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 trigger before the original action take place, and can't prevent it to happen.
76 This 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 (does not include array/hash subscripts and slices).
114 This one is triggered each time the value of the variable changes (includes array/hash subscripts and slices).
120 This magic is a little special : it is called when the 'size' or the 'length' of the variable has to be known by Perl.
121 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>).
122 The callback has then to return the length as an integer.
128 This magic is invoked when a container variable is reset, i.e. when an array or a hash is emptied.
129 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>).
135 This one can be considered as an object destructor.
136 It happens when the variable goes out of scope (with the exception of global scope), but not when it is undefined.
142 This magic only applies to tied arrays and hashes.
143 It fires when you try to access or change their elements.
144 It is available on your perl iff C<MGf_COPY> is true.
150 Invoked when the variable is cloned across threads.
151 Currently not available.
157 When this magic is set on a variable, all subsequent localizations of the variable will trigger the callback.
158 It is available on your perl iff C<MGf_LOCAL> is true.
162 The following actions only apply to hashes and are available iff C<VMG_UVAR> is true.
163 They are referred to as C<uvar> magics.
171 This magic happens each time an element is fetched from the hash.
177 This one is called when an element is stored into the hash.
183 This magic fires when a key is tested for existence in the hash.
189 This last one triggers when a key is deleted in the hash, regardless of whether the key actually exists in it.
193 You can refer to the tests to have more insight of where the different magics are invoked.
195 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).
196 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.
204 XSLoader::load(__PACKAGE__, $VERSION);
211 get => sub { my ($ref, $data [, $op]) = @_; ... },
212 set => sub { my ($ref, $data [, $op]) = @_; ... },
213 len => sub { my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen; },
214 clear => sub { my ($ref, $data [, $op]) = @_; ... },
215 free => sub { my ($ref, $data [, $op]) = @_, ... },
216 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
217 local => sub { my ($ref, $data [, $op]) = @_; ... },
218 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
219 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
220 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
221 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
223 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ]
225 This function creates a 'wizard', an opaque type that holds the magic information.
226 It takes a list of keys / values as argument, whose keys can be :
234 The numerical signature.
235 If not specified or undefined, a random signature is generated.
236 If the signature matches an already defined magic, then the existant magic object is returned.
242 A code reference to a private data constructor.
243 It is called each time this magic is cast on a variable, and the scalar returned is used as private data storage for it.
244 C<$_[0]> is a reference to the magic object and C<@_[1 .. @_-1]> are all extra arguments that were passed to L</cast>.
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 references to the corresponding magic callbacks.
251 You don't have to specify all of them : the magic associated with undefined entries simply won't be hooked.
252 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).
254 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<$num == VMG_OP_INFO_OBJECT>.
255 Both have a performance hit, but just getting the name is lighter than getting the op object.
257 Other arguments are specific to the magic hooked :
265 When the variable is an array or a scalar, C<$_[2]> contains the non-magical length.
266 The callback can return the new scalar or array length to use, or C<undef> to default to the normal length.
272 C<$_[2]> is a either a copy or an alias of the current key, which means that it is useless to try to change or cast magic on it.
273 C<$_[3]> is an alias to the current element (i.e. the value).
277 C<fetch>, C<store>, C<exists> and C<delete>
279 C<$_[2]> is an alias to the current key.
280 Nothing prevents you from changing it, but be aware that there lurk dangerous side effects.
281 For example, it may righteously be readonly if the key was a bareword.
282 You can get a copy instead by passing C<< copy_key => 1 >> to L</wizard>, which allows you to safely assign to C<$_[2]> in order to e.g. redirect the action to another key.
283 This however has a little performance drawback because of the copy.
287 All the callbacks are expected to return an integer, which is passed straight to the perl magic API.
288 However, only the return value of the C<len> callback currently holds a meaning.
292 # A simple scalar tracer
293 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
294 set => sub { print STDERR "set to ${$_[0]}\n" },
295 free => sub { print STDERR "${$_[0]} was deleted\n" }
297 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.
302 croak 'Wrong number of arguments for wizard()' if @_ % 2;
304 my @keys = qw/sig data op_info get set len clear free/;
305 push @keys, 'copy' if MGf_COPY;
306 push @keys, 'dup' if MGf_DUP;
307 push @keys, 'local' if MGf_LOCAL;
308 push @keys, qw/fetch store exists delete copy_key/ if VMG_UVAR;
309 my $ret = eval { _wizard(map $opts{$_}, @keys) };
311 $err =~ s/\sat\s+.*?\n//;
319 With this tool, you can manually generate random magic signature between SIG_MIN and SIG_MAX inclusive.
320 That's the way L</wizard> creates them when no signature is supplied.
322 # Generate a signature
329 This accessor returns the magic signature of this wizard.
332 my $sig = getsig $wiz;
336 cast [$@%&*]var, [$wiz|$sig], ...
338 This function associates C<$wiz> magic to the variable supplied, without overwriting any other kind of magic.
339 You can also supply the numeric signature C<$sig> instead of C<$wiz>.
340 It returns true on success or when C<$wiz> magic is already present, C<0> on error, and C<undef> when no magic corresponds to the given signature (in case C<$sig> was supplied).
341 All extra arguments specified after C<$wiz> are passed to the private data constructor.
342 If the variable isn't a hash, any C<uvar> callback of the wizard is safely ignored.
344 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
346 die 'error' unless cast $x, $wiz;
348 The C<var> argument can be an array or hash value.
349 Magic for those behaves like for any other scalar, except that it is dispelled when the entry is deleted from the container.
