Variable::Magic - Associate user-defined magic to variables from Perl.
VERSION
- Version 0.08
+ Version 0.23
SYNOPSIS
use Variable::Magic qw/wizard cast dispell/;
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
+ 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.
+ Magic differs from tieing and overloading in several ways :
+
+ * Magic isn't copied on assignation (as for blessed references) : you
+ attach it to variables, not values.
+
+ * It doesn't replace the original semantics : magic callbacks trigger
+ before the original action take place, and can't prevent it to
+ happen.
+
+ * It's mostly invisible at the Perl level : magical and non-magical
+ variables cannot be distinguished with "ref", "reftype" or another
+ trick.
+
+ * It's notably faster, since perl's way of handling magic is lighter
+ by nature, and there's no need for any method resolution.
+
The operations that can be overloaded are :
- "get"
+ * "get"
+
This magic is invoked when the variable is evaluated (does not
include array/hash subscripts and slices).
- "set"
+ * "set"
+
This one is triggered each time the value of the variable changes
(includes array/hash subscripts and slices).
- "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
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 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"
+ * "free"
+
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.
- "copy"
+ * "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"
+ * "dup"
+
Invoked when the variable is cloned across threads. Currently not
available.
- "local"
+ * "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.
- The following actions only applies to hashes and are available iff
+ The following actions only apply to hashes and are available iff
"VMG_UVAR" is true. They are referred to as "uvar" magics.
- "fetch"
+ * "fetch"
+
This magic happens each time an element is fetched from the hash.
- "store"
+ * "store"
+
This one is called when an element is stored into the hash.
- "exists"
+ * "exists"
+
This magic fires when a key is tested for existence in the hash.
- "delete"
+ * "delete"
+
This last one triggers when a key is deleted in the hash, regardless
of whether the key actually exists in it.
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.
+ * 5.6.x
+
+ *p14416* : 'copy' and 'dup' magic.
+
+ * 5.9.3
- 5.9.3
- 'len' magic is no longer called when pushing an element into a magic
- array.
- *p26569* : 'local' magic.
+ *p25854* : 'len' magic is no longer called when pushing an element
+ into a magic array.
- 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.
+ *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.
CONSTANTS
"SIG_MIN"
When this constant is true, you can use the "fetch,store,exists,delete"
callbacks on hashes.
+ "VMG_COMPAT_ARRAY_PUSH_NOLEN"
+ True for perls that don't call 'len' magic when you push an element in a
+ magical array.
+
+ "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
+ True for perls that call 'clear' magic when undefining magical arrays.
+
+ "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
+ True for perls that don't call 'len' magic when taking the "length" of a
+ magical scalar.
+
+ "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.
+
FUNCTIONS
"wizard"
wizard sig => ...,
len => sub { my ($ref, $data, $len) = @_; ... ; return $newlen; },
clear => sub { my ($ref, $data) = @_; ... },
free => sub { my ($ref, $data) = @_, ... },
- copy => sub { my ($ref, $data, $elt) = @_; ... },
+ copy => sub { my ($ref, $data, $key, $elt) = @_; ... },
local => sub { my ($ref, $data) = @_; ... },
fetch => sub { my ($ref, $data, $key) = @_; ... },
store => sub { my ($ref, $data, $key) = @_; ... },
information. It takes a list of keys / values as argument, whose keys
can be :
- "sig"
+ * "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"
+ * "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".
- "get", "set", "len", "clear", "free", "copy", "local", "fetch", "store",
- "exists" and "delete"
+ * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
+ "store", "exists" and "delete"
+
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, $_[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). In the
special case of "len" magic and when the variable is an array, $_[2]
- contains its normal length. "copy" magic receives the current
- element (i.e. the value) in $_[2]. $_[2] is also the current key in
- "fetch", "store", "exists" and "delete" callbacks.
+ contains its normal length. $_[2] is the current key in "copy",
+ "fetch", "store", "exists" and "delete" callbacks, although for
+ "copy" it may just be a copy of the actual key so it's useless to
+ (for example) cast magic on it. "copy" magic also receives the
+ current element (i.e. the value) in $_[3].
# A simple scalar tracer
my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
magic is already present, 0 on error, and "undef" when no magic
corresponds to the given signature (in case $sig was supplied). All
extra arguments specified after $wiz are passed to the private data
- constructor.
+ constructor. If the variable isn't a hash, any "uvar" callback of the
+ wizard is safely ignored.
# Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
my $x;
"MGf_LOCAL" and "VMG_UVAR" are also only exported on request. They are
all exported 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 assignation. 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.
+
DEPENDENCIES
perl 5.7.3.
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 #perl @ FreeNode (Prof_Vince).
+ 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 2007-2008 Vincent Pit, all rights reserved.
projects / perl / modules / Variable-Magic.git / blobdiff