Variable::Magic - Associate user-defined magic to variables from Perl.
VERSION
- Version 0.01
+ Version 0.49
SYNOPSIS
- use Variable::Magic qw/wizard cast dispell/;
-
- my $wiz = wizard set => sub { print STDERR "now set to $_[0]!\n" };
- my $a = 1;
- cast $a, $wiz;
- $a = 2; # "now set to 2!"
- dispell $a, $wiz;
- $a = 3 # (nothing)
+ use Variable::Magic qw<wizard cast VMG_OP_INFO_NAME>;
+
+ { # A variable tracer
+ my $wiz = wizard(
+ set => sub { print "now set to ${$_[0]}!\n" },
+ free => sub { print "destroyed!\n" },
+ );
+
+ my $a = 1;
+ cast $a, $wiz;
+ $a = 2; # "now set to 2!"
+ } # "destroyed!"
+
+ { # A hash with a default value
+ my $wiz = wizard(
+ data => sub { $_[1] },
+ fetch => sub { $_[2] = $_[1] unless exists $_[0]->{$_[2]}; () },
+ store => sub { print "key $_[2] stored in $_[-1]\n" },
+ copy_key => 1,
+ op_info => VMG_OP_INFO_NAME,
+ );
+
+ my %h = (_default => 0, apple => 2);
+ cast %h, $wiz, '_default';
+ print $h{banana}, "\n"; # "0" (there is no 'banana' key in %h)
+ $h{pear} = 1; # "key pear stored in helem"
+ }
DESCRIPTION
- Magic is Perl way of enhancing objects. This mechanism let the user add
- extra data to any variable and overload 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.
+ Magic is Perl's way of enhancing variables. This mechanism lets the user
+ add extra data to any variable and hook syntactical operations (such as
+ access, assignment or destruction) that can be applied to it. With this
+ module, you can add your own magic to any variable without having to
+ write a single line of XS.
+
+ You'll realize that these magic variables look a lot like tied
+ variables. It's not surprising, as tied variables are implemented as a
+ special kind of magic, just like any 'irregular' Perl variable : scalars
+ like $!, $( or $^W, the %ENV and %SIG hashes, the @ISA array, "vec()"
+ and "substr()" lvalues, threads::shared variables... They all share the
+ same underlying C API, and this module gives you direct access to it.
+
+ Still, the magic made available by this module differs from tieing and
+ overloading in several ways :
+
+ * It isn't copied on assignment.
+
+ You attach it to variables, not values (as for blessed references).
+
+ * It doesn't replace the original semantics.
+
+ Magic callbacks usually get triggered before the original action
+ takes place, and can't prevent it from happening. This also makes
+ catching individual events easier than with "tie", where you have to
+ provide fallbacks methods for all actions by usually inheriting from
+ the correct "Tie::Std*" class and overriding individual methods in
+ your own class.
+
+ * It's type-agnostic.
+
+ The same magic can be applied on scalars, arrays, hashes, subs or
+ globs. But the same hook (see below for a list) may trigger
+ differently depending on the the type of the variable.
+
+ * It's mostly invisible at the Perl level.
+
+ Magical and non-magical variables cannot be distinguished with
+ "ref", "tied" or another trick.
+
+ * It's notably faster.
+
+ Mainly because perl's way of handling magic is lighter by nature,
+ and because there's no need for any method resolution. Also, since
+ you don't have to reimplement all the variable semantics, you only
+ pay for what you actually use.
The operations that can be overloaded are :
- "get"
- This magic is invoked when the variable is evaluated (does not
- include array/hash subscripts and slices).
+ * "get"
+
+ This magic is invoked when the variable is evaluated. It is never
+ called for arrays and hashes.
+
+ * "set"
- "set"
- This one is triggered each time the value of the variable changes
- (includes array/hash subscripts and slices).
+ This one is triggered each time the value of the variable changes.
+ It is called for array subscripts and slices, but never for hashes.
+
+ * "len"
- "len"
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 ("for", "map" or "grep"). The
+ also on array assignment and loops ("for", "map" or "grep"). The
callback has then to return the length as an integer.
