Variable::Magic - Associate user-defined magic to variables from Perl.
VERSION
- Version 0.10
+ Version 0.30
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 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.
+
+ * 5.8.9
+
+ *p28160* : Integration of *p25854* (see below).
+
+ *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.
- 5.9.3
- 'len' magic is no longer called when pushing an element into a magic
- array.
- *p26569* : 'local' magic.
+ *p31473* : 'clear' magic wasn't invoked when undefining an array.
+ The bug is fixed as of this version.
- 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*.
CONSTANTS
"SIG_MIN"
True for perls that don't call 'len' magic when you push 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_LISTASSIGN_COPY"
- True for perls that call 'copy' magic on list assignments. Implies that
- "MGf_COPY" is true.
+ "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.
+
+ "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.
FUNCTIONS
"wizard"
- wizard sig => ...,
- data => sub { ... },
- get => sub { my ($ref, $data) = @_; ... },
- set => sub { my ($ref, $data) = @_; ... },
- len => sub { my ($ref, $data, $len) = @_; ... ; return $newlen; },
- clear => sub { my ($ref, $data) = @_; ... },
- free => sub { my ($ref, $data) = @_, ... },
- copy => sub { my ($ref, $data, $elt) = @_; ... },
- local => sub { my ($ref, $data) = @_; ... },
- fetch => sub { my ($ref, $data, $key) = @_; ... },
- store => sub { my ($ref, $data, $key) = @_; ... },
- exists => sub { my ($ref, $data, $key) = @_; ... },
- delete => sub { my ($ref, $data, $key) = @_; ... }
+ wizard sig => ...,
+ data => sub { ... },
+ get => sub { my ($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"
+ * "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"
- Code references to corresponding magic callbacks. You don't have to
- specify all of them : the magic associated with undefined entries
+ * "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). 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.
+ (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"
+
+ 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.
+
+ * "copy"
+
+ $_[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).
+
+ * "fetch", "store", "exists" and "delete"
+
+ $_[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
+ 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.
# A simple scalar tracer
my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
my $x;
die 'error' unless cast $x, $wiz;
+ 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, [$wiz|$sig]
first.
DEPENDENCIES
- perl 5.7.3.
+ perl 5.8.
Carp (standard since perl 5), XSLoader (standard since perl 5.006).
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.
+ Copyright 2007-2009 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