Variable::Magic(3pm) User Contributed Perl Documentation Variable::Magic(3pm)NAMEVariable::Magic - Associate user-defined magic to variables from Perl.
VERSION
Version 0.62
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 is 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 :
· Magic is not copied on assignment.
You attach it to variables, not values (as for blessed references).
· Magic does not replace the original semantics.
Magic callbacks usually get triggered before the original action
takes place, and cannot 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.
· Magic is multivalued.
You can safely apply different kinds of magics to the same
variable, and each of them will be invoked successively.
· Magic is 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 type of the variable.
· Magic is invisible at Perl level.
Magical and non-magical variables cannot be distinguished with
"ref", "tied" or another trick.
· Magic is notably faster.
Mainly because perl's way of handling magic is lighter by nature,
and because there is 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 magic is called each time the value of the variable changes.
It is called for array subscripts and slices, but never for hashes.
· len
This magic only applies to arrays (though it used to also apply to
scalars), and is triggered when the 'size' or the 'length' of the
variable has to be known by Perl. This is typically the magic
involved when an array is evaluated in scalar context, but also on
array assignment and loops ("for", "map" or "grep"). The length is
returned from the callback as an integer.
Starting from perl 5.12, this magic is no longer called by the
"length" keyword, and starting from perl 5.17.4 it is also no
longer called for scalars in any situation, making this magic only
meaningful on arrays. You can use the constants
"VMG_COMPAT_SCALAR_LENGTH_NOLEN" and "VMG_COMPAT_SCALAR_NOLEN" to
see if this magic is available for scalars or not.
· 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 magic is called when a variable is destroyed as the result of
going out of scope (but not when it is undefined). It behaves
roughly like Perl object destructors (i.e. "DESTROY" methods),
except that exceptions thrown from inside a free callback will
always be propagated to the surrounding code.
· copy
When applied to tied arrays and hashes, this magic fires when you
try to access or change their elements.
Starting from perl 5.17.0, it can also be applied to closure
prototypes, in which case the magic will be called when the
prototype is cloned. The "VMG_COMPAT_CODE_COPY_CLONE" constant is
true when your perl support this feature.
· dup
This magic is invoked when the variable is cloned across threads.
It is 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 if and only if "MGf_LOCAL" is true.
The following actions only apply to hashes and are available if and
only if "VMG_UVAR" is true. They are referred to as uvar magics.
· fetch
This magic is invoked 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 magic is triggered 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 object 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 in scalar context each time the magic is cast onto a
variable, with $_[0] being a reference to this variable and @_[1 ..
@_-1] being all extra arguments that were passed to "cast". The
scalar returned from this call is then attached to the variable and
can be retrieved later with "getdata".
· "get", "set", "len", "clear", "free", "copy", "local", "fetch",
"store", "exists" and "delete"
Code (or string) references to the respective magic callbacks. You
don't have to specify all of them : the magic corresponding to
undefined entries will simply not be hooked.
When those callbacks are executed, $_[0] is a reference to the
magic variable and $_[1] is the associated private data (or "undef"
when no private data constructor is supplied with the wizard).
Other arguments depend on which kind of magic is involved :
· len
$_[2] contains the natural, non-magical length of the
variable (which can only be a scalar or an array as len
magic is only relevant for these types). The callback is
expected to return the new scalar or array length to use,
or "undef" to default to the normal length.
· copy
When the variable for which the magic is invoked is an
array or an hash, $_[2] is a either an alias or a copy of
the current key, and $_[3] is an alias to the current
element (i.e. the value). Since $_[2] might be a copy, it
is useless to try to change it or cast magic on it.
Starting from perl 5.17.0, this magic can also be called
for code references. In this case, $_[2] is always "undef"
and $_[3] is a reference to the cloned anonymous
subroutine.
· fetch, store, exists and delete
$_[2] is an alias to the current key. Note that $_[2] may
rightfully be readonly if the key comes from a bareword,
and as such it is unsafe to assign to it. You can ask for
a copy instead by passing "copy_key => 1" to "wizard"
which, at the price of a small performance hit, allows you
to safely assign to $_[2] in order to e.g. redirect the
action to another key.
Finally, if "op_info => $num" is also passed to "wizard", then one
extra element is appended to @_. Its nature depends on the value
of $num :
· "VMG_OP_INFO_NAME"
$_[-1] is the current op name.
· "VMG_OP_INFO_OBJECT"
$_[-1] is the "B::OP" object for the current op.
Both result in a small performance hit, but just getting the name
is lighter than getting the op object.
These callbacks are always executed in scalar context. The
returned value is coerced into a signed integer, which is then
passed straight to the perl magic API. However, note that perl
currently only cares about the return value of the len magic
callback and ignores all the others. Starting with Variable::Magic
0.58, a reference returned from a non-len magic callback will not
be destroyed immediately but will be allowed to survive until the
end of the statement that triggered the magic. This lets you use
this return value as a token for triggering a destructor after the
original magic action takes place. You can see an example of this
technique in the cookbook.
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 magic is never called during global destruction, as
there is no way to ensure that the wizard object and the callback were
not destroyed before the variable.
Here is 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, @args
This function associates $wiz magic to the supplied variable, without
overwriting any other kind of magic. It returns true on success or
when $wiz magic is already attached, and croaks on error. When $wiz
provides a data constructor, it is called just before magic is cast
onto the variable, and it receives a reference to the target variable
in $_[0] and the content of @args in @_[1 .. @args]. Otherwise, @args
is ignored.
# Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
my $x;
cast $x, $wiz, 1;
The "var" argument can be an array or hash value. Magic for these
scalars behaves like for any other, 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 $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
If you want to handle the possible deletion of the 'TZ' entry, you must
also specify store magic.
"getdata"
getdata [$@%&*]var, $wiz
This accessor fetches the private data associated with the magic $wiz
in the variable. It croaks when $wiz does 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 data attached to $wiz in $x, or undef if $wiz
# did 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 if and only if the copy magic is available. This is
the case for perl 5.7.3 and greater, which is ensured by the
requirements of this module.
"MGf_DUP"
Evaluates to true if and only if the dup magic is available. This is
the case for perl 5.7.3 and greater, which is ensured by the
requirements of this module.
"MGf_LOCAL"
Evaluates to true if and only if the local magic is available. This is
the case for perl 5.9.3 and greater.
"VMG_UVAR"
When this constant is true, you can use the fetch, store, exists and
delete magics 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_SCALAR_NOLEN"
True for perls that don't call len magic on scalars. Implies
"VMG_COMPAT_SCALAR_LENGTH_NOLEN".
"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 magic when you delete an element
from a hash in void context.
"VMG_COMPAT_CODE_COPY_CLONE"
True for perls that call copy magic when a magical closure prototype is
cloned.
"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 if and only if this module could have been built with thread-
safety features enabled.
"VMG_FORKSAFE"
True if and only if this module could have been built with fork-safety
features enabled. This is always true except on Windows where it is
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";
}
$$data;
}
}
{
BEGIN { *ud = \&Magical::UserData::ud }
my $cb;
$cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
ud(&$cb) = 'world';
$cb->(); # Hello, world!
}
Recursively cast magic on datastructures
"cast" can be called from any magical callback, and in particular from
"data". This allows you to recursively cast magic on datastructures :
my $wiz;
$wiz = wizard data => sub {
my ($var, $depth) = @_;
$depth ||= 0;
my $r = ref $var;
if ($r eq 'ARRAY') {
&cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
} elsif ($r eq 'HASH') {
&cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
}
return $depth;
},
free => sub {
my ($var, $depth) = @_;
my $r = ref $var;
print "free $r at depth $depth\n";
();
};
{
my %h = (
a => [ 1, 2 ],
b => { c => 3 }
);
cast %h, $wiz;
}
When %h goes out of scope, this prints something among the lines of :
free HASH at depth 0
free HASH at depth 1
free SCALAR at depth 2
free ARRAY at depth 1
free SCALAR at depth 3
free SCALAR at depth 3
Of course, this example does nothing with the values that are added
after the "cast".
Delayed magic actions
Starting with Variable::Magic 0.58, the return value of the magic
callbacks can be used to delay the action until after the original
action takes place :
my $delayed;
my $delayed_aux = wizard(
data => sub { $_[1] },
free => sub {
my ($target) = $_[1];
my $target_data = &getdata($target, $delayed);
local $target_data->{guard} = 1;
if (ref $target eq 'SCALAR') {
my $orig = $$target;
$$target = $target_data->{mangler}->($orig);
}
return;
},
);
$delayed = wizard(
data => sub {
return +{ guard => 0, mangler => $_[1] };
},
set => sub {
return if $_[1]->{guard};
my $token;
cast $token, $delayed_aux, $_[0];
return \$token;
},
);
my $x = 1;
cast $x, $delayed => sub { $_[0] * 2 };
$x = 2;
# $x is now 4
# But note that the delayed action only takes place at the end of the
# current statement :
my @y = ($x = 5, $x);
# $x is now 10, but @y is (5, 5)
PERL MAGIC HISTORY
The places where magic is invoked have changed a bit through perl
history. Here is 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 was not 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
In order to hook hash operations with magic, you need at least perl
5.10.0 (see "VMG_UVAR").
If you want to store a magic object in the private data slot, you will
not be able to recover the magic with "getdata", since magic is not
copied by assignment. You can work around this gotcha by storing a
reference to the magic object instead.
If you define a wizard with free magic and cast it on itself, it
results in a memory cycle, so this destructor will not be called when
the wizard is freed.
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 (core since perl 5), XSLoader (since 5.6.0).
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>. 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::MagicCOPYRIGHT & LICENSE
Copyright 2007,2008,2009,2010,2011,2012,2013,2014,2015,2016,2017
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 v5.26.1 2017-11-11 Variable::Magic(3pm)