- "clear"
+ * "clear"
+
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 reset is a
- result of the undefine (e.g. for an array).
+ 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 history).
- "free"
- This last 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.
+ * "free"
- 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).
+ This one can be considered as an object destructor. It happens when
+ the variable goes out of scope, but not when it is undefined.
-CONSTANTS
- "SIG_MIN"
- The minimum integer used as a signature for user-defined magic.
+ * "copy"
+
+ 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 "MGf_COPY" is true.
+
+ * "dup"
+
+ Invoked when the variable is cloned across threads. Currently not
+ available.
+
+ * "local"
+
+ 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 "MGf_LOCAL" is true.
- "SIG_MAX"
- The maximum integer used as a signature for user-defined magic.
+ The following actions only apply to hashes and are available iff
+ "VMG_UVAR" is true. They are referred to as "uvar" magics.
- "SIG_NBR"
- SIG_NBR = SIG_MAX - SIG_MIN + 1
+ * "fetch"
+
+ This magic happens each time an element is fetched from the hash.
+
+ * "store"
+
+ This one is called when an element is stored into the hash.
+
+ * "exists"
+
+ This magic fires when a key is tested for existence in the hash.
+
+ * "delete"
+
+ This last one triggers when a key is deleted in the hash, regardless
+ of whether the key actually exists in it.
+
+ You can refer to the tests to have more insight of where the different
+ magics are invoked.
FUNCTIONS
"wizard"
- wizard sig => .., data => ..., get => .., set => .., len => .., clear => .., free => ..
+ wizard(
+ data => sub { ... },
+ get => sub { my ($ref, $data [, $op]) = @_; ... },
+ set => sub { my ($ref, $data [, $op]) = @_; ... },
+ len => sub {
+ my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen
+ },
+ clear => sub { my ($ref, $data [, $op]) = @_; ... },
+ free => sub { my ($ref, $data [, $op]) = @_, ... },
+ copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
+ local => sub { my ($ref, $data [, $op]) = @_; ... },
+ fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
+ store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
+ exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
+ delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
+ copy_key => $bool,
+ op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
+ )
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 :
- 'sig'
- The numerical signature. If not specified or undefined, a random
- signature is generated.
+ * "data"
- 'data'
- A code reference to a private data constructor. It will be called
- each time this magic is cast on a variable, and the scalar returned
- will be used as private data storage for it.
+ A code (or string) 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. $_[0] is a
+ reference to the magic object and @_[1 .. @_-1] are all extra
+ arguments that were passed to "cast".
- 'get', 'set', 'len', 'clear' and 'free'
- 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. When the magic variable is an array or a
- hash, $_[0] is a reference to it, but directly references it
- otherwise. $_[1] is the private data (or "undef" when no private
- data constructor was supplied). In the special case of "len" magic
- and when the variable is an array, $_[2] contains its normal length.
+ * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
+ "store", "exists" and "delete"
- # A simple scalar tracer
- my $wiz = wizard get => sub { print STDERR "got $_[0]\n" },
- set => sub { print STDERR "set to $_[0]\n" },
- free => sub { print STDERR "$_[0] was deleted\n" }
+ Code (or string) references to the 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, $_[0]
+ is always a reference to the magic object and $_[1] is always the
+ private data (or "undef" when no private data constructor was
+ supplied).
+
+ Moreover, when you pass "op_info => $num" to "wizard", the last
+ element of @_ will be the current op name if "$num ==
+ VMG_OP_INFO_NAME" and a "B::OP" object representing the current op
+ if "$num == VMG_OP_INFO_OBJECT". Both have a performance hit, but
+ just getting the name is lighter than getting the op object.
+
+ Other arguments are specific to the magic hooked :
+
+ * "len"
- "gensig"
- With this tool, you can manually generate random magic signature between
- SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
- no signature is supplied.
+ When the variable is an array or a scalar, $_[2] contains
+ the non-magical length. The callback can return the new
+ scalar or array length to use, or "undef" to default to the
+ normal length.
