MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR
590 VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID
591 VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID
592 VMG_COMPAT_ARRAY_UNDEF_CLEAR
593 VMG_COMPAT_SCALAR_LENGTH_NOLEN
596 VMG_THREADSAFE VMG_FORKSAFE
597 VMG_OP_INFO_NAME VMG_OP_INFO_OBJECT
600 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
601 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
605 If 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.
606 The only way to address this would be to return a reference.
608 If 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.
610 In order to define magic on hash members, you need at least L<perl> 5.10.0 (see L</VMG_UVAR>)
617 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.
619 L<Carp> (standard since perl 5), L<XSLoader> (standard since perl 5.006).
621 Copy tests need L<Tie::Array> (standard since perl 5.005) and L<Tie::Hash> (since 5.002).
623 Some uvar tests need L<Hash::Util::FieldHash> (standard since perl 5.009004).
625 Glob tests need L<Symbol> (standard since perl 5.002).
627 Threads tests need L<threads> and L<threads::shared>.
631 L<perlguts> and L<perlapi> for internal information about magic.
633 L<perltie> and L<overload> for other ways of enhancing objects.
637 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
639 You can contact me by mail or on C<irc.perl.org> (vincent).
643 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>. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
647 You can find documentation for this module with the perldoc command.
649 perldoc Variable::Magic
651 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>.
653 =head1 COPYRIGHT & LICENSE
655 Copyright 2007,2008,2009,2010,2011 Vincent Pit, all rights reserved.
657 This program is free software; you can redistribute it and/or modify it
658 under the same terms as Perl itself.
662 1; # End of Variable::Magic