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__, $VERSION);
211 wizard data => sub { ... },
212 get => sub { my ($ref, $data [, $op]) = @_; ... },
213 set => sub { my ($ref, $data [, $op]) = @_; ... },
214 len => sub { my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen; },
215 clear => sub { my ($ref, $data [, $op]) = @_; ... },
216 free => sub { my ($ref, $data [, $op]) = @_, ... },
217 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
218 local => sub { my ($ref, $data [, $op]) = @_; ... },
219 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
220 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
221 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
222 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
224 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ]
226 This function creates a 'wizard', an opaque type that holds the magic information.
227 It takes a list of keys / values as argument, whose keys can be :
235 A code reference to a private data constructor.
236 It is called each time this magic is cast on a variable, and the scalar returned is used as private data storage for it.
237 C<$_[0]> is a reference to the magic object and C<@_[1 .. @_-1]> are all extra arguments that were passed to L</cast>.
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 references to the corresponding magic callbacks.
244 You don't have to specify all of them : the magic associated with undefined entries simply won't be hooked.
245 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).
247 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<$num == VMG_OP_INFO_OBJECT>.
248 Both have a performance hit, but just getting the name is lighter than getting the op object.
250 Other arguments are specific to the magic hooked :
258 When the variable is an array or a scalar, C<$_[2]> contains the non-magical length.
259 The callback can return the new scalar or array length to use, or C<undef> to default to the normal length.
265 C<$_[2]> is a either a copy or an alias of the current key, which means that it is useless to try to change or cast magic on it.
266 C<$_[3]> is an alias to the current element (i.e. the value).
270 C<fetch>, C<store>, C<exists> and C<delete>
272 C<$_[2]> is an alias to the current key.
273 Nothing prevents you from changing it, but be aware that there lurk dangerous side effects.
274 For example, it may rightfully be readonly if the key was a bareword.
275 You can get a copy instead by passing C<< copy_key => 1 >> to L</wizard>, which allows you to safely assign to C<$_[2]> in order to e.g. redirect the action to another key.
276 This however has a little performance drawback because of the copy.
280 All the callbacks are expected to return an integer, which is passed straight to the perl magic API.
281 However, only the return value of the C<len> callback currently holds a meaning.
285 # A simple scalar tracer
286 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
287 set => sub { print STDERR "set to ${$_[0]}\n" },
288 free => sub { print STDERR "${$_[0]} was deleted\n" }
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.
295 croak 'Wrong number of arguments for wizard()' if @_ % 2;
297 my @keys = qw/data op_info get set len clear free/;
298 push @keys, 'copy' if MGf_COPY;
299 push @keys, 'dup' if MGf_DUP;
300 push @keys, 'local' if MGf_LOCAL;
301 push @keys, qw/fetch store exists delete copy_key/ if VMG_UVAR;
302 my $ret = eval { _wizard(map $opts{$_}, @keys) };
304 $err =~ s/\sat\s+.*?\n//;
312 cast [$@%&*]var, $wiz, ...
314 This function associates C<$wiz> magic to the variable supplied, without overwriting any other kind of magic.
315 It returns true on success or when C<$wiz> magic is already present, and croaks on error.
316 All extra arguments specified after C<$wiz> are passed to the private data constructor in C<@_[1 .. @_-1]>.
317 If the variable isn't a hash, any C<uvar> callback of the wizard is safely ignored.
319 # Casts $wiz onto $x, and pass '1' to the data constructor.
323 The C<var> argument can be an array or hash value.
324 Magic for those behaves like for any other scalar, except that it is dispelled when the entry is deleted from the container.
325 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 :
328 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
330 If you want to overcome the possible deletion of the C<'TZ'> entry, you have no choice but to rely on C<store> uvar magic.
334 getdata [$@%&*]var, $wiz
336 This accessor fetches the private data associated with the magic C<$wiz> in the variable.
337 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.
339 # Get the attached data, or undef if the wizard does not attach any.
340 my $data = getdata $x, $wiz;
344 dispell [$@%&*]variable, $wiz
346 The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable.
347 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.
350 die 'no such magic in $x' unless dispell $x, $wiz;