- # Generate a signature
- my $sig = gensig;
+ * "copy"
- "getsig"
- getsig $wiz
+ $_[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. $_[3] is an alias to the current element (i.e.
+ the value).
- This accessor returns the magic signature of this wizard.
+ * "fetch", "store", "exists" and "delete"
- # Get $wiz signature
- my $sig = getsig $wiz;
+ $_[2] is an alias to the current key. Nothing prevents you
+ from changing it, but be aware that there lurk dangerous
+ side effects. For example, it may rightfully be readonly if
+ the key was a bareword. You can get a copy instead by
+ passing "copy_key => 1" to "wizard", which allows you to
+ safely assign to $_[2] in order to e.g. redirect the action
+ to another key. This however has a little performance
+ drawback because of the copy.
+
+ All the callbacks are expected to return an integer, which is passed
+ straight to the perl magic API. However, only the return value of
+ the "len" callback currently holds a meaning.
+
+ Each callback can be specified as :
+
+ * a code reference, which will be called as a subroutine.
+
+ * a string reference, where the string denotes which subroutine is to
+ be called when magic is triggered. If the subroutine name is not
+ fully qualified, then the current package at the time the magic is
+ invoked will be used instead.
+
+ * a reference to "undef", in which case a no-op magic callback is
+ installed instead of the default one. This may especially be helpful
+ for 'local' magic, where an empty callback prevents magic from being
+ copied during localization.
+
+ Note that "free" callbacks are *never* called during global destruction,
+ as there's no way to ensure that the wizard and the "free" callback
+ weren't destroyed before the variable.
+
+ Here's a simple usage example :
+
+ # A simple scalar tracer
+ my $wiz = wizard(
+ get => sub { print STDERR "got ${$_[0]}\n" },
+ set => sub { print STDERR "set to ${$_[0]}\n" },
+ free => sub { print STDERR "${$_[0]} was deleted\n" },
+ );
"cast"
- cast [$@%&*]var, $wiz
+ cast [$@%&*]var, $wiz, ...
This function associates $wiz magic to the variable supplied, without
overwriting any other kind of magic. It returns true on success or when
- $wiz magic is already present, and false on error.
+ $wiz magic is already present, and croaks on error. All extra arguments
+ specified after $wiz are passed to the private data constructor in @_[1
+ .. @_-1]. If the variable isn't a hash, any "uvar" callback of the
+ wizard is safely ignored.
- # Casts $wiz to $x
+ # Casts $wiz onto $x, and pass '1' to the data constructor.
my $x;
- die 'error' unless cast $x, $wiz;
+ cast $x, $wiz, 1;
+
+ The "var" argument can be an array or hash value. Magic for those
+ behaves like for any other scalar, except that it is dispelled when the
+ entry is deleted from the container. For example, if you want to call
+ "POSIX::tzset" each time the 'TZ' environment variable is changed in
+ %ENV, you can use :
+
+ use POSIX;
+ cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
+
+ If you want to overcome the possible deletion of the 'TZ' entry, you
+ have no choice but to rely on "store" uvar magic.
"getdata"
getdata [$@%&*]var, $wiz
This accessor fetches the private data associated with the magic $wiz in
- the variable. "undef" is returned when no such magic or data is found.
+ the variable. It croaks when $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.
+
+ # Get the attached data, or undef if the wizard does not attach any.
+ my $data = getdata $x, $wiz;
"dispell"
dispell [$@%&*]variable, $wiz
- dispell [$@%&*]variable, $sig
The exact opposite of "cast" : it dissociates $wiz magic from the
- variable. You can also pass the magic signature as the second argument.
- True is returned on success, and false on error or when no magic
- represented by $wiz could be found in the variable.
+ variable. This function returns true on success, 0 when no magic
+ represented by $wiz could be found in the variable, and croaks if the
+ supplied wizard is invalid.
+
+ # Dispell now.
+ die 'no such magic in $x' unless dispell $x, $wiz;
+
+CONSTANTS
+ "MGf_COPY"
+ Evaluates to true iff the 'copy' magic is available.
