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. 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. With this module, you can add your own magic to any variable without the pain of the C API.
40 Magic differs from tieing and overloading in several ways :
46 Magic isn't copied on assignation (as for blessed references) : you attach it to variables, not values.
50 It doesn't replace the original semantics : magic callbacks trigger before the original action take place, and can't prevent it to happen.
54 It's mostly invisible at the Perl level : magical and non-magical variables cannot be distinguished with C<ref>, C<reftype> or another trick.
58 It's notably faster, since perl's way of handling magic is lighter by nature, and there's no need for any method resolution.
62 The operations that can be overloaded are :
70 This magic is invoked when the variable is evaluated (does not include array/hash subscripts and slices).
76 This one is triggered each time the value of the variable changes (includes array/hash subscripts and slices).
82 This magic is a little special : it is called when the 'size' or the 'length' of the variable has to be known by Perl. 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>). The callback has then to return the length as an integer.
88 This magic is invoked when the variable is reset, such as when an array is emptied. 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>).
94 This one can be considered as an object destructor. It happens when the variable goes out of scope (with the exception of global scope), but not when it is undefined.
100 This magic only applies to tied arrays and hashes. It fires when you try to access or change their elements. It is available on your perl iff C<MGf_COPY> is true.
106 Invoked when the variable is cloned across threads. Currently not available.
112 When this magic is set on a variable, all subsequent localizations of the variable will trigger the callback. It is available on your perl iff C<MGf_LOCAL> is true.
116 The following actions only apply to hashes and are available iff C<VMG_UVAR> is true. They are referred to as C<uvar> magics.
124 This magic happens each time an element is fetched from the hash.
130 This one is called when an element is stored into the hash.
136 This magic fires when a key is tested for existence in the hash.
142 This last one triggers when a key is deleted in the hash, regardless of whether the key actually exists in it.
146 You can refer to the tests to have more insight of where the different magics are invoked.
148 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).
150 =head1 PERL MAGIC HISTORY
152 The places where magic is invoked have changed a bit through perl history. Here's a little list of the most recent ones.
160 I<p14416> : 'copy' and 'dup' magic.
166 I<p25854> : 'len' magic is no longer called when pushing an element into a magic array.
168 I<p26569> : 'local' magic.
174 I<p31064> : Meaningful 'uvar' magic.
176 I<p31473> : 'clear' magic wasn't invoked when undefining an array. The bug is fixed as of this version.
182 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.
188 I<p32969> : 'len' magic is no longer invoked when calling C<length> with a magical scalar.
196 The minimum integer used as a signature for user-defined magic.
200 The maximum integer used as a signature for user-defined magic.
204 SIG_NBR = SIG_MAX - SIG_MIN + 1
208 Evaluates to true iff the 'copy' magic is available.
212 Evaluates to true iff the 'dup' magic is available.
216 Evaluates to true iff the 'local' magic is available.
220 When this constant is true, you can use the C<fetch,store,exists,delete> callbacks on hashes.
222 =head2 C<VMG_COMPAT_ARRAY_PUSH_NOLEN>
224 True for perls that don't call 'len' magic when you push an element in a magical array.
226 =head2 C<VMG_COMPAT_ARRAY_UNDEF_CLEAR>
228 True for perls that call 'clear' magic when undefining magical arrays.
230 =head2 C<VMG_COMPAT_SCALAR_LENGTH_NOLEN>
232 True for perls that don't call 'len' magic when taking the C<length> of a magical scalar.
234 =head2 C<VMG_PERL_PATCHLEVEL>
236 The perl patchlevel this module was built with, or C<0> for non-debugging perls.
238 =head2 C<VMG_THREADSAFE>
240 True iff this module could have been built with thread-safety features enabled.
248 XSLoader::load(__PACKAGE__, $VERSION);
255 get => sub { my ($ref, $data) = @_; ... },
256 set => sub { my ($ref, $data) = @_; ... },
257 len => sub { my ($ref, $data, $len) = @_; ... ; return $newlen; },
258 clear => sub { my ($ref, $data) = @_; ... },
259 free => sub { my ($ref, $data) = @_, ... },
260 copy => sub { my ($ref, $data, $key, $elt) = @_; ... },
261 local => sub { my ($ref, $data) = @_; ... },
262 fetch => sub { my ($ref, $data, $key) = @_; ... },
263 store => sub { my ($ref, $data, $key) = @_; ... },
264 exists => sub { my ($ref, $data, $key) = @_; ... },
265 delete => sub { my ($ref, $data, $key) = @_; ... }
267 This function creates a 'wizard', an opaque type that holds the magic information. It takes a list of keys / values as argument, whose keys can be :
275 The numerical signature. If not specified or undefined, a random signature is generated. If the signature matches an already defined magic, then the existant magic object is returned.
281 A code reference to a private data constructor. It is called each time this magic is cast on a variable, and the scalar returned is used as private data storage for it. C<$_[0]> is a reference to the magic object and C<@_[1 .. @_-1]> are all extra arguments that were passed to L</cast>.
