1 package Variable::Magic;
12 Variable::Magic - Associate user-defined magic to variables from Perl.
27 use Variable::Magic qw/wizard cast dispell/;
29 my $wiz = wizard set => sub { print STDERR "now set to ${$_[0]}!\n" };
32 $a = 2; # "now set to 2!"
38 Magic is Perl way of enhancing objects.
39 This mechanism lets the user add extra data to any variable and hook syntaxical operations (such as access, assignment or destruction) that can be applied to it.
40 With this module, you can add your own magic to any variable without having to write a single line of XS.
42 You'll realize that these magic variables look a lot like tied variables.
43 It's not surprising, as tied variables are implemented as a special kind of magic, just like any 'irregular' Perl variable : scalars like C<$!>, C<$(> or C<$^W>, the C<%ENV> and C<%SIG> hashes, the C<@ISA> array, C<vec()> and C<substr()> lvalues, L<thread::shared> variables...
44 They all share the same underlying C API, and this module gives you direct access to it.
46 Still, the magic made available by this module differs from tieing and overloading in several ways :
52 It isn't copied on assignment.
54 You attach it to variables, not values (as for blessed references).
58 It doesn't replace the original semantics.
60 Magic callbacks trigger before the original action take place, and can't prevent it to happen.
61 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.
67 The same magic can be applied on scalars, arrays, hashes, subs or globs.
68 But the same hook (see below for a list) may trigger differently depending on the the type of the variable.
72 It's mostly invisible at the Perl level.
74 Magical and non-magical variables cannot be distinguished with C<ref>, C<tied> or another trick.
80 Mainly because perl's way of handling magic is lighter by nature, and because there's no need for any method resolution.
81 Also, since you don't have to reimplement all the variable semantics, you only pay for what you actually use.
85 The operations that can be overloaded are :
93 This magic is invoked when the variable is evaluated (does not include array/hash subscripts and slices).
99 This one is triggered each time the value of the variable changes (includes array/hash subscripts and slices).
105 This magic is a little special : it is called when the 'size' or the 'length' of the variable has to be known by Perl.
106 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>).
107 The callback has then to return the length as an integer.
113 This magic is invoked when the variable is reset, such as when an array is emptied.
114 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>).
120 This one can be considered as an object destructor.
121 It happens when the variable goes out of scope (with the exception of global scope), but not when it is undefined.
127 This magic only applies to tied arrays and hashes.
128 It fires when you try to access or change their elements.
129 It is available on your perl iff C<MGf_COPY> is true.
135 Invoked when the variable is cloned across threads.
136 Currently not available.
142 When this magic is set on a variable, all subsequent localizations of the variable will trigger the callback.
143 It is available on your perl iff C<MGf_LOCAL> is true.
147 The following actions only apply to hashes and are available iff C<VMG_UVAR> is true.
148 They are referred to as C<uvar> magics.
156 This magic happens each time an element is fetched from the hash.
162 This one is called when an element is stored into the hash.
168 This magic fires when a key is tested for existence in the hash.
174 This last one triggers when a key is deleted in the hash, regardless of whether the key actually exists in it.
178 You can refer to the tests to have more insight of where the different magics are invoked.
180 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).
188 XSLoader::load(__PACKAGE__, $VERSION);
195 get => sub { my ($ref, $data [, $op]) = @_; ... },
196 set => sub { my ($ref, $data [, $op]) = @_; ... },
197 len => sub { my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen; },
198 clear => sub { my ($ref, $data [, $op]) = @_; ... },
199 free => sub { my ($ref, $data [, $op]) = @_, ... },
200 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
201 local => sub { my ($ref, $data [, $op]) = @_; ... },
202 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
203 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
204 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
205 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
207 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ]
209 This function creates a 'wizard', an opaque type that holds the magic information.
210 It takes a list of keys / values as argument, whose keys can be :
218 The numerical signature.
219 If not specified or undefined, a random signature is generated.
220 If the signature matches an already defined magic, then the existant magic object is returned.
226 A code reference to a private data constructor.
227 It is called each time this magic is cast on a variable, and the scalar returned is used as private data storage for it.