+
+ "MGf_DUP"
+ Evaluates to true iff the 'dup' magic is available.
+
+ "MGf_LOCAL"
+ Evaluates to true iff the 'local' magic is available.
+
+ "VMG_UVAR"
+ When this constant is true, you can use the "fetch,store,exists,delete"
+ callbacks on hashes. Initial VMG_UVAR capability was introduced in perl
+ 5.9.5, with a fully functional implementation shipped with perl 5.10.0.
+
+ "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
+ True for perls that don't call 'len' magic when taking the "length" of a
+ magical scalar.
+
+ "VMG_COMPAT_ARRAY_PUSH_NOLEN"
+ True for perls that don't call 'len' magic when you push an element in a
+ magical array. Starting from perl 5.11.0, this only refers to pushes in
+ non-void context and hence is false.
+
+ "VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID"
+ True for perls that don't call 'len' magic when you push in void context
+ an element in a magical array.
+
+ "VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
+ True for perls that don't call 'len' magic when you unshift in void
+ context an element in a magical array.
+
+ "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
+ True for perls that call 'clear' magic when undefining magical arrays.
+
+ "VMG_COMPAT_HASH_DELETE_NOUVAR_VOID"
+ True for perls that don't call 'delete' uvar magic when you delete an
+ element from a hash in void context.
+
+ "VMG_COMPAT_GLOB_GET"
+ True for perls that call 'get' magic for operations on globs.
+
+ "VMG_PERL_PATCHLEVEL"
+ The perl patchlevel this module was built with, or 0 for non-debugging
+ perls.
+
+ "VMG_THREADSAFE"
+ True iff this module could have been built with thread-safety features
+ enabled.
+
+ "VMG_FORKSAFE"
+ True iff this module could have been built with fork-safety features
+ enabled. This will always be true except on Windows where it's false for
+ perl 5.10.0 and below .
+
+ "VMG_OP_INFO_NAME"
+ Value to pass with "op_info" to get the current op name in the magic
+ callbacks.
+
+ "VMG_OP_INFO_OBJECT"
+ Value to pass with "op_info" to get a "B::OP" object representing the
+ current op in the magic callbacks.
+
+COOKBOOK
+ Associate an object to any perl variable
+ This technique can be useful for passing user data through limited APIs.
+ It is similar to using inside-out objects, but without the drawback of
+ having to implement a complex destructor.
+
+ {
+ package Magical::UserData;
+
+ use Variable::Magic qw<wizard cast getdata>;
+
+ my $wiz = wizard data => sub { \$_[1] };
+
+ sub ud (\[$@%*&]) : lvalue {
+ my ($var) = @_;
+ my $data = &getdata($var, $wiz);
+ unless (defined $data) {
+ $data = \(my $slot);
+ &cast($var, $wiz, $slot)
+ or die "Couldn't cast UserData magic onto the variable";
+ }
+ $$data;
+ }
+ }
+
+ {
+ BEGIN { *ud = \&Magical::UserData::ud }
+
+ my $cb;
+ $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
+
+ ud(&$cb) = 'world';
+ $cb->(); # Hello, world!
+ }
+
+ Recursively cast magic on datastructures
+ "cast" can be called from any magical callback, and in particular from
+ "data". This allows you to recursively cast magic on datastructures :
+
+ my $wiz;
+ $wiz = wizard data => sub {
+ my ($var, $depth) = @_;
+ $depth ||= 0;
+ my $r = ref $var;
+ if ($r eq 'ARRAY') {
+ &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
+ } elsif ($r eq 'HASH') {
+ &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
+ }
+ return $depth;
+ },
+ free => sub {
+ my ($var, $depth) = @_;
+ my $r = ref $var;
+ print "free $r at depth $depth\n";
+ ();
+ };
+
+ {
+ my %h = (
+ a => [ 1, 2 ],
+ b => { c => 3 }
+ );
+ cast %h, $wiz;
+ }
+
+ When %h goes out of scope, this will print something among the lines of
+ :
+
+ free HASH at depth 0
+ free HASH at depth 1
+ free SCALAR at depth 2
+ free ARRAY at depth 1
+ free SCALAR at depth 3
+ free SCALAR at depth 3
+
+ Of course, this example does nothing with the values that are added
+ after the "cast".