285 C<get>, C<set>, C<len>, C<clear>, C<free>, C<copy>, C<local>, C<fetch>, C<store>, C<exists> and C<delete>
287 Code references to corresponding magic callbacks. You don't have to specify all of them : the magic associated with undefined entries simply won't be hooked. 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). In the special case of C<len> magic and when the variable is an array, C<$_[2]> contains its normal length. C<$_[2]> is the current key in C<copy>, C<fetch>, C<store>, C<exists> and C<delete> callbacks, although for C<copy> it may just be a copy of the actual key so it's useless to (for example) cast magic on it. C<copy> magic also receives the current element (i.e. the value) in C<$_[3]>.
291 # A simple scalar tracer
292 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
293 set => sub { print STDERR "set to ${$_[0]}\n" },
294 free => sub { print STDERR "${$_[0]} was deleted\n" }
299 croak 'Wrong number of arguments for wizard()' if @_ % 2;
301 my @cbs = qw/sig data get set len clear free/;
302 push @cbs, 'copy' if MGf_COPY;
303 push @cbs, 'dup' if MGf_DUP;
304 push @cbs, 'local' if MGf_LOCAL;
305 push @cbs, qw/fetch store exists delete/ if VMG_UVAR;
306 my $ret = eval { _wizard(map $opts{$_}, @cbs) };
308 $err =~ s/\sat\s+.*?\n//;
316 With this tool, you can manually generate random magic signature between SIG_MIN and SIG_MAX inclusive. That's the way L</wizard> creates them when no signature is supplied.
318 # Generate a signature
325 This accessor returns the magic signature of this wizard.
328 my $sig = getsig $wiz;
332 cast [$@%&*]var, [$wiz|$sig], ...
334 This function associates C<$wiz> magic to the variable supplied, without overwriting any other kind of magic. You can also supply the numeric signature C<$sig> instead of C<$wiz>. It returns true on success or when C<$wiz> magic is already present, C<0> on error, and C<undef> when no magic corresponds to the given signature (in case C<$sig> was supplied). All extra arguments specified after C<$wiz> are passed to the private data constructor. If the variable isn't a hash, any C<uvar> callback of the wizard is safely ignored.
336 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
338 die 'error' unless cast $x, $wiz;
342 getdata [$@%&*]var, [$wiz|$sig]
344 This accessor fetches the private data associated with the magic C<$wiz> (or the signature C<$sig>) in the variable. 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<$wiz> magic from the variable. You can also pass the magic signature C<$sig> as the second argument. 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).
355 # Dispell now. If $wiz isn't a signature, undef can't be returned.
356 die 'no such magic or error' unless dispell $x, $wiz;
360 The functions L</wizard>, L</gensig>, L</getsig>, L</cast>, L</getdata> and L</dispell> are only exported on request. All of them are exported by the tags C<':funcs'> and C<':all'>.
362 The constants L</SIG_MIN>, L</SIG_MAX>, L</SIG_NBR>, L</MGf_COPY>, L</MGf_DUP>, L</MGf_LOCAL> and L</VMG_UVAR> are also only exported on request. They are all exported by the tags C<':consts'> and C<':all'>.
366 use base qw/Exporter/;
370 'funcs' => [ qw/wizard gensig getsig cast getdata dispell/ ],
371 'consts' => [ qw/SIG_MIN SIG_MAX SIG_NBR MGf_COPY MGf_DUP MGf_LOCAL VMG_UVAR/,
372 qw/VMG_COMPAT_ARRAY_PUSH_NOLEN VMG_COMPAT_ARRAY_UNDEF_CLEAR/,
373 qw/VMG_COMPAT_SCALAR_LENGTH_NOLEN/,
374 qw/VMG_PERL_PATCHLEVEL/,
377 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
378 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
382 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. The only way to address this would be to return a reference.
384 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.
390 L<Carp> (standard since perl 5), L<XSLoader> (standard since perl 5.006).
392 Copy tests need L<Tie::Array> (standard since perl 5.005) and L<Tie::Hash> (since 5.002).
394 Some uvar tests need L<Hash::Util::FieldHash> (standard since perl 5.009004).
396 Glob tests need L<Symbol> (standard since perl 5.002).
398 Threads tests need L<threads> and L<threads::shared>.
402 L<perlguts> and L<perlapi> for internal information about magic.
404 L<perltie> and L<overload> for other ways of enhancing objects.
408 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
410 You can contact me by mail or on C<irc.perl.org> (vincent).
414 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.
418 You can find documentation for this module with the perldoc command.
420 perldoc Variable::Magic
422 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Variable-Magic>.
424 =head1 COPYRIGHT & LICENSE
426 Copyright 2007-2008 Vincent Pit, all rights reserved.
428 This program is free software; you can redistribute it and/or modify it
429 under the same terms as Perl itself.
433 1; # End of Variable::Magic