356 Evaluates to true iff the 'copy' magic is available.
360 Evaluates to true iff the 'dup' magic is available.
364 Evaluates to true iff the 'local' magic is available.
368 When this constant is true, you can use the C<fetch,store,exists,delete> callbacks on hashes.
370 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN>
372 True for perls that don't call 'len' magic when you push an element in a magical array.
373 Starting from perl 5.11.0, this only refers to pushes in non-void context and hence is false.
375 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID>
377 True for perls that don't call 'len' magic when you push in void context an element in a magical array.
379 =head2 C<VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID>
381 True for perls that don't call 'len' magic when you unshift in void context an element in a magical array.
383 =head2 C<VMG_COMPAT_ARRAY_UNDEF_CLEAR>
385 True for perls that call 'clear' magic when undefining magical arrays.
387 =head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN>
389 True for perls that don't call 'len' magic when taking the C<length> of a magical scalar.
391 =head2 C<VMG_PERL_PATCHLEVEL>
393 The perl patchlevel this module was built with, or C<0> for non-debugging perls.
395 =head2 C<VMG_THREADSAFE>
397 True iff this module could have been built with thread-safety features enabled.
399 =head2 C<VMG_FORKSAFE>
401 True iff this module could have been built with fork-safety features enabled.
402 This will always be true except on Windows where it's false for perl 5.10.0 and below .
404 =head2 C<VMG_OP_INFO_NAME>
406 Value to pass with C<op_info> to get the current op name in the magic callbacks.
408 =head2 C<VMG_OP_INFO_OBJECT>
410 Value to pass with C<op_info> to get a C<B::OP> object representing the current op in the magic callbacks.
414 =head2 Associate an object to any perl variable
416 This can be useful for passing user data through limited APIs.
419 package Magical::UserData;
421 use Variable::Magic qw/wizard cast getdata/;
423 my $wiz = wizard data => sub { \$_[1] };
425 sub ud (\[$@%*&]) : lvalue {
427 my $data = &getdata($var, $wiz);
428 unless (defined $data) {
430 $data = &getdata($var, $wiz);
431 die "Couldn't cast UserData magic onto the variable" unless defined $data;
438 BEGIN { *ud = \&Magical::UserData::ud }
441 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
444 $cb->(); # Hello, world!
447 =head2 Recursively cast magic on datastructures
449 C<cast> can be called from any magical callback, and in particular from C<data>.
450 This allows you to recursively cast magic on datastructures :
453 $wiz = wizard data => sub {
454 my ($var, $depth) = @_;
458 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
459 } elsif ($r eq 'HASH') {
460 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
465 my ($var, $depth) = @_;
467 print "free $r at depth $depth\n";
479 When C<%h> goes out of scope, this will print something among the lines of :
483 free SCALAR at depth 2
484 free ARRAY at depth 1
485 free SCALAR at depth 3
486 free SCALAR at depth 3
488 Of course, this example does nothing with the values that are added after the C<cast>.
490 =head1 PERL MAGIC HISTORY
492 The places where magic is invoked have changed a bit through perl history.
493 Here's a little list of the most recent ones.
501 I<p14416> : 'copy' and 'dup' magic.
507 I<p28160> : Integration of I<p25854> (see below).
509 I<p32542> : Integration of I<p31473> (see below).
515 I<p25854> : 'len' magic is no longer called when pushing an element into a magic array.
517 I<p26569> : 'local' magic.
523 I<p31064> : Meaningful 'uvar' magic.
525 I<p31473> : 'clear' magic wasn't invoked when undefining an array.
526 The bug is fixed as of this version.
532 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.
538 I<p32969> : 'len' magic is no longer invoked when calling C<length> with a magical scalar.
540 I<p34908> : 'len' magic is no longer called when pushing / unshifting an element into a magical array in void context.
541 The C<push> part was already covered by I<p25854>.
543 I<g9cdcb38b> : 'len' magic is called again when pushing into a magical array in non-void context.
549 The functions L</wizard>, L</cast>, L</getdata> and L</dispell> are only exported on request.
550 All of them are exported by the tags C<':funcs'> and C<':all'>.
552 All the constants are also only exported on request, either individually or by the tags C<':consts'> and C<':all'>.
556 use base qw/Exporter/;
560 'funcs' => [ qw/wizard cast getdata dispell/ ],
562 qw/MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR/,
563 qw/VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID/,
564 qw/VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID/,
565 qw/VMG_COMPAT_ARRAY_UNDEF_CLEAR/,
566 qw/VMG_COMPAT_SCALAR_LENGTH_NOLEN/,
567 qw/VMG_PERL_PATCHLEVEL/,
568 qw/VMG_THREADSAFE VMG_FORKSAFE/,
569 qw/VMG_OP_INFO_NAME VMG_OP_INFO_OBJECT/
572 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
573 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
577 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.
578 The only way to address this would be to return a reference.
580 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.
586 L<Carp> (standard since perl 5), L<XSLoader> (standard since perl 5.006).
588 Copy tests need L<Tie::Array> (standard since perl 5.005) and L<Tie::Hash> (since 5.002).
590 Some uvar tests need L<Hash::Util::FieldHash> (standard since perl 5.009004).
592 Glob tests need L<Symbol> (standard since perl 5.002).
594 Threads tests need L<threads> and L<threads::shared>.
598 L<perlguts> and L<perlapi> for internal information about magic.
600 L<perltie> and L<overload> for other ways of enhancing objects.
604 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
606 You can contact me by mail or on C<irc.perl.org> (vincent).
610 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.
614 You can find documentation for this module with the perldoc command.
616 perldoc Variable::Magic
618 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>.
620 =head1 COPYRIGHT & LICENSE
622 Copyright 2007-2009 Vincent Pit, all rights reserved.
624 This program is free software; you can redistribute it and/or modify it
625 under the same terms as Perl itself.
629 1; # End of Variable::Magic