+
+PERL MAGIC HISTORY
+ The places where magic is invoked have changed a bit through perl
+ history. Here's a little list of the most recent ones.
+
+ * 5.6.x
+
+ *p14416* : 'copy' and 'dup' magic.
+
+ * 5.8.9
+
+ *p28160* : Integration of *p25854* (see below).
- # Dispell now
- die 'no such magic or error' unless dispell $x, $wiz;
+ *p32542* : Integration of *p31473* (see below).
+
+ * 5.9.3
+
+ *p25854* : 'len' magic is no longer called when pushing an element
+ into a magic array.
+
+ *p26569* : 'local' magic.
+
+ * 5.9.5
+
+ *p31064* : Meaningful 'uvar' magic.
+
+ *p31473* : 'clear' magic wasn't invoked when undefining an array.
+ The bug is fixed as of this version.
+
+ * 5.10.0
+
+ Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
+ 'copy' magic on hash stores for (non-tied) hashes that also have
+ 'uvar' magic.
+
+ * 5.11.x
+
+ *p32969* : 'len' magic is no longer invoked when calling "length"
+ with a magical scalar.
+
+ *p34908* : 'len' magic is no longer called when pushing / unshifting
+ an element into a magical array in void context. The "push" part was
+ already covered by *p25854*.
+
+ *g9cdcb38b* : 'len' magic is called again when pushing into a
+ magical array in non-void context.
EXPORT
- The functions "wizard", "gensig", "getsig", "cast", "getdata" and
- "dispell" are only exported on request. All of them are exported by the
- tags ':funcs' and ':all'.
+ The functions "wizard", "cast", "getdata" and "dispell" are only
+ exported on request. All of them are exported by the tags ':funcs' and
+ ':all'.
- The constants "SIG_MIN", "SIG_MAX" and "SIG_NBR" are also only exported
- on request. They are all exported by the tags ':consts' and ':all'.
+ All the constants are also only exported on request, either individually
+ or by the tags ':consts' and ':all'.
+
+CAVEATS
+ If you store a magic object in the private data slot, the magic won't be
+ accessible by "getdata" since it's not copied by assignment. The only
+ way to address this would be to return a reference.
+
+ If you define a wizard with a "free" callback and cast it on itself,
+ this destructor won't be called because the wizard will be destroyed
+ first.
+
+ In order to define magic on hash members, you need at least perl 5.10.0
+ (see "VMG_UVAR")
DEPENDENCIES
+ perl 5.8.
+
+ A C compiler. This module may happen to build with a C++ compiler as
+ well, but don't rely on it, as no guarantee is made in this regard.
+
Carp (standard since perl 5), XSLoader (standard since perl 5.006).
- Tests use Symbol (standard since perl 5.002).
+ Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
+ (since 5.002).
+
+ Some uvar tests need Hash::Util::FieldHash (standard since perl
+ 5.009004).
+
+ Glob tests need Symbol (standard since perl 5.002).
+
+ Threads tests need threads and threads::shared.
SEE ALSO
perlguts and perlapi for internal information about magic.
+ perltie and overload for other ways of enhancing objects.
+
AUTHOR
- Vincent Pit, "<perl at profvince.com>"
+ Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
+
+ You can contact me by mail or on "irc.perl.org" (vincent).
BUGS
Please report any bugs or feature requests to "bug-variable-magic at
perldoc Variable::Magic
+ Tests code coverage report is available at
+ <http://www.profvince.com/perl/cover/Variable-Magic>.
+
COPYRIGHT & LICENSE
- Copyright 2007 Vincent Pit, all rights reserved.
+ Copyright 2007,2008,2009,2010,2011,2012 Vincent Pit, all rights
+ reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
projects / perl / modules / Variable-Magic.git / blobdiff