228 C<$_[0]> is a reference to the magic object and C<@_[1 .. @_-1]> are all extra arguments that were passed to L</cast>.
232 C<get>, C<set>, C<len>, C<clear>, C<free>, C<copy>, C<local>, C<fetch>, C<store>, C<exists> and C<delete>
234 Code references to the corresponding magic callbacks.
235 You don't have to specify all of them : the magic associated with undefined entries simply won't be hooked.
236 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).
238 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<$num == VMG_OP_INFO_OBJECT>.
239 Both have a performance hit, but just getting the name is lighter than getting the op object.
241 Other arguments are specific to the magic hooked :
249 When the variable is an array or a scalar, C<$_[2]> contains the non-magical length.
250 The callback can return the new scalar or array length to use, or C<undef> to default to the normal length.
256 C<$_[2]> is a either a copy or an alias of the current key, which means that it is useless to try to change or cast magic on it.
257 C<$_[3]> is an alias to the current element (i.e. the value).
261 C<fetch>, C<store>, C<exists> and C<delete>
263 C<$_[2]> is an alias to the current key.
264 Nothing prevents you from changing it, but be aware that there lurk dangerous side effects.
265 For example, it may righteously be readonly if the key was a bareword.
266 You can get a copy instead by passing C<< copy_key => 1 >> to L</wizard>, which allows you to safely assign to C<$_[2]> in order to e.g. redirect the action to another key.
267 This however has a little performance drawback because of the copy.
271 All the callbacks are expected to return an integer, which is passed straight to the perl magic API.
272 However, only the return value of the C<len> callback currently holds a meaning.
276 # A simple scalar tracer
277 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
278 set => sub { print STDERR "set to ${$_[0]}\n" },
279 free => sub { print STDERR "${$_[0]} was deleted\n" }
284 croak 'Wrong number of arguments for wizard()' if @_ % 2;
286 my @keys = qw/sig data op_info get set len clear free/;
287 push @keys, 'copy' if MGf_COPY;
288 push @keys, 'dup' if MGf_DUP;
289 push @keys, 'local' if MGf_LOCAL;
290 push @keys, qw/fetch store exists delete copy_key/ if VMG_UVAR;
291 my $ret = eval { _wizard(map $opts{$_}, @keys) };
293 $err =~ s/\sat\s+.*?\n//;
301 With this tool, you can manually generate random magic signature between SIG_MIN and SIG_MAX inclusive.
302 That's the way L</wizard> creates them when no signature is supplied.
304 # Generate a signature
311 This accessor returns the magic signature of this wizard.
314 my $sig = getsig $wiz;
318 cast [$@%&*]var, [$wiz|$sig], ...
320 This function associates C<$wiz> magic to the variable supplied, without overwriting any other kind of magic.
321 You can also supply the numeric signature C<$sig> instead of C<$wiz>.
322 It returns true on success or when C<$wiz> magic is already present, C<0> on error, and C<undef> when no magic corresponds to the given signature (in case C<$sig> was supplied).
323 All extra arguments specified after C<$wiz> are passed to the private data constructor.
324 If the variable isn't a hash, any C<uvar> callback of the wizard is safely ignored.
326 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
328 die 'error' unless cast $x, $wiz;
330 The C<var> argument can be an array or hash value.
331 Magic for those behaves like for any other scalar, except that it is dispelled when the entry is deleted from the container.
332 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 :
335 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
337 If you want to overcome the possible deletion of the C<'TZ'> entry, you have no choice but to rely on C<store> uvar magic.
341 getdata [$@%&*]var, [$wiz|$sig]
343 This accessor fetches the private data associated with the magic C<$wiz> (or the signature C<$sig>) in the variable.
344 C<undef> is returned when no such magic or data is found, or when C<$sig> does not represent a current valid magic object.
346 # Get the attached data.
347 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
351 dispell [$@%&*]variable, [$wiz|$sig]
353 The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable.
354 You can also pass the magic signature C<$sig> as the second argument.
355 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).
357 # Dispell now. If $wiz isn't a signature, undef can't be returned.