350 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 :
353 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
355 If you want to overcome the possible deletion of the C<'TZ'> entry, you have no choice but to rely on C<store> uvar magic.
357 C<cast> can be called from any magical callback, and in particular from C<data>.
358 This allows you to recursively cast magic on datastructures :
363 my ($var, $depth) = @_;
367 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
368 } elsif ($r eq 'HASH') {
369 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
374 my ($var, $depth) = @_;
376 print "free $r at depth $depth\n";
388 When C<%h> goes out of scope, this will print something among the lines of :
392 free SCALAR at depth 2
393 free ARRAY at depth 1
394 free SCALAR at depth 3
395 free SCALAR at depth 3
397 Of course, this example does nothing with the values that are added after the C<cast>.
401 getdata [$@%&*]var, [$wiz|$sig]
403 This accessor fetches the private data associated with the magic C<$wiz> (or the signature C<$sig>) in the variable.
404 C<undef> is returned when no such magic or data is found, or when C<$sig> does not represent a current valid magic object.
406 # Get the attached data.
407 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
411 dispell [$@%&*]variable, [$wiz|$sig]
413 The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable.
414 You can also pass the magic signature C<$sig> as the second argument.
415 True is returned on success, C<0> on error or when no magic represented by C<$wiz> could be found in the variable, and C<undef> when no magic corresponds to the given signature (in case C<$sig> was supplied).
417 # Dispell now. If $wiz isn't a signature, undef can't be returned.
418 die 'no such magic or error' unless dispell $x, $wiz;
424 The minimum integer used as a signature for user-defined magic.
428 The maximum integer used as a signature for user-defined magic.
432 SIG_NBR = SIG_MAX - SIG_MIN + 1
436 Evaluates to true iff the 'copy' magic is available.
440 Evaluates to true iff the 'dup' magic is available.
444 Evaluates to true iff the 'local' magic is available.
448 When this constant is true, you can use the C<fetch,store,exists,delete> callbacks on hashes.
450 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN>
452 True for perls that don't call 'len' magic when you push an element in a magical array.
454 =head2 C<VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID>
456 True for perls that don't call '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 'clear' magic when undefining magical arrays.
462 =head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN>
464 True for perls that don't call 'len' magic when taking the C<length> of a magical scalar.
466 =head2 C<VMG_PERL_PATCHLEVEL>
468 The perl patchlevel this module was built with, or C<0> for non-debugging perls.
470 =head2 C<VMG_THREADSAFE>
472 True iff this module could have been built with thread-safety features enabled.
474 =head2 C<VMG_OP_INFO_NAME>
476 Value to pass with C<op_info> to get the current op name in the magic callbacks.
478 =head2 C<VMG_OP_INFO_OBJECT>
480 Value to pass with C<op_info> to get a C<B::OP> object representing the current op in the magic callbacks.
482 =head1 PERL MAGIC HISTORY
484 The places where magic is invoked have changed a bit through perl history.
485 Here's a little list of the most recent ones.
493 I<p14416> : 'copy' and 'dup' magic.
499 I<p28160> : Integration of I<p25854> (see below).
501 I<p32542> : Integration of I<p31473> (see below).
507 I<p25854> : 'len' magic is no longer called when pushing an element into a magic array.
509 I<p26569> : 'local' magic.
515 I<p31064> : Meaningful 'uvar' magic.
517 I<p31473> : 'clear' magic wasn't invoked when undefining an array.
518 The bug is fixed as of this version.
524 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.
530 I<p32969> : 'len' magic is no longer invoked when calling C<length> with a magical scalar.
532 I<p34908> : 'len' magic is no longer called when pushing / unshifting an element into a magical array in void context.
533 The C<push> part was already covered by I<p25854>.
539 The functions L</wizard>, L</gensig>, L</getsig>, L</cast>, L</getdata> and L</dispell> are only exported on request.
540 All of them are exported by the tags C<':funcs'> and C<':all'>.
542 All the constants are also only exported on request, either individually or by the tags C<':consts'> and C<':all'>.
546 use base qw/Exporter/;
550 'funcs' => [ qw/wizard gensig getsig cast getdata dispell/ ],
552 qw/SIG_MIN SIG_MAX SIG_NBR MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR/,
553 qw/VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID VMG_COMPAT_ARRAY_UNDEF_CLEAR VMG_COMPAT_SCALAR_LENGTH_NOLEN/,
554 qw/VMG_PERL_PATCHLEVEL/,
556 qw/VMG_OP_INFO_NAME VMG_OP_INFO_OBJECT/
559 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
560 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
564 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.
565 The only way to address this would be to return a reference.
567 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.
573 L<Carp> (standard since perl 5), L<XSLoader> (standard since perl 5.006).
575 Copy tests need L<Tie::Array> (standard since perl 5.005) and L<Tie::Hash> (since 5.002).
577 Some uvar tests need L<Hash::Util::FieldHash> (standard since perl 5.009004).
579 Glob tests need L<Symbol> (standard since perl 5.002).
581 Threads tests need L<threads> and L<threads::shared>.
585 L<perlguts> and L<perlapi> for internal information about magic.
587 L<perltie> and L<overload> for other ways of enhancing objects.
591 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
593 You can contact me by mail or on C<irc.perl.org> (vincent).
597 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.
601 You can find documentation for this module with the perldoc command.
603 perldoc Variable::Magic
605 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>.
607 =head1 COPYRIGHT & LICENSE
609 Copyright 2007-2009 Vincent Pit, all rights reserved.
611 This program is free software; you can redistribute it and/or modify it
612 under the same terms as Perl itself.
616 1; # End of Variable::Magic