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 let the user add extra data to any variable and hook syntaxical operations (such as access, assignation or destruction) that can be applied to it.
40 With this module, you can add your own magic to any variable without the pain of the C API.
42 Magic differs from tieing and overloading in several ways :
48 Magic isn't copied on assignation (as for blessed references) : you attach it to variables, not values.
52 It doesn't replace the original semantics : magic callbacks trigger before the original action take place, and can't prevent it to happen.
56 It's mostly invisible at the Perl level : magical and non-magical variables cannot be distinguished with C<ref>, C<reftype> or another trick.
60 It's notably faster, since perl's way of handling magic is lighter by nature, and there's no need for any method resolution.
64 The operations that can be overloaded are :
72 This magic is invoked when the variable is evaluated (does not include array/hash subscripts and slices).
78 This one is triggered each time the value of the variable changes (includes array/hash subscripts and slices).
84 This magic is a little special : it is called when the 'size' or the 'length' of the variable has to be known by Perl.
85 Typically, it's the magic involved when an array is evaluated in scalar context, but also on array assignation and loops (C<for>, C<map> or C<grep>).
86 The callback has then to return the length as an integer.
92 This magic is invoked when the variable is reset, such as when an array is emptied.
93 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>).
99 This one can be considered as an object destructor.
100 It happens when the variable goes out of scope (with the exception of global scope), but not when it is undefined.
106 This magic only applies to tied arrays and hashes.
107 It fires when you try to access or change their elements.
108 It is available on your perl iff C<MGf_COPY> is true.
114 Invoked when the variable is cloned across threads.
115 Currently not available.
121 When this magic is set on a variable, all subsequent localizations of the variable will trigger the callback.
122 It is available on your perl iff C<MGf_LOCAL> is true.
126 The following actions only apply to hashes and are available iff C<VMG_UVAR> is true.
127 They are referred to as C<uvar> magics.
135 This magic happens each time an element is fetched from the hash.
141 This one is called when an element is stored into the hash.
147 This magic fires when a key is tested for existence in the hash.
153 This last one triggers when a key is deleted in the hash, regardless of whether the key actually exists in it.
157 You can refer to the tests to have more insight of where the different magics are invoked.
159 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).
161 =head1 PERL MAGIC HISTORY
163 The places where magic is invoked have changed a bit through perl history.
164 Here's a little list of the most recent ones.
172 I<p14416> : 'copy' and 'dup' magic.
178 I<p28160> : Integration of I<p25854> (see below).
180 I<p32542> : Integration of I<p31473> (see below).
186 I<p25854> : 'len' magic is no longer called when pushing an element into a magic array.
188 I<p26569> : 'local' magic.
194 I<p31064> : Meaningful 'uvar' magic.
196 I<p31473> : 'clear' magic wasn't invoked when undefining an array.
197 The bug is fixed as of this version.
203 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.
209 I<p32969> : 'len' magic is no longer invoked when calling C<length> with a magical scalar.
211 I<p34908> : 'len' magic is no longer called when pushing / unshifting an element into a magical array in void context.
212 The C<push> part was already covered by I<p25854>.
220 The minimum integer used as a signature for user-defined magic.
224 The maximum integer used as a signature for user-defined magic.
228 SIG_NBR = SIG_MAX - SIG_MIN + 1
232 Evaluates to true iff the 'copy' magic is available.
236 Evaluates to true iff the 'dup' magic is available.
240 Evaluates to true iff the 'local' magic is available.
244 When this constant is true, you can use the C<fetch,store,exists,delete> callbacks on hashes.
246 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN>
248 True for perls that don't call 'len' magic when you push an element in a magical array.
250 =head2 C<VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID>
252 True for perls that don't call 'len' magic when you unshift in void context an element in a magical array.
254 =head2 C<VMG_COMPAT_ARRAY_UNDEF_CLEAR>
256 True for perls that call 'clear' magic when undefining magical arrays.
258 =head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN>
260 True for perls that don't call 'len' magic when taking the C<length> of a magical scalar.
262 =head2 C<VMG_PERL_PATCHLEVEL>
264 The perl patchlevel this module was built with, or C<0> for non-debugging perls.
266 =head2 C<VMG_THREADSAFE>
268 True iff this module could have been built with thread-safety features enabled.
270 =head2 C<VMG_OP_INFO_NAME>
272 Value to pass with C<op_info> to get the current op name in the magic callbacks.
274 =head2 C<VMG_OP_INFO_OBJECT>
276 Value to pass with C<op_info> to get a C<B::OP> object representing the current op in the magic callbacks.
284 XSLoader::load(__PACKAGE__, $VERSION);
291 get => sub { my ($ref, $data [, $op]) = @_; ... },
292 set => sub { my ($ref, $data [, $op]) = @_; ... },
293 len => sub { my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen; },
294 clear => sub { my ($ref, $data [, $op]) = @_; ... },
295 free => sub { my ($ref, $data [, $op]) = @_, ... },
296 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
297 local => sub { my ($ref, $data [, $op]) = @_; ... },
298 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
299 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
300 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
301 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
303 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ]
305 This function creates a 'wizard', an opaque type that holds the magic information.
306 It takes a list of keys / values as argument, whose keys can be :
314 The numerical signature.
315 If not specified or undefined, a random signature is generated.
316 If the signature matches an already defined magic, then the existant magic object is returned.
322 A code reference to a private data constructor.
323 It is called each time this magic is cast on a variable, and the scalar returned is used as private data storage for it.
324 C<$_[0]> is a reference to the magic object and C<@_[1 .. @_-1]> are all extra arguments that were passed to L</cast>.
