phpman > perldoc > Variable::Magic(3pm)

NAME
    Variable::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::Magic

COPYRIGHT & 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.