1 package Variable::Magic;
12 Variable::Magic - Associate user-defined magic to variables from Perl.
27 use Variable::Magic qw/wizard cast VMG_OP_INFO_NAME/;
30 my $wiz = wizard set => sub { print "now set to ${$_[0]}!\n" },
31 free => sub { print "destroyed!\n" };
35 $a = 2; # "now set to 2!"
38 { # A hash with a default value
39 my $wiz = wizard data => sub { $_[1] },
40 fetch => sub { $_[2] = $_[1] unless exists $_[0]->{$_[2]}; () },
41 store => sub { print "key $_[2] stored in $_[-1]\n" },
43 op_info => VMG_OP_INFO_NAME;
45 my %h = (_default => 0, apple => 2);
46 cast %h, $wiz, '_default';
47 print $h{banana}, "\n"; # "0", because the 'banana' key doesn't exist in %h
48 $h{pear} = 1; # "key pear stored in helem"
53 Magic is Perl way of enhancing objects.
54 This mechanism lets the user add extra data to any variable and hook syntaxical operations (such as access, assignment or destruction) that can be applied to it.
55 With this module, you can add your own magic to any variable without having to write a single line of XS.
57 You'll realize that these magic variables look a lot like tied variables.
58 It's not surprising, as tied variables are implemented as a special kind of magic, just like any 'irregular' Perl variable : scalars like C<$!>, 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...
59 They all share the same underlying C API, and this module gives you direct access to it.
61 Still, the magic made available by this module differs from tieing and overloading in several ways :
67 It isn't copied on assignment.
69 You attach it to variables, not values (as for blessed references).
73 It doesn't replace the original semantics.
75 Magic callbacks usually trigger before the original action take place, and can't prevent it to happen.
76 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 The same magic can be applied on scalars, arrays, hashes, subs or globs.
83 But the same hook (see below for a list) may trigger differently depending on the the type of the variable.
87 It's mostly invisible at the Perl level.
89 Magical and non-magical variables cannot be distinguished with C<ref>, C<tied> or another trick.
95 Mainly because perl's way of handling magic is lighter by nature, and because there's no need for any method resolution.
96 Also, since you don't have to reimplement all the variable semantics, you only pay for what you actually use.
100 The operations that can be overloaded are :
108 This magic is invoked when the variable is evaluated.
109 It is never called for arrays and hashes.
115 This one is triggered each time the value of the variable changes.
116 It is called for array subscripts and slices, but never for hashes.
122 This magic is a little special : it is called when the 'size' or the 'length' of the variable has to be known by Perl.
123 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>).
124 The callback has then to return the length as an integer.
130 This magic is invoked when the variable is reset, such as when an array is emptied.
131 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>).
137 This one can be considered as an object destructor.
138 It happens when the variable goes out of scope, but not when it is undefined.
144 This magic only applies to tied arrays and hashes.
145 It fires when you try to access or change their elements.
146 It is available on your perl iff C<MGf_COPY> is true.
152 Invoked when the variable is cloned across threads.
153 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 iff C<MGf_LOCAL> is true.
164 The following actions only apply to hashes and are available iff C<VMG_UVAR> is true.
165 They are referred to as C<uvar> magics.
173 This magic happens 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 last one triggers 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.
197 To prevent any clash between different magics defined with this module, an unique numerical signature is attached to each kind of magic (i.e. each set of callbacks for magic operations).
198 At the C level, magic tokens owned by magic created by this module have their C<< mg->mg_private >> field set to C<0x3891> or C<0x3892>, so please don't use these magic (sic) numbers in other extensions.
206 XSLoader::load(__PACKAGE__, $VERSION);
213 get => sub { my ($ref, $data [, $op]) = @_; ... },
214 set => sub { my ($ref, $data [, $op]) = @_; ... },
215 len => sub { my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen; },
216 clear => sub { my ($ref, $data [, $op]) = @_; ... },
217 free => sub { my ($ref, $data [, $op]) = @_, ... },
218 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
219 local => sub { my ($ref, $data [, $op]) = @_; ... },
220 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
221 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
222 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
223 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
225 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ]
227 This function creates a 'wizard', an opaque type that holds the magic information.
228 It takes a list of keys / values as argument, whose keys can be :
236 The numerical signature.
237 If not specified or undefined, a random signature is generated.
238 If the signature matches an already defined magic, then the existant magic object is returned.
244 A code reference to a private data constructor.
245 It is called each time this magic is cast on a variable, and the scalar returned is used as private data storage for it.
246 C<$_[0]> is a reference to the magic object and C<@_[1 .. @_-1]> are all extra arguments that were passed to L</cast>.