328 C<get>, C<set>, C<len>, C<clear>, C<free>, C<copy>, C<local>, C<fetch>, C<store>, C<exists> and C<delete>
330 Code references to the corresponding magic callbacks.
331 You don't have to specify all of them : the magic associated with undefined entries simply won't be hooked.
332 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).
334 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<$num == VMG_OP_INFO_OBJECT>.
335 Both have a performance hit, but just getting the name is lighter than getting the op object.
337 Other arguments are specific to the magic hooked :
345 When the variable is an array or a scalar, C<$_[2]> contains the non-magical length.
346 The callback can return the new scalar or array length to use, or C<undef> to default to the normal length.
352 C<$_[2]> is a either a copy or an alias of the current key, which means that it is useless to try to change or cast magic on it.
353 C<$_[3]> is an alias to the current element (i.e. the value).
357 C<fetch>, C<store>, C<exists> and C<delete>
359 C<$_[2]> is an alias to the current key.
360 Nothing prevents you from changing it, but be aware that there lurk dangerous side effects.
361 For example, it may righteously be readonly if the key was a bareword.
362 You can get a copy instead by passing C<< copy_key => 1 >> to L</wizard>, which allows you to safely assign to C<$_[2]> in order to e.g. redirect the action to another key.
363 This however has a little performance drawback because of the copy.
367 All the callbacks are expected to return an integer, which is passed straight to the perl magic API.
368 However, only the return value of the C<len> callback currently holds a meaning.
372 # A simple scalar tracer
373 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
374 set => sub { print STDERR "set to ${$_[0]}\n" },
375 free => sub { print STDERR "${$_[0]} was deleted\n" }
380 croak 'Wrong number of arguments for wizard()' if @_ % 2;
382 my @keys = qw/sig data op_info get set len clear free/;
383 push @keys, 'copy' if MGf_COPY;
384 push @keys, 'dup' if MGf_DUP;
385 push @keys, 'local' if MGf_LOCAL;
386 push @keys, qw/fetch store exists delete copy_key/ if VMG_UVAR;
387 my $ret = eval { _wizard(map $opts{$_}, @keys) };
389 $err =~ s/\sat\s+.*?\n//;
397 With this tool, you can manually generate random magic signature between SIG_MIN and SIG_MAX inclusive.
398 That's the way L</wizard> creates them when no signature is supplied.
400 # Generate a signature
407 This accessor returns the magic signature of this wizard.
410 my $sig = getsig $wiz;
414 cast [$@%&*]var, [$wiz|$sig], ...
416 This function associates C<$wiz> magic to the variable supplied, without overwriting any other kind of magic.
417 You can also supply the numeric signature C<$sig> instead of C<$wiz>.
418 It returns true on success or when C<$wiz> magic is already present, C<0> on error, and C<undef> when no magic corresponds to the given signature (in case C<$sig> was supplied).
419 All extra arguments specified after C<$wiz> are passed to the private data constructor.
420 If the variable isn't a hash, any C<uvar> callback of the wizard is safely ignored.
422 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
424 die 'error' unless cast $x, $wiz;
428 getdata [$@%&*]var, [$wiz|$sig]
430 This accessor fetches the private data associated with the magic C<$wiz> (or the signature C<$sig>) in the variable.
431 C<undef> is returned when no such magic or data is found, or when C<$sig> does not represent a current valid magic object.
433 # Get the attached data.
434 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
438 dispell [$@%&*]variable, [$wiz|$sig]
440 The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable.
441 You can also pass the magic signature C<$sig> as the second argument.
442 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).
444 # Dispell now. If $wiz isn't a signature, undef can't be returned.
445 die 'no such magic or error' unless dispell $x, $wiz;
449 The functions L</wizard>, L</gensig>, L</getsig>, L</cast>, L</getdata> and L</dispell> are only exported on request.
450 All of them are exported by the tags C<':funcs'> and C<':all'>.
452 The constants L</SIG_MIN>, L</SIG_MAX>, L</SIG_NBR>, L</MGf_COPY>, L</MGf_DUP>, L</MGf_LOCAL> and L</VMG_UVAR> are also only exported on request.
453 They are all exported by the tags C<':consts'> and C<':all'>.
457 use base qw/Exporter/;
461 'funcs' => [ qw/wizard gensig getsig cast getdata dispell/ ],
463 qw/SIG_MIN SIG_MAX SIG_NBR MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR/,
464 qw/VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID VMG_COMPAT_ARRAY_UNDEF_CLEAR VMG_COMPAT_SCALAR_LENGTH_NOLEN/,
465 qw/VMG_PERL_PATCHLEVEL/,
467 qw/VMG_OP_INFO_NAME VMG_OP_INFO_OBJECT/
470 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
471 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
477 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 assignation.
478 The only way to address this would be to return a reference.
480 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.
486 L<Carp> (standard since perl 5), L<XSLoader> (standard since perl 5.006).
488 Copy tests need L<Tie::Array> (standard since perl 5.005) and L<Tie::Hash> (since 5.002).
490 Some uvar tests need L<Hash::Util::FieldHash> (standard since perl 5.009004).
492 Glob tests need L<Symbol> (standard since perl 5.002).
494 Threads tests need L<threads> and L<threads::shared>.
498 L<perlguts> and L<perlapi> for internal information about magic.
500 L<perltie> and L<overload> for other ways of enhancing objects.
504 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
506 You can contact me by mail or on C<irc.perl.org> (vincent).
510 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.
514 You can find documentation for this module with the perldoc command.
516 perldoc Variable::Magic
518 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>.
520 =head1 COPYRIGHT & LICENSE
522 Copyright 2007-2009 Vincent Pit, all rights reserved.
524 This program is free software; you can redistribute it and/or modify it
525 under the same terms as Perl itself.
529 1; # End of Variable::Magic