Variable::Magic - Associate user-defined magic to variables from Perl.
VERSION
- Version 0.35
+ Version 0.46
SYNOPSIS
- use Variable::Magic qw/wizard cast VMG_OP_INFO_NAME/;
+ use Variable::Magic qw<wizard cast VMG_OP_INFO_NAME>;
{ # A variable tracer
my $wiz = wizard set => sub { print "now set to ${$_[0]}!\n" },
}
DESCRIPTION
- Magic is Perl way of enhancing objects. This mechanism lets the user add
- extra data to any variable and hook syntaxical operations (such as
+ 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.
* It doesn't replace the original semantics.
- Magic callbacks usually trigger before the original action take
- place, and can't prevent it to happen. 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.
+ 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.
* "get"
- This magic is invoked when the variable is evaluated (does not
- include array/hash subscripts and slices).
+ This magic is invoked when the variable is evaluated. It is never
+ called for arrays and hashes.
* "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"
FUNCTIONS
"wizard"
- wizard sig => ...,
- data => sub { ... },
+ wizard data => sub { ... },
get => sub { my ($ref, $data [, $op]) = @_; ... },
set => sub { my ($ref, $data [, $op]) = @_; ... },
len => sub { my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen; },
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. If the signature matches an already defined
- magic, then the existant magic object is returned.
-
* "data"
- 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. $_[0] is a reference to the
- magic object and @_[1 .. @_-1] are all extra arguments that were
- passed to "cast".
+ 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", "free", "copy", "local", "fetch",
"store", "exists" and "delete"
- Code 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).
+ 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 ==
$_[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 righteously be readonly if
+ 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
straight to the perl magic API. However, only the return value of
the "len" callback currently holds a meaning.
- # 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" }
+ Each callback can be specified as a code or a string reference, in which
+ case the function denoted by the string will be used as the callback.
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.
- "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.
+ Here's a simple usage example :
- # Generate a signature
- my $sig = gensig;
-
- "getsig"
- getsig $wiz
-
- This accessor returns the magic signature of this wizard.
-
- # Get $wiz signature
- my $sig = getsig $wiz;
+ # 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|$sig], ...
+ cast [$@%&*]var, $wiz, ...
This function associates $wiz magic to the variable supplied, without
- overwriting any other kind of magic. You can also supply the numeric
- signature $sig instead of $wiz. It returns true on success or when $wiz
- magic is already present, and croaks on error or when no magic
- corresponds to the given signature (in case a $sig was supplied). 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.
+ overwriting any other kind of magic. It returns true on success or when
+ $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 onto $x, and pass '1' to the data constructor.
my $x;
If you want to overcome the possible deletion of the 'TZ' entry, you
have no choice but to rely on "store" uvar magic.
- "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, $wiz;
- }
-
- When %h goes out of scope, this will print something among the lines of
- :
-
- free HASH at depth 0
- free HASH at depth 1
- free SCALAR at depth 2
- free ARRAY at depth 1
- free SCALAR at depth 3
- free SCALAR at depth 3
-
- Of course, this example does nothing with the values that are added
- after the "cast".
-
"getdata"
- getdata [$@%&*]var, [$wiz|$sig]
+ getdata [$@%&*]var, $wiz
- This accessor fetches the private data associated with the magic $wiz
- (or the signature $sig) in the variable. It croaks when $wiz or $sig do
- not represent a current valid magic object attached to the variable, and
- returns "undef" when the wizard has no data constructor or when the data
- is actually "undef".
+ This accessor fetches the private data associated with the magic $wiz in
+ 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, [$wiz|$sig]
+ dispell [$@%&*]variable, $wiz
The exact opposite of "cast" : it dissociates $wiz magic from the
- variable. You can also pass the magic signature $sig as the second
- argument. This function returns true on success, 0 when no magic
- represented by $wiz or $sig could be found in the variable, and croaks
- if the supplied wizard or signature is invalid.
+ 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
- "SIG_MIN"
- The minimum integer used as a signature for user-defined magic.
-
- "SIG_MAX"
- The maximum integer used as a signature for user-defined magic.
-
- "SIG_NBR"
- SIG_NBR = SIG_MAX - SIG_MIN + 1
-
"MGf_COPY"
Evaluates to true iff the 'copy' magic is available.
"VMG_UVAR"
When this constant is true, you can use the "fetch,store,exists,delete"
- callbacks on hashes.
+ 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_ARRAY_PUSH_NOLEN"
True for perls that don't call 'len' magic when you push an element in a
- magical array.
+ 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
True for perls that don't call 'len' magic when taking the "length" of a
magical scalar.
+ "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.
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.
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.
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'.
All the constants are also only exported on request, either individually
or by the tags ':consts' and ':all'.
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).
Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
<http://www.profvince.com/perl/cover/Variable-Magic>.
COPYRIGHT & LICENSE
- Copyright 2007-2009 Vincent Pit, all rights reserved.
+ Copyright 2007,2008,2009,2010,2011 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