358 die 'no such magic or error' unless dispell $x, $wiz;
364 The minimum integer used as a signature for user-defined magic.
368 The maximum integer used as a signature for user-defined magic.
372 SIG_NBR = SIG_MAX - SIG_MIN + 1
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.
390 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN>
392 True for perls that don't call 'len' magic when you push an element in a magical array.
394 =head2 C<VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID>
396 True for perls that don't call 'len' magic when you unshift in void context an element in a magical array.
398 =head2 C<VMG_COMPAT_ARRAY_UNDEF_CLEAR>
400 True for perls that call 'clear' magic when undefining magical arrays.
402 =head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN>
404 True for perls that don't call 'len' magic when taking the C<length> of a magical scalar.
406 =head2 C<VMG_PERL_PATCHLEVEL>
408 The perl patchlevel this module was built with, or C<0> for non-debugging perls.
410 =head2 C<VMG_THREADSAFE>
412 True iff this module could have been built with thread-safety features enabled.
414 =head2 C<VMG_OP_INFO_NAME>
416 Value to pass with C<op_info> to get the current op name in the magic callbacks.
418 =head2 C<VMG_OP_INFO_OBJECT>
420 Value to pass with C<op_info> to get a C<B::OP> object representing the current op in the magic callbacks.
422 =head1 PERL MAGIC HISTORY
424 The places where magic is invoked have changed a bit through perl history.
425 Here's a little list of the most recent ones.
433 I<p14416> : 'copy' and 'dup' magic.
439 I<p28160> : Integration of I<p25854> (see below).
441 I<p32542> : Integration of I<p31473> (see below).
447 I<p25854> : 'len' magic is no longer called when pushing an element into a magic array.
449 I<p26569> : 'local' magic.
455 I<p31064> : Meaningful 'uvar' magic.
457 I<p31473> : 'clear' magic wasn't invoked when undefining an array.
458 The bug is fixed as of this version.
464 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.
470 I<p32969> : 'len' magic is no longer invoked when calling C<length> with a magical scalar.
472 I<p34908> : 'len' magic is no longer called when pushing / unshifting an element into a magical array in void context.
473 The C<push> part was already covered by I<p25854>.
479 The functions L</wizard>, L</gensig>, L</getsig>, L</cast>, L</getdata> and L</dispell> are only exported on request.
480 All of them are exported by the tags C<':funcs'> and C<':all'>.
482 All the constants are also only exported on request, either individually or by the tags C<':consts'> and C<':all'>.
486 use base qw/Exporter/;
490 'funcs' => [ qw/wizard gensig getsig cast getdata dispell/ ],
492 qw/SIG_MIN SIG_MAX SIG_NBR MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR/,
493 qw/VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID VMG_COMPAT_ARRAY_UNDEF_CLEAR VMG_COMPAT_SCALAR_LENGTH_NOLEN/,
494 qw/VMG_PERL_PATCHLEVEL/,
496 qw/VMG_OP_INFO_NAME VMG_OP_INFO_OBJECT/
499 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
500 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
504 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.
505 The only way to address this would be to return a reference.
507 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.
513 L<Carp> (standard since perl 5), L<XSLoader> (standard since perl 5.006).
515 Copy tests need L<Tie::Array> (standard since perl 5.005) and L<Tie::Hash> (since 5.002).
517 Some uvar tests need L<Hash::Util::FieldHash> (standard since perl 5.009004).
519 Glob tests need L<Symbol> (standard since perl 5.002).
521 Threads tests need L<threads> and L<threads::shared>.
525 L<perlguts> and L<perlapi> for internal information about magic.
527 L<perltie> and L<overload> for other ways of enhancing objects.
531 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
533 You can contact me by mail or on C<irc.perl.org> (vincent).
537 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.
541 You can find documentation for this module with the perldoc command.
543 perldoc Variable::Magic
545 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>.
547 =head1 COPYRIGHT & LICENSE
549 Copyright 2007-2009 Vincent Pit, all rights reserved.
551 This program is free software; you can redistribute it and/or modify it
552 under the same terms as Perl itself.
556 1; # End of Variable::Magic