Variable::Magic(3) User Contributed Perl Documentation Variable::Magic(3)NAME
Variable::Magic - Associate user-defined magic to variables from Perl.
VERSION
Version 0.47
SYNOPSIS
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'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. It is never
called for arrays and hashes.
· "set"
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"
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 assignment and loops ("for", "map" or
"grep"). The callback has then to return the length as an integer.
· "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"
This one can be considered as an object destructor. It happens
when the variable goes out of scope, but not when it is undefined.
· "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.
The following actions only apply to hashes and are available iff
"VMG_UVAR" is true. They are referred to as "uvar" magics.
· "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(
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 :
· "data"
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 (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"
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 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 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.
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, ...
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 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;
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. 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
The exact opposite of "cast" : it dissociates $wiz magic from the
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).
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", "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'.
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).
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>", <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
rt.cpan.org", or through the web interface at
http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic
<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.
SUPPORT
You can find documentation for this module with the perldoc command.
perldoc Variable::Magic
Tests code coverage report is available at
http://www.profvince.com/perl/cover/Variable-Magic
<http://www.profvince.com/perl/cover/Variable-Magic>.
COPYRIGHT & LICENSE
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.
perl v5.12.5 2011-10-27 Variable::Magic(3)