250 C<get>, C<set>, C<len>, C<clear>, C<free>, C<copy>, C<local>, C<fetch>, C<store>, C<exists> and C<delete>
252 Code references to the corresponding magic callbacks.
253 You don't have to specify all of them : the magic associated with undefined entries simply won't be hooked.
254 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).
256 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<$num == VMG_OP_INFO_OBJECT>.
257 Both have a performance hit, but just getting the name is lighter than getting the op object.
259 Other arguments are specific to the magic hooked :
267 When the variable is an array or a scalar, C<$_[2]> contains the non-magical length.
268 The callback can return the new scalar or array length to use, or C<undef> to default to the normal length.
274 C<$_[2]> is a either a copy or an alias of the current key, which means that it is useless to try to change or cast magic on it.
275 C<$_[3]> is an alias to the current element (i.e. the value).
279 C<fetch>, C<store>, C<exists> and C<delete>
281 C<$_[2]> is an alias to the current key.
282 Nothing prevents you from changing it, but be aware that there lurk dangerous side effects.
283 For example, it may rightfully be readonly if the key was a bareword.
284 You can get a copy instead by passing C<< copy_key => 1 >> to L</wizard>, which allows you to safely assign to C<$_[2]> in order to e.g. redirect the action to another key.
285 This however has a little performance drawback because of the copy.
289 All the callbacks are expected to return an integer, which is passed straight to the perl magic API.
290 However, only the return value of the C<len> callback currently holds a meaning.
294 # A simple scalar tracer
295 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
296 set => sub { print STDERR "set to ${$_[0]}\n" },
297 free => sub { print STDERR "${$_[0]} was deleted\n" }
299 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.
304 croak 'Wrong number of arguments for wizard()' if @_ % 2;
306 my @keys = qw/sig data op_info get set len clear free/;
307 push @keys, 'copy' if MGf_COPY;
308 push @keys, 'dup' if MGf_DUP;
309 push @keys, 'local' if MGf_LOCAL;
310 push @keys, qw/fetch store exists delete copy_key/ if VMG_UVAR;
311 my $ret = eval { _wizard(map $opts{$_}, @keys) };
313 $err =~ s/\sat\s+.*?\n//;
321 With this tool, you can manually generate random magic signature between SIG_MIN and SIG_MAX inclusive.
322 That's the way L</wizard> creates them when no signature is supplied.
324 # Generate a signature
331 This accessor returns the magic signature of this wizard.
334 my $sig = getsig $wiz;
338 cast [$@%&*]var, [$wiz|$sig], ...
340 This function associates C<$wiz> magic to the variable supplied, without overwriting any other kind of magic.
341 You can also supply the numeric signature C<$sig> instead of C<$wiz>.
342 It returns true on success or when C<$wiz> magic is already present, and croaks on error or when no magic corresponds to the given signature (in case a C<$sig> was supplied).
343 All extra arguments specified after C<$wiz> are passed to the private data constructor in C<@_[1 .. @_-1]>.
344 If the variable isn't a hash, any C<uvar> callback of the wizard is safely ignored.
346 # Casts $wiz onto $x, and pass '1' to the data constructor.
350 The C<var> argument can be an array or hash value.
351 Magic for those behaves like for any other scalar, except that it is dispelled when the entry is deleted from the container.
352 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 :
355 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
357 If you want to overcome the possible deletion of the C<'TZ'> entry, you have no choice but to rely on C<store> uvar magic.
359 C<cast> can be called from any magical callback, and in particular from C<data>.
360 This allows you to recursively cast magic on datastructures :
365 my ($var, $depth) = @_;
369 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
370 } elsif ($r eq 'HASH') {
371 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
376 my ($var, $depth) = @_;
378 print "free $r at depth $depth\n";
390 When C<%h> goes out of scope, this will print something among the lines of :
394 free SCALAR at depth 2
395 free ARRAY at depth 1
396 free SCALAR at depth 3
397 free SCALAR at depth 3
399 Of course, this example does nothing with the values that are added after the C<cast>.
403 getdata [$@%&*]var, [$wiz|$sig]
405 This accessor fetches the private data associated with the magic C<$wiz> (or the signature C<$sig>) in the variable.
406 It croaks when C<$wiz> or C<$sig> 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.
408 # Get the attached data, or undef if the wizard does not attach any.
409 my $data = getdata $x, $wiz;
413 dispell [$@%&*]variable, [$wiz|$sig]
415 The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable.
416 You can also pass the magic signature C<$sig> as the second argument.
417 This function returns true on success, C<0> when no magic represented by C<$wiz> or C<$sig> could be found in the variable, and croaks if the supplied wizard or signature is invalid.
420 die 'no such magic in $x' unless dispell $x, $wiz;
426 The minimum integer used as a signature for user-defined magic.
430 The maximum integer used as a signature for user-defined magic.
434 SIG_NBR = SIG_MAX - SIG_MIN + 1
438 Evaluates to true iff the 'copy' magic is available.
442 Evaluates to true iff the 'dup' magic is available.
446 Evaluates to true iff the 'local' magic is available.
450 When this constant is true, you can use the C<fetch,store,exists,delete> callbacks on hashes.
452 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN>
454 True for perls that don't call 'len' magic when you push an element in a magical array.
456 =head2 C<VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID>
458 True for perls that don't call 'len' magic when you unshift in void context an element in a magical array.
460 =head2 C<VMG_COMPAT_ARRAY_UNDEF_CLEAR>
462 True for perls that call 'clear' magic when undefining magical arrays.
464 =head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN>
466 True for perls that don't call 'len' magic when taking the C<length> of a magical scalar.
468 =head2 C<VMG_PERL_PATCHLEVEL>
470 The perl patchlevel this module was built with, or C<0> for non-debugging perls.
472 =head2 C<VMG_THREADSAFE>
474 True iff this module could have been built with thread-safety features enabled.
476 =head2 C<VMG_FORKSAFE>
478 True iff this module could have been built with fork-safety features enabled.
479 This will always be true except on Windows where it's false for perl 5.10.0 and below .
481 =head2 C<VMG_OP_INFO_NAME>
483 Value to pass with C<op_info> to get the current op name in the magic callbacks.
485 =head2 C<VMG_OP_INFO_OBJECT>
487 Value to pass with C<op_info> to get a C<B::OP> object representing the current op in the magic callbacks.
489 =head1 PERL MAGIC HISTORY
491 The places where magic is invoked have changed a bit through perl history.
492 Here's a little list of the most recent ones.
500 I<p14416> : 'copy' and 'dup' magic.
506 I<p28160> : Integration of I<p25854> (see below).
508 I<p32542> : Integration of I<p31473> (see below).
514 I<p25854> : 'len' magic is no longer called when pushing an element into a magic array.
516 I<p26569> : 'local' magic.
522 I<p31064> : Meaningful 'uvar' magic.
524 I<p31473> : 'clear' magic wasn't invoked when undefining an array.
525 The bug is fixed as of this version.
531 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.
537 I<p32969> : 'len' magic is no longer invoked when calling C<length> with a magical scalar.
539 I<p34908> : 'len' magic is no longer called when pushing / unshifting an element into a magical array in void context.
540 The C<push> part was already covered by I<p25854>.
546 The functions L</wizard>, L</gensig>, L</getsig>, L</cast>, L</getdata> and L</dispell> are only exported on request.
547 All of them are exported by the tags C<':funcs'> and C<':all'>.
549 All the constants are also only exported on request, either individually or by the tags C<':consts'> and C<':all'>.
553 use base qw/Exporter/;
557 'funcs' => [ qw/wizard gensig getsig cast getdata dispell/ ],
559 qw/SIG_MIN SIG_MAX SIG_NBR MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR/,
560 qw/VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID VMG_COMPAT_ARRAY_UNDEF_CLEAR VMG_COMPAT_SCALAR_LENGTH_NOLEN/,
561 qw/VMG_PERL_PATCHLEVEL/,
562 qw/VMG_THREADSAFE VMG_FORKSAFE/,
563 qw/VMG_OP_INFO_NAME VMG_OP_INFO_OBJECT/
566 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
567 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
571 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.
572 The only way to address this would be to return a reference.
574 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.
580 L<Carp> (standard since perl 5), L<XSLoader> (standard since perl 5.006).
582 Copy tests need L<Tie::Array> (standard since perl 5.005) and L<Tie::Hash> (since 5.002).
584 Some uvar tests need L<Hash::Util::FieldHash> (standard since perl 5.009004).
586 Glob tests need L<Symbol> (standard since perl 5.002).
588 Threads tests need L<threads> and L<threads::shared>.
592 L<perlguts> and L<perlapi> for internal information about magic.
594 L<perltie> and L<overload> for other ways of enhancing objects.
598 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
600 You can contact me by mail or on C<irc.perl.org> (vincent).
604 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.
608 You can find documentation for this module with the perldoc command.
610 perldoc Variable::Magic
612 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>.
614 =head1 COPYRIGHT & LICENSE
616 Copyright 2007-2009 Vincent Pit, all rights reserved.
618 This program is free software; you can redistribute it and/or modify it
619 under the same terms as Perl itself.
623 1; # End of Variable::Magic