"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 :

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;

"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